PHARMACOKINETICS OF PHENYLBUTAZONE IN NEONATAL FOALS

Single doses (2.2 mg/kg of body weight) of phenylbutazone (pst) were a dministered IV to 6 neonatal horses (5 to 17 hours old at time of dosi ng). Plasma concentrations of pst and its metabolite oxyphenbutazone w ere monitored serially for 120 hours after drug administration. Pharma cokinetic variables were calculated, using 1- and 2-compartment open m odels. Descriptive equations from the best model for each foal were th en used to derive model-independent variables describing pst dispositi on. Median volume of distribution at steady-state was 0.274 L/kg (rang e, 0.190 to 0.401 L/kg). Median terminal half-life was 7.4 (6.4 to 22. 1) hours, and median total plasma clearance of PBZ for foals in this s tudy was 0.018 L/kg/h (range, 0.013 to 0.038 L/kg/h). Volume of distri bution was larger, half-life was longer, and total clearance was lower , compared with similar values reported for administration of PBZ to a dult horses.