Abstract

To better understand the relationship between the chemical structure and biological fate of dietary polyphenols, the hepatic metabolic stability and intestinal absorption of methylated polyphenols, in comparison with unmethylated polyphenols, were investigated in pooled human liver S9 fraction and human colon adenocarcinoma (Caco-2) cells. Consistent with previous in vivo studies, the two well known unmethylated polyphenols resveratrol (3,5,4′-trihydroxystilbene) and quercetin (3,5,7,3′,4′-pentahydroxyflavone) were rapidly eliminated by the S9 fraction in the presence of the appropriate cofactors for conjugation and oxidation. In contrast, the methylated flavones, i.e., 7-methoxyflavone, 7,4′-dimethoxyflavone, 5,7-dimethoxyflavone, and 5,7,4′-trimethoxyflavone, were relatively stable, indicating high resistance to hepatic metabolism. The corresponding unmethylated flavones, i.e., 7-hydroxyflavone, 7,4′-dihydroxyflavone, chrysin (5,7-dihydroxyflavone), and apigenin (5,7,4′-trihydroxyflavone), were rapidly eliminated because of extensive glucuronidation and/or sulfation just as resveratrol and quercetin were. The rate of intestinal absorption was evaluated using Caco-2 cells grown in porous inserts. The methylated flavones showed approximately 5- to 8-fold higher apparent permeability (P_app, 22.6–27.6 × 10^–6 cm s^–1) of apical to basolateral flux than the unmethylated flavones (P_app, 3.0–7.8 × 10^–6 cm s^–1). The lower P_app values for the unmethylated flavones correlated with their extensive metabolism in the Caco-2 cells. Thus, combined use of the hepatic S9 fraction and Caco-2 cells will be useful for predicting the oral bioavailability of dietary polyphenols. The higher hepatic metabolic stability and intestinal absorption of the methylated polyphenols make them more favorable than the unmethylated polyphenols to be developed as potential cancer chemopreventive agents.