Purpose: Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) are primarily effective against BRCA1/2-mutated breast and ovarian cancers but resistance due to reversion of mutated BRCA1/2 and other mechanisms is common. Based on previous reports demonstrating a functional role for DNMT1 in DNA repair (DDR) and our previous studies demonstrating DNMTis ability to resensitize tumors to primary therapies, we hypothesized that combining a DNMTi with PARPi would sensitize PARPi resistant breast and ovarian cancers to PARPi therapy, independent of BRCA status. Experimental Design: Breast and ovarian cancer cell lines (BRCA- wildtype/-mutant) were treated with PARPi talazoparib and DNMTi guadecitabine. Effects on cell survival, reactive oxygen species (ROS) accumulation, and cAMP levels were examined. In vivo, mice bearing either BRCA-proficient breast or ovarian cancer cells were treated with talazoparib and guadecitabine, alone or in combination. Tumor progression, gene expression and overall survival were analyzed. Results: Combination guadecitabine and talazoparib synergized to enhance PARPi efficacy, irrespective of BRCA mutation status. Co-administration of guadecitabine with talazoparib increased accumulation of ROS, promoted PARP activation and further sensitized, in a cAMP/PKA-dependent manner, breast and ovarian cancer cells to PARPi. In addition, DNMTi enhanced PARP 'trapping' by talazoparib. Guadecitabine plus talazoparib decreased xenograft tumor growth and increased overall survival in BRCA-proficient high-grade serous ovarian and triple negative breast cancer models. Conclusions: The novel combination of the next generation DNMTi guadecitabine and the first-in-class PARPi talazoparib inhibited breast and ovarian cancers harboring either wildtype- or mutant-BRCA, supporting further clinical exploration of this drug combination in PARPi-resistant cancers.
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