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

Drug-Drug-Gene Interactions in Cardiovascular Medicine

Cardiovascular disease remains a leading cause of both morbidity and mortality worldwide. It is widely accepted that both concomitant medications (drug-drug interactions, DDIs) and genomic factors (drug-gene interactions, DGIs) can influence cardiovascular drug-related efficacy and safety outcomes. Although thousands of DDI and DGI (aka pharmacogenomic) studies have been published to date, the literature on drug-drug-gene interactions (DDGIs, cumulative effects of DDIs and DGIs) remains scarce. Moreover, multimorbidity is common in cardiovascular disease patients and is often associated with polypharmacy, which increases the likelihood of clinically relevant drug-related interactions. These, in turn, can lead to reduced drug efficacy, medication-related harm (adverse drug reactions, longer hospitalizations, mortality) and increased healthcare costs. To examine the extent to which DDGIs and other interactions influence efficacy and safety outcomes in the field of cardiovascular medicine, we review current evidence in the field. We describe the different categories of DDIs and DGIs before illustrating how these two interact to produce DDGIs and other complex interactions. We provide examples of studies that have reported the prevalence of clinically relevant interactions and the most implicated cardiovascular medicines before outlining the challenges associated with dealing with these interactions in clinical practice. Finally, we provide recommendations on how to manage the challenges including but not limited to expanding the scope of drug information compendia, interaction databases and clinical implementation guidelines (to include clinically relevant DDGIs and other complex interactions) and work towards their harmonization; better use of electronic decision support tools; using big data and novel computational techniques; using clinically relevant endpoints, preemptive genotyping; ensuring ethnic diversity; and upskilling of clinicians in pharmacogenomics and personalized medicine.

Pharmacogenetics of drug-drug interaction and drug-drug-gene interaction: a systematic review on CYP2C9, CYP2C19 and CYP2D6

Currently, most guidelines on drug-drug interaction (DDI) neither consider the potential effect of genetic polymorphism in the strength of the interaction nor do they account for the complex interaction caused by the combination of DDI and drug-gene interaction (DGI) where there are multiple biotransformation pathways, which is referred to as drug-drug-gene interaction (DDGI). In this systematic review, we report the impact of pharmacogenetics on DDI and DDGI in which three major drug-metabolizing enzymes - CYP2C9, CYP2C19 and CYP2D6 - are central. We observed that several DDI and DDGI are highly gene-dependent, leading to a different magnitude of interaction. Precision drug therapy should take pharmacogenetics into account when drug interactions in clinical practice are expected.

Serum flecainide S/R ratio reflects the CYP2D6 genotype and changes in CYP2D6 activity

The aims of this study were to clarify whether the ratio of S- to R-flecainide (S/R ratio) in the serum flecainide concentration was associated with the stereoselectivity of flecainide metabolism, and to investigate the effects of the cytochrome P450 (CYP) 2D6 (CYP2D6) genotype and CYP2D6 inhibitor on the serum flecainide S/R ratio. In vitro studies using human liver microsomes and cDNA-expressed CYP isoforms suggested that variability in the serum flecainide S/R ratio was associated with the stereoselectivity of CYP2D6-mediated flecainide metabolism. We examined the serum flecainide S/R ratio in 143 patients with supraventricular tachyarrhythmia. The S/R ratio was significantly lower in intermediate metabolizers and poor metabolizers (IMs/PMs) than in extensive metabolizers (EMs) identified by the CYP2D6 genotype. The cut-off value for the S/R ratio to allow the discrimination between CYP2D6 EMs and IMs/PMs was 0.99. The S/R ratio in patients with co-administration of bepridil, a potent CYP2D6 inhibitor, was lower than 0.99, regardless of the CYP2D6 genotype status. Other factors, including age, sex, body weight, and renal function, did not affect the serum flecainide S/R ratio. This study suggests that the serum flecainide S/R ratio reflects the CYP2D6 genotype and changes in CYP2D6 activity on co-administration of a CYP2D6 inhibitor.

Cytochrome P450-2D6 Genotype Definition May Improve Therapy for Paroxysmal Atrial Fibrillation A Case of Syncope Following "Pill-in-the-Pocket" Quinidine plus Propafenone

Classes 1A, 1C and III anti-arrhythmics may be ineffective or induce adverse events including potentially fatal arrhythmias when administered in recommended doses. Serum levels of these medications vary widely during conventional dosing due in large part to variations in cytochrome P450-2D6 isoenzyme activity which metabolizes most antiarrhythmics in addition to over 25% of other commonly prescribed medications. 2D6 activity is also profoundly inhibited by some antiarrhythmics and other commonly used medications and varies widely between the individuals of all populations, a pattern which has resulted in separation of subjects into 4 phenotypes and genotypes consisting of poor metabolizers (PM), intermediate metabolizers (IM), efficient metabolizers (EM), and ultra-rapid metabolizers (UM). Patients with a phenotype PM classification almost universally are also genotype PM due to the possession of two inactive 2D6 alleles, with this PM pattern often inducing supratherapeutic and toxic antiarrhythmic blood levels during conventional antiarrhythmic therapy. UM individuals have supranormal levels of 2D6 activity often created by the presence of 3 or more active alleles which often induce subtherapeutic and ineffective drug levels during antiarrhythmic administration in conventional doses. We searched for evidence relating Cytochrome P450-2D6 phenotypes or genotypes to antiarrhythmic metabolism in order to judge whether this analysis might contribute to improved safety and effectiveness of antiarrhythmic medications commonly utilized in the treatment of atrial fibrillation. The available evidence strongly supported these possibilities. We also describe a patient in whom knowledge of his IM/PM CYP2D6 genotype might have prevented the only episode of syncope and myocardial stunning which developed during his 28 years of "Pill-in-a-Pocket" therapy.

Stereoselective analysis of flecainide enantiomers using reversed-phase liquid chromatography for assessing CYP2D6 activity

Stereoselective analyses of flecainide enantiomers were performed using reversed-phase high-performance liquid chromatography (HPLC) equipped with a polysaccharide-based chiral column (Chiralpak AS-RH) and fluorescence detector. Excitation and emission wavelengths were set at 300 and 370 nm, respectively. Flecainide enantiomers in serum and urine were extracted using diethyl ether. The mobile phase solution, comprising 0.1 m potassium hexafluorophosphate and acetonitrile (65:35, v/v), was pumped at a flow rate of 0.5 mL/min. The recoveries of flecainide enantiomers were greater than 94%, with the coefficients of variation (CVs) <6%. The calibration curves of flecainide enantiomers in serum and urine were linear in the concentration range 5-500 ng/mL and 0.75-15 µg/mL (r > 0.999), respectively. CVs in intra-day and inter-day assays were 1.8-5.8 and 3.4-7.5%, respectively. In a pharmacokinetic study, the ratios of (S)- to (R)-flecainide (S/R ratio) in the area under the curve and the amount of flecainide enantiomers excreted in urine were lower in a subject carrying CYP2D6*10/*10 than in subjects carrying CYP2D6*1/*2. The S/R ratio of trough serum flecainide concentration ranged from 0.79 to 1.16 in patients receiving oral flecainide. The present HPLC method can be used to assess hepatic flecainide metabolism in a pharmacokinetic study and therapeutic drug monitoring.

Evaluating the safety and efficacy of dextromethorphan/quinidine in the treatment of pseudobulbar affect

Pseudobulbar affect (PBA) is a common manifestation of brain pathology associated with many neurological diseases, including amyotrophic lateral sclerosis, Alzheimer's disease, stroke, multiple sclerosis, Parkinson's disease, and traumatic brain injury. PBA is defined by involuntary and uncontrollable expressed emotion that is exaggerated and inappropriate, and also incongruent with the underlying emotional state. Dextromethorphan/quinidine (DM/Q) is a combination product indicated for the treatment of PBA. The quinidine component of DM/Q inhibits the cytochrome P450 2D6-mediated metabolic conversion of dextromethorphan to its active metabolite dextrorphan, thereby increasing dextromethorphan systemic bioavailability and driving the pharmacology toward that of the parent drug and away from adverse effects of the dextrorphan metabolite. Three published efficacy and safety studies support the use of DM/Q in the treatment of PBA; significant effects were seen on the primary end point, the Center for Neurologic Study-Lability Scale, as well as secondary efficacy end points and quality of life. While concentration-effect relationships appear relatively weak for efficacy parameters, concentrations of DM/Q may have an impact on safety. Some special safety concerns exist with DM/Q, primarily because of the drug interaction and QT prolongation potential of the quinidine component. However, because concentrations of dextrorphan (which is responsible for many of the parent drug's side effects) and quinidine are lower than those observed in clinical practice with these drugs administered alone, some of the perceived safety issues may not be as relevant with this low dose combination product. However, since patients with PBA have a variety of other medical problems and are on numerous other medications, they may not tolerate DM/Q adverse effects, or may be at risk for drug interactions. Some caution is warranted when initiating DM/Q treatment, particularly in patients with underlying risk factors for torsade de pointes and in those receiving medications that may interact with DM/Q.

Impact of genetic polymorphism on drug-drug interactions mediated by cytochromes: a general approach

Currently, quantitative prediction of the impact of genetic polymorphism and drug-drug interactions mediated by cytochromes, based on in vivo data, is made by two separate methods and restricted to a single cytochrome. We propose a unified approach for describing the combined impact of drug-drug interactions and genetic polymorphism on drug exposure. It relies on in vivo data and uses the following three characteristic parameters: one for the victim drug, one for the interacting drug, and another for the genotype. These parameters are known for a wide range of drugs and genotypes. The metrics of interest are the ratio of victim drug area under the curve (AUC) in patients with genetic variants taking both drugs, to the AUC in patients with either variant or wild-type genotype taking the victim drug alone. The approach was evaluated by external validation, comparing predicted and observed AUC ratios found in the literature. Data were found for 22 substrates, 30 interacting drugs, and 38 substrate-interacting drug couples. The mean prediction error of AUC ratios was 0.02, and the mean prediction absolute error was 0.38 and 1.34, respectively. The model may be used to predict the variations in exposure resulting from a number of drug-drug-genotype combinations. The proposed approach will help (1) to identify comedications and population at risk, (2) to adapt dosing regimens, and (3) to prioritize the clinical pharmacokinetic studies to be done.

Effects of CYP2D6 genotypes on age-related change of flecainide metabolism: involvement of CYP1A2-mediated metabolism

AIMS

The aim of this study was to clarify the effects of CYP2D6 genotype on age-related change in flecainide metabolism in patients with supraventricular tachyarrhythmias. An in vitro study using microsomes was performed to identify other CYPs responsible for age-related change in flecainide metabolism.

METHODS

The study population comprised 111 genotyped patients: CYP2D6-homozygous extensive metabolizers (hom-EMs, n= 34), heterozygous EMs (het-EMs, n= 56), and intermediate and poor metabolizers (IMs/PMs, n= 21). Serum concentrations of flecainide and its metabolites [m-O-dealkylated flecainide (MODF) and m-O-dealkylated lactam of flecainide] were determined by use of a high-performance liquid chromatography. Metabolic ratio (MR) was expressed as serum concentrations of flecainide to its metabolites. In vitro formation of MODF was examined in human liver microsomes and cDNA-expressed CYP isoforms.

RESULTS

MR was higher in elderly patients (> or =70 years) than in middle-aged patients (<70 years). The increase of MR in elderly patients differed among CYP2D6 genotypes: 1.6-fold in het-EMs [4.3, 95% confidence interval (CI) 2.8, 5.7 vs. 2.7, 95% CI 2.3, 3.1, P < 0.05], 1.5-fold in IMs/PMs (6.0, 95% CI 4.5, 7.6 vs. 4.1, 95% CI 2.9, 5.4, P < 0.05), and no change in hom-EMs. The in vitro study using microsomes revealed that both CYP2D6 and CYP1A2 were involved in the formation of MODF. MODF formation in CYP2D6 PM microsomes increased as CYP1A2 activity increased.

CONCLUSIONS

The results suggest that patients with poor CYP2D6-mediated metabolism (het-EMs and IMs/PMs) showed age-related reduction in flecainide metabolism because metabolism was taken over by CYP1A2, whose activity decreases with age.

Depressive effect of an antidepressant: therapeutic failure of venlafaxine in a case lacking CYP2D6 activity

Understanding the mechanisms of drug metabolism and interactions can help to prevent side-effects. Not only drug interactions, environmental factors, disease processes and ageing are factors in the inter-individual metabolic capacity variance but also genetic factors probably play an important role, as is illustrated in the case presented. Besides therapeutic drug monitoring, genotyping some important cytochrome P450 (CYP450) enzymes was of additional value in explaining why the patient developed severe adverse effects and, moreover, did not experience any therapeutical effect of venlafaxine. Results indicated that the patient was a poor metabolizer for CYP2D6, the most important phase I enzyme to metabolize venlafaxine. This corroborates that polymorphisms in the CYP450 gene influence the metabolic activity of the corresponding enzymes, thus affecting the subsequent serum drug levels and their metabolites. This case highlights the potential benefit of both clinical and genetic risk stratification (pharmacogenetics) prior to treatment, either for setting the individual dose or for making a decision about using a particular drug.

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.

Pharmacokinetic interaction of flecainide and paroxetine in relation to the CYP2D6*10 allele in healthy Korean subjects

AIMS

The objectives were to evaluate the effect of CYP2D6 genetic polymorphism on the pharmacokinetics of flecainide, and also on the extent of drug interaction with paroxetine as a CYP2D6 inhibitor after a single oral administration in healthy subjects.

METHODS

An open-label, two-period, single-sequence, cross-over study was performed in 21 healthy Korean male volunteers (seven for CYP2D6*1/*1 or *1/*2, group 1; seven for CYP2D6*1/*10, group 2; seven for CYP2D6*10/*10 or *10/*36, group 3). Subjects were administered 200 mg of flecainide on day 1. After a 7-day wash-out period, subjects were administered 20 mg of paroxetine from day 8 to 14, and 200 mg of flecainide on day 15. Blood sampling was performed up to 72 h after flecainide administration.

RESULTS

Terminal elimination half-life and mean residence time (MRT) were significantly different among three genotype groups after a single oral administration of flecainide (P = 0.021, 0.011, respectively). Area under the concentration-time curve, terminal elimination half-life and MRT increased significantly after paroxetine co-administration only in groups 1 and 2.

CONCLUSIONS

This study reports that the extent of drug interaction between flecainide and paroxetine is influenced by the CYP2D6*10 allele in healthy subjects, which is frequent in Asians.

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.

THEINTEGRATION OFPHARMACOKINETICS ANDPHARMACODYNAMICS: Understanding Dose-Response

Pharmacokinetic (PK) and pharmacodynamic (PD) studies have proven to be powerful and instructive tools, particularly in elucidating important aspects of human pharmacology. Nevertheless, they remain imperfect tools in that they only allow researchers to indirectly extrapolate, through computational modeling, the dynamic processes of drug action. Furthermore, neither tool alone provides a complete nor necessarily relevant picture of drug action. This review explores the utility and applications of PK and PD in the study of drugs, provides examples of lessons learned from their application to studies of human pharmacology, points out some of their limitations, and advances the thesis that these tools ideally should be employed together in an integrated approach. As we continue to apply these tools across the continuum of age and disease, they provide a powerful means to enhance our understanding of drug action, drug interactions, and intrinsic host factors that influence pharmacologic response.

Drug-induced pneumonitis and heart failure simultaneously associated with venlafaxine

Two cases of interstitial pneumonia with cardiac failure developing in patients treated with the new antidepressant venlafaxine are presented. A strong relationship between the development of the patients' illness and the initiation of venlafaxine treatment was identified. The cytochrome P (CYP) 450 system is involved in the metabolism of venlafaxine, suggesting that alterations in the drug metabolic clearance might be, at least in part, responsible for the development of drug-induced damage in these cases. This might occur either as a consequence of a genetic factor or concomitant drug therapy with an inhibitor of the related CYP system. After identifying the causative agent in the first case, withdrawal of the antidepressant together with corticosteroid treatment led to a favorable outcome. In the other case, the multiorgan failure became fatal. These cases highlight a hitherto undescribed association of an adverse lung reaction and heart failure due to venlafaxine.

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.

Pharmacogenetic diagnostics of cytochrome P450 polymorphisms in clinical drug development and in drug treatment

The current use and future perspectives of molecular genetic characterisation of cytochrome P450 enzymes (CYP) for drug development and drug treatment are summarised. CYP genes are highly polymorphic and the enzymes play a key role in the elimination of the majority of drugs from the human body. Frequent variants of some enzymes, CYP2A6, 2C9, 2C19 and 2D6, should be analysed in participants of clinical trials whenever these enzymes may play a role. It is suggested that a CYP genotype certificate is handed out to the volunteers or patients to avoid replicate analyses, and to allow that this information is available for future research and also for treatment with eventually needed drugs. Guidelines on what CYP alleles have to be analysed in drug development, as well as on analytical validation and CYP genotype data handling will be required. Treatment with several drugs may be improved by prior genotyping. The concepts and problems of CYP genotype-based clinical dose recommendations are presented and illustrated for selected drugs. The requirement for prospective trials on the medical and economic benefits of routine CYP genotyping is emphasised. Specific operationally defined recommendations dependent on genotype are a prerequisite for such studies and this review presents tentative CYP genotype-based dose recommendations systematically calculated from published data. Because of the multiplicity of factors involved, these doses will not be the optimal doses for each given individual, but should be more adequate than doses generally recommended for an average total population. Those CYP alleles and polymorphically metabolised drugs which are currently most interesting in drug development and drug treatment are reviewed, and more complete information is available from websites cited in this article.

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.

Therapeutic drug monitoring: antiarrhythmic drugs

Antiarrhythmic agents are traditionally classified according to Vaughan Williams into four classes of action. Class I antiarrhythmic agents include most of the drugs traditionally thought of as antiarrhythmics, and have as a common action, blockade of the fast-inward sodium channel on myocardium. These agents have a very significant toxicity, and while they are being used less, therapeutic drug monitoring (TDM) does significantly increase the safety with which they can be administered. Class II agents are antisympathetic drugs, particularly the beta-adrenoceptor blockers. These are generally safe agents which do not normally require TDM. Class III antiarrhythmic agents include sotalol and amiodarone. TDM can be useful in the case of amiodarone to monitor compliance and toxicity but is generally of little value for sotalol. Class IV antiarrhythmic drugs are the calcium channel blockers verapamil and diltiazem. These are normally monitored by haemodynamic effects, rather than using TDM. Other agents which do not fall neatly into the Vaughan Williams classification include digoxin and perhexiline. TDM is very useful for monitoring the administration (and particularly the safety) of both of these agents.

Pharmacological importance of stereochemical resolution of enantiomeric drugs

Drug enantiomers have identical properties in an achiral environment, but should be considered as different chemical compounds. This is because they often differ considerably in potency, pharmacological activity and pharmacokinetic profile, since the modules with which they interact in biological systems are also optically active. Within biological systems, the metabolism of one isomer may be via a different pathway or occur at a different rate from that of the other isomer. Preferential binding of one isomer to plasma proteins may cause differences in circulating free drug and hence alter concentrations at active sites. Interactions of both isomers may differ at the active sites through which pharmacological action is mediated. Actions and levels of activity of the stereoisomers in vivo may also differ. All the pharmacological activity may reside in a single enantiomer, whereas several possibilities exist for the other enantiomer-- it may be inactive, have a qualitatively different effect, an antagonistic effect or produce greater toxicity. Two isomers may have nearly identical qualitative pharmacological activity, qualitatively similar pharmacological activity but quantitatively different potency, or qualitatively different pharmacological activity. To avoid adverse effects and optimise the therapeutic value of enantiomeric drugs, it is necessary that methods for the resolution of racemates be evolved and devolved to determine isomeric purity, establish the effectiveness of isomers of the drug, and detect the presence of an enantiomer with lower therapeutic activity and undesirable adverse effects. Even if a drug is given as a pure enantiomer, methods to discriminate between enantiomers are required because racemisation can occur both in vitro and in vivo. Methods developed for resolution of drug enantiomers should facilitate routine testing of single isomers and their metabolites, studies of pharmacological, toxicological and clinical effectiveness, routine analysis of racemates, pure enantiomers or intermediates in manufacturing processes, and investigation of the potential for inversion of an enantiopure drug substance during the early stages of drug development and therapeutic drug monitoring.

Adverse effects of class I antiarrhythmic drugs

Class I antiarrhythmic drugs are characterised by their ability to block the fast inward sodium current in cardiac muscle tissue. However, at the same time, they can be responsible for various effects involving other organs and systems. Although some of these effects can be helpful in specific situations, most of them, such as their pro-arrhythmic propensity, are deleterious. Some of the adverse effects of class I antiarrhythmic drugs are directly linked to sodium-channel blockade (conduction disorders haemodynamic perturbations, and digestive and neurological effects), while others are linked to other specific pharmacological properties (e.g. atropinic, or alpha- or beta-adrenergic blockade) or to nonspecific properties (idiosyncratic hypersensitivity, and haematological, dermatological or hepatic reactions). Other adverse effects are associated with complex interactions between class I antiarrhythmics and individual predisposing factors, trigger mechanisms and physiological factors (including concomitant drug treatment). These numerous variations and interactions within a specific environment and underlying disorder might be of pharmacological or/and pharmacokinetic origin, making analysis of the true liability of the class I drugs very difficult when adverse effects occur.

Stereoselective disposition of carvedilol is determined by CYP2D6

Carvedilol is a mixed alpha- and beta-adrenergic receptor antagonist that is administered as a racemic mixture. Although the two isomers are equally potent as alpha 1-blockers the S(-)-isomer is principally responsible for the beta blockade of carvedilol. To determine the role of pharmacogenetics in the metabolism of carvedilol we studied nine extensive metabolizers of both debrisoquin and mephenytoin, seven poor metabolizers of debrisoquin but extensive metabolizers of mephenytoin, and three poor metabolizers of mephenytoin but extensive metabolizers of debrisoquin. The clearance of R-carvedilol was significantly lower than S-carvedilol in both debrisoquin phenotypes. Poor metabolizers of debrisoquin had a significantly lower clearance of R-carvedilol than extensive metabolizers of debrisoquin. The partial metabolic clearance of carvedilol to the two ring-hydroxylated metabolites 4- and 5-hydroxyphenyl carvedilol were significantly reduced in poor metabolizers of debrisoquin. No effect of mephenytoin phenotype on carvedilol kinetics was observed. Thus carvedilol is stereoselectively metabolized in humans, and the clearance of S-carvedilol is higher than that of R-carvedilol. In poor metabolizers of debrisoquin the clearance of R-carvedilol is further reduced, resulting in higher plasma concentrations and perhaps greater alpha-blockade.

Flecainide

Flecainide is a Class IC antiarrhythmic agent whose primary electrophysiologic effect is a slowing of conduction in a wide range of cardiac tissues. It is well absorbed and effective in suppressing isolated premature ventricular contractions (PVCs) or nonsustained ventricular arrhythmia but has only a modest efficacy when electrophysiologic testing is used as an endpoint. Its adverse effect on mortality in the CAST trial suggested a propensity to proarrhythmia--a phenomenon to which the Class IC agents appear particularly prone. Despite the applicability of the CAST study only to patients with a prior myocardial infarction, there has been a shift away from flecainide in ventricular arrhythmia, but the low noncardiac side effect profile of the agent allows for its continued use in a wide variety of supraventricular arrhythmias.

Genetically determined adverse drug reactions involving metabolism

Genetic factors represent an important source of interindividual variation in drug response. Relatively few adverse drug effects with a pharmacodynamic basis are known, and most of the well characterised inherited traits take the form of genetic polymorphisms of drug metabolism. Monogenic control of N-acetylation, S-methylation and cytochrome P450-catalysed oxidation of drugs can have important clinical consequences. Individuals who inherit an impaired ability to perform one or more of these reactions may be at an increased risk of concentration-related toxicity. There is a strong case for phenotyping before starting treatment with a small number of drugs that are polymorphically N-acetylated or S-methylated. However, the issue of clinical significance is perhaps most relevant for the debrisoquine oxidation polymorphism, which is mediated by cytochrome CYP2D6 and which determines the pharmacokinetics of many commonly used drugs. Phenotypic poor metabolisers of debrisoquine (8% of Caucasian populations) taking standard doses of some tricyclic antidepressants, neuroleptics or antiarrhythmic drugs may be particularly prone to adverse reactions. Similarly, clinically relevant drug interactions between these drugs and other substrates of cytochrome CYP2D6 may occur in the majority of the population who are extensive metabolisers. However, it is clear that in the majority of cases there is a need for controlled prospective studies to determine clinical significance. Accordingly, routine debrisoquine phenotyping or genotyping before beginning drug treatment is difficult to justify at present, although it may be helpful in individual cases. When prescribing drugs whose metabolism is polymorphic alone or in combination, careful titration of the dose in both phenotypic groups is prudent. In some cases it will be preferable to use alternative therapy to avoid the risk of adverse drug reactions.

Stereoselective and isozyme-selective drug interactions

A surprisingly large number of marketed drugs are racemic mixtures. The pharmacokinetic literature on racemic drugs contains a vast amount of information on drug-drug interactions derived from the measurement of total drug concentrations in plasma and urine. The appreciation of the role of stereochemistry in drug interactions with racemic warfarin resulted in a long-overdue scientific rigor being applied to the study of drug interactions. It also compelled us to recognize that much of the literature was uninterpretable. A better understanding of oxidative metabolism, particularly the complexity of the cytochrome P-450 family of enzymes, has also strengthened the scientific basis of drug interactions. We now recognize that investigators and clinicians must consider both stereoselectivity and isozyme selectivity in the study of drug interactions to understand the nature of the interaction so as to more effectively use new and potent drugs.