Diindolylmethane in Treating Patients With Nonmetastatic Prostate Cancer That Has Not Responded To Previous Hormone Therapy

DISEASE CHARACTERISTICS: * Histologically proven adenocarcinoma of the prostate * Prostate specific antigen (PSA)-only failure after local therapy (surgery, radiation therapy, brachytherapy, or cryotherapy) * Rising PSA despite androgen-deprivation therapy with castrate levels of testosterone (\< 50 ng/dL) * Two successive rising PSA levels at least 1 week apart * PSA ≥ 5 ng/mL * Patients with a history of combined hormonal therapy must continue luteinizing-hormone releasing-hormone agonist treatment but must demonstrate rising PSA after anti-androgen withdrawal * No evidence of distant metastasis by bone scan and CT scan * No known brain metastases requiring active therapy PATIENT CHARACTERISTICS: * ECOG performance status ≤ 3 * Life expectancy ≥ 12 weeks * Absolute neutrophil count ≥ 1,500/mm\^3 * Platelet count ≥ 100,000/mm\^3 * Hemoglobin ≥ 8.0 g/dL * Total bilirubin ≤ 1.5 times upper limit of normal (ULN) * SGOT and/or SGPT ≤ 2.5 times ULN AND alkaline phosphatase normal OR alkaline phosphatase ≤ 4 times ULN AND SGOT and/or SGPT normal * Creatinine clearance ≥ 60 mL/min OR creatinine normal * Fertile patients must use effective contraception * None of the following conditions within the past 6 months: * Myocardial infarction * Severe or unstable angina * Symptomatic congestive heart failure * Cerebrovascular accident or transient ischemic attack * Coronary/peripheral artery bypass grafting * No other severe acute or chronic medical or psychiatric condition or laboratory abnormality that would preclude study participation PRIOR CONCURRENT THERAPY: * See Disease Characteristics * At least 28 days since prior radiotherapy * At least 28 days since prior investigational agents for treatment of prostate cancer * At least 4 weeks since prior flutamide * At least 6 weeks since prior bicalutamide * No other concurrent antineoplastic agents * No concurrent warfarin-related anticoagulants * No concurrent proton-pump inhibitor drugs for gastroesophageal reflux disease (e.g., rabeprazole, esomeprazole magnesium, lansoprazole, omeprazole, or pantoprazole sodium) * No concurrent micronutrient supplements or dietary soy products * One daily multivitamin allowed