Mediator kinase inhibition reverses castration resistance of advanced prostate cancer

Mediator kinases CDK19 and CDK8, pleiotropic regulators of transcriptional reprogramming, are differentially regulated by androgen signaling but both kinases are upregulated in castration-resistant prostate cancer (CRPC). Genetic or pharmacological inhibition of CDK8 and CDK19 reverses the castration-resistant phenotype and restores the sensitivity of CRPC xenografts to androgen deprivation in vivo. Prolonged CDK8/19 inhibitor treatment combined with castration not only suppresses the growth of CRPC xenografts but also induces tumor regression and cures. Transcriptomic analysis revealed that Mediator kinase inhibition amplifies and modulates the effects of castration on gene expression, disrupting CRPC adaptation to androgen deprivation. Mediator kinase inactivation in tumor cells also affects stromal gene expression, indicating that Mediator kinase activity in CRPC molds the tumor microenvironment. The combination of castration and Mediator kinase inhibition downregulates the MYC pathway, and Mediator kinase inhibition suppresses a MYC-driven CRPC tumor model even without castration. CDK8/19 inhibitors show efficacy in patient-derived xenograft models of CRPC, and a gene signature of Mediator kinase activity correlates with tumor progression and overall survival in clinical samples of metastatic CRPC. These results indicate that Mediator kinases mediate androgen-independent in vivo growth of CRPC, supporting the development of CDK8/19 inhibitors for the treatment of this presently incurable disease.