Pharmacokinetics of oxytetracycline in goats: modifications induced by a long-acting formulation

The pharmacokinetics of oxytetracycline were studied in goats, after the intravenous and intramuscular injection of a conventional and long-acting formulation. The antibiotic was distributed according to an open two-compartment model. The apparent volume of distribution (Vz) and the central compartment volume (Vc) were 1.443 litres/kg and 0.453 litre/kg, respectively, and the total body clearance was 0.156 litre/kg/hour. The mean half-lives (T1/2 lambda z) of the conventional formulation after intravenous and intramuscular administration were six hours 28 minutes and 10 hours 38 minutes, respectively, whereas the long-acting formulation had half-lives of six hours 36 seconds and 29 hours, respectively, after intravenous and intramuscular injection. From the results of these single administrations two intramuscular dosage regimens can be proposed that achieve minimum concentrations of over 0.5 mg/litre (the minimum inhibitory concentration for most susceptible pathogens): with the conventional formulation by administering an initial dose of 10 mg/kg and a maintenance dose of 8.5 mg/kg every 24 hours, and with the long-acting formulation by administering an initial dose of 20 mg/kg and a maintenance dose of 14 mg/kg every 48 hours.