Synergy between retigabine and GABA in modulating the convulsant site of the GABAA receptor complex

Glial KCNQ K+ channels control neuronal output by regulating GABA release from glia in C. elegans

KCNQs are voltage-gated K+ channels that control neuronal excitability and are mutated in epilepsy and autism spectrum disorder (ASD). KCNQs have been extensively studied in neurons, but their function in glia is unknown. Using voltage, calcium, and GABA imaging, optogenetics, and behavioral assays, we show here for the first time in Caenorhabditis elegans (C. elegans) that glial KCNQ channels control neuronal excitability by mediating GABA release from glia via regulation of the function of L-type voltage-gated Ca2+ channels. Further, we show that human KCNQ channels have the same role when expressed in nematode glia, underscoring conservation of function across species. Finally, we show that pathogenic loss-of-function and gain-of-function human KCNQ2 mutations alter glia-to-neuron GABA signaling in distinct ways and that the KCNQ channel opener retigabine exerts rescuing effects. This work identifies glial KCNQ channels as key regulators of neuronal excitability via control of GABA release from glia.

New developments and opportunities in drugs being trialed for amyotrophic lateral sclerosis from 2020 to 2022

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder that primarily affects motor neurons in the brain and spinal cord. In the recent past, there have been just two drugs approved for treatment, riluzole and edaravone, which only prolong survival by a few months. However, there are many novel experimental drugs in development. In this review, we summarize 53 new drugs that have been evaluated in clinical trials from 2020 to 2022, which we have classified into eight mechanistic groups (anti-apoptotic, anti-inflammatory, anti-excitotoxicity, regulated integrated stress response, neurotrophic factors and neuroprotection, anti-aggregation, gene therapy and other). Six were tested in phase 1 studies, 31 were in phase 2 studies, three failed in phase 3 studies and stopped further development, and the remaining 13 drugs were being tested in phase 3 studies, including methylcobalamin, masitinib, MN-166, verdiperstat, memantine, AMX0035, trazodone, CNM-Au8, pridopidine, SLS-005, IONN363, tofersen, and reldesemtiv. Among them, five drugs, including methylcobalamin, masitinib, AMX0035, CNM-Au8, and tofersen, have shown potent therapeutic effects in clinical trials. Recently, AMX0035 has been the third medicine approved by the FDA for the treatment of ALS after riluzole and edaravone.

The Influence of AA29504 on GABAA Receptor Ligand Binding Properties and Its Implications on Subtype Selectivity

The unique pharmacological properties of δ-containing γ-aminobutyric acid type A receptors (δ-GABAARs) make them an attractive target for selective and persistent modulation of neuronal excitability. However, the availability of selective modulators targeting δ-GABAARs remains limited. AA29504 ([2-amino-4-(2,4,6-trimethylbenzylamino)-phenyl]-carbamic acid ethyl ester), an analog of K+ channel opener retigabine, acts as an agonist and a positive allosteric modulator (Ago-PAM) of δ-GABAARs. Based on electrophysiological studies using recombinant receptors, AA29504 was found to be a more potent and effective agonist in δ-GABAARs than in γ2-GABAARs. In comparison, AA29504 positively modulated the activity of recombinant δ-GABAARs more effectively than γ2-GABAARs, with no significant differences in potency. The impact of AA29504's efficacy- and potency-associated GABAAR subtype selectivity on radioligand binding properties remain unexplored. Using [3H]4'-ethynyl-4-n-propylbicycloorthobenzoate ([3H]EBOB) binding assay, we found no difference in the modulatory potency of AA29504 on GABA- and THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol)-induced responses between native forebrain GABAARs of wild type and δ knock-out mice. In recombinant receptors expressed in HEK293 cells, AA29504 showed higher efficacy on δ- than γ2-GABAARs in the GABA-independent displacement of [3H]EBOB binding. Interestingly, AA29504 showed a concentration-dependent stimulation of [3H]muscimol binding to γ2-GABAARs, which was absent in δ-GABAARs. This was explained by AA29504 shifting the low-affinity γ2-GABAAR towards a higher affinity desensitized state, thereby rising new sites capable of binding GABAAR agonists with low nanomolar affinity. Hence, the potential of AA29504 to act as a desensitization-modifying allosteric modulator of γ2-GABAARs deserves further investigation for its promising influence on shaping efficacy, duration and plasticity of GABAAR synaptic responses.

Drug treatment of epilepsy: From serendipitous discovery to evolutionary mechanisms

Epilepsy is a chronic brain disorder caused by abnormal firing of neurons. Up to now, using antiepileptic drugs is the main method of epilepsy treatment. The development of antiepileptic drugs lasted for centuries. In general, most agents entering clinical practice act on the balance mechanisms of brain "excitability-inhibition". More specifically, they target voltage-gated ion channels, GABAergic transmission and glutamatergic transmission. In recent years, some novel drugs representing new mechanisms of action have been discovered. Although there are about 30 available drugs in the market, it is still in urgent need of discovering more effective and safer drugs. The development of new antiepileptic drugs is into a new era: from serendipitous discovery to evolutionary mechanism-based design. This article presents an overview of drug treatment of epilepsy, including a series of traditional and novel drugs.

The Kv7 Modulator, Retigabine, is an Efficacious Antiseizure Drug for Delayed Treatment of Organophosphate-induced Status Epilepticus

Benzodiazepines are the primary treatment option for organophosphate (OP)-induced status epilepticus (SE), but these antiseizure drugs (ASDs) lose efficacy as treatment is delayed. In the event of a mass civilian or military exposure, significant treatment delays are likely. New ASDs that combat benzodiazepine-resistant, OP-induced SE are critically needed, particularly if they can be efficacious after a long treatment delay. This study evaluated the efficacy of the Kv7 channel modulator, retigabine, as a novel therapy for OP-induced SE. Adult, male rats were exposed to soman or diisopropyl fluorophosphate (DFP) to elicit SE and monitored by electroencephalogram (EEG) recording. Retigabine was administered alone or adjunctive to midazolam (MDZ) at delays of 20- or 40-min in the soman model, and 60-min in the DFP model. Following EEG recordings, rats were euthanized and brain tissue was collected for Fluoro-Jade B (FJB) staining to quantify neuronal death. In the DFP model, MDZ + 15 mg/kg retigabine suppressed seizure activity and was neuroprotective. In the soman model, MDZ + 30 mg/kg retigabine suppressed seizures at 20- and 40-min delays. Without MDZ, 15 mg/kg retigabine provided partial antiseizure and neuroprotectant efficacy in the DFP model, while 30 mg/kg without MDZ failed to attenuate soman-induced SE. At 60 mg/kg, retigabine without MDZ strongly reduced seizure activity and neuronal degeneration against soman-induce SE. This study demonstrates the antiseizure and neuroprotective efficacy of retigabine against OP-induced SE. Our data suggest retigabine could be a useful adjunct to standard-of-care and has potential for use in the absence of MDZ.

KCNQ3 is the principal target of retigabine in CA1 and subicular excitatory neurons

Retigabine is a first-in-class potassium channel opener approved for patients with epilepsy. Unfortunately, several side effects have limited its use in clinical practice, overshadowing its beneficial effects. Multiple studies have shown that retigabine acts by enhancing the activity of members of the voltage-gated KCNQ (Kv7) potassium channel family, particularly the neuronal KCNQ channels KCNQ2-KCNQ5. However, it is currently unknown whether retigabine's action in neurons is mediated by all KCNQ neuronal channels or by only a subset. This knowledge is necessary to elucidate retigabine's mechanism of action in the central nervous system and its adverse effects and to design more effective and selective retigabine analogs. In this study, we show that the action of retigabine in excitatory neurons strongly depends on the presence of KCNQ3 channels. Deletion of Kcnq3 severely limited the ability of retigabine to reduce neuronal excitability in mouse CA1 and subiculum excitatory neurons. In addition, we report that in the absence of KCNQ3 channels, retigabine can enhance CA1 pyramidal neuron activity, leading to a greater number of action potentials and reduced spike frequency adaptation; this finding further supports a key role of KCNQ3 channels in mediating the action of retigabine. Our work provides new insight into the action of retigabine in forebrain neurons, clarifying retigabine's action in the nervous system.NEW & NOTEWORTHY Retigabine has risen to prominence as a first-in-class potassium channel opener approved by the Food and Drug Administration, with potential for treating multiple neurological disorders. Here, we demonstrate that KCNQ3 channels are the primary target of retigabine in excitatory neurons, as deleting these channels greatly diminishes the effect of retigabine in pyramidal neurons. Our data provide the first indication that retigabine controls neuronal firing properties primarily through KCNQ3 channels.

Activation of KCNQ4 as a Therapeutic Strategy to Treat Hearing Loss

Potassium voltage-gated channel subfamily q member 4 (KCNQ4) is a voltage-gated potassium channel that plays essential roles in maintaining ion homeostasis and regulating hair cell membrane potential. Reduction of the activity of the KCNQ4 channel owing to genetic mutations is responsible for nonsyndromic hearing loss, a typically late-onset, initially high-frequency loss progressing over time. In addition, variants of KCNQ4 have also been associated with noise-induced hearing loss and age-related hearing loss. Therefore, the discovery of small compounds activating or potentiating KCNQ4 is an important strategy for the curative treatment of hearing loss. In this review, we updated the current concept of the physiological role of KCNQ4 in the inner ear and the pathologic mechanism underlying the role of KCNQ4 variants with regard to hearing loss. Finally, we focused on currently developed KCNQ4 activators and their pros and cons, paving the way for the future development of specific KCNQ4 activators as a remedy for hearing loss.

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

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

Neuropathic pain and Kv7 voltage-gated potassium channels: The potential role of Kv7 activators in the treatment of neuropathic pain

Neuropathic pain conditions severely and chronically affect the quality of life in a large human population, but the pain conditions are not adequately managed due to poor understanding of their underlying mechanisms. There is a pressing need for further research into this field to help develop effective and nonaddictive medications to treat neuropathic pain. This article first describes general clinical classification of pain, types and symptoms of neuropathic pain, and current practices of clinical management for neuropathic pain. This is followed by a discussion of various cellular and molecular mechanisms responsible for the development and maintenance of neuropathic pain. In this review, we highlight the loss of function of Kv7 voltage-gated potassium as a mechanism of neuropathic pain and the potential use of Kv7 channel activator as subsequent treatment.

Design, synthesis and evaluation of substituted piperidine based KCNQ openers as novel antiepileptic agents

Epilepsy is a kind of disease with complicated pathogenesis. KCNQ (Kv7) is a voltage dependent potassium channel that is mostly associated with epilepsy and thus becomes an important target in the treatment of epilepsy. In this paper, a series of substituted piperidine derivatives targeting KCNQ were designed and synthesized by using scaffold hopping and active substructure hybridization. Compounds were evaluated by fluorescence-based thallium influx assay, Rb⁺ flow assay and electrophysiological patch-clamp assay. Results showed that some compounds possessed more potent potassium channel opening activity than Retigabine. More significantly, compound 11 was found to have good pharmacokinetic profiles in vivo.

Efficacy of Retigabine on Acute Limbic Seizures in Adult Rats

Background and Purpose:

The efficacy of retigabine (RGB), a positive allosteric modulator of K+ channels indicated for adjunct treatment of partial seizures, was studied in two adult models of kainic acid (KA)-induced status epilepticus to determine it’s toleratbility.

Methods:

Retigabine was administered systemiclly at high (5 mg/kg) and low (1–2 mg/kg) doses either 30 min prior to or 2 hr after KA-induced status epilepticus. High (1 µg/µL) and low (0.25 µg/µL) concentrations of RGB were also delivered by intrahippocampal microinjection in the presence of KA.

Results:

Dose-dependent effects of RGB were observed with both models. Lower doses increased seizure behavior latency and reduced the number of single spikes and synchronized burst events in the electroencephalogram (EEG). Higher doses worsened seizure behavior, produced severe ataxia, and increased spiking activity. Animals treated with RGB that were resistant to seizures did not exhibit significant injury or loss in GluR1 expression; however if stage 5–6 seizures were reached, typical hippocampal injury and depletion of GluR1 subunit protein in vulernable pyramidal fields occurred.

Conclusions:

RGB was neuroprotective only if seizures were significantly attenuated. GluR1 was simultaneously suppressed in the resistant granule cell layer in presence of RGB which may weaken excitatory transmission. Biphasic effects observed herein suggest that the human dosage must be carefully scrutinized to produce the optimal clinical response.

Discovery of a novel Kv7 channel opener as a treatment for epilepsy

Facilitating activation, or delaying inactivation, of the native Kv7 channel reduces neuronal excitability, which may be beneficial in controlling spontaneous electrical activity during epileptic seizures. In an effort to identify a compound with such properties, the structure-activity relationship (SAR) and in vitro ADME for a series of heterocyclic Kv7.2-7.5 channel openers was explored. PF-05020182 (2) demonstrated suitable properties for further testing in vivo where it dose-dependently decreased the number of animals exhibiting full tonic extension convulsions in response to corneal stimulation in the maximal electroshock (MES) assay. In addition, PF-05020182 (2) significantly inhibited convulsions in the MES assay at doses tested, consistent with in vitro activity measure. The physiochemical properties, in vitro and in vivo activities of PF-05020182 (2) support further development as an adjunctive treatment of refractory epilepsy.

Retigabine calms seizure-induced behavior following status epilepticus

In adult rats, intraperitoneal injection of kainate (KA) results in sustained status epilepticus and persistent behavioral comorbidities such as hyperexcitability, anxiety, and altered response to environmental cues. Intrahippocampal KA also results in sustained status epilepticus and continuous high frequency oscillations in the electroencephalograph (EEG), although subsequent behavioral side effects are unknown. We hypothesized that retigabine, a recently discovered anticonvulsant and potent positive modulator of Kv7 channels, may attenuate seizure-induced behavioral abnormalities. Status epilepticus was induced by administration of KA either intraperitoneally (15 mg/kg) or by single intrahippocampal injection (1.0 μg/0.5 μL). After 24 h, half of systemically KA-treated animals that reached stage 6 seizures were injected once daily with retigabine (5 mg/kg) for 14 continuous days. All groups underwent three behavioral tests--capture and handling, open field, and elevated plus maze--24 h following the last retigabine treatment and were sacrificed at 25-28 days. In the capture and handling test, systemic KA treatment resulted in frisky behavior and resistance to capture with wild attempts to escape during the 1st, 2nd, and 3rd weeks of the observation period. In contrast, these behaviors were attenuated in KA+retigabine-treated animals. In the open-field test, KA-treated animals spent more time in the center zone, but KA+retigabine-treated rats had greater overall activity compared with those having vehicle, KA, or retigabine-only treatment. In the elevated plus maze, KA+retigabine-treated animals traveled greater distances in open and closed arms (proximal and distal) compared with controls, also signifying anxiety reduction. Retigabine-only-treated rats traveled more in the open proximal arms compared with controls, indicating increased hyperlocomotion in normotensive rats. Although treatment with KA+retigabine resulted in anxiolytic-like effects in all three behavioral tasks compared with vehicle, this group did not significantly differ from systemically KA-treated rats in most measurements in open-field and elevated plus maze tasks, suggesting that retigabine may also cause hyperlocomotion unrelated to anxiety level. Despite that intrahippocampal KA-treated rats displayed comparable seizure behavior, epileptiform activity, and hippocampal injury, their behavior resembled the controls, suggesting that molecular and subsequent cellular changes are also partially responsible for anxiolytic-like effects and that these results are likely independent of the hippocampus.

Activation of peripheral KCNQ channels attenuates inflammatory pain

Background

Refractory chronic pain dramatically reduces the quality of life of patients. Existing drugs cannot fully achieve effective chronic pain control because of their lower efficacy and/or accompanying side effects. Voltage-gated potassium channels (KCNQ) openers have demonstrated their analgesic effect in preclinical and clinical studies, and are thus considered to be a potential therapeutic target as analgesics. However, these drugs exhibit a narrow therapeutic window due to their imposed central nerve system (CNS) side effects. To clarify the analgesic effect by peripheral KCNQ channel activation, we investigated whether the analgesic effect of the KCNQ channel opener, retigabine, is inhibited by intracerebroventricular (i.c.v.) administration of the KCNQ channel blocker, 10, 10-bis (4-Pyridinylmethyl)-9(10H) -anthracenone dihydrochloride (XE-991) in rats.

Results

Oral administration (p.o.) of retigabine showed an anticonvulsant effect on maximal electronic seizures and an analgesic effect on complete Freund’s adjuvant-induced thermal hyperalgesia. However, impaired motor coordination and reduced exploratory behavior were also observed at the analgesic doses of retigabine. Administration (i.c.v.) of XE-991 reversed the retigabine-induced anticonvulsant effect, impaired motor coordination and reduced exploratory behavior but not the analgesic effect. Moreover, intraplantar administration of retigabine or an additional KCNQ channel opener, N-(6-Chloro-pyridin-3-yl)-3,4-difluoro-benzamide (ICA-27243), inhibited formalin-induced nociceptive behavior.

Conclusions

Our findings suggest that the peripheral sensory neuron is the main target for KCNQ channel openers to induce analgesia. Therefore, peripheral KCNQ channel openers that do not penetrate the CNS may be suitable analgesic drugs as they would prevent CNS side effects.

Ezogabine (KCNQ2/3 channel opener) prevents delayed activation of meningeal nociceptors if given before but not after the occurrence of cortical spreading depression

We proposed recently that induction of delayed activation of trigeminovascular neurons by cortical spreading depression (CSD) can explain the delayed onset of headache after the migraine aura ("aura"). This prompted us to search for ways to block the neuronal activation by CSD - a preclinical correlate of an attempt to find a drug that can block the initiation of headache when administered shortly after onset of aura (i.e., preemptively). Because migraine headache and epileptic seizures are comorbid chronic neurological disorders characterized by hyperexcitable brain networks, we began the search for such goal with an M-type potassium channel opener. We opted to use ezogabine, recently approved by the FDA as adjunctive treatment of partial onset seizures in adults, because it is a selective KCNQ2/3 channel opener. When CSD was induced before ezogabine injection (8.25 mg/kg, i.p.), 40% (6/15) of the units doubled their firing rate about 45 min later for about 95 min. Similarly, when CSD was induced before vehicle was injected (4% DMSO, 0.5% methylcellulose), 50% (3/6) of the units doubled their firing rate about 30 min later for about 120 min. When CSD was triggered 1h after ezogabine injection, it activated only 8% of the units. By itself, ezogabine injection resulted in a 30% attenuation of ongoing firing in all 10 control units. Thus, activation of KCNQ2/3 channels during the aura is unlikely to preempt the onset of headache but may reduce the incidence of migraine if given during prodromes that precede the headache by hours. Given the mechanistic similarities between migraine aura and epileptic seizures, it may be worthwhile to determine whether preemptive administration of ezogabine can prevent oncoming seizures in patients whose warning signs precede their seizures by more than an hour.

Characterization of a novel high-potency positive modulator of K(v)7 channels

K(v)7 channel activators decrease neuronal excitability and might potentially treat neuronal hyperexcitability disorders like epilepsy and mania. Here we introduce NS15370 ((2-(3,5-difluorophenyl)-N-[6-[(4-fluorophenyl)methylamino]-2-morpholino-3-pyridyl]acetamide)hydrochloride, an in vitro high-potency chemical analogue of retigabine, without effects on GABA(A) receptors. NS15370 activates recombinant homo- and heteromeric K(v)7.2-K(v)7.5 channels in HEK293 cells at sub-micromolar concentrations (EC₅₀~100 nM, as quantified by a fluorescence based Tl⁺-influx assay). In voltage clamp experiments NS15370 exhibits a complex, concentration-dependent mode-of-action: At low concentrations it accelerates voltage-dependent activation rates, slows deactivations, and increases steady-state current amplitudes. Quantified by the peak-tail current method, the V½ value of the steady-state activation curve is shifted towards hyperpolarized potentials at concentrations ~100 times lower than retigabine. However, in contrast to retigabine, NS15370 also introduces a distinct time-dependent current decrease, which eventually, at higher concentrations, causes suppression of the current at depolarized potentials, and an apparent "cross-over" of the voltage-activation curve. In brain slices, NS15370 hyperpolarizes and increases spike frequency adaptation of hippocampal CA1 neurons and the compound reduces the autonomous firing of dopaminergic neurons in the substantia-nigra pars compacta. NS15370 is effective in rodent models of hyperexcitability: (i) it yields full protection against mouse 6 Hz seizures and rat amygdala kindling discharges, two models of partial epilepsia; (ii) it reduces (+)-MK-801 hydrogen maleate (MK-801)-induced hyperactivity as well as chlordiazepoxide (CDP)+d-amphetamine (AMP)-induced hyperactivity, models sensitive to classic anti-psychotic and anti-manic treatments, respectively. Our findings with NS15370 consolidate neuronal K(v)7 channels as targets for anti-epileptic and psychiatric drug development.

Clinical utility of adjunctive retigabine in partial onset seizures in adults

In ~30% of epileptic patients, full seizure control is not possible, which is why the search for novel antiepileptic drugs continues. Retigabine exhibits a mechanism of action that is not shared by the available antiepileptic drugs. This antiepileptic enhances potassium currents via Kv7.2–7.3 channels, which very likely results from destabilization of a closed conformation or stabilization of the open conformation of the channels. Generally, the pharmacokinetics of retigabine are linear and the drug undergoes glucuronidation and acetylation. Results from clinical trials indicate that, in the form of an add-on therapy, retigabine proves an effective drug in refractory epileptic patients. The major adverse effects of the add-on treatment are dizziness, somnolence, and fatigue. This epileptic drug is also considered for other conditions – neuropathic pain, affective disorders, stroke, or even Alzheimer’s disease.

The mechanism of action of retigabine (ezogabine), a first-in-class K+ channel opener for the treatment of epilepsy

The pharmacologic profile of retigabine [RTG (international nonproprietary name); ezogabine, EZG (U.S. adopted name)], is different from all currently approved antiepileptic drugs (AEDs). Its primary mechanism of action (MoA) as a positive allosteric modulator of KCNQ2-5 (K(v) 7.2-7.5) ion channels defines RTG/EZG as the first neuronal potassium (K(+)) channel opener for the treatment of epilepsy. KCNQ2-5 channels are predominantly expressed in neurons and are important determinants of cellular excitability, as indicated by the occurrence of human genetic mutations in KCNQ channels that underlie inheritable disorders including, in the case of KCNQ2/3, the syndrome of benign familial neonatal convulsions. In vitro pharmacologic studies demonstrate that the most potent action of RTG/EZG is at KCNQ2-5 channels, particularly heteromeric KCNQ2/3. Furthermore, mutagenesis and modeling studies have pinpointed the RTG/EZG binding site to a hydrophobic pocket near the channel gate, indicating how RTG/EZG can stabilize the open form of KCNQ2-5 channels; the absence of this site in KCNQ1 also provides a clear explanation for the inbuilt selectivity RTG/EZG has for potassium channels other than the KCNQ cardiac channel. KCNQ channels are active at the normal cell resting membrane potential (RMP) and contribute a continual hyperpolarizing influence that stabilizes cellular excitability. The MoA of RTG/EZG increases the number of KCNQ channels that are open at rest and also primes the cell to retort with a larger, more rapid, and more prolonged response to membrane depolarization or increased neuronal excitability. In this way, RTG/EZG amplifies this natural inhibitory force in the brain, acting like a brake to prevent the high levels of neuronal action potential burst firing (epileptiform activity) that may accompany sustained depolarizations associated with the initiation and propagation of seizures. This action to restore physiologic levels of neuronal activity is thought to underlie the efficacy of RTG/EZG as an anticonvulsant in a broad spectrum of preclinical seizure models and in placebo-controlled trials in patients with partial epilepsy. In this article, we consider the pharmacologic characteristics of RTG/EZG at the receptor, cellular, and network levels as a means of understanding the novel and efficacious MoA of this new AED as defined in both preclinical and clinical research.

Profile of ezogabine (retigabine) and its potential as an adjunctive treatment for patients with partial-onset seizures

Epilepsy is a common disease with significant morbidity and mortality. Approximately one-third of patients with epilepsy are refractory to available seizure medications, emphasizing the need to develop better drugs with novel mechanisms of action. Ezogabine, also known as retigabine, is a new potential adjunctive treatment for adults with intractable partial seizures. Ezogabine has a unique mechanism of action consisting of activating KCNQ2/3 (Kv7) potassium channels. Ezogabine has undergone a number of Phase II and III trials demonstrating efficacy at 600,900 and 1200 mg/day in a dose-dependent fashion. The most common adverse events with ezogabine are central nervous system effects, particularly dizziness and somnolence. Urologic symptoms, particularly urinary retention, represent a rare but unique side effect of ezogabine. Ezogabine is predominantly metabolized via glucuronidation. Its half-life is 8 hours, suggesting a need for three-times-a-day administration. Ezogabine exhibits minimal interactions with other seizure medications, except possibly lamotrigine. Ezogabine has potential for clinical applications in other medical conditions beyond epilepsy, such as neuropathic pain, neuromyotonia, and bipolar disease, but these are based primarily on experimental models.

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.

Pharmacological characterization of a novel positive modulator at alpha 4 beta 3 delta-containing extrasynaptic GABA(A) receptors

The in vitro and in vivo pharmacological effects of [2-amino-4-(2,4,6-trimethylbenzylamino)-phenyl]-carbamic acid ethyl ester (AA29504), which is a close analogue of retigabine, have been investigated. AA29504 induced a rightward shift of the activation threshold at cloned KCNQ2, 2/3 and 4 channels expressed in Xenopus oocytes, with a potency 3-4fold lower than retigabine. AA29504 (1 muM) had no agonist activity when tested at alpha(1)beta(3)gamma(2s) or alpha(4)beta(3)delta GABA(A) receptors expressed in Xenopus oocytes, but left-shifted the EC(50) for GABA and gaboxadol (THIP) at both receptors. The maximum GABA response at alpha(1)beta(3)gamma(2s) receptors was unchanged by AA29504 (1 muM), but increased 3-fold at alpha(4)beta(3)delta receptors. In slices prepared from the prefrontal cortex of adult rats AA29504 had no effect alone on the average IPSC or the tonic current in layer II/III pyramidal neurons, but potentiated the effect of gaboxadol on both phasic and tonic currents. Thus, the effects of gaboxadol could be positively modulated by AA29504. Systemic administration of AA29504 at doses relevant for modulating GABA transmission produced anxiolytic effects and reduced motor coordination consistent with activity at GABA(A) receptors. We conclude that AA29504 exerts a major action via alpha(4)beta(3)delta-containing GABA(A) receptors, which will be important for interpreting its effect in vivo.

Enhancing m currents: a way out for neuropathic pain?

Almost three decades ago, the M current was identified and characterized in frog sympathetic neurons (Brown and Adams, 1980). The years following this discovery have seen a huge progress in the understanding of the function and the pharmacology of this current as well as on the structure of the underlying ion channels. Therapies for a number of syndromes involving abnormal levels of excitability in neurons are benefiting from research on M currents. At present, the potential of M current openers as analgesics for neuropathic pain is under discussion. Here we offer a critical view of existing data on the involvement of M currents in pain processing. We believe that enhancement of M currents at the site of injury may become a powerful strategy to alleviate pain in some peripheral neuropathies.

Electrophysiological and molecular analysis of Kv7/KCNQ potassium channels in the inferior colliculus of adult guinea pig

Neurons located in the inferior colliculus (IC) are on the path which processes acoustic information converging from the peripheral auditory system and to be sent through ascending pathways to superior structures. Previous in vitro recordings from early stage animals suggest that voltage-gated K channels underlie distinct neuronal discharge patterns observed in the IC. In this study, using reverse transcriptase quantitative polymerase chain reaction, we show the presence of a voltage-gated K channel family (Kv7/KCNQ) in the central nucleus of the IC (ICc) of the adult guinea pig. Whole-cell recordings from neurons in the nucleus were also made in slices from mature animals, and the action of specific openers and blockers demonstrated on the firing patterns. Our results indicate that mRNA from all members of the Kv7 family of channels are expressed in the ICc, but at different levels, and provide evidence that these channels can modulate neuronal excitability in this nucleus.

N-(6-chloro-pyridin-3-yl)-3,4-difluoro-benzamide (ICA-27243): a novel, selective KCNQ2/Q3 potassium channel activator

KCNQ2 (Kv7.2) and KCNQ3 (Kv7.3) are voltage-gated K(+) channel subunits that underlie the neuronal M current. In humans, mutations in these genes lead to a rare form of neonatal epilepsy (Biervert et al., 1998; Singh et al., 1998), suggesting that KCNQ2/Q3 channels may be attractive targets for novel antiepileptic drugs. In the present study, we have identified the compound N-(6-chloro-pyridin-3-yl)-3,4-difluoro-benzamide (ICA-27243) as a selective activator of the neuronal M current and KCNQ2/Q3 channels. In SH-SY5Y human neuroblastoma cells, ICA-27243 produced membrane potential hyperpolarization that could be prevented by coadministration with the M-current inhibitors 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone dihydrochloride (XE-991) and linopirdine. ICA-27243 enhanced both (86)Rb(+) efflux (EC(50) = 0.2 microM) and whole-cell currents in Chinese hamster ovary cells stably expressing heteromultimeric KCNQ2/Q3 channels (EC(50) = 0.4 microM). Activation of KCNQ2/Q3 channels was associated with a hyperpolarizing shift of the voltage dependence of channel activation (V((1/2)) shift of -19 mV at 10 microM). In contrast, ICA-27243 was less effective at activating KCNQ4 and KCNQ3/Q5 and was selective over a wide range of neurotransmitter receptors and ion channels such as voltage-dependent sodium channels and GABA-gated chloride channels. ICA-27243 (1-10 microM) was found to reversibly suppress seizure-like activity in an ex vivo hippocampal slice model of epilepsy and demonstrated in vivo anticonvulsant activity (ED(50) = 8.4 mg/kg) in the mouse maximal electroshock epilepsy model. In conclusion, ICA-27243 represents the first member of a novel chemical class of selective KCNQ2/Q3 activators with anticonvulsant-like activity in experimental models of epilepsy.

Antinociceptive effect of methyleugenol on formalin-induced hyperalgesia in mice

The effects of methyleugenol, an essential oil isolated from Asiasari radix, on antinociception were examined using the formalin test in mice. Oral administration of 10 mg/kg methyleugenol significantly decreased the duration of licking and biting behavior in the second phase without affecting that of the first phase, as did diclofenac, a non-steroidal anti-inflammatory drug. Methyleugenol also inhibited pain-related behaviors induced by intrathecal injection of N-methyl-d-aspartic acid (NMDA), while diclofenac did not affect these behaviors. These effects of methyleugenol were suppressed by bicuculline, a gamma-aminobutyric acid(A) (GABA(A)) antagonist. Muscimol, a GABA(A) agonist, displays the same action as methyleugenol with respect to the formalin test and NMDA-induced behaviors. Methyleugenol did not affect cyclooxygenase-1 and -2 activities. These results suggest that the antinociceptive effect of methyleugenol on the second phase of formalin-induced pain may be due to the inhibition of NMDA receptor-mediated hyperalgesia via GABA(A) receptors.

The KCNQ Channel Opener Retigabine Inhibits the Activity of Mesencephalic Dopaminergic Systems of the Rat

Homo- and heteromeric complexes of KCNQ channel subunits are the molecular correlate of the M-current, a neuron-specific voltage-dependent K(+) current with a well established role in control of neural excitability. We investigated the effect of KCNQ channel modulators on the activity of dopaminergic neurons in vitro and in vivo in the rat ventral mesencephalon. The firing of dopaminergic neurons recorded in mesencephalic slices was robustly inhibited in a concentration-dependent manner by the KCNQ channel opener N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester (retigabine). The effect of retigabine persisted in the presence of tetrodotoxin and simultaneous blockade of GABA(A) receptors, small-conductance calcium-activated K(+) (SK) channels, and hyperpolarization-activated (I(h)) channels, and it was potently reversed by the KCNQ channel blocker 4-pyridinylmethyl-9(10H)-anthracenone (XE991), indicating a direct effect on KCNQ channels. Likewise, in vivo single unit recordings from dopaminergic neurons revealed a prominent reduction in spike activity after systemic administration of retigabine. Furthermore, retigabine inhibited dopamine synthesis and c-Fos expression in the striatum under basal conditions. Retigabine completely blocked the excitatory effect of dopamine D(2) autoreceptor antagonists. Again, the in vitro and in vivo effects of retigabine were completely reversed by preadministration of XE991. Dual immunocytochemistry revealed that KCNQ4 is the major KCNQ channel subunit expressed in all dopaminergic neurons in the mesolimbic and nigrostriatal pathways. Collectively, these observations indicate that retigabine negatively modulates dopaminergic neurotransmission, likely originating from stimulation of mesencephalic KCNQ4 channels.

Diverse mechanisms of antiepileptic drugs in the development pipeline

There is a remarkable array of new chemical entities in the current antiepileptic drug (AED) development pipeline. In some cases, the compounds were synthesized in an attempt improve upon the activity of marketed AEDs. In other cases, the discovery of antiepileptic potential was largely serendipitous. Entry into the pipeline begins with the demonstration of activity in one or more animal screening models. Results from testing in a panel of such models provide a basis to differentiate agents and may offer clues as to the mechanism. Target activity may then be defined through cell-based studies, often years after the initial identification of activity. Some pipeline compounds are believed to act through conventional targets, whereas others are structurally novel and may act by novel mechanisms. Follow-on agents include the levetiracetam analogs brivaracetam and seletracetam that act as SV2A-ligands; the valproate-like agents valrocemide, valnoctamide, propylisopropyl acetamide, and isovaleramide; the felbamate analog flurofelbamate, a dicarbamate, and the unrelated carbamate RWJ-333369; the oxcarbazepine analog licarbazepine, which probably acts as a use-dependent sodium channel blockers, and its prodrug acetate BIA 2-093; various selective partial benzodiazepine receptor agonists, including ELB139, which is a positive allosteric modulator of alpha3-containing GABA(A) receptors. A variety of AEDs that may act through novel targets are also in clinical development: lacosamide, a functionalized amino acid; talampanel, a 2,3-benzodiazepine selective noncompetitive AMPA receptor antagonist; NS1209, a competitive AMPA receptor antagonist; ganaxolone, a neuroactive steroid that acts as a positive modulator of GABA(A) receptors; retigabine, a KCNQ potassium channel opener with activity as a GABA(A) receptor positive modulator; the benzanilide KCNQ potassium channel opener ICA-27243 that is more selective than retigabine; and rufinamide, a triazole of unknown mechanism.

Central nervous system depressant action of flavonoid glycosides

The pharmacological effects on the central nervous system (CNS) of a range of available flavonoid glycosides were explored and compared to those of the glycosides 2S-hesperidin and linarin, recently isolated from valeriana. The glycosides 2S-neohesperidin, 2S-naringin, diosmin, gossipyn and rutin exerted a depressant action on the CNS of mice following i.p. injection, similar to that found with 2S-hesperidin and linarin. We demonstrate in this work that these behavioural actions, as measured in the hole board, thiopental induced sleeping time and locomotor activity tests, are unlikely to involve a direct action on gamma-aminobutyric acid type A (GABA(A)) receptors. The corresponding aglycones were inactive, pointing to the importance of the sugar moieties in the glycosides in their CNS depressant action following systemic administration. The pharmacological properties of the flavonoid glycosides studied here, in addition to our previous results with hesperidin and linarin, opens a promising new avenue of research in the field.

Effects of M-current modulators on the excitability of immature rat spinal sensory and motor neurones

M-currents have been shown to control neuronal excitability in a variety of central and peripheral neurones. Here we studied the effects of specific M-current modulators on the excitability of spinal neurones and their response to synaptic activation. Experiments were performed in vitro using the hemisected spinal cord from 7- to 11-day-old rats. Intracellular recordings were obtained from lumbar deep dorsal horn and motor neurones. Neuronal excitability was assessed by applying outward current pulses and synaptic responses were elicited by activation of a lumbar dorsal root. The M-current antagonist 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone (XE-991) and the agonist retigabine were superfused at 10 microM. Retigabine produced hyperpolarization and a large decrease in the excitability of motor (7/7) and dorsal horn neurones (11/12). The effects of retigabine were fully reversed by XE-991. XE-991 induced depolarization of most neurones tested and a large increase in the excitability of motor neurones (7/7) but only a weak increase in the excitability of a proportion of dorsal horn neurones (4/10). The effects of XE-991 were partly reversed by retigabine. Consistent with their effects on neuronal excitability, retigabine showed a general depressant effect on synaptic transmission, whereas XE-991 showed the opposite tendency to potentiate responses to dorsal root stimulation, particularly in motor neurones. The results show that retigabine can depress spinal excitability and the transmission of nociceptive information. Results also indicate a post-synaptic expression of functional M-currents in most motor neurones and a considerable proportion of deep dorsal horn neurones.

Urodynamic effects of the K+ channel (KCNQ) opener retigabine in freely moving, conscious rats

PURPOSE

Retigabine is a novel anticonvulsant drug that not only augments gamma-aminobutyric acid mechanisms, but also opens voltage gated K+ channels (KCNQ). In this study we investigated the effects of retigabine on detrusor activity in rats.

MATERIALS AND METHODS

To conscious, female Sprague-Dawley rats undergoing continuous cystometry retigabine was given intravenously (0.5, 1 and 5 mg/kg(-1)). The KCNQ channel blocker linopirdine was given intravenously (2 mg/kg(-1)) 5 minutes prior to retigabine (1 mg/kg(-1)). In addition, retigabine was given intracerebroventricularly (1, 5 and 10 microg) and intravesically (100, 500 and 1,000 ng ml(-1)). The effects of the drug (intravesical administration) on capsaicin induced bladder overactivity were also tested.

RESULTS

Retigabine given intravenously (1 mg/kg(-1)) decreased baseline and maximal bladder pressures, increased voided and infused volumes, and increased voiding intervals. Retigabine (10 microg) given intracerebroventricularly decreased baseline pressure and increased voided and infused volumes as well as voiding intervals. However, bladder pressures were not significantly affected. Intravesical retigabine (1,000 ng.ml(-1)) decreased maximal bladder pressure, increased voided and infused volumes, and increased voiding intervals. Given intravesically for 30 minutes prior to intravesical capsaicin (30 microM) instillation retigabine (1,000 ng.ml(-1)) decreased the detrusor overactivity induced by capsaicin. The KNCQ channel blocker linopirdine (2 mg/kg(-1)) completely blocked the effects of intravenous retigabine (1 mg/kg(-1)).

CONCLUSIONS

Retigabine given intravenously, intracerebroventricularly and intravesically increased micturition volume and voiding intervals and, when given intravesically, it decreased capsaicin induced detrusor overactivity, suggesting that KCNQ channels can be interesting targets for drugs aiming at micturition control. Retigabine may be a candidate to test as a treatment for detrusor overactivity in humans.

Pharmacological characterisation of acid-induced muscle allodynia in rats

Previous studies have shown that repeated injections of acidic saline, given into the lateral gastrocnemius muscle of rats, results in a bilateral reduction in withdrawal threshold to tactile stimulation of the hindpaws. We have now characterised this model of muscoskeletal pain pharmacologically, by evaluating the antinociceptive effects of various analgesics after systemic administration. The micro-opioid receptor agonist morphine (3 and 6 mg/kg) produced a particularly prolonged antiallodynic effect. The glutamate receptor antagonists ([8-methyl-5-(4-(N,N-dimethylsulfamoyl)phenyl)-6,7,8,9,-tetrahydro-1H-pyrrolo[3,2-h]-iso-quinoline-2,3-dione-3-O-(4-hydroxybutyric acid-2-yl)oxime] NS1209 and ketamine (6 and 15 mg/kg, respectively), the KCNQ K(+) channel openers retigabine and flupirtine (10 and 20 mg/kg, respectively) and the Na(+) channel blocker mexiletine (37.5 mg/kg) also significantly increased paw withdrawal threshold, although to a lesser degree than morphine. In contrast, the anticonvulsant lamotrigine (30 mg/kg), the cyclooxygenase-2 inhibitor carprofen (15 mg/kg) and the benzodiazepine diazepam (3 mg/kg) were ineffective. All antinociceptive effects were observed at nonataxic doses as determined by the rotarod test. These results suggest that in this model, muscle-mediated pain can be alleviated by various analgesics with differing mechanisms of action, and that once established ongoing inflammation does not appear to contribute to this process.

A four-ligand hypercube model to quantify allosteric interactions within the GABAA receptor complex

The aim of this study was to investigate the couplings between various binding sites on the GABA(A) receptor complex. We investigated combinations of three test compounds: (1) GABA (gamma-aminobutyric acid), (2) Org 20549 [(2 beta 3 alpha 5 alpha)-21hydroxy-3Hydroxy-2(4morpholinyl)pregnan-20one methane-sulphonate)], a neuroactive steroid and (3) retigabine (D-23129, N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester), a new antiepileptic drug. Receptor-binding assays were conducted using rat brain membranes. [3H]TBOB ([3H]-t-butyl-bicyclo-ortho-benzoate) was the tracer ligand. All three test compounds inhibited the binding of [3H]TBOB with EC(50)'s of 4.0, 98 and 23 microM, respectively. Isobolic analysis of the combination data showed that the three compounds act in synergy in displacing [3H]TBOB. These interactions could be described and quantified by a hypercube model in which each of the three test compounds and [3H]TBOB bind to different, allosterically coupled sites such that each of the test compounds allosterically displaces the tracer [3H]TBOB and allosterically enhances the affinity of any other test compound by a factor 4.4. The simultaneous binding of any two ligands enhances the affinity of the third by a factor 9. These results may contribute to the understanding of individual variability in drug responses and to the discussion about rational polytherapy.