Small-molecule inhibitors of PARP are thought to mediate their antitumor effects 
as catalytic inhibitors that block repair of DNA single-strand breaks (SSB). 
However, the mechanism of action of PARP inhibitors with regard to their effects 
in cancer cells is not fully understood. In this study, we show that PARP 
inhibitors trap the PARP1 and PARP2 enzymes at damaged DNA. Trapped PARP-DNA 
complexes were more cytotoxic than unrepaired SSBs caused by PARP inactivation, 
arguing that PARP inhibitors act in part as poisons that trap PARP enzyme on 
DNA. Moreover, the potency in trapping PARP differed markedly among inhibitors 
with niraparib (MK-4827) > olaparib (AZD-2281) >> veliparib (ABT-888), a pattern 
not correlated with the catalytic inhibitory properties for each drug. We also 
analyzed repair pathways for PARP-DNA complexes using 30 genetically altered 
avian DT40 cell lines with preestablished deletions in specific DNA repair 
genes. This analysis revealed that, in addition to homologous recombination, 
postreplication repair, the Fanconi anemia pathway, polymerase β, and FEN1 are 
critical for repairing trapped PARP-DNA complexes. In summary, our study 
provides a new mechanistic foundation for the rational application of PARP 
inhibitors in cancer therapy.