Pharmacokinetic interactions between repeated dose phenylbutazone and gentamicin in the horse

This study examined the pharmacokinetics of steady-state phenylbutazone and single bolus intravenous gentamicin when administered together in the horse. The trial design was completed as a cross-over with seven thoroughbred horses. In the first phase each horse received 2.2 mg/kg gentamicin intravenously. After a 2-week washout, each horse received 4.4 mg/kg phenylbutazone intravenously every 24 h for 5 days. On the fourth day each horse received gentamicin as before. Plasma was harvested for gentamicin concentration determination by fluorescence polarization immunoassay and for phenylbutazone concentration determination by high-performance liquid chromatography. All gentamicin data were best approximated by a two-compartment open model using sequential, weighted non-linear regression. Pharmacokinetic parameters were calculated using model-dependent formulae. Phenylbutazone data were analysed by non-compartmental methods. Phenylbutazone induced a 49% increase in the rate of gentamicin return to the central compartment from peripheral tissues (k21) (P < 0.05) and there was a trend to a 24% increase in k12 (P = 0.052). The gentamicin elimination half-life was decreased 23% and the Vd(urea) was reduced by 26%. No induction by gentamicin of changes in phenylbutazone pharmacokinetics were detected. In summary, phenylbutazone induced changes to the rate and extent of distribution and elimination of gentamicin. Therefore, care should be exercised in the use of aminoglycosides in equine patients concurrently maintained on phenylbutazone.