Dual PI3K/mTOR Inhibitor NVP-BEZ235 Sensitizes Docetaxel in Castration Resistant Prostate Cancer

Effective therapeutic strategies that can achieve long-term improvement in patients with castration resistant prostate cancer are urgently needed. We recently reported that the activated PI3K/Akt/mTOR signaling pathway induced by docetaxel explains resistance to docetaxel in castration resistant prostate cancer. In this study we explored the efficacy of NVP-BEZ235, a dual PI3K and mTORC1/2 inhibitor, for docetaxel resistant castration resistant prostate cancer.We used the 2 human castration resistant prostate cancer cell lines C4-2 and C4-2AT6. At our laboratory C4-2AT6 cells were established from C4-2 under androgen ablated treatment for 6 months. We investigated the efficacy of NVP-BEZ235 monotherapy and NVP-BEZ235 combined with docetaxel in vitro and in vivo.Increased phosphorylated Akt in C4-2AT6 cells was significantly inhibited by NVP-BEZ235 in a dose and time dependent manner. WST cell proliferation assay results in C4-2AT6 cells revealed that combined administration of NVP-BEZ235 and docetaxel had significant, synergistically greater cytotoxicity than NVP-BEZ235 or docetaxel monotherapy. Combined NVP-BEZ235 (40 mg/kg) and docetaxel (4 mg/kg) in vivo in a castrated mouse xenograft model inhibited C4-2AT6 tumor growth to a greater degree than in the monotherapy groups. Also, NVP-BEZ235 showed significant efficacy with docetaxel at a low concentration in vivo, suggesting that NVP-BEZ235 effectively decreased resistance to docetaxel.Results suggest that inhibition of the PI3K/Akt/mTOR signaling pathway by NVP-BEZ235 can overcome docetaxel resistance in human castration resistant prostate cancer. Our findings provide a molecular basis for the clinical use of combined administration of NVP-BEZ235 and docetaxel in patients with castration resistant prostate cancer.