Statement of Changes in Beneficial Ownership (Form 4)

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b). Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

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

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1. Name and Address of Reporting Person * Heron Patrick J 2. Issuer Name and Ticker or Trading Symbol MBX Biosciences, Inc. [MBX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director __X__ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) , 1001 PAGE MILL RD, BUILDING 4, SUITE B 3. Date of Earliest Transaction (Month/Day/Year) 09/26/2025 (Street) PALO ALTO, CA 94304 4. If Amendment, Date Original Filed (Month/Day/Year) 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 (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 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 Common Stock 09/26/2025 P 666,666 A $18 5,219,440 I By Frazier Life Sciences X, L.P.(1)

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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 (Month/Day/Year)

3A. Deemed Execution Date, if any (Month/Day/Year)

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 (Month/Day/Year)

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

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Heron Patrick J 1001 PAGE MILL RD, BUILDING 4, SUITE B PALO ALTO, CA 94304	X	X
FHMLS X, L.P. FRAZIER LIFE SCIENCES MANAGEMENT, L.P. 1001 PAGE MILL RD, BUILDING 4, SUITE B PALO ALTO, CA 94304		X
FHMLS X, L.L.C. FRAZIER LIFE SCIENCES MANAGEMENT, L.P. 1001 PAGE MILL RD, BUILDING 4, SUITE B PALO ALTO, CA 94304		X
Topper James N FRAZIER LIFE SCIENCES MANAGEMENT, L.P. 1001 PAGE MILL RD, BUILDING 4, SUITE B PALO ALTO, CA 94304		X
Frazier Life Sciences X, L.P. FRAZIER LIFE SCIENCES MANAGEMENT, L.P. 1001 PAGE MILL RD, BUILDING 4, SUITE B PALO ALTO, CA 94304		X

Signatures

/s/ Steve R. Bailey, Attorney-in-Fact For Patrick J. Heron		09/30/2025
**Signature of Reporting Person		Date
By Steve R. Bailey, CFO of FHMLS X, L.L.C., GP of FHMLS X, L.P., GP of Frazier Life Sciences X, L.P.		09/30/2025
**Signature of Reporting Person		Date
By Steve R. Bailey, CFO of FHMLS X, L.L.C., GP of FHMLS X, L.P.		09/30/2025
**Signature of Reporting Person		Date
By Steve R. Bailey, CFO of FHMLS X, L.L.C.		09/30/2025
**Signature of Reporting Person		Date
/s/ Steve R. Bailey, Attorney-in-Fact For James N. Topper		09/30/2025
**Signature of Reporting Person		Date

Explanation of Responses:

*	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).
(1)	The shares reported herein are held of record by Frazier Life Sciences X, L.P. FHMLS X, L.P. is the general partner of Frazier Life Sciences X, L.P. and FHMLS X, L.L.C. is the general partner of FHMLS X, L.P. James N. Topper and Patrick J. Heron are the sole managing members of FHMLS X, L.L.C. and share voting and investment power over the shares held by Frazier Life Sciences X, L.P. Dr. Topper and Mr. Heron disclaim beneficial ownership of such securities except to the extent of their pecuniary interest therein.

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