PHARMACOKINETICS OF ENROFLOXACIN AFTER SINGLE INTRAVENOUS, INTRAMUSCULAR AND SUBCUTANEOUS INJECTIONS IN LACTATING COWS

Five Ayrshire cows were given enrofloxacin (5 mg/kg body weight) intra venously (i,v.), intramuscularly (i.m.) and subcutaneously (s.c.), The antimicrobial activity was measured in milk and serum samples using t he agar-diffusion technique. High-performance liquid chromatography (H PLC) assay was used to study the extent of metabolism of enrofloxacin to ciprofloxacin. Analysis of the serum concentration-time data was ba sed on statistical moment theory, Mean t(1/2 beta) of antimicrobial ac tivity in serum was 1.7, 5.9 and 5.6 h after i.v., i.m. and s.c. admin istration, respectively. Both i.m, and s.c. routes were associated wit h a marked flip-flop phenomenon, Based on HPLC analysis of serum sampl es, the half-lives of enrofloxacin and ciprofloxacin were approximatel y the same. A marked proportion of enrofloxacin was metabolized to cip rofloxacin. The enrofloxacin fraction bound in vitro to serum proteins was 36-45 %. About 0.2% of the total enrofloxacin dose was found in m ilk during the first 24 h and the amount transferred did not depend on the route of administration. Based on the HPLC data, enrofloxacin con centration in milk was parallel to that in serum, while ciprofloxacin was concentrated in milk, After i,v, injection, the peak concentration of enrofloxacin in milk was reached between 0.7 and 1.3 h but occurre d much later for ciprofloxacin (t(max) 5-8 h). After i.m. and s.c. adm inistration the concentration-time curves for both enrofloxacin and ci profloxacin in milk were shallow and there were no obvious peaks.