Tivozanib + Enzalutamide in Adv Prostate Cancer

Patients will be treated in treatment cycles of 28 days (4 weeks), during which time they will take tivozanib once a day for 21 days (3 weeks) followed by a one week break from treatment. The other medication, enzalutamide, will be taken every day throughout each cycle. Patients will be given a Study Drug Administration Diary to keep a brief record of medication administration, and to record any side effects or symptoms. Patients will be seen in the clinic at the beginning of each cycle (every 4 weeks). During each visit they will have the following procedures: medical history, vital sign measurements, complete physical examination, performance status, routine blood tests, urine sample, prostate specific antigen (PSA) test, assessment of tumor, review of study drug administration diary, and review of current medications. About four weeks after stopping the study drug patients will be asked to return to the research clinic for a final study visit. The following procedures will be done: medical history, vital sign measurements, brief physical examination, electrocardiogram, review of other medications used since the last visit, routine blood tests, urine sample and a review of any changes in health.