Prostate cancer (PCa) represents an important target for chemoprevention given its prolonged natural history and high prevalence. Epidemiologic and laboratory data suggest that vitamin D and genistein (soy isoflavone) may decrease PCa progression. The effect of vitamin D on prostate epithelial cell proliferation and differentiation is well documented and genistein may augment this affect through inhibition of the CYP24 enzyme, which is responsible for intracellular vitamin D metabolism. In addition, both genistein and vitamin D inhibit the intraprostatic synthesis of prostaglandin E2, an important mediator of inflammation. The objectives of this prospective multicenter trial were to compare prostate tissue calcitriol levels and down-stream related biomarkers in men with localized prostate cancer randomized to receive cholecalciferol and genistein versus placebo cholecalciferol and placebo genistein during the pre-prostatectomy period.Men undergoing radical prostatectomy were randomly assigned to one of two treatment groups: (1) cholecalciferol (vitamin D) 200,000 IU as one dose at study entry plus genistein (G-2535), 600 mg daily or (2) placebo cholecalciferol day 1 and placebo genistein PO daily for 21-28 days prior to radical prostatectomy. Serum and tissue analyses were performed and side-effects recorded.A total of 15 patients were enrolled, 8 in the placebo arm and 7 in the vitamin D + genistein (VD + G) arm. All patients were compliant and completed the study. No significant differences in side effect profiles were noted. Utilization of the VD + G trended toward increased calcitriol serum concentrations when compared to placebo (0.104 ± 0.2 vs. 0.0013 ± 0.08; p=0.08); however, prostate tissue levels did not increase. Calcidiol levels did not change (p=0.5). Immunohistochemistry for marker analyses using VECTRA automated quantitation revealed a increase in AR expression (p=0.04) and a trend toward increased TUNEL staining (p=0.1) in prostate cancer tissues in men randomized to receive VD + G compared to placebo.In this first study testing the combination of a single, large dose of cholecalciferol and daily genistein, the agents were well tolerated. While an increase in AR expression suggesting differentiation was observed, it is difficult to draw firm conclusions regarding the bioactivity of the combination given the sample size.