DEFA14A

DEFA14A 1 defa_14a_2026.htm DEFA14A DEFA14A

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the Securities

Exchange Act of 1934 (Amendment No. )

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Filed by a Party other than the Registrant ☐

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☐ Preliminary Proxy Statement

☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐ Definitive Proxy Statement

☒ Definitive Additional Materials

☐ Soliciting Material Pursuant to §240.14a-12

MBX BIOSCIENCES, INC.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

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Your Vote Counts! MBX BIOSCIENCES, INC. 2026 Annual MeetingVote by June 3, 2026 11:59 PM ET MBX BIOSCIENCES, INC. 11711 N. MERIDIAN STREET, SUITE 300CARMEL, INDIANA 46032 V93043-P47757 You invested in MBX BIOSCIENCES, INC. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on June 4, 2026. Get informed before you vote View the Annual Report, Notice and Proxy Statement online at www.proxyvote.com OR you can receive a free paper or emailcopy of the material(s) by requesting prior to May 21, 2026. If you would like to request a copy of the material(s) for thisand/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email tosendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. For complete information and to vote, visit www.ProxyVote.comControl # Smartphone users Point your camera here and vote without entering a control number Vote Virtually at the Meeting* June 4, 2026 8:00 a.m. ET Virtually at: www.virtualshareholdermeeting.com/MBX2026 *Please check the meeting materials for any special requirements for meeting attendance.

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Board Recommends Voting Items 1. Election of two Class II Directors, each to serve until the Company’s 2029 Annual Meeting of Stockholders and until such person’s successor is duly elected and qualified, or until their earlier death, resignation or removal: Nominees: 1a. Patrick J. Heron For 1b. Edward T. Mathers For 2. Ratification of the appointment of Ernst & Young LLP as the Company’s Independent Registered Public Accounting Firm for the Fiscal Year Ending December 31, 2026. For NOTE: Such other business as may properly come before the meeting or any adjournment thereof. Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Delivery Settings”. V93044-P47757

SEC Form DEFA14A filed by MBX Biosciences Inc.