Sec Form 4/A Filing - Mandelia Ashish @ Mersana Therapeutics, Inc. - 2024-01-13

FORM 4/A 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 Mandelia Ashish 2. Issuer Name and Ticker or Trading Symbol Mersana Therapeutics, Inc. [ MRSN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) VP, Chief Accounting Officer (Last) (First) (Middle) C/O MERSANA THERAPEUTICS, INC., 840 MEMORIAL DRIVE 3. Date of Earliest Transaction (MM/DD/YY) 01/13/2024 (Street) CAMBRIDGE, MA02139 4. If Amendment, Date Original Filed (MM/DD/YY) 01/17/2024 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 25,973 ( 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

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Mandelia Ashish C/O MERSANA THER APEUTICS, INC. 840 MEMORIAL DRIVE CAMBRIDGE, MA02139 VP, Chief Accounting Officer Signatures /s/ Alejandra Carvajal, Attorney-in-Fact 02/16/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On January 17, 2024, the Reporting Person filed a Form 4 reporting the vesting of restricted stock units ("RSU"s) between January 13, 2024 and January 15, 2024 and the sale of shares of common stock on January 16, 2024 to satisfy the Reporting Person's tax withholding obligations in connection with the service-based vesting and settlement of the RSUs (the "Original Form 4"). Due to an inadvertent arithmetic error, the number of shares beneficially owned by the Reporting Person following the receipt of the RSUs that vested on January 13, 2024 was incorrectly reported, which error also impacted the beneficial ownership figures reported following the subsequent transactions reported in the Original Form 4. This Form 4/A is being filed to correct the number of shares held by the Reporting Person as of January 16, 2024 after all transactions reported in the Original Form 4.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
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