RATIONALE: Vitamin D and soy extract may be effective in lowering prostate-specific antigen (PSA) levels in patients with recurrent prostate cancer that has not responded to previous treatment.

PURPOSE: This phase II trial is studying how well giving vitamin D together with soy supplements works in treating patients with recurrent prostate cancer.

OBJECTIVES:

• Test the response of biochemically recurrent prostate cancer to a combination of cholecalciferol (i.e., vitamin D) and soy isoflavones (i.e., soy extract) after failed definitive local therapy as determined by PSA response.

OUTLINE: Patients receive oral cholecalciferol twice daily and a soy supplement (i.e., soy bar or shake) once daily. Treatment continues for 3-12 months in the absence of disease progression or unacceptable toxicity.

Blood samples are obtained at baseline and periodically during study to measure serum PSA, serum calcium, plasma cholecalciferol, and plasma soy isoflavone levels. Blood samples are also analyzed for expression of cholecalciferol receptor, p21, and p27 in peripheral blood lymphocytes as surrogate markers of the actions of cholecalciferol and genistein. Protein expression is assessed by immunoblot analysis of cell lysates as well as quantitative polymerase chain reaction.

Patients complete a toxicity questionnaire once each month to assess for cholecalciferol and soy supplementation toxicities and symptoms of hypercalcemia.

After completion of study therapy, patients are followed every 3 months for 1 year.

Inclusion:

• Age > 18 years
• Histologically confirmed adenocarcinoma of the prostate
• Biochemical relapse following definitive therapy by ASTRO criteria (PSA with 3 consecutive rising measurements separated by at least one month) and minimum PSA ≥ 1.0 ng/mL
• PSA doubling time of ≥ 6 months, as demonstrated by 3 PSA measurements obtained ≥ 2 months apart
• No hormonal therapy in 6 months prior to enrollment
• ECOG performance status 0-2
• Life expectancy > 3 months
• At least 2 years since prior definitive radiotherapy
• No concurrent cholecalciferol, calcium, or soy supplements
• Absolute granulocyte count ≥ 1,000/mm³
• Platelet count ≥ 100,000/mm³
• Hemoglobin ≥ 9.0 g/dL
• Creatinine ≤ 2.0 mg/dL
• Total bilirubin ≤ 2.0 mg/dL
• Calcium > 8.5 mg/dL and < 10.5 mg/dL
• Testosterone ≥ 150 ng/dL

Exclusion:

• No clinically evident brain metastases
• Concurrent cholecalciferol, calcium, or soy supplements
• Concurrent chemotherapy with nonstudy drugs
• Serious medical illness that would limit survival to < 3 months, or psychiatric condition that would preclude giving informed consent
• Other malignancy except nonmelanoma skin cancer, adequately treated stage I or II cancer from which the patient is currently in complete remission, or any other cancer from which the patient has been disease-free for the past 5 years
• Active, uncontrolled bacterial, viral, or fungal infection
• Hemorrhagic disorder
• Evidence of metastatic disease by bone scan or CT scan
• History of hypercalcemia
• More History of exposure to other phytotherapeutics, including PC-SPES and Saw Palmetto, within the last year.

up to 3 years

only the most severe of each type of adverse event during treatment was recorded.