Bioavailability and pharmacokinetics of phenylbutazone in the cow

Abstract

Phenylbutazone was administered orally and intravenously at a dose of 5 mg/kg to healthy cows and to cows positive for bovine leukaemia virus (BLV). Pharmacokinetic parameters and bioavailability were investigated. No differences were seen in the parameters between the healthy and BLV positive animals. The biological half‐lives ranged from 31.4 to 82.1 h after intravenous administration and from 38.6 to 78.2 h after oral administration. The mean value of the apparent volume of distribution was 0.09 litres/kg. Total body clearance ranged from 0.78 to 1.94 ml/kg/h. The average systemic availability was 67.5% with a wide variation among the animals (range 41.9–95.5%). Based on the disposition kinetics and on the therapeutic concentration range suggested in man, an oral loading dose ranging from 10 to 20 mg/kg and a daily maintenance dose ranging from 2.5 to 5 mg/kg can be proposed.