Factors affecting absorption of non-steroidal anti-inflammatory agents in the horse

The absorption of orally administered phenylbutazone (5 mg/kg) was studied in 10 thoroughbreds, eight ponies and four pony foals. Large variations in area under the curve (AUC) and peak plasma concentrations were found both within an animal and within groups of animals. Administration of phenylbutazone (5 mg/kg) following an overnight fast resulted in no difference among the three groups of animals with respect to AUC (0 to 24 hours), mean (+/- sd) values of which were 132 +/- 68, 107 +/- 48 and 98 +/- 6, respectively. Absorption characteristics of two oral phenylbutazone preparations (Equipalazone; Arnolds and Prodynam; Leo Laboratories) (5 mg/kg) given after an overnight fast were similar. Feeding before drug administration decreased AUC and peak plasma concentration and extended the range of the time taken to attain the latter. Repeated twice daily administration of phenylbutazone (5 mg/kg) resulted in more rapid absorption following the morning than the afternoon dose. Investigations with meclofenamic acid (4.4 mg/kg) also resulted in highest peak plasma concentrations and AUC following overnight fast. It was found that absorption was slower with intramuscular injections of phenylbutazone (2.5 mg/kg) than after oral administration in the fasted animal.