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

Background: Enflicoxib 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. Objective: The aim of this work was to explore enflicoxib biotransformation in vitro in humans, rats and dogs, and to determine its metabolic pathways. Method: Different 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. Results: Enflicoxib 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. Conclusion: The 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.