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Pharmacological particulars
Pharmacotherapeutic group: Antibacterials for systemic use, Combinations of sulphonamides and trimethoprim, incl. derivatives, sulfadiazine and trimethoprim.
ATCvet code: QJ01EW10
Pharmacodynamic properties
Sulfadiazine is a bacteriostatic antibiotic belonging to the sulphonamide group which acts by interference with the synthesis of nucleic acids. Trimethoprim is a reductase inhibitor which also interferes with the synthesis of bacterial nucleic acids. Trimethoprim and sulfadiazine each have a bacteriostatic action, but together they have a synergistic bactericidal effect by intervening in two consecutive steps of the bacterial folate metabolism.
The combination of trimethoprim and sulfadiazine has a broad antibacterial spectrum for both gram positive and gram negative bacteria. 'Chromosomal mutation and plasmid-mediated resistance are described for sulphonamides and its combinations. Resistance is widespread among bacteria isolated from animals reflecting extensive use over time. There is complete cross-resistance between sulphonamides.'
Pharmacokinetic properties
At the recommended dosage for horses of 30 mg of the active substances together (i.e. 25 mg sulfadiazine and 5 mg trimethoprim) per kg body weight mean maximum plasma concentrations obtained after a single dose are about 13 micrograms/ml of sulfadiazine and approximately 1.0 micrograms/ml of trimethoprim after 2.3 and 1.7 hours respectively. The plasma half-life is approximately 7 hours for sulfadiazine and about 3 hours for trimethoprim. Both substances are metabolized in the liver; sulfadiazine by acetylation and glucuronidation and trimethoprim by hydroxylation and glucuronidation. Excretion is primarily by the kidney and only to a lesser extent in the faeces.