Mexiletine: Antiarrhythmic mechanisms, emerging clinical applications and mortality

Mexiletine, a class Ib antiarrhythmic drug, exhibits its major antiarrhythmic effect via inhibition of the fast and late Na+ currents in myocardial tissues that are dependent on the opening of Na+ channels for their excitation. Through a comprehensive examination of mexiletine's therapeutic benefits and potential risks, we aim to provide valuable insights that reinforce its role as a vital therapeutic option for patients with ventricular arrhythmias, long QT syndrome, and other heart rhythm disorders. This review will highlight the current understandings of the antiarrhythmic effects and rationales for recent off-label use and address the mortality and proarrhythmic effects of mexiletine utilizing published basic and clinical studies over the past five decades.

hERG stereoselective modulation by mexiletine-derived ureas: Molecular docking study, synthesis, and biological evaluation

Long QT syndrome (LQTS) is a disorder of cardiac electrophysiology resulting in life-threatening arrhythmias; nowadays, only a few drugs are available for the management of LQTS. Focusing our attention on LQT2, one of the most common subtypes of LQTS caused by mutations in the human ether-à-go-go-related gene (hERG), in the present work, the stereoselectivity of the recently discovered mexiletine-derived urea 8 was investigated on the hERG potassium channel. According to preliminary in silico predictions, in vitro studies revealed a stereoselective behavior, with the meso form showing the greatest hERG opening activity. In addition, functional studies on guinea pig isolated left atria, aorta, and ileum demonstrated that 8 does not present any cardiac or intestinal liability in our ex vivo studies. Due to its overall profile, (R,S)-8 paves the way for the design and development of a new series of compounds potentially useful in the treatment of both congenital and drug-induced forms of LQTS.

Beneficial effects of chronic mexiletine treatment in a human model of SCN5A overlap syndrome

AIMS

SCN5A mutations are associated with various cardiac phenotypes, including long QT syndrome type 3 (LQT3), Brugada syndrome (BrS), and cardiac conduction disease (CCD). Certain mutations, such as SCN5A-1795insD, lead to an overlap syndrome, with patients exhibiting both features of BrS/CCD [decreased sodium current (INa)] and LQT3 (increased late INa). The sodium channel blocker mexiletine may acutely decrease LQT3-associated late INa and chronically increase peak INa associated with SCN5A loss-of-function mutations. However, most studies have so far employed heterologous expression systems and high mexiletine concentrations. We here investigated the effects of a therapeutic dose of mexiletine on the mixed phenotype associated with the SCN5A-1795insD mutation in HEK293A cells and human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs).

METHODS AND RESULTS

To assess only the chronic effects on trafficking, HEK293A cells transfected with wild-type (WT) SCN5A or SCN5A-1795insD were incubated for 48 h with 10 µm mexiletine followed by wash-out, which resulted in an increased peak INa for both SCN5A-WT and SCN5A-1795insD and an increased late INa for SCN5A-1795insD. Acute re-exposure of HEK293A cells to 10 µm mexiletine did not impact on peak INa but significantly decreased SCN5A-1795insD late INa. Chronic incubation of SCN5A-1795insD hiPSC-CMs with mexiletine followed by wash-out increased peak INa, action potential (AP) upstroke velocity, and AP duration. Acute re-exposure did not impact on peak INa or AP upstroke velocity, but significantly decreased AP duration.

CONCLUSION

These findings demonstrate for the first time the therapeutic benefit of mexiletine in a human cardiomyocyte model of SCN5A overlap syndrome.

Functional characterization and identification of a therapeutic for a novel SCN5A-F1760C variant causing type 3 long QT syndrome refractory to all guideline-directed therapies

BACKGROUND

Pathogenic variants in the SCN5A-encoded Nav1.5 sodium channel cause type 3 long QT syndrome (LQT3). We present the case of an infant with severe LQT3 who was refractory to multiple pharmacologic therapies as well as bilateral stellate ganglionectomy. The patient's novel variant, p.F1760C-SCN5A, involves a critical residue of the Nav1.5's local anesthetic binding domain.

OBJECTIVE

The purpose of this study was to characterize functionally the p.F1760C-SCN5A variant using TSA-201 and patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs).

METHODS

Whole-cell patch clamp was used to assess p.F1760C-SCN5A associated sodium currents with/without lidocaine (Lido), flecainide, and phenytoin (PHT) in TSA-201 cells. p.F1760C-SCN5A and CRISPR-Cas9 variant-corrected isogenic control (IC) iPSC-CMs were generated. FluoVolt voltage dye was used to measure the action potential duration (APD) with/without mexiletine or PHT.

RESULTS

V1/2 of inactivation was right-shifted significantly in F1760C cells (-72.2 ± 0.7 mV) compared to wild-type (WT) cells (-86.3 ± 0.9 mV; P <.0001) resulting in a marked increase in window current. F1760C increased sodium late current 2-fold from 0.18% ± 0.04% of peak in WT to 0.49% ± 0.07% of peak in F1760C (P = .0005). Baseline APD to 90% repolarization (APD90) was increased markedly in F1760C iPSC-CMs (601 ± 4 ms) compared to IC iPSC-CMs (423 ± 15 ms; P <.0001). However, 4-hour treatment with 10 μM mexiletine failed to shorten APD90, and treatment with 5μM PHT significantly decreased APD90 of F1760C iPSC-CMs (453 ± 6 ms; P <.0001).

CONCLUSION

PHT rescued electrophysiological phenotype and APD of a novel p.F1760C-SCN5A variant. The antiepileptic drug PHT may be an effective alternative therapeutic for the treatment of LQT3, especially for variants that disrupt the Lido/mexiletine binding site.

Indications for mexiletine in the new ESC guidelines and beyond

INTRODUCTION

Mexiletine is a class IB sodium-channel blocker. Unlike class IA or IC antiarrhythmic drugs, mexiletine rather shortens than prolongs action potential duration; therefore, it is less associated with proarrhythmic effects.

AREAS COVERED

Recently, new European Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death were published, including a reappraisal of some established older antiarrhythmic drugs.

EXPERT OPINION

Mexiletine offers a first-line, genotype-specific treatment strategy for LQT3 patients as emphasized by the most recent guidelines. Besides this recommendation, current study reports suggest that in therapy-refractory ventricular tachyarrhythmias and electrical storms adjunctive mexiletine treatment may offer the possibility of stabilizing patients with or without concomitant interventional therapy such as catheter ablation.

Protein 14-3-3 Influences the Response of the Cardiac Sodium Channel Nav1.5 to Antiarrhythmic Drugs

The cardiac sodium channel Nav1.5 is a key contributor to the cardiac action potential, and dysregulations in Nav1.5 can lead to cardiac arrhythmias. Nav1.5 is a target of numerous antiarrhythmic drugs (AADs). Previous studies identified the protein 14-3-3 as a regulator of Nav1.5 biophysical coupling. Inhibition of 14-3-3 can remove the Nav1.5 functional coupling and has been shown to inhibit the dominant-negative effect of Brugada syndrome mutations. However, it is unknown whether the coupling regulation is involved with AADs' modulation of Nav1.5. Indeed, AADs could reveal important structural and functional information about Nav1.5 coupling. Here, we investigated the modulation of Nav1.5 by four classic AADs, quinidine, lidocaine, mexiletine, and flecainide, in the presence of 14-3-3 inhibition. The experiments were carried out by high-throughput patch-clamp experiments in an HEK293 Nav1.5 stable cell line. We found that 14-3-3 inhibition can enhance acute block by quinidine, whereas the block by other drugs was not affected. We also saw changes in the use- and dose-dependency of quinidine, lidocaine, and mexiletine when inhibiting 14-3-3. Inhibiting 14-3-3 also shifted the channel activation toward hyperpolarized voltages in the presence of the four drugs studied and slowed the recovery of inactivation in the presence of quinidine. Our results demonstrated that the protein 14-3-3 and Nav1.5 coupling could impact the effects of AADs. Therefore, 14-3-3 and Nav1.5 coupling are new mechanisms to consider in the development of drugs targeting Nav1.5. SIGNIFICANCE STATEMENT: The cardiac sodium channel Nav1.5 is a target of commonly used antiarrhythmic drugs, and Nav1.5 function is regulated by the protein 14-3-3. The present study demonstrated that the regulation of Nav1.5 by 14-3-3 influences Nav1.5's response to antiarrhythmic drugs. This study provides detailed information about how 14-3-3 differentially regulated Nav1.5 functions under the influence of different drug subtypes. These findings will guide future molecular studies investigating Nav1.5 and antiarrhythmic drugs outcomes.

Blockers of Skeletal Muscle Nav1.4 Channels: From Therapy of Myotonic Syndrome to Molecular Determinants of Pharmacological Action and Back

The voltage-gated sodium channels represent an important target for drug discovery since a large number of physiological processes are regulated by these channels. In several excitability disorders, including epilepsy, cardiac arrhythmias, chronic pain, and non-dystrophic myotonia, blockers of voltage-gated sodium channels are clinically used. Myotonia is a skeletal muscle condition characterized by the over-excitability of the sarcolemma, resulting in delayed relaxation after contraction and muscle stiffness. The therapeutic management of this disorder relies on mexiletine and other sodium channel blockers, which are not selective for the Na_v1.4 skeletal muscle sodium channel isoform. Hence, the importance of deepening the knowledge of molecular requirements for developing more potent and use-dependent drugs acting on Na_v1.4. Here, we review the available treatment options for non-dystrophic myotonia and the structure-activity relationship studies performed in our laboratory with a focus on new compounds with potential antimyotonic activity.

[Impact of late sodium current inhibition on cardiac electrophysiology parameters and ventricular arrhythmias in isolated Langendorff perfused rabbit hearts with short QT interval]

Objective: To determine the electrophysiological effects and related mechanisms of late sodium current inhibitors on hearts with short QT intervals. Methods: The electrophysiological study was performed on isolated Langendorff perfused rabbit hearts. A total of 80 New Zealand White rabbits were used and 34 hearts without drug treatment were defined as control group A, these hearts were then treated with I_KATP opener pinacidil, defined as pinacidil group A. Then, 27 hearts from pinacidil group A were selected to receive combined perfusion with sodium channel inhibitors or quinidine, a traditional drug used to treat short QT syndrome, including ranolazine combined group (n=9), mexiletine combined group (n=9), and quinidine combined group (n=9). Nineteen out of the remaining 46 New Zealand rabbits were selected as control group B (no drug treatments, n=19), and then treated with pinacidil, defined as pinacidil group B (n=19). The remaining 27 rabbits were treated with sodium inhibitors or quinidine alone, including ranolazine alone group (n=9), mexiletine alone group (n=9), and quinidine alone group (n=9). Electrocardiogram (ECG) physiological parameters of control group A and pinacidil group A were collected. In control group B and pinacidil group B, programmed electrical stimulation was used to induce ventricular arrhythmias and ECG was collected. ECG physiological parameters and ventricular arrhythmia status of various groups were analyzed. The concentrations of pinacidil, ranolazine, mexiletine and quinidine used in this study were 30, 10, 30 and 1 μmol/L, respectively. Results: Compared with control group A, the QT interval, 90% of the repolarization in epicardial and endocardial monophasic action potential duration (MAPD₉₀-Epi, MAPD₉₀-Endo) was shortened, the transmural dispersion of repolarization (TDR) was increased, and the effective refractor period (ERP) and post-repolarization refractoriness (PRR) were reduced in pinacidil group A (all P<0.05). Compared with the pinacidil group A, MAPD₉₀-Epi, MAPD₉₀-Endo, QT interval changes were reversed in quinidine combined group and mexiletine combined group (all P<0.05), but not in ranolazine combined group. All these three drugs reversed the pinacidil-induced increases of TDR and the decreases of ERP and PRR. The induced ventricular arrhythmia rate was 0 in control group B, and increased to 10/19 (χ²=13.6, P<0.05) in pinacidil group B during programmed electrical stimulation. Compared with the pinacidil group B, incidences of ventricular arrhythmia decreased to 11% (1/9), 11% (1/9) and 0 (0/9) (χ²=4.5, 4.5, 7.4, P<0.05) respectively in ranolazine group, mexiletine group and quinidine group. Conclusions: Inhibition of late sodium current does not increase but even decreases the risk of malignant arrhythmia in hearts with a shortened QT interval. The antiarrhythmic mechanism might be associated with the reversal of the increase of TDR and the decrease of refractoriness (including both ERP and PRR) of hearts with shortened QT interval.

Bioisosteric Modification of To042: Synthesis and Evaluation of Promising Use-Dependent Inhibitors of Voltage-Gated Sodium Channels

Three analogues of To042, a tocainide-related lead compound recently reported for the treatment of myotonia, were synthesized and evaluated in vitro as skeletal muscle sodium channel blockers possibly endowed with enhanced use-dependent behavior. Patch-clamp experiments on hNav1.4 expressed in HEK293 cells showed that N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine, the aryloxyalkyl bioisostere of To042, exerted a higher use-dependent block than To042 thus being able to preferentially block the channels in over-excited membranes while preserving healthy tissue function. It also showed the lowest active transport across BBB according to the results of P-glycoprotein (P-gp) interacting activity evaluation and the highest cytoprotective effect on HeLa cells. Quantum mechanical calculations and dockings gave insights on the most probable conformation of the aryloxyalkyl bioisostere of To042 in solution and the target residues involved in the binding, respectively. Both approaches indicated the conformations that might be adopted in both the unbound and bound state of the ligand. Overall, N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine exhibits an interesting toxico-pharmacological profile and deserves further investigation.

Human iPSC-derived cardiomyocytes and pyridyl-phenyl mexiletine analogs

In the United States, approximately one million individuals are hospitalized every year for arrhythmias, making arrhythmias one of the top causes of healthcare expenditures. Mexiletine is currently used as an antiarrhythmic drug but has limitations. The purpose of this work was to use normal and Long QT syndrome Type 3 (LQTS3) patient-derived human induced pluripotent stem cell (iPSC)-derived cardiomyocytes to identify an analog of mexiletine with superior drug-like properties. Compared to racemic mexiletine, medicinal chemistry optimization of substituted racemic pyridyl phenyl mexiletine analogs resulted in a more potent sodium channel inhibitor with greater selectivity for the sodium over the potassium channel and for late over peak sodium current.

Computational prediction of drug response in short QT syndrome type 1 based on measurements of compound effect in stem cell-derived cardiomyocytes

Short QT (SQT) syndrome is a genetic cardiac disorder characterized by an abbreviated QT interval of the patient's electrocardiogram. The syndrome is associated with increased risk of arrhythmia and sudden cardiac death and can arise from a number of ion channel mutations. Cardiomyocytes derived from induced pluripotent stem cells generated from SQT patients (SQT hiPSC-CMs) provide promising platforms for testing pharmacological treatments directly in human cardiac cells exhibiting mutations specific for the syndrome. However, a difficulty is posed by the relative immaturity of hiPSC-CMs, with the possibility that drug effects observed in SQT hiPSC-CMs could be very different from the corresponding drug effect in vivo. In this paper, we apply a multistep computational procedure for translating measured drug effects from these cells to human QT response. This process first detects drug effects on individual ion channels based on measurements of SQT hiPSC-CMs and then uses these results to estimate the drug effects on ventricular action potentials and QT intervals of adult SQT patients. We find that the procedure is able to identify IC50 values in line with measured values for the four drugs quinidine, ivabradine, ajmaline and mexiletine. In addition, the predicted effect of quinidine on the adult QT interval is in good agreement with measured effects of quinidine for adult patients. Consequently, the computational procedure appears to be a useful tool for helping predicting adult drug responses from pure in vitro measurements of patient derived cell lines.

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.

Pharmacological profiling of JME-173, a novel mexiletine derivative combining dual anti-inflammatory/anti-spasmodic functions and limited action in Na+ channels

Existing evidence suggests that the local anaesthetic mexiletine can be beneficial for patients with asthma. However, caution is required since anaesthesia of the airways inhibits protective bronchodilator neuronal reflexes, limiting applications in conditions of hyperirritable airways. Here, we describe the synthesis of a new series of mexiletine analogues, which were screened for reduced activity in Na⁺ channels and improved smooth muscle relaxant effects, that were evaluated using the patch-clamp technique and an isolated tracheal organ bath, respectively. JME-173 (1-(4-bromo-3,5-dimethylphenoxy)propan-2-amine) was the most effective among the four mexiletine analogues investigated. JME-173 was then studied in vivo using a murine model of lung inflammation induced by cigarette smoke (CS) and in vitro using neutrophil chemotaxis and mast cell degranulation assays. Finally, the JME-173 pharmacokinetic profile was assessed using HPLC-MS/MS bioanalytical method. JME-173 directly inhibited IL-8 (CXCL8)- and FMLP-induced human neutrophil chemotaxis and allergen-induced mast cell degranulation. After oral administration 1 h before CS exposure, JME-173 (50 mg/kg) strongly reduced the increased number of macrophages and neutrophils recovered in the bronchoalveolar effluent without altering lymphocyte counts. Pharmacokinetic experiments of JME-173 (10 mg/kg, orally) showed values of maximum concentration (C_max), maximum time (T_max), area under the blood concentration-time curve (AUC_0-t) and area under the blood concentration-time curve from 0-Inf (AUC_0-inf) of 163.3 ± 38.3 ng/mL, 1.2 ± 0.3 h, 729.4 ± 118.3 ng*h/ml and 868.9 ± 117.1 ng*h/ml (means ± S.E.M.), respectively. Collectively, these findings suggest that JME-173 has the potential to be an effective oral treatment for diseases associated with bronchoconstriction and inflammation.

Complex interactions in a novel SCN5A compound mutation associated with long QT and Brugada syndrome: Implications for Na+ channel blocking pharmacotherapy for de novo conduction disease

Background

The SCN5A mutation, P1332L, is linked to a malignant form of congenital long QT syndrome, type 3 (LQT3), and affected patients are highly responsive to the Na⁺ channel blocking drug, mexiletine. In contrast, A647D is an atypical SCN5A mutation causing Brugada syndrome. An asymptomatic male with both P1332L and A647D presented with varying P wave/QRS aberrancy and mild QTc prolongation which did not shorten measurably with mexiletine.

Objective

We characterized the biophysical properties of P1332L, A647D and wild-type (WT) Na⁺ channels as well as their combinations in order to understand our proband’s phenotype and to guide mexilitine therapy.

Methods

Na⁺ channel biophysics and mexilitine-binding kinetics were assessed using heterologous expression studies in CHO-K1 cells and human ventricular myocyte modeling.

Results

Compared to WT, P1332L channels displayed a hyperpolarizing shift in inactivation, slower inactivation and prominent late Na⁺ currents (I_Na). While A647D had no effect on the biophysical properties of I_Na, it reduced peak and late I_Na density when co-expressed with either WT or P1332L. Additionally, while P1332L channels had greater sensitivity to block by mexiletine compared to WT, this was reduced in the presence of A647D. Modelling studies revealed that mixing P1332L with A647D channels, action potential durations were shortened compared to P1332L, while peak I_Na was reduced compared to either A647D coexpressing with WT or WT alone.

Conclusions

While A647D mitigates the lethal LQT3 phenotype seen with P1332L, it also reduces mexilitine sensitivity and decreases I_Na density. These results explain our proband’s mild repolarization abnormality and prominent conduction defect in the atria and ventricles, but also suggest that expression of P1332L with A647D yields a novel disease phenotype for which mexiletine pharmacotherapy is no longer suitable.

Thromboelastographic assessment of the impact of mexiletine on coagulation abnormalities induced by air or normal saline intravenous injections in conscious rats

BACKGROUND

Thromboelastography (TEG) in venous air embolism (VAE) has been poorly studied. We induced coagulation abnormalities by VAE in a rat model, assessed by TEG with and without mexiletine, a lidocaine analogue local anesthetic.

METHODS

Twenty-three Sprague Dawley rats instrumented under isoflurane anesthesia and allowed to recover five days prior to the experiments were randomized into three experimental groups: 1) VAE (n = 6); 2) VAE and mexiletine (n = 9); and 3) normal saline (NS) alone (control group, n = 8). Blood samples were collected at baseline, one hour (h) and 24 h in all groups and analyzed by TEG to record the R, K, angle α and MA parameters.

RESULTS

In Group 1, VAE decreased significantly R at 1 h (31%), K at 1 h (59%) and 24 h (34%); α increased significantly at 1 h (30%) and 24 h (22%). While R returned to baseline values within 24 h, K, MA and α did not. In group-2 (Mexiletine + VAE), K and R decreased at 1 h (48% and 29%, respectively) and at 24 h the changes were non-significant. Angle α increased at 1 h (28%) and remained increased for 24 h (25%). In group 3 (NS), only R was temporarily affected. MA increased significantly at 24 h only in the VAE alone group.

CONCLUSION

As expected, VAE produced a consistent and significant hypercoagulable response diagnosed/confirmed by TEG. Mexiletine prevented the MA elevation seen with VAE and corrected R and K time at 24 h, whereas angle α remained unchanged. Mexiletine seemed to attenuate the hypercoagulability associated with VAE in this experiment. These results may have potential clinical applications and deserve further investigation.

Mexiletine co-injected with clonidine increases the quality and duration of cutaneous analgesia in response to skin pinpricks in the rat

The goal of the experimental design was to assess the cutaneous analgesic effect of mexiletine by co-injection with clonidine. The effect of nociceptive block was evaluated according to the inhibition of the cutaneous trunci muscle reflex (CTMR) in response to skin pinpricks in rats. The dose-related analgesic effect of mexiletine alone or mexiletine co-administrated with clonidine was constructed after subcutaneous injection. Subcutaneous injections of mexiletine elicited dose-related cutaneous analgesia. Compared with mexiletine (1.8μmol), adding clonidine to mexiletine (1.8μmol) solutions for skin nociceptive block potentiated and prolonged the action (p<0.01). Mexiletine (6μmol) combined with clonidine extended the duration of cutaneous analgesia when compared with mexiletine (6μmol) alone (p<0.01). Co-administration of clonidine increases the potency and extends the duration of cutaneous analgesia by mexiletine, and the minimal dose of clonidine to intensify the analgesic effect is 0.06μmol.

Electrocardiographic Biomarkers for Detection of Drug-Induced Late Sodium Current Block

BACKGROUND

Drugs that prolong the heart rate corrected QT interval (QTc) on the electrocardiogram (ECG) by blocking the hERG potassium channel and also block inward currents (late sodium or L-type calcium) are not associated with torsade de pointes (e.g. ranolazine and verapamil). Thus, identifying ECG signs of late sodium current block could aid in the determination of proarrhythmic risk for new drugs. A new cardiac safety paradigm for drug development (the "CiPA" initiative) will involve the preclinical assessment of multiple human cardiac ion channels and ECG biomarkers are needed to determine if there are unexpected ion channel effects in humans.

METHODS AND RESULTS

In this study we assess the ability of eight ECG morphology biomarkers to detect late sodium current block in the presence of QTc prolongation by analyzing a clinical trial where a selective hERG potassium channel blocker (dofetilide) was administered alone and then in combination with two late sodium current blockers (lidocaine and mexiletine). We demonstrate that late sodium current block has the greatest effect on the heart-rate corrected J-Tpeak interval (J-Tpeakc), followed by QTc and then T-wave flatness. Furthermore, J-Tpeakc is the only biomarker that improves detection of the presence of late sodium current block compared to using QTc alone (AUC: 0.83 vs. 0.72 respectively, p<0.001).

CONCLUSIONS

Analysis of the J-Tpeakc interval can differentiate drug-induced multichannel block involving the late sodium current from selective hERG potassium channel block. Future methodologies assessing drug effects on cardiac ion channel currents on the ECG should use J-Tpeakc to detect the presence of late sodium current block.

TRIAL REGISTRATION

NCT02308748 and NCT01873950.

Comparison of Gating Properties and Use-Dependent Block of Nav1.5 and Nav1.7 Channels by Anti-Arrhythmics Mexiletine and Lidocaine

Mexiletine and lidocaine are widely used class IB anti-arrhythmic drugs that are considered to act by blocking voltage-gated open sodium currents for treatment of ventricular arrhythmias and relief of pain. To gain mechanistic insights into action of anti-arrhythmics, we characterized biophysical properties of Na_v1.5 and Na_v1.7 channels stably expressed in HEK293 cells and compared their use-dependent block in response to mexiletine and lidocaine using whole-cell patch clamp recordings. While the voltage-dependent activation of Na_v1.5 or Na_v1.7 was not affected by mexiletine and lidocaine, the steady-state fast and slow inactivation of Na_v1.5 and Na_v1.7 were significantly shifted to hyperpolarized direction by either mexiletine or lidocaine in dose-dependent manner. Both mexiletine and lidocaine enhanced the slow component of closed-state inactivation, with mexiletine exerting stronger inhibition on either Na_v1.5 or Na_v1.7. The recovery from inactivation of Na_v1.5 or Na_v1.7 was significantly prolonged by mexiletine compared to lidocaine. Furthermore, mexiletine displayed a pronounced and prominent use-dependent inhibition of Na_v1.5 than lidocaine, but not Na_v1.7 channels. Taken together, our findings demonstrate differential responses to blockade by mexiletine and lidocaine that preferentially affect the gating of Na_v1.5, as compared to Na_v1.7; and mexiletine exhibits stronger use-dependent block of Na_v1.5. The differential gating properties of Na_v1.5 and Na_v1.7 in response to mexiletine and lidocaine may help explain the drug effectiveness and advance in new designs of safe and specific sodium channel blockers for treatment of cardiac arrhythmia or pain.

Functional characterization of a novel frameshift mutation in the C-terminus of the Nav1.5 channel underlying a Brugada syndrome with variable expression in a Spanish family

Introduction

We functionally analyzed a frameshift mutation in the SCN5A gene encoding cardiac Na⁺ channels (Nav1.5) found in a proband with repeated episodes of ventricular fibrillation who presented bradycardia and paroxysmal atrial fibrillation. Seven relatives also carry the mutation and showed a Brugada syndrome with an incomplete and variable expression. The mutation (p.D1816VfsX7) resulted in a severe truncation (201 residues) of the Nav1.5 C-terminus.

Methods and Results

Wild-type (WT) and mutated Nav1.5 channels together with hNavβ1 were expressed in CHO cells and currents were recorded at room temperature using the whole-cell patch-clamp. Expression of p.D1816VfsX7 alone resulted in a marked reduction (≈90%) in peak Na⁺ current density compared with WT channels. Peak current density generated by p.D1816VfsX7+WT was ≈50% of that generated by WT channels. p.D1816VfsX7 positively shifted activation and inactivation curves, leading to a significant reduction of the window current. The mutation accelerated current activation and reactivation kinetics and increased the fraction of channels developing slow inactivation with prolonged depolarizations. However, late I_Na was not modified by the mutation. p.D1816VfsX7 produced a marked reduction of channel trafficking toward the membrane that was not restored by decreasing incubation temperature during cell culture or by incubation with 300 μM mexiletine and 5 mM 4-phenylbutirate.

Conclusion

Despite a severe truncation of the C-terminus, the resulting mutated channels generate currents, albeit with reduced amplitude and altered biophysical properties, confirming the key role of the C-terminal domain in the expression and function of the cardiac Na⁺ channel.

An improved synthesis of m-hydroxymexiletine, a potent mexiletine metabolite

m-Hydroxymexiletine (MHM), a minor metabolite of the class IB anti-arrhythmic drug mexiletine, is about two fold more potent than the parent compound on human cardiac voltage-gated sodium channels (hNav1.5), and equipotent to mexiletine on human skeletal-muscle voltage-gated sodium channels (hNav1.4). Herein, an alternative and simplified synthesis of this promising compound has been accomplished. This route, as well as being more efficient, has the advantage, over the first, to avoid the use of oxidizing agents, such as the meta-chloroperoxybenzoic acid.

Stable Expression and Characterization of Human PN1 and PN3 Sodium Channels

Nociceptive transduction in inflammatory and neuropathic pain involves peripherally expressed voltage-gated sodium channels, such as tetrodotoxin (TTX)-sensitive PN1 and TTX-resistant PN3. We generated recombinant cell lines stably expressing the human PN1 and PN3 sodium channels in Chinese hamster ovary (CHO) cells using inducible expression vectors. The PN1 and PN3 cDNAs were isolated from human adrenal gland and heart poly(A)+ RNAs, respectively. The recombinant human PN1 currents exhibited rapid activation and inactivation kinetics and were blocked by TTX with a half-maximal inhibitory concentration (IC50) of 32.6 nM. The human PN3 channel expressed in stable transfectants showed TTX-resistant inward currents with slow inactivation kinetics. The IC50 value for TTX was 73.3 microM. The voltage-dependence of activation of the PN3 channel was shifted to the depolarizing direction, compared to that of the PN1 channel. Lidocaine and mexiletine exhibited tonic and use-dependent block of PN1 and PN3 channels. The PN1 channel was more susceptible to inhibition by mexiletine than PN3. These results suggest that stable transfectants expressing the human PN1 and PN3 sodium channels will be useful tools to define subtype selectivity for sodium channel blockers.

Attenuating effect of mexiletine hydrochloride on herpetic pain in mice infected with herpes simplex virus

The influence of mexiletine hydrochloride on herpes-related pain responses was examined using mice infected with herpes virus. BALB/c mice were inoculated with herpes simplex virus (HSV; 1 times 106 plaque-forming units) on the right hind paw, and the contralateral hind paw was without inoculation. The changes in nociceptive threshold were examined using electric von fray meter. BALB/c mice inoculated with HSV showed a decrease in nociceptive threshold. Intraperitoneal administration of mexiletine prevented the decrease in nociceptive threshold dose-dependently in HSV-inoculated mice, which was firstly observed at a dose of 15.0 mg kg−1, and peaked at doses more than 17.5 mg kg−1. This antinociceptive effect of mexiletine attained peaks at 60–90 min after administration and declined gradually to non-treated levels by 150 min. Intraperitoneal administration of mexiletine at a dose of 17.5 mg kg−1 (but not 10.0 mg kg−1) caused significant increase in β-endorphin levels in the mid brain and hypothalamus of HSV-inoculated mice. However, mexiletine scarcely affected noradrenaline (norepinephrine) levels in the pons and medulla oblongata, even when HSV-inoculated mice were treated with 17.5 mg kg−1 mexiletine. These results strongly suggested that mexiletine exerts antinociceptive effects on herpes-related pain through enhancement of β-endorphin levels in the central nervous system in HSV-inoculated mice. It is also suggested that mexiletine will be a good candidate for an antinociceptive drug in the treatment of acute herpetic pain in man.

Comparison of the effect of mexiletine and methylprednisolone on neural function and histopathological damage after transient spinal cord ischemia in rabbits

AIM

The purpose of this study was to investigate the effect of mexiletine on the neural function and histopathological changes after ischemic spinal cord injury in rabbits. We also compared the effect of mexiletine to that of methylprednisolone.

MATERIAL AND METHODS

Twenty six male New Zealand white rabbits were randomly divided into six groups. Group 1; sham operated group (n=3) underwent only the surgical exposure of infrarenal aorta. Group 2 (n=4) received neither intravenous (iv) nor intraperitoneal medication but the infrarenal aorta was cross-clamped. Group 3 (n=5) received intravenous infusion of 20 ml/kg/h normal saline. Group 4 (n=5) received 30 mg/kg intravenous methylprednisolone. Group 5 (n=3) received intraperitoneal 20mg/kg/h normal saline. Group 6 (n=6) received 50mg/kg mexiletine intraperitoneally. Temporary spinal cord ishemia was induced by infrarenal aortic occlusion for 25 minutes and followed by reperfusion. The neural status was scored using the Tarlov criteria at 24 hours after reperfusion. Immediately after the neurological scoring, the spinal cords of all animals were removed for histopathological study.

RESULTS

Histopathological examination scores were significantly higher in group 6 compared to group 2 (p < 0.05).

CONCLUSION

Mexiletine can significantly ameloriate the neural function and prevent histopathological damage after transient spinal cord ischemia in rabbits. This is the first research that investigates the neuron=protective effect of mexiletine in a spinal cord ischemia model.

The preventive effect of mexiletine on cerebral ischemic injury following experimental middle cerebral artery occlusion

AIM

Previous studies demonstrated that mexiletine has some important features in the prevention of ischemic brain injury such as sodium and calcium canal blockage and free radical occurrence. Our aim was to investigate the effects of mexiletine on ischemic brain injury.

MATERIAL AND METHODS

Experiments were performed on 30 adult male Sprague- Dawley rats (285-425 g). Left middle cerebral artery occlusion following microcraniectomy and simultaneous bilateral carotid artery occlusion were performed. Three different treatments were included in this study: (a) "naïve" control group (no drug applied; n = 10); (b) "sham surgery" control group (only saline was applied; n = 10); and a (c) "treatment group (n = 10) where mexiletine was applied. After 24 h from ischemic insult, all rats were decapitated and prepared for immunocytochemical and histopathological analyses. Cerebral infarct volumes were calculated and compared using ANOVA and a Post- Hoc Bonferroni test in each group statistically.

RESULTS

The results showed statistically significant differences between the treatment (81.98 +/- 12.58 mm?), control (121.57 +/- 11.41 mm?) and sham (116.08 +/- 12.36 mm?) groups (p < 0,0001), respectively.

CONCLUSION

Mexiletine should be considered as an alternative medication for prevention and treatment of ischemic brain injury due to its multipotent effects.

[Central and peripheral mechanisms in antinociception: current and future perspectives]

As it is well known opioids are the most powerful drugs used for acute and chronic pain, although, their several serious side effects, such as respiratory depression, mental clouding, constipation, and tolerance dependence producing capacity, as well as large interpatient variability in responses limit their safe everyday use. Furthermore, the treatment of certain types of pain (e.g. neuropathic pain) is not very satisfactorily managed. Consequently, there is a continuous need to find analgesics efficient against chronic neuropathic pain and avoid these side actions and still retain opioid like potency. There are several possible way to find new targets for these purposes. Recently opioid receptors have been identified on peripheral processes of sensory neurons. These findings provide new insights into intrinsic mechanisms of pain control and suggest innovative strategies for developing drugs and alternative approaches to pain treatment. In the effort to discover better analgesic drugs for chronic pain, attention is being paid to specific ion channels at the periphery, include members of transient receptor potential family (TRPV1, capsaicin receptors), as well as P2x receptors, sensitive to purines released from tissue injury. A special tetradotoxin-resistant, voltage dependent type of sodium channel is associated with dorsal root ganglia neurons is blocked by mexiletine, used in chronic pain. A synthetic peptide analogue of marine snail toxin ziconitine blocks N-type calcium channels. GABA and NMDA receptors are also involved in the antinociceptive actions of gabapentin and ketamine, respectively. Furthermore nicotine and analogues (epibatidine) induce analgesia through nicotinic ACh receptors. We studied mostly the peripheral targets of hydrophilic heterocyclic opioids in antinociceptive processes.

Pharmacologic evaluation of pressor and visceromotor reflex responses to bladder distension

AIMS

Several mechanisms that are involved in acute rat bladder nociception were examined. The nociceptive response was measured by analyzing both cardiovascular and visceromotor reflex responses to urinary bladder distension. The contributions of micro-opioid receptor, kappa-opioid receptor, sodium channels, muscarinic receptors, and cyclooxygenase, were explored with morphine, U50,488, mexiletine, oxybutynin, and naproxen, respectively.

METHODS

Female Sprague-Dawley rats were acutely instrumented with jugular venous, carotid arterial, and bladder cannulas. Needle electrodes were placed directly into the abdominal musculature to measure myoelectrical activity subsequent to repeated phasic urinary bladder distension (60 mmHg for 20 sec in 3 min intervals) under 1% isoflurane. Drugs were administered by i.v. bolus injection 2 min prior to distension.

RESULTS

The analgesics morphine (ID50 0.69 mg/kg), U50,488 (1.34 mg/kg), and mexiletine (2.60 mg/kg) significantly inhibited the visceromotor reflex response to noxious urinary bladder distension. Oxybutynin also attenuated reflex responses to noxious urinary bladder distension to 41% of the maximal pressor response and 32% of the control visceromotor reflex response (3.01 and 5.05 mg/kg), respectively, indicating a role of muscarinic receptors in bladder nociception. Naproxen did not attenuate the pressor response, but moderately inhibited visceromotor reflex to 45% of control at 30 mg/kg (P < 0.05).

CONCLUSIONS

Current results using the rat urinary bladder distension model are consistent with previous research demonstrating a role of the analgesics (morphine, U50,488, and mexiletine) in the inhibition of visceral nociceptive transmission. The utility of the reflex responses to urinary bladder distension may provide a method useful to examine mechanisms which target the bladder sensory pathway.

Attenuation of microtubule associated protein-2 degradation after mild head injury by mexiletine and calpain-2 inhibitor

The objective of the study was to address the early effects of mild, closed, head injuries on neuronal stability and the prevention of microtubule-associated protein-2 (MAP-2) degradation by mexiletine and calpain-2 inhibitor. Twenty-four rats were divided into four groups: control group (1); trauma group without treatment (2); mexiletine-pretreated and subjected to trauma group (3); trauma subjected and then calpain-2 inhibitor received group (4). All animals were subjected to mild, closed, head trauma. Frontal lobes were removed and processed for staining and immunofluorescent labelling of MAP-2 cytoskeletal proteins, which were evaluated by confocal microscopy in serial optical sections showing the three dimensional cytoarchitecture of affected areas. MAP-2 decoration in almost all neurons obtained from traumatized brain regions drastically diminished, while minute filamentous and granular profiles in axons and/or dendrites were retained together implying a massive degradation/depolymerization of microtubules. In contrast, in mexiletine-pretreated animals, MAP-2 positivity in axonal and perikaryonal profiles was fairly retained, which clearly depicts the protective role of mexiletine after trauma. Compared with mexiletine-pretreated group, calpain-2 inhibitor treated group displayed a less well-preserved MAP-2 expression. Mexiletine can prevent cytoskeletal structure and protein degradation after mild head trauma. Calpain-2 inhibitor prevents protein degradation, but cytoskeletal organization is better preserved with mexiletine.

The effects of Na movement on surgical myocardial protection: the role of the Na+-H+ exchange system and Na-channel in the development of ischemia and reperfusion injury

OBJECTIVES

We investigated whether the Na+-H+ exchange inhibitor, HOE642 (Hoe), and/or the Na channel blocker, mexiletine (Mex), enhance a cardioprotective effect on St. Thomas' Hospital cardioplegic solution (STS) to clarify the mechanism by which intracellular Na+ is accumulated after cardioplegic arrest.

MATERIALS AND METHODS

Isolated working rat hearts were perfused with Krebs-Henseleit bicarbonate buffer (KHBB). The hearts were then arrested with STS and subjected to normothermic global ischemia (30 min). This was followed by Langendorff reperfusion (15 min) and then a working reperfusion (20 min). In study A, we added Hoe (5, 10, and 20 microM), Mex (70 microM), or a combination of Hoe (20 microM) and Mex (70 microM), to STS. In study B, we added Hoe (20 microM), Mex (70 microM), or a combination of Hoe (20 microM) and Mex (70 microM) to KHBB during the first 3 min of Langendorff reperfusion.

RESULTS

In study A, the addition of Hoe (10 and 20 microM) to STS showed a significantly greater postischemic recovery of cardiac output compared to the control group [63.1+/-5.7% (10 microM), 62.7+/-4.7% (20 microM), and 55.5+/-4.6% (control), respectively]. The postischemic recovery of cardiac output was significantly greater in the group of the combined addition (Hoe and Mex) to STS than that in the control, 20 microM Hoe, 70 microM Mex groups [70.3+/-3.7 (Hoe and Mex), 55.5+/-4.6% (control), 62.7+/-4.7% (Hoe 20 microM), and 60.2+/-4.7% (Mex 70 microM), respectively]. The myocardial water content in the postischemic period was 565.1+/-29.1, 525.8+/-2.9, 509.4+/-19.6, and 532.2+/-20.1; it was 497.3+/-9.1 mL/100 g dry weight in the control; and 10 microM Hoe, 20 microM Hoe, and 70 microM Mex in the combined use groups. In study B, there was no significant difference in the postischemic recovery of cardiac output in all experimental groups.

CONCLUSION

The combined use of the Na+-H+ exchange inhibitor and Na+ channel blocker during cardioplegia may achieve a superior cardioprotective effect on myocardial damage because of ischemia and reperfusion.

Differential Contribution of Sodium Channel Subtypes to Action Potential Generation in Unmyelinated Human C-type Nerve Fibers

Background

Multiple voltage-dependent sodium channels (Na(v)) contribute to action potentials and excitability of primary nociceptive neurons. The aim of the current study was to characterize subtypes of Na(v) that contribute to action potential generation in peripheral unmyelinated human C-type nerve fibers.

Methods

Registration of C-fiber compound action potentials and determination of membrane threshold was performed by a computerized threshold tracking program. Nerve fibers were stimulated with a 1-ms current pulse either alone or after a small ramp current lasting 300 ms.

Results

Compound C-fiber action potentials elicited by supramaximal 1-ms current pulses were rather resistant to application of tetrodotoxin (30-90 nM). However, the same concentrations of tetrodotoxin strongly reduced the peak height and elevated membrane threshold of action potentials evoked at the end of a 300-ms current ramp. A similar effect was observed during application of lidocaine and mexiletine (50 microM each).

Conclusions

These data indicate that more than one type of Na(v) contributes to the generation of action potentials in unmyelinated human C-type nerve fibers. The peak height of an action potential produced by a short electrical impulse is dependent on the activation of tetrodotoxin-resistant ion channels. In contrast, membrane threshold and action potential peak height at the end of a slow membrane depolarization are regulated by a subtype of Na(v) with high sensitivity to low concentrations of tetrodotoxin, lidocaine, and mexiletine. The electrophysiologic and pharmacologic characteristics may indicate the functional activity of the Na(v) 1.7 subtype of voltage-dependent sodium channels.

Gating properties of SCN5A mutations and the response to mexiletine in long-QT syndrome type 3 patients

BACKGROUND

Mexiletine (Mex) has been proposed as a gene-specific therapy for patients with long-QT syndrome type 3 (LQT3) caused by mutations in the cardiac sodium channel gene (SCN5A). The degree of QT shortening and the protection from arrhythmias vary among patients harboring different mutations. We tested whether the clinical response to Mex in LQT3 could be predicted by the biophysical properties of the different mutations.

METHODS AND RESULTS

We identified 4 SCN5A mutations in 5 symptomatic LQT3 patients with different responses to Mex (6 to 8 mg . kg(-1) . d(-1)). We classified the mutations as sensitive to Mex (P1332L, R1626P; >/=10% of QTc shortening and QTc <500 ms or no arrhythmias) or insensitive to Mex (S941N, M1652R; negligible or no QTc shortening and sudden death). We measured Na(+) current from HEK 293 cells transfected with wild-type (WT) or mutant Nav1.5. All mutations showed impaired inactivation of Na(+) current, but the mutations identified in patient responders to Mex (P1332L, R1626P) showed a hyperpolarizing shift of V(1/2) of steady-state inactivation. Furthermore, Mex produced use-dependent block with the order R1626P=P1332L>S941N=WT>M1652R, suggesting that Mex-sensitive mutants present prolonged recovery from Mex block.

CONCLUSIONS

We propose that voltage dependence of channel availability and shifts of V(1/2) of steady-state inactivation correlate with the clinical response observed in LQT3 patients. This supports the view that the response to Mex is mutation specific and that in vitro testing may help to predict the response to therapy in LQT3.

The Cutaneous Analgesic Effect of Class I Antiarrhythmic Drugs

BACKGROUND

Local anesthetics, when applied to nerves, produce reversible loss of sensation by blocking Na+ channels. Because all Class I antiarrhythmic drugs are Na+ channel blockers, theoretically, they may have local anesthetic effects. In this study, we sought to define the cutaneous local anesthetic actions of three Class I antiarrhythmic drugs.

METHODS

Using a subcutaneous infiltration model in rats, the potencies and durations of action of quinidine (Class IA), mexiletine (IB), and flecainide (IC) were determined and compared with the actions of lidocaine and bupivacaine. Saline injection was used as control.

RESULTS

Three Class I antiarrhythmic drugs produced a dose-related cutaneous analgesia with ranking of potencies of bupivacaine > flecainide > quinidine > mexiletine > lidocaine (P < 0.05 for the differences among drugs). On an equipotent basis, the ranking of durations of action was flecainide > quinidine and bupivacaine > mexiletine and lidocaine (P < 0.05 for the differences among drugs).

CONCLUSION

Three Class I antiarrhythmic drugs, quinidine (IA), mexiletine (IB), and flecainide (IC) have a local anesthetic effect on cutaneous analgesia.

Effects of Mexiletine on Electrical Field Stimulation-Induced Contractile Responses in the Ipsilateral and Contralateral Vasa Deferentia after Unilateral Testicular Torsion/Detorsion

AIM

To investigate testicular torsion-induced changes on the electrical field stimulation (EFS)-induced contractions in rabbit vasa deferentia and to evaluate the effect of mexiletine.

METHODS

18 male New Zealand albino rabbits were used in this experiment. Rabbits were divided into three groups: (1) control group (n = 6); (2) torsion group (n = 6), and (3) mexiletine group (n = 6). In the control group, vasa deferentia on both sides were harvested. In the torsion and mexiletine groups, the left testes of the rabbits were subjected to 720 degrees of clockwise torsion for 2 h and then detorsion was performed. In the mexiletine group, 50 mg/kg i.p. mexiletine was administered 1 h before detorsion. Following 24 h of the torsion, vasa deferentia on both sides were harvested and 2-cm strips including both the prostatic and epididymal portions were prepared to record EFS-induced contractions.

RESULTS

Testicular torsion caused a significant inhibition in both phases of EFS-induced biphasic contractions of the ipsi- and contralateral vasa deferentia. Mexiletine treatment did not affect these inhibitory responses. Torsion/detorsion of the spermatic cord did not alter exogenously applied noradrenaline-induced contractions in both vasa deferentia. However, KCl-induced contractions diminished significantly in ipsilateral vas deferens of the torsion group and mexiletine restored this inhibition.

CONCLUSIONS

Unilateral testicular torsion/detorsion leads to inhibition in both phases of EFS-induced biphasic contractions of the ipsi- and contralateral vasa deferentia by causing a defect in presynaptic nerve transmission. However, mexiletine has no effect on this inhibition. Inhibition of the KCl-induced contractions in the ipsilateral vas deferens, which indicates postsynaptic tissue damage, is restored by administering mexiletine 1 h prior to detorsion.

Evaluation of the pharmacological activity of the major mexiletine metabolites on skeletal muscle sodium currents

BACKGROUND AND PURPOSE

Mexiletine (Mex), an orally effective antiarrhythmic agent used to treat ventricular arrhythmias, has also been found to be effective for myotonia and neuropathic pain. It is extensively metabolized in humans but little information exists about the pharmacodynamic properties of its metabolites.

EXPERIMENTAL APPROACH

To determine their contribution to the clinical activity of Mex, p-hydroxy-mexiletine (PHM), hydroxy-methyl-mexiletine (HMM), N-hydroxy-mexiletine (NHM) (phase I reaction products) and N-carbonyloxy beta-D-glucuronide (NMG) (phase II reaction product) were tested on sodium currents (I(Na)) of frog skeletal muscle fibres. Sodium currents were elicited with depolarizing pulses from different holding potentials (HP=-140, -100, -70 mV) and stimulation frequencies (0.25, 0.5, 1, 2, 5, 10 Hz) using the vaseline-gap voltage-clamp method.

KEY RESULTS

All the hydroxylated derivatives blocked the sodium channel in a voltage- and use-dependent manner. The PHM, HMM and NHM metabolites were up to 10-fold less effective than the parent compound. However, HMM showed a greater use-dependent behaviour (10 Hz), compared to Mex and the other metabolites. Similar to Mex, these products behaved as inactivating channel blockers. Conjugation with glucuronic acid (NMG) resulted in almost complete abolition of the pharmacological activity of the parent compound.

CONCLUSIONS AND IMPLICATIONS

Thus, although less potent, the phase I metabolites tested demonstrated similar pharmacological behaviour to Mex and might contribute to its clinical profile.

Potent analgesic effects of a putative sodium channel blocker M58373 on formalin-induced and neuropathic pain in rats

M58373, 4-[2-(4-hydroxy-4-{[N-(4-isopropoxyphenyl)-N-methylamino]methyl}piperidin-1-yl)ethyl]benzonitrile monohydrochloride, is a novel compound, which has an inhibitory activity on neurotoxin binding to the site 2 of voltage-gated sodium channels. In this study, we investigated the effects of M58373 on substance P release from sensory neurons in vitro and pain behaviors/responses in rats, compared with mexiletine. M58373 (1-10 microM) inhibited veratridine-induced release of substance P from dorsal root ganglion cells. In the formalin test, oral M58373 (0.3-10 mg/kg) reduced the time spent in nociceptive behaviors only in the late phase. In the neuropathic pain model, oral M58373 (1-10 mg/kg) attenuated mechanical allodynia and heat hyperalgesia in the nerve-injured paw without affecting normal responses in the uninjured paw. In contrast, oral mexiletine (10-100 mg/kg) had a narrow therapeutic dose range in both models because of the adverse effects on the central nervous system. These results suggest that M58373 is a favorable prototype for novel anti-neuropathic pain agents.

Partial expression defect for the SCN5A missense mutation G1406R depends on splice variant background Q1077 and rescue by mexiletine

Mutations in the cardiac Na(+) channel gene SCN5A cause loss of function and underlie arrhythmia syndromes. SCN5A in humans has two splice variants, one lacking a glutamine at position 1077 (Q1077del) and one containing Q1077. We investigated the effect of splice variant background on loss of function and rescue for G1406R, a mutation reported to cause Brugada syndrome. Mutant and wild-type (WT) channels in both backgrounds were transfected into HEK-293 cells and incubated for up to 72 h with and without mexiletine. At 8 h, neither current nor cell surface expression was observed for the mutant in either background, but both were present in WT channels. At 24 h, small (<10% compared with WT) currents were noted and accompanied by cell surface expression. At 48 h, current density was approximately 40% of WT channels for the mutant in the Q1077del variant background but remained at <10% of WT channels in Q1077. Current levels were stable by 72 h. Coexpression with beta(1)- or beta(3)-subunits or insertion of the polymorphism H558R in the background did not significantly affect current expression. Mexiletine restored current density of the mutant channel in both backgrounds to nearly WT levels. The mutant channels also showed a negative shift in inactivation, slower recovery, and enhanced slow inactivation, consistent with a loss of function phenotype. These data show that a trafficking defect may be partial and time dependent and may differ with the splice variant background. Also, expression defects and gating abnormalities may contribute to loss of function for the same mutation.

Effect of mexiletine on vincristine-induced painful neuropathy in mice

In the present study, we examined the effect of mexiletine on vincristine-induced thermal hyperalgesia in mice. Mice were intraperitoneally treated with vincristine at a dose of 0.05 mg/kg one day after the measurement of the pre-drug latency in the tail-flick test, and then treated with a dose of 0.125 mg/kg twice a week for 6 weeks. In vincristine-treated mice, a significant decrease in tail-flick latency developed at 6 weeks after treatment. Pretreatment with mexiletine, at doses of 3, 10 and 30 mg/kg, i.p., dose-dependently increased the tail-flick latency in vincristine-treated mice. A significant reduction of the tail-flick latency was observed when the tail-flick latency was examined 60 min after i.t. administration of NG-nitro-L-arginine methyl ester (L-NAME, 30 nmol), a nitric oxide synthase (NOS) inhibitor, in naive mice. This L-NAME-induced thermal hyperalgesia was dose-dependently attenuated by pretreatment with mexiletine (10 and 30 mg/kg, i.p.), 10 min before the injection of L-NAME. The duration of nociceptive behavioral response induced by fenvalerate, at a dose of 0.1 microg, i.t., was significantly increased by pretreatment with L-NAME (30 nmol, i.t.). Intrathecal pretreatment with L-arginine (300 pmol) significantly reversed the L-NAME-induced enhancement of fenvalerate-induced nociceptive responses. The present study demonstrates that systemic mexiletine can effectively attenuate vincristine-induced thermal hyperalgesia. Furthermore, these results suggest that blockade of nitric oxide-induced enhancement of nociceptive transmission, in which tetrodotoxin-resistant sodium channels play an important role, may participate in the antinociceptive effect of mexiletine on vincristine-induced thermal hyperalgesia.

A common cardiac sodium channel variant associated with sudden infant death in African Americans, SCN5A S1103Y

Thousands die each year from sudden infant death syndrome (SIDS). Neither the cause nor basis for varied prevalence in different populations is understood. While 2 cases have been associated with mutations in type Valpha, cardiac voltage-gated sodium channels (SCN5A), the "Back to Sleep" campaign has decreased SIDS prevalence, consistent with a role for environmental influences in disease pathogenesis. Here we studied SCN5A in African Americans. Three of 133 SIDS cases were homozygous for the variant S1103Y. Among controls, 120 of 1,056 were carriers of the heterozygous genotype, which was previously associated with increased risk for arrhythmia in adults. This suggests that infants with 2 copies of S1103Y have a 24-fold increased risk for SIDS. Variant Y1103 channels were found to operate normally under baseline conditions in vitro. As risk factors for SIDS include apnea and respiratory acidosis, Y1103 and wild-type channels were subjected to lowered intracellular pH. Only Y1103 channels gained abnormal function, demonstrating late reopenings suppressible by the drug mexiletine. The variant appeared to confer susceptibility to acidosis-induced arrhythmia, a gene-environment interaction. Overall, homozygous and rare heterozygous SCN5A missense variants were found in approximately 5% of cases. If our findings are replicated, prospective genetic testing of SIDS cases and screening with counseling for at-risk families warrant consideration.

Neuroprotective effect of mexiletine in the central nervous system of diabetic rats

Both experimental and clinical studies suggests that oxidative stress plays an important role in the pathogenesis of diabetes mellitus type 1 and type 2. Hyperglycaemia leads to free radical generation and causes neural degeneration. In the present study we investigated the possible neuroprotective effect of mexiletine against streptozotocin-induced hyperglycaemia in the rat brain and spinal cord. 30 adult male Wistar rats were divided into three groups: control, diabetic, and diabetic-mexiletine treated group. Diabetes mellitus was induced by a single injection of streptozotocin (60 mg/kg body weight). Mexiletine (50 mg/kg) was injected intraperitoneally every day for six weeks. After 6 weeks the brain, brain stem and cervical spinal cord of the rats were removed and the hippocampus, cortex, cerebellum, brain stem and spinal cord were dissected for biochemical analysis (the level of Malondialdehide [MDA], Nitric Oxide [NO], Reduced Glutathione [GSH], and Xanthine Oxidase [XO] activity). MDA, XO and NO levels in the hippocampus, cortex, cerebellum, brain stem and spinal cord of the diabetic group increased significantly, when compared with control and mexiletine groups (P < 0.05). GSH levels in the hippocampus, cortex, cerebellum, brain stem and spinal cord of the diabetic group decreased significantly when compared with control and mexiletine groups (P < 0.05). This study demonstrates that mexiletine protects the neuronal tissue against the diabetic oxidative damage.

Class Ic antiarrhythmics block human skeletal muscle Na channel during myotonia-like stimulation

Flecainide, a class Ic antiarrhythmic drug, has been anecdotally reported to improve myotonia, but little is known about its kinetics on human skeletal muscle sodium channels applicable in vivo. Here we explored the anti-myotonic action of flecainide for human skeletal muscle sodium channels heterologously expressed in cultured cells. Flecainide blocked sodium channels in a highly state-dependent manner with 20-fold difference in IC(50) between use-dependent and tonic blocks. When pulses of brief depolarization simulating myotonia were applied from a holding potential of -90 mV, flecainide at therapeutic concentrations significantly blocked sodium currents. Flecainide slowed the time course of recovery but most channels recovered from block within 10-20 s. In contrast to mexiletine, flecainide did not markedly block sodium current during prolonged depolarization, suggesting an open-channel blocking action. Considering the slow recovery from block and the specific action against repetitive depolarization, flecainide may represent a potent therapeutic agent for myotonia.

Acetazolamide acts directly on the human skeletal muscle chloride channel

Acetazolamide, a carbonic anhydrase inhibitor, is used empirically in neuromuscular diseases with episodic ataxia, weakness, and myotonia, although not all of the mechanisms responsible for its therapeutic effects are understood. To elucidate whether acetazolamide acts directly on the human skeletal muscle voltage-gated chloride channel (ClC-1), which is associated with myotonia, we evaluated the effects of acetazolamide on ClC-1 expressed in cultured mammalian cells, using whole-cell recording. Acetazolamide significantly shifted the voltage dependency of the open probability (P(o)) toward negative potentials in a dose-dependent manner, resulting in an increase of chloride conductance at voltages near the resting membrane potential. This effect was attenuated when using a pipette solution containing 30 mmol/L Hepes. These results suggest that acetazolamide can influence the voltage-dependent opening gate of ClC-1 through a mechanism related to intracellular acidification by inhibiting carbonic anhydrase, and that the therapeutic effects of acetazolamide in neuromuscular diseases may be mediated by activation of ClC-1.

Analysis of human Nav1.8 expressed in SH-SY5Y neuroblastoma cells

The tetrodotoxin-resistant voltage-gated sodium channel alpha-subunit Nav1.8 is expressed in nociceptors and has been implicated in chronic pain. Difficulties of heterologous expression have so far precluded analysis of the pharmacological properties of human Nav1.8. To address this we have introduced human Nav1.8 in neuroblastoma SH-SY5Y cells. Voltage-clamp analysis showed that human Nav1.8 generated an inward tetrodotoxin-resistant sodium current with an activating threshold around -50 mV, half maximal activation at -11+/-3 mV and a reversal potential of 67+/-4 mV. These properties closely match those of the endogenous rat tetrodotoxin-resistant sodium current in dorsal root ganglia suggesting that the expressed human channel is in a near physiological conformation. Human Nav1.8 was resistant to tetrodotoxin and activated by the pyrethroid toxin deltamethrin. Both voltage-activated and deltamethrin-activated human Nav1.8 were inhibited by the sodium channel blockers BIII 890 CL, NW-1029, and mexiletine. Inhibition of Nav1.8 by these compounds may underlie their known analgesic effects in animal models.

Comparison of the effects of four Na+ channel analgesics on TTX-resistant Na+ currents in rat sensory neurons and recombinant Nav1.2 channels

Na(+) channel blockers are highly effective analgesics. Among the neuronal Na(+) channel subtypes, Nav1.8 is discussed to be of importance for certain pain states, and Nav1.8-preferring Na(+) channel blockers should be able to relief pain without causing severe effects (due to the restricted expression of this channel type). In this study, the effects of four Na(+) channel blockers on rat tetrodotoxin-resistant (TTX-r) Na(+) channels (representing mostly Nav1.8) in sensory neurons were investigated using the patch-clamp technique in the voltage-clamp configuration, and compared with those on cells heterologously expressing Nav1.2 alpha subunits. The compounds were lidocaine, mexiletine, benzocaine, and ambroxol, which are clinically used to treat pain after local or systemic administration. The four compounds inhibited resting TTX-r channels concentration-dependently, with ambroxol being the most effective (IC(50) value: 34.3 microM), and benzocaine being the weakest (IC(50) value: 1,901 microM). All compounds shifted steady-state inactivation curves to more negative values. Ambroxol blocked resting TTX-r channels more potently than Nav1.2, the opposite was the case for lidocaine, mexiletine and benzocaine. Based on the drugs' potencies found in this study, and the published information on clinically achievable plasma levels, the amount of Na(+) channel block to induce analgesia after systemic administration was estimated.

Varicella zoster virus induces neuropathic changes in rat dorsal root ganglia and behavioral reflex sensitisation that is attenuated by gabapentin or sodium channel blocking drugs

Reactivation of latent varicella zoster virus (VZV) within sensory trigeminal and dorsal root ganglia (DRG) neurons produces shingles (zoster), often accompanied by a chronic neuropathic pain state, post-herpetic neuralgia (PHN). PHN persists despite latency of the virus within human sensory ganglia and is often unresponsive to current analgesic or antiviral agents. To study the basis of varicella zoster-induced pain, we have utilised a recently developed model of chronic VZV infection in rodents. Immunohistochemical analysis of DRG following VZV infection showed the presence of a viral immediate early gene protein (IE62) co-expressed with markers of A- (neurofilament-200; NF-200) and C- (peripherin) afferent sensory neurons. There was increased expression of neuropeptide Y (NPY) in neurons co-expressing NF-200. In addition, there was an increased expression of alpha2delta1 calcium channel, Na(v)1.3 and Na(v)1.8 sodium channels, the neuropeptide galanin and the nerve injury marker, Activating Transcription Factor-3 (ATF-3) as determined by Western blotting in DRG of VZV-infected rats. VZV infection induced increased behavioral reflex responsiveness to both noxious thermal and mechanical stimuli ipsilateral to injection (lasting up to 10 weeks post-infection) that is mediated by spinal NMDA receptors. These changes were reversed by systemic administration of gabapentin or the sodium channel blockers, mexiletine and lamotrigine, but not by the non-steroidal anti-inflammatory agent, diclofenac. This is the first time that the profile of VZV infection-induced phenotypic changes in DRG has been shown in rodents and reveals that this profile appears to be broadly similar (but not identical) to changes in other neuropathic pain models.

Preferred mexiletine block of human sodium channels with IVS4 mutations and its pH-dependence

The effects of extracellular pH (6.2, 7.4 and 8.2) and 0.1 mM mexiletine, a channel blocker of the lidocaine type, are studied on two mutations of the fourth voltage sensor of the Nav1.4 sodium channel, R1448H/C. The fast inactivated channel state to which mexiletine preferentially binds is destabilized by the mutations. By contrast to the expected low response of R1448H/C carriers, mexiletine is particularly effective in preventing exercise-induced stiffness and paralysis from which these patients suffer. Our measurements performed in the whole-cell mode on stably transfected HEK cells show for the first time that the mutations strikingly accelerate closed-state inactivation and, as steady-state fast inactivation is shifted to more negative potentials, stabilize the fast inactivated channel state in the potential range around the resting potential. At pH 7.4 and 8.2, the phasic mexiletine block is larger for R1448C (55%) and R1448H (47%) than for wild-type channels (31%) due to slowed recovery from block (tau is approximately 520 ms for R1448C versus 270 ms for wild-type at pH 7.4) although the recovery from inactivation is slightly faster for the mutants (tau is approximately 1.9 ms for R1448C versus 3.8 ms for wild-type at pH 7.4). At pH 6.2, recovery from block is relatively fast (tau is approximately 35 ms for R1448H/C and 14 ms for wild-type) and thus shows no use-dependence. We conclude that enhanced closed-state inactivation expands the concept of a mutation-induced uncoupling of channel inactivation from activation to a new potential range and that the higher mexiletine efficacy in R1448H/C carriers compared to other myotonic patients offers a pharmacogenetic strategy for mutation-specific treatment.

Influence of mexiletine on action potential discharge and conduction in nodose Adelta afferent neurons innervating guinea pig isolated trachea

Local anesthetics are among the most effective peripherally acting antitussives. A complete understanding of their pharmacological properties in airway afferent neurons associated with the cough reflex has been hampered by an incomplete understanding of the contribution of various classes of afferent neuron to cough. The aim of this study was to evaluate the influence of the antitussive local anesthetic mexiletine on nodose ganglion-derived vagal afferent Adelta-fibers innervating guinea pig trachea. This distinct subtype of airway sensory neuron was recently shown to be involved in evoking cough in anaesthetized guinea pigs. The current findings demonstrate that a concentration of mexiletine sufficient to inhibit citric acid- or mechanically-induced action potential initiation at the nerve ending did not block action potential conduction along axons. These findings are indicative of differences in sensitivity to local anesthetics of highly specialized regions of afferent neurons involved in initiation or conduction of impulses.

Novel cyclopentane dicarboxamide sodium channel blockers as a potential treatment for chronic pain

A series of new voltage-gated sodium channel blockers were prepared based on the screening lead succinic diamide BPBTS. Replacement of the succinimide linker with the more rigid cyclic 1,2-trans-diamide linker was well tolerated. N-Methylation on the biphenylsulfonamide side of the amide moiety significantly reduced the clearance rate in rat pharmacokinetic studies.

Mexiletine treatment—induced inhibition of caspase-3 activation and improvement of behavioral recovery after spinal cord injury

Object. It has been demonstrated in several experimental studies that apoptosis contributes to cellular damage after spinal cord injury (SCI). During apoptosis dying cells secrete additional mediators of apoptosis such as cytokines and free radicals which have additional toxic effects and exacerbate neuronal death. The aim of this laboratory study was to investigate the effects of mexiletine on caspase-3 activation and functional recovery and compare its post-SCI effectiveness with methylprednisolone.

Methods. The rats were divided into five groups. Animals in the trauma group underwent traumatic interventions after laminectomy. Spinal cord contusion injury was produced using the weight-drop method. Animals in treatment groups received a single dose of methylprednisolone sodium succinate (Group C), single dose of mexiletine (Group D), or vehicle solution (saline; Group E) intraperitoneally immediately after injury. Hind-limb functions were assessed using the inclined plane technique and caspase-3 activity in tissue samples was measured 24 hours after SCI. Traumatic injury was found to increase tissue caspase-3 activity. In both treatment groups the drug prevented an increase in caspase-3 activity. Mexiletine treatment improved early behavioral recovery after SCI.

Conclusions. The results obtained in this study demonstrated that mexiletine treatment inhibits caspase-3 activation and preserve/restore better neuronal function compared with methylprednisolone after experimental SCI.

Effect of sodium channel blocker (mexiletine) on pathological ectopic firing pattern in a rat chronic constriction nerve injury model

We studied the efficacy of mexiletine as a sodium channel blocker for neuropathic pain by investigating the effect of mexiletine on the pathological ectopic firing pattern in a chronic constriction nerve injury (CCI) model. The experiment was conducted with 60 male Wistar rats. The CCI model was created by loosely ligating the sciatic nerve. After breeding 7 days, the frequency and pattern of ectopic firing antidromically recorded from the sural nerve and the amplitude of antidromic sensory nerve-evoked potential were analyzed. The CCI rats were given an intravenous injection of normal saline and mexiletine (5 or 15 mg/kg). Mexiletine significantly suppressed spontaneous firing frequency, an on-off firing pattern that consisted of cyclic bursting spikes and ectopic firing generation under the hypoxic condition. Mexiletine did not influence the amplitude of A-delta component in the antidromic sensory nerve-evoked potential. Mexiletine suppressed ectopic firing by blocking activity of the abnormal sodium channel at the nerve-injured site and dorsal root ganglion without blocking nerve conduction. This study suggests that mexiletine is useful for treating neuropathic pain in peripheral neuropathy.

Inhibitory effects of lidocaine and mexiletine on vasorelaxation mediated by adenosine triphosphate-sensitive K+ channels and the role of kinases in the porcine coronary artery

BACKGROUND

Effects of antiarrhythmic drugs on coronary vasodilation mediated by K channels have not been studied. Modulator roles of protein kinase C and tyrosine kinase in the activity of K channels have also been unclear in the coronary artery. The current study examined whether lidocaine and mexiletine in the porcine coronary artery modify the vasorelaxation mediated by adenosine triphosphate-sensitive K channels via activation of protein kinase C and tyrosine kinase.

METHODS

Porcine coronary arteries without endothelium were suspended for isometric force recording, and vasorelaxation to levcromakalim (10 to 10 m) was obtained. Changes in membrane potentials produced by levcromakalim (10 m) were also recorded.

RESULTS

Glibenclamide completely abolished vasorelaxation as well as hyperpolarization in response to levcromakalim. Lidocaine and mexiletine significantly reduced these responses. Calphostin C, Go 6976, genistein, and erbstatin A partly restored vasorelaxation or hyperpolarization in response to levcromakalim in arteries treated with mexiletine but not in those with lidocaine, whereas these inhibitors did not alter the vasorelaxation to levcromakalim. Phorbol 12-myristate 13-acetate produced reduction of vasorelaxation in response to levcromakalim, which is recovered by calphostin C or Go 6976.

CONCLUSIONS

Therefore, lidocaine and mexiletine inhibit vasorelaxation mediated by the activation of adenosine triphosphate-sensitive K channels in the coronary artery. Protein kinase C and tyrosine kinase seem to have roles in the inhibitory effect of mexiletine but not in that of lidocaine. Class Ib antiarrhythmic drugs may reduce coronary vasodilation mediated by adenosine triphosphate-sensitive K channels via the differential modulator effects on these kinases.