Abstract 2948: Combination treatment of PARP inhibitor, BMN 673 and DNMT inhibitor, Azacytidine: A potential therapy for BRCA negative and positive, triple negative breast cancers (2015)

Abstract Poly-ADP-ribose polymerase inhibitors (PARPis) represent one of the most exciting recent developments in cancer therapy and have shown efficacy in a minority (5-10%) of triple negative (estrogen, progesterone and herceptin receptor negative) breast cancers (TNBCs) with inherited mutations in breast cancer genes BRCA1/2 (BRCA-ve). In PARPi clinical trials, approximately of 60% BRCA-ve patients showed responses, while a significant proportion did not. In addition, sporadic TNBCs with intact BRCA1/2 have failed PARPi therapy. This suggests the urgent need for additional novel therapies for breast cancer. Our studies are underpinned by 4 important findings: (1) the development of a new class of potent PARPis exemplified by BMN 673, (2) the mechanism of cytotoxicity of PARPis, including BMN 673, is attributed to its ability to trap PARP-DNA complexes in chromatin, (3) unpublished findings by our group that PARP interacts with epigenetic factors such as DNMT1 in a high molecular weight complex on chromatin in response to DNA damage and (4) DNMT inhibitors (DNMTis) such as FDA approved Azacytidine (AzaC) covalently trap DNMTs in DNA as their mechanism of action. Therefore, the interaction of PARP and DNMT1 suggest a therapeutic opportunity for combining PARPis and DNMTis in an effort to enhance the effects of PARPi therapy. We first set out to determine whether a combination of these two drugs would enhance trapping of PARP-DNA complexes in chromatin, leading to increased cytotoxicity of TNBC. We treated MDA-MB-231 with BMN 673 (10-100nM) or AzaC (150nM) alone, or the two drugs in combination, and measured PARP trapping in chromatin extracts following induction of DNA damage by radiation (2Gy) after 72hrs. As expected, BMN 673 treatment leads to PARP trapping as determined by western blotting of proteins from chromatin extracts. Interestingly, treatment with AzaC treatment alone also leads to the presence of PARP in chromatin. Importantly, AzaC in combination with BMN 673 treatment shows an at least a 2-fold increase in PARP trapping, compared to the individual drug treatments. To determine whether PARP trapping correlates with cytotoxicity and cell death, BRCA+ve TNBC lines MDA-MB-231 and MDA-MB-468 and BRCA-ve TNBC lines SUM149PT and HCC1937 cells were treated with the drugs alone and in combination and placed in colony forming assays. Compared with single drug treatments, the combination treatment showed a significant (p&amp;lt;0.05) decrease in colony formation in both BRCA positive and negative cell lines indicating BMN 673 could potentially be used in combination with AzaC to improve treatment of TNBCs. In vivo xenograft studies are underway to determine the efficacy of the drug combination compared with single treatments and vehicle controls. Citation Format: Khadiza Chowdhury, Rena G. Lapidus, Eun Yong Choi, Stephen Baylin, Feyruz V. Rassool. Combination treatment of PARP inhibitor, BMN 673 and DNMT inhibitor, Azacytidine: A potential therapy for BRCA negative and positive, triple negative breast cancers. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2948. doi:10.1158/1538-7445.AM2015-2948