Notice of Proposed Sale · 144

MBX Biosciences, Inc.

MBXNASDAQEQUITYCurrent

Notice of Proposed Sale

Filed Aug 4, 2026Accepted Aug 4, 2026, 4:44 PM EDTFiling CIK 1673991Accession 0001959173-26-005629

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Structured filing — 144

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Notice of proposed sale. The proposed amounts below do not establish that a sale occurred.

Proposed-sale notice

Issuer: MBX Biosciences, Inc.
Issuer CIK: 0001776111
SEC file number: 001-42272
Phone: 3179893100
Person for whose account securities are to be sold: Hawryluk P. Kent
Relationships to issuer: Officer · Director
Street: 11711 N. MERIDIAN STREET
Street (continued): SUITE 300
City: CARMEL
State / country code: IN
Postal code: 46032

Securities proposed for sale

Securities proposed for sale · 1 reported row
Security	Proposed units	Aggregate market value	Units outstanding	Approximate sale date	Exchange	Broker / market maker
Common	936	61,563.43	47,597,536	08/04/2026	NASDAQ	Fidelity Brokerage Services LLC

Broker / market maker — proposed row 1, entry 1

Name: Fidelity Brokerage Services LLC
Street: 900 Salem Street
City: Smithfield
State / country code: RI
Postal code: 02917

Acquisition of securities to be sold

Acquisition of securities to be sold · 1 reported row
Security	Acquired	Nature of acquisition	Acquired from	Amount acquired	Gift transaction	Payment date	Nature of payment
Common	08/03/2026	Restricted Stock Vesting	Issuer	936	N	08/03/2026	Compensation

Sales reported in the preceding three months

Sales reported in the preceding three months · 1 reported row
Seller	Security	Sale date	Amount sold	Gross proceeds
Peter Kent Hawryluk	Common	05/06/2026	607	17,933.51

Seller — prior-sale row 1, entry 1

Name: Peter Kent Hawryluk
Street: 11711 N. Meridian Street
Street (continued): Suite 300
City: Carmel
State / country code: IN
Postal code: 46032

Prior-sale reporting

Nothing to report for the preceding three months: N

Remarks

Sale includes an amount necessary to cover a tax obligation resulting from the settlement of a vested equity award distribution.

Notice signature

Notice date: 08/04/2026
Signed: /s/ Daniel Tucci, as a duly authorized representative of Fidelity Brokerage Services LLC, as attorney-in-fact for Peter Hawryluk

Company context

We are a clinical-stage biopharmaceutical company focused on the discovery and development of novel precision peptide therapies for the treatment of endocrine and metabolic disorders. Our company was founded by global leaders with a transformative approach to peptide drug design and development. Leveraging this expertise, we designed our proprietary Precision Endocrine Peptide™, or PEP™, platform to overcome the key limitations of unmodified and modified peptide therapies and to improve clinical outcomes and simplify disease management for patients. Our PEPs are selectively engineered to have optimized pharmaceutical properties, including extended time-action profiles and consistent drug concentrations with low peak-to-trough concentration ratios, consistent exposure to target tissues, and less frequent dosing. We are advancing a pipeline of novel candidates for endocrine and metabolic disorders with clinically validated targets, established endpoints for regulatory approval, significant unmet medical needs and large potential market opportunities. Our product candidates and programs include:

Current securities

MBXNASDAQ · EQUITY · Current

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