On February 25, 2026, MBX Biosciences, Inc. and Richard Bartram entered into a separation agreement pursuant to which Mr. Bartram will resign as Chief Financial Officer of the Company. He will continue his employment with the Company as its Chief Financial Officer through March 15, 2026. Mr. Bartram has agreed to serve as a consultant to the Company following his resignation to assist with the transition, under the terms of a consulting agreement entered into with the Company that will become effective upon Mr. Bartram?s resignation.

John Smither has been appointed as the Company?s Interim Chief Financial Officer, effective March 16, 2026. Mr. Smither, age 73, has over 40 years of experience in finance and accounting in the biotechnology industry. Mr. Smither previously served as the Chief Financial Officer of Arcutis Biotherapeutics, Inc. from May 2019 until March 2021, and again as Interim Chief Financial Officer from September 2023 to April 2024, and the Chief Financial Officer of Sienna Biopharmaceuticals, Inc. from April 2018 until March 2019.

Mr. Smither also served as the interim Chief Financial Officer at Kite Pharma (a Gilead Sciences, Inc. company) from October 2017 until April 2018 during its integration with Gilead. Mr. Smither presently serves on the board of directors of NewAmsterdam Pharma Company N.V. as chair of its audit committee and Genelux Corporation serving as chair of its compensation committee and as a member of its audit committee. Mr. Smither also served on the board of directors and audit committee chair of eFFECTOR Therapeutics, Inc. from March 2018 to September 2023 and Applied Molecular Transport, Inc. from January 2022 to December 2023.

Mr. Smither also served as a member of the nomination and corporate governance committee of eFFECTOR Therapeutics and the compensation committee of Applied Molecular Transport. Mr. Smither previously served on the board of directors of Achaogen, Inc. and Principia Biopharma Inc. and as the chair of their audit committees. Mr. Smither also has 15 years?

experience as a practicing CPA (inactive), including time spent as an audit partner with Ernst & Young LLP.