Abstract 4759: Integral role of the PI3K-AKT-mTOR pathway in DDR-mediated antitumor actions of PARP inhibitor in triple-negative breast carcinogenesis

Abstract

Abstract Background: The PI3K pathway, in addition to its pro-proliferative and anti-apoptotic effects on tumor cells, is known to contribute to DNA-damage repair (DDR). We hypothesized that GDC-0980, a dual PI3K-mTOR inhibitor, would potentiate an anti-tumor effect in BRCA-competent TNBC cells when combined with the PARP inhibitor, ABT888 and carboplatin. Methods: Athymic mice bearing BRCA-competent TNBC xenograft tumors were treated with GDC-0980 alone or in combination with ABT888 and carboplatin. Mechanism-based in vitro studies (MTT, CelltiterGLO, flow cytometry-based live/dead cell, cell cycle analyses, anchorage independent soft-agar, anchorage dependent 3D ON-TOP, apoptosis, cell signaling markers assays) using a panel of 5-7 BRCA-wt /mutant TNBC cell lines were performed. Results: GDC-0980 treatment led to DNA damage (increased pgH2AX; WB, IF), gain in PAR and a subsequent sensitization of BRCA-competent TNBC cells to ABT888 plus carboplatin in a time-dependent manner. The treatment (1) decreased proliferation signals (pAKT, pP70S6K, p4EBP1, pS6RP), PAR/PARP, PAR/pgH2AX, live/dead cell ratios, cell cycle progression and clonogenic 3D growth, and (2) increased apoptosis markers (cl-caspase3, 9, BIM, cl-PARP, and annexinV positivity). These effects were more pronounced in PTEN-null MDA-MB468 than in RAS/RAF mutated MDA-MB231 cells. Three-dimensional projection movie showed that (1) GDC-0980 alone attenuated nuclear pγH2AXS139 foci in MDA-MB468 cells at 24 hours and (2) cytoplasmic cl-caspase3 increased in GDC-0980+ABT888+carboplatin treated MDA-MB468 cells at 72 hours. Finally, GDC-0980 in combination with ABT888 plus carboplatin blocked the growth of established xenograft tumors by 80-90% with a concomitant decrease in tumor Ki67, cl-caspse3, pVEGFR, CD31, p4EBP1, and pS6RP IHC-levels. Conclusion: This is the first mechanism-based study to demonstrate the inhibition of DDR as another mode of action of GDC-0980. Our data revealed that in a BRCA-competent model, GDC-0980 enhanced the antitumor efficacy of ABT888 in the presence of carboplatin by inhibiting the DDR system in conjunction with inhibition of the PI3K-mTOR pathway mediated pro-proliferative and anti-apoptotic signals. Citation Format: Brian Leyland-Jones, Jennifer Carlson, Yuliang Sun, Lori Friedman, Pradip De, Nandini Dey. Integral role of the PI3K-AKT-mTOR pathway in DDR-mediated antitumor actions of PARP inhibitor in triple-negative breast carcinogenesis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4759. doi:10.1158/1538-7445.AM2014-4759