Withdrawal Time of Phenylbutazone Administered to Healthy Horses

Equine medication regulation rules have been established to prevent a horse's performance being altered after the administration of prohibited substances or approved therapeutic medications used for legitimate treatment. Phenylbutazone (PBZ) is a nonsteroidal anti-inflammatory drug widely used in equine medicine and is one of the most frequently identified medications in the anti-doping controls. The aim of this study was to evaluate plasma elimination kinetic of PBZ in healthy horses after both intravenous and oral administration of two commercial products. Phenylbutazone was administered to horses intravenously or orally at a dose rate of 4.4 mg/kg once daily for five consecutive days. Blood (10 mL) samples were collected before the last dose (t0); and then after 1, 3, 6, 9, 12, and 24 hours; and every 12 hours for 10 days after treatment. Plasma levels of PBZ and its major metabolite oxyphenbutazone were measured by high performance liquid chromatography with ultraviolet detector (limit of detection [LOD] = 0.005 μg/mL). The detection time obtained in this study by both intravenous and oral PBZ administration by using a LOD of 0.005 μg/mL was 84 hours. Using the dose regimen reported here, a withdrawal time of 5 days could be a correct approach to be adopted for the use of PBZ in performance horses.