Soy Protein 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