DISEASE CHARACTERISTICS: Histologically confirmed androgen independent prostate cancer that is progressing under castrate levels of testosterone Progression documented in one of the following ways: A minimum of 3 rising PSA values from baseline obtained 1 week or more apart or 2 measurements 2 weeks or more apart New osseous lesions on bone scan Greater than 25% increase in bidimensionally measurable soft tissue disease, or the appearance of new sites of disease Patients on an antiandrogen must have shown progression off of the antiandrogen prior to study Testosterone no greater than 50 ng/mL No residual proctitis following radiotherapy No active CNS or epidural primary tumor or metastases