Metformin in overcoming enzalutamide resistance in castration-resistant prostate cancer.

Androgen deprivation is the standard treatment for patients with prostate cancer. However, the disease eventually progresses as castration-resistant prostate cancer (CRPC). Enzalutamide, an androgen receptor inhibitor, is a typical drug for treating CRPC and with continuous reliance on the drug, can lead to enzalutamide resistance. This highlights the necessity for developing novel therapeutic targets to combat the gain of resistance. Metformin has been recently investigated for its potential antitumorigenic effects in many cancer types. In this study, we used enzalutamide and metformin in combination to explore the possible rescued efficacy of enzalutamide in the treatment of enzalutamide-resistant CRPC. We first tested the effects of this combination treatment on cell viability, drug synergy, and cell proliferation in enzalutamide-resistant CRPC cell lines. After combination treatment, we observed a decrease in cell proliferation and viability as well as a synergistic effect of both enzalutamide and metformin in vitro. Following these results, we sought to explore how combination treatment affected mitochondrial fitness using mitochondrial stress test analysis and mitochondrial membrane potential shifts due to metformin's action in inhibiting complex I of oxidative phosphorylation. We employed 2 different strategies for in vivo testing using 22Rv1 and LuCaP35CR xenograft models. Finally, RNA sequencing revealed a potential link in the downregulation of rat sarcoma-mitogen-activated protein kinase signaling following combination treatment. SIGNIFICANCE STATEMENT: Increasing evidence suggests that oxidative phosphorylation might play a critical role in the development of resistance to cancer therapy. This study showed that targeting oxidative phosphorylation with metformin can enhance the efficacy of enzalutamide in castration-resistant prostate cancer in vitro.