Sionna Therapeutics (SION) 10% Owner RA Capital Sells 3.45M Shares

What Happened
RA Capital Management, L.P. (a reported 10% owner) sold a total of 3,448,811 shares of Sionna Therapeutics (SION) in open-market transactions on September 17, 2026, generating aggregate proceeds of approximately $21,835,018. The filing shows two reported dispositions: 3,415,011 shares at $6.32 per share (proceeds ~$21,582,870) and 33,800 shares at a weighted-average $7.46 per share (proceeds ~$252,148). This was a sale (not a purchase or option exercise).

Key Details

• Transaction date: 2026-09-17; Form 4 filed 2026-09-21 (filed within the standard two-business-day window).
• Prices and amounts:
  • 3,415,011 shares sold at $6.32 — ~$21,582,870
  • 33,800 shares sold at a weighted-average $7.46 (individual trades ranged $7.30–$7.68) — ~$252,148
  • Total sold: 3,448,811 shares for ~ $21.84M
• Shares owned after transaction: not specified in the provided filing excerpt.
• Footnotes of note:
  • F1: RA Capital is the investment manager for several funds and disclaims beneficial ownership except for pecuniary interest.
  • F2–F6: indicate which reported holdings were held directly by the Fund, Nexus Fund, Nexus Fund III, and a separately managed account.
  • F3: the $7.46 price is a weighted average across multiple trades (range $7.30–$7.68); full execution-level breakdown is available on request from the filer or SEC staff.
• Remark: Dr. Joshua Resnick, a Partner at the Adviser, serves on Sionna’s board of directors.

Context

• This filing reflects institutional selling by a 10% owner (an adviser managing multiple funds and an account), which is often portfolio rebalancing or fund-related activity rather than an individual executive signal. Retail investors should note sales are common and not necessarily indicative of company prospects.
• The transaction was an open-market sale (S code); there is no indication of option exercises, awards, gifts, or tax-withholding in this report.