The present invention relates to methods of treating cancer in a patient by administering a compound useful as an inhibitor of ATR protein kinase in combination with a compound useful as an inhibitor of Chkl protein kinase. The aforementioned combination displays a surprising synergistic effect in treating cancer despite the targeted protein kinases being within the same biological pathway. Moreover, the present invention also relates to methods of treating cancer by administering a compound useful as an inhibitor of ATR protein kinase； administering a compound useful as an inhibitor of Chkl protein kinase； as well as administering a DNA damaging agent to a patient. The compounds utilized in this invention are represented by formula (I) and formula (II): wherein the variables are as defined herein.
