Abstract 675: Dual inhibition of the androgen receptor and PI-3K/mTor pathways has significant antitumor activity in castrate-resistant prostate cancer

Abstract

Abstract Inhibition of the androgen receptor (AR) results in activation of the PI-3K/mTOR pathway in PTEN deficient prostate cancer cells by reciprocal feedback inhibition (Carver BS, 2011). Enzalutamide or PI-3K/mTor inhibition result in feedback activation and combined inhibition abrogated this effect. We explored AR inhibition in CRPC cell lines with low, intermediate and high AR expression in the context of PI-3K/mTor inhibition to identify biomarkers of response and resistance. We utilized no/low (DU145, PC3), intermediate (LNCaP) and high (22Rv1,VCap) AR expressing prostate cancer (PC) cell lines to evaluate anti-tumor activity with 4 pan-PI3K/mTOR inhibitors (GDC-0980, GDC-0941, LY294002, PF-04691502), enzalutamide and abiraterone as single agents or various in combinations at clinically relevant doses. Cytotoxic activity of enzalutamide was more potent than abiraterone (IC50's was < 1µM and > 10µM) in LNCaP cells, however, the opposite was seen in DU145 and PC-3 cell lines. High AR expressing cells (22Rv1, VCap) were more sensitive to AR inhibition. PI-3K/mTor SMIs GDC-0980, GDC-0941 or PF-04691502 had IC50's in the range 50-500nM while LY294002 had an IC50 16-24 µM except for DU145. This cell line had increased IC50 values comparatively for PI-3K/mTor SMIs GDC-0980, GDC-0941 or PF-04691502 (1.5-3.75 µM) and LY29002 (48.5 µM). The combination index (CI) isobologram method indicated significant synergism for PI-3K/mTOR SMIs plus enzalutamide or abiraterone in all PC cell lines irrespective of AR status. Combination treatments were associated with increased apoptosis (Annexin-V staining and PARP-cleavage assay) in LNCaP and VCaP > 22Rv1 > DU145 and PC3. Cell cycle analysis by flow cytometry demonstrated that enzalutamide induces G1 arrest in LNCaP and DU145 cells and G2/M arrest in PC3 cells. PI-3K inhibition induces S arrest in LNCaP and DU145 and G2/M arrest in PC3 cells. The combination of enzalutamide plus PI-3K inhibition enhanced S arrest in LNCaP while in DU145 and PC3, G0/G1 arrest was prominent. PI-3K SMI plus enzalutamide completely inhibited pAkt308/473, pS6 and p4EBP1 in LNCaP cells, pAkt473 in PC3 cells and pS6 and p4EBP1 in 22Rv1 cells. Enzalutamide alone or in combination with PI-3K inhibition also suppressed AR expression in 22Rv1 cells. A 22Rv1 mouse xenograft model is evaluating safety and efficacy of the combination. This is a novel therapeutic strategy for CRPC patients to be evaluated in a clinical trial. Citation Format: Daruka Mahadevan, Carla Morales, Laurence S. Cooke, Bradley Somer, Wenqing Qi. Dual inhibition of the androgen receptor and PI-3K/mTor pathways has significant antitumor activity in castrate-resistant prostate cancer. [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 675. doi:10.1158/1538-7445.AM2014-675