0001453687-21-000069 | 10-Q | XBRL Viewer

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Cover page - shares	3 Months Ended
Cover page - shares	Mar. 31, 2021	May 07, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
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	65 Grove Street,
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	Non-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		113,193,597
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001453687

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 125,407	$ 138,685
Marketable securities	22,405	0
Accounts receivable	8,342	7,224
Prepaid expenses and other current assets	6,971	5,434
Total current assets	163,125	151,343
Property and equipment, net	1,301	1,395
Right-of-use asset, net	10,676	10,948
Long-term restricted cash	1,379	1,379
Other assets	265	370
Total assets	176,746	165,435
Current liabilities:
Accounts payable	594	443
Accrued expenses	6,847	8,146
Loan payable	460	0
Lease liability	942	908
Deferred revenue	75,764	72,050
Total current liabilities	84,607	81,547
Non-current liabilities:
Loan payable, net of current portion	24,551	24,793
Lease liability	9,403	9,647
Deferred revenue	32,301	38,746
Warrant liabilities	45,455	28,708
Total liabilities	196,317	183,441
Commitments and contingencies (Note 17)
Stockholders’ (deficit) equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding at March 31, 2021 and December 31, 2020	0	0
Common stock, $0.0001 par value; 200,000,000 shares authorized; 112,977,004 and 108,071,249 shares issued and outstanding as of March 31, 2021 and December 31, 2020, respectively	11	11
Additional paid-in capital	414,214	391,175
Accumulated deficit	(429,226)	(404,629)
Accumulated other comprehensive loss	(4,570)	(4,563)
Total stockholders’ (deficit) equity	(19,571)	(18,006)
Total liabilities and stockholders’ (deficit) equity	$ 176,746	$ 165,435

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
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 (in shares)	200,000,000	200,000,000
Common stock, shares issued	112,977,004	108,071,249
Common stock, shares outstanding	112,977,004	108,071,249

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 11,050	$ 0
Operating expenses:
Research and development	13,004	14,724
General and administrative	5,204	4,098
Total operating expenses	18,208	18,822
Loss from operations	(7,158)	(18,822)
Investment income	12	240
Foreign currency transaction gain, net	7	82
Interest expense	(711)	(273)
Change in fair value of warrant liabilities	(16,747)	(846)
Other income (expense), net	0	(1)
Net loss	(24,597)	(19,620)
Other comprehensive loss:
Foreign currency translation adjustment	(6)	(60)
Unrealized (losses) on marketable securities	(1)	0
Total comprehensive loss	$ (24,604)	$ (19,680)
Net loss per share:
Basic and diluted (in dollars per share)	$ (0.22)	$ (0.21)
Weighted average common shares outstanding:
Basic and diluted (in shares)	110,742,150	94,723,513

Consolidated Statements of Changes in Stockholders’ (Deficit) Equity - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2019		86,325,547
Beginning balance at Dec. 31, 2019	$ 8,397	$ 9	$ 348,664	$ (335,753)	$ (4,523)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		78,583
Issuance of common stock under Employee Stock Purchase Plan	114		114
Issuance of common stock upon exercise of options (in shares)		5,128
Issuance of common stock upon exercise of options	3		3
Issuance of vested restricted stock units (in shares)		10,937
Issuance of vested restricted stock units	0
Issuance of common stock through at-the-market offering, net (in shares)		598,977
Issuance of common stock through at-the-market offering, net	1,141		1,141
Other financing fees	(147)		(147)
Stock-based compensation expense	1,409		1,409
Currency translation adjustment	(60)				(60)
Unrealized (losses) on marketable securities	0
Net loss	(19,620)			(19,620)
Ending balance (in shares) at Mar. 31, 2020		87,019,172
Ending balance at Mar. 31, 2020	$ (8,763)	$ 9	351,184	(355,373)	(4,583)
Beginning balance (in shares) at Dec. 31, 2020	108,071,249	108,071,249
Beginning balance at Dec. 31, 2020	$ (18,006)	$ 11	391,175	(404,629)	(4,563)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)		34,696
Issuance of common stock under Employee Stock Purchase Plan	72		72
Issuance of common stock upon exercise of options (in shares)		153,278
Issuance of common stock upon exercise of options	244		244
Issuance of vested restricted stock units (in shares)		10,937
Issuance of vested restricted stock units	0
Issuance of common stock through at-the-market offering, net (in shares)		4,706,844
Issuance of common stock through at-the-market offering, net	20,943		20,943
Stock-based compensation expense	1,780		1,780
Currency translation adjustment	(6)				(6)
Unrealized (losses) on marketable securities	(1)				(1)
Net loss	$ (24,597)			(24,597)
Ending balance (in shares) at Mar. 31, 2021	112,977,004	112,977,004
Ending balance at Mar. 31, 2021	$ (19,571)	$ 11	$ 414,214	$ (429,226)	$ (4,570)

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net loss	$ (24,597)	$ (19,620)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	207	231
Amortization of premiums and discounts on marketable securities	14	0
Non-cash lease expense	272	366
Loss on disposal of property and equipment	0	1
Stock-based compensation expense	1,780	1,409
Non-cash interest expense	387	123
Warrant liabilities revaluation	16,747	846
Changes in operating assets and liabilities:
Accounts receivable	(1,118)	5,000
Prepaid expenses, deposits and other assets	(1,512)	(61)
Accounts payable	151	836
Deferred revenue	(2,728)	0
Accrued expenses and other liabilities	(1,736)	(829)
Net cash (used in) operating activities	(12,133)	(11,698)
Cash flows from investing activities
Purchases of marketable securities	(22,420)	0
Purchases of property and equipment	(25)	(135)
Net cash (used in) investing activities	(22,445)	(135)
Cash flows from financing activities
Repayments of principal on outstanding debt	0	(2,100)
Net proceeds from issuance of common stock	20,991	1,141
Issuance costs paid for December 2019 financing	0	(4,381)
Other financing fees	0	(147)
Proceeds from exercise of stock options	244	3
Proceeds from issuance of common stock under Employee Stock Purchase Plan	72	114
Net cash provided by (used in) financing activities	21,307	(5,370)
Effect of exchange rate changes on cash	(7)	(84)
Net change in cash, cash equivalents, and restricted cash	(13,278)	(17,287)
Cash, cash equivalents, and restricted cash at beginning of period	140,064	91,551
Cash, cash equivalents, and restricted cash at end of period	126,786	74,264
Supplement cash flow information
Cash paid for interest	494	232
Noncash investing and financing activities
Purchase of property and equipment not yet paid	10	10
Equity offering costs in accrued liabilities	48	42
Unrealized (losses) on marketable securities	$ (1)	$ 0

Nature of the Business and Basis of Presentation	3 Months Ended
Nature of the Business and Basis of Presentation	Mar. 31, 2021
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., or the Company was incorporated in Delaware on December 10, 2007, and is based in Watertown, Massachusetts. The Company is a clinical-stage biopharmaceutical company leveraging its ImmTOR™ immune tolerance platform with the goals of amplifying the efficacy of biologics, including enabling the re-dosing of life-saving gene therapies, and restoring self-tolerance in autoimmune diseases. The Company’s ImmTOR platform encapsulates rapamycin, also known as sirolimus, an immunomodulator, in biodegradable nanoparticles and is designed to induce antigen-specific immune tolerance. The Company believes ImmTOR has the potential to enhance the efficacy without compromising the safety of biologic therapies, improve product candidates under development, and enable novel therapeutic modalities. 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. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three months ended March 31, 2021 and 2020 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission, or 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, or U.S. GAAP, have been condensed or omitted pursuant to such rules and regulations. 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, 2020 included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 12, 2021. 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 March 31, 2021 and consolidated results of operations and cash flows for the three months ended March 31, 2021. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021. 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, private placements 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 platform, 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 March 31, 2021, the Company’s cash, cash equivalents, restricted cash and marketable securities were $149.2 million, of which $1.4 million was restricted cash related to lease commitments and $0.3 million was held by its Russian subsidiary designated solely for use in its operations. The Company believes the cash, cash equivalents, restricted cash and marketable securities as of March 31, 2021 will enable it to fund its operating expenses and capital expenditure requirements into the second quarter of 2023. The Company has incurred losses and negative cash flows from operating activities since inception. As of March 31, 2021, the Company had an accumulated deficit of $429.2 million. 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. At this time, there is significant uncertainty relating to the trajectory of the COVID-19 pandemic and the impact of related responses. Any impact of COVID-19 on the Company’s business, revenues, results of operations and financial condition will largely depend on future developments, which are highly uncertain and cannot be predicted with confidence, such as the duration of the pandemic, travel restrictions and social distancing in the United States and other countries, business closures or business disruptions, supply chain disruptions, the ultimate impact on financial markets and the global economy, and the effectiveness of actions taken in the United States and other countries to contain and treat the disease. 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 March 31, 2021, 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	3 Months Ended
Summary of Significant Accounting Policies	Mar. 31, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies There have been no material changes to the Company’s significant accounting policies during the three months ended March 31, 2021, as compared to the significant accounting policies disclosed in Note 2 – Summary of Significant Accounting Policies included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020. Recent Accounting Pronouncements Recently Adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740) - Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The Company adopted the new standard effective January 1, 2021, and there was no impact on its consolidated financial statements. In October 2020, the FASB issued ASU 2020-10, Codification Improvements, which updates various codification topics by clarifying or improving disclosure requirements to align with the SEC’s regulations. The Company adopted the new standard effective January 1, 2021, and there was no impact on its consolidated financial statements. Not Yet Adopted In August 2020, the FASB issued ASU 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815 – 40). ASU 2020-06 simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2023, with early adoption permitted no earlier than January 1, 2021. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. 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.

Marketable Securities

3 Months Ended

Mar. 31, 2021

Investments, Debt and Equity Securities [Abstract]

Marketable Securities

The following table summarizes the marketable securities held as of March 31, 2021 (in thousands):


	Amortized cost		Unrealized gains		Unrealized losses		Fair value
March 31, 2021

Corporate bonds	$	2,032	$	—	$	(1)	$	2,031
Commercial paper	20,374		—		—		20,374

Total	$	22,406	$	—	$	(1)	$	22,405

All marketable securities held at March 31, 2021 had maturities of less than 12 months when purchased and are classified as short-term marketable securities on the accompanying consolidated balance sheet. During the three months ended March 31, 2021, there were no marketable securities adjusted for other than temporary declines in fair value.

As of December 31, 2020, the Company held no marketable securities.

Net Loss Per Share

3 Months Ended

Mar. 31, 2021

Earnings Per Share [Abstract]

Net Loss Per Share

The Company has reported a net loss for the three months ended March 31, 2021 and 2020. 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 March 31,
	2021		2020
Numerator:
Net loss attributable to common stockholders	$	(24,597)	$	(19,620)
Denominator:
Weighted-average common shares and pre-funded warrants outstanding—basic and diluted	110,742,150		94,723,513
Net loss per share attributable to common stockholders —basic and diluted	$	(0.22)	$	(0.21)

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:


	March 31,
	2021	2020
Stock options to purchase common stock	10,187,394	7,745,936
Unvested restricted stock units	544,313	170,313
Warrants to purchase common stock	12,378,016	23,084,120


Total	23,109,723	31,000,369

Fair Value Measurements

3 Months Ended

Mar. 31, 2021

Fair Value Disclosures [Abstract]

Fair Value Measurements

The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2021 and December 31, 2020 (in thousands):


	March 31, 2021
	Total		(Level 1)		(Level 2)		(Level 3)
Assets:
Money market funds	$	58,158	$	58,158	$	—	$	—
Marketable securities:
Corporate bonds	2,031		—		2,031		—
Commercial paper	20,374		—		20,374		—
Total assets	$	80,563	$	58,158	$	22,405	$	—

Liabilities:
Warrant liabilities	$	45,455	$	—	$	—	$	45,455
Total liabilities	$	45,455	$	—	$	—	$	45,455


	December 31, 2020
	Total		(Level 1)		(Level 2)		(Level 3)
Assets:
Money market funds	$	80,576	$	80,576	$	—	$	—


Total assets	$	80,576	$	80,576	$	—	$	—

Liabilities:
Warrant liabilities	$	28,708	$	—	$	—	$	28,708
Total liabilities	$	28,708	$	—	$	—	$	28,708

There were no transfers within the fair value hierarchy during the three months ended March 31, 2021 or the year ended December 31, 2020.

Cash and Cash Equivalents

As of March 31, 2021 and December 31, 2020, the money market funds were classified as cash and cash equivalents on the accompanying consolidated balance sheets as they mature within 90 days from the date of purchase.

As of March 31, 2021, the Company had restricted cash balances relating to a secured letter of credit in connection with its lease for the Company’s headquarters (see Note 8 included elsewhere in this Quarterly Report). The Company’s consolidated statement of cash flows includes the following as of March 31, 2021 and 2020 (in thousands):


	March 31,
	2021		2020
Cash and cash equivalents	$	125,407	$	72,606
Short-term restricted cash	—		279
Long-term restricted cash	1,379		1,379
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	126,786	$	74,264

Marketable Securities

As of March 31, 2021, marketable securities classified as Level 2 within the valuation hierarchy consist of corporate bonds and commercial paper. Marketable securities represent holdings of available-for-sale marketable debt securities in accordance with the Company’s investment policy. The Company estimates the fair values of these marketable securities by taking into consideration valuations that include market pricing based on real-time trade data for the same or similar securities, and other observable inputs. The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to the earliest call date for premiums or to maturity for discounts.

Loans Payable

At March 31, 2021, given the recent issuance of the Term A Loan under the 2020 Term Loan, the Company believes the carrying value approximates the fair value of the loan.

Common Warrants

In December 2019, the Company issued common warrants in connection with a private placement of common shares. Pursuant to the terms of the common warrants, the Company could be required to settle the common warrants in cash in the event of certain acquisitions of the Company and, as a result, the common warrants are required to be measured at fair value and reported as a liability on the balance sheet. The Company recorded the fair value of the common warrants upon issuance using the Black-Scholes valuation model and is required to revalue the common warrants at each reporting date with any changes in fair value recorded in the statement of operations. The valuation of the common warrants is considered under Level 3 of the fair value hierarchy due to the need to use assumptions in the valuation that are both significant to the fair value measurement and unobservable. The significant unobservable inputs used in the fair value measurement of the warrant liabilities were the volatility rate and the estimated term of the warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement. The change in

the fair value of the Level 3 warrant liability is reflected in the statement of operations for the three months ended March 31, 2021 and 2020.

The estimated fair value of warrants is determined using Level 3 inputs inherent in the Black-Scholes simulation valuation.

Estimated fair value of the underlying stock. The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period.

Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption.

Dividend rate. The dividend rate is based on the historical rate, which the Company anticipates will remain at zero.

Expected life. The expected life of the warrants is assumed to be equivalent to their remaining contractual term which expires on December 23, 2024.

Volatility. The Company estimates stock price volatility based on the Company’s historical volatility and the historical volatility of peer companies for a period of time commensurate with the expected remaining life of the warrants.

A summary of the Black-Scholes pricing model assumptions used to record the fair value of the warrant liability is as follows:


	March 31,
	2021
Risk-free interest rate	0.35	%
Dividend yield	—
Expected life (in years)	3.73
Expected volatility	101.13	%

Changes in Level 3 Liabilities Measured at Fair Value on a Recurring Basis

The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 10), for the three months ended March 31, 2021 (in thousands):


	Warrant liabilities
Fair value as of December 31, 2020	$	28,708

Change in fair value	16,747
Fair value as of March 31, 2021	$	45,455

- 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

3 Months Ended

Mar. 31, 2021

Property, Plant and Equipment [Abstract]

Property and Equipment

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


	March 31,		December 31,
	2021		2020
Laboratory equipment	$	4,429	$	4,427
Computer equipment and software	694		532
Leasehold improvements	38		38
Furniture and fixtures	327		327
Office equipment	163		163
Construction in process	30		163
Total property and equipment	5,681		5,650
Less accumulated depreciation	(4,380)		(4,255)
Property and equipment, net	$	1,301	$	1,395

Depreciation expense was $0.1 million and $0.2 million for the three months ended March 31, 2021 and 2020, respectively.

Accrued Expenses

3 Months Ended

Mar. 31, 2021

Payables and Accruals [Abstract]

Accrued Expenses

Accrued expenses consist of the following (in thousands):


	March 31,		December 31,
	2021		2020
Payroll and employee related expenses	$	1,840	$	3,049

Collaboration and licensing	1,350		1,350
Accrued patent fees	471		534
Accrued external research and development costs	2,010		2,029
Accrued professional and consulting services	802		798

Accrued interest	170		170

Other	204		216
Accrued expenses	$	6,847	$	8,146

Leases

3 Months Ended

Mar. 31, 2021

Leases [Abstract]

Leases

65 Grove Street Lease

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, or the Headquarters Lease. As part of the Headquarters Lease, the Company incurred $0.8 million in non-reimbursable construction costs. The lease began in March 2020, consistent with when the Company took control of the office space, and the lease term is 8 years. The discount rate of 8.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods. In connection with the Headquarters Lease, the Company secured a letter of credit from Silicon Valley Bank, or SVB, for $1.4 million, recognized as long-term restricted cash, as of March 31, 2021 and December 31, 2020, respectively, which automatically renews each year.

Moscow, Russia Lease

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

As of March 31, 2021 and December 31, 2020, the components of the operating leases were as follows (in thousands):


	March 31,	December 31,
		2021		2020
Assets:
Right-of-use asset, net		$	10,676	$	10,948

Liabilities:
Current operating lease liabilities		$	942	$	908
Non-current operating lease liabilities		9,403		9,647
Total operating lease liabilities		$	10,345	$	10,555

For the three months ended March 31, 2021 and 2020 the components of lease costs were as follows (in thousands):


	March 31,
		2021		2020
Operating lease cost		$	444	$	472
Variable lease cost		288		199
Short-term lease cost		3		2
Total lease cost		$	735	$	673

The maturity of the Company’s operating lease liabilities as of March 31, 2021 were as follows (in thousands):


	March 31,
	2021
2021 (remainder)	$	1,368
2022	1,866
2023	1,922
2024	1,980
2025	2,039
Thereafter	4,945
Total future minimum lease payments	14,120
Less imputed interest	3,775
Total operating lease liabilities	$	10,345

The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands):


	March 31,
	2021		2020
Cash paid for amounts included in the measurement of lease liabilities:	$	444	$	487

Other than the initial recording of the right-of-use asset and lease liability for the Headquarters Lease, which is non-cash, the changes in the Company’s right-of-use asset and lease liability for the three months ended March 31, 2021 and 2020 are reflected in the non-cash lease expense and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows.

The following summarizes additional information related to operating leases:


	March 31,
	2021		2020
Weighted-average remaining lease term	7.1 years		8.0 years
Weighted-average discount rate	8.9	%	8.9	%

Debt

3 Months Ended

Mar. 31, 2021

Debt Disclosure [Abstract]

Debt

2020 Term Loan

On August 31, 2020, the Company entered into a term loan of up to $35.0 million, or the 2020 Term Loan, consisting of term loans in an aggregate amount of $25.0 million, or the Term A Loan, and term loans in an aggregate amount of $10.0 million, or the Term B Loan, governed by a loan and security agreement, or the Loan Agreement, between the Company and Oxford Finance LLC, or Oxford, as Collateral Agent and a Lender, and SVB, as a Lender. The Term A Loan was funded in full on August 31, 2020, or the Funding Date.

The Term B Loan will be available, subject to Collateral Agent’s discretion and customary terms and conditions, during the period commencing on the date the Company has delivered to the Collateral Agent and the Lenders evidence: (i) the Company or one of the Company’s collaboration partners has enrolled its first patient for a Phase 1 clinical trial evaluating the treatment of methylmalonic acidemia, or MMA, and (ii) the Company has enrolled the first patient in each of two Phase 3 pivotal trials evaluating SEL-212, or the Second Draw Period Milestone, and ending on the earliest of (i) the date which is 30 days following the date the Second Draw Period Milestone is achieved, (ii) September 30, 2021 (iii) and the occurrence of an event of default, other than an event of default that has been waived in writing by Collateral Agent and the Lenders in their sole discretion, with such period referred to as the Second Draw Period.

The 2020 Term Loan will mature on August 1, 2025. Each advance under the Term Loan accrues interest at a floating per annum rate equal to the greater of (a) 7.90%, and (b) the lesser of (x) the sum of (i) the prime rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, and (ii) 4.65% and (y) 10.00%. The Term Loan provides for interest-only payments on a monthly basis until April 1, 2022; provided however, if the Company has delivered to the Collateral Agent and the Lenders prior to September 30, 2021 evidence that Borrower has achieved the Second Draw Period Milestone, the Term Loan provides for interest-only payments on a monthly basis until October 1, 2022. Thereafter, amortization payments will be payable monthly in equal installments of principal and interest to fully amortize the outstanding principal over the remaining term of the loan, subject to recalculation upon a change in the prime rate. The Company may prepay the Term Loan in full but not in part provided that the Company (i) provides ten days’ prior written notice to Collateral Agent, (ii) pays on the date of such prepayment (A) all outstanding principal plus

accrued and unpaid interest, and (B) a prepayment fee of between 3.0% and 1.0% of the aggregate original principal amount advanced by the lender depending on the timing of the prepayment. Amounts outstanding during an event of default are payable upon SVB’s demand and shall accrue interest at an additional rate of 5.0% per annum of the past due amount outstanding. At the end of the loan term (whether at maturity, by prepayment in full or otherwise), the Company shall make a final payment to the lender in the amount of 9.0% of the aggregate original principal amount advanced by the lender. The final payment fee totaling $2.3 million is recorded as a loan discount.

The 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 the Collateral Agent a negative pledge with respect to its intellectual property.

The Loan Agreement contains customary covenants and representations, including but not limited to financial reporting obligations and limitations on dividends, indebtedness, collateral, investments, distributions, transfers, mergers or acquisitions, taxes, corporate changes, deposit accounts, and subsidiaries. The Loan Agreement also contains other customary provisions, such as expense reimbursement, non-disclosure obligations as well as indemnification rights for the benefit of the Collateral Agent.

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 change, 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 of at least $0.5 million, or the occurrence of any default under any agreement or obligation of the Company involving indebtedness in excess of $0.5 million. If an event of default occurs, the Collateral Agent is entitled to take enforcement action, including acceleration of amounts due under the Loan Agreement.

The Company incurred $0.4 million in debt issuance costs in connection with the closing of the 2020 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 Company assessed all terms and features of the 2020 Term Loan 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 2020 Term Loan, including any put, call, and contingent features. The Company determined that the interest rate collar and prepayment call option did not require bifurcation; whereas the contingent put option and default (contingent) interest rate feature met bifurcation criteria resulting in immaterial amounts.

Warrants

On August 31, 2020, in connection with the 2020 Term A Loan, the Company issued warrants to the Lenders to purchase an aggregate of 196,850 shares of its common stock at an exercise price equal to $2.54 per share. In accordance with ASC 815-40, these warrants are classified as permanent equity in the accompanying consolidated balance sheets and will expire ten years from the date of issuance. The initial grant date fair value of the warrants was $0.4 million as determined by the Black-Scholes valuation model and recorded to shareholders' equity, with the SVB portion allocated to the reacquisition price of the 2017 Term Loan and the Oxford fair value portion as a loan discount to the Term A Loan.

Additionally, on August 31, 2020, pursuant to the terms of a Warrant Side Letter agreement among the Company and the Lenders, the Company agreed to issue to the Lenders, on the date the Company draws the Term B Loan and in accordance with each party’s respective pro rata share with respect to the Term B Loan, one or more warrants to purchase an aggregate number of shares of its common stock that is equal to $200,000 divided by the average closing price of the Company’s common stock on The Nasdaq Stock Market LLC for the ten consecutive trading days ending the day before such issuance, rounded down to the nearest whole number of shares, and having an exercise price equal to the Term B Warrant Price.

Payoff

On the Funding Date, the Company entered into a payoff letter with SVB, pursuant to which the Company utilized $13.7 million of the 2020 Term Loan to pay off all outstanding obligations under the previous term loan, consisting of the principal payment, final prepayment and accrued interest. During the three months ended September 30, 2020, the Company recognized a loss on extinguishment of debt in the amount of $0.5 million determined as the difference between the reacquisition price and carrying value at August 31, 2020.

As of March 31, 2021 and December 31, 2020, the outstanding principal balance under the 2020 Term Loan was $25.0 million.

Future minimum principal and interest payments on the 2020 Term Loan as of March 31, 2021 are as follows (in thousands):


2021 (remainder)	$	1,509
2022	7,343
2023	8,611
2024	8,027
2025	7,274
Total minimum debt payments	32,764
Less: Amount representing interest	(5,514)
Less: Debt discount and deferred charges	(2,239)
Less: Current portion of loan payable	(460)
Loan payable, net of current portion	$	24,551

Equity

3 Months Ended

Mar. 31, 2021

Equity [Abstract]

Equity

Equity Financings

August 2020 Shelf Registration Statement

On August 6, 2020, the Company filed an updated universal shelf registration statement on Form S-3 (Reg. No. 333-241692) with the SEC to sell an aggregate amount of up to $200.0 million of certain of its securities. The shelf registration statement was declared effective by the SEC on August 14, 2020.

“At-the-Market” Offerings

In August 2017, the Company entered into a sales agreement, or the 2017 Sales Agreement, with Jefferies LLC, as sales agent, to sell shares of its common stock with an aggregate value of up to $50.0 million in an “at the market offering.” On August 6, 2020, concurrent with the filing of the updated shelf registration statement, the Company entered into a sales agreement, or the 2020 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 $50.0 million in an “at the market offering.” The 2017 Sales Agreement terminated pursuant to its terms in August 2020.

Sales of common stock, if any, pursuant to the 2020 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 Stock 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 2020 Sales Agreement at any time.

During the year ended December 31, 2020, the Company sold 1,069,486 shares of its common stock pursuant to the 2020 and 2017 Sales Agreements at an average price of approximately $2.16 per share for aggregate net proceeds of $2.1 million, after deducting commissions and other transaction costs.

During the three months ended March 31, 2021, the Company sold 4,706,844 shares of its common stock pursuant to the 2020 Sales Agreement at an average price of approximately $4.59 per share for aggregate net proceeds of $20.9 million, after deducting commissions and other transaction costs.

June 2020 Sobi Stock Purchase

On June 11, 2020, the Company entered into a stock purchase agreement with Sobi, pursuant to which the Company sold an aggregate of 5,416,390 shares of its common stock at a purchase price equal to $4.6156 per share, which represented 120% of the 10-day volume-weighted average price of the Company’s common stock prior to signing, for aggregate gross proceeds of $25.0 million, or the Sobi Private Placement. The closing of the Sobi Private Placement occurred on July 31, 2020. The shares of common stock acquired in the Sobi Private Placement are subject to a one-year lock-up from closing, during which time Sobi is prohibited from selling or otherwise disposing of such shares.

In accordance with ASC 815, this forward sale treatment qualified as equity classification as the shares are not within the scope of ASC 480. The gross proceeds of $25.0 million were determined to include a premium to the fair value of the Company’s shares as of July 28, 2020 of approximately $14.5 million. As a result, such amount was included in the transaction price for revenue recognition of the Sobi License. See Note 12 for details.

Also on June 11, 2020, the Company entered into a registration rights agreement (as amended by that certain letter agreement, dated as of November 4, 2020) with Sobi, pursuant to which the Company agreed to prepare and file a registration statement with respect to the resale of the shares of common stock acquired in the Sobi Private Placement. The Company will be required to file this resale registration statement within 30 days following receipt by the Company of a written request from

Sobi to file such resale registration statement, and to have the registration statement declared effective within 10 business days after the SEC informs the Company that no review of such resale registration statement will be made or that the SEC has no further comments on such resale registration statement.

December 2019 Financing

On December 18, 2019, the Company entered into a securities purchase agreement, or the 2019 Purchase Agreement, with a group of institutional investors and certain members of the board of directors. Pursuant to the 2019 Purchase Agreement, the Company sold an aggregate of 37,634,883 shares of its common stock at a purchase price of $1.46 per share, warrants to purchase an aggregate of 22,988,501 shares of common stock at a purchase price of $0.125 per share underlying each common warrant, and pre-funded warrants to purchase an aggregate of 8,342,128 shares of common stock at a purchase price of $1.46 per share, all with five year terms, or the 2019 PIPE. The closing of the 2019 PIPE occurred on December 23, 2019. The exercise price of the pre-funded warrants is $0.0001 per share and the exercise price for the common warrants is $1.46 per share. In the event of a certain sale of the Company, the terms of the common warrants require us to make a payment to such common warrant holders based on a Black-Scholes valuation (using variables as specified in the warrants). This provision does not apply to the pre-funded warrants. Therefore, the Company is required to account for the common warrants as liabilities and record them at fair value, while the pre-funded warrants met the criteria to be classified as permanent equity.

The Company recorded the fair value of the common warrants of $40.7 million upon issuance using the Black-Scholes valuation model. Issuance costs were allocated between the equity component with an offset to additional paid-in capital and the liability component recorded as expense on a relative fair value basis. Total net proceeds from the equity offering was $65.6 million, after deducting transaction costs and commissions of $4.4 million which was paid in the three months ended March 31, 2020.

The common warrants were revalued as of March 31, 2021 at $45.5 million. During the three months ended March 31, 2021 and 2020, the Company recorded losses on the increases in the fair value of the warrants of $(16.7) million and $(0.8) million, respectively, in the unaudited consolidated statements of operations.

Warrants

During the three months ended March 31, 2021 and 2020, there were no warrants issued, exercised, or canceled.


	Number of Warrants
	Equity classified	Liability classified	Total	Weighted average exercise price





Outstanding at March 31, 2021	292,469	12,085,547	12,378,016	$	1.60

Common Stock

As of March 31, 2021, the Company had 200,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 112,977,004 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 March 31, 2021, 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 ended
	March 31, 2021	December 31, 2020
Exercise of common warrants	12,378,016	12,378,016
Shares available for future stock incentive awards	7,719,817	4,916,374
RSUs reserved for issuance	98,750	—
Unvested restricted stock units	544,313	87,500
Outstanding common stock options	10,187,394	7,775,249
Total	30,928,290	25,157,139

Stock Incentive Plans

3 Months Ended

Mar. 31, 2021

Share-based Payment Arrangement [Abstract]

Stock Incentive Plans

The Company maintains the 2008 Stock Incentive Plan, or 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.

In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 1,210,256 shares of common stock for future issuance under the 2016 Plan and ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remain subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expire, lapse or terminate become available under the 2016 Plan as shares available for future grants.

Additionally, pursuant to the terms of the 2016 Plan, the Board is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In January 2021 and 2020, the number of shares of common stock that may be issued under the 2016 Plan was increased by 4,322,850 and 3,453,022 shares, respectively. As of March 31, 2021, 3,248,520 shares remain available for future issuance under the 2016 Plan.

In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan was adopted by the Board without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 1,175,000 shares of its common stock for issuance. In March 2019, the Board approved the amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. As of March 31, 2021, there are 1,358,333 shares available for future grant under the 2018 Inducement Incentive Award Plan.

Stock-based Compensation Expense

Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):


	Three Months Ended March 31,
	2021		2020
Research and development	$	754	$	623
General and administrative	1,026		786
Total stock-based compensation expense	$	1,780	$	1,409

Stock Options

Employees

The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:


	Three Months Ended March 31,
	2021			2020
Risk-free interest rate	0.44		%	1.73		%
Dividend yield	—			—
Expected term	5.28			6.08
Expected volatility	83.26		%	87.96		%
Weighted-average fair value of common stock	$	3.13		$	2.30

The weighted average grant date fair value of stock options granted to employees during the three months ended March 31, 2021 and 2020 was $1.99 and $1.69, respectively.

As of March 31, 2021 total unrecognized compensation expense related to unvested employee stock options was $11.8 million, which is expected to be recognized over a weighted average period of 2.7 years.

Non-employee consultants

As of March 31, 2021, there was no unrecognized compensation expense related to non-employee consultants stock options.

The following table summarizes the stock option activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan:


				Weighted-average
				remaining	Aggregate
	Number of	Weighted-average		contractual term	intrinsic value
	options	exercise price ($)		(in years)	(in thousands)
Employees








Outstanding at December 31, 2020	7,302,176	$	3.98	8.43	$	4,456
Granted	2,628,000	$	3.13
Exercised	(153,278)	$	1.47
Forfeited	(62,577)	$	5.19
Outstanding at March 31, 2021	9,714,321	$	3.78	8.65	$	15,665

Vested at March 31, 2021	3,064,130	$	5.46	7.63	$	4,313
Vested and expected to vest at March 31, 2021	8,920,548	$	3.88	8.59	$	14,231

Non-employee consultants








Outstanding at December 31, 2020	473,073	$	5.89	5.23	$	86
Granted	—	$	—
Exercised	—	$	—
Forfeited	—	$	—
Outstanding at March 31, 2021	473,073	$	5.89	4.98	$	366

Vested at March 31, 2021	462,186	$	5.82	4.92	$	366
Vested and expected to vest at March 31, 2021	473,073	$	5.89	4.98	$	366

Restricted Stock Units

In January 2021, the Company granted 369,800 restricted stock awards to employees under the 2016 Plan which will vest over a four year term.

In addition, during the first quarter of 2021, the Company awarded 197,500 restricted stock units to executives under the 2016 Plan, of which 98,750 were determined to be granted as of the award date consistent with ASC 718. The remaining 98,750 restricted stock units do not have a defined performance metric as of the award date, resulting in the restricted stock units being reserved for future issuance as of March 31, 2021. These restricted stock units will vest in two equal installments on the dates

an applicable performance condition is achieved, on or prior to December 31, 2021. If the performance conditions are not satisfied on or prior to December 31, 2021, the restricted stock units will be forfeited for no consideration.

The restricted stock units granted had a weighted average fair value of $2.99 per share based on the closing price of the Company’s common stock on the date of grant. The restricted stock units were valued at approximately $1.4 million on their grant date. 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% for restricted stock awards to employees based on historical attrition trends.

Unrecognized compensation expense for all restricted stock units was $1.5 million as of March 31, 2021, which is expected to be recognized over a weighted average period of 3.1 years.

The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan:


	Number of shares	Weighted average grant date fair value ($)
Unvested at December 31, 2020	87,500	$	6.03
Granted	468,550	2.99
Vested	(10,937)	6.03
Reserved for issuance	98,750	2.99
Forfeited	(800)	2.99
Unvested at March 31, 2021	643,063	$	3.35

Employee Stock Purchase Plan

In June 2016, the Company approved the 2016 Employee Stock Purchase Plan, or the ESPP, which authorized 173,076 shares of common stock for future issuance under the ESPP to participating employees. In January 2021 and 2020, the number of shares of common stock authorized for issuance under the ESPP was increased by 1,080,711 shares and 863,254 shares, respectively. During the three months ended March 31, 2021, the Company issued 34,696 shares of common stock under the ESPP. As of March 31, 2021, 2,546,464 shares remain available for future issuance under the ESPP.

For each of the three months ended March 31, 2021 and 2020, the Company recognized less than $0.1 million of stock-based compensation expense under the ESPP.

Revenue Arrangements

3 Months Ended

Mar. 31, 2021

Revenue from Contract with Customer [Abstract]

Revenue Arrangements

Swedish Orphan Biovitrum

License and Development Agreement

On June 11, 2020, the Company and Sobi entered into the Sobi License. Pursuant to the Sobi License, the Company has agreed to grant Sobi an exclusive, worldwide (except as to Greater China) license to develop, manufacture and commercialize the Company’s SEL-212 drug candidate, which is currently in development for the treatment of chronic refractory gout. The SEL-212 drug candidate is a pharmaceutical composition containing a combination of SEL-037, or the Compound, and ImmTOR. Pursuant to the Sobi License, in consideration of the license, Sobi agreed to pay the Company a one-time, up-front payment of $75.0 million. Sobi has also agreed to make milestone payments totaling up to $630.0 million to the Company upon the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier.

Pursuant to the Sobi License, the Company has agreed to supply (at cost) quantities of the Compound and ImmTOR as necessary for completion of the two Phase 3 clinical trials of SEL-212 (DISSOLVE I and DISSOLVE II) and a 6-month placebo extension. The Company is required to supply quantities of the Compound until all rights to the Compound and any materials needed to manufacture the Compound are transferred to Sobi. Sobi has agreed to reimburse the Company for all budgeted costs incurred to complete development of SEL-212, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, except for any costs of additional development activities required that are related to ImmTOR and that are unrelated to SEL-212. Sobi will have control and responsibility over all regulatory filings, including any investigational drug applications (IND), biologics license applications (BLA), and marketing authorization applications (MAA) relating to the licensed product.

The transactions contemplated by the Sobi License were consummated on July 28, 2020 following the expiration or termination of the required waiting periods under the Hart-Scott-Rodino Antitrust Improvements Act of 1976. Sobi may terminate the Sobi License for any reason upon 180 days’ written notice to the Company, whereby all rights granted under the Sobi License would revert back to the Company. In addition, if Sobi were to terminate the Sobi License, the Company has the option to obtain a license to all patents and know-how necessary to exploit SEL-212 in existence as of the termination date from Sobi in return for making an equitable royalty payment to Sobi.

Additionally, on June 11, 2020, the Company entered into the Sobi Purchase Agreement in connection with the Sobi License. The closing of the Sobi Private Placement occurred on July 31, 2020, following the closing of the transactions contemplated under the Sobi License. See Note 10 for details.

The Company determined that the Sobi License represents a service arrangement under the scope of ASC 606. In addition, given the Sobi License and Sobi Purchase Agreement were executed contemporaneously and negotiated as a package with a single commercial objective, the Company will account for the two agreements as a single contract. The term of the Sobi License commenced upon the effective date of July 28, 2020 and will continue on a product-by-product basis until the royalty terms for each country have expired. The royalty term for a given product begins upon the first commercial sale of the product in a country and ends at the later of ten years from the first commercial sale, expiration of the last valid patent claim covering the product and expiration of all regulatory exclusivity periods for the product in a country. Given the reversion of the rights under the Sobi License represents a penalty in substance for a termination by Sobi, the contract term would remain the stated term of the Sobi License.

The Company determined that the Sobi License contains three distinct performance obligations due to the nature of the promises in the contract, which includes conducting the Phase 3 DISSOLVE trials, Sobi’s option to set-up a second source supplier, and a combined obligation comprised of the delivery of the license to SEL-212, transfer of the know-how and the manufacturing and delivery of SEL-212 supply for development, or the Combined License Obligation. As the set-up of a second source supplier is optional for Sobi and the Company will be reimbursed at cost for its efforts in the subsequent set-up and technology transfer, the option for this future service was determined to be at a significant and incremental discount to its standalone selling price and treated as a material right in the arrangement, namely a distinct performance obligation.

In determining the transaction price, the Company concluded the upfront payment of $75.0 million and the $5.0 million development milestone associated with the dosing of the first patient in the Phase 3 DISSOLVE trials will be included in the transaction price. All other development milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. 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 the evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company and probability of success criteria is estimated. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved. In accordance with ASC 606, the Company will only recognize revenue associated with sales-based milestones and royalties when the subsequent sales thresholds are reached and underlying sales occur, respectively. In connection with the Sobi Purchase Agreement, the Company determined that the gross proceeds of $25.0 million from the Sobi Private Placement included a premium to the fair value of the Company’s shares as of July 28, 2020 equal to approximately $14.5 million. The premium amount will be included in the transaction price for revenue recognition. The Company will estimate and include in the transaction price the total reimbursements to be received from Sobi for both the manufacturing and delivery of the Compound and ImmTOR as well as conducting the Phase 3 DISSOLVE trials. The Company determined that a significant financing component does not exist in its arrangement with Sobi.

The Company allocated the transaction price based on the relative standalone selling prices of the three distinct performance obligations. The Company estimated the standalone selling price of conducting the Phase 3 DISSOLVE trials by forecasting its anticipated costs and applying a margin reflective of the industry. The Company must determine the standalone selling price of the second source supplier option by determining the discount given to Sobi multiplied by the likelihood that Sobi will exercise the option in the future. Similar to the Phase 3 program estimate, the Company estimated the discount of the option by forecasting the set-up costs and applying a margin that is reflective of the industry. As the Company will be providing the set-up and technology transfer services and the future supply at cost, the discount of the option is equal to the margin amount. The Company considered discussions with Sobi as well as probability of regulatory success of SEL-212 in determining the likelihood of exercise. The Company estimated the standalone selling price of the Combined License Obligation by utilizing a discounted cash flow model.

The Company determined that the delivery of the supply to Sobi best represents the pattern of delivery of the Combined License Obligation as the supply is essential to the utility of the license and know-how. The Company will recognize the revenue allocated to the Combined License Obligation by utilizing the output method. The Company estimated the total supply of the Compound and ImmTOR to be required during the clinical trial period and will recognize revenue as this supply is shipped for use in the clinical trials. The Company will recognize the revenue allocated to the conducting of the Phase 3 DISSOLVE trials obligation by utilizing the input method. The Company estimated the total budgeted costs to be incurred over

the Phase 3 DISSOLVE trials and will recognize revenue as these costs are incurred. The Company’s costs best represent the pattern of transfer as these will capture all performance of the trials completed to date and can be readily measured. The Company will recognize the revenue allocated to the second source supplier option when the future services and goods are transferred.

As of March 31, 2021 and December 31, 2020, the Company recorded $72.0 million and $68.3 million, respectively, as a short-term contract liability and $17.8 million and $24.2 million, respectively, as a long-term contract liability, representing deferred revenue associated with this agreement.

In addition, as of March 31, 2021 the Company has recorded $1.3 million of contract assets related to incremental costs that would not have been incurred if the Sobi License had not been obtained, of which $1.0 million is presented in prepaid expenses and other current assets and $0.3 million is presented in other assets in the accompanying unaudited consolidated balance sheets. Amortization of contract assets was less than $0.1 million for the three months ended March 31, 2021.

As of March 31, 2021 and December 31, 2020, the Company recorded a total outstanding receivable of $8.3 million and $6.9 million, respectively, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi. Revenue of $11.1 million related to the Sobi License was recognized during the three months ended March 31, 2021.

Sarepta Therapeutics, Inc.

Research License and Option Agreement

On June 13, 2020, the Company and Sarepta entered into a Research License and Option Agreement, or the Sarepta Agreement. Pursuant to the Sarepta Agreement, the Company agreed to grant Sarepta a license under the Company’s intellectual property rights covering the Company’s antigen-specific biodegradable nanoparticle encapsulating ImmTOR to research and evaluate ImmTOR in combination with Sarepta’s adeno-associated virus gene therapy technology, or gene editing technology, using viral or non-viral delivery, to treat Duchenne Muscular Dystrophy and certain Limb-Girdle Muscular Dystrophy subtypes, or the Indications. Sarepta will have an option term of 24 months during which it can opt-in to obtain an exclusive license to further develop and commercialize the Product to treat at least one Indication, with a potential to extend the option term for an additional fee. The Company will supply ImmTOR to Sarepta for clinical supply on a cost-plus basis.

Sarepta paid a $2.0 million up-front payment to the Company upon signing of the Sarepta Agreement, and the Company is eligible to receive additional preclinical payments during the option term. If Sarepta opts-in to an exclusive license agreement, the Company could receive option exercise payments per Indication upon execution of the exclusive license, and the Company would be entitled to significant development and commercial milestone payments and tiered royalties ranging from the mid-to-high single digits based on net sales.

Pursuant to the Sarepta Agreement, the Company determined the Sarepta Agreement represents a service arrangement under the scope of ASC 606, with a 24 month contract duration. Given the reversion of the rights under the Sarepta Agreement represents a penalty in substance for a termination by Sarepta, the contract term would remain the stated term of the Sarepta Agreement.

The Company determined that the Sarepta Agreement and supply obligation including the delivery of the research license, the licensed know-how, the manufactured supply and delivery of materials represent a single promise and performance obligation to be transferred to Sarepta over time due to the nature of the promises in the contract. The delivery of the manufactured supply is the predominant promise within the arrangement, as it is essential to the utility of the licensed intellectual property. As such, consideration in the initial transaction price will be allocated to the single performance obligation based on the contractual price.

In determining the transaction price, the Company concluded the payment associated with all the performance milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. 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 study milestones is outside the control of the Company and probability of success criteria is estimated. As of March 31, 2021 and December 31, 2020, all milestones were constrained. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved. The Company will recognize the revenue associated with the upfront payment and combined single performance obligation utilizing the output method, over the 24 month term as the manufactured supply is delivered to Sarepta.

The Company also determined the option to enter into a future commercial license agreement and extend the term of the option does not represent a material right since it was not priced at an incremental discount. Sarepta may terminate the Sarepta Agreement for any reason upon 30 days’ written notice to the Company. The Sarepta Agreement contains other customary terms and conditions, including representations and warranties, covenants, termination, and indemnification obligations in favor of each party. During the year ended December 31, 2020, the Company and Sarepta entered into two amendments relating to an

additional feasibility study. During the three months ended March 31, 2021, the Company and Sarepta entered into a third amendment relating to feasibility study research plan. None of the amendments executed through March 31, 2021 had a material impact on deferred revenue or revenue recognition.

At each of March 31, 2021 and December 31, 2020, the Company recorded $2.0 million as a short-term contract liability representing deferred revenue associated with this agreement. No revenue related to the Sarepta Agreement was recognized during the three months ended March 31, 2021 and 2020.

Asklepios Biopharmaceutical, Inc.

License Agreement for Pompe Disease

On December 17, 2019, the Company and AskBio entered into a license agreement, or the AskBio License Agreement. Pursuant to the AskBio License Agreement, AskBio has exercised its option to exclusively license the Company’s intellectual property rights covering the Company’s ImmTOR platform to research, develop, and commercialize certain AAV gene therapy products utilizing ImmTOR, and targeting the GAA gene, or derivatives thereof, to treat Pompe Disease.

Pursuant to the AskBio License Agreement and ancillary documents, AskBio agreed to pay to the Company upfront fees of an aggregate of $7.0 million. Assuming successful development and commercialization, the Company could receive up to an additional $237.0 million in development, regulatory, and sales milestone payments. If commercialized, the Company would be eligible to receive tiered royalties on global net sales at percentages ranging from mid-to-high single digits. 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 until the expiration of the later of (i) ten years after the first commercial sale and (ii) expiration of the last to expire valid claim on patents covering the licensed product.

Pursuant to the AskBio License Agreement, the Company will supply AskBio with its ImmTOR platform, or the Supply Obligation, and AskBio will be responsible for all preclinical, clinical and commercial manufacture and supply of licensed products (other than ImmTOR) and carry out all other activities related to the research, development, and commercialization of licensed products at its sole expense, including all regulatory activities related thereto.

The Company determined that the AskBio License Agreement and Supply Obligation represent a single promise and performance obligation. This is because AskBio cannot derive benefit from the license without the simultaneous transfer of the patent protected ImmTOR supply. Therefore, the License Obligation and Supply Obligation represent the only promise in the arrangement and are combined as a single performance obligation.

In determining the transaction price, the Company concluded that the future development milestones, regulatory milestones, sales milestones, 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. Consideration related to sales-based milestones as well as royalties on net sales upon commercialization by AskBio, will be recognized when the related sales occur, as they were determined to relate predominantly to the intellectual property granted to AskBio and, therefore, have also been excluded from the transaction price in accordance with the royalty recognition constraint. As of March 31, 2021 and December 31, 2020, 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 total initial transaction price of the contract on the effective date was $7.0 million, comprised of a $2.0 million initial up-front payment upon agreement of terms, and a $5.0 million initial up-front execution fee.

At each of March 31, 2021 and December 31, 2020, the Company recorded $1.7 million as short-term and $5.3 million as a long-term, representing deferred revenue associated with this agreement. Revenue will be recognized over the period in which the particles are delivered. No revenue related to the AskBio License Agreement was recognized during the three months ended March 31, 2021 and 2020 as no deliveries were made during the periods.

Spark Therapeutics, Inc.

Spark License Agreement

The disclosures relating to the Company’s license and option agreement, or the Spark License Agreement, with Spark pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the ImmTOR platform reported in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 which was filed with the SEC on March 12, 2021 have not materially changed since the Company filed such report.

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). As of March 31, 2021, the aggregate amount of the transaction price allocated to remaining performance obligations was $108.1 million.

Contract Balances from Contracts with Customers (Sobi, Sarepta, AskBio, and Spark)

The following table presents changes in the Company’s contract liabilities during the three months ended March 31, 2021 (in thousands):


	Balance at						Balance at
	beginning of period		Additions		Deductions		end of period
Three Months Ended March 31, 2021


Contract liabilities:
Deferred revenue	$	110,796	$	—	$	(2,730)	$	108,066
Total contract liabilities	$	110,796	$	—	$	(2,730)	$	108,066

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

Related-Party Transactions	3 Months Ended
Related-Party Transactions	Mar. 31, 2021
Related Party Transactions [Abstract]
Related-Party Transactions	Related-Party Transactions Consulting Services The Company incurred expenses for consulting services provided by its founders totaling less than $0.1 million during each of the three months ended March 31, 2021 and 2020. The Company entered into consulting agreements with its founders to serve on its Scientific Advisory Board, effective January 1, 2020 to December 31, 2021, under which they will be paid quarterly for their services.

Collaboration Agreements	3 Months Ended
Collaboration Agreements	Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Collaboration Agreements	Collaboration Agreements Asklepios Biopharmaceutical, Inc. Feasibility Study and License Agreement In August 2019, the Company entered into a feasibility study and license agreement with AskBio, or the AskBio Collaboration Agreement. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain AAV gene therapy products utilizing the Company’s ImmTOR platform 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 Collaboration Agreement, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AskBio’s AAV gene therapy, or MMA-101, for the treatment of MMA, to mitigate the formation of neutralizing anti-AAV capsid antibodies, or 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 Collaboration Agreement will expire. The SEL-399 program combines an empty AAV capsid (EMC-101), which is an AAV capsid containing no transgene, with ImmTOR and is being conducted in partnership with AskBio. Building on the preclinical data the Company has generated showing ImmTOR’s effect on mitigating or reducing the formation of neutralizing antibodies to AAV gene therapies, the Company has commenced a clinical trial of SEL-399 in healthy adult volunteers in Belgium. The goal of the SEL-399 clinical trial is to demonstrate the appropriate dose of ImmTOR in humans to mitigate the formation of antibodies to AAV capsids used in gene therapies. 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 Collaboration Agreement, AskBio is responsible for manufacturing the AAV capsids and AAV vectors and the Company is responsible for manufacturing ImmTOR. The AskBio Collaboration Agreement 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 Collaboration Agreement and have identified the following promises in the arrangement (1) conducting research and development activities to develop and commercialize products under the collaboration, or 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, or the IP Rights, for the purpose of performing the POC Studies, or the Research License, (3) granting an exclusive, nontransferable, worldwide license to the IP Rights for use in certain indications, or the Collaboration License, (4) providing manufactured supply of preclinical and clinical ImmTOR, or the Manufactured Supply, (5) participation on identified steering committees responsible for the oversight of the collaboration, or 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 (see discussion of the option exercise in Note 12). 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 the Company’s condensed consolidated statements of operations. For each of the three months ended March 31, 2021 and 2020, the Company recognized $1.2 million of collaboration expense under the AskBio Collaboration Agreement in which actual costs incurred by both parties approximate a 50% cost share. 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 collaborators as reductions to R&D expense. Massachusetts Institute of Technology In November 2008, the Company entered into an exclusive patent license agreement, or the MIT License, with the Massachusetts Institute of Technology, or MIT, under which 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. In June 2020, the Company entered into a Fifth Amendment, or the MIT Amendment, to the MIT License, which is effective as of May 15, 2020. Pursuant to the MIT Amendment, certain of the Company’s diligence obligations were extended, including a diligence obligation to commence a Phase 3 trial for a licensed product by a specific date in the second quarter of 2021. Additionally, certain of the Company’s development and regulatory milestones and payments upon achievement of such milestones were adjusted. As of March 31, 2021, and in connection with the execution of the Spark License Agreement, the Company has made contractual payments pursuant to the MIT 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 three months ended March 31, 2021. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement, or the 3SBio License, with Shenyang Sunshine Pharmaceutical Co., Ltd., or 3SBio. The Company has paid to 3SBio an aggregate of $7.0 million in upfront and milestone-based payments under the 3SBio License as of March 31, 2021. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $15.0 million for products containing the Company’s ImmTOR platform.

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2021
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 months ended March 31, 2021 and 2020, 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. 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 have occurred previously, or 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. As of December 31, 2020, the Company completed a Section 382 study. The Company has determined that all of the $234.5 million of net operating losses are available in the future, with approximately $36.5 million of that total limited under Section 382 and therefore available for future use through 2028. The Company applies ASC 740, Income Taxes to uncertain tax positions. As of March 31, 2021 and December 31, 2020, the Company had no unrecognized tax benefits or related interest and penalties accrued. 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. The statute of limitations for assessment by the United States 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.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Defined Contribution Plan	3 Months Ended
Defined Contribution Plan	Mar. 31, 2021
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution PlanThe Company maintains a defined contribution plan, or the 401(k) Plan, under Section 401(k) of the Internal Revenue Code. 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 March 31, 2021 and 2020.

- Definition

The entire disclosure for retirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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+ Details

Commitments and Contingencies	3 Months Ended
Commitments and Contingencies	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of March 31, 2021, 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. On August 4, 2020, a putative stockholder of Selecta filed a stockholder derivative action, purportedly on behalf of Selecta and against certain current and former members of the Company’s Board of Directors, as well as one affiliated company owned by a current board member, in the Court of Chancery of the State of Delaware, namely Franchi v. Barabe, et al. The complaint alleges that the individual defendants breached their fiduciary duties and committed corporate waste when they authorized a private placement transaction, announced on December 19, 2019, at a price allegedly below fair value. The complaint further alleges that the four defendant directors who participated in the private placement were unjustly enriched in connection with the transaction. On September 25, 2020, the defendants filed a motion to dismiss the lawsuit. On November 6, 2020, the plaintiff filed an amended complaint, and the defendants filed a second motion to dismiss on January 8, 2021. On December 31, 2020, the Company received a litigation demand letter from two other putative stockholders relating to the same private placement transaction. On April 12, 2021, the Court of Chancery in the State of Delaware granted a motion to stay the litigation pending a review by a Special Committee appointed by the Company’s Board of Directors. At this time, the Company has not accrued a liability for this matter, as any liability has been determined to be either not estimable or not probable. 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.

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events “At-the-Market” Offerings Subsequent to March 31, 2021 through May 13, 2021, the Company sold 206,593 shares of its common stock pursuant to the 2020 Sales Agreement, at an average price of approximately $4.51 per share for aggregate net proceeds of $0.9 million, after deducting commissions and other transaction costs. Milestone Event On April 13, 2021, the Company was notified by Sarepta of the achievement of the milestone event related to the completion of a non-clinical study for Duchenne muscular dystrophy and certain limb-girdle muscular dystrophies under the Company’s Research License and Option Agreement. Under the terms of the Agreement, Sarepta is obligated to pay a non-refundable, non-creditable milestone payment of $3.0 million to the Company within 60 days of Sarepta’s receipt of the related invoice from the Company. The AskBio Collaboration Agreement On April 29, 2021, the Company was notified by AskBio that it intended to opt-out of development of the MMA indication. Consequently, the Company will assume all rights to the MMA program and intends to continue to progress the MMA-101 and ImmTOR combination through clinical development.

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2021
Accounting Policies [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740) - Simplifying the Accounting for Income Taxes. ASU 2019-12 simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The Company adopted the new standard effective January 1, 2021, and there was no impact on its consolidated financial statements. In October 2020, the FASB issued ASU 2020-10, Codification Improvements, which updates various codification topics by clarifying or improving disclosure requirements to align with the SEC’s regulations. The Company adopted the new standard effective January 1, 2021, and there was no impact on its consolidated financial statements. Not Yet Adopted In August 2020, the FASB issued ASU 2020-06, Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815 – 40). ASU 2020-06 simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2023, with early adoption permitted no earlier than January 1, 2021. The Company is assessing the impact this standard will have on its consolidated financial statements and disclosures. 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.

Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2021

Investments, Debt and Equity Securities [Abstract]

Summary of Available-for-Sale Marketable Securities

The following table summarizes the marketable securities held as of March 31, 2021 (in thousands):


	Amortized cost		Unrealized gains		Unrealized losses		Fair value
March 31, 2021

Corporate bonds	$	2,032	$	—	$	(1)	$	2,031
Commercial paper	20,374		—		—		20,374

Total	$	22,406	$	—	$	(1)	$	22,405

Net Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2021

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 March 31,
	2021		2020
Numerator:
Net loss attributable to common stockholders	$	(24,597)	$	(19,620)
Denominator:
Weighted-average common shares and pre-funded warrants outstanding—basic and diluted	110,742,150		94,723,513
Net loss per share attributable to common stockholders —basic and diluted	$	(0.22)	$	(0.21)

Schedule of Potential Common Shares Issuable Upon Conversion of Warrants

Potential dilutive common share equivalents consist of the following:


	March 31,
	2021	2020
Stock options to purchase common stock	10,187,394	7,745,936
Unvested restricted stock units	544,313	170,313
Warrants to purchase common stock	12,378,016	23,084,120


Total	23,109,723	31,000,369

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2021

Fair Value Disclosures [Abstract]

Summary of Assets Measured at Fair Value on a Recurring Basis

The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of March 31, 2021 and December 31, 2020 (in thousands):


	March 31, 2021
	Total		(Level 1)		(Level 2)		(Level 3)
Assets:
Money market funds	$	58,158	$	58,158	$	—	$	—
Marketable securities:
Corporate bonds	2,031		—		2,031		—
Commercial paper	20,374		—		20,374		—
Total assets	$	80,563	$	58,158	$	22,405	$	—

Liabilities:
Warrant liabilities	$	45,455	$	—	$	—	$	45,455
Total liabilities	$	45,455	$	—	$	—	$	45,455


	December 31, 2020
	Total		(Level 1)		(Level 2)		(Level 3)
Assets:
Money market funds	$	80,576	$	80,576	$	—	$	—


Total assets	$	80,576	$	80,576	$	—	$	—

Liabilities:
Warrant liabilities	$	28,708	$	—	$	—	$	28,708
Total liabilities	$	28,708	$	—	$	—	$	28,708

Schedule of Fair Value Measurement Inputs and Valuation Techniques

A summary of the Black-Scholes pricing model assumptions used to record the fair value of the warrant liability is as follows:


	March 31,
	2021
Risk-free interest rate	0.35	%
Dividend yield	—
Expected life (in years)	3.73
Expected volatility	101.13	%

Schedule Of Changes In The Warrant Liabilities

The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 10), for the three months ended March 31, 2021 (in thousands):


	Warrant liabilities
Fair value as of December 31, 2020	$	28,708

Change in fair value	16,747
Fair value as of March 31, 2021	$	45,455

Schedule of Cash and Cash Equivalents

The Company’s consolidated statement of cash flows includes the following as of March 31, 2021 and 2020 (in thousands):


	March 31,
	2021		2020
Cash and cash equivalents	$	125,407	$	72,606
Short-term restricted cash	—		279
Long-term restricted cash	1,379		1,379
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	126,786	$	74,264

Schedule of Restricted Cash and Cash Equivalents

The Company’s consolidated statement of cash flows includes the following as of March 31, 2021 and 2020 (in thousands):


	March 31,
	2021		2020
Cash and cash equivalents	$	125,407	$	72,606
Short-term restricted cash	—		279
Long-term restricted cash	1,379		1,379
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$	126,786	$	74,264

Property and Equipment (Tables)

3 Months Ended

Mar. 31, 2021

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

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


	March 31,		December 31,
	2021		2020
Laboratory equipment	$	4,429	$	4,427
Computer equipment and software	694		532
Leasehold improvements	38		38
Furniture and fixtures	327		327
Office equipment	163		163
Construction in process	30		163
Total property and equipment	5,681		5,650
Less accumulated depreciation	(4,380)		(4,255)
Property and equipment, net	$	1,301	$	1,395

Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2021

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):


	March 31,		December 31,
	2021		2020
Payroll and employee related expenses	$	1,840	$	3,049

Collaboration and licensing	1,350		1,350
Accrued patent fees	471		534
Accrued external research and development costs	2,010		2,029
Accrued professional and consulting services	802		798

Accrued interest	170		170

Other	204		216
Accrued expenses	$	6,847	$	8,146

Leases (Tables)

3 Months Ended

Mar. 31, 2021

Leases [Abstract]

Schedule of Lessee, Components Of Operating Lease

As of March 31, 2021 and December 31, 2020, the components of the operating leases were as follows (in thousands):


	March 31,	December 31,
		2021		2020
Assets:
Right-of-use asset, net		$	10,676	$	10,948

Liabilities:
Current operating lease liabilities		$	942	$	908
Non-current operating lease liabilities		9,403		9,647
Total operating lease liabilities		$	10,345	$	10,555

Schedule of Lease, Cost

For the three months ended March 31, 2021 and 2020 the components of lease costs were as follows (in thousands):


	March 31,
		2021		2020
Operating lease cost		$	444	$	472
Variable lease cost		288		199
Short-term lease cost		3		2
Total lease cost		$	735	$	673

The following summarizes additional information related to operating leases:


	March 31,
	2021		2020
Weighted-average remaining lease term	7.1 years		8.0 years
Weighted-average discount rate	8.9	%	8.9	%

Schedule of Lessee, Operating Lease, Liability, Maturity

The maturity of the Company’s operating lease liabilities as of March 31, 2021 were as follows (in thousands):


	March 31,
	2021
2021 (remainder)	$	1,368
2022	1,866
2023	1,922
2024	1,980
2025	2,039
Thereafter	4,945
Total future minimum lease payments	14,120
Less imputed interest	3,775
Total operating lease liabilities	$	10,345

Schedule of Operating Lease, Lease Income

The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands):


	March 31,
	2021		2020
Cash paid for amounts included in the measurement of lease liabilities:	$	444	$	487

Debt (Tables)

3 Months Ended

Mar. 31, 2021

Debt Disclosure [Abstract]

Schedule of Future Minimum Payments on the Term Loans

Future minimum principal and interest payments on the 2020 Term Loan as of March 31, 2021 are as follows (in thousands):


2021 (remainder)	$	1,509
2022	7,343
2023	8,611
2024	8,027
2025	7,274
Total minimum debt payments	32,764
Less: Amount representing interest	(5,514)
Less: Debt discount and deferred charges	(2,239)
Less: Current portion of loan payable	(460)
Loan payable, net of current portion	$	24,551

Equity (Tables)

3 Months Ended

Mar. 31, 2021

Equity [Abstract]

Schedule of Stockholders' Equity Note, Warrants or Rights


	Number of Warrants
	Equity classified	Liability classified	Total	Weighted average exercise price





Outstanding at March 31, 2021	292,469	12,085,547	12,378,016	$	1.60

Schedule of Authorized Shares of Common Stock for Future Issuance

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


	Period ended
	March 31, 2021	December 31, 2020
Exercise of common warrants	12,378,016	12,378,016
Shares available for future stock incentive awards	7,719,817	4,916,374
RSUs reserved for issuance	98,750	—
Unvested restricted stock units	544,313	87,500
Outstanding common stock options	10,187,394	7,775,249
Total	30,928,290	25,157,139

- 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.

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+ Details

Stock Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2021

Share-based Payment Arrangement [Abstract]

Schedule of Stock-Based Compensation

Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):


	Three Months Ended March 31,
	2021		2020
Research and development	$	754	$	623
General and administrative	1,026		786
Total stock-based compensation expense	$	1,780	$	1,409

Schedule of Weighted Average Assumptions Used


	Three Months Ended March 31,
	2021			2020
Risk-free interest rate	0.44		%	1.73		%
Dividend yield	—			—
Expected term	5.28			6.08
Expected volatility	83.26		%	87.96		%
Weighted-average fair value of common stock	$	3.13		$	2.30

Summary of Options Activity

The following table summarizes the stock option activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan:


				Weighted-average
				remaining	Aggregate
	Number of	Weighted-average		contractual term	intrinsic value
	options	exercise price ($)		(in years)	(in thousands)
Employees








Outstanding at December 31, 2020	7,302,176	$	3.98	8.43	$	4,456
Granted	2,628,000	$	3.13
Exercised	(153,278)	$	1.47
Forfeited	(62,577)	$	5.19
Outstanding at March 31, 2021	9,714,321	$	3.78	8.65	$	15,665

Vested at March 31, 2021	3,064,130	$	5.46	7.63	$	4,313
Vested and expected to vest at March 31, 2021	8,920,548	$	3.88	8.59	$	14,231

Non-employee consultants








Outstanding at December 31, 2020	473,073	$	5.89	5.23	$	86
Granted	—	$	—
Exercised	—	$	—
Forfeited	—	$	—
Outstanding at March 31, 2021	473,073	$	5.89	4.98	$	366

Vested at March 31, 2021	462,186	$	5.82	4.92	$	366
Vested and expected to vest at March 31, 2021	473,073	$	5.89	4.98	$	366

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

The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan:


	Number of shares	Weighted average grant date fair value ($)
Unvested at December 31, 2020	87,500	$	6.03
Granted	468,550	2.99
Vested	(10,937)	6.03
Reserved for issuance	98,750	2.99
Forfeited	(800)	2.99
Unvested at March 31, 2021	643,063	$	3.35

Revenue Arrangements (Tables)

3 Months Ended

Mar. 31, 2021

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 three months ended March 31, 2021 (in thousands):


	Balance at						Balance at
	beginning of period		Additions		Deductions		end of period
Three Months Ended March 31, 2021


Contract liabilities:
Deferred revenue	$	110,796	$	—	$	(2,730)	$	108,066
Total contract liabilities	$	110,796	$	—	$	(2,730)	$	108,066

Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents, restricted cash and marketable securities	$ 149,200
Restricted cash and cash equivalents	1,400
Accumulated deficit	429,226	$ 404,629
Russian subsidiary
Cash and Cash Equivalents [Line Items]
Cash maintained in Russian bank accounts	$ 300

Marketable Securities - Summary of Available-for-Sale Marketable Securities (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 22,406,000
Unrealized gains	0
Unrealized losses	(1,000)
Fair value	22,405,000	$ 0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,032,000
Unrealized gains	0
Unrealized losses	(1,000)
Fair value	2,031,000
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost	20,374,000
Unrealized gains	0
Unrealized losses	0
Fair value	$ 20,374,000

Marketable Securities - Narrative (Details) - USD ($)	3 Months Ended
Marketable Securities - Narrative (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Marketable securities adjusted for other than temporary declines in fair value	$ 0
Marketable securities:	$ 22,405,000	$ 0

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net loss attributable to common stockholders	$ (24,597)	$ (19,620)
Denominator:
Weighted‑average common shares outstanding—basic and diluted (in shares)	110,742,150	94,723,513
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.22)	$ (0.21)

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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+ Details

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Potential common shares
Total (in shares)	23,109,723	31,000,369
Stock options to purchase common stock
Potential common shares
Total (in shares)	10,187,394	7,745,936
Unvested restricted stock units
Potential common shares
Total (in shares)	544,313	170,313
Warrants to purchase common stock
Potential common shares
Total (in shares)	12,378,016	23,084,120

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Assets:
Marketable securities:	$ 22,405,000	$ 0
Liabilities:
Warrant liabilities	45,455,000	28,708,000
Corporate bonds
Assets:
Marketable securities:	2,031,000
Commercial paper
Assets:
Marketable securities:	20,374,000
Recurring
Assets:
Total assets	80,563,000	80,576,000
Liabilities:
Warrant liabilities	45,455,000	28,708,000
Total liabilities	45,455,000	28,708,000
Recurring \| (Level 1)
Assets:
Total assets	58,158,000	80,576,000
Liabilities:
Warrant liabilities	0	0
Total liabilities	0	0
Recurring \| (Level 2)
Assets:
Total assets	22,405,000	0
Liabilities:
Warrant liabilities	0	0
Total liabilities	0	0
Recurring \| (Level 3)
Assets:
Total assets	0	0
Liabilities:
Warrant liabilities	45,455,000	28,708,000
Total liabilities	45,455,000	28,708,000
Recurring \| Corporate bonds
Assets:
Marketable securities:	2,031,000
Recurring \| Corporate bonds \| (Level 1)
Assets:
Marketable securities:	0
Recurring \| Corporate bonds \| (Level 2)
Assets:
Marketable securities:	2,031,000
Recurring \| Corporate bonds \| (Level 3)
Assets:
Marketable securities:	0
Recurring \| Commercial paper
Assets:
Marketable securities:	20,374,000
Recurring \| Commercial paper \| (Level 1)
Assets:
Marketable securities:	0
Recurring \| Commercial paper \| (Level 2)
Assets:
Marketable securities:	20,374,000
Recurring \| Commercial paper \| (Level 3)
Assets:
Marketable securities:	0
Recurring \| Money market funds
Assets:
Money market funds	58,158,000	80,576,000
Recurring \| Money market funds \| (Level 1)
Assets:
Money market funds	58,158,000	80,576,000
Recurring \| Money market funds \| (Level 2)
Assets:
Money market funds	0	0
Recurring \| Money market funds \| (Level 3)
Assets:
Money market funds	$ 0	$ 0

Fair Value Measurements - Assumptions used to record the fair value of the warrants (Details) - Valuation Technique, Option Pricing Model	Mar. 31, 2021
Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0035
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0
Expected life (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	3 years 8 months 23 days
Expected volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	1.0113

Fair Value Measurements - Change in the warrant liabilities (Details) - Warrant liabilities $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value as of December 31, 2020	$ 28,708
Change in fair value	16,747
Fair value as of March 31, 2021	$ 45,455

Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
Fair Value Measurements - Narrative (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers, net	$ 0	$ 0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, liability, transfers, net	$ 0	$ 0

Fair Value Measurements - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Cash and cash equivalents	$ 125,407	$ 138,685	$ 72,606
Short-term restricted cash	0		279
Long-term restricted cash	1,379	1,379	1,379
Total cash, cash equivalents, and restricted cash shown in the consolidated statement of cash flows	$ 126,786	$ 140,064	$ 74,264	$ 91,551

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 5,681	$ 5,650
Less accumulated depreciation	(4,380)	(4,255)
Property and equipment, net	1,301	1,395
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	4,429	4,427
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	694	532
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	38	38
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	327	327
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	163	163
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 30	$ 163

Property and Equipment - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 0.1	$ 0.2

Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 1,840	$ 3,049
Collaboration and licensing	1,350	1,350
Accrued patent fees	471	534
Accrued external research and development costs	2,010	2,029
Accrued professional and consulting services	802	798
Accrued interest	170	170
Other	204	216
Accrued expenses	$ 6,847	$ 8,146

Leases - Narrative (Details) $ in Thousands	1 Months Ended	3 Months Ended
Leases - Narrative (Details) $ in Thousands	Jul. 31, 2019 USD ($) ft²	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Operating Leased Assets [Line Items]
Lessee-paid construction costs		$ 735	$ 673
Operating lease, weighted average discount rate, percent		8.90%	8.90%
65 Grove Street, Watertown,MA
Operating Leased Assets [Line Items]
Area of additional office space leased \| ft²	25,078
Lessee-paid construction costs	$ 800
Term of contract	8 years
Letter of credit
Operating Leased Assets [Line Items]
Long-term restricted cash		$ 1,400		$ 1,400

Leases - Components of Operating Leases (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets:
Right-of-use asset, net	$ 10,676	$ 10,948
Liabilities:
Current operating lease liabilities	942	908
Non-current operating lease liabilities	9,403	9,647
Total operating lease liabilities	$ 10,345	$ 10,555

Leases - Components of lease costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Leases [Abstract]
Operating lease cost	$ 444	$ 472
Variable lease cost	288	199
Short-term lease cost	3	2
Total lease cost	$ 735	$ 673

Leases - Operating Lease, Liability, Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Leases [Abstract]
2021 (remainder)	$ 1,368
2022	1,866
2023	1,922
2024	1,980
2025	2,039
Thereafter	4,945
Total future minimum lease payments	14,120
Less imputed interest	3,775
Lease liabilities	$ 10,345	$ 10,555

Leases - Other Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities:	$ 444	$ 487

Leases - Lease Term and Discount Rate (Details)	Mar. 31, 2021	Mar. 31, 2020
Leases [Abstract]
Operating lease, weighted average remaining lease term	7 years 1 month 6 days	8 years
Operating lease, weighted average discount rate, percent	8.90%	8.90%

Debt - Term Loans (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Aug. 31, 2020	Mar. 31, 2021	Sep. 30, 2020	Dec. 31, 2020
Line of Credit Facility [Line Items]
Final payment fee		$ 2,300
Class of warrant or right, number of securities called by warrants or rights (in shares)	196,850
Exercise price (in dollars per share)	$ 2.54	$ 1.60
Class of warrant or right, expiration period	10 years
Adjustments to additional paid in capital, warrant issued	$ 400
2020 Term Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	35,000
Debt instrument, additional accrued interest in the event of default		0.050
Debt instrument, redemption price, percentage of principal amount redeemed		9.00%
Debt instrument, default, bankruptcy, or insolvency value of triggering accelerated redemption	500
Debt issuance costs, line of credit arrangements (less than)	400
Term loan facility		$ 25,000		$ 25,000
Term A Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	25,000
Term B Loans
Line of Credit Facility [Line Items]
Maximum borrowing capacity	10,000
2017 Term Loans
Line of Credit Facility [Line Items]
Repayments of lines of credit	13,700
Loss on extinguishment of debt			$ 500
Warrants to purchase common stock \| Term B Loans
Line of Credit Facility [Line Items]
Sale common stock	$ 200,000
Maximum \| 2020 Term Loans
Line of Credit Facility [Line Items]
Debt instrument, prepayment fee, percentage		0.030
Minimum \| 2020 Term Loans
Line of Credit Facility [Line Items]
Debt instrument, prepayment fee, percentage		0.010
Debt Instrument, Redemption, Period One \| 2020 Term Loans
Line of Credit Facility [Line Items]
Basis spread on variable rate		7.90%
Debt Instrument, Redemption, Period Two \| 2020 Term Loans
Line of Credit Facility [Line Items]
Basis spread on variable rate		4.65%
Debt Instrument, Redemption, Period Three \| 2020 Term Loans
Line of Credit Facility [Line Items]
Basis spread on variable rate		10.00%

Debt - Schedule of Future Minimum Payments on the Term Loans (Details) - 2017 Term Loans $ in Thousands	Mar. 31, 2021 USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2021 (remainder)	$ 1,509
2022	7,343
2023	8,611
2024	8,027
2025	7,274
Total minimum debt payments	32,764
Less: Amount representing interest	(5,514)
Less: Debt discount and deferred charges	(2,239)
Less: Current portion of loan payable	(460)
Loan payable, net of current portion	$ 24,551

Equity - Narrative (Details)					1 Months Ended	3 Months Ended		12 Months Ended
Equity - Narrative (Details)	Aug. 06, 2020 USD ($)	Jul. 28, 2020 USD ($)	Jun. 11, 2020 USD ($) $ / shares shares	Dec. 18, 2019 USD ($) $ / shares shares	Aug. 31, 2017 USD ($)	Mar. 31, 2021 USD ($) vote $ / shares shares	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / shares shares	Aug. 31, 2020 $ / shares
Subsidiary, Sale of Stock [Line Items]
Shares issued, value						$ 20,991,000	$ 1,141,000
Class of warrant or right, outstanding (in shares) \| shares						12,378,016,000
Exercise price (in dollars per share) \| $ / shares						$ 1.60			$ 2.54
Warrant liabilities						$ 45,455,000		$ 28,708,000
Common stock, shares authorized (in shares) \| shares						200,000,000		200,000,000
Common stock, par value (in dollars per share) \| $ / shares						$ 0.0001		$ 0.0001
Common stock, shares issued \| shares						112,977,004		108,071,249
Common stock, shares outstanding \| shares						112,977,004		108,071,249
Number of votes per share that common stockholders are entitled to \| vote						1
Dividends declared or paid on common stock						$ 0
Exercise of common warrants
Subsidiary, Sale of Stock [Line Items]
Class of warrant or right, outstanding (in shares) \| shares						292,469,000
August 2020 Shelf Registration Statement
Subsidiary, Sale of Stock [Line Items]
Sale common stock	$ 200,000,000.0
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Sale common stock					$ 50,000,000.0
Sale of stock, number of shares issued in transaction (in shares) \| shares						4,706,844		1,069,486
Share price (in dollars per share) \| $ / shares						$ 4.59		$ 2.16
Shares issued, value						$ 20,900,000		$ 2,100,000
2020 Sale Agreement - At-the-Market Offering
Subsidiary, Sale of Stock [Line Items]
Sale common stock					$ 50,000,000.0
SOBI Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Sale common stock			$ 25,000,000.0
Sale of stock, number of shares issued in transaction (in shares) \| shares			5,416,390
Share price (in dollars per share) \| $ / shares			$ 4.6156
Sale of stock, percentage of ten-day volume weighted average price of common stock			120.00%
Lock-up from closing, term						1 year
Redemption premium		$ 14,500,000
December 2019 Financing
Subsidiary, Sale of Stock [Line Items]
Sale common stock				$ 65,600,000
Sale of stock, number of shares issued in transaction (in shares) \| shares				37,634,883
Share price (in dollars per share) \| $ / shares				$ 1.46
Class of warrant or right, outstanding (in shares) \| shares				22,988,501
Warrant Issued				$ 40,700,000
Issuance costs							4,400,000
Warrant liabilities						$ 45,500,000
Warrant, increase (decrease) in equity, amount						$ (16,700,000)	$ (800,000)
December 2019 Financing \| Pre-Funded Warrant
Subsidiary, Sale of Stock [Line Items]
Class of warrant or right, number of warrants exercised (in shares) \| shares				8,342,128
Exercise price (in dollars per share) \| $ / shares				$ 0.0001
December 2019 Financing \| Common Warrant
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) \| $ / shares				0.125
Exercise price (in dollars per share) \| $ / shares				$ 1.46
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized (in shares) \| shares						200,000,000
Common stock, par value (in dollars per share) \| $ / shares						$ 0.0001

Equity - Warranty Activity (Details) - $ / shares shares in Thousands	Mar. 31, 2021	Aug. 31, 2020
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	12,378,016
Exercise price (in dollars per share)	$ 1.60	$ 2.54
Exercise of common warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	292,469
Exercise of common warrants
Class of Warrant or Right [Line Items]
Class of warrant or right, outstanding (in shares)	12,085,547

Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details) - shares	Mar. 31, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Total (in shares)	30,928,290	25,157,139
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	7,719,817	4,916,374
RSUs reserved for issuance
Class of Stock [Line Items]
Total (in shares)	98,750	0
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	544,313	87,500
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	10,187,394	7,775,249
Exercise of common warrants
Class of Stock [Line Items]
Total (in shares)	12,378,016	12,378,016

Stock Incentive Plans - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended
Stock Incentive Plans - Narrative (Details) - USD ($)	Jan. 31, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Mar. 25, 2019	Sep. 25, 2018	Jun. 21, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		30,928,290		25,157,139
Stock‑based compensation expense		$ 1,780,000	$ 1,409,000
Unrecognized compensation expense related to unvested non-employee stock options		$ 0
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)		7,719,817		4,916,374
Estimated forfeitures rate		10.00%
Weighted average grant date fair value of stock options (in dollars per share)		$ 1.99	$ 1.69
Unrecognized compensation expense related to unvested employee stock options		$ 11,800,000
Weighted average period for recognition		2 years 8 months 12 days
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants in period (in dollars per share)		$ 2.99
Grants in period (in shares)		468,550
Reserved for issuance (in shares)		98,750
Reserved for issuance (in dollars per share)		$ 2.99
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)							1,210,256
Number of shares authorized, increase (in shares)		4,322,850	3,453,022
Common stock authorized and reserved for future issuance (in shares)		3,248,520
2016 Plan \| Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested in period		$ 1,400,000
Grants in period (in shares)	369,800	98,750
Awarded in period (in shares)		197,500
Reserved for issuance (in shares)		98,750
Award vesting term		4 years
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)		1,358,333
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]
Weighted average period for recognition		3 years 1 month 6 days
Unrecognized compensation expense		$ 1,500,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized, increase (in shares)		1,080,711	863,254
Common stock authorized and reserved for future issuance (in shares)		2,546,464					173,076
Stock‑based compensation expense		$ 100,000	$ 100,000

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,780	$ 1,409
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	754	623
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,026	$ 786

Stock Incentive Plans - Schedule of Assumptions (Details) - Shares available for future stock incentive awards - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.44%	1.73%
Dividend yield	0.00%	0.00%
Expected term	5 years 3 months 10 days	6 years 29 days
Expected volatility	83.26%	87.96%
Weighted-average fair value of common stock (in dollars per share)	$ 3.13	$ 2.30

Stock Incentive Plans - Summary of Options Activity (Details) - 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Employees
Number of Options
Beginning balance (in shares)	7,302,176
Granted (in shares)	2,628,000
Exercised (in shares)	(153,278)
Forfeited (in shares)	(62,577)
Ending balance (in shares)	9,714,321	7,302,176
Vested (in shares)	3,064,130
Vested and expected to vest (in shares)	8,920,548
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 3.98
Granted (in dollars per share)	3.13
Exercised (in dollars per share)	1.47
Forfeited (in dollars per share)	5.19
Ending balance (in dollars per share)	3.78	$ 3.98
Vested (in dollars per share)	5.46
Vested and expected to vest (in dollars per share)	$ 3.88
Weighted-average remaining contractual term
Outstanding, term	8 years 7 months 24 days	8 years 5 months 4 days
Vested, term	7 years 7 months 17 days
Vested and expected to vest, term	8 years 7 months 2 days
Aggregate intrinsic value
Outstanding	$ 15,665	$ 4,456
Vested	4,313
Vested and expected to vest	$ 14,231
Non-employee consultants
Number of Options
Beginning balance (in shares)	473,073
Granted (in shares)	0
Exercised (in shares)	0
Forfeited (in shares)	0
Ending balance (in shares)	473,073	473,073
Vested (in shares)	462,186
Vested and expected to vest (in shares)	473,073
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 5.89
Granted (in dollars per share)	0
Exercised (in dollars per share)	0
Forfeited (in dollars per share)	0
Ending balance (in dollars per share)	5.89	$ 5.89
Vested (in dollars per share)	5.82
Vested and expected to vest (in dollars per share)	$ 5.89
Weighted-average remaining contractual term
Outstanding, term	4 years 11 months 23 days	5 years 2 months 23 days
Vested, term	4 years 11 months 1 day
Vested and expected to vest, term	4 years 11 months 23 days
Aggregate intrinsic value
Outstanding	$ 366	$ 86
Vested	366
Vested and expected to vest	$ 366

Stock Incentive Plans - Restricted Stock Units (Details) - Unvested restricted stock units	3 Months Ended
	Mar. 31, 2021 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested at beginning of period (in shares) \| shares	87,500
Grants in period (in shares) \| shares	468,550
Vested in period (in shares) \| shares	(10,937)
Reserved for issuance (in shares) \| shares	98,750
Forfeited in period (in shares) \| shares	(800)
Unvested at end of period (in shares) \| shares	643,063
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Unvested at beginning of period (in dollars per share) \| $ / shares	$ 6.03
Grants in period (in dollars per share) \| $ / shares	2.99
Vested in period (in dollars per share) \| $ / shares	6.03
Reserved for issuance (in dollars per share) \| $ / shares	2.99
Forfeited in period (in dollars per share) \| $ / shares	2.99
Unvested at end of period (in dollars per share) \| $ / shares	$ 3.35

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Jun. 21, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	30,928,290		25,157,139
Stock-based compensation expense	$ 1,780	$ 1,409
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	2,546,464			173,076
Number of shares authorized, increase (in shares)	1,080,711	863,254
Shares of common stock issued to employees under the ESPP (in shares)	34,696
Stock-based compensation expense	$ 100	$ 100

Revenue Arrangements - Narrative (Details)					3 Months Ended
Revenue Arrangements - Narrative (Details)	Jul. 28, 2020 USD ($)	Jun. 13, 2020 USD ($)	Jun. 11, 2020 USD ($) obligation	Dec. 17, 2019 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Disaggregation of Revenue [Line Items]
Short-term contract liability					$ 75,764,000		$ 72,050,000
Long-term contract liability					32,301,000		38,746,000
Contract assets amortization (less than)					100,000
Accounts receivable					8,342,000		7,224,000
Revenue recognized					2,730,000
Remaining performance obligation					108,100,000
SOBI Purchase Agreement
Disaggregation of Revenue [Line Items]
Sale common stock			$ 25,000,000.0
Redemption premium	$ 14,500,000
Short-term contract liability					72,000,000.0		68,300,000
Long-term contract liability					17,800,000		24,200,000
Contract assets					1,300,000
Accounts receivable					8,300,000		6,900,000
Revenue recognized					11,100,000
SOBI Purchase Agreement \| Prepaid Expenses and Other Current Assets
Disaggregation of Revenue [Line Items]
Contract assets					1,000,000.0
SOBI Purchase Agreement \| Other Current Assets
Disaggregation of Revenue [Line Items]
Contract assets					300,000
Collaborative Arrangement
Disaggregation of Revenue [Line Items]
Upfront cash payment			75,000,000.0
Sales milestone payments			$ 630,000,000.0
License and option agreement, written day notice period before cancellation			180 days
Period after first commercial sale when the Company is eligible to receive royalties			10 years
Number of obligations \| obligation			3
Development milestone			$ 5,000,000.0
Sarepta Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
Upfront cash payment		$ 2,000,000.0
License and option agreement, written day notice period before cancellation		30 days
Short-term contract liability					2,000,000.0		2,000,000.0
Revenue recognized					0	$ 0
Option term		24 months
License Agreement For Pompe Disease
Disaggregation of Revenue [Line Items]
Upfront cash payment				$ 7,000,000.0
Sales milestone payments				$ 237,000,000.0
Period after first commercial sale when the Company is eligible to receive royalties				10 years
Short-term contract liability					1,700,000		1,700,000
Long-term contract liability					5,300,000		$ 5,300,000
Revenue recognized					$ 0	$ 0
Initial up-front cash payment				$ 2,000,000.0
Initial upfront execution fee payment				$ 5,000,000.0

Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Contract liabilities:
Deferred revenue, beginning of period	$ 110,796
Contract liabilities, beginning of period	110,796
Deferred revenue, additions	0
Contract liabilities, additions	0
Deferred revenue, deductions	(2,730)
Contract liabilities, deductions	(2,730)
Deferred revenue, end of period	108,066
Contract liabilities, end of period	$ 108,066

Related-Party Transactions - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Founders
Related Party Transaction [Line Items]
Consulting services expenses (less than)	$ 0.1	$ 0.1

Collaboration Agreements - AskBio (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Operating expenses	$ 18,208	$ 18,822
AskBio License
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Operating expenses		$ 1,200
Cost share percentage	50.00%

Collaboration Agreements - MIT (Narrative) (Details) - Spark License Agreement	3 Months Ended
	Mar. 31, 2021 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 Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details) - 3SBio License $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate amount of upfront and milestone-based payments	$ 7.0
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing ImmTOR platform	$ 15.0

Income Taxes - Narrative (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Operating loss carryforwards, carried forward indefinitely	$ 234,500,000
Net operating losses available for use through 2028	36,500,000
Unrecognized tax benefits	$ 0	$ 0

Defined Contribution Plan - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Retirement Benefits [Abstract]
Vesting period	4 years
Employer contribution made	$ 0.1	$ 0.1

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended
	Apr. 13, 2021	May 13, 2021	Aug. 31, 2017
Sarepta Therapeutics, Inc. \| Forecast
Subsequent Event [Line Items]
Milestone payment	$ 3.0
2020 Sale Agreement - At-the-Market Offering
Subsequent Event [Line Items]
Sale common stock			$ 50.0
Subsequent Event \| 2020 Sale Agreement - At-the-Market Offering
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		206,593
Share price (in dollars per share)		$ 4.51
Sale common stock		$ 0.9