The role of genetically determined polymorphic drug metabolism in the beta-blockade produced by propafenone

CYP2D6 mutations and therapeutic outcome in schizophrenic patients

STUDY OBJECTIVE

To investigate whether a relationship exists between the most common known cytochrome P450 (CYP) isozyme 2D6 mutations and schizophrenia. Because most antipsychotic and antidepressant agents interact with CYP2D6, we also investigated clinical outcomes in schizophrenic poor metabolizers (PMs) and extensive metabolizers (EMs).

DESIGN

Prospective, observational study.

SETTING

Two psychiatric hospitals and a university-affiliated nonpsychiatric hospital.

SUBJECTS

Thirty-nine consecutive schizophrenic patients (POP 1), 89 schizophrenics of French Canadian origin (POP 2), and 384 healthy French Canadians (POP 3).

INTERVENTION

All study subjects were genotyped for CYP2D6 mutant alleles. POP 1 patients were evaluated before and after 21 or more days of treatment with antipsychotic drugs metabolized at least in part by CYP2D6.

MEASUREMENTS AND MAIN RESULTS

Whole blood was collected to determine CYP2D6 alleles *1, *3, *4, *5, *6, and *7 using standard restriction fragment length polymorphisms and polymerase chain reaction techniques. In comparison, CYP2D6 genotypes were determined in POP 2 and POP 3. Twenty-three (59.0%) of 39 patients in POP 1 were genotypically EM homozygotes, 15 (38.4%) were EM heterozygotes, and 1 (2.6%) was a PM. Similar genotype distributions were determined in POP 2 and in POP 3. Genotype distributions for all three populations were in Hardy-Weinberg equilibrium (p>0.05), and there was no significant difference among them (p=0.857). In POP 1, no differences were seen among genotypes in disease symptom severity, number and severity of adverse drug effects, or attitudes toward drug treatment at baseline and at the end of the study. In fact, all patients improved significantly during their hospital stay (all p<0.05), although independent of the CYP2D6 genotype.

CONCLUSION

Common CYP2D6 mutant alleles were not associated with schizophrenia or with disease symptoms, antipsychotic-related adverse effects, or attitudes toward treatment.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

Effects of propafenone and 5-hydroxy-propafenone on hKv1.5 channels

1. The goal of this study was to analyse the effects of propafenone and its major metabolite, 5-hydroxy-propafenone, on a human cardiac K+ channel (hKv1.5) stably expressed in Ltk- cells and using the whole-cell configuration of the patch-clamp technique. 2. Propafenone and 5-hydroxy-propafenone inhibited in a concentration-dependent manner the hKv1.5 current with K(D) values of 4.4+/-0.3 microM and 9.2+/-1.6 microM, respectively. 3. Block induced by both drugs was voltage-dependent consistent with a value of electrical distance (referenced to the cytoplasmic side) of 0.17+/-0.55 (n=10) and 0.16+/-0.81 (n=16). 4. The apparent association (k) and dissociation (l) rate constants for propafenone were (8.9+/-0.9) x 10(6) M(-1) s(-1) and 39.5+/-4.2 s(-1), respectively. For 5-hydroxy-propafenone these values averaged (2.3+/-0.3) x 10(6) M(-1) s(-1) and 21.4+/-3.1 s(-1), respectively. 5. Both drugs reduced the tail current amplitude recorded at -40 mV after 250 ms depolarizing pulses to +60 mV, and slowed the deactivation time course resulting in a 'crossover' phenomenon when the tail currents recorded under control conditions and in the presence of each drug were superimposed. 6. Both compounds induced a small but statistically significant use-dependent block when trains of depolarizations at frequencies between 0.5 and 3 Hz were applied. 7. These results indicate that propafenone and its metabolite block hKv1.5 channels in a concentration-, voltage-, time- and use-dependent manner and the concentrations needed to observe these effects are in the therapeutical range.

Survival in a case of life-threatening flecainide overdose

Flecainide acetate is a potent class Ic anti-arrhythmic drug with major sodium channel blocking actions. On the surface electrocardiogram this results in QTc interval prolongation. Overdose with class Ic drugs (< 0.1% of total intoxications) is uncommon, but management is difficult and the mortality high [1]. Serious flecainide overdose is characterised by ventricular tachyarrhythmias, severe bradycardia and variable degrees of atrioventricular block. This report describes a case of life-threatening flecainide overdose in a previously fit individual, resulting in a combination of cardiac disturbances. The treatment options and management are discussed.

Intravenous and oral propafenone for treatment of tachycardia in infants and children: pharmacokinetics and clinical response

To elucidate contribution of an active metabolite to overall clinical responses to propafenone, steady-state disposition of propafenone and its active metabolite and the clinical responses to treatment were examined in pediatric patients receiving intravenous or oral propafenone. There were more than ten-fold interindividual differences in apparent clearance, resulting in a wide range of the steady-state trough plasma concentrations of propafenone. The active metabolite, 5-hydroxypropafenone, was detected in four of the six patients receiving oral propafenone; however, two neonates receiving oral propafenone and all eight receiving intravenous propafenone had no detectable levels of 5-hydroxypropafenone in plasma. In nine patients for whom electrocardiographic (ECG) data were available, the PQ interval was significantly increased, whereas the QRS duration and the QTc interval were not. There was no close relationship between plasma concentrations of propafenone or 5-hydroxypropafenone and ECG parameters. Lack of good correlation between serum concentrations and clinical response precludes using a serum-concentration targeting strategy with propafenone therapy.

Assessment of individual CYP2D6 activity in extensive metabolizers with renal failure: comparison of sparteine and dextromethorphan

OBJECTIVES

To examine whether the variability of CYP2D6 activity in patients with chronic renal failure can be assessed, particularly among subjects with the extensive metabolizer phenotype, by use of standard in vivo indexes of CYP2D6 activity derived from oral administration of dextromethorphan and sparteine.

METHODS

A single 100 mg oral dose of sparteine and a single 40 mg oral dose of dextromethorphan were administered on two occasions to 12 patients with chronic renal failure (creatinine clearance ranging from 20 to 70 ml/min) and 12 age- and sex-matched healthy subjects. Sparteine clearances, sparteine metabolic ratio, and urinary recovery of dextrorphan were calculated. Patients and healthy control subjects were not selected on the basis of their CYP2D6 phenotypes.

RESULTS

Chronic renal failure was associated with a decrease in sparteine partial metabolic clearance to dehydrosparteine (median of 322 ml/min and range of 62 to 670 ml/min in patients with renal failure versus median of 635 ml/min and range of 77 to 1276 ml/min in normal subjects; p < 0.02). Sparteine apparent oral clearance (p < 0.03) and renal clearance (p < 0.001) decreased in patients with renal failure. However, sparteine metabolic ratio was not significantly altered in patients with renal failure and showed that all patients were extensive metabolizers of sparteine. Although fractional urinary excretion of dextrorphan decreased in patients with renal failure (median, 24.4%; range, 9.7% to 55.9%) compared with control (median, 47.5%; range, 24.1% to 72.1%) (p = 0.02), it also showed that all subjects were extensive metabolizers of dextromethorphan. The amount of dextromethorphan excreted in urine correlated with creatinine clearance independently from CYP2D6 activity measured as sparteine partial metabolic clearance. However, it did not correlate with sparteine metabolic ratio or with fractional urinary excretion of dehydrosparteine.

CONCLUSION

Assessment of CYP2D6 activity by use of dextromethorphan and sparteine is possible in extensive metabolizer patients with chronic renal failure. However, in these subjects, dextromethorphan and sparteine do not reflect CYP2D6 activity in the same way.

Propafenone: an effective agent for the management of supraventricular arrhythmias

Propafenone is a sodium channel blocking antiarrhythmic drug. It also has beta-adrenergic, potassium channel, and weak calcium channel blocking activity. The drug is metabolized in the liver with rates dependent on the debrisoquin phenotype. The saturable metabolism results in nonlinear pharmacokinetics. The metabolites retain sodium channel blocking activity but little beta-adrenergic blocking activity. Both controlled and noncontrolled studies have documented its efficacy in a variety of supraventricular arrhythmias. Intravenous propafenone is effective in converting atrial fibrillation to normal sinus rhythm. Chronic oral administration decreases the frequency of recurrence of atrial fibrillation and paroxysmal supraventricular tachycardia. The drug is particularly effective in the Wolff-Parkinson-White syndrome. The drug may produce SA block in patients with underlying sinus node dysfunction. Propafenone has comparatively few noncardiac side effects. It is a useful primary drug or an alternative to more commonly used drugs used for the treatment of supraventricular arrhythmias.

Propafenone in a usual dose produces severe side-effects: the impact of genetically determined metabolic status on drug therapy

We report the case of an elderly lady presenting with dizziness, a head injury resulting from a fall and bradycardia. Propafenone 150 mg t.i.d. had been prescribed for atrial fibrillation with tachyarrhythmia, induced by hyperthyroidism, 18 months earlier. A toxic concentration of parent propafenone, and no 5-hydroxy metabolite, was detected in a plasma sample. Symptoms disappeared after the discontinuation of propafenone. The poor metaboliser (PM) phenotype of sparteine/debrisoquine was assumed and subsequently confirmed by phenotyping (sparteine test) and genotyping (allele-specific polymerase chain reaction). The PM phenotype is common in European populations, with a prevalence of about 7%. If drugs with narrow therapeutic ranges undergo genetically polymorphic metabolism, toxicity may arise even with recommended doses. Individualization of doses is required to avoid adverse effects.

Propafenone overdose

Propafenone is an infrequently used class IC antiarrhythmic drug. We report our experience with a patient who overdosed with propafenone and developed coma, seizures, bradycardia, hypotension, and conduction delay. The clinical manifestations and management of this patient are discussed in light of the known pharmacology of propafenone and compared with the limited number of cases that appear in the literature.

An enantiomer-enantiomer interaction of (S)- and (R)-propafenone modifies the effect of racemic drug therapy

BACKGROUND

Therapy with racemic compounds produces effects that can be attributed to both (S)- and (R)-enantiomers. Here we have tested the hypothesis that an enantiomer-enantiomer interaction would modulate the effects of treatment with a racemate, the antiarrhythmic propafenone. Previous studies have shown that while the enantiomers of propafenone exert similar sodium channel-blocking (QRS widening) effects, it is the (S)-enantiomer that produces beta-blockade; moreover, we have demonstrated recently that (R)-propafenone inhibits the metabolism of (S)-propafenone in vitro.

METHODS AND RESULTS

This single-blind, randomized study compared the effects of (R/S)-, (S)-, (R)-propafenone (150 mg q 6 hours for 4 days) and placebo on QRS duration (delta QRS) and on maximum exercise heart rate (delta HRmax), an index of beta-blockade. The clearance of (S)-propafenone was significantly lower (-55 +/- 24%, P < .001) during treatment with (R/S)-propafenone than with the (S)-enantiomer alone, and delta HRmax was significantly altered during (R/S)-propafenone (-8.8 +/- 6.6 beats per minute; P < .01) and during (S)-propafenone (-4.3 +/- 4.8 beats per minute; P < .01) but not during (R)-propafenone (-1.8 +/- 6.4 beats per minute) or placebo (0.3 +/- 7.1 beats per minute). In contrast, (R/S)-, (S)-, and (R)-propafenone all prolonged QRS compared with placebo.

CONCLUSIONS

These data indicate that (R)-propafenone impairs the disposition of (S)-propafenone in humans. As a result, the beta-blocking effects of 150 mg of racemic propafenone (75 mg of the [S]-enantiomer) were more pronounced than those of 150 mg of (S)-propafenone alone. Thus, the effects of racemic drug therapy are not necessarily those predicted by summation of the effects of the individual enantiomers.

Usefulness of propafenone for supraventricular arrhythmias in infants and children

The relation between propafenone dose, serum level, electrocardiographic parameters, antiarrhythmic drug efficacy and adverse effects was studied in 47 children with symptomatic supraventricular arrhythmias aged 1 day to 10.3 years (median 2.2 months) with a mean follow-up of 14.3 months. Propafenone trough serum levels were measured using gas chromatography. Oral propafenone (mean dose 353 mg/m2/day) was effective in 41 of the 47 patients (87.2%). Serum levels did not differ between patients responding and not responding to propafenone (0.45 +/- 0.40 vs 0.36 +/- 0.41 mg/liter). PR interval and QRS complex duration increased more significantly with propafenone dose increments (p < 0.001), than with propafenone serum levels (p < 0.05). At successful treatment PR interval and QRS complex were prolonged by a mean of 19.2 and 20.5% compared with pretreatment status. Five patients exhibited unexpected marked QRS complex prolongation (50 to 200%) despite low propafenone dosage (< 300 mg/m2/day) and level ranging from 0.05 to 1.33 mg/liter. Three patients (6.1%) were suspected of being "poor" metabolizers of propafenone. Mild chronic elevation of serum liver enzymes was observed in 5 patients treated with a larger dose (mean 448 mg/m2/day, p < 0.001). No proarrhythmia was noted on serial Holter monitors. One patient with Wolff-Parkinson-White syndrome and a normal heart had cardiac arrest after aspiration. Serial monitoring of PR interval and QRS complex duration was more useful for proper dosage adjustment than propafenone serum levels. Serum liver enzymes should be closely monitored when using higher propafenone doses. Malignant proarrhythmia could not be excluded in the 1 patient with cardiac arrest.

Clinical significance of genetic influences on cardiovascular drug metabolism

Inherited differences in metabolism may be responsible for individual variability in the efficacy of drugs and the occurrence of adverse drug reactions. Among the cardiovascular drugs reported to exhibit genetic polymorphism are debrisoquine, sparteine, some beta-adrenoceptor antagonists, flecainide, encainide, propafenone, nifedipine, procainamide, and hydralazine. The implications of genetic differences in the metabolism of these drugs for cardiovascular therapeutics is the subject of this review.

Genetic basis for a lower prevalence of deficient CYP2D6 oxidative drug metabolism phenotypes in black Americans

Debrisoquin hydroxylase (CYP2D6) is a cytochrome P450 enzyme that catalyzes the metabolism of > 30 commonly prescribed medications. Deficiency in CYP2D6 activity, inherited as an autosomal recessive trait, was found to be significantly less common in American blacks (1.9%) than whites (7.7%). To determine the genetic basis for this difference, inactivating CYP2D6 mutations were assessed by allele-specific PCR amplification and RFLP analyses of genomic DNA from 126 unrelated whites and 127 unrelated blacks. Blacks had a twofold lower frequency (8.5 versus 23%, P = 0.001) of the CYP2D6(B) mutation (point mutation at intron 3/exon 4 splice site), while complete deletion of the CYP2D6 gene (5.5% blacks, 2.4% whites), and the CYP2D6(A) mutation (single nucleotide deletion in exon 5; 0.24% blacks, 1.4% whites) were not different between the two groups. The prevalence of heterozygous genotypes was significantly lower in blacks (25 versus 42% of extensive metabolizers, P = 0.009), consistent with the observed prevalence of the deficient trait in blacks and whites. We conclude that the same CYP2D6 mutations lead to a loss of functional expression in blacks and whites, but American blacks have a lower prevalence of the deficient trait due to a lower frequency of the CYP2D6(B) mutation. This could explain racial differences in drug effects and disease risk.

Genetically Determined Polymorphisms in Drug Metabolism

Many factors can influence the metabolism and disposition of drugs. Genetically determined differences in an individual's capacity to metabolize drugs are known causes of interindividual and interethnic variabilities in drug disposition and response. In general, a poor metabolizer for a specific metabolic pathway would likely develop adverse effects, and an extensive metabolizer for the same metabolic pathway might have less than optimal response. Although there are different types of polymorphism in drug metabolism, polymorphisms in debrisoquine-type oxidation, S-mephenytoin oxidation, and N-acetylation have been the most extensively studied. This article will present the basic concepts of pharmacogenetics, review the major types of metabolic polymorphisms, outline ways to determine phenotyping and genotyping differences in metabolizing enzyme activities, and discuss how these differences relate to drug metabolism, response, and toxicity. When evaluating drug response and adverse reactions in individual patients, an awareness of genetic differences in metabolic capacities would help contribute to optimization in drug therapy.

Reversible protective effect of propafenone or flecainide during atrial fibrillation in patients with an accessory atrioventricular connection

In 34 patients with a symptomatic accessory atrioventricular connection the reversible protective effect of orally administered flecainide (300 mg/day) and of propafenone (900 mg/day) in control of ventricular response during atrial fibrillation by exercise was assessed. The study consisted of three sections of 1 week each: an initial treatment phase during which propafenone or flecainide was administered, a drug-free phase, and a period of crossover to treatment with the other drug. At the end of each phase, transesophageal stimulation was performed during physical exercise to induce atrial fibrillation episodes: the goal was to control the persistence of drug effectiveness. At rest, the mean and shortest R-R interval during the period of induced atrial fibrillation in patients who were treated with flecainide or propafenone increased significantly as compared with the drug-free period. On the other hand, at maximum exercise levels no difference in both shortest and mean R-R intervals during atrial fibrillation was observed between patients who were treated with flecainide and those who were treated with propafenone, as well as between flecainide treatment and the drug-free period, whereas a slightly significant difference persisted with propafenone treatment (p less than 0.05). In addition, at maximum exercise levels no significant difference in the number of preexcited QRS complexes among the three treatments was noted. The data from this study suggests that a reversible protective effect against rapid ventricular rate as the result of an episode of atrial fibrillation exists during exercise in patients with a symptomatic accessory atrioventricular connection who are treated with flecainide or propafenone.

Beta-blocking effect of propafenone based on spectral analysis of heart rate variability

RR variability was analyzed in 15 patients with ventricular arrhythmias to evaluate whether the antiarrhythmic action of propafenone is associated with alteration of neural control mechanisms. Before drug administration, spectral analysis of RR variability was characterized by 2 major components at low and high frequency, which are considered to reflect sympathetic and parasympathetic modulation of the heart period. After propafenone (600 to 900 mg/day), there was a marked reduction in RR variance (826 +/- 184 to 412 +/- 77 ms2; p < 0.05), although the mean RR interval was unchanged. The drug significantly reduced the low-frequency component (52 +/- 6 to 28 +/- 4 nu) and augmented the high-frequency component (39 +/- 6 to 55 +/- 5 nu). As a result, the low-/high-frequency ratio (an index of sympathovagal balance) decreased from 2.0 +/- 0.4 to 0.6 +/- 0.1. A positive correlation between serum levels and drug-induced changes in the low-frequency component was also observed. Furthermore, the increase in the low-frequency component induced by tilt (53 +/- 5 to 79 +/- 3 nu) was markedly attenuated after drug administration (27 +/- 5 to 54 +/- 7 nu). Thus, propafenone administration is associated with changes in spectral components that are consistent with a beta-blocking effect of the drug.

Stereoselective genetically-determined interaction between chronic flecainide and quinidine in patients with arrhythmias

1. Recent reports have indicated a role for the P450IID6 polymorphism in the stereoselective disposition of single doses of the antiarrhythmic flecainide. 2. In this study, we evaluated the effects of adding low dose quinidine, a potent inhibitor of P450IID6, to chronic flecainide therapy in patients with arrhythmias. 3. In five extensive metabolizer patients, quinidine significantly reduced the clearance of R-(-)-flecainide, from 395 +/- 121 (s.d.) to 335 +/- 88 ml min-1. This change was attributable to a decrease in metabolic clearance, was accompanied by decreased formation of the two major metabolites of flecainide and was not observed in a poor metabolizer subject. The renal clearance of R-(-)-flecainide rose significantly. 4. Quinidine did not alter the clearance of S-(+)-flecainide. 5. The pharmacologic effects of flecainide therapy (QRS widening, % arrhythmia suppression) were slightly, but not significantly, increased. 6. In extensive metabolizer patients receiving chronic flecainide, increased plasma concentrations will develop if P450IID6 is inhibited.

Identification of propafenone metaboliser phenotype from plasma and urine excretion data

The aim of the study was to validate a test based on analyses of urine to identify the propafenone metaboliser phenotype during routine chronic therapy. Twenty seven patients chronically treated with propafenone were studied. A debrisoquine test was performed in 10. Propafenone and its metabolites in plasma and urine were measured by HPLC. Propafenone, 5-hydroxypropafenone and N-depropylpropafenone concentrations in plasma were 1.09, 0.182 and 0.101 ng.ml-1, respectively. Total recovery of the administered dose in urine was 30.7%. Two patients were identified as PM, based on the result of the debrisoquine test (log D/4OHD of 1.26 and 1.36). This finding was confirmed by the propafenone metabolic ratio in urine, but the plasma data did not permit clearcut separation of the phenotypes. Propafenone/5-hydroxypropafenone in plasma was not a good predictor of metabolizer phenotype. Although the number of patients who completed all three tests was limited, it is concluded that analysis of propafenone/5-hydroxypropafenone in urine collected between two consecutive doses at steady-state is more practical than the debrisoquine test and more specific than determining the propafenone/5-hydroxypropafenone ratio in plasma, for identification of the propafenone metaboliser phenotypes.

Differences in phenytoin biotransformation and susceptibility to congenital malformations: a review

The clinical variability of teratogenic response to fetal drug exposure has been well documented. Metabolic differences in biotransformation have been shown to extend to multiple drugs and may involve many steps in drug metabolism with alterations of key intermediates. Although metabolic differences have been reported to be associated with complications of medication use, it has only recently been appreciated that such differences also may be associated in the unborn with the potential for the disruption of normal embryologic development and the production of congenital malformations. It has long been suspected that the teratogenicity of phenytoin may be mediated not only by the parent compound, but also by toxic intermediary metabolites that are produced during the biotransformation of the parent compound. Recent work elucidating differences in isoenzyme forms of cytochrome P-450 enzyme systems, glutathione, and microsomal epoxide hydrolase has provided increased interest in the multiple individual pharmacogenetic differences that may be significant factors affecting increased susceptibility to birth defects in individuals and families with fetal exposure to phenytoin.

Clinical course and outcome in class IC antiarrhythmic overdose

120 cases of class IC antiarrhythmic overdose, including propafenone, flecainide, ajmaline and prajmaline overdose, were evaluated with respect to clinical course, therapy and outcome. Whereas drug overdose in general has an overall mortality of less than 1%, intoxication with antiarrhythmic drugs of class IC was associated with a mean mortality of 22.5%. Nausea, which occurred within the first 30 minutes after ingestion, was the earliest symptom. Spontaneous vomiting probably led to self-detoxication in about half the patients. Cardiac symptoms including bradycardia and, less frequently, tachyrhythmia occurred after about 30 minutes to 2 hours. Therapeutic measures included administration of activated charcoal, gastric lavage and a saline laxative, catecholamines, and in some patients, hypertonic sodium bicarbonate, insertion of a transvenous pacemaker and hemoperfusion. Fatal outcome was mainly due to cardiac conduction disturbances progressing to electromechanical dissociation or asystolia. Resuscitation, which had to be performed in 29 patients, was successful in only two of them. No correlation was found between fatal outcome, the type of antiarrhythmic, and ingested dose. Since a specific treatment is not available and resuscitive procedures including sodium bicarbonate and insertion of a pacemaker are of limited therapeutic value, early diagnosis and primary detoxification are most important for prevention of fatal outcome.