X0306 4 2021-10-12 1 0001745020 Theseus Pharmaceuticals, Inc. THRX 0001757917 Omega Fund VI, L.P. 888 BOYLSTON STREET, SUITE 1111 BOSTON MA 02199 0 0 0 1 Former 10% owner 0001826584 Omega Fund VI GP Manager, Ltd. 888 BOYLSTON STREET, SUITE 1111 BOSTON MA 02199 0 0 0 1 Former 10% owner 0001361712 Stampacchia Otello 888 BOYLSTON STREET, SUITE 1111 BOSTON MA 02199 0 0 0 1 Former 10% owner 0001722050 Nessi Claudio 888 BOYLSTON STREET, SUITE 1111 BOSTON MA 02199 0 0 0 1 Former 10% owner 0001652863 Paster Anne-Mari 888 BOYLSTON STREET, SUITE 1111 BOSTON MA 02199 0 0 0 1 Former 10% owner Common Stock 2021-10-12 4 C 0 523979 A 523979 D Common Stock 2021-10-12 4 P 0 187500 16.00 A 711479 D Series B Preferred Stock 2021-10-12 4 C 0 693152 D Common Stock 523979 0 D Omega Fund VI, L.P. ("Omega VI") held 693,152 shares of the Issuer's Series B Preferred Stock. On October 12, 2021, the Series B Preferred Stock automatically converted into 523,979 shares of the Issuer's Common Stock on a 1-for-1.32286 reverse split basis, immediately prior to and in connection with the closing of a public offering of the Issuer's securities pursuant to an effective registration statement under the Securities Act of 1933, as amended (the "Securities Act") and without payment of further consideration. The reported securities are beneficially owned by Omega Fund VI, L.P. ("Omega VI"). The reported securities may be deemed to be beneficially owned by each of Omega Fund VI GP, L.P. ("Omega VI GP"), as the general partner of Omega VI, and Omega Fund VI GP Manager, Ltd. ("Omega VI GP Manager"), as the general partner of Omega VI GP. Otello Stampacchia, Claudio Nessi and Anne-Mari Paster are all the shareholders and directors of Omega VI GP Manager and have shared voting and investment power over the shares held by Omega VI and, as a result, may each be deemed to beneficially own the reported securities. Each of such individuals disclaims beneficial ownership of the shares held by Omega VI. As of October 12, 2021 and after giving effect to the transactions reported above, Omega VI holds less than 10% of the outstanding shares of the Issuer's Common Stock. /s/ Anne-Mari Paster, as an authorized signatory of each Reporting Person 2021-10-14