Clinical pharmacology and antiarrhythmic efficacy of encainide in patients with chronic ventricular arrhythmias

Abstract

We determined the pharmacokinetics, efficacy and therapeutic plasma concentration of encainide, a new antiarrhythmic drug that affects His-Purkinje conduction but not ventricular refractoriness. Nine patients with frequent and complex premature ventricular complexes were studied in a 3-day double-blind protocol. Each day, each patient received 75 mg of i.v. or oral encainide or placebo. Frequent blood samples for encainide plasma concentration determination and continuous ambulatory ECGs were obtained. There was a marked intersubject variation in bioavailability (mean 42 +/- 24%, range 7.4-82%), clearance (13.2 +/- 5.6 ml/min/kg, range 3.75-22.1 ml/min/kg) and half-life (3.4 +/- 1.7 hours i.v., 2.5 +/- 0.8 hours oral). Eight of nine patients had more than 90% suppression of premature ventricular complexes for 3-36 hours. Minimal antiarrhythmic plasma concentration was higher (39 +/- 54 ng/ml, range 3.5-170 ng/ml) after i.v. dosing than after oral dosing (14 +/- 16 ng/ml, range 1.5-48 ng/ml), suggesting an active metabolite after oral dosing in many patients. Minimal side effects were seen despite high peak plasma concentrations (range 794-1556 ng/ml i.v., 36-495 ng/ml oral). The minimal ratio of toxic to therapeutic plasma concentration ranged from 4.3-326 (median 23) after oral dosing. Antiarrhythmic action was associated with an 11-44% widening of the QRS complex that was not associated with other adverse effects. We conclude that encainide effectively suppresses ventricular arrhythmias. Despite a variable bioavailability, high clearance and short half-life, its wide ratio of toxic to therapeutic concentration and probable active metabolite permit a long duration of action, which should allow a reasonable dose schedule in most patients during chronic oral dosing.