SCHEDULE 13G/A 0001673991 XXXXXXXX LIVE 1 Common Stock, par value $0.0001 per share 09/30/2025 0001776111 MBX Biosciences, Inc. 55287L101 11711 N. Meridian Street Suite 300 Carmel IN 46032 Rule 13d-1(d) P. Kent Hawryluk X1 1995759 0 1995759 0 1995759 N 4.3 IN The percentage of class was calculated based on (i) 44,902,302 shares of Common Stock outstanding as of November 3, 2025, as disclosed in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 6, 2025, plus (ii) an aggregate of 1,066,085 shares of Common Stock issuable upon exercise of stock options exercisable within 60 days of September 30, 2025, which are deemed outstanding pursuant to SEC Rule 13-3(d)(1)(i) only for purposes of computing the percentage ownership of the Reporting Person. MBX Biosciences, Inc. 11711 N. Meridian Street, Suite 300, Carmel, Indiana 46032 P. Kent Hawryluk (the "Reporting Person"). The business address of the Reporting Person is c/o MBX Biosciences, Inc., 11711 N. Meridian Street, Suite 300, Carmel, Indiana 46032. The Reporting Person is a citizen of the United States of America. Y The Reporting Person may be deemed to be the beneficial owner of 1,995,759 shares of Common Stock, which includes: (i) 481,397 shares of Common Stock, $0.0001 par value per share (the "Common Stock") held directly by the Reporting Person, (ii) 1,066,085 shares of Common Stock issuable upon exercise of stock options exercisable within 60 days of September 30, 2025 held directly by the Reporting Person, and (iii) 448,277 shares of Common Stock held by the P. Kent Hawryluk Revocable Trust dated January 25, 2011, of which the Reporting Person serves as trustee. The percentage of class was calculated based on (i) 44,902,302 shares of Common Stock outstanding as of November 3, 2025, as disclosed in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 6, 2025, plus (ii) an aggregate of 1,066,085 shares of Common Stock issuable upon exercise of stock options exercisable within 60 days of September 30, 2025, which are deemed outstanding pursuant to SEC Rule 13-3(d)(1)(i) only for purposes of computing the percentage ownership of the Reporting Person. 4.3% 1,995,759 0 1,995,759 0 N Y Y Y Y Y Y P. Kent Hawryluk /s/ P. Kent Hawryluk 11/14/2025