Inhibition of cytochrome P450 enzymes involved in ketamine metabolism by use of liver microsomes and specific cytochrome P450 enzymes from horses, dogs, and humans.
OBJECTIVE
To identify and characterize cytochrome P450 enzymes (CYPs) responsible for the metabolism of racemic ketamine in 3 mammalian species in vitro by use of chemical inhibitors and antibodies.


METHODS
Human, canine, and equine liver microsomes and human single CYP3A4 and CYP2C9 and their canine orthologs.


METHODS
Chemical inhibitors selective for human CYP enzymes and anti-CYP antibodies were incubated with racemic ketamine and liver microsomes or specific CYPs. Ketamine N-demethylation to norketamine was determined via enantioselective capillary electrophoresis.


RESULTS
The general CYP inhibitor 1-aminobenzotriazole almost completely blocked ketamine metabolism in human and canine liver microsomes but not in equine microsomes. Chemical inhibition of norketamine formation was dependent on inhibitor concentration in most circumstances. For all 3 species, inhibitors of CYP3A4, CYP2A6, CYP2C19, CYP2B6, and CYP2C9 diminished N-demethylation of ketamine. Anti-CYP3A4, anti-CYP2C9, and anti-CYP2B6 antibodies also inhibited ketamine N-demethylation. Chemical inhibition was strongest with inhibitors of CYP2A6 and CYP2C19 in canine and equine microsomes and with the CYP3A4 inhibitor in human microsomes. No significant contribution of CYP2D6 to ketamine biotransformation was observed. Although the human CYP2C9 inhibitor blocked ketamine N-demethylation completely in the canine ortholog CYP2C21, a strong inhibition was also obtained by the chemical inhibitors of CYP2C19 and CYP2B6. Ketamine N-demethylation was stereoselective in single human CYP3A4 and canine CYP2C21 enzymes.


CONCLUSIONS
Human-specific inhibitors of CYP2A6, CYP2C19, CYP3A4, CYP2B6, and CYP2C9 diminished ketamine N-demethylation in dogs and horses. To address drug-drug interactions in these animal species, investigations with single CYPs are needed.
Division of Veterinary Pharmacology & Toxicology, Vetsuisse Faculty, University of Bern, 3012 Bern, Switzerland.
Lone D
LD
Mössner
Andrea
A
Schmitz
Regula
R
Theurillat
Wolfgang
W
Thormann
Meike
M
Mevissen
D000700:Analgesics / Q000737:chemistry / Q000378:metabolism*; D000778:Anesthetics, Dissociative / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D019138:Antibodies, Blocking / Q000737:chemistry / Q000494:pharmacology; D001711:Biotransformation; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004285:Dogs; D019075:Electrophoresis, Capillary; D004791:Enzyme Inhibitors / Q000378:metabolism / Q000494:pharmacology; D005260:Female; D006736:Horses; D006801:Humans; D007649:Ketamine / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism*; D008297:Male; D008745:Methylation; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism; D013045:Species Specificity; D013237:Stereoisomerism
D000700:Analgesics; D000778:Anesthetics, Dissociative; D019138:Antibodies, Blocking; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D007649:Ketamine; D003577:Cytochrome P-450 Enzyme System; C033419:norketamine