15632
title and abstract
The CYP2B6*6 Allele Significantly Alters the N-demethylation of Ketamine Enantiomers In Vitro.
Ketamine is primarily metabolized to norketamine by hepatic cytochrome P450 (CYP) 2B6 and CYP3A4-mediated N-demethylation. However, the relative contribution from each enzyme remains controversial. The CYP2B6*6 allele is associated with reduced enzyme expression and activity that may lead to interindividual variability in ketamine metabolism. We examined the N-demethylation of individual ketamine enantiomers using human liver microsomes (HLMs) genotyped for the CYP2B6*6 allele, insect cell expressed recombinant CYP2B6 and CYP3A4 enzymes and COS-1 cell expressed recombinant CYP2B6.1 and CYP2B6.6 protein variant. Effects of CYP-selective inhibitors on norketamine formation were also determined in HLMs. The two-enzyme Michaelis-Menten model best fitted the HLM kinetic data. The Km value for the high affinity enzyme and the low affinity enzyme were similar to those for the expressed CYP2B6 and CYP3A4, respectively. The intrinsic clearance for both ketamine enantiomers by the high affinity enzyme in HLMs with CYP2B6*1/*1 genotype were at least 2-fold and 6-fold higher, respectively, than those for CYP2B6*1/*6 genotype and CYP2B6*6/*6 genotype. The Vmax and Km values for CYP2B6.1 were approximately 160% and 70% of those for CYP2B6.6, respectively. ThioTEPA (CYP2B6 inhibitor, 25 μM) and the monoclonal antibody against CYP2B6 but not troleandomycin (CYP3A4 inhibitor, 25 μM) or the monoclonal antibody against CYP3A4 inhibited ketamine N-demethylation at clinically relevant concentrations. The degree of inhibition was significantly reduced in HLMs with the CYP2B6*6 allele (gene-dose P<0.05). These results indicate a major role of CYP2B6 in ketamine N-demethylation in vitro and a significant impact of the CYP2B6*6 allele on enzyme-ketamine binding and catalytic activity.