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SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Name of Issuer)

(Title of Class of Securities)

(CUSIP Number)

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

SCHEDULE 13D/A 0001072613-25-000107 0001906837 XXXXXXXX LIVE 1 Common Stock, $0.00001 par value 11/13/2025 false 0002040807 59267L107 Metsera, Inc. 3 World Trade Center 175 Greenwich Street New York NY 10007 Mark McDonnell (773) 380-6600 ARCH Venture Management, LLC 8755 W. Higgins Road, Suite 1025 Chicago IL 60631 0001906837 N ARCH Venture Fund XII, L.P. WC N DE 0 0 0 0 0 N 0 PN 0002006772 N ARCH Venture Fund XIII, L.P. WC N DE 0 0 0 0 0 N 0 PN 0001979765 N ARCH Venture Partners XII, L.P. AF N DE 0 0 0 0 0 N 0 PN 0002016083 N ARCH Venture Partners XIII, L.P. WC N DE 0 0 0 0 0 N 0 PN 0001979548 N ARCH Venture Partners XII, LLC AF N DE 0 0 0 0 0 N 0 OO 0002016082 N ARCH Venture Partners XIII, LLC AF N DE 0 0 0 0 0 N 0 OO 0001219042 N Robert Nelsen AF N X1 0 0 0 0 0 N 0 IN 0001219039 N Keith Crandell AF N X1 0 0 0 0 0 N 0 IN 0001569248 N Kristina Burow AF N X1 0 0 0 0 0 N 0 IN 0001229592 N Steven Gillis AF N X1 0 0 0 0 0 N 0 IN 0001202769 N Paul Berns AF N X1 0 0 0 0 0 N 0 IN Common Stock, $0.00001 par value Metsera, Inc. 3 World Trade Center 175 Greenwich Street New York NY 10007 This statement relates to the Common Stock, $0.00001 par value per share (the "Common Stock"), of Metsera, Inc. (the "Issuer") having its principal executive office at 3 World Trade Center, 175 Greenwich Street, New York, NY, 10007 USA. Item 5 of this Schedule 13D is amended and supplemented as follows: Pursuant to the Agreement and Plan of Merger dated September 21, 2025, as amended on November 7, 2025 (the "Merger Agreement"), by and among the Issuer, Pfizer Inc., a Delaware corporation ("Parent"), and Mayfair Merger Sub, Inc., a Delaware corporation and a wholly owned subsidiary of Parent ("Merger Sub"), Merger Sub merged with and into the Issuer, with the Issuer continuing as the surviving corporation and a wholly owned subsidiary of 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 Isser was converted automatically into the right to receive (i) cash in an amount equal to $65.60 per share without interest, net of all applicable withholding taxes, plus (ii) one contractual contingent value right representing the right to receive contingent payments 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 Parent and Equiniti Trust Company, LLC, dated November 13, 2025. Regarding the number of shares as to which such person has: (i) sole power to vote or to direct the vote: See line 7 of cover sheets. (ii) shared power to vote or to direct the vote: See line 8 of cover sheets. (iii) sole power to dispose or to direct the disposition: See line 9 of cover sheets. (iv) shared power to dispose or to direct the disposition: See line 10 of cover sheets. Except as set forth above, none of the Reporting Persons has effected any transaction in the Common Stock during the last 60 days. No other person is known to have the right to receive or the power to direct the receipt of dividends from, or any proceeds from the sale of, shares beneficially owned by any of the Reporting Persons. On November 13, 2025, as a result of the transactions described in Item 5(a) hereof, the Reporting Persons ceased to be the beficial owners of 5% or more of the Common Stock. ARCH Venture Fund XII, L.P. ARCH Venture Partners XII, L.P. its General Partner 11/14/2025 ARCH Venture Partners XII, LLC its General Partner 11/14/2025 /s/ Mark McDonnell Mark McDonnell as Attorney-in-Fact for Robert Nelsen, Managing Director 11/14/2025 ARCH Venture Fund XIII, L.P. ARCH Venture Partners XIII, LLC its General Partner 11/14/2025 /s/ Mark McDonnell Mark McDonnell as Attorney-in-Fact for Keith Crandell, Managing Director 11/14/2025 ARCH Venture Partners XII, L.P. ARCH Venture Partners XII, LLC its General Partner 11/14/2025 /s/ Mark McDonnell Mark McDonnell as Attorney-in-Fact for Robert Nelsen, Managing Director 11/14/2025 ARCH Venture Partners XIII, L.P. ARCH Venture Partners XIII, LLC its General Partner 11/14/2025 /s/ Mark McDonnell Mark McDonnell as Attorney-in-Fact for Keith Crandell, Managing Director 11/14/2025 ARCH Venture Partners XII, LLC /s/ Mark McDonnell Mark McDonnell as Attorney-in-Fact for Robert Nelsen, Managing Director 11/14/2025 ARCH Venture Partners XIII, LLC /s/ Mark McDonnell Mark McDonnell as Attorny-in-Fact for Keith Crandell, Managing Director 11/14/2025 Robert Nelsen /s/ Mark McDonnell Mark McDonnell as Attorny-in-Fact for Robert Nelsen 11/14/2025 Keith Crandell /s/ Mark McDonnell Mark McDonnell as Attorny-in-Fact for Keith Crandell 11/14/2025 Kristina Burow /s/ Mark McDonnell Mark McDonnell as Attorny-in-Fact for Kristina Burow 11/14/2025 Steven Gillis /s/ Mark McDonnell Mark McDonnell as Attorny-in-Fact for Steven Gillis 11/14/2025 Paul Berns /s/ Mark McDonnell Mark McDonnell as Attorny-in-Fact for Paul Berns 11/14/2025