SCHEDULE 13G/A 0001903360-24-000002 0001903360 XXXXXXXX LIVE 1 Common Stock 09/30/2025 0001776111 MBX Biosciences 55287L101 11711 North Meridian Street Suite 300 Carmel X1 46032 Rule 13d-1(c) Wellington Biomedical Innovation Master Investors (Cayman) II, L.P. DE 0 2017394 0 2017394 2017394.00 4.5 PN MBX Biosciences 11711 North Meridian Street, Suite 300, Carmel IN 46032 Wellington Biomedical Innovation Master Investors (Cayman) II, L.P. c/o Wellington Management Company LLP, 280 Congress Street, Boston MA 02210 Delaware Y 2,017,394 4.50 0 2,017,394 0 2,017,394 Y 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. Wellington Biomedical Innovation Master Investors (Cayman) II, L.P. Ihsan Speede 11/12/2025 Wellington Alternative Investments LLC; Its General Partner Signature: Ihsan Speede Name/Title: Authorized Person Date: 11/12/2025