Involvement of CYP1A2 in mexiletine metabolism

Arylboronic Acid Deborylation Deuteration via Synergistic Thiol, Lewis Base, and Photoredox Catalysis

A mild method for the deborylation deuteration of arylboronic acids with D₂O, mediated by the synergistic combination of a thiol, a Lewis base, and photoredox catalysis, is reported. This reaction showed a broad substrate scope, excellent deuterium incorporation, and functional group tolerance. Therefore, this method is practical for the site-selective D-labeling of bioactive molecules and drug molecules.

In vitro study on the effect of peucedanol on the activity of cytochrome P450 enzymes

CONTEXT

Peucedanol is a major extract of Peucedanum japonicum Thunb. (Apiaceae) roots, which is a commonly used herb in paediatrics. Its interaction with cytochrome P450 enzymes (CYP450s) would lead to adverse effects or even failure of therapy.

OBJECTIVE

The interaction between peucedanol and CYP450s was investigated.

MATERIALS AND METHODS

Peucedanol (0, 2.5, 5, 10, 25, 50, and 100 μM) was incubated with eight human liver CYP isoforms (CYP1A2, 2A6, 3A4, 2C8, 2C9, 2C19, 2D6, and 2E1), in pooled human liver microsomes (HLMs) for 30 min with specific inhibitors as positive controls and untreated HLMs as negative controls. The enzyme kinetics and time-dependent study (0, 5, 10, 15, and 30 min) were performed to obtain corresponding parameters in vitro.

RESULTS

Peucedanol significantly inhibited the activity of CYP1A2, 2D6, and 3A4 in a dose-dependent manner with IC50 values of 6.03, 13.57, and 7.58 μM, respectively. Peucedanol served as a non-competitive inhibitor of CYP3A4 with a Ki value of 4.07 μM and a competitive inhibitor of CYP1A2 and 2D6 with a Ki values of 3.39 and 6.77 μM, respectively. Moreover, the inhibition of CYP3A4 was time-dependent with the Ki/Kinact value of 5.44/0.046 min/μM.

DISCUSSION AND CONCLUSIONS

In vitro inhibitory effect of peucedanol on the activity of CYP1A2, 2A6, and 3A4 was reported in this study. As these CYPs are involved in the metabolism of various drugs, these results implied potential drug-drug interactions between peucedanol and drugs metabolized by CYP1A2, 2D6, and 3A4, which needs further in vivo validation.

Agrimoniin inhibits the activity of CYP1A2, 2D6, and 3A4 in human liver microsomes

The effect of agrimoniin on the activity of cytochrome P450 (CYP450) enzymes would induce drug-drug interaction, which leads to adverse effects or even failure of therapy.Agrimoniin was incubated with the specific substrates of eight human liver CYP isoforms in pooled human liver microsomes. The enzyme kinetics and time-dependent study were performed to obtain kinetic parameters and characteristics in vitro.Agrimoniin significantly inhibited the activity of CYP1A2, 2D6, and 3A4 in a dose-dependent manner with IC₅₀ values of 6.26, 9.35, and 8.30 μM, respectively. Agrimoniin served as a non-competitive inhibitor of CYP3A4 and a competitive inhibitor of CYP1A2 and 2D6. Moreover, the incubation time also significantly affected the inhibition of CYP3A4.In vitro inhibitory effect of agrimoniin on the activity of CYP1A2, 2A6, and 3A4 was reported in this study. The potential drug-drug interactions between agrimoniin and drugs metabolised by CYP1A2, 2D6, and 3A4 should be paid special attention.

Mexiletine in Myotonic Dystrophy Type 1: A Randomized, Double-Blind, Placebo-Controlled Trial

OBJECTIVE

To assess mexiletine's long-term safety and effect on 6-minute walk distance in a well-defined cohort of patients with myotonic dystrophy type 1 (DM1).

METHODS

We performed a randomized, double-blind, placebo-controlled trial of mexiletine (150 mg 3 times daily) to evaluate its efficacy and safety in a homogenous cohort of adult ambulatory patients with DM1. The primary outcome was change in 6-minute walk distance at 6 months. Secondary outcomes included changes in hand grip myotonia, strength, swallowing, forced vital capacity, lean muscle mass, Myotonic Dystrophy Health Index scores, and 24-hour Holter and ECG results at 3 and 6 months.

RESULTS

Forty-two participants were randomized and 40 completed the 6-month follow-up (n = 20 in both groups). No significant effects of mexiletine were observed on 6-minute walk distance, but hand grip myotonia was improved with mexiletine treatment. There were no differences between the mexiletine and placebo groups with respect to the frequency or type of adverse events. Changes in PR, QRS, and QTc intervals were similar in mexiletine- and placebo-treated participants.

CONCLUSIONS

There was no benefit of mexiletine on 6-minute walk distance at 6 months. Although mexiletine had a sustained positive effect on objectively measured hand grip myotonia, this was not seen in measures reflecting participants' perceptions of their myotonia. No effects of mexiletine on cardiac conduction measures were seen over the 6-month follow-up period.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that for ambulatory patients with DM1, mexiletine does not significantly change 6-minute walk distance at 6 months.

Sacubitril/Valsartan and Mexiletine: A Proarrhythmic Combination?

The uptake of sacubitril/valsartan since the PARADIGM study confirmed its beneficial effects on outcomes over enalapril in chronic systolic heart failure has inevitably led to potential interactions with co-prescribed medications in real-world patients. We report two cases that raise the possibility of an interaction between sacubitril/valsartan and the class Ib anti-arrhythmic mexiletine resulting in proarrhythmic effects. We discuss the pharmacokinetics of both agents and posit potential mechanistic interactions that suggest caution should be used and careful monitoring for (ventricular) arrhythmias applied in patients receiving sacubitril/valsartan and mexiletine.

Tobacco smoking and its drug interactions with comedications involving CYP and UGT enzymes and nicotine

Tobacco smoking is a global public health threat causing several illnesses including cardiovascular disease (Myocardial infarction), cerebrovascular disease (Stroke), peripheral vascular disease (Claudication), chronic obstructive pulmonary disease, asthma, reduced female infertility, sexual dysfunction in men, different types of cancer and many other diseases. It has been estimated in 2015 that approximately 1.3 billion people smoke, around the globe. Use of medications among smokers is more common, nowadays. This review is aimed to identify the medications affected by smoking, involving Cytochrome P450 (CYP) and uridine diphosphate-glucuronosyltransferases (UGTs) enzymes and Nicotine. Polycyclic aromatic hydrocarbons (PAHs) of tobacco smoke have been associated with the induction of CYP enzymes such as CYP1A1, CYP1A2 and possibly CYP2E1 and UGT enzymes. The drugs metabolized by CYP1A1, CYP1A2, CYP2E1 and UGT enzymes might be affected by tobacco smoking and the smokers taking medications metabolized by those enzymes, may need higher doses due to decreased plasma concentrations through enhanced induction by PAHs of tobacco smoke. The prescribers and the pharmacists are required to be aware of medications affected by tobacco smoking to prevent the toxicity-associated complications during smoking cessation.

Interactions of Mexiletine with Novel Antiepileptic Drugs in the Maximal Electroshock Test in Mice: An Isobolographic Analysis

The aim of the study was to evaluate precisely the type of interactions between mexiletine (an antiarrhythmic drug) and four new generation antiepileptic drugs: lamotrigine, oxcarbazepine, topiramate and pregabalin in the maximal electroshock test in mice (MES). The isobolographic analysis was used to assess the nature of interactions between the tested drugs. Total brain concentrations of antiepileptics were also measured to detect possible pharmacokinetic interactions. The results obtained indicated that the mixture of mexiletine and pregabalin at the fixed ratios of 1:1 and 3:1 led to supra-additive interaction in terms of seizure suppression, while the proportion of 1:3 occurred additive. Synergism was also demonstrated for the combination of mexiletine and topiramate in all three proportions. Combinations of mexiletine with lamotrigine and mexiletine with oxcarbazepine were found to be additive. Adverse-effect profiles of mexiletine, antiepileptics and drug combinations were evaluated in the chimney test (motor coordination) and step-through passive-avoidance task (long-term memory). Mexiletine and drug combinations did not impair long-term memory. Moreover, all combinations of mexiletine with lamotrigine, oxcarbazepine and topiramate had no significant effect on motor coordination. However, the results from the chimney test indicated that pregabalin, administered alone at its ED₅₀ dose from the MES-test, significantly impaired motor performance. Similar adverse effects were observed when mexiletine was co-administered with pregabalin at the fixed-dose ratio combinations of 1:1 and 1:3. However, reduction of pregabalin dose at the fixed ratio of 3:1 seems to prevent significant motor impairment. The results may indicate that mexiletine can be considered as an adjunctive drug in antiepileptic treatment, particularly in patients with concomitant cardiac arrhythmia.

Therapeutic drug monitoring of mexiletine at a large academic medical center

Introduction:

The therapeutic trough range for mexiletine (0.8–2 mcg/mL) was largely established in the setting of arrhythmia prophylaxis following myocardial infarction.

Objective:

Describe the usage patterns of serum mexiletine concentrations and the impact of these concentrations on mexiletine dosing in modern practice for ventricular arrhythmia treatment.

Methods:

A single-center, retrospective analysis was conducted using the electronic medical record to identify serum mexiletine concentrations drawn between December 2004 and December 2014. The primary endpoint was the incidence of mexiletine concentrations drawn as troughs. Secondary outcomes included the incidence of mexiletine concentrations that prompted a dose change, association between adverse events and elevated concentrations, and association between baseline characteristics and mexiletine concentrations.

Results:

A total of 237 individual concentrations were included for analysis with 109 (46.0%) drawn appropriately as trough concentrations. Only 31 (13.1%) of the 237 concentrations drawn prompted a dose change. Mexiletine was primarily used for the treatment of ventricular arrhythmias (96.2%), and 108 (45.6%) concentrations were drawn in an effort to assess efficacy. The median concentration was statistically different between patients with and without an adverse event (0.8 vs 0.7 mcg/mL, respectively; p = 0.017), but may not represent a clinical significance. Patients with hepatic dysfunction had higher median concentrations compared to those without hepatic dysfunction (1.30 vs 1.07 mcg/mL; p = 0.01).

Conclusion:

Mexiletine concentrations are often drawn at inappropriate times and seldom influence a dose change. This study suggests that routine monitoring of mexiletine concentrations may not be necessary; however, therapeutic drug monitoring may be considered in patients with hepatic dysfunction or to confirm mexiletine absorption in patients where this represents a concern.

The role of cytochrome P450 pharmacogenomics in chronic non-cancer pain patients

INTRODUCTION

Pharmacogenomics is the field that studies an individualized treatment approach for patients' medication regimen that can impact drug safety, productivity, and personalized health care. Pharmacogenomics characterizes the genetic differences in metabolic pathways which can affect a patient's individual responses to drug treatments. Areas covered: The various responses to pharmacological agents are mainly determined by the different types of genetic variants of the CYP450. CYP2D6 polymorphism is well known for its variation in the metabolism of drugs from many therapeutic arenas, including some analgesic drugs such as codeine, hydromorphone, oxycodone and tramadol. Allele combinations determine the phenotypic expression, characterized as either: extensive metabolizer, intermediate metabolizer, ultra-rapid metabolizer and poor metabolizer. Expert opinion: The Human Genome Project (HGP) revolutionized the future of medicine and the way health care providers approach individualized patient treatment, and chronic pain management is one of those areas. The key findings in the literature appear to be related to the CYP2D6 expression and its high polymorphism influencing the metabolism of opioid medications, and the impact of that on the patient's therapeutic outcome thus exemplifying the importance of genetic testing for CYP2D6 in the process of physician therapeutic decision making.

Influence of experimental diabetes and insulin treatment on the enantioselective pharmacokinetics of mexiletine and its metabolites

This study evaluates the influence of streptozotocin-induced diabetes on the kinetic disposition and metabolism of mexiletine (MEX) enantiomers in rats. Animals in the control (n = 6 for each blood collection time), diabetic (single intravenous dosage of 45 mg·(kg body mass)(-1) of streptozotocin), and insulin-treated groups (diabetic rats treated daily with 2 IU insulin) received by gavage a single dose of 10 mg·(kg body mass)(-1) racemic MEX. MEX enantiomers and the metabolites hydroxymethylmexiletine (HMM) and p-hydroxymexiletine PHM) were analyzed by LC-MS/MS. Statistical analysis was based on a serial sacrifice design, and parameter estimation was performed using a Bayesian modeling procedure. Area under the curve (AUC) for the (-)-(R) enantiomers of MEX, HMM, and PHM did not differ between the control and diabetic groups. However, AUC for (+)-(S)-MEX and (+)-(S)-HMM were lower in the diabetic than in the control group. Insulin treatment recovered glucose levels to normal and the (+)-(S)-MEX AUC and (+)-(S)-HMM AUC became similar to the AUCs observed in the nondiabetic animals.

Considering CYP1A2 phenotype and genotype for optimizing the dose of olanzapine in the management of schizophrenia

INTRODUCTION

Schizophrenia, a mental disorder, is a debilitating condition which typically strikes young people in their early 20's. Antipsychotic medications are widely prescribed for the treatment of schizophrenia however a balancing act is necessary to provide the correct dose to each patient. It is suggested that a large number of patients discontinue antipsychotic pharmacotherapy because the treatments provided do not always reduce the positive symptoms of the disease, while many have adverse effects on the patients. This implies that neither the incorrect drug nor the optimal dosage for that patient is achieved.

AREAS COVERED

The current review investigates variability in response to olanzapine with a specific focus on the common intrinsic and extrinsic factors that influence both olanzapine and CYP1A2 activity. Furthermore, the authors discuss the utilization of phenotyping and genotyping of CYP1A2 and their potential utility in clinical practice for olanzapine dosing regimens. The authors also consider the potential of pharmacometrics compared to pharmacogenomics as a tool to personalize medicine.

EXPERT OPINION

Careful consideration must be given to the impact of a genetic variant on the disposition of a drug prior to implementing genetic 'tests' to determine response. CYP1A2 phenotypic assessment can yield important information regarding the disposition of olanzapine; however, it relies on the accuracy of the metric and the minimal impact of other metabolic pathways. The application of pharmacometrics provides an effective method to establish covariates that significantly influence olanzapine disposition which can incorporate phenotype and/or genotype.

Scutellarin inhibits cytochrome P450 isoenzyme 1A2 (CYP1A2) in rats

Scutellarin is the most important flavone glycoside in the herbal drug Erigeron breviscapus (Vant.) Hand.-Mazz. It is used frequently in the clinic to treat ischemic vascular diseases in China. However, the direct relationship between scutellarin and cytochrome P450 (CYP450) is unclear. The present study investigated the in vitro and in vivo effects of scutellarin on cytochrome P450 1A2 (CYP 1A2) metabolism. According to in vitro experiments, scutellarin (10-250 µM) decreased the formation of 4-acetamidophenol in a concentration-dependent manner, with an IC₅₀ value of 108.20 ± 0.657 µM. Furthermore, scutellarin exhibited a weak mixed-type inhibition against the activity of CYP1A2 in rat liver microsomes, with a K(i) value of 95.2 µM. Whereas in whole animal studies, scutellarin treatment for 7 days (at 5, 15, 30 mg/kg, i.p.) decreased the clearance (CL), and increased the T(1/2) (at 15, 30 mg/kg, i.p.), it did not affect the V(d) of phenacetin. Scutellarin treatment (at 5, 15, 30 mg/kg, i.p.) increased the AUC(0-∞) by 14.3%, 67.3% and 159.2%, respectively. Scutellarin at 30 mg/kg also weakly inhibited CYP1A2 activity, in accordance with our in vitro study. Thus, the results indicate that CYP1A2 is inhibited directly, but weakly, by scutellarin in vivo, and provide useful information on the safe and effective use of scutellarin in clinical practice.

Structure, function, regulation and polymorphism and the clinical significance of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a number of clinical drugs (e.g., clozapine, tacrine, tizanidine, and theophylline; n > 110), a number of procarcinogens (e.g., benzo[a]pyrene and aromatic amines), and several important endogenous compounds (e.g., steroids). CYP1A2 is subject to reversible and/or irreversible inhibition by a number of drugs, natural substances, and other compounds. The CYP1A gene cluster has been mapped on to chromosome 15q24.1, with close link between CYP1A1 and 1A2 sharing a common 5'-flanking region. The human CYP1A2 gene spans almost 7.8 kb comprising seven exons and six introns and codes a 515-residue protein with a molecular mass of 58,294 Da. The recently resolved CYP1A2 structure has a relatively compact, planar active site cavity that is highly adapted for the size and shape of its substrates. The architecture of the active site of 1A2 is characterized by multiple residues on helices F and I that constitutes two parallel substrate binding platforms on either side of the cavity. A large interindividual variability in the expression and activity of CYP1A2 has been observed, which is largely caused by genetic, epigenetic and environmental factors (e.g., smoking). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. Induction or inhibition of CYP1A2 may provide partial explanation for some clinical drug interactions. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of them have been associated with altered drug clearance and response and disease susceptibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

Fast prediction of cytochrome P450 mediated drug metabolism

Cytochrome P450 mediated metabolism of drugs is one of the major determinants of their kinetic profile, and prediction of this metabolism is therefore highly relevant during the drug discovery and development process. A new rule-based method, based on results from density functional theory calculations, for predicting activation energies for aliphatic and aromatic oxidations by cytochromes P450 is developed and compared with several other methods. Although the applicability of the method is currently limited to a subset of P450 reactions, these reactions describe more than 90 % of the metabolites. The rules employed are relatively few and general, and when combined with solvent-accessible surface area calculations to account for steric accessibility, the method gives a major P450 metabolite as first-ranked position for 75 % of the substrates, and ranked in the top three for 90 % of substrates for a set of 20 substrates. In combination with docking, it can predict isoform-specific metabolism, and we apply this on CYP1A2 with very good results on 81 substrates, for which we find a major metabolite ranked in the top three for 90 % of the substrates (100 % in the training set and 87 % in the larger test set).

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.

Drug-induced changes in P450 enzyme expression at the gene expression level: a new dimension to the analysis of drug-drug interactions

Drug-drug interactions (DDIs) caused by direct chemical inhibition of key drug-metabolizing cytochrome P450 enzymes by a co-administered drug have been well documented and well understood. However, many other well-documented DDIs cannot be so readily explained. Recent investigations into drug and other xenobiotic-mediated expression changes of P450 genes have broadened our understanding of drug metabolism and DDI. In order to gain additional information on DDI, we have integrated existing information on drugs that are substrates, inhibitors, or inducers of important drug-metabolizing P450s with new data on drug-mediated expression changes of the same set of cytochrome P450s from a large-scale microarray gene expression database of drug-treated rat tissues. Existing information on substrates and inhibitors has been updated and reorganized into drug-cytochrome P450 matrices in order to facilitate comparative analysis of new information on inducers and suppressors. When examined at the gene expression level, a total of 119 currently marketed drugs from 265 examined were found to be cytochrome P450 inducers, and 83 were found to be suppressors. The value of this new information is illustrated with a more detailed examination of the DDI between PPARalpha agonists and HMG-CoA reductase inhibitors. This paper proposes that the well-documented, but poorly understood, increase in incidence of rhabdomyolysis when a PPARalpha agonist is co-administered with a HMG-CoA reductase inhibitor is at least in part the result of PPARalpha-induced general suppression of drug metabolism enzymes in liver. The authors believe this type of information will provide insights to other poorly understood DDI questions and stimulate further laboratory and clinical investigations on xenobiotic-mediated induction and suppression of drug metabolism.

CYP2A13 metabolizes the substrates of human CYP1A2, phenacetin, and theophylline

Human cytochrome CYP2A13 shows overlapping substrate specificity with CYP2A6, catalyzing the metabolism of coumarin, nicotine, cotinine, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. Recently, it was found that CYP2A13 could catalyze the metabolic activations of 4-aminobiphenyl and aflatoxin B(1), which are known to be catalyzed by human CYP1A2. In the present study, we investigated the substrate specificity of CYP2A13. It was shown that CYP2A13 could catalyze ethoxyresorufin O-deethylation, methoxyresorufin O-demethylation, and phenacetin O-deethylation, which are used as marker activities for human CYP1A2. Although the intrinsic clearances (V(max)/K(m)) of the two former reactions by CYP2A13 were much lower than that of CYP1A2, the value of the last reaction by CYP2A13 was 2-fold higher than that of CYP1A2. Of particular interest was that CYP2A13 has higher affinity toward phenacetin than CYP1A2. In contrast, CYP2A6 hardly catalyzed these reactions, although the amino acid identity with CYP2A13 is as high as 93.5%. Furthermore, we found that CYP2A13 can catalyze theophylline 8-hydroxylation and 3-demethylation, which are known to be mainly catalyzed by human CYP1A2, although the intrinsic clearances were approximately one-tenth that of CYP1A2. CYP2A13 would not contribute to the systemic clearance of these drugs because CYP2A13 is hardly expressed in human liver. However, it may play a role in metabolism in local tissues such as lung or trachea. In conclusion, the results of the present study could extend our understanding of the substrate specificity of CYP2A13.

Drug interactions and smoking: raising awareness for acute and critical care providers

Because the prevalence of smoking in the United States remains significantly high, it is important to determine a patient's smoking status and perform a complete medication history to assess for potential drug interactions with smoking. Tobacco smoke can increase the hepatic metabolism and can oppose the pharmacologic effects of certain drugs. This article reviews the clinically significant drug interactions, resulting primarily from the induction of cytochrome P450 enzymes by tobacco smoke, of which all acute and critical care providers need to be aware when making therapeutic decisions and recommendations.

Catalytic roles of CYP2D6.10 and CYP2D6.36 enzymes in mexiletine metabolism: In vitro functional analysis of recombinant proteins expressed in Saccharomyces cerevisiae

Cytochrome P450 2D6 (CYP2D6) metabolizes approximately one-third of the medicines in current clinical use and exhibits genetic polymorphism with interindividual differences in metabolic activity. To precisely investigate the effect of CYP2D6*10B and CYP2D6*36 frequently found in Oriental populations on mexiletine metabolism in vitro, CYP2D6 proteins of wild-type (CYP2D6.1) and variants (CYP2D6.10 and CYP2D6.36) were heterologously expressed in yeast cells and their mexiletine p- and 2-methyl hydroxylation activities were determined. Both variant CYP2D6 enzymes showed a drastic reduction of CYP2D6 holo- and apoproteins compared with those of CYP2D6.1. Mexiletine p- and 2-methyl hydroxylation activities on the basis of the microsomal protein level at the single substrate concentration (100 microM) of variant CYP2D6s were less than 6% for CYP2D6.10 and 1% for CYP2D6.36 of those of CYP2D6.1. Kinetic analysis for mexiletine hydroxylation revealed that the affinity toward mexiletine of CYP2D6.10 and CYP2D6.36 was reduced by amino acid substitutions. The Vmax and Vmax/Km values of CYP2D6.10 on the basis of the microsomal protein level were reduced to less than 10% of those of CYP2D6.1, whereas the values on the basis of functional CYP2D6 level were comparable to those of CYP2D6.1. Although it was impossible to estimate the kinetic parameters for the mexiletine hydroxylation of CYP2D6.36, the metabolic ability toward mexiletine was considered to be poorer not only than that of CYP2D6.1 but also than that of CYP2D6.10. The same tendency was also observed in kinetic analysis for bufuralol 1''-hydroxylation as a representative CYP2D6 probe. These findings suggest that CYP2D6*36 has a more drastic impact on mexiletine metabolism than CYP2D6*10.

Effect of Congestive Heart Failure on Mexiletine Pharmacokinetics in a Japanese Population

OBJECTIVE

The goal of this study was to evaluate the influence of congestive heart failure (CHF) on the clearance of mexiletine.

METHODS

The mexiletine clearance/bioavailability (CL/F) ratio was estimated in 584 inpatients receiving mexiletine therapy. The study population consisted of 210 patients with CHF [CHF group; 116 inpatients with New York Heart Association (NYHA) class I-II (group NYHA I-II) CHF and 94 inpatients with NYHA class III-IV (group NYHA III-IV) CHF] and 374 inpatients without CHF (Non-CHF group). Serum levels of mexiletine were determined by high performance liquid chromatography (HPLC).

RESULTS

Mexiletine clearance was significantly lower in the CHF group when compared with the Non-CHF group (0.264+/-0.093 vs. 0.393+/-0.082 l/h/kg, mean+/-S.D., p<0.05). Further, the CL/F ratio was 50% lower in group NYHA III-IV when compared with the Non-CHF group, and the CL/F ratio tended to change in inverse proportion to NYHA class.

CONCLUSION

CHF status significantly affects mexiletine clearance. Therefore, dose adjustments and careful monitoring are likely required in CHF patients receiving mexiletine.

Inhibitory effects of psychotropic drugs on mexiletine metabolism in human liver microsomes: prediction of in vivo drug interactions

Mexiletine, an anti-arrhythmic agent, is used for the control of ventricular arrhythmias and for neuropathic pain from cancer or diabetes mellitus. It is sometimes used together with psychotropic drugs in patients with depression, schizophrenia or sleep disorder. It is metabolized mainly by cytochrome P450 (CYP) 2 D 6 and, to a minor extent, by CYP1A2. To predict possible drug interactions between mexiletine and psychotropic drugs, the inhibitory effects of 14 psychotropic drugs (phenytoin, carbamazepine, fluvoxamine, paroxetine, fluoxetine, citalopram, sertraline, imipramine, desipramine, haloperidol, thioridazine, olanzapine, etizolam, and quazepam) on mexiletine metabolism in human liver microsomes were determined. Fluoxetine (Ki=0.6+/- 0.1 microM), sertraline (Ki=7.6+/- 0.8 microM) and desipramine (Ki=3.2+/- 0.5 microM) competitively inhibited the mexiletine p-hydroxylation in human liver microsomes. Thioridazine (Kis=0.5+/- 0.2 microM; Kii =3.6+/-1.6 microM) and paroxetine (Kis=1.7+/- 0.7 microM; Kii=3.6+/- 0.9 microM) exhibited a mixed-type inhibition (competitive and non-competitive) toward mexiletine p-hydroxylation in human liver microsomes. The changes of the in vivo clearance of mexiletine by the psychotropic drugs were predicted by 1+(I/Ki) using the in vitro Ki and unbound inhibitor concentrations in liver. The values were calculated as 2.4 for paroxetine, 5.5 for fluoxetine, 1.1 for sertraline, 2.8 for desipramine and 2.2 for thioridazine. In addition, paroxetine exhibited a mechanism-based inactivation with Ki=0.7 microM and Kinact=0.15 min(-1). The present study predicted the possibility of drug interactions between mexiletine and paroxetine, fluoxetine, desipramine, and thioridazine in clinical use.

Assessment of CYP1A2 Activity in Clinical Practice: Why, How, and When?

The cytochrome P450 enzyme CYP1A2 mediates the rate-limiting step in the metabolism of many drugs including theophylline, clozapine, and tacrine as well as in the bioactivation of procarcinogens. CYP1A2 activity shows both pronounced intra- and interindividual variability, which is, among other factors, related to smoking causing enzyme induction, to drug intake and to dietary factors which may result in induction or inhibition. In contrast to these exogenous factors, genetic influences on enzyme activity seem to be less pronounced. Therefore, phenotyping of CYP1A2, i.e. the determination of the actual activity of the enzyme in vivo, represents a useful approach both for scientific and clinical applications. CYP1A2 is almost exclusively expressed in the liver. Since liver tissue cannot be obtained for direct phenotyping, a probe drug which is metabolized by CYP1A2 has to be given. Proposed probe drugs include caffeine, theophylline, and melatonin. Caffeine is most often used because of the predominating role of CYP1A2 in its overall metabolism and the excellent tolerability. Various urinary, plasma, saliva, and breath based CYP1A2 caffeine metrics have been applied. While caffeine clearance is considered as the gold standard, the salivary or plasma ratio of paraxanthine to caffeine in a sample taken approximately 6 hr after a defined dose of caffeine is a more convenient, less expensive but also fully validated CYP1A2 phenotyping metric. CYP1A2 phenotyping is applied frequently in epidemiologic and drug-drug interaction studies, but its clinical use and usefulness remains to be established.

Effects of Ciprofloxacin on the Stereoselective Disposition of Mexiletine in Man

Mexiletine is extensively metabolized in man, with less than 10% of the dose being excreted unchanged in urine. Clinical drug-drug interaction studies as well as in vitro drug metabolism studies suggest that CYP1A2, in addition to CYP2D6, is involved in the metabolism of mexiletine in man. Therefore, the objective of the study was to determine whether potential inhibition of CYP1A2 by the quinolone antibiotic agent ciprofloxacin would alter the stereoselective disposition of mexiletine. Nineteen healthy men (10 smokers and 9 nonsmokers) received a single 200-mg oral dose of racemic mexiletine hydrochloride on 2 occasions: once alone and once during concomitant administration of ciprofloxacin 750 mg BID (starting 3 days before and up to 2 days after the administration of mexiletine). Serial blood and urine samples were collected for 48 hours, and pharmacokinetic parameters were derived. Total clearances of R-(-)- and S-(+)-mexiletine were 42% and 63% higher in smokers compared with nonsmokers (P < 0.05). This observation is in agreement with increased clearance of mexiletine under conditions of increased CYP1A2 activity. On the other hand, ciprofloxacin administration only marginally decreased R-(-)- and S-(+)-mexiletine clearances (2 to 5 L/h; P < 0.05) secondary to a decrease in mexiletine nonrenal clearance. In conclusion, the increase in mexiletine nonrenal clearance in smokers and its decrease during the combined administration of ciprofloxacin confirm the role of CYP1A2 in the overall clearance of the drug. Nevertheless, results obtained in this study suggest that no major drug interaction is to be expected during the concomitant administration of ciprofloxacin and mexiletine in patients.

Predicting drug clearance from recombinantly expressed CYPs: intersystem extrapolation factors

1. Recombinantly expressed human cytochromes P450 (rhCYPs) have been underused for the prediction of human drug clearance (CL). 2. Differences in intrinsic activity (per unit CYP) between rhCYP and human liver enzymes complicate the issue and these discrepancies have not been investigated systematically. We define intersystem extrapolation factors (ISEFs) that allow the use of rhCYP data for the in vitro-in vivo extrapolation of human drug CL and the variance that is associated with interindividual variation of CYP abundance due to genetic and environmental effects. 3. A large database (n = 451) of metabolic stability data has been compiled and used to derive ISEFs for the most commonly used expression systems and CYP enzymes. 4. Statistical models were constructed for the ISEFs to determine major covariates in order to optimize experimental design to increase prediction accuracy. 5. Suggestions have been made for the conduct of future studies using rhCYP to predict human drug clearance.

Thein vitro metabolism of desglymidodrine, an active metabolite of prodrug midodrine by human liver microsomes

The human cytochrome P450 (CYP) isoforms catalyzing the oxidation metabolism of desglymidodrine (DMAE), an active metabolite of midodrine, were studied. Recombinant human CYP2D6, 1A2 and 2C19 exhibited appreciable catalytic activity with respect to the 5'-O-demethylation of DMAE. The O-demethylase activity by the recombinant CYP2D6 was much higher than that of other CYP isoforms. Quinidine (a selective inhibitor of CYP2D6) inhibited the O-demethylation of DMAE in pooled human microsomes by 86%, while selective inhibitors for other forms of CYP did not show any appreciable effect. Although the activity of CYP2D6 was almost negligible in the PM microsomes, the O-demethylase activity of DMAE was found to be maintained by about 25% of the pooled microsomes. Furafylline (a selective inhibitor of CYP1A2) inhibited the M-2 formation in the PM microsomes by 57%. The treatment of pooled microsomes with an antibody against CYP2D6 inhibited the formation of M-2 by about 75%, whereas that of the PM microsomes did not show drastic inhibition. In contrast, the antibody against CYP1A2 suppressed the activity by 40 to 50% in the PM microsomes. These findings suggest that CYP2D6 have the highest catalytic activity of DMAE 5'-O-demethylation in human liver microsomes, followed by CYP1A2 to a small extent.

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.

Role of specific cytochrome P450 enzymes in the N-oxidation of the antiarrhythmic agent mexiletine

1. Mexiletine is extensively metabolized in man by C- and N-oxidation and the aim of the present study was to characterize major cytochrome P450 enzyme(s) involved in the formation of N-hydroxymexiletine. 2. Incubations with genetically engineered microsomes indicated that the formation rate of N-hydroxymexiletine was highest in the presence of microsomes expressing high levels of either CYP1A2 or CYP2E1 and the formation of N-hydroxymexiletine by human liver microsomes was inhibited about 40% by antibodies directed against CYP1A1/1A2 or CYP2E1. Additional incubations demonstrated that formation of N-hydroxymexiletine was decreased 47 and 51% by furafylline, 40 microm and 120 microm, respectively, and decreased 55 and 67% by alpha-naphthoflavone, 1 microm and 3 microm, respectively (all p < 0.05 versus control). 3. The formation rate of N-hydroxymexiletine in human liver microsomes was highly correlated with CYP2B6 (RS-mexiletine, r = 0.7827; R-(-)-enantiomer, r = 0.7034; S-(+)-enantiomer, r = 0.7495), CYP2E1 (S-(+)-enantiomer, r = 0.7057) and CYP1A2 (RS-mexiletine, r = 0.5334; S-(+)-enantiomer, r = 0.6035). 4. In conclusion, we have demonstrated that CYP1A2 is a major human cytochrome P450 enzyme involved in the formation of N-hydroxymexiletine. However, other cytochrome P450 enzymes (CYP2E1 and CYP2B6) also appear to play a role in the N-oxidation of this drug.

Lack of interaction between amiodarone and mexiletine in cardiac arrhythmia patients

Amiodarone has pharmacokinetic interactions with various therapeutic agents, including phenytoin, flecainide, and cyclosporine. Mexiletine is metabolized by CYP2D6 and CYP1A2. The objective of this study is to evaluate the effect of amiodarone on the pharmacokinetics of mexiletine through its inhibition of various cytochrome P450 (CYP) subtypes. In a series of 181 inpatients with supraventricular tachyarrhythmias, 26 inpatients received mexiletine and amiodarone therapy (MEX + AMD group), and the others received mexiletine therapy (MEX group). In 10 inpatients of the MEX + AMD group, the mexiletine clearance (CL(MEX)/F) before and after coadministration of amiodarone was compared. CL(MEX)/F was also compared in the MEX and MEX + AMD groups after the start of amiodarone therapy. Serum mexiletine, amiodarone, and desethylamiodarone concentrations were measured by an HPLC method. The CL(MEX)/F was estimated by the Bayesian method using population pharmacokinetic analysis. There was no significant difference in CL(MEX)/F before and after 1-month coadministration of amiodarone in 10 inpatients of the MEX + AMD group. Although serum amiodarone and desethylamiodarone concentrations gradually increased with time after the start of amiodarone therapy in these patients, CL(MEX)/F showed no change at 3 and 5 months after the start of amiodarone therapy. There was no significant difference in CL(MEX)/F of the MEX group and the MEX + AMD group. The results suggest that the pharmacokinetics of mexiletine is not affected by amiodarone in patients with cardiac arrhythmias.

Evaluation of mexiletine clearance in a Japanese population

OBJECTIVE

To evaluate mexiletine clearance in a Japanese population and to clarify the roles of CYP2D6 and CYP1A2 in mexiletine disposition.

METHODS

Concentrations of serum and urinary mexiletine and its metabolites were determined and mexiletine clearances were estimated in 334 inpatients receiving mexiletine therapy. Concentrations of mexiletine and its metabolites in serum and urine samples were determined by HPLC.

RESULTS

Although interindividual variation of mexiletine clearance was small, the effect of age on mexiletine clearance was comparatively large. Mexiletine clearance in patients with dilated cardiomyopathy (DCM) was decreased when compared with other diagnoses (Non-DCM). The fractional contents of p-hydroxymexiletine (POH) and 2-hydroxymexiletine (OHMEX) in urine amounted to approximately 50%. Almost all of the POH was conjugated, whereas less than one-third of the OHMEX was conjugated. Although no significant differences in POH and OHMEX were observed between patients with DCM and those without, a trend toward an increase in conjugation pathway of DCM patients was observed.

CONCLUSIONS

The interindividual variation of mexiletine clearance was small, while the effect of age on the mexiletine clearance in Non-DCM was comparatively large. A significant difference in mexiletine clearance between patients with DCM and those with Non-DCM was observed. Therefore, when mexiletine is administered to patients with DCM, careful monitoring is needed.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

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.

Clinically significant pharmacokinetic interactions between dietary caffeine and medications

Caffeine from dietary sources (mainly coffee, tea and soft drinks) is the most frequently and widely consumed CNS stimulant in the world today. Because of its enormous popularity, the consumption of caffeine is generally thought to be safe and long term caffeine intake may be disregarded as a medical problem. However, it is clear that this compound has many of the features usually associated with a drug of abuse. Furthermore, physicians should be aware of the possible contribution of dietary caffeine to the presenting signs and symptoms of patients. The toxic effects of caffeine are extensions of their pharmacological effects. The most serious caffeine-related CNS effects include seizures and delirium. Other symptoms affecting the cardiovascular system range from moderate increases in heart rate to more severe cardiac arrhythmia. Although tolerance develops to many of the pharmacological effects of caffeine, tolerance may be overwhelmed by the nonlinear accumulation of caffeine when its metabolism becomes saturated. This might occur with high levels of consumption or as the result of a pharmacokinetic interaction between caffeine and over-the-counter or prescription medications. The polycyclic aromatic hydrocarbon-inducible cytochrome P450 (CYP) 1A2 participates in the metabolism of caffeine as well as of a number of clinically important drugs. A number of drugs, including certain selective serotonin reuptake inhibitors (particularly fluvoxamine), antiarrhythmics (mexiletine), antipsychotics (clozapine), psoralens, idrocilamide and phenylpropanolamine, bronchodilators (furafylline and theophylline) and quinolones (enoxacin), have been reported to be potent inhibitors of this isoenzyme. This has important clinical implications, since drugs that are metabolised by, or bind to, the same CYP enzyme have a high potential for pharmacokinetic interactions due to inhibition of drug metabolism. Thus, pharmacokinetic interactions at the CYP1A2 enzyme level may cause toxic effects during concomitant administration of caffeine and certain drugs used for cardiovascular, CNS (an excessive dietary intake of caffeine has also been observed in psychiatric patients), gastrointestinal, infectious, respiratory and skin disorders. Unless a lack of interaction has already been demonstrated for the potentially interacting drug, dietary caffeine intake should be considered when planning, or assessing response to, drug therapy. Some of the reported interactions of caffeine, irrespective of clinical relevance, might inadvertently cause athletes to exceed the urinary caffeine concentration limit set by sports authorities at 12 mg/L. Finally, caffeine is a useful and reliable probe drug for the assessment of CYP1A2 activity, which is of considerable interest for metabolic studies in human populations.

Inhibitory effects of amiodarone and its N-deethylated metabolite on human cytochrome P450 activities: prediction of in vivo drug interactions

AIMS

To predict the drug interactions of amiodarone and other drugs, the inhibitory effects and inactivation potential for human cytochrome P450 (CYP) enzymes by amiodarone and its N-dealkylated metabolite, desethylamiodarone were examined.

METHODS

The inhibition or inactivation potency of amiodarone and desethylamiodarone for human CYP activities were investigated using microsomes from B-lymphoblastoid cell lines expressing CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. The in vivo drug interactions of amiodarone and desethylamiodarone were predicted in vitro using the 1+Iu/Ki values.

RESULTS

Amiodarone weakly inhibited CYP2C9, CYP2D6, and CYP3A4-mediated activities with Ki values of 45.1-271.6 microm. Desethylamiodarone competitively inhibited the catalytic activities of CYP2D6 (Ki=4.5 microm ) and noncompetitively inhibited CYP2A6 (Ki=13.5 microm ), CYP2B6 (Ki=5.4 microm ), and CYP3A4 (Ki=12.1 microm ). The catalytic activities of CYP1A1 (Ki=1.5 microm, alpha=5.7), CYP1A2 (Ki=18.8 microm, alpha=2.6), CYP2C9 (Ki=2.3 microm, alpha=5.9), and CYP2C19 (Ki=15.7 microm, alpha=4.5) were inhibited by desethylamiodarone with mixed type. The 1+Iu/Ki values of desethylamiodarone were higher than those of amiodarone. Amiodarone inactivated CYP3A4, while desethylamiodarone inactivated CYP1A1, CYP1A2, CYP2B6, and CYP2D6.

CONCLUSIONS

The interactions between amiodarone and other drugs might occur via the inhibition of CYP activities by its N-dealkylated metabolite, desethylamiodarone, rather than by amiodarone itself. In addition, the inactivation of CYPs by desethylamiodarone as well as by amiodarone would also contribute to the drug interactions.

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.

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.