The intramuscular bioavailability of a phenylbutazone preparation in the horse

The plasma concentrations of phenylbutazone (PBZ) and its major metabolites, oxyphenbutazone (OPBZ) and gamma-OH-phenylbutazone (OHPBZ) were determined for up to 72 h in six horses, following intravenous (i.v.) and intramuscular (i.m.) administration of 4 g phenylbutazone, 20 ml Phenylarthrite Ventoquinol (Vetoquinol Spécialités Pharmaceutiques Vétérinaires, Magny-Vernois, 70200 Lure, France). After i.v. dosing the plasma disposition was best described by a two-compartment open model. The hydroxylated metabolites OPBZ and OHPBZ were present in detectable concentrations for 72 h and 48 h, respectively. After 36 h the OPBZ concentrations exceeded plasma PBZ concentrations. The plasma disposition following i.m. injection could be described by a one-compartment open model. The hydroxylated metabolites OPBZ and OHPBZ were present in detectable concentrations for 72 h and 36 h, respectively. Only after 72 h was the concentration of OPBZ in plasma higher than the concentration of PBZ. The mean i.m. bioavailability of phenylbutazone was calculated to be 91.7 +/- 10.1%.