Serum and milk concentrations of several sulphonamides and their N4-acetyl metabolites following oral administration to cows.

Serum and milk concentrations of sulphonamides following oral administration of three commercial sulphonamide products to dairy cows were studied. The fate of the N4-acetyl derivatives of the compounds was also monitored. Sulfadimidine was found to have the slowest oral absorption rate compared to sulfametoxypyridazine, sulfa-phenazole and sulfanilamide. Sulfaphenazole was least absorbed. Excretion of the different sulfonamides into the milk was by passive diffusion and was best for the least ionized sulfanilamide followed by sulfadimidine and sulfamethoxypyridazine. The most ionized sulfaphenazole was not found in milk. Sulfanilamides was found to be readily acetylated (and sulfaphenazole to a lesser degree). N4-acetyl-sulfanilamide seemed to be actively secreted from blood into milk in the cow.