The effects of fentanyl on the minimum alveolar concentration of isoflurane and characterization of cytochrome P450 mediated xenobiotic metabolism in the horse.

摘要

Opioids are potent analgesic agents often used in veterinary medicine for intra and postoperative pain management. When administered with potent inhalation anesthetics, opioids, such as fentanyl, have been shown to reduce the inhalant concentration required for a given depth of anesthesia. The effect on the minimum alveolar concentration (MAC) of inhalation anesthetics, such as isoflurane has been determined for humans and many species of small animals, however a limited range of plasma fentanyl concentrations have been assessed in horses. Due to its potential use for pain management in equine medicine, it is also important to further characterize the pharmacology of fentanyl in the horse. In the horse, the major metabolite of fentanyl is PMA, while in other species, it is norfentanyl. Due to species differences in metabolism and the possible pharmacological and toxicological implications of drugs, such as fentanyl, it is important to characterize the metabolic fate of therapeutic compounds in the individual species of interest. Substantial gaps exist in our knowledge of the metabolic clearance of therapeutic and non-therapeutic agents in horses. The need for more rational approaches in the use of therapeutic agents in this species necessitates additional studies on the spectrum, content and catalytic activities of equine cytochrome P450 monooxygenases (CYP450). In this study, we have begun to characterize drug metabolism in the equine liver, creating a library of recombinant CYP450s for use in vitro , which is capable of providing data on the spectrum of likely metabolites of therapeutic and non-therapeutic drugs. A cDNA library was constructed using mRNA isolated from equine liver tissue. The library was screened for CYP450s using standard molecular biology techniques. Isolated CYP450 genes were submitted to the CYP450 nomenclature committee for name designation. Recombinant enzymes, including CYP2C92 and CYP2D50 were expressed using a baculovirus expression system. These enzymes were then characterized using human isoform selective substrates and metabolic turnover compared to that of the human orthologues. The results of this study demonstrate substantial interspecies variability in metabolism of substrates by CYP2C and CYP2D orthologues in the horse and human and support the need to fully characterize this enzyme system in equids.