Disposition of sulfonamides in food-producing animals IV: Pharmacokinetics of sulfamethazine in cattle following administration of an intravenous dose and three oral dosage forms.

The plasma and urine data obtained following intravenous administration of sulfamethazine to cattle were fit to a one-compartment pharmacokinetic model with a half-life of elimination of 9 hr and a volume of distribution of 0.35 liter/kg. Sulfamethazine was eliminated by excretion of unchanged sulfamethazine (18%) into urine and by formation of three metabolites subsequently excreted into urine. Sulfamethazine also was administered as a solution, a rapid-release bolus, and a sustained-release bolus. The change in the urinary metabolic pattern with different routes of administration suggested that first-pass metabolism was occurring during the absorption process. The absorption half-life was 6 hr. The absorption process for the two solid boluses kinetically appeared to include a dissolution step.