Pharmacokinetics of the bovine formulation of enrofloxacin (Baytril 100) in horses.

Following approval of a concentrated injectable formulation of enrofloxacin for cattle (Baytril 100 Injectable, Bayer Corp. Agricultural Division, Shawnee Mission, KS), equine practitioners have started administering this preparation both parenterally and orally to horses, despite the lack of pharmacokinetic data in this species. Six healthy horses received enrofloxacin at 7.5 mg/kg both orally and intravenously, with the sequence being randomly assigned and at least 1 week of washout allowed between administrations. Blood samples were collected from each horse at various intervals after drug administration to study the pharmacokinetic profile of this product. As concentration-dependent antimicrobials, fluorinated quinolones such as enrofloxacin are most efficacious when the inhibitory quotient is at least 8. In this study, based on inhibitory quotients between 8 and 10, the bovine injectable formulation of enrofloxacin administered to horses intravenously or orally once daily at 7.5 mg/kg may be effective against susceptible organisms whose minimum inhibitory concentration for 90% of the isolates is lower than 0.5 microg/ml.