Insider Filing: Frazier Entities Raise MBX Stake to 5.22M Shares

Rhea-AI Filing Summary

MBX Biosciences Form 4 summary: On 09/26/2025 an affiliated investor group acquired 666,666 shares of MBX common stock at $18.00 per share. After the reported purchase the group beneficially owned 5,219,440 shares. The shares are held of record by Frazier Life Sciences X, L.P., with FHMLS X, L.P., FHMLS X, L.L.C., James N. Topper and Patrick J. Heron having shared voting and investment power; Heron is identified as a reporting person and a director and 10% owner. The filing is signed by an attorney-in-fact on behalf of the reporting parties.

Positive

• Insider/affiliate accumulation: Reported acquisition of 666,666 shares at $18 signals increased stake by the Frazier group.
• Clear ownership chain disclosed: Filing specifies record holder (Frazier Life Sciences X, L.P.) and controlling entities/individuals (FHMLS entities, Topper, Heron).

Negative

• None.

Insights

Securities Analyst

TL;DR: An affiliated investor increased its indirect stake by 666,666 shares at $18, modestly raising its ownership to 5.22M shares.

The transaction is an open-market or private purchase reported on Form 4 showing an increase in beneficial ownership by an organized investor group led through Frazier entities. The purchase price of $18 and the post-transaction holding of 5,219,440 shares are clear, and the filing discloses the chain of control among the Frazier entities and two named individuals who share voting and investment power. For investors this is a disclosure of insider/affiliate accumulation rather than a disposition; the filing itself does not include context such as percentage ownership of outstanding shares or intent.

Corporate Governance Specialist

TL;DR: Governance disclosure clarifies indirect ownership and voting power among related Frazier entities and two individuals.

The Form 4 provides the required identification and relationship information: reporting persons are directors and 10% owners, and the explanation section maps record holder to controlling entities. Signatures are provided via attorney-in-fact. The filing meets Section 16 disclosure mechanics but does not state any change in board composition or governance arrangements beyond ownership reporting.

SEC Form 4

FORM 4

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
or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL

OMB Number:			3235-0287
Estimated average burden
hours per response:			0.5

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

1. Name and Address of Reporting Person^*

Heron Patrick J

(Last) (First) (Middle)

1001 PAGE MILL RD, BUILDING 4, SUITE B

(Street)

PALO ALTO CA 94304

(City) (State) (Zip)

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)

3. Date of Earliest Transaction (Month/Day/Year)
09/26/2025

4. If Amendment, Date of 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

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)

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 Securities Underlying Derivative Security (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: 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

1. Name and Address of Reporting Person^*

Heron Patrick J

(Last) (First) (Middle)

1001 PAGE MILL RD, BUILDING 4, SUITE B

(Street)

PALO ALTO CA 94304

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

X	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

FHMLS X, L.P.

(Last) (First) (Middle)

FRAZIER LIFE SCIENCES MANAGEMENT, L.P.

1001 PAGE MILL RD, BUILDING 4, SUITE B

(Street)

PALO ALTO CA 94304

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

FHMLS X, L.L.C.

(Last) (First) (Middle)

FRAZIER LIFE SCIENCES MANAGEMENT, L.P.

1001 PAGE MILL RD, BUILDING 4, SUITE B

(Street)

PALO ALTO CA 94304

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

Topper James N

(Last) (First) (Middle)

FRAZIER LIFE SCIENCES MANAGEMENT, L.P.

1001 PAGE MILL RD, BUILDING 4, SUITE B

(Street)

PALO ALTO CA 94304

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

1. Name and Address of Reporting Person^*

Frazier Life Sciences X, L.P.

(Last) (First) (Middle)

FRAZIER LIFE SCIENCES MANAGEMENT, L.P.

1001 PAGE MILL RD, BUILDING 4, SUITE B

(Street)

PALO ALTO CA 94304

(City) (State) (Zip)

Relationship of Reporting Person(s) to Issuer

	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

Explanation of Responses:

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.

	/s/ Steve R. Bailey, Attorney-in-Fact For Patrick J. Heron	09/30/2025
	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
	By Steve R. Bailey, CFO of FHMLS X, L.L.C., GP of FHMLS X, L.P.	09/30/2025
	By Steve R. Bailey, CFO of FHMLS X, L.L.C.	09/30/2025
	/s/ Steve R. Bailey, Attorney-in-Fact For James N. Topper	09/30/2025
	** Signature of Reporting Person	Date
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).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.

FAQ

What transaction did MBX reporting persons disclose on Form 4?

The filing reports an acquisition of 666,666 shares of MBX common stock on 09/26/2025 at a price of $18.00 per share.

How many MBX shares are beneficially owned after the reported transaction?

Following the reported purchase the filing shows beneficial ownership of 5,219,440 shares by the reporting group.

Who holds the shares of MBX and who controls them?

The shares are held of record by Frazier Life Sciences X, L.P.; control and voting/investment power are shared by FHMLS X, L.P., FHMLS X, L.L.C., James N. Topper and Patrick J. Heron.

What roles do the reporting persons have with MBX?

Patrick J. Heron is identified as a director and a 10% owner; the filing lists other Frazier-related entities and individuals with director and 10% owner designations.

Was the Form 4 signed and when?

The filing includes signatures by Steve R. Bailey, acting as attorney-in-fact, with signature dates shown as 09/30/2025.