NaV1.4 is a voltage-gated sodium channel subtype that is predominantly expressed in skeletal muscle cells. It is essential for producing action potentials and stimulating muscle contraction, and mutations in NaV1.4 can cause various muscle disorders. The discovery of the cryo-EM structure of NaV1.4 in complex with β1 has opened new possibilities for designing drugs and toxins that target NaV1.4. In this review, we summarize the current understanding of channelopathies, the binding sites and functions of chemicals including medicine and toxins that interact with NaV1.4. These substances could be considered novel candidate compounds or tools to develop more potent and selective drugs targeting NaV1.4. Therefore, studying NaV1.4 pharmacology is both theoretically and practically meaningful.

Congenital myotonia represents a rare group of genetically inherited conditions. It can be either autosomal dominant (Thomsen) or autosomal recessive (Becker). It is characterized by muscular hypertrophy, proximal weakness, and myotonia, or impaired relaxation after contraction. These are due to mutations in the CLC1 gene. A 14-year-old male child presented with complaints of gradually progressive weakness for five years. Weakness was more pronounced in the proximal muscle groups. The weakness worsened after rest and improved with activity. This led to absenteeism and affected his school performance. Clinical examination showed generalized muscular hypertrophy with pronounced hypertrophy of the calf muscles. A neurological examination showed significant myotonia and impaired relaxation after making a fist. The diagnosis of myotonia was confirmed by electromyography, which produced a dive-bomber sound on insertion. Next-generation sequencing revealed a homozygous eight-base pair insertion in exon 19 of the CLCN1 gene. This mutation has not been reported in the existing literature for myotonia congenita. The child was started on mexiletine and improved significantly. Presently, the patient is on regular medications and doing well on follow-up. Though rare, congenital myotonia is an important cause of neuromuscular weakness. It can be easily diagnosed with a thorough clinical examination and routine testing for myotonia in all children with weakness. The treatment is relatively simple and can give the patient significant relief. Myotonia can be easily diagnosed clinically, and pharmacotherapy and proper monitoring can remarkably improve patients\' quality of life.

Despite impressive developments in the field of ventricular arrhythmias, there is still a relevant number of patients with ventricular arrhythmias who require antiarrhythmic drug therapy and may, e.g., in otherwise drug and/or ablation refractory situations, benefit from agents known for decades, such as mexiletine. Through its capability of blocking fast sodium channels in cardiomyocytes, it has played a minor to moderate antiarrhythmic role throughout the recent decades. Nevertheless, certain patients with structural heart disease suffering from drug-refractory, i.e., mainly amiodarone refractory ventricular arrhythmias, as well as those with selected forms of congenital long QT syndrome (LQTS) may nowadays still benefit from mexiletine. Here, we outline mexiletine\'s cellular and clinical electrophysiological properties. In addition, the application of mexiletine may be accompanied by various potential side effects, e.g., nausea and tremor, and is limited by several drug-drug interactions. Thus, we shed light on the current therapeutic role of mexiletine for therapy of ventricular arrhythmias and discuss clinically relevant aspects of its indications based on current evidence.

UNASSIGNED: Myotonic disorders, such as non-dystrophic myotonias (NDMs) and myotonic dystrophies (DMs) are characterized by a delay in muscle relaxation after a contraction stimulus. There is general consensus that protocols to treat myotonia need to be implemented.
UNASSIGNED: Mexiletine is the only pharmacological agent approved for the symptomatic treatment of myotonia in adult patients with NDM and is considered to be the first-line treatment for DMs; however, its production in Italy was halted in 2022 making its availability to patients problematic.
UNASSIGNED: A panel of 8 Italian neurologists took part in a two-round Delphi panel between June and October 2022, analyzing the current use of mexiletine in Italian clinical practice.
UNASSIGNED: The panelists assist 1126 patients (69% DM type1, 18% NDM and 13% DM type2). Adult NDM patients receive, on average, 400-600 mg of mexiletine hydrochloride (HCl) while adult DM patients receive 100-600 mg, per day in the long-term. The severity of symptoms is considered the main reason to start mexiletine treatment for both NDM and DM patients. Mexiletine is reckoned to have a clinical impact for both NDM and DM patients, but currently drug access is problematic.
UNASSIGNED: Mexiletine treatment is recognized to have a role in the reduction of the symptomatic burden for NDM and DM patients. Patient management could be improved by facilitating access to therapy and developing new drug formulations.

Brugada Syndrome (BrS) is a rare inherited cardiac arrhythmia causing potentially fatal ventricular tachycardia or fibrillation, mainly occurring during rest or sleep in young individuals without heart structural issues. It increases the risk of sudden cardiac death, and its characteristic feature is an abnormal ST segment elevation on the ECG. While BrS has diverse genetic origins, a subset of cases can be conducted to mutations in the SCN5A gene, which encodes for the Nav1.5 sodium channel. Our study focused on three novel SCN5A mutations (p.A344S, p.N347K, and p.D349N) found in unrelated BrS families. Using patch clamp experiments, we found that these mutations disrupted sodium currents: p.A344S reduced current density, while p.N347K and p.D349N completely abolished it, leading to altered voltage dependence and inactivation kinetics when co-expressed with normal channels. We also explored the effects of mexiletine treatment, which can modulate ion channel function. Interestingly, the p.N347K and p.D349N mutations responded well to the treatment, rescuing the current density, while p.A344S showed a limited response. Structural analysis revealed these mutations were positioned in key regions of the channel, impacting its stability and function. This research deepens our understanding of BrS by uncovering the complex relationship between genetic mutations, ion channel behavior, and potential therapeutic interventions.

Non-dystrophic myotonia (NDM) is a group of rare mono-genetic muscle disorders caused by skeletal muscle sodium or chloride channelopathies. These disorders are characterized by high muscle tone and the inability of the muscles to relax spontaneously after voluntary contraction. Myotonia congenita refers to a form of NDM that typically manifests during the later stages of childhood. It occurs as a result of genetic mutations affecting the chloride channels found in the sarcolemma membrane of skeletal muscles. Here, we present a case series of two male siblings born out of third-degree consanguineous union ages 10 and eight years, respectively, who presented with proximal muscle weakness and the characteristic \"Herculean body\" appearance. They demonstrated characteristic clinical diagnostic signs of myotonia. The diagnosis of myotonia congenita was confirmed through distinctive electromyography (EMG) findings, which were further supported by genetic testing revealing a homozygous mutation c.1445G>A in exon 13 of the CLCN1 gene, indicating autosomal recessive inheritance. This uncommon condition exhibits characteristic clinical manifestations and classical EMG findings, which are difficult to disregard once encountered. Genetic tests serve as a valuable tool to validate the diagnosis.

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

BACKGROUND: The present study aimed to compare the effects of the combined administration of two adjuvants, dopamine and phenylephrine, on the cutaneous analgesic effect and duration of mexiletine in rats.
METHODS: Nociceptive blockage was evaluated by the inhibition of response to skin pinpricks in rats via the cutaneous trunci muscle reflex (CTMR). After subcutaneous injection, the analgesic activities of mexiletine in the absence and presence of either dopamine or phenylephrine were assessed. Each injection was standardized into 0.6 ml with a mixture of drugs and saline.
RESULTS: Subcutaneous injections of mexiletine successfully induced dose-dependent cutaneous analgesia in rats. The results revealed that rats injected with 1.8 μmol mexiletine exhibited 43.75% blockage (%MPE), while rats injected with 6.0 μmol mexiletine showed 100% blockage. Co-application of mexiletine (1.8 or 6.0 μmol) with dopamine (0.06, 0.60, or 6.00 μmol) elicited full sensory block (%MPE). Sensory blockage ranged from 81.25% to 95.83% in rats injected with mexiletine (1.8 μmol) and phenylephrine (0.0059 or 0.0295 μmol), and complete subcutaneous analgesia was observed in rats injected with mexiletine (1.8 μmol) and a higher concentration of phenylephrine (0.1473 μmol). Furthermore, mexiletine at 6.0 μmol completely blocked nociception when combined with any concentration of phenylephrine, while 0.1473 μmol phenylephrine alone exhibited 35.417% subcutaneous analgesia. The combined application of dopamine (0.06/0.6/6 μmol) and mexiletine (1.8/6 μmol) resulted in increased %MPE, complete block time, full recovery time, and AUCs compared to the combined application of phenylephrine (0.0059 and 0.1473 μmol) and mexiletine (1.8/6 μmol) (p < 0.001).
CONCLUSIONS: Dopamine is superior to phenylephrine in improving sensory blockage and enhancing the duration of nociceptive blockage by mexiletine.

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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.