Adverse effects from metoprolol are not generally associated with oxidation status

CYP2D6 Genetic Variation and Beta-Blocker Maintenance Dose in Patients with Heart Failure

PURPOSE

This study examined whether a CYP2D6 polymorphism (CYP2D6*4) was related to beta-blocker maintenance dose in patients with heart failure.

METHODS

Logistic regression modeling was utilized in a retrospective chart-review analysis of heart-failure patients (60% Male, 90% of European descent) to assess whether CYP2D6*4 (non-functional CYP2D6 allele present in 1 of 5 individuals of European descent) is associated with maintenance dose of carvedilol (n = 65) or metoprolol (n = 33).

RESULTS

CYP2D6*4 was associated with lower maintenance dose of metoprolol (OR 0.13 [95% CI 0.02-0.75] p = 0.023), and a trend was observed between CYP2D6*4 and higher maintenance dose of carvedilol (OR 2.94 [95% CI 0.84-10.30] p = 0.093). None of the patients that carried CYP2D6*4 achieved the recommended target dose of metoprolol (200 mg/day).

CONCLUSION

Consistent with the role of CYP2D6 in the metabolism of metoprolol, the tolerated maintenance dose of metoprolol was lower in CYP2D6*4 carriers compared to non-carriers. Consistent with the role of CYP2D6 in activation of carvedilol, tolerated maintenance dose of carvedilol was higher in CYP2D6*4 carriers compared to non-carriers. Further investigation is warranted to ascertain the potential of CYP2D6 as a potential predictive biomarker of beta-blocker maintenance dose in heart failure patients.

Safe and effective medicines for all: is personalized medicine the answer?

An improvement in drug treatment and clinical outcome is one of the major challenges in clinical medicine. The development of evidence-based standards of care has led to a significant improvement, but, by definition, strictly standardized cohorts in clinical trials have to ignore individual differences. Personalized medicine is defined as the application of genomic and molecular data to better target the delivery of healthcare, facilitate the discovery and clinical testing of new products, and help determine a person's predisposition to a particular disease or condition. After the deciphering of the human genome, however, the high expectations in individualized medicine were not always fulfilled. However, personalized medicine has become indispensable in the treatment of malignant diseases and there is increasing evidence for its benefit in other areas. This article outlines the impact of pharmacogenetics and pharmacogenomics, especially with regard to personalized medicine, in major medical indications and reflects the obstacles and chances taken in current daily practice.

CYP2D6 genotype and its relationship with metoprolol dose, concentrations and effect in patients with systolic heart failure

The aims of this study were to examine the relationships between CYP2D6 genotype and metoprolol dose, S- and R-metoprolol concentrations and clinical effects in patients with systolic heart failure. Data were obtained for 52 subjects, of which 27 had 2 functional alleles (24/27, CYP2D6*1/*1), 22 had 1 functional allele (18/22, CYP2D6*1/*4) and 3 had no functional alleles (CYP2D6*4/*4). Median dose-adjusted concentrations of S-metoprolol (active) were 6.3- and 3.2-fold higher in subjects with zero or one functional allele (P=0.016 and P=0.006), respectively, compared with subjects with two functional alleles. For the R-enantiomer (inactive), these concentrations were 10.7- and 3.7-fold higher (P=0.013 and P=0.003), respectively. Despite clear gene-concentration differences, no relationships between CYP2D6 genotype and dose or clinical effects could be shown. Although the number with no functional alleles was too small (n=3) to show effects, in patients with 1 functional allele other sources of variance are likely to be obscuring differences in clinical effects.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Extension of a pilot study: impact from the cytochrome P450 2D6 polymorphism on outcome and costs associated with severe mental illness

The influence of cytochrome P450 2D6 (CYP2D6) genetic variability was examined in psychiatric inpatients by evaluating adverse drug events (ADEs), hospital stays, and total costs over a 1-year period in an extension of a previously published brief report. One hundred consecutive psychiatric patients from Eastern State Hospital in Lexington, Kentucky, were genotyped for CYP2D6 expression. ADEs were evaluated by a neurologic rating scale, modified Udvalg for Kliniske Undersogelser Side Effect Rating Scale, or chart review. Information on total hospitalization days and total costs were gathered for a 1-year period. Forty-five percent of the patients received medications that were primarily dependent on the CYP2D6 enzyme for their elimination. When the analysis was restricted to just those patients in each group receiving medication heavily dependent on the CYP2D6 enzyme, the following were observed: (1) a trend toward greater numbers of ADEs from medications as one moved from the group with ultrarapid CYP2D6 activity (UM) to the group with absent CYP2D6 activity (PM); (2) the cost of treating patients with extremes in CYP2D6 activity (UM and PM) was on average $4,000 to $6,000 per year greater than the cost of treating patients in the efficient metabolizer (EM) and intermediate metabolizer (IM) groups; and (3) total duration of hospital stay was more pronounced for those in CYP2D6 PM group. Variance of hospital stays and costs calculated from these preliminary data suggests that 1,500 to 2,000 patients must be evaluated over at least a 1-year period to determine whether the CYP2D6 genetic variation significantly alters the duration of hospital stay and costs.

Influence of debrisoquine oxidation phenotype on exercise tolerance and subjective fatigue after metoprolol and atenolol in healthy subjects

1. The effects of single doses of metoprolol 50 mg, metoprolol 100 mg and atenolol 100 mg on exercise tolerance were compared with placebo in a double-blind random cross-over study in 12 healthy subjects. Nine subjects were extensive metabolisers of debrisoquine, and three were poor metabolisers. 2. Three hours after dosing beta-adrenoceptor blocker treatments significantly reduced exercise heart rate, prolonged time to complete exercise, and increased subjective fatigue measured by visual analogue scale. 3. Scores for subjective fatigue did not correlate with reduction in exercise heart rate or prolongation of exercise time. Exercise time prolongation was weakly but not significantly correlated with exercise heart rate reduction. 4. When compared with placebo, prolongation of exercise time and increased fatigue with metoprolol were not significantly related to debrisoquine oxidation phenotype or to the debrisoquine/4-hydroxydebrisoquine (D/4OH-D) ratio. 5. When metoprolol responses were compared with those for atenolol, changes in exercise time and fatigue scores were significantly related to oxidation phenotype. For metoprolol 100 mg, poor metabolisers required 20.8 s longer to complete exercise (P less than 0.05) and had higher fatigue scores by 78% (P less than 0.05) as compared with extensive metabolisers.(ABSTRACT TRUNCATED AT 400 WORDS)

Side effects of beta-blocker treatments as related to the central nervous system

During the last decade beta-adrenoceptor antagonists have become one of the first-line treatments for hypertension. Generally, they have been shown to be safe with a low frequency of serious side effects. However, minor subjective symptoms, usually considered to be CNS-related, have been reported for all beta-blockers used. Thus, all beta-blockers on the market seem to have a high benefit:risk ratio; independent of their physicochemical properties and pharmacodynamic profile, however, they seem to cause CNS-related side effects to about the same extent. These minor side effects, the mechanisms of which are unclear, consist of subtle effects on general well being, decreased initiative, a depressed frame of mind, and disturbed sleep. Generally, however, beta-blockers in therapeutic dosages do not affect the qualitative functions of the brain. The results so far available have been obtained primarily by using objective methods. Further comparison has now been initiated using documented subjective methods to investigate whether the objectively documented differences are of any clinical relevance to the patient's quality of life. Although it cannot be claimed with certainty, nonselective beta-blockers seem to cause CNS-related side effects to a greater extent than beta 1-selective blockers. Differences in the degree of hydrophilicity of the beta-blocker are apparently of no clinical relevance in this respect. Rather, the plasma concentration of the beta-blocking drug (degree of beta-blockade) seems to be the major determinant of whether or not CNS-related symptoms appear in susceptible patients.

A pharmacokinetic and pharmacodynamic comparison of metoprolol CR/ZOK with a conventional slow release preparation

The plasma concentration-time profile and hemodynamic effects of metoprolol after the administration of metoprolol CR/ZOK, a multiple-unit controlled release formulation and a conventional slow release formulation, metoprolol Durules (Astra, AB Hässle, Mölndal, Sweden) once daily, were investigated in 12 healthy men. Data were collected over one 24-hour dose interval at steady state after 5 days of treatment. The study was randomized, three-way, crossover double-blind comparison of metoprolol CR/ZOK 200 mg, metoprolol Durules 200 mg and placebo. The reduction in exercise heart rate compared with placebo treatment was used as a measure of beta 1-blockade. The metoprolol plasma concentration-time profile during treatment with metoprolol CR/ZOK was smooth and uniform, showing a more constant release profile than that obtained with metoprolol Durules. This was demonstrated by the significantly longer time period during which the plasma concentration exceeded 75% of the maximum concentration (T75), for metoprolol CR/ZOK compared with metoprolol Durules (P less than .01). The Fluctuation Index of plasma metoprolol concentration was significantly smaller for metoprolol CR/ZOK than for metoprolol Durules (P less than .001). The pharmacokinetic differences between the formulations also produced differences in the time profiles of exercise heart rate. The percentage fluctuation in exercise heart rate over the dose interval tended to be smaller for metoprolol CR/ZOK. At the start and the end of the dosing interval, the CR/ZOK formulation was significantly more effective (P less than .01). These results indicate that Metoprolol CR/ZOK has a more sustained time profile of beta 1-blockade at steady state.(ABSTRACT TRUNCATED AT 250 WORDS)

The genetic polymorphism of debrisoquine/sparteine metabolism--clinical aspects

It has been established that the metabolism of more than twenty drugs, including antiarrhythmics, beta-adrenoceptor antagonists, antidepressants, opiates and neuroleptics is catalyzed by cytochrome P-450dbl. The activity of this P-450 isozyme is under genetic rather than environmental control. This article discusses the therapeutic implications for each of the classes of drugs affected by this genetic polymorphism in drug metabolism. Not only are the problems associated with poor metabolizers who are unable to metabolize the compounds discussed, but it is also emphasized that it is difficult to attain therapeutic plasma concentrations for some drugs in high activity extensive metabolizers.

Clinical significance of the sparteine/debrisoquine oxidation polymorphism

The sparteine/debrisoquine oxidation polymorphism results from differences in the activity of one isozyme of cytochrome P450, the P450db1 (P450 IID1). The oxidation of more than 20 clinically useful drugs has now been shown to be under similar genetic control to that of sparteine/debrisoquine. The clinical significance of this polymorphism may be defined by the value of phenotyping patients before treatment. The clinical significance of such polymorphic elimination of a particular drug can be analyzed in three steps: first, does the kinetics of active principle of a drug depend significantly on P450db1?; second, is the resulting pharmacokinetic variability of any clinical importance?; and third, can the variation in response be assessed by direct clinical or paraclinical measurements? It is concluded from such an analysis that, in general, the sparteine/debrisoquine oxidation polymorphism is of significance in patient management only for those drugs for which plasma concentration measurements are considered useful and for which the elimination of the drug and/or its active metabolite is mainly determined by P450db1. At present, this applies to tricyclic antidepressants and to certain neuroleptics (e.g. perphenazine and thioridazine) and antiarrhythmics (e.g. propafenone and flecainide). Phenotyping should be introduced in to clinical routine under strictly controlled conditions to afford a better understanding of its potentials and limitations. The increasing knowledge of specific substrates and inhibitors of P450db1 allows precise predictions of drug-drug interactions. At present, the strong inhibitory effect of neuroleptics on the metabolism of tricyclic antidepressants represents the best clinically documented and most relevant example of such an interaction.

Adverse drug reaction reporting and retrospective phenotyping for oxidation polymorphism

A genetically determined impairment in the ability to oxidase sparteine and debrisoquine also affects the oxidation of several other drugs. This impairment in oxidation may result in accumulation of the associated drugs and in an increased susceptibility to adverse reactions from these drugs. Dunedin houses the New Zealand national centre for the collation and study of adverse drug reactions. Included among the reporting schemes is an intensified monitoring system for newly released drugs, in which physicians report all clinical events occurring during treatment with the drugs under surveillance. The centre thus has available extensive records of names and addresses of prescribers and patients who have been reported as experiencing an adverse event while receiving drug therapy. We investigated the association between genetically poor oxidation of sparteine and adverse reactions to drugs selected as possibly sharing the sparteine/debrisoquine oxidation pathway; these included perhexiline, metoprolol, debrisoquine, piroxicam, mianserin and nifedipine. A kit containing instructions, a sparteine capsule and a container for urine collection was sent to physicians who reported adverse reactions or events to one of the above drugs for forwarding to the patient. It appeared possible, after assays of returned urine for sparteine and its metabolites, that adverse reactions to nifedipine were associated with genetically poor oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

The genetic polymorphism of sparteine metabolism

The formation of the two major metabolites of the antiarrhythmic and oxytocic drug sparteine (2- and 5-dehydrosparteine) exhibits a genetic polymorphism. Two phenotypes, extensive (EM) and poor metabolizers (PM) are observed in the population. The frequency of the PM phenotype in various populations (Caucasian and Japanese) ranges from 2.3 to 9%. The metabolism of sparteine is determined by two allelic genes at a single gene locus. PM subjects are homozygous for an autosomal recessive gene. The metabolism of sparteine is predominantly under genetic control as treatment with drugs such as antipyrine and rifampicin known to induce oxidative drug metabolism elicited only marginal changes in sparteine metabolism. The formation of 2-dehydrosparteine in human liver microsomes from EM and PM subjects showed a more than 40-fold difference in Km between EM and PM subjects. However, Vmax-values were almost identical in both groups. These data indicate that the basis of the differences in oxidative capacity between EM and PM subjects is more likely to be due to a variant isozyme with defective catalytic properties than to a decreased amount of the isozyme.

The polymorphic oxidation of beta-adrenoceptor antagonists. Clinical pharmacokinetic considerations

Wide variability in response to some drugs such as debrisoquine can be attributed largely to genetic polymorphism of their oxidative metabolism. Most beta-blockers undergo extensive oxidation. Anecdotal reports of high plasma concentrations of certain beta-blockers in poor metabolisers (PMs) of debrisoquine have claimed that the oxidation of these drugs is under polymorphic control. Subsequently, controlled studies have shown that debrisoquine oxidation phenotype is a major determinant of the metabolism, pharmacokinetics and some of the pharmacological actions of metoprolol, bufuralol, timolol and bopindolol. The poor metaboliser phenotype is associated with increased plasma drug concentrations, a prolongation of elimination half-life and more intense and sustained beta-blockade. Phenotypic differences have also been observed in the pharmacokinetics of the enantiomers of metoprolol and bufuralol. In vivo and in vitro studies have identified some of the metabolic pathways which are subject to the defect, viz. alpha-hydroxylation and O-demethylation of metoprolol and 1'- and possibly 4- and 6-hydroxylation of bufuralol. In contrast, the overall pharmacokinetics and pharmacodynamics of propranolol, which is also extensively oxidised, are not related to debrisoquine polymorphism, although 4'-hydroxypropranolol formation is lower in poor metabolisers. As anticipated, the disposition of atenolol which is eliminated predominantly unchanged by the kidney and in the faeces, is unrelated to debrisoquine phenotype. The clinical significance of impaired elimination of beta-blockers is not clear. If standard doses of beta-blockers are used in poor metabolisers, these subjects may be susceptible to concentration-related adverse reactions and they may also require less frequent dosing for control of angina pectoris.

Genetically determined variability in acetylation and oxidation. Therapeutic implications

The clinical significance of two separate genetic polymorphisms which alter drug metabolism, acetylation and oxidation is discussed, and methods of phenotyping for both acetylator and polymorphic oxidation status are reviewed. Particular reference is made to the dapsone method, which provides a simple means of distinguishing fast and slow - and possibly intermediate - acetylators, and to the sparteine method which allows a clear separation of oxidation phenotypes. Although acetylation polymorphism has been known for some time, definite indications for phenotyping are few. It is doubtful whether acetylator phenotype makes a significant difference to the outcome in most isoniazid treatment regimens, and peripheral neuropathy from isoniazid in slow acetylators is easily overcome by pyridoxine administration. However, in comparison with rapid acetylators, slow acetylators receiving isoniazid have an increased susceptibility to phenytoin toxicity, and perhaps also to carbamazepine toxicity. It is also possible that rapid acetylators receiving isoniazid attain higher serum fluoride concentrations from enflurane and similar anaesthetics than do similarly treated slow acetylators. Thus, when drug interactions of these types are suspected, phenotyping for acetylator status may be advisable. If routine monitoring of serum procainamide and N-acetylprocainamide concentrations is practised, phenotyping of subjects prior to therapy with these agents should not be necessary. Although acetylator phenotype influences serum concentrations of hydralazine, when this drug is given in combination with other drugs acetylator phenotype has not been shown to influence the therapeutic response. Slow acetylator phenotype along with female gender and the presence of HLA-DR antigens appear to be risk factors in the development of hydralazine-induced systemic lupus erythematosus (SLE). Determination of acetylator phenotype may therefore help determine susceptibility to this adverse reaction. In the case of sulphasalazine, adult slow acetylators require a lower daily dose of the drug than fast acetylators in order to maintain ulcerative colitis in remission without significant side effects. It is therefore advisable to determine acetylator phenotype prior to sulphasalazine therapy. Work on the association of acetylation polymorphism with various disease states is also reviewed. It is possible that a higher incidence of bladder cancer is associated with slow acetylation phenotype - especially in individuals exposed to high levels of arylamines. The question as to whether idiopathic SLE is more common in slow acetylators remains unresolved. There appears to be no difference between fa