SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 1)

Metsera, Inc.

(Name of Issuer)

Common Stock, $0.00001 par value

(Title of Class of Securities)

59267L107

(CUSIP Number)

Mark McDonnell
ARCH Venture Management, LLC, 8755 W. Higgins Road, Suite 1025
Chicago, IL, 60631
(773) 380-6600

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

11/13/2025

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box. Checkbox not checked

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

ARCH Venture Fund XII, L.P.

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

ARCH Venture Fund XIII, L.P.

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

ARCH Venture Partners XII, L.P.

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

ARCH Venture Partners XIII, L.P.

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

ARCH Venture Partners XII, LLC

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

ARCH Venture Partners XIII, LLC

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

Robert Nelsen

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

Keith Crandell

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

Kristina Burow

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

Steven Gillis

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

CUSIP No.

59267L107

Name of reporting person

Paul Berns

Check the appropriate box if a member of a Group (See Instructions)

(a)

(b)

SEC use only

Source of funds (See Instructions)

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

Checkbox not checked

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

Aggregate amount beneficially owned by each reporting person

0.00

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in Row (11)

0 %

Type of Reporting Person (See Instructions)

SCHEDULE 13D

Item 1.

Security and Issuer

(a)

Title of Class of Securities:

Common Stock, $0.00001 par value

(b)

Name of Issuer:

Metsera, Inc.

(c)

Address of Issuer's Principal Executive Offices:

3 World Trade Center, 175 Greenwich Street, New York, NEW YORK , 10007.

Item 1 Comment:

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.

Interest in Securities of the Issuer

(a)

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.

(b)

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.

(c)

Except as set forth above, none of the Reporting Persons has effected any transaction in the Common Stock during the last 60 days.

(d)

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.

(e)

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.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

ARCH Venture Fund XII, L.P.

Signature:	ARCH Venture Partners XII, L.P.
Name/Title:	its General Partner
Date:	11/14/2025

Signature:	ARCH Venture Partners XII, LLC
Name/Title:	its General Partner
Date:	11/14/2025

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorney-in-Fact for Robert Nelsen, Managing Director
Date:	11/14/2025

ARCH Venture Fund XIII, L.P.

Signature:	ARCH Venture Partners XIII, LLC
Name/Title:	its General Partner
Date:	11/14/2025

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorney-in-Fact for Keith Crandell, Managing Director
Date:	11/14/2025

ARCH Venture Partners XII, L.P.

Signature:	ARCH Venture Partners XII, LLC
Name/Title:	its General Partner
Date:	11/14/2025

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorney-in-Fact for Robert Nelsen, Managing Director
Date:	11/14/2025

ARCH Venture Partners XIII, L.P.

Signature:	ARCH Venture Partners XIII, LLC
Name/Title:	its General Partner
Date:	11/14/2025

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorney-in-Fact for Keith Crandell, Managing Director
Date:	11/14/2025

ARCH Venture Partners XII, LLC

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorney-in-Fact for Robert Nelsen, Managing Director
Date:	11/14/2025

ARCH Venture Partners XIII, LLC

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorny-in-Fact for Keith Crandell, Managing Director
Date:	11/14/2025

Robert Nelsen

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorny-in-Fact for Robert Nelsen
Date:	11/14/2025

Keith Crandell

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorny-in-Fact for Keith Crandell
Date:	11/14/2025

Kristina Burow

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorny-in-Fact for Kristina Burow
Date:	11/14/2025

Steven Gillis

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorny-in-Fact for Steven Gillis
Date:	11/14/2025

Paul Berns

Signature:	/s/ Mark McDonnell
Name/Title:	Mark McDonnell as Attorny-in-Fact for Paul Berns
Date:	11/14/2025

Amendment: SEC Form SCHEDULE 13D/A filed by Metsera Inc.