RATIONALE: Soy protein supplement may prevent or delay the development of prostate cancer in patients who have elevated prostate-specific antigen (PSA) levels.

PURPOSE: Randomized phase II trial to determine the effectiveness of soy protein supplement in preventing prostate cancer in patients who have elevated PSA levels.

OBJECTIVES:

• Compare the reduction in the rate of prostatic cellular proliferation in patients with an elevated PSA (5 to 10 ng/mL) and a negative biopsy for prostate cancer when treated with daily soy protein supplements vs placebo.
• Compare the effect of these regimens on additional biomarkers of prostate cancer (PSA, high-grade prostate intraepithelial neoplasia, induction of apoptosis, sex steroid receptor expression, and loss of glutathione S-transferase-pi) in these patients.
• Compare the effect of these regimens on quality of life, including urinary and sexual function, in these patients.

OUTLINE: This is a randomized, double-blind, placebo-controlled, multicenter study. Patients are stratified according to race (Caucasian vs African American). After 2 weeks of daily oral placebo, patients are randomized to 1 of 2 arms.

• Arm I: Patients receive oral soy protein supplement daily for 12 months.
• Arm II: Patients receive oral placebo daily for 12 months. Quality of life is assessed at baseline and at 6 and 12 months.

PROJECTED ACCRUAL: A total of 160 patients (80 per arm) will be accrued for this study within 12 months.

DISEASE CHARACTERISTICS:

• Histologically confirmed absence of prostate cancer

  • Atypical adenomatous hyperplasia or prostatic intraepithelial neoplasia (high-grade or low-grade) allowed
  • Abnormal baseline transrectal ultrasound and digital rectal exam allowed
  • Biopsy may be before or after study entry, but must be within the past 90 days

• PSA 5-10 ng/mL

PATIENT CHARACTERISTICS:

Age:

• 50 and over

Performance status:

• ECOG 0-2

Life expectancy:

• Not specified

Hematopoietic:

• Not specified

Hepatic:

• Not specified

Renal:

• Not specified

Other:

• No known allergy to soy protein or milk protein
• No invasive cancer within the past 5 years except non-melanoma skin cancer

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• Not specified

Chemotherapy:

• Not specified

Endocrine therapy:

• No prior hormonal therapy
• No prior finasteride
• No concurrent hormonal therapy
• No concurrent finasteride

Radiotherapy:

• Not specified

Surgery:

• At least 6 months since prior transurethral resection of the prostate
• No prior orchiectomy
• No concurrent orchiectomy

Other:

• No other concurrent soy products