Three phase elimination of oxytetracycline in veal calves; the presence of an extended terminal elimination phase.

The pharmacokinetics of oxytetracycline were studied after both intravenous (i.v.) and intramuscular (i.m.) administration to a group of five veal calves. Blood samples were taken frequently during the terminal elimination phase in order to calculate a reliable elimination rate constant. Because of the low limit of quantification of the method of analysis used, oxytetracycline plasma concentrations could be monitored over a 12-day period of time. After the intravenous administration of oxytetracycline, data were fitted according a three-compartment model. After i.m. administration, plasma-concentration-time curves could best be described by a two-compartment model. It was demonstrated that a very slow terminal elimination phase was present both after i.v. and i.m. administration with a half-life of approximately 95 h. The data show that this phase cannot be explained by slow absorption from the injection site and that release of oxytetracycline incorporated into bone is not a likely explanation.