Possible contribution of encainide metabolites to the long-term antiarrhythmic efficacy of encainide

Encainide dosing in patients with severe renal dysfunction: report of a case and literature review

Dosage of encainide for patients with lethal ventricular arrhythmias is based on pharmacodynamic effects and efficacy of arrhythmia suppression, coupled with metabolizer phenotype and extent of renal and hepatic dysfunction. Decreased clearance in patients with renal dysfunction necessitates a reduction in dosage to avoid toxic and dose-related proarrhythmic effects. This case represents a patient with severe renal dysfunction and sustained ventricular tachycardia who achieved electrophysiologically guided suppression of induced ventricular tachycardia at a steady-state encainide dose of only 25 mg daily, significantly lower than package insert or compendial recommendations for initial dosage in patients with renal insufficiency. Documented "therapeutic" metabolite concentrations correlated to electrophysiologic response. Literature review illustrates the complexity of encainide dosage in such individuals and underscores the need for therapeutic drug monitoring to individualize dosage.

Pharmacokinetic interactions with calcium channel antagonists (Part II)

Since calcium channel antagonists are a diverse class of drugs frequently administered in combination with other agents, the potential for clinically significant pharmacokinetic drug interactions exists. These interactions occur most frequently via altered hepatic blood flow and impaired hepatic enzyme activity. Part I of the article, which appeared in the previous issue of the Journal, dealt with interactions between calcium antagonists and marker compounds, theophylline, midazolam, lithium, doxorubicin, oral hypoglycaemics and cardiac drugs. Part II examines interactions with cyclosporin, anaesthetics, carbamazepine and cardiovascular agents.

Intravenous 3-methoxy-O-desmethyl-encainide in reentrant supraventricular tachycardia: a randomized double-blind placebo-controlled trial in patients undergoing EP study

Encainide is an agent effective in atrioventricular and atrioventricular nodal reentrant tachycardia. The metabolites O-desmethyl encainide and 3-methoxy-O-desmethyl encainide (MODE) are responsible for the clinical effects of encainide in most patients. In this study, intravenous MODE was evaluated in eight patients with reentrant supraventricular tachycardia undergoing electrophysiological testing. After tachycardia was induced at least twice to ensure reproducibility, MODE (30 micrograms/kg/min x 15 min, then 7.5 micrograms/kg/min) or placebo was administered in a double-blind fashion. If tachycardia remained inducible, the infusion was unblinded; in nonresponding subjects who received placebo, MODE was then administered. Placebo was ineffective in 3/3 patients. MODE prevented tachycardia induction in 5/8 patients and increased the tachycardia cycle length from 302 +/- 38 to 413 +/- 67 msec in the other three. At a mean concentration of 774 +/- 229 ng/ml, MODE prolonged PR, AH, HV, QRS, and QT intervals, right ventricular and accessory pathway effective refractory periods, and slowed or blocked antegrade accessory pathway conduction. Changes in intracardiac conduction were rate independent between cycle lengths 400 to 600 msec, while changes in ventricular effective refractory periods were most pronounced at rapid pacing rates. No adverse effects, hemodynamic changes, or conduction disturbances occurred. Thus, MODE can modify or suppress induction of reentrant atrioventricular or atrioventricular nodal tachycardia. The study design used here is well suited for the evaluation of newer antiarrhythmic agents by electrophysiological testing.

Electrophysiologic and electrocardiographic effects, efficacy and safety of encainide in malignant ventricular arrhythmias associated with coronary artery disease

The electrophysiologic and electrocardiographic effects, efficacy and safety of encainide were evaluated in 48 patients with coronary artery disease undergoing programmed stimulation for ventricular tachyarrhythmias. The study group included 41 men and 7 women, aged 64 +/- 8 years (mean +/- standard deviation), who had presented with nonsustained ventricular tachycardia (VT) (4 patients), sustained VT (32), ventricular fibrillation (8) or unexplained syncope (4). The left ventricular ejection fraction averaged 34 +/- 13%. The arrhythmias induced at the baseline, drug-free electrophysiologic study included nonsustained VT in 8 patients, sustained VT in 35 and ventricular fibrillation in 5. All patients had failed greater than or equal to 1 class IA and a combination of class IA and IB agents (mean 2.2 +/- 1.1 drugs) before encainide. Oral encainide was given in a mean daily dose of 80 +/- 11 mg for greater than or equal to 3 days before repeat programmed stimulation. Encainide was discontinued before follow-up electrophysiologic testing in 5 patients due to spontaneous development of new sustained VT. Of the 43 patients undergoing electropharmacologic testing with encainide, 5 had no inducible arrhythmia. In 9 patients VT was inducible by fewer extrastimuli, in 2 patients a previously stable VT required cardioversion, whereas in 28 patients VT remained inducible by the same number of or more extrastimuli. Thus, encainide prevented the induction of VT or ventricular fibrillation in 5 of 48 patients (10%), while it had a possible proarrhythmic effect in 15 patients (31%). Of the 5 patients without inducible VT administered long-term encainide therapy, 1 returned within 4 weeks of hospital discharge with VT recurrence.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and metabolism of encainide

The metabolism of encainide occurs in the liver and is polymorphically distributed according to the same genetic factor that determines the 4-hydroxylation of debrisoquine. Over 90% of patients are extensive metabolizers (EM) in whom the oral bioavailability of encainide is only 30% because of extensive first-pass metabolism. In EMs, elimination t1/2 is about 2.5 hours, with a systemic clearance of 1.8 l/min. The plasma concentrations of the major metabolites O-desmethyl-encainide (ODE) and 3-methoxy-O-desmethyl-encainide (3-MODE) are higher than those of encainide and have antiarrhythmic activity. The remaining patients (less than 10%) are poor metabolizers (PM), in whom the oral bioavailability is near 88% with an elimination t1/2 of 8-11 hours and a systemic clearance of 0.2 l/min. Encainide plasma concentrations are 10- to 20-fold higher than in EMs, but considerably less ODE and no 3-MODE is formed by the PMs. The conversion to the N-desmethyl-encainide (NDE) metabolite seems to be similar in both metabolizer groups, and plasma protein binding of encainide of 70-78% is also similar. During long-term treatment, the antiarrhythmic metabolites of encainide accumulate in the plasma, so that the relationships between the effect and plasma concentration on encainide, ODE, and 3-MODE are not always obvious. Minimally effective plasma concentrations appear to be approximately 300 ng/ml of encainide, 35 ng/ml of ODE, and 100 ng/ml of 3-MODE. Dose adjustment is necessary in patients with decreased kidney function, but not in patients with cirrhosis, in whom the plasma levels of metabolites appear to be comparable to those in normal subjects.

Encainide-induced encephalopathy in a patient with chronic renal failure

A report of encainide-induced encephalopathy in a patient with chronic renal failure is presented. Drug encephalopathy has been previously reported with various agents, but not with encainide. The patient improved after withdrawal of encainide.

Antiarrhythmic efficacy, clinical electrophysiology, and pharmacokinetics of 3-methoxy-O-desmethyl encainide (MODE) in patients with inducible ventricular tachycardia or fibrillation

In most patients, the clinical effects of therapy with encainide are mediated by the generation of the active metabolites O-desmethyl encainide and 3-methoxy-O-desmethyl encainide (MODE). Data from in vitro and animal studies have indicated that MODE has electrophysiologic and pharmacokinetic features that make its further evaluation desirable; in earlier studies, we found that MODE suppressed chronic high-frequency nonsustained ventricular arrhythmias at plasma concentrations of 50-160 ng/ml. We now report the clinical electrophysiology, antiarrhythmic activity, and pharmacokinetics of MODE in 17 patients with inducible ventricular tachyarrhythmias (VTs) in whom programmed electrical stimulation was performed before drug administration and after one or two sequences of loading and maintenance infusions of MODE. Because the relation between plasma concentration and effect had been incompletely defined, a dose-titration approach was adopted: available pharmacokinetic data were used to construct loading and maintenance infusion regimens that were predicted to attain low plasma concentrations in initial patients while higher infusion rates were evaluated in subsequent patients. MODE prevented VT induction in three of 17 patients and VT cycle length was increased by greater than or equal to 100 msec in a further seven of 17; most responses to MODE occurred at plasma concentrations greater than 556 ng/ml (greater than 1 SD above mean plasma MODE during encainide therapy). Response to MODE did not predict subsequent response to oral therapy with encainide. MODE increased intracardiac conduction times, QT intervals during atrial and ventricular pacing, and right ventricular effective refractory periods (RVERP); changes in RVERP were most prominent at rapid pacing rates, while changes in intracardiac conduction were rate-independent at cycle lengths between 400 and 600 msec. Plasma MODE concentrations measured during electrophysiology study correlated well with those predicted by the pharmacokinetic simulations (r = 0.91, p less than 0.001). Serial plasma sampling after programmed electrical stimulation indicated a minimum MODE elimination half-life of 8.2 +/- 5.4 hours. Side effects were confined to three instances of asymptomatic conduction system depression in subjects with latent conduction system disturbances. We conclude that MODE slows intracardiac conduction, delays repolarization, and can suppress or substantially modify inducible VT. Moreover, it was only with the adoption of the dose-titration strategy that we were able to safely demonstrate that plasma MODE concentrations higher than those routinely observed during encainide therapy were required to substantially alter cardiac electrophysiology.

Sinus node dysfunction and sudden cardiac death following treatment with encainide

Encainide, a class Ic drug, is generally thought of as having little effect on sinus node function. In this article, we present the clinical course and electrophysiological findings of a patient who had cardiac arrest after 1 week of encainide therapy for ventricular extrasystoles. No ventricular tachyarrhythmias were induced during programmed ventricular stimulation (baseline study and while receiving encainide therapy). Prior to encainide therapy, sinus node function was normal, but clinical observations after admission for cardiac arrest and subsequent electrophysiological study revealed that encainide had caused striking impairments in sinus node function. During a 6-month follow-up without antiarrhythmic drug treatment, this patient has had an uneventful course. We concluded that encainide can cause severe and life-threatening sinus node dysfunction.

Resolution of enantiomers of the antiarrhythmic drug encainide and its major metabolites by chiral derivatization and high-performance liquid chromatography

Commercially available chiral columns were unable to provide adequate resolution of enantiomers of the antiarrhythmic drug encainide or its major metabolites. The homochiral derivatizing agent, (-)-menthyl chloroformate, was found to react at the tertiary piperidine nitrogen of racemic encainide providing two menthyl carbamate diastereomers. The individual diastereomers could be separated with baseline resolution on normal-phase high-performance liquid chromatography on a silica column. Structures of the derivatives were confirmed by electron impact mass spectrometry and 1H NMR spectroscopy. The method was adapted for the chiral analysis of the major metabolites of encainide. The limit of sensitivity for racemic encainide was 10 ng on column and it was possible to detect a mixture containing (+)- and (-)-encainide in a ratio of 1:99. Preliminary studies indicated that (-)-encainide was O-demethylated to a greater extent than the (+)-enantiomer by rat liver microsomes.

Comparison of twice daily with thrice daily administered encainide for benign or potentially lethal ventricular arrhythmias

The antiarrhythmic efficacy of encainide administered 2 (group A) or 3 times daily (group B) was evaluated in a randomized, placebo-controlled trial involving 101 patients with benign or potentially malignant ventricular arrhythmias. In group A, encainide was titrated at dosages of 35, 50 and 75 mg twice daily and in group B at dosages of 25, 35 and 50 mg 3 times daily. Drug efficacy, as judged by repeated ambulatory monitoring, was defined as greater than 75% reduction in ventricular premature complexes combined with a greater than 90% abolition of pairs and runs of nonsustained ventricular tachycardia. In group A, 27 of 52 patients (52%) had their arrhythmia suppressed by the drug compared with 34 of 49 (69%) in group B (difference not significant). There was a trend toward better arrhythmia control in group B if a total daily dose of greater than 100 mg was necessary for arrhythmia suppression. Side effects were frequent in both groups (24 vs 28%, difference not significant). Thus, encainide administered twice daily effectively suppresses ventricular arrhythmias in this patient population.

Safety considerations and dosing guidelines for encainide in supraventricular arrhythmias

The safety issues relevant to treatment with encainide in patients with supraventricular arrhythmia were reviewed based on 349 patients enrolled in clinical trials in the United States and Europe. Although 20% of patients had a history of congestive heart failure, cardiomegaly, or cardiomyopathy at entry, there was no case of new or worsened heart failure. There were 5 cases (1.4%) of proarrhythmia in adults, reflecting a worsening of the arrhythmia being treated or of a coexisting ventricular arrhythmia. The profile of drug-related adverse effects was comparable to that previously reported, causing discontinuance in 6% of patients. The effects most often seen were dizziness, visual disturbance, headache, nausea and vertigo. Only 1 patient had clinically significant abnormal laboratory values, possibly reflecting hepatocellular injury in conjunction with viral hepatitis. Most responders received a daily dose of 75 to 200 mg/day, generally given in 3 divided doses. Encainide has a very favorable safety profile for use in the treatment of supraventricular arrhythmias.

Initial dosage selection of antiarrhythmic therapy

The success or failure of antiarrhythmic drug treatment depends, in part, on the selection of the initial dosage. Too low a dosage can lead to unnecessary (and frequently life-endangering) delays in achievement of arrhythmia suppression. Conversely, an excessively high dosage can lead to intolerable toxicity and cessation of treatment. The recommended approach to therapy is to begin with a relatively low dosage, i.e., the lowest dosage with a reasonable chance of producing a favorable response, and titrating the dose upward as needed. Dose titration should be guided by clinical response and, when appropriate, concentrations of the drug and any active metabolites in the plasma. In situations frequently encountered in practice, however, the initial dosage must be modified because of interindividual differences in drug disposition. These changes in drug pharmacokinetics can arise from a variety of factors, including disease processes (e.g., congestive heart failure, cirrhosis and renal failure), concomitant medications (e.g., hepatic enzyme inducers such as phenytoin and inhibitors such as amiodarone), drug formulation, protein binding and inherited drug metabolism capacity. Knowledge of these factors can help the clinician to avoid potential pitfalls in initial dosage selection and can enhance the changes of successful drug treatment of arrhythmias.

The natural history of benign and potentially malignant ventricular arrhythmias with special reference to nonsustained ventricular tachycardia

Ambulatory ECG recordings are routinely used to identify patients at increased risk of sudden cardiac death and to monitor changes in ventricular arrhythmias during antiarrhythmic drug therapy. The arrhythmia frequency established during the initial baseline has previously been reported to change during a second placebo monitoring period in patients with non-life-threatening ventricular arrhythmias, but the extent to which this applies to patients with nonsustained ventricular tachycardia has not been examined. To extend these observations to patients with potentially lethal ventricular arrhythmias, we studied 53 patients enrolled in one of two investigational antiarrhythmic drug trials that introduced a second single-blind placebo period (placebo-pulse) an average of 16 months after successful arrhythmia suppression. Thirty-eight of the 53 patients had runs of nonsustained ventricular tachycardia recorded during the initial baseline (placebo I) period, with 63% averaging greater than or equal to 10 runs per day. There was a marked reduction in the arrhythmia frequencies between the two placebo periods: 55% for ventricular premature beats, and 77% for pairs (p less than 0.001, respectively). Of the 38 patients with nonsustained ventricular tachycardia, there was a 72% reduction (892 +/- 531 vs 245 +/- 18 runs of VT/day, placebo I vs II; p = 0.0001), with 32% having total suppression of nonsustained ventricular tachycardia during the second placebo period. The results of this trial extend our previous observations of long-term spontaneous changes in arrhythmia frequency to patients with symptomatic, potentially lethal ventricular arrhythmia and support the recommendation for periodic reassessment of baseline arrhythmia frequency to determine the continued need for antiarrhythmic therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

New antiarrhythmic drugs

While controversy still exists as to the precise indications for the treatment of all forms of ventricular arrhythmia, advances in the number and, more importantly, type of antiarrhythmic drugs can provide the clinician with a rational basis for selecting antiarrhythmic drug therapy. A host of new agents with different pharmacokinetic and electrophysiological actions are now available, and can be compared or contrasted to conventional antiarrhythmic agents such as quinidine, procainamide, disopyramide, lignocaine (lidocaine) and bretylium. This review summarises the electrophysiological, haemodynamic, pharmacokinetic, and efficacy and safety data of mexiletine, tocainide, flecainide, encainide, propafenone, amiodarone, sotalol, pirmenol, cibenzoline (cifenline) and ethmozine (moracizine, moricizine), and aims to provide a basis on which clinicians can compare and contrast these agents and form an algorithm for selection of antiarrhythmic drug therapy in the treatment of patients with ventricular arrhythmias.

Newer antiarrhythmic drugs

The effective management of cardiac arrhythmias remains a major challenge in cardiovascular therapeutics. The management of arrhythmias encompasses a wide spectrum of supraventricular and ventricular tachyarrhythmias occurring in patients with various cardiac diagnoses and different degrees of myocardial dysfunction. A number of the newer antiarrhythmic drugs that have either recently been released or appear promising are reviewed in this article. Drugs are described with respect to their basic pharmacology, electrophysiologic actions, pharmacokinetics and metabolism, hemodynamics, antiarrhythmic effects, side effects, interactions, indications, and dosage.

Antiarrhythmic activity, electrocardiographic effects and pharmacokinetics of the encainide metabolites O-desmethyl encainide and 3-methoxy-O-desmethyl encainide in man

Although encainide is an effective antiarrhythmic agent, plasma concentrations and pharmacologic effects are not well correlated. One explanation is the generation of active metabolites: while in most patients (extensive metabolizers; EMs) concentrations of the metabolites O-desmethyl encainide (ODE) and 3-methoxy-O-desmethyl encainide (3MODE) are higher than those of encainide, a small subset (poor metabolizers; PMs) lack the ability to extensively biotransform encainide. Considerable data from studies in vitro and animal studies, as well as indirect evidence in patients, indicate that ODE and 3MODE produce the effects seen during long-term encainide therapy in EMs. We now report the initial direct evaluation of the pharmacologic actions of these metabolites of encainide in man. Nine patients with ventricular arrhythmias, seven of the EM phenotype and two of the PM phenotype, were studied. Chronic high-frequency ventricular arrhythmias were suppressed by encainide therapy in seven of nine; monitoring arrhythmia frequency during withdrawal of encainide allowed definition of plasma concentrations of encainide and metabolites associated with arrhythmia suppression. Intravenous infusions of both ODE and 3MODE suppressed chronic ventricular arrhythmias, while infusions of placebo had no effect. ODE clearance was a function of metabolizer phenotype, with higher clearance (mean 914 ml/min; range 554 to 1,314) in EMs than in PMs (434, 298 ml/min); moreover, 3MODE was detected during ODE infusions in all seven EMs but in neither PM. 3MODE clearance was more uniform (mean 289 ml/min in EMs [range 180-410] vs 300 and 78 ml/min in the two PMs) and ODE was not detected in any subject during 3MODE infusion. Encainide itself was not detected after any infusion of ODE or 3MODE. During withdrawal of encainide therapy, ODE plasma concentration at the time of arrhythmia recurrence was 55 +/- 40 ng/ml (mean +/- SD), while ODE by infusion was effective at a concentration of 37 +/- 15 ng/ml. Similarly, plasma concentration of 3MODE at the time of arrhythmia recurrence after withdrawal of chronic encainide was 116 +/- 35 ng/ml and that during 3MODE infusion was 105 +/- 50 ng/ml. While both compounds prolonged QRS duration, ODE was the more potent, increasing QRS by 9.2 +/- 1.6% per 100 ng/ml vs 1.2 +/- 0.5% per 100 ng/ml for 3MODE. On the other hand, 3MODE prolonged the corrected JT interval by 1.9 +/- 0.6% per 100 ng/ml, while ODE shortened it by 2.7 +/- 1.9% per 100 ng/ml.(ABSTRACT TRUNCATED AT 400 WORDS)

Encainide-induced hyperglycemia

Twenty-three patients were treated for at least one month with encainide, a new antiarrhythmic drug. No patient was treated for hyperglycemia prior to encainide therapy. During encainide administration, five episodes of marked hyperglycemia (serum glucose level greater than or equal to 200 mg/dl) developed in four patients. (One patient received encainide twice.) The mean pretreatment glucose level was 190 +/- 69 mg/dl and rose to 397 +/- 163 mg/dl after one month of encainide therapy in patients in whom hyperglycemia developed (p less than 0.025). The glucose level was 111 +/- 27 mg/dl in nonhyperglycemic patients before encainide administration and 108 +/- 22 mg/dl after one month of encainide therapy (p = NS). There was no difference in age or encainide dosage between hyperglycemic and nonhyperglycemic patients. Treatment for hyperglycemia was given during four of the five encainide treatment periods in hyperglycemic patients. Encainide was discontinued in each of the five hyperglycemic episodes; therapeutic requirements for hyperglycemia markedly decreased. Hypoglycemic reactions to insulin occurred in two patients when encainide was stopped. Thus, encainide exacerbates hyperglycemia in some patients. These patients usually have mild hyperglycemia not requiring therapy before administration of encainide but may require insulin while receiving encainide. Treatment requirements for hyperglycemia decrease following withdrawal of encainide. The mechanism of this effect and the consequences of long-term encainide therapy on glucose metabolism are unknown.

Encainide: its electrophysiologic and antiarrhythmic effects, pharmacokinetics, and safety

Encainide is a class IC antiarrhythmic agent that has been under clinical investigation for the last decade. Laboratory and clinical studies have demonstrated it to be a potent suppressor of ventricular extrasystoles. It is effective in approximately one-half of patients with malignant ventricular arrhythmias. The preliminary experience in patients with supraventricular arrhythmias indicates that the drug is particularly effective in arrhythmias associated with an accessory pathway. Side effects most commonly include blurred vision, nausea, heart block, and proarrhythmic effects. The hemodynamic effect of oral encainide are insignificant in patients with well-preserved left ventricular function. Despite minimal myocardial depression in patients with left ventricular dysfunction, there is the potential for worsening of heart failure. Encainide has a short half-life of 3 hours, but has 2 active metabolites with longer half-lives. No clinically significant drug interaction has been demonstrated with encainide therapy.

Pharmacologic causes of arrhythmogenic actions of antiarrhythmic drugs

All currently known antiarrhythmic agents can induce or worsen arrhythmias. Inappropriate dosage selection, mistakenly based on pharmacokinetic data from "normal" subjects, may result in adverse reactions when an antiarrhythmic drug is given to patients. Unexpected variations in drug clearance can increase plasma concentration of antiarrhythmic agents and aggravate arrhythmias. Changes in the rate of drug metabolism by the liver, e.g., due to cessation of alcohol or drugs that induce hepatic metabolism, can reduce drug clearance, making a previously well-tolerated dose toxic. Another possible explanation for adverse drug reactions is nonlinear protein binding. Recently, genetic determinants of drug metabolism have been identified as explanations of interindividual variations in drug responsiveness. Finally, the interactions of antiarrhythmic agents may also lead to aggravation of arrhythmias. A better understanding of the pharmacology of antiarrhythmic agents can reduce, if not prevent, the occurrence of potentially lethal proarrhythmic events.

Disposition kinetics of encainide and metabolites

Interpretation of plasma concentration data during encainide therapy is predicated on an understanding of the role of active metabolites during treatment. In over 90% of patients, encainide is rapidly biotransformed to O-desmethyl encainide (ODE) and 3-methoxy-O-desmethyl encainide (3-MODE), which persist in plasma hours after encainide itself is undetectable. This metabolism occurs in the liver, and encainide clearance is sufficiently high that a significant first-pass effect is seen during oral therapy (bioavailability 30 +/- 7%). In these extensive metabolizers, ODE and 3-MODE appear to mediate the arrhythmia suppression and electrocardiographic changes seen during encainide therapy. In less than 10% of patients, a genetic defect prevents expression of the enzyme responsible for the rapid biotransformation of encainide. In this poor metabolizer subset, the systemic clearance of encainide is 10-fold lower than in extensive metabolizers (0.18 +/- .002 vs 1.9 +/- 0.2 liters/min), the first-pass effect is virtually absent (bioavailability 83% to 88%), plasma concentrations are higher and an antiarrhythmic effect may be seen at usual encainide doses. Minimally effective plasma concentrations appear to be 35 ng/ml (ODE), 100 ng/ml (3-MODE) and 300 ng/ml (encainide), making ODE one of the most potent sodium channel blockers yet used in man. The elimination half-life of encainide is 2.3 +/- 0.3 hours in extensive metabolizer patients. Despite this rapid elimination, encainide can be administered every 8 to 12 hours in both extensive and poor metabolizer subsets; this is because of slowly eliminated metabolites in extensive metabolizers and slower elimination of encainide itself (11.3 +/- 0.3 hours) in poor metabolizers.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of encainide and its metabolites on energy requirements for defibrillation

Encainide, a class IC antiarrhythmic agent, has been associated with proarrhythmic responses of ventricular tachycardia and fibrillation requiring defibrillation in patients. We examined the short-term effects of intravenous encainide and its two major metabolites, O-demethyl-encainide (ODE) and 3-methoxy-ODE (MODE), on the energy requirements for successful defibrillation in 25 pentobarbital-anesthetized, open-chest dogs. Truncated exponential (60% tilt) defibrillation shocks were administered through right atrial spring and left ventricular epicardial patch electrodes identical to those used in man with the automatic implantable defibrillator. At baseline multiple shocks of varying energy were applied to construct curves of percent successful defibrillation as a function of energy (DF curves) for each animal. Encainide, ODE, or MODE was then infused in loading and maintenance doses to achieve QRS widening of 20% to 50%. Saline was administered to animals serving as controls. Determination of the DF curve was repeated, after which the infusion was discontinued. After 1 hr washout period, an additional DF curve was constructed. The data were analyzed by logistic regression, and the energies required for 50% successful defibrillation (E50) were compared. No significant differences existed between the four groups in body or heart weight, extent of QRS widening, or baseline E50 values. After administration of encainide and ODE, the E50 increased by 129 +/- 43% (p less than .001) and 76 +/- 34% (p less than .005), respectively. Return of E50 toward baseline was observed after the washout periods in both groups (p less than .025), demonstrating the reversibility of the drugs' effects.(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.

The changing base line of complex ventricular arrhythmias. A new consideration in assessing long-term antiarrhythmic drug therapy

Initial base-line electrocardiograms are used to assess the efficacy of treatment for ventricular arrhythmias. This approach assumes that in the absence of treatment the frequency of arrhythmia would remain constant. To test the validity of this assumption, we studied 26 clinically stable patients with symptomatic but not life-threatening ventricular arrhythmias, during two periods of placebo treatment separated by a mean of 17 months. As compared with the initial placebo period, there were significant reductions in ventricular premature depolarizations (50 per cent), pairs (65 per cent), and ventricular tachycardia (83 per cent) during the second period of placebo administration (P less than or equal to 0.05 for all comparisons). Over one third of the patients gave the appearance of receiving successful therapy during the second placebo period, even when the reported spontaneous variability of ventricular arrhythmia was taken into consideration. If unrecognized, these long-term spontaneous changes in the frequency of arrhythmia could result in continuation of unnecessary and potentially toxic therapy and lead to incorrect conclusions regarding the efficacy of antiarrhythmic drugs in clinical trials. We therefore recommend that the frequency of arrhythmia be reassessed annually in the absence of treatment in patients similar to those in our study. These recommendations should not be applied to patients with life-threatening ventricular arrhythmias.

Long-term efficacy and safety of oral encainide in the treatment of chronic ventricular ectopic activity: relationship to plasma concentrations--a French multicenter trial

To establish long-term efficacy and safety of encainide, 48 patients with chronic premature ventricular contractions (PVCs) underwent 6 months of therapy with encainide. Twenty-four-hour ambulatory ECGs were obtained at baseline for each daily dosage of 75 mg, 150 mg, and 225 mg of encainide during the in-hospital titration period and at the end of the first and sixth months during the follow-up period. There was a significant reduction in the median hourly total PVC rates from 480.6 at baseline to 2.0 at the end of the titration period with the highest dosage and to 22.1 at the last visit of the chronic dosing period. Nearly total suppression of PVCs was observed in 56% of patients at the end of the titration period and in 30% at the end of the 6-month follow-up period. The most common side effects were vertigo, vision disturbance, and headache. PR, QRS, and QTc intervals showed consistent significant increases from baseline during the various encainide trial periods. Encainide may have worsened ventricular arrhythmia in four patients who received more than 200 mg of encainide daily. Plasma concentrations of encainide and encainide metabolites showed wide interpatient variation, and no relationship was found between antiarrhythmic efficacy and plasma levels of encainide, O-demethyl-encainide, or 3-methoxy-O-demethyl-encainide.

Chronic lorcainide therapy for symptomatic premature ventricular complexes: efficacy, pharmacokinetics and evidence for norlorcainide antiarrhythmic effect

Chronic premature ventricular complexes (PVCs) have been effectively suppressed by oral lorcainide as reported in previous short-term studies. The plasma level-effect relation of lorcainide may be affected by the possible cardioactivity of norlorcainide, a metabolite that accumulates after repeated oral doses. This study evaluated the long-term efficacy of lorcainide in suppressing chronic symptomatic PVCs, and examined the relation of arrhythmia suppression to plasma concentrations of lorcainide and norlorcainide. Fourteen patients were treated with lorcainide, 200 to 400 mg/day, 12 of whom achieved nearly complete suppression of arrhythmias after treatment for 1 year. Chronic lorcainide treatment was well tolerated; no patient discontinued treatment because of adverse effects. Lorcainide and norlorcainide plasma concentrations remained stable after the first week of therapy. Antiarrhythmic activity persisted throughout the year. Upon drug withdrawal, the mean lorcainide washout half-life was 14.3 +/- 3.7 hours and the mean norlorcainide washout half-life was 31.9 +/- 8.9 hours. The return of arrhythmias occurred well after the lorcainide plasma concentration had decreased to subtherapeutic levels, suggesting an antiarrhythmic effect of norlorcainide. Thus, long-term lorcainide therapy is effective in treating chronic symptomatic PVCs and is well tolerated by most patients. The metabolite norlorcainide appears to have antiarrhythmic activity independent of lorcainide.

Clinical pharmacokinetics of the newer antiarrhythmic agents

This article reviews clinical pharmacokinetic data on 8 new antiarrhythmic agents. Some of these drugs have been studied extensively while others are relatively new, with incomplete data due to limited evaluation. Amiodarone is a class III antiarrhythmic drug which is effective in treating many atrial and ventricular arrhythmias that are refractory to other drugs. Amiodarone accumulates extensively in tissues and its disposition characteristics are best described by models with 3 and 4 compartments. Its apparent volume of distribution is very large (1300 to 11,000L) and its elimination half-life very long (53 days). A delay of up to 28 days from of treatment to onset of antiarrhythmic effect may be observed, and the antiarrhythmic effect may persist for weeks to months following cessation of therapy. Clinically significant drug interactions have been observed with amiodarone and warfarin, digoxin, quinidine and procainamide. Encainide is a class Ic antiarrhythmic drug. Although it has a short elimination half-life (1 to 3h), 2 major metabolites with antiarrhythmic effects accumulate in the plasma of patients during long term therapy. Plasma concentrations of O-demethyl encainide appear to correlate with the antiarrhythmic effect. Flecainide, another class Ic antiarrhythmic agent, has an elimination half-life of 14 hours which makes it suitable for twice daily dosing. Flecainide elimination is prolonged in patients with low output heart failure. Significant drug interactions with digoxin and cimetidine have been reported. Lorcainide is also a class Ic antiarrhythmic drug, the bioavailability of which is nonlinear. Clearance of the drug is reduced during long term therapy. A major active metabolite, norlorcainide, accumulates in the plasma of patients during long term therapy and its concentration exceeds that of lorcainide by a factor of 2. The elimination half-lives of lorcainide (9h) and norlorcainide (28h) allow for once or twice daily dosing. Mexiletine, a class Ib antiarrhythmic drug, is structurally similar to lignocaine (lidocaine). A sustained release formulation provides effective plasma concentrations when administered twice daily. The apparent volume of distribution of mexiletine is 5.0 to 6.6 L/kg, and the elimination half-life varies from 6 to 12 hours in normal subjects and from 11 to 17 hours in cardiac patients. Mexilitine is extensively metabolised but the metabolites are not pharmacologically active. Renal elimination of mexiletine is pH dependent. Drugs which induce hepatic metabolism significantly alter the pharmacokinetics of mexiletine.(ABSTRACT TRUNCATED AT 400 WORDS)

Therapy with investigational antiarrhythmic drugs

The investigational antiarrhythmic agents available for use in this country are predominantly class I drugs with local anesthetic membrane effects. These drugs are often used successfully to control arrhythmias refractory to treatment with the standard antiarrhythmic drugs. Side effects often limit their use, and particular attention needs to be paid to their cardiac side effects, such as exacerbation of arrhythmia or enhanced conduction defects.

New directions in antiarrhythmic drug therapy

Cardiac arrhythmia causing sudden cardiac death is a serious worldwide public health problem. Antiarrhythmic agents have been available for therapy, but the conventional agents cause a high degree of intolerable side effects. The recent development of many new experimental antiarrhythmic agents has increased our capacity to effectively treat cardiac arrhythmias. Using a multifaceted approach of programmed electrical stimulation studies, drug level determinations, exercise testing and 24-hour ambulatory Holter monitoring, it can reasonably be decided which patient needs therapy and if therapy is going to be effective. Both aspects of the sudden death equation, ectopy frequency (triggering mechanism) and the ability to propagate sustained ventricular tachycardia (substrate), may be examined. Careful follow-up is needed to determine continued drug efficacy and the presence of side effects that may compromise patient compliance with therapy. If side effects intervene that may cause continued therapy to be intolerable, changing the antiarrhythmic agent, as opposed to decreasing the dosage to an ineffective range, may be appropriate. A comprehensive approach to arrhythmia management may begin to reduce the high incidence of sudden death due to fatal arrhythmias.