Other Events. On August 27, 2026, the Company announced the dosing of the first patient in its pivotal Phase 3 oPTimize clinical trial evaluating once-weekly canvuparatide for the treatment of chronic hypoparathyroidism (HP). A summary of the trial design is as follows: Phase 3 oPTimize trial design The Phase 3 oPTimize trial is a global, multicenter, randomized, double-blind, placebo-controlled study evaluating the efficacy and safety of once-weekly canvuparatide in adults with chronic HP, f…

Other Events. On August 6, 2026, the Company provided an update on its MBX 4291 phase 1 clinical trial. The Company remains on track to report data from the first cohort of the 12-week multiple ascending dose (Part C) in the fourth quarter of 2026. The first cohort in Part C is evaluating a 30 mg (once-weekly times four) plus 120 mg (once-monthly) plus 180 mg (once-monthly) titration regimen in adult participants with obesity (body mass index ≥30). Forward-Looking Statements This Current Repo…

of this Current Report on Form 8-K (including Exhibit 99.1 attached hereto), is intended to be furnished and shall not be deemed "filed" for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), or otherwise subject to the liabilities of that section, and shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such filing.

CEO — Kent P. Hawryluk: Dr. Kent P. Hawryluk is stepping away from his role as CEO with a successor named.

Other Events. On June 12, 2026, the Company announced one-year data from the Phase 2 Avail™ and OLE clinical trials of canvuparatide in adult patients with chronic HP. The update is summarized below. Key Findings from the 12-Week Avail™ Phase 2 Trial and One-Year OLE Responder Rate • At 12 Weeks: As previously reported, 63% of canvuparatide-treated patients (30/48) achieved the primary composite endpoint compared with 31% of placebo-treated patients (5/16) (p=0.042). The primary endpoint was…

Annual board election with no specific departures or promotions mentioned.

of this Current Report on Form 8-K (including Exhibit 99.1 attached hereto), is intended to be furnished and shall not be deemed "filed" for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), or otherwise subject to the liabilities of that section, and shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such filing.

of this Current Report on Form 8-K (including Exhibit 99.1 attached hereto), is intended to be furnished and shall not be deemed "filed" for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), or otherwise subject to the liabilities of that section, and shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such filing.

Other Events. On March 9, 2026, the Company announced completion of an End-of-Phase 2 ("EOP2") meeting with the U.S. Food and Drug Administration ("FDA"). Based on feedback from the FDA, the Company plans to advance once-weekly canvuparatide into a Phase 3 trial for the treatment of chronic hypoparathyroidism ("HP") in the third quarter of 2026. The Phase 3 double-blind placebo-controlled trial will enroll approximately 160 patients randomized in a 3:1 ratio to receive canvuparatide or placeb…

Entry into a Material Definitive Agreement. On February 24, 2026, MBX Biosciences, Inc. (the “Company”) entered into a lease agreement (the “Lease Agreement") with 5 Burlington Woods, LLC ("5 Burlington Woods") to lease approximately 13,642 square feet of new office and laboratory space in Burlington, MA ("the Premises"). The Lease Agreement provides an estimated lease commencement date for the Premises of May 1, 2026 ("Lease Commencement Date"), with the rent commencing five months from the…

Chief Financial Officer — Richard Bartram: Mr. Bartram resigned as Chief Financial Officer with a transition period and consulting agreement.

Other Events. Sales Pursuant to At-the-Market ("ATM") Offering Program On February 4, 2026, MBX Biosciences, Inc. (the “Company”), closed on the sale and issuance of an aggregate of 2,250,986 shares of its common stock (the "Shares") at a volume weighted average price per share of $38.76 for gross proceeds of approximately $87.1 million. The Shares were sold pursuant to the Company's Open Market Sale Agreement SM with Jefferies, LLC ("Jefferies") dated November 6, 2025 and a shelf registratio…

Director — Laurie Stelzer: Ms. Laurie Stelzer was appointed as a director and chairperson of the Audit Committee.

of this Form 8-K is unaudited and preliminary and does not present all information necessary for an understanding of the Company’s financial condition as of December 31, 2025. The audit of the Company’s consolidated financial statements for the year ended December 31, 2025 is ongoing and could result in changes to the information set forth above.

Other Events. Future Development Plans On January 11, 2026, the Company announced that it expects to expand its obesity portfolio in 2026 with two additional obesity candidates, with nominations anticipated in the second quarter and third quarter of 2026. Based on the Company’s current operating plan, the Company estimates that its cash, cash equivalents and marketable securities as of December 31, 2025 will be sufficient to fund its operations into 2029. The Company has based this estimate o…

of this Current Report on Form 8-K (including Exhibit 99.1 attached hereto), is intended to be furnished and shall not be deemed "filed" for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), or otherwise subject to the liabilities of that section, and shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such filing.

Director — James M. Cornelius: Mr. Cornelius resigned from the Board and was succeeded by Steven Hoerter as executive chairperson.

Entry into a Material Definitive Agreement. On September 24, 2025, MBX Biosciences, Inc. (the “Company”) entered into an Underwriting Agreement (the “Underwriting Agreement”) with J.P. Morgan Securities LLC and Jefferies LLC, as representatives of the several underwriters (collectively, the “Underwriters”), related to an underwritten public offering (the “Offering”) of 11,108,055 shares of the Company’s common stock, par value $0.0001 per share (“Common Stock”), at a purchase price per share…

Other Events. On September 22, 2025, the Company announced topline data from its Phase 2 Avail™ clinical trial of canvuparatide in adult patients with chronic HP. The topline results are summarized below. All patients (n=64) completed the 12-week study, and 94% of patients elected to enroll in the six month open-label extension (OLE) to further evaluate longer-term outcomes with once-weekly canvuparatide. Phase 2 Avail Topline Results 12-week and 6-month Responder Rates: • At 12 Weeks: The pr…

contains “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934, each as amended. The words “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “intend,” “may,” “plan,” “potential,” “predict,” “project,” “should,” “target,” “would” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. Th…

of this Current Report on Form 8-K (including Exhibit 99.1 attached hereto), is intended to be furnished and shall not be deemed "filed" for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), or otherwise subject to the liabilities of that section, and shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such filing.

contains “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934, each as amended. The words “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “intend,” “may,” “plan,” “potential,” “predict,” “project,” “should,” “target,” “would” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. Th…

Annual board election with no specific departures or promotions mentioned.

of this Current Report on Form 8-K (including Exhibit 99.1 attached hereto), is intended to be furnished and shall not be deemed "filed" for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), or otherwise subject to the liabilities of that section, and shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such filing.

Director — Steven L. Hoerter: Appointment of a new independent director to the board.