SCHEDULE 13G/A

MBX Biosciences Inc

Common Stock · CUSIP: 55287L101

Event date: 2025-09-30

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

• Wellington Management Group LLP

  Shares: 4,823,336.00 · Class: 10.8%

• Wellington Group Holdings LLP

  Shares: 4,823,336.00 · Class: 10.8%

• Wellington Investment Advisors Holdings LLP

  Shares: 4,823,336.00 · Class: 10.8%

• Wellington Management Company LLP

  Shares: 4,823,336.00 · Class: 10.6%

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