X0306 4 2021-10-12 1 0001745020 Theseus Pharmaceuticals, Inc. THRX 0001595725 Rock Springs Capital Management LP 650 SOUTH EXETER ST., SUITE 1070 BALTIMORE MD 21202 0 0 1 0 0001735393 Rock Springs Capital Master Fund LP C/O WALKERS CORPORATE LIMITED, CAYMAN CO CENTER 27 HOSPITAL ROAD GEORAGE TOWN E9 KY-1-9008 CAYMAN ISLANDS 0 0 1 0 0001735397 Rock Springs Capital LLC 650 SOUTH EXETER ST., SUITE 1070 BALTIMORE MD 2102 0 0 1 0 Common Stock 2021-10-12 4 C 0 349320 A 349320 I See Footnote Common Stock 2021-10-12 4 C 0 87329 A 87329 I See Footnote Common Stock 2021-10-12 4 P 0 250000 16.00 A 599320 I See Footnote Series B Convertible Preferred Stock 2021-10-12 4 C 0 349320 D Common Stock 349320 0 I See Footnote Series B Convertible Preferred Stock 2021-10-12 4 C 0 87329 D Common Stock 87329 0 I See Footnote The total represents shares received upon conversion of shares of Series B Convertible Preferred Stock. The reported security converted into the issuer's Common Stock on a one-for-one basis automatically immediately prior to the closing of the issuer's initial public offering without payment of further consideration. The shares have no expiration date. The securities reported herein are held of record by Rock Springs Capital Master Fund LP ("Master Fund"). Rock Springs Capital LLC ("RSC") is the general partner of Rock Springs Capital Management LP ("RSCM") which is the investment manager to Master Fund. RSC and RSCM may therefore be deemed to have or share beneficial ownership of the shares held directly by Master Fund. The securities reported herein are held of record by Four Pines Master Fund LP ("Four Pines"). RSC is the general partner of RSCM which is the investment manager to Four Pines. RSC and RSCM may therefore be deemed to have or share beneficial ownership of the shares held directly by Four Pines. Represents shares purchased in the issuer's initial public offering. By: /s/ Mark Bussard, Authorized Signatory, Rock Springs Capital Management LP 2021-10-14 By: /s/ Mark Bussard, Authorized Signatory, Rock Springs Capital Master Fund LP 2021-10-14 By: /s/ Mark Bussard, Authorized Signatory, Rock Springs Capital LLC 2021-10-14