Soy isoflavones in the treatment of prostate cancer

Epidemiological studies suggest an inverse association between soy intake and prostate cancer risk. We have previously shown that soy isoflavones induce apoptosis and inhibit growth of both androgen-sensitive and androgen-independent prostate cancer cell lines. We conducted a pilot study in patients with advanced prostate cancer to determine the clinical efficacy of soy isoflavones in this population. Patients with prostate cancer were enrolled on the study if they had either newly diagnosed and untreated disease under watchful waiting with rising PSA or had increasing serum PSA following local therapy (Groups I-II) or had failed hormone therapy (Group III). WE administered 100 mg of soy isoflavone (NovasoyO) orally twice daily for three to six months. Forty-one patients were enrolled (22 in groups I-II and 19 in group III). Median PSA level was 13.3 ng/ml. Thirty-nine patients were assessed for response. Soy isoflavones were given for a median of 5.5 (range 0.8-6) months. Although there were no decreases in PSA, stabilization of the PSA occurred in 83% of patients in hormone-sensitive (group II) and 35% of hormone-refractory (group III) patients. There was a decrease in the velocity of serum PSA in the whole group (p=0.01) with the velocity decreasing from 14% to 6% in Group II (p=0.21) and from 31 % to 9% in Group III (p=0.05) following the soy isoflavones. Serum genistein and daidzein levels increased during supplementation from 0.11mM to 0.65 mM (p=0.00002) and from 0.11 moM to 0.51 moM (p=0.00001), respectively. These data suggest that soy isoflavones may benefit patients with prostate cancer.