MBX Biosciences, Inc.

(a)

As of the date of this Annual Report on Form 10-K, we intend to hold our 2025 Annual meeting of Stockholders (the "2025 Annual Meeting") on or about June 5, 2025 at 8 a.m. local time virtually. We are providing the following disclosure in accordance with our Amended and Restated Bylaws (the "Bylaws") and Rule 14a-8 under the Exchange Act.

Bylaws Advance Notice Deadline for Submission of Stockholder Proposals and Director Nominations

Pursuant to our Bylaws, since the 2025 Annual Meeting is the first Annual Meeting following our initial public offering, for notice of stockholder proposals submitted outside of Rule 14a-8 of the Exchange Act and director nominations to be timely, they must be so received not later than the later of (i) the close of business on the 90th day before the 2025 Annual Meeting; or (ii) the close of business on the 10th day following the day on which public announcement of the date of the 2025 Annual Meeting is first made by us. As this is our first public disclosure of the date of the 2025 Annual Meeting, to be considered timely, stockholder proposals submitted outside of Rule 14a-8 of the Exchange Act and director nominations, in each case intended to be brought before the 2025 Annual Meeting, must be received no later than the close of business on March 27, 2025. Any such stockholder proposals and director nominations must be directed to our Corporate Secretary at our corporate offices at MBX Biosciences, Inc., 11711 N. Meridian Street, Suite 300, Carmel, Indiana. Such stockholder proposals and director nominations must also comply with the advance notice provisions contained in Section 2 of our Bylaws.

Rule 14a-8 Deadline for the Submission of Stockholder Proposals

As we did not hold an annual meeting in 2024, pursuant to Rule 14a-8(e)(2) under the Exchange Act, the deadline for the receipt of any stockholder proposals submitted pursuant to Rule 14a-8 of the Exchange Act for inclusion in the Company’s proxy materials for the 2025 Annual Meeting would be a reasonable time before the company begins to print and send its proxy materials. We have determined that March 27, 2025 is a reasonable time before we expect to begin to print and distribute its proxy materials for the 2025 Annual Meeting, and that any stockholder proposals must be received on or before the close of business on that day. Such proposals must be directed to our Corporate Secretary at our corporate offices at MBX Biosciences, Inc., 11711 N. Meridian Street, Suite 300, Carmel, Indiana. Such proposals must also comply with Rule 14a-8 of the Exchange Act.

(b)Trading Plans

During the quarter ended December 31, 2024, no director or officer (as defined in Rule 16a-1(f) of the Exchange Act) adopted or terminated any Rule 10b5-1 trading arrangements or non-Rule 10b5-1 trading arrangements (in each case, as defined in Item 408(a) of Regulation S-K).