PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent biologic response modifiers Chemotherapy: No other concurrent chemotherapy Endocrine therapy: See Disease Characteristics At least 2 weeks since prior corticosteroids No concurrent exogenous corticosteroids except for documented CNS metastases or adrenal insufficiency Concurrent hormones for nondisease conditions allowed (e.g., insulin for diabetes mellitus) Concurrent continuation of monthly hormonal therapy with an LHRH agonist or DES required Radiotherapy: Prior radiation to symptomatic metastatic site allowed provided it is not the only measurable or evaluable lesion At least 2 weeks since prior radiotherapy and recovered No concurrent radiotherapy No concurrent palliative radiotherapy except for whole brain irradiation for documented CNS disease or impending spinal cord compression Surgery: See Disease Characteristics At least 3 weeks since prior surgery and recovered Other: No other concurrent treatment for prostate cancer except pain palliation