Enzalutamide is an androgen receptor antagonist (it blocks the activity of the male sex hormones). Prostate cancers are initially dependent on the male hormone testosterone for growth. Hormonal therapies that lower testosterone or block the ability of testosterone to act at the level of the prostate cancer are currently among the most effective treatments for prostate cancers taht have spread to other body organs (metastasized). The effectiveness of hormonal treatments, however, is not permanent, and over time many prostate cancers progress in spite of these treatments. Enzalutamide is a drug that has been proven to help delay the progression of advanced prostate cancer on average for about 8 months.