Effects of synthetic and naturally occurring flavonoids on benzo[a]pyrene metabolism by hepatic microsomes prepared from rats treated with cytochrome P-450 inducers.

Activity-directed fractionation of Trifolium pratense resulted in isolation of the isoflavone biochanin A, a potent inhibitor of metabolic activation of the carcinogen benzo[a]pyrene (B[a]P) in cells in culture. To determine the structural features required for maximal inhibition of cytochrome P-450 mediated metabolism of B[a]P, the inhibitory potencies of 23 flavonoids on metabolism of B[a]P to water-soluble derivatives were examined in liver S-9 homogenate from rats induced with Aroclor 1254. Flavones were much more efficient inhibitors than their corresponding isoflavone or flavanone analogs. Most flavonols were as effective inhibitors as their flavone analogs with the exception of kaempferide. Flavones with two hydroxyl or two methoxyl groups at positions 5 and 7 were the most active. Although all eight flavonoids tested effectively inhibited B[a]P metabolism by beta-naphthoflavone-induced microsomes, none were very effective inhibitors of B[a]P metabolism by phenobarbitol-induced microsomes, and only three were effective inhibitors of B[a]P metabolism by microsomes from non-induced rats. These results indicate that flavones or flavonols that contain free 5- and 7-hydroxyls are potent inhibitors of P-450 induced by beta-naphthoflavone (P-450IA1 and/or P-450IA2) and may potentially be useful as chemopreventive agents against hydrocarbon-induced carcinogenesis.