Characterization of the binding of [3H]-SB-204269, a radiolabelled form of the new anticonvulsant SB-204269, to a novel binding site in rat brain membranes

Cx43 hemichannels contribute to astrocyte-mediated toxicity in sporadic and familial ALS

Our results demonstrate that connexin 43 hemichannels are the conduits for amyotrophic lateral sclerosis (ALS) astrocyte-mediated motor neuron toxicity and disease spread, acting as a common mechanism that can target both familial ALS and sporadic ALS populations. Furthermore, our present work provides proof of principle that tonabersat, as a drug already studied in clinical trials for other indications, could serve as a potential ALS therapeutic.

Connexin 43 (Cx43) gap junctions and hemichannels mediate astrocyte intercellular communication in the central nervous system under normal conditions and contribute to astrocyte-mediated neurotoxicity in amyotrophic lateral sclerosis (ALS). Here, we show that astrocyte-specific knockout of Cx43 in a mouse model of ALS slows disease progression both spatially and temporally, provides motor neuron (MN) protection, and improves survival. In addition, Cx43 expression is up-regulated in human postmortem tissue and cerebrospinal fluid from ALS patients. Using human induced pluripotent stem cell–derived astrocytes (hiPSC-A) from both familial and sporadic ALS, we establish that Cx43 is up-regulated and that Cx43-hemichannels are enriched at the astrocyte membrane. We also demonstrate that the pharmacological blockade of Cx43-hemichannels in ALS astrocytes using GAP 19, a mimetic peptide blocker, and tonabersat, a clinically tested small molecule, provides neuroprotection of hiPSC-MN and reduces ALS astrocyte-mediated neuronal hyperexcitability. Extending the in vitro application of tonabersat with chronic administration to SOD1^G93A mice results in MN protection with a reduction in reactive astrocytosis and microgliosis. Taking these data together, our studies identify Cx43 hemichannels as conduits of astrocyte-mediated disease progression and a pharmacological target for disease-modifying ALS therapies.

Reactive Astrocytes in Brain Metastasis

Brain metastasis, the secondary growth of malignant cells within the central nervous system (CNS), exceeds the incidence of primary brain tumors (i.e., gliomas) by tenfold and are seemingly on the rise owing to the emergence of novel targeted therapies that are more effective in controlling extracranial disease relatively to intracranial lesions. Despite the fact that metastasis to the brain poses a unmet clinical problem, with afflicted patients carrying significant morbidity and a fatal prognosis, our knowledge as to how metastatic cells manage to adapt to the tissue environment of the CNS remains limited. Answering this question could pave the way for novel and more specific therapeutic modalities in brain metastasis by targeting the specific makeup of the brain metastatic niche. In regard to this, astrocytes have emerged as the major host cell type that cancer cells encounter and interact with during brain metastasis formation. Similarly to other CNS disorders, astrocytes become reactive and respond to the presence of cancer cells by changing their phenotype and significantly influencing the outcome of disseminated cancer cells within the CNS. Here, we summarize the current knowledge on the contribution of reactive astrocytes in brain metastasis by focusing on the signaling pathways and types of interactions that play a crucial part in the communication with cancer cells and how these could be translated into innovative therapies.

Carcinoma-astrocyte gap junctions promote brain metastasis by cGAMP transfer

Brain metastasis represents a substantial source of morbidity and mortality in various cancers, and is characterized by high resistance to chemotherapy. Here we define the role of the most abundant cell type in the brain, the astrocyte, in promoting brain metastasis. Breast and lung cancer cells express protocadherin 7 (PCDH7) to favor the assembly of carcinoma-astrocyte gap junctions composed of connexin 43 (Cx43). Once engaged with the astrocyte gap-junctional network, brain metastatic cancer cells employ these channels to transfer the second messenger cGAMP to astrocytes, activating the STING pathway and production of inflammatory cytokines IFNα and TNFα. As paracrine signals, these factors activate the STAT1 and NF-κB pathways in brain metastatic cells, which support tumour growth and chemoresistance. The orally bioavailable modulators of gap junctions meclofenamate and tonabersat break this paracrine loop, and we provide proof-of-principle for the applicability of this therapeutic strategy to treat established brain metastasis.

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.

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.

Treatment of juvenile myoclonic epilepsy

Drug treatment of juvenile myoclonic epilepsy (JME) is mainly based on clinical experience and prospective and retrospective studies, with little evidence from randomized clinical trials. There are almost no head-to-head comparisons between old and new antiepileptic drugs (AEDs). Valproate is the drug of the first choice in men with JME. In women, lamotrigine (LTG) should be preferred regarding teratogenicity and side effects of valproate. Levetiracetam (LEV) is also effective. Recent data suggest that it may soon be used as first line treatment. Some AEDs can aggravate JME. In addition to AEDs, nonpharmacological treatments are important in JME. JME usually requires lifelong treatment because seizures nearly always return after withdrawal of therapy.

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.

Treatment of juvenile myoclonic epilepsy

Drug treatment of juvenile myoclonic epilepsy (JME) is mainly based on clinical experience and prospective and retrospective studies, with little evidence from randomized clinical trials. There are almost no head-to-head comparisons between old and new antiepileptic drugs (AEDs). Valproate is the drug of the first choice in men with JME. In women, lamotrigine (LTG) should be preferred regarding teratogenicity and side effects of valproate. Levetiracetam (LEV) is also effective. Recent data suggest that it may soon be used as first line treatment. Some AEDs can aggravate JME. In addition to AEDs, nonpharmacological treatments are important in JME. JME usually requires lifelong treatment because seizures nearly always return after withdrawal of therapy.

Treatment of myoclonic seizures in patients with juvenile myoclonic epilepsy

Drug treatment of Juvenile myoclonic epilepsy (JME) is mainly based on clinical experience and prospective and retrospective studies, with little evidence from randomized clinical trials. There are no head-to-head comparisons between old and new antiepileptic drugs (AEDs) and no drugs licensed specifically for JME. Valproate is unquestionably the drug of the first choice in men with JME. In women, lamotrigine should be preferred regarding teratogenicity and side effects of valproate. In addition, levetiracetam and topiramate are effective and can be use in combination or as second line treatment. Some AEDs can aggravate JME. In addition of AEDs, non-pharmacological treatments are important in JME. JME usually require lifelong treatment because seizures nearly always return after withdrawal of therapy.

A delayed class of BOLD waveforms associated with spreading depression in the feline cerebral cortex can be detected and characterised using independent component analysis (ICA)

An application of independent component analysis to blood oxygenation level- dependent MRI (BOLD-MRI) results was used to detect cerebrovascular changes that followed the initiation of cortical spreading depression (CSD) in feline brain. The cortical images were obtained from a horizontal plane at 28 s intervals before, and for 1.4-1.75 h after, KCl dissolved in agar (KCl/agar) had been directly applied to the left suprasylvian gyrus of 13 anesthetized cats for 10 min. It successfully resolved, for the first time, a novel class of prolonged, and delayed, biphasic BOLD waveforms. These were larger in amplitude ( approximately 20%), longer lasting and more delayed in onset (13-33 min) than the brief propagating (90 s) BOLD increase ( approximately 4%) already known to be associated with CSD on earlier occasions. Furthermore, such changes occurred in localized regions on the hemisphere ipsilateral to the site of stimulus application in 4 out of 5 control subjects rather than themselves generating propagating waves. Finally, the biphasic waveforms were consistently abolished in the 4 experimental animals studied following the i.v. administration of sumatriptan (0.3 mg kg(-1)), an antimigraine 5-HT(1B/1D) agonist, 15 min before the application of the transient stimulus. They were abolished in 2 out of 4 animals following the intraperitoneal (i.p.) administration of SB-220453 (tonabersat: 10 mg kg(-1), 90 min before stimulus application), a novel anticonvulsant that has recently been reported to inhibit CSD. ICA has thus been successful in detecting a novel localized, as opposed to propagating, signal of potential physiological significance hidden in complex BOLD- MRI data, whose sensitivity to sumatriptan may relate it to the cerebrovascular changes reported in the headache phase of migraine.

Binding characteristics of [3H]ucb 30889 to levetiracetam binding sites in rat brain

Levetiracetam (2S-(2-oxo-1-pyrrolidinyl)butanamide, KEPPRA, a novel antiepileptic drug, has been shown to bind to a specific binding site located in brain (levetiracetam binding site [Eur. J. Pharmacol. 286 (1995) 137]). However, [3H]levetiracetam displayed only micromolar affinity for these sites making it an unsuitable probe for further characterization. The present study describes the binding properties of an analogue of levetiracetam: [3H]ucb 30889, (2S)-2-[4-(3-azidophenyl)-2-oxopyrrolidin-1-yl]butanamide. [3H]ucb 30889 binds reversibly to specific binding sites in rat brain. Kinetics at 4 degrees C were biphasic with half-times of association and dissociation of, respectively, 3 and 4 min for the fast component and 47 and 61 min for the slow component. [3H]ucb 30889 saturation binding curves were compatible with the labelling of a homogenous population of binding sites having a B(max) of 4496+/-790 fmol/mg protein (mean+/-S.D., n=5) and a K(d) of 62+/-20 nM (mean+/-S.D., n=5), a 20-fold increase in affinity compared to [3H]levetiracetam. Competition binding curves with ligands known to interact with levetiracetam binding sites and tissue distribution restricted to the brain indicated that [3H]ucb 30889 and [3H]levetiracetam bind to the same site. Although levetiracetam binding sites and GABA(A) (gamma-aminobutyric acid) receptors share some ligands such as pentobarbital and pentylenetetrazol, experiments performed with [35S]TBPS (tert-butyl-bicyclo[2.2.2]phosporothionate), a probe for the GABA(A) Cl(-) channel do not support the hypothesis that levetiracetam binding sites are part of the GABA(A) receptor complex. Preliminary autoradiography studies in rat brain revealed that [3H]ucb 30889 labels specific sites in all brain regions and that this binding is concentration-dependently displaced by levetiracetam.

The design of 8,8-dimethyl[1,6]naphthyridines as potential anticonvulsant agents

Starting from a series of 7-linked tetrahydroisoquinoline derivatives, as exemplified by SB-270664, a new series of 8,8-dimethylnaphthyridine compounds has been identified. SAR studies around these attractive leads have provided compounds such as 12 which display excellent anticonvulsant activity and an encouraging pharmacokinetic profile in vivo.

Novel anticonvulsant medications in development

Epilepsy is currently the most prevalent neurological disorder worldwide. Pharmacological therapy remains the cornerstone of epilepsy treatment, however, refractory epilepsy is still a significant clinical problem despite the release of the second generation of anticonvulsants. Anticonvulsant treatment failures may result from lack of efficacy and presence of significant side effects. One rationale for incomplete effectiveness of the currently available anticonvulsants is that they were identified using the same classical models and therefore work largely by the same actions. These mechanisms fail to consider variations in the pathophysiological process that results in epilepsy, nor have they been shown to prevent the process of developing epilepsy (epileptogenesis). The next generation of anticonvulsants has taken into account the shortcomings of existing agents and attempted to improve on the currently available treatments using rationale drug design. This group of investigational anticonvulsants may be broadly classified as possessing one or more of the following: 1) increased tolerability through improvement in drug chemical structure or better delivery to the site of action, 2) new mechanisms (or combinations of mechanisms) of action, 3) improved pharmacokinetic properties. This article will discuss the next generation of anticonvulsants (carabersat, CGX-1007, fluorofelbamate, harkoseride, losigamone, pregabalin, retigabine, safinamide, SPD-421, talampanel, valrocemide) and the possible populations in which they would be clinically useful.

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

The Sixth Eilat Conference on New Antiepileptic Drugs (AEDs) took place in Taormina, Sicily, Italy from 7th to 11th April, 2002. Basic scientists, clinical pharmacologists and neurologists from 27 countries attended the conference, whose main themes included dose-response relationships with conventional and recent AEDs, teratogenic effects of conventional and recent AEDs, update on clinical implications of AED metabolism, prevention of epileptogesis, and seizure aggravation by AEDs. According to tradition, the central part of the conference was devoted to a review of AEDs in development, as well to updates on AEDs, which have been marketed in recent years. This article summarizes the information presented on drugs in preclinical and clinical development, including carabersat (SB-204269), CGX-1007 (Conantokin-G), pregabalin, retigabine (D-23129), safinamide, SPD421 (DP-VPA), SPM 927, talampanel and valrocemide (TV 1901). Updates on fosphenytoin, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, zonisamide, new formulations of valproic acid, and the antiepileptic vagal stimulator device are also presented.

Future prospects for the drug treatment of epilepsy

Great progress has been made in the last 150 years in the pharmacological management of epilepsy, and, despite the increasing number of technological advances available, antiepileptic drugs (AEDs) remain the mainstay of treatment for the vast majority of patients with epilepsy. This review looks at possible avenues of development in the drug treatment of epilepsy. The strengths and weaknesses of those AEDs which are currently licensed are examined, and ways in which their use may be improved are discussed (e.g. rational combinations, use of new formulations). Potentially new targets that may allow the development of effective treatments are highlighted (neuroimmunological manipulation, decreasing inherent drug resistance mechanisms, and modification of adenosine neurotransmission), and a summary of the most promising AEDs currently in development is provided [e.g. carabersat, ganaxolone, harkoseride, MDL 27192, safinamide (NW 1015), pregabalin, retigabine, talampanel, valrocemide, losigamone and BIA 2093].

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.

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

1. The effects of tonabersat (SB-220453) were evaluated on trigeminal nerve ganglion stimulation-induced sensory-autonomic neurovascular reflexes in the anaesthetized cat. Comparisons were made to intravenous administration of carabersat (SB-204269), and to valproate, gabapentin and lamotrigine following intraduodenal administration. 2. There were no effects on resting blood pressure, heart rate, carotid blood flow or carotid vascular resistance for any compound evaluated. 3. Trigeminal nerve ganglion stimulation increased carotid blood flow by 65% and reduced vascular resistance by 41% with minimal effect on blood pressure (< 10%) and no effect on heart rate. Intravenous infusion of tonabersat or carabersat (both 3.4 micromol h(-1)) produced time related reductions in stimulation-induced responses with a maximal inhibition (relative to control) of 30 +/- 7% (n=4), at 240 min for tonabersat and 33+/-4% (n=3) at 180 min for carabersat. Tonabersat (11.5 micromol h(-1)) produced a similar inhibitory effect (32 +/- 9%, n=4) after 120 min of infusion. 4. Following intraduodenal administration of tonabersat, the maximal inhibition of nerve stimulation-induced responses was 55 +/- 4% at 120 min (n=4) for tonabersat 10 mg kg(-1), and 24+/-2% after 180 min for 1 mg kg(-1) (n=4). 5. Intraduodenal administration of sodium valproate (10 or 100 mg kg(-1) n=4/group) had no effect on neurovascular reflexes. Maximal inhibition of nerve ganglion-stimulated reductions in carotid vascular resistance were observed at 150 min for lamotrigine (50 mg kg(-1), 52+/-12%, n=4) and gabapentin (100 mg kg(-1), 17+/-13%, n=3). Lamotrigine 10 mg kg(-1) produced 22+/-11% (n=3) inhibition after 180 min. 6. These data demonstrate blockade of trigeminal parasympathetic reflexes with tonabersat, carabersat and other anticonvulsants. These agents may therefore have therapeutic benefit in conditions where this type of reflex is evident.

Use of the radioligand [(125)I]-SB-217644 in the characterisation and solubilisation of the novel binding site for the anticonvulsant SB-204269 in rat brain membranes and cell lines

SB-204269 (trans-(+)-6-acetyl-4S-(4-fluorobenzoylamino)-3, 4-dihydro-2,2-dimethyl-2H-benzo[b]pyran-3R-ol) shows anticonvulsant activity in a range of animal seizure models, with a high therapeutic index and a lack of side-effects. We have previously reported the characterisation of a novel binding site for [(3)H]-SB-204269 in rat forebrain, which has a unique profile unrelated to other known anticonvulsant sites of action. We now describe the use of a [(125)I]-labelled form of SB-217644 (trans-6-acetyl-4S-(3-iodobenzoylamino)-3,4-dihydro-2, 2-dimethyl-2H-benzo[b]pyran-3R-ol), an analogue of SB-204269, for studies on this novel binding site. In rat forebrain membranes, [(125)I]-SB-217644 shows a similar binding profile to that of [(3)H]-SB-204269, with a maximum specific binding capacity (B(max)) of 286+/-12 fmol/mg protein, but has twenty-fold higher affinity (K(d) value 1.7+/-0.1 nM). The high affinity and high specific activity of [(125)I]-SB-217644 allowed it to be used for detection and characterisation of the detergent-solubilised form of the binding site. Specific [(125)I]-SB-217644 binding to cholate-solubilised rat cerebellum showed a K(d) value of 2.7+/-0.3 nM and a B(max) value of 55+/-11 fmol/mg protein, with a 7.3+/-0.3% yield of solubilised binding sites. [(125)I]-SB-217644 was also used in whole-cell binding assays for investigation of the properties of the novel binding site in a range of cell lines. Both rat brain neuronal and glial primary cultures and several CNS-related cell lines were found to have levels of specific [(125)I]-SB-217644 binding similar to those present in rat forebrain membranes. The solubilisation of this novel binding site, and the ability to quantify and characterise it in solubilised tissues and whole cells using [(125)I]-SB217644, will allow further studies towards the ultimate identification of the molecular target of SB-204269.

Evaluation of a series of anticonvulsant 1,2,3,4-tetrahydroisoquinolinyl-benzamides

SAR studies around a series of N-(tetrahydroisoquinolinyl)-2-methoxybenzamides, identified by high-throughput screening at the novel SB-204269 binding site, have provided compounds such as 13, 29-33 with high affinity and excellent anticonvulsant activity in animal models.

Repetitive cortical spreading depression in a gyrencephalic feline brain: inhibition by the novel benzoylamino-benzopyran SB-220453

Transient cortical depolarization is implicated in the pathology of migraine. SB-220453 is a potent anti-convulsant which inhibits neurogenic inflammation and cortical spreading depression (SD)-evoked nitric oxide release via a novel but unknown mechanism. This study further investigates the effects of SB-220453 on generation and propagation of repetitive SD in the anaesthetized cat. Vehicle or SB-220453 1, 3 or 10 mg/kg was administered intraperitoneally 90 min prior to induction of SD in the suprasylvian gyrus (SG). Changes in d.c. potential were recorded in the SG and the adjacent marginal gyrus (MG). In vehicle-treated animals (n = 7), a brief exposure (6 min) to KCl induced a median (25-75% range) number of five (four to six) and three (two to four) depolarizations over a duration of 55 min (32-59 min) and 51 min (34-58 min) in the SG and MG, respectively. SB-220453 produced dose-related inhibition of the number of events and period of repetitive SD activity. SB-220453 also reduced SD-induced repetitive pial vasodilatation but had no effect on resting haemodynamics. However, when SD events were observed in the presence of SB-220453, it had no effect on metabolic coupling. These results show that SB-220453 produces marked inhibition of repetitive SD in the anaesthetized cat. SB-220453 may therefore have therapeutic potential in treatment of SD-like activity in migraine.

Solubilisation of a novel anticonvulsant binding site from pig cortical membranes

The present study describes the solubilisation of the novel anticonvulsant, SB-204269, binding site from pig cortical membranes. Throughout the study the binding of a close analogue of this compound, [125I]-SB-217644 (trans 6- Acetyl-4S-(3-iodobenzoylamino)-3,4-dihydro-2,2-dimethyl-2H-benzo[b ]pyran-3R-ol) was used to monitor the success of the solubilisation procedure. [125I]-SB-217644 was an ideal mechanistic tool for quantifying the binding to this novel anticonvulsant site, with a high specific activity and affinity (K(D) of 3 nmol/l). Optimum conditions for the solubilisation of this anticonvulsant binding site were investigated using a multifactorial experimental design to assess a large number of variables. Detergent type, detergent-protein ratio, absence of Mg2+ and temperature were deemed to be important factors. However, the increases observed in binding site specific activity were minimal compared with those achieved for yields. Maximum percentage yields of binding activity (25%) were achieved with a low concentration of the zwitterionic detergent, CHAPS, in the presence of a low protein concentration. This yield was further enhanced on combining mixtures of detergents. The highest recovery (37%) was achieved with a 50:50 (v:v; 1.5 x critical micelle concentration) mixture of the ionic detergent, sodium cholate, and the non-ionic detergent, MEGA-10. In summary, we report the successful solubilisation of a novel anticonvulsant binding site, identified by its selective affinity for SB-204269 and its analogues. The recovery of nearly 40% of the target binding sites from the starting material should provide a good starting point for the purification of this protein.

Lack of effect of the novel anticonvulsant SB-204269 on voltage-dependent currents in neurones cultured from rat hippocampus

The novel anticonvulsant SB-204269 inhibits epileptiform afterdischarges induced by high K+ in rat hippocampal slices. Its effects on voltage-gated Na+ currents, measured from cultured hippocampal neurones using whole cell patch clamp, were compared to the effects of existing anticonvulsants. SB-204269 produced no significant tonic block of Na+ currents nor any voltage-dependent and frequency-dependent block at doses 50 to 500 fold higher than its anticonvulsant EC50 of 0.2 microM. In contrast, lamotrigine, phenytoin and carbamazepine at 50 microM, blocked Na+ currents in a voltage-dependent manner. SB-204269 also had no effect on action potential discharges evoked by elevating external K+. These data suggest that direct blockade of voltage-gated channels does not contribute to the anticonvulsant properties of SB-204269 and further support the hypothesis that this compound has a novel mechanism of action.

Identification of (-)-cis-6-acetyl-4S-(3-chloro-4-fluoro-benzoylamino)- 3,4-dihydro-2,2-dimethyl-2H-benzo[b]pyran-3S-ol as a potential antimigraine agent

Optimisation of novel cis- and trans-4-(substituted-amido)benzopyran-3-ol derivatives has led to the identification of SB-220453 20 with an in vivo pre-clinical CNS profile predictive of potential antimigraine activity.

Identification of a series of 1,2,3,4-tetrahydroisoquinolinyl-benzamides with potential anticonvulsant activity

A series of N-(tetrahydroisoquinolinyl)-2-methoxybenzamides was identified by high-throughput screening at the novel SB-204269 binding site. SAR studies have provided compounds 4 and 14 with high affinity and good anticonvulsant activity in animal models.

Profile of SB-204269, a mechanistically novel anticonvulsant drug, in rat models of focal and generalized epileptic seizures

1. Earlier optimization of structure-activity relationships in a novel series of 4-(benzoylamino)-benzopyrans, led to the discovery of SB-204269 (trans-(+)-6-acetyl-4S-(4-fluorobenzoylamino)-3,4-dihydro-2, 2-dimethyl-2H-benzo[b]pyran-3R-ol, hemihydrate), a potent orally-active anticonvulsant in the mouse maximal electroshock seizure threshold (MEST) test. 2. Studies have now been undertaken to determine the effects of SB-204269 in a range of seizure models and tests of neurological deficits in rats. In addition, the compound has been evaluated in a series of in vitro mechanistic assays. 3. SB-204269 proved to be an orally-effective anticonvulsant agent, at doses (0.1-30 mg Kg-1) devoid of overt behavioural depressant properties, in models of both electrically (MEST and maximal electroshock (MEST)) and chemically (i.v. pentylenetetrazol (PTZ) infusion)-evoked tonic extension seizures. However, the compound did not inhibit PTZ-induced myoclonic seizures at doses up to 30 mg kg-1, p.o. 4. SB-204269 also selectively reduced focal electrographic seizure activity in an in vitro elevated K+ rat hippocampal slice model at concentrations (0.1-10 microM) that had no effect on normal synaptic activity and neuronal excitability. 5. In all of these seizure models, SB-204269 was equivalent or better than the clinically established antiepileptic drugs carbamazepine and lamotrigine, in terms of anticonvulsant potency and efficacy. 6. Unlike SB-204269, the corresponding trans 3S,4R enantiomer, SB-204268, did not produce marked anticonvulsant effects, an observation in accord with previous findings for other related pairs of trans enantiomers in the benzopyran series. 7. In the rat accelerating rotarod test, a sensitive paradigm for the detection of neurological deficits such as sedation and motor incoordination, SB-204269 was inactive even at doses as high as 200 mg kg-1, p.o. This was reflected in the excellent therapeutic index (minimum significantly effective dose in the rotarod test/ED50 in the MES test) for SB-204269 of > 31, as compared to equivalent values of only 7 and 13 for carbamazepine and lamotrigine, respectively. 8. At concentrations (> or = 10 microM) well above those required to produce anticonvulsant activity in vivo (i.e. 0.1 microM in brain), SB-204269 did not interact with many of the well known mechanistic targets for established antiepileptic drugs (e.g. Na+ channels or GABAergic neurotransmission). Subsequent studies have shown that the anticonvulsant properties of SB-204269 are likely to be mediated by a novel stereospecific binding site present in the CNS. 9. The overall efficacy profile in rodent seizure models, together with a minimal liability for inducing neurological impairment and an apparently unique mechanism of action, highlight the therapeutic potential of SB-204269 for the treatment of refractory partial and generalized tonic-clonic seizures.