SCHEDULE 13D/A 0001193125-24-223402 0001863769 XXXXXXXX LIVE 1 Common Stock, $0.0001 par value per share 08/18/2025 false 0001776111 55287L101 MBX Biosciences, Inc.

11711 N. Meridian Street, Suite 300 Carmel IN 46032

Steve R. Bailey (206) 621-7200 601 Union Street, Suite 3200 Seattle WA 98101 0001863769 N Frazier Life Sciences Public Fund, L.P. b WC N DE 0.00 1103920.00 0.00 1103920.00 1103920.00 N 3.3 PN The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001879466 N FHMLSP, L.P. b AF N DE 0.00 1103920.00 0.00 1103920.00 1103920.00 N 3.3 PN The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001879465 N FHMLSP, L.L.C. b AF N DE 0.00 1103920.00 0.00 1103920.00 1103920.00 N 3.3 OO The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001913804 N Frazier Life Sciences Public Overage Fund, L.P. b WC N DE 0.00 325653.00 0.00 325653.00 325653.00 N 1.0 PN The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001963247 N FHMLSP Overage, L.P. b AF N DE 0.00 325653.00 0.00 325653.00 325653.00 N 1.0 PN The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001963248 N FHMLSP Overage, L.L.C. b AF N DE 0.00 325653.00 0.00 325653.00 325653.00 N 1.0 OO The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001790879 N Frazier Life Sciences X, L.P. b WC N DE 0.00 4552774.00 0.00 4552774.00 4552774.00 N 13.6 PN The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001790880 N FHMLS X, L.P. b AF N DE 0.00 4552774.00 0.00 4552774.00 4552774.00 N 13.6 PN The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001790811 N FHMLS X, L.L.C. b AF N DE 0.00 4552774.00 0.00 4552774.00 4552774.00 N 13.6 OO The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001911592 N Frazier Life Sciences XI, L.P. b WC N DE 0.00 3000.00 0.00 3000.00 3000.00 N 0.0 PN The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001911580 N FHMLS XI, L.P. b AF N DE 0.00 3000.00 0.00 3000.00 3000.00 N 0.0 PN The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001911623 N FHMLS XI, L.L.C. b AF N DE 0.00 3000.00 0.00 3000.00 3000.00 N 0.0 OO The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001341382 N James N. Topper b AF N X1 0.00 4552774.00 0.00 4552774.00 4552774.00 N 13.6 IN The Aggregate Amount represents 4,552,774 shares of Common Stock held directly by Frazier Life Sciences X, L.P. The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001365617 N Patrick J. Heron b AF N X1 4323.00 4552774.00 4323.00 4552774.00 4557097.00 N 13.6 IN The Aggregate Amount represents (i) 4,323 shares of Common Stock that are issuable upon the exercise of options held directly by Patrick J. Heron, and (ii) 4,552,774 shares of Common Stock held directly by Frazier Life Sciences X, L.P. The Percent of Class is calculated based on 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 7, 2025. 0001397906 N Albert Cha b AF N X1 0.00 0.00 0.00 0.00 0.00 N 0.0 IN 0001865123 N James Brush b AF N X1 0.00 0.00 0.00 0.00 0.00 N 0.0 IN 0001702527 N Daniel Estes b AF N X1 0.00 0.00 0.00 0.00 0.00 N 0.0 IN Common Stock, $0.0001 par value per share MBX Biosciences, Inc. 11711 N. Meridian Street, Suite 300 Carmel IN 46032 This Amendment No. 1 ("Amendment No. 1") to Schedule 13D amends the statement on Schedule 13D filed on September 23, 2024 (the "Original Schedule 13D" and this Amendment No. 1, the "Schedule 13D"). Except as otherwise specified in Amendment No. 1, all items in the Original Schedule 13D are unchanged. All capitalized terms used in this Amendment No. 1 and not otherwise defined herein have the meanings ascribed to such terms in the Original Schedule 13D. Prior to the Issuer's initial public offering (the "IPO"), in a series of private transactions, (i) FLS X purchased from the Issuer 18,922,852 shares of Series A Preferred Stock ("Series A Stock") for an aggregate approximate purchase price of $13,000,000 and 28,297,265 shares of Series B Preferred Stock ("Series B Stock") for an aggregate approximate purchase price of $25,065,525, (ii) FLSPF purchased from the Issuer 7,497,087 shares of Series C Preferred Stock ("Series C Stock"), for an aggregate approximate purchase price of $7,722,000, and (iii) FLSPOF purchased from the Issuer 2,211,650 shares of Series C Stock for an aggregate approximate purchase price of $2,278,000. Upon closing of the IPO, the shares of Series A Stock, Series B Stock and Series C Stock held by FLS X, FLSPF and FLSPOF automatically converted into shares of Common Stock of the Issuer on a 12.0221-to-1 basis (the "Conversion), resulting in (i) FLS X holding a total of 3,927,774 shares of Common Stock, (ii) FLSPF holding a total of 623,608 shares of Common Stock, and (iii) FLSPOF holding a total of 183,965 shares of Common Stock, in each case on an as-converted basis following the Conversion. In addition, at the time of the IPO, (i) FLS X purchased an aggregate of 625,000 shares of Common Stock of the Issuer at the IPO price of $16.00 per share, (ii) FLSPF purchased an aggregate of 480,312 shares of Common Stock of the Issuer at the IPO price of $16.00 per share, (iii) FLSPOF purchased an aggregate of 141,688 shares of Common Stock of the Issuer at the IPO price of $16.00 per share and (iv) FLS XI purchased an aggregate of 3,000 shares of Common Stock of the Issuer at the IPO price of $16.00 per share (items (i)-(iv) together, the "IPO Purchases"). As of the date of this filing, (i) FLS X holds 4,552,774 shares of the Issuer's Common Stock (the "FLS X Shares"); (ii) FLSPF holds 1,103,920 shares of the Issuer's Common Stock (the "FLSPF Shares"); (iii) FLSPOF holds 325,653 shares of the Issuer's Common Stock (the "FLSPOF Shares"); and (iv) FLS XI holds 3,000 shares of the Issuer's Common Stock (the "FLS XI Shares"). The working capital of FLSPF, FLSPOF, FLS X and FLS XI was the source of the funds for the purchase of the FLSPF Shares, the FLSPOF Shares, the FLS X Shares and the FLS XI Shares. No part of the purchase price of the FLSPF Shares, the FLSPOF Shares, the FLS X Shares or the FLS XI Shares was represented by funds or other consideration borrowed or otherwise obtained for the purpose of acquiring, holding, trading or voting the FLSPF Shares, the FLSPOF Shares, the FLS X Shares or the FLS XI Shares. The information contained in Rows 7, 8, 9, 10, 11, and 13 of each Reporting Person's cover page to this Schedule 13D (including the footnotes thereto) is incorporated by reference into this Item 5. FLSPF directly holds 1,103,920 shares of the Issuer's Common Stock (the "FLSPF Shares"). FHMLSP, L.P. is the general partner of FLSPF and the general partner of FHMLSP, L.P. is FHMLSP, L.L.C., which is managed by an investment committee of four that acts by majority vote. Accordingly, no members of such committee are attributed beneficial ownership of the securities directly held by FLSPF, and this Schedule 13D corrects any previous attribution to, and overstatement of beneficial ownership by, such members as reflected in any prior Statements. FLSPOF directly holds 325,653 shares of the Issuer's Common Stock (the "FLSPOF Shares"). FHMLSP Overage, L.P. is the general partner of FLSPOF and the general partner of FHMLSP Overage, L.P. is FHMLSP Overage, L.L.C., which is managed by an investment committee of four that acts by majority vote. Accordingly, no members of such committee are attributed beneficial ownership of the securities directly held by FLSPOF, and this Schedule 13D corrects any previous attribution to, and overstatement of beneficial ownership by, such members as reflected in any prior Statements. FLS X directly holds 4,552,774 shares of the Issuer's Common Stock (the "FLS X Shares"). FHMLS X, L.P. is the general partner of FLS X and FHMLS X, L.L.C. is the general partner of FHMLS X, L.P. Heron and Topper are the members of FHMLS X, L.L.C. and therefore share voting and investment power over the FLS X Shares. FLS XI directly holds 3,000 shares of the Issuer's Common Stock (the "FLS XI Shares"). FHMLS XI, L.P. is the general partner of FLS XI and the general partner of FHMLS XI, L.P. is FHMLS XI, L.L.C., which is managed by an investment committee of three that acts by majority vote. Accordingly, no members of such committee are attributed beneficial ownership of the securities directly held by FLS XI, and this Schedule 13D corrects any previous attribution to, and overstatement of beneficial ownership by, such members as reflected in any prior Statements. Except as specifically stated herein, the filing of this Schedule 13D shall not be construed as an admission that any Reporting Person or any of the foregoing is, for the purposes of Section 13(d) and/or Section 13(g) of the Act or otherwise, the beneficial owner of any securities covered by this Schedule 13D or a member of a "group" with any other person. The information contained in Rows 7, 8, 9, 10, 11, and 13 of each Reporting Person's cover page to this Schedule 13D (including the footnotes thereto) is incorporated by reference into this Item 5. Exhibit 10.1 Second Amended and Restated Investors' Rights Agreement (incorporated by reference to Exhibit 4.1 to the Issuer's Registration Statement on Form S-1 filed with the Commission on August 23, 2024). Exhibit 99.1 Joint Filing Agreement (incorporated by reference to Exhibit A to Schedule 13D filed on September 23, 2024). Frazier Life Sciences Public Fund, L.P. /s/ Steve R. Bailey 08/20/2025 FHMLSP, L.P. /s/ Steve R. Bailey 08/20/2025 FHMLSP, L.L.C. /s/ Steve R. Bailey 08/20/2025 Frazier Life Sciences Public Overage Fund, L.P. /s/ Steve R. Bailey 08/20/2025 FHMLSP Overage, L.P. /s/ Steve R. Bailey 08/20/2025 FHMLSP Overage, L.L.C. /s/ Steve R. Bailey 08/20/2025 Frazier Life Sciences X, L.P. /s/ Steve R. Bailey 08/20/2025 FHMLS X, L.P. /s/ Steve R. Bailey 08/20/2025 FHMLS X, L.L.C. /s/ Steve R. Bailey 08/20/2025 Frazier Life Sciences XI, L.P. /s/ Steve R. Bailey 08/20/2025 FHMLS XI, L.P. /s/ Steve R. Bailey 08/20/2025 FHMLS XI, L.L.C. /s/ Steve R. Bailey 08/20/2025 James N. Topper /s/ Steve R. Bailey 08/20/2025 Patrick J. Heron /s/ Steve R. Bailey 08/20/2025 Albert Cha /s/ Steve R. Bailey 08/20/2025 James Brush /s/ Steve R. Bailey 08/20/2025 Daniel Estes /s/ Steve R. Bailey 08/20/2025