Abstract 4682: Compensatory activation of alternative DNA repair pathways following exposure to the PARP Inhibitor ABT-888 in vitro and in patients with solid malignancies

Abstract

Abstract PARP inhibitors are being developed with the hope of inhibiting base excision repair in tumor cells following DNA insult. They do so by inhibiting poly ADP-ribose polymerase-1(PARP-1) which plays an important role in the repair of single-stranded DNA (ssDNA) breaks. Although some PARP inhibitors have been shown to suppress its target at safely achievable concentrations in humans, mono-therapy with these agents is generally not able to induce responses. The compensatory effect of alternative repair pathways, including., homologous recombination (HR) repair, nucleotide excision repair (NER), among others, could potentially be responsible for this lack of effect. Examples of HR repair proteins include those involved in the Fanconi Anemia/BRCA pathway and for NER, the ERCC proteins. We performed Western immunoblot analysis to evaluate the expression level of PAR, Fanconi Anemia D2, ERCC1 and beta-actin proteins in the human cancer cell lines H1299 (lung) and MDA-MB-231(breast) following exposure to the PARP inhibitor ABT-888 (veliparib). The expression level of these protein was also evaluated in human peripheral blood mononuclear cells (PBMCs) obtained from consenting patients with advanced solid malignancies participating in a dose escalation trial with ABT-888, as monotherapy or in combination with mitomycin C. PAR protein level was reduced at 6 hours post ABT-888 exposure (5μM) and maintained at low levels through 48 hours in the H1299 and MDA-MB-231cancer cells. However FANCD2 and ERCC1 protein expression was elevated simultaneously in these cancer cells post ABT-888. Similarly, FANCD2 and ERCC1 protein levels increased in the PBMCs of 22 patients analyzed following ABT-888 as a single agent (n=12) or in combination with mitomycin C (n=10). These results confirm that ABT888 can inhibit its target PAR expression both in cancer cells and patients’ PBMCs. However, our results also show that ABT-888 up-regulates homologous recombination (HR) and nucleotide excision repair (NER) pathways, which have the potential of resulting in not only resistance to the antitumor effect of ABT-888, but also in decreasing the effect of DNA targeting cytotoxic chemotherapy when combined with isolated PARP inhibition. It is plausible that cancers with defective FA or NER pathways, which are incapable of mounting a compensatory response, would represent a better target for ABT-888 alone or in combination with DNA targeted cytotoxic chemotherapy. Experiments aimed to evaluate this notion are underway. The potential role of other compensatory repair mechanisms also merit further investigation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4682. doi:1538-7445.AM2012-4682