X0202 SCHEDULE 13G 0000078003 XXXXXXXX LIVE Common Stock par value $0.0001 per share 10/08/2026 0002139097 TREX BIO, INC. 89531N100 269 East Grand Avenue Suite 300 South San Francisco CA 94080 Rule 13d-1(c) PFIZER INC. (13-5315170) DE 700000.00 1265066.00 700000.00 1265066.00 1965066.00 N 7.2 CO 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. PFIZER VENTURES (US) LLC (82-5524518) DE 0.00 1265066.00 0.00 1265066.00 1265066.00 N 4.6 OO 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. TREX BIO, INC. 269 East Grand Avenue, Suite 300, South San Francisco, California, 94080 PFIZER INC. 66 Hudson Boulevard East, New York, NY 10001 Delaware, US Y See the respective responses of the Reporting Persons to Item 9 on the attached cover pages. See the respective responses of the Reporting Persons to Item 11 on the attached cover pages. See the respective responses of the Reporting Persons to Item 5 on the attached cover pages. See the respective responses of the Reporting Persons to Item 6 on the attached cover pages. See the respective responses of the Reporting Persons to Item 7 on the attached cover pages. See the respective responses of the Reporting Persons to Item 8 on the attached cover pages. Y N Y Y Y Y N 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. Exhibit 99.1 Joint Filing Agreement, dated as of October 16, 2026, by and between Pfizer and PVUS. PFIZER INC. (13-5315170) /s/ Madelyn D. Purcell Madelyn D. Purcell/Assistant Secretary 10/16/2026 PFIZER VENTURES (US) LLC (82-5524518) /s/ Madelyn D. Purcell Madelyn D. Purcell/Assistant Secretary 10/16/2026