Pharmacokinetics and antinociceptive effects of oral tramadol hydrochloride administration in Greyhounds

Objective To determine the pharmacokinetics of tramadol, the active metabolite O-desmethyltramadol, and the metabolites N-desmethyltramadol and N,O-didesmethyltramadol after oral tramadol administration and to determine the antinociceptive effects of the drug in Greyhounds. Animals-6 healthy 2- to 3-year-old Greyhounds (3 male and 3 female), weighing 25.5 to 41.1 kg. Procedures-A mean dose of 9.9 mg of tramadol HCl/kg was administered PO as whole tablets. Blood samples were obtained prior to and at various points after administration to measure plasma concentrations of tramadol and its metabolites via liquid chromatography with mass spectrometry. Antinociceptive effects were determined by measurement of pain-pressure thresholds with a von Frey device. Results-Tramadol was well tolerated, and a significant increase in pain-pressure thresholds was evident 5 and 6 hours after administration. The mean maximum plasma concentrations of tramadol, O-desmethyltramadol, N-desmethyltramadol, and N,O-didesmethyltramadol were 215.7, 5.7, 379.1, and 2372 ng/mL, respectively. The mean area-under-the-curve values for the compounds were 592, 16, 1,536, and 1,013 h.ng/mL, respectively. The terminal half-lives of the compounds were 1.1, 1.4, 2.3, and 3.6 hours, respectively. Tramadol was detected in urine 5 days, but not 7 days, after administration. Conclusions and Clinical Relevance-Oral tramadol administration yielded antinociceptive effects in Greyhounds, but plasma concentrations of tramadol and O-desmethyltramadol were lower than expected. Compared with the approved dose (100 mg, PO) in humans, a mean dose of 9.9 mg/kg, PO resulted in similar tramadol but lower O-desmethyltramadol plasma concentrations in Greyhounds. (Am J Vet Res 2011;72:256-262)