X0202 SCHEDULE 13G 0001346824 XXXXXXXX LIVE Common Stock, par value $0.001 per share 03/31/2026 0002093101 Atrium Therapeutics, Inc. 04965N104 10578 Science Center Drive, Suite 125 San Diego CA 92121 Rule 13d-1(d) RA Capital Management, L.P. DE 0.00 864102.00 0.00 864102.00 864102.00 5.6 IA PN Peter Kolchinsky X1 0.00 864102.00 0.00 864102.00 864102.00 5.6 HC IN Rajeev Shah X1 0.00 864102.00 0.00 864102.00 864102.00 5.6 HC IN RA Capital Healthcare Fund, L.P. DE 0.00 864102.00 0.00 864102.00 864102.00 5.6 PN Atrium Therapeutics, Inc. 10578 Science Center Drive, Suite 125, San Diego, CA, 92121. The names of the persons filing this report (collectively, the "Reporting Persons") are: RA Capital Management, L.P. ("RA Capital") Peter Kolchinsky Rajeev Shah RA Capital Healthcare Fund, L.P. (the "Fund") The Reporting Persons expressly disclaim status as a "group" for purposes of this Schedule 13G. The address of the principal business office of each of the Reporting Persons is: c/o RA Capital Management, L.P., 200 Berkeley Street, 18th Floor, Boston MA 02116 RA Capital and the Fund are Delaware limited partnerships. Dr. Kolchinsky and Mr. Shah are United States citizens. Y Row 9 of each Reporting Person's cover page to this Schedule 13G sets forth the aggregate number of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference. The Fund directly holds 864,102 shares of Common Stock. RA Capital Healthcare Fund GP, LLC is the general partner of the Fund. 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 and may be deemed a beneficial owner, for purposes of Section 13(d) of the Act, of any securities of the Issuer held by the Fund. The Fund has delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in the Fund's portfolio, including the shares of the Issuer's Common Stock reported herein. Because the Fund has divested voting and investment power over the reported securities it holds and may not revoke that delegation on less than 61 days' notice, the Fund disclaims beneficial ownership of the securities it holds for purposes of 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 13 other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13G 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. Row 11 of each Reporting Person's cover page to this Schedule 13G sets forth the percentages of the securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference. The percentage set forth in each Row 11 is based upon 15,514,996 shares of the Issuer's Common Stock that were outstanding as of February 26, 2026, as reported in the Issuer's Current Report on Form 8-K, filed with the Securities and Exchange Commission (the "SEC") on February 26, 2026. Row 5 of each Reporting Person's cover page to this Schedule 13G sets forth the sole power to vote or to direct the vote of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference. Row 6 of each Reporting Person's cover page to this Schedule 13G sets forth the shared power to vote or to direct the vote of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference. Row 7 of each Reporting Person's cover page to this Schedule 13G sets forth the sole power to dispose or to direct the disposition of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference. Row 8 of each Reporting Person's cover page to this Schedule 13G sets forth the shared power to dispose or to direct the disposition of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference. Y Y Y Y Y Y Exhibit 99.1 Joint Filing Agreement RA Capital Management, L.P. /s/ Peter Kolchinsky 05/15/2026 Peter Kolchinsky /s/ Peter Kolchinsky 05/15/2026 Rajeev Shah /s/ Rajeev Shah 05/15/2026 RA Capital Healthcare Fund, L.P. /s/ Peter Kolchinsky 05/15/2026