Received for publication May 23, 2007.
Revised August 13, 2007.
Accepted for publication August 13, 2007.

Bioavailable flavonoids: CYP-mediated metabolism of methoxyflavones

Abstract

Methoxylated flavones were recently shown to be promising cancer chemopreventive agents. Their high metabolic stability compared to the hydroxylated analogs was demonstrated 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 towards oxidative metabolism was investigated with human liver microsomes and recombinant cytochrome P450 isoforms. Among fifteen 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^-1 kg^-1). Of the fully methylated compounds, 5,7- dimethoxyflavone and 5-methoxyflavone were the most stable (Cl_int 13 and 18 ml min^-1 kg^-1, 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^-1 kg^-1, respectively),emphasizing the importance of the positions of the methoxy substituents in the flavone ring system. Among the five CYP 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.

Key words: anticancer agents, bioavailability, CYP1A, CYP3A, cytochrome P450 catalyzed oxidations, HPLC, liver microsomes