MBX Biosciences (MBX) CFO John Smither files initial Form 3

Rhea-AI Filing Summary

MBX Biosciences, Inc. reported that Chief Financial Officer John W. Smither filed an initial Form 3 as an officer of the company. The filing lists him as Chief Financial Officer and does not report any purchases, sales, gifts, or other transactions in MBX Biosciences securities.

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SEC Form 3

FORM 3

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF 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^*

Smither John W

(Last) (First) (Middle)

11711 N. MERIDIAN STREET

SUITE 300

(Street)

CARMEL INDIANA 46032

(City) (State) (Zip)

UNITED STATES

(Country)

2. Date of Event Requiring Statement (Month/Day/Year)
03/16/2026

3. Issuer Name and Ticker or Trading Symbol
MBX Biosciences, Inc. [ MBX ]

3a. Foreign Trading Symbol

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

4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

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

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 Beneficially Owned
1. Title of Security (Instr. 4)	2. Amount of Securities Beneficially Owned (Instr. 4)	3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	4. Nature of Indirect Beneficial Ownership (Instr. 5)

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
	Date Exercisable	Expiration Date	Title	Amount or Number of Shares

Explanation of Responses:

Remarks:

Exhibit 24 - Power of Attorney

No securities are beneficially owned.

	/s/ P. Kent Hawryluk, attorney-in-fact	03/18/2026
	** 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 5 (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.
* Form 3: SEC 1473 (03-26)

FAQ

What does the MBX Biosciences (MBX) Form 3 filing report?

The Form 3 reports an initial insider ownership filing for MBX Biosciences. It identifies John W. Smither as Chief Financial Officer but does not list any stock purchases, sales, or other transactions in MBX Biosciences securities in this particular filing.

Who is the insider named in the MBX Biosciences (MBX) Form 3?

The insider named is John W. Smither, who is reported as an officer of MBX Biosciences with the title Chief Financial Officer. The filing focuses on his status as a reporting person rather than detailing specific trades or derivative positions.

Does the MBX Biosciences (MBX) Form 3 show any stock trades?

No, the Form 3 does not show any trades. The transaction summary indicates zero purchases, zero sales, and no derivative exercises, gifts, tax withholdings, or restructuring transactions involving MBX Biosciences securities for John W. Smither in this filing.

Are there any derivative securities reported for MBX Biosciences (MBX) in this Form 3?

No derivative securities are reported in this Form 3. The derivative summary is empty and the exercise-related fields in the transaction summary are zero, indicating no options, warrants, or other derivatives are listed for John W. Smither here.

What role does John W. Smither hold at MBX Biosciences (MBX) in this filing?

John W. Smither is identified as Chief Financial Officer of MBX Biosciences. The Form 3 shows he is an officer, not a director or ten percent owner, and serves as an initial beneficial ownership report without transaction activity.