Sec Form 4 Filing - EcoR1 Capital, LLC @ Mersana Therapeutics, Inc. - 2024-12-19

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 EcoR1 Capital, LLC 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) 357 TEHAMA STREET #3 3. Date of Earliest Transaction (MM/DD/YY) 12/19/2024 (Street) SAN FRANCISCO, CA94103 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 12/19/2024 J( 2 ) 8,036,688 ( 3 ) D 11,328,000 ( 4 ) I See Note ( 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 Pre-Funded Warrants $ 0.0001 12/19/2024 J( 2 ) 8,036,688 ( 5 ) 12/20/2024 ( 6 ) Common Stock 8,036,688 ( 7 ) ( 2 ) 8,036,688 ( 8 ) I See Note ( 1 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other EcoR1 Capital, LLC 357 TEHAMA STREET #3 SAN FRANCISCO, CA94103 X Signatures /s/ 12/23/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The reporting persons are EcoR1 Capital, LLC ("EcoR1"), Oleg Nodelman and EcoR1 Capital Fund Qualified, L.P. ("Qualified Fund"). EcoR1 is the general partner and investment adviser of private funds, including Qualified Fund (the "Funds"). Mr. Nodelman is the manager and controlling owner of EcoR1. EcoR1 is filing this Form 4 for itself, Mr. Nodelman and Qualified Fund. The filers are filing this Form 4 jointly, but not as a group, and each expressly disclaims membership in a group within the meaning of Rule 13d-5(b) under the Securities Exchange Act of 1934. The Funds hold these securities directly for the benefit of their investors. EcoR1 may be deemed to indirectly beneficially own them as the investment adviser to the Funds. Mr. Nodelman may be deemed to indirectly beneficially own them as the control person of EcoR1. The filers disclaim beneficial ownership of the securities except to the extent of their respective pecuniary interests therein. ( 2 )The Funds exchanged 8,036,688 shares of Common Stock for pre-funded warrants to acquire 8,036,688 shares of Common Stock ( 3 )Qualified Fund disposed of 7,578,596 shares of Common Stock disposed of in this exchange ( 4 )After this transaction, Qualified Fund owns 10,682,303 shares of Common Stock. ( 5 )Qualified Fund acquired 7,578,596 of the pre-funded warrants acquired in this exchange. ( 6 )The pre-funded warrants do not have an expiration date. ( 7 )Each pre-funded warrant is exercisable to purchase one share of Common Stock, except that a holder of a pre-funded warrant will not be entitled to exercise any pre-funded warrant if, upon giving effect or immediately prior to such exercise, such exercise would result in (i) the aggregate number of shares of Common Stock beneficially owned by such holder (together with its affiliates) exceeding 9.99% of the number of shares of Common Stock outstanding immediately after giving effect to such exercise, or (ii) the combined voting power of Company securities beneficially owned by such holder (together with its affiliates) exceeding 9.99% of the combined voting power of all Company securities outstanding immediately after giving effect to such exercise, as such percentage ownership is determined in accordance with the terms of the pre-funded warrants. ( 8 )After this transaction, Qualified Fund owned 7,578,596 of the pre-funded warrants.

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