Equine cytochrome P450 2B6 — Genomic identification, expression and functional characterization with ketamine

Stereoselective methadone disposition after administration of racemic methadone to anesthetized Shetland ponies assessed by capillary electrophoresis

The enantioselectivity of the pharmacokinetics of methadone was investigated in anesthetized Shetland ponies after a single intravenous (0.5 mg/kg methadone hydrochloride; n = 6) or constant rate infusion (0.25 mg/kg bolus followed by 0.25 mg/kg/h methadone hydrochloride; n = 3) administration of racemic methadone. Plasma concentrations of l‐methadone and d‐methadone and their major metabolites, l‐ and d‐2‐ethylidene‐1,5‐dimethyl‐3,3‐diphenylpyrrolidine (EDDP), respectively, were analyzed by CE with highly sulfated γ‐cyclodextrin as chiral selector and electrokinetic analyte injection from liquid/liquid extracts prepared at alkaline pH. In both trials, the d‐methadone concentrations were lower than those of l‐methadone and the d‐EDDP levels were lower than those of L‐EDDP. For the case of a single intravenous bolus injection, the plasma concentration versus time profile of methadone enantiomers was analyzed with a two‐compartment pharmacokinetic model. l‐methadone showed a slower elimination rate constant, a lower body clearance, and a smaller steady‐state volume of distribution than d‐methadone. d‐methadone and d‐EDDP were eliminated faster than their respective l‐enantiomers. This is the first study that outlines that the disposition of racemic methadone administered to anesthetized equines is enantioselective.

Bioanalysis of drugs and their metabolites by chiral electromigration techniques (2010-2020)

The further development and application of capillary electromigration techniques for the enantioselective determination of drugs and their metabolites in body fluids, tissues, and in vitro preparations during the 2010 to 2020 time period continued to proof their usefulness and attractiveness in bioanalysis. This review discusses the principles and important aspects of capillary electrophoresis- based chiral drug bioassays, provides a survey of the assays reported during the past 10 years and presents an overview of the key achievements encountered in that time period. For systems with charged chiral selectors, special attention is paid on assays that feature field-amplified sample injection to enable the determination of ppb levels of analytes and optimized online incubation procedures for the rapid assessment of a metabolic pathway. Applications discussed encompass the pharmacokinetics of drug enantiomers in vivo and in vitro, the impact of inhibitors on metabolic steps, the elucidation of the stereoselectivity of drug metabolism in vivo and in vitro, and drug enantiomers in toxicological, forensic, and doping analysis.

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. V_max values calculated for CYP3A94 and CYP3A97 were higher than for CYP3A95. The lowest K_m 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.

Heterologous expression of equine CYP3A94 and investigation of a tunable system to regulate co-expressed NADPH P450 oxidoreductase levels

The activity of cytochrome P450 enzymes depends on the enzyme NADPH P450 oxidoreductase (POR). The aim of this study was to investigate the activity of the equine CYP3A94 using a system that allows to regulate the POR protein levels in mammalian cells. CYP3A94 and the equine POR were heterologously expressed in V79 cells. In the system used, the POR protein regulation is based on a destabilizing domain (DD) that transfers its instability to a fused protein. The resulting fusion protein is therefore degraded by the ubiquitin-proteasome system (UPS). Addition of “Shield-1” prevents the DD fusion protein from degradation. The change of POR levels at different Shield-1 concentrations was demonstrated by cytochrome c reduction, Western immunoblot analysis, and immunocytochemistry. The alteration of CYP3A94 activity was investigated using a substrate (BFC) known to detect CYP3A4 activity. Equine CYP3A94 was demonstrated to be metabolically active and its activity could be significantly elevated by co-expression of POR. Cytochrome c reduction was significantly increased in V79-CYP3A94/DD-POR cells compared to V79-CYP3A94 cells. Surprisingly, incubation with different Shield-1 concentrations resulted in a decrease in POR protein shown by Western immunoblot analysis. Cytochrome c reduction did not change significantly, but the CYP3A94 activity decreased more than 4-fold after incubation with 500 nM and 1 µM Shield-1 for 24 hours. No differences were obtained when V79-CYP3A94 POR cells with and without Shield-1 were compared. The basal activity levels of V79-CYP3A94/DD-POR cells were unexpectedly high, indicating that DD/POR is not degraded without Shield-1. Shield-1 decreased POR protein levels and CYP3A94 activity suggesting that Shield-1 might impair POR activity by an unknown mechanism. Although regulation of POR with the pPTuner system could not be obtained, the cell line V79-CYP3A94/DD-POR system can be used for further experiments to characterize the equine CYP3A94 since the CYP activity was significantly enhanced with co-expressed POR.

Gene and protein expression and cellular localisation of cytochrome P450 enzymes of the 1A, 2A, 2C, 2D and 2E subfamilies in equine intestine and liver

BACKGROUND

Among the cytochrome P450 enzymes (CYP), families 1-3 constitute almost half of total CYPs in mammals and play a central role in metabolism of a wide range of pharmaceuticals. This study investigated gene and protein expression and cellular localisation of CYP1A, CYP2A, CYP2C, CYP2D and CYP2E in equine intestine and liver. Real-time polymerase chain reaction (RT-PCR) was used to analyse gene expression, western blot to examine protein expression and immunohistochemical analyses to investigate cellular localisation.

RESULTS

CYP1A and CYP2C were the CYPs with the highest gene expression in the intestine and also showed considerable gene expression in the liver. CYP2E and CYP2A showed the highest gene expression in the liver. CYP2E showed moderate intestinal gene expression, whereas that of CYP2A was very low or undetectable. For CYP2D, rather low gene expression levels were found in both intestine and the liver. In the intestine, CYP gene expression levels, except for CYP2E, exhibited patterns resembling those of the proteins, indicating that intestinal protein expression of these CYPs is regulated at the transcriptional level. For CYP2E, the results showed that the intestinal gene expression did not correlate to any visible protein expression, indicating that intestinal protein expression of this CYP is regulated at the post-transcriptional level. Immunostaining of intestine tissue samples showed preferential CYP staining in enterocytes at the tips of intestinal villi in the small intestine. In the liver, all CYPs showed preferential localisation in the centrilobular hepatocytes.

CONCLUSIONS

Overall, different gene expression profiles were displayed by the CYPs examined in equine intestine and liver. The CYPs present in the intestine may act in concert with those in the liver to affect the oral bioavailability and therapeutic efficiency of substrate drugs. In addition, they may play a role in first-pass metabolism of feed constituents and of herbal supplements used in equine practice.