Abstract 544: Dual PI3K/mTOR inhibitor, GDC-0980 sensitizes BRCA-competent triple negative breast cancer cells to ABT888 in the presence of carboplatin by impairing DNA-damage repair: A proof of concept combination of PI3K/mTOR inhibitor with PARP inhibitor .
Authors: Dey Nandini , Carlson Jennifer , Sun Yuliang , Friedman Lori , De Pradip , Leyland-Jones Brian 
Published: August 2014

Abstract
Background: PARP is a promising target in the Triple Negative subset of Breast Cancer, the most aggressive subset characterized by poor prognosis and limited option for targeted therapy. The PI3K pathway, in addition to its pro-proliferative effects on tumor cells, also contributes to the repair of DSB in both a kinase-dependent and independent manner (Kumar, 2010) directly by controlling relevant enzymes (Friedman, 2009) or indirectly by regulating the expression of repair-related proteins. PI3K inhibition impairs BRCA1/2 expression in BRCA-proficient cells leading to the DNA damage, and HR impairment (Juvekar, 2012; Ibrahim, 2012). Purpose: We hypothesize that an additional inhibition of HR and NHEJ by GDC-0980 in conjunction with the inhibition of classical PI3K-mTOR survival signals, will induce more effective anti-proliferative and pro-apoptotic signals in BRCA-competent TNBC cells when combined with PARP inhibitor (impaired DNA-SSB-repair), and carboplatin (increased DNA-DSB). Methods: We tested effects of ABT888, and GDC-0980 (alone, and combination), on cell survival/ proliferation in a panel of 5-7 BRCA-wt and mutant TNBC cell lines using MTT assay, CelltiterGLO, and cell cycle analyses (PI & EDU). Clonogenic growth was tested by 3D ON-TOP assay and soft-agar assay. Apoptosis (Annexin V, Apoptosis Array), and cell signaling marker(s) (WB) were investigated. Results: The results show that, (1) the range of EC50s for GDC-0980 in cells (HCC70, HCC1143, HCC1937, MDA-MB231, MDA468, BT20) varied from 0.2 μM to 5 μM; PIK3CA mutated (H1047R) BT20 cells exhibited the lowest EC50, while RAS/RAF mutated MDA-MB231 cells exhibited the highest EC50, (2) PTEN-null and ATM kinase mutated MDA-MB468 cells were the most sensitive to ABT888 in the dose-response study by clonogenic assay (ABT888 failed to show a response by MTT assay), (3) out of three doses of GDC-0980 (200 nM, 100 nM, and 50 nM) tested in combination with ABT888 (10μM) plus carboplatin (10 μM), BRCA-competent MDA-MB468 cells were found to be more sensitive to the combination at 50 nM GDC-0980 than BRCA-competent MDA-MB231 cells, (4) GDC-0980 treatment (50nM) decreased pAKT-S473 and pP70S6K, and (5) GDC-0980 treatment time-dependently increased cleaved-caspase 3, 9, and cleaved-PARP in cells. Conclusion: Data indicate that GDC-0980 enhances the activity of ABT888, in the presence of carboplatin. We are currently pursuing the studies to delineate the mechanistic relationship between the anti-proliferative, pro-apoptotic, and anti-DNA-damage-repair effects of GDC-0980 in BRCA-competent TNBC cell lines in combination with ABT888 plus carboplatin, the results of which will be presented in the meeting.
Citation Format: Nandini Dey, Jennifer Carlson, Yuliang Sun, Lori Friedman, Pradip De, Brian Leyland-Jones. Dual PI3K/mTOR inhibitor, GDC-0980 sensitizes BRCA-competent triple negative breast cancer cells to ABT888 in the presence of carboplatin by impairing DNA-damage repair: A proof of concept combination of PI3K/mTOR inhibitor with PARP inhibitor . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 544. doi:10.1158/1538-7445.AM2013-544