Tramadol Steady-State Pharmacokinetics of Immediate-Release Capsules and Sustained-Release Tablets in Dogs

Tramadol is a veterinary analgesic for dogs. In this study, the steady-state pharmacokinetics of a sustained-release (SR) tablet (Tramagetic OD®) and immediate-release capsules (IR) were compared. In a crossover design, six dogs received five doses of IR 50 mg four times a day (qid), or two doses of SR 200 mg once a day (sid). Eight blood samples were collected per dog, per formulation, up to 6 and 24 h after the last dose, respectively. Serum concentrations of tramadol and its metabolites were measured with LC-MS/MS. Metabolite M1 levels were below the lower limit of quantification (LLOQ) in all samples. The non-compartmental analysis of the time–concentration data showed a later Tmax with the SR formulation (median 6.00 h (3.00–9.00)) and a lower Cmax/D (median 7.74 µg/L/mg/kg (0.09–25.3)) compared to the IR formulation (median Tmax 1.75 h (0.75–2.00) and median Cmax/D 11.1 µg/L/mg/kg (4.8–70.4)). AUCtau/D after SR administration was 55.5 h × kg × µg/L/mg (0–174.1) compared to 29.8 h × kg × µg/L/mg (12.2–140.8) after IR administration. The terminal elimination half-lives were 2.38 h (1.77–6.22) and 1.70 h (0.95–2.11) for the SR and IR formulations, respectively. Strong conclusions cannot be drawn from this study because of the high percentage of samples that were below LLOQ and the great interindividual variability, but these results suggest that Tramagetic OD can be administered less frequently in dogs.