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A phase 2 study of enzalutamide monotherapy in patients with hormone naïve prostate cancer demonstrated high prostate specific antigen response rates at 25 weeks, 1 year and 2 years with minimal effects on total body bone mineral density and favorable safety. In this followup analysis we evaluated enzalutamide antitumor activity and safety at 3 years.In a single arm analysis 67 patients with hormone naïve prostate cancer and noncastrate testosterone (230 ng/dl or greater) received enzalutamide 160 mg per day orally until disease progression or unacceptable toxicity. The primary end point was the prostate specific antigen response (80% or greater decline from baseline).No patients discontinued treatment during year 3. Of 42 patients with prostate specific antigen assessments at 3 years 38 (90.5%, 95% CI 77.4–97.3) maintained a prostate specific antigen response. Of 26 patients with metastases at baseline 17 (65.4%) had a complete or partial response as the best overall response during 3 years. In patients who completed the 3-year visit minimal mean changes from baseline were observed in total body bone mineral density or bone mineral density of the femoral neck, trochanter, spine L1–L4 or forearm (range –2.7% to –0.1%). At 3 years total body fat had increased a mean of 16.5%, total lean body mass had decreased a mean of –6.5% and global health status had minimally decreased from baseline. Common adverse events were gynecomastia, fatigue, hot flush and nipple pain.Enzalutamide antitumor activity was maintained in patients with hormone naïve prostate cancer at 3 years. Overall bone mineral density, global health status and safety results were similar to those at 2 years.