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

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 stereoselectivity on the pharmacokinetics and pharmacodynamics of antiarrhythmic drugs

Many antiarrhythmic drugs introduced into the market during the past three decades have a chiral centre in their structure and are marketed as racemates. Most of these agents, including disopyramide, encainide, flecainide, mexiletine, propafenone and tocainide, belong to class I antiarrhythmics, whereas verapamil is a class IV antiarrhythmic agent. Except for encainide and flecainide, there is substantial stereoselectivity in one or more of the pharmacological actions of chiral antiarrhythmics, with the activity of enantiomers differing by as much as 100-fold or more for some of these drugs. The absorption of chiral antiarrhythmics appears to be nonstereoselective. However, their distribution, metabolism and renal excretion usually favour one enantiomer versus the other. In terms of distribution, plasma protein binding is stereoselective for most of these drugs, resulting in up to two-fold differences between the enantiomers in their unbound fractions in plasma and volume of distribution. For disopyramide, stereoselective plasma protein binding is further complicated by nonlinearity in the binding at therapeutic concentrations. Hepatic metabolism plays a significant role in the elimination of these antiarrhythmics, accounting for >90% of the elimination of mexiletine, propafenone and verapamil. Additionally, in most cases, significant stereoselectivity is observed in different pathways of metabolism of these drugs. For some drugs, such as propafenone and verapamil, the stereoselectivity in metabolism is further complicated by nonlinearity in one or more of the metabolic pathways. Further, the metabolism of a number of chiral antiarrhythmics, such as mexiletine, propafenone, encainide and flecainide, cosegregates with debrisoquine/sparteine hydroxylation phenotype. Therefore, it is not surprising that a wide interindividual variability exists in the metabolism of these drugs. Excretion of the unchanged enantiomers in urine is an important pathway for the elimination of disopyramide, flecainide and tocainide. The renal clearances of both disopyramide and flecainide exceed the filtration rate for these drugs, suggesting the involvement of active tubular secretion. However, the stereoselectivity in the renal clearance of these drugs, if any, is minimal. Similarly, there is no stereoselectivity in the renal clearance of tocainide, a drug that undergoes tubular reabsorption in addition to glomerular filtration. Overall, substantial stereoselectivity has been observed in both the pharmacokinetics and pharmacodynamics of chiral antiarrhythmic agents. Because the effects of these drugs are related to their plasma concentrations, this information is of special clinical relevance.

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.

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)

Proarrhythmia, cardiac arrest and death in young patients receiving encainide and flecainide. The Pediatric Electrophysiology Group

The potential for proarrhythmic responses to the class IC sodium channel-blocking drugs encainide and flecainide has not been well described in young patients. Therefore, data were retrospectively collected from 36 institutions regarding 579 young patients who were administered encainide or flecainide for treatment of supraventricular tachycardias (encainide 86 patients, flecainide 369 patients) or ventricular arrhythmias (encainide 21 patients, flecainide 103 patients) to assess the frequency of proarrhythmia, cardiac arrest and death during therapy (adverse events). The two drugs were similar in regard to efficacy (flecainide 71.4%, encainide 59.8%) and rate of proarrhythmic responses (flecainide 7.4%; encainide 7.5%). However, patients receiving encainide more frequently experienced cardiac arrest (encainide 7.5% vs. flecainide 2.3%, p less than 0.05) or died during treatment (encainide 7.5% vs. flecainide 2.1%, p less than 0.05). Detailed data were provided for 44 patients experiencing one or more adverse events. Patient age, previous drug trials, concomitant therapy and days of inpatient monitoring were similar for patients receiving encainide or flecainide. However, echocardiographic left ventricular shortening before treatment was lower among patients receiving encainide (0.23 +/- 0.09) than among those receiving flecainide (0.34 +/- 0.06, p less than 0.05). Plasma drug concentrations were rarely elevated. Cardiac arrest (12 patients) and deaths (13 patients) occurred predominantly among patients with underlying heart disease, particularly among patients receiving flecainide for supraventricular tachycardia (8.3% vs. 0.3%, p less than 0.001). Fifteen patients with an ostensibly normal heart and normal ventricular function experienced proarrhythmia during treatment for supraventricular tachycardia, but only 3 of the 15 had a cardiac arrest or died. The relatively high incidence of adverse events should be considered when contemplating treatment with encainide or flecainide, particularly among patients with underlying heart disease.

Minimal effective concentration values of propafenone and 5-hydroxy-propafenone in acute and chronic therapy

We evaluated the antiarrhythmic efficacy and the minimal effective concentrations of propafenone and its metabolite 5-hydroxy-propafenone during a) acute intravenous infusion (1.5 mg/kg in bolus followed by 45 minutes infusion), b) an acute oral single-dose test (450 mg), and c) 14-day chronic therapy (300 mg tid) followed by a washout. Oxidative metabolism was assessed by a debrisoquine oral test in every patient. Eleven patients with stable ventricular premature beats (VPBs) greater than or equal to 300/hr and Lown class greater than or equal to 3 completed the study. The main results emphasized a certain discrepancy between the clinical effect of the acute intravenous infusion (efficacy in 5 out of 11 patients) and of the acute oral test and chronic therapy (efficacy in 11/11), with a time lag of the ECG changes during the acute intravenous infusion. The minimal effective concentrations were lower after acute oral administration compared with chronic treatment both for propafenone (200 +/- 189 ng/ml vs. 492 +/- 530 ng/ml; p less than 0.05) and for 5-hydroxy-propafenone (82 +/- 40 ng/ml vs. 149 +/- 80 ng/ml; p less than 0.02). A linear correlation was demonstrated between drug/metabolite ratios of propafenone and debrisoquine, either after acute oral (r = 0.91) or after chronic administration (r = 0.84). The pharmacokinetics of propafenone was nonlinear and showed wide interindividual variations.(ABSTRACT TRUNCATED AT 250 WORDS)

Recent antiarrhythmic drugs

Clinical failure of antiarrhythmic drugs often occurs in practice. Therefore, there is a need for new, effective and long-acting drugs with a wide therapeutic range and a low level of toxicity. Most new class I compounds block the fast sodium ion inward current of myocardial cells. According to their effects on the recovery kinetics of the sodium ion channel, these drugs are classified into 3 groups: IA (intermediate--cibenzoline, pirmenol, hydroxy-3-S-dihydroquinidine, quinacainol); IB (fast--tocainide, moricizine); IC (slow--flecainide, encainide, propafenone, lorcainide, indecainide, recainam and penticainide). Class IC drugs greatly depress intracardiac conduction and are the most potent antiarrhythmic compounds able to suppress ventricular premature beats. However, it is doubtful that long-term suppression of ventricular arrhythmias will improve survival of the patients. Some new drugs have been developed belonging to other classes: class II, esmolol, a new ultrashort-acting beta blocker; class III, N-acetyl-procainamide and sotalol, which prolong duration of the action potential and increase ventricular refractoriness; class IV, the mixed sodium ion-calcium ion-potassium ion antagonist, bepridil. The pharmacologic properties and the clinical effects of these new antiarrhythmic drugs are reviewed. However, future therapeutic trends will depend on the results of large multicenter clinical secondary prevention trials such as the Cardiac Arrhythmia Suppression Trial. New antiarrhythmic drugs with original electrophysiologic profiles and minimal adverse effects must prove their ability not only to suppress arrhythmias but also to reduce sudden cardiac death rate.

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.

Depression of maximum rate of depolarization of guinea-pig ventricular action potentials by metabolites of encainide

1. Standard microelectrode methods have been used to record action potentials from guinea-pig ventricular myocardium and dog Purkinje fibres, and to study the effects of the two major metabolites of encainide, O-desmethyl encainide (ODE) and 3-methoxy-O-desmethyl encainide (MODE). 2. In concentrations similar to those found in patients during chronic encainide therapy, neither ODE nor MODE produced significant depression of maximum rate of depolarization (Vmax) of action potentials in unstimulated tissue. Repetitive stimulation, however, was associated with depression of Vmax which increased with increasing driving rates (rate-dependent block, RDB). At the fastest rate studied (interstimulus interval = 300 ms) ODE 1 microM depressed Vmax by 47.5 +/- 5.7% and MODE 1 microM, reduced Vmax by 52.2 +/- 12%. 3. The onset and offset kinetics of this rate-dependent block were very slow. Full development of RDB during a train required over 100 action potentials and the time constants of recovery of Vmax from RDB were 86.4 +/- 37 s for ODE and 100.4 +/- 18 s for MODE. The amount of RDB and its rate of onset increased with drug concentration. The recovery time constants were independent of inter-stimulus interval or drug concentration. Both metabolites also produced rate-dependent depression of conduction velocity in canine Purkinje fibres, but no evidence of selective depression of conduction of interpolated premature potentials was seen. 4. Early afterdepolarizations occurred spontaneously in three preparations in the presence of MODE, 1 microM and one preparation in ODE, 1 microM. 5. It is concluded that these metabolites of encainide may play a role in producing both its antiarrhythmic and its proarrhythmic effects.

Improved high-performance liquid chromatographic assay for encainide and its metabolites in human body fluids

Methods reported previously for the determination of encainide and its metabolites in biological fluids have not been extensively described and evaluated. We report an improved high-performance liquid chromatographic assay for the quantification of these compounds in plasma and urine with complete estimation of the accuracy and reproducibility of the analytical method. The major improvements consist of: (1) the use of ethaverine as an appropriate internal standard; (2) the use of the salting-out technique which improves the extraction recovery for the metabolites of encainide and the sensitivity of the assay; and (3) a shift of the ultraviolet absorption wavelength from 254 to 270 nm to increase the selectivity of the detection.

Role of the electrocardiogram in determining electrophysiologic end points of drug therapy

Electrocardiographic monitoring can be a useful adjunct to antiarrhythmic therapy, since the electrocardiogram is a simple indicator of net cardiac drug effect, irrespective of factors such as pharmacokinetic variability, drug-metabolite interactions or intraindividual variability in drug sensitivity. Changes associated with antiarrhythmic drug therapy include markers of sodium channel block, such as increased QRS or sinus-ectopic coupling intervals and increased QT interval, a marker of action potential prolongation. Electrocardiographic changes can serve 3 purposes: They can correlate with arrhythmia suppression, they may be a guide to impending drug toxicity, and they can indicate the presence of an antiarrhythmic drug at some electrophysiologically active site in the heart. This latter indication may be used as an assessment of compliance, as a clue to drug-drug interactions that may lower antiarrhythmic drug concentrations or preparatory to electrophysiologic testing when it is desirable to avoid testing patients who have no demonstrable drug effect. Drug-induced changes in the microelectrophysiologic environment may sometimes fail to express themselves on the surface electrocardiogram. Overall, however, the electrocardiogram is an inexpensive, readily available tool to monitor net antiarrhythmic drug effects on the heart. Monitoring of the electrocardiogram should, therefore, be an integral part of managing antiarrhythmic drug therapy in patients with arrhythmias.

Role of plasma concentration monitoring in the evaluation of response to antiarrhythmic drugs

Plasma concentration monitoring of antiarrhythmic agents is valuable, but it is often misused or overemphasized in therapeutic decision-making. There are strict requirements for its appropriate use that are often not met--for both the newer and even the conventional antiarrhythmic drugs. For maximum value, there must be a reliable, accurate relation between the plasma drug concentration and drug action, a relation closer than that between dosage and drug action. The time of sample collection is important--most guidelines are based on "trough" plasma concentrations measured after steady-state equilibrium has been achieved. The use of an accurate, sensitive and specific assay is crucial to the value of plasma concentration monitoring guidelines. However, for agents having active metabolites, monitoring the concentration of only the parent drug can be misleading and limits (but does not necessarily eliminate) the value of plasma concentration monitoring guidelines for these agents. Plasma concentration monitoring of most antiarrhythmic agents is of value for certain specific purposes: to determine compliance to antiarrhythmic therapy, to detect and analyze possible drug interactions, to assess the benefit to risk ratio for increasing the dose of a particular antiarrhythmic agent, to maintain a stable drug effect in the presence of a patient's changing clinical condition and, to a limited extent, to assess the role of an agent in causing an adverse drug reaction. The importance of understanding the assay methods currently in use, as well as how plasma concentration monitoring of individual antiarrhythmic agents is affected by the presence of active metabolites, optical isomers differing in their activity and variations in protein binding, is essential in interpreting data obtained from plasma concentration monitoring.