Inhibition of cytochrome P450 2B6 activity by kava extracts

Kava is a beverage prepared from the rhizomes and roots of the tropical evergreen plant Piper methysticum. Kava contains 6 major kavalactones. Legacy studies found that kavalactones did not inhibit human cytochrome P450s in vitro at relevant concentrations, and kava did not inhibit the major P450s in humans in vivo and was devoid of clinically significant drug interactions. Nevertheless, CYP2B6 was never evaluated. Recent investigation identified that (+)-dihydromethysticin, 1 of the 6 natural kavalactones, was an inhibitor of CYP2B6 in vitro. This investigation evaluated the effects of kava extract and (+)-dihydromethysticin on expressed CYP2B6 activity-both wild-type CYP2B6.1 and active allelic variant CYP2B6.4. Effects on human liver microsomal CYP2B6 were also evaluated. Two independent kava extracts inhibited expressed and human liver microsomal CYP2B6 activity, in a concentration-dependent manner, assessed using the CYP2B6 probe substrates 7-ethoxy-4-trifluoromethyIcoumarin O-deethylation and S-ketamine N-demethylation. K_i values for CYP2B6 inhibition by (+)-dihydromethysticin alone and in kava extracts were 0.01-0.05 μM. Kava and (+)-dihydromethysticin interaction with expressed CYP2B6.1 and CYP2B6.4 generated a difference spectrum consistent with formation of a metabolite-inhibitor complex. (+)-Dihydromethysticin underwent NADPH-dependent metabolism by expressed CYP2B6 and human liver microsomes. These results show that kava extracts inhibit CYP2B6, an effect attributed to (+)-dihydromethysticin, which appears to act as a mechanism-based inhibitor. Because kava consumption achieves plasma (+)-dihydromethysticin concentrations exceeding the inhibitory K_i, there is a potential risk of in vivo CYP2B6-mediated herb-drug interactions. Such potential merits clinical drug-herb interaction studies using standard CYP2B6 substrate probes. SIGNIFICANCE STATEMENT: Kava extracts are widely used as botanical supplements for managing various health conditions. Kava extracts showed concentration-dependent inhibition of CYP2B6 in both expressed systems and human liver microsomes. Inhibition was attributed to (+)-dihydromethysticin. Spectral and metabolism studies suggested that CYP2B6 catalyzed the formation of a metabolite-inhibitor complex with (+)-dihydromethysticin alone and present in the extracts. Typical kava consumption yields plasma (+)-dihydromethysticin concentrations that exceed the inhibitory K_i; thus, showing a clear potential for clinically relevant in vivo CYP2B6-mediated herb-drug interactions.