Tonabersat (SB-220453) a novel benzopyran with anticonvulsant properties attenuates trigeminal nerve-induced neurovascular reflexes

Blockade of connexin hemichannels with tonabersat protects against mild hypoxic ischemic brain injury in neonatal rats

BACKGROUND AND PURPOSE

There is growing evidence that infants with mild hypoxic-ischemic (HI) encephalopathy have increased risk of brain injury and adverse neurodevelopmental outcomes. Currently, there is no approved treatment for these infants. It was previously shown that blocking connexin 43 hemichannels is neuroprotective in models of moderate to severe HI injury. However, it is yet to be established whether these channels play a role in the evolution of mild HI brain injury, and whether blocking these channels after mild HI is neuroprotective.

METHODS

HI was induced in postnatal day 10 rats of both sexes by right carotid artery ligation followed by 80 min of hypoxia in 8% oxygen. Pups receiving HI were randomised to receive intraperitoneal injections of either saline, vehicle (2-hydroxypropyl-beta-cyclodextrin polyethylene glycol-400), or tonabersat (2 mg/kg), at 60 min, 24 h, and 48 h after hypoxia. Seven days after HI, brains were harvested for measurement of volume loss and histological analysis.

RESULTS

HI resulted in a significant reduction in hemispheric, hippocampal, and white matter volumes, which were significantly attenuated after treatment with tonabersat. HI was also associated with a significant reduction in numbers of neurons in the CA1 and CA3 hippocampal regions, a reduction in the numbers of oligodendrocytes in the corpus callosum, and an increase in the number of astrocytes in both regions, which were significantly attenuated by tonabersat treatment. There were no differences in rectal temperatures between tonabersat- and vehicle-treated rat pups.

CONCLUSIONS

Blockade of connexin hemichannels with tonabersat significantly reduced mild HI injury in the hippocampus and white matter, without causing hypothermia.

An Update: Pathophysiology of Migraine

Migraine is the most common disabling primary headache globally. Attacks typically present with unilateral throbbing headache and associated symptoms including, nausea, multisensory hypersensitivity, and marked fatigue. In this article, the authors address the underlying neuroanatomical basis for migraine-related headache, associated symptomatology, and discuss key clinical and preclinical findings that indicate that migraine likely results from dysfunctional homeostatic mechanisms. Whereby, abnormal central nervous system responses to extrinsic and intrinsic cues may lead to increased attack susceptibility.

Design, synthesis and docking studies of novel benzopyrone derivatives as anticonvulsants

A series of coumarin derivatives 6-8, 9a-h, 11 and 13a, b -16a, b was synthesized and screened for their anticonvulsant profile. Screening of these analogues using the 'gold standard methods' revealed variable anticonvulsant potential with remarkable effects observed particularly in chemically-induced seizure test. Compounds 6, 7, 13b disclosed the highest potency among the series with 100% protection against scPTZ. Quantification study confirmed that compound 6 (ED₅₀ 0.238 mmol/kg) was the most active congener in the scPTZ model and was approximately 1.5 folds more potent than ethosuximide as reference drug Meanwhile, in the MES test, candidate drugs exhibited mild to moderate anticonvulsant efficacy, the highest of which was compound 14a, imparting 50% protection at 2.1 mmol/kg, followed by other compounds with activity ranging from 14 to 33%, as compared to diphenylhydantoin. Additionally, all candidate compounds were screened for acute neurotoxicity using the rotarod method to identify motor impairment, where almost all compounds passed the test. Further neurochemical investigation was performed to unravel the effect of the most active compound (6) on GABA level in mouse brain, where a significant elevation was evident by 4 and 1.4 folds with respect to that of the control and reference groups at p < 0.05. Molecular modeling study using Discovery Studio program was performed, where compound 6 exhibited good binding interaction with γ-aminobutyric acid aminotransferase (GABA-AT) enzyme and this was consistent with the attained experimental results.

Connexin Hemichannel Block Using Orally Delivered Tonabersat Improves Outcomes in Animal Models of Retinal Disease

Increased Connexin43 hemichannel opening is associated with inflammasome pathway activation and inflammation in a range of pathologies including ocular disorders, such as age-related macular degeneration (AMD) and diabetic retinopathy (DR). In this study, the effect on retinal function and morphology of clinically safe doses of orally delivered tonabersat, a small molecule connexin hemichannel blocker, was investigated in the light-damaged retina animal model of dry AMD and in a spontaneous rat model of DR. Clinical parameters (fundus imaging, optical coherence tomography (OCT), and electroretinography) and inflammatory markers (immunohistochemistry for Iba-1 microglial marker, astrocyte marker glial fibrillary acidic protein, and Connexin43 protein expression) were assessed. Tonabersat treatment reduced inflammation in the retina in parallel with preservation of retinal photoreceptor function when assessed up to 3 months post light damage in the dry AMD model. In the DR model, clinical signs, including the presence of aneurysms confirmed using Evans blue dye perfusion, were reduced after daily tonabersat treatment for 2 weeks. Inflammation was also reduced and retinal electrical function restored. Tonabersat regulates assembly of the inflammasome (NLRP3) through Connexin43 hemichannel block, with the potential to reduce inflammation, restore vascular integrity and improve anatomical along with some functional outcomes in retinal disease.

Inhibition of gap junctional intercellular communication by an anti-migraine agent, flunarizine

Gap junctions (GJs), which consist of proteins called connexins, are intercellular channels that allow the passage of ions, second messengers, and small molecules. GJs and connexins are considered as emerging therapeutic targets for various diseases. Previously, we screened numerous compounds using our recently developed iodide yellow fluorescent protein gap junctional intercellular communication (I-YFP GJIC) assay and found that flunarizine (FNZ), used for migraine prophylaxis and as an add-on therapy for epilepsy, inhibits GJIC in LN215 human glioma cells. In this study, we confirmed that FNZ inhibits GJIC using the I-YFP GJIC assay. We demonstrated that FNZ inhibits GJ activities via a mechanism that is independent of calcium channels and dopaminergic D2, histaminergic H1, or 5-HT receptors. In addition, we showed that FNZ significantly increases connexin 43 (Cx43) phosphorylation on the cell surface, but does not alter the total amount of Cx43. The beneficial effects of FNZ on migraines and epilepsy might be related to GJ inhibition.

Side effects associated with current and prospective antimigraine pharmacotherapies

INTRODUCTION

Migraine is a neurovascular disorder. Current acute specific antimigraine pharmacotherapies target trigeminovascular 5-HT1B/1D, 5-HT1F and CGRP receptors but, unfortunately, they induce some cardiovascular and central side effects that lead to poor treatment adherence/compliance. Therefore, new antimigraine drugs are being explored. Areas covered: This review considers the adverse (or potential) side effects produced by current and prospective antimigraine drugs, including medication overuse headache (MOH) produced by ergots and triptans, the side effects observed in clinical trials for the new gepants and CGRP antibodies, and a section discussing the potential effects resulting from disruption of the cardiovascular CGRPergic neurotransmission. Expert opinion: The last decades have witnessed remarkable developments in antimigraine therapy, which includes acute (e.g. triptans) and prophylactic (e.g. β-adrenoceptor blockers) antimigraine drugs. Indeed, the triptans represent a considerable advance, but their side effects (including nausea, dizziness and coronary vasoconstriction) preclude some patients from using triptans. This has led to the development of the ditans (5-HT1F receptor agonists), the gepants (CGRP receptor antagonists) and the monoclonal antibodies against CGRP or its receptor. The latter drugs represent a new hope in the antimigraine armamentarium, but as CGRP plays a role in cardiovascular homeostasis, the potential for adverse cardiovascular side effects remains latent.

Tonabersat Prevents Inflammatory Damage in the Central Nervous System by Blocking Connexin43 Hemichannels

The cis benzopyran compound tonabersat (SB-220453) has previously been reported to inhibit connexin26 expression in the brain by attenuating the p38-mitogen-activated protein kinase pathway. We show here that tonabersat directly inhibits connexin43 hemichannel opening. Connexin43 hemichannels have been called "pathological pores" based upon their role in secondary lesion spread, edema, inflammation, and neuronal loss following central nervous system injuries, as well as in chronic inflammatory disease. Both connexin43 hemichannels and pannexin channels released adenosine triphosphate (ATP) during ischemia in an in vitro ischemia model, but only connexin43 hemichannels contributed to ATP release during reperfusion. Tonabersat inhibited connexin43 hemichannel-mediated ATP release during both ischemia and reperfusion phases, with direct channel block confirmed using electrophysiology. Tonabersat also reduced connexin43 gap junction coupling in vitro, but only at higher concentrations, with junctional plaques internalized and degraded via the lysosomal pathway. Systemic delivery of tonabersat in a rat bright-light retinal damage model (a model for dry age-related macular degeneration) resulted in significantly improved functional outcomes assessed using electroretinography. Tonabersat also prevented thinning of the retina, especially the outer nuclear layer and choroid, assessed using optical coherence tomography. We conclude that tonabersat, already given orally to over 1000 humans in clinical trials (as a potential treatment for, and prophylactic treatment of, migraine because it was thought to inhibit cortical spreading depression), is a connexin hemichannel inhibitor and may have the potential to be a novel treatment of central nervous system injury and chronic neuroinflammatory disease.

A Double-Blind Study of SB-220453 (Tonerbasat) in the Glyceryltrinitrate (GTN) Model of Migraine

The need for experimental migraine models increases as therapeutic options widen. In the present study, we investigated SB-220453 for efficacy in the glyceryltrinitrate (GTN) human experimental migraine model. SB-220453 is a novel benzopyran compound, which in animal models inhibits neurogenic inflammation, blocks propagation of spreading depression and inhibits trigeminal nerve ganglion stimulation-induced carotid vasodilatation. We included 15 patients with migraine without aura in a randomized double-blind crossover study. SB-220453 40 mg or placebo was followed by a 20-min GTN infusion. Headache, scored 0-10, was registered for 12 h, and fulfillment of International Headache Society (IHS) criteria was recorded until 24 h. Four subjects had a hypotensive episode after SB-220453 plus GTN but none after GTN alone. The reaction was unexpected, since animal models and previous human studies had shown no vascular or sympaticolytic activity with SB-220453. The study was terminated prematurely due to this interaction. GTN was consistent in producing headache and migraine that resembled the patients' usual spontaneous migraine. Nine patients had GTN on both study days. Peak headache score showed a trend towards reduction after SB-220453 compared with placebo (median 4 vs. 7, P = 0.15). However, no reduction was seen in the number of subjects experiencing delayed headache (8 vs. 8), number of subjects reporting migraine (6 vs. 8), migraine attacks fulfilling IHS criteria 1.1 or 1.7 (6 vs. 7) or IHS 1.1 alone (4 vs. 5). SB-220453 had no significant pre-emptive anti-migraine activity compared with placebo in this human model of migraine. Interaction between SB-220453 and GTN was discovered. This is important for the future development of the compound and underlines the usefulness of experimental migraine models.

New therapeutic approaches for the prevention and treatment of migraine

The management of patients with migraine is often unsatisfactory because available acute and preventive therapies are either ineffective or poorly tolerated. The acute treatment of migraine attacks has been limited to the use of analgesics, combinations of analgesics with caffeine, ergotamines, and the triptans. Successful new approaches for the treatment of acute migraine target calcitonin gene-related peptide (CGRP) and serotonin (5-hydroxytryptamine, 5-HT1F) receptors. Other approaches targeting the transient receptor potential vanilloid (TRPV1) receptor, glutamate, GABAA receptors, or a combination of 5-HT1B/1D receptors and neuronal nitric oxide synthesis have been investigated but have not been successful in clinical trials thus far. In migraine prevention, the most promising new approaches are humanised antibodies against CGRP or the CGRP receptor. Non-invasive and invasive neuromodulation approaches also show promise as both acute and preventive therapies, although further studies are needed to define appropriate candidates for these therapies and optimum protocols for their use.

Influence of drugs on gap junctions in glioma cell lines and primary astrocytes in vitro

Gap junctions (GJs) are hemichannels on cell membrane. Once they are intercellulary connected to the neighboring cells, they build a functional syncytium which allows rapid transfer of ions and molecules between cells. This characteristic makes GJs a potential modulator in proliferation, migration, and development of the cells. So far, several types of GJs are recognized on different brain cells as well as in glioma. Astrocytes, as one of the major cells that maintain neuronal homeostasis, express different types of GJs that let them communicate with neurons, oligodendrocytes, and endothelial cells of the blood brain barrier; however, the main GJ in astrocytes is connexin 43. There are different cerebral diseases in which astrocyte GJs might play a role. Several drugs have been reported to modulate gap junctional communication in the brain which can consequently have beneficial or detrimental effects on the course of treatment in certain diseases. However, the exact cellular mechanism behind those pharmaceutical efficacies on GJs is not well-understood. Accordingly, how specific drugs would affect GJs and what some consequent specific brain diseases would be are the interests of the authors of this chapter. We would focus on pharmaceutical effects on GJs on astrocytes in specific diseases where GJs could possibly play a role including: (1) migraine and a novel therapy for migraine with aura, (2) neuroautoimmune diseases and immunomodulatory drugs in the treatment of demyelinating diseases of the central nervous system such as multiple sclerosis, (3) glioma and antineoplastic and anti-inflammatory agents that are used in treating brain tumors, and (4) epilepsy and anticonvulsants that are widely used for seizures therapy. All of the above-mentioned therapeutic categories can possibly affect GJs expression of astrocytes and the role is discussed in the upcoming chapter.

Involvement of gap junction channels in the pathophysiology of migraine with aura

Migraine is a common, recurrent, and disabling primary headache disorder with a genetic component which affects up to 20% of the population. One third of all patients with migraine experiences aura, a focal neurological disturbance that manifests itself as visual, sensitive or motor symptoms preceding the headache. In the pathophysiology of migraine with aura, activation of the trigeminovascular system from the meningeal vessels mediates migraine pain via the brainstem and projections ascend to the thalamus and cortex. Cortical spreading depression (CSD) was proposed to trigger migraine aura and to activate perivascular trigeminal nerves in the cortex. Quinine, quinidine and the derivative mefloquine are able to inhibit CSD suggesting an involvement of neuronal connexin36 channels in CSD propagation. More recently, CSD was shown to induce headache by activating the trigeminovascular system through the opening of stressed neuronal Pannexin1 channels. A novel benzopyran compound, tonabersat, was selected for clinical trial on the basis of its inhibitory activity on CSD and neurogenic inflammation in animal models of migraine. Interestingly, in the time course of animal model trials, tonabersat was shown to inhibit trigeminal ganglion (TGG) neuronal-glial cell gap junctions, suggesting that this compound could prevent peripheral sensitization within the ganglion. Three clinical trials aimed at investigating the effectiveness of tonabersat as a preventive drug were negative, and conflicting results were obtained in other trials concerning its ability to relieve attacks. In contrast, in another clinical trial, tonabersat showed a preventive effect on attacks of migraine with aura but had no efficacy on non-aura attacks. Gap junction channels seem to be involved in several ways in the pathophysiology of migraine with aura and emerge as a new promising putative target in treatment of this disorder.

Efficacy and mechanism of anticonvulsant drugs in migraine

Anticonvulsants represent one of the main substance classes used for the preventive treatment of migraine. Efficacy has been demonstrated in randomized placebo-controlled trials for topiramate and valproic acid including divalproex sodium. In the case of topiramate, efficacy has recently been proven for chronic migraine and even medication overuse headache, questioning the established concept of medication withdrawal. However, preventive treatment with anticonvulsants is frequently hampered by side effects that occasionally require treatment discontinuation. In addition, these data indicate that some anticonvulsant drugs are effective in migraine, while a number are clearly not useful. Effective anticonvulsants, such as topiramate and valproate, target nociceptive trigeminovascular and trigeminothalamic dural pathways or mechanisms involved in cortical spreading depression. Dissecting out how the anticonvulsants that do not work differ mechanistically from those that do will almost certainly provide avenues through which one can develop new treatments to bring to patients with migraine.

Drug-likeness analysis of traditional Chinese medicines: prediction of drug-likeness using machine learning approaches

Quantitative or qualitative characterization of the drug-like features of known drugs may help medicinal and computational chemists to select higher quality drug leads from a huge pool of compounds and to improve the efficiency of drug design pipelines. For this purpose, the theoretical models for drug-likeness to discriminate between drug-like and non-drug-like based on molecular physicochemical properties and structural fingerprints were developed by using the naive Bayesian classification (NBC) and recursive partitioning (RP) techniques, and then the drug-likeness of the compounds from the Traditional Chinese Medicine Compound Database (TCMCD) was evaluated. First, the impact of molecular physicochemical properties and structural fingerprints on the prediction accuracy of drug-likeness was examined. We found that, compared with simple molecular properties, structural fingerprints were more essential for the accurate prediction of drug-likeness. Then, a variety of Bayesian classifiers were constructed by changing the ratio of drug-like to non-drug-like molecules and the size of the training set. The results indicate that the prediction accuracy of the Bayesian classifiers was closely related to the size and the degree of the balance of the training set. When a balanced training set was used, the best Bayesian classifier based on 21 physicochemical properties and the LCFP_6 fingerprint set yielded an overall leave-one-out (LOO) cross-validated accuracy of 91.4% for the 140,000 molecules in the training set and 90.9% for the 40,000 molecules in the test set. In addition, the RP classifiers with different maximum depth were constructed and compared with the Bayesian classifiers, and we found that the best Bayesian classifier outperformed the best RP model with respect to overall prediction accuracy. Moreover, the Bayesian classifier employing structural fingerprints highlights the important substructures favorable or unfavorable for drug-likeness, offering extra valuable information for getting high quality lead compounds in the early stage of the drug design/discovery process. Finally, the best Bayesian classifier was used to predict the drug-likeness of 33,961 compounds in TCMCD. Our calculations show that 59.37% of the molecules in TCMCD were identified as drug-like molecules, indicating that traditional Chinese medicines (TCMs) are therefore an excellent source of drug-like molecules. Furthermore, the important structural fingerprints in TCMCD were detected and analyzed. Considering that the pharmacology of TCMCD and MDDR (MDL Drug Data Report) was linked by the important common structural features, the potential pharmacology of the compounds in TCMCD may therefore be annotated by these important structural signatures identified from Bayesian analysis, which may be valuable to promote the development of TCMs.

[Gap junctional intercellular communication: a new mechanism in pathophysiology of migraine with aura. Therapeutic applications]

Migraine is a common, recurrent and disabling primary headache disorder, which affects up to 20% of the population. About a third of patients with migraine have attacks with aura, a focal neurological disturbance that manifests itself as visual, sensitive or motor symptoms. Cortical spreading depression, a wave of electrical activity that moves across the cerebral cortex through neuronal-glial cell gap junctions, would be involved in the triggering of migraine aura. Moreover, cortical spreading depression activates perivascular trigeminal afferents in the neocortex, that through central and peripheral reflex, cause inflammatory reaction in the meninges to generate the headache. Tonabersat, a novel benzopyran compound, was selected for clinical trial on the basis of its inhibitory activity on cortical spreading depression and neurogenic inflammation in animal models of migraine. Moreover, tonabersat inhibited trigeminal ganglion neuronal-glial cell gap junctions, suggesting that this compound could prevent peripheral sensitization within the ganglion. In clinical trial, tonabersat showed a preventive effect on attacks of migraine with aura but had no efficacy on non-aura attacks and in the acute treatment of migraine. In conclusion, neuronal-glial cell gap junctional intercellular communication seems to be involved in the pathophysiology of migraine with aura and is emerging as a new promising therapeutic target for prophylactic treatment of patients with chronic attacks.

New and investigational antiepileptic drugs

In the last fifteen years, new antiepileptic medications have been offered for the treatment of patients with epilepsy. Nevertheless, despite optimal medical treatment, up to 30% of patients still experience recurrent seizures and the challenge for new, more efficacious and better-tolerated drugs continues. New antiepileptic drugs include the evolution of pre-existing drugs and new compounds identified through the investigation of additional molecular targets, such as SV2A synaptic vesicle protein, voltage-gated potassium channels, ionotropic and metabotropic glutamate receptors, and gap junctions. This paper reviews the available information on various classes of molecules that are in the pipeline as well as on the innovative approaches to the treatment of epilepsy.

Pharmacology

Headache treatment has been based primarily on experiences with non-specific drugs such as analgesics, non-steroidal anti-inflammatory drugs, or drugs that were originally developed to treat other diseases, such as beta-blockers and anticonvulsant medications. A better understanding of the basic pathophysiological mechanisms of migraine and other types of headache has led to the development over the past two decades of more target-specific drugs. Since activation of the trigeminovascular system and neurogenic inflammation are thought to play important roles in migraine pathophysiology, experimental studies modeling those events successfully predicted targets for selective development of pharmacological agents to treat migraine. Basically, there are two fundamental strategies for the treatment of migraine, abortive or preventive, based to a large degree on the frequency of attacks. The triptans, which exhibit potency towards selective serotonin (5-hydroxytryptamine, 5-HT) receptors expressed on trigeminal nerves, remain the most effective drugs for the abortive treatment of migraine. However, numerous preventive medications are currently available that modulate the excitability of the nervous system, particularly the cerebral cortex. In this chapter, the pharmacology of commercially available medications as well as drugs in development that prevent or abort headache attacks will be discussed.

New drugs for migraine

After the triptans, a calcitonin gene-related peptide blocker (telcagepant) is the first acute medicine that has been developed primarily for treatment of acute migraine. Otherwise, the new drugs have been developed first for other purposes, like anticonvulsants, antihypertensives and antidepressants used for migraine prophylaxis. For acute attacks, a new way to administer a traditional drug like dihydroergotamine is under way, and documentation of efficacy in migraine has been gained for some commonly used painkillers and anti-inflammatory drugs, and for some herbal extracts. Based on insights into the basic pathophysiological mechanisms of the disorder, some drugs have been developed which seem promising in early phase II studies (NOS inhibitors and 5HT1F-receptor agonists). In the future, development and enhancements of existing medicines must be accompanied by increased efforts to develop truly new migraine drugs based on knowledge of the pathophysiology if one wishes to reduce substantially the great burden migraine poses on patients and society.

Tonabersat, a novel gap-junction modulator for the prevention of migraine

Migraine is a common, recurrent, primary headache disorder associated with significant morbidity as well as high direct and indirect costs. Despite its impact, only a proportion of migraineurs who meet criteria for prophylactic treatment take preventive medication. Antiepileptic drugs and beta-blockers are among the most used preventive therapies, but their exact mechanisms of action in migraine prophylaxis are unknown. Recent research has pointed to the role of cortical spreading depression in the genesis of migraine aura and pain, with neuronal-glial gap junctions playing a prominent part in cortical spreading depression. Tonabersat is a unique compound with demonstrated activity as a gap-junction inhibitor in animal studies. In preclinical and clinical trials, tonabersat was well tolerated, with no cardiovascular effects; the pharmacokinetic profile suggested its usefulness in the prophylaxis of migraine.

Efficacy and safety of tonabersat, a gap-junction modulator, in the acute treatment of migraine: a double-blind, parallel-group, randomized study

The ability of tonabersat to relieve the symptoms of migraine attacks with or without aura was evaluated in a randomized, double-blind, placebo-controlled, multicentre, parallel-group study. Patients received 20 or 40 mg of tonabersat, or 50 mg of sumatriptan (positive control), or placebo at the onset of a moderate or severe attack. Headache intensity, relief and recurrence were recorded for 24 h after dosing. On the basis of primary or secondary efficacy measures, tonabersat did not provide a clinically or statistically significant advantage over placebo. Tonabersat generally was well tolerated and had no effect on vital signs, electrocardiogram recordings or laboratory values. The lack of efficacy may be a function of the slow absorption of tonabersat. As a consequence of slow absorption, daily administration of tonabersat as prophylaxis for migraine attacks is under investigation in ongoing studies.

Neurological mechanisms of migraine: potential of the gap-junction modulator tonabersat in prevention of migraine

Migraine is a neurovascular disorder characterized by recurrent episodic headaches, and is caused by abnormal processing of sensory information due to peripheral and/or central sensitization. The exact pathophysiological mechanism underlying migraine is not fully understood; however, cortical spreading depression (CSD) is thought to provide the basis for migraine aura and may serve as a trigger of migraine pain. CSD depends on neuronal-glial cell communication, which is mediated by intercellular transfer of messengers through connexin-containing gap junctions, as well as messengers released into the extracellular space by non-junctional connexin-containing hemichannels. These processes are believed to be important in peripheral sensitization within the trigeminal ganglion and to lead to central sensitization. The novel benzopyran compound tonabersat binds selectively to a unique site in the brain. In preclinical studies, tonabersat markedly reduced CSD and CSD-associated events and inhibited gap-junction communication between neurons and satellite glial cells in the trigeminal ganglion. Together, these findings suggest that tonabersat should have clinical application in preventing migraine attacks.

Randomized, Double-Blind, Placebo-Controlled, Proof-of-Concept Study of the Cortical Spreading Depression Inhibiting Agent Tonabersat in Migraine Prophylaxis

Tonabersat is a novel putative migraine prophylactic agent with an unique stereospecific binding site in the brain. Tonabersat has been shown, in animal models, to inhibit experimentally induced cortical spreading depression, the likely underlying mechanism for migraine aura, and cerebrovascular responses to trigeminal nerve stimulation. The aim was to study the potential for tonabersat as a migraine preventive. A randomized, double-blind, placebo-controlled, multicentre, parallel group study recruited patients with migraine with and without aura experiencing between two and six migraine attacks per month. After a 1-month baseline they received tonabersat 20 mg daily for 2 weeks and 40 mg daily for a further 10 weeks. The primary end-point was the change in mean number of migraine headache days between the third month and the baseline period in the intention-to-treat population comparing the placebo ( n = 65) and tonabersat ( n = 58) groups. At the primary end-point there was a 1.0-day (95% confidence interval −0.33, 2.39; P = 0.14) difference in reduction in migraine days between tonabersat and placebo. There were 10 secondary efficacy end-points, of which two were statistically significant. In month 3 of treatment, the responder rate, defined as a 50% reduction in migraine attacks, was 62% for tonabersat and 45% for placebo ( P < 0.05), and the rescue medication use was reduced in the tonabersat group compared with placebo by 1.8 days ( P = 0.02). Placebo responses were particularly high for all end-points. At least one treatment-emergent adverse event was reported in the tonabersat group in 61% of patients compared with 51% in the placebo group; none was worrisome. Placebo responses were unexpectedly high in this trial, complicating straightforward interpretation of the study results. The good tolerability and promising efficacy results support further exploration of higher doses of tonabersat in larger controlled trials.

Effects of tonabersat on migraine with aura: a randomised, double-blind, placebo-controlled crossover study

BACKGROUND

Migraine with aura is thought likely to be caused by cortical spreading depression (CSD). Tonabersat inhibits CSD, and we therefore investigated whether tonabersat has a preventive effect in migraine with aura.

METHODS

In this randomised, double-blind, placebo-controlled crossover trial, 40 mg tonabersat once daily was compared with matched placebo in patients who had at least one aura attack per month during the past 3 months. Randomisation was by computer-generated list. Patients kept a detailed diary to enable objective diagnosis of each attack as migraine with aura, migraine without aura, or other type of headache. Primary endpoints were a reduction in aura attacks with or without headache and a reduction in migraine headache days with or without an aura. Analysis was per protocol. This trial is registered, number NCT00332007.

FINDINGS

39 patients were included in the study, of whom 31 were included in the statistical analysis of efficacy. Median (IQR) attacks of aura were reduced from 3.2 (1.0-5.0) per 12 weeks on placebo to 1.0 (0-3.0) on tonabersat (p=0.01), whereas the other primary outcome measure, median migraine headache days with or without aura, was not significantly different between placebo and tonabersat groups (3.0 days in each group; p=0.09). Tonabersat was well tolerated but overall had more side-effects than placebo.

INTERPRETATION

Tonabersat showed a preventive effect on attacks of migraine aura but no efficacy on non-aura attacks, in keeping with its known inhibitory effect on CSD. The results support the theory that auras are caused by CSD and that this phenomenon is not involved in attacks without aura.

FUNDING

Minster Pharmaceuticals; Lundbeck Foundation.

Tonabersat inhibits trigeminal ganglion neuronal-satellite glial cell signaling

BACKGROUND

Sensitization and activation of trigeminal neurons are implicated in the underlying pathology of migraine, acute sinusitis, and allergic rhinitis. Cell bodies of trigeminal neurons that provide sensory innervation of the dura and nasal mucosa reside in the trigeminal ganglion in association with satellite glial cells where they communicate via gap junctions. Gap junctions, channels formed by connexins, modulate the excitability state of both neurons and glia under pathological conditions. Tonabersat, a compound being tested as an antimigraine drug, is thought to block gap junction activity.

OBJECTIVE

To investigate the cellular events within trigeminal ganglia that may account for the significant comorbidity of migraine and rhinosinusitis and determine the effect of tonabersat on neuron-satellite glia communication.

METHODS

Sprague Dawley rats injected with True Blue were used to localize neuronal cell bodies in the ganglion and study neuron-glia signaling via gap junctions in the trigeminal ganglion. Dye coupling studies were conducted under basal conditions and in response to tumor necrosis factor-alpha injection into the whisker pad and/or capsaicin injection into the eyebrow. Changes in connexin 26 and active p38 levels were determined by immunohistochemistry. In addition, the effect of tonabersat prior to chemical stimulation on gap junction activity and expression of connexins and active p38 was investigated.

RESULTS

Injection of tumor necrosis factor-alpha, a cytokine implicated in the pathology of acute sinusitis and allergic rhinitis, into the V2 region was shown to lower the amount of capsaicin required to stimulate neurons located in the V1 region of the ganglion. While injection of tumor necrosis factor-alpha into the whisker pad or capsaicin injection into the eyebrow alone did not cause increased dye movement, the combination of both stimuli greatly increased neuron-satellite glia communication via gap junctions in both V1 and V2 regions. The change in gap junction activity was accompanied by increased expression of connexin 26 and active p38 levels in both neurons and satellite glia in V1 and V2 regions. Pretreatment with tonabersat inhibited gap junction communication between neurons and satellite glia and blocked the increase in connexin 26 and active p38 levels in response to injection of both tumor necrosis factor-alpha (V2) and capsaicin (V1).

CONCLUSIONS

We propose that increased levels of tumor necrosis factor-alpha, as reported during acute sinusitis and allergic rhinitis, reduces the amount of capsaicin necessary to stimulate V1 neurons that leads to cellular changes in both V1 and V2 regions. The cellular events observed in this study may help to explain, in part, the significant comorbidity reported with migraine and rhinosinusitis. In addition, we have provided evidence to suggest that tonabersat can prevent increased neuron-satellite glia signaling and, thus, may be useful in the treatment of migraine, acute sinusitis, and allergic rhinitis.

Progress report on new antiepileptic drugs: a summary of the Ninth Eilat Conference (EILAT IX)

The Ninth Eilat Conference on New Antiepileptic Drugs (AEDs)-EILAT IX, took place in Sitges, Barcelona from the 15th to 19th of June 2008. Over 300 basic scientists, clinical pharmacologists and neurologists from 25 countries attended the conference, whose main themes included old and new AEDs in generalized epilepsies, novel formulations and routes of administration of AEDs, common targets and mechanisms of action of drugs for treating epilepsy and other central nervous system (CNS) disorders, and opportunities and perspectives in new AED discovery. Consistent with previous formats of this conference, a large part of the programme was devoted to a review of AEDs in development, as well as updates on AEDs introduced since 1989. Unlike previous EILAT manuscripts, the current (EILAT IX) manuscript focuses only on the preclinical and clinical pharmacology of AEDs that are currently in development. These include brivaracetam, carisbamate (RWJ-333369), 2-deoxy-d-glucose, eslicarbazepine acetate (BIA-2-093), ganaxolone, huperzine, JZP-4, lacosamide, NAX-5055, propylisopropylacetamide (PID), retigabine, T-2000, tonabersat, valrocemide and YKP-3089. The CNS efficacy of these compounds in anticonvulsant animal models as well as other disease model systems are presented in first and second tables and their proposed mechanisms of action are summarized in the third table.

Emerging therapies for migraine

Migraine is a common disabling brain disorder that--considering its clinical and economic impact--is understudied and in need of additional management options. Currently, treatments are classified as preventive or acute-attack therapies, although it is expected that this distinction will become blurred over time. The gap-junction blocker tonabersat, an inducible nitric oxide synthase (NOS) inhibitor and botulinum toxin A are all being investigated in clinical trials as preventive therapies. Device-based approaches using neurostimulation of the occipital nerve have provided promising results, whereas the first study of patent foramen ovale closure for migraine prevention produced disappointing results. Calcitonin gene-related peptide receptor antagonists, vanilloid TRPV1 receptor antagonists and NOS inhibitors are all being investigated in clinical trials for acute migraine. There is much cause for optimism in this area of neurology and considerable benefit awaits our patients.

Recent advances in understanding migraine mechanisms, molecules and therapeutics

Migraine is a complex, disabling disorder of the brain that manifests itself as attacks of often severe, throbbing head pain with sensory sensitivity to light, sound and head movement. There is a clear familial tendency to migraine, which has been well defined in a rare autosomal dominant form of familial hemiplegic migraine (FHM). FHM mutations so far identified include those in CACNA1A (P/Q voltage-gated Ca(2+) channel), ATP1A2 (N(+)-K(+)-ATPase) and SCN1A (Na(+) channel) genes. Physiological studies in humans and studies of the experimental correlate--cortical spreading depression (CSD)--provide understanding of aura, and have explored in recent years the effect of migraine preventives in CSD. Therapeutic developments in migraine have come by targeting the trigeminovascular system, with the most-recent being the proof-of-principle study of calcitonin gene-related peptide (CGRP) receptor antagonists in acute migraine. To understand the basic pathophysiology of migraine, brain imaging studies have firmly established reproducible changes in the brainstem in regions that include areas that are involved in sensory modulation. These data lead to the view that migraine is a form of sensory dysmodulatio--a system failure of normal sensory processing.

Molecular targets for antiepileptic drug development

This review considers how recent advances in the physiology of ion channels and other potential molecular targets, in conjunction with new information on the genetics of idiopathic epilepsies, can be applied to the search for improved antiepileptic drugs (AEDs). Marketed AEDs predominantly target voltage-gated cation channels (the alpha subunits of voltage-gated Na+ channels and also T-type voltage-gated Ca2+ channels) or influence GABA-mediated inhibition. Recently, alpha2-delta voltage-gated Ca2+ channel subunits and the SV2A synaptic vesicle protein have been recognized as likely targets. Genetic studies of familial idiopathic epilepsies have identified numerous genes associated with diverse epilepsy syndromes, including genes encoding Na+ channels and GABA(A) receptors, which are known AED targets. A strategy based on genes associated with epilepsy in animal models and humans suggests other potential AED targets, including various voltage-gated Ca2+ channel subunits and auxiliary proteins, A- or M-type voltage-gated K+ channels, and ionotropic glutamate receptors. Recent progress in ion channel research brought about by molecular cloning of the channel subunit proteins and studies in epilepsy models suggest additional targets, including G-protein-coupled receptors, such as GABA(B) and metabotropic glutamate receptors; hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits, responsible for hyperpolarization-activated current Ih; connexins, which make up gap junctions; and neurotransmitter transporters, particularly plasma membrane and vesicular transporters for GABA and glutamate. New information from the structural characterization of ion channels, along with better understanding of ion channel function, may allow for more selective targeting. For example, Na+ channels underlying persistent Na+ currents or GABA(A) receptor isoforms responsible for tonic (extrasynaptic) currents represent attractive targets. The growing understanding of the pathophysiology of epilepsy and the structural and functional characterization of the molecular targets provide many opportunities to create improved epilepsy therapies.

Migraine: emerging treatment options for preventive and acute attack therapy

This review discusses emerging treatments of migraine in the context of what is now available. At present, patients are treated with a range of acute attack medicines or preventive treatments, with many having significant drawbacks. Important unmet needs are acute attack treatments that act by exclusively neural mechanisms with no vascular effects, and effective, well tolerated preventive medicines. Calcitonin gene-related peptide receptor antagonist, vanilloid receptor antagonists and nitric oxide synthase inhibitors are all in clinical trials for acute migraine. Tonaberset (a gap-junction blocker), an inducible nitric oxide synthase inhibitor and botulinum toxin A are in clinical trials for preventive therapy. Device-based approaches using neurostimulation of the occipital nerve are being studied, although the first study of patent foramen ovale closure for migraine prevention failed.

Can we Develop Neurally Acting Drugs for the Treatment of Migraine?

Serotonin (5-HT)(1B/1D) receptor agonists, which are also known as triptans, represent the most important advance in migraine therapeutics in the four millennia that the condition has been recognized. The vasoconstrictive activity of triptans produced a small clinical penalty in terms of coronary vasoconstriction but also raised an enormous intellectual question: to what extent is migraine a vascular problem? Functional neuroimaging and neurophysiological studies have consistently developed the theme of migraine as a brain disorder and, therefore, demanded that the search for neurally acting antimigraine drugs should be undertaken. The prospect of non-vasoconstrictor acute migraine therapies, potential targets for which are discussed here, offers a real opportunity to patients and provides a therapeutic rationale that places migraine firmly in the brain as a neurological problem, where it undoubtedly belongs.

Emerging drugs in migraine treatment

Ergot alkaloids have been the mainstay of acute migraine therapy for most of the 20th century. They have been supplanted by sumatriptan-like drugs ('triptans'), which, while keeping some of the ergotś mechanisms of action, show improved safety profiles due to their increased receptor selectivity. However, triptans are still far from being perfect drugs: they can constrict human coronary arteries at therapeutic doses and, therefore, are contra-indicated in the presence of cardiovascular disease. Another problem with these agents is recurrence of moderate-to-severe pain within 24 h of initial headache relief. While mechanism-driven drug design has led to the development of various novel, albeit still imperfect, acute antimigraine medications, only a few new prophylactic agents have been made available to migraine clinicians. The efficacy of most, if not all of them has been discovered serendipitously. This is probably due to the fact that, while the pathophysiology of a migraine attack is now reasonably understood, the mechanisms leading to an attack are still mostly unknown. This update analyses the profile of some antimigraine drugs in clinical trials, their mode of action and their potential advantages or drawbacks over already available agents.

Prophylactic migraine therapy: Emerging treatment options

In this paper, new treatment options for migraine prevention are reviewed. An overview about migraine pathophysiology is provided and current indications for migraine prevention and new and upcoming preventive medications are discussed briefly. Data are presented on topiramate, levetiracetam, zonisamide, botulinim toxin, tizanidine, nefazodone, lisinopril, candesartan, carabersat, petasites, and coenzyme Q.

Diffusion-weighted MRI used to detect in vivo modulation of cortical spreading depression: comparison of sumatriptan and tonabersat

Spreading cortical depolarization and depression of electroencephalographic activity (SD) may underlie the aura and spreading neurovascular events of migraine. Cortical depolarization may also precipitate the progressive development of cerebral pathology following ischemia. However, data on SD in the human brain are sparse, most likely reflecting the technical difficulties involved in performing such clinical studies. We have previously shown that the transient cerebral water disturbances during SD can be quantitatively investigated in the gyrencephalic brain using repetitive diffusion-weighted magnetic resonance imaging (DWI). To investigate whether DWI could detect modulation of the spatiotemporal properties of SD in vivo, the effects of the antimigraine drug sumatriptan (0.3 mg/kg iv) and the novel anticonvulsant tonabersat (10 mg/kg ip) were evaluated in the cat brain. Supporting previous findings, sumatriptan did not affect the numbers of events (range, 4-8), the duration of SD activity (39.8 +/- 4.4 min, mean +/- SEM), and event velocity (2.2 +/- 0.4 mm min(-1)); tonabersat significantly reduced SD event initiation (range, 0-3) and duration (13.2 +/- 5.0 min) and increased primary event velocity (5.4 +/- 0.7 mm min(-1)). However, both drugs significantly decreased, by >50%, the spatial extent of the first KCl-evoked SD event, and sumatriptan significantly increased event propagation across the suprasylvian sulcus (5.5 +/- 0.6 vs 2.4 +/- 0.4 events in controls). These results demonstrate (1) the feasibility of using DWI to evaluate therapeutic effects on SD, and (2) that sumatriptan may directly modulate the spatial distribution of SD activity in the gyrencephalic brain.