Glycyrrhizin and glycyrrhetinic acid in the human plasma after administration of FM-100 (the powder obtained from acidic fraction of methanolic extract of Glycyrrhiza) were determined by capillary gas chromatography-selected ion monitoring by using [2H5]glycyrrhetinic acid methyl ester as an internal standard. Plasma was subjected to direct hydrolysis, and the resulting mixture was adsorbed on a polyamide column. Glycyrrhetinic acid was eluted with methanol, and the eluant was further purified by a Sep-Pak Silica cartridge and a silicagel column after the addition of the internal standard. Glycyrrhetinic acid, a metabolite of glycyrrhizin, was directly extracted with AcOEt from the plasma with high recovery. Glycyrrhetinic acid extracted was converted to its methyl ester-O-trimethylsilyl ether derivative by treating with diazomethane and then with N, O-bis-(trimethylsilyl) trifluoroacetamide. Monitoring of the molecular ion indicated that the quantitation limit for glycyrrhizin was 1 ng/ml plasma. After oral administration of 400 mg of FM-100, plasma levels of glycyrrhizin and glycyrrhetinic acid were shown with a maximum at 8-14 h post dose period. For the study of bioavailability of glycyrrhizin, each subject was indispensable to fast in order to avoid the effect of natural glycyrrhizin from many food additives for more than 10 h before and after administration of FM-100, respectively. This method was useful for the study on pharmacokinetics of glycyrrhizin and glycyrrhetinic acid, because of having sufficient sensitivity and specificity for objective compounds.
