Abstract LB-A18: Combination therapy of carmustine and selenite targets AR full length and AR variants: New hope for patients with castration resistant prostate cancer

The major obstacle for successful treatment of advanced prostate cancer is the development of resistance to androgen ablation therapy leading to castration resistant prostate cancer (CRPC). Evidence strongly suggests that the androgen receptor (AR) and AR variants in CRPC build resistance to anti-cancer drugs, including next-generation therapies such as abiraterone and MDV3100 that target AR signaling. Despite established AR antagonists, AR signaling remains a central mechanism of CRPC progression, involving AR gene amplification, reactivation of AR/AR variants, and generation of constitutively active AR-V7 variant. Therefore, there is an urgent need for anticancer drugs that can target these crucial signaling pathways to trigger apoptosis and growth inhibition in CRPC. We have previously shown that the combination treatment of carmustine and selenite effectively induces apoptosis in AR-independent PC-3 cells (Cancer Manag Res. 2012; 4: 383-95). Since there are no drugs available as of today to inhibit AR/AR variants, we tested whether the combination of carmustine and selenite could induce apoptosis and inhibit growth of CRPC cells (22Rv1) in in vitro and in vivo models and if this response is mediated by targeting AR/AR variants and AR-V7 in tandem with multiple signaling pathways. CRPC (22Rv1 and PC-3) and normal prostate epithelial cell lines were used to study the efficacy of carmustine and selenite in combination. Anchorage-independent growth of 22Rv1 and PC-3 cells was determined by a soft-agar assay. In vivo antitumor efficacy of the combination agent with therapeutically relevant doses was studied in a 22Rv1 xenograft mouse model. AR/AR variants, PSA and multiple signaling pathway (AKT, anti-apoptotic proteins, Bcl-2 and Mcl-1; cell cycle regulator, p27; and markers of proliferation, c-Myc and cyclin-D1) proteins were determined by western analysis. Combination of carmustine and selenite treatment completely abolished the protein expression of AR/AR variants, AR-V7, PSA in a time dependent manner, substantially decreased cell viability, growth, and induced apoptosis in 22Rv1 cells. In addition, the combination treatment dramatically induced p-H2AX, p27 while strongly inhibiting pAKT, Bcl-2, Mcl-1, c-Myc and cyclin-D1 in 22Rv1 cells without causing genotoxicity in normal prostate epithelial cells. In addition, anchorage-independent growth of 22Rv1 and PC-3 cells was completely inhibited. Consistent with in vitro results, combination treatment induced apoptosis and eradicated 22Rv1 xenograft tumors in nude mice correlating with dramatic inhibition of AR/AR variants, AR-V7, PSA, and Bcl-2 in xenograft tumors. A significant reduction in toxicity was revealed by an observed decrease in body weight loss. Whereas, individual agent treatment showed only partial effect both in vitro and in vivo. This is the first study to demonstrate that the combination of carmustine and selenite treatment completely inhibits AR/AR variants, AR-V7, and induces apoptosis in CRPC which is highly correlated with tumor growth inhibition in vitro and in-vivo. Our findings indicate that the combination of carmustine and selenite can be the next-generation therapy for successful treatment, survival, and improved quality of life of patients with castration resistant prostate cancer. Source of funding: HFHS-H10100 Citation Format: Vijayalakshmi Thamilselvan, Mani Menon, Sivagnanam Thamilselvan. Combination therapy of carmustine and selenite targets AR full length and AR variants: New hope for patients with castration resistant prostate cancer. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr LB-A18.