Pharmacokinetics, N1-glucuronidation and N4-acetylation of sulfamethomidine in humans

Sulfamethomidine metabolism was studied in 6 volunteers. In humans, only N1-glucuronidation and N4-acetylation take place, leading to the final double conjugate N4-acetylsulfamethomidine N1-glucuronide. The N1-glucuronides were directly measured by high pressure liquid chromatography. Fast and slow acetylators show a similar half-life for sulfamethomidine (26 +/- 6 h) and its conjugates sulfamethomidine (26 +/- 6 h) and N4-acetylsulfamethomidine (36 +/- 16 h). Approximately 50-60% of the oral dose of sulfamethomidine is excreted in the urine, leaving 40-50% for excretion into bile and faeces. The main metabolite of sulfamethomidine is its N1-glucuronide, which accounts for 36 +/- 7% of the dose, followed by N4-acetylsulfamethomidine (16 +/- 8%). N1-glucuronidation results in a 75% decrease in protein binding of sulfamethomidine. N4-acetylsulfamethomidine and its N1-glucuronide showed the same high protein binding of 99%. The renal clearance of N4-acetylsulfamethomidine is 7.9 +/- 2.2 ml/min and approximately 20 times as high as that of the parent drug (0.46 +/- 0.16 ml/min). Total body clearance of sulfamethomidine is 4.5 +/- 0.9 ml/min and the volume of distribution in steady state 10.6 +/- 1.7 1. No measurable plasma concentrations of the N1-glucuronides from sulfamethomidine are found in plasma. This may be explained by renal glucuronidation after active tubular reabsorption.