4-Hydroxy-5-methoxy-2,3-dihydro-1H-[1,3]benzodioxolo[5,6-c]pyrrolo[1,2-f]-phenanthridium chloride (NK314) is a benzo[c] phenanthridine alkaloid that inhibits topoisomerase IIα, leading to the generation of DNA double-strand breaks (DSBs) and activating the G(2) checkpoint pathway. The purpose of the present studies was to investigate the DNA intercalating properties of NK314, to evaluate the DNA repair mechanisms activated in cells that may lead to resistance to NK314, and to develop mechanism-based combination strategies to maximize the antitumor effect of the compound. A DNA unwinding assay indicated that NK314 intercalates in DNA, a property that likely cooperates with its ability to trap topoisomerase IIα in its cleavage complex form. The consequence of this is the formation of DNA DSBs, as demonstrated by pulsed-field gel electrophoresis and H2AX phosphorylation. Clonogenic assays demonstrated a significant sensitization in NK314-treated cells deficient in DNA-dependent protein kinase (DNA-PK) catalytic subunit, Ku80, ataxia telangiectasia mutated (ATM), BRCA2, or XRCC3 compared with wild-type cells, indicating that both nonhomologous end-joining and homologous recombination DNA repair pathways contribute to cell survival. Furthermore, both the DNA-PK inhibitor 8-(4-dibenzothienyl)-2-(4-morpholinyl)-4H-1-benzopyran-4-one (NU7441) and the ATM inhibitor 2-(4-morpholinyl)-6-(1-thianthrenyl)-4H-pyran-4-one (KU55933) significantly sensitized cells to NK314. We conclude that DNA-PK and ATM contribute to cell survival in response to NK314 and could be potential targets for abrogating resistance and maximizing the antitumor effect of NK314.