Disposition of sulfonamides in food-producing animals: pharmacokinetics of sulfamethazine in lambs.

Previously reported plasma and urine concentrations of unchanged sulfamethazine and 3 metabolites following intravenous administration of sodium sulfamethazine to young ewe lamb were fitted to a linear pharmacokinetic model in which sulfamethazine itself obeyed 1-compartment phamacokinetics. The average rate constant for overall elimination of sulfamethazine was 0.096 +/- 0.023 hours-1, corresponding to a biological half-life of 7.2 +/- 1.7 hours. The results of residue analysis of 8 tissues obtained at slaughter showed that tissue and plasma concentrations and urine output of unchanged sulfamethazine fell parallel throughout the experiment. The results indicate that determination of the plasma concentration or urinary output of sulfamethazine can be substituted for tissue residue analysis to determine contamination of carcasses above specified tolerance limits.