Form 4 Filing by Thomas Frank E for Spero Therapeutics, Inc.

This filing details transactions by Thomas Frank E, a director at Spero Therapeutics, Inc. On June 23, 2026, Mr. Thomas acquired 10,000 shares of Common Stock and 20,000 shares underlying a Stock Option. The Common Stock acquisition, represented by restricted stock units (RSUs), vests on June 23, 2027, contingent upon continued service. The stock option also vests on June 23, 2027, with an exercise price of $2.15 per share. Following these transactions, Mr. Thomas directly owns 85,000 shares of Common Stock and 20,000 shares underlying the stock option. The transactions were reported on Form 4, signed by Maegan Deare as Attorney-in-Fact.