Effects of a Soybean Isoflavone Mixture on Carcinogenesis in Prostate and Seminal Vesicles of F344 Rats

Several epidemiological studies have suggested an inverse association between the risk of prostate cancer and intake of soybeans and their products. In vitro data pointing to possible anti‐carcinogenic properties of the soybean isoflavone, genistein, led us to investigate the chemopreventive potential of soybean isoflavones in a rat carcinogenesis model induced by 3,2′‐dimethyl‐4‐aminobi‐phenyl (DMAB) and testosterone propionate (TP). Animals received DMAB s.c. injections at 2‐week intervals for the first 20 weeks and implanted silicon tubes containing 40 mg of TP, replaced at 6‐week intervals throughout the experiment. The soybean isoflavone mixture consisting of 74% genistein and 21% daidzein was mixed in basal diet (AIN‐76A) at concentrations of 100 and 400 ppm and fed to F344 male rats throughout the experiment. Rats treated with carcinogens and administered isoflavone mixture at 100 and 400 ppm developed adenocarcinomas at incidences of 35% and 29%, respectively, in the prostate and seminal vesicles, whereas the figure was 60% for those maintained on control diet. Feeding of the isoflavone mixture at 100 and 400 ppm significantly inhibited the number of argyrophilic nucleolar organizer regions (AgNORs) in adenocarcinomas of the accessory sex glands as compared to those of rats fed control diet. No influence on the development of neoplastic lesions originating in other organs was noted. The results of this study provide evidence that soybean isoflavones may have potential as chemopreventive agents against carcinogenesis in the prostate.