X0202 SCHEDULE 13G 0001595849 XXXXXXXX LIVE Common Stock, par value $0.001 per share 06/30/2026 0002093101 Atrium Therapeutics, Inc. 04965N104 3020 CALLAN ROAD SAN DIEGO CA 92121 Rule 13d-1(b) Sessa Capital IM, L.P. DE 0.00 1500000.00 0.00 1500000.00 1500000.00 N 8.8 IA PN John Petry X1 0.00 1500000.00 0.00 1500000.00 1500000.00 N 8.8 HC IN Atrium Therapeutics, Inc. 10578 Science Center Drive, Suite 125, San Diego, CA, 92121. This statement is filed by: (i) Sessa Capital IM, L.P. ("Sessa IM"), which serves as the investment manager to certain investment funds and accounts (the "Sessa Funds"), with respect to shares of common stock, par value $0.001 per share ("Common Stock"), of Atrium Therapeutics, Inc., a Delaware corporation (the "Company"), held by the Sessa Funds; and (ii) John Petry, who serves as the sole member and manager of Sessa Capital IM GP, LLC ("Sessa IM GP"), the general partner of Sessa IM, with respect to the shares of Common Stock held by the Sessa Funds. The filing of this statement should not be construed as an admission that any Reporting Person is, for purposes of Section 13 of the Act, the beneficial owner of the securities reported herein. The address of the business office of each of the Reporting Persons and Sessa IM GP is 888 Seventh Avenue, 30th Floor, New York, NY 10019. Sessa IM is a Delaware limited partnership. Sessa IM GP is a Delaware limited liability company. Mr. Petry is a United States citizen. N IA HC The information required by Item 4(a) is set forth in Row 9 of the cover page for each of the Reporting Persons and is incorporated herein by reference. The percentage set forth in this Schedule 13G is calculated based upon an aggregate of 17,105,643 shares of Common Stock outstanding as of May 5, 2026, as reported in the Company's Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2026, filed with the Securities and Exchange Commission on May 14, 2026. 8.8% The information required by Item 4(c)(i) is set forth in Row 5 of the cover page for each of the Reporting Persons and is incorporated herein by reference. The information required by Item 4(c)(ii) is set forth in Row 6 of the cover page for each of the Reporting Persons and is incorporated herein by reference. The information required by Item 4(c)(iii) is set forth in Row 7 of the cover page for each of the Reporting Persons and is incorporated herein by reference. The information required by Item 4(c)(iv) is set forth in Row 8 of the cover page for each of the Reporting Persons and is incorporated herein by reference. Y N See Item 2(a). Sessa Capital (Master), L.P., a Sessa Fund, has the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, more than 5% of the Common Stock of the Company. Y Y Y N By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11. Exhibit 99.1: Joint Filing Agreement Sessa Capital IM, L.P. /s/ John Petry 08/14/2026 John Petry /s/ John Petry 08/14/2026