Material Event (Form 8-K)

Item 8.01 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 AgreementSM with Jefferies, LLC ("Jefferies") dated November 6, 2025 and a shelf registration statement on Form S-3 (File No. 333-291308) previously filed by the Company and declared effective by the Securities and Exchange Commission (the "SEC") on December 8, 2025, which included a prospectus supplement relating to the Company's at the market offering program. The Company currently intends to use the net proceeds from this sale for costs associated with the development of its clinical-stage programs, its discovery research and development activities and additional clinical development, and for general corporate purposes, working capital and capital expenditures. The Company may also use a portion of the remaining net proceeds and its existing cash, cash equivalents and marketable securities to in-license, acquire, or invest in complementary businesses, technologies, products or assets. Following these sales, the Company may offer and sell additional shares of its common stock having an aggregate offering price of up to $62.9 million from time to time through Jefferies pursuant to the Open Market Sale AgreementSM.

This Current Report on Form 8-K contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995, as amended. Forward-looking statements can be identified by words such as "anticipate," "believe," "continue," "could," "estimate," "expect," "future," "goal," "intend," "look forward to," "may," "plan," "potential," "predict," "project," "seek," "strategy," "should," "target," "will," "would" and similar expressions regarding future periods. These forward-looking statements include, but are not limited to, statements regarding the Company's intended use of the sale proceeds. Any forward-looking statements in this 8-K are based on management's current expectations and beliefs and are subject to a number of risks, uncertainties and important factors that may cause actual events or results to differ materially from those expressed or implied by any forward-looking statements contained in this 8-K, including, without limitation, those risks and uncertainties set forth more fully under the caption "Risk Factors" in the Company's prospectus supplement included in the Company's registration statement on Form S-3 filed with the SEC on November 6, 2025, the Company's most recent Annual Report on Form 10-K filed with the SEC and subsequent filings with the SEC. The Company cautions you not to place undue reliance on any forward-looking statements, which speak only as of the date they are made. The Company disclaims any obligation to publicly update or revise any such statements to reflect any change in expectations or in events, conditions or circumstances on which any such statements may be based, or that may affect the likelihood that actual results will differ from those set forth in the forward-looking statements. Any forward-looking statements contained in this 8-K represent the Company's views only as of the date hereof and should not be relied upon as representing its views as of any subsequent date.