Amendment to Schedule 13D Filing for ACUMEN PHARMACEUTICALS, INC.

This filing is an amendment (Amendment No. 4) to the Schedule 13D originally filed on July 16, 2021, by RA Capital Management, L.P. and related entities. The amendment concerns the beneficial ownership of common stock in ACUMEN PHARMACEUTICALS, INC. (the "Issuer"). On March 13, 2026, the Issuer entered into a securities purchase agreement with institutional accredited investors, including RA Capital Healthcare Fund, L.P. (the "Fund"), for the private placement of 10,833,331 shares of common stock at $3.30 per share. The Fund purchased 6,060,606 shares for $19,999,999.80, funded by its working capital. This transaction closed on March 16, 2026. The filing also details a Registration Rights Agreement entered into in connection with this private placement, which provides for the resale of the shares. RA Capital Management, L.P. and its affiliates now beneficially own approximately 29.5% of the Issuer's common stock, representing 21,101,669 shares.