Pharmacokinetics of enrofloxacin and its metabolite ciprofloxacin in freshwater crocodiles (Crocodylus siamensis) after intravenous and intramuscular administration

To the best of the authors' knowledge, pharmacokinetic information to establish suitable therapeutic plans for freshwater crocodiles is limited. Therefore, the purpose of this study was to clarify the pharmacokinetic characteristics of enrofloxacin (ENR) in freshwater crocodiles, Crocodylus siamensis, following single intravenous and intramuscular administration at a dosage of 5 mg/kg body weight (b.w.). Blood samples were collected at assigned times up to 168 hr. The plasma concentrations of ENR and its metabolite ciprofloxacin (CIP) were measured by liquid chromatography tandem-mass spectrometry. The concentrations of ENR and CIP in the plasma were quantified up to 144 hr after both the administrations. The half-life was long (43-44 hr) and similar after both administrations. The absolute i.m. bioavailability was 82.65% and the binding percentage of ENR to plasma protein ranged from 9% to 18% with an average of 10.6%. Percentage of CIP (plasma concentrations) was 15.9% and 19.9% after i.v. and i.m. administration, respectively. Based on the pharmacokinetic data, susceptibility break point and PK-PD indexes, i.m. single administration of ENR at a dosage of 5 mg/kg b.w. might be appropriate for treatment of susceptible bacteria (MIC > 1 μg/mL) in freshwater crocodiles, C. siamensis.