Poly-adeninosinediphosphate-ribose polymerase inhibitors as sensitizers for therapeutic treatments in human tumor and blood mononuclear cells

Abstract

14024 Background: Poly(ADP-ribose) (PAR) polymerase-1 (PARP-1) is a molecular sensor of DNA breaks that facilitates DNA repair and controls genomic stability. Treatment with single doses of ABT-888, a novel potent PARP-1 inhibitor, reduced PAR levels in peripheral blood mononuclear cells (PBMCs) and tumor biopsies in an on-going Phase-0 trial at the NIH Clinical Center. As a corollary to this study, we investigated whether ABT-888 can act as a sensitizer for radiation therapy and chemotherapy in human cancer cell culture, xenograft tumors and PBMCs to support future combination clinical trials. Methods: Inhibition of PARP-1 by ABT-888 was determined by a quantitative PAR chemiluminescence immunoassay validated for the Phase 0 trial. Since gamma-H2AX (?-H2AX) is a marker of DNA damage, we also developed and validated ?H2AX assays to monitor the effects of PARP-1 inhibition during treatment with Topo I inhibitors and radiation. Human monocytic leukemia (THP-1) and breast carcinoma (MCF-7) cell lines were treated with Topo I inhibitors including indenoisoquinoline, camptothecin and topotecan or irradiated with 0.5 to 10 Gy in the presence of ABT 888. We further evaluated these effects in human blood ex vivo to confirm the observations made in cell culture. Results: We found that ABT-888 inhibited PAR, but did not significantly increase DNA damage. Combination of ABT-888 with a Topo I inhibitor produced over 275% increase of DNA damage in THP-1 leukemia cells compared to indenoisoquinoline alone. ?H2AX foci per cell were 9.5 ± 0.8 in MCF-7 treated with 0.5 Gy/50 nM ABT-888 in comparison to 4.0 ± 0.6 with radiation alone. When whole blood was treated in the presence of ABT-888, camptothecin- induced DNA damage in PBMCs was also increased 2–3 fold, with maximum ?H2AX expression at 2 hours post treatment. Conclusions: We conclude that ABT-888 is a highly potent PARP-1 inhibitor that can enhance the DNA damaging effects of chemotherapy and radiation therapy of human cancer. Funded by NCI Contract N01-CO-12400. No significant financial relationships to disclose.