Pharmacokinetics, metabolism, and renal clearance of sulfadiazine, sulfamerazine, and sulfamethazine and of their N4-acetyl and hydroxy metabolites in calves and cows.

The effect of molecular structure on the drug disposition and protein binding in plasma and milk, the urinary recovery, and the renal clearance of sulfadiazine, sulfamerazine, and sulfamethazine and of their N4-acetyl and hydroxy derivatives were studied in calves and cows. Sulfadiazine was highly acetylated and was slightly hydroxylated. Sulfamerazine and sulfamethazine were hydroxylated predominantly at the methyl group of the pyrimidine side chain; hydroxylation of the pyrimidine ring itself was more extensive for sulfamethazine than for sulfamerazine. At dosages between 100 and 200 mg/kg of body weight, sulfamethazine had a capacity-limited elimination pattern, which was not observed for sulfadiazine or sulfamerazine. The concentrations of the parent sulfonamide and its metabolites in plasma and milk were parallel, the latter being lower. Metabolite concentrations in milk were at least 8 times lower than those of the parent drug. Metabolism speeds drug elimination, producing compounds with renal clearance values higher than those of the parent drug. The effect on the metabolism and renal clearance of methyl substitution in the pyrimidine side chain is discussed.