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Background  PARP inhibitors have shown promising clinical results in cancer patients carrying  BRCA1/2  mutations. Their clinical efficacy could logically be influenced by PARP1 protein levels in patient tumors.    Methods  We screened three cohorts of patients with ovarian cancer, totaling 313 samples, and evaluated PARP1 protein expression by immunohistochemistry with further validation by western blotting.    Results  We observed that up to 60 % of tumors showed little PARP1 protein expression. In serous ovarian tumors, comparing intratumoral PARP1 expression between chemo-naïve and post-chemotherapy patients revealed a decrease in intratumoral PARP1 following chemotherapy in all three cohorts (immunohistochemistry:  p  < 0.001, n = 239; western blot:  p  = 0.012, n = 74). The findings were further confirmed in a selection of matched samples from the same patients before and after chemotherapy.    Conclusion  Our data suggest that patients should be screened for PARP1 expression prior to therapy with PARP inhibitors. Further, the observed reduction of intratumoral PARP1 post-chemotherapy suggests that treating chemo-naïve patients with PARP inhibitors prior to the administration of chemotherapy, or concurrently, might increase the responsiveness to PARP1 inhibition. Thus, a change in the timing of PARP inhibitor administration may be warranted for future clinical trials.    Electronic supplementary material  The online version of this article (doi:10.1186/s12916-015-0454-9) contains supplementary material, which is available to authorized users.

Chemotherapy reduces PARP1 in cancers of the ovary: implications for future clinical trials involving PARP inhibitors