Antiepileptogenic and antiictogenic effects of retigabine under conditions of rapid kindling: an ontogenic study

Characteristics of Epileptiform Spike-wave Discharges and Chronic Histopathology in Controlled Cortical Impact Model of Sprague-Dawley Rats

Post-traumatic epilepsy (PTE) is a serious complication that can occur following traumatic brain injury (TBI). Sustained secondary changes after TBI promote the process of PTE. Here, we aim to evaluate changes in behavior, electrocorticogram, and histomorphology in rats following chronic TBI models. We observed intensive 7-8 Hz spike-wave-discharges (SWDs) at frontal recording sites and quantified them in SD rats with different degrees of TBI and compared them with age-matched sham rats to evaluate the association between SWDs and injury severity. Notably, although SWDs were even presented in the sham group, the number and duration of events were much lower than those in the TBI groups. SWDs have numerous similarities to absence seizures, such as abrupt onset, termination, and lack of postictal suppression, which may be the nonconvulsive characteristics of PTE. Retigabine, a novel antiepileptic drug, is ineffective in reducing SWDs. In addition, we examined chronic histopathological changes in TBI rats. Rats subjected to moderate and severe TBI exhibited significantly impaired neurological function, which was accompanied by marked cortical injury, hippocampus deformation, reactive gliosis, and mossy fiber sprouting. Long-term progressive structural changes in the brain are one of the characteristics of epileptogenesis after TBI. Our study provided the potential value of epileptiform SWDs in reflecting the nonconvulsive characteristic of PTE and highlighted the vital role of chronic pathological changes, such as reactive gliosis, in promoting the epileptogenesis following TBI.

Anticonvulsant effects of the aqueous and methanol extracts from the stem bark of Psychotria camptopus Verdc. (Rubiacaea) in rats

ETHNOPHARMACOLOGY RELEVANCE

The decoction from the stem bark of Psychotria camptopus (Rubiaceae) is used in the Cameroonian pharmacopoeia to treat neurological pathologies including epilepsy.

AIM

The present work was undertaken to study the anticonvulsant properties of the aqueous (AE) and methanol (ME) extracts from the stem bark of P. camptopus in acute models of epileptic seizures in Wistar rats.

METHOD

AE and ME were obtained by decoction and maceration of the stem bark powder in water and methanol, respectively. They were tested orally at the doses of 40, 80 and 120 mg/kg, on the latency of onset and duration of epileptic seizures induced by pentylene tetrazole (PTZ, 70 mg/kg, i.p.). The kinetic effect of both extracts at 120 mg/kg was evaluated. Their effects on diazepam (50 mg/kg) induced sleep and strychnine (STR, 2.5 mg/kg, i.p.) induced seizures were determined. ME was further tested on picrotoxin (PIC, 7.5 mg/kg, i.p.) and thiosemicarbazide (TSC, 50 mg/kg, i.p.) induced seizure models. The phytochemical composition of ME was assessed using LC-MS method, as well as its acute toxicity.

RESULTS

AE and ME significantly (p < 0.001) reduced the duration of seizures in both PTZ and STR models. Their maximal effect was observed at 1 h after administration, though their effect at 120 mg/kg was maintained (p < 0.05) up to 24 h post-treatment. Both extracts significantly (p < 0.01) reduced sleep duration. ME significantly (p < 0.001) increased the latency of rat death on PIC-induced convulsions. In TSC rats, ME significantly (p < 0.001) delayed the latency to the first convulsion, and decreased the duration and frequency of convulsions. ME showed no acute toxicity while its phytochemical screening revealed the presence of two flavonoids (Rutin and Butin), two triterpenoid saponins (Psycotrianoside B and Bauerenone) and four alkaloids (10-Hydroxy-antirhine, 10-hydroxy-iso-deppeaninol, Emetine and Hodkinsine). In conclusion, AE and ME from the stem bark of P. camptopus have comparable anticonvulsant properties. The effect of ME is likely due to the presence of flavonoids and alkaloid and the activation of GABA pathway. These results further justify and support the use of P. camptopus in traditional medicine for the treatment of epilepsy.

Evaluation of subchronic administration of antiseizure drugs in spontaneously seizing rats

OBJECTIVE

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

METHODS

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

RESULTS

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

SIGNIFICANCE

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

Functional responses of the hippocampus to hyperexcitability depend on directed, neuron-specific KCNQ2 K+ channel plasticity

M-type (KCNQ2/3) K⁺ channels play dominant roles in regulation of active and passive neuronal discharge properties such as resting membrane potential, spike-frequency adaptation, and hyper-excitatory states. However, plasticity of M-channel expression and function in nongenetic forms of epileptogenesis are still not well understood. Using transgenic mice with an EGFP reporter to detect expression maps of KCNQ2 mRNA, we assayed hyperexcitability-induced alterations in KCNQ2 transcription across subregions of the hippocampus. Pilocarpine and pentylenetetrazol chemoconvulsant models of seizure induction were used, and brain tissue examined 48 hr later. We observed increases in KCNQ2 mRNA in CA1 and CA3 pyramidal neurons after chemoconvulsant-induced hyperexcitability at 48 hr, but no significant change was observed in dentate gyrus (DG) granule cells. Using chromogenic in situ hybridization assays, changes to KCNQ3 transcription were not detected after hyper-excitation challenge, but the results for KCNQ2 paralleled those using the KCNQ2-mRNA reporter mice. In mice 7 days after pilocarpine challenge, levels of KCNQ2 mRNA were similar in all regions to those from control mice. In brain-slice electrophysiology recordings, CA1 pyramidal neurons demonstrated increased M-current amplitudes 48 hr after hyperexcitability; however, there were no significant changes to DG granule cell M-current amplitude. Traumatic brain injury induced significantly greater KCNQ2 expression in the hippocampal hemisphere that was ipsilateral to the trauma. In vivo, after a secondary challenge with subconvulsant dose of pentylenetetrazole, control mice were susceptible to tonic-clonic seizures, whereas mice administered the M-channel opener retigabine were protected from such seizures. This study demonstrates that increased excitatory activity promotes KCNQ2 upregulation in the hippocampus in a cell-type specific manner. Such novel ion channel expressional plasticity may serve as a compensatory mechanism after a hyperexcitable event, at least in the short term. The upregulation described could be potentially leveraged in anticonvulsant enhancement of KCNQ2 channels as therapeutic target for preventing onset of epileptogenic seizures.

Activation of KCNQ Channels Suppresses Spontaneous Activity in Dorsal Root Ganglion Neurons and Reduces Chronic Pain after Spinal Cord Injury

A majority of people who have sustained spinal cord injury (SCI) experience chronic pain after injury, and this pain is highly resistant to available treatments. Contusive SCI in rats at T10 results in hyperexcitability of primary sensory neurons, which contributes to chronic pain. KCNQ channels are widely expressed in nociceptive dorsal root ganglion (DRG) neurons, are important for controlling their excitability, and their activation has proven effective in reducing pain in peripheral nerve injury and inflammation models. The possibility that activators of KCNQ channels could be useful for treating SCI-induced chronic pain is strongly supported by the following findings. First, SCI, unlike peripheral nerve injury, failed to decrease the functional or biochemical expression of KCNQ channels in DRG as revealed by electrophysiology, real-time quantitative polymerase chain reaction, and Western blot; therefore, these channels remain available for pharmacological targeting of SCI pain. Second, treatment with retigabine, a specific KCNQ channel opener, profoundly decreased spontaneous activity in primary sensory neurons of SCI animals both in vitro and in vivo without changing the peripheral mechanical threshold. Third, retigabine reversed SCI-induced reflex hypersensitivity, adding to our previous demonstration that retigabine supports the conditioning of place preference after SCI (an operant measure of spontaneous pain). In contrast to SCI animals, naïve animals showed no effects of retigabine on reflex sensitivity or conditioned place preference by pairing with retigabine, indicating that a dose that blocks chronic pain-related behavior has no effect on normal pain sensitivity or motivational state. These results encourage the further exploration of U.S. Food and Drug Administration-approved KCNQ activators for treating SCI pain, as well as efforts to develop a new generation of KCNQ activators that lack central side effects.

Pro-epileptogenic effects of viral-like inflammation in both mature and immature brains

BACKGROUND

Infectious encephalitides are most often associated with acute seizures during the infection period and are risk factors for the development of epilepsy at later times. Mechanisms of viral encephalitis-induced epileptogenesis are poorly understood. Here, we evaluated the contribution of viral encephalitis-associated inflammation to ictogenesis and epileptogenesis using a rapid kindling protocol in rats. In addition, we examined whether minocycline can improve outcomes of viral-like brain inflammation.

METHODS

To produce viral-like inflammation, polyinosinic-polycytidylic acid (PIC), a toll-like receptor 3 (TLR3) agonist, was applied to microglial/macrophage cell cultures and to the hippocampus of postnatal day 13 (P13) and postnatal day 74 (P74) rats. Cell cultures permit the examination of the inflammation induced by PIC, while the in vivo setting better suits the analysis of cytokine production and the effects of inflammation on epileptogenesis. Minocycline (50 mg/kg) was injected intraperitoneally for 3 consecutive days prior to the kindling procedure to evaluate its effects on inflammation and epileptogenesis.

RESULTS

PIC injection facilitated kindling epileptogenesis, which was evident as an increase in the number of full limbic seizures at both ages. Furthermore, in P14 rats, we observed a faster seizure onset and prolonged retention of the kindling state. PIC administration also led to an increase in interleukin 1β (IL-1β) levels in the hippocampus in P14 and P75 rats. Treatment with minocycline reversed neither the pro-epileptogenic effects of PIC nor the increase of IL-1β in the hippocampus in both P14 and P75 rats.

CONCLUSIONS

Hippocampal injection of PIC facilitates rapid kindling epileptogenesis at both P14 and P75, suggesting that viral-induced inflammation increases epileptogenesis irrespective of brain maturation. Minocycline, however, was unable to reverse the increase of epileptogenesis, which might be linked to its absence of effect on hippocampal IL-1β levels at both ages.

Effect of a single dose of retigabine in cortical excitability parameters: A cross-over, double-blind placebo-controlled TMS study

BACKGROUND

Antiepileptic drugs (AEDs) decrease the occurrence of epileptic seizures and modulate cortical excitability through several mechanisms that likely interact. The modulation of brain excitability by AEDs is believed to reflect their antiepileptic action(s) and could be used as a surrogate marker of their efficacy. Transcranial magnetic stimulation (TMS) is one of the best noninvasive methods to study cortical excitability in human subjects. Specific TMS parameters can be used to quantify the various mechanisms of action of AEDs. A new AED called retigabine increases potassium efflux by changing the conformation of KCNQ 2-5 potassium channels, which leads to neuronal hyperpolarisation and a decrease in excitability.

HYPOTHESIS

The purpose of this study is to investigate the effect of retigabine on cortical excitability. Based on the known mechanisms of action of retigabine, we hypothesized that the oral intake of retigabine would increase the resting motor threshold (RMT).

METHODS

Fifteen healthy individuals participated in a placebo-controlled, double-blind, randomised, clinical trial (RCT). The primary outcome measure was the RMT quantified before and after oral intake of retigabine. Several secondary TMS outcome measures were acquired.

RESULTS

The mean RMT, active motor threshold (AMT) and intensity to obtain a 1mV peak-to-peak amplitude potential (SI1mV) were significantly increased after retigabine intake compared to placebo (RMT: P=0.039; AMT: P=0.014; SI1mV: P=0.019). No significant differences were found for short-interval intracortical inhibition/intracortical facilitation (SICI/ICF), long-interval intracortical inhibition (LICI) or short-interval intracortical facilitation (SICF).

CONCLUSION

A single dose of retigabine increased the RMT, AMT and S1mV in healthy individuals. No modulating intracortical facilitation or inhibition was observed. This study provides the first in vivo demonstration of the modulating effects of retigabine on the excitability of the human brain, and the results are consistent with the data showing that retigabine hyperpolarizes neurons mainly by increasing potassium conductance.

Profile of retigabine-induced neuronal apoptosis in the developing rat brain

OBJECTIVE

Acute neonatal exposure to some, but not all, anticonvulsant drugs induces a profound increase in neuronal apoptosis in rats. Phenobarbital and phenytoin induce apoptosis at a therapeutically relevant dose range, lamotrigine and carbamazepine do so only at supratherapeutic doses or in polytherapy, and valproate does so even at subtherapeutic doses. Levetiracetam is devoid of pro-apoptotic effects. Retigabine, a new-generation drug, acts uniquely by enhancing the M-type potassium current. Because its safety profile in developing animals is unstudied, we sought to determine if retigabine would induce apoptosis.

METHODS

Postnatal day (P) 7 rat pups were treated with retigabine (5-30 mg/kg), vehicle (saline), or comparator drugs (phenobarbital, lamotrigine, levetiracetam, or carbamazepine). Cell death was assessed using amino-cupric-silver staining. A separate group of animals was treated repeatedly (three times over 24 h) with retigabine (15 mg/kg) or vehicle. To establish a pharmacokinetic profile for retigabine, we measured plasma and brain levels after drug treatment.

RESULTS

Consistent with prior studies from our group and others, we found phenobarbital-induced cell death throughout thalamus, nucleus accumbens, and several neocortical areas. By contrast, levetiracetam, lamotrigine, and carbamazepine were found to have no appreciable apoptotic effect on the aforementioned structures. Acute (single) exposure to retigabine, even at doses of 30 mg/kg, was also without effect on apoptosis. However, repeated (three times) exposure to retigabine triggered apoptosis in a subset of brain areas. The half-life of retigabine in plasma was 2.5 h, with appreciable concentrations reached in the brain within 1 h of administration.

SIGNIFICANCE

These data demonstrate that retigabine, like many other anticonvulsant drugs, is capable of triggering neuronal apoptosis in the developing rat brain. Unlike other drugs, repeated dosing of retigabine was necessary to induce this effect. This may be due to its shorter half-life as compared to other drugs, such as phenobarbital.

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.

Perspectives on treatment options for mesial temporal lobe epilepsy with hippocampal sclerosis

INTRODUCTION

Mesial temporal lobe epilepsy associated with hippocampal sclerosis (MTLE-HS) is a syndrome that is often refractory to drug treatment. The effects on specific syndromes are not currently available from the pre-marketing clinical development of new AEDs; this does not allow the prediction of whether new drugs will be more effective in the treatment of some patients.

AREAS COVERED

We have reviewed all the existing literature relevant to the understanding of a potential effectiveness in MTLE-HS patients for the latest AEDs, namely brivaracetam, eslicarbazepine, lacosamide, perampanel and retigabine also including the most relevant clinical data and a brief description of their pharmacological profile. Records were identified using predefined search criteria using electronic databases (e.g., PubMed, Cochrane Library Database of Systematic Reviews). Primary peer-reviewed articles published up to the 15 June 2015 were included.

EXPERT OPINION

All the drugs considered have the potential to be effective in the treatment of MTLE-HS; in fact, they possess proven efficacy in animal models; currently considered valuable tools for predicting drug efficacy in TLE. Furthermore, for some of these (e.g., lacosamide and eslicarbazepine) data are already available from post-marketing studies while brivaracetam acting on SV2A like levetiracetam might have the same potential effectiveness with the possibility to be more efficacious considering its ability to inhibit voltage gated sodium channels; finally, perampanel and retigabine are very effective drugs in animal models of TLE.

Anti-ictogenic and antiepileptogenic properties of brivaracetam in mature and immature rats

OBJECTIVE

Brivaracetam (BRV) is a new antiepileptic drug candidate rationally designed for high affinity and selectivity for the synaptic vesicle protein 2A. This study explored anti-ictogenic and antiepileptogenic effects of BRV in rats at different stages of development.

METHODS

Using a rapid kindling model in P14, P21, P28, and P60 rats, we studied two doses of BRV: 10 and 100 mg/kg injected intraperitoneally 30 min before afterdischarge assessment. We also assessed blood and brain concentrations of BRV 30 min after the injection.

RESULTS

BRV 100 mg/kg significantly increased the afterdischarge threshold (ADT) at all ages, whereas BRV at 10 mg/kg increased ADT in P60, P28, and P21 rats. BRV also shortens the afterdischarge duration (ADD), achieving statistical significance with 10 and 100 mg/kg at P60 and with 100 mg/kg at P21. At P60, BRV increases the number of stimulations required to achieve a stage 4-5 seizure in a dose-dependent manner. At P28 and P21, BRV increased the number of stimulations required to develop a stage 4-5 seizure in a dose-dependent manner with almost complete elimination of stage 4-5 seizures. In contrast, at P14, BRV had no effect on the number of stage 4-5 seizures. An age-related decrease in blood and brain concentrations of BRV was observed 30 min after injection of BRV 10 mg/kg, whereas with 100 mg/kg there were no significant age-correlated differences in brain and serum BRV concentrations.

SIGNIFICANCE

BRV exerted dose-dependent anti-ictogenic effects from P60 to P14 independent of brain maturation. BRV also exhibited antiepileptogenic effects at P60, whereas this effect need to be further evaluated at P28 and P21. We did not observe any effect on epileptogenesis at P14 at either dose.

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.

Efficacy of retigabine in adjunctive treatment of partial onset seizures in adults

Objective

To evaluate efficacy and tolerability of retigabine (ezogabine, US adopted name) in the adjunctive treatment of partial-onset seizures in adults. Retigabine is the first anticonvulsant in its class, decreasing neuronal excitability by opening voltage-gated potassium channels.

Methods

MEDLINE and EMBASE were systematically searched using search terms retigabine and ezogabine for randomized controlled trials published from 1980 through August 17, 2013. Additionally, articles relating to pharmacology, pharmacokinetics, tolerability and interactions were examined for inclusion. Published abstracts and websites of the Food and Drug Administration and European Medication Agency were reviewed for additional relevant information.

Results

One phase IIb and two phase III trials were identified. Retigabine has been reported to have dose dependent efficacy in adjunctive treatment of resistant partial-onset seizures in adults in doses of 600, 900 and 1200 mg/day. Similar to other anticonvulsants, the most common adverse events were central nervous system related. Retigabine has several unique adverse events compared to other anticonvulsants: urinary retention and, with extended use, pigment changes to the skin and retina. Retigabine is metabolized by glucuronidation and acetylation. There are few drug interactions with retigabine.

Conclusions

Retigabine has been shown to have efficacy when used as adjunctive therapy in partial-onset seizures. It has a novel mechanism of action, activation of voltage-gated potassium channels. It has less drug interactions than many other anticonvulsants because it is not metabolized through the P-450 system. Its place in therapy has yet to be determined, especially with recent reports of pigment discoloration of skin and the retina with extended use.

Efficacy and tolerability exposure-response relationship of retigabine (ezogabine) immediate-release tablets in patients with partial-onset seizures

BACKGROUND

Retigabine (international nonproprietary name)/ezogabine (United States adopted name) is an antiepileptic drug (AED) that enhances KCNQ (Kv7) potassium channel activity.

OBJECTIVES

The aim of this study was to explore the relationship between retigabine/ezogabine systemic exposure and efficacy and adverse events (AEs) of retigabine/ezogabine from Phase III clinical trials.

METHODS

Data were combined from Studies 301 and 302, which were both randomized, double-blind, placebo-controlled, multicenter, parallel-group studies with similar inclusion and exclusion criteria. All patients had partial-onset seizures and were receiving 1 to 3 concomitant AEDs. Systemic exposure was predicted for each patient as the average steady-state AUC0-τ during the 12-week maintenance phase, based on a population pharmacokinetic model developed for retigabine/ezogabine. Efficacy end points included reduction in total partial-seizure frequency from baseline and probability of ≥50% reduction from baseline in seizure frequency. The probabilities of occurrence of 6 AEs were also evaluated.

RESULTS

AUC0-τ values increased linearly over the 600- to 1200-mg/d dose range. Over the entire AUC0-τ range, the probability of efficacy was greater than that for any AE. The slopes of the exposure-response relationship for probability of dizziness and abnormal coordination were similar to that for efficacy, whereas the slopes for dysarthria, somnolence, tremor, and blurred vision were shallower, indicating that the probability of these events occurring was less affected than the probability of efficacy by increases in retigabine/ezogabine AUC0-τ.

CONCLUSIONS

Based on the summary statistics of pharmacokinetic parameters, systemic exposure to retigabine/ezogabine increased linearly with dose (600-1200 mg/d). Population pharmacokinetics and pharmacodynamics showed that the probability of efficacy and AEs increased with increasing systemic retigabine/ezogabine exposure, and the probability of efficacy was higher than the probability of any of the AEs. The 35%-50% between-patient variability and overlap between retigabine/ezogabine dose levels in AUC0-τ values indicate that, as with other AEDs, doses should be individually titrated based on a balance between efficacy and tolerability.

Carbamazepine, but not valproate, displays pharmacoresistance in lamotrigine-resistant amygdala kindled rats

The voltage gated sodium channel (VGSC) blocker lamotrigine (LTG), when administered during kindling acquisition, leads to the development of resistance to LTG. The present study aimed to assess whether LTG-resistant amygdala-kindled rats display subsequent resistance to the VGSC blocker carbamazepine (CBZ) and the broad-spectrum antiepileptic drug (AED) sodium valproate (VPA). Two groups of male Sprague Dawley rats received either 0.5% methylcellulose (MC) or LTG (5mg/kg, i.p.) 1h before each amygdala kindling stimulation. Treatments were stopped once both the groups were fully kindled. Two days later, both groups were challenged with a higher dose of LTG (15mg/kg, i.p.) to verify LTG-resistance in the experimental group (i.e., LTG-pretreated rats). The efficacy of CBZ and VPA was then evaluated in both groups. A higher dose of LTG blocked fully kindled seizures in the vehicle-treated rats but not seizures in the LTG-treated group. The mean seizure score, of the control group (1.2±0.3) was significantly lower (P<.05) than that of the LTG-treated population (3.5±0.7; n=8). A lower percent of the population in the control group was observed to display a generalized stage 4-5 seizure compared to the experimental group (i.e., those that received LTG during kindling acquisition) (28.5% vs. 62%, respectively). Interestingly, CBZ (10, 20, and 40mg/kg) displayed a dose-dependent anticonvulsant effect in the vehicle-kindled group, but was less effective in LTG-treated animals. In contrast, VPA (300mg/kg) effectively blocked the behavioral seizure and decreased the afterdischarge duration (ADD) in both vehicle and LTG groups. These findings suggest that the LTG-resistant, amygdala-kindled rat may represent a novel model of pharmacoresistant epilepsy.

Anticonvulsant effect of retigabine during postnatal development in rats

Retigabine is a new-generation antiepileptic drug that exerts therapeutic action through the activation of KCNQ channel dependent M-type potassium currents. While retigabine has been extensively studied in adult animals using a wide variety of seizure models, its effects in developing animals have not been examined. There has only been one previous report of retigabine efficacy in juvenile rats (Mazarati et al., 2008), which examined efficacy against kindled seizures and did not examine ages younger than postnatal day (P) 14. To determine the efficacy of retigabine during brain development we pretreated rats with retigabine (0-30 mg/kg) at three ages corresponding to the neonatal period through late childhood/early adolescence (i.e., P7, P14, or P25). Seizures were induced 30 min later using a chemoconvulsant (pentylenetetrazol, PTZ) model, which has been widely used to determine anticonvulsant efficacy of many other antiepileptic drugs in neonatal animals. In a dose and age-dependent manner, retigabine reduced the severity of PTZ evoked seizures, increased the latency to seizure onset, and decreased the incidence of full maximal seizures. The minimum effective dose was found to be 5mg/kg for P7 animals, 2.5mg/kg for P14 animals, and 1mg/kg for P25 animals. These findings allow a direct comparison between retigabine and previously studied antiepileptic drugs against PTZ seizures during development, and provide the first report of the effective dose range of retigabine in neonatal animals.

Novel animal models of pediatric epilepsy

When mimicking epileptic processes in a laboratory setting, it is important to understand the differences between experimental models of seizures and epilepsy. Because human epilepsy is defined by the appearance of multiple spontaneous recurrent seizures, the induction of a single acute seizure without recurrence does not constitute an adequate epilepsy model. Animal models of epilepsy might be useful for various tasks. They allow for the investigation of pathophysiological mechanisms of the disease, the evaluation, or the development of new antiepileptic treatments, and the study of the consequences of recurrent seizures and neurological and psychiatric comorbidities. Although clinical relevance is always an issue, the development of models of pediatric epilepsies is particularly challenging due to the existence of several key differences in the dynamics of human and rodent brain maturation. Another important consideration in modeling pediatric epilepsy is that "children are not little adults," and therefore a mere application of models of adult epilepsies to the immature specimens is irrelevant. Herein, we review the models of pediatric epilepsy. First, we illustrate the differences between models of pediatric epilepsy and models of the adulthood consequences of a precipitating insult in early life. Next, we focus on new animal models of specific forms of epilepsies that occur in the developing brain. We conclude by emphasizing the deficiencies in the existing animal models and the need for several new models.

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.

Neuronal potassium channel openers in the management of epilepsy: role and potential of retigabine

Despite the availability of over 20 antiepileptic drugs, about 30% of epileptic patients do not achieve seizure control. Thus, identification of additional molecules targeting novel molecular mechanisms is a primary effort in today's antiepileptic drug research. This paper reviews the pharmacological development of retigabine, an antiepileptic drug with a novel mechanism of action, namely the activation of voltage-gated potassium channels of the Kv7 subfamily. These channels, which act as widespread regulators of intrinsic neuronal excitability and of neurotransmitter-induced network excitability changes, are currently viewed among the most promising targets for anticonvulsant pharmacotherapy. In particular, the present work reviews the pathophysiological role of Kv7 channels in neuronal function, the molecular mechanisms involved in the Kv7 channel-opening action of retigabine, the activity of retigabine in preclinical in vitro and in vivo studies predictive of anticonvulsant activities, and the clinical status of development for this drug as an add-on treatment for pharmacoresistant epilepsy. Particular efforts are devoted to highlighting the potential advantages and disadvantages of retigabine when compared with currently available compounds, in order to provide a comprehensive assessment of its role in therapy for treatment-resistant epilepsies.

Inflammation enhances epileptogenesis in the developing rat brain

In many experimental systems, proinflammatory stimuli exhibit proconvulsant properties. There are also accumulating data suggesting that inflammation may contribute to epileptogenesis in experimental models as well as in humans. Using two different models (Lithium-pilocarpine induced-status epilepticus (SE) and rapid kindling), we address this issue in the developing brain. Using P14 Wistar rat pups, we showed that inflammation induced by LPS results, after SE, into a more severe disease in adulthood. The main histological feature was an active gliosis that was observed only when inflammation and SE was combined. The use of a kindling model at P14, a model where seizure progress without any neurodegeneration, permits to show that systemic inflammation is responsible of an enhancement of epileptogenesis. The role of inflammation should be further explored in immature brain to identify therapeutic targets that may be relevant to clinical practice where the association of inflammation and epileptic events is common.

Inflammation induced by LPS enhances epileptogenesis in immature rat and may be partially reversed by IL1RA

Inflammatory signaling in the central nervous system (CNS) has been shown to exacerbate both seizure activity and seizure-induced neuronal injury. However, it has not been firmly established whether neurodegeneration is a prerequisite of proconvulsant effect of neuroinflammation, or whether the latter may facilitate seizures without involving neuronal injury. We examined effects of inflammation in the rapid kindling model, where seizure progression occurs in the absence of neurodegeneration. P14 male Wistar rats were subjected to a rapid kindling procedure: 60 electrical stimulations of the hippocampus delivered every 5 min at the current that had been established to induce afterdischarge. Lipopolysaccharide (LPS) was injected (50 microg/kg, i.p., 2 h prior to the rapid kindling protocol [RKP]); IL-1Ra was injected (25 mg/kg, i.p., 2 h prior to the RKP). The effects of treatments were examined on baseline hippocampal excitability, on the progression of rapid kindling, and on the retention of rapid kindling. LPS increased baseline hippocampal excitability, evident as the decrease of hippocampal ADT. LPS also increased kindling progression. Twenty-four hours after the completion of kindling procedure, LPS-treated animals exhibited increased excitability as compared with saline-treated kindling controls. The kindling progression was blocked by IL1RA when given in combination with LPS. IL1RA was able to reverse the effect of LPS on afterdischarge duration (ADD) while IL1RA alone decreased ADT. We showed that inflammation provoked by LPS enhanced rapid kindling epileptogenesis in immature rat brains. IL1RA was also able to mitigate this augmentation of epileptogenesis enhanced by LPS.

Evaluation of development-specific targets for antiepileptogenic therapy using rapid kindling

We used the method of rapid hippocampal kindling to assess the potential antiepileptogenic efficacy of a number of anticonvulsant medications. This method afforded a higher throughput than methods based on traditional kindling or post-status epilepticus models of epileptogenesis. This "compressed epileptogenesis" model also permitted the study of age-dependent pharmacologic targets, and distinguished among antiepileptic drugs (AEDs) on the basis of their age-specific antiepileptogenic efficacy. We found retigabine to be the most effective anticonvulsant therapy during early development. Topiramate seemed most effective further along development, whereas some drugs did not demonstrate an age-specific effect. The method also reproduced some of the paradoxical pharmacologic findings previously shown with lamotrigine. Although the utility of this model for screening the antiepileptogenic therapies requires further validation, it introduces the ability to undertake development-specific testing and a more rapid throughput than conventional methods.

Anticonvulsant effects of the selective melatonin receptor agonist ramelteon

OBJECTIVE

The endogenous hormone melatonin has previously been shown to exert anticonvulsant effects in a variety of experimental models. Accordingly, we asked whether ramelteon, a synthetic and selective melatonin receptor agonist, might also possess anticonvulsant and/or antiepileptogenic properties.

METHODS

The effects of ramelteon (30 or 100 mg/kg intraperitoneally twice daily for 5 days) were evaluated in two animal models of epilepsy. In the rat rapid kindling model, baseline hippocampal afterdischarge properties, kindling progression, and hippocampal excitability in kindled animals were measured. Anti-ictogenic efficacy was assessed after acute administration in untreated kindled rats. In the spontaneously epileptic Kcna1-null mouse model, we determined seizure frequency and periodicity using continuous video/EEG monitoring over 72 hours. Further, circadian rest-activity rhythms in ramelteon-treated animals were studied with actigraphy.

RESULTS

In kindled animals, ramelteon reversed kindling-induced hippocampal excitability; however, it did not modify baseline afterdischarge properties, the progression and establishment of the kindled state in the rapid kindling model. However, in Kcna1-null mice, ramelteon (200 mg/kg/day) significantly attenuated seizure periodicity and frequency and improved circadian rest-activity rhythms compared with control animals.

CONCLUSIONS

The selective melatonin receptor agonist ramelteon possesses anticonvulsant properties in a chronic epilepsy model. Our findings provide further support for melatonin receptors being potential novel targets for anticonvulsant drug development.

Bumetanide inhibits rapid kindling in neonatal rats

PURPOSE

To examine the effects of bumetanide, a selective blocker of Na+-K+-2Cl- cotransporter (NKCC1), on hippocampal excitability and rapid kindling in immature rats.

METHODS

Studies were performed in Wistar rats of three ages: postnatal day 11 (P11, neonatal), P14 (postneonatal), and P21 (preadolescent). Bumetanide (0.2, 0.5, 2.5 mg/kg) was given intraperitoneally 20 min prior to the beginning of the studies. Hippocampal excitability was examined by measuring threshold and duration of afterdischarge, which had been elicited by electrical stimulation of ventral hippocampus. Kindling procedure consisted of 80 electrical stimulations of ventral hippocampus, delivered every 5 min.

RESULTS

At P11, bumetanide (0.5 mg/kg) increased the baseline hippocampal afterdischarge threshold and shortened the afterdischarge duration. Bumetanide delayed the occurrence, and reduced the number of full motor seizures during kindling, and prevented the development of kindling-induced enhanced seizure susceptibility in a majority of animals. At P14, bumetanide (0.5 mg/kg) induced no significant antiepileptic effects, although suppression of hippocampal excitability and inhibition of kindling were observed in a subset of animals. At P21, bumetanide (0.2; 2.5 mg/kg) exerted no effects on hippocampal excitability and kindling progression.

DISCUSSION

The obtained results provide further evidence that bumetanide may be beneficial for treating neonatal seizures, and that NKCC1 represents a potential target for antiepileptic interventions in the immature brain.