Bioavailability and pharmacokinetics of sulphadiazine, N4-acetylsulphadiazine and trimethoprim following intravenous and intramuscular administration of a sulphadiazine/trimethoprim combination in sheep.

The combination of sulphadiazine and trimethoprim is extensively used in farm animal species; however, there are no data concerning its pharmacokinetics after intramuscular administration in sheep. Twelve rams of the Chios breed were used to study the disposition of sulphadiazine, its metabolite N4-acetylsulphadiazine and trimethoprim after intravenous (i.v.) and intramuscular (i.m.) administration of a sulphadiazine/trimethoprim (5:1) combination in sheep. Sulphadiazine bioavailability (+/-SD) was 69.00%+/-10.51%. The half-life of the terminal phase (4.10+/-0.58 h after i. v., and 4.03+/-0.31 h after i.m. administration) was significantly higher than the respective value for trimethoprim (0.59+/-0.19 h) after i.v. administration. The maintenance of a constant plasma concentration ratio after i.v. administration was therefore impossible. The acetylation capacity in sheep, determined by the AUC ratio between N4-acetylsulphadiazine and the parent compound, sulphadiazine, was very low (less than 4%). The most remarkable finding of this study was that trimethoprim was not detected in sheep plasma after i.m. injection. In conclusion, according to the findings of the present study, following i.v. administration of the sulphadiazine/trimethoprim combination, trimethoprim can be considered as the limiting factor for any possible synergistic effect, and the i.m. route cannot be recommended in sheep.