Clinical importance of metabolites of antiarrhythmic drugs

Common and uncommon cytochrome P450 reactions related to metabolism and chemical toxicity

Cytochrome P450 (P450) enzymes catalyze a variety of reactions and convert chemicals to potentially reactive products as well as make compounds less toxic. Most of the P450 reactions are oxidations. The majority of these can be rationalized in the context of an FeO(3+) intermediate and odd electron abstraction/rebound mechanisms; however, other iron-oxygen complexes are possible and alternate chemistries can be considered. Another issue regarding P450-catalyzed reactions is the delineation of rate-limiting steps in the catalytic cycle and the contribution to reaction selectivity. In addition to the rather classical oxidations, P450s also catalyze less generally discussed reactions including reduction, desaturation, ester cleavage, ring expansion, ring formation, aldehyde scission, dehydration, ipso attack, one-electron oxidation, coupling reactions, rearrangement of fatty acid and prostaglandin hydroperoxides, and phospholipase activity. Most of these reactions are rationalized in the context of high-valent iron-oxygen intermediates and Fe(2+) reductions, but others are not and may involve acid-base catalysis. Some of these transformations are involved in the bioactivation and detoxication of xenobiotic chemicals.

Relation of blood level and metabolites to the antiarrhythmic effectiveness of encainide

Encainide is a potent new antiarrhythmic agent with 2 major active metabolites and 2 distinct phenotypes for metabolism, extensive (approximately 92%) and nonextensive (8%). Encainide is an active compound with close correlation of plasma levels with antiarrhythmic effectiveness and electrocardiographic changes in nonextensive metabolizers. Its metabolites, O-demethyl-encainide and 3-methoxy-O-demethyl-encainide, are active against experimental and clinical arrhythmias. They have longer half-lives than and equal or greater potency than the parent compound. All 3 compounds contribute to the antiarrhythmic profile in extensive metabolizers. There is no readily apparent relation between encainide and its metabolites, blood levels and efficacy because of the complexity of the 3 active compounds and individual variation in pharmacokinetic and arrhythmia responsiveness. Encainide has been given for up to 2 years in 140 patients with sustained ventricular tachycardia or ventricular fibrillation. The survival curves are similar to historical control data from patients reported by Graboys and Swerdlow. The survival curves for long-term administration in patients with frequent ventricular premature complexes (greater than 30/min) are comparable to data from Califf. While these data must be viewed cautiously, it seems fair to conclude that encainide is as effective as any combination of drugs for preventing sudden death in patients with life-threatening ventricular arrhythmias.

Dose-response range of encainide for benign and potentially lethal ventricular arrhythmias

A multicenter, 2-week, double-blind, placebo-controlled, parallel group study was performed to determine the dose-response relation of encainide administered 3 times daily and to determine its onset of action. To be included in the study, patients with benign or potentially lethal ventricular arrhythmias were required to have an average of at least 30 ventricular premature complexes (VPCs) per hour on 48-hour Holter monitoring after a 48-hour washout period without antiarrhythmic drug treatment. Patients were randomly assigned to receive either placebo or 10, 25 or 50 mg of encainide 3 times daily (tid) for 2 weeks. Of the 125 patients who entered the study, 122 were available for efficacy analysis. Efficacy was determined using 24-hour Holter monitoring on days 1, 7 and 14. There was no difference in frequency of VPCs or of ventricular tachycardia events in the placebo and 10-mg-tid encainide arms. At doses of 25 and 50 mg of tid, encainide was effective in suppressing VPCs and in reducing the number of episodes of ventricular tachycardia. A positive dose-response relation was identified. The onset of effect of encainide was apparent at 3 hours and lasted for 24 hours with tid dosing. No difference in on-therapy conditions were found among the 4 study arms. No patients were discontinued from the study because of electrocardiographic changes. There was no statistically significant change in vital signs or physical examination data. In 1 patient an elevated serum glucose level developed. No symptomatic proarrhythmic events occurred and none required discontinuation of study medication.(ABSTRACT TRUNCATED AT 250 WORDS)

Encainide: a new antiarrhythmic agent

Encainide is classified as a type Ic antiarrhythmic agent. Absorption is essentially complete, but bioavailability is variable because of first-pass metabolism. Two metabolic phenotypes, extensive and poor metabolizers, have been identified. O-demethyl encainide and 3-methoxy-O-demethyl encainide are active metabolites of encainide and contribute significantly to its antiarrhythmic effect. In clinical trials, encainide has been shown to be highly effective in suppressing premature ventricular contractions and ventricular tachyarrhythmias. The drug is useful in treating ventricular arrhythmias refractory to other agents. Encainide is also moderately effective in supraventricular arrhythmias involving an accessory pathway. It is highly effective in cases of Wolff-Parkinson-White syndrome, where the accessory pathway has a short refractory period. Common adverse effects of encainide are dizziness, visual disturbances, nausea, and headache. Encainide appears to be a safe and effective antiarrhythmic agent with few adverse effects and negligible hemodynamic effects. Encainide may be a useful agent for ventricular and supraventricular arrhythmias, particularly those refractory to other agents.