Sec Form 3 Filing - Frazier Life Sciences X, L.P. @ MBX Biosciences, Inc. - 2024-09-12

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Frazier Life Sciences X, L.P. 2. Issuer Name and Ticker or Trading Symbol MBX Biosciences, Inc. [ MBX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 601 UNION STREET, SUITE 3200 3. Date of Earliest Transaction (MM/DD/YY) 09/12/2024 (Street) SEATTLE, WA98101 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Series A Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 1,574,005 D ( 2 ) Series B Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 2,353,769 D ( 2 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Frazier Life Sciences X, L.P. 601 UNION STREET, SUITE 3200 SEATTLE, WA98101 X FHMLS X, L.P. 601 UNION STREET, SUITE 3200 SEATTLE, WA98101 X FHMLS X, L.L.C. 601 UNION STREET, SUITE 3200 SEATTLE, WA98101 X Topper James N 601 UNION STREET, SUITE 3200 SEATTLE, WA98101 X Signatures Frazier Life Sciences X, L.P., By: FHMLS X, L.P., its general partner, By: FHMLS X, L.L.C., its general partner, By: /s/ Steve R. Bailey, Chief Financial Officer 09/12/2024 Signature of Reporting Person Date FHMLS X, L.P., By: FHMLS X, L.L.C., its general partner, By: /s/ Steve R. Bailey, Chief Financial Officer 09/12/2024 Signature of Reporting Person Date FHMLS X, L.L.C., By: /s/ Steve R. Bailey, Chief Financial Officer 09/12/2024 Signature of Reporting Person Date /s/ Steve R. Bailey, Attorney-in-Fact for James N. Topper 09/12/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Each share of Series A Convertible Preferred Stock, Series B Convertible Preferred Stock and Series C Convertible Preferred Stock (collectively, the "Preferred Stock") is convertible into Common Stock on a one-for-12.0221 basis at any time at the option of the holder, and will automatically convert into the number of shares shown in Column 3 immediately prior to the closing of the Issuer's initial public offering. The Preferred Stock has no expiration date. ( 2 )These securities are held of record by Frazier Life Sciences X, L.P. ("FLS X"). 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. Patrick Heron and James Topper, M.D., Ph.D., are the sole managing members of FHMLS X, L.L.C. and share voting and investment power of the securities held by FLS X, and as a result, may be deemed to have beneficial ownership over such securities. Mr. Heron and Dr. Topper disclaim beneficial ownership of such securities except to the extent of their pecuniary interest therein.

Remarks: Exhibit 24 - Power of Attorney

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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