Disposition of Enrofloxacin (Baytril®) into the Udder after Intravenous and Intra‐Arterial Injections into Dairy Cows

Enrofloxacin (Baytril) was injected into arteries supplying the udder of dairy cows. The idea was to avoid primary distribution, metabolism and elimination and thus to deliver the drug to the target organ at higher concentration. Enrofloxacin injected into the abdominal aorta or the external iliac artery resulted in high initial enrofloxacin retention by the udder and high milk concentrations. Injection of enrofloxacin into the abdominal aorta resulted in 2.2 times higher milk peak concentration of the drug than intravenous injection into the jugular vein. Injection of the drug into one of the two external iliac arteries allowed drug concentrations of milk from the udder halves to be compared: when enrofloxacin was injected into the right external iliac artery, the peak milk enrofloxacin concentration from the right udder half was 4.8 times that of the left udder half. The bulk of enrofloxacin was absorbed from the milk compartment of the udder before the next regular milking 6.5 h later. By this time, the metabolite ciprofloxacin had accumulated in milk. Pharmacokinetic values for enrofloxacin and its metabolite ciprofloxacin are given separately for serum and milk whey following three intravascular dosing routes.