Pharmacokinetics and urinary excretion following 10 mg/kg-1 intravenous dose and in vitro plasma protein binding of azithromycin were investigated in water buffalo (Bubalus bubalis L.) calves. Plasma concentrations of azithromycin were determined by microbiological assay using Koccoria rhizophia as test organism. The drug levels above MIC were detected up to 36 h in plasma and the disposition followed three compartment open model. Azithromycin was rapidly distributed from central to peripheral compartment as evidenced by short distribution half-life ( 1=0.04 ± 0.002 h) and high values of K12 (6.44 ± 0.30 h-1) and K13 (7.36 ± 0.34 h-1). Large steady state volume of distribution (6.18 ± 0.17 L/kg-1) obtained in this study indicated excellent distribution of azithromycin in this animal. The elimination half-life, AUC and total body clearance were 35.5 ± 1.25 h, 71.9 ± 0.65 g/ml-1/h and 0.14 ± 0.001 L/kg-1/h-1, respectively. About 11.5 ± 2.49 % of administered drug was bound to plasma proteins and approximately 5.64 ± 2.51 % of the administered dose was recovered in urine within 24 h. A convenient intravenous (I.V.) dosage regimen for azithromycin in water buffalo calves would be 14 mg/kg-1 repeated by 11 mg/kg-1 at 72 h intervals.

Key words: azithromycin, pharmacokinetics, protein binding, urinary excretion, water buffalo