SCHEDULE 13D/A

MBX Biosciences Inc

Common Stock, $0.0001 par value per share · CUSIP: 55287L101

Event date: 2025-09-26

Reporting persons may disclose overlapping interests. Their amounts are shown individually and are not added into a filing total.

Filing-level shares

Unavailable

Percent of class

Unavailable

• Frazier Life Sciences Public Fund, L.P.Reported CIK 0001863769

  Shares: 1,103,920.00 · Class: 2.5%

• FHMLSP, L.P.Reported CIK 0001879466

  Shares: 1,103,920.00 · Class: 2.5%

• FHMLSP, L.L.C.Reported CIK 0001879465

  Shares: 1,103,920.00 · Class: 2.5%

• Frazier Life Sciences Public Overage Fund, L.P.Reported CIK 0001913804

  Shares: 325,653.00 · Class: 0.7%

• FHMLSP Overage, L.P.Reported CIK 0001963247

  Shares: 325,653.00 · Class: 0.7%

• FHMLSP Overage, L.L.C.Reported CIK 0001963248

  Shares: 325,653.00 · Class: 0.7%

• Frazier Life Sciences X, L.P.Reported CIK 0001790879

  Shares: 5,219,440.00 · Class: 11.7%

• FHMLS X, L.P.Reported CIK 0001790880

  Shares: 5,219,440.00 · Class: 11.7%

• FHMLS X, L.L.C.Reported CIK 0001790811

  Shares: 5,219,440.00 · Class: 11.7%

• Frazier Life Sciences XI, L.P.Reported CIK 0001911592

  Shares: 3,000.00 · Class: 0.0%

• FHMLS XI, L.P.Reported CIK 0001911580

  Shares: 3,000.00 · Class: 0.0%

• FHMLS XI, L.L.C.Reported CIK 0001911623

  Shares: 3,000.00 · Class: 0.0%

• James N. TopperReported CIK 0001341382

  Shares: 5,219,440.00 · Class: 11.7%

• Patrick J. HeronReported CIK 0001365617

  Shares: 5,224,095.00 · Class: 11.7%

Amounts are as disclosed in this filing. Schedule type alone does not establish investment intent.