KU59403 is a potent and selective ATM (Ataxia telangiectasia mutated) inhibitor with with potential anticancer activity. KU59403 was not cytotoxic to human cancer cell lines (SW620, LoVo, HCT116, and MDA-MB-231) per se but significantly increased the cytotoxicity of topoisomerase I and II poisons: camptothecin, etoposide, and doxorubicin. KU59403 significantly enhanced the antitumor activity of topoisomerase poisons in mice bearing human colon cancer xenografts (SW620 and HCT116) at doses that were nontoxic alone and well-tolerated in combination