Pharmacokinetics of enrofloxacin administered intravenously and orally to foals

Abstract

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-∞) was 48.54 ± 10.46 µg · h/ml, clearance was 103.72 ± 0.06 ml/kg/h, halflife (t_1/2β) was 17.10 ± 0.09 hours, and apparent volume of distribution was 2.49 ± 0.43 L/kg. For oral administration, AUC_0-∞ was 58.47 ± 16.37 µg · h/ml, t_1/2β was 18.39 ± 0.06 hours, maximum concentration (C_max) was 2.12 ± 00.51 µg/ml, time to C_max was 2.20 ± 2.17 hours, mean absorption time was 2.09 ± 0.51 hours, and bioavailability was 42 ± 0.42%. Conclusions and Clinical Relevance—Compared with adult horses given 5 mg of enrofloxacin/kg IV, foals have higher AUC_0-∞, longer t_1/2β, 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. (Am J Vet Res 2000;61: 706–709)