Comparative In vitro Metabolism of Enflicoxib in Dogs, Rats, and Humans: Main Metabolites and Proposed Metabolic Pathways.

Abstract

BackgroundEnflicoxib is a non-steroidal anti-inflammatory drug of the coxib family characterized by a long-lasting pharmacological activity that has been attributed to its active metabolite E-6132.ObjectivesThe aim of this work was to explore enflicoxib biotransformation in vitro in humans, rats and dogs, and to determine its metabolic pathways.MethodsDifferent in vitro test systems were used, including hepatocytes and liver and non-hepatic microsomes. The samples were incubated with enflicoxib and/or any of its metabolites at 37°C for different times depending on the test system. The analyses were performed by liquid chromatography coupled with either radioactivity detection or high-resolution mass spectrometry.ResultsEnflicoxib was efficiently metabolized by cytochrome P-450 into three main phase I metabolites: M8, E-6132, and M7. The non-active hydroxy-pyrazoline metabolite M8 accounted for most of the fraction metabolized in all the three species. The active pyrazol metabolite E-6132 showed a slow formation rate, especially in dogs, whereas metabolite M7 was a secondary metabolite formed by oxidation of M8. In hepatocytes, diverse phase II metabolite conjugates were formed, including enflicoxib glucuronide, M8 glucuronide, E-6132 glucuronide, M7 glucuronide, and M7 sulfate. Metabolite E-6132 was most probably eliminated by a unique glucuronidation reaction at a very low rate.ConclusionThe phase I metabolism of enflicoxib was qualitatively very similar among rats, humans and dogs. The low formation and glucuronidation rates of the active enflicoxib metabolite E-6132 in dogs are postulated as key factors underlying the mechanism of its long-lasting pharmacokinetics and enflicoxib's overall sustained efficacy.