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Cover page - shares	9 Months Ended
Cover page - shares	Sep. 30, 2019	Nov. 04, 2019
Cover page.
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2019
Document Transition Report	false
Entity File Number	001-37798
Entity Registrant Name	Selecta Biosciences, Inc
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1622110
Entity Address, Address Line One	480 Arsenal Way
Entity Address, City or Town	Watertown
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02472
City Area Code	617
Local Phone Number	923-1400
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	SELB
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Smaller Reporting Company	true
Entity Emerging Growth Company	true
Entity Extended Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		48,196,387
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001453687
Current Fiscal Year End Date	--12-31

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash, cash equivalents, and restricted cash	$ 34,513	$ 37,403
Prepaid expenses and other current assets	1,606	4,673
Total current assets	36,119	42,076
Property and equipment, net	1,452	2,127
Right of use asset, net (Note 8)	594
Long-term restricted cash	1,379	279
Total assets	39,544	44,482
Current liabilities:
Accounts payable	1,478	1,100
Accrued expenses	5,316	11,700
Loan payable, current portion	20,927	21,385
Lease liability, current portion (Note 8)	734
Deferred revenue, current portion	1,023	959
Total current liabilities	29,478	35,144
Non‑current liabilities:
Deferred rent and lease incentive		34
Deferred revenue, net of current portion	14,981	13,818
Other long‑term liabilities	0	904
Total liabilities	44,459	49,900
Commitments and contingencies (Note 17)
Stockholders’ equity (deficit):
Preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding at September 30, 2019 and December 31, 2018, respectively	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized; 48,196,387 and 22,471,776 shares issued and outstanding as of September 30, 2019 and December 31, 2018, respectively	5	3
Additional paid-in capital	320,478	279,539
Accumulated deficit	(320,865)	(280,403)
Accumulated other comprehensive loss	(4,533)	(4,557)
Total stockholders’ equity (deficit)	(4,915)	(5,418)
Total liabilities and stockholders’ equity (deficit)	$ 39,544	$ 44,482

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	48,196,387	22,471,776
Common stock, shares outstanding	48,196,387	22,471,776

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 0	$ 0	$ 23	$ 0
Operating expenses:
Research and development	8,104	11,885	27,591	37,431
General and administrative	3,690	4,056	12,317	13,092
Total operating expenses	11,794	15,941	39,908	50,523
Loss from operations	(11,794)	(15,941)	(39,885)	(50,523)
Investment income	184	295	707	829
Foreign currency transaction (loss), net	7	26	(33)	97
Interest expense	(388)	(384)	(1,184)	(1,099)
Other (expense), net	(3)	3	(67)	11
Net Income (Loss) Available to Common Stockholders, Basic	(11,994)	(16,001)	(40,462)	(50,685)
Net loss	(11,994)	(16,001)	(40,462)	(50,685)
Other comprehensive loss:
Foreign currency translation adjustment	(5)	(42)	24	(113)
Unrealized gain on securities	(3)	1	0	16
Total comprehensive loss	$ (12,002)	$ (16,042)	$ (40,438)	$ (50,782)
Net loss per share:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.26)	$ (0.71)	$ (0.94)	$ (2.27)
Weighted average common shares outstanding:
Basic and diluted (in shares)	46,407,846	22,403,954	43,265,909	22,368,574

Consolidated Statements of Changes in Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common stock	Additional paid-In Capital	Stock option receivable	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2017		22,343,254
Beginning balance at Dec. 31, 2017	$ 51,814	$ 3	$ 273,128	$ 0	$ (216,897)	$ (4,420)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		6,586
Issuance of common stock under Employee Stock Purchase Plan	51		51
Stock‑based compensation expense	1,153		1,153
Currency translation adjustment	19					19
Unrealized gains (losses) on securities	3					3
Net loss	(15,888)				(15,888)
Ending balance (in shares) at Mar. 31, 2018		22,349,840
Ending balance at Mar. 31, 2018	38,982	$ 3	274,332	0	(230,955)	(4,398)
Beginning balance (in shares) at Dec. 31, 2017		22,343,254
Beginning balance at Dec. 31, 2017	51,814	$ 3	273,128	0	(216,897)	(4,420)
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	(113)
Unrealized gains (losses) on securities	16
Net loss	(50,685)
Ending balance (in shares) at Sep. 30, 2018		22,426,493
Ending balance at Sep. 30, 2018	7,402	$ 3	277,721	(53)	(265,752)	(4,517)
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting principle	1,830				1,830
Beginning balance (in shares) at Mar. 31, 2018		22,349,840
Beginning balance at Mar. 31, 2018	38,982	$ 3	274,332	0	(230,955)	(4,398)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)		46,343
Issuance of common stock upon exercise of options	311		311
Stock‑based compensation expense	1,245		1,245
Currency translation adjustment	(90)					(90)
Unrealized gains (losses) on securities	12					12
Net loss	(18,796)				(18,796)
Ending balance (in shares) at Jun. 30, 2018		22,396,183
Ending balance at Jun. 30, 2018	21,664	$ 3	275,888	0	(249,751)	(4,476)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		18,152
Issuance of common stock under Employee Stock Purchase Plan	145		145
Issuance of common stock upon exercise of options (in shares)		12,158
Issuance of common stock upon exercise of options	109		162	(53)
Stock‑based compensation expense	1,526		1,526
Currency translation adjustment	(42)					(42)
Unrealized gains (losses) on securities	1					1
Net loss	(16,001)				(16,001)
Ending balance (in shares) at Sep. 30, 2018		22,426,493
Ending balance at Sep. 30, 2018	$ 7,402	$ 3	277,721	$ (53)	(265,752)	(4,517)
Beginning balance (in shares) at Dec. 31, 2018	22,471,776	22,471,776
Beginning balance at Dec. 31, 2018	$ (5,418)	$ 3	279,539		(280,403)	(4,557)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		11,943
Issuance of common stock under Employee Stock Purchase Plan	20		20
Issuance of common stock upon exercise of options (in shares)		115,600
Issuance of common stock upon exercise of options	145		145
Issuance of common stock, net of issuance costs (in shares)		22,188,706
Issuance of common stock, net of issuance costs of $344	30,942	$ 2	30,940
Stock‑based compensation expense	1,180		1,180
Currency translation adjustment	22					22
Unrealized gains (losses) on securities	2					2
Net loss	(12,074)				(12,074)
Ending balance (in shares) at Mar. 31, 2019		44,788,025
Ending balance at Mar. 31, 2019	$ 14,819	$ 5	311,824		(292,477)	(4,533)
Beginning balance (in shares) at Dec. 31, 2018	22,471,776	22,471,776
Beginning balance at Dec. 31, 2018	$ (5,418)	$ 3	279,539		(280,403)	(4,557)
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	24
Unrealized gains (losses) on securities	0
Net loss	$ (40,462)
Ending balance (in shares) at Sep. 30, 2019	48,196,387	48,196,387
Ending balance at Sep. 30, 2019	$ (4,915)	$ 5	320,478		(320,865)	(4,533)
Beginning balance (in shares) at Mar. 31, 2019		44,788,025
Beginning balance at Mar. 31, 2019	14,819	$ 5	311,824		(292,477)	(4,533)
Increase (Decrease) in Stockholders' Equity
Stock‑based compensation expense	1,251		1,251
Currency translation adjustment	7					7
Unrealized gains (losses) on securities	1					1
Net loss	(16,394)				(16,394)
Issuance of common stock through at-the-market offering, net (in shares)		164,926
Issuance of common stock through at-the-market offering, net	372		372
Ending balance (in shares) at Jun. 30, 2019		44,952,951
Ending balance at Jun. 30, 2019	56	$ 5	313,447		(308,871)	(4,525)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		5,262
Issuance of common stock under Employee Stock Purchase Plan	8		8
Issuance of common stock upon exercise of options (in shares)		10,000
Issuance of common stock upon exercise of options	5		5
Stock‑based compensation expense	1,303		1,303
Currency translation adjustment	(5)					(5)
Unrealized gains (losses) on securities	(3)					(3)
Net loss	(11,994)				(11,994)
Issuance of restricted stock units (in shares)		50,000
Issuance of restricted stock units	0
Issuance of common stock through private placement, net (in shares)		3,178,174
Issuance of common stock through private placement, net of issuance costs of $37	$ 5,715		5,715
Ending balance (in shares) at Sep. 30, 2019	48,196,387	48,196,387
Ending balance at Sep. 30, 2019	$ (4,915)	$ 5	$ 320,478		$ (320,865)	$ (4,533)

Consolidated Statements of Changes in Stockholders’ Equity (Deficit) - (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2019	Mar. 31, 2019
Statement of Stockholders' Equity [Abstract]
Issuance costs	$ 37	$ 344

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (40,462)	$ (50,685)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	522	789
Amortization of premiums (accretion of discounts) on investments	(154)	(101)
(Gain) loss on disposal of property and equipment	67	(85)
Stock‑based compensation expense	3,734	3,924
Non‑cash interest expense	338	354
Net realized losses on investments	(1)	0
Changes in operating assets and liabilities:
Prepaid expenses, deposits and other assets	4,078	(2,816)
Accounts payable	378	(369)
Deferred revenue	(13)	810
Accrued expenses and other liabilities	(7,046)	1,766
Net cash used in operating activities	(38,559)	(46,413)
Cash flows from investing activities
Receipts from the maturity of short-term investments	16,350	41,655
Purchases of short-term investments	(18,188)	(15,598)
Sale of short term investments	1,992	0
Purchases of property and equipment	(10)	(516)
Proceeds from the sale of property and equipment	94	105
Net cash provided by investing activities	238	25,646
Cash flows from financing activities
Repayments of principal on outstanding debt	(700)	0
Net proceeds from issuance of common stock	30,942	0
Proceeds from exercise of stock options	150	421
Proceeds from issuance of common stock under Employee Stock Purchase Plan	28	196
Net cash provided by financing activities	36,507	617
Effect of exchange rate changes on cash	24	(113)
Net change in cash, cash equivalents, and restricted cash	(1,790)	(20,263)
Cash, cash equivalents, and restricted cash at beginning of period	37,682	71,027
Cash, cash equivalents, and restricted cash at end of period	35,892	50,764
Supplement cash flow information
Cash paid for interest	1,652	833
Noncash investing and financing activities
Purchase of property and equipment not yet paid	0	310
Unrealized gain (loss) on marketable securities	0	16
At-The-Market Offering
Cash flows from financing activities
Net proceeds from issuance of common stock	372	$ 0
Private Placement
Cash flows from financing activities
Net proceeds from issuance of common stock	$ 5,715

Nature of the Business and Basis of Presentation	9 Months Ended
Nature of the Business and Basis of Presentation	Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	Nature of the Business and Basis of Presentation Selecta Biosciences, Inc. (the “Company”) was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a clinical-stage biotechnology company focused on unlocking the full potential of biologic therapies based on its immune tolerance technology (ImmTOR™) platform. The Company plans to combine ImmTOR with a range of biologic therapies for rare and serious diseases that require new treatment options due to high immunogenicity of existing therapies. Since inception, the Company has devoted its efforts principally to research and development of its technology and product candidates, recruiting management and technical staff, acquiring operating assets, and raising capital. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. The accompanying financial statements have been prepared on a basis that assumes the Company is a going concern, and does not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classifications of liabilities that may result from any uncertainty related to its ability to continue as a going concern. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three and nine months ended September 30, 2019 and 2018 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2018 included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 15, 2019 (the “Annual Report on Form 10-K”). The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments that are necessary for a fair statement of the Company’s financial position as of September 30, 2019 and consolidated results of operations and cash flows for the three and nine months ended September 30, 2019. Such adjustments are of a normal and recurring nature. The results of operations for the three and nine months ended September 30, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019. Liquidity and Management's Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through the initial public offering of its common stock, a private placement of its common stock, issuances of common and preferred stock, debt, research grants and research collaborations. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, all of the Company's revenue has been collaboration and grant revenue. The Company has devoted substantially all of its financial resources and efforts to developing its ImmTOR technology, identifying potential product candidates and conducting preclinical studies and its clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any ImmTOR-enabled therapies. As of September 30, 2019, the Company’s cash, cash equivalents and restricted cash were $35.9 million, of which $0.4 million was held by its Russian subsidiary designated solely for use in its operations. The Company has incurred losses and negative cash flows from operating activities since inception. As of September 30, 2019 and December 31, 2018, the Company had an accumulated deficit of $320.9 million and $280.4 million, respectively. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research, development of its product candidates, conducting preclinical studies and clinical trials, and its administrative organization. The Company will require substantial additional financing to fund its operations and to continue to execute its strategy, and the Company will pursue a range of options to secure additional capital. These conditions raise substantial doubt about its ability to continue as a going concern within one year after the date that the financial statements are issued. Management is actively exploring licenses and other strategic collaborations that have the potential to provide non-dilutive capital and/or accelerate the development of new or existing product candidates incorporating the Company’s ImmTOR technology. Additionally, the Company may seek to fund its operations through issuances of equity and/or other securities. If the Company enters into strategic collaborations and alliances, which may include existing collaboration partners, the Company may have to relinquish valuable rights to its technologies or product candidates, or grant licenses on terms that are not favorable to the Company. To the extent that the Company raises additional capital through the sale of equity, the ownership interest of its existing shareholders will be diluted and other preferences may be necessary that adversely affect the rights of existing shareholders. The Company requires additional external sources of capital to complete the ongoing head-to-head Phase 2 COMPARE trial against KRYSTEXXA® and additional capital to conduct the planned Phase 3 clinical program for SEL-212. If the Company is unable to raise sufficient capital through strategic collaborations and/or the sale of equity or other securities, it intends to curtail expenses contemplated by the current operating plan, and the Company may be required to delay, limit, reduce or terminate its product development efforts or grant rights to develop and market product candidates that it would otherwise prefer to develop and market itself. Because of the uncertainty in securing additional capital and the insufficient amount of capital resources at September 30, 2019, management has concluded that substantial doubt exists with respect to the Company's ability to continue as a going concern within one year after the date of the filing of this Quarterly Report on Form 10-Q. All amounts due under the 2017 Term Loan (see Note 9) have been classified as a current liability as of September 30, 2019 due to the considerations discussed above and the assessment that the material adverse change clause under the 2017 Term Loan is not within the Company's control. The Company has not been notified of an event of default by the Lender as of the date of the filing of this Quarterly Report on Form 10-Q. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through September 30, 2019, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Principles of Consolidation

The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated.

Foreign Currency

The functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ deficit. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss.

Use of Estimates

The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, accounting for stock-based compensation and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.

Segment Information

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases.

Cash Equivalents, Short-term Investments and Restricted Cash

Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying consolidated balance sheets. Investments with less than one year until maturity are classified as short term, while investments with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.

Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the three months ended September 30, 2019, there were de minimis realized losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value.

As of September 30, 2019, the Company has restricted cash balances relating to secured letters of credit in connection with its current Headquarters Lease and New Headquarters Lease (as defined in Note 8). The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows:



	September 30,
	2019		2018
Cash and cash equivalents	$	34,234	$	50,485
Restricted cash	279		—
Long-term restricted cash	1,379		279
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	35,892	$	50,764

Concentrations of Credit Risk and Off‑Balance Sheet Risk

Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, short-term deposits and investments, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of September 30, 2019, the Company maintained approximately $0.4 million in Russian bank accounts, all of which was held in U.S. dollars.

The Company did not have any off-balance sheet arrangements as of September 30, 2019 and December 31, 2018.

Fair Value of Financial Instruments

The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, and common stock warrants. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At September 30, 2019, the carrying amount of the Company's loan payable approximates its estimated fair value due to the short-term nature of the instrument.

Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below:

Level 1—Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date.

Level 2—Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability.

Level 3—Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date.

To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of the Company's loan payable was determined using Level 2 inputs.

Fair value is a market‑based measure considered from the perspective of a market participant rather than an entity‑specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. There were no transfers within the fair value hierarchy during the nine months ended September 30, 2019 or the year ended December 31, 2018.

Property and Equipment

Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use.

Impairment of Long‑Lived Assets

The Company reviews long‑lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available, which is at the entity level ("asset group"). An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. Based on management's evaluation, the fair value of the asset group, measured as the market capitalization of the Company exceeds its carrying value, and for this reason the Company did not recognize any material impairment losses during the nine months ended September 30, 2019 and 2018.

Debt Issuance Costs

Debt issuance costs and fees paid to lenders are classified as a debt discount and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred.

Accumulated Other Comprehensive Income (Loss)

Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non‑owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses on available-for-sale securities.

The components of accumulated other comprehensive income (loss), net of tax, were as follows (in thousands):



	Foreign currency translation adjustment			Unrealized gains (losses) on available-for-sale securities		Accumulated other comprehensive income (loss)
Balance at December 31, 2018	$	(4,557	)	$	—	$	(4,557	)
Other comprehensive income (loss) during the period	$	24		$	—	$	24
Balance at September 30, 2019	$	(4,533	)	$	—	$	(4,533	)

Revenue Recognition

Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC 606, a customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Spark (see Note 12) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term.

Collaboration and Grant Revenue: The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Grants and license agreements with customers are accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, Collaborative Arrangements (ASC 808) and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). The Company early adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, which provides guidance on evaluating certain transactions between collaborative arrangement participants. If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC 730 and records reimbursements from counterparties as an offset to the related costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success.

The terms of the Company’s arrangements typically include one or more of the following: (i) up-front fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of R&D expenses; and (v) profit/loss sharing arising from co-promotion arrangements.

Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations.

Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes.

Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations.

Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint.

Research and Development Costs

Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.

Clinical Trial Costs

Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued

liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs.

Income Taxes

The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized.

The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more‑likely‑than‑not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions.

Warrants

The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity, and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock. Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or warrants that must or may require settlement by issuing variable number of shares.

If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date.

Stock‑Based Compensation

The Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest.

Net Loss Per Share

The Company has reported losses since inception and has computed basic net loss per share by dividing net loss by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same.

Contingent Liabilities

The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies. A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter. As of September 30, 2019 and

December 31, 2018, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows.

Leases

Under ASC 842 which was adopted January 1, 2019, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets, lease liabilities and, if applicable, long-term lease liabilities. The Company elected not to recognize leases with a term less than one year on its balance sheet. Operating lease right-of-use (ROU) assets and their corresponding lease liabilities are recorded based on the present value of lease payments over the expected remaining lease term. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment.

In accordance with the guidance in ASU 2016-02, components of a lease should be split into three categories: lease components (e.g. land, building, etc.), non-lease components (e.g. common area maintenance, consumables, etc.), and non-components (e.g. property taxes, insurance, etc.) Then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, the Company elected the practical expedient to not separate lease and non-lease components. The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense. See Note 8 for details.

Under prior guidance ASC 840, rent expense and lease incentives from operating leases were recognized on a straight‑line basis over the lease term. The difference between rent expense recognized and rental payments was recorded as deferred rent in the accompanying consolidated balance sheets.

Recent Accounting Pronouncements

Recently Adopted

In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. (“ASU”) 2016-02, Leases (Topic 842). ASU 2016-02 requires lessees to recognize most leases on their balance sheet as a right-of-use asset and a lease liability. Leases are classified as either operating or finance based on criteria similar to current lease accounting, with the classification affecting the pattern and classification of expense recognition in the statement of operations.

Subsequently, in July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements (ASU 2018-11), which includes certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. Among these amendments is the option to not restate comparative periods presented in the financial statements. The Company has elected this transition approach, using a cumulative-effect adjustment on the effective date of the standard, with comparative periods presented in accordance with the existing guidance in ASC 840.

Pursuant to the guidance under ASU 2016-02, the Company elected certain available expedients by electing the transition package of practical expedients permitted with ASU 2016-02, which allows the Company the option not to reassess previous accounting conclusions around, (i) whether expired or existing contracts contain leases, (ii) lease classification for any expired or existing leases, and (iii) the treatment of initial direct costs for any existing leases. The Company also made an accounting policy election to exclude leases with an initial term of 12 months or less from their balance sheet.

The Company adopted the new standard as of the required effective date of January 1, 2019 resulting in the recognition of a net additional lease liability and right-of-use asset. The standard did not impact the Company's consolidated net loss. See Note 8 for details.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (ASU 2018-18), which provides guidance on whether certain transactions between collaborative arrangement participants should be accounted for with revenue under Topic 606. The new standard is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years with early adoption permitted. The Company early adopted the new standard effective September 30, 2019, and there was no impact on its consolidated financial statements. See Note 14 for details.

Not Yet Adopted

In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments. Subsequently, in November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments-Credit Losses. ASU 2016-13 requires entities to measure all expected credit losses for most financial assets held at the reporting date based on an expected loss model which includes historical experience, current conditions, and reasonable and supportable forecasts. ASU 2016-13 also requires enhanced disclosures to help financial

statement users better understand significant estimates and judgments used in estimating credit losses. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2022, with early adoption permitted. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13) which changes the fair value measurement disclosure requirements of ASC 820. Entities will no longer be required to disclose the amount of, and reasons for, transfers between Level 1 and Level 2 of the fair value hierarchy, the policy of timing of transfers between levels of the fair value hierarchy and the valuation processes for Level 3 fair value measurements. This ASU is effective for public entities for fiscal years beginning after December 15, 2019, with early adoption permitted. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures.

In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, (ASU 2018-15). The amendments in this update align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The new standard will be effective beginning January 1, 2020 and early adoption is permitted. The Company is currently evaluating the potential impact ASU 2018-15 may have on its financial position upon adoption.

Available-for-Sale Marketable Securities	9 Months Ended
Available-for-Sale Marketable Securities	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale Marketable Securities	Available-for-Sale Marketable SecuritiesAs of September 30, 2019, and December 31, 2018, the Company did not have available-for-sale marketable securities.

Net Loss Per Share

9 Months Ended

Sep. 30, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

The Company has reported a net loss for the three and nine months ended September 30, 2019, and 2018, and for this reason basic and diluted net loss per share are the same for all periods presented. The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data):



	Three Months Ended September 30,						Nine Months Ended September 30,
	2019			2018			2019			2018
Numerator:
Net loss attributable to common stockholders	$	(11,994	)	$	(16,001	)	$	(40,462	)	$	(50,685	)
Denominator:
Weighted‑average common shares outstanding—basic and diluted	46,407,846			22,403,954			43,265,909			22,368,574
Net loss per share attributable to common stockholders —basic and diluted	$	(0.26	)	$	(0.71	)	$	(0.94	)	$	(2.27	)

All potential dilutive common shares have been excluded from the computation of the diluted net loss per share for all periods presented, as the effect would have been anti-dilutive. Potential dilutive common share equivalents consist of the following:



	September 30,
	2019	2018
Stock options to purchase common stock	5,354,645	3,188,169
Unvested restricted stock units	225,000	40,000
Stock warrants to purchase common stock	95,619	176,432
Total	5,675,264	3,404,601

Fair Value Measurements

9 Months Ended

Sep. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

The tables below present information about the Company’s financial assets that are measured and carried at fair value as of September 30, 2019 and December 31, 2018, and indicate the level within the fair value hierarchy where each measurement is

classified. Below is a summary of assets measured at fair value on a recurring basis (in thousands):



	September 30, 2019
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	6,878	$	—	$	—	$	6,878
Total cash equivalents	$	6,878	$	—	$	—	$	6,878



	December 31, 2018
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	10,123	$	—	$	—	$	10,123
Total cash equivalents	$	10,123	$	—	$	—	$	10,123

At each of September 30, 2019 and December 31, 2018, cash and cash equivalent investments were held in money market funds maturing within 90 days from the date of purchase.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment

9 Months Ended

Sep. 30, 2019

Property, Plant and Equipment [Abstract]

Property and Equipment

Property and equipment consists of the following (in thousands):



	September 30,			December 31,
	2019			2018
Laboratory equipment	$	4,909		$	5,379
Computer equipment and software	477			561
Leasehold improvements	278			278
Furniture and fixtures	235			247
Office equipment	135			135
Construction in process	2			79
Total property and equipment	6,036			6,679
Less accumulated depreciation	(4,584		)	(4,552		)
Property and equipment, net	$	1,452		$	2,127

Depreciation expense was $0.2 million and $0.2 million for the three months ended September 30, 2019 and 2018, respectively. For the nine months ended September 30, 2019 and 2018, depreciation expense was $0.5 million and $0.8 million, respectively.

Accrued Expenses

9 Months Ended

Sep. 30, 2019

Payables and Accruals [Abstract]

Accrued Expenses

Accrued expenses consist of the following (in thousands):



	September 30,		December 31,
	2019		2018
Payroll and employee related expenses	$	1,780	$	2,497
Current portion of deferred rent and lease incentive	—		117
Collaboration and licensing	—		1,222
Accrued patent fees	565		736
Accrued external research and development costs	1,760		5,344
Accrued professional and consulting services	793		994
Accrued grant refund	—		175
Accrued interest	96		106
Other	322		509
Accrued expenses	$	5,316	$	11,700

Leases

9 Months Ended

Sep. 30, 2019

Leases [Abstract]

Leases

On January 1, 2019, the Company adopted Topic 842 using the modified retrospective approach. The Company recorded operating lease assets (right-of-use assets) of $1.6 million and operating lease liabilities of $1.8 million and reversed a lease liability of $0.2 million related to straight-line rent and incentives. There was no impact to accumulated deficit upon

adoption of Topic 842. The underlying assets of the Company’s leases are primarily office space. The Company determines if an arrangement qualifies as a lease at its inception.

As a practical expedient permitted under Topic 842, the Company has elected to account for the lease and non-lease components as a single lease component for all leases of which it is the lessee. Lease payments, which may include lease and non-lease components, are included in the measurement of the Company’s lease liabilities to the extent that such payments are either fixed amounts or variable amounts that depend on a rate or index as stipulated in the lease contract.

When the Company cannot readily determine the rate implicit in the lease, the Company determines its incremental borrowing rate by using the rate of interest that it would have to pay to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. On January 1, 2019, the discount rate used on existing operating leases at adoption, which had remaining lease terms of 15 months, was 10.0%. For new or renewed leases starting in 2020, the discount rate is determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods.

The Company enters into lease agreements with terms generally ranging from 2-8 years. Some of the Company’s lease agreements include Company options to either extend and/or early terminate the lease, the costs of which are included in its operating lease liabilities to the extent that such options are reasonably certain of being exercised. Leases with renewal options allow the Company to extend the lease term typically between 1 and 5 years. When determining the lease term, renewal options reasonably certain of being exercised are included in the lease term. When determining if a renewal option is reasonably certain of being exercised, the Company considers several economic factors, including but not limited to, the significance of leasehold improvements incurred on the property, whether the asset is difficult to replace, underlying contractual obligations, or specific characteristics unique to that particular lease that would make it reasonably certain that the Company would exercise such option. Renewal and termination options were generally not included in the lease term for the Company’s existing operating leases.

The Company has a non‑cancellable operating lease for its laboratory and office space located at 480 Arsenal Way, Watertown, Massachusetts ("Headquarters Lease"). As part of the Headquarters Lease agreement, the landlord provided the Company a tenant improvement allowance of up to $0.7 million, which the Company fully utilized during 2012. The leasehold improvements are capitalized as a component of property and equipment. In connection with the Headquarters Lease, the Company secured a letter of credit for $0.3 million which renews automatically each year and is classified in restricted cash.

In August 2016, the Company signed an amendment to the Headquarters Lease, which extends the term through March 31, 2020. Leases with an initial term of 12 months or less are not recorded on the balance sheet; the Company recognizes lease expense for these leases on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of Topic 842, lease and non-lease components are combined.

In October 2017, the Company entered into a lease for approximately 5,100 square feet of additional office space located at 75 North Beacon Street, Watertown, Massachusetts (the “75 North Beacon Lease”). On January 11, 2019, the Company vacated 75 North Beacon Street, Watertown, MA and consolidated all employees at its corporate headquarters at 480 Arsenal Way, Watertown, MA. The right of use asset carrying amount of $0.2 million attributable to the 75 North Beacon Lease was written down to zero during the first quarter of 2019.

The Company has a month‑to‑month facility agreement for its Moscow, Russia office. Rent expense is recognized as incurred.

In July 2019, the Company entered into a lease for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts (the “New Headquarters Lease”). The Company estimates that it will incur $0.8 million in non-reimbursable lessee-paid construction costs for lessor assets. None of these costs were incurred as of September 30, 2019. Lease commencement is expected to occur in March 2020, and the expected lease term is 8 years, therefore the right of use asset and lease liability is not recorded as of September 30, 2019. Rent commencement is expected to occur in May 2020, and the base rent for the first year is $0.2 million per month. The total minimum rental commitments for the New Headquarters Lease are $15.8 million. In connection with the New Headquarters Lease, the Company secured a letter of credit from Silicon Valley Bank for $1.4 million which renews automatically each year.

The Company's total minimum rental commitments for the New Headquarters Lease as of September 30, 2019 are as follows (in thousands):



	September 30,
	2019
2020	$	1,191
2021	1,811
2022	1,865
2023	1,921
2024	1,979
Thereafter	7,027
Total New Headquarters Lease commitment	$	15,794

Rent expense for the three months ended September 30, 2019 and 2018 was $0.5 million and $0.5 million, respectively. Rent expense for the nine months ended September 30, 2019 and 2018 was $1.5 million and $1.5 million, respectively.

For the three and nine months ended September 30, 2019, the components of lease costs were as follows (in thousands):



	Three Months Ended September 30,		Nine Months Ended September 30,
	2019		2019
Operating lease expense	$	341	$	1,023
Variable lease expense	211		625
Short-term lease expense	3		14
Total lease expense	$	555	$	1,662

The maturity of the Company's operating lease liabilities as of September 30, 2019 were as follows (in thousands):



	September 30,
Operating leases:	2019
2019 (Remainder)	$	374
2020	375
Total future minimum lease payments	$	749
Less imputed interest	15
Total operating lease liabilities	$	734
Included in the condensed consolidated balance sheet:
Current operating lease liabilities	$	734
Non-current operating lease liabilities	—
Total operating lease liabilities	$	734

The following information represents supplemental disclosure for the statement of cash flows related to operating leases (in thousands):



	Nine Months Ended September 30,
Operating leases:	2019
Supplemental Cash Flows Information
Operating cash flows from operating leases	$	1,108

The changes in the Company’s right of use asset and lease liability for the nine months ended September 30, 2019 are reflected in the changes in prepaid expenses, deposits and other assets and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows.

The following summarizes additional information related to operating leases:



	September 30,
Operating leases:	2019
Weighted-average remaining lease term	0.5 years
Weighted-average discount rate	10	%

Debt

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

Debt

2017 Term Loan

On September 12, 2017, the Company entered into a term loan facility of up to $21.0 million (the “2017 Term Loan”) with Silicon Valley Bank, a California corporation (“SVB”). The 2017 Term Loan is governed by a loan and security agreement, dated September 12, 2017, between the Company and SVB (the “Loan Agreement”). The 2017 Term Loan was funded in full on September 13, 2017 (the “Funding Date”).

On the Funding Date, the Company entered into a payoff letter with SVB, pursuant to which SVB utilized $10.0 million of the 2017 Term Loan to pay off all outstanding obligations under the 2015 Term Loan. The Company recognized a loss on extinguishment of debt in the amount of $0.7 million during the three months ended September 30, 2017.

The Company incurred less than $0.1 million in debt issuance costs in connection with the closing of the 2017 Term Loan. Debt issuance costs are presented in the consolidated balance sheet as a direct deduction from the associated liability and amortized to interest expense over the term of the related debt.

The 2017 Term Loan will mature on February 1, 2022. Each advance under the 2017 Term Loan accrues interest at a floating per annum rate equal to one-half of one percent above the prime rate (as published in the money rates section of The Wall Street Journal). The 2017 Term Loan provided for interest-only payments monthly until August 31, 2019. On September 1, 2019, the Company began making amortization payments on the Term Loan, which will continue to be payable monthly in equal installments of principal and variable interest to fully amortize the outstanding principal over the remaining term of the loan. The monthly interest is subject to recalculation upon a change in the prime rate. The Company may prepay the 2017 Term Loan in full but not in part provided that the Company (i) provides five business days’ prior written notice to SVB, (ii) pays on the date of such prepayment for all outstanding principal plus accrued and unpaid interest, 1% if prepaid after the second anniversary.

Amounts outstanding during an event of default are payable upon SVB’s demand and shall accrue interest at an additional rate of 4.0% per annum of the past due amount outstanding. The events of default under the Loan Agreement include, but are not limited to, the Company’s failure to make any payments of principal or interest under the Loan Agreement or other transaction documents, the Company’s breach or default in the performance of any covenant under the Loan Agreement or other transaction documents, the occurrence of a material adverse effect, the Company making a false or misleading representation or warranty in any material respect under the Loan Agreement, the Company’s insolvency or bankruptcy, any attachment or judgment on the Company’s assets in excess of approximately $0.3 million, or the occurrence of any default under any agreement or obligation of the Company involving indebtedness in excess of approximately $0.3 million. If an event of default occurs, SVB is entitled to take enforcement action, including acceleration of amounts due under the Loan Agreement.

The 2017 Term Loan is secured by a lien on substantially all of the assets of the Company, other than intellectual property, provided that such lien on substantially all assets includes any rights to payments and proceeds from the sale, licensing or disposition of intellectual property. The Company has also granted SVB a negative pledge with respect to its intellectual property.

The 2017 Term Loan does not include any financial covenants. The 2017 Term Loan requires a final payment fee of 5% on the aggregate principal amounts borrowed upon repayment at maturity, on a prepayment date, or upon default. The final payment fee totaling $1.1 million is recorded as a loan discount. Under the 2017 Term Loan, the Company is not required to maintain a minimum cash balance. All deposits in operating, depository and securities accounts are required to be maintained with SVB in an amount equal to the lessor of (i) 100% of the dollar value owed or (ii) 105% of the dollar amount of the then outstanding obligations. In addition, the 2017 Term Loan contains a subjective acceleration clause whereby in an event of default, an immediate acceleration of repayment occurs if there is a material impairment of the lenders’ lien or the value of the collateral, a material adverse change in the business condition or operations, or a material uncertainty exists that any portion of the loan may not be repaid.

The Company assessed all terms and features of the 2017 Term Loan in order to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the 2017 Term Loan, including any put and call features. The Company determined that all features of the 2017 Term Loan were clearly and closely associated with the debt host and did not require bifurcation as a derivative liability, or the fair value of the embedded feature was immaterial to the Company's consolidated financial statements. The Company reassesses the identified features on a quarterly basis to determine if they require bifurcation.

As of September 30, 2019 and December 31, 2018, the outstanding principal balance under the 2017 Term Loan was $20.3 million and $21.0 million, respectively.

Future minimum principal and interest payments on the 2017 Term Loan as of September 30, 2019 are as follows (in thousands):



2019 (Remainder)	3,066
2020	9,163
2021	8,692
2022	1,752
Total minimum debt payments	$	22,673
Less: Amount representing interest	(1,323		)
Less: Debt discount and deferred charges	(423		)
Less: Current portion of loan payable	(20,927		)
Loan payable, net of current portion	$	—

All amounts due under the 2017 Term Loan have been classified as a current liability as of September 30, 2019 due to the considerations discussed in Note 1 and the assessment that the material adverse change clause under the 2017 Term Loan is not within the Company's control. The Company has not been notified of an event of default by SVB as of the date of the filing of this Quarterly Report on Form 10-Q.

During each of the three months ended September 30, 2019 and 2018, the Company recognized $0.4 million of interest expense related to the 2017 Term Loan. During the nine months ended September 30, 2019 and 2018, interest expense was $1.2 million and $1.1 million, respectively.

Stockholders' Equity

9 Months Ended

Sep. 30, 2019

Equity [Abstract]

Stockholders' Equity

Private Placement

On August 19, 2019, the Company sold 3,178,174 shares of its common stock pursuant to a stock purchase agreement (the "2019 Private Placement") to individual investors, including certain executive officers and members of the board of directors of the Company for aggregate net proceeds of approximately $5.7 million, after deducting transaction costs, at a purchase price equal to $1.81 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on August 19, 2019. The shares in the 2019 Private Placement were issued as “restricted securities” (as defined in Rule 144 of the Securities Act) and carry no registration rights that require or permit the filing of any registration statement.

2019 Public Offering

On January 25, 2019, the Company completed an underwritten public offering (the “2019 Follow-On”) of 20,000,000 shares of its common stock at a public offering price of $1.50 per share. On January 29, 2019, an additional 2,188,706 shares were sold at a public offering price of $1.50 per share pursuant to the underwriters’ exercise of an over allotment option. The total net proceeds from the offering were $30.9 million, after deducting underwriting discounts, transaction costs and commissions.

2017 Shelf Registration Statement

On August 10, 2017, the Company filed a universal shelf registration statement on Form S-3 with the SEC to sell an aggregate amount of up to $200.0 million of certain of our securities. The shelf registration statement was declared effective by the SEC on August 28, 2017.

“At-the-Market” Offering

Concurrent with the filing of the shelf registration statement, the Company entered into a sales agreement (the “Sales Agreement”) with Jefferies LLC, as sales agent, pursuant to which the Company may, from time to time, issue and sell common stock with an aggregate value of up to $50 million in an "at-the-market" offering.

Sales of common stock, if any, pursuant to the Sales Agreement, may be made in sales deemed to be an “at the market offering” as defined in Rule 415(a) of the Securities Act, including sales made directly through the Nasdaq Global Market or on any

other existing trading market for the Company’s common stock. The Company intends to use the proceeds from the offering for working capital and other general corporate purposes. The Company may suspend or terminate the Sales Agreement at any time.

During the three months ended June 30, 2019, the Company sold 164,926 shares of its common stock pursuant to the Sales Agreement for aggregate net proceeds of approximately $0.4 million, after deducting commissions and other transaction costs. There were no sales made under the Sale Agreement during three months ended September 30, 2019.

PIPE Financing

On June 26, 2017, the Company entered into a securities purchase agreement (the "Institutional Purchase Agreement") with a select group of institutional investors (the “Institutional Investors”) and a securities purchase agreement with Timothy A. Springer, Ph.D., a member of the board of directors (the "Springer Purchase Agreement") for a private placement of the Company's securities (the "2017 PIPE"). The closing of the 2017 PIPE occurred on June 27, 2017.

Pursuant to the Institutional Purchase Agreement, the Company sold an aggregate of 2,750,000 shares of its common stock at a purchase price equal to $16.00 per share. Pursuant to the Springer Purchase Agreement, the Company sold to Dr. Springer an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Global Market on June 23, 2017, and warrants to purchase up to 79,130 shares of common stock (“Warrant Shares”), exercisable at $17.71 per Warrant Share, and with a term of five years. The purchase price for each warrant was equal to $0.125 for each Warrant Share, consistent with Nasdaq Global Market requirements for an “at the market” offering. Under the terms of the Common Stock Purchase Warrant, the warrants can be settled in unregistered shares. The Warrant Shares qualify for equity classification. The fair value of the allocated proceeds was determined on the relative fair value basis.

After deducting for placement agent fees and offering expenses, the aggregate net proceeds from the 2017 PIPE were approximately $47.1 million.

On June 27, 2017, in connection with the 2017 PIPE, the Company entered into a Registration Rights Agreement (the “Registration Rights Agreement”) with the Institutional Investors and Dr. Springer. Pursuant to the Registration Rights Agreement, the Company agreed to prepare and file a registration statement with the SEC within 20 days after the closing of the 2017 PIPE for purposes of registering the resale of the shares of common stock issued and sold in the 2017 PIPE (the “Shares”), the Warrant Shares, and any shares of common stock issued as a dividend or other distribution with respect to the Shares or Warrant Shares. The 2017 PIPE registration statement was declared effective by the SEC on July 21, 2017.

The Company agreed to indemnify the Institutional Investors and Dr. Springer, their officers, directors, members, employees and agents, successors and assigns under the registration statement from certain liabilities and to pay all fees and expenses (excluding any legal fees of the selling holder(s), and any underwriting discounts and selling commissions) incident to the Company’s obligations under the Registration Rights Agreement.

Initial Public Offering

On June 21, 2016, the Company completed its IPO and issued and sold 5,000,000 shares of common stock at a price to the public of $14.00 per share for net proceeds of $60.8 million after deducting underwriting discounts and commissions and offering expenses. On July 25, 2016, 289,633 additional shares of the Company’s common stock were sold to the underwriters pursuant to the exercise of their option to purchase additional shares of common stock at a price to the public of $14.00 per share resulting in additional net proceeds of approximately $3.7 million after deducting underwriting discounts, commissions and offering expenses, bringing the total IPO net proceeds to $64.5 million. Upon the closing of the Company's IPO on June 27, 2016, all outstanding shares of its convertible preferred stock automatically converted into 10,126,118 shares of the Company’s common stock. In addition, at this time, the warrants to purchase shares of the Company’s Series D and Series E convertible preferred stock were converted into warrants to purchase shares of the Company’s common stock.

Common Stock

As of September 30, 2019, the Company had 200,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 48,196,387 shares issued and outstanding. The voting, dividend and liquidation rights of the common stockholders are subject to and qualified by the rights, powers and preferences of the preferred stock. The common stock has the following characteristics:

Voting

The common stockholders are entitled to one vote for each share of common stock held with respect to all matters voted on by the stockholders of the Company.

Dividends

The common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. Through September 30, 2019, no dividends have been declared or paid on common stock.

Liquidation

Upon liquidation of the Company, the common stockholders are entitled to receive all assets of the Company available for distribution to such stockholders.

Reserved Shares

The Company has authorized shares of common stock for future issuance as follows:



	Period ending
	September 30, 2019	December 31, 2018
Exercise of common warrants	95,619	95,619
Shares available for future stock incentive awards	3,207,042	1,586,925
Unvested restricted stock units	225,000	175,000
Outstanding common stock options	5,354,645	4,093,979
Total	8,882,306	5,951,523

Stock Incentive Plans

9 Months Ended

Sep. 30, 2019

Share-based Payment Arrangement [Abstract]

Stock Incentive Plans

Stock Options

The Company maintains the 2008 Stock Incentive Plan (the “2008 Plan”) for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non‑qualified stock option and restricted stock awards as determined by the Board. At inception of the 2008 Plan, a total of 2,213,412 shares of common stock were authorized for grants under the 2008 Plan. The Company ceased granting awards under the 2008 Plan upon the effectiveness of the 2016 Plan (as defined below); however, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan and the applicable 2008 Plan agreement. Shares subject to awards that were granted under the 2008 Plan and that expire, lapse or terminate following the effectiveness of the 2016 Plan become available under the 2016 Plan as shares available for future grants. All unvested stock options granted under the 2008 Plan may be exercised into restricted stock subject to forfeiture upon termination prior to vesting.

On June 7, 2016, the Company’s stockholders approved the 2016 Incentive Award Plan (the “2016 Plan”), which became effective June 21, 2016. The 2016 Plan provides for the granting of incentive and non‑qualified stock option, restricted stock and other stock and cash-based awards as determined by the Board. Shares subject to awards that are granted under the 2016 Plan and that expire, lapse or terminate are available for future grants under the 2016 Plan. At inception of the 2016 Plan, a total of 1,210,256 shares of common stock were authorized for future issuance under the 2016 Plan. The number of shares of common stock that may be issued under the 2016 Plan automatically increases on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Board. During the nine months ended September 30, 2019 and 2018, the number of shares of common stock that may be issued under the 2016 Plan was increased by 898,871 shares and 893,730 shares, respectively. As of September 30, 2019, 1,709,636 shares remain available for future issuance under the 2016 Plan.

The 2008 Plan and 2016 Plan provide that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the Company's common stock on the grant date for participants who own 10% or less of the total combined voting power of the Company, and not less than 110% for participants who own more than 10% of the Company’s voting power. Options and restricted stock awards granted under the 2008 Plan and 2016 Plan vest over periods as determined by the Board, which are generally four years and, for options, with terms that generally expire ten years from the grant date.

The Company’s 2018 Employment Inducement Incentive Award Plan (the “Inducement Incentive Award Plan”), which was adopted by the Board on September 25, 2018 without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules ("Rule 5635(c)(4)"), provides for the grant of equity-based awards in the form of non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and other stock or cash based awards. In accordance with Rule 5635(c)(4), awards under the Inducement Incentive Award Plan may only be made to a newly hired employee who has not previously been a member of the Board, or an employee who is being rehired following a bona fide period of non-employment by the Company, as a material inducement to the employee’s entering into employment with the Company. The Company reserved 1,175,000 shares of its common stock for issuance under the Inducement Incentive Award Plan. On March 25, 2019, the Board approved the amendment and restatement of the Inducement Incentive Award Plan to

reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. As of September 30, 2019, there are 750,000 shares available for future grant under the Inducement Incentive Award Plan.

The fair value of each option award was estimated on the grant date using the Black‑Scholes option pricing model. Expected volatilities are based on historical volatilities from guideline companies because the Company's common stock has not traded for a period that is at least equal to the expected term of its stock option awards. The Company uses the “simplified” method to estimate the expected life of options granted and are expected to be outstanding. The risk‑free interest rate used is the rate for a U.S. Treasury zero coupon issue with a remaining life consistent with the options expected life on the grant date. The Company has not paid and does not expect to pay in the foreseeable future, any cash dividends. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% based on historical attrition trends. The Company records stock‑based compensation expense only on awards that are expected to vest.

The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:



	Nine Months Ended September 30,
	2019			2018
Risk-free interest rate	2.14		%	2.82		%
Dividend yield	—			—
Expected term	6.05			6.04
Expected volatility	87.79		%	85.45		%
Weighted-average fair value of common stock	$	2.17		$	12.38

The weighted average grant date fair value of stock options granted to employees during the nine months ended September 30, 2019 and 2018 was $1.60 and $9.03, respectively. The aggregate intrinsic value of stock options exercised during the nine months ended September 30, 2019 and 2018 was $0.1 million and $0.4 million, respectively.

As of September 30, 2019, total unrecognized compensation expense related to unvested employee stock options was $9.1 million, which is expected to be recognized over a weighted average period of 2.8 years.

The estimated grant date fair values of non-employee stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:



	Nine Months Ended September 30,
	2019		2018
Risk-free interest rate	2.07	%	2.70	%
Dividend yield	—		—
Expected life (in years)	5.49		5.85
Expected volatility	88.46	%	85.95	%

The weighted average grant date fair value of stock options granted to non-employees during the nine months ended September 30, 2019 and 2018 was $1.46 and $8.71, respectively.

As of September 30, 2019, total unrecognized compensation expense related to unvested non‑employee stock options was $0.8 million, which is expected to be recognized over a weighted average period of 1.5 years.

The following table summarizes the activity under the 2008 Plan, 2016 Plan, and Inducement Incentive Award Plan:



					Weighted‑average
					remaining	Aggregate
	Number of		Weighted-average		contractual term	intrinsic value
	options		exercise price ($)		(in years)	(in thousands)
Employee Awards
Outstanding at December 31, 2018	3,681,575		$	9.49	7.77	$	300
Granted	2,389,255		$	2.17
Exercised	(125,600	)	$	1.20
Forfeited	(1,040,169	)	$	9.99
Outstanding at September 30, 2019	4,905,061		$	6.03	8.62	$	94

Vested at September 30, 2019	1,180,429		$	10.72	6.49	$	57
Vested and expected to vest at September 30, 2019	4,551,446		$	6.23	8.55	$	87

Non‑Employee Awards
Outstanding at December 31, 2018	412,404		$	6.44	6.42	$	28
Granted	50,000		$	2.04
Exercised	—		$	—
Forfeited	(12,820	)	$	0.47
Outstanding at September 30, 2019	449,584		$	6.12	6.29	$	—

Vested at September 30, 2019	266,736		$	5.72	4.37	$	—
Vested and expected to vest at September 30, 2019	449,584		$	6.12	6.29	$	—

Restricted Stock Units

During the first quarter of 2019, the Company awarded 100,000 restricted stock units under the Inducement Incentive Award Plan, of which 50,000 were determined to be granted and 50,000 were reserved for issuance consistent with ASC 718. The granted restricted stock units had a fair value of $2.29 per share based on the closing price of the Company’s common stock on the date of grant. These restricted stock units were valued at approximately $0.1 million, and will vest on the date an applicable performance condition is achieved on or prior to December 31, 2020. If the performance condition is not satisfied on or prior to December 31, 2020, the restricted stock units will be forfeited for no consideration.

The reserved 50,000 restricted stock units did not have defined performance criteria until August 6, 2019, at which time, were both granted and vested. These restricted stock units had a fair value of $1.65 per share. The restricted stock units were valued at approximately $0.1 million and fully expensed as of September 30, 2019.

Unrecognized compensation expense is $0.9 million as of September 30, 2019, which is expected to be recognized over a weighted average period of 3.0 years.

The following table summarizes the status of the Company’s restricted stock units:



	Number of Shares (#)	Weighted Average Fair Value ($)
Unvested at December 31, 2018	175,000	$	6.03
Granted	100,000	1.97
Vested	50,000	1.65
Forfeited	—	—
Unvested at September 30, 2019	225,000	$	5.20

Employee Stock Purchase Plan

On June 7, 2016, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the “ESPP”), which became effective June 21, 2016. The ESPP is intended to qualify as an "employee stock purchase plan" under Section 423 of the Internal Revenue Code of 1986 with the purpose of providing employees with an opportunity to purchase the Company's common stock through accumulated payroll deductions.

Under the ESPP, the Company has set two six-month offering periods during each calendar year, one beginning March 1^st and the other beginning September 1^st of each calendar year, during which employees may elect to have up to 25% of their eligible compensation deducted on each payday on an after-tax basis for use in purchasing the Company's common stock on the last trading day of each offering period, subject to limits imposed by the Internal Revenue Code. The purchase price of the shares may not be less than 85% of the fair market value on the first or last trading day of the offering period, whichever is lower. The first ESPP offering period began on March 1, 2017.

At inception of the ESPP, a total of 173,076 shares of common stock were authorized and reserved for future issuance under the ESPP. The number of shares of common stock that may be issued under the ESPP will automatically increase on the first day of each calendar year, beginning in 2017 and ending in and including 2026, by an amount equal to the lesser of: (i) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (ii) such smaller number of shares as is determined by the Company’s Board of Directors. During the nine months ended September 30, 2019 and 2018, the number of shares of common stock that may be issued under the ESPP was increased by 224,717 shares and 223,432 shares, respectively. During the nine months ended September 30, 2019, the Company issued 17,205 shares of common stock under the ESPP. As of September 30, 2019, 747,406 shares remain available for future issuance under the ESPP.

For each of the nine months ended September 30, 2019 and 2018, the Company recognized less than $0.1 million of stock-based compensation expense under the ESPP.

The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):



	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	504	$	709	$	1,468	$	1,842
General and administrative	799		776		2,266		2,082
Total stock-based compensation expense	$	1,303	$	1,485	$	3,734	$	3,924

Revenue Arrangements

9 Months Ended

Sep. 30, 2019

Revenue from Contract with Customer [Abstract]

Revenue Arrangements

Spark Therapeutics, Inc.

Spark License Agreement

In December 2016, the Company entered into a License and Option Agreement (“Spark License Agreement”) with Spark Therapeutics, Inc. (“Spark”) pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the ImmTOR technology. The Spark License Agreement provides Spark with certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property, allowing Spark to develop and commercialize gene therapies in combination with ImmTOR for an initial identified target.

In addition to an upfront cash payment of $10.0 million under the Spark License Agreement, additional payments of an aggregate of $5.0 million in two payments of $2.5 million each were paid within twelve months of December 2, 2016 (“Contract Date”). The first of the two additional payments was scheduled to be made on or before May 31, 2017 (the “May 2017 License Payment”) (see “Spark Letter Agreement” below) and the second was made on October 31, 2017. Spark may also exercise options to research, develop and commercialize gene therapies utilizing the ImmTOR technology for up to four additional targets. The Company is eligible to receive a variable fee up to $2.0 million for each additional target option elected, dependent on the incidence of the applicable indication. As per the agreement, the election period in which Spark can exercise additional targets is a term of three years from the Contract Date, which term expires on December 1, 2019.

Assuming successful development and commercialization, the Company could receive up to an additional $65.0 million in development and regulatory milestone payments and $365.0 million in commercialization milestone payments for each indication. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-single to low-double digits, all of which apply on a target-by-target basis. Under the terms of the agreement, the Company will be eligible to receive these royalties commencing on the first commercial sale of the licensed product and terminating upon the later of (i) ten years after the first commercial sale, (ii) expiration of the last to expire valid claim on patents covering the jointly invented field specific improvements, or (iii) the expiration of regulatory exclusivity in the applicable country for the licensed product.

The Spark License Agreement may be terminated by Spark for convenience upon ninety days’ notice. Either party may terminate the Spark License Agreement on a target-by-target basis for material breach with respect to such target.

In December 2016, the Company also entered into a Share Purchase Agreement (the “Spark Purchase Agreement”) with Spark. Pursuant to the Spark Purchase Agreement, the Company sold 197,238 shares of the Company’s common stock to Spark for gross proceeds of $5.0 million, or $25.35 per share of common stock, at an initial closing (the “Initial Closing”). The purchase price per share represents an amount equal to 115% of the average daily volume weighted average price (“VWAP”) of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Contract Date. Under the Spark Purchase Agreement, Spark agreed not to dispose of any of the Initial Closing Shares or any Acquisition Right Shares that it may acquire until January 1, 2018 and, thereafter, transfers are contractually subject to volume limitations applicable to an “affiliate” under Rule 144 of the Securities Act.

Beyond the Initial Closing, the Spark Purchase Agreement contemplated potential future sales of shares by the Company to Spark as follows:


•	First Acquisition Right. During the period beginning on May 1, 2017 and ending on June 1, 2017, Spark had the right (the “First Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million. See "Spark Letter Agreement" below.

•

Second Acquisition Right. During the period beginning on October 1, 2017 and ending on November 1, 2017, Spark had the right (the “Second Acquisition Right”) to purchase a number of shares of common stock equal to an aggregate price of $5.0 million. On October 31, 2017 Spark exercised this right and purchased 205,254 shares of common stock from the Company for $5.0 million, or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date.

The First Acquisition Rights and Second Acquisition Rights are collectively referred to herein as the “Acquisition Rights”.

In connection with the Spark License Agreement and Spark Purchase Agreement, the Company has made contractual payments defined in the MIT license agreement (see Note 14) totaling $2.2 million for the MIT sub-license provided to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement.

The terms of the Spark Purchase Agreement and the Spark License Agreement were negotiated at the same time between the parties and the terms of the Spark Purchase Agreement are referenced in the Spark License Agreement in multiple sections. The pricing and terms of the agreements are unique and must be considered in contemplation with each other. There are provisions within the Spark License Agreement that link to the Spark Purchase Agreement related to provisions that constitute a material breach of the license agreement. Therefore, the Company concluded that the two agreements must be combined and evaluated as a single agreement. While the Spark Purchase Agreement and the Spark License agreement are considered to be a single agreement, the Company determined that the purchase of common stock and future acquisition rights are not within the scope of ASC 606. The Company determined that the initial purchase of common stock combined with the embedded future stock Acquisition Rights had a fair value of $2.7 million and this amount was recorded in equity as of the effective date. The remaining $2.3 million of cash received in exchange for the stock and acquisition rights is included in allocable consideration, as this represents the premium paid by Spark on the purchase of common stock, and should be allocated to the remaining performance obligations.

The Company identified the following promises at the inception of the agreement: (1) certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property and a license to conduct certain research activities under the collaboration, (the "License Obligation"), (2) options to research, develop and commercialize gene therapies utilizing the ImmTOR technology for up to four additional target therapy options, (the "Option Obligation"), (3) manufactured supply of preclinical and clinical ImmTOR, (the "Supply Obligation") at a discount, and (4) option to purchase manufactured supply of commercial ImmTOR, (the "Commercial Supply Obligation") at fair value. In consideration for these promises, the Company received an upfront payment of $15.0 million. In addition, the Company is eligible to receive additional payments of up to $35.0 million based on the achievement by Spark of future specified development milestones, up to $30.0 million based on the achievement by Spark of future specified regulatory milestones, up to $110.0 million based on the achievement by Spark of future specified commercial milestones, and up to $255.0 million based on the achievement by Spark of future specified sales milestones. The Company will also be eligible to receive tiered royalty payments that reach low double-digits based on future net sales for the duration of the royalty term.

The Company determined that the License Obligation was not capable of being distinct from the Supply Obligation. This is because Spark cannot derive benefit from the license without the simultaneous transfer of the preclinical and clinical supply. Therefore, the License Obligation and Supply Obligation are combined as a single performance obligation (the “Combined License and Supply Obligation”). The Company also determined that the Option Obligation, which includes the related Supply Obligation, provides the customer with a material right and is considered a performance obligation in the arrangement since it was priced at an incremental discount. The Company determined that the optional Commercial Supply Obligation does not provide the customer with a material right and is not considered to be a performance obligation because Spark can derive benefit from the Combined License and Supply Obligation without the delivery of the Commercial Supply Obligation and is

not at an incremental discount. Therefore, the Company determined that the Spark agreement contains five distinct performance obligations: the Combined License and Supply Obligation, and the four separate Option Obligations.

In determining the transaction price, the Company considered the future development milestones, regulatory milestones, commercial milestones, sales milestone, and sales royalties all represent variable consideration. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company. Separately, any consideration related to sales-based milestones as well as royalties on net sales upon commercialization by Spark, will be recognized when the related sales occur as they were determined to relate predominantly to the intellectual property granted to Spark and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of September 30, 2019, all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur.

The Company determined that the up-front payment of $12.3 million ($15.0 million, less fair value of the equity totaling $2.7 million as discussed above) was included in the transaction price and was allocated to the performance obligations based on the Company’s best estimate of their relative stand-alone selling prices. The Company allocated $7.1 million to the Combined License and Supply Obligation and $5.2 million to the discount on the Option Obligation ($1.3 million for each option) using the relative standalone selling price method to each obligation. The standalone selling price for the Combined License and Supply Obligation was determined using a discounted cash flow model. The standalone selling price for the Option Obligation was determined based on the fair value of the license minus the strike price of the option (the probability of exercise was included in the valuation) as well as the estimated discount of the Supply Obligation.

The estimated proceeds to be received from the sale of the Supply Obligation was also included in the transaction price for the Combined License and Supply Obligation. The total consideration allocated to the Combined License and Supply Obligation will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term which is estimated to be approximately four years. The discount associated with the Option Obligation, along with the proceeds to be received upon exercise and estimated sale of the Supply Obligation, will be recognized when each of the options are exercised, over the related expected deliveries of its supply. If the options expire without exercise, the related deferred revenue associated with each option will be recognized upon expiration (December 1, 2019).

The Company recognized other assets of $2.6 million related to the incremental costs relating to the payments to MIT that would not have been incurred if the contract with Spark had not been obtained. Under the Company's existing license agreement with MIT (see Note 14), in the event the Company sublicenses the MIT patents to a third party, it will be required to remit to MIT a percentage (ranging from 10% to 30%) of sublicense income. The Company concluded that the payments made to MIT were analogous to sales commissions and represented the cost to obtain a contract, which were evaluated under ASC 340-40-25-1. Such amounts were capitalized as they were both incremental and recoverable. However, upon further review, the Company noted that the amounts paid to MIT represent the cost to fulfill a contract rather than a cost to obtain a contract as they represent the costs that were incurred in order to fulfill their supply obligations under the Spark License Agreement. Therefore, the incremental payments to MIT represent part of the cost to fulfill the contract. When determining the appropriate accounting guidance for the costs to fulfill a customer contract, ASC 340-40-25-6 indicates that any other applicable literature should be considered first. Since the intellectual property is being used exclusively for research and development, the accounting for the MIT costs were previously accounted for under ASC 730-10. Since all of the payments to MIT related to the underlying intellectual property, that does not have alternative future use, such amounts should not have been capitalized, and instead remained recorded as research and development expense.

The Company evaluated the impact of the error on previously issued financial statements included in the Form 10-Q for the quarters ended March 31, 2018, June 30, 2018 and September 30, 2018, noting that the impact was not material to the balance sheet, statement of operations or cash flows. However, correcting the error would be material to the quarterly trends in the statement of operations to expense the amount in the fourth quarter. Therefore, such amounts were corrected by reducing current and long-term assets by $0.2 million and $2.4 million, respectively and increasing accumulated deficit by $2.6 million as of January 1, 2018 by reversing the amounts initially recorded in transition as is shown in the first quarter 2018 Stockholders' Equity (Deficit) statement. There was no impact to the consolidated statements of operations or cash flows for any of the quarters previously filed and no impact to any of the previously issued annual financial statements.

During the nine months ended September 30, 2019, there were two deliveries resulting in less than $0.1 million of revenue recognized. No revenue related to the Spark License Agreement was recognized during the twelve months ended December 31, 2018.

As of September 30, 2019, there was a contract liability of $15.9 million representing deferred revenue associated with this agreement. A total of $1.0 million is presented as current and $14.9 million is presented as noncurrent in the accompanying consolidated balance sheet. As of December 31, 2018, there was $14.7 million of deferred revenue related to this agreement.

Spark Letter Agreement

On June 6, 2017, the Company and Spark entered into a letter agreement (the “Letter Agreement”), pursuant to which the parties agreed that Spark would make the May 2017 License Payment by June 6, 2017. The May 2017 License Payment was received, and recorded as a liability as of June 30, 2017, of which some or all may potentially constitute the reimbursement described below. The parties also agreed that Spark would be deemed to have delivered notice on May 31, 2017 exercising its right to purchase the shares pursuant to the First Acquisition Right. The Letter Agreement further outlines a cost reimbursement arrangement, pursuant to which the Company agreed to reimburse Spark for all costs and expenses, including the cost of materials provided by the Company, associated with the preclinical research and toxicology studies being performed by Spark for any licensed products for a specified amount of time (the “Reimbursement Period”), in an amount not to exceed $2.5 million.

Consistent with the First Acquisition Right, Spark purchased 324,362 shares of common stock pursuant to the Spark Purchase Agreement, as amended by the Letter Agreement, for an aggregate purchase price of $5.0 million, or $15.41 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily volume weighted average price (“VWAP”) of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the First Acquisition Right notification date. At the initial contract assessment, the Company allocated $2.7 million to equity (representing the fair value of the initial purchase of common stock combined with the embedded future stock Acquisition Rights). Upon exercise of the First Acquisition Right, the Company recorded the purchase amount to stockholders’ equity.

The Company determined that the Letter Agreement resulted in a modification to the original agreement. The amount received totaling $2.5 million and the reimbursements pursuant to the Letter Agreement totaling $2.5 million were both included in the transaction price, and a liability was recorded for the amount expected to be repaid. As repayments were made, the underlying liability was reduced. To the extent that an amount was expected to be applied towards the clinical supply obligation, the analysis of variable consideration was updated accordingly.

On October 31, 2017, Spark paid the Company a $2.5 million milestone payment pursuant to the Spark License Agreement, which was included in the transaction price and allocated to the performance obligations using the relative standalone selling price. In addition, Spark exercised the Second Acquisition Right set forth in Section 2.4 of the Spark Purchase Agreement and purchased 205,254 shares of common stock from the Company for $5.0 million, or $24.36 per share of common stock. The purchase price per share represents an amount equal to 115.0% of the average daily VWAP of the common stock during the thirty consecutive calendar days leading up to and ending on the day prior to the Second Acquisition Right notification date.

On June 5, 2019, the term of the Reimbursement Period under the Letter Agreement expired. As of September 30, 2019, the Company updated its estimate of variable consideration included in the transaction price to include $1.2 million of unpaid reimbursements to Spark. As of September 30, 2019, the transaction price totaled $18.3 million, consisting of $11.6 million allocated to the Combined License and Supply Obligation and $6.7 million allocated to the discount on the Option Obligation ($1.7 million for each option).

Skolkovo Foundation

The Company has received grant funding from the Russia-based Development Fund of New Technologies Development and Commercialization Center ("Skolkovo"). From grant inception through September 30, 2019, the Company received $2.0 million from Skolkovo.

Based on the guidance in ASC 606, the Company concluded that the entire $2.0 million of grant funds received from Skolkovo is variable consideration. Although the Company believes it has an enforceable right to the amounts received, there is risk that an audit could result in the Company needing to refund certain amounts back to Skolkovo, resulting in variability in the transaction price. The Company utilized the “expected value” approach in determining the amount that can be recognized. The Company estimated that it will be entitled to revenue of $1.8 million from the Skolkovo grant, and recorded this amount. The remainder of $0.2 million was recorded as a contract liability.

During the year ended December 31, 2018, the Company made a decision to cease work relating to the Skolkovo grant. As a result, Skolkovo performed a formal review of project expenses incurred by the Company. Skolkovo concluded that the Company should (i) return unused grant funds to Skolkovo in the amount of less than $0.1 million and (ii) reimburse $0.1 million of costs deemed to have been overspent relative to the cost share requirement stipulated in the grant.

As of September 30, 2019, a contract liability of $0.1 million remains on the balance sheet and will not be recognized as revenue until the expiration of the three-year audit period, expected April 2021, or sooner, if resolution is reached with Skolkovo or there is a change in the estimate.

Transaction Price Allocated to Future Performance Obligations

Remaining performance obligations represent the transaction price of contracts for which work has not been performed (or has been partially performed) and excludes unexercised contract options. As of September 30, 2019, the aggregate amount of the transaction price allocated to remaining performance obligations was $9.3 million. The Company expects to recognize revenue on approximately 8.8% of the remaining performance obligations over the next 12 months.

Contract Balances from Contracts with Customers

The following table presents changes in the Company’s contract liabilities during the nine months ended September 30, 2019 (in thousands):



	Balance at							Balance at
	Beginning of Period		Additions		Deductions			End of Period
Nine Months Ended September 30, 2019
Contract liabilities:
Deferred revenue	$	14,777	$	1,250	$	(23	)	$	16,004
Other liabilities	2,126		—		(2,126		)	—
Total contract liabilities	$	16,903	$	1,250	$	(2,149	)	$	16,004

Related-Party Transactions

9 Months Ended

Sep. 30, 2019

Related-Party Transactions

Related‑Party Transactions

In August 2019, the Company completed the 2019 Private Placement (as described in Note 10). The following table sets forth the number of shares of common stock purchased in the 2019 Private Placement by executive officers and members of the board of directors of the Company (and related parties thereto) and holders of more than 5% of the Company’s common stock.



Name	Shares of Common Stock Purchased	Total Purchase Price
Timothy A. Springer, Ph.D.	1,600,000	$	2,896,000.00
TAS Partners, LLC (affiliate of Timothy A. Springer, Ph.D.	1,100,000	$	1,991,000.00
Elona Kogan, J.D.	82,872	$	149,998.32
Patrick Zenner	55,248	$	99,998.88
Takashi Kei Kishimoto, Ph.D.	50,000	$	90,500.00
Carsten Brunn, Ph.D.	41,436	$	74,999.16
Scott D. Myers	41,436	$	74,999.16
Stephen Smolinski	13,812	$	24,999.72

In January 2019, the Company completed the 2019 Follow-On (as described in Note 10). The following table sets forth the number of shares of common stock purchased in the 2019 Follow-On by members of the board of directors of the Company (and related parties thereto) and holders of more than 5% of the Company’s common stock:



Name	Shares of Common Stock Purchased	Total Purchase Price
Timothy A. Springer, Ph.D.	4,000,000	$	6,000,000.00
Entities affiliated with NanoDimension	1,666,666	$	2,499,999.00
Entities affiliated with OrbiMed Advisors	1,333,333	$	1,999,999.50
Entities affiliated with Polaris	666,666	$	999,999.00
SAF-BND Trust (affiliate of Omid Farokhzad, M.D.)	83,333	$	124,999.50
Chafen Lu (Timothy A. Springer’s wife)	66,666	$	99,999.00
Jed Springer (Timothy A. Springer’s brother)	1,000	$	1,500.00

During the fourth quarter of 2018, the Company entered into an amended consulting agreement with Dr. Omid Farokhzad, a member of its Board of Directors. The term of the amendment to the Consulting Agreement is April 1, 2018 to December 31,

2019, which extends the original consulting term for an additional nine months from March 31, 2019. Compensation for the Amendment includes a $85,000 payment for the period beginning January 1, 2019 and ending December 31, 2019. The $85,000 will be paid quarterly across the contract term in arrears beginning March 31, 2019. Included within this agreement, a stock option award of 75,000 shares was granted, with a weighted average grant date fair value of $4.35.

The Company incurred expenses for consulting services provided by its founders during the three months ended September 30, 2019 and 2018 totaling $0.1 million and $0.1 million, respectively, and $0.4 million and $0.3 million during the nine months ended September 30, 2019 and 2018, respectively.

Collaboration and License Agreements	9 Months Ended
Collaboration and License Agreements	Sep. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Collaboration and License Agreements	Collaboration and License Agreements Asklepios Biopharmaceutical, Inc. On August 6, 2019, the Company entered into a Feasibility Study and License Agreement with Asklepios Biopharmaceutical, Inc. (“AskBio”), which is referred to as the AskBio License. Pursuant to the AskBio License, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain adeno-associated virus (“AAV”) gene therapy products utilizing the Company’s ImmTOR technology to enable re-dosing of such AAV gene therapy products to treat serious rare and orphan genetic diseases for which there is a significant unmet medical need. Pursuant to the AskBio License, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AAV for the treatment of methylmalonic acidemia (“MMA”), based on the Company’s product candidate SEL-302, to mitigate the formation of neutralizing anti-AAV capsid antibodies (the “POC Studies”). If the POC Studies are successful, or the parties otherwise elect to do so, the parties will proceed with a collaboration to pursue the development and commercialization of AAV gene therapy product candidates utilizing ImmTOR for the treatment of certain agreed serious rare and orphan genetic diseases. If the POC Studies fail to demonstrate a proof of concept, and the parties do not mutually agree in writing to proceed with the collaboration, the AskBio License will expire. The Company and AskBio will share responsibility for the research, development and commercialization of products developed under this collaboration. The parties will also share research, development and commercialization costs equally for all collaboration products, but with a right of either party to opt out of certain products, and thereby no longer be required to share costs for such products. Each party will receive a percentage of net profits for each product sold under the collaboration equal to the percentage of shared costs borne by such party in the development of such product. Pursuant to the AskBio License, AskBio is responsible for manufacturing the AAV capsids and AAV vectors and the Company is responsible for manufacturing ImmTOR. The AskBio License is considered to be within the scope of ASC 808, as both parties are active participants and exposed to the risks and rewards of the collaborative activity. The Company evaluated the terms of the AskBio License and have identified the following promises in the arrangement (1) conducting research and development activities to develop and commercialize products under the collaboration, (the “R&D Services”), (2) granting a non-exclusive, non-transferable, royalty-free, fully paid up, worldwide license to certain intellectual property of the Company, (the “IP Rights”) for the purpose of performing the POC Studies, (the “Research License”), (3) granting an exclusive, nontransferable, worldwide license to the IP Rights for use in certain indications (the” Collaboration License”), (4) providing manufactured supply of preclinical and clinical ImmTOR, (the “Manufactured Supply”), (5) participation on identified steering committees responsible for the oversight of the collaboration, (the “JSC Participation”), and (6) granting an exclusive option to obtain a license under the IP Rights to research, develop and commercialize Licensed Products. The Company determined that the R&D Services, Research License, Collaboration License, Manufactured Supply, and JSC Participation were not capable of being distinct, and therefore must be combined into a single performance obligation. Therefore, promises (1) through (5) identified above were combined into a single performance obligation. Furthermore, the Company evaluated the Option Agreement and determined that it does not provide AskBio with a material right under ASC 606 as the option was not priced at a discount. The Company noted that AskBio did not meet the definition of a customer within the scope of ASC 606 for any distinct performance obligations as the Company concluded that such items were not an output of the Company’s ordinary activities. As such, the Company determined that the entire arrangement would be accounted for within the scope of ASC 808. In accordance with ASC 808, collaboration expenses are recognized within R&D expense and selling, general and administrative expense on our condensed consolidated statements of operations. For the nine months ended September 30, 2019, the Company did not recognize any collaboration expense under the AskBio License. Under certain collaborative arrangements, the Company is entitled to reimbursement of certain R&D expense. Activities under collaborative arrangements for which the Company is entitled to reimbursement are considered to be collaborative activities under the scope of ASC 808. For these units of account, the Company does not analogize to ASC 606 or recognize revenue. Rather, the Company analogizes to the guidance in ASC 730, which requires that reimbursements from counterparties be recognized as an offset to the related costs. In accordance with ASC 730, the Company records reimbursement payments received from collaboration partners as reductions to R&D expense. Massachusetts Institute of Technology On November 25, 2008, the Company entered into an Exclusive Patent License agreement with the Massachusetts Institute of Technology (“MIT”), which is referred to as the Exclusive Patent License. The Company received an exclusive royalty‑bearing license to utilize patents held by MIT in exchange for upfront consideration and annual license maintenance fees. Such fees are expensed as incurred and have not been material to any period presented. As of September 30, 2019, and in connection with the execution of the Spark License Agreement, the Company has made contractual payments pursuant to the Exclusive Patent License totaling $2.2 million for the sublicense granted to Spark, and $0.4 million relative to the calculated premium paid by Spark for the equity investments made under the Spark Purchase Agreement. The Company made no additional payments during the nine months ended September 30, 2019. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement with Shenyang Sunshine Pharmaceutical Co., Ltd. (“3SBio”), which is referred to as the 3SBio License. The Company has paid to 3SBio an aggregate of $3.0 million in upfront and milestone-based payments under the 3SBio License. Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. In May 2016, the Company entered into a license agreement with the Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (collectively, “MEE”), which is referred to as the MEE License. On September 16, 2019, in accordance with the terms of the MEE License, the Company notified MEE of its intention to terminate the MEE License, effective December 15, 2019. Through September 30, 2019, the Company paid a total of $0.4 million in license fees due under the MEE License. License fees less than $0.1 million are accrued under the MEE License as of September 30, 2019.

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company provides for income taxes under ASC 740. Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. For the three and nine months ended September 30, 2019 and 2018, the Company did not record a current or deferred income tax expense or benefit. The Company has provided a full valuation allowance against its net deferred tax assets, as the Company believes that it is more likely than not that the deferred tax assets will not be realized. During the third quarter of 2019, the Company completed a Section 382 study, noting that an ownership change has not occurred since November 2011, therefore, the deferred tax assets related to the federal and Massachusetts net operating losses and credit carryforwards are not currently limited. However, utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 and 383 of the Internal Revenue Code due to ownership change limitations that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company applies ASC 740 to uncertain tax positions. As of the adoption date of January 1, 2010 and through September 30, 2019, the Company had no unrecognized tax benefits or related interest and penalties accrued. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statement of operations. The Company has not, as of yet, conducted a study of its research and development credit carryforwards. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed, and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. As a result, there would be no impact to the consolidated balance sheets, statements of operations and comprehensive loss, or cash flows if an adjustment was required. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities is open for tax years since inception. The Company files income tax returns in the United States and Massachusetts. There are currently no federal, state or foreign audits in progress. Upon adoption of ASC 842, a deferred tax liability was recorded for the right-of-use asset. The deferred tax asset for the lease liability and the deferred tax asset for the lease incentives was reversed, with no impact to the valuation allowance or deferred tax expense.

Defined Contribution Plan	9 Months Ended
Defined Contribution Plan	Sep. 30, 2019
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution Plan The Company maintains a defined contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) Plan”). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. All matching contributions vest ratably over 4 years and participant contributions vest immediately. Contributions by the Company totaled less than $0.1 million during each of the three months ended September 30, 2019 and 2018, and $0.1 million and $0.2 million during each of the nine months ended September 30, 2019 and 2018, respectively.

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of September 30, 2019, the Company had operating lease agreements for offices in Watertown, MA. See Note 8 for additional information regarding the Company's leases. Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non‑performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect the Company's business, financial position, results of operations or cash flows.

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2019
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta RUS, LLC (“Selecta (RUS)”), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts Security Corporation. All significant intercompany accounts and transactions have been eliminated.
Foreign Currency	Foreign Currency The functional currency of Selecta (RUS) is the Russian ruble. Assets and liabilities of Selecta (RUS) are translated at period-end exchange rates, while revenues and expenses are translated at average exchange rates for the period. Translation gains and losses are reflected in accumulated other comprehensive loss within stockholders’ deficit. Foreign currency transaction gains or losses are reflected in the consolidated statements of operations and comprehensive loss.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: revenue recognition, accounting for stock-based compensation and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Segment Information	Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, the Company’s Chief Executive Officer, in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment, the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases.
Cash Equivalents, Short-term Investments and Restricted Cash	Cash Equivalents, Short-term Investments and Restricted Cash Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying consolidated balance sheets. Investments with less than one year until maturity are classified as short term, while investments with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the three months ended September 30, 2019, there were de minimis realized losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value.
Concentrations of Credit Risk and Off-Balance Sheet Risk	Concentrations of Credit Risk and Off‑Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, short-term deposits and investments, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and therefore bear minimal interest rate risk. As an integral part of operating its Russian subsidiary, the Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, and common stock warrants. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. At September 30, 2019, the carrying amount of the Company's loan payable approximates its estimated fair value due to the short-term nature of the instrument. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three‑level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1—Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2—Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3—Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of the Company's loan payable was determined using Level 2 inputs. Fair value is a market‑based measure considered from the perspective of a market participant rather than an entity‑specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated using the straight‑line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use.
Impairment of Long-Lived Assets	Impairment of Long‑Lived Assets The Company reviews long‑lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available, which is at the entity level ("asset group"). An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group.
Debt Issuance Costs	Debt Issuance Costs Debt issuance costs and fees paid to lenders are classified as a debt discount and are recorded as a direct deduction from the face amount of the related debt. Issuance costs paid to third parties that are the direct result of the debt issuance are capitalized as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the interest method and recorded as interest expense. Costs and fees paid to third parties are expensed as incurred.
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non‑owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net loss and (ii) all components of comprehensive loss other than net loss, referred to as other comprehensive loss. Other comprehensive loss is comprised of foreign currency translation adjustments and the unrealized gains and losses on available-for-sale securities.
Revenue Recognition	Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC 606, a customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract, including whether they are distinct in the context of the contract; (iii) determine the transaction price, including the constraint on variable consideration; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. If a promised good or service is not distinct, it is combined with other performance obligations. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For example, certain performance obligations associated with Spark (see Note 12) will be satisfied over time, and revenue will be recognized using the output method, based on the proportion of actual deliveries to the total expected deliveries over the initial term. Collaboration and Grant Revenue: The Company currently generates its revenue through grants, collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Grants and license agreements with customers are accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, Collaborative Arrangements (ASC 808) and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). The Company early adopted ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, which provides guidance on evaluating certain transactions between collaborative arrangement participants. If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC 730 and records reimbursements from counterparties as an offset to the related costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The terms of the Company’s arrangements typically include one or more of the following: (i) up-front fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of R&D expenses; and (v) profit/loss sharing arising from co-promotion arrangements. Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other performance obligations in the contract. For licenses that are combined with other performance obligations, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint.
Research and Development Costs	Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.
Clinical Trial Costs	Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancelable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs.
Income Taxes	Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more‑likely‑than‑not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions.
Warrants	Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity, and then in accordance with ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock. Under ASC 480, warrants are considered liability classified if the warrants are mandatorily redeemable, obligate the issuer to settle the warrants or the underlying shares by paying cash or other assets, or warrants that must or may require settlement by issuing variable number of shares. If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date.
Stock-Based Compensation	Stock‑Based Compensation The Company accounts for all stock‑based compensation granted to employees and non‑employees using a fair value method. Stock‑based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight‑line basis, net of estimated forfeitures. The Company reduces recorded stock‑based compensation for estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock‑based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest.
Net Loss Per Share	Net Loss Per Share The Company has reported losses since inception and has computed basic net loss per share by dividing net loss by the weighted average number of common shares outstanding for the period. The Company has computed diluted net loss per common share after considering all potentially dilutive common shares, including stock options, convertible preferred stock, and warrants outstanding during the period except where the effect of including such securities would be antidilutive. Because the Company has reported net losses since inception, these potential common shares have been anti‑dilutive and basic and diluted loss per share have been the same.
Contingent Liabilities	Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies. A provision is recorded when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. With respect to legal matters, provisions are reviewed and adjusted to reflect the impact of negotiations, estimated settlements, legal rulings, advice of legal counsel and other information and events pertaining to a particular matter.
Leases	Leases Under ASC 842 which was adopted January 1, 2019, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets, lease liabilities and, if applicable, long-term lease liabilities. The Company elected not to recognize leases with a term less than one year on its balance sheet. Operating lease right-of-use (ROU) assets and their corresponding lease liabilities are recorded based on the present value of lease payments over the expected remaining lease term. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term, an amount equal to the lease payments in a similar economic environment. In accordance with the guidance in ASU 2016-02, components of a lease should be split into three categories: lease components (e.g. land, building, etc.), non-lease components (e.g. common area maintenance, consumables, etc.), and non-components (e.g. property taxes, insurance, etc.) Then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, the Company elected the practical expedient to not separate lease and non-lease components. The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update No. (“ASU”) 2016-02, Leases (Topic 842). ASU 2016-02 requires lessees to recognize most leases on their balance sheet as a right-of-use asset and a lease liability. Leases are classified as either operating or finance based on criteria similar to current lease accounting, with the classification affecting the pattern and classification of expense recognition in the statement of operations. Subsequently, in July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements (ASU 2018-11), which includes certain amendments to ASU 2016-02 intended to provide relief in implementing the new standard. Among these amendments is the option to not restate comparative periods presented in the financial statements. The Company has elected this transition approach, using a cumulative-effect adjustment on the effective date of the standard, with comparative periods presented in accordance with the existing guidance in ASC 840. Pursuant to the guidance under ASU 2016-02, the Company elected certain available expedients by electing the transition package of practical expedients permitted with ASU 2016-02, which allows the Company the option not to reassess previous accounting conclusions around, (i) whether expired or existing contracts contain leases, (ii) lease classification for any expired or existing leases, and (iii) the treatment of initial direct costs for any existing leases. The Company also made an accounting policy election to exclude leases with an initial term of 12 months or less from their balance sheet. The Company adopted the new standard as of the required effective date of January 1, 2019 resulting in the recognition of a net additional lease liability and right-of-use asset. The standard did not impact the Company's consolidated net loss. See Note 8 for details. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (ASU 2018-18), which provides guidance on whether certain transactions between collaborative arrangement participants should be accounted for with revenue under Topic 606. The new standard is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years with early adoption permitted. The Company early adopted the new standard effective September 30, 2019, and there was no impact on its consolidated financial statements. See Note 14 for details. Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments. Subsequently, in November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments-Credit Losses. ASU 2016-13 requires entities to measure all expected credit losses for most financial assets held at the reporting date based on an expected loss model which includes historical experience, current conditions, and reasonable and supportable forecasts. ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2022, with early adoption permitted. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13) which changes the fair value measurement disclosure requirements of ASC 820. Entities will no longer be required to disclose the amount of, and reasons for, transfers between Level 1 and Level 2 of the fair value hierarchy, the policy of timing of transfers between levels of the fair value hierarchy and the valuation processes for Level 3 fair value measurements. This ASU is effective for public entities for fiscal years beginning after December 15, 2019, with early adoption permitted. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, (ASU 2018-15). The amendments in this update align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The new standard will be effective beginning January 1, 2020 and early adoption is permitted. The Company is currently evaluating the potential impact ASU 2018-15 may have on its financial position upon adoption.

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Schedule of Cash and Cash Equivalents

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the consolidated statement of cash flows:



	September 30,
	2019		2018
Cash and cash equivalents	$	34,234	$	50,485
Restricted cash	279		—
Long-term restricted cash	1,379		279
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	35,892	$	50,764

Schedule of Restricted Cash and Cash Equivalents



	September 30,
	2019		2018
Cash and cash equivalents	$	34,234	$	50,485
Restricted cash	279		—
Long-term restricted cash	1,379		279
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	35,892	$	50,764

Components of Accumulated Other Comprehensive Income (Loss), Net of Tax

The components of accumulated other comprehensive income (loss), net of tax, were as follows (in thousands):



	Foreign currency translation adjustment			Unrealized gains (losses) on available-for-sale securities		Accumulated other comprehensive income (loss)
Balance at December 31, 2018	$	(4,557	)	$	—	$	(4,557	)
Other comprehensive income (loss) during the period	$	24		$	—	$	24
Balance at September 30, 2019	$	(4,533	)	$	—	$	(4,533	)

Net Loss Per Share (Tables)

9 Months Ended

Sep. 30, 2019

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss Per Share

The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per‑share data):



	Three Months Ended September 30,						Nine Months Ended September 30,
	2019			2018			2019			2018
Numerator:
Net loss attributable to common stockholders	$	(11,994	)	$	(16,001	)	$	(40,462	)	$	(50,685	)
Denominator:
Weighted‑average common shares outstanding—basic and diluted	46,407,846			22,403,954			43,265,909			22,368,574
Net loss per share attributable to common stockholders —basic and diluted	$	(0.26	)	$	(0.71	)	$	(0.94	)	$	(2.27	)

Schedule of Potential Common Shares Issuable Upon Conversion of Warrants

Potential dilutive common share equivalents consist of the following:



	September 30,
	2019	2018
Stock options to purchase common stock	5,354,645	3,188,169
Unvested restricted stock units	225,000	40,000
Stock warrants to purchase common stock	95,619	176,432
Total	5,675,264	3,404,601

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2019

Fair Value Disclosures [Abstract]

Summary of Assets Measured at Fair Value on a Recurring Basis

Below is a summary of assets measured at fair value on a recurring basis (in thousands):



	September 30, 2019
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	6,878	$	—	$	—	$	6,878
Total cash equivalents	$	6,878	$	—	$	—	$	6,878



	December 31, 2018
	(Level 1)		(Level 2)		(Level 3)		Total
Cash equivalents:
Money market funds	$	10,123	$	—	$	—	$	10,123
Total cash equivalents	$	10,123	$	—	$	—	$	10,123

Property and Equipment (Tables)

9 Months Ended

Sep. 30, 2019

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consists of the following (in thousands):



	September 30,			December 31,
	2019			2018
Laboratory equipment	$	4,909		$	5,379
Computer equipment and software	477			561
Leasehold improvements	278			278
Furniture and fixtures	235			247
Office equipment	135			135
Construction in process	2			79
Total property and equipment	6,036			6,679
Less accumulated depreciation	(4,584		)	(4,552		)
Property and equipment, net	$	1,452		$	2,127

Accrued Expenses (Tables)

9 Months Ended

Sep. 30, 2019

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):



	September 30,		December 31,
	2019		2018
Payroll and employee related expenses	$	1,780	$	2,497
Current portion of deferred rent and lease incentive	—		117
Collaboration and licensing	—		1,222
Accrued patent fees	565		736
Accrued external research and development costs	1,760		5,344
Accrued professional and consulting services	793		994
Accrued grant refund	—		175
Accrued interest	96		106
Other	322		509
Accrued expenses	$	5,316	$	11,700

Leases (Tables)

9 Months Ended

Sep. 30, 2019

Leases [Abstract]

Schedule of Minimum Rental Commitments

The Company's total minimum rental commitments for the New Headquarters Lease as of September 30, 2019 are as follows (in thousands):



	September 30,
	2019
2020	$	1,191
2021	1,811
2022	1,865
2023	1,921
2024	1,979
Thereafter	7,027
Total New Headquarters Lease commitment	$	15,794

Schedule of Lease, Cost

The following information represents supplemental disclosure for the statement of cash flows related to operating leases (in thousands):



	Nine Months Ended September 30,
Operating leases:	2019
Supplemental Cash Flows Information
Operating cash flows from operating leases	$	1,108

The following summarizes additional information related to operating leases:



	September 30,
Operating leases:	2019
Weighted-average remaining lease term	0.5 years
Weighted-average discount rate	10	%

For the three and nine months ended September 30, 2019, the components of lease costs were as follows (in thousands):



	Three Months Ended September 30,		Nine Months Ended September 30,
	2019		2019
Operating lease expense	$	341	$	1,023
Variable lease expense	211		625
Short-term lease expense	3		14
Total lease expense	$	555	$	1,662

Schedule of Lessee, Operating Lease, Liability, Maturity

The maturity of the Company's operating lease liabilities as of September 30, 2019 were as follows (in thousands):



	September 30,
Operating leases:	2019
2019 (Remainder)	$	374
2020	375
Total future minimum lease payments	$	749
Less imputed interest	15
Total operating lease liabilities	$	734
Included in the condensed consolidated balance sheet:
Current operating lease liabilities	$	734
Non-current operating lease liabilities	—
Total operating lease liabilities	$	734

Debt (Tables)

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

Schedule of Future Minimum Payments on the Term Loans

Future minimum principal and interest payments on the 2017 Term Loan as of September 30, 2019 are as follows (in thousands):



2019 (Remainder)	3,066
2020	9,163
2021	8,692
2022	1,752
Total minimum debt payments	$	22,673
Less: Amount representing interest	(1,323		)
Less: Debt discount and deferred charges	(423		)
Less: Current portion of loan payable	(20,927		)
Loan payable, net of current portion	$	—

Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2019

Equity [Abstract]

Schedule of Authorized Shares of Common Stock for Future Issuance

The Company has authorized shares of common stock for future issuance as follows:



	Period ending
	September 30, 2019	December 31, 2018
Exercise of common warrants	95,619	95,619
Shares available for future stock incentive awards	3,207,042	1,586,925
Unvested restricted stock units	225,000	175,000
Outstanding common stock options	5,354,645	4,093,979
Total	8,882,306	5,951,523

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 505
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-Paragraph 2
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 505
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-Section 50
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 235
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-Subparagraph (SX 210.4-08(d))
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Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 14: http://fasb.org/us-gaap/role/ref/legacyRef
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+ Details

Stock Incentive Plans (Tables)

9 Months Ended

Sep. 30, 2019

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of Weighted Average Assumptions Used



	Nine Months Ended September 30,
	2019			2018
Risk-free interest rate	2.14		%	2.82		%
Dividend yield	—			—
Expected term	6.05			6.04
Expected volatility	87.79		%	85.45		%
Weighted-average fair value of common stock	$	2.17		$	12.38

Summary of Activity Under the 2008 Plan and the 2016 Plan

The following table summarizes the activity under the 2008 Plan, 2016 Plan, and Inducement Incentive Award Plan:



					Weighted‑average
					remaining	Aggregate
	Number of		Weighted-average		contractual term	intrinsic value
	options		exercise price ($)		(in years)	(in thousands)
Employee Awards
Outstanding at December 31, 2018	3,681,575		$	9.49	7.77	$	300
Granted	2,389,255		$	2.17
Exercised	(125,600	)	$	1.20
Forfeited	(1,040,169	)	$	9.99
Outstanding at September 30, 2019	4,905,061		$	6.03	8.62	$	94

Vested at September 30, 2019	1,180,429		$	10.72	6.49	$	57
Vested and expected to vest at September 30, 2019	4,551,446		$	6.23	8.55	$	87

Non‑Employee Awards
Outstanding at December 31, 2018	412,404		$	6.44	6.42	$	28
Granted	50,000		$	2.04
Exercised	—		$	—
Forfeited	(12,820	)	$	0.47
Outstanding at September 30, 2019	449,584		$	6.12	6.29	$	—

Vested at September 30, 2019	266,736		$	5.72	4.37	$	—
Vested and expected to vest at September 30, 2019	449,584		$	6.12	6.29	$	—

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award

The following table summarizes the status of the Company’s restricted stock units:



	Number of Shares (#)	Weighted Average Fair Value ($)
Unvested at December 31, 2018	175,000	$	6.03
Granted	100,000	1.97
Vested	50,000	1.65
Forfeited	—	—
Unvested at September 30, 2019	225,000	$	5.20

Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock



	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	504	$	709	$	1,468	$	1,842
General and administrative	799		776		2,266		2,082
Total stock-based compensation expense	$	1,303	$	1,485	$	3,734	$	3,924

Non-employee stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of Weighted Average Assumptions Used



	Nine Months Ended September 30,
	2019		2018
Risk-free interest rate	2.07	%	2.70	%
Dividend yield	—		—
Expected life (in years)	5.49		5.85
Expected volatility	88.46	%	85.95	%

Revenue Arrangements (Tables)

9 Months Ended

Sep. 30, 2019

Revenue from Contract with Customer [Abstract]

Schedule of Changes in Contract Liabilities

The following table presents changes in the Company’s contract liabilities during the nine months ended September 30, 2019 (in thousands):



	Balance at							Balance at
	Beginning of Period		Additions		Deductions			End of Period
Nine Months Ended September 30, 2019
Contract liabilities:
Deferred revenue	$	14,777	$	1,250	$	(23	)	$	16,004
Other liabilities	2,126		—		(2,126		)	—
Total contract liabilities	$	16,903	$	1,250	$	(2,149	)	$	16,004

Related-Party Transactions - (Tables)

9 Months Ended

Sep. 30, 2019

Schedule of Related Party Transactions

The following table sets forth the number of shares of common stock purchased in the 2019 Follow-On by members of the board of directors of the Company (and related parties thereto) and holders of more than 5% of the Company’s common stock:



Name	Shares of Common Stock Purchased	Total Purchase Price
Timothy A. Springer, Ph.D.	4,000,000	$	6,000,000.00
Entities affiliated with NanoDimension	1,666,666	$	2,499,999.00
Entities affiliated with OrbiMed Advisors	1,333,333	$	1,999,999.50
Entities affiliated with Polaris	666,666	$	999,999.00
SAF-BND Trust (affiliate of Omid Farokhzad, M.D.)	83,333	$	124,999.50
Chafen Lu (Timothy A. Springer’s wife)	66,666	$	99,999.00
Jed Springer (Timothy A. Springer’s brother)	1,000	$	1,500.00

Private Placement

Schedule of Related Party Transactions

The following table sets forth the number of shares of common stock purchased in the 2019 Private Placement by executive officers and members of the board of directors of the Company (and related parties thereto) and holders of more than 5% of the Company’s common stock.



Name	Shares of Common Stock Purchased	Total Purchase Price
Timothy A. Springer, Ph.D.	1,600,000	$	2,896,000.00
TAS Partners, LLC (affiliate of Timothy A. Springer, Ph.D.	1,100,000	$	1,991,000.00
Elona Kogan, J.D.	82,872	$	149,998.32
Patrick Zenner	55,248	$	99,998.88
Takashi Kei Kishimoto, Ph.D.	50,000	$	90,500.00
Carsten Brunn, Ph.D.	41,436	$	74,999.16
Scott D. Myers	41,436	$	74,999.16
Stephen Smolinski	13,812	$	24,999.72

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ 320,865	$ 280,403
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	35,892	$ 37,682	$ 50,764	$ 71,027
Russian subsidiary \| Unrestricted cash
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, and restricted cash	$ 400

Summary of Significant Accounting Policies - Segment Information (Details)	9 Months Ended
	Sep. 30, 2019 segment
Accounting Policies [Abstract]
Number of operating segments	1

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 34,234		$ 50,485
Restricted cash	279		0
Long-term restricted cash	1,379		279
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$ 35,892	$ 37,682	$ 50,764	$ 71,027

Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details) $ in Millions	Sep. 30, 2019 USD ($)
Russian subsidiary
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 0.4

Summary of Significant Accounting Policies - Property and Equipment (Details)	9 Months Ended
	Sep. 30, 2019
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Laboratory equipment, software and office equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years

Summary of Significant Accounting Policies - Impairment of Long‑Lived Assets (Details) - USD ($)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Impairment of intangible assets, finite-lived	$ 0	$ 0

Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Income (Loss), Net of Tax (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
AOCI Attributable to Parent, Net of Tax
Beginning balance	$ (5,418)
Ending balance	(4,915)
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,557)
Other comprehensive income (loss) during the period	24
Ending balance	(4,533)
Unrealized gains (losses) on available-for-sale securities
AOCI Attributable to Parent, Net of Tax
Beginning balance	0
Other comprehensive income (loss) during the period	0
Ending balance	0
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance	(4,557)
Other comprehensive income (loss) during the period	24
Ending balance	$ (4,533)

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss attributable to common stockholders	$ (11,994)	$ (16,001)	$ (40,462)	$ (50,685)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)	46,407,846	22,403,954	43,265,909	22,368,574
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.26)	$ (0.71)	$ (0.94)	$ (2.27)

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Potential common shares
Total (in shares)	5,675,264	3,404,601
Stock options to purchase common stock
Potential common shares
Total (in shares)	5,354,645	3,188,169
Unvested restricted stock units
Potential common shares
Total (in shares)	225,000	40,000
Stock warrants to purchase common stock
Potential common shares
Total (in shares)	95,619	176,432

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 6,878	$ 10,123
(Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	6,878	10,123
(Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
(Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	6,878	10,123
Money market funds \| (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	6,878	10,123
Money market funds \| (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	0	0
Money market funds \| (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 0	$ 0

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 6,036	$ 6,679
Less accumulated depreciation	(4,584)	(4,552)
Property and equipment, net	1,452	2,127
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	4,909	5,379
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	477	561
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	278	278
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	235	247
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	135	135
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 2	$ 79

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 200	$ 200	$ 522	$ 789

Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 1,780	$ 2,497
Current portion of deferred rent and lease incentive	0	117
Collaboration and licensing	0	1,222
Accrued patent fees	565	736
Accrued external research and development costs	1,760	5,344
Accrued professional and consulting services	793	994
Accrued grant refund	0	175
Accrued interest	96	106
Other	322	509
Accrued expenses	$ 5,316	$ 11,700

Leases - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended			9 Months Ended
Leases - Narrative (Details) $ in Thousands	Jan. 01, 2019 USD ($)	Jul. 31, 2019 USD ($) ft²	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Oct. 31, 2017 ft²	Dec. 31, 2012 USD ($)
Operating Leased Assets [Line Items]
Right of use asset, net			$ 594			$ 594
Lease liabilities			734			734
Operating lease, remaining lease term	15 months
Discount rate	10.00%
Lessee-paid construction costs			555			1,662
Operating lease, expense			500		$ 500	1,500	$ 1,500
Lessee, Operating Lease, Liability, Payments, Due			$ 749			$ 749
75 North Beacon, Watertown, MA
Operating Leased Assets [Line Items]
Right of use asset, net	$ 200			$ 0
Area of additional office space leased \| ft²								5,100
65 Grove Street, Watertown,MA
Operating Leased Assets [Line Items]
Term of contract			8 years			8 years
Area of additional office space leased \| ft²		25,078
Lessee-paid construction costs		$ 800
Operating lease, expense				$ 200
Lessee, Operating Lease, Liability, Payments, Due		15,800
Letter of credit
Operating Leased Assets [Line Items]
Restricted cash		$ 1,400	$ 300			$ 300
Minimum
Operating Leased Assets [Line Items]
Term of contract			2 years			2 years
Renewal term			1 year			1 year
Maximum
Operating Leased Assets [Line Items]
Term of contract			8 years			8 years
Renewal term			5 years			5 years
Maximum \| Laboratory and office space
Operating Leased Assets [Line Items]
Leasehold Improvements, Gross									$ 700
Accounting Standards Update 2016-02
Operating Leased Assets [Line Items]
Right of use asset, net	1,600
Lease liabilities	1,800
Deferred rent and incentives	$ 200

Leases - Lease Commitments (Details) - 65 Grove Street, Watertown,MA $ in Thousands	Sep. 30, 2019 USD ($)
Lessee, Lease, Description [Line Items]
2020	$ 1,191
2021	1,811
2022	1,865
2023	1,921
2024	1,979
Thereafter	7,027
Total New Headquarters Lease commitment	$ 15,794

Leases - Components of lease costs (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Operating lease expense	$ 341	$ 1,023
Variable lease expense	211	625
Short-term lease expense	3	14
Total lease expense	$ 555	$ 1,662

Leases - Operating Lease, Liability, Maturity (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Leases [Abstract]
2019 (Remainder)	$ 374
2020	375
Total future minimum lease payments	749
Less imputed interest	15
Lease liabilities	734
Current operating lease liabilities	734
Non-current operating lease liabilities	$ 0

Leases - Other Information (Details) $ in Thousands	9 Months Ended
Leases - Other Information (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Leases [Abstract]
Operating cash flows from operating leases	$ 1,108

Leases - Lease Term and Discount Rate (Details)	Sep. 30, 2019
Leases [Abstract]
Operating lease, weighted average remaining lease term	15 days
Operating lease, weighted average discount rate, percent	10.00%

Debt - Term Loans (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Sep. 12, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Line of Credit Facility [Line Items]
Minimum assets trigger				$ 300
Minimum amount in excess of indebtedness trigger				300
Interest expense related to the Term Loan		$ 388	$ 384	1,184	$ 1,099
2017 Term Loans
Line of Credit Facility [Line Items]
Term loan facility	$ 21,000	20,300		$ 20,300		$ 21,000
Loss on extinguishment of debt			700
Debt issuance costs, line of credit arrangements (less than)	$ 100
Interest rate per annum				4.00%
Final payment fee (as a percent)				5.00%
Final payment fee		1,100		$ 1,100
Debt instrument, basis spread on variable rate				100.00%
Interest expense related to the Term Loan		$ 400	$ 400	$ 1,200	$ 1,100
2017 Term Loans \| Base Rate
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.05%
2017 Term Loans \| Outstanding Balances
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate				105.00%
2017 Term Loans \| Prepaid after second anniversary
Line of Credit Facility [Line Items]
Prepayment fee (as a percent)		1.00%		1.00%
Term Loans
Line of Credit Facility [Line Items]
Repayments of debt	$ 10,000

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - 2017 Term Loans $ in Thousands	Sep. 30, 2019 USD ($)
Year ended December 31,
2019 (Remainder)	$ 3,066
2020	9,163
2021	8,692
2022	1,752
Total minimum debt payments	22,673
Less: Amount representing interest	(1,323)
Less: Debt discount and deferred charges	(423)
Less: Current portion of loan payable	(20,927)
Loan payable, net of current portion	$ 0

Stockholders' Equity - Narrative (Details)								3 Months Ended	9 Months Ended
Stockholders' Equity - Narrative (Details)	Aug. 19, 2019 USD ($) $ / shares shares	Jan. 29, 2019 shares	Jan. 25, 2019 $ / shares shares	Aug. 10, 2017 USD ($)	Jun. 27, 2017 USD ($) $ / shares shares	Jul. 25, 2016 USD ($) $ / shares shares	Jun. 21, 2016 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) shares	Sep. 30, 2019 USD ($) vote $ / shares shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 $ / shares shares	Jun. 27, 2016 shares
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares									48,196,387
Sale common stock \| $									$ 30,900,000
Shares issued, value \| $									$ 30,942,000	$ 0
Common stock, par value (in dollars per share) \| $ / shares									$ 0.0001		$ 0.0001
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions \| $						$ 64,500,000
Convertible preferred stock, shares issued upon conversion (in shares) \| shares												10,126,118
Common stock, shares authorized (in shares) \| shares									200,000,000		200,000,000
Number of votes per share that common stockholders are entitled to \| vote									1
Dividends declared or paid on common stock \| $									$ 0
Private Placement
Subsidiary, Sale of Stock [Line Items]
Shares issued, value \| $									5,715,000
Private Placement \| Director
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares	3,178,174
Proceeds from issuance of common stock, net \| $	$ 5,700,000
Share price (in dollars per share) \| $ / shares	$ 1.81
Public Stock Offering
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares			20,000,000
Share price (in dollars per share) \| $ / shares			$ 1.50
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares		2,188,706				289,633
Share price (in dollars per share) \| $ / shares						$ 14.00
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions \| $						$ 3,700,000
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares								164,926
Sale common stock \| $									50,000,000
Public securities offering, amount authorized \| $				$ 200,000,000.0
Shares issued, value \| $								$ 400,000	$ 372,000	$ 0
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares					2,750,000
Share price (in dollars per share) \| $ / shares					$ 16.00
Shares issued, value \| $					$ 47,100,000
Common stock, par value (in dollars per share) \| $ / shares									$ 0.0001
Common stock, shares authorized (in shares) \| shares									200,000,000
Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) \| $ / shares					$ 17.71
Springer Purchase Agreement \| Common stock
Subsidiary, Sale of Stock [Line Items]
Stock issued (in shares) \| shares					338,791
Exercise of common warrants
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares					79,130
Exercise price (in dollars per share) \| $ / shares					$ 17.71
Warrants and rights outstanding, term					5 years
Share price (in dollars per share) \| $ / shares					$ 0.125
IPO
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares							5,000,000
Share price (in dollars per share) \| $ / shares							$ 14.00
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions \| $							$ 60,800,000

Stockholders' Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Total (in shares)	8,882,306		5,951,523
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	3,207,042		1,586,925
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	225,000	50,000	175,000
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	5,354,645		4,093,979
Exercise of common warrants
Class of Stock [Line Items]
Total (in shares)	95,619		95,619

Stock Incentive Plans - Narrative (Details) $ / shares in Units, $ in Thousands		3 Months Ended			9 Months Ended
	Jun. 21, 2016 shares	Sep. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Mar. 25, 2019 shares	Dec. 31, 2018 $ / shares shares	Sep. 25, 2018 shares	Dec. 31, 2008 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		8,882,306			8,882,306			5,951,523
Stock‑based compensation expense \| $		$ 1,303		$ 1,485	$ 3,734	$ 3,924
Shares issued, value \| $			$ 30,942
Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		3,207,042			3,207,042			1,586,925
Estimated forfeitures rate					10.00%
Weighted average grant date fair value of stock options (in dollars per share) \| $ / shares					$ 1.60	$ 9.03
Aggregate intrinsic value of stock options exercised \| $		$ 100		$ 400	$ 100	$ 400
Unrecognized compensation expense related to unvested employee stock options \| $		9,100			$ 9,100
Weighted average period for recognition					2 years 9 months 18 days
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value of stock options (in dollars per share) \| $ / shares					$ 1.46	$ 8.71
Unrecognized compensation expense related to unvested non-employee stock options \| $		$ 800			$ 800
Unrecognized compensation expense related to unvested non-employee stock options, period					1 year 6 months
Granted (in dollars per share) \| $ / shares					$ 4.35
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		225,000	50,000		225,000			175,000
Grants in period (in shares)			50,000		100,000
Reserved for issuance (in shares)					50,000
Vested in period (in dollars per share) \| $ / shares					$ 1.65
Grants in period (in dollars per share) \| $ / shares			$ 2.29		1.97
Shares issued (in dollars per share) \| $ / shares		$ 5.20			$ 5.20			$ 6.03
Shares issued, value \| $					$ 100
The 2008 Plan and the 2016 Plan \| Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock upon exercise of options (in shares)					125,600
Aggregate intrinsic value of stock options exercised \| $		$ 87			$ 87
Forfeited (in shares)					1,040,169
Granted (in shares)					2,389,255
Granted (in dollars per share) \| $ / shares					$ 2.17
The 2008 Plan and the 2016 Plan \| Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock upon exercise of options (in shares)					0
Aggregate intrinsic value of stock options exercised \| $		$ 0			$ 0
Forfeited (in shares)					12,820
Granted (in shares)					50,000	0
Granted (in dollars per share) \| $ / shares					$ 2.04
2008 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)										2,213,412
2008 Plan \| Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term					4 years
Award expiration term					10 years
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)		1,210,256			1,210,256
Automatic increase in the number of shares that may be issued (as a percent)					0.04
Number of shares authorized, increase (in shares)					898,871	893,730
Common stock authorized and reserved for future issuance (in shares)		1,709,636			1,709,636
2016 Plan \| Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term					4 years
Award expiration term					10 years
2016 Plan \| Employee stock option grants \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)					10.00%
2016 Plan \| Employee stock option grants \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Combined voting power of participants (as a percent)					10.00%
2016 Plan \| Employee stock option grants \| Tranche One \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)					100.00%
2016 Plan \| Employee stock option grants \| Tranche Two \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of incentive stock options (not less than)					110.00%
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)		750,000			750,000
Common stock authorized and reserved for future issuance (in shares)							2,000,000		1,175,000
Employment Inducement Incentive Award Plan \| Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in shares)			100,000
Weighted average period for recognition					3 years
Unrecognized compensation expense \| $		$ 900			$ 900
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Automatic increase in the number of shares that may be issued (as a percent)	0.01
Number of shares authorized, increase (in shares)					224,717	223,432
Common stock authorized and reserved for future issuance (in shares)	173,076	747,406			747,406
Exercise price of incentive stock options (not less than)	85.00%
Shares issued (in shares)					17,205
Stock‑based compensation expense \| $					$ 100	$ 100

Stock Incentive Plans - Schedule of Assumptions (Details) - $ / shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Employee stock option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.14%	2.82%
Dividend yield	0.00%	0.00%
Expected term	6 years 18 days	6 years 14 days
Expected volatility	87.79%	85.45%
Weighted-average fair value of common stock (in dollars per share)	$ 2.17	$ 12.38
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.07%	2.70%
Dividend yield	0.00%	0.00%
Expected term	5 years 5 months 26 days	5 years 10 months 6 days
Expected volatility	88.46%	85.95%

Stock Incentive Plans - Summary of Activity Under the 2008 Plan and the 2016 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Employee Awards
Aggregate intrinsic value
Vested and expected to vest	$ 100	$ 400
Non‑Employee Awards
Weighted-average exercise price
Granted (in dollars per share)	$ 4.35
The 2008 Plan and the 2016 Plan \| Employee Awards
Number of Options
Beginning balance (in shares)	3,681,575
Granted (in shares)	2,389,255
Exercised (in shares)	(125,600)
Forfeited (in shares)	(1,040,169)
Ending balance (in shares)	4,905,061		3,681,575
Vested (in shares)	1,180,429
Vested and expected to vest (in shares)	4,551,446
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 9.49
Granted (in dollars per share)	2.17
Exercised (in dollars per share)	1.20
Forfeited (in dollars per share)	9.99
Ending balance (in dollars per share)	6.03		$ 9.49
Vested (in dollars per share)	10.72
Vested and expected to vest (in dollars per share)	$ 6.23
Weighted-average remaining contractual term
Outstanding, term	8 years 7 months 13 days		7 years 9 months 7 days
Vested, term	6 years 5 months 26 days
Vested and expected to vest, term	8 years 6 months 18 days
Aggregate intrinsic value
Outstanding	$ 94		$ 300
Vested	57
Vested and expected to vest	$ 87
The 2008 Plan and the 2016 Plan \| Non‑Employee Awards
Number of Options
Beginning balance (in shares)	412,404
Granted (in shares)	50,000	0
Exercised (in shares)	0
Forfeited (in shares)	(12,820)
Ending balance (in shares)	449,584		412,404
Vested (in shares)	266,736
Vested and expected to vest (in shares)	449,584
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 6.44
Granted (in dollars per share)	2.04
Exercised (in dollars per share)	0
Forfeited (in dollars per share)	0.47
Ending balance (in dollars per share)	6.12		$ 6.44
Vested (in dollars per share)	5.72
Vested and expected to vest (in dollars per share)	$ 6.12
Weighted-average remaining contractual term
Outstanding, term	6 years 3 months 14 days		6 years 5 months 1 day
Vested, term	4 years 4 months 13 days
Vested and expected to vest, term	6 years 3 months 14 days
Aggregate intrinsic value
Outstanding	$ 0		$ 28
Vested	0
Vested and expected to vest	$ 0

Stock Incentive Plans - Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Stock‑based compensation expense	$ 1,303		$ 1,485	$ 3,734	$ 3,924
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Shares outstanding at December 31, 2018		175,000		175,000
Grants in period (in shares)		50,000		100,000
Vested in period (in shares)				50,000
Forfeited in period (in shares)				0
Shares outstanding at March 31, 2019	225,000			225,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Shares outstanding at December 31, 2018 (in dollars per share)		$ 6.03		$ 6.03
Grants in period (in dollars per share)		$ 2.29		1.97
Vested in period (in dollars per share)				1.65
Forfeited in period (in dollars per share)				0
Shares outstanding at March 31, 2019 (in dollars per share)	$ 5.20			$ 5.20

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details) $ in Thousands			3 Months Ended		9 Months Ended
	Jun. 21, 2016 shares	Jun. 20, 2016	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) offering_period shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			8,882,306		8,882,306		5,951,523
Stock-based compensation expense \| $			$ 1,303	$ 1,485	$ 3,734	$ 3,924
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of offering periods under the ESPP \| offering_period					2
Term of each offering period		6 months
Eligible compensation withheld for use (up to) (as a percent)	25.00%
Purchase price of shares (not less than) (as a percent)	85.00%
Common stock authorized and reserved for future issuance (in shares)	173,076		747,406		747,406
Number of shares of common stock outstanding (as a percent)	0.01
Number of shares authorized, increase (in shares)					224,717	223,432
Shares of common stock issued to employees under the ESPP (in shares)					17,205
Stock-based compensation expense \| $					$ 100	$ 100

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,303	$ 1,485	$ 3,734	$ 3,924
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	504	709	1,468	1,842
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 799	$ 776	$ 2,266	$ 2,082

Revenue Arrangements - Narrative (Details)					1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended	40 Months Ended
Revenue Arrangements - Narrative (Details)	Jun. 05, 2019 USD ($)	Oct. 31, 2017 USD ($) $ / shares shares	Jun. 06, 2017 USD ($) $ / shares shares	Dec. 02, 2016 USD ($) obligation contract target	Dec. 31, 2016 USD ($) shares	Sep. 30, 2019 USD ($) agreement	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) agreement shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)	Sep. 30, 2017 $ / shares	Nov. 25, 2008
Disaggregation of Revenue [Line Items]
Number of additional targets \| target				4
Sale of stock, number of shares issued in transaction (in shares) \| shares								48,196,387
Sale common stock								$ 30,900,000
Deferred revenue related to agreement						$ 16,004,000		16,004,000		$ 16,903,000
Deferred revenue, current portion						1,023,000		1,023,000		959,000
Deferred revenue, net of current portion						14,981,000		14,981,000		13,818,000
Remaining performance obligation						9,300,000		9,300,000
Current assets						(36,119,000)		(36,119,000)		(42,076,000)
Accumulated deficit						320,865,000		320,865,000		280,403,000
Revenue recognized								23,000
Adoption of new accounting principle											$ 1,830,000
Grant and collaboration revenue						0	$ 0	23,000	$ 0
Other liabilities						0		0		2,126,000
Spark Letter Agreement
Disaggregation of Revenue [Line Items]
Sale common stock		$ 2,500,000
Premium of average daily VWAP (as a percent)		115.00%
Deferred revenue related to agreement		$ 2,500,000
Aggregate purchase price			$ 5,000,000.0
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement (in shares) \| shares		205,254	324,362
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights				$ 2,700,000
Aggregate purchase price per share (in dollar per share) \| $ / shares		$ 24.36	$ 15.41
Reimbursement invoices	$ 1,200,000							18,300,000
First Acquisition Right
Disaggregation of Revenue [Line Items]
Aggregate purchase price		$ 5,000,000.0
Skolkovo
Disaggregation of Revenue [Line Items]
Deferred revenue related to agreement						100,000		100,000				$ 200,000
Payments received											2,000,000.0
Grants from entities, unused grant funds							100,000
Overspent costs							100,000
Second Acquisition Right
Disaggregation of Revenue [Line Items]
Share price (in dollars per share) \| $ / shares		$ 24.36
Premium of average daily VWAP (as a percent)		115.00%
Aggregate price of shares Spark will have the right to purchase		$ 5,000,000.0
Spark License Agreement
Disaggregation of Revenue [Line Items]
Upfront cash payment				10,000,000.0
Aggregate additional payments due within twelve months				$ 5,000,000.0
Term loan number of payments \| contract				2
Two payments due within 12 months of contract date				$ 2,500,000
Number of additional targets \| target				4
Variable fee the Company is eligible to receive for each additional target option elected				$ 2,000,000.0
Election period to exercise additional targets				3 years
Additional development and regulatory milestone payments				$ 65,000,000.0
Commercialization milestone payments				365,000,000.0
Sales milestone payments				$ 255,000,000.0
Period after first commercial sale when the Company is eligible to receive royalties				10 years
Period for prior written notice to terminate license				90 days
Sale of stock, number of shares issued in transaction (in shares) \| shares					197,238
Sale common stock					$ 5,000,000.0
Share price (in dollars per share) \| $ / shares													$ 25.35
Premium of average daily VWAP (as a percent)													115.00%
Deferred revenue related to agreement						15,900,000	$ 14,700,000	15,900,000	$ 14,700,000
Deferred revenue, current portion						1,000,000.0		1,000,000.0
Deferred revenue, net of current portion						$ 14,900,000		$ 14,900,000
Number of agreements combined and evaluated into a single agreement \| agreement						2		2
Fair value of initial purchase of common stock combined with the embedded future stock Acquisition Rights				$ 2,700,000
Remaining performance obligation				12,300,000		$ 2,300,000		$ 2,300,000
Performance obligation per each option				1,300,000		1,700,000		1,700,000
Incremental costs of obtaining a contract				2,600,000
Spark License Agreement \| MIT
Disaggregation of Revenue [Line Items]
License and option agreement, additional sub-license payment received liability								2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement								400,000
Spark License Agreement \| Consideration for Promises
Disaggregation of Revenue [Line Items]
Upfront cash payment				15,000,000.0
Spark License Agreement \| Development Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments				35,000,000.0
Spark License Agreement \| Regulatory Milestones
Disaggregation of Revenue [Line Items]
Additional development and regulatory milestone payments				30,000,000.0
Spark License Agreement \| Commercial Milestones
Disaggregation of Revenue [Line Items]
Commercialization milestone payments				$ 110,000,000.0
Spark License Agreement \| License and Supply Obligation
Disaggregation of Revenue [Line Items]
Number of obligations \| obligation				5
Remaining performance obligation				$ 7,100,000		11,600,000		11,600,000
Spark License Agreement \| Discount on Option Obligation
Disaggregation of Revenue [Line Items]
Number of obligations \| obligation				4
Remaining performance obligation				$ 5,200,000		6,700,000		6,700,000
Accumulated deficit
Disaggregation of Revenue [Line Items]
Adoption of new accounting principle											$ 1,830,000
Accumulated deficit \| ASU 2014-09 \| Skolkovo
Disaggregation of Revenue [Line Items]
Adoption of new accounting principle												$ 1,800,000
Grant Revenue
Disaggregation of Revenue [Line Items]
Grant and collaboration revenue						100,000				$ 0
Restatement Adjustment \| Error Recognized Upon the Initial Adoption of ASC 606
Disaggregation of Revenue [Line Items]
Current assets						200,000		200,000
Long-term assets						2,400,000		2,400,000
Accumulated deficit						$ 2,600,000		$ 2,600,000
Minimum \| MIT
Disaggregation of Revenue [Line Items]
Percentage of sublicense income														10.00%
Maximum \| MIT
Disaggregation of Revenue [Line Items]
Percentage of sublicense income														30.00%

Revenue Arrangements - Transaction Price Allocated to Future Performance Obligation (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 02, 2016
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 9.3
Remaining performance obligation, percentage	8.80%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period	12 months
Spark License Agreement
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 2.3	$ 12.3
Spark License Agreement \| License and Supply Obligation
Revenue from Contract with Customer [Abstract]
Remaining performance obligation	$ 11.6	$ 7.1
Spark License Agreement \| License and Supply Obligation \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2016-12-03
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, period		4 years

Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
Contract liabilities:
Deferred revenue, beginning of period	$ 14,777
Other liabilities, beginning of period	2,126
Contract liabilities, beginning of period	16,903
Deferred revenue, additions	1,250
Other liabilities, additions	0
Contract liabilities, additions	1,250
Deferred revenue, deductions	(23)
Other liabilities, deductions	(2,126)
Contract liabilities, deductions	(2,149)
Deferred revenue, end of period	16,004
Other liabilities, end of period	0
Contract liabilities, end of period	$ 16,004

Related-Party Transactions - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Director \| Quarterly payment
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction			$ 85,000
Founders
Related Party Transaction [Line Items]
Consulting services expenses	$ 100,000	$ 100,000	$ 400,000	$ 300,000
Non-employee stock options
Related Party Transaction [Line Items]
Options granted (in shares)			75,000
Granted (in dollars per share)			$ 4.35

Related-Party Transactions - Number of Shares of Common Stock Purchased (Details) - USD ($)		9 Months Ended
	Aug. 19, 2019	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Stock issued (in shares)		48,196,387
Shares issued, value		$ 30,942,000	$ 0
Timothy A. Springer, Ph.D. \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		4,000,000
Shares issued, value		$ 6,000,000.00
Entities affiliated with NanoDimension
Related Party Transaction [Line Items]
Stock issued (in shares)		1,666,666
Shares issued, value		$ 2,499,999.00
Entities affiliated with OrbiMed Advisors
Related Party Transaction [Line Items]
Stock issued (in shares)		1,333,333
Shares issued, value		$ 1,999,999.50
Entities affiliated with Polaris
Related Party Transaction [Line Items]
Stock issued (in shares)		666,666
Shares issued, value		$ 999,999.00
SAF-BND Trust (affiliate of Omid Farokhzad, M.D.)
Related Party Transaction [Line Items]
Stock issued (in shares)		83,333
Shares issued, value		$ 124,999.50
Chafen Lu (Timothy A. Springer’s wife)
Related Party Transaction [Line Items]
Stock issued (in shares)		66,666
Shares issued, value		$ 99,999.00
Jed Springer (Timothy A. Springer’s brother)
Related Party Transaction [Line Items]
Stock issued (in shares)		1,000
Shares issued, value		$ 1,500.00
Private Placement
Related Party Transaction [Line Items]
Shares issued, value		$ 5,715,000
Private Placement \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)	3,178,174
Private Placement \| Timothy A. Springer, Ph.D. \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		1,600,000
Shares issued, value		$ 2,896,000.00
Private Placement \| TAS Partners, LLC (affiliate of Timothy A. Springer, Ph.D. \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		1,100,000
Shares issued, value		$ 1,991,000.00
Private Placement \| Elona Kogan, J.D. \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		82,872
Shares issued, value		$ 149,998.32
Private Placement \| Patrick Zenner \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		55,248
Shares issued, value		$ 99,998.88
Private Placement \| Takashi Kei Kishimoto, Ph.D. \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		50,000
Shares issued, value		$ 90,500.00
Private Placement \| Carsten Brunn, Ph.D. \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		41,436
Shares issued, value		$ 74,999.16
Private Placement \| Scott D. Myers \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		41,436
Shares issued, value		$ 74,999.16
Private Placement \| Stephen Smolinski \| Director
Related Party Transaction [Line Items]
Stock issued (in shares)		13,812
Shares issued, value		$ 24,999.72

Collaboration and License Agreements - MIT (Narrative) (Details) - Spark License Agreement	9 Months Ended
	Sep. 30, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	$ 0
MIT
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contractual payments defined in the Exclusive Patent License agreement	2,200,000
License and option agreement, payments made relative to calculated premium paid for initial equity investment made under the purchase agreement	$ 400,000

Collaboration and License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) $ in Millions	May 31, 2014 USD ($)
3SBio License
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate amount of upfront and milestone-based payments	$ 3.0

Collaboration and License Agreements - Massachusetts Eye and Ear Infirmary and The Schepens Eye Research Institute, Inc. (Narrative) (Details) - MEE $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
License fees due under agreement	$ 0.4
Accrued license fees	$ 0.1

Income Taxes (Details)	Sep. 30, 2019 USD ($)
Income Tax Disclosure [Abstract]
Unrecognized tax benefits	$ 0

Defined Contribution Plan - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Retirement Benefits [Abstract]
Vesting period			4 years
Employer contribution made	$ 0.1	$ 0.1	$ 0.1	$ 0.2