Synergistic cytotoxicity of histone deacetylase, poly-ADP ribose polymerase inhibitors and decitabine in breast and ovarian cancer cells: Therapeutic implications.

e15193 Background: Innovative strategies are needed for breast and ovarian cancer. We explored the synergistic cytotoxicity of histone deacetylase inhibitors (HDACi), poly(ADP-ribose) polymerase inhibitors (PARPi), and decitabine (DAC, D) in breast and ovarian cancer cells. The drugs affect epigenetic regulation and DNA repair, making them promising anticancer therapies. Methods: Breast (MDAMB231 and MCF-7) and ovarian (HEY-T30 and SKOV-3) cells were exposed to HDACi (panobinostat, Pano, P; vorinostat; Vori, V), PARPi (talazoparib, TLZ, T; olaparib, Ola, O) and DAC. Synergism was assessed by MTT and clonogenic assays. Changes in DNA damage response/repair markers were assessed with Western blotting. Results: The combinations of HDACi, PARPi, and DAC had synergistic effects in all cell lines, evidenced by combination index <1. Clonogenic assay confirmed sensitivity of all cell lines to the 3-drug combinations (Pano, TLZ, DAC; Pano, Ola, DAC; Vori, TLZ, DAC; Vori, Ola, DAC). Cell proliferation was inhibited by 50% - 70%, and Annexin V positivity was 51% - 58% in all cells exposed to the combinations. Western blots showed inhibited protein PARylation, cleaved caspase 3 and PARP1 (suggestive of apoptosis), and down-regulated c-MYC. The 3-drug combinations induced more DNA damage (increased phosphorylation of ƔH2AX) than the individual drugs, impaired DNA repair (decreased ATM, BRCA1, ATRX proteins; Artemis), and altered gene expression (reduced NuRD complex subunits). DNA damage caused by HDACis and DAC may confer increased cellular dependence on protein PARylation, increasing sensitivity to combined HDACi, PARPi, and DAC. The P values comparing the inhibitory effects of each drug combination with that of individual drugs (MTT assay) are shown in Table ( P ≤ .05, statistically significant). Conclusions: The results support exploring HDACi, PARPi, and DAC combinations to overcome drug resistance and improve patient outcomes, and carefully designed clinical trials are warranted in breast and ovarian cancer. Table: see text