Abstract
Ketamine is primarily metabolized to norketamine by hepatic CYP2B6 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 Michaelis-Menten constants (Km) 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. N,N′N′-triethylenethiophosphoramide (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.
Footnotes
- Received February 25, 2013.
- Accepted April 2, 2013.
This work was supported by University of Adelaide, School of Medical Sciences funding, Adelaide Graduate Fee Scholarship; the Australian Research Council Australian Research Fellowship [Grant DP110100297]; FTT Fricker Research Fellowship [Medical Endowment Funds, University of Adelaide]; and the Robert Bosch Foundation, Stuttgart, Germany.
This work was previously presented at the following conferences: Somogyi AA (2012) Impact of CYP2B6*6 genotype on ketamine human pharmacokinetics and microsomal metabolism. 19th International Symposium on Microsomes and Drug Oxidations and 12th European ISSX Meeting; 2012 June 17–21; Noordwijk aan Zee, The Netherlands; and Li Y (2012) CYP2B6*6 mutation significantly reduces in vitro N-demethylation of ketamine enantiomers. Joint ASCEPT-APSA 2012 Conference; 2012 December 2–5; Sydney, NSW, Australia.
- Copyright © 2013 by The American Society for Pharmacology and Experimental Therapeutics
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