Effect of injection site on the bioavailability of an oxytetracycline formulation in ruminant calves.

The oxytetracycline (OTC) disposition was studied in a group of six calves following the administration of an oxytetracycline-10 per cent formulation (i) intravenously (i.v.), (ii) subcutaneously (s.c.) in the lateral neck, and intramuscularly (i.m.) in (iii) the lateral neck, (iv) the shoulder (M. triceps brachii), and (v) the buttock (M. semitendineus). The dose levels used for the intravenous route and other routes were respectively 17.0 +/- 2.3 and 18.3 +/- 1.25 mg OTC/kg. The peak OTC concentrations (Cmax) were achieved with the s.c. and i.m. routes between 4 and 8 hours after injection, the highest being found after application in the shoulder (Cmax:6.9 +/- 0.82 microgram/ml plasma). The Cmax for the s.c. and other i.m. routes in application was similar to each other, ranging from 5.0 to 5.5 micrograms/ml plasma. For different points in time after injection the partial bioavailability was calculated. At 52 h post injection (p.i.) maximal bioavailability was observed for the i.m. shoulder route, viz. 98.1 +/- 7.0 per cent of the administered dose, while at 76 h p.i. similar bioavailabilities were achieved for the i.m. neck and shoulder route, namely 93.3 +/- 8.9 and 99.4 +/- 4.2 per cent, respectively. The lowest bioavailability (83.1 +/- 13.4 per cent) was obtained following the i.m. buttock route at 76 p.i. An obvious irritating effect was observed after s.c. application in the neck an di.m. injection in the buttock, which had disappeared at 5 days p.i. It is assumed that the longer persistence of OTC in plasma resulting with the latter two routes of administration was due to this irritation effect.