Bioavailable flavonoids: cytochrome P450-mediated metabolism of methoxyflavones.

Methoxylated flavones were recently shown to be promising cancer chemopreventive agents. Their high metabolic stability compared with the hydroxylated analogs was shown in our laboratory using the human hepatic S9 fraction with cofactors for glucuronidation, sulfation, and oxidation. In the present study, the resistance of methoxylated flavones toward oxidative metabolism was investigated with human liver microsomes and recombinant cytochrome P450 (P450) isoforms. Among 15 methoxylated flavones investigated, the two partially methylated compounds, tectochrysin and kaempferide, were among the most susceptible to microsomal oxidation (Cl(int) 283 and 82 ml/min/kg). Of the fully methylated compounds, 5,7-dimethoxyflavone and 5-methoxyflavone were the most stable (Cl(int) 13 and 18 ml/min/kg, respectively), whereas 4'-methoxyflavone, 3'-methoxyflavone, 5,4'-dimethoxyflavone, and 7,3'-dimethoxyflavone were the least stable (Cl(int) 161, 140, 119, and 92 ml/min/kg, respectively), emphasizing the importance of the positions of the methoxy substituents in the flavone ring system. Among the five P450 isoforms tested, CYP1A1 showed the highest rate of metabolism of fully methylated compounds, followed by CYP1A2 and CYP3A4. CYP2C9 and CYP2D6 gave minimal disappearance of the parent compound. Finally, in incubations with hepatic S9 fraction with cofactors for oxidation and both conjugation reactions, partially methylated flavones, as expected, were much less metabolically stable than fully methylated flavones, confirming that oxidative demethylation is the rate-limiting metabolic reaction for fully methylated flavones only. In summary, the rate of oxidative metabolism of methoxylated flavones, mainly involving CYP1A1 and CYP1A2, varied widely, even between compounds with very similar structures.