Placental transfer and renal clearance of sulphamethoxazole and its metabolite N4-acetylsulphamethoxazole in a pregnant ewe.

Sulphamethoxazole and its metabolite, N4-acetylsulphamethoxazole, were shown to cross the placenta of a pregnant ewe. The plasma concentration of N4-acetylsulphamethoxazole increased in the foetus due to the limited elimination properties of the immature kidney. The amniotic fluid concentration of sulphamethoxazole and N4-acetylsulphamethoxazole were low, amounting to 2.0 and 0.46 micrograms/ml respectively. Protein binding of the sulphonamides was slightly higher in the foetus than in the ewe.