Veliparib dihydrochloride (also known as ABT888 dihydrochloride; ABT-888 dihydrochloride) is a novel and potent inhibitor of PARP1 and PARP2 [Poly (ADP-Ribose) Polymerase] with Ki of 5.2 nM and 2.9 nM in cell-free assays, respectively. It has chemosensitizing and antitumor activities, and is inactive against SIRT2. ABT-888 has demonstrated excellent in vivo efficacy in a broad spectrum of preclinical tumor models in combination with a variety of cytotoxic agents. ABT-888 is also active on microsatellite instability (MSI) cell lines harboring mutations in both MRE11 and RAD50 genes compared to microsatellite stable (MSS) cell lines (wild-type for both genes). ABT-888 has 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.