X0306 4 2021-12-21 0 0001745020 Theseus Pharmaceuticals, Inc. THRX 0001242992 CLACKSON TIMOTHY P C/O THESEUS PHARMACEUTICALS, INC. 245 MAIN STREET CAMBRIDGE MA 02142 1 1 0 0 President and CEO Common Stock 2021-12-21 4 P 0 2600 9.7621 A 348530 D The price reported is a weighted average price of shares purchased. These shares were purchased in multiple transaction at prices ranging from $9.65 to $9.89, inclusive. The reporting person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price. /s/ Timothy Clackson 2021-12-22