Dose-dependent changes in sulfamethazine kinetics in rapid and slow isoniazid acetylators

The pharmacokinetics of sulfamethazine (SMZ) was studied in 44 healthy volunteers. After an oral dose of 10 mg/kg we found the usual distribution of rapid and slow acetylators. The differences in the t1/2s between the phenotypes were not due to variations in renal clearance. Repeat studies at this dose showed very little intraindividual variation in t1/2s. After a 40 mg/kg dose there was an increase in the t1/2 of SMZ in every subject. The implications of these results on methods of phenotyping are discussed.