RATIONALE: Chemoprevention therapy is the use of certain drugs to try to prevent or delay the development of early cancer. Soy isoflavones may be effective in delaying the development of early prostate cancer. PURPOSE: This randomized phase II trial is studying different regimens of soy isoflavones to compare how well they work in treating patients who are undergoing radical prostatectomy for stage I or stage II prostate cancer (adenocarcinoma).
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DISEASE CHARACTERISTICS: * Histologically confirmed adenocarcinoma of the prostate * Stage T1c or T2 * Disease confined to the prostate gland * Planning to undergo radical prostatectomy within the next 3-4 weeks PATIENT CHARACTERISTICS: Age * 18 and over Performance status * Not specified Life expectancy * Not specified Hematopoietic * Not specified Hepatic * ALT and AST less than 2 times upper limit of normal (ULN) * Alkaline phosphatase less than 2 times ULN Renal * Not specified Other * Fertile patients must use effective barrier contraception * Medically cleared for surgery * No concurrent thyroid disease PRIOR CONCURRENT THERAPY: Biologic therapy * No prior biological therapy for prostate cancer * No concurrent biological agents Chemotherapy * No prior chemotherapy for prostate cancer * No concurrent chemotherapy Endocrine therapy * No prior hormonal therapy for prostate cancer * No concurrent thyroid hormone replacement medication * No concurrent hormonal therapy Radiotherapy * Not specified Surgery * See Disease Characteristics Other * At least 3 months since prior high-dose nutritional supplements * No concurrent regular use (more than once weekly) of soy products greater than 50 g of soy protein or 50 mg of soy isoflavone * No concurrent high-dose nutritional supplements * Standard-dose single multivitamin tablet (e.g., Centrum™) allowed * No concurrent herbs * No concurrent soy foods * No other concurrent isoflavone supplements * No other concurrent antineoplastic agents