144: Filer Information

Filer CIK

0001723729

Filer CCC

XXXXXXXX

Is this a LIVE or TEST Filing?

LIVE

TEST

Submission Contact Information

Name

Phone

E-Mail Address

144: Issuer Information

Name of Issuer

MBX Biosciences, Inc.

SEC File Number

001-42272

Address of Issuer

11711 N. MERIDIAN STREET
SUITE 300
CARMEL
INDIANA
46032

Phone

3179893100

Name of Person for Whose Account the Securities are To Be Sold

Bartram Richard

See the definition of "person" in paragraph (a) of Rule 144. Information is to be given not only as to the person for whose account the securities are to be sold but also as to all other persons included in that definition. In addition, information shall be given as to sales by all persons whose sales are required by paragraph (e) of Rule 144 to be aggregated with sales for the account of the person filing this notice.

Relationship to Issuer

Former Officer

144: Securities Information

Title of the Class of Securities To Be Sold

Name and Address of the Broker

Number of Shares or Other Units To Be Sold

Aggregate Market Value

Number of Shares or Other Units Outstanding

Approximate Date of Sale

Name the Securities Exchange

Common

Fidelity Brokerage Services LLC
900 Salem Street
Smithfield � RI � 02917

10833

321273.40

47597536

06/09/2026

NASDAQ

Furnish the following information with respect to the acquisition of the securities to be sold and with respect to the payment of all or any part of the purchase price or other consideration therefor:

144: Securities To Be Sold

Title of the Class

Date you Acquired

Nature of Acquisition Transaction

Name of Person from Whom Acquired

Is this a Gift?

Date Donor Acquired

Amount of Securities Acquired

Date of Payment

Nature of Payment *

Common

06/09/2026

Stock Option Exercise

Issuer

10833

06/09/2026

Cash

* If the securities were purchased and full payment therefor was not made in cash at the time of purchase, explain in the table or in a note thereto the nature of the consideration given. If the consideration consisted of any note or other obligation, or if payment was made in installments describe the arrangement and state when the note or other obligation was discharged in full or the last installment paid.

Furnish the following information as to all securities of the issuer sold during the past 3 months by the person for whose account the securities are to be sold.

144: Securities Sold During The Past 3 Months

Name and Address of Seller

Title of Securities Sold

Date of Sale

Amount of Securities Sold

Gross Proceeds

Richard B. Bartram
11711 N. Meridian Street, Suite 300
Carmel � IN � 46032

Common

03/16/2026

10000

300471.00

Richard B. Bartram
11711 N. Meridian Street, Suite 300
Carmel � IN � 46032

Common

03/25/2026

1061

30217.49

Richard B. Bartram
11711 N. Meridian Street, Suite 300
Carmel � IN � 46032

Common

04/22/2026

10423

341377.72

Richard B. Bartram
11711 N. Meridian Street, Suite 300
Carmel � IN � 46032

Common

05/01/2026

5000

159000.00

Richard B. Bartram
11711 N. Meridian Street
Suite 300
Carmel � IN � 46032

Common

05/07/2026

10000

322050.00

Richard B. Bartram
11711 N. Meridian Street
Suite 300
Carmel � IN � 46032

Common

05/08/2026

10000

379562.50

Richard B. Bartram
11711 N. Meridian Street
Suite 300
Carmel � IN � 46032

Common

05/11/2026

22500

991901.00

Richard B. Bartram
11711 N. Meridian Street
Suite 300
Carmel � IN � 46032

Common

05/13/2026

15000

554295.50

144: Remarks and Signature

Remarks

Date of Notice

06/09/2026

ATTENTION:

The person for whose account the securities to which this notice relates are to be sold hereby represents by signing this notice that he does not know any material adverse information in regard to the current and prospective operations of the Issuer of the securities to be sold which has not been publicly disclosed. If such person has adopted a written trading plan or given trading instructions to satisfy Rule 10b5-1 under the Exchange Act, by signing the form and indicating the date that the plan was adopted or the instruction given, that person makes such representation as of the plan adoption or instruction date.

Signature

/s/ Gerardo Ibarra-Munoz, as a duly authorized representative of Fidelity Brokerage Services LLC, as attorney-in-fact for Richard B. Bartram

144: Filer Information

Submission Contact Information

144: Issuer Information

144: Securities Information

144: Securities To Be Sold

144: Securities Sold During The Past 3 Months

144: Remarks and Signature

ATTENTION:

ATTENTION: Intentional misstatements or omission of facts constitute Federal Criminal Violations (See 18 U.S.C. 1001)

SEC Form 144 filed by MBX Biosciences Inc.