The metabolism of mexiletine in relation to the debrisoquine/sparteine-type polymorphism of drug oxidation

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.

Interactions of rosiglitazone and anti-arrhythmic drugs in animal model

BACKGROUND

Diabetes increases the risk of vascular problems by two times compared with a healthy individual, with deposition of fats in blood vessel and this includes cardiovascular disease. The treatment regimens for patients suffering from both diseases generally include prolonged use of anti-diabetic drugs for diabetes and anti-arrhythmic drugs for cardiac arrhythmias.

AIM

The aim of the study is to compare the influence of Mexiletine and Disopyramide on the pharmacodynamics (PDs) of Rosiglitazone in normal and diabetic rats.

MATERIALS AND METHODS

The study was conducted in normal rats and diabetic induced rats (with Alloxan monohydrate 100 mg/kg body weight). Albino rats weighing between 160 and 280 g were administered oral doses of Rosiglitazone 0.72 mg/kg, Mexiletine 36 mg/kg, or Disopyramide 18 mg/kg of bodyweight and their combination, with 1 week of washout between treatments. Eighteen rats were divided into three sub-sets with six rats in each sub-set. After 4 days, the blood glucose was estimated to confirm the diabetes. The Analysis of Covariance (ANCOVA) using MedCalc(®) software Version 11.6.1.0 was performed to analyze mean change in blood glucose between treatments with body weight as co-variable and treatment as factor for normal and diabetic rats.

RESULTS

No statistically significant difference in mean change in blood glucose between Rosiglitazone in comparison with Rosiglitazone + Mexiletine or Rosiglitazone + Disopyramide was observed in normal and diabetic rats (P = 0.606). The maximum mean change in blood glucose for Rosiglitazone and Rosiglitazone + Mexiletine or Rosiglitazone + Disopyramide was observed at 1 h and 8 h in normal and diabetic rats. The post hoc analysis showed baseline correction method has increased the reliability of the results (P < 0.001).

CONCLUSION

The study concludes that PD activity of Rosiglitazone was not affected by the anti-arrhythmic drugs. This study introduced a new statistical methodology for analyzing the blood glucose endpoint.

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.

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.

Stereospecific synthesis of ?para-hydroxymexiletine? and sodium channel blocking activity evaluation

Both enantiomers of "para-hydroxymexiletine" (PHM), one of the main metabolites of mexiletine, were synthesized and fully characterized. Properties of (R)- and (S)-PHM, in terms of blocking potency and stereoselectivity on frog skeletal muscle Na(+) channels, were evaluated. The presence of a hydroxy group on the aryloxy moiety in the 4-position, as in PHM, reduced potency with respect to mexiletine in reducing I(Na max). However, PHM showed clear use-dependent behavior similar to that of mexiletine and, in contrast with what is observed with the parent compound, maintained its stereoselectivity during the use-dependent block. Chirality 16:72-78, 2004.

Impact of CYP2D6*10 on mexiletine pharmacokinetics in healthy adult volunteers

OBJECTIVE

In vitro studies with human liver microsomes have suggested that the oxidative conversion of mexiletine (MX) to its metabolites is catalyzed by CYP2D6 and is significantly impaired in microsomes with the CYP2D6*10/*10 genotype. Therefore, we examined the influence of the CYP2D6*10 allele on MX pharmacokinetics in Japanese subjects.

METHODS

Subjects with CYP2D6*1/*1 (group *1/*1; n=5), CYP2D6*10/*10 (group *10/*10; n=6) and CYP2D6*5/*10 (group *5/*10; n=4) genotypes received a single 200-mg dose of MX. Plasma and urinary levels of MX and its metabolites ( p-hydroxymexiletine (PHM), hydroxymethylmexiletine (HMM) and N-hydroxymexiletine (NHM)) were determined by means of high-performance liquid chromatography.

RESULTS

Mean area under the concentration-time curve (AUC) and t(1/2) of MX were significantly ( P<0.05) higher in the CYP2D6*10/*5 group (AUC 11.23+/-3.05 micro g.h/ml; t(1/2) 15.5+/-3.2 h) than in the CYP2D6*1/*1 (AUC 5.53+/-1.01 micro g.h/ml; t(1/2) 8.1+/-1.6 h) and CYP2D6*10/*10 (AUC 7.32+/-2.36 micro g.h/ml; t(1/2) 10.8+/-2.8 h) groups, but there was no significant difference between the CYP2D6*1/*1 and CYP2D6*10/*10 groups. The maximum plasma concentration of MX was not significantly different among the three groups. The values of urinary excretion of PHM and HMM in the CYP2D6*1/*1 group were significantly ( P<0.05) higher than those in the CYP2D6*10/*10 and CYP2D6*5/*10 groups, but there was no significant difference in that of NHM among the three groups. Clearance of MX in the CYP2D6*5/*10 subjects was comparable to that in the poor metabolizers described previously.

CONCLUSION

The present findings demonstrated that carriers of the CYP2D6*10 allele showed a decreased clearance of MX. Subjects with CYP2D6*5/ *10 showed significantly ( P<0.05) increased plasma levels of MX, and homozygotes for CYP2D6*10 also showed an increase, although to a lesser extent. Thus, the CYP2D6*10 allele plays an important role in MX pharmacokinetics.

Mexiletine carbonyloxy beta-D-glucuronide: a novel metabolite in human urine

1. The study was performed to isolate and characterize a glucuronic acid conjugate of mexiletine that releases mexiletine on acid hydrolysis from urine samples obtained from healthy volunteers following a single oral dose of mexiletine. 2. The [M-H]- ion of the isolated metabolite was observed at m/z 398 in the negative electrospray ionization mass spectrum. This mass number was 44 higher than that of the product generated when mexiletine is subjected to direct glucuronidation. In positive-ion mode, collision-induced dissociation of the quasimolecular ion [M+NH4]+, m/z 417, gave product ions at m/z 224, 180 and 58. These mass spectral data indicated that the metabolite contained a carbonyloxy moiety in its structure in addition to mexiletine and a glucuronic acid moiety. 3. The presence of this carbonyloxy moiety was further supported by the following chemical reactions. When the metabolite was hydrolysed with an aqueous solution of 1 M sodium hydroxide at room temperature, mexiletine was released, whereas the N-methoxycarbonyl derivative of mexiletine was obtained after treatment of the metabolite with methanolic sodium hydroxide solution. 4. The results indicated that the structure of the isolated metabolite was the N-carbonyloxy beta-D-glucuronic acid conjugate of mexiletine.

CYP2D in the Brain

CYP2D1, 2D2, 2D3, and 2D4 are major CYP2D isoforms expressed in the rat. In humans, only CYP2D6 is expressed. In rat brain, the mRNA for CYP2D4 is most abundant in cerebellum, striatum, pons and medulla oblongata. In human brain, CYP2D6 mRNA expression was detected in all regions with highest levels observed in cerebellum. CYP2D isoforms are involved in the metabolism of not only xenobiotics such as antidepressants, beta-adrenergic blockers, antiarrhysthmics, and antihypertensives, but also endogenous compounds such as trace amine and neurosteroids. Among 11 isoforms of human recombinant P450s, only CYP2D6 exhibited an ability to efficiently convert tyramine which exists in the brain, to dopamine. CYP2D4 and CYP2D6 which are the predominant CYP2D isoforms in the rat and human brain, respectively, possess 21-hydroxylation activity for both progesterone and allopregnanolone. CYP2D4, not P450c21, works as a steroid 21-hydroxylase in the brain. These results suggested that CYP2D in the brain may be involved in the metabolism of neuronal amines and steroids and in the regulation of the central nervous system.

Influence of the CYP2D6*10 allele on the metabolism of mexiletine by human liver microsomes

AIMS

To study the influence of CYP2D6*10 on the formation of p-hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM) using microsomes from human liver of known genotypes.

METHODS

Microsomes from human livers of genotype CYP2D6*1/*1 (n = 5), *1/*10 (n = 6) and *10/*10 (n = 6) were used in this study. The formation of PHM and HMM was determined by high-performance liquid chromatography.

RESULTS

The formation rates of PHM and HMM were decreased by more than 50% and 85% in CYP2D6*1/*10 and *10/*10 microsomes, respectively, compared with *1/*1 microsomes.

CONCLUSIONS

The metabolism of mexiletine to form PHM and HMM appears to be impaired to a significant extent in human liver microsomes from hetero- and homozygotes of CYP2D6*10.

Clinical pharmacokinetics of mexiletine

Mexiletine, a class Ib antiarrhythmic agent, is rapidly and completely absorbed following oral administration with a bioavailability of about 90%. Peak plasma concentrations following oral administration occur within 1 to 4 hours and a linear relationship between dose and plasma concentration is observed in the dose range of 100 to 600 mg. Mexiletine is weakly bound to plasma proteins (70%). Its volume of distribution is large and varies from 5 to 9 L/kg in healthy individuals. Mexiletine is eliminated slowly in humans (with an elimination half-life of 10 hours). It undergoes stereoselective disposition caused by extensive metabolism. Eleven metabolites of mexiletine are presently known, but none of these metabolites possesses any pharmacological activity. The major metabolites are hydroxymethyl-mexiletine, p-hydroxy-mexiletine, m-hydroxy-mexiletine and N-hydroxy-mexiletine. Formation of hydroxymethyl-mexiletine, p-hydroxy-mexiletine and m-hydroxy-mexiletine is genetically determined and cosegregates with polymorphic debrisoquine 4-hydroxylase [cytochrome P450 (CYP) 2D6] activity. On the other hand, CYP1A2 seems to be implicated in the N-oxidation of mexiletine. Various physiological, pathological, pharmacological and environmental factors influence the disposition of mexiletine. Myocardial infarction, opioid analgesics, atropine and antacids slow the rate of absorption, whereas metoclopramide enhances it. Rifampicin (rifampin), phenytoin and cigarette smoking significantly enhance the rate of elimination of mexiletine, whereas ciprofloxacin, propafenone and liver cirrhosis decrease it. Cimetidine, ranitidine, fluconazole and omeprazole do not modify the disposition of mexiletine. Conversely, mexiletine is known to alter the disposition of other drugs, such as caffeine and theophylline. Factors affecting the elimination of mexiletine may be clinically important and dosage adjustments are often necessary.

Stereoselective disposition of the antiarrhythmic agent mexiletine during the concomitant administration of caffeine

Caffeine consumption is extensive in industrialized countries and its role in drug-drug interactions is often overlooked. CYP1A2, the major cytochrome P450 isoform involved in the metabolism of caffeine, has also been implicated in the formation of N-hydroxymexiletine, the major metabolite of mexiletine. Therefore, the objective of this study was to assess the effects of a clinically relevant dosage of caffeine on the stereoselective disposition of mexiletine. Fourteen healthy volunteers--10 extensive metabolizers (EMs) and 4 poor metabolizers (PMs) of CYP2D6--received a single 200 mg oral dose of racemic mexiletine hydrochloride on two occasions (1 week apart): once by itself and once during administration of caffeine (100 mg four times daily). Serial blood and urine samples were collected and pharmacokinetic parameters were estimated. Although the total clearance of mexiletine was not significantly altered by the coadministration of caffeine in EMs and PMs, a stereoselective decrease (16% in EMs and 14% in PMs) in the urinary recovery of N-hydroxymexiletine from the R-(-)-enantiomer was observed. Also, the partial metabolic clearance of R-(-)-mexiletine to N-hydroxymexiletine glucuronide was reduced from 126 +/- 48 mL/min to 106 +/- 32 mL/min and 152.6 (73.4-196.2) mL/min to 109 (77-127) mL/min by the coadministration of caffeine in EMs and PMs, respectively. Consequently, the R/S ratio for urinary recovery and the partial metabolic clearance of mexiletine to N-hydroxymexiletine were 28% lower during the coadministration of caffeine. In conclusion, data obtained in this study indicate that coadministration of caffeine does not lead to clinically significant changes in mexiletine plasma concentrations. However, results obtained suggest that CYP1A2 is involved in the formation of N-hydroxymexiletine.

Mexiletine. A review of its therapeutic use in painful diabetic neuropathy

Mexiletine is an orally active local anaesthetic agent which is structurally related to lidocaine (lignocaine) and has been used for alleviating neuropathic pain of various origins. Mexiletine has been evaluated in several randomised, placebo-controlled trials in patients with painful diabetic neuropathy. The drug decreased mean visual analogue scale (VAS) pain ratings in all studies that used this measure, although in only 2 studies was this effect significantly greater than the often substantial responses seen with placebo. The clinical significance of these decreases is not clear. Statistically significant (vs placebo) reductions in VAS pain ratings were observed in 16 patients receiving mexiletine 10 mg/kg/day for 10 weeks in 1 study and in nocturnal (but not diurnal) pain in 31 patients receiving mexiletine 675 mg/day for 3 weeks in another. Retrospective analysis of another study revealed that mexiletine recipients (225 to 675 mg/day) who described their pain as stabbing, burning or formication on the pain-rating-index-total instrument of the McGill Pain Questionnaire, experienced statistically significant reductions in VAS pain scores after 5 weeks, compared with placebo recipients. Mexiletine generally did not have a significant influence on the quality of sleep in patients with diabetic neuropathy. In Japanese patients, statistically significant reductions in subjective pain ratings were achieved with mexiletine 300 mg/day in 1 study and with 450 mg/day in a further study. In controlled trials, the frequency of adverse events in patients receiving mexiletine for painful diabetic neuropathy ranged from 13.5 to 50%. Gastrointestinal complaints, of which nausea was the most frequent, were the most common adverse events in mexiletine recipients. Central nervous system complaints were uncommon, but included: sleep disturbance, headache, shakiness, dizziness and tiredness. Serious cardiac arrhythmias have not been reported in patients receiving mexiletine for painful diabetic neuropathy; however, transient tachycardia and palpitations have been reported. There are significant differences in the metabolism of mexiletine between people who have cytochrome P450 2D6 [CYP2D6; extensive metabolisers (EMs)] and those who lack this isoenzyme [poor metabolisers (PMs)]. EMs, but not PMs, are susceptible to drug interactions between mexiletine and drugs that inhibit CYP2D6 (e.g. quinidine). Moreover, mexiletine inhibits CYP2D6-mediated metabolism of metoprolol and cytochrome P450 1A2-mediated metabolism of theophylline. Phenytoin and rifampicin (rifampin) induce the metabolism of mexiletine. Clearance of mexiletine is impaired in patients with hepatic, but not renal, dysfunction. Hence, dosage adjustments may be necessary in patients with liver disease.

CONCLUSIONS

Tricyclic antidepressants (TCAs) are the agents of choice for painful diabetic neuropathy; however, they are ineffective in approximately 50% of patients and are generally not well tolerated. Mexiletine is an alternative agent for the treatment of painful diabetic neuropathy in patients who have not had a satisfactory response to, or cannot tolerate, TCAs and/or other drugs.

Involvement of CYP1A2 in mexiletine metabolism

AIMS

Mexiletine has been reported to be hydroxylated by cytochrome P450 2D6 (CYP2D6) in humans. However, the involvement of CYP1A2 in the metabolism of mexiletine has been proposed based on the interaction with theophylline which is mainly metabolized by CYP1A2. The aim of this study was to clarify the role of human CYP1A2 in mexiletine metabolism.

METHODS

Human CYP isoforms involved in mexiletine metabolism were investigated using microsomes from human liver and B-lymphoblastoid cells expressing human CYPs. The contributions of CYP1A2 and CYP2D6 to mexiletine metabolism were estimated by the relative activity factor (RAF).

RESULTS

Mexiletine p- and 2-hydroxylase activities in human liver microsomes were inhibited by ethoxyresorufin and furafylline as well as quinidine. Mexiletine p- and 2-hydroxylase activities in microsomes from nine human livers correlated significantly with bufuralol 1'-hydroxylase activity (r = 0.907, P < 0.001 and r = 0.886, P < 0.01, respectively). Microsomes of B-lymphoblastoid cells expressing human CYP1A2 exhibited lower mexiletine p- and 2-hydroxylase activities than those expressing human CYP2D6. It was estimated by RAF that the major isoform involved in mexiletine metabolism was CYP2D6, and the contribution of CYPIA2 to both mexiletine p- and 2-hydroxylase activities was 7-30% in human liver microsomes. However, the Km values of the expressed CYP1A2 (approximately 15 microM) were almost identical with those of the expressed CYP2D6 (approximately 22 microM) and human liver microsomes.

CONCLUSIONS

Mexiletine is a substrate of CYP1A2. The data obtained in this study suggest that the interaction of mexiletine with theophylline might be due to competitive inhibition of CYP1A2.

Stereoselective metabolism of mexiletine in Chagasic women with ventricular arrhythmias

Following a week of racemic mexiletine HCl at 200 mg tid (2x100 mg capsules), stereoselective aliphatic hydroxylation was studied in eight Chagasic women with chronic ventricular arrhythmias (52-67 yrs) with no history of renal or hepatic diseases. Blood samples were collected at dose interval up to 24 h of drug administration. Plasma concentrations of R(-) and S(+) mexiletine (MEX) and its metabolite hydroxymethylmexiletine (HMM) were determined by HPLC after derivatization with chiral reagent. The differences between R(-) and S(+) enantiomers were compared by paired t-test. Results are mean (95% CI). The following differences (p < 0.05) between R(-) and S(+) enantiomers, respectively, were found: MEX AUCss(0-8) 2.34 (1.84-2.85) vs 2.55 (1.97-3.13) microg.ml(-1) x h(-1); MEX CL/f 11.27 (7.77-14.77) vs 10.46 (7.18-13.74)ml.min(-1).Kg(-1); HMM Cmax 38.26 (24.3-52.22) vs 16.73 (10.1-23.29)ng.ml(-1); HMM Tmax 4.71 (2.67-6.76) vs 3.29 (1.24-5.33) h and HMM AUCss(0-8) 253.50 (165.39-341.61) vs 103.70 (69.51-137.90)ng.ml(-1).h(-1). The AUCss(0-8) ratio R(-)/S(+) for MEX was 0.93 (0.87-0.98) while for HMM was 2.50 (2.16-2.85). Distribution of MEX and HMM enantiomers were not significantly different. In this study we demonstrate that kinetic disposition of mexiletine exhibits stereoselectivity in vivo and that aliphatic hydroxylation is favored for R(-) mexiletine in Chagasic women with ventricular arrhythmias.

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.

High-performance liquid chromatographic determination of mexiletine enantiomers in plasma using direct and indirect enantioselective separations

Two methods were developed for the determination of mexiletine enantiomers in plasma samples suitable for studies on the stereoselective disposition of this drug. Both methods used fluorescence detection to improve sensitivity and selectivity. The direct enantioselective separation was based on the chiral resolution of mexiletine-2-naphthamide derivatives on a Chiralcel OJ column. The calibration curves were linear over the concentration range 50-500 ng/ml for each enantiomer; therefore the method can be used only for therapeutic monitoring, drug interaction and multiple dose pharmacokinetic studies. The indirect method was based on the formation of diastereomers using o-phthaldialdehyde and N-acetyl-L-cysteine reagents. The diastereomers were resolved on a reversed-phase RP-18 column. The method proved to be suitable for single or multiple dose pharmacokinetic studies based on the low quantification limit (1 ng/ml) and the broader linear range (1-1000 ng/ml) obtained.

Prediction of CYP2D6-mediated polymorphic drug metabolism (sparteine type) based on in vitro investigations

Discovery of genetic polymorphism in drug metabolism has contributed a great deal to understanding the variability in dose-concentration relationships introduced by genetic factors, thereby elucidating the mechanisms responsible for unexpected drug reactions. This knowledge should find its way into clinical practice in order to make therapy more efficient and safe. Moreover, genetic factors in drug metabolism should be taken into account during drug development. Therefore, in vitro methods for identifying the metabolic pattern of new compounds during early stages of drug development should be improved. This review summarizes in vitro methods available to identify genetic polymorphism in drug oxidation, in particular the CYP2D6-related polymorphism.

Determination of the enantiomeric composition of mexiletine and its four hydroxylated metabolites in urine by enantioselective capillary gas chromatography

The enantiomers of mexiletine and four of its hydroxylated metabolites were directly separated by capillary gas chromatography using a heptakis(6-O-tert-butyl-dimethylsilyl-2,3-di-O-methyl)-beta- cyclodextrin column. The method was applied to the analysis of urine samples from cancer patients who were treated with racemic mexiletine as part of a study of the use of mexiletine in the relief of neuropathic pain. Samples analyzed before and after deconjugation of the urine with beta-glucuronidase/arylsulfatase showed a high stereoselectivity in the formation and conjugation of these compounds.

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.

Metabolic polymorphisms

Polymorphisms have been detected in a variety of xenobiotic-metabolizing enzymes at both the phenotypic and genotypic level. In the case of four enzymes, the cytochrome P450 CYP2D6, glutathione S-transferase mu, N-acetyltransferase 2 and serum cholinesterase, the majority of mutations which give rise to a defective phenotype have now been identified. Another group of enzymes show definite polymorphism at the phenotypic level but the exact genetic mechanisms responsible are not yet clear. These enzymes include the cytochromes P450 CYP1A1, CYP1A2 and a CYP2C form which metabolizes mephenytoin, a flavin-linked monooxygenase (fish-odour syndrome), paraoxonase, UDP-glucuronosyltransferase (Gilbert's syndrome) and thiopurine S-methyltransferase. In the case of a further group of enzymes, there is some evidence for polymorphism at either the phenotypic or genotypic level but this has not been unambiguously demonstrated. Examples of this class include the cytochrome P450 enzymes CYP2A6, CYP2E1, CYP2C9 and CYP3A4, xanthine oxidase, an S-oxidase which metabolizes carbocysteine, epoxide hydrolase, two forms of sulphotransferase and several methyltransferases. The nature of all these polymorphisms and possible polymorphisms is discussed in detail, with particular reference to the effects of this variation on drug metabolism and susceptibility to chemically-induced diseases.

Influence of debrisoquine hydroxylation phenotype on the pharmacokinetics of mexiletine

Marked interindividual variation has been observed in the pharmacokinetics of the antiarrhythmic agent mexiletine. The fact that its urinary excretion is dependent on urinary pH may account, in part, for such variation. The influence that genetic differences in hepatic metabolism of the debrisoquine-type may have on mexiletine pharmacokinetics was considered in this study. The pharmacokinetics and urinary excretion of mexiletine (250 mg administered intravenously) were investigated in 5 rapid extensive metabolisers (EM), 5 slow EM and 5 poor metabolisers (PM) of debrisoquine, under conditions of controlled urinary pH. Mexiletine disposition kinetics was found to be altered in PM individuals. These subjects showed higher total area under the curve (AUC), (15.7 versus 8.16 micrograms.h.ml-1) prolonged elimination half-lives (in serum and urine) (serum: 18.5 versus 11.6 h, urine: 19.2 versus 11.7 h) and lower total clearance values compared with EM (216 versus 450 ml.min-1). In this respect, slow EM individuals generally presented intermediate values of those pharmacokinetic parameters. A higher incidence of adverse-effects was also observed among slow EM and PM subjects. It is concluded that genetic differences in mexiletine oxidation of the debrisoquine-type have an influence on its observed pharmacokinetic variability. The clinical consequences are discussed.

The role of individual human cytochromes P450 in drug metabolism and clinical response

Recent advances in the study of human cytochromes P450 by protein purification, molecular cloning techniques and analysis of polymorphisms has led to increased understanding of the role of the various forms in the metabolism of clinically important drugs. In particular, the substrate specificity of one form, CYP2D6, is well established. CYP2D6 shows polymorphism, with 5-10% of Caucasians (poor metabolizers) not expressing this enzyme. The molecular basis of this deficiency is now well understood and methods for the detection of poor metabolizers are discussed, as well as the effect of the polymorphism on drug metabolism. Substrate specificities and possible polymorphisms in other cytochromes P450 are also discussed.