Pharmacokinetics of oxytetracycline in young cattle: comparison of conventional vs long-acting formulations
Toutain, P.L.; Raynaud, J.P.
American Journal of Veterinary Research 44(7): 1203-1209
Pharmacokinetics of a long-acting formulation of the antibacterial agent oxytetracycline (OTC) was studied by young cattle after i.v. and i.m. injections (20 mg/kg of body weight). Oxytetracycline tissue concentrations were also determined after i.m. injection. Comparisons with the conventional formulation of OTC (using different administration schedules) were made, using the same total dosage after assessing serum OTC concentrations, areas under the curve and peripheral drug quantities. The i.m. administration of the long-acting formulation was followed by a rapid (60-90 min) achievement of maximal serum concentrations (4 .mu.g/ml) which then persisted for an additional 12 h. Thereafter, serum concentrations decreased; a half-life of 21.83 h was calculated, the length of which was attributed to a combination of continued absorption and elimination. A bioavailability of 51.5% was determined. Two rates of absorption of the drug from the i.m. site of administration were indicated. From analogic simulation, it was apparent that 14% of the dose was rapidly absorbed with a half-life of absorption of 48 min. The remaining 37.5% was absorbed more slowly with a half-life of 18.1 h. Drug concentrations in the tissues decreased exponentially with time. Lung concentrations closely approximated serum values and were maintained at > 0.5 .mu.g/ml for .apprx. 65 h. Higher and more persistent values were found in the kidney and liver tissue. The long-acting formulation after single i.m. administration produced higher quantities in the peripheral compartment in comparison with that after daily administration of the conventional formulation, using 2 dosage schedules, as determined by analogic computation. Evidently, the long-acting formulation was an effective way to administer OTC and removed the need for repeated daily treatments.