Etoricoxib, a selective inhibitor of
cyclooxygenase-2, is used in the treatment of many inflammatory diseases and dental
pain in humans. The aim of this study was to determine the pharmacokinetics and metabolism of
etoricoxib in horses. Six horses weighing an average of 475 ± 25 kg were administered a single oral dose of
etoricoxib at 1 mg/kg
body weight. The results show that the drug reached a maximum concentration of 505.2 ± 67.8 ng/mL in 48 minutes after administration. The elimination half-life was calculated to be 10.20 ± 1.30 hours. Mass spectrometric analysis confirmed that
etoricoxib is metabolized in horses via the oxidation of its 6'-methyl group to form a
hydroxyl methyl
etoricoxib which can further be oxidized to form either an
acid or be glucuronidated. In addition, the 1'-N terminal of 6'-hydroxymethyl metabolite is oxidized to form the corresponding 1'-N
oxide metabolite. The present results have clearly demonstrated that
etoricoxib is mainly excreted in urine as metabolites. From these data, it is also possible to postulate a detection time for the metabolites which in turn can assist in the control of illegal use of the drug in horse racing.