Disposition kinetics of encainide and metabolites

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Impact of food on the bioavailability of encainide

The bioavailability of drugs that undergo extensive presystemic hepatic metabolism may be increased by concomitant ingestion with food. The effect of food on the bioavailability of encainide, a class IC antiarrhythmic agent, was evaluated in 14 healthy subjects in this randomized crossover study. The subjects received encainide 35 mg every 8 hours for 7 days and were randomized to receive their test dose of encainide with food or after an overnight fast. Encainide area-under-the-concentration versus time curve (AUCs) were detectable in 3 of 14 subjects after fasting and in 7 of 14 after feeding. Although food increased the mean encainide AUC by more than threefold, this increase did not reach statistical significance because of the large number of subjects with indeterminate encainide AUCs. Food did significantly increase the AUC of O-demethyl-encainide (ODE), but not the AUC of methoxy-O-demethyl-encainide (MODE). Despite the increase in ODE AUC, no significant effect on the surface electrocardiogram 2 hours after dose administration could be detected. Food may increase the bioavailability of encainide and one of its active metabolites (ODE). The clinical relevance of this pharmacodynamic effect warrants further evaluation.

Encainide overdose in an infant

Ingestion of as little as a single tablet of encainide resulted in life-threatening ventricular arrhythmias in a child. Insertion of an intraosseous line permitted prompt delivery of medications and fluids. Prehospital care providers must be aware that apparently trivial amounts of some adult dosage forms can be toxic to small children.

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.

Phenotypic debrisoquine 4-hydroxylase activity among extensive metabolizers is unrelated to genotype as determined by the Xba-I restriction fragment length polymorphism

1. The major pathway for 4-hydroxylation of debrisoquine in man is polymorphic and under genetic control. More than 90% of subjects (extensive metabolizers, EMs) have active debrisoquine 4-hydroxylase (cytochrome P450IID6) while in the remainder (poor metabolizers, PMs), cytochrome P450IID6 activity is greatly impaired. 2. Within the EM group, cytochrome P450IID6-mediated metabolism of a range of substrates varies widely. Some of this intra-phenotype non-uniformity may be explained by the presence of two subsets of subjects with different genotypes (heterozygotes and homozygotes). 3. Cytochrome P450IID6 substrates have not differentiated between these two genotypes. However, a restriction fragment length polymorphism (RFLP) which identifies mutant alleles of cytochrome P450IID6 locus has been described and can definitively assign genotype in some heterozygous EM subjects. 4. In this study, we used RFLP analysis and encainide as a model substrate to determine if non-uniformity in cytochrome P450IID6 activity among EMs is related to genotype. We tested the hypothesis that heterozygotes exhibit intermediate metabolic activity and that homozygous dominants exhibit the highest activity. We proposed encainide as a useful substrate for this purpose since cytochrome P450IID6 catalyzes not only its biotransformation to O-desmethyl encainide (ODE) but also the subsequent metabolism of ODE to 3-methoxy-O-desmethyl encainide (MODE). 5. A single 50 mg oral dose of encainide was administered to 139 normal volunteers and 14 PMs were identified. Urinary ratios among encainide, ODE and MODE in the remaining 125 EM subjects revealed a wide range of cytochrome P450IID6 activity. However, Southern blotting of genomic DNA digested with XbaI identified obligate heterozygotes in both extremes of all ratio distributions.(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.

Advances in oral anti-arrhythmic therapy: implications for the anaesthetist

Surgical patients often are receiving antiarrhythmic therapy. Thus, because anaesthetic agents can affect cardiac function and may interact with concurrent antiarrhythmic medications, the anaesthetist should be aware of the electrophysiology associated with dysrhythmias and their management. Tocainide, flecainide, mexiletine, encainide and amiodarone have been introduced recently and each has an unique pattern of bioavailability, metabolism and toxicity. Patients treated with these drugs need special concern as they have abnormal cardiovascular systems and may be at increased risk for perioperative morbidity. In addition, unexpected untoward reactions and toxicity can result from interactions of anaesthetic agents and these drugs. This review discusses normal cardiac electrophysiology, common dysrhythmias and the electrophysiological effects of the newer oral antiarrhythmic drugs.

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.

Encainide

Encainide is a class IC antiarrhythmic agent having little or no effect on action-potential duration or maximum diastolic potential but decreasing the maximum rate of phase O depolarization as well as increasing atrial and ventricular effective refractory periods. In intact animals or humans, encainide increases the AH, PR, QRS, and H-V intervals while not affecting the sinus node cycle length or JT interval. QT interval increases only by the concomitant increase in the QRS interval. Encainide is metabolized to O-demethyl encainide (ODE) and 3-methoxy-ODE (MODE), both of which are also antiarrhythmics with similar pharmacology to encainide. Encainide and its metabolites have little negative inotropic activity and ancillary pharmacology. Consequently, encainide has little or no effect on hemodynamic variables in patients with either normal or compromised cardiac function. The drug is well tolerated, with side effects being mainly those associated with its local anesthetic activity such as blurred vision and dizziness. Encainide is particularly effective in patients with excessive premature ventricular complexes (PVCs) and less so in patients with sustained ventricular tachycardia (VT). Like all antiarrhythmics, encainide may aggravate or precipitate new arrhythmias (proarrhythmia). The overall incidence of proarrhythmia is about 10%, with less occurring in patients with PVCs and more in those with sustained VT; also, the incidence of proarrhythmia is higher in patients with underlying heart disease. Encainide is also effective for the treatment of supra-ventricular arrhythmias, including atrial fibrillation, PSVT (both PAF as well as reentry of the nodal or W-P-W type), and ectopic atrial tachycardia. Its dosage and role in antiarrhythmic therapy are discussed.

Electrophysiologic and clinical effects of oral encainide in paroxysmal atrioventricular node reentrant tachycardia

The electrophysiologic effects of oral encainide (75 to 150 mg daily) were evaluated in 14 patients (6 male and 8 female, aged 49 +/- 9 years) with atrioventricular (AV) node reentrant tachycardia of the slow-fast type. The patients were studied in control conditions and after 2 to 12 days of treatment. Encainide increased the AH interval from 67 +/- 10 to 82 +/- 23 ms (p less than 0.02). Anterograde Wenckebach cycle length was increased in three patients, reduced in four, unchanged in one; it was not measurable in the remaining patients because tachycardia was induced. Retrograde Wenckebach periodicity increased from 307 +/- 71 to 401 +/- 92 ms (p less than 0.005) in all nine patients in whom it was measurable; complete retrograde block was observed in one patient. At the control study, tachycardia was induced in all patients, with a mean cycle length of 341 +/- 50 ms; after encainide, tachycardia was inducible in only 1 patient, with an increase in cycle length from 270 to 320 ms; in the other patients, tachycardia was not inducible because of a lack of retrograde (11 patients) or anterograde (2 patients) conduction. The mean plasma concentrations of encainide and its metabolites O-demethyl-encainide and 3-methoxy-O-demethyl-encainide measured in 13 patients during the repeat study were 161 +/- 304, 128 +/- 100 and 95 +/- 85 ng/ml, respectively; three poor metabolizers who presented a high concentration of the parent compound were observed in this series. All patients were discharged on encainide at a mean daily dose of 112 +/- 39 mg.(ABSTRACT TRUNCATED AT 250 WORDS)

New antiarrhythmic agents in pediatrics

This article reviews recent developments in the pharmacologic management of arrhythmias in children and provides specific information about six newer antiarrhythmic agents. With the current increase in recognition, frequency, and complexity of rhythm disturbances in children, pediatricians can expect to encounter children on these antiarrhythmic medications in their practices with increasing frequency.

Previously unreported adverse reaction to encainide

We describe a patient with recurrent atrial fibrillation who suffered a previously unreported adverse reaction to encainide therapy manifested by fever, chills, diaphoresis and myalgia. The patient had a similar response upon rechallenging with encainide, which resolved on discontinuation of therapy.

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.

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.

New antiarrhythmic drugs: tocainide, mexiletine, flecainide, encainide, and amiodarone

Tocainide, mexiletine, flecainide, encainide, and amiodarone are antiarrhythmic agents that have recently been approved by the Food and Drug Administration for general use in the treatment of ventricular arrhythmias. All five agents are effective in the treatment of patients with ventricular arrhythmias, whereas encainide, flecainide, and amiodarone are also useful in patients with supraventricular arrhythmias and the Wolff-Parkinson-White syndrome (although not yet approved for these indications). Tocainide and mexiletine are similar to lidocaine and are as effective as quinidine in patients with ventricular arrhythmias. Encainide and flecainide are superior to quinidine for the control of ventricular ectopic beats and as effective as quinidine for patients with ventricular tachycardia. Amiodarone is the most effective agent available for treating patients with ventricular tachycardia, but it is also the most toxic antiarrhythmic agent and should be used only when other antiarrhythmic drugs have not been effective or tolerated.

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.