Sec Form 3 Filing - NEXTECH CROSSOVER I SCSP @ Mersana Therapeutics, Inc. - 2024-12-20

FORM 3 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 NEXTECH CROSSOVER I SCSP 2. Issuer Name and Ticker or Trading Symbol Mersana Therapeutics, Inc. [ MRSN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 8 RUE LOU HEMMER, 3. Date of Earliest Transaction (MM/DD/YY) 12/20/2024 (Street) SENNINGERBERG, N4L-1748 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ 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 12,067,246 D ( 1 )

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 NEXTECH CROSSOVER I SCSP 8 RUE LOU HEMMER SENNINGERBERG, N4L-174 8 X Nextech Crossover I GP S.a r.l. 8 RUE LOU HEMMER SENNINGERBERG, N4L-1748 X Charoub Ian C/O NEXTECH INVEST 8 RUE LOU HEMMER SENNINGERBERG, N4L-1748 X Constantinides Costas C/O NEXTECH INVEST 8 RUE LOU HEMMER SENNINGERBERG, N4L-1748 X Sgobbo Rocco C/O NEXTECH INVEST 8 RUE LOU HEMMER SENNINGERBERG, N4L-1748 X Signatures Nextech Crossover I SCSp, By Nextech Crossover I GP S.a r.l., its General Partner, By /s/ Ian Charoub and /s/ Costas Constantinides, Managers 12/30/2024 Signature of Reporting Person Date Nextech Crossover I GP S.a r.l., By /s/ Ian Charoub and /s/ Costas Constantinides, Managers 12/30/2024 Signature of Reporting Person Date /s/ Ian Charoub 12/30/2024 Signature of Reporting Person Date /s/ Costas Constantinides 12/30/2024 Signature of Reporting Person Date /s/ Rocco Sgobbo 12/30/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The securities are held of record by Nextech Crossover I SCSp ("Nextech Crossover LP"). Nextech Crossover I GP S.a. r.l. ("Nextech Crossover GP") is the general partner of Nextech Crossover LP and Ian Charoub, Costas Constantinides and Rocco Sgobbo are Managers of Nextech Crossover GP. Nextech Crossover GP and Messrs. Charoub, Constantinides and Sgobbo may be deemed to share voting and investment power with respect to the securities reported herein and disclaim beneficial ownership over such securities, except to the extent of its or his respective pecuniary interest therein, if any.

Remarks: The Reporting Persons became the beneficial owners of greater than 10% of the Issuer's outstanding common stock as a result of a decrease in the number of shares outstanding, as described in the Issuer's Current Report on Form 8-K filed with the SEC on December 19, 2024, and not as a result of the acquisition of any securities by the Reporting Persons.

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).

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