Compared to procainamide in an animal arrhythmic model, the antiarrhythmic potency of the N-acetylated metabolite of procainamide (NAPA) was 92% with respect to dose and 70% with respect to plasma level. The antiarrhythmic effects of combinations of the drugs were additive. Measurements of procainamide and NAPA plasma levels needed to suppress ventricular extrasystoles suggested that both compounds are nearly equipotent in patients as well. The average plasma level required for arrhythmia control in these patients was equivalent to 5.1 mcg/ml procainamide. Since patients on long-term procainamide therapy have plasma concentrations of NAPA that are usually comparable to, and occasionally greater than, their procainamide levels, dose regiments based on procainamide levels alone need revision to include consideration of the levels of this metabolite.

Absorption of a single oral dose of N-acetylprocainamide (NAPA) was studied in 3 normal subjects. Approximately 85 % of the oral dose was absorbed and peak plasma NAPA concentrations were reached in 45 to 90 min. In 2 subjects, NAPA was absorbed at a fast initial rate, then more slowly, prolonging the apparent elimination phase half-life. Absolute bioavailability was determined by a new stable isotope method that entailed intravenous injection of NAPA-13 C at the same time that an unlabeled NAPA capsule was given orally. Plasma levels and urine excretion of both compounds were determined by mass fragmentography. Bioavailability was assessed by deconvoluting the plasma level vs time curves resulting from intravenous and oral drug administration, and also by comparing the relative percentage of NAPA and NAPA-13 C excreted unchanged in the 24 hr after simultaneous administration.

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The pharmacokinetics of procainamide (PA) and N-acetylprocainamide (NAPA) were compared in 3 normal subjects after simultaneous intraveous 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 L/kg for PA and 1.46 L/kg for NAPA. However, the intercompartmental clearances of NAPA were slower than those of PA. In these normal subjects, the average elimination t1/2 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 ml/min. Mean renal clearances of PA (346.7 ml/min) and of NAPA (199.5 ml/min) 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.

Oral administration ofa 1.S-gm dose of N-acetylprocainamide (NAPA) to 9 patients with premature ventricular contractions (PVCs) confirmed previous indirect evidence that this metabolite of procainamide has antiarrhythmic efficacy and potency comparable to those of procainamide. Although the mechanism by which NAPA acts as an antiarrhythmic drug is not known, it was found that the 6 patients with coupled PVCs responded to NAPA therapy and that the 3 patients without coupled PVCs failed to respond. Coupling interval prolongation also occurred during NAP A therapy in 4 of the 6 responding patients. These observations suggest that NAP A may terminate coupled PVCs by slowing and then interrupting conduction of re-entrant impulses, as has been proposed for procainamide. NAPA plasma concentrations of 7.4-17.2 f,Lglml were well tolerated by the patients and produced an average fall of 3 mm Hg in mean arterial pressure and a 7.6% mean increase in corrected QT interval.