We studied the behavior, fate and metabolism of sulfadiazine (SDZ) a prominent sulfonamide after applying the ¹⁴C-labeled veterinary drug to fattening pigs.
SDZ was applied on 4 consecutive days to pigs according to the recommended dosage of the producer. Daily sampling of the manure started one day after the first application for ten days. Primarily, the excretion kinetic was determined and the metabolites were identified via ¹⁴C-detection and high resolution LC-MS/MS measurement. The main metabolites acetyl-sulfadiazine (AC-SDZ) and 4-hydroxy-sulfadiazine (4-OH-SDZ) and two hitherto unidentified minor metabolites were found.

Simultaneously the application to pigs has been carried out with ¹⁴C-labeled and non-labeled SDZ in order to obtain manure for outdoor experiments. The excretion kinetics of SDZ and its metabolites was monitored both, for the radioactive and non-labeled-SDZ. After 10 days of sampling, more than 96% of the pharmaceutical was excreted by the pigs. In the final composite manure, the pharmaceutical residue consist of SDZ (~44%) and its metabolites (28% 4-OH-SDZ, 24% AC-SDZ, 4% minor metabolites). The results reveal the need for the determination of biotransformation products of veterinary medicines as less than 50% is excreted as parent compound.

The manure obtained after application of SDZ to pigs was distributed to the experimental field site of the Institute. SDZ and its metabolites are rapidly sorbed and/or degraded in soil. The mechanism of the sorption/desorption process was subject to further laboratory investigations. The tendency of SDZ for the establishment of bound residues could be shown.

The results of this study demonstrate that fate and effect of metabolites have to be known. Though the parent compound is generally more potent than its metabolites, the latter may still be biological active and its impact may therefore be underestimated if only the parent compound is considered.