Clinical pharmacokinetics of tramadol and main metabolites in horses undergoing orchiectomy

Background: Tramadol is a synthetic codeine analogue used as an analgesic in human and veterinary medicine. It is not approved for use in horses, but could represent a valid tool for pain treatment in this species.Objectives: The serum pharmacokinetic profile and urinary excretion of tramadol and its metabolites (O-desmethyltramadol [M1], N-desmethyltramadol [M2] and N,O-desmethyltramadol [M5]) was investigated in a multidrug anaesthetic and analgesic approach for orchiectomy in horses. The evaluation of the degree of cardiovascular stability, the intraoperative effect and postoperative analgesia obtained by the visual analogue scale are also reported.Animal and methods: Tramadol (4 mg/kg BW) was administered intravenously to eight male yearlings as a bolus over 60 seconds, 5 min after intubation and 15 min prior to surgery. Drug quantification was performed in serum and urine for tramadol, M1, M2 and M5 by high-performance liquid chromatography with fluorimetric detection.Results: Mean tramadol concentra...