Oxidative metabolism of encainide: polymorphism, pharmacokinetics and clinical considerations

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.

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.

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)

An examination of a possible pharmacokinetic interaction between nifedipine and antipyrine

The influence of 2 weeks oral intake of nifedipine (2 x 20 mg) on the oxidative metabolism of antipyrine was investigated in 12 normal volunteers, who had 1050 mg antipyrine solution orally before and after the course of nifedipine. There were no statistically significant differences in the saliva pharmacokinetic parameters of antipyrine on both occassions. However, the metabolite profile of antipyrine in urine showed a significant reduction in the amount of norantipyrine excreted after compared to that before nifedipine administration (16.5 vs 19.6%). This may have implications for drugs that share a similar demethylation pathway with norantipyrine.

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.

The effects of encainide versus diltiazem on the oxidative metabolic pathways of antipyrine

The effects of diltiazem and encainide on the pharmacokinetics and metabolism of antipyrine were compared in nine healthy male volunteers. Diltiazem 90 mg every 8 hours for 5 days decreased the oral clearance of antipyrine from 2.34 to 1.86 L/hour (p less than 0.05) and increased half-life from 12.7 to 15.9 hours (p less than 0.05). Diltiazem reduced the formation rate constants for 3-hydroxymethylantipyrine by 27% (p less than 0.05) and 4-hydroxyantipyrine by 37% (p less than 0.05). There was also a 21% reduction in the formation rate constant for norantipyrine (0.05 less than p less than 0.10). Encainide 25 mg every 8 hours for 5 days had no apparent effect on the oral clearance or half-life of antipyrine, or on the formation rate constants for metabolites of antipyrine. In contrast to a previously published report in rats, encainide, unlike diltiazem, does not inhibit the oxidative metabolism of antipyrine in humans.

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.

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.

Disposition of 3-hydroxyquinidine in patients receiving initial intravenous quinidine gluconate for electrophysiology testing of ventricular tachycardia

The formation rate constant and elimination rate constant for 3-hydroxyquinidine were determined in eight patients with ventricular tachycardia. These two parameters (mean +/- SD) were found to be 0.784 +/- 0.202 and 0.042 +/- 0.058 h-1, respectively. Coefficients of determination for the computer-generated line of best fit for serum concentration-time data were 0.986 +/- 0.008. Patients received two infusions of quinidine gluconate 5 mg/kg over 30 minutes separated by a 20-30 minute electrophysiologic testing period. Unbound and total 3-hydroxyquinidine concentrations were also determined. Among the eight patients, 3-hydroxyquinidine was 61.9 percent bound. Studies in healthy volunteers had shown 50 percent binding. Linear regression of unbound and total 3-hydroxyquinidine was described by the equation Y = 0.3814X-1.448, r = 0.813. Although half-lives of 3.5-12.4 hours had been reported in healthy volunteers, prolonged half-lives were observed in all but two of our arrhythmia patients.

Genetic polymorphism in drug oxidation

Of the two clearly established drug oxidation polymorphisms, only the one referred to as debrisoquine polymorphism affects many drugs. The only known polymorphic substrates of mephenytoin hydroxylase are mephenytoin and mephobarbital. Relatively recently discovered drug substrates of debrisoquine hydroxylase are propafenone, diltiazem, and codeine. The list of substrates contains 28 items. The fate of slightly less than half of these is clinically affected in poor metabolizers, and several of the latter drugs are no longer marketed. There are many reasons why a failure of metabolism may not alter the fate of a drug sufficiently to affect its clinical use. Of interest and clinical importance is the inhibition of debrisoquine hydroxylase by inhibitors such as quinidine and by some neuroleptics; also the simultaneous use of two substrates has led to serious toxicity by mutual metabolic inhibition. The study of these oxidation polymorphisms has been instructive not only for formal pharmacogenetics but also for the understanding of problems of therapy in patients without genetic defects.

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.

Enantioselectivity of 4-hydroxylation in extensive and poor metabolizers of debrisoquine

Debrisoquine (DQ) has no chiral centre, but hydroxylation in position 4 leads to formation of an asymmetric carbon centre with two possible enantiomers, their absolute configuration being R(-) and S(+)-4-hydroxydebrisoquine (4-OHDQ). Since the absolute stereochemistry of the 4-hydroxylation of DQ in man is unknown, the enantioselectivity of this process was studied in panels of extensive (EM) and poor metabolizers (PM) of DQ. In EM subjects 4-hydroxylation of DQ leads almost exclusively to the formation of S(+)-4-OHDQ. In contrast, PM subjects were not only characterized by a decreased total 4-OHDQ formation but also a marked loss of enantioselectivity in product formation. Between 5 to 36% of total 4-OHDQ was excreted as R(-)-4-OHDQ.

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)

Genetic variation in the human hepatic cytochrome P-450 system

Studies in rodents indicate that the cytochrome P-450 system consists of a superfamily of heme proteins, produced by clusters of structural genes on different chromosomes. Equivalent P-450s of different species show more homologies than members of different P-450 families within a species. The Ah receptor serves the induction of members of one of the cytochrome families. The human structural gene for the methylcholanthrene-inducible P1-450 is located on Chromosome 15. This gene has been completely sequenced. The human Ah receptor is also measurable. New methods to measure inducibility in man involve new lymphocyte bioassays and mRNA determinations, while in vivo biotransformation studies of caffeine allow estimates of the state of induction. Structural genes for phenobarbital-inducible cytochromes have been localized to Chromosome 19. The deficiency of biotransformation of debrisoquine and sparteine continues to be explored intensely. Linkage studies indicate the gene for the variable cytochrome P-450 to be located on Chromosome 22. The deficiency is more likely due to structural variation than absence of the cytochrome. Inhibiting drugs can mimic the genetic defect. Many pharmacological and toxicological consequences of the deficiency have been defined. The main characteristics of the genetic deficiencies affecting the metabolisms of mephenytoin, phenytoin, tolbutamide, nifedipine and of methyl cysteine were outlined briefly.

Drug interaction studies and encainide use in renal and hepatic impairment

The effect of encainide administration on steady-state plasma digoxin levels was evaluated in 17 patients receiving stable doses of digoxin. A paired t test, comparing plasma digoxin levels (mean +/- standard error) before encainide therapy (1.05 +/- 0.14 ng/ml) and after 2 weeks of encainide, 100 mg/day (1.03 +/- 0.11 ng/ml) or 200 mg/day (1.2 +/- 0.2 ng/ml), indicates no significant (p greater than 0.05) change in digoxin levels. These results were confirmed in a second study of 10 patients with severe congestive heart failure. Also, no difference in efficacy of either drug was observed and changes in dosing of digoxin were not required. Plasma concentrations of encainide and its 2 major metabolites, O-demethyl encainide (ODE) and 3-methoxy-O-demethyl encainide, significantly increased by 31.6%, 43.1% and 35.6% after concomitant cimetidine administration in 13 healthy adult men receiving 75 mg/day of encainide. However, a retrospective evaluation of 33 patients receiving both drugs did not reveal any clinically significant interactions. Retrospective evaluation of patients enrolled in clinical studies who received concomitant digoxin (268), antiarrhythmics (118), anticoagulants (78), antidiabetics (40), antipsychotics (23), beta blockers (88), calcium-channel blockers (24) or diuretics (229) did not reveal any clinically significant interactions with encainide. Similarly, in vitro protein binding studies did not reveal any clinically significant interactions with encainide or its major metabolites. Six patients with moderate to severe renal impairment (creatinine clearance 10 to 38 ml/min) received 25 mg of encainide, 3 times/day, for 7 doses. Plasma encainide, ODE and 3-methoxy-O-demethyl concentrations were similar to those observed in normal subjects who had received twice the dose of encainide, and steady-state apparent oral clearance of encainide was reduced by 66% with renal impairment. Based on these data it is recommended that in patients with moderate to severe renal impairment encainide be initiated at one-third the normal dose, or 25 mg once a day. Doses may be elevated in small increments at 1-week intervals if needed for efficacy. The effect of hepatic impairment on the pharmacokinetics of encainide was studied in 7 patients with clinically documented cirrhosis. Compared with normal subjects studied using a similar protocol, the plasma concentrations of encainide were elevated significantly due to a 6-fold decrease in oral clearance. However, since plasma concentrations of the active metabolite ODE were correspondingly lower, specific encainide dosing instructions for patients with hepatic impairment are not indicated.