In-vitro metabolism of glycyrrhetinic acid by human and rat liver microsomes and its interactions with six CYP substrates.

Objectives: Glycyrrhetinic acid is the main metabolite of glycyrrhizin and the main active component of Licorice root. This study was designed to investigate the in-vitro metabolism of glycyrrhetinic acid by liver microsomes and to examine possible metabolic interactions that glycyrrhetinic acid may have with other cytochrome P450 (CYP) substrates.

Methods: Glycyrrhetinic acid was incubated with rat liver microsomes (RLM) and human liver microsomes (HLM). Liquid chromatography tandem mass spectrometry was used for glycyrrhetinic acid or substrates identification and quantification.

Key Findings: The K(m) and V(max) values for HLM are 33.41 µm and 2.23 nmol/mg protein/min, respectively; for RLM the K(m) and V(max) were 24.24µm and 6.86 nmol/mg protein/min, respectively. CYP3A4 is likely to be the major enzyme responsible for glycyrrhetinic acid metabolism in HLM while CYP2C9 and CYP2C19 are considerably less active. Other human CYP isoforms have minimal or no activity toward glycyrrhetinic acid. The interactions of glycyrrhetinic acid and six CYP substrates, such as phenacetin, diclofenac, (S)-mephenytoin, dextromethorphan, chlorzoxazone and midazolam were also investigated. The inhibitory action of glycyrrhetinic acid was observed in CYP2C9 for 4-hydroxylation of diclofenac, CYP2C19 for 4'-hydroxylation of (S)-mephenytoin and CYP3A4 for 1'-hydroxylation of midazolam with half maximal inhibitory concentration (IC50) values of 4.3-fold, 3.8-fold and 9.6-fold higher than specific inhibitors in HLM, respectively. However, glycyrrhetinic acid showed relatively little inhibitory effect (IC50>400 µm) on phenacetin O-deethylation, dextromethorphan O-demethylation and chlorzoxazone 6-hydroxylation.

Conclusions: The study indicated that CYP3A4 is likely to be the major enzyme responsible for glycyrrhetinic acid metabolism in HLM while CYP2C9 and CYP2C19 are considerably less active. The results suggest that glycyrrhetinic acid has the potential to interact with a wide range of xenobiotics or endogenous chemicals that are CYP2C9, CYP2C19 and CYP3A4 substrates.