RATIONALE: Soy protein may help prevent prostate cancer recurrence in patients who have undergone surgery for prostate cancer.

PURPOSE: This randomized phase II/III trial is studying how well soy protein works and compares it to a placebo in preventing recurrent cancer in patients who have undergone surgery for stage II prostate cancer.

OBJECTIVES:

• Determine whether soy protein isolate reduces the PSA failure rate and time to PSA failure within 2 years following radical prostatectomy in patients who are at high risk for prostate cancer recurrence.
• Determine the effects of soy protein isolate on intermediate biomarkers of steroid hormone axis (testosterone, estradiol, and SHBG) and thyroid activity (T3 and T4).
• Determine the effects of soy protein isolate on intermediate biomarkers of apoptosis (soluble Fas and Fas-ligand), angiogenesis (VEGF and bFGF), antioxidant activity (8-isoprostanes), and IGF axis (IGF-1 and IGFBP-3).
• Compare patients who are equol producers to those who are non-producers.

OUTLINE: This is a multicenter study. Patients are stratified according to hospital/clinic site (NYU vs UIC or Jesse Brown VA Medical Center vs other sites), number of high-risk characteristics (1 vs > 1), and race (African American vs non-African American [i.e., non-Hispanic White, Hispanic, Asian, and other]). Patients are randomized to 1 of 2 treatment arms.

• Arm I: Patients receive an oral soy protein isolate beverage once daily.
• Arm II: Patients receive an oral casein placebo beverage once daily. Treatment in both arms continues for up to 2 years in the absence of disease progression or unacceptable toxicity.

Blood samples are collected periodically for biomarker laboratory studies. Samples are analyzed to measure PSA levels by Tosoh PSA assay; cholesterol levels; isoflavone and equol concentrations by HPLC and ESA; and indicators of steroid hormone axis (testosterone, estradiol, and SHBG), indicators of thyroid activity (T3 and T4), indicators of apoptosis (soluble Fas and Fas-ligand), indicators of angiogenesis (VEGF and bFGF), indicators of oxidative stress (8-isoprostanes), and indicators of IGF axis (IGF-1 and IGFBP-3) by ELISA.

DISEASE CHARACTERISTICS:

• Diagnosis of prostate cancer

  • Clinically localized (T1c or T2) disease

• At high risk for recurrence, as defined by ≥ 1 of the following:

  • Preoperative PSA > 20.0 ng/mL
  • Seminal vesicle invasion
  • Extracapsular extension (excluding the bladder neck)
  • Positive surgical margins (excluding apical margins)
  • Micrometastases in any removed pelvic lymph nodes
  • Final Gleason score of ≥ 8

• Must have undergone radical prostatectomy for prostate cancer within the past 4 months

• Must have an undetectable PSA (< 0.07 ng/mL) at baseline, as measured by Tosoh PSA assay

• No clinical evidence of locally recurrent or metastatic disease

PATIENT CHARACTERISTICS:

• No significant intake of soy (i.e., more than once a week) at baseline including, but not limited to, any of the following:

  • Vegetarians who regularly consume soy products (e.g., tofu)
  • Individuals with customary Asian dietary habits, including regular intake of soy products
  • Individuals who use soy-based milk replacements

• No anemia, iron deficiency problems, or subclinical iron deficiency at baseline

• No diabetes

• No thyroid disease

• No requirement for a sodium-free diet

• No substantive tendency to be constipated (i.e., ≥ grade 2 constipation experienced regularly)

• No medical problem that would preclude the consumption of the soy containing beverage powder, including allergies against soy (or milk protein)

• No concurrent major disease, including major mental disease or major substance abuse problems

• No significant side effects from medication

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics
• No concurrent radiotherapy or hormonal therapy
• No other concurrent adjuvant therapy for prostate cancer