Population pharmacokinetics of enrofloxacin and its active metabolite ciprofloxacin in ill cows.

This study was performed in 105 ill cows to determine the best practical individualized dose of enrofloxacin after i.m. (2.5 mg/kg) single-dose administration. Samples were collected from each cow at random time to ensure the percentage of samples distributed equally in the absorption phase, distribution phase, and elimination phase of the drug. Drug concentrations were determined by high-performance liquid chromatography with fluorometric detector, analyzed by population pharmacokinetic (PPK) modeling with NONMEM. The concentration-time data for enrofloxacin in plasma and ciprofloxacin were fitted to the one-compartment model with first-order absorption and elimination. The final covariate model indicated that body weight and daily milk productions have significant influence on clearance (CL) of enrofloxacin and ciprofloxacin, and the volume (V) of distribution of enrofloxacin. The typical PPK parameters were K(a) = 3.33 h(-1), CL = 1.25 L/h/kg, and V = 2.98 L/kg of enrofloxacin, and the interindividual variability for CL and V were 20.2% and 24.3%, respectively, the population mean estimates of K(a), CL, and V for ciprofloxacin were 1.12 h(-1), 2.36 L/h/kg, 8.20 L/kg, respectively, and their interindividual variability was 36.9%, 15.8% and 14.1%, respectively.