Simultaneous determination of enrofloxacin and its primary metabolite ciprofloxacin in bovine milk and plasma by ion-pairing liquid chromatography.

A simple and sensitive high-performance liquid chromatographic method has been developed for the simultaneous determination of enrofloxacin and ciprofloxacin in bovine milk and plasma. Sample preparation consisted of mixing equal volumes of milk or plasma with acetonitrile-0.1 M sodium hydroxide (1:1, v/v), followed by ultrafiltration through 3000 Da molecular mass cut-off filters. Separation of these two fluoroquinolones in milk or plasma ultrafiltrate was accomplished by ion-pairing liquid chromatography using a reversed-phase analytical column eluted with acetonitrile-methanol-water. Ultraviolet absorbance of the column effluent was monitored over the 230-350 nm range with a photodiode-array detector (lambda max 278 nm). Recoveries of enrofloxacin from bovine milk and plasma were 92-107% and 80-84%, respectively. Recoveries of ciprofloxacin from bovine milk and plasma were 92-105% and 73-75%, respectively. The limit of detection for the two compounds was 5 ng/ml. Enrofloxacin was administered intravenously to a lactating cow at a dose of 2.5 mg/kg. Enrofloxacin was detected in milk within 15 min after injection and the metabolite ciprofloxacin rapidly appeared in plasma and milk. Both enrofloxacin and ciprofloxacin were below the limit of detection (5 ng/ml) by 48 h after drug administration.