Pharmacokinetics of enrofloxacin administered intravenously and orally to foals

Objective-To determine the pharmacokinetics of enrofloxacin administered IV and orally to foals. Animals-5 clinically normal foals. Procedure-A 2-dose cross-over trial with IV and oral administration was performed. Enrofloxacin was administered once IV (5 mg/kg of body weight) to 1-week-old foals, followed by 1 oral administration (10 mg/kg) after a 7-day washout period. Blood samples were collected for 48 hours after the single dose IV and oral administrations and analyzed for plasma enrofloxacin and ciprofloxacin concentrations by use of high-performance liquid chromatography. Results-For IV administration, mean +/- SD total area under the curve (AUC(0-infinity)) was 48.54 +/- 10.46 mu g.h/ml, clearance was 103.72 +/- 0.06 ml/kg/h, half-life (t(1/2 beta)) was 17.10 +/- 0.09 hours, and apparent volume of distribution was 2.49 +/- 0.43 L/kg. For oral administration, AUC(0-infinity) was 58.47 +/- 16.37 mu g.h/ml, t(1/2 beta) was 18.39 1 0.06 hours, maximum concentration (C-max) was 3.12 +/- 00.51 mu g/ml, time to C-max was 2.20 +/- 2.17 hours, mean absorption time was 2.09 +/- 0.51 hours, and bioavailability was 42 +/- 0.4296. Conclusions and Clinical Relevance-Compared with adult horses given 5 mg of enrofloxacin/kg IV, foals have higher AUC(0-infinity) longer t(1/2 beta) and lower clearance. Concentration of ciprofloxacin was negligible. Using a target C-max to minimum inhibitory concentration ratio of 1:8 to 1:10, computer modeling suggests that 2.5 to 10 mg of enrofloxacin/kg administered every 24 hours would be effective in foals, depending on minimum inhibitory concentration of the pathogen.