Co-treatment with BEZ235 Enhances Sensitivity of BRCA1-negative Breast Cancer Cells to Olaparib

The poly(ADP-ribose) polymerase (PARP) inhibitor, olaparib has been reported as having preferential anti-proliferative effects on breast cancer 1 (BRCA1)-deficient breast and ovarian cancer cells and was recently approved by the US Food and Drug Administration (FDA) for advanced, BRCA1-mutated ovarian cancer. Herein, we show that BEZ235, a protein kinase inhibitor, enhanced the tumor cell-killing effect of olaparib in BRCA1-mutated breast cancer cells in vitro. BEZ235 reduced olaparib-induced phosphorylation of p53 binding protein 1 (53BP1) and 53BP1 foci formation, as well as phosphorylation of AKT (S473). Long-term colony-formation assay revealed more strong synergistic effects of this combination in SUM149PT and MDA-MB-468 breast cancer cell lines. BEZ235 treatment combined with olaparib may be a candidate for effective therapeutic treatment of BRCA1-mutated breast cancer.