Role of the equine CYP3A94, CYP3A95 and CYP3A97 in ketamine metabolism in presence of medetomidine, diazepam and methadone studied by enantioselective capillary electrophoresis.

The anesthetic ketamine is often combined with analgesics and benzodiazepines in equine medicine. Therefore, drug-drug interactions are possible. Enzyme kinetics for ketamine N-demethylation were determined using equine CYP3A94, CYP3A95 and CYP3A97, and the effect of medetomidine, diazepam and methadone on the ketamine metabolism was studied in vitro. Ketamine was incubated with the CYPs or equine liver microsomes (ELM) alone or in presence of medetomidine, diazepam and/or methadone for different times. Norketamine levels were determined using enantioselective capillary electrophoresis (CE) with highly sulfated γ-cyclodextrin as chiral selector. The three equine CYPs were demonstrated to be involved in ketamine N-demethylation and the kinetics can be described with the Michaelis-Menten model. Vmax values calculated for CYP3A94 and CYP3A97 were higher than for CYP3A95. The lowest Km value was found for CYP3A94. In contrast to diazepam and methadone, the α2-recepor agonist medetomidine diminished the norketamine formation significantly in CYP3A94 and CYP3A97. In ELM, increasing concentrations of diazepam inhibited the norketamine formation. Despite the differences in ketamine N-demethylation in combination with diazepam and methadone, the effect is unlikely to be of clinical relevance because ketamine and the other drugs do not have a small therapeutic margin.