Evaluation of antiseizure drug efficacy and tolerability in the rat lamotrigine-resistant amygdala kindling model

Targeting Kv7 Potassium Channels for Epilepsy

Voltage-gated Kv7 potassium channels, particularly Kv7.2 and Kv.7.3 channels, play a critical role in modulating susceptibility to seizures, and mutations in genes that encode these channels cause heterogeneous epilepsy phenotypes. On the basis of this evidence, activation of Kv7.2 and Kv.7.3 channels has long been considered an attractive target in the search for novel antiseizure medications. Ezogabine (retigabine), the first Kv7.2/3 activator introduced in 2011 for the treatment of focal seizures, was withdrawn from the market in 2017 due to declining use after discovery of its association with pigmentation changes in the retina, skin, and mucosae. A novel formulation of ezogabine for pediatric use (XEN496) has been recently investigated in children with KCNQ2-related developmental and epileptic encephalopathy, but the trial was terminated prematurely for reasons unrelated to safety. Among novel Kv7.2/3 openers in clinical development, azetukalner has shown dose-dependent efficacy against drug-resistant focal seizures with a good tolerability profile and no evidence of pigmentation-related adverse effects in early clinical studies, and it is now under investigation in phase III trials for the treatment of focal seizures, generalized tonic-clonic seizures, and major depressive disorder. Another Kv7.2/3 activator, BHV-7000, has completed phase I studies in healthy subjects, with excellent tolerability at plasma drug concentrations that exceed the median effective concentrations in a preclinical model of anticonvulsant activity, but no efficacy data in patients with epilepsy are available to date. Among other Kv7.2/3 activators in clinical development as potential antiseizure medications, pynegabine and CB-003 have completed phase I safety and pharmacokinetic studies, but results have not been yet reported. Overall, interest in targeting Kv7 channels for the treatment of epilepsy and for other indications remains strong. Future breakthroughs in this area could come from exploitation of mechanistic differences in the action of Kv7 activators, and from the development of molecules that combine Kv7 activation with other mechanisms of action.

Enhancement of Glutamate Uptake as Novel Antiseizure Approach: Preclinical Proof of Concept

OBJECTIVE

Excitotoxicity is a common hallmark of epilepsy and other neurological diseases associated with elevated extracellular glutamate levels. Thus, here, we studied the protective effects of (R)-AS-1, a positive allosteric modulator (PAM) of glutamate uptake in epilepsy models.

METHODS

(R)-AS-1 was evaluated in a range of acute and chronic seizure models, while its adverse effect profile was assessed in a panel of standard tests in rodents. The effect of (R)-AS-1 on glutamate uptake was assessed in COS-7 cells expressing the transporter. WAY 213613, a selective competitive EAAT2 inhibitor, was used to probe the reversal of the enhanced glutamate uptake in the same transporter expression system. Confocal microscopy and Western blotting analyses were used to study a potential influence of (R)-AS-1 on GLT-1 expression in mice.

RESULTS

(R)-AS-1 showed robust protection in a panel of animal models of seizures and epilepsy, including the maximal electroshock- and 6 Hz-induced seizures, corneal kindling, mesial temporal lobe epilepsy, lamotrigine-resistant amygdala kindling, as well as seizures induced by pilocarpine or Theiler's murine encephalomyelitis virus. Importantly, (R)-AS-1 displayed a favorable adverse effect profile in the rotarod, the minimal motor impairment, and the Irwin tests. (R)-AS-1 enhanced glutamate uptake in vitro and this effect was abolished by WAY 213613, while no influence on GLT-1 expression in vivo was observed after repeated treatment.

INTERPRETATION

Collectively, our results show that (R)-AS-1 has favorable tolerability and provides robust preclinical efficacy against seizures. Thus, allosteric enhancement of EAAT2 function could offer a novel therapeutic strategy for treatment of epilepsy and potentially other neurological disorders associated with glutamate excitotoxicity. ANN NEUROL 2025;97:344-357.

Assessing the Effectiveness of Eslicarbazepine Acetate in Reducing Audiogenic Reflex Seizures in the GASH/Sal Model of Epilepsy

Eslicarbazepine acetate (ESL) is a third-generation antiepileptic drug indicated as monotherapy for adults with newly diagnosed epilepsy and as adjunctive therapy for the treatment of partial seizures. Our aim was to assess the effectiveness and safety of both acute and repeated ESL administration against reflex audiogenic seizures, as shown by the Genetic Audiogenic Seizures Hamster from Salamanca (GASH/Sal). Animals were subject to the intraperitoneal administration of ESL, applying doses of 100, 150 and 200 mg/kg for the acute study, whereas a daily dose of 100 mg/kg was selected for the subchronic study, which lasted 14 days. In both studies, the anticonvulsant effect of the therapy was evaluated using neuroethological methods. To assess the safety of the treatment, behavioral tests were performed, hematological and biochemical liver profiles were obtained, and body weight was monitored. In addition, the ESL levels in blood were measured after the acute administration of a 200 mg/kg dose. Treatment with ESL caused a reduction in seizure severity. No statistically significant differences were detected between the selected doses or between the acute or repeated administration of the drug. To summarize, the intraperitoneal administration of ESL is safe and shows an anticonvulsant effect in the GASH/Sal.

Is there a biochemical basis for purinergic P2X3 and P2X4 receptor antagonists to be considered as anti-seizure medications?

Patients with epilepsy require improved medications. Purinergic receptors were identified as late as 1976 and are slowly emerging as potential drug targets for the discovery of antiseizure medications. While compounds interacting with these receptors have been approved for use as medicines (e.g., gefapixant for cough) and continue to be explored for a number of diseases (e.g., pain, cancer), there have been no purinergic receptor antagonists that have been advanced for epilepsy. There are very few studies on the channel conducting receptors, P2X3 and P2X4, that suggest their possible role in seizure generation or control. However, the limited data available provides some compelling reasons to believe that they could be valuable antiseizure medication drug targets. The data implicating P2X3 and P2X4 receptors in epilepsy includes the role played by ATP in neuronal excitability and seizures, receptor localization, increased receptor expression in epileptic brain, the involvement of these receptors in seizure-associated inflammation, crosstalk between these purinergic receptors and neuronal processes involved in seizures (GABAergic and glutamatergic neurotransmission), and the significant attenuation of seizures and seizure-like activity with P2X receptor blockade. The discovery of new and selective antagonists for P2X3 and P2X4 receptors is ongoing, armed with new structural data to guide rational design. The availability of safe, brain-penetrant compounds will likely encourage the clinical exploration of epilepsy as a disease entity.

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.

Revisiting the concept of drug-resistant epilepsy: A TASK1 report of the ILAE/AES Joint Translational Task Force

Despite progress in the development of anti-seizure medications (ASMs), one third of people with epilepsy have drug-resistant epilepsy (DRE). The working definition of DRE, proposed by the International League Against Epilepsy (ILAE) in 2010, helped identify individuals who might benefit from presurgical evaluation early on. As the incidence of DRE remains high, the TASK1 workgroup on DRE of the ILAE/American Epilepsy Society (AES) Joint Translational Task Force discussed the heterogeneity and complexity of its presentation and mechanisms, the confounders in drawing mechanistic insights when testing treatment responses, and barriers in modeling DRE across the lifespan and translating across species. We propose that it is necessary to revisit the current definition of DRE, in order to transform the preclinical and clinical research of mechanisms and biomarkers, to identify novel, effective, precise, pharmacologic treatments, allowing for earlier recognition of drug resistance and individualized therapies.

Animal Models of Drug-Resistant Epilepsy as Tools for Deciphering the Cellular and Molecular Mechanisms of Pharmacoresistance and Discovering More Effective Treatments

In the last 30 years, over 20 new anti-seizure medicines (ASMs) have been introduced into the market for the treatment of epilepsy using well-established preclinical seizure and epilepsy models. Despite this success, approximately 20-30% of patients with epilepsy have drug-resistant epilepsy (DRE). The current approach to ASM discovery for DRE relies largely on drug testing in various preclinical model systems that display varying degrees of ASM drug resistance. In recent years, attempts have been made to include more etiologically relevant models in the preclinical evaluation of a new investigational drug. Such models have played an important role in advancing a greater understanding of DRE at a mechanistic level and for hypothesis testing as new experimental evidence becomes available. This review provides a critical discussion of the pharmacology of models of adult focal epilepsy that allow for the selection of ASM responders and nonresponders and those models that display a pharmacoresistance per se to two or more ASMs. In addition, the pharmacology of animal models of major genetic epilepsies is discussed. Importantly, in addition to testing chemical compounds, several of the models discussed here can be used to evaluate other potential therapies for epilepsy such as neurostimulation, dietary treatments, gene therapy, or cell transplantation. This review also discusses the challenges associated with identifying novel therapies in the absence of a greater understanding of the mechanisms that contribute to DRE. Finally, this review discusses the lessons learned from the profile of the recently approved highly efficacious and broad-spectrum ASM cenobamate.

miR-9a-5p expression is decreased in the hippocampus of rats resistant to lamotrigine: A behavioural, molecular and bioinformatics assessment

The major obstacle in developing new treatment strategies for refractory epilepsy is the complexity and poor understanding of its mechanisms. Utilizing the model of lamotrigine-resistant seizures, we evaluated whether changes in the expression of sodium channel subunits are responsible for the diminished responsiveness to lamotrigine (LTG) and if miRNAs, may also be associated. Male rats were administered LTG (5 mg/kg) before each stimulation during kindling acquisition. Challenge stimulation following LTG exposure (30 mg/kg) was performed to confirm resistance in fully kindled rats. RT-PCR was used to measure the mRNA levels of sodium channel subunits (SCN1A, SCN2A, and SCN3A) and miRNAs (miR-155-5p, miR-30b-5p, miR-137-3p, miR-342-5p, miR-301a-3p, miR-212-3p, miR-9a-5p, and miR-133a-3p). Western blot analysis was utilized to measure Nav1.2 protein, and bioinformatics tools were used to perform target prediction and enrichment analysis for miR-9a-5p, the only affected miRNA according to the responsiveness to LTG. Amygdala kindling seizures downregulated Nav1.2, miR-137-3p, miR-342-5p, miR-155-5p, and miR-9a-5p as well as upregulated miR-212-3p. miR-9a-5p was the only molecule decreased in rats resistant to LTG. The bioinformatic assessment and disease enrichment analysis revealed that miR-9a-5p targets expressed with high confidence in the hippocampus are the most significantly associated with epilepsy. Due to the miR-9a-5p dysregulation, major pathways affected are neurotrophic processes, neurotransmission, inflammatory response, cell proliferation and apoptosis. Interaction network analysis identified LTG target SCN2A as interacting with highest number of genes regulated by miR-9-5p. Further studies are needed to propose specific genes and miRNAs responsible for diminished responsiveness to LTG. miR-9a-5p targets, like KCNA4, KCNA2, CACNB2, SCN4B, KCNC1, should receive special attention in them.

Frontline Sodium Channel-Blocking Antiseizure Medicine Use Promotes Future Onset of Drug-Resistant Chronic Seizures

The mechanisms of treatment-resistant epilepsy remain unclear. We have previously shown that frontline administration of therapeutic doses of lamotrigine (LTG), which preferentially inhibits the fast-inactivation state of sodium channels, during corneal kindling of mice promotes cross-resistance to several other antiseizure medicines (ASMs). However, whether this phenomenon extends to monotherapy with ASMs that stabilize the slow inactivation state of sodium channels is unknown. Therefore, this study assessed whether lacosamide (LCM) monotherapy during corneal kindling would promote future development of drug-resistant focal seizures in mice. Male CF-1 mice (n = 40/group; 18-25 g) were administered an anticonvulsant dose of LCM (4.5 mg/kg, i.p.), LTG (8.5 mg/kg, i.p.), or vehicle (0.5% methylcellulose) twice daily for two weeks during kindling. A subset of mice (n = 10/group) were euthanized one day after kindling for immunohistochemical assessment of astrogliosis, neurogenesis, and neuropathology. The dose-related antiseizure efficacy of distinct ASMs, including LTG, LCM, carbamazepine, levetiracetam, gabapentin, perampanel, valproic acid, phenobarbital, and topiramate, was then assessed in the remaining kindled mice. Neither LCM nor LTG administration prevented kindling: 29/39 vehicle-exposed mice were kindled; 33/40 LTG-exposed mice were kindled; and 31/40 LCM-exposed mice were kindled. Mice administered LCM or LTG during kindling became resistant to escalating doses of LCM, LTG, and carbamazepine. Perampanel, valproic acid, and phenobarbital were less potent in LTG- and LCM-kindled mice, whereas levetiracetam and gabapentin retained equivalent potency across groups. Notable differences in reactive gliosis and neurogenesis were also appreciated. This study indicates that early, repeated administration of sodium channel-blocking ASMs, regardless of inactivation state preference, promotes pharmacoresistant chronic seizures. Inappropriate ASM monotherapy in newly diagnosed epilepsy may thus be one driver of future drug resistance, with resistance being highly ASM class specific.

Eslicarbazepine, but Not Lamotrigine or Ranolazine, Shows Anticonvulsant Efficacy in Carbamazepine-Resistant Rats Developed by Window-Pentylenetetrazole Kindling

Approximately 30% of epileptic patients develop Drug-Resistant Epilepsy. Based on evidence that shows a loss of efficacy in some sodium channel blocker antiseizure drugs in epilepsy, we focus our study on assessing the anticonvulsant efficacy of different sodium channel blockers on carbamazepine (CBZ)-resistant seizures generated using the window-pentylenetetrazole (PTZ) kindling model to verify whether one of these drugs presents some anticonvulsant effect that could have potential therapeutic use. Wistar rats were treated with a subthreshold dose of PTZ (35 mg/kg) three times/week. Fully kindled rats were then treated with a single dose of CBZ (40 mg/kg i.p.) at 2, 9 and 16 days after their last kindling stimulation to obtain CBZ-resistant rats. Right after, sodium channel blockers were tested for anticonvulsant action (lamotrigine, 30 mg/kg i.p.; eslicarbazepine, 150 or 300 mg/kg i.p.; ranolazine, 10, 20 or 40 mg/kg i.p.). Behavioral parameters included severity, latency or duration of convulsions. Our data showed for the first time directly that eslicarbazepine does have an anticonvulsant effect over CBZ-resistant seizures, while lamotrigine shows drug resistance and ranolazine demonstrates severe seizure worsening. It is of potential therapeutic relevance that eslicarbazepine could be useful to control seizures resistant to common sodium channel blockers such as CBZ.

Epileptic seizures and link to memory processes

Epileptogenesis is a complex and not well understood phenomenon. Here, we explore the hypothesis that epileptogenesis could be "hijacking" normal memory processes, and how this hypothesis may provide new directions for epilepsy treatment. First, we review similarities between the hypersynchronous circuits observed in epilepsy and memory consolidation processes involved in strengthening neuronal connections. Next, we describe the kindling model of seizures and its relation to long-term potentiation model of synaptic plasticity. We also examine how the strengthening of epileptic circuits is facilitated during the physiological slow wave sleep, similarly as episodic memories. Furthermore, we present studies showing that specific memories can directly trigger reflex seizures. The neuronal hypersynchrony in early stages of Alzheimer's disease, and the use of anti-epileptic drugs to improve the cognitive symptoms in this disease also suggests a connection between memory systems and epilepsy. Given the commonalities between memory processes and epilepsy, we propose that therapies for memory disorders might provide new avenues for treatment of epileptic patients.

Spontaneous recurrent seizures in an intra-amygdala kainate microinjection model of temporal lobe epilepsy are differentially sensitive to antiseizure drugs

The discovery and development of novel antiseizure drugs (ASDs) that are effective in controlling pharmacoresistant spontaneous recurrent seizures (SRSs) continues to represent a significant unmet clinical need. The Epilepsy Therapy Screening Program (ETSP) has undertaken efforts to address this need by adopting animal models that represent the salient features of human pharmacoresistant epilepsy and employing these models for preclinical testing of investigational ASDs. One such model that has garnered increased interest in recent years is the mouse variant of the Intra-Amygdala Kainate (IAK) microinjection model of mesial temporal lobe epilepsy (MTLE). In establishing a version of this model, several methodological variables were evaluated for their effect(s) on pertinent quantitative endpoints. Although administration of a benzodiazepine 40 min after kainate (KA) induced status epilepticus (SE) is commonly used to improve survival, data presented here demonstrates similar outcomes (mortality, hippocampal damage, latency periods, and 90-day SRS natural history) between mice given midazolam and those that were not. Using a version of this model that did not interrupt SE with a benzodiazepine, a 90-day natural history study was performed and survival, latency periods, SRS frequencies and durations, and SRS clustering data were quantified. Finally, an important step towards model adoption is to assess the sensitivities or resistances of SRSs to a panel of approved and clinically used ASDs. Accordingly, the following ASDs were evaluated for their effects on SRSs in these mice: phenytoin (20 mg/kg, b.i.d.), carbamazepine (30 mg/kg, t.i.d.), valproate (240 mg/kg, t.i.d.), diazepam (4 mg/kg, b.i.d.), and phenobarbital (25 and 50 mg/kg, b.i.d.). Valproate, diazepam, and phenobarbital significantly attenuated SRS frequency relative to vehicle controls at doses devoid of observable adverse behavioral effects. Only diazepam significantly increased seizure freedom. Neither phenytoin nor carbamazepine significantly altered SRS frequency or freedom under these experimental conditions. These data demonstrate that SRSs in this IAK model of MTLE are pharmacoresistant to two representative sodium channel-inhibiting ASDs (phenytoin and carbamazepine) and partially sensitive to GABA receptor modulating ASDs (diazepam and phenobarbital) or a mixed-mechanism ASD (valproate). Accordingly, this model is being incorporated into the NINDS-funded ETSP testing platform for treatment resistant epilepsy.

Pharmacoresponsiveness of spontaneous recurrent seizures and the comorbid sleep disorder of epileptic Kcna1-null mice

Drug resistant epilepsy affects ∼30% of people with epilepsy and is associated with epilepsy syndromes with frequent and multiple types of seizures, lesions or cytoarchitectural abnormalities, increased risk of mortality and comorbidities such as cognitive impairment and sleep disorders. A limitation of current preclinical models is that spontaneous seizures with comorbidities take time to induce and test, thus making them low-throughput. Kcna1-null mice exhibit all the characteristics of drug resistant epilepsy with spontaneous seizures and comorbidities occurring naturally; thus, we aimed to determine whether they also demonstrate pharmacoresistanct seizures and the impact of medications on their sleep disorder comorbidity. In this exploratory study, Kcna1-null mice were treated with one of four conventional antiseizure medications, carbamazepine, levetiracetam, phenytoin, and phenobarbital using a moderate throughput protocol (vehicle for 2 days followed by 2 days of treatment with high therapeutic doses selected based on published data in the 6 Hz model of pharmacoresistant seizures). Spontaneous recurrent seizures and vigilance states were recorded with video-EEG/EMG. Carbamazepine, levetiracetam and phenytoin had partial efficacy (67%, 75% and 33% were seizure free, respectively), whereas phenobarbital was fully efficacious and conferred seizure freedom to all mice. Thus, seizures of Kcna1-null mice appear to be resistant to three of the drugs tested. Levetiracetam failed to affect sleep architecture, carbamazepine and phenytoin had moderate effects, and phenobarbital, as predicted, restored sleep architecture. Data suggest Kcna1-null mice may be a moderate throughput model of drug resistant epilepsy useful in determining mechanisms of pharmacoresistance and testing novel therapeutic strategies.

MicroRNAs in the development of resistance to antiseizure drugs and their potential as biomarkers in pharmacoresistant epilepsy

Although many new antiseizure drugs have been developed in the past decade, approximately 30%-40% of patients remain pharmacoresistant. There are no clinical tools or guidelines for predicting therapeutic response in individual patients, leaving them no choice other than to try all antiseizure drugs available as they suffer debilitating seizures with no relief. The discovery of predictive biomarkers and early identification of pharmacoresistant patients is of the highest priority in this group. MicroRNAs (miRNAs), a class of short noncoding RNAs negatively regulating gene expression, have emerged in recent years in epilepsy, following a broader trend of their exploitation as biomarkers of various complex human diseases. We performed a systematic search of the PubMed database for original research articles focused on miRNA expression level profiling in patients with drug-resistant epilepsy or drug-resistant precilinical models and cell cultures. In this review, we summarize 17 publications concerning miRNAs as potential new biomarkers of resistance to antiseizure drugs and their potential role in the development of drug resistance or epilepsy. Although numerous knowledge gaps need to be filled and reviewed, and articles share some study design pitfalls, several miRNAs dysregulated in brain tissue and blood serum were identified independently by more than one paper. These results suggest a unique opportunity for disease monitoring and personalized therapeutic management in the future.

The ups and downs of alkyl-carbamates in epilepsy therapy: How does cenobamate differ?

Since 1955, several alkyl-carbamates have been developed for the treatment of anxiety and epilepsy, including meprobamate, flupirtine, felbamate, retigabine, carisbamate, and cenobamate. They have each enjoyed varying levels of success as antiseizure drugs; however, they have all been plagued by the emergence of serious and sometimes life-threatening adverse events. In this review, we compare and contrast their predominant molecular mechanisms of action, their antiseizure profile, and where possible, their clinical efficacy. The preclinical, clinical, and mechanistic profile of the prototypical γ-aminobutyric acidergic (GABAergic) modulator phenobarbital is included for comparison. Like phenobarbital, all of the clinically approved alkyl-carbamates share an ability to enhance inhibitory neurotransmission through modulation of the GABA_A receptor, although the specific mechanism of interaction differs among the different drugs discussed. In addition, several alkyl-carbamates have been shown to interact with voltage-gated ion channels. Flupirtine and retigabine share an ability to activate K⁺ currents mediated by KCNQ (Kv7) K⁺ channels, and felbamate, carisbamate, and cenobamate have been shown to block Na⁺ channels. In contrast to other alkyl-carbamates, cenobamate seems to be unique in its ability to preferentially attenuate the persistent rather than transient Na⁺ current. Results from recent randomized controlled clinical trials with cenobamate suggest that this newest antiseizure alkyl-carbamate possesses a degree of efficacy not witnessed since felbamate was approved in 1993. Given that ceno-bamate's mechanistic profile is unique among the alkyl-carbamates, it is not clear whether this impressive efficacy reflects an as yet undescribed mechanism of action or whether it possesses a unique synergy between its actions at the GABA_A receptor and on persistent Na⁺ currents. The high efficacy of cenobamate is, however, tempered by the risk of serious rash and low tolerability at higher doses, meaning that further safety studies and clinical experience are needed to determine the true clinical value of cenobamate.

Drug Resistance in Epilepsy: Clinical Impact, Potential Mechanisms, and New Innovative Treatment Options

Epilepsy is a chronic neurologic disorder that affects over 70 million people worldwide. Despite the availability of over 20 antiseizure drugs (ASDs) for symptomatic treatment of epileptic seizures, about one-third of patients with epilepsy have seizures refractory to pharmacotherapy. Patients with such drug-resistant epilepsy (DRE) have increased risks of premature death, injuries, psychosocial dysfunction, and a reduced quality of life, so development of more effective therapies is an urgent clinical need. However, the various types of epilepsy and seizures and the complex temporal patterns of refractoriness complicate the issue. Furthermore, the underlying mechanisms of DRE are not fully understood, though recent work has begun to shape our understanding more clearly. Experimental models of DRE offer opportunities to discover, characterize, and challenge putative mechanisms of drug resistance. Furthermore, such preclinical models are important in developing therapies that may overcome drug resistance. Here, we will review the current understanding of the molecular, genetic, and structural mechanisms of ASD resistance and discuss how to overcome this problem. Encouragingly, better elucidation of the pathophysiological mechanisms underpinning epilepsies and drug resistance by concerted preclinical and clinical efforts have recently enabled a revised approach to the development of more promising therapies, including numerous potential etiology-specific drugs ("precision medicine") for severe pediatric (monogenetic) epilepsies and novel multitargeted ASDs for acquired partial epilepsies, suggesting that the long hoped-for breakthrough in therapy for as-yet ASD-resistant patients is a feasible goal. SIGNIFICANCE STATEMENT: Drug resistance provides a major challenge in epilepsy management. Here, we will review the current understanding of the molecular, genetic, and structural mechanisms of drug resistance in epilepsy and discuss how the problem might be overcome.

Evaluation of subchronic administration of antiseizure drugs in spontaneously seizing rats

OBJECTIVE

Approximately 30% of patients with epilepsy do not experience full seizure control on their antiseizure drug (ASD) regimen. Historically, screening for novel ASDs has relied on evaluating efficacy following a single administration of a test compound in either acute electrical or chemical seizure induction. However, the use of animal models of spontaneous seizures and repeated administration of test compounds may better differentiate novel compounds. Therefore, this approach has been instituted as part of the National Institute of Neurological Disorders and Stroke Epilepsy Therapy Screening Program screening paradigm for pharmacoresistant epilepsy.

METHODS

Rats were treated with intraperitoneal kainic acid to induce status epilepticus and subsequent spontaneous recurrent seizures. After 12 weeks, rats were enrolled in drug screening studies. Using a 2-week crossover design, selected ASDs were evaluated for their ability to protect against spontaneous seizures, using a video-electroencephalographic monitoring system and automated seizure detection. Sixteen clinically available compounds were administered at maximally tolerated doses in this model. Dose intervals (1-3 treatments/d) were selected based on known half-lives for each compound.

RESULTS

Carbamazepine (90 mg/kg/d), phenobarbital (30 mg/kg/d), and ezogabine (15 mg/kg/d) significantly reduced seizure burden at the doses evaluated. In addition, a dose-response study of topiramate (20-600 mg/kg/d) demonstrated that this compound reduced seizure burden at both therapeutic and supratherapeutic doses. However, none of the 16 ASDs conferred complete seizure freedom during the testing period at the doses tested.

SIGNIFICANCE

Despite reductions in seizure burden, the lack of full seizure freedom for any ASD tested suggests that this screening paradigm may be useful for testing novel compounds with potential utility in pharmacoresistant epilepsy.

The current approach of the Epilepsy Therapy Screening Program contract site for identifying improved therapies for the treatment of pharmacoresistant seizures in epilepsy

The Epilepsy Therapy Screening Program (ETSP), formerly known as the Anticonvulsant Screening Program (ASP), has played an important role in the preclinical evaluation of many of the antiseizure drugs (ASDs) that have been approved by the FDA and thus made available for the treatment of seizures. Recent changes to the animal models used at the contract site of the ETSP at the University of Utah have been implemented in an attempt to better model the unmet clinical needs of people with pharmacoresistant epilepsy and thus identify improved therapies. In this review, we describe the changes that have occurred over the last several years in the screening approach used at the contract site and, in particular, detail the pharmacology associated with several of the animal models and assays that are either new to the program or have been recently characterized in more depth. There is optimism that the refined approach used by the ETSP contract site, wherein etiologically relevant models that include those with spontaneous seizures are used, will identify novel, potentially disease modifying therapies for people with pharmacoresistant epilepsy and those at risk for developing epilepsy. 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'.

The Positive Allosteric Modulator of α2/3-Containing GABAA Receptors, KRM-II-81, Is Active in Pharmaco-Resistant Models of Epilepsy and Reduces Hyperexcitability after Traumatic Brain Injury

The imidizodiazepine, 5-(8-ethynyl-6-(pyridin-2-yl)-4H-benzo[f]imidazo[1,5-a][1,4]diazepin-3-yl)oxazole (KRM-II-81), is selective for α2/3-containing GABAA receptors. KRM-II-81 dampens seizure activity in rodent models with enhanced efficacy and reduced motor-impairment compared with diazepam. In the present study, KRM-II-81 was studied in assays designed to detect antiepileptics with improved chances of impacting pharmaco-resistant epilepsies. The potential for reducing neural hyperactivity weeks after traumatic brain injury was also studied. KRM-II-81 suppressed convulsions in corneal-kindled mice. Mice with kainate-induced mesial temporal lobe seizures exhibited spontaneous recurrent hippocampal paroxysmal discharges that were significantly reduced by KRM-II-81 (15 mg/kg, orally). KRM-II-81 also decreased convulsions in rats undergoing amygdala kindling in the presence of lamotrigine (lamotrigine-insensitive model) (ED50 = 19 mg/kg, i.p.). KRM-II-81 reduced focal and generalized seizures in a kainate-induced chronic epilepsy model in rats (20 mg/kg, i.p., three times per day). In mice with damage to the left cerebral cortex by controlled-cortical impact, enduring neuronal hyperactivity was dampened by KRM-II-81 (10 mg/kg, i.p.) as observed through in vivo two-photon imaging of layer II/III pyramidal neurons in GCaMP6-expressing transgenic mice. No notable side effects emerged up to doses of 300 mg/kg KRM-II-81. Molecular modeling studies were conducted: docking in the binding site of the α1β3γ2L GABAA receptor showed that replacing the C8 chlorine atom of alprazolam with the acetylene of KRM-II-81 led to loss of the key interaction with α1His102, providing a structural rationale for its low affinity for α1-containing GABAA receptors compared with benzodiazepines such as alprazolam. Overall, these findings predict that KRM-II-81 has improved therapeutic potential for epilepsy and post-traumatic epilepsy. SIGNIFICANCE STATEMENT: We describe the effects of a relatively new orally bioavailable small molecule in rodent models of pharmaco-resistant epilepsy and traumatic brain injury. KRM-II-81 is more potent and generally more efficacious than standard-of-care antiepileptics. In silico docking experiments begin to describe the structural basis for the relative lack of motor impairment induced by KRM-II-81. KRM-II-81 has unique structural and anticonvulsant effects, predicting its potential as an improved antiepileptic drug and novel therapy for post-traumatic epilepsy.