Sionna Therapeutics SEC Filings

Sionna Therapeutics (SION) disclosed insider activity on a Form 4. A director reported open‑market sales of common stock on 10/10, 10/13, 10/14, and 10/15/2025, executed under Rule 10b5‑1 trading plans adopted on February 25, 2025 by Atlas Venture Fund XI, L.P. and Atlas Venture Opportunity Fund II, L.P.

Examples: on 10/15, Atlas Venture Fund XI, L.P. sold 65,726 shares at a weighted average price of $30.37 (range $29.95–$30.94) and 30,032 shares at $31.12 (range $30.95–$31.30). Atlas Venture Opportunity Fund II, L.P. sold 13,795 shares at $30.37 and 6,303 shares at $31.12. Following the reported transactions, indirect holdings were 2,619,185 shares (Atlas Venture Fund XI, L.P.) and 751,664 shares (Atlas Venture Opportunity Fund II, L.P.).

Sionna Therapeutics, Inc. submitted a Form 144 reporting a proposed sale of 3,511 common shares with an aggregate market value of $99,536.85, representing part of the holder's holding against 44,139,823 shares outstanding. The filing names Merrill Lynch as the broker and lists an approximate sale date of 10/10/2025. The securities were acquired in a private placement on 03/04/2024 and paid for in cash.

The filing also discloses multiple securities sales by affiliated funds over the prior three months: Atlas Venture Opportunity Fund II and Atlas Venture Fund XI executed several transactions between 09/29/2025 and 10/06/2025, with individual gross proceeds ranging from $1,048.25 to $1,644,065.58. The filer certifies no undisclosed material adverse information and includes the standard Rule 144 attestation language.

Sionna Therapeutics filed a Form 144 notifying a proposed sale of 16,727 shares of common stock through Merrill Lynch on 10/10/2025 with an aggregate market value of $474,210.45. The shares were acquired in a private placement on 03/04/2024 and were paid for in cash. The filing also lists multiple open-market sales by affiliated funds in the prior three months, including several trades between 09/29/2025 and 10/06/2025 totaling tens of thousands of shares and gross proceeds in the low millions.

The filer certifies they know of no material nonpublic information and notes the statutory warning about false statements. The notice identifies Merrill Lynch as the broker and NASDAQ as the exchange for the planned sale.

Insider sales by a director were disclosed for Sionna Therapeutics (SION). The Form 4 shows Director Bruce Booth reported multiple sales under Rule 10b5-1 trading plans on 10/03/2025 and 10/06/2025

On 10/03/2025 the filing reports sales of 31,198 and 6,548 shares at a weighted price around $30. On 10/06/2025 it reports sales of 9,753 and 2,047 shares at a weighted price around $29.95. The reported beneficial ownership after the transactions is 2,780,301 shares held by Atlas Venture Fund XI, L.P. and 785,480 shares held by Atlas Venture Opportunity Fund II, L.P.

The filing notes the shares sold were held indirectly by the two Atlas Venture funds and that the reporting person disclaims beneficial ownership except for any pecuniary interest. The transactions were executed pursuant to trading plans adopted on 02/25/2025 and the Form is signed by an attorney-in-fact on 10/07/2025.

Sionna Therapeutics, Inc. (SION) filed a Form 144 reporting a proposed sale of 2,047 common shares through Merrill Lynch with an aggregate market value of $61,430.47. The filing lists total outstanding shares as 44,139,823, and an approximate sale date of 10/06/2025. The shares were acquired in a private placement on 03/04/2024 for cash.

The notice also discloses recent related sales by affiliated funds: Atlas Venture Opportunity Fund II and Atlas Venture Fund XI sold multiple blocks between 09/29/2025 and 10/03/2025, including a 54,637-share sale generating $1,644,065.58. The filer certifies no undisclosed material adverse information and includes the standard Rule 10b5-1 and signature attestations.

Sionna Therapeutics, Inc. (SION) filed a Form 144 notice reporting a proposed sale of 9,753 common shares valued at $292,687.53 to be sold through Merrill Lynch on 10/06/2025 on NASDAQ. The shares were acquired in a private placement on 03/04/2024 and paid for in cash.

The filing also lists recent open-market sales by related funds between 09/29/2025 and 10/03/2025, totaling approximately $3,499,940.95 across several transactions. The filer affirms no undisclosed material adverse information and includes the standard certification language regarding Rule 10b5-1 trading plans.

Sionna Therapeutics filed a Form 144 notifying a proposed sale of 31,198 common shares acquired in a private placement on 03/04/2024. The filing reports an aggregate market value of $914,101.40 and lists the approximate sale date as 10/03/2025, with Merrill Lynch named as the broker and the securities listed on NASDAQ. The company shows 44,139,823 shares outstanding. The filing also discloses recent sales by related funds in late September–early October 2025 totaling multiple transactions (largest: 54,637 shares for $1,644,065.58 on 09/29/2025).

Caroline Stark Beer, Chief Business Officer of Sionna Therapeutics, Inc. (SION), was granted a non‑qualified stock option to purchase 255,000 shares of common stock with an exercise price of $28.47. The grant was reported with a transaction date of 10/01/2025 and the filing was signed on 10/03/2025. The option becomes exercisable under a vesting schedule where 25% of the shares vest on September 8, 2026, with the remaining shares vesting in 36 equal monthly installments thereafter, contingent on continued service. The option expires on 09/30/2035.

Sionna Therapeutics, Inc. (SION) filed an Initial Statement of Beneficial Ownership (Form 3) for Caroline Stark Beer, who is identified as the company's Chief Business Officer and an officer/director. The form shows the reportable event date of 10/01/2025 and is signed on 10/03/2025. The filing explicitly states: No securities are beneficially owned by the reporting person, and no non‑derivative or derivative holdings are reported.