In vitro and in vivo metabolic activation of rhein and characterization of glutathione conjugates derived from rhein.

Rhein (RH), 4,5-dihydroxyanthrauinone-2-carboxylic acid, is found in rhubarb (Dahuang), a traditional herbal medicine. RH has reportedly demonstrated multiple pharmacologic properties. Previous studies have also shown that RH induced hepatotoxicity, but the mechanisms of the adverse effect remain unknown. The major objective of the present study was to study the metabolic pathways of RH in order to identify potential reactive metabolites. One mono-hydroxylation metabolite (M1) was detected in urine and bile of rats given RH. M1 was also observed in rat and human liver microsomal incubations after exposure to RH. A total of three (GSH) conjugates (M2, M3 and M5) were detected in bile of rats treated with RH. We concluded that M2-M3 were directly derived from parent compound RH through spontaneous reaction with GSH. M5 was derived from M1 by reaction with GSH, which required cytoslic GSTs. M5 was further metabolized to the corresponding NAC conjugate (mercapturic acid) and was excreted in urine. P450 2C9 was mainly involved in the oxidation of RH.