Apurinic/apyrimidinic endonuclease-1 (Ape-1) is involved in base excision repair, cleaving the phosphodiester linkage that is 5’ to the abasic site to produce a single strand break. DNA base excision repair pathway inhibitor is a cell-permeable inhibitor of Ape-1 (IC50 = 3-11 µM). Although not toxic to cells by itself at 200 µM, it enhances the cytotoxicity of certain DNA alkylating agents, including temozolomide, when added at this concentration. DNA base excision repair pathway inhibitor has little or no effect on radiation-induced cell survival.