[The concentration changes of different phenylbutazone formulations in horse plasma]

In a study in the horse, the disposition, the pharmacokinetic parameters and the absorption rates of 3 formulations of phenylbutazone (injection solution, powder and paste suspension) have been determined. After i.v. injection, the half-life time of phenylbutazone has been determined to be 6.6-6.7 h. After oral administration, the absorption of phenylbutazone was found to be faster after administration via stomach tube than after direct application into the mouth. The absorption rat constant of the paste suspension was found to be higher than that of the powder (1.797-2.304 h-1 vs. 0.656-1.197 h-1). The maximal plasma concentrations were attained within 0.5-3 hours with all formulations and were dose-dependent. The elimination of phenylbutazone was not dependent on the dose and formulation applied. The bioavailability of the studied powder and paste formulation was calculated to be 55.1-96.7%. Generally, the bioavailability was higher following administration of the drug via stomach tube. In some horses, the disposition pattern was atypical and delayed absorption was found. This may be due to adsorption of phenylbutazone on the food with subsequent release in the large intestine. The minimal therapeutic level of 5 micrograms/ml was attained 0.2-0.8 hours after administration of the drug and lasted 8.2-22 hours depending on the dose applied. The paste suspension of phenylbutazone may provide a good therapeutic tool for horses, which normally refuse to take powder supplemented food. At the end the authors report on their good clinical results in using Butasan-Paste with 30 ill horses.