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X0202 SCHEDULE 13D/A 0001193125-24-223402 0001863769 XXXXXXXX LIVE 3 Common Stock, $0.0001 par value per share 08/06/2026 false 0001776111 55287L101 MBX BIOSCIENCES, INC. 11711 N. Meridian Street, Suite 300 Carmel IN 46032 Jennifer Martin (206) 451-8040 1700 Seventh Ave, Suite 1120 Seattle WA 98101 0001863769 N Frazier Life Sciences Public Fund, L.P. b WC N DE 0.00 1429573.00 0.00 1429573.00 1429573.00 N 3.0 PN The shares listed in rows 8, 10 and 11 include shares that were acquired from Frazier Life Sciences Public Overage Fund, L.P. pursuant to a merger between Frazier Life Sciences Public Overage Fund, L.P. and Frazier Life Sciences Public Fund, L.P. The percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 0001879466 N FHMLSP, L.P. b AF N DE 0.00 1429573.00 0.00 1429573.00 1429573.00 N 3.0 PN The shares listed in rows 8, 10 and 11 include shares that were acquired from Frazier Life Sciences Public Overage Fund, L.P. pursuant to a merger between Frazier Life Sciences Public Overage Fund, L.P. and Frazier Life Sciences Public Fund, L.P. The percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 0001879465 N FHMLSP, L.L.C. b AF N DE 0.00 1429573.00 0.00 1429573.00 1429573.00 N 3.0 OO The shares listed in rows 8, 10 and 11 include shares that were acquired from Frazier Life Sciences Public Overage Fund, L.P. pursuant to a merger between Frazier Life Sciences Public Overage Fund, L.P. and Frazier Life Sciences Public Fund, L.P. The percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 0001790879 N Frazier Life Sciences X, L.P. b WC N DE 0.00 5219440.00 0.00 5219440.00 5219440.00 N 10.8 PN The percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 0001790880 N FHMLS X, L.P. b AF N DE 0.00 5219440.00 0.00 5219440.00 5219440.00 N 10.8 PN The percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 0001790811 N FHMLS X, L.L.C. b AF N DE 0.00 5219440.00 0.00 5219440.00 5219440.00 N 10.8 OO The percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 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 percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 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 percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 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 percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 0001341382 N James N. Topper b AF N X1 0.00 5219440.00 0.00 5219440.00 5219440.00 N 11.7 IN The Aggregate Amount represents 5,219,440 shares of Common Stock held directly by Frazier Life Sciences X, L.P. The percentage listed in row 13 is calculated based on 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026. 0001365617 N Patrick J. Heron b AF N X1 23981.00 5219440.00 23981.00 5219440.00 5243421.00 N 11.7 IN The Aggregate Amount represents (i) 23,981 shares of Common Stock that are issuable upon the exercise of options held directly by Patrick J. Heron, and (ii) 5,219,440 shares of Common Stock held directly by Frazier Life Sciences X, L.P. The percentage listed in row 13 is calculated based on the sum of (i) 48,160,096 shares of the Issuer's Common Stock outstanding as of August 3, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2026, and (ii) 23,981 shares of Common Stock that are issuable upon the exercise of options held directly by Patrick J. Heron Common Stock, $0.0001 par value per share MBX BIOSCIENCES, INC. 11711 N. Meridian Street, Suite 300 Carmel IN 46032 This Amendment No. 3 ("Amendment No. 3") to Schedule 13D amends the statement on Schedule 13D filed on September 23, 2024 (the "Original Schedule 13D") as amended on August 20, 2025 and September 30, 2025 (the "Prior Amendments", and together with the Original Schedule 13D and this Amendment No. 3, the "Schedule 13D"). Except as otherwise specified in this Amendment No. 3, all items in the Original Schedule 13D, as amended by the Prior Amendments, are unchanged. All capitalized terms used in this Amendment No. 3 and not otherwise defined herein have the meanings ascribed to such terms in the Original Schedule 13D, as amended by the Prior Amendments. The entities and persons filing this statement (collectively, the "Reporting Persons") are: Frazier Life Sciences Public Fund, L.P. ("FLSPF") FHMLSP, L.P. FHMLSP, L.L.C. Frazier Life Sciences X, L.P. ("FLS X") FHMLS X, L.P. FHMLS X, L.L.C. Frazier Life Sciences XI, L.P. ("FLS XI") FHMLS XI, L.P. FHMLS XI, L.L.C. James N. Topper ("Topper") Patrick J. Heron ("Heron" and together with Topper, the "Members") The address of the principal place of business for each of the Reporting Persons is: c/o Frazier Life Sciences Management, L.P. 1001 Page Mill Rd, Building 4, Suite 200B Palo Alto, CA 94304 Item 2(c) is hereby amended and restated in its entirety to read as follows: FLSPF is a venture capital fund concentrating in life sciences and related fields. The sole business of FHMLSP, L.P. is to serve as general partner of FLSPF. The sole business of FHMLSP, L.L.C. is to serve as general partner of FHMLSP, L.P. FLS X is a venture capital fund concentrating in life sciences and related fields. The sole business of FHMLS X, L.P. is to serve as general partner of FLS X. The sole business of FHMLS X, L.L.C. is to serve as general partner of FHMLS X, L.P. FLS XI is a venture capital fund concentrating in life sciences and related fields. The sole business of FHMLS XI, L.P. is to serve as general partner of FLS XI. The sole business of FHMLS XI, L.L.C. is to serve as general partner of FHMLS XI, L.P. The principal business of the Members is to manage FLSPF, FHMLSP, L.P., FHMLSP, L.L.C., FLS X, FHMLS X, L.P., FHMLS X, L.L.C., FLS XI, FHMLS XI, L.P., FHMLS XI, L.L.C. and a number of affiliated partnerships with similar businesses. Item 2(f) is hereby amended and restated in its entirety to read as follows: FLSPF - Delaware, U.S.A. FHMLSP, L.P. - Delaware, U.S.A. FHMLSP, L.L.C. - Delaware, U.S.A. FLS X - Delaware, U.S.A. FHMLS X, L.P. - Delaware, U.S.A. FHMLS X, L.L.C. - Delaware, U.S.A. FLS XI - Delaware, U.S.A. FHMLS XI, L.P. - Delaware, U.S.A. FHMLS XI, L.L.C. - Delaware, U.S.A. Topper - United States Citizen Heron - United States Citizen Item 3 of the Schedule 13D is hereby amended to incorporate Item 5(c) hereof and to replace the last paragraph with the following: The working capital of FLSPF, FLS X and FLS XI was the source of the funds for the purchase of the FLSPF Shares, the FLS X Shares and the FLS XI Shares. No part of the purchase price of the FLSPF 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 FLS X Shares or the FLS XI Shares. Item 4 is hereby amended to remove references to "FLSPOF" and the "FLSPOF Shares" from the first paragraph. 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 1,429,573 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. FLS X directly holds 5,219,440 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. 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. Item 5(d) is hereby amended to remove references to the "FLSPOF Shares" from the paragraph. 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 Frazier Life Sciences Public Fund, L.P. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLSP, L.L.C., GP of FHMLSP, L.P., GP of Frazier Life Sciences Public Fund, L.P. 08/10/2026 FHMLSP, L.P. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLSP, L.L.C., GP of FHMLSP, L.P. 08/10/2026 FHMLSP, L.L.C. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLSP, L.L.C. 08/10/2026 Frazier Life Sciences X, L.P. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLS X, L.L.C., GP of FHMLS X, L.P., GP of Frazier Life Sciences X, L.P. 08/10/2026 FHMLS X, L.P. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLS X, L.L.C., GP of FHMLS X, L.P. 08/10/2026 FHMLS X, L.L.C. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLS X, L.L.C. 08/10/2026 Frazier Life Sciences XI, L.P. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLS XI, L.L.C., GP of FHMLS XI, L.P., GP of Frazier Life Sciences XI, L.P. 08/10/2026 FHMLS XI, L.P. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLS XI, L.L.C., GP of FHMLS XI, L.P. 08/10/2026 FHMLS XI, L.L.C. /s/ Jennifer Martin By Jennifer Martin, CFO of FHMLS XI, L.L.C. 08/10/2026 James N. Topper /s/ Jennifer Martin By Jennifer Martin, Attorney-in-Fact for James N. Topper, pursuant to Power of Attorney 08/10/2026 Patrick J. Heron /s/ Jennifer Martin By Jennifer Martin, Attorney-in-Fact for Patrick J. Heron, pursuant to Power of Attorney 08/10/2026