Epidemiologic and experimental data suggest that soy consumption may prevent prostate cancer and be beneficial for men with prostate cancer. Soy intake and risk of prostate cancer are inversely correlated; soy isoflavones inhibit growth of prostate cancer cells and reduce prostate carcinogenesis in animal models. We tested the hypothesis that soy consumption reduces biochemical recurrence after radical prostatectomy in a randomized controlled clinical trial with a soy protein supplement versus a casein-based placebo in men at increased risk for PSA failure in the first 2 years after radical prostatectomy. PSA was tested at 2-month intervals in year 1 and every 3 months in year 2. Eligibility criteria were: Gleason sum of >8, extra-capsular extension, seminal vesicle invasion, positive surgical margins, positive lymph nodes, and/or a preoperative PSA of >20 ng/ml. Biochemical recurrence was defined a priori as reaching a PSA value of α0.07 ng/ml, confirmed twice. A two-year PSA failure rate in eligible subjects of approximately 30% was expected, based on data from NYU and previous literature. With a planned sample size of 128 evaluable subjects per arm, the study had 80% power to detect a 50% reduction in PSA failure rate at a 2-sided significance level of 0.05. The soy protein isolate and placebo (generously provided by Solae LCC, St Louis, MO) were identical in composition, except for the protein source (19.2-19.8 g protein/day); the soy product provided daily 23.5 mg genistein and 40.9 mg total isoflavones (aglycone equivalents); the placebo was devoid of any soy-specific constituents. Accrual did not reach the intended level and 172 subjects were randomized and enrolled, of whom 142 were evaluable (completed two years on study or developed confirmed recurrence within two years). Soy protein consumption did not alter recurrence rate or time-to-recurrence (TTR). Of the 74 evaluable subjects in the soy arm 22 (30%) recurred as did 22 (32%) of the 68 subjects in the placebo arm recurred. The mean TTR was 40.7 + 29.3 (SD) weeks in the soy group and 44.5 + 28.0 weeks in men on placebo. Statistical analysis on intention-to-treat basis confirmed the absence of an effect on these primary endpoints. The drop-out rate was 17% and there were <5% missing values, none of which impacted the primary endpoints. Compliance by self-report was excellent and is being confirmed by serum isoflavone levels. There were no treatment-related adverse events, except a few cases of constipation. We previously ruled out effects of the intervention on anti-thyroid activity and iron status in a subset of subjects. Thus, soy consumption is safe, but unlikely to benefit men after radical prostatectomy, although the results of this study do not rule out preventive effects of soy prior to prostate cancer development. (Supported by NIH grants CA27790, CA166195 & RR029879 and the Prevent Cancer Foundation)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3563. doi:1538-7445.AM2012-3563