[Form 4] MBX Biosciences, Inc. Insider Trading Activity

Rhea-AI Filing Summary

MBX Biosciences (MBX) reported an insider transaction by its President & CEO and Director on 10/24/2025. The executive exercised a stock option (Code M) to acquire 182,859 shares of common stock at an exercise price of $3.25 per share. Following the transaction, beneficial ownership stood at 664,256 shares held directly and 468,277 shares held indirectly through a revocable trust. The filing lists 129,708 derivative securities (options) remaining post-transaction. The option began vesting monthly from 11/07/2022 and is scheduled to fully vest by 11/07/2026, with early-exercise permitted subject to an issuer repurchase right.

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).

Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

1. Name and Address of Reporting Person^*

Hawryluk P. Kent

(Last) (First) (Middle)

C/O MBX BIOSCIENCES, INC.

11711 N. MERIDIAN STREET, SUITE 300

(Street)

CARMEL IN 46032

(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		10% Owner
X	Officer (give title below)		Other (specify below)
	President & CEO

3. Date of Earliest Transaction (Month/Day/Year)
10/24/2025

4. If Amendment, Date of Original Filed (Month/Day/Year)

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	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	10/24/2025		M		182,859	A	$3.25	664,256	D
Common Stock								468,277	I	By Trust(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
Stock option (right to buy)	$3.25	10/24/2025		M			182,859	11/07/2022(2)	11/06/2032	Common Stock	182,859	$0	129,708	D

Explanation of Responses:

1. Shares held by the P. Kent Hawryluk Revocable Trust dated January 25, 2011, of which the Reporting Person serves as trustee. The Reporting Person disclaims beneficial ownership except to the extent of any pecuniary interest therein, and this report shall not be deemed an admission that such shares are beneficially owned by him for Section 16 or any other purpose.

2. 1/48th of the shares underlying this option vest in substantially equal monthly installments on each monthly anniversary of November 7, 2022, subject to the Reporting Person's continued service on each such vesting date, until the option vests in full on November 7, 2026. The shares underlying this option became exercisable in full on November 7, 2022, pursuant to an early-exercise provision, subject to an Issuer right of repurchase.

	/s/ Richard Bartram, attorney-in-fact	10/28/2025
	** 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 did MBX (MBX) disclose in this Form 4?

The President & CEO exercised an option (Code M) to acquire 182,859 shares at an exercise price of $3.25 on 10/24/2025.

How many MBX shares does the insider hold after the transaction?

Post-transaction holdings are 664,256 shares directly and 468,277 shares indirectly via a revocable trust.

What derivative securities remain after the exercise?

The filing shows 129,708 stock options remaining following the reported transaction.

What is the vesting schedule of the exercised option?

The option vests at 1/48th monthly from 11/07/2022 until full vesting on 11/07/2026, with early exercise allowed subject to a repurchase right.

What does the indirect ownership represent for MBX?

Indirect shares are held by the P. Kent Hawryluk Revocable Trust; the insider disclaims beneficial ownership beyond pecuniary interest.

What was the transaction code and price for the MBX option exercise?

Transaction code was M (option exercise) at an exercise price of $3.25 per share.