Form 4: Sionna Director's Funds Sell $3M in Stock

Funds associated with Sionna Therapeutics Director Bruce Booth sold 88,878 shares of common stock for approximately $3.04 million on October 21, 2025, under pre-arranged Rule 10b5-1 trading plans.

Summary

• Bruce Booth, a Director of Sionna Therapeutics, Inc. (SION), reported the sale of common stock by affiliated entities.
• Atlas Venture Fund XI, L.P. sold a total of 73,460 shares of Sionna Therapeutics common stock on October 21, 2025, at weighted average prices ranging from $32.69 to $36.42 per share.
• Atlas Venture Opportunity Fund II, L.P. sold a total of 15,418 shares of Sionna Therapeutics common stock on October 21, 2025, at weighted average prices ranging from $32.69 to $36.42 per share.
• The total number of shares sold across both funds was 88,878, amounting to an approximate total value of $3,040,751.18.
• These transactions were executed pursuant to Rule 10b5-1 trading plans adopted by Atlas Venture Fund XI, L.P. and Atlas Venture Opportunity Fund II, L.P. on February 25, 2025.
• Following these transactions, Atlas Venture Fund XI, L.P. beneficially owns 2,309,035 shares, and Atlas Venture Opportunity Fund II, L.P. beneficially owns 686,567 shares.
• Bruce Booth disclaims beneficial ownership of these securities held by the funds, except to the extent of his pecuniary interest therein, if any.

Sentiment

Score: 5

Explanation: A Form 4 filing disclosing an insider sale is a factual report of a transaction and does not inherently carry positive or negative sentiment. The sale was conducted under a pre-arranged 10b5-1 trading plan, which is a common practice for insiders to manage their holdings.

Industry Context

This filing is a standard disclosure of an insider stock transaction, common for directors and significant shareholders. Such transactions are typically pre-arranged through Rule 10b5-1 plans to avoid accusations of trading on material non-public information, indicating a planned divestment rather than a reaction to immediate news.

Stakeholder Impact

• Shareholders may interpret the sale by an affiliated fund of a director as a signal, though the presence of a 10b5-1 plan suggests it was pre-scheduled and not based on immediate new information. The impact is generally minor unless the sale is unusually large or unexpected in context.

Key Dates

Keywords

Sionna Therapeutics, SION, Form 4, Insider Trading, Stock Sale, Bruce Booth, Atlas Venture, 10b5-1 Plan