INVESTIGATIONS ON THE CHANGES OF CONCENTR ATIONS OF DIFFERENT PHENYLBUTAZONE FORMULA IN HORSE PLASMA

In a study in the horse, the disposition, the pharmacokinetic paramete rs and the absorption rates of 3 formulations of phenylbutazone (injec tion solution, powder and paste suspension) have been determined. Afte r i.v. injection, the half-life time of phenylbutazone has been determ ined to be 6.6-6.7 h. After oral administration, the absorption of phe nylbutazone was found to be faster after administration via stomach tu be than after direct application into the mouth. The absorption rat co nstant 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 dep endent 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 wa s atypical and delayed absorption was found. This may be due to adsorp tion of phenylbutazone on the food with subsequent release in the larg e intestine. The minimal therapeutic level of 5 mu g/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 the ir good clinical results in using Butasan(R)-Paste with 30 ill horses.