Acute, subchronic and chronic safety studies with genistein in rats.

Genistein is a phytoestrogen that occurs naturally in the diet, especially in soy based foods. There is wide spread interest in phytoestrogens as chemopreventive agents for a variety of diseases and cancers based on epidemiologic evidence. Although soy, and its constituents such as genistein, have been consumed at high levels in several Asian populations without apparent adverse effects, concern has been raised about potential adverse effects due to the estrogenic and other activities. Safety studies with genistein were conducted in the Wistar rat including two acute studies, two subchronic (4 weeks and 13 weeks) and a chronic 52-week dietary admix study. In the acute studies, genistein had a low order of toxicity. In the three repeated dose safety studies at doses up to 500 mg/kg/day, genistein was well tolerated. In all of the studies, decreased food consumption and body weight gain were observed at 500 mg/kg/day. The main hematological findings were decreased red blood cell parameters at 500 mg/kg/day with a compensatory increase in reticulocytes. For clinical chemistry, with the exception of a slight increase in gamma glutamyl transferase in male and female rats at the high dose, there were a number of other minor changes considered not toxicologically significant. At necropsy, there were relatively few macroscopic changes; in the 52-week study, dilation of the uterus with fluid at the high dose and cysts of the ovaries in treated animals were observed. Organ weight changes in male rats at the high dose of 500 mg/kg/day included increased kidney, spleen, adrenal and testes weights and for females included, increased liver, kidney, spleen, ovary and uterus weights. After 4 and 13 weeks of treatment with genistein, there were no treatment related histopathologic findings. After 26 and 52 weeks of treatment, histological changes were seen in the female reproductive organs (ovaries and uterus), and in males (epididymides and prostate), and bone, kidneys, heart, liver and spleen in both sexes. After 52 weeks of treatment of males, vacuolation of the epididymal epithelium at 500 mg/kg/day and inflammation of the prostate were recorded at a higher incidence at 50 and 500 mg/kg/day. In females, cytological changes in the uterus, squamous metaplasia at 50 and 500 mg/kg/day and hyperplasia at 500 mg/kg/day were observed. Furthermore, hydrometra of the uterus and findings in the vagina consisting of anestric or diestrus vaginal mucosa with vaginal mucification, hyperplastic epithelium and multifocal cystic degeneration were noted at 500 mg/kg/day. Atrophy of the ovaries increased in severity in animals at 50 and 500 mg/kg/day. Osteopetrosis (hyperostosis) was observed in male and female rats at 50 and 500 mg/kg/day along with a compensatory increase in extramedullary hemopoiesis in the spleen; females were more affected than males. Hepatocellular hypertrophy and minimal bile duct proliferation were recorded at a higher incidence in animals at 500 mg/kg/day. It is concluded that almost all of the treatment related findings in these studies are related to the estrogenic properties of genistein as a phytoestrogen and would be expected to occur with a compound with estrogenic activity. The hormonally related changes were considered to be functional in nature and thus not adverse effects. Most of the findings in these studies were limited to the high dose of 500 mg/kg/day and were reversible. The few findings observed at 50 mg/kg/day were relatively minor and in view of the functional (hormonally mediated) nature of the effects, were considered not adverse effects. The increased incidence of minimal bile duct proliferation and slightly increased gamma glutamyl transferase are indicative of a mild hepatic effect at the high dose of 500 mg/kg/day. The no observed adverse effect level (NOAEL) of genistein is considered to be 50 mg/kg/day based on the presence of mild hepatic effects at the high dose of 500 mg/kg/day. The no observed effect level (NOEL) is considered to be 5 mg/kg/day based on the hormonally induced functional changes at higher doses.