TRex Bio: Pfizer reports 7.2% ownership stake

Pfizer Inc.'s reported position includes 700,000 shares under sole voting and dispositive power and 1,265,066 under shared power.

Rhea-AI Filing Summary

TRex Bio, Inc. common stock is the subject of a Schedule 13G reporting Pfizer Inc.'s beneficial ownership of 1,965,066 shares, or 7.2% of the class. Pfizer Inc. reported sole voting and dispositive power over 700,000 shares and shared voting and dispositive power over 1,265,066 shares. Pfizer Ventures (US) LLC reported beneficial ownership of 1,265,066 shares, or 4.6%, with shared voting and dispositive power. The ownership percentages are based on 27,445,682 shares outstanding as of October 8, 2026.

Key Figures

Pfizer Inc. beneficial ownership: 1,965,066 shares Pfizer Inc. ownership of class: 7.2% Pfizer Inc. sole voting and dispositive power: 700,000 shares +4 more

7 metrics

Pfizer Inc. beneficial ownership 1,965,066 shares Reported in the ownership disclosure

Pfizer Inc. ownership of class 7.2% Based on shares outstanding as of October 8, 2026

Pfizer Inc. sole voting and dispositive power 700,000 shares Reported in the ownership disclosure

Pfizer Inc. shared voting and dispositive power 1,265,066 shares Reported in the ownership disclosure

Pfizer Ventures (US) LLC beneficial ownership 1,265,066 shares Reported in the ownership disclosure

Pfizer Ventures (US) LLC ownership of class 4.6% Based on shares outstanding as of October 8, 2026

Shares outstanding 27,445,682 shares As of October 8, 2026

Key Terms

beneficially owned, Sole Voting Power, Shared Dispositive Power

3 terms

beneficially owned financial

"Amount beneficially owned"

Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.

Sole Voting Power financial

"Sole Voting Power 700,000.00"

Sole voting power is the exclusive right to cast votes attached to a shareholder’s stock without needing approval from anyone else. Like holding the only remote control for a TV, it lets that holder decide corporate matters such as board members, mergers, and policy changes, making it important to investors because it concentrates control and can strongly influence a company’s strategy and the value of its shares.

Shared Dispositive Power financial

"Shared Dispositive Power 1,265,066.00"

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

How many TRXB shares did Pfizer Inc. report owning?

Pfizer Inc. reported beneficial ownership of 1,965,066 shares, equal to 7.2% of the class. Its reported voting and dispositive powers include 700,000 shares under sole power and 1,265,066 shares under shared power.

How many TRXB shares did Pfizer Ventures (US) LLC report owning?

Pfizer Ventures (US) LLC reported beneficial ownership of 1,265,066 shares, or 4.6% of the class, with shared voting and dispositive power.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Available on EDGAR 10/16/2026 - 06:02 PM EDT Accepted by SEC EDGAR 10/16/2026 - 06:02 PM EDT Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

TREX BIO, INC.

(Name of Issuer)

Common Stock par value $0.0001 per share

(Title of Class of Securities)

89531N100

(CUSIP Number)

10/08/2026

(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)

schemaVersion:

SCHEDULE 13G

CUSIP Number(s): 89531N100

1

Names of Reporting Persons

PFIZER INC. (13-5315170)

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 700,000.00
6	Shared Voting Power 1,265,066.00
7	Sole Dispositive Power 700,000.00
8	Shared Dispositive Power 1,265,066.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,965,066.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

7.2 %

12

Type of Reporting Person (See Instructions)

CO

Comment for Type of Reporting Person: Percent of class outstanding represented by the amount in row (11) is based on 27,445,682 shares of Common Stock outstanding as of October 8, 2026 as set forth in the Issuer's prospectus filed with the United States Securities and Exchange Commission pursuant to Rule 424(b) under the Securities Act of 1933, as amended, on October 8, 2026.

SCHEDULE 13G

CUSIP Number(s): 89531N100

1

Names of Reporting Persons

PFIZER VENTURES (US) LLC (82-5524518)

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 1,265,066.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 1,265,066.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,265,066.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

4.6 %

12

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person: Percent of class outstanding represented by the amount in row (11) is based on 27,445,682 shares of Common Stock outstanding as of October 8, 2026 as set forth in the Issuer's prospectus filed with the United States Securities and Exchange Commission pursuant to Rule 424(b) under the Securities Act of 1933, as amended, on October 8, 2026.

SCHEDULE 13G

Item 1.
(a)	Name of issuer: TREX BIO, INC.
(b)	Address of issuer's principal executive offices: 269 East Grand Avenue, Suite 300, South San Francisco, California, 94080
Item 2.
(a)	Name of person filing: PFIZER INC.
(b)	Address or principal business office or, if none, residence: 66 Hudson Boulevard East, New York, NY 10001
(c)	Citizenship: Delaware, US
(d)	Title of class of securities: Common Stock par value $0.0001 per share
(e)	CUSIP Number(s): 89531N100
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: See the respective responses of the Reporting Persons to Item 9 on the attached cover pages.
(b)	Percent of class: See the respective responses of the Reporting Persons to Item 11 on the attached cover pages.
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: See the respective responses of the Reporting Persons to Item 5 on the attached cover pages.
	(ii) Shared power to vote or to direct the vote: See the respective responses of the Reporting Persons to Item 6 on the attached cover pages.
	(iii) Sole power to dispose or to direct the disposition of: See the respective responses of the Reporting Persons to Item 7 on the attached cover pages.
	(iv) Shared power to dispose or to direct the disposition of: See the respective responses of the Reporting Persons to Item 8 on the attached cover pages.
Item 5.	Ownership of 5 Percent or Less of a Class.
	Not Applicable
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	Not Applicable
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	Not Applicable
Item 8.	Identification and Classification of Members of the Group.
	Not Applicable
Item 9.	Notice of Dissolution of Group.
	Not Applicable

Item 10.

Certifications:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11.

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.

PFIZER INC. (13-5315170)

Signature:	/s/ Madelyn D. Purcell
Name/Title:	Madelyn D. Purcell/Assistant Secretary
Date:	10/16/2026

PFIZER VENTURES (US) LLC (82-5524518)

Signature:	/s/ Madelyn D. Purcell
Name/Title:	Madelyn D. Purcell/Assistant Secretary
Date:	10/16/2026

Exhibit Information

Exhibit 99.1 Joint Filing Agreement, dated as of October 16, 2026, by and between Pfizer and PVUS.