Lacosamide Inhibition of Nav1.7 Voltage-Gated Sodium Channels: Slow Binding to Fast-Inactivated States

Arbidol, an antiviral drug, identified as a sodium channel blocker with anticonvulsant activity

BACKGROUND AND PURPOSE

Sodium channel blockers (SCBs) have traditionally been utilized as anti-seizure medications by primarily targeting the inactivation process. In a drug discovery project aiming at finding potential anticonvulsants, we have identified arbidol, originally an antiviral drug, as a potent SCB. In order to evaluate its anticonvulsant potential, we have thoroughly examined its biophysical properties as well as its effects on animal seizure models.

EXPERIMENTAL APPROACH

Patch clamp recording was used to investigate the electrophysiological properties of arbidol, as well as the binding and unbinding kinetics of arbidol, carbamazepine and lacosamide. Furthermore, we evaluated the anticonvulsant effects of arbidol using three different seizure models in male mice.

KEY RESULTS

Arbidol effectively suppressed neuronal epileptiform activity by blocking sodium channels. Arbidol demonstrated a distinct mode of action by interacting with both the fast and slow inactivation of Nav1.2 channels compared with carbamazepine and lacosamide. A kinetic study suggested that the binding and unbinding rates might be associated with the specific characteristics of these three drugs. Arbidol targeted the classical binding site of local anaesthetics, effectively inhibited the gain-of-function effects of Nav1.2 epileptic mutations and exhibited varying degrees of anticonvulsant effects in the maximal electroshock model and subcutaneous pentylenetetrazol model but had no effect in the pilocarpine-induced status epilepticus model.

CONCLUSIONS AND IMPLICATIONS

Arbidol shows promising potential as an anticonvulsant agent, providing a unique mode of action that sets it apart from existing SCBs.

Persistent sodium currents in neurons: potential mechanisms and pharmacological blockers

Persistent sodium current (INaP) is an important activity-dependent regulator of neuronal excitability. It is involved in a variety of physiological and pathological processes, including pacemaking, prolongation of sensory potentials, neuronal injury, chronic pain and diseases such as epilepsy and amyotrophic lateral sclerosis. Despite its importance, neither the molecular basis nor the regulation of INaP are sufficiently understood. Of particular significance is a solid knowledge and widely accepted consensus about pharmacological tools for analysing the function of INaP and for developing new therapeutic strategies. However, the literature on INaP is heterogeneous, with varying definitions and methodologies used across studies. To address these issues, we provide a systematic review of the current state of knowledge on INaP, with focus on mechanisms and effects of this current in the central nervous system. We provide an overview of the specificity and efficacy of the most widely used INaP blockers: amiodarone, cannabidiol, carbamazepine, cenobamate, eslicarbazepine, ethosuximide, gabapentin, GS967, lacosamide, lamotrigine, lidocaine, NBI-921352, oxcarbazepine, phenytoine, PRAX-562, propofol, ranolazine, riluzole, rufinamide, topiramate, valproaic acid and zonisamide. We conclude that there is strong variance in the pharmacological effects of these drugs, and in the available information. At present, GS967 and riluzole can be regarded bona fide INaP blockers, while phenytoin and lacosamide are blockers that only act on the slowly inactivating component of sodium currents.

Inhibition of resurgent Na+ currents by rufinamide

Na+ channels are essential for the genesis of action potentials in most neurons. After opening by membrane depolarization, Na+ channels enter a series of inactivated states (e.g. the fast, intermediate, and slow inactivated states; or If, Ii, and Is). The inactivated Na+ channel may recover via the open state upon membrane repolarization, giving rise to "resurgent" Na+ currents which could be critical for densely repetitive or burst discharges. We incubated CHO-K1 cells transfected with human NaV1.7 cDNA and measured resurgent currents with whole-cell patch recordings. We found Ii is the major inactivated state responsible for the genesis of resurgent currents. Rufinamide, in therapeutic concentrations, could selectively bind to Ii to slow the recovery process and dose-dependently inhibit resurgent currents. The other Na+ channel-inhibiting antiseizure medications (ASM), such as phenytoin and lacosamide (selectively binds to If and Is, separately), fail to show a similar inhibitory effect in clinically relevant concentrations. Resurgent currents are decreased with lengthening of the prepulse, presumably because of redistribution of the channel from Ii to If. Rufinamide could accentuate the decrease to mimic a use-dependent inhibitory effect. The molecular action of slowing of recovery from inactivation by binding to Ii also explains the highly correlative inhibitory effect of rufinamide on both transient and resurgent Na+ currents. The modest but correlative inhibition of both currents may make a novel synergistic effect and thus strong-enough suppression of pathological repetitive and especially burst discharges. Rufinamide may thus have a unique spectrum of therapeutic applications for disorders with excessive neural excitabilities.

Molecular Pharmacology of Selective NaV1.6 and Dual NaV1.6/NaV1.2 Channel Inhibitors that Suppress Excitatory Neuronal Activity Ex Vivo

Voltage-gated sodium channel (NaV) inhibitors are used to treat neurological disorders of hyperexcitability such as epilepsy. These drugs act by attenuating neuronal action potential firing to reduce excitability in the brain. However, all currently available NaV-targeting antiseizure medications nonselectively inhibit the brain channels NaV1.1, NaV1.2, and NaV1.6, which potentially limits the efficacy and therapeutic safety margins of these drugs. Here, we report on XPC-7724 and XPC-5462, which represent a new class of small molecule NaV-targeting compounds. These compounds specifically target inhibition of the NaV1.6 and NaV1.2 channels, which are abundantly expressed in excitatory pyramidal neurons. They have a > 100-fold molecular selectivity against NaV1.1 channels, which are predominantly expressed in inhibitory neurons. Sparing NaV1.1 preserves the inhibitory activity in the brain. These compounds bind to and stabilize the inactivated state of the channels thereby reducing the activity of excitatory neurons. They have higher potency, with longer residency times and slower off-rates, than the clinically used antiseizure medications carbamazepine and phenytoin. The neuronal selectivity of these compounds is demonstrated in brain slices by inhibition of firing in cortical excitatory pyramidal neurons, without impacting fast spiking inhibitory interneurons. XPC-5462 also suppresses epileptiform activity in an ex vivo brain slice seizure model, whereas XPC-7224 does not, suggesting a possible requirement of Nav1.2 inhibition in 0-Mg2+- or 4-AP-induced brain slice seizure models. The profiles of these compounds will facilitate pharmacological dissection of the physiological roles of NaV1.2 and NaV1.6 in neurons and help define the role of specific channels in disease states. This unique selectivity profile provides a new approach to potentially treat disorders of neuronal hyperexcitability by selectively downregulating excitatory circuits.

Safety, Tolerability, and Dose-Limiting Toxicity of Lacosamide in Patients With Painful Chronic Pancreatitis: Protocol for a Phase 1 Clinical Trial to Determine Safety and Identify Side Effects

BACKGROUND

Chronic abdominal pain is the hallmark symptom of chronic pancreatitis (CP), with 50% to 80% of patients seeking medical attention for pain control. Although several management options are available, outcomes are often disappointing, and opioids remain a mainstay of therapy. Opioid-induced hyperalgesia is a phenomenon resulting in dose escalation, which may occur partly because of the effects of opioids on voltage-gated sodium channels associated with pain. Preclinical observations demonstrate that the combination of an opioid and the antiseizure drug lacosamide diminishes opioid-induced hyperalgesia and improves pain control.

OBJECTIVE

In this phase 1 trial, we aim to determine the safety, tolerability, and dose-limiting toxicity of adding lacosamide to opioids for the treatment of painful CP and assess the feasibility of performance of a pilot study of adding lacosamide to opioid therapy in patients with CP. As an exploratory aim, we will assess the efficacy of adding lacosamide to opioid therapy in patients with painful CP.

METHODS

Using the Bayesian optimal interval design, we will conduct a dose-escalation trial of adding lacosamide to opioid therapy in patients with painful CP enrolled in cohorts of size 3. The initial dose will be 50 mg taken orally twice a day, followed by incremental increases to a maximum dose of 400 mg/day, with lacosamide administered for 7 days at each dose level. Adverse events will be documented according to Common Terminology Criteria for Adverse Events (version 5.0).

RESULTS

As of December 2023, we have currently enrolled 6 participants. The minimum number of participants to be enrolled is 12 with a maximum of 24. We expect to publish the results by March 2025.

CONCLUSIONS

This trial will test the feasibility of the study design and provide reassurance regarding the tolerability and safety of opioids in treating painful CP. It is anticipated that lacosamide will prove to be safe and well tolerated, supporting a subsequent phase 2 trial assessing the efficacy of lacosamide+opioid therapy in patients with painful CP, and that lacosamide combined with opiates will lower the opioid dose necessary for pain relief and improve the safety profile of opioid use in treating painful CP.

TRIAL REGISTRATION

Clinicaltrials.gov NCT05603702; https://clinicaltrials.gov/study/NCT05603702.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

PRR1-10.2196/50513.

Current advances in rodent drug-resistant temporal lobe epilepsy models: Hints from laboratory studies

Anti-seizure drugs (ASDs) are the first choice for the treatment of epilepsy, but there is still one-third of patients with epilepsy (PWEs) who are resistant to two or more appropriately chosen ASDs, named drug-resistant epilepsy (DRE). Temporal lobe epilepsy (TLE), a common type of epilepsy usually associated with hippocampal sclerosis (HS), shares the highest proportion of drug resistance (approximately 70%). In view of the key role of the temporal lobe in memory, emotion, and other physiological functions, patients with drug-resistant temporal lobe epilepsy (DR-TLE) are often accompanied by serious complications, and surgical procedures also yield extra considerations. The exact mechanisms for the genesis of DR-TLE remain unillustrated, which makes it hard to manage patients with DR-TLE in clinical practice. Animal models of DR-TLE play an irreplaceable role in both understanding the mechanism and searching for new therapeutic strategies or drugs. In this review article, we systematically summarized different types of current DR-TLE models, and then recent advances in mechanism investigations obtained in these models were presented, especially with the development of advanced experimental techniques and tools. We are deeply encouraged that novel strategies show great therapeutic potential in those DR-TLE models. Based on the big steps reached from the bench, a new light has been shed on the precise management of DR-TLE.

The effect of lacosamide in peripheral neuropathic pain: A randomized, double-blind, placebo-controlled, phenotype-stratified trial

BACKGROUND

Neuropathic pain is common and difficult to treat. The sodium channel blocker lacosamide is efficacious in animal models of pain, but its effect on neuropathic pain in humans is inconclusive.

METHODS

In a multicentre, randomized, double-blinded placebo-controlled phenotype stratified trial, we examined if lacosamide produced better pain relief in patients with the irritable nociceptor phenotype compared to those without. The primary outcome was the change in daily average pain from baseline to last week of 12 weeks of treatment. Secondary and tertiary outcomes included pain relief, patient global impression of change and presence of 30% and 50% pain reduction.

RESULTS

The study was prematurely closed with 93 patients included and 63 randomized to lacosamide or placebo in a 2:1 ratio, of which 49 fulfilled the per protocol criteria and was used for the primary objective. We did not find a better effect of lacosamide in patients with the irritable nociceptor phenotype, the 95% CI for the primary objective was 0.41 (-1.2 to 2.0). For all patients randomized, lacosamide had no effect on the primary outcome, but significantly more patients were responders to lacosamide than during placebo, with an NNT of 4.0 (95% CI 2.3-16.1) and 5.0 (95% CI 2.8-24.5) for 30% and 50% pain reduction respectively. We did not identify any predictors for response. Lacosamide was generally well tolerated.

CONCLUSION

We could not confirm that lacosamide was more efficacious in patients with the irritable nociceptor type, but the study was prematurely closed, so we cannot exclude a small difference.

SIGNIFICANCE

Treatment of neuropathic pain is often a trial and error process. Little is known about which patient benefit from which kind of medication. The sodium channel blocker lacosamide shows variable effect on neuropathic pain. Pain sensory phenotype, as defined by quantitative sensory testing, did not predict response to treatment with lacosamide.

Using Human-Induced Pluripotent Stem Cell Derived Neurons on Microelectrode Arrays to Model Neurological Disease: A Review

In situ physiological signals of in vitro neural disease models are essential for studying pathogenesis and drug screening. Currently, an increasing number of in vitro neural disease models are established using human-induced pluripotent stem cell (hiPSC) derived neurons (hiPSC-DNs) to overcome interspecific gene expression differences. Microelectrode arrays (MEAs) can be readily interfaced with two-dimensional (2D), and more recently, three-dimensional (3D) neural stem cell-derived in vitro models of the human brain to monitor their physiological activity in real time. Therefore, MEAs are emerging and useful tools to model neurological disorders and disease in vitro using human iPSCs. This is enabling a real-time window into neuronal signaling at the network scale from patient derived. This paper provides a comprehensive review of MEA's role in analyzing neural disease models established by hiPSC-DNs. It covers the significance of MEA fabrication, surface structure and modification schemes for hiPSC-DNs culturing and signal detection. Additionally, this review discusses advances in the development and use of MEA technology to study in vitro neural disease models, including epilepsy, autism spectrum developmental disorder (ASD), and others established using hiPSC-DNs. The paper also highlights the application of MEAs combined with hiPSC-DNs in detecting in vitro neurotoxic substances. Finally, the future development and outlook of multifunctional and integrated devices for in vitro medical diagnostics and treatment are discussed.

Effect of Lacosamide on Interictal Epileptiform Discharges in Pediatric Patients With Newly Diagnosed Focal Epilepsy

BACKGROUND

The purpose of this study was to determine the efficacy of lacosamide (LCM) on interictal epileptiform discharges (IEDs) and evaluate the relationships between IEDs and seizure outcome in pediatric patients with focal epilepsy.

METHODS

Patient inclusion criteria included (1) newly diagnosed focal epilepsy with unknown etiology; and (2) electroencephalogram recorded twice (before and after starting LCM) under the same conditions. The difference between the highest number of IEDs over five successive minutes (IEDs/5 min) and the location of IEDs was determined. Seizure outcome was evaluated one year after achieving the maintenance dose of LCM. Responders were identified as showing a ≥50% reduction in the pre-LCM seizure frequency.

RESULTS

Of 22 patients, 10 showed an increase in IEDs/5 min after starting LCM. The median IEDs/5 min before and after starting LCM was not significantly different, at 1.5 (interquartile range: 0, 31.75) and 10.5 (0, 80.5), respectively. No relationship was identified between the difference in IEDs/5 min and seizure outcome. Patients with multiple regional or diffuse IEDs had significantly poorer seizure outcome compared with patients without those IEDs (P = 0.036 and P = 0.039, respectively). Of 10 patients with single regional IEDs, a tendency of IEDs to disappear was observed between patients with frontal and non-frontal IEDs.

CONCLUSION

The effects of LCM on the number of IEDs may be unrelated to seizure outcome. LCM may be ineffective at improving seizure outcomes in patients with multiple regional or diffuse IEDs.

Understanding Lamotrigine's Role in the CNS and Possible Future Evolution

The anti-epileptic drug lamotrigine (LTG) has been widely used to treat various neurological disorders, including epilepsy and bipolar disorder. However, its precise mechanism of action in the central nervous system (CNS) still needs to be determined. Recent studies have highlighted the involvement of LTG in modulating the activity of voltage-gated ion channels, particularly those related to the inhibition of neuronal excitability. Additionally, LTG has been found to have neuroprotective effects, potentially through the inhibition of glutamate release and the enhancement of GABAergic neurotransmission. LTG's unique mechanism of action compared to other anti-epileptic drugs has led to the investigation of its use in treating other CNS disorders, such as neuropathic pain, PTSD, and major depressive disorder. Furthermore, the drug has been combined with other anti-epileptic drugs and mood stabilizers, which may enhance its therapeutic effects. In conclusion, LTG's potential to modulate multiple neurotransmitters and ion channels in the CNS makes it a promising drug for treating various neurological disorders. As our understanding of its mechanism of action in the CNS continues to evolve, the potential for the drug to be used in new indications will also be explored.

Somatic and terminal CB1 receptors are differentially coupled to voltage-gated sodium channels in neocortical neurons

Endogenous cannabinoid signaling is vital for important brain functions, and the same pathways can be modified pharmacologically to treat pain, epilepsy, and posttraumatic stress disorder. Endocannabinoid-mediated changes to excitability are predominantly attributed to 2-arachidonoylglycerol (2-AG) acting presynaptically via the canonical cannabinoid receptor, CB1. Here, we identify a mechanism in the neocortex by which anandamide (AEA), another major endocannabinoid, but not 2-AG, powerfully inhibits somatically recorded voltage-gated sodium channel (VGSC) currents in the majority of neurons. This pathway involves intracellular CB1 that, when activated by anandamide, decreases the likelihood of recurrent action potential generation. WIN 55,212-2 similarly activates CB1 and inhibits VGSC currents, indicating that this pathway is also positioned to mediate the actions of exogenous cannabinoids on neuronal excitability. The coupling between CB1 and VGSCs is absent at nerve terminals, and 2-AG does not block somatic VGSC currents, indicating functional compartmentalization of the actions of two endocannabinoids.

The therapeutic effects of lacosamide on epilepsy-associated comorbidities

Epilepsy is a chronic neurological disorder associated with severe social and psychological effects, and most epilepsy patients often report at least one comorbidity. Accumulating evidence have suggested that lacosamide, a new generation of anti-seizure medications, may exhibit efficacy in the management of both epilepsy and its related comorbidities. Therefore, this narrative review aimed to elucidate the recent advancements regarding the therapeutic role of lacosamide in epilepsy-associated comorbidities. The possible pathophysiological mechanisms between epilepsy and epilepsy-associated comorbidities have been also partially described. Whether lacosamide improves cognitive and behavioral functions in patients with epilepsy has not been conclusively established. Some studies support that lacosamide may alleviate anxiety and depression in epilepsy patients. In addition, lacosamide has been found to be safe and effective in the treatment of epilepsy in people with intellectual disabilities, epilepsy of cerebrovascular etiology, and epilepsy associated with brain tumors. Moreover, lacosamide treatment has demonstrated fewer side effects on other systems. Hence, future larger and higher quality clinical studies are needed to further explore both the safety and efficacy of lacosamide in the treatment of epilepsy-associated comorbidities.

Effects of Mexiletine and Lacosamide on Nerve Excitability in Healthy Subjects: A Randomized, Double-Blind, Placebo-Controlled, Crossover Study

Selective voltage-gated sodium channel blockers are of growing interest as treatment for pain. For drug development of such compounds, it would be critical to have a biomarker that can be used for proof-of-mechanism. We aimed to evaluate whether drug-induced changes in sodium conductance can be detected in the peripheral nerve excitability profile in 18 healthy subjects. In a randomized, double-blind, 3-way crossover study, effects of single oral doses of 333 mg mexiletine and 300 mg lacosamide were compared with placebo. On each study visit, motor and sensory nerve excitability measurements of the median nerve were performed (predose; and 3 and 6 hours postdose) using Qtrac. Treatment effects were calculated using an analysis of covariance (ANCOVA) with baseline as covariate. Mexiletine and lacosamide had significant effects on multiple motor and sensory nerve excitability variables. Depolarizing threshold electrotonus (TEd₄₀ (40-60 ms)) decreased by mexiletine (estimated difference (ED) -1.37% (95% confidence interval (CI): -2.20, -0.547; P = 0.002) and lacosamide (ED -1.27%, 95% CI: -2.10, -0.443; P = 0.004) in motor nerves. Moreover, mexiletine and lacosamide decreased superexcitability (less negative) in motor nerves (ED 1.74%, 95% CI: 0.615, 2.87; P = 0.004, and ED 1.47%, 95% CI: 0.341, 2.60; P = 0.013, respectively). Strength-duration time constant decreased after lacosamide in motor- (ED -0.0342 ms, 95% CI: -0.0571, -0.0112; P = 0.005) and sensory nerves (ED -0.0778 ms, 95% CI: -0.116, -0.0399; P < 0.001). Mexiletine and lacosamide significantly decrease excitability of motor and sensory nerves, in line with their suggested mechanism of action. Results of this study indicate that nerve excitability threshold tracking can be an effective pharmacodynamic biomarker. The method could be a valuable tool in clinical drug development.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na⁺ and Ca²⁺ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K⁺ channels. Because they display limited central side effects, peripherally restricted Na⁺ and Ca²⁺ channel blockers and K⁺ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Na_v1.3, Na_v1.7, Na_v1.8, Ca_v3.2, and HCN2 and activators of K_v7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K⁺ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na⁺ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca²⁺ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing “pain” as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Lacosamide Inhibition of NaV1.7 Channels Depends on its Interaction With the Voltage Sensor Domain and the Channel Pore

Lacosamide, developed as an anti-epileptic drug, has been used for the treatment of pain. Unlike typical anticonvulsants and local anesthetics which enhance fast-inactivation and bind within the pore of sodium channels, lacosamide enhances slow-inactivation of these channels, suggesting different binding mechanisms and mode of action. It has been reported that lacosamide's effect on Na_V1.5 is sensitive to a mutation in the local anesthetic binding site, and that it binds with slow kinetics to the fast-inactivated state of Na_V1.7. We recently showed that the Na_V1.7-W1538R mutation in the voltage-sensing domain 4 completely abolishes Na_V1.7 inhibition by clinically-achievable concentration of lacosamide. Our molecular docking analysis suggests a role for W1538 and pore residues as high affinity binding sites for lacosamide. Aryl sulfonamide sodium channel blockers are also sensitive to substitutions of the W1538 residue but not of pore residues. To elucidate the mechanism by which lacosamide exerts its effects, we used voltage-clamp recordings and show that lacosamide requires an intact local anesthetic binding site to inhibit Na_V1.7 channels. Additionally, the W1538R mutation does not abrogate local anesthetic lidocaine-induced blockade. We also show that the naturally occurring arginine in Na_V1.3 (Na_V1.3-R1560), which corresponds to Na_V1.7-W1538R, is not sufficient to explain the resistance of Na_V1.3 to clinically-relevant concentrations of lacosamide. However, the Na_V1.7-W1538R mutation conferred sensitivity to the Na_V1.3-selective aryl-sulfonamide blocker ICA-121431. Together, the W1538 residue and an intact local anesthetic site are required for lacosamide's block of Na_V1.7 at a clinically-achievable concentration. Moreover, the contribution of W1538 to lacosamide inhibitory effects appears to be isoform-specific.

Cinacalcet inhibition of neuronal action potentials preferentially targets the fast inactivated state of voltage-gated sodium channels

Voltage-gated sodium channel (VGSC) activation is essential for action potential generation in the brain. Allosteric calcium-sensing receptor (CaSR) agonist, cinacalcet, strongly and ubiquitously inhibits VGSC currents in neocortical neurons via an unidentified, G-protein-dependent inhibitory molecule. Here, using whole-cell patch VGSC clamp methods, we investigated the voltage-dependence of cinacalcet-mediated inhibition of VGSCs and the channel state preference of cinacalcet. The rate of inhibition of VGSC currents was accelerated at more depolarized holding potentials. Cinacalcet shifted the voltage-dependence of both fast and slow inactivation of VGSC currents in the hyperpolarizing direction. Utilizing a simple model, the voltage-dependence of VGSC current inhibition may be explained if the affinity of the inhibitory molecule to the channel states follows the sequence: fast-inactivated > slow-inactivated > resting. The state dependence of VGSC current inhibition contributes to the non-linearity of action potential block by cinacalcet. This dynamic and abundant signaling pathway by which cinacalcet regulates VGSC currents provides an important voltage-dependent mechanism for modulating central neuronal excitability.

Ion-Channel Antiepileptic Drugs: An Analytical Perspective on the Therapeutic Drug Monitoring (TDM) of Ezogabine, Lacosamide, and Zonisamide

The term seizures includes a wide array of different disorders with variable etiology, which currently represent one of the most important classes of neurological illnesses. As a consequence, many different antiepileptic drugs (AEDs) are currently available, exploiting different activity mechanisms and providing different levels of performance in terms of selectivity, safety, and efficacy. AEDs are currently among the psychoactive drugs most frequently involved in therapeutic drug monitoring (TDM) practices. Thus, the plasma levels of AEDs and their metabolites are monitored and correlated to administered doses, therapeutic efficacy, side effects, and toxic effects. As for any analytical endeavour, the quality of plasma concentration data is only as good as the analytical method allows. In this review, the main techniques and methods are described, suitable for the TDM of three AEDs belonging to the class of ion channel agents: ezogabine (or retigabine), lacosamide, and zonisamide. In addition to this analytical overview, data are provided, pertaining to two of the most important use cases for the TDM of antiepileptics: drug–drug interactions and neuroprotection activity studies. This review contains 146 references.

Novel and Emerging Electrophysiological Biomarkers of Diabetic Neuropathy and Painful Diabetic Neuropathy

PURPOSE

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes. Small and large peripheral nerve fibers can be involved in DPN. Large nerve fiber damage causes paresthesia, sensory loss, and muscle weakness, and small nerve fiber damage is associated with pain, anesthesia, foot ulcer, and autonomic symptoms. Treatments for DPN and painful DPN (pDPN) pose considerable challenges due to the lack of effective therapies. To meet these challenges, there is a major need to develop biomarkers that can reliably diagnose and monitor progression of nerve damage and, for pDPN, facilitate personalized treatment based on underlying pain mechanisms.

METHODS

This study involved a comprehensive literature review, incorporating article searches in electronic databases (Google Scholar, PubMed, and OVID) and reference lists of relevant articles with the authors' substantial expertise in DPN. This review considered seminal and novel research and summarizes emerging biomarkers of DPN and pDPN that are based on neurophysiological methods.

FINDINGS

From the evidence gathered from 145 papers, this submission describes emerging clinical neurophysiological methods with potential to act as biomarkers for the diagnosis and monitoring of DPN as well as putative future roles as predictors of response to antineuropathic pain medication in pDPN. Nerve conduction studies only detect large fiber damage and do not capture pathology or dysfunction of small fibers. Because small nerve fiber damage is prominent in DPN, additional biomarkers of small nerve fiber function are needed. Activation of peripheral nociceptor fibers using laser, heat, or targeted electrical stimuli can generate pain-related evoked potentials, which are an objective neurophysiological measure of damage along the small fiber pathways. Assessment of nerve excitability, which provides a surrogate of axonal properties, may detect alterations in function before abnormalities are detected by nerve conduction studies. Microneurography and rate-dependent depression of the Hoffmann-reflex can be used to dissect underlying pain-generating mechanisms arising from the periphery and spinal cord, respectively. Their role in informing mechanistic-based treatment of pDPN as well as facilitating clinical trials design is discussed.

IMPLICATIONS

The neurophysiological methods discussed, although currently not practical for use in busy outpatient settings, detect small fiber and early large fiber damage in DPN as well as disclosing dominant pain mechanisms in pDPN. They are suited as diagnostic and predictive biomarkers as well as end points in mechanistic clinical trials of DPN and pDPN.

Pharmacology of lacosamide: From its molecular mechanisms and pharmacokinetics to future therapeutic applications

Epilepsy is one of the most common brain disorders, affecting more than 50 million people worldwide. Although its treatment is currently symptomatic, the last generation of anti-seizure drugs is characterized by better pharmacokinetic profiles, efficacy, tolerability and safety. Lacosamide is a third-generation anti-seizure drug that stands out due to its good efficacy and safety profile. It is used with effectiveness in the treatment of partial-onset seizures with or without secondary generalization, primary generalized tonic-clonic seizures and off-label in status epilepticus. Despite scarcely performed until today, therapeutic drug monitoring of lacosamide is proving to be advantageous by allowing the control of inter and intra-individual variability and promoting a successful personalized therapy, particularly in special populations. Herein, the pharmacology, pharmacokinetics, and clinical data of lacosamide were reviewed, giving special emphasis to the latest molecular investigations underlying its mechanism of action and therapeutic applications in pathologies besides epilepsy. In addition, the pharmacokinetic characteristics of lacosamide were updated, as well as current literature concerning the high pharmacokinetic variability observed in special patient populations and that must be considered during treatment individualization.

Characterization of Vixotrigine, a Broad-Spectrum Voltage-Gated Sodium Channel Blocker

Voltage-gated sodium channels (Navs) are promising targets for analgesic and antiepileptic therapies. Although specificity between Nav subtypes may be desirable to target specific neural types, such as nociceptors in pain, many broadly acting Nav inhibitors are clinically beneficial in neuropathic pain and epilepsy. Here, we present the first systematic characterization of vixotrigine, a Nav blocker. Using recombinant systems, we find that vixotrigine potency is enhanced in a voltage- and use-dependent manner, consistent with a state-dependent block of Navs. Furthermore, we find that vixotrigine potently inhibits sodium currents produced by both peripheral and central nervous system Nav subtypes, with use-dependent IC₅₀ values between 1.76 and 5.12 μM. Compared with carbamazepine, vixotrigine shows higher potency and more profound state-dependent inhibition but a similar broad spectrum of action distinct from Nav1.7- and Nav1.8-specific blockers. We find that vixotrigine rapidly inhibits Navs and prolongs recovery from the fast-inactivated state. In native rodent dorsal root ganglion sodium channels, we find that vixotrigine shifts steady-state inactivation curves. Based on these results, we conclude that vixotrigine is a broad-spectrum, state-dependent Nav blocker. SIGNIFICANCE STATEMENT: Vixotrigine blocks both peripheral and central voltage-gated sodium channel subtypes. Neurophysiological approaches in recombinant systems and sensory neurons suggest this block is state-dependent.

Differential effect of lacosamide on Nav1.7 variants from responsive and non-responsive patients with small fibre neuropathy

Small fibre neuropathy is a common pain disorder, which in many cases fails to respond to treatment with existing medications. Gain-of-function mutations of voltage-gated sodium channel Na_v1.7 underlie dorsal root ganglion neuronal hyperexcitability and pain in a subset of patients with small fibre neuropathy. Recent clinical studies have demonstrated that lacosamide, which blocks sodium channels in a use-dependent manner, attenuates pain in some patients with Na_v1.7 mutations; however, only a subgroup of these patients responded to the drug. Here, we used voltage-clamp recordings to evaluate the effects of lacosamide on five Na_v1.7 variants from patients who were responsive or non-responsive to treatment. We show that, at the clinically achievable concentration of 30 μM, lacosamide acts as a potent sodium channel inhibitor of Na_v1.7 variants carried by responsive patients, via a hyperpolarizing shift of voltage-dependence of both fast and slow inactivation and enhancement of use-dependent inhibition. By contrast, the effects of lacosamide on slow inactivation and use-dependence in Na_v1.7 variants from non-responsive patients were less robust. Importantly, we found that lacosamide selectively enhances fast inactivation only in variants from responders. Taken together, these findings begin to unravel biophysical underpinnings that contribute to responsiveness to lacosamide in patients with small fibre neuropathy carrying select Na_v1.7 variants.

Real world, open label experience with lacosamide against acute painful oxaliplatin-induced peripheral neurotoxicity

We report the outcome of a pilot, open-label study that tested the potential of lacosamide (200 mg/bi.d) as an effective and safe symptomatic treatment against acute painful oxaliplatin-induced peripheral neurotoxicity (OXAIPN). Lacosamide was introduced in 18 colorectal cancer patients with evidence of clinically significant acute, painful OXAIPN after infusion of the third course (T1) of oxaliplatin-based chemotherapy (FOLFOX4) and was maintained until completion of all 12 courses (T4). The OXA-Neuropathy Questionnaire (OXA-NQ) was used to record the severity of acute OXAIPN; the PI-NRS estimated the severity of neuropathic pain, while the chronic OXAIPN was graded with TNSc. The EuroQOL (EQ-5D) instrument was also applied. The Patient Global Impression of Change (PGIC) scale measured the lacosamide-attributed perception of change. LCM-responders were considered those with ≥50% reduction in PI-NRS and OXA-NQ scores at T4, compared to T1. Patients experienced on T1 a median number of acute OXAIPN symptoms of 4 and had a median neuropathic pain severity score of 6, which was strongly related to lower quality of life, according to EQ-VAS (P < .001). At T4, 12 patients (66.7%) were classified as responders. A significant clinical improvement was documented in the severity of acute OXAIPN and neuropathic pain in relation to lacosamide (P < .001) at T4 compared to T1, which was associated with improved EQ-VAS scores (P < .001). Twelve patients scored PGIC ≥5 (lacosamide-attributed) at T4. There were no incidences of early drop-outs for safety reasons. Lacosamide appears to be an effective and well-tolerated symptomatic treatment against acute, painful OXAIPN.

Lidocaine Binding Enhances Inhibition of Nav1.7 Channels by the Sulfonamide PF-05089771

PF-05089771 is an aryl sulfonamide Nav1.7 channel blocker that binds to the inactivated state of Nav1.7 channels with high affinity but binds only weakly to channels in the resting state. Such aryl sulfonamide Nav1.7 channel blockers bind to the extracellular surface of the S1-S4 voltage-sensor segment of homologous Domain 4, whose movement is associated with inactivation. This binding site is different from that of classic sodium channel inhibitors like lidocaine, which also bind with higher affinity to the inactivated state than the resting state but bind at a site within the pore of the channel. The common dependence on gating state with distinct binding sites raises the possibility that inhibition by aryl sulfonamides and by classic local anesthetics might show an interaction mediated by their mutual state dependence. We tested this possibility by examining the state-dependent inhibition by PF-05089771 and lidocaine of human Nav1.7 channels expressed in human embryonic kidney 293 cells. At -80 mV, where a small fraction of channels are in an inactivated state under drug-free conditions, inhibition by PF-05089771 was both enhanced and speeded in the presence of lidocaine. The results suggest that lidocaine binding to the channel enhances PF-05089771 inhibition by altering the equilibrium between resting states (with D4S4 in the inner position) and inactivated states (with D4S4 in the outer position). The gating state-mediated interaction between the compounds illustrates a principle applicable to many state-dependent agents. SIGNIFICANCE STATEMENT: The results show that lidocaine enhances the degree and rate of inhibition of Nav1.7 channels by the aryl sulfonamide compound PF-05089771, consistent with state-dependent binding by lidocaine increasing the fraction of channels presenting a high-affinity binding site for PF-05089771 and suggesting that combinations of agents targeted to the pore-region binding site of lidocaine and the external binding site of aryl sulfonamides may have synergistic actions.

Mechanisms of action of currently used antiseizure drugs

Antiseizure drugs (ASDs) prevent the occurrence of seizures; there is no evidence that they have disease-modifying properties. In the more than 160 years that orally administered ASDs have been available for epilepsy therapy, most agents entering clinical practice were either discovered serendipitously or with the use of animal seizure models. The ASDs originating from these approaches act on brain excitability mechanisms to interfere with the generation and spread of epileptic hyperexcitability, but they do not address the specific defects that are pathogenic in the epilepsies for which they are prescribed, which in most cases are not well understood. There are four broad classes of such ASD mechanisms: (1) modulation of voltage-gated sodium channels (e.g. phenytoin, carbamazepine, lamotrigine), voltage-gated calcium channels (e.g. ethosuximide), and voltage-gated potassium channels [e.g. retigabine (ezogabine)]; (2) enhancement of GABA-mediated inhibitory neurotransmission (e.g. benzodiazepines, tiagabine, vigabatrin); (3) attenuation of glutamate-mediated excitatory neurotransmission (e.g. perampanel); and (4) modulation of neurotransmitter release via a presynaptic action (e.g. levetiracetam, brivaracetam, gabapentin, pregabalin). In the past two decades there has been great progress in identifying the pathophysiological mechanisms of many genetic epilepsies. Given this new understanding, attempts are being made to engineer specific small molecule, antisense and gene therapies that functionally reverse or structurally correct pathogenic defects in epilepsy syndromes. In the near future, these new therapies will begin a paradigm shift in the treatment of some rare genetic epilepsy syndromes, but targeted therapies will remain elusive for the vast majority of epilepsies until their causes are identified. This article is part of the special issue entitled 'New Epilepsy Therapies for the 21st Century - From Antiseizure Drugs to Prevention, Modification and Cure of Epilepsy'.

Lacosamide in patients with Nav1.7 mutations-related small fibre neuropathy: a randomized controlled trial

Symptomatic treatment of neuropathic pain in small fibre neuropathy is often disappointing. The finding of voltage-gated sodium channel mutations in small fibre neuropathy (with mutations in SCN9A, encoding for Nav1.7) being most frequently reported suggest a specific target for therapy. The anticonvulsant lacosamide acts on Nav1.3, Nav1.7, and Nav1.8. The aim of this study was to evaluate the efficacy, safety, and tolerability of lacosamide as a potential treatment for pain in Nav1.7-related small fibre neuropathy. The Lacosamide-Efficacy-'N'-Safety in SFN (LENSS) was a randomized, placebo-controlled, double-blind, crossover-design study. Subjects were recruited in the Netherlands between November 2014 and July 2016. Patients with Nav1.7-related small fibre neuropathy were randomized to start with lacosamide followed by placebo or vice versa. In both 8-week treatment phases, patients received 200 mg two times a day (BID), preceded by a titration period, and ended by a tapering period. The primary outcome was efficacy, defined as the proportion of patients with 1-point average pain score reduction compared to baseline using the Pain Intensity Numerical Rating Scale. The trial is registered with ClinicalTrials.gov, number NCT01911975. Twenty-four subjects received lacosamide, and 23 received placebo. In 58.3% of patients receiving lacosamide, mean average pain decreased by at least 1 point, compared to 21.7% in the placebo group [sensitivity analyses, odds ratio 5.65 (95% confidence interval: 1.83-17.41); P = 0.0045]. In the lacosamide group, 33.3% reported that their general condition improved versus 4.3% in the placebo group (P-value = 0.0156). Additionally, a significant decrease in daily sleep interference, and in surface pain intensity was demonstrated. No significant changes in quality of life or autonomic symptoms were found. Lacosamide was well tolerated and safe in use. This study shows that lacosamide has a significant effect on pain, general wellbeing, and sleep quality. Lacosamide was well tolerated and safe, suggesting that it can be used for pain treatment in Nav1.7-related small fibre neuropathy.

Preparation, Characterization, and Stability Evaluation of Taste-Masking Lacosamide Microparticles

Lacosamide (LCM) is a third-generation antiepileptic drug. Selective action of the drug on voltage-gated sodium channels reduces side effects. Oral administration of LCM shows good pharmacokinetic profile. However, the bitter taste of LCM is a barrier to the development of oral formulations. In this study, we aimed to prepare encapsulated LCM microparticles (MPs) for masking its bitter taste. Encapsulated LCM MPs were prepared with Eudragit^® E100 (E100), a pH-dependent polymer, by spray drying. Three formulations comprising different ratios of LCM and E100 (3:1, 1:1, and 1:3) were prepared. Physicochemical tests showed that LCM was in an amorphous state in the prepared formulations, and they were not miscible. LCM-E100 (1:3) had a rough surface due to surface enrichment of LCM. Increased E100 ratio in LCM-E100 MPs resulted in better taste-making effectiveness: LCM-E100 (1:1) and LCM-E100 (1:3) showed good taste-masking effectiveness, while LCM-E100 (3:1) could not mask the bitter taste of LCM. Dissolution results of the prepared formulations showed good correlation with taste-masking effectiveness. Nevertheless, high E100 ratio reduced the stability of the prepared formulations. Especially the difference in initial dissolution profile observed for LCM-E100 (1:3) indicated rapid reduction in taste-masking effectiveness and surface recrystallization. Therefore, LCM-E100 formulation in the ratio of 1:1 was selected as the best formulation with good taste-masking effectiveness and stability.

Impaired chromaffin cell excitability and exocytosis in autistic Timothy syndrome TS2-neo mouse rescued by L-type calcium channel blockers

KEY POINTS

Tymothy syndrome (TS) is a multisystem disorder featuring cardiac arrhythmias, autism and adrenal gland dysfunction that originates from a de novo point mutation in the gene encoding the Cav1.2 (CACNA1C) L-type channel. To study the role of Cav1.2 channel signals in autism, the autistic TS2-neo mouse has been generated bearing the G406R point-mutation associated with TS type-2. Using heterozygous TS2-neo mice, we report that the G406R mutation reduces the rate of inactivation and shifts leftward the activation and inactivation of L-type channels, causing marked increase of resting Ca²⁺ influx ('window' Ca²⁺ current). The increased 'window current' causes marked reduction of Na_V channel density, switches normal tonic firing to abnormal burst firing, reduces mitochondrial metabolism, induces cell swelling and decreases catecholamine release. Overnight incubations with nifedipine rescue Na_V channel density, normal firing and the quantity of catecholamine released. We provide evidence that chromaffin cell malfunction derives from altered Cav1.2 channel gating.

ABSTRACT

L-type voltage-gated calcium (Cav1) channels have a key role in long-term synaptic plasticity, sensory transduction, muscle contraction and hormone release. A point mutation in the gene encoding Cav1.2 (CACNA1C) causes Tymothy syndrome (TS), a multisystem disorder featuring cardiac arrhythmias, autism spectrum disorder (ASD) and adrenal gland dysfunction. In the more severe type-2 form (TS2), the missense mutation G406R is on exon 8 coding for the IS6-helix of the Cav1.2 channel. The mutation causes reduced inactivation and induces autism. How this occurs and how Cav1.2 gating-changes alter cell excitability, neuronal firing and hormone release on a molecular basis is still largely unknown. Here, using the TS2-neo mouse model of TS we show that the G406R mutation altered excitability and reduced secretory activity in adrenal chromaffin cells (CCs). Specifically, the TS2 mutation reduced the rate of voltage-dependent inactivation and shifted leftward the activation and steady-state inactivation of L-type channels. This markedly increased the resting 'window' Ca²⁺ current that caused an increased percentage of CCs undergoing abnormal action potential (AP) burst firing, cell swelling, reduced mitochondrial metabolism and decreased catecholamine release. The increased 'window' Ca²⁺ current caused also decreased Na_V channel density and increased steady-state inactivation, which contributed to the increased abnormal burst firing. Overnight incubation with the L-type channel blocker nifedipine rescued the normal AP firing of CCs, the density of functioning Na_V channels and their steady-state inactivation. We provide evidence that CC malfunction derives from the altered Cav1.2 channel gating and that dihydropyridines are potential therapeutics for ASD.

Pain relief in a neuropathy patient by lacosamide: Proof of principle of clinical translation from patient-specific iPS cell-derived nociceptors

BACKGROUND

Small fiber neuropathy (SFN) is a severe and disabling chronic pain syndrome with no causal and limited symptomatic treatment options. Mechanistically based individual treatment is not available. We report an in-vitro predicted individualized treatment success in one therapy-refractory Caucasian patient suffering from SFN for over ten years.

METHODS

Intrinsic excitability of human induced pluripotent stem cell (iPSC) derived nociceptors from this patient and respective controls were recorded on multi-electrode (MEA) arrays, in the presence and absence of lacosamide. The patient's pain ratings were assessed by a visual analogue scale (10: worst pain, 0: no pain) and treatment effect was objectified by microneurography recordings of the patient's single nerve C-fibers.

FINDINGS

We identified patient-specific changes in iPSC-derived nociceptor excitability in MEA recordings, which were reverted by the FDA-approved compound lacosamide in vitro. Using this drug for individualized treatment of this patient, the patient's pain ratings decreased from 7.5 to 1.5. Consistent with the pain relief reported by the patient, microneurography recordings of the patient's single nerve fibers mirrored a reduced spontaneous nociceptor (C-fiber) activity in the patient during lacosamide treatment. Microneurography recordings yielded an objective measurement of altered peripheral nociceptor activity following treatment.

INTERPRETATION

Thus, we are here presenting one example of successful patient specific precision medicine using iPSC technology and individualized therapeutic treatment based on patient-derived sensory neurons.

Enhancing inactivation rather than reducing activation of Nav1.7 channels by a clinically effective analgesic CNV1014802

The Nav1.7 channel represents a promising target for pain relief. In the recent decades, a number of Nav1.7 channel inhibitors have been developed. According to the effects on channel kinetics, these inhibitors could be divided into two major classes: reducing activation or enhancing inactivation. To date, however, only several inhibitors have moved forward into phase 2 clinical trials and most of them display a less than ideal analgesic efficacy, thus intensifying the controversy regarding if an ideal candidate should preferentially affect the activation or inactivation state. In the present study, we investigated the action mechanisms of a recently clinically confirmed inhibitor CNV1014802 using both electrophysiology and site-directed mutagenesis. We found that CNV1014802 inhibited Nav1.7 channels through stabilizing a nonconductive inactivated state. When the cells expressing Nav1.7 channels were hold at 70 mV or 120 mV, the half maximal inhibitory concentration (IC50) values (with 95% confidence limits) were 1.77 (1.20-2.33) and 71.66 (46.85-96.48) μmol/L, respectively. This drug caused dramatic hyperpolarizing shift of channel inactivation but did not affect activation. Moreover, CNV1014802 accelerated the onset of inactivation and delayed the recovery from inactivation. Notably, application of CNV1014802 (30 μmol/L) could rescue the Nav1.7 mutations expressed in CHO cells that cause paroxysmal extreme pain disorder (PEPD), thereby restoring the impaired inactivation to those of the wild-type channel. Our study demonstrates that CNV1014802 enhances the inactivation but does not reduce the activation of Nav1.7 channels, suggesting that identifying inhibitors that preferentially affect inactivation is a promising approach for developing drugs targeting Nav1.7.

Mutations in SCN3A cause early infantile epileptic encephalopathy

OBJECTIVE

Voltage-gated sodium (Na⁺ ) channels underlie action potential generation and propagation and hence are central to the regulation of excitability in the nervous system. Mutations in the genes SCN1A, SCN2A, and SCN8A, encoding the Na⁺ channel pore-forming (α) subunits Nav1.1, 1.2, and 1.6, respectively, and SCN1B, encoding the accessory subunit β₁ , are established causes of genetic epilepsies. SCN3A, encoding Nav1.3, is known to be highly expressed in brain, but has not previously been linked to early infantile epileptic encephalopathy. Here, we describe a cohort of 4 patients with epileptic encephalopathy and heterozygous de novo missense variants in SCN3A (p.Ile875Thr in 2 cases, p.Pro1333Leu, and p.Val1769Ala).

METHODS

All patients presented with treatment-resistant epilepsy in the first year of life, severe to profound intellectual disability, and in 2 cases (both with the variant p.Ile875Thr), diffuse polymicrogyria.

RESULTS

Electrophysiological recordings of mutant channels revealed prominent gain of channel function, with a markedly increased amplitude of the slowly inactivating current component, and for 2 of 3 mutants (p.Ile875Thr and p.Pro1333Leu), a leftward shift in the voltage dependence of activation to more hyperpolarized potentials. Gain of function was not observed for Nav1.3 variants known or presumed to be inherited (p.Arg1642Cys and p.Lys1799Gln). The antiseizure medications phenytoin and lacosamide selectively blocked slowly inactivating over transient current in wild-type and mutant Nav1.3 channels.

INTERPRETATION

These findings establish SCN3A as a new gene for infantile epileptic encephalopathy and suggest a potential pharmacologic intervention. These findings also reinforce the role of Nav1.3 as an important regulator of neuronal excitability in the developing brain, while providing additional insight into mechanisms of slow inactivation of Nav1.3. Ann Neurol 2018;83:703-717.

Modulation of sodium channels as pharmacological tool for pain therapy-highlights and gaps

Voltage-gated sodium channels are crucially involved in the transduction and transmission of nociceptive signals and pathological pain states. In the past decades, a lot of effort has been spent examining and characterizing biophysical properties of the different sodium channels and their role in signaling pathways. Several gains of function mutations of the sodium channels Nav1.7, Nav1.8, and Nav1.9 are associated with pain disorders. Due to their critical role in nociceptive pathways voltage-gated sodium channels are regarded interesting targets for pharmacological pain treatment. However we still need to fill the gap that exists in the translation of efficacy in preclinical in vitro experiments and in models of pain into the clinic. This review summarizes biological and electrophysiological properties of voltage-gated sodium channels and aims to discuss limitations and promising pharmacological strategies in sodium channel research in the context of pain therapy.

Mechanisms of Drug Binding to Voltage-Gated Sodium Channels

Voltage-gated sodium (Na⁺) channels are expressed in virtually all electrically excitable tissues and are essential for muscle contraction and the conduction of impulses within the peripheral and central nervous systems. Genetic disorders that disrupt the function of these channels produce an array of Na⁺ channelopathies resulting in neuronal impairment, chronic pain, neuromuscular pathologies, and cardiac arrhythmias. Because of their importance to the conduction of electrical signals, Na⁺ channels are the target of a wide variety of local anesthetic, antiarrhythmic, anticonvulsant, and antidepressant drugs. The voltage-gated family of Na⁺ channels is composed of α-subunits that encode for the voltage sensor domains and the Na⁺-selective permeation pore. In vivo, Na⁺ channel α-subunits are associated with one or more accessory β-subunits (β₁-β₄) that regulate gating properties, trafficking, and cell-surface expression of the channels. The permeation pore of Na⁺ channels is divided in two parts: the outer mouth of the pore is the site of the ion selectivity filter, while the inner cytoplasmic pore serves as the channel activation gate. The cytoplasmic lining of the permeation pore is formed by the S6 segments that include highly conserved aromatic amino acids important for drug binding. These residues are believed to undergo voltage-dependent conformational changes that alter drug binding as the channels cycle through the closed, open, and inactivated states. The purpose of this chapter is to broadly review the mechanisms of Na⁺ channel gating and the models used to describe drug binding and Na⁺ channel inhibition.