Poly(ADP-ribose) polymerases (PARP1, -2, and -3) play important roles in DNA 
damage repair. As such, a number of PARP inhibitors are undergoing clinical 
development as anticancer therapies, particularly in tumors with DNA repair 
deficits and in combination with DNA-damaging agents. Preclinical evidence 
indicates that PARP inhibitors potentiate the cytotoxicity of DNA alkylating 
agents. It has been proposed that a major mechanism underlying this activity is 
the allosteric trapping of PARP1 at DNA single-strand breaks during base 
excision repair; however, direct evidence of allostery has not been reported. 
Here the data reveal that veliparib, olaparib, niraparib, and talazoparib 
(BMN-673) potentiate the cytotoxicity of alkylating agents. Consistent with 
this, all four drugs possess PARP1 trapping activity. Using biochemical and 
cellular approaches, we directly probe the trapping mechanism for an allosteric 
component. These studies indicate that trapping is due to catalytic inhibition 
and not allostery. The potency of PARP inhibitors with respect to trapping and 
catalytic inhibition is linearly correlated in biochemical systems but is 
nonlinear in cells. High-content imaging of γH2Ax levels suggests that this is 
attributable to differential potentiation of DNA damage in cells. Trapping 
potency is inversely correlated with tolerability when PARP inhibitors are 
combined with temozolomide in mouse xenograft studies. As a result, PARP 
inhibitors with dramatically different trapping potencies elicit comparable in 
vivo efficacy at maximum tolerated doses. Finally, the impact of trapping on 
tolerability and efficacy is likely to be context specific.
IMPLICATIONS: Understanding the context-specific relationships of trapping and 
catalytic inhibition with both tolerability and efficacy will aid in determining 
the suitability of a PARP inhibitor for inclusion in a particular clinical 
regimen.