THE INTRAMUSCULAR BIOAVAILABILITY OF A PHENYLBUTAZONE PREPARATION IN THE HORSE

The plasma concentrations of phenylbutazone (PBZ) and its major metabo lites, 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(R) Ventoquinol (Vetoquinol Specialites Pharmaceutiques Veterinaires, Magny-Vernois, 70200 Lure, France). After i.v. dosing th e plasma disposition was best described by a two-compartment open mode l. The hydroxylated metabolites OPBZ and OHPBZ were present in detecta ble concentrations for 72 h and 48 h, respectively. After 36 h the OPB Z concentrations exceeded plasma PBZ concentrations. The plasma dispos ition 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 aft er 72 h was the concentration of OPBZ in plasma higher than the concen tration of PBZ. The mean i.m. bioavailability of phenylbutazone was ca lculated to be 91.7 +/- 10.1%.