Targeting the Androgen Receptor Confers In Vivo Cross-resistance Between Enzalutamide and Docetaxel, But Not Cabazitaxel, in Castration-resistant Prostate Cancer.

Treatment options for metastatic castration-resistant prostate cancer (CRPC) have evolved with the established benefit of the novel androgen receptor (AR)-targeted agents abiraterone and enzalutamide in the prechemotherapy setting. However, concerns regarding cross-resistance between the taxanes docetaxel and cabazitaxel and these AR-targeted agents have arisen, and the optimal drug treatment sequence is unknown. We investigated the in vivo efficacy of docetaxel and cabazitaxel in enzalutamide-resistant CRPC, and mechanisms of cross-resistance between these agents. Castrated mice harboring enzalutamide-resistant tumors and enzalutamide-naïve tumors were treated with docetaxel and cabazitaxel. Tumor growth kinetics, AR nuclear localization, AR-regulated gene expression, Ki67 expression, and serum levels of prostate-specific antigen, docetaxel, and cabazitaxel were analyzed. Docetaxel inhibited tumor growth, AR nuclear localization, and AR-regulated gene expression in enzalutamide-naïve tumors, but did not in enzalutamide-resistant tumors, demonstrating in vivo cross-resistance. By contrast, cabazitaxel remained highly effective in enzalutamide-resistant tumors and demonstrated superior antitumor activity compared to docetaxel, independent of the AR pathway. These findings demonstrate that the AR pathway is able to confer in vivo cross-resistance between enzalutamide and docetaxel, but not cabazitaxel, in CRPC.