Synergistic antitumor effect of NVP-BEZ235 and sunitinib on docetaxel-resistant human castration-resistant prostate cancer cells

According to recent studies, mTOR (mammalian target of rapamycin) inhibitor and tyrosine kinase inhibitor (TKI) can be used as combinational agents to enhance the antitumor effect or overcome resistance to one of the agents. In the present study, we investigated the synergistic interaction between NVP-BEZ235, a PI3K (phosphoinositide 3-kinase)/mTOR dual inhibitor, and sunitinib, a TKI, in castration-resistant prostate cancer (CRPC) cells with docetaxel resistance. Prostate cancer cells with different sensitivities to hormones and docetaxel levels were exposed to escalating doses of NVP-BEZ235 alone and in combination with sunitinib. The synergy between NVP-BEZ235 and sunitinib was determined by the combination index, threedimensional model, and clonogenic assays. Flow cytometry and western blot analysis of proteins related to apoptosis and cell survival axis were performed. The combination of NVPBEZ235 and sunitinib caused a significant synergistic antitumor effect over a wide range of doses in docetaxelresistant CRPC cells. Furthermore, the IC₅₀ (half-maximal inhibitory concentration) of NVP-BEZ235 and sunitinib was reduced by 7.8-fold and 6.6-fold, respectively. The three-dimensional synergy analysis resulted in a synergy volume of 182.47 μM/ml²%, indicating a strong synergistic effect of combination therapy. Combination therapy caused an induction of caspase-dependent apoptosis in docetaxelresistant CRPC cells. Adding sunitinib did not produce any additional effect on the NVP-BEZ235-mediated inhibition of PI3K/AKT/mTOR phosphorylation. In conclusion, combining NVP-BEZ235, a dual PI3K/mTOR inhibitor, with sunitinib can synergistically potentiate the antitumor effect in CRPC cells after docetaxel failure though induction of caspase-dependent apoptosis.