X0609 4 2026-04-14 0 0002036042 Sionna Therapeutics, Inc. SION 0001357522 Thompson Peter A. false C/O SIONNA THERAPEUTICS, INC. 21 HICKORY DRIVE SUITE 500 WALTHAM MA 02451 1 0 0 0 1 Common Stock 2026-04-14 4 S 0 47814 44.51 D 3482888 I See footnotes Common Stock 2026-04-15 4 S 0 235863 44.64 D 3247025 I See footnotes Common Stock 2026-04-16 4 S 0 226906 45.34 D 3020119 I See footnotes These securities were sold pursuant to a 10b5-1 plan. These securities are held of record by OrbiMed Private Investments VIII, LP ("OPI VIII"). OrbiMed Capital GP VIII LLC ("GP VIII") is the general partner of OPI VIII and OrbiMed Advisors LLC ("OrbiMed Advisors") is the managing member of GP VIII. By virtue of such relationships, GP VIII and OrbiMed Advisors may be deemed to have voting and investment power with respect to the shares held by OPI VIII and as a result may be deemed to have beneficial ownership of such shares. OrbiMed Advisors exercises investment and voting power through a management committee comprised of Carl L. Gordon, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by OPI VIII. The Reporting Person is a member of OrbiMed Advisors. Each of the Reporting Person, OrbiMed Advisors, and GP VIII disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its pecuniary interest therein, if any. This report shall not be deemed an admission that any such entity or person is a beneficial owner of such securities for purposes of Section 16 of the Exchange Act, or for any other purpose. /s/ Peter A. Thompson 2026-04-16