Abstract 5542: Synergistic inhibition of growth of triple-negative breast cancer cells by co-targeting PARP and HDAC

Triple-negative breast cancers (TNBC) have a poor prognosis after it has spread beyond the breast. Existing therapies remain inadequate. Despite similarities with BRCA-mutated breast cancers (such as high sensitivity to cisplatin reflecting defects in DNA repair pathways), Poly ADP Ribose Polymerase (PARP) inhibitors failed to improve prognosis of non-BRCA-mutated TNBC. Histone DeACetylase (HDAC) inhibitors are targeted therapies that can modulate expression of genes by affecting chromatin conformation, including down-regulation of genes encoding proteins involved in Homologous Recombination (HR). A previous study noted that the HDAC inhibitor, vorinostat, can induce DNA damage which normal but not cancer cells can repair. Thus, our goal was to determine if HDAC inhibitors could sensitize TNBC cells to PARP inhibitors. We studied the effect of olaparib in combination with vorinostat in 8 TNBC cell lines. Median IC50 for olaparib and for vorinostat were 146µM and 6.4µM, respectively. Remarkedly, the combination of 2µM of vorinostat decreased the median IC50 for olaparib by 3.47-fold (from 146µM to 45µM, p=.033). Further studies showed the drug combination was synergistic for seven of eight cell lines with an average Combination Index (CI) of 0.62 (± 0.20), using CompuSyn software. Synergism was also observed with belinostat (another HDAC inhibitor) when combined with olaparib. Furthermore, co-treatment with vorinostat and olaparib drastically inhibited clonogenic growth of TNBC cells in soft-agar. Also, olaparib (10µM) and vorinostat (2µM) markedly increased G2 arrest (46%±0.8 after 48 hours, p=.033) and significantly increased the percentage of apoptotic cells (17%±1.91 cells positive as measured by Annexin V - Propidium Iodine staining after 48 hours, p=.022), compared to either drug alone using MDA-MB-157 TNBC cells. Similar trends were observed in two other TNBC lines (MDA-MB-231 and HCC1937). Regarding mechanisms involved in this observed synergism of the two drugs: #1 expression of several proteins involved in HR (RAD51, Chk2 and BRCA1) were decreased; #2 P-H2AX expression markedly increased with combination treatment suggesting the two drugs caused greater DNA breaks. As a result, the P-H2AX/RAD51 ratio increased when the two drugs were used together (occurring in both BRCA1-wt and -mutant TNBC lines). Our results provide evidence that treatment with a HDAC inhibitor can sensitize TNBC cell lines to olaparib in BRCA-mutated as well as in BRCA-wt cell