Patrick J. Heron - 26 Sep 2025 Form 4 Insider Report for MBX Biosciences, Inc. (MBX)

Reporting relationship: Director, 10%+ Owner
Issuer ticker: MBX
Reported as of: 26 Sep 2025
Net transaction value: +$11,999,988
SEC form: 4
Accepted by SEC: 30 Sep 2025, 16:59:56 UTC
Prior SEC filing: 25 Jun 2025
Next SEC filing: 27 Jan 2026
Source filing: View source filing
Reporting owner signature: /s/ Steve R. Bailey, Attorney-in-Fact For Patrick J. Heron

Key filing fact

Patrick J. Heron filed Form 4 for MBX Biosciences, Inc. (MBX) on 30 Sep 2025.

Key facts

This page summarizes Patrick J. Heron's Form 4 filing for MBX Biosciences, Inc. (MBX).
1 reported transaction and 0 derivative rows are listed below.
Accepted by SEC: 30 Sep 2025, 16:59.

Change

Previous filing in this sequence was filed on 25 Jun 2025.
Current net transaction value: +$11,999,988.

Research use

This tells you what this filing adds before you inspect full transaction and derivative tables.
You can trace every row back to the original SEC filing document.

Evidence

Filed on Form 4

Ownership activity is grounded in SEC Form 4 disclosures.

View source filing

Reporting Owners (5)

CIK 0001365617 Primary reporting owner

Heron Patrick J

Relationship: Director, 10%+ Owner
Address: 1001 PAGE MILL RD, BUILDING 4, SUITE B, PALO ALTO
Signature: /s/ Steve R. Bailey, Attorney-in-Fact For Patrick J. Heron
Signature date: 30 Sep 2025

CIK 0001790880

FHMLS X, L.P.

Relationship: 10%+ Owner
Address: FRAZIER LIFE SCIENCES MANAGEMENT, L.P., 1001 PAGE MILL RD, BUILDING 4, SUITE B, PALO ALTO
Signature: 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.
Signature date: 30 Sep 2025

CIK 0001790811

FHMLS X, L.L.C.

Relationship: 10%+ Owner
Address: FRAZIER LIFE SCIENCES MANAGEMENT, L.P., 1001 PAGE MILL RD, BUILDING 4, SUITE B, PALO ALTO
Signature: By Steve R. Bailey, CFO of FHMLS X, L.L.C., GP of FHMLS X, L.P.
Signature date: 30 Sep 2025

CIK 0001341382

Topper James N

Relationship: 10%+ Owner
Address: FRAZIER LIFE SCIENCES MANAGEMENT, L.P., 1001 PAGE MILL RD, BUILDING 4, SUITE B, PALO ALTO
Signature: By Steve R. Bailey, CFO of FHMLS X, L.L.C.
Signature date: 30 Sep 2025

CIK 0001790879

Frazier Life Sciences X, L.P.

Relationship: 10%+ Owner
Address: FRAZIER LIFE SCIENCES MANAGEMENT, L.P., 1001 PAGE MILL RD, BUILDING 4, SUITE B, PALO ALTO
Signature: /s/ Steve R. Bailey, Attorney-in-Fact For James N. Topper
Signature date: 30 Sep 2025

Reported non-derivative transactions

Shares, units, or other non-derivative securities reported in this filing.

MBX transaction

Common Stock

Purchase

Transaction value: $11,999,988
Shares: +666,666
Change %: +15%
Price: $18.00
Shares after: 5,219,440
Date: 26 Sep 2025
Ownership: By Frazier Life Sciences X, L.P.
Footnotes: F1

^* marks a reported price that did not pass the local price check.

Additional SEC filing notes

Filing notes and footnotes

Explanation of responses 1 footnote

Footnote F1

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.