John W. Smither - 03 Aug 2026 Form 4 Insider Report for MBX Biosciences, Inc. (MBX)

Reporting relationship: Chief Financial Officer
Issuer ticker: MBX
Reported as of: 03 Aug 2026
Net transaction value: $0
SEC form: 4
Accepted by SEC: 04 Aug 2026, 08:51:27 UTC
Prior SEC filing: 02 Jul 2026
Next SEC filing: 04 Aug 2026
Source filing: View source filing
Reporting owner signature: /s/ John W. Smither

Key filing fact

John W. Smither filed Form 4 for MBX Biosciences, Inc. (MBX) on 04 Aug 2026.

Key facts

This page summarizes John W. Smither's Form 4 filing for MBX Biosciences, Inc. (MBX).
2 reported transactions and 1 derivative row are listed below.
Accepted by SEC: 04 Aug 2026, 08:51.

Change

Previous filing in this sequence was filed on 02 Jul 2026.
Current net transaction value: $0.

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 (1)

CIK 0001559583 Primary reporting owner

Smither John W

Relationship: Chief Financial Officer
Address: 11711 N. MERIDIAN STREET, SUITE 300, CARMEL
Signature: /s/ John W. Smither
Signature date: 04 Aug 2026

Reported non-derivative transactions

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

MBX transaction

Common Stock

Award

Transaction value
Shares: +28,000
Change %
Price: $0.000000^*
Shares after: 28,000
Date: 03 Aug 2026
Ownership: Direct
Footnotes: F1

Reported derivative securities

Options, warrants, convertible securities, or similar derivative positions disclosed in the filing.

MBX transaction Derivative

Stock option (right to buy)

Award

Transaction value
Shares: +130,000
Change %
Price: $0.000000^*
Shares after: 130,000
Date: 03 Aug 2026
Ownership: Direct
Underlying class: Common Stock
Underlying amount: 130,000
Exercise price: $64.41
Footnotes: F2

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

Additional SEC filing notes

Filing notes and footnotes

Explanation of responses 2 footnotes

Footnote F1

Represents the grant of restricted stock units ("RSUs"). 25% of the shares subject to the RSU grant will vest annually over four years, on each July 13, 2027, 2028, 2029 and 2030, subject to continued service on each such vesting date.

Footnote F2

The options will vest and become exercisable with 25% of the shares subject to the award vesting on July 13, 2027 and the remainder vesting in 36 equal monthly installments thereafter, subject to continued service on each such vesting date.

SEC remarks

Exhibit 24 - Power of Attorney