In vitro absorption and metabolism of a citrus chemopreventive agent, auraptene, and its modifying effects on xenobiotic enzyme activities in mouse livers.

We previously reported that auraptene (7-geranyloxycoumarin, AUR), widely occurring in citrus fruit, is a structurally novel type of effective cancer-preventive agent, as manifested in several rodent models. However, its bio-availability and metabolism in biological systems have yet to be investigated. In the present study, we examined the chemical stability of AUR at pH 1.57 and 37 degrees C (as a stomach digestion model) and observed its stoichiometric conversion to umbelliferone [7-hydroxycoumarin, UMB; half-life (t1/2) = 15 h; 7-ethoxycoumarin (ETC) was stable for 24 h]. Differentiated Caco-2 cells, a human colorectal adenocarcinoma cell line, were used as a small intestine model. ETC permeated the basolateral (portal vein) side of Caco-2 cells in a time-dependent manner; AUR slightly permeated the cells, but with an intracellular accumulation. Epoxyauraptene and UMB were detected when AUR was treated with the rat liver S-9 mixture. ETC was also converted to UMB, but its t1/2 of two hours was much shorter than that of AUR (> 24 h). This suggests that AUR, bearing a geranyloxyl side chain, is a relatively metabolism-resistant substrate for cytochrome P-450 enzymes and, thus, is stable in the liver compared with ETC. Oral administration of AUR by gavage at 50-200 mg/kg body wt dose dependently induced glutathione S-transferase (GST) activity in mouse livers without affecting cytochrome P-450 activity. Using 10 coumarin-related compounds, we found that only those coumarins having a 7-alkyloxyl group induced GST, but not cytochrome P-450, activity. The present study presumes that AUR accumulates in the epithelial cells of the small intestine and then gradually permeates into the portal vein. Stable localizability of AUR in the colon and liver may be associated with the induction of GST activity, which is important as the action mechanism for suppression of rodent chemical carcinogenesis.