Common Stock, $0.0001 par value per share

Acumen Pharmaceuticals, Inc.

1210-1220 Washington St., Suite 210, Newton, MA, 02465

This Amendment No. 3 is being filed on behalf of RA Capital Management, L.P. ("RA Capital"), Peter Kolchinsky, Rajeev Shah, and RA Capital Healthcare Fund, L.P. (the "Fund"), who are collectively referred to herein as the "Reporting Persons." The agreement among the Reporting Persons to file this 13D/A jointly in accordance with Rule 13d-1(k) of the Securities Exchange Act of 1934, as amended (the "Act"), is attached hereto as Exhibit 99.1. The Reporting Persons' ownership of the Issuer's securities includes (i) 13,043,179 shares of common stock held directly by the Fund; (ii) 1,300,034 shares of common stock held by RA Capital Nexus Fund II, L.P. (the "Nexus Fund II"); (iii) 588,850 shares of common stock held by a separately managed account (the "Account"); and a total of 84,000 shares underlying vested stock options (right to buy) held by Dr. Laura Stoppel for the benefit of RA Capital. RA Capital Healthcare Fund GP, LLC is the general partner of the Fund and RA Capital Nexus Fund II GP, LLC is the general partner of the Nexus Fund II. The general partner of RA Capital is RA Capital Management GP, LLC, of which Dr. Kolchinsky and Mr. Shah are the controlling persons. RA Capital serves as investment adviser for the Fund, the Nexus Fund II and the Account and may be deemed a beneficial owner, for purposes of Section 13(d) of the Securities Exchange Act of 1934 (the "Act"), of any securities of the Issuer held by the Fund, the Nexus Fund II or the Account. The Fund and the Nexus Fund II have delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in the Fund's and the Nexus Fund II's portfolio, including the shares of the Issuer's common stock reported herein. Because the Fund and the Nexus Fund II have divested themselves of voting and investment power over the reported securities they hold and may not revoke that delegation on less than 61 days' notice, the Fund and the Nexus Fund II disclaim beneficial ownership of the securities they hold for purposes of Section 13(d) of the Act and therefore disclaim any obligation to report ownership of the reported securities under Section 13(d) of the Act. As managers of RA Capital, Dr. Kolchinsky and Mr. Shah may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Issuer beneficially owned by RA Capital. RA Capital, Dr. Kolchinsky, and Mr. Shah disclaim beneficial ownership of the securities reported in this Schedule 13D/A other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13D/A shall not be deemed an admission that either RA Capital, Dr. Kolchinsky, or Mr. Shah is the beneficial owner of such securities for any other purpose.

The address of the principal business office of each of the Reporting Persons is 200 Berkeley Street, 18th Floor, Boston, MA 02116.

The Fund is a private investment vehicle. RA Capital provides investment management services to the Fund, the Nexus Fund II and the Account. The principal occupation of Dr. Kolchinsky and Mr. Shah is investment management.

During the last five years, none of the Reporting Persons has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree of final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

See Item 6 of the cover pages.

Rows 11 and 13 of each Reporting Person's cover page to this Schedule 13D/A set forth the aggregate number of shares of common stock and percentages of the shares of common stock beneficially owned by such Reporting Person and are incorporated by reference. The percentage set forth in each row 13 is based upon 60,573,425 shares of common stock outstanding as of March 24, 2025, as reported in the Issuer's Annual Report on Form 10-K filed with the SEC on March 27, 2025, and giving effect to stock options, to the extent exercisable within 60 days hereof, as referenced herein.

Rows 7 through 10 of each Reporting Person's cover page to this Schedule 13D/A set forth the number of shares of common stock as to which such Reporting Person has the sole or shared power to vote or direct the vote and sole or shared power to dispose or to direct the disposition and are incorporated by reference.

No transactions with respect to the securities of the Issuer have been effected during the past sixty days by any Reporting Person. The percentage ownership reported in this amendment reflect an increase in the number of outstanding shares of the Issuer.

Except as set forth below, none of the Reporting Persons has effected any transactions with respect to the securities of the Issuer since the most recent amendment to this Schedule 13D/A.

Not applicable.