X0202 SCHEDULE 13G/A 0001346824-26-000145 0001346824 XXXXXXXX LIVE 1 Common Stock, par value $0.001 per share 06/30/2026 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 0.00 0.00 0.00 0.00 0.0 IA PN Peter Kolchinsky X1 0.00 0.00 0.00 0.00 0.00 0.0 HC IN Rajeev Shah X1 0.00 0.00 0.00 0.00 0.00 0.0 HC IN RA Capital Healthcare Fund, L.P. DE 0.00 0.00 0.00 0.00 0.00 0.0 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/A. 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/A sets forth the aggregate number of securities of the Issuer beneficially owned by such Reporting Person as of June 30, 2026 and is incorporated by reference. Row 11 of each Reporting Person's cover page to this Schedule 13G/A sets forth the percentages of the securities of the Issuer beneficially owned by such Reporting Person as of June 30, 2026 and is incorporated by reference. Row 5 of each Reporting Person's cover page to this Schedule 13G/A 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 June 30, 2026 and is incorporated by reference. Row 6 of each Reporting Person's cover page to this Schedule 13G/A 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 June 30, 2026 and is incorporated by reference. Row 7 of each Reporting Person's cover page to this Schedule 13G/A 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 June 30, 2026 and is incorporated by reference. Row 8 of each Reporting Person's cover page to this Schedule 13G/A 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 June 30, 2026 and is incorporated by reference. N Y Y Y Y Y Y Exhibit 99.1 Joint Filing Agreement (incorporated by reference to Exhibit 99.1 to Schedule 13G filed on May 15, 2026) RA Capital Management, L.P. /s/ Peter Kolchinsky 08/14/2026 Peter Kolchinsky /s/ Peter Kolchinsky 08/14/2026 Rajeev Shah /s/ Rajeev Shah 08/14/2026 RA Capital Healthcare Fund, L.P. /s/ Peter Kolchinsky 08/14/2026