X0609 4 2026-05-08 0001776111 MBX Biosciences, Inc. MBX 0002030246 Azoulay Salomon false C/O MBX BIOSCIENCES, INC. 11711 N. MERIDIAN STREET, SUITE 300 CARMEL IN 46032 false true false false Chief Medical Officer true Common Stock 2026-05-08 4 M false 55000 10.46 A 69769 D Common Stock 2026-05-08 4 S false 55000 38.38 D 14769 D Common Stock 2026-05-08 4 M false 15003 16 A 29772 D Common Stock 2026-05-08 4 S false 15003 38.30 D 14769 D Stock option (right to buy) 10.46 2026-05-08 4 M false 55000 0 A 2034-08-02 Common Stock 55000 194540 D Stock option (right to buy) 16 2026-05-08 4 M false 15003 0 A 2034-09-12 Common Stock 15003 30008 D The sales reported on this Form 4 represent the exercise and sale of common stock options by the reporting person pursuant to an approved Rule 10b5-1 trading plan adopted on January 27, 2026. The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $35.00 to $41.26, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote. The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $35.00 to $41.26, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote. 25% of the shares underlying this option vested and became exercisable on June 24, 2025, with the remaining 75% vesting in thirty-six (36) substantially equal monthly installments thereafter, subject to the Reporting Person's continued service on each such vesting date. 25% of the shares underlying this option vested and became exercisable on September 12, 2025, with the remaining 75% vesting in thirty-six (36) substantially equal monthly installments thereafter, subject to the Reporting Person's continued service on each such vesting date. /s/ P. Kent Hawryluk, attorney-in-fact 2026-05-11