Abstract P1-03-04: Role of PTEN and BRCA1 as determinants of synergy for the combination of vistusertib with carboplatin and olaparib in TNBC

Introduction: Platinum agents are being used in combination with targeted agents in advanced triple-negative breast cancer (TNBC) (See K. Gelmon et al., 2012). Additionally inhibition of PARP is also being considered as a “targeted” therapy for TNBC (Anders CK et al., 2010). PARP inhibitor (i), Lynparza (olaparib,AstraZeneca)met the primary endpoint of a Phase III trial in which Lynparza was compared to physician's choice of a standard of care chemotherapy in patients with HER2-negative metastatic BC harboring germline BRCA1/2 mutations (BRCAm). Based on cBioportal data analyses and experimental studies we and others have reported that more than 30% PTEN loss in TNBC leads to activation/upregulation of the PI3K pathway (Nature. 2012; Ellis and Perou, 2013; Dey et al., 2012; De et al., 2016; Reed and Shokat 2017;). In line with active kinase profiling, genetic and pharmacological data which defined mTOR as an important target in TNBC (Montero JC et al., 2012), we have demonstrated that mTORi has anti-tumor activity in TNBC (De et al., 2014). Aim: These studies focused on exploring the synergy of biology-based targeted drugs PARPi (olaparib, O), mTOR kinase (vistusertib, V), and platinum (C) in TNBC models. Method: TNBC cells of multiple genetic backgrounds were used to test the combination(s) on proliferation and apoptosis by monitoring growth and using real time Annexin V reagent in a microscopy-based assay (Essen IncuCyte Zoom). Flow cytometric analysis of cell cycle progression by PI staining was also used. Long term clonogenic 3D growth was monitored in matrigel. Results: In BRCAm/PTEN null Sum149 and HCC1937 cells and BRCA wild type (wt)/PTEN null MDA-MB-468 the addition of (V) to (O) and (C) enhanced apoptosis induction and further slowed growth. In Sum149 cells, single agent V treatment induced G1 arrest while O plus C or the triple combination increased S phase accumulation. In MDA-MB-468 cells G1 arrest was seen with V alone and in the triplet. In BRCA wt/PTEN null HCC70 cells V decreased cell proliferation and induced G1 arrest. In the HCC70 model, the addition of O plus C did not synergize with V. In BT20, a BRCA wt/PTEN wt but PI3KCA mutant cell line, no effect on proliferation or apoptosis was seen in the O plus C treated arms. V slowed cell proliferation and increased G1 arrest in a dose-dependent manner. As expected in a BRCA wt/PTEN wt, but RAS active mutant cell line MDA-MB-231 this treatment combination was not effective and was used as an internal negative control. Based on ratios of the normalized slopes of proliferation curves (for triplet), the cells were graded in terms of synergy as SUM149>MDA-MB-468>HCC1937 and HCC70>BT20>MDA-MB231. Treatment with the triplet had the largest effect on reducing 3D colony formation and size as compared to control over single or double treatment. Summary: Here, we present the effect of the combination of vistusertib with olaparib plus carboplatin in several TNBC models. Our data demonstrate that increased effectiveness of the triple combination is seen in cells harboring BRCA1 and PTEN-null mutations. The mechanistic role of these two targets on determining this synergy is being worked out and will be presented at the meeting. Citation Format: Carlson JH, O'Connor MJ, De P, Dey N, Leyland-Jones B. Role of PTEN and BRCA1 as determinants of synergy for the combination of vistusertib with carboplatin and olaparib in TNBC [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-03-04.