Procainamide and N‐acetylprocainamide kinetics investigated simultaneously with stable isotope methodology

Abstract

The pharmacokinetics of procainamide (PA) and N-acetylprocainamide (NAPA) were compared in 3 normal subjects after simultaneous intravenous injection of PA and NAPA-₁₃C. The distribution kinetics of both compounds were modeled with a 3-compartment mamillary system, and it was found that their steady-state distribution volumes were not significantly different, averaging 1.41 Llkg for PA and 1.46 Llkg for NAPA. However, the intercompartmental clearances of NAPA were slower than those of PA. In these normal subjects, the average elimination tl/2 and total elimination clearance for PA were 2.5 hr and 589.8 ml/min, and for NAPA were 6.2 hr and 233.7 mllmin. Mean renal clearances of PA (346.7 ml/min) and of NAPA (199.5 mllmin) exceeded the usual rate of glomerular filtration, which suggests that both compounds are eliminated in part by renal tubular secretion. All subjects were phenotypic rapid acetylators of isoniazid and converted approximately one fourth of the administered PA dose to NAPA-₁₂C. The fate of 15.4% of the administered PA and 14.5% of the administered NAPA-₁₃C was not determined.