Pharmacokinetics of a long-acting chloramphenicol formulation administered by intramuscular and subcutaneous routes in cattle.

The pharmacokinetic properties of a long-acting formulation of chloramphenicol were determined in six yearling cattle after a single intravenous (i.v.) administration (40 mg/kg body weight) and after two sequential subcutaneous (s.c.) or intramuscular (i.m.) administrations (90 mg/kg/48 h). The two extravascular routes were studied during a crossover trial for a bioequivalence test. After i.v. administration, the plasma concentration-time graph was characteristic of a two-compartment open model. Mean values were a half-life of 4.1 h, a volume of distribution of 0.86 l/kg and a body clearance of 0.128 l/kg/h. Plasma concentrations of chloramphenicol following i.m. and s.c. administrations increased slowly to a broad peak at 10-15 micrograms/ml between 9 and 12 h. Bioavailability was 19.1% after i.m. injection and 12.4% after s.c. administration. The extent of absorption from the two routes did not differ significantly. The rate of absorption was significantly lower after s.c. application than it was after i.m. injection. The time necessary for the plasma concentration to exceed 5 micrograms/ml was the same for the two routes. Thus, i.m. and s.c. routes are bioequivalent.