Simultaneous determination of 18α‐ and 18β‐glycyrrhetic acid in human plasma by LC‐ESI‐MS and its application to pharmacokinetics

A highly sensitive and selective LC‐ESI‐MS was developed, validated for the simultaneous determination of 18α‐glycyrrhetic acid (α‐GA) and 18β‐glycyrrhetic acid (β‐GA) for pharmacokinetic studies in healthy subjects. Sample preparation was performed by liquid–liquid extraction with ethyl acetate and the separations were achieved using a C18 column with the mobile phase composed of 10 mmol/L ammonium acetate solution–methanol–acetonitrile (40:36:24, v/v/v) at a flow rate of 1 mL/min. The internal standard was honokiol and the epimers were quantified using a single quadrupole mass spectrometer employing ESI in the negative ion mode. The separation factor, α, was 1.512 for α‐ and β‐GA. The standard curves were linear for both epimers with coefficients of determination (r ≥ 0.9998) over the concentration range of 1–150 ng/mL. The precision and accuracy were ≤ 4.33 and ≤ 6.57%, respectively. The mean plasma extraction recoveries were 82.23 ± 1.91 and 84.29 ± 2.09% for α‐GA and β‐GA, respectively. The assay was successfully applied to evaluation of the pharmacokinetic properties of α‐GA and β‐GA from 19 volunteers who had received oral administration of diammonium glycyrrhizinate capsules. The initial data suggest that glycyrrhizin metabolizes to glycyrrhetic acid fairly slowly and the elimination of α‐GA is slower than that of β‐GA.