UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 2 )*

Cartesian Therapeutics, Inc.

(Name of Issuer)

Common Stock, $0.0001 par value

(Title of Class of Securities)

816212302

(CUSIP Number)

William R. Kolb

Daniel S. Clevenger

Foley Hoag LLP

155 Seaport Boulevard

Boston, MA 02210

Tel: (617) 832-1000

Fax: (617) 832-7000

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

October 11, 2024

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box. ☐

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

This Amendment No. 2 to the joint statement on Schedule 13D with respect to the common stock, par value $0.0001 (the “Common Stock”), of Cartesian Therapeutics, Inc., a Delaware corporation (the “Issuer”), filed by Murat Kalayoglu, Seven One Eight Three Four Irrevocable Trust (the “Trust”), Elizabeth Hoge and Sinan Kalayoglu (collectively, the “Reporting Persons”) on November 22, 2023, as amended by Amendment No. 1 filed by the Reporting Persons on April 10, 2024 (such joint statement, as previously amended and as further amended herein, the “Schedule 13D”), amends the Schedule 13D as follows:

On October 11, 2024, pursuant to a Notice of Optional Conversion delivered to the Company by the Trust pursuant to Section 6.02 of the Certificate of Designation, as amended by the Amendment to the Certificate of Designation, 45,551.190 shares of Series A Preferred Stock held by the Trust were converted into 1,518,373 shares of Common Stock.

On October 11, 2024, pursuant to a Notice of Optional Conversion delivered to the Company by the Trust pursuant to Section 6.02 of the Certificate of Designation, as amended by the Amendment to the Certificate of Designation, 45,551.190 shares of Series A Preferred Stock held by the Trust were converted into 1,518,373 shares of Common Stock.

(a, b) The percentages of beneficial ownership reported in this Item 5, and on each Reporting Person’s cover page to this Schedule 13D, are based on a total of 23,896,525 shares of Common Stock issued and outstanding as of October 2, 2024, as advised by the Issuer. All of the share numbers reported herein are as of October 11, 2024, unless otherwise indicated. Each Reporting Person’s cover page to this Schedule 13D is incorporated by reference into this Item 5(a, b).

In aggregate, the Reporting Persons have voting and dispositive power over 5,058,026 shares of Common Stock, representing approximately 19.9% of such class of securities. The securities reported herein exclude (a) 1,880,075 shares of Common Stock issuable upon conversion of shares of Series A Preferred Stock held by the Trust, but which would exceed the Conversion Limit, (b) 5,278 shares of Common Stock underlying stock options held by Murat Kalayoglu that are not exercisable within 60 days of the date hereof and (c) 5,933 shares of Common Stock underlying restricted stock units held by Murat Kalayoglu that will vest more than 60 days after the date hereof.

The beneficial ownership of each Reporting Person is as follows:

Pursuant to a right of substitution, Murat Kalayoglu has the right to acquire from the Trust the shares of Common Stock and Series A Preferred Stock held by the Trust in exchange for assets with an equal value to such shares. Accordingly, the shares of Common Stock beneficially owned by the Trust are included in his holdings. Murat Kalayoglu has the sole power to vote and dispose of the shares of Common Stock beneficially owned by him (as described above). The Trust has the sole power to vote and dispose of the shares of Common Stock beneficially owned by it (as described above). Each of Elizabeth Hoge and Sinan Kalayoglu has the shared power to vote and dispose of the shares of Common Stock beneficially owned by such person (as described above).

(c) No Reporting Person effected any transaction in the Common Stock from August 16, 2024 (the date 60 days prior to the filing of this Schedule 13D) to October 15, 2024.

(d) No person other than the Reporting Persons is known to have the right to receive or the power to direct the receipt of dividends from or the proceeds from the sale of shares of the Common Stock.

(e) Not applicable.

[signature page follows]

Signature

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.