Abstract

NBI-921352 is the first highly isoform selective inhibitor of Na_V1.6 voltage-gated sodium channels (Na_Vs) to enter clinical development for epilepsy. Nonselective inhibitors of Na_Vs have long been a mainstay of epilepsy pharmacotherapy. Many such Na_V inhibitors are available, but they are not mechanistically well differentiated. They all inhibit via the same highly conserved binding site, and they inhibit the distinct Na_V isoforms equivalently. This broad Na_V inhibition limits the utility of these drugs, as dose levels required to reduce seizures can cause tolerability issues in both the central nervous and cardiovascular systems. The rationale to develop NBI-921352 was to take a precision medicine approach to developing novel Na_V inhibitors. This approach has been validated preclinically, both in scn8a gain-of-function mutant cell lines and in Scn8a gain-of-function mice. NBI-921352 should improve tolerability by stripping away effects mediated by inhibition of the Na_Vs of cardiac cells (Na_V1.5), muscle cells (Na_V1.4), and inhibitory interneurons (Na_V1.1). Despite the selective nature of NBI-921352, the compound retains the ability to reduce neural excitability and increases seizure resistance in rodents. NBI-921352 is currently entering clinical trials to establish whether the improved properties observed in rodents translate to human epilepsy patients

Introduction

Drugs that inhibit voltage-gated sodium channels, such as phenytoin, carbamazepine, and lacosamide, are among the most widely used anticonvulsants. Many analogs of these compounds have been developed to progressively improve the pharmacodynamic properties and specificity for sodium channels. The mechanism of action of sodium channel inhibitors has been extensively reviewed, including an excellent summary in the fourth edition of this volume (Catterall, 2012). A limitation common to all of the currently used anticonvulsant sodium channel inhibitors is a relatively small therapeutic index. In general, the adverse effects seen with these compounds are a result of their lack of sodium channel selectivity. This is not unexpected because sodium channels are widely expressed in both excitatory and inhibitory neurons in the central nervous system (CNS) and play a key role in the activation of action potentials. The currently available sodium channel agents are non-isoform selective inhibitors and thus have similar potency for all sodium channel isoforms, including those in both excitatory and inhibitory circuits. All the agents produce state-dependent inhibition such that the inactivated states are stabilized, reducing the availability of channel opening during conditions that cause hyperexcitability. This profile favors inhibition of rapidly firing cells, as is encountered in epilepsy, but inhibitory interneurons can also fire rapidly. Ideally, one would strive to reduce the neuronal firing of the excitatory and not the inhibitory neurons. Moreover, the currently used sodium channel inhibitors have similar potency for both neuronal and muscle sodium channels. Inhibition of Na_V1.4 and Na_V1.5, the isoforms prevalent in skeletal and cardiac muscle, respectively, does not offer therapeutic benefit but does bring additional risk.

A strategy for enhancing the efficacy and therapeutic index of epileptic agents is to create sodium channel inhibitors that preferentially exist in excitatory neuronal circuits. Studies on channelopathies, especially from studies of severe myoclonic epilepsy of infancy, also known as SMEI or Dravet syndrome, add credence to this strategy. Three isoforms of voltage-gated sodium channels comprise the vast majority of those in the adult CNS: Na_V1.1, Na_V1.2, and Na_V1.6. Electrophysiological and mouse genetic studies show that Dravet syndrome is caused by loss-of-function mutations in Na_V1.1 and concomitant loss of excitability in inhibitory GABAergic neurons (Catterall, 2012). Thus, inhibition of Na_V1.1 is likely counterproductive to antiseizure activity. A compound that selectively inhibits Na_V1.6 and/or Na_V1.2 and spares Na_V1.1 and Na_V1.5 could have superior efficacy and a better therapeutic index.

Another group of severe childhood epilepsy syndromes is attributed to de novo gain-of-function (GOF) mutations in SCN8A, the gene that codes for Na_V1.6 (Veeramah et al., 2012; Meisler, 2019). As in Dravet syndrome, understanding of the channelopathy is derived from genetic studies in both humans and mice. The most severe of the SCN8A-related epilepsy syndromes (SCN8A-RES) is SCN8A developmental and epileptic encephalopathy (SCN8A-DEE) (Johannesen et al., 2019; Gardella and Moller, 2019). Since decreased Na_V1.1 currents and increased Na_V1.6 currents both cause seizure syndromes, it follows that a selective Na_V1.6 inhibitor could rebalance the excitatory inhibitory balance and provide therapeutic benefit in epilepsy. We hypothesize that a selective inhibitor of Na_V1.6 could provide a safer and more effective treatment for patients with SCN8A-RES and might also be efficacious in more common forms of epilepsy.

As shown in Figure 72–2, our earlier studies developing selective inhibitors of Na_V1.7 served as a paradigm for targeting Na_V1.6 and provided confidence that the appropriate selectivity among sodium channel isoforms might be achieved (Ahuja et al., 2015; Clairfeuille et al., 2020). The Na_V inhibitors in use as antiseizure medicines bind to the pore domain of the channels, a site that is highly conserved among the isoforms. Time- and voltage-dependent inhibition can be also obtained by binding to an extracellular site on the domain IV voltage sensor (VSD4), and this site differs substantially among Na_V1.6, Na_V1.1, and Na_V1.5. An extensive medicinal-chemistry effort focused on the VSD4 binding site produced NBI-921352 (formerly known as XEN901), a potent and isoform-selective inhibitor of Na_V1.6 channels (Johnson et al., 2022).

Figure 72–2.

Targeting domain IV voltage sensor (VSD4) enables highly selective inhibitors. Therapeutically used Na_V inhibitors bind in pore domain to a promiscuous, low-affinity binding site. Aryl sulfonamide inhibitors bind to a distinct extracellular binding site (more...)

We explored the properties of NBI-921352 in three preclinical rodent-seizure models. One model evaluated antiseizure activity in mice genetically engineered to carry a patient-identified heterozygous variant in the SCN8A gene coding for gain-of-function Na_V1.6 channels (N1768D) that predispose the mice to seizures (Veeramah et al., 2012; Hammer et al., 2016; Wagnon et al., 2015). N1768D channels have impaired voltage-dependent inactivation gating that results in persistent sodium currents and enhanced resurgent currents (Veeramah et al., 2012). Because Na_V1.6 channels are highly expressed in the neurons of the brain, and particularly so in excitatory pyramidal neurons, the increased Na⁺ flux leads to mice that are predisposed to seizures. Many, but not all, of the mice will develop spontaneous seizures between 6 weeks and 6 months after birth. In addition, the transgenic mice have a lower threshold for electrically induced seizures, and we evaluated the efficacy of Na_V1.6 inhibitors to prevent seizures by stimuli that only provoke seizures in transgenic mice.

In addition to evaluating antiseizure activity in this genetically modified mouse, the selective Na_V1.6 inhibitors were evaluated in the direct current maximal electroshock seizure (DC-MES) models in wild-type mice and rats, assays that have been used in the development of the currently used antiseizure medicine (ASM) Na_V inhibitors. NBI-921352 is highly effective in these models and is a promising agent for treating both the childhood SCN8A-RES epilepsies as well as adult epilepsies with unknown etiologies (Johnson et al., 2022).

In this chapter, we will examine the rationale in support of creating Na_V1.6 selective inhibitors for the treatment of epilepsy as well as preclinical data supporting this approach with our novel Na_V1.6 selective inhibitor NBI-921352.

Na_V Cellular and Subcellular Distribution

Central Nervous System

There are four Na_V isoforms that predominate in the central nervous system, Na_V1.1, Na_V1.2, Na_V1.3, and Na_V1.6. Early in development, Na_V1.2 and Na_V1.3 are the most prevalent isoforms. Na_V1.3 is largely embryonic, with high expression prenatally and perinatally. In the first year of life (or weeks in mice), Na_V1.3 is largely replaced by Na_V1.1. As shown in Figure 72–1, Na_V1.1 is particularly important in inhibitory interneurons, where it supports action-potential firing and GABAergic signaling.

Figure 72–1.

NaV isoforms have diverse cellular and subcellular expression patterns. Schematic showing Na_V localization. Na_V1.1 predominates in inhibitory interneurons. Na_V1.2 and Na_V1.6 are more prevalent in excitatory glutamatergic pyramidal neurons. Na_V1.2 is (more...)

Na_V1.2 is highly expressed throughout life but is particularly dominant at early stages of development. As shown in Figure 72–1, Na_V1.6 expression is low at birth and increases in the first year of life, displacing Na_V1.2 from the distal axon initial segments of many neurons as well as at the nodes of Ranvier (Sole and Tamkun, 2020). Na_V1.2 remains the most abundant Na_V in the dendrites and the proximal axon initial segment. This temporal and spatial differentiation of Na_V isoform expression leads to diverse physiological roles as well as diverse roles in pathological conditions.

Genetic Channelopathies Guide a Preferred Selectivity Profile for Antiseizure Medications

Designing a New Class of Isoform-Selective Sodium Channel Inhibitors

Based on the known risks of decreasing Na_V currents through Na_V1.1, Na_V1.4, and Na_V1.5, we set out to create a selective inhibitor of Na_V1.6. We predicted that such a compound would be safer, and perhaps more efficacious, than nonselective ASMs, since the proconvulsant effect of inhibiting Na_V1.1 would be avoided. This approach could provide better tolerated and more effective treatment for patients with SCN8A-RES and might also be efficacious in more common forms of epilepsy. An extensive medicinal-chemistry effort produced many compounds with diverse properties and selectivity profiles. NBI-921352 combines potent inhibition of Na_V1.6, high selectivity against off-target Na_Vs, and favorable pharmaceutical properties. NBI-921352 is the first selective inhibitor of Na_V1.6 channel currents to enter clinical development for epilepsy (Neurocrine., 2021; Johnson et al., 2022).

Most small-molecule inhibitors of sodium channels bind to the intracellular pore of the channel. Upon binding, they can impair Na⁺ flow through open channels directly, but they also stabilize nonconductive inactivated states of the channel. A fundamental challenge of this binding site is that it is well conserved across Na_V isoforms and, hence, does not readily lend itself to isoform-selective inhibition.

Multiple drug discovery projects in the pharmaceutical industries have targeted selective Na_V channels inhibition, usually Na_V1.3, Na_V1.7, or Na_V1.8 for pain indications, and numerous molecules have been identified that bind with some selectivity. Some of these molecules have the desired antiepileptic selectivity profile for targeting Na_V1.6 and Na_V1.2, but sparing Na_V1.1 and Na_V1.5 (McCormack et al., 2013). These compounds also block Na_V1.7, but that would not be expected to impact their performance for epilepsy. The selective profile of these Na_V1.7 targeted molecules is made possible by the fact that they target a distinct binding site on the extracellular side of the channel voltage sensor, the domain IV voltage sensor (Ahuja et al., 2015). This site is more diverse across channel isoforms than the canonical binding site for sodium channel inhibitors. The biggest challenge for these Na_V1.7 molecules as potential ASMs is that they were designed to be peripherally restricted (to improve tolerability) and, hence, are excluded from the brain. Thus, these molecules are trapped in the peripheral circulation where they cannot impact epilepsy.

We set out to target this domain IV voltage-sensor (VSD4) binding site to enable selectivity, but to do so with molecules that can cross the blood–brain barrier and target Na_Vs in the brain. After many iterations of medicinal chemistry, we created compounds that balance potency, isoform selectivity, and brain penetrance.

NBI-921352, the First Isoform-Selective Inhibitor of Na_V1.6

As shown in Figure 72–3A, NBI-921352 potently inhibits human Na_V1.6 channel currents with an inhibitory concentration 50% (IC₅₀) of 0.0514 µM. Inhibition of other human Na_V1.X isoforms requires >100-fold higher concentrations of NBI-921352. NBI-921352 was 756-fold selective for hNa_V1.6 versus Na_V1.1, 134-fold for Na_V1.2, 276-fold for Na_V1.7, and >583-fold for Na_V1.3, Na_V1.4, and Na_V1.5.

Figure 72–3.

NBI-921352 is selective for NaV1.6 relative to other isoforms in both human (A) and mouse (B) channels. (C) NBI-921352 potently inhibits both resurgent and persistent currents in a patient-identified gain-of-function variant of Na_V1.6, but it is a weak (more...)

We expressed point mutations in the putative binding site for our Na_V1.6 inhibitors and defined the key residues for potency (see Fig 72–4; other data not shown). These studies suggested that the binding pocket for NBI-921352 in VSD IV is well conserved between the human and rodent orthologs and suggested that the potency would be similar among these species. We confirmed this by assessing the potency of NBI-921352 in the mouse Na_V isoforms most highly expressed in the brain, Na_V1.6, Na_V1.1, and Na_V1.2. As shown in Figure 72–3B, the potency and selectivity in mouse Na_V channels closely paralleled that seen in the human orthologs with an IC₅₀ of 0.058 µM for mNa_V1.6. NBI-921352 was 709-fold selective for Na_V1.6 versus Na_V1.1, and 191-fold for Na_V1.2. NBI-921352 potently inhibits both human and mouse Na_V1.6 channels, and it does so at concentrations much lower than are needed to inhibit any of the other channel isoforms tested.

The species-independent potency and selectivity profile of NBI-921352 is in stark contrast to VSD IV binding compounds developed to target Na_V1.7 and Na_V1.3. Because of species-specific amino acid differences, compounds like PF-04856264 (hNa_V1.7 selective) and ICA-121431 (hNa_V1.3 and hNa_V1.1 selective) exhibit strongly species-specific binding. This results in markedly different Na_V isoform-selectivity profiles in humans versus rodents for those peripherally restricted compounds (McCormack et al., 2013). Species-selectivity issues can make preclinical mouse studies challenging. NBI-921352 is potent and Na_V1.6 selective among both human and mouse isoforms and is, therefore, suited to both preclinical and human pharmacology.

Like Nonselective Na_V Inhibitor ASMs That Bind the Pore Domain, NBI-921352 Is a State-Dependent Inhibitor

Most small-molecule inhibitors of Na_V channels bind preferentially to open and or inactivated states (Bean et al., 1983; Courtney et al., 1978; Strichartz, 1976). State-dependent inhibition leads to higher affinity of the drug for the target channel under depolarized conditions, where the channel is more frequently in open or inactivated states—like when neurons are firing rapidly in a seizure. State dependence is a common feature of Na_V inhibitor ASMs and has been suggested to improve the therapeutic margins for those compounds.

Supraphysiologic hyperpolarizations of the membrane potential (–120 mV) push most channels to reside in the resting (closed) state. Brief depolarizations to measure Na_V1.6 current from –120 mV result in weak inhibition of the channels (IC₅₀ of 36.3 µM). Holding the membrane potential more depolarized (–45 mV) promotes channels to transition to open and inactivated states. Brief hyperpolarizations allow rapid recovery from inactivation for channels that are not bound to drug followed by a short 20 ms test depolarization. NBI-921352 inhibits the reopening of inactivated channels much more efficiently than closed channels. As shown in Figure 72–3C, the IC₅₀ of NBI-921352 for inactivated channels is 0.051 µM, demonstrating NBI-921352 is even more state dependent than currently available Na_V ASMs, with a 750-fold preference for activated (open or inactivated) states relative to rested state closed channels.

NBI-921352 Inhibits Persistent and Resurgent Currents from Mutant Na_V1.6 Channels

Elevated persistent and or resurgent currents are believed to underlie or contribute to the pathology of many sodium channel related pathologies, and Na_V1.6 is a source of persistent and resurgent currents (Mason et al., 2019; Pan and Cummins, 2020; Potet et al., 2020; Tidball et al., 2020; Zaman et al., 2019). In most conditions, Na_V channels inactivate rapidly and nearly completely after opening. The small persistent currents in healthy neurons have been proposed to be primarily carried by Na_V1.6. Persistent currents can be larger in pathologic conditions (Lampl et al., 1998; Schwindt and Crill, 1980). The persistent currents exhibited by many GOF mutations in Na_V1.6 that disrupt inactivation, including N1768D, are believed to be important contributors to their tendency to cause hyperexcitability and seizures in patients (Tidball et al., 2020; Wagnon et al., 2015). Thus, inhibition of persistent currents is a desirable feature. As shown in Figure 72–3C, NBI-921352 inhibits N1768D Na_V1.6 persistent currents (measured as the non-inactivating current 10 ms after initiation of the depolarizing test pulse) with a similar potency as for open and inactivated wild-type Na_V1.6 channels with an IC₅₀ of 0.0593 µM.

Resurgent currents occur after repolarization following a strong depolarization as channels redistribute between closed, open, and inactivated states (Raman and Bean, 1997). These resurgent currents are enhanced in many SCN8A-RES variants and have also been suggested to drive pathologic hyperexcitability (Raman et al., 1997; Pan and Cummins, 2020). NBI-921352 also effectively inhibits resurgent currents from N1768D channels with an IC₅₀ of 0.0373 µM.

NBI-921352 Inhibits Electrically Induced Seizures in Scn8a N1768D/+ Mice

A selective inhibitor of Na_V1.6 should lend itself to the treatment of disease states caused by pathologic gain of function mutations of SCN8A. Hence, we examined the ability of NBI-921352 to inhibit electrically induced seizures in mice with a patient-identified gain-of-function variant in the Scn8a gene encoding Na_V1.6. N1768D is a variant of Na_V1.6 identified in the first-reported SCN8A-DEE patient (Veeramah et al., 2012). Subsequently, genetically modified mice bearing the same variant (Scn8a^N1768D/+) were created and found to be seizure prone, producing a mouse model with a phenotype resembling that observed in SCN8A-DEE patients (Wagnon et al., 2015). To assess the ability of NBI-921352 to engage Na_V1.6 channels in vivo, we designed a modified version of the 6Hz psychomotor seizure assay in Scn8a^N1768D/+ mice (Barton et al., 2001, Focken et al., 2019). A low-level current stimulation evokes robust generalized tonic-clonic seizures (GTC) with hind-limb extension in Scn8a^N1768D/+ mice, but not in their wild-type littermates.

As shown in Figure 72–5, oral administration of NBI-921352 prior to electrical stimulation prevented induction of GTC with hind-limb extension in Scn8a^N1768D/+ mice in a concentration-dependent-manner.

Figure 72–5.

NBI-921352 increases seizure resistance in rodents. NBI-921352 reduces induced seizure incidence at concentrations more than 50-fold lower than those of three currently used NaV inhibitor ASMs (left). Similar brain concentrations of NBI-921352 are also (more...)

Figure 72–4.

Comparison of NBI-921352 potency on heterologously expressed human Na_V1.6 and patient-identified variants of Na_V1.6. Bars show the fold change in IC₅₀ for the variant channel relative to WT (Variant IC₅₀ / WT IC₅₀). Values less than one mean that NBI-921352 (more...)

Concentration Dependence of NBI-921352 in Comparison to Common ASMs

Translating dose dependence between species is challenging, as there are many complications that make cross-species allometric predictions uncertain. Consequently, we use plasma concentration or, better yet, target tissue concentration as a means of comparing compound activity. To accomplish this, all animals are euthanized immediately after seizure assessment, and the concentration of the test compound is determined in the plasma and brain tissue from each animal tested. As shown in Figure 72–5, the average concentrations for each dose group were used to generate brain concentration versus efficacy relationships. The efficacy of Na_V ASMs is driven by CNS exposure, and the tolerability of Na_V inhibitor ASMs is generally limited by CNS signs. Therefore, we focus on brain concentrations. The brain EC₅₀ (the concentration at which a half-maximal effect was observed) for preventing electrically induced seizures for NBI-921352 was 0.065 µM.

Currently available Na_V inhibitor ASMs required higher brain concentrations to achieve efficacy in this model. The brain EC₅₀s for carbamazepine, phenytoin, and lacosamide were 9.4 µM, 18 µM, and 3.3 µM, respectively. Lacosamide failed to provide full protection at the concentrations tested, and higher concentrations were not tolerated by the mice. Thus, NBI-921352 was 50-fold more potent than lacosamide, 144-fold more potent that carbamazepine, and 277-fold more potent than phenytoin. The improved potency of NBI-921352 in vivo is consistent with its greater potency for Na_V1.6 channels in vitro.

NBI-921352 Inhibits Electrically Induced Seizures in Wild-Type Mice and Rats

Na_V1.6 is an important mediator of neuronal excitability, even in animals without gain-of-function mutations, and selective inhibition of Na_V1.6 has been suggested as a potential target for other epilepsies involving mutations, such as SCN8A-RES, the proline-rich transmembrane protein 2 related epilepsies (PRRT2-RES), and Dravet syndrome, as well as for more common idiopathic seizures (Fruscione et al., 2018). We wondered whether NBI-921352 might have broader application in epilepsy in other syndromes of neural hyperexcitability. To begin to explore this idea, we assessed NBI-921352 in a maximal electroshock assay induced by a direct-current electrical stimulus (DC-MES) in wild-type mice and rats. MES assays have been widely used to predict therapeutic efficacy of ASMs, including nonselective Na_V inhibitor ASMs. Figure 72–5, right panel, shows that NBI-921352 protects wild-type rodents from DC-MES-induced seizures at brain concentrations that overlap those needed to protect mutant Scn8a^N1768D/+ mice in the 6Hz assay. This suggests that NBI-921352 may be more broadly efficacious and may effectively prevent seizures, even in populations where no mutations have compromised Na_V1.6 function (Johnson et al., 2022).

Efficacious Concentrations of NBI-921352 Are Well Separated from Concentrations That Provoke Behavioral Signs

The intent of creating a highly selective Na_V1.6 antagonist was to make a better antiseizure medication. If a selective compound approach works, it should provide the efficacy of a classic, nonselective, sodium channel inhibitor drug, but prevent the adverse events caused by inhibiting other sodium channels and non-sodium channel targets. If sparing Na_V1.1 and other off-target interactions does, in fact, reduce adverse events, then even higher receptor occupancy of Na_V1.6 might be achievable, and this could further improve efficacy.

As shown in Figure 72–6, to evaluate our hypothesis, we compared the plasma concentrations required for seizure control (plasma EC₅₀) with the minimal plasma concentration at which behavioral changes associated with the limits of tolerability were observed. We made this comparison both for NBI-921352 and for several widely used nonselective Na_V inhibitor ASMs: carbamazepine, phenytoin, and lacosamide.

Figure 72–6.

Selective inhibition improves tolerability. Rat efficacy compared to acute tolerability for NBI-921352 is markedly higher than for other NaV inhibitor ASMs. Plasma concentration versus efficacy data is shown for the rat DC-MES assay for NBI-921352, carbamazepine, (more...)

NBI-921352 was well tolerated in these studies up to a plasma concentration of 71 µM. Dividing this concentration by the plasma EC₅₀ of 0.15 µM in the rat DC-MES study results in a behavioral-signs concentration (BSC)/plasma EC₅₀ ratio of 473-fold. The same calculation was repeated for the established ASMs. The minimal plasma concentrations provoking behavioral signs of intolerability for carbamazepine, phenytoin, and lacosamide were 110 µM, 11 µM, and 123 µM, respectively. Their plasma EC₅₀s for seizure control were 30 µM, 4.5 µM, and 19.6 µM, respectively. Figure 75–7E shows BSC/plasma EC₅₀ ratios for carbamazepine (3.7-fold), phenytoin (2.4-fold), and lacosamide (6.3-fold). These data indicate that increasing Na_V1.6 selectivity can dramatically improve the tolerability of Na_V inhibitors in mouse-seizure models.

In Vitro Na_V1.6 Inhibition Predicts In Vivo Efficacy

Both Na_V1.2 and Na_V1.6 play critical roles in the firing of excitatory glutamatergic pyramidal neurons, despite differentiated profiles at the level of subcellular localization. Na_V1.2 is more concentrated in the dendrites and proximal axon initial segment (AIS) (Spratt et al., 2019; Thompson et al., 2020). Na_V1.6 is dominant in the distal AIS and nodes of Ranvier (Akin et al., 2015). Na_V1.2 has been suggested to be an important mediator of backpropagating signals and integration of incoming signals, while Na_V1.6 is more critical for action potentials propagating to downstream synapses (Ben-Shalom et al., 2017; Sanders et al., 2018).

De novo, it was unclear whether Na_V1.2 selective compounds, Na_V1.6 selective, or dual inhibitors would be the more fruitful approach. As shown in Figure 72–7, a comparison of efficacious brain concentrations in both Scn8a^N1768D/+ mice and wild-type mice seizure assays to the in vitro Na_V potency suggests that the potency on Na_V1.6 channels is the best predictor of efficacy for these seizure models. Na_V1.2 was loosely correlated with efficacy, and Na_V1.1 potency appeared unrelated to efficacy.

Figure 72–7.

Na_V1.6 inhibition in vitro predicts efficacy in vivo. Each point represents a distinct compound tested in a mouse in vivo efficacy assay. Upper panels represent activity in the Scn8aN1768D/+ 6Hz assay. Lower panels display data from the mouse DC-MES (more...)

It is likely that the ideal selectivity profile may be impacted by the seizure model assessed or, in the clinic, the specific etiology of the seizure syndrome. For example, SCN2A-related epilepsy syndromes (SCN2A-RES) may benefit from Na_V1.2 inhibition, since, in most cases, SCN2A-RES is associated with gain-of-function variants of Na_V1.2. Nonetheless, our data suggest that inhibition of Na_V1.6 in isolation is sufficient to impact neural excitability and to reduce the tendency for seizures in preclinical models.

Conclusions

The goal of ASMs is complete inhibition of seizures without adverse events. Dramatic reductions in seizure frequency are certainly welcome, but quality of life is greatly improved with complete suppression. Currently available Na_V ASMs have dose-limiting side effects. Our studies indicate that NBI-921352 is very promising because it is highly isoform-selective for Na_V1.6 and is well tolerated at brain concentrations well above those that suppress seizures.

A priori, it was unclear whether inhibition of multiple Na_V isoforms was necessary to achieve seizure suppression. Our studies indicate that inhibition of Na_V1.6 alone is sufficient, at least for the models that we have tested. The pharmacokinetic-pharmacodynamic (PK-PD) studies that we have conducted suggest that modest levels of inhibition of Na_V1.6 are adequate to achieve seizure control and reestablish the balance between inhibitory and excitatory circuits in the CNS.

For SCN8A-RES patients, where disease is explicitly linked to excess Na_V1.6 current, we anticipate that not only seizures but also other comorbidities that erode quality of life may be improved by a precision-medicine approach with NBI-92152. NBI-921352 is currently in development by Neurocrine Biosciences and is undergoing Phase II proof-of-concept trials for SCN8A-DEE (Neurocrine, 2019, 2021).

References

1. Ahuja, S., Mukund, S., Deng, L., Khakh, K., Chang, E., Ho, H., Shriver, S., Young, C., Lin, S., Johnson, J. P., Jr., Wu, P., Li, J., Coons, M., Tam, C., Brillantes, B., Sampang, H., Mortara, K., Bowman, K. K., Clark, K. R., Estevez, A., Xie, Z., Verschoof, H., Grimwood, M., Dehnhardt, C., Andrez, J. C., Focken, T., Sutherlin, D. P., Safina, B. S., Starovasnik, M. A., Ortwine, D. F., Franke, Y., Cohen, C. J., Hackos, D. H., Koth, C. M. & Payandeh, J.  2015. Structural basis of Nav1.7 inhibition by an isoform-selective small-molecule antagonist.  Science, 350, aac5464. [PubMed: 26680203]
2. Akin, E. J., Sole, L., Dib-Hajj, S. D., Waxman, S. G. & Tamkun, M. M.  2015. Preferential targeting of Nav1.6 voltage-gated Na+ channels to the axon initial segment during development.  PLoS One, 10, e0124397. [PMC free article: PMC4398423] [PubMed: 25874799]
3. Alsaloum, M., Higerd, G. P., Effraim, P. R. & Waxman, S. G.  2020. Status of peripheral sodium channel blockers for non-addictive pain treatment.  Nat Rev Neurol, 16, 689–705. [PubMed: 33110213]
4. Barton, M. E., Klein, B. D., Wolf, H. H. & White, H. S.  2001. Pharmacological characterization of the 6 Hz psychomotor seizure model of partial epilepsy.  Epilepsy Res, 47, 217–227. [PubMed: 11738929]
5. Bean, B. P., Cohen, C. J. & Tsien, R. W.  1983. Lidocaine block of cardiac sodium channels.  J Gen Physiol, 81, 613–642. [PMC free article: PMC2216565] [PubMed: 6306139]
6. Ben-Shalom, R., Keeshen, C. M., Berrios, K. N., An, J. Y., Sanders, S. J. & Bender, K. J.  2017. Opposing effects on NaV1.2 function underlie differences between SCN2A variants observed in individuals with autism spectrum disorder or infantile seizures.  Biol Psychiatry, 82, 224–232. [PMC free article: PMC5796785] [PubMed: 28256214]
7. Boerma, R. S., Braun, K. P., Van De Broek, M. P., Van Berkestijn, F. M., Swinkels, M. E., Hagebeuk, E. O., Lindhout, D., Van Kempen, M., Boon, M., Nicolai, J., De Kovel, C. G., Brilstra, E. H. & Koeleman, B. P.  2016. Remarkable phenytoin sensitivity in four children with SCN8A-related epilepsy: A molecular neuropharmacological approach.  Neurotherapeutics, 13, 192–197. [PMC free article: PMC4720675] [PubMed: 26252990]
8. Braakman, H. M., Verhoeven, J. S., Erasmus, C. E., Haaxma, C. A., Willemsen, M. H. & Schelhaas, H. J.  2017. Phenytoin as a last-resort treatment in SCN8A encephalopathy.  Epilepsia Open, 2, 343–344. [PMC free article: PMC5862112] [PubMed: 29588963]
9. Burgess, D. L., Kohrman, D. C., Galt, J., Plummer, N. W., Jones, J. M., Spear, B. & Meisler, M. H.  1995. Mutation of a new sodium channel gene, Scn8a, in the mouse mutant “motor endplate disease.”  Nat Genet, 10, 461–465. [PubMed: 7670495]
10. Campuzano, O., Sarquella-Brugada, G., Cesar, S., Arbelo, E., Brugada, J. & Brugada, R.  2020. Update on genetic basis of Brugada syndrome: monogenic, polygenic or oligogenic?  Int J Mol Sci, 21. [PMC free article: PMC7582739] [PubMed: 32998306]
11. Cannon, S. C.  2018. Sodium channelopathies of skeletal muscle.  Handb Exp Pharmacol, 246, 309–330. [PMC free article: PMC5866235] [PubMed: 28939973]
12. Catterall, W. A.  2012. Sodium Channel Mutations and Epilepsy. In: Th, Noebels, J. L., Avoli, M., Rogawski, M. A., Olsen, R. W. & Delgado-Escueta, A. V. (eds.) Jasper’s Basic Mechanisms of the Epilepsies. Bethesda (MD).
13. Catterall, W. A., Kalume, F. & Oakley, J. C.  2010. NaV1.1 channels and epilepsy.  J Physiol, 588, 1849–1859. [PMC free article: PMC2901973] [PubMed: 20194124]
14. Cheah, C. S., Westenbroek, R. E., Roden, W. H., Kalume, F., Oakley, J. C., Jansen, L. A. & Catterall, W. A.  2013. Correlations in timing of sodium channel expression, epilepsy, and sudden death in Dravet syndrome.  Channels (Austin), 7, 468–472. [PMC free article: PMC4042481] [PubMed: 23965409]
15. Chen, Q., Kirsch, G. E., Zhang, D., Brugada, R., Brugada, J., Brugada, P., Potenza, D., Moya, A., Borggrefe, M., Breithardt, G., Ortiz-Lopez, R., Wang, Z., Antzelevitch, C., O’brien, R. E., Schulze-Bahr, E., Keating, M. T., Towbin, J. A. & Wang, Q.  1998. Genetic basis and molecular mechanism for idiopathic ventricular fibrillation.  Nature, 392, 293–296. [PubMed: 9521325]
16. Claes, L., Del-Favero, J., Ceulemans, B., Lagae, L., Van Broeckhoven, C. & De Jonghe, P.  2001. De novo mutations in the sodium-channel gene SCN1A cause severe myoclonic epilepsy of infancy.  Am J Hum Genet, 68, 1327–1332. [PMC free article: PMC1226119] [PubMed: 11359211]
17. Clairfeuille, T., Buchholz, K. R., Li, Q., Verschueren, E., Liu, P., Sangaraju, D., Park, S., Noland, C. L., Storek, K. M., Nickerson, N. N., Martin, L., Dela Vega, T., Miu, A., Reeder, J., Ruiz-Gonzalez, M., Swem, D., Han, G., Deponte, D. P., Hunter, M. S., Gati, C., Shahidi-Latham, S., Xu, M., Skelton, N., Sellers, B. D., Skippington, E., Sandoval, W., Hanan, E. J., Payandeh, J. & Rutherford, S. T.  2020. Structure of the essential inner membrane lipopolysaccharide-PbgA complex.  Nature, 584, 479–483. [PubMed: 32788728]
18. Courtney, K. R., Kendig, J. J. & Cohen, E. N.  1978. The rates of interaction of local anesthetics with sodium channels in nerve.  J Pharmacol Exp Ther, 207, 594–604. [PubMed: 712641]
19. Cox, J. J., Reimann, F., Nicholas, A. K., Thornton, G., Roberts, E., Springell, K., Karbani, G., Jafri, H., Mannan, J., Raashid, Y., Al-Gazali, L., Hamamy, H., Valente, E. M., Gorman, S., Williams, R., Mchale, D. P., Wood, J. N., Gribble, F. M. & Woods, C. G.  2006. An SCN9A channelopathy causes congenital inability to experience pain.  Nature, 444, 894–898. [PMC free article: PMC7212082] [PubMed: 17167479]
20. Escayg, A., Macdonald, B. T., Meisler, M. H., Baulac, S., Huberfeld, G., An-Gourfinkel, I., Brice, A., Leguern, E., Moulard, B., Chaigne, D., Buresi, C. & Malafosse, A.  2000. Mutations of SCN1A, encoding a neuronal sodium channel, in two families with GEFS+2.  Nat Genet, 24, 343–345. [PubMed: 10742094]
21. FDA. 2021. Drug Safety Communication: Studies show increased risk of heart rhythm problems with seizure and mental health medicine lamotrigine (Lamictal) in patients with heart disease.
22. Focken, T., Burford, K., Grimwood, M. E., Zenova, A., Andrez, J. C., Gong, W., Wilson, M., Taron, M., Decker, S., Lofstrand, V., Chowdhury, S., Shuart, N., Lin, S., Goodchild, S. J., Young, C., Soriano, M., Tari, P. K., Waldbrook, M., Nelkenbrecher, K., Kwan, R., Lindgren, A., De Boer, G., Lee, S., Sojo, L., Devita, R. J., Cohen, C. J., Wesolowski, S. S., Johnson, J. P., Jr., Dehnhardt, C. M. & Empfield, J. R.  2019. Identification of CNS-Penetrant Aryl Sulfonamides as Isoform-Selective NaV1.6 Inhibitors with Efficacy in Mouse Models of Epilepsy.  J Med Chem, 62, 9618–9641. [PubMed: 31525968]
23. Fruscione, F., Valente, P., Sterlini, B., Romei, A., Baldassari, S., Fadda, M., Prestigio, C., Giansante, G., Sartorelli, J., Rossi, P., Rubio, A., Gambardella, A., Nieus, T., Broccoli, V., Fassio, A., Baldelli, P., Corradi, A., Zara, F. & Benfenati, F.  2018. PRRT2 controls neuronal excitability by negatively modulating Na+ channel 1.2/1.6 activity.  Brain, 141, 1000–1016. [PMC free article: PMC5888929] [PubMed: 29554219]
24. Gardella, E. & Moller, R. S.  2019. Phenotypic and genetic spectrum of SCN8A-related disorders, treatment options, and outcomes.  Epilepsia, 60 Suppl 3, S77–S85. [PubMed: 31904124]
25. Gennaro, E., Veggiotti, P., Malacarne, M., Madia, F., Cecconi, M., Cardinali, S., Cassetti, A., Cecconi, I., Bertini, E., Bianchi, A., Gobbi, G. & Zara, F.  2003. Familial severe myoclonic epilepsy of infancy: truncation of Nav1.1 and genetic heterogeneity.  Epileptic Disord, 5, 21–25. [PubMed: 12773292]
26. Goldberg, Y. P., Macfarlane, J., Macdonald, M. L., Thompson, J., Dube, M. P., Mattice, M., Fraser, R., Young, C., Hossain, S., Pape, T., Payne, B., Radomski, C., Donaldson, G., Ives, E., Cox, J., Younghusband, H. B., Green, R., Duff, A., Boltshauser, E., Grinspan, G. A., Dimon, J. H., Sibley, B. G., Andria, G., Toscano, E., Kerdraon, J., Bowsher, D., Pimstone, S. N., Samuels, M. E., Sherrington, R. & Hayden, M. R.  2007. Loss-of-function mutations in the Nav1.7 gene underlie congenital indifference to pain in multiple human populations.  Clin Genet, 71, 311–319. [PubMed: 17470132]
27. Goldberg, Y. P., Pimstone, S. N., Namdari, R., Price, N., Cohen, C., Sherrington, R. P. & Hayden, M. R.  2012. Human Mendelian pain disorders: a key to discovery and validation of novel analgesics.  Clin Genet, 82, 367–373. [PubMed: 22845492]
28. Goodwin, G. & Mcmahon, S. B.  2021. The physiological function of different voltage-gated sodium channels in pain.  Nat Rev Neurosci, 22, 263–274. [PubMed: 33782571]
29. Hains, B. C., Klein, J. P., Saab, C. Y., Craner, M. J., Black, J. A. & Waxman, S. G.  2003. Upregulation of sodium channel Nav1.3 and functional involvement in neuronal hyperexcitability associated with central neuropathic pain after spinal cord injury.  J Neurosci, 23, 8881–8892. [PMC free article: PMC6740400] [PubMed: 14523090]
30. Hammer, M. F., Wagnon, J. L., Mefford, H. C. & Meisler, M. H.  2016. SCN8A-Related Epilepsy with Encephalopathy.  In:  Adam, M. P., Ardinger, H. H., Pagon, R. A., Wallace, S. E., Bean, L. J. H., Stephens, K. & Amemiya, A. (eds.) GeneReviews((R)). Seattle (WA).
31. Hawkins, N. A., Martin, M. S., Frankel, W. N., Kearney, J. A. & Escayg, A.  2011. Neuronal voltage-gated ion channels are genetic modifiers of generalized epilepsy with febrile seizures plus.  Neurobiol Dis, 41, 655–660. [PMC free article: PMC3035952] [PubMed: 21156207]
32. Huang, J., Vanoye, C. G., Cutts, A., Goldberg, Y. P., Dib-Hajj, S. D., Cohen, C. J., Waxman, S. G. & George, A. L., Jr. 2017. Sodium channel NaV1.9 mutations associated with insensitivity to pain dampen neuronal excitability.  J Clin Invest, 127, 2805–2814. [PMC free article: PMC5490760] [PubMed: 28530638]
33. Inglis, G. A. S., Wong, J. C., Butler, K. M., Thelin, J. T., Mistretta, O. C., Wu, X., Lin, X., English, A. W. & Escayg, A.  2020. Mutations in the Scn8a DIIS4 voltage sensor reveal new distinctions among hypomorphic and null Nav 1.6 sodium channels.  Genes Brain Behav, 19, e12612. [PubMed: 31605437]
34. Johannesen, K. M., Gardella, E., Encinas, A. C., Lehesjoki, A. E., Linnankivi, T., Petersen, M. B., Lund, I. C. B., Blichfeldt, S., Miranda, M. J., Pal, D. K., Lascelles, K., Procopis, P., Orsini, A., Bonuccelli, A., Giacomini, T., Helbig, I., Fenger, C. D., Sisodiya, S. M., Hernandez-Hernandez, L., Krithika, S., Rumple, M., Masnada, S., Valente, M., Cereda, C., Giordano, L., Accorsi, P., Burki, S. E., Mancardi, M., Korff, C., Guerrini, R., Von Spiczak, S., Hoffman-Zacharska, D., Mazurczak, T., Coppola, A., Buono, S., Vecchi, M., Hammer, M. F., Varesio, C., Veggiotti, P., Lal, D., Brunger, T., Zara, F., Striano, P., Rubboli, G. & Moller, R. S.  2019. The spectrum of intermediate SCN8A-related epilepsy.  Epilepsia, 60, 830–844. [PubMed: 30968951]
35. Johnson, J. P., Focken, T., Khakh, K., Tari, P. K., Dube, C., Goodchild, S. J., Andrez, J. C., Bankar, G., Bogucki, D., Burford, K., Chang, E., Chowdhury, S., Dean, R., De Boer, G., Decker, S., Dehnhardt, C., Feng, M., Gong, W., Grimwood, M., Hasan, A., Hussainkhel, A., Jia, Q., Lee, S., Li, J., Lin, S., Lindgren, A., Lofstrand, V., Mezeyova, J., Namdari, R., Nelkenbrecher, K., Shuart, N. G., Sojo, L., Sun, S., Taron, M., Waldbrook, M., Weeratunge, D., Wesolowski, S., Williams, A., Wilson, M., Xie, Z., Yoo, R., Young, C., Zenova, A., Zhang, W., Cutts, A. J., Sherrington, R. P., Pimstone, S. N., Winquist, R., Cohen, C. J. & Empfield, J. R.  2022. NBI-921352, a first-in-class, NaV1.6 selective, sodium channel inhibitor that prevents seizures in Scn8a gain-of-function mice, and wild-type mice and rats.  Elife, 11. [PMC free article: PMC8903829] [PubMed: 35234610]
36. Jurkat-Rott, K., Holzherr, B., Fauler, M. & Lehmann-Horn, F.  2010. Sodium channelopathies of skeletal muscle result from gain or loss of function.  Pflugers Arch, 460, 239–248. [PMC free article: PMC2883924] [PubMed: 20237798]
37. Lampl, I., Schwindt, P. & Crill, W.  1998. Reduction of cortical pyramidal neuron excitability by the action of phenytoin on persistent Na+ current.  J Pharmacol Exp Ther, 284, 228–237. [PubMed: 9435183]
38. Liu, Y., Schubert, J., Sonnenberg, L., Helbig, K. L., Hoei-Hansen, C. E., Koko, M., Rannap, M., Lauxmann, S., Huq, M., Schneider, M. C., Johannesen, K. M., Kurlemann, G., Gardella, E., Becker, F., Weber, Y. G., Benda, J., Moller, R. S. & Lerche, H.  2019. Neuronal mechanisms of mutations in SCN8A causing epilepsy or intellectual disability.  Brain, 142, 376–390. [PubMed: 30615093]
39. Mantegazza, M. & Broccoli, V.  2019. SCN1A/NaV 1.1 channelopathies: Mechanisms in expression systems, animal models, and human iPSC models.  Epilepsia, 60 Suppl 3, S25–S38. [PubMed: 31904127]
40. Martin, M. S., Tang, B., Papale, L. A., Yu, F. H., Catterall, W. A. & Escayg, A.  2007. The voltage-gated sodium channel Scn8a is a genetic modifier of severe myoclonic epilepsy of infancy.  Hum Mol Genet, 16, 2892–2899. [PubMed: 17881658]
41. Mason, E. R., Wu, F., Patel, R. R., Xiao, Y., Cannon, S. C. & Cummins, T. R.  2019. Resurgent and gating pore currents induced by de novo SCN2A epilepsy mutations.  eNeuro, 6. [PMC free article: PMC6795554] [PubMed: 31558572]
42. Mccormack, K., Santos, S., Chapman, M. L., Krafte, D. S., Marron, B. E., West, C. W., Krambis, M. J., Antonio, B. M., Zellmer, S. G., Printzenhoff, D., Padilla, K. M., Lin, Z., Wagoner, P. K., Swain, N. A., Stupple, P. A., De Groot, M., Butt, R. P. & Castle, N. A.  2013. Voltage sensor interaction site for selective small molecule inhibitors of voltage-gated sodium channels.  Proc Natl Acad Sci U S A, 110, E2724–E2732. [PMC free article: PMC3718154] [PubMed: 23818614]
43. Meisler, M. H.  2019. SCN8A encephalopathy: Mechanisms and models.  Epilepsia, 60 Suppl 3, S86–S91. [PMC free article: PMC6953611] [PubMed: 31904118]
44. Neurocrine. 2019. Neurocrine biosciences and Xenon Pharmaceuticals announce agreement to develop first-in-class treatments for epilepsy.  Press Release.
45. Neurocrine. 2021. Study to evaluate NBI-921352 as adjunctive therapy in Subjects With SCN8A Developmental and Epileptic Encephalopathy Syndrome (SCN8A-DEE).  ClinicalTrials.gov
46. Pan, Y. & Cummins, T. R.  2020. Distinct functional alterations in SCN8A epilepsy mutant channels.  J Physiol, 598, 381–401. [PMC free article: PMC7216308] [PubMed: 31715021]
47. Potet, F., Egecioglu, D. E., Burridge, P. W. & George, A. L., Jr. 2020. GS-967 and eleclazine block sodium channels in human induced pluripotent stem cell-derived cardiomyocytes.  Mol Pharmacol, 98, 540–547. [PubMed: 32938719]
48. Raman, I. M. & Bean, B. P.  1997. Resurgent sodium current and action potential formation in dissociated cerebellar Purkinje neurons.  J Neurosci, 17, 4517–4526. [PMC free article: PMC6573347] [PubMed: 9169512]
49. Raman, I. M., Sprunger, L. K., Meisler, M. H. & Bean, B. P.  1997. Altered subthreshold sodium currents and disrupted firing patterns in Purkinje neurons of Scn8a mutant mice.  Neuron, 19, 881–891. [PubMed: 9354334]
50. Roden, D. M., George, A. L., Jr. & Bennett, P. B.  1995. Recent advances in understanding the molecular mechanisms of the long QT syndrome.  J Cardiovasc Electrophysiol, 6, 1023–1031. [PubMed: 8589871]
51. Royeck, M., Horstmann, M. T., Remy, S., Reitze, M., Yaari, Y. & Beck, H.  2008. Role of axonal NaV1.6 sodium channels in action potential initiation of CA1 pyramidal neurons.  J Neurophysiol, 100, 2361–2380. [PubMed: 18650312]
52. Samad, O. A., Tan, A. M., Cheng, X., Foster, E., Dib-Hajj, S. D. & Waxman, S. G.  2013. Virus-mediated shRNA knockdown of Na(v)1.3 in rat dorsal root ganglion attenuates nerve injury-induced neuropathic pain.  Mol Ther, 21, 49–56. [PMC free article: PMC3538302] [PubMed: 22910296]
53. Sanders, S. J., Campbell, A. J., Cottrell, J. R., Moller, R. S., Wagner, F. F., Auldridge, A. L., Bernier, R. A., Catterall, W. A., Chung, W. K., Empfield, J. R., George, A. L., Jr., Hipp, J. F., Khwaja, O., Kiskinis, E., Lal, D., Malhotra, D., Millichap, J. J., Otis, T. S., Petrou, S., Pitt, G., Schust, L. F., Taylor, C. M., Tjernagel, J., Spiro, J. E. & Bender, K. J.  2018. Progress in understanding and treating SCN2A-mediated disorders.  Trends Neurosci, 41, 442–456. [PMC free article: PMC6015533] [PubMed: 29691040]
54. Sasaki, R., Takano, H., Kamakura, K., Kaida, K., Hirata, A., Saito, M., Tanaka, H., Kuzuhara, S. & Tsuji, S.  1999. A novel mutation in the gene for the adult skeletal muscle sodium channel alpha-subunit (SCN4A) that causes paramyotonia congenita of von Eulenburg.  Arch Neurol, 56, 692–696. [PubMed: 10369308]
55. Schwindt, P. & Crill, W.  1980. Role of a persistent inward current in motoneuron bursting during spinal seizures.  J Neurophysiol, 43, 1296–1318. [PubMed: 6246221]
56. Sole, L. & Tamkun, M. M.  2020. Trafficking mechanisms underlying Nav channel subcellular localization in neurons.  Channels (Austin), 14, 1–17. [PMC free article: PMC7039628] [PubMed: 31841065]
57. Spratt, P. W. E., Ben-Shalom, R., Keeshen, C. M., Burke, K. J., Jr., Clarkson, R. L., Sanders, S. J. & Bender, K. J.  2019. The autism-associated gene Scn2a contributes to dendritic excitability and synaptic function in the prefrontal cortex.  Neuron, 103, 673–685. [PMC free article: PMC7935470] [PubMed: 31230762]
58. Strichartz, G.  1976. Molecular mechanisms of nerve block by local anesthetics.  Anesthesiology, 45, 421–441. [PubMed: 9844]
59. Thompson, C. H., Ben-Shalom, R., Bender, K. J. & George, A. L.  2020. Alternative splicing potentiates dysfunction of early-onset epileptic encephalopathy SCN2A variants.  J Gen Physiol, 152. [PMC free article: PMC7054859] [PubMed: 31995133]
60. Tidball, A. M., Lopez-Santiago, L. F., Yuan, Y., Glenn, T. W., Margolis, J. L., Clayton Walker, J., Kilbane, E. G., Miller, C. A., Martina Bebin, E., Scott Perry, M., Isom, L. L. & Parent, J. M.  2020. Variant-specific changes in persistent or resurgent sodium current in SCN8A-related epilepsy patient-derived neurons.  Brain, 143, 3025–3040. [PMC free article: PMC7780473] [PubMed: 32968789]
61. Veeramah, K. R., O’brien, J. E., Meisler, M. H., Cheng, X., Dib-Hajj, S. D., Waxman, S. G., Talwar, D., Girirajan, S., Eichler, E. E., Restifo, L. L., Erickson, R. P. & Hammer, M. F.  2012. De novo pathogenic SCN8A mutation identified by whole-genome sequencing of a family quartet affected by infantile epileptic encephalopathy and SUDEP.  Am J Hum Genet, 90, 502–510. [PMC free article: PMC3309181] [PubMed: 22365152]
62. Wagnon, J. L., Korn, M. J., Parent, R., Tarpey, T. A., Jones, J. M., Hammer, M. F., Murphy, G. G., Parent, J. M. & Meisler, M. H.  2015. Convulsive seizures and SUDEP in a mouse model of SCN8A epileptic encephalopathy.  Hum Mol Genet, 24, 506–515. [PMC free article: PMC4275076] [PubMed: 25227913]
63. Wengert, E. R., Tronhjem, C. E., Wagnon, J. L., Johannesen, K. M., Petit, H., Krey, I., Saga, A. U., Panchal, P. S., Strohm, S. M., Lange, J., Kamphausen, S. B., Rubboli, G., Lemke, J. R., Gardella, E., Patel, M. K., Meisler, M. H. & Moller, R. S.  2019. Biallelic inherited SCN8A variants, a rare cause of SCN8A-related developmental and epileptic encephalopathy.  Epilepsia, 60, 2277–2285. [PMC free article: PMC6842408] [PubMed: 31625145]
64. Yin, R., Liu, D., Chhoa, M., Li, C. M., Luo, Y., Zhang, M., Lehto, S. G., Immke, D. C. & Moyer, B. D.  2016. Voltage-gated sodium channel function and expression in injured and uninjured rat dorsal root ganglia neurons.  Int J Neurosci, 126, 182–192. [PubMed: 25562420]
65. Zaman, T., Abou Tayoun, A. & Goldberg, E. M.  2019. A single-center SCN8A-related epilepsy cohort: clinical, genetic, and physiologic characterization.  Ann Clin Transl Neurol, 6, 1445–1455. [PMC free article: PMC6689675] [PubMed: 31402610]