Study of Enzalutamide in Patients With Castration-resistant Prostate Cancer

After the screening procedures confirm that the patient is able to participate in the study, * The patient will have the first of two required prostate biopsies prior to starting study treatment. * The patient will be given a prescription for study drug and a study drug-dosing diary for each treatment cycle. Each treatment cycle lasts 28 days (4 weeks), during which time the patient will be taking the study drug once daily. The diary will also include special instructions for taking the study drug. The study drug (enzalutamide) should be taken orally (by mouth) at home. On Day 1 of each cycle (+/- 4 days), the following procedures will be performed in clinic: * A medical history. * A Physical examination * Performance status * Blood tests (2-3 tablespoons). * The patient will be asked about medications they are currently taking, including over-the counter medications, herbal remedies, vitamins, and supplements. * The patient will be asked about any disease-related symptoms they are experiencing. * The patient will receive a new supply and the medication diary will be reviewed. Every 12 weeks (+/-1 week) the following procedures will be performed in clinic: * Blood tests. A small sample of the patient's blood (about 1-2 tablespoons) will be collected. This blood will be collected for specialized laboratory tests. * A (CT) scan of the patient's chest and a CT or MRI scan of abdomen, and pelvis and a bone scan will be performed every 12 weeks (+/- 1 week) while the patient is on active treatment. If your baseline CT of the patient's chest does not show disease, the investigator may not asked for this to be repeated. End of Study Visit in clinic: * A medical history. * A physical examination * Performance status * Blood tests (2-3 tablespoons) * The patient will be asked about any disease-related symptoms If the patient completed at least 4 cycles of enzalutamide, the patient will undergo a second of two required biopsies.