Sec Form 4 Filing - Kurtoglu Metin @ Cartesian Therapeutics, Inc. - 2024-04-08

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Kurtoglu Metin 2. Issuer Name and Ticker or Trading Symbol Cartesian Therapeutics, Inc. [ RNAC] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Chief Technology Officer (Last) (First) (Middle) C/O CARTESIAN THERAPEUTICS, INC., 704 QUINCE ORCHARD ROAD 3. Date of Earliest Transaction (MM/DD/YY) 04/08/2024 (Street) GAITHERSBURG, MD20878 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 51,033 ( 1 ) D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option (Right to Buy) $ 46.77 04/08/2024 M 6,414.682 ( 2 ) 11/06/2026 Series A Non-Voting Convertible Preferred Stock 6,414.682 ( 3 ) 0 D Stock Option (Right to Buy) $ 1.41 04/08/2024 M 213,820 ( 4 ) 11/06/2026 Common Stock 213,820 ( 1 ) ( 5 ) 213,820 ( 1 ) D Stock Option (Right to Buy) $ 107.59 04/08/2024 M 427.645 ( 2 ) 04/25/2031 Series A Non-Voting Convertible Preferred Stock 427.645 ( 3 ) 0 D Stock Option (Right to Buy) $ 3.23 04/08/2024 M 14,254 ( 4 ) 04/25/2031 Common Stock 14,254 ( 1 ) ( 5 ) 14,254 ( 1 ) D Stock Option (Right to Buy) $ 107.59 04/08/2024 M 427.646 ( 6 ) 01/15/2033 Series A Non-Voting Convertible Preferred Stock 427.646 ( 3 ) 0 D Stock Option (Right to Buy) $ 3.23 04/08/2024 M 14,254 ( 7 ) 02/29/2032 Common Stock 14,254 ( 1 ) ( 5 ) 14,254 ( 1 ) D Stock Option (Right to Buy) $ 19.656 ( 8 ) 01/01/2034 Common Stock 65,499 ( 1 ) 65,499 ( 1 ) D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Kurtoglu Metin C/O CARTESIAN THERAPEUTICS, INC. 704 QUINCE ORCHARD ROAD GAITHERSBURG, MD20878 Chief Technology Officer Signatures /s/ Matthew Bartholomae, Attorney-in-Fact for Metin Kurtoglu 04/10/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On April 4, 2024, the issuer effected a 1-for-30 reverse stock split (the "Reverse Stock Split") of its common stock ("Common Stock"). The amount of securities reported on this Form 4 has been adjusted to reflect the Reverse Stock Split. ( 2 )On March 27, 2024, the issuer held a special meeting of stockholders (the "Special Meeting"). At the Special Meeting, the stockholders of the issuer approved the Reverse Stock Split and the conversion of the issuer's Series A Non-Voting Convertible Preferred Stock ("Series A Preferred Stock") into shares of Common Stock. On April 8, 2024, the conversion of the majority of the shares of Series A Preferred Stock occurred automatically (the "Automatic Conversion"). The remaining shares of Series A Preferred Stock remain subject to certain beneficial ownership limitations described in the issuer's filings with the Securities and Exchange Commission. Prior to the Automatic Conversion, the option was fully exercisable. ( 3 )On November 13, 2023, the issuer acquired the private Delaware corporation which was then known as Cartesian Therapeutics, Inc. ("Old Cartesian") in accordance with the terms of an Agreement and Plan of Merger, dated November 13, 2023. Options to purchase Old Cartesian common stock held by the reporting person were converted into options to purchase shares of Series A Preferred Stock in connection with the acquisition. ( 4 )Following the Automatic Conversion, the option is fully exercisable. ( 5 )Following the Automatic Conversion, options previously exercisable for shares of Series A Preferred Stock may be exercised solely for shares of Common Stock. ( 6 )Prior to the Automatic Conversion, the option was exercisable for 213.823 shares of Series A Preferred Stock and would have become exercisable for the remaining 213.823 shares of Series A Preferred Stock in two equal tranches on March 1, 2025 and 2026. ( 7 )Following the Automatic Conversion, the option is currently exercisable for 7,127 shares of Common Stock and becomes exercisable for the remaining 7,127 shares of Common Stock in two equal tranches on March 1, 2025 and 2026. ( 8 )This option vests as to 25% on January 2, 2025. The remainder of the underlying shares will vest in three equal annual installments thereafter so that the underlying shares will be fully vested on January 2, 2028.

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