Sec Form 4 Filing - ARCH Venture Partners XII, LLC @ Metsera, Inc. - 2025-11-13

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 ARCH Venture Partners XII, LLC 2. Issuer Name and Ticker or Trading Symbol Metsera, Inc. [ MTSR] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 8755 W. HIGGINS ROAD, SUITE 1025, 3. Date of Earliest Transaction (MM/DD/YY) 11/13/2025 (Street) CHICAGO, IL60631 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 11/13/2025( 1 )( 2 ) D( 1 )( 2 ) 18,503,128 D 0 I See Footnote ( 3 ) Common Stock 11/13/2025( 1 )( 2 ) D( 1 )( 2 ) 8,313,680 D 0 I See Footnote ( 4 )

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 ARCH Venture Partners XII, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X ARCH Venture Fund XII, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X ARCH Venture Partners XII, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X ARCH Venture Fund XIII, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X ARCH Venture Partners XIII, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X ARCH Venture Partners XIII, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X CRANDELL KEITH C/O ARCH VENTURE PARTNERS IX, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X GILLIS STEVEN C/O ARCH VENTURE PARTNERS 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X NELSEN ROBERT 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X X Signatures ARCH Venture Fund XII, L.P. By: ARCH Venture Partners XII, L.P., its General Partner By: ARCH Venture Partners XII, LLC, its General Partner By: /s/ Mark McDonnell, as Attorney-in-Fact 11/14/2025 Signature of Reporting Person Date ARCH Venture Partners XII, L.P. By: ARCH Venture Partners XII, LLC, its General Partner By: /s/ Mark McDonnell, as Attorney-in-Fact 11/14/2025 Signature of Reporting Person Date ARCH Venture Partners XII, LLC By: /s/ Mark McDonnell, as Attorney-in-Fact 11/14/2025 Signature of Reporting Person Date ARCH Venture Fund XIII, L.P. By: ARCH Venture Partners XIII, L.P., its General Partner By: ARCH Venture Partners XIII, LLC, its General Partner By: /s/ Mark McDonnell, as Attorney-in-Fact 11/14/2025 Signature of Reporting Person Date ARCH Venture Partners XIII, L.P. By: ARCH Venture Partners XIII, LLC, its General Partner By: /s/ Mark McDonnell, as Attorney-in-Fact 11/14/2025 Signature of Reporting Person Date ARCH Venture Partners XIII, LLC By: /s/ Mark McDonnell, as Attorney-in-Fact 11/14/2025 Signature of Reporting Person Date /s/ Keith Crandell, By: Mark McDonnell, attorney-in-fact 11/14/2025 Signature of Reporting Person Date /s/ Steven Gillis, By: Mark McDonnell, attorney-in-fact 11/14/2025 Signature of Reporting Person Date /s/ Robert Nelsen, By: Mark McDonnell, attorney-in-fact 11/14/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Pursuant to the Agreement and Plan of Merger dated September 21, 2025, as amended on November 7, 2025 (the "Merger Agreement"), by and among Metsera, Inc. (the "Company"), Pfizer Inc., a Delaware corporation ("Parent"), and Mayfair Merger Sub, Inc., a Delaware corporation and a wholly owned subsidiary of the Parent (the "Merger Sub"), the Merger Sub merged with and into the Company, with the Company continuing as the surviving corporation and a wholly owned subsidiary of the Parent (the "Merger"). At the Effective Time of the Merger (as defined in the Merger Agreement), each issued and outstanding share of common stock, par value $0.00001 per share of the Company (the "Common Stock") was converted automatically into the right to receive (i) cash in an amount equal to $65.60 per share without interest (the "Closing Amount"), net of all applicable withholding taxes, plus ( 2 )(Continued from footnote 1) (ii) one contractual contingent value right representing the right to receive contingent payments (a "CVR") in cash, without interest, upon the achievement of certain specified milestones, in accordance with the terms and conditions of the contingent value rights agreement entered into by the Parent and Equiniti Trust Company, LLC, dated November 13, 2025 (collectively, the "Merger Consideration"). ( 3 )Represents shares held directly by ARCH Venture Fund XII, L.P ("ARCH Venture Fund XII"). ARCH Venture Partners XII, L.P. ("AVP XII LP"), as the sole general partner of ARCH Venture Fund XII, may be deemed to beneficially own the shares held by ARCH Venture Fund XII. ARCH Venture Partners XII, LLC ("AVP XII LLC"), as the sole general partner of AVP XII LP, may be deemed to beneficially own the shares held by ARCH Venture Fund XII. AVP XII LP and AVP XII LLC disclaim beneficial ownership except to the extent of any pecuniary interest therein. As members of the investment committee of AVP XII LLC, each of Kristina M. Burow, Keith Crandell, Steven Gillis and Robert Nelsen (the "AVP XII Committee Members") may also be deemed to share the power to direct the disposition and vote of the ARCH Venture Fund XII shares. Each AVP XII Committee Member disclaims beneficial ownership except to the extent of any pecuniary interest therein. ( 4 )Represents shares held directly by ARCH Venture Fund XIII, L.P. ("ARCH Venture Fund XIII"). ARCH Venture Partners XIII, L.P. ("AVP XIII LP"), as the sole general partner of ARCH Venture Fund XIII, may be deemed to beneficially own the shares held by ARCH Venture Fund XIII. ARCH Venture Partners XIII, LLC ("AVP XIII LLC"), as the sole general partner of AVP XIII LP, may be deemed to beneficially own the shares held by ARCH Venture Fund XIII. AVP XIII LP and AVP XIII LLC disclaim beneficial ownership except to the extent of any pecuniary interest therein. As members of the investment committee of AVP XIII LLC, each of Paul L. Berns, Kristina M. Burow, Keith Crandell and Robert Nelsen (the "AVP XIII Committee Members") may also be deemed to share the power to direct the disposition and vote of the ARCH Venture Fund XIII shares. Each AVP XIII Committee Member disclaims beneficial ownership except to the extent of any pecuniary interest therein.

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).

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