A poly(ADP-ribose) polymerase (PARP) -1 and -2 inhibitor with chemosensitizing and antitumor activities. With no antiproliferative effects as a single agent at therapeutic concentrations, ABT-888 inhibits PARPs, thereby inhibiting DNA repair and potentiating the cytotoxicity of DNA-damaging agents. PARP nuclear enzymes are activated by DNA single or double strand breaks, resulting in the poly(ADP-ribosyl)ation of other nuclear DNA binding proteins involved in DNA repair; poly(ADP-ribosyl)ation contributes to efficient DNA repair and to survival of proliferating cells exposed to mild genotoxic stresses as induced by as oxidants, alkylating agents or ionizing radiation.