An inhibitor of ATM kinase (IC50s = 0.04 and 0.57 nM in enzyme and cell-based assays, respectively); selective for ATM over other PIKK family kinases including DNA-PK, mTOR, PI3Kα, PI3Kβ, PI3Kγ, and PI3Kδ (IC50s = 0.14, 0.20, 0.32, 1.8, 1.1, and 0.27 μM, respectively, in enzyme assays); enhances the antitumor activity of irinotecan in an SW620 colon cancer mouse xenograft model at 20 mg/kg and olaparib in an HBCx-10 breast cancer PDX mouse model at 5 mg/kg, leading to tumor regression in both models