Toxicokinetics of the phytoestrogen daidzein in female DA/Han rats

Female DA/Han rats were given the phytoestrogen daidzein, either intravenou sly(10 mg/kg b.w.) or orally by gavage (10 or 100 mg/kg b.w.). The plasma c oncentration-time curve determined after i.v. administration of daidzein wa s fitted to a triexponential model, resulting in a final half-life (gamma - phase) of approximately 4 h. The oral bioavailability of 10 mg daidzein/kg was 9.7%, while that of 100 mg/kg was 2.2%; the higher dose (100 mg/kg) was apparently absorbed to a four- to fivefold lower extent than the smaller d ose. The plasma concentration-time curves after oral administration of daid zein Lo female DA/Han rats revealed pronounced interindividual differences and multiple peaks, pointing to extensive enterohepatic circulation and/or protracted absorption from the gastrointestinal tract. As shown in a separa te experiment with bile duct-cannulated rats, daidzein (i.p. 10 mg/kg b.w.) is efficiently excreted with bile: glucuronide/sulfate metabolites amounti ng to approximately 30% of the dose in 8 h. Conjugates were also the main c irculating metabolites upon i.v. or gavage administration of daidzein, indi cating efficient phase II metabolism in female DA/Han rats. Since only few data have been published on tissue levels of isoflavones, their concentrati ons were measured in various organs and compared to plasma levels determine d at the time the animals were killed, with one exception 32 or 48 h after rats had received a single dose of daidzein (i.v. or per os). As expected, the daidzein concentrations depended upon dose and administration route. De spite notable differences in the absolute amounts of total daidzein (free p lus hydrolyzed conjugates), the levels were usually three- to fivefold high er in liver and kidney than in plasma; in most samples of uteri, the concen trations were similar, or up to twofold higher, than the respective plasma levels. These data point to an uptake and storage of isoflavones and metabo lites in tissues. Experimental toxicokinetics appear to be a relevant subje ct that should be integrated into assessments of toxicological data for end ocrine modulators.