Simultaneous determination of enrofloxacin and ciprofloxacin in animal biological fluids by high-performance liquid chromatography. Application in pharmacokinetic studies in pig and rabbit.

A simple and rapid high-performance liquid chromatographic method for the simultaneous determination of enrofloxacin and ciprofloxacin has been developed in pig and rabbit samples. Solid-phase extraction was applied from samples on a C18 cartridge using a mixture of methanol-hydrochloric acid (98:2, v/v). Analytical separation was performed on a C18 column with UV detection at 277 nm under gradient conditions. The mobile phase was a mixture of orthophosphoric acidtriethylamine-acetonitrile. The method has been validated for both molecules in pig and rabbit plasma and adapted for rabbit tissue-cage fluid (TCF). The assay is specific and reproducible within the both drugs and mean recoveries for ciprofloxacin and enrofloxacin, respectively, were 92+/-6% and 90+/-5% for pig plasma over the range used. Mean recoveries for enrofloxacin were 108+/-9% and 102+/-7% for rabbit plasma and TCF, respectively, over the range used. The suitability of the assay for pharmacokinetic studies was determined by measuring enrofloxacin and ciprofloxacin concentrations either in pig plasma after administration of a single intravenous 5 mg/kg dose of enrofloxacin or in rabbit plasma and TCF during a 24 h infusion of enrofloxacin at a rate of 1.25 mg/kg per hour.