Pharmacokinetics, Plasma Protein Binding and Dosage of Oxytetracycline in Cattle and Horses

Abstract

In 5 adult cows and 5 adult horses the disappearance of oxytetracycline (OT) from blood plasma after intravenous injection of 2.5 mg/kg body weight was followed by determining plasma concentrations with a fluorescence assay for OT. The binding of OT to plasma proteins in the 2 species was measured by means of equilibrium dialysis. The results are expressed as the rate constant (k2) and half time (t’2) for elimination of the drug, and binding capacity (β) and dissociation constant of protein-drug-complex (Kβ) for its protein binding. From these figures a set of ideal initial and maintenance doses for differently susceptible micro-organisms and different dosage intervals are calculated, with a safety factor of 5 for cattle and horses. These doses are applicable in the case of intravenous administration and are probably reasonable approximations also for intramuscular injection. It is also shown how these doses must be altered if different dosage intervals and a different safety factor are chosen. It is concluded that the currently recommended doses of OT for horses and cows could well underestimate the amount of OT necessary for the successful treatment of infections due to bacteria of average or low sensitivity.