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 (AUC0-∞) was 48.54 ± 10.46
µg · h/ml, clearance was 103.72 ± 0.06 ml/kg/h, halflife
(t1/2β) was 17.10 ± 0.09 hours, and apparent volume
of distribution was 2.49 ± 0.43 L/kg. For oral
administration, AUC0-∞ was 58.47 ± 16.37 µg · h/ml,
t1/2β was 18.39 ± 0.06 hours, maximum concentration
(Cmax) was 2.12 ± 00.51 µg/ml, time to Cmax 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 AUC0-∞, longer t1/2β, and lower
clearance. Concentration of ciprofloxacin was negligible.
Using a target Cmax 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: