Pharmacokinetics and bioavailability of the isoflavone biochanin A in rats.

Biochanin A (BCA) is a dietary isoflavone present in legumes, most notably red clover, and in many herbal dietary supplements. BCA has been reported to have chemopreventive properties and is metabolized to the isoflavone genistein (GEN), BCA conjugates, and GEN conjugates. The metabolites may contribute to the chemopreventive effects of BCA. The absorption, metabolism, and disposition of BCA have not been determined in rats. Our objective was to evaluate the pharmacokinetics and metabolism of BCA in rats. Male Sprague-Dawley rats were administered BCA by intravenous injection (1 and 5 mg/kg), by intraperitoneal injection (5 and 50 mg/kg), and orally (5 and 50 mg/kg). Plasma and bile samples were enzymatically hydrolyzed in vitro to determine conjugate concentrations for BCA and GEN. Equilibrium dialysis was used to determine protein binding. The BCA and GEN concentrations in plasma, urine, and bile were determined by liquid chromatography-tandem mass spectrometry (LC/MS/MS). The pharmacokinetic parameters of BCA were analyzed by noncompartmental analysis. Significant levels of BCA conjugates and GEN conjugates were detected in plasma and bile. Both BCA and GEN were found to have a high clearance and a large apparent volume of distribution; the bioavailability of both was poor (<4%). Reentry peaks were evident after oral administration of both BCA and GEN, suggesting enterohepatic cycling. The free fraction of BCA in rat plasma was 1.5%. A 2-compartment model that included both linear and nonlinear clearance terms and enterohepatic recirculation best described the plasma data. This represents the first evaluation of the dose-dependent pharmacokinetics and metabolism of BCA in rats.