A 75-year-old male with metastatic prostate cancer returns to your clinic to discuss management. He has been managed with androgen deprivation therapy since being diagnosed 3 years ago with metastatic prostate cancer to bone (Gleason 5+4=9 with presenting PSA of 29.68 ng/ml). He has tolerated androgen deprivation therapy well and remains asymptomatic from his bony metastatic disease. He has no visceral metastases and no lymph node involvement. His last several PSA's have steadily increased and he now presents to discuss additional therapy for castrate-resistant prostate cancer. His past medical history includes atrial fibrillation for which he is on warfarin. Which of the following is the best next step in management?