Methylation protects dietary flavonoids from rapid hepatic metabolism

5,7‐Dimethoxyflavone (5,7‐DMF) and 3′,4′‐dimethoxyflavone (3′,4′‐DMF), dietary methoxylated flavones, have been shown to inhibit tobacco carcinogen benzo[a]pyrene (BaP)‐induced DNA adducts and CYP1A1 and/or CYP1B1 expression in human hepatocytes (Hep G2) and oral epithelial cells (SCC‐9), therefore, have been suggested to be potential chemopreventive agents. However, little is known about their bioavailability. In the present study, the metabolic profiles of 5,7‐DMF and 3′,4′‐DMF, in comparison with galangin (5,7,4′‐trihydroxyflavone), were investigated in human liver microsomes, S9 fractions and freshly plated hepatocytes. Galangin was extensively glucuronidated (70%) followed by sulfation in S9 fractions, consistent with a previous report. In sharp contrast, 5,7‐DMF and 3′,4′‐DMF were metabolically much more stable than galangin in S9 fractions, as evidenced by more than 80% remaining unchanged after incubation with the cofactors for conjugation and oxidation, i.e. UDPGA, PAPS and NADPH. Incubation of human liver microsomes with NADPH demonstrated that all three compounds, i.e. 5,7‐DMF, 3′,4′‐DMF and galangin, were substrates of CYP oxidation with intrinsic clearance (CL int ) of 13, 64 and 38 ml/min/kg, respectively. Further experiments in freshly plated human hepatocytes showed complete depletion of galangin after 2 hr incubations, as well as only about 30% depletion of 3′,4′‐DMF, and less than 10% depletion of 5,7‐DMF after 4 hr incubations. These observations clearly indicate that methoxylated flavonoids may have high oral bioavailability, compared to their unmethylated forms. Supported by DOD (N6311602MD200), NIH (GM55561) and AICR (02A095).