Identification of mechanisms of resistance to treatment with abiraterone acetate or enzalutamide in patients with castration-resistant prostate cancer (CRPC)

BACKGROUND Two androgen receptor (AR)-targeted therapies, enzalutamideand abiraterone acetateplus prednisone ( abiraterone), have been approved for the treatment of metastatic castration-resistant prostate cancer (CRPC). Many patients respond to these agents, but both de novo and acquired resistance are common. The authors characterized resistant phenotypes that emerge after treatment with abirateroneor enzalutamide. METHODS Patients who received abirateroneor enzalutamidein the course of routine clinical care were consented for serial blood collection. A proprietary system (CellSearch) was used to enumerate and enrich circulating tumor cells(CTCs). RNA-sequencing ( RNA-seq) was performed on pools of up to 10 epithelial cell adhesion molecule(EpCAM)-positive/CD45-negative CTCs. The impact of gene expression changes observed in CTCs between patients who responded or were resistant to abiraterone/ enzalutamidetherapies was further explored in a model cell line system. RESULTS RNA-seqdata from CTCs identified mutations commonly associated with CRPC as well as novel mutations, including several in the ligand-binding domain of AR that could facilitate escape from AR-targeted agents. Ingenuity pathway analysisof differentially regulated genes identified the transforming growth factor β (TGFβ) and cyclin D1 (CCND1) signaling pathways as significantly upregulated in drug-resistant CTCs. Transfection experiments using enzalutamide-sensitive and enzalutamide-resistant LNCaPcells confirmed the involvement of SMADfamily member 3, a key mediator of the TGFβ pathway, and of CCND1 in resistance to enzalutamidetreatment. CONCLUSIONS The current results indicate that RNA-seqof CTCs representing abirateroneand enzalutamidesensitive and resistant states can identify potential mechanisms of resistance. Therapies targeting the downstream signaling mediated by SMADfamily member 3 (SMAD3) and CCND1, such as cyclin-dependent kinase 4/ cyclin-dependent kinase 6inhibitors, could provide new therapeutic options for the treatment of antiandrogen-resistant disease. Cancer 2017. © 2017 American Cancer Society.