The impact of perampanel and targeting AMPA transmission on anti-seizure drug discovery

1. Introduction

Epilepsy is one of the most common chronic neurological disorder affecting around 70 million people worldwide. Although long-term seizure control can be achieved in most cases [1], about 30% of the patients continue to have seizures despite adequate antiepileptic therapy [2]. Notably, the burden of drug-resistant epilepsy has remained stable over the last decades and still represents a major health issue considering the association of uncontrolled epilepsy with an increased risk of morbidity, social disability, stigma, poor quality of life, and mortality. Thus, the search for antiepileptic drugs (AEDs) with novel mechanisms of action and potential broad spectrum of efficacy, alongside with high tolerability and safe profile, continues [3]. This editorial explores the discovery of perampanel (PER) and the glutamatergic excitatory neurotransmission as a promising area of research in the field of epilepsy.

2. Glutamatergic and excitatory pathways in epilepsy

2.1. Glutamate receptors and seizures

Glutamate is the main excitatory neurotransmitter in the central nervous system. Being the key mediator of the excitatory connectivity between pyramidal neurons and inter-neuronal network through the generation of excitatory post-synaptic potentials (EPSPs), glutamate is crucial in triggering, maintaining, and spreading epileptic activity [4]. Accordingly, sustained increase in extracellular glutamate concentrations has been observed in the hippocampus of patients with epilepsy before and during seizure activity [5].

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors are the main post-synaptic glutamate receptors to mediate the fast-synaptic excitatory neurotransmission and the fast component of the EPSP. Hypersensitivity, upregulation, and increased density of AMPA receptors have been demonstrated in hippocampal and neocortical tissues obtained from patients with epilepsy, and altered expression and phosphorylation of AMPA receptors have been pointed out in epileptogenesis. N-methyl-D-aspartate (NMDA) receptors need to be activated by depolarization and glutamate binding, contribute to a later and slower component of the EPSP, and are mostly involved in calcium-dependent responses underlying synaptic plasticity [6]. Finally, kainate receptors are the less frequently encountered type of ionotropic glutamate receptor. They are located at pre-synaptic level where regulate neurotransmitter release, and their role in the pathophysiology of epileptic activity has to be fully understood, yet.

2.2. Targeting the excitatory neurotransmission

The block of NMDA receptors abolished late bursting without stopping the initial seizure-like discharge in guinea pig hippocampal slices, while blocking AMPA receptors eliminated the seizure-like discharges [7]. In addition, antagonists of NMDA receptors have shown seizure protection in some in vivo seizure models, including the pentylenetetrazol (PTZ)-induced clonic seizures, maximal electroshock (MES) test, and reflex seizure models, while they were ineffective against fully kindled seizures and did not suppress or eliminate epileptiform activity in most in vitromodels [8]. Conversely, antagonists of AMPA receptors displayed a broader spectrum of anticonvulsant activity and were not associated with the troubling neurobehavioral side effects seen with NMDA antagonists [8]. The disappointing results obtained with NMDA receptor antagonists and the favorable findings observed by antagonizing AMPA pathways have prompted interest in AMPA receptors as a target for epilepsy research and selective antagonists of AMPA receptors as promising therapeutic options.

3. Targeting AMPA receptors: perampanel efficacy and pharmacological profile

3.1. Perampanel: from discovery to synergetic drug interactions

Perampanel (2-(2-oxo-1-phenyl-5-pyridin-2-yl-1,2-dihydropyridin-3-yl)benzonitrile) is a first-in-class high-selective, non-competitive antagonist of AMPA receptors. To date, PER has been approved as monotherapy for focal onset seizures, with or without secondary generalization in the USA, and as adjunctive therapy for focal onset seizures with or without secondary generalization in patients aged 12 years and older in more than 55 countries. PER has also been authorized for primary generalized tonic-clonic seizures in the USA, Canada, Europe, Asia, and Russia in patients aged 12 years and older.

PER is the first approved AED to provide an allosteric negative modulation of postsynaptic excitatory neurotransmission through the selective antagonism of glutamate AMPA receptors. Non-competitive antagonists are likely to have improved antiseizure activity in comparison to competitive ones in hyper-excitatory states. As non-competitive antagonists bind at a different site than glutamate, the antagonism is maintained even in the presence of high glutamate concentrations, as it occurs during seizures. Competitive antagonists are required at higher doses to obtain equivalent protection and, thus, are more likely to induce side effects and be less tolerated.

Historically, many issues hindered the development of AMPA receptor antagonists, including poor blood-brain barrier permeability, short half-lives, and structure-related toxicity [9]. By searching for novel compounds with different chemical structures to overcome the challenges encountered with the early compounds, the basic structure of PER was discovered through modern methods of medicinal chemistry and focused discovery program. Two high throughput screening assays, namely the rat cortical neuron AMPA-induced cell death and tritiated AMPA binding assays, were used to identify a pyridine core as the preferred scaffold, detect, and eliminate those compounds acting as competitive antagonists, and start the optimization process, which resulted in a series of novel 1,3,5-triaryl-1H-pyridin-2-one derivatives and culminated in the discovery of PER [10]. In preclinical studies, PER exhibited antiseizure activity in both in vitro assays and in vivo models. In cultures of rat cortical neurons, PER inhibited the AMPA-induced increases in intracellular calcium concentration in a dose-dependent fashion. In animal models of partial and generalized epilepsy, PER showed a broad-spectrum anti-seizure activity [9]. PER protected mice from tonic-clonic generalized seizures in MES and audiogenic seizures tests, and from absence or myoclonic seizures in the PTZ-induced seizures model with median effective doses (ED50) comparable or lower than those reported with carbamazepine (CBZ) or valproate (VPA). PER also inhibited seizures in the 6 Hz electroshock-induced test, a model of refractory psychomotor seizures, with only a slight elevation of the ED50 (from 2.1 to 2.8 mg/kg) when the stimulus intensity raised from 32 to 44 mA. Conversely, oral PER at doses of 1, 3, and 10 mg/kg did not result effective against absences in genetic rat model of absence epilepsy [4,9]. In the temporal lobe epilepsy model (amygdala kindling), PER significantly elevated the after-discharge threshold (ADT) and reduced motor seizure duration, seizure severity and after-discharge duration when an electrical stimulus intensity higher than the ADT was applied. These findings suggested that PER could have inhibitory effect on the propagation of seizures and provided the rationale for its efficacy in secondary generalized seizures [4,9].

Pharmacodynamic and pharmacokinetic interactions of PER with other AEDs have been explored providing useful insights into the potential ‘rational polytherapy.’ In a rat amygdala kindling model, high-dose PER, but not low-dose PER, levetiracetam, lamotrigine, CBZ or VPA reduced seizure duration and score compared with vehicle alone. However, when low-dose PER was combined with any of the four AEDs, seizure severity parameters were significantly reduced compared with the respective drug alone. In many cases, there were also significant reductions than with low-dose PER alone. Efficacy may have been slightly affected by variations in PER plasma levels, which increased with levetiracetam or lamotrigine and decreased with CBZ or VPA co-administration [11]. Pronounced threshold increases have been also observed following co-administration of zonisamide and PER, revealing a synergistic interaction between the two drugs. Rotarod data suggested that the combination has a favorable tolerability profile when zonisamide is combined with PER at low doses, and analysis of plasma levels argued against a pharmacokinetic nature of the synergism [12].

4. Expert opinion

Combining compounds with different pharmacokinetic and pharmacodynamic properties may improve effectiveness and tolerability, enable more individualized treatment and, hence, offer benefits to those patients who do not respond or tolerate current therapies. As the first selective AMPA receptor antagonist, PER markedly renewed the interest in the rational polytherapy.

The improvement in the understanding of AMPA-mediated pathways and the discovery of PER opened new avenues and frontiers in the treatment of seizures. AMPA receptors play a key role in the pathogenesis of refractory status epilepticus (SE). Indeed, the transition from self-limiting to self-sustaining seizures is characterized by the internalization of the ε – subunit of γ-aminobutyric acid (GABA) receptors, with a decreased likelihood for GABAergic drugs to bind their target, and the externalization of AMPA receptors [13]. Plasticity of AMPA receptors composition in excitatory synapses, with an over-expression of calcium-permeable GluA2-lacking AMPA receptors, also occurs. Targeting AMPA transmission may be a promising strategy to overcome refractoriness and time-dependent pharmaco-resistance to benzodiazepines in SE. In a diazepam-refractory lithium-pilocarpine-induced SE model, non-competitive AMPA antagonists, namely PER and GYKI52466, were effective to terminate seizures [14]. Different PER formulation, including the intravenous form, should be made available to test its therapeutic potential in refractory SE in clinical practice [13].

Due to the role of increased extracellular glutamate and altered glutamate homeostasis in brain tumor cell migration and invasion and tumor-related epileptogenesis, AMPA receptors antagonists may have a role either to counteract tumor growth or reduce epileptiform activity. Of similar interest is the neuroprotective potential of AMPA antagonism in traumatic brain injury. In a rat model, PER decreased brain oedema and contusion volume, reduced apoptosis, and oxidative stress, and prevented neurologic consequences through anti-inflammatory activity [15]. Hyperactivation of AMPA receptors has been also reported in early stages of epileptogenesis after hypoxia-induced neonatal seizures and phosphorylation of GluA1 at S831 and S845 is suggested to be involved in epileptogenesis in early life seizures [4].

PER efficacy in refractory seizures has been established in pivotal trials, and ongoing efficacy demonstrated by post-marketing evidence. The drug is completely absorbed and exhibits a half-life that allows once-daily administration in doses up to 12 mg/day. Drug interactions are minimal, but increased doses may be required when given in association with strong inducers of cytochrome P450 enzymes, including other AEDs that exhibit this property. The most common adverse effects reportedin clinical trials and real-world everyday practice are dizziness and somnolence. In particular, psychiatric, and behavioral adverse events have been documented in both adult and pediatric patients, including those with no corresponding diagnostic history. Further research will be required to better define the place of PER and next-generation AMPA-receptor antagonists in the treatment of focal-onset seizures and identify specific patient population targets.

Declaration of interest

P Striano has received fees for participating on the advisory boards of Eisai, GW Pharma and Kolfarma Srl. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This manuscript was not funded.