Enzalutamide (brand name Xtandi) is an orally bioavailable, organic, non-steroidal small molecule targeting the androgen receptor (AR) with potential antineoplastic activity. It was developed at UCLA and marketed by the pharmaceutical company Medivation for the treatment of metastatic castration-resistant prostate cancer. Through a mechanism that is reported to be different from other approved AR antagonists, enzalutamide inhibits the activity of prostate cancer cell ARs, which may result in a reduction in prostate cancer cell proliferation and, correspondingly, a reduction in the serum prostate specific antigen (PSA) level. AR over-expression in prostate cancer represents a key mechanism associated with prostate cancer hormone resistance.