A pharmacokinetic study of phenobarbital in mature horses after oral dosing

Ravis, W.R., Duran, S.H., Pedersoli, W.M. & Schumacher, J. A pharmacokinetic study of phenobarbital in mature horses after oral dosing. J. vet. Pharmacol. Therap.10, 283–289. The pharmacokinetics of phenobarbital were determined in six mature horses after a single oral dose. Horses were administered a 5.5 mg/kg of body weight oral dose of phenobarbital tablets. Based on the combined evaluation of i.v. and oral results, phenobarbital displayed two-compartment pharmacokinetics in the horse with a terminal half-life of 19.0 ± 4.4 (mean ± SD) h. This half-life is considerably shorter than those reported for dogs and humans. The steady-state volume of distribution (Vdss/F) and the total body clearance (Cl1/F) of phenobarbital were 0.753 ± 0.115 1/kg and 27.9 ± 9.2 ml/h/kg, respectively. The average extent of oral absorption was 101% with a range of 76 to 124% among the six horses. Examination of the absorption kinetics demonstrated a biphasic absorption process in four horses with a rapid absorption followed by a slower absorption phase. The mean residence time (MRT) was 36.9 ± 4.1 h and the mean residence time for oral absorption (MRTabs) was 11.3 h. Based on the results of the present study, an oral dosing regimen of 11 mg/kg of body weight every 24 h can be recommended.