Phase 1 obesity trial begins for MBX Biosciences drug (NASDAQ: MBX)

Rhea-AI Filing Summary

MBX Biosciences, Inc. filed a current report describing progress in its obesity program. On September 4, 2025, the company dosed the first participant in a Phase 1 clinical trial of MBX 4291, a long-acting GLP-1/GIP receptor co-agonist prodrug being developed for the treatment of obesity.

The Phase 1 study is a randomized, double-blind, placebo-controlled, first-in-human trial in adults with obesity. It includes a single ascending dose part with five cohorts of eight participants and a four-week multiple ascending dose part with three cohorts of eight participants, all using a 3:1 randomization of MBX 4291 to placebo. After these parts, the company plans additional multiple-dose cohorts over 12 weeks in up to two groups of 30 participants each, using a 2:1 randomization.

Participants are followed for several weeks after dosing to evaluate safety, tolerability, pharmacokinetics, and pharmacodynamics. MBX Biosciences anticipates topline Phase 1 results in 2027 and highlights that these plans and timelines are forward-looking and subject to clinical, regulatory, and funding risks.

Insights

Biotech clinical development analyst

MBX advances obesity candidate MBX 4291 into first-in-human testing.

MBX Biosciences has started a Phase 1 trial for MBX 4291, described as a long-acting GLP-1/GIP receptor co-agonist prodrug for obesity. Moving into a randomized, double-blind, placebo-controlled, first-in-human study indicates the program has cleared preclinical and regulatory gates needed to test in adults with obesity.

The design uses single and multiple ascending dose parts, followed by planned 12-week multiple-dose cohorts in larger groups, with follow-up periods of up to 120 days after first dosing. This structure is typical for characterizing safety, tolerability, pharmacokinetics, and pharmacodynamics before any later-stage efficacy-focused trials.

The company anticipates topline Phase 1 results in 2027, but explicitly notes that these are forward-looking statements subject to risks in research and development, regulatory approvals, dependence on third parties, funding needs, and competition. Actual timelines and outcomes will depend on how the ongoing and planned trial stages proceed.

0001776111false00017761112025-09-042025-09-04

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): September 04, 2025

MBX Biosciences, Inc.

(Exact name of Registrant as Specified in Its Charter)

Delaware			001-42272	84-1882872
(State or Other Jurisdiction of Incorporation)			(Commission File Number)	(IRS Employer Identification No.)
11711 N. Meridian Street Suite 300
Carmel, Indiana				46032
(Address of Principal Executive Offices)				(Zip Code)

Registrant’s Telephone Number, Including Area Code: (317) 659-0200

Not Applicable

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Title of each class		Trading Symbol(s)		Name of each exchange on which registered
Common Stock, $0.0001 par value per share		MBX		Nasdaq Global Select Market

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company ☒

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Item 7.01 Regulation FD Disclosure.

On September 4, 2025, MBX Biosciences, Inc. (the "Company") issued a press release (the "Press Release") titled “MBX Biosciences Doses First Participant in Phase 1 Trial of MBX 4291 for the Treatment of Obesity.” A copy of the Press Release is being furnished as Exhibit 99.1 to this Current Report on Form 8-K.

The information included under Item 7.01 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.

Item 8.01 Other Events.

On September 4, 2025, the Company announced the first dose of the first participant in a phase 1 trial of for MBX 4291, a long-acting glucagon-like peptide-1 (GLP-1)/ glucose-dependent insulinotropic polypeptide (GIP) receptor co-agonist prodrug for the treatment of obesity. Topline results of the Phase 1 clinical trial of MBX 4291 are anticipated in 2027, and a summary of the trial design is as follows:

Phase 1 trial design

The Phase 1 trial is a randomized, double-blind, placebo-controlled first-in-human study to evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamics of single and multiple ascending doses of MBX 4291 in adult participants with obesity. The Phase 1 trial will include:

Part A Single Ascending Dose (SAD): single ascending doses of MBX 4291 or matching placebo will be administered in 5 cohorts consisting of 8 participants each. Participants will be randomized on Study Day 1 to receive MBX 4291 or matching placebo in a 3:1 ratio. Participants in each cohort will be followed for 63 days after the single study intervention administration.

Part B Multiple Ascending Dose (MAD) (4 weeks): multiple ascending doses of MBX 4291 or matching placebo will be administered in 3 cohorts consisting of 8 participants each. Participants will be randomized on Study Day 1 to receive MBX 4291 or matching placebo in a 3:1 ratio. Participants in each cohort will receive a total of 4 study intervention administrations one week apart and will be followed for 71 days after the first study intervention administration.

Following completion of Parts A and B, the Company plans to evaluate multiple ascending doses of MBX 4291, or matching placebo, administered over 12 weeks in up to two cohorts consisting of 30 participants each in a 2:1 randomization ratio. Participants are expected to receive up to a total of 12 study intervention administrations one week or one week and one month apart with increasing doses of MBX 4291 and will be followed for 120 days after the first dose.

More information on the trial can be found at www.clinicaltrials.gov, identifier NCT07142707.

The disclosure under this Item 8.01 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. These forward-looking statements include, but are not limited to, express or implied statements regarding: the expected timing of topline results of a Phase 1 trial of MBX 4291 in 2027.

Forward-looking statements are based on management’s current expectations and are subject to risks and uncertainties that could negatively affect MBX Biosciences’ business, operating results, financial condition, and stock value. Factors that could cause actual results to differ materially from those currently anticipated include: risks relating to the Company’s research and development activities; MBX Biosciences’ ability to execute on its strategy including obtaining the requisite regulatory approvals on the expected timeline, if at all; uncertainties relating to preclinical and clinical development activities; the Company’s dependence on third parties to conduct clinical trials, manufacture its product candidates and develop and commercialize its product candidates, if approved; MBX Biosciences’ ability to attract, integrate and retain key personnel; risks related to the Company’s financial condition and need for substantial additional funds in order to complete development activities and commercialize a product candidate, if approved; risks related to regulatory developments and approval processes of the U.S. Food and Drug Administration and comparable foreign regulatory authorities; risks related to establishing and maintaining MBX Biosciences’ intellectual property protections; and risks related to the competitive landscape for MBX Biosciences’ product candidates; as well as other risks described in “Risk Factors,” in MBX Biosciences’ Annual Report on Form 10-K for the year ended December 31, 2024 filed with the Securities and Exchange Commission (SEC), as well as subsequent filings with the SEC. MBX Biosciences expressly disclaims any obligation or undertaking to release publicly any updates or revisions to any forward-looking statements contained herein to reflect any change in its expectations or any changes in events, conditions or circumstances on which any such statement is based, except as required by law, and claims the protection of the safe harbor for forward-looking statements contained in the Private Securities Litigation Reform Act of 1995.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits.

Exhibit No.	Description
99.1	Press Release Issued by MBX Biosciences, Inc. on September 4, 2025
104	Cover Page Interactive Data File (embedded within the Inline XBRL document)

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

			MBX BIOSCIENCES, INC.
Date:	September 4, 2025	By:	/s/ P. Kent Hawryluk
			President and Chief Executive Officer (Principal Executive Officer)

FAQ

What did MBX Biosciences (MBX) report in this Form 8-K?

MBX Biosciences reported that on September 4, 2025 it dosed the first participant in a Phase 1, first-in-human clinical trial of MBX 4291 for the treatment of obesity and furnished a related press release as an exhibit.

What is MBX 4291 in MBX Biosciences obesity program?

MBX 4291 is described as a long-acting glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptor co-agonist prodrug being developed by MBX Biosciences for the treatment of obesity.

How is the Phase 1 trial of MBX 4291 designed?

The Phase 1 trial is a randomized, double-blind, placebo-controlled, first-in-human study in adults with obesity. It includes a single ascending dose part with five cohorts of eight participants and a four-week multiple ascending dose part with three cohorts of eight participants, all with a 3:1 randomization to MBX 4291 or placebo.

What additional dosing is planned after the initial Phase 1 parts for MBX 4291?

After completing the initial single and multiple ascending dose parts, MBX Biosciences plans to evaluate multiple ascending doses of MBX 4291 or placebo over 12 weeks in up to two cohorts of 30 participants each, using a 2:1 randomization, with up to 12 administrations and follow-up of 120 days after the first dose.

When does MBX Biosciences expect topline results from the MBX 4291 Phase 1 trial?

MBX Biosciences states that topline results from the Phase 1 clinical trial of MBX 4291 are anticipated in 2027, while emphasizing this is a forward-looking statement subject to various development and regulatory risks.

Where can investors find more information on the MBX 4291 Phase 1 trial?

The company notes that more information on the MBX 4291 Phase 1 trial can be found at www.clinicaltrials.gov under the identifier NCT07142707.

What key risks does MBX Biosciences highlight related to MBX 4291 and its development plans?

The company cites risks related to research and development activities, obtaining regulatory approvals, uncertainties in preclinical and clinical development, reliance on third parties for trials and manufacturing, financial condition and funding needs, regulatory processes in the U.S. and abroad, protection of intellectual property, and the competitive landscape for its product candidates.