Thompson Peter A. 4

What Happened
Peter A. Thompson, a director of Sionna Therapeutics (SION), disposed of a total of 510,583 shares in three open‑market sales executed April 14–16, 2026. The sales were: 47,814 shares at $44.51 (≈ $2,128,201) on 2026‑04‑14; 235,863 shares at $44.64 (≈ $10,528,924) on 2026‑04‑15; and 226,906 shares at $45.34 (≈ $10,287,918) on 2026‑04‑16. Total gross proceeds across the three transactions were about $22.95 million. These sales were made pursuant to a 10b5‑1 trading plan (prearranged plan), which typically indicates the trades were scheduled in advance rather than spontaneous.

Key Details

• Transaction dates and prices:
  • 2026-04-14: Sold 47,814 @ $44.51 — $2,128,201
  • 2026-04-15: Sold 235,863 @ $44.64 — $10,528,924
  • 2026-04-16: Sold 226,906 @ $45.34 — $10,287,918
• Total shares sold: 510,583; total proceeds ≈ $22,945,043.
• Shares owned after transaction: Not specified in this Form 4.
• Footnotes:
  • F1: Sales were made pursuant to a 10b5‑1 plan (preplanned trading).
  • F2/F3: The reported securities are held of record by OrbiMed Private Investments VIII, LP (an institutional vehicle). OrbiMed Advisors and its GP may be deemed to have voting/investment power; the reporting person is a member of OrbiMed Advisors. Those parties disclaim beneficial ownership except to the extent of any pecuniary interest.
• Filing/timeliness: Form filed 2026‑04‑16 (no late‑filing flag indicated on the form).

Context
These were sales, not purchases, and were executed under a 10b5‑1 plan, which generally signals preplanned disposition rather than an immediate opinion about company prospects. The reporting person is linked to an institutional investor (OrbiMed vehicle) rather than acting solely as an individual insider; the filing includes disclaimers about beneficial ownership. For retail investors, purchases by insiders tend to be more informative than routine, planned sales; this Form 4 documents a large, prearranged sale by an entity-affiliated director.