A 72-year-old man with a history of hypertension and diabetes mellitus presents with bone pain in the lumbar spine and pelvis for 3 weeks. PSA is markedly elevated at 145 ng/mL. Serum alkaline phosphatase is 89 U/L (normal 30–120). Bone scan shows multiple hot spots in the lumbar vertebrae, pelvis, and femur. Biopsy of a pelvic lesion confirms adenocarcinoma of the prostate. Serum testosterone is 450 ng/dL (normal 300–1000). What is the most appropriate first-line hormonal agent for this patient?