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Abstract
1. 14C-Sulphamethazine (4-amino-N-(4,6-dimethyl-2-pyrimidinyl)benzene-[U-14C]-sulphonamide; 220 mg/kg of body weight) was given orally or i.v. to lactating dairy cows. Milk collected from 0–48 h after dosing accounted for 2.0% (oral dose) and 1.1% (i.v. dose) of the total 14C-activity administered.
2. Sulphamethazine accounted for 70-79% (oral dose) and 54–75% (i.v. dose) of the total 14C in milk samples collected from 0–48 h after dosing. N4-acetylsulphamethazine accounted for 1–2% (oral dose) and 1–4% (i.v. dose) of the 14C in milk.
3. The major 14C-labelled metabolite in the milk was isolated and identified as the N4-lactose conjugate of sulphamethazine, a unique type of metabolite not previously reported. This metabolite accounted for 10–14% (oral dose) and 9–20% (i.v. dose) of the 14C-activity in the milk collected from 0–48 h after dosing with 14C-sulphamethazine.
4. N4-lactose conjugates of sulphapyridine, sulphamerazine, sulphathiazole, sulphadimethoxine and sulphaquinoxaline were present in the milk from cows orally dosed with these five sulphonamide drugs.