The influence of the sparteine/debrisoquin phenotype on the disposition of flecainide

The Clinical Significance of the Pharmacogenetics of Cardiovascular Medications

Inter-individual variability in the response to numerous drugs can be traced to a number of sources. One source of variability in drug response is the variability associated with the metabolic capacity of an individual. The component of metabolic capacity that will be the focus of this article is that determined by heredity. Pharmacogenetics is frequently referred to as the study of the effects of heredity on the disposition and response to medications. This article will review the pharmacokinetic and pharmacodynamic significance of pharmacogenetics as it pertains to a select number of cardiovascular agents. The enzyme systems responsible for drug metabolism discussed in this article will be limited to the P-450IID6 and N-acetylation pathways. Given the extensive use of cardiovascular agents in clinical practice that are affected by this genetic polymorphism, it is important for the practicing pharmacist to be aware of this phenomenon and its implications. Hopefully, the knowledge gained from this article will help practicing pharmacists to appreciate the clinical significance of polymorphic drug metabolism and provide a basis for the application of this knowledge to a variety of practice settings.

Narrow therapeutic index drugs: a clinical pharmacological consideration to flecainide

Purpose

The therapeutic index (TI) is the range of doses at which a medication is effective without unacceptable adverse events. Drugs with a narrow TI (NTIDs) have a narrow window between their effective doses and those at which they produce adverse toxic effects. Generic drugs may be substituted for brand-name drugs provided that they meet the recommended bioequivalence (BE) limits. However, an appropriate range of BE for NTIDs is essential to define due to the potential for ineffectiveness or adverse events. Flecainide is an antiarrhythmic agent that has the potential to be considered an NTID. This review aims to evaluate the literature surrounding guidelines on generic substitution for NTIDs and to evaluate the evidence for flecainide to be considered an NTID.

Methods

A review of recommendations from various regulatory authorities regarding BE and NTIDs, and publications regarding the NTID characteristics of flecainide, was carried out.

Results

Regulatory authorities generally recommend reduced BE limits for NTIDs. Some, but not all, regulatory authorities specify flecainide as an NTID. The literature review demonstrated that flecainide displays NTID characteristics including a steep drug dose–response relationship for safety and efficacy, a need for therapeutic drug monitoring of pharmacokinetic (PK) or pharmacodynamics measures and intra-subject variability in its PK properties.

Conclusions

There is much evidence for flecainide to be considered an NTID based on both preclinical and clinical data. A clear understanding of the potential of proarrhythmic effects or lack of efficacy, careful patient selection and regular monitoring are essential for the safe and rational administration of flecainide.

Electronic supplementary material

The online version of this article (doi:10.1007/s00228-015-1832-0) contains supplementary material, which is available to authorized users.

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.

Time-dependent responses to provocative testing with flecainide in the diagnosis of Brugada syndrome

BACKGROUND

Time-dependent variability of electrocardiogram (ECG) in patients with Brugada syndrome could affect the interpretation of provocative testing.

OBJECTIVE

The aim of this study was to characterize ECG changes during and after flecainide infusion.

METHODS

We studied 59 consecutive patients. The ECG was continuously analyzed during the first 30 minutes of provocative testing, and a single ECG was recorded 60 minutes later. We analyzed CYP2D6 and CYP3A5 variants affecting flecainide metabolism and performed blinded measurements at lead II.

RESULTS

At baseline, ECG patterns were classified as follows: type II in 31 patients (53%), type III in 15 (25%), and normal ECG in 13 (22%). Because of induction of type I ECG, the percentage of responders progressively increased with longer recording time periods (6.8% in 10 minutes vs 11.9% in 20-30 minutes vs 18.6% in 90 minutes; P < .01). Four patients displayed a late response, which was evidenced 90 minutes after the initiation of provocative testing. QRS width differentially increased between responders and nonresponders (P < .01), with a maximum QRS width of 110 ms during the first 30 minutes being effective for identifying possible late responders (sensitivity 100%; specificity 85.6%; positive predictive value 88%; negative predictive value 100%). The incidence of CYP2D6 variants was lower in late responders than in early or delayed responders (0% vs 75% vs 100%; P = .04), while a homogeneous distribution of CYP3A5*3/*3 was observed in our population.

CONCLUSION

Response to flecainide exhibits time-dependent variability of ECG patterns and intervals. Longer periods of ECG recording increase the recognition probability of type I ECG.

Estimation of the duration after methamphetamine injection using a pharmacokinetic model in suspects who caused fatal traffic accidents

When the population parameters of drug pharmacokinetics in the human body system are known, the time-course of a certain drug in an individual can generally be estimated by pharmacokinetics. In the present two cases where methamphetamine abusers were suspected to have inflicted mortalities in traffic accidents, the time-elapse or duration immediately after methamphetamine injection to the time when the accidents occurred became points of contention. In each case, we estimated the time-course of blood methamphetamine after the self-administration in the suspects using a 2-compartment pharmacokinetic model with known pharmacokinetic parameters from the literatures. If the injected amount can be determined to a certain extent, it is easy to calculate the average time-elapse after injection by referring to reference values. However, there is considerable individual variability in the elimination rate based on genetic polymorphism and a considerably large error range in the estimated time-elapse results. To minimize estimation errors in such cases, we also analyzed genotype of CYP2D6, which influenced methamphetamine metabolism. Estimation based on two time-point blood samples would usefully benefit legal authorities in passing ruling sentences in cases involving similar personalities and circumstances as those involved in the present study.

Usefulness of postmortem biochemistry in forensic pathology: illustrative case reports

The aim of this work is to present some practical, postmortem biochemistry applications to illustrate the usefulness of this discipline and reassert the importance of carrying out biochemical investigations as an integral part of the autopsy process. Five case reports are presented pertaining to diabetic ketoacidosis in an adult who was not known to suffer from diabetes and in presence of multiple psychotropic substances; fatal flecainide intoxication in a poor metabolizer also presenting an impaired renal function; diabetic ketoacidosis showing severe postmortem changes; primary aldosteronism presented with intracranial hemorrhage and hypothermia showing severe postmortem changes. The cases herein presented can be considered representative examples of the importance of postmortem biochemistry investigations, which may provide significant information useful in determining the cause of death in routine forensic casework or contribute to understanding the pathophysiological mechanisms involved in the death process.

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.

Assessment of serum flecainide trough levels in patients with tachyarrhythmia

The reported therapeutic range for trough flecainide concentration is 200–1000 ng mL−1. Severe adverse events, such as ventricular arrhythmias, have occurred occasionally in patients whose serum flecainide exceeded 1000 ng mL−1. However, the lower limit remains controversial. We have evaluated blood flecainide concentrations in patients with tachyarrhythmia who received the drug to control palpitation. We measured the flecainide trough levels and incidence and frequency of palpitation of 44 outpatients receiving oral flecainide (150–300 mg daily). Mean serum flecainide trough concentrations differed significantly between patients with (n = 14) and without (n = 30) palpitation (259.5 ± 85.2 vs 462.2 ± 197.7 ng mL−1, P &lt; 0.01). The frequency of palpitation decreased as the serum flecainide concentration increased. The incidence of palpitation was 65% at serum flecainide concentrations &lt; 300 ng mL−1 and 11% at ≥ 300 ng mL−1. QRS values were increased significantly in patients with serum flecainide ≥ 300 ng mL−1 compared with &lt; 300 ng mL−1 (0.110 ± 0.016s vs 0.093 ± 0.019s, P &lt; 0.05). We concluded that to control paroxysm in patients receiving flecainide for tachyarrhythmia serum flecainide concentrations should be maintained at ≥ 300 ng mL−1.

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.

Gender-associated differences in pharmacokinetics and anti-arrhythmic effects of flecainide in Japanese patients with supraventricular tachyarrhythmia

OBJECTIVE

We examined the effect of gender-associated differences in pharmacokinetics on the anti-arrhythmic effects of flecainide in Japanese patients with supraventricular tachyarrhythmia and in healthy subjects.

METHODS

The study population comprised 72 outpatients (52 males and 20 females) treated with oral flecainide for supraventricular tachyarrhythmias. Serum flecainide concentrations were determined by use of high-performance liquid chromatography. The anti-arrhythmic efficacy of flecainide was assessed for at least 2 months through evaluation of symptomatology, electrocardiograms, and Holter monitoring. Pharmacokinetics of flecainide after a single 50-mg dose was examined in 14 healthy subjects (7 males and 7 females).

RESULTS

The daily dose of flecainide did not differ between males and females (2.87 +/- 0.68 versus 2.92 +/- 0.90 mg/kg). The serum flecainide concentration was significantly lower in males than in females (315 +/- 151 versus 408 +/- 184 ng/mL, P < 0.05). Clinically relevant efficacy of flecainide was achieved significantly (P < 0.05) less often in male patients (31 of 52; 60%) than in female patients (19 of 20; 95%). We confirmed that nonrenal clearance of flecainide among healthy subjects was significantly higher in males than in females (0.77 +/- 0.16 versus 0.57 +/- 0.06 L h(-1) kg(-1), P < 0.05).

CONCLUSIONS

Our results suggest that the anti-arrhythmic efficacy of flecainide differed between males and females because of gender-associated differences in pharmacokinetics.

Impact of serum flecainide level on control of atrial fibrillation in a case treated with cardiac resynchronization therapy

A 72-year-old male patient with dilated cardiomyopathy was treated with oral flecainide (100 mg/day) for persistent atrial fibrillation (AF) that could not be converted to sinus rhythm by electrical cardioversion. Initiation of flecainide treatment provided sinus rhythm without prolongation of QRS and QTc, bradycardia and first-degree atrioventricular block at a serum flecainide level of 438 ng/mL. Then, he received cardiac resynchronization therapy (CRT). Dose reduction to 50 mg/day because of stabilization of heart rate after CRT produced AF at a serum flecainide level of 270 ng/mL. Electrical cardioversion did not restore the AF to a sinus or pacing rhythm. Dose escalation of flecainide (to 100 mg/day) restored the pacing rhythm at a serum flecainide level of 401 ng/mL. This case suggests that in the Japanese population, serum flecainide level should be maintained at >300 ng/mL to control AF even after effective CRT.

Relevance of Genetics and Genomics for Prevention and Treatment of Cardiovascular Disease

Atherosclerotic cardiovascular disease (CVD) is a major health problem in the United States and around the world. Evidence accumulated over decades convincingly demonstrates that family history in a parent or a sibling is associated with atherosclerotic CVD, manifested as coronary heart disease, stroke, and/or peripheral arterial disease. Although there are several mendelian disorders that contribute to CVD, most common forms of CVD are believed to be multifactorial and to result from many genes, each with a relatively small effect working alone or in combination with modifier genes and/or environmental factors. The identification and the characterization of these genes and their modifiers would enhance prediction of CVD risk and improve prevention, treatment, and quality of care. This scientific statement describes the approaches researchers are using to advance understanding of the genetic basis of CVD and details the current state of knowledge regarding the genetics of myocardial infarction, atherosclerotic CVD, hypercholesterolemia, and hypertension. Current areas of interest and investigation--including gene-environment interaction, pharmacogenetics, and genetic counseling--are also discussed. The statement concludes with a list of specific recommendations intended to help incorporate usable knowledge into current clinical and public health practice, foster and guide future research, and prepare both researchers and practitioners for the changes likely to occur as molecular genetics moves from the laboratory to clinic.

Attention à l'emploi de la flécaïnide chez l'hémodialysé

Cardiac arrhythmias are frequent by hemodialyzed patients and may lead to the prescription of antiarrhythmic drugs among which flecainide. However there is for this latter a wide interindividual variability in the dose-plasma concentration relationship because of a genetically controlled polymorphic metabolism. That explains why it is not possible to adjust dose only to renal clearance. It becomes even more difficult at hemodialysis stage, especially if hepatic insufficiency and/or certain medications join to it. So this antiarrhythmic drug has to be cautiously initiated and plasma concentrations have to be carefully monitored to avoid side effects, sometimes crippling as in our observation, sometimes as cardiac complications.

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.

Effect of CYP2D6 genotype on flecainide pharmacokinetics in Japanese patients with supraventricular tachyarrhythmia

OBJECTIVE

To examine the effect of CYP2D6 genotype on the pharmacokinetics of flecainide, we conducted a population pharmacokinetic analysis of the data collected during routine therapeutic drug monitoring of Japanese patients with supraventricular tachyarrhythmia.

METHODS

Population analysis was performed on retrospective data from 58 patients with normal kidney and liver function treated with oral flecainide for supraventricular tachyarrhythmia. Serum concentrations of flecainide were determined by high-performance liquid chromatography. CYP2D6 genotyping for extensive metabolizer (EM), intermediate metabolizer (IM) and poor metabolizer (PM) alleles was conducted by allele-specific polymerase chain reaction (PCR) and stepdown PCR. WinNonMix was used to estimate oral clearance (CL/F) of flecainide with a one-compartment model for first-order absorption.

RESULTS

Body weight, age, sex, serum creatinine concentration (Scr), and CYP2D6 genotype influenced flecainide pharmacokinetics. The CL/F was affected by age (30% reduction in > or =70 years old) and sex (24% reduction in females). The ratios of CL/F for the five CYP2D6 genotypes were: 1.00 (EM/EM), 0.89 (EM/IM), 0.84 (EM/PM), 0.79 (IM/IM), 0.73 (IM/PM). A model including these five covariates reduced the interpatient variability of CL/F from 32.9% (base model) to 17.8%. Using a Bayesian method we estimated that the CL/F in IMs was significantly lower than in homozygous EMs (0.25+/-0.05 l h(-1) kg(-1) vs. 0.37+/-0.08 l h(-1) kg(-1), P<0.05) among male patients under 70 years old.

CONCLUSIONS

CYP2D6 genotype, even in IMs, as well as body weight, age, sex, and Scr influence flecainide pharmacokinetics in Japanese patients with supraventricular tachyarrhythmia.

Absorption kinetics and pharmacodynamics of two oral dosage forms of flecainide in patients with an episode of paroxysmal atrial fibrillation

OBJECTIVES

The objectives were to study the absorption kinetics and pharmacodynamics of two oral formulations of flecainide in patients with atrial fibrillation (AF) and to assess the relationship between pharmacokinetic parameters and the efficacy in restoring sinus rhythm.

METHODS

The data of 54 patients included in a randomised, open, parallel-group study were used. Patients received an oral solution containing 300 mg flecainide and 20 mg cisapride or three tablets each containing 100 mg flecainide. The pharmacokinetic profile of flecainide was fitted using a one-compartment model with lag-time and first-order absorption.

RESULTS

The tablets gave a maximum concentration (C (max\ fit)) of 0.43+/-0.14 mg/l at 2.37+/-1.20 h. The oral solution resulted in a much faster peak concentration at 1.05+/-0.71 h (P<0.0001). The C (max\ fit) of the oral solution of 0.60+/-0.17 mg/l was higher (P=0.0002) than that of the tablets, and interindividual variabilities of C (max\ fit) were 28% and 33%, respectively. The absorption rate constant (ka) of the oral solution was twofold larger (P<0.0001). A higher ka (P=0.04) and a duration of AF less than 24 h (P=0.006) increased the probability of cardioversion. If atrial fibrillation lasted less than 24 h, only ka (P=0.016) was obtained as a significant variable in multivariate analysis. The linear models of QRS interval changes versus flecainide concentrations of both formulations had similar slopes with similar interindividual variabilities.

CONCLUSIONS

The probability of cardioversion after an oral loading dose of flecainide in patients with AF is dependent on ka. Rapid loading of the effect compartment, i.e. the atria, appears to be critical to reach cardioversion. Higher flecainide serum concentrations and a more rapid absorption does not increase interindividual variability of pharmacokinetics and pharmacodynamics, which is important when safety is considered.

Simultaneous determination of serum flecainide and its metabolites by using high performance liquid chromatography

Simultaneous determination of serum flecainide and its oxidative metabolites was carried out by using high performance liquid chromatography (HPLC) equipped with conventional octadecylsilyl silica (ODS) column and fluorescence detector. Flecainide and its metabolites, m-O-dealkylated flecainide (MODF) and m-O-dealkylated lactam of flecainide (MODLF) in serum were extracted with ethyl acetate. The recoveries of flecainide, MODF and MODLF were greater than 92, 93, and 60% with the coefficient of variations (CVs) less than 3.2, 5.8, and 5.3%, respectively. The calibration curves were linear at the concentration range of 50-1500 ng/mL for flecainide and 10-500 ng/mL for MODF and MODLF (r > 0.999). The CVs for intra-day assay were 2.7-5.3% for flecainide, 3.0-4.2% for MODF, and 3.7-4.3% for MODLF, respectively. The CVs for inter-day assay were 7.0-8.4% for flecainide, 3.3-6.7% for MODF, and 4.4-7.7% for MODLF, respectively. This assay method can be used for assessing the metabolic ability of flecainide in the patients with tachyarrhythmia.

Pharmacogenetics of cytochrome p4502D6: genetic background and clinical implication

Interindividual differences in the pharmacokinetics of a number of drugs are often due to hereditary polymorphisms of drug-metabolizing enzymes. Most important is cytochrome p4502D6 (CYP2D6), also known as debrisoquine/sparteine hydroxylase. It catalyzes hydroxylation or demethylation of more than 20% of drugs metabolized in the human liver, such as neuroleptics, antidepressants, some beta-blockers and many others like codeine. About 7%-10% of Caucasians lack any CYP2D6 activity due to deletions and frame-shift or splice-site mutations of the gene. About 1%-3% of Middle-Europeans, but up to 29% of Ethiopians display gene duplications, leading to elevated so-called ultrarapid metabolization rates. Meanwhile there is now a much better understanding of the genetic background of poor, intermediate, extensive and ultrarapid metabolizers, enabling a more precise DNA genotyping-based prediction of plasma levels. Since there is evidence that deteriorated drug elimination partly accounts for drug side-effects, CYP2D6 genotyping could contribute to an individualized and therefore optimized drug therapy.

Gender, ethnicity, and genes in cardiovascular disease. Part 2: implications for pharmacotherapy

Women are underrepresented in clinical trials. Lower doses of beta-blockers are required for Southeast Asians. ACE and ARB's are teratogenic in the second trimester. Torsades de Pointes is more common in women related to a longer QT-interval. Lower dose OCPs decrease the risk of MI, stroke and thrombosis. HRTs are not effective for CAD prevention.

Evaluation of a novel method to estimate absolute bioavailability of drugs from oral data

The goal of this investigation was to evaluate the performance of a novel method allowing estimation of absolute bioavailability from oral data only. In contrast to the traditional method, which compares areas under the drug concentration time curves after oral and intravenous administration in subjects with normal renal function, the novel method uses total and renal clearance values following oral administration from subjects with varying renal functions to estimate bioavailability. The novel method can also provide estimates for nonrenal clearance.Published data on total clearance and renal clearance of drugs obtained from subjects with variable renal functions were collected, the novel method applied, estimates of bioavailability and nonrenal clearance obtained and compared with reported estimates by the traditional methods. In addition computations were performed to assess various factors that could possibly affect the reliability of the novel method. The results indicated that the novel method provides accurate estimates for bioavailability of drugs meeting the prerequisites: linear kinetics, predominant renal excretion in normals, absence of metabolic polymorphism and independence of bioavailability and nonrenal clearance from renal function. The average (standard deviation) of the prediction error and bias of the bioavailability estimates by the novel method was 7.8 (6.0) and -1.4 (9.8)%, respectively. The estimates for nonrenal clearance by the novel method were less accurate. The computations confirmed that the estimates by the novel method are sensitive to renal-function dependent changes in nonrenal clearance and bioavailability and also depend on the extent of renal excretion of a drug. In conclusion, the novel method's main use is to diagnose absence or presence of changes in bioavailability and non-renal clearance of drugs in populations with varying renal function.

Principles and Clinical Application of Assessing Alterations in Renal Elimination Pathways

Drugs and metabolites are eliminated from the body by metabolism and excretion. The kidney makes the major contribution to excretion of unchanged drug and also to excretion of metabolites. Net renal excretion is a combination of three processes - glomerular filtration, tubular secretion and tubular reabsorption. Renal function has traditionally been determined by measuring plasma creatinine and estimating creatinine clearance. However, estimated creatinine clearance measures only glomerular filtration with a small contribution from active secretion. There is accumulating evidence of poor correlation between estimated creatinine clearance and renal drug clearance in different clinical settings, challenging the 'intact nephron hypothesis' and suggesting that renal drug handling pathways may not decline in parallel. Furthermore, it is evident that renal drug handling is altered to a clinically significant extent in a number of disease states, necessitating dosage adjustment not just based on filtration. These observations suggest that a re-evaluation of markers of renal function is required. Methods that measure all renal handling pathways would allow informed dosage individualisation using an understanding of renal excretion pathways and patient characteristics. Methodologies have been described to determine individually each of the renal elimination pathways. However, their simultaneous assessment has only recently been investigated. A cocktail of markers to measure simultaneously the individual renal handling pathways have now been developed, and evaluated in healthy volunteers. This review outlines the different renal elimination pathways and the possible markers that can be used for their measurement. Diseases and other physiological conditions causing altered renal drug elimination are presented, and the potential application of a cocktail of markers for the simultaneous measurement of drug handling is evaluated. Further investigation of the effects of disease processes on renal drug handling should include people with HIV infection, transplant recipients (renal and liver) and people with rheumatoid arthritis. Furthermore, changes in renal function in the elderly, the effect of sex on renal function, assessment of living kidney donors prior to transplantation and the investigation of renal drug interactions would also be potential applications. Once renal drug handling pathways are characterised in a patient population, the implications for accurate dosage individualisation can be assessed. The simultaneous measurement of renal function elimination pathways of drugs and metabolites has the potential to assist in understanding how renal function changes with different disease states or physiological conditions. In addition, it will further our understanding of fundamental aspects of the renal elimination of drugs.

The gastroprokinetic and antiemetic drug metoclopramide is a substrate and inhibitor of cytochrome P450 2D6

Metoclopramide is increasingly prescribed for conditions previously treated with cisapride, but its metabolic enzymology and drug interactions are poorly understood. Using human liver microsomes (HLMs) and recombinant human cytochromes P450 (P450), we identified the major route of metoclopramide oxidation and the P450 isoforms involved. We also documented the ability of metoclopramide to inhibit the P450 system, using isoform-specific substrate reaction probes of CYP1A2, 2C19, 2C9, 2D6, 2E1, and 3A4. Metoclopramide was predominantly N-dealkylated to monodeethylmetoclopramide, a metabolite that has not so far been described in humans. Formation rate of this metabolite followed Michaelis-Menten kinetics (K(m), 68 +/- 16 microM; V(max), 183 +/- 57 pmol/min/mg of protein; n = 3 HLMs). Of the isoform-specific inhibitors tested, 1 microM quinidine was a potent inhibitor of metoclopramide (25 microM) monodeethylation [by an average of 58.2%; range, approximately 38% (HL09-14-99) to 78.7% (HL161)] with K(i) values highly variable among the HLMs tested (K(i), mean +/- S.D., 2.7 +/- 2.8 microM; range, 0.15 microM in HL66, 2.4 microM in HL09-14-99, and 5.7 microM in HLD). Except troleandomycin, which inhibited metoclopramide metabolism in only one HLM (by approximately 23% in HL09-14-99), the effect of other inhibitors was minimal. Among the recombinant human P450 isoforms examined, monodeethylmetoclopramide was formed at the highest rate by CYP2D6 (V = 4.5 +/- 0.3 pmol/min/pmol of P450) and to a lesser extent by CYP1A2 (0.97 +/- 0.15 pmol/min/pmol of P450). The K(m) value derived (approximately 53 microM) was close to that from HLMs (68 microM). Metoclopramide is a potent inhibitor of CYP2D6 at therapeutically relevant concentrations (K(i) = 4.7 +/- 1.3 microM), with negligible effect on other isoforms tested. Further inhibition of CYP2D6 was observed when metoclopramide was preincubated with HLMs and NADPH-generating system before the substrate probe was added (maximum rate of inactivation, K(inact) = 0.02 min(-1), and the concentration required to achieve the half-maximal rate of inactivation, K'(i) = 0.96 microM), suggesting mechanism-based inhibition. Metoclopramide elimination is likely to be slowed in poor metabolizers of CYP2D6 or in patients taking inhibitors of this isoform, whereas metoclopramide itself could reduce the clearance of CYP2D6 substrate drugs.

Phenytoin intoxication induced by fluvoxamine

A patient had phenytoin intoxication after administration of fluvoxamine, a selective serotonin reuptake inhibitor. The serum concentration of phenytoin increased dramatically from 16.6 to 49.1 microg/mL when fluvoxamine was coadministered, although the daily dosage of phenytoin and other drugs had not changed. During phenytoin and fluvoxamine treatment, ataxia, a typical side effect of phenytoin, was observed. The genotypes of CYP2C9 and 2C19, the enzymes responsible for phenytoin metabolism, were homozygous for the wild-type alleles (CYP2C9*1/*1 and 2C19*1/ *1). The interaction may be a result of inhibition of both CYP2C9 and 2C19 by fluvoxamine.

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.

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.

Human variability and noncancer risk assessment- An analysis of the default uncertainty factor

A 10-fold uncertainty factor is used for noncancer risk assessments to allow for possible interindividual differences between humans in the fate of the chemical in the body (kinetics) and target organ sensitivity (dynamics). Analysis of a database on the variability in each of these aspects is consistent with an even subdivision of the 10-fold factor into 10(0.5) (3.16) for kinetics and 10(0.5) (3.16) for dynamics. Analysis of the number of subjects in a normally and log-normally distributed population which would not be covered by factors of 3.16 supports this subdivision and also the use of a 10-fold factor to allow for both aspects. Analysis of kinetic data for subgroups of the population indicates that the standard default value of 3.16 for kinetics will not be adequate for all routes of elimination and all groups of the population. A scheme is proposed which would allow the selection of appropriate default uncertainty factors based on knowledge of the biological fate and effects of the chemical under review. Copyright 1998 Academic Press.

Determination of dextromethorphan metabolic phenotype by salivary analysis with a reference to genotype in Chinese patients receiving renal hemodialysis

BACKGROUND

The polymorphic metabolism of debrisoquin and sparteine by cytochrome P450IID6 (CYP2D6) is genetically determined. Determination of the CYP2D6 metabolic phenotype with conventional urine analytic methods is not feasible in anuric patients with renal failure. The possibility of using salivary analysis, with dextromethorphan as a probe drug, to determine the CYP2D6 metabolic phenotype in patients with renal failure was evaluated.

METHODS AND RESULTS

One hundred four Chinese patients with renal failure were recruited. All 104 patients were receiving hemodialysis. Saliva was collected before and at 3 hours after each patient took a capsule of dextromethorphan hydrobromide (30 mg). Four patients were excluded because of insufficient samples of saliva. The distribution of logarithms of the metabolic ratios (log[MR]) in the 100 patients appeared to be normal. Administration of quinidine sulfate (200 mg twice daily) to nine of the patients significantly and markedly increased the dextromethorphan metabolic ratios. The metabolic ratios of nine patients pretreated with quinidine were higher than any of the 100 patients with renal failure who did not receive quinidine pretreatment. A metabolic ratio of 33 separated these two groups. Genomic deoxyribonucleic acid was extracted from whole blood in a subset of patients. Polymerase chain reaction (PCR)-based methods were used to detect the CYP2D6 and B mutant genes. Mutant B alleles (which are common in white poor metabolizers) of CYP2D6 genes were not detected in any of the 47 subjects tested. A PCR-based test of cytosine (C188) to thymine (T188) polymorphism at 188 base pairs in exon 1 of CYP2D6 genes was performed in 61 patients. Subjects who were homozygous for C188 had significantly (p = 0.0067) lower log[MR] values than those who were homozygous for T188.

CONCLUSIONS

Determination of dextromethorphan metabolic ratios in saliva is feasible in patients with renal failure requiring hemodialysis. All subjects in this study appeared to be "extensive metabolizer" phenotype for CYP2D6, and no poor metabolizer was identified. From the results with quinidine pretreatment, a metabolic ratio of 33 is suggested to be a tentative antimode for identification of poor metabolizers in patients with renal failure.

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.

Genetic differences in drug disposition

Genetic polymorphisms of drug metabolizing enzymes are well recognized. This review presents molecular mechanisms, ontogeny and clinical implications of genetically determined intersubject variation in some of these enzymes. Included are the polymorphic enzymes N-acetyl transferase, cytochromes P4502D6 and 2C, which have been well described in humans. Information regarding other Phase I and Phase II polymorphic pathways, such as glutathione and methyl conjugation and alcohol and acetaldehyde oxidation continues to increase and are also discussed. Genetic factors effecting enzyme activity are frequently important determinants of the disposition of drugs and their efficacy and toxicity. In addition, associations between genetic differences in these enzymes and susceptibility to carcinogens and teratogens have been reported. Ultimately, the application of knowledge regarding these genetic factors of enzyme activity may guide medical therapy and minimize xenobiotic-induced disease.

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.

The effect of a low dose of quinidine on the disposition of flecainide in healthy volunteers

We have studied the effects of quinidine on ECG intervals and on the pharmacokinetics of flecainide and its two metabolites in 6 healthy men in an open randomized crossover study. Flecainide acetate (150 mg) was given as a constant rate i.v. infusion over 30 min. Quinidine (50 mg orally), given the previous evening, did not change the volume of distribution of flecainide (7.9 vs 7.4 l.kg-1), but significantly increased its half-life (8.8 vs 10.7 h). This was attributable to a reduction in total clearance (10.6 vs 8.1 ml.min-1 x kg-1), most of it being accounted for by a reduction in non-renal clearance (7.2 vs 5.2 ml.min-1 x kg-1). The excretion of the metabolites of flecainide over 48 h was significantly reduced. These findings suggest that quinidine inhibits the first step of flecainide metabolism, although it may also reduce its renal clearance, but to a lesser extent (3.5 vs 2.9 ml.min-1 x kg-1). The effects of flecainide on ECG intervals were not altered by quinidine. Thus, quinidine tends to shift extensive metabolizer status for flecainide towards poor metabolizer status and may also alter its renal excretion.

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.

Flecainide: evidence of non-linear kinetics

The pharmacokinetics of flecainide has been studied in 12 patients with ventricular arrhythmias, both after single administration and during chronic treatment. Both the half-life and the AUC were significantly increased during chronic treatment. This suggests that the kinetics of flecainide might be non-linear also in patients with normal kidney and liver function. The increase in plasma flecainide levels during chronic treatment could not be predicted, so close monitoring of its plasma levels is advisable.

Pharmacokinetics of newer drugs in patients with renal impairment (Part II)

Cardiovascular diseases occur frequently in patients with renal failure. Any pharmacokinetic impairment in these diseases should be considered when individualizing drug therapy. The pharmacokinetics of new cardiovascular drugs in uraemic patients are reviewed: alpha- and beta-blocking agents, ACE inhibitors, centrally acting antihypertensive agents, calcium antagonists, antiarrhythmic agents and inotropic agents. Guidelines are proposed for adjustment of dosage regimens as a function of renal impairment. Renal or extrarenal elimination of drugs and their metabolites, and the activity of the latter, are taken into account. The disposition of new drugs such as flestolol, alacepril, delapril, propafenone, milrinone or enoximone, is not well documented in patients with renal failure. Further characterizations of the elimination of these compounds are needed and the potential therapeutic or toxic effects of the metabolites require evaluation to determine whether the dosage needs to be adjusted. Until such investigations are performed, those drugs should not be used in uraemic patients; if no therapeutic alternative is available, clinical controls are necessary at regular intervals. Relationships between pharmacological or therapeutic effects and drug plasma concentrations should be evaluated for such long term use drugs. The knowledge of a plasma concentration therapeutic window is important to provide information which will be useful in determining appropriate drug dosage in renal failure.

Codeine O-demethylation: rat strain differences and the effects of inhibitors

The oxidative metabolism of more than 20 drugs (e.g. sparteine, debrisoquine, dextromethorphan) is mediated by cytochrome P450IID6. Codeine O-demethylation to morphine was recently demonstrated to co-segregate with the polymorphic metabolism of debrisoquine and dextromethorphan. The female Dark-Agouti rat (DA) is an animal model for the poor metabolizer phenotype (PM) using debrisoquine or dextromethorphan as substrates. Studies were carried out to evaluate codeine metabolism in liver microsomes from female DA and Sprague-Dawley (SD) rats. The intrinsic clearance of codeine to morphine was 10-fold lower in DA rats due to a 5-fold higher Km (287 vs 49 microM) and a 2-fold lower Vmax (48 vs 94 nmol/mg/hr). Nineteen drugs were tested for inhibition of codeine O-demethylation. The four most potent competitive inhibitors were dextromethorphan (Ki = 2.53 microM), propafenone (Ki = 0.58 microM), racemic methadone (Ki = 0.3 microM) and quinine (Ki = 0.07 microM). The differences in morphine formation from codeine between SD and DA rats and the inhibition results show that this animal model appears to be a suitable model for the human EM and PM phenotypes, respectively. These strains could be used to study the pharmacodynamic consequences of the genetic polymorphism in codeine O-demethylation, and the effects of metabolic inhibitors. The outcome of these studies could impact on the therapy of pain control.

Polymorphic flecainide disposition under conditions of uncontrolled urine flow and pH

The pharmacokinetics of R- and S-flecainide have been determined in five poor (PM) and five extensive (EM) metabolisers of sparteine/debrisoquine under conditions of uncontrolled urine flow and pH. The half-lives of R- and S-flecainide in PMs (R 19.3 h; S 16.1 h) were approximately twice those observed in EMs (R 8.8 h; S 9.1 h). The apparent oral clearances of R- and S-flecainide were lower in PMs (R 313 ml.min-1; S 379 ml.min-1) than in EMs (R 783 ml.min-1; S 828 ml.min-1). The renal clearance, however, was comparable for both enantiomers in both EMs and PMs, and therefore the phenotypic differences in flecainide disposition observed must be due to differences in metabolic clearance. The nonrenal clearance of both enantiomers was significantly lower in poor (R 123 ml.min-1; S 201 ml.min-1) relative to extensive metabolisers (R 533 ml.min-1; S 586 ml.min-1). The partial clearance to the two major metabolites meta-O-dealkylated flecainide (MODF) and the meta-O-dealkylated lactam of flecainide (MODLF) was significantly lower in poor (62 ml.min-1) than extensive (267 ml.min-1) metabolisers. The impairment in flecainide metabolism in poor metabolisers of sparteine/debrisoquine has therefore been confirmed. Under conditions reflecting the clinical situation the difference in disposition between EMs and PMs would be considerable. However, it may be predicted that at standard doses concentrations greater than 1000 ng.ml-1 would not be attained in the PMs studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Enzyme immunoassay for flecainide

In order to develop an enzyme immunoassay (ELISA) for measurement of plasma flecainide levels, we first synthesized flecainide-hemisuccinate-BSA as immunogen. A highly specific antiserum was then raised in rabbits. Enzyme-labelled flecainide was prepared by the mixed anhydride method using hemifumarate-flecainide and horse radish peroxidase. The limit of detection was 10 ng/ml, the method was linear up to 1,000 micrograms/l. The intra and inter-assay variation was 5.1-7.8%. Cross reactivity with various metabolites of flecainide or some chemically related drugs was less than 0.1%. By contrast the cross reaction with some structural analogues achieved 25%. The importance of parts of the aromatic ring structure in immunogenic properties of this drug is discussed.

Genetic polymorphism of sparteine/debrisoquine oxidation: a reappraisal

Polymorphic oxidation of the sparteine/debrisoquine-type has been shown to account for much of the interindividual variation in the metabolism, pharmacokinetics and pharmacodynamics of an increasing number of drugs, including some antiarrhythmic, antidepressant and beta-adrenoceptor antagonist agents. Impaired hydroxylation of these drugs results from the absence of the enzyme cytochrome P450IID6 in the livers of poor metabolisers, who constitute 6% to 10% of Caucasian populations. The clinical importance of the phenomenon has to be explored further and for most sparteine/debrisoquine-related substrates there is a need for controlled prospective studies to define the consequences to the patient of impaired or enhanced drug oxidation.

Flecainide acetate dose-concentration relationship in cardiac arrhythmias: influence of heart failure and amiodarone

The trough concentration-dose (C/D) ratio of flecainide was prospectively studied in 78 patients with various cardiac arrhythmias. After the removal of two outlier values, no influence of body weight on C/D ratio was evidenced. Coadministration of amiodarone, and, moreover, the presence of heart failure increase the C/D ratio, from 2.01 +/- 0.78 to 2.55 +/- 0.37 and 2.9 +/- 1.19 ng/ml/mg, respectively (p less than 0.001 by two-factor analysis of variance). The presence of both heart failure and amiodarone therapy increases the C/D ratio to 3.88 +/- 1.07 ng/ml/mg. A single loading oral dose (30 mg/kg) of amiodarone increased C/D measured at the sixth hour in nine patients from 2.27 +/- 0.50 to 2.57 +/- 0.73 ng/ml/mg (p less than 0.05). The trough C/D ratio increased more during chronic treatment from 2.03 +/- 0.86 to 2.92 +/- 1.32 ng/ml/mg (p less than 0.05). Thus, a dosage reduction of flecainide (of 50% in some cases) is mandatory, in case of heart failure or the combination with amiodarone therapy, to obtain a plasma level of the drug that is similar to those observed in patients with a normal heart and without amiodarone therapy. The flecainide-amiodarone interaction seems time dependent.