Objectives: Epidemiological studies have shown significant relationships between the use of dietary components and prostate cancer incidence and mortality. Large studies of primary prevention, which confirm these findings, are desirable but costly and difficult to design. The present tertiary prevention study reports on the effect of a dietary supplement in comparison with placebo on the rate of increase of prostate-specific antigen (PSA).
Methods: 49 patients with a history of prostate cancer and rising PSA levels after radical prostatectomy (n = 34) or radiotherapy (n = 15) participated in a randomised, double-blind, placebo-controlled crossover study of a dietary supplement. Ethical approval of the protocol was obtained. Treatment periods of 10 weeks were separated by a 4-week washout period. The supplement consisted of soy, isoflavones, lycopene, silymarin and antioxidants as main ingredients. Changes in the rate of increase of PSA (PSA slope and doubling time) were the primary parameters of efficacy. Analyses according to intention to treat (ITT) and per protocol (PP) were carried out.
Results: Baseline parameters did not differ between randomised groups. Five participants were lost to follow-up, however 46 could be evaluated in an ITT analysis. PP analysis could be performed in 42 men with at least 5 PSA measurements. Per protocol analysis showed a significant decrease in PSA slope (p = 0.030) and (2)log PSA slope (p = 0.041). This translates into a 2.6 fold increase in the PSA doubling time from 445 to 1150 days for the supplement and placebo periods. No treatment-based changes in safety parameters were observed during the study.
Conclusions: The soy-based dietary supplement utilised in this study was shown to delay PSA progression after potentially curative treatment in a significant fashion. More extensive studies of the supplement may be indicated.