An inhibitor of ATR (Ki = 50 = 19 nM in pancreatic cancer cells in vitro); reduces the IC50 of SN-38 by at least 8-fold in vitro when used at a concentration of 80 nM; delays the growth of radiation-treated PSN-1 pancreatic cancer xenografts in mice by 2-3-fold relative to radiation alone (60 mg/kg); enhances the tumoricidal effects of cisplatin against primary human lung cancer xenografts; enhances the effects of irinotecan against COLO 205 colorectal tumors