As filed with the Securities and Exchange Commission on September 22, 2025. Registration No. 333- UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Form S-1 REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 MBX Biosciences, Inc. (Exact name of registrant as specified in its charter) (State or other jurisdiction of incorporation or organization) Delaware 2834 84-1882872 (Primary Standard Industrial Classification Code Number) MBX Biosciences, Inc. 11711 N. Meridian Street, Suite 3 ...