Characterization of KCNQ5/Q3 potassium channels expressed in mammalian cells

In vivo profile of ICA-27243 [N-(6-chloro-pyridin-3-yl)-3,4-difluoro-benzamide], a potent and selective KCNQ2/Q3 (Kv7.2/Kv7.3) activator in rodent anticonvulsant models

Openers or activators of neuronal KCNQ2/Q3 potassium channels decrease neuronal excitability and may provide benefit in the treatment of disorders of neuronal excitability such as epilepsy. In the present study, we evaluate the effects of ICA-27243 [N-(6-chloro-pyridin-3-yl)-3,4-difluoro-benzamide], an orally bioavailable, potent, and selective KCNQ2/Q3 opener, in a broad range of rodent seizure models. ICA-27243 was effective against maximal electroshock (MES) and pentylenetetrazole (PTZ)-induced seizures in both rats (MES, ED(50) = 1.5 mg/kg p.o.; PTZ, ED(50) = 2.2 mg/kg p.o.) and mice (MES, ED(50) = 8.6 mg/kg p.o.; PTZ, ED(50) = 3.9 mg/kg p.o.) in the rat amygdala kindling model of partial seizures (full protection from seizure at 9 mg/kg p.o.) and in the 6-Hz model of psychomotor seizures in mice (active at 10 mg/kg i.p.). Antiseizure efficacy in all models was observed at doses significantly less than those shown to effect open-field locomotor activity (rat ED(50) = 40 mg/kg p.o.) or ability to remain on a Rotorod (no effect in rat at doses up to 100 mg/kg p.o.). There was no evidence of cognition impairment as measured in the Morris water maze in the rat (10 and 30 mg/kg p.o.), nor was there evidence of the development of tolerance after multiple doses of ICA-27243. Our findings suggest that selective KCNQ2/Q3 opening activity in the absence of effects on KCNQ3/Q5 or GABA-activated channels may be sufficient for broad-spectrum antiepileptic activity in rodents.

Bimodal effects of the Kv7 channel activator retigabine on vascular K+ currents

BACKGROUND AND PURPOSE

This study investigated the functional and electrophysiological effects of the Kv7 channel activator, retigabine, on murine portal vein smooth muscle.

EXPERIMENTAL APPROACH

KCNQ gene expression was determined by reverse transcriptase polymerase chain reaction (RT-PCR) and immunocytochemical experiments. Whole cell voltage clamp and current clamp were performed on isolated myocytes from murine portal vein. Isometric tension recordings were performed on whole portal veins. K+ currents generated by KCNQ4 and KCNQ5 expression were recorded by two-electrode voltage clamp in Xenopus oocytes.

KEY RESULTS

KCNQ1, 4 and 5 were expressed in mRNA derived from murine portal vein, either as whole tissue or isolated myocytes. Kv7.1 and Kv7.4 proteins were identified in the cell membranes of myocytes by immunocytochemistry. Retigabine (2-20 microM) suppressed spontaneous contractions in whole portal veins, hyperpolarized the membrane potential and augmented potassium currents at -20 mV. At more depolarized potentials, retigabine and flupirtine, decreased potassium currents. Both effects of retigabine were prevented by prior application of the K(v)7 blocker XE991 (10 muM). Recombinant KCNQ 4 or 5 channels were only activated by retigabine or flupirtine.

CONCLUSIONS AND IMPLICATIONS

The Kv7 channel activators retigabine and flupirtine have bimodal effects on vascular potassium currents, which are not seen with recombinant KCNQ channels. These results provide support for KCNQ4- or KCNQ5-encoded channels having an important functional impact in the vasculature.

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.

Retigabine: in partial seizures

Retigabine has anticonvulsant properties that appear to be primarily mediated by opening neuronal voltage-gated potassium channels. This action has been shown in neuronal KCNQ2/3 and KCNQ3/5 potassium channels. In addition to this unique action, retigabine also potentiates GABA-evoked currents in cortical neurons at high concentrations. When used as adjunctive therapy in patients with partial seizures, retigabine 600-1200 mg/day (200-400 mg three times daily) was associated with significant linear dose-dependent reductions in monthly seizure frequency compared with placebo in a large 16-week randomised phase II trial. Median monthly seizure frequency decreased from baseline by up to 35% among patients in the retigabine treatment arms compared with 13% in the placebo group. Retigabine 1200 mg/day was also significantly more effective than retigabine 600 mg/day. Responder rates, defined as the proportion of patients with > or = 50% reduction in seizure frequency, were significantly higher among patients in the retigabine 900 and 1200 mg/day groups than in those who received placebo. CNS-related adverse events were the most commonly reported treatment-emergent adverse events associated with retigabine in clinical trials. Across all three retigabine groups in the large phase II trial, somnolence (20.3%), dizziness (14.6%), confusion (12.3%) and speech disorder (11.3%) were the most frequent CNS-related adverse events.

KCNQ/M-currents contribute to the resting membrane potential in rat visceral sensory neurons

The M-current is a slowly activating, non-inactivating potassium current that has been shown to be present in numerous cell types. In this study, KCNQ2, Q3 and Q5, the molecular correlates of M-current in neurons, were identified in the visceral sensory neurons of the nodose ganglia from rats through immunocytochemical studies. All neurons showed expression of each of the three proteins. In voltage clamp studies, the cognition-enhancing drug linopirdine (1-50 microM) and its analogue, XE991 (10 microM), quickly and irreversibly blocked a small, slowly activating current that had kinetic properties similar to KCNQ/M-currents. This current activated between -60 and -55 mV, had a voltage-dependent activation time constant of 208 +/- 12 ms at -20 mV, a deactivation time constant of 165 +/- 24 ms at -50 mV and V1/2 of -24 +/- 2 mV, values which are consistent with previous reports for endogenous M-currents. In current clamp studies, these drugs also led to a depolarization of the resting membrane potential at values as negative as -60 mV. Flupirtine (10-20 microM), an M-current activator, caused a 3-14 mV leftward shift in the current-voltage relationship and also led to a hyperpolarization of resting membrane potential. These data indicate that the M-current is present in nodose neurons, is activated at resting membrane potential and that it is physiologically important in regulating excitability by maintaining cells at negative voltages.

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.

Ionic mechanisms of autorhythmic firing in rat cerebellar Golgi cells

Although Golgi cells (GoCs), the main type of inhibitory interneuron in the cerebellar granular layer (GL), are thought to play a central role in cerebellar network function, their excitable properties have remained unexplored. GoCs fire rhythmically in vivo and in slices, but it was unclear whether this activity originated from pacemaker ionic mechanisms. We explored this issue in acute cerebellar slices from 3-week-old rats by combining loose cell-attached (LCA) and whole-cell (WC) recordings. GoCs displayed spontaneous firing at 1-10 Hz (room temperature) and 2-20 Hz (35-37 degrees C), which persisted in the presence of blockers of fast synaptic receptors and mGluR and GABAB receptors, thus behaving, in our conditions, as pacemaker neurons. ZD 7288 (20 microM), a potent hyperpolarization-activated current (Ih) blocker, slowed down pacemaker frequency. The role of subthreshold Na+ currents (INa,sub) could not be tested directly, but we observed a robust TTX-sensitive, non-inactivating Na+ current in the subthreshold voltage range. When studying repolarizing currents, we found that retigabine (5 microM), an activator of KCNQ K+ channels generating neuronal M-type K+ (IM) currents, reduced GoC excitability in the threshold region. The KCNQ channel antagonist XE991 (5 microM) did not modify firing, suggesting that GoC IM has low XE991 sensitivity. Spike repolarization was followed by an after-hyperpolarization (AHP) supported by apamin-sensitive Ca2+-dependent K+ currents (I(apa)). Block of I(apa) decreased pacemaker precision without altering average frequency. We propose that feed-forward depolarization is sustained by Ih and INa,sub, and that delayed repolarizing feedback involves an IM-like current whose properties remain to be characterized. The multiple ionic mechanisms shown here to contribute to GoC pacemaking should provide the substrate for fine regulation of firing frequency and precision, thus influencing the cyclic inhibition exerted by GoCs onto the cerebellar GL.

Molecular determinants of KCNQ (Kv7) K+ channel sensitivity to the anticonvulsant retigabine

Epilepsy is caused by an electrical hyperexcitability in the CNS. Because K+ channels are critical for establishing and stabilizing the resting potential of neurons, a loss of K+ channels could support neuronal hyperexcitability. Indeed, benign familial neonatal convulsions, an autosomal dominant epilepsy of infancy, is caused by mutations in KCNQ2 or KCNQ3 K+ channel genes. Because these channels contribute to the native muscarinic-sensitive K+ current (M current) that regulates excitability of numerous types of neurons, KCNQ (Kv7) channel activators would be effective in epilepsy treatment. A compound exhibiting anticonvulsant activity in animal seizure models is retigabine. It specifically acts on the neuronally expressed KCNQ2-KCNQ5 (Kv7.2-Kv7.5) channels, whereas KCNQ1 (Kv7.1) is not affected. Using the differential sensitivity of KCNQ3 and KCNQ1 to retigabine, we constructed chimeras to identify minimal segments required for sensitivity to the drug. We identified a single tryptophan residue within the S5 segment of KCNQ3 and also KCNQ2, KCNQ4, and KCNQ5 as crucial for the effect of retigabine. Furthermore, heteromeric KCNQ channels comprising KCNQ2 and KCNQ1 transmembrane domains (attributable to transfer of assembly properties from KCNQ3 to KCNQ1) are retigabine insensitive. Transfer of the tryptophan into the KCNQ1 scaffold resulted in retigabine-sensitive heteromers, suggesting that the tryptophan is necessary in all KCNQ subunits forming a functional tetramer to confer drug sensitivity.

The role of Gilbert's syndrome and frequent NAT2 slow acetylation polymorphisms in the pharmacokinetics of retigabine

Retigabine (RGB) is an investigational antiepileptic drug, which undergoes extensive UGT1A1, 1A9 and 1A4-mediated N-glucuronidation and N-acetylation. The mono-acetylated metabolite of RGB has some pharmacological activity and is denoted AWD21-360. We investigated whether the pharmacokinetics (PK) of RGB and AWD21-360 are altered in subjects with Gilbert's syndrome (GS) and/or with frequent N-acetyltransferase 2 (NAT2) slow acetylator (SA) polymorphisms. Based on consistent genotyping and phenotyping screening results, 37 Caucasian subjects (21-46 years; 31 men, six women) were assigned to one of the following groups: (1) absence of GS (non-GS)/rapid acetylator (RA) (N=11); (2) GS/RA (N=8); (3) non-GS/SA (N=11); (4) GS/SA (N=7). Subjects received single and multiple (b.i.d.) 200-mg oral RGB doses over 5 days. Blood samples were collected up to 60 h after dosing for plasma PK of RGB and AWD21-360. Group comparisons were performed by ANOVA. Single-dose PK of RGB and AWD21-360 and multiple-dose PK of RGB did not differ significantly between groups. After multiple dose treatment, RA subjects showed a significantly higher total exposure to AWD21-360 of about 32% (95% CI 101.9-172.5) relative to SA subjects (P=0.0362). The UGT1A1 metabolic capacity (i.e. presence or absence of GS), however, did not significantly affect the overall exposure to AWD21-360. The results indicate that the PK of RGB is unaltered in individuals with GS, in subjects with NAT2 SA status, and in carriers of both variants, whereas the total exposure to AWD21-360 is significantly related to the RA or SA status of subjects. Results further suggest that metabolic switching to the mono-acetylated metabolite AWD21-360 may partially compensate for the impaired glucuronidation capacity in GS subjects. RGB treatment showed no significant differences in tolerability and safety between groups.

Retigabine: chemical synthesis to clinical application

Retigabine [D23129; N-(2-amino-4-(4-fluorobenzylamino)-phenyl)carbamic acid ethyl ester] is an antiepileptic drug with a recently described novel mechanism of action that involves opening of neuronal K(V)7.2-7.5 (formerly KCNQ2-5) voltage-activated K(+) channels. These channels (primarily K(V)7.2/7.3) enable generation of the M-current, a subthreshold K(+) current that serves to stabilize the membrane potential and control neuronal excitability. In this regard, retigabine has been shown to have a broad-spectrum of activity in animal models of electrically-induced (amygdala-kindling, maximal electroshock) and chemically-induced (pentylenetetrazole, picrotoxin, NMDA) epileptic seizures. These encouraging results suggest that retigabine may also prove useful in the treatment of other diseases associated with neuronal hyperexcitability. Neuropathic pain conditions are characterized by pathological changes in sensory pathways, which favor action potential generation and enhanced pain transmission. Although sometimes difficult to treat with conventional analgesics, antiepileptics can relieve some symptoms of neuropathic pain. A number of recent studies have reported that retigabine can relieve pain-like behaviors (hyperalgesia and allodynia) in animal models of neuropathic pain. Neuronal activation within several key structures within the CNS can also be observed in various animal models of anxiety. Moreover, amygdala-kindled rats, which have a lowered threshold for neuronal activation, also display enhanced anxiety-like responses. Retigabine dose-dependently reduces unconditioned anxiety-like behaviors when assessed in the mouse marble burying test and zero maze. Early clinical studies have indicated that retigabine is rapidly absorbed and distributed, and is resistant to first pass metabolism. Tolerability is good in humans when titrated up to its therapeutic dose range (600-1200 mg/day). No tolerance, dependence or withdrawal potential has been reported, although adverse effects can include mild dizziness, headache, nausea and somnolence. Thus, retigabine may prove to be useful in the treatment of a diverse range of disease states in which neuronal hyperexcitability is a common underlying factor.

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.

Meclofenamic acid and diclofenac, novel templates of KCNQ2/Q3 potassium channel openers, depress cortical neuron activity and exhibit anticonvulsant properties

The voltage-dependent M-type potassium current (M-current) plays a major role in controlling brain excitability by stabilizing the membrane potential and acting as a brake for neuronal firing. The KCNQ2/Q3 heteromeric channel complex was identified as the molecular correlate of the M-current. Furthermore, the KCNQ2 and KCNQ3 channel alpha subunits are mutated in families with benign familial neonatal convulsions, a neonatal form of epilepsy. Enhancement of KCNQ2/Q3 potassium currents may provide an important target for antiepileptic drug development. Here, we show that meclofenamic acid (meclofenamate) and diclofenac, two related molecules previously used as anti-inflammatory drugs, act as novel KCNQ2/Q3 channel openers. Extracellular application of meclofenamate (EC(50) = 25 microM) and diclofenac (EC(50) = 2.6 microM) resulted in the activation of KCNQ2/Q3 K(+) currents, heterologously expressed in Chinese hamster ovary cells. Both openers activated KCNQ2/Q3 channels by causing a hyperpolarizing shift of the voltage activation curve (-23 and -15 mV, respectively) and by markedly slowing the deactivation kinetics. The effects of the drugs were stronger on KCNQ2 than on KCNQ3 channel alpha subunits. In contrast, they did not enhance KCNQ1 K(+) currents. Both openers increased KCNQ2/Q3 current amplitude at physiologically relevant potentials and led to hyperpolarization of the resting membrane potential. In cultured cortical neurons, meclofenamate and diclofenac enhanced the M-current and reduced evoked and spontaneous action potentials, whereas in vivo diclofenac exhibited an anticonvulsant activity (ED(50) = 43 mg/kg). These compounds potentially constitute novel drug templates for the treatment of neuronal hyperexcitability including epilepsy, migraine, or neuropathic pain.

(S)-N-[1-(4-cyclopropylmethyl-3,4-dihydro-2H-benzo[1,4]oxazin-6-yl)-ethyl]-3-(2-fluoro-phenyl)-acrylamide is a potent and efficacious KCNQ2 opener which inhibits induced hyperexcitability of rat hippocampal neurons

(S)-N-[1-(4-Cyclopropylmethyl-3,4-dihydro-2H-benzo[1,4]oxazin-6-yl)-ethyl]-3-(2-fluoro-phenyl)-acrylamide ((S)-2) was identified as a potent and efficacious KCNQ2 opener. This compound demonstrated significant activity in reducing neuronal hyperexcitability in rat hippocampal slices, and the inhibition mediated by (S)-2 was reversed by the KCNQ blocker linopirdine.

The anti-hyperalgesic activity of retigabine is mediated by KCNQ potassium channel activation

Retigabine (N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester) has a broad anticonvulsant spectrum and is currently in clinical development for epilepsy. The compound has an opening effect on neuronal KCNQ channels. At higher concentrations an augmentation of gamma-aminobutyric acid (GABA) induced currents as well as a weak blocking effect on sodium and calcium currents were observed. The goal of this study was to characterise the activity of retigabine in models of acute and neuropathic pain and to investigate if the potassium channel opening effect of retigabine contributes to its activity. Retigabine was tested in mice and rats in the tail flick model of acute pain and in the nerve ligation model with tight ligation of the 5th spinal nerve (L5) using both thermal and tactile stimulation. While retigabine like gabapentin had almost no analgesic effect in mice it showed some analgesic effects in rats in the tail flick model. These effects could not be antagonised with linopirdine, a selective KCNQ potassium channel blocker, indicating a different mode of action for this activity. In L5-ligated rats retigabine significantly and dose-dependently elevated the pain threshold and prolonged the withdrawal latency after tactile and thermal stimulation, respectively. In the L5 ligation model with thermal stimulation retigabine 10 mg/kg p.o. was as effective as 100 mg/kg gabapentin or 10 mg/kg tramadol. The L5 model with tactile stimulation was used to test the role of the KCNQ potassium channel opening effect of retigabine. If retigabine 10 mg/kg p.o. was administered alone it was as effective as tramadol 10 mg/kg p.o. in elevating the pain threshold. Linopirdine (1 and 3 mg/kg i.p.) had nearly no influence on neuropathic pain response. If we administered both retigabine and linopirdine the effect of retigabine was abolished or diminished depending on the dose of linopirdine used.In summary, retigabine is effective in predictive models for neuropathic pain. The activity is comparable to tramadol and is present at lower doses compared with gabapentin. Since the anti-allodynic effect can be inhibited by linopirdine we can conclude that the potassium channel opening properties of retigabine are critically involved in its ability to reduce neuropathic pain response.

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.

Localization of KCNQ5 in the normal and epileptic human temporal neocortex and hippocampal formation

The KCNQ family of voltage-dependent non-inactivating K+ channels is composed of five members, four of which (KCNQ2-5) are expressed in the CNS and are responsible for the M-current. Mutations in either KCNQ2 or KCNQ3 lead to a hereditary form of dominant generalized epilepsy. Using specific antisera to the KCNQ2, KCNQ3 and KCNQ5 subunits, we found that KCNQ3 co-immunoprecipitated with KCNQ2 and KCNQ5 subunits, but no association was detected between KCNQ2 and KCNQ5. Intense KCNQ5 immunoreactivity was found to be widely distributed throughout the temporal neocortex and the hippocampal formation. In these structures, both pyramidal and non-pyramidal neurons and a population of glial cells in the white matter expressed the KCNQ5 subunit. In the sclerotic areas of the CA fields of epileptic patients, a marked loss of KCNQ5 immunoreactive pyramidal neurons was found in relation with the loss of neurons in these regions. However, in the regions adjacent to the sclerotic areas, the distribution and intensity of KCNQ5 immunostaining was apparently normal. The widespread distribution of KCNQ5 subunits, its persistence in pharmacoresistant epilepsy, along with the significant role of the M-current in the control of neuronal excitability, makes this protein a possible target for the development of anticonvulsant drugs.

Stoichiometry of expressed KCNQ2/KCNQ3 potassium channels and subunit composition of native ganglionic M channels deduced from block by tetraethylammonium

KCNQ2 and KCNQ3 potassium-channel subunits can form both homomeric and heteromeric channels; the latter are thought to constitute native ganglionic M channels. We have tried to deduce the stoichiometric contributions of KCNQ2 and KCNQ3 subunits to currents generated by the coexpression of KCNQ2 and KCNQ3 cDNA plasmids in Chinese hamster ovary (CHO) cells, and to native M currents in dissociated rat superior cervical ganglion (SCG) neurons, by comparing the block of these currents produced by tetraethylammonium (TEA) with the block of currents generated by a tandem KCNQ3/2 construct. TEA concentration-inhibition curves against coexpressed KCNQ2 plus KCNQ3 currents, and against native M currents in SCG neurons from 6-week-old [postnatal day 45 (P45)] rats, were indistinguishable from those for the expressed tandem construct, and fully accorded with a 1:1 stoichiometry. Inhibition curves in neurons from younger (P17) rats could be better fitted assuming an additional small proportion of current carried by KCNQ2 homomultimers. Single-cell PCR yielded signals for KCNQ2, KCNQ3, and KCNQ5 mRNAs in all SCG neurons tested from both P17 and P45 rats. Quantitative PCR of whole-ganglion mRNA revealed stable levels of KCNQ2 and KCNQ5 mRNA between P7 and P45, but excess and incrementing levels of KCNQ3 mRNA. Increasing levels of KCNQ3 protein between P17 and P45 were confirmed by immunocytochemistry. We conclude that coexpressed KCNQ2 plus KCNQ3 cDNAs generate channels with 1:1 (KCNQ2:KCNQ3) stoichiometry in CHO cells and that native M channels in SCG neurons adopt the same conformation during development, assisted by the increased expression of KCNQ3 mRNA and protein.

Voltage-independent KCNQ4 currents induced by (+/-)BMS-204352

The compound BMS-204352 has been targeted for use against acute ischemic stroke, due to its activation of the large-conductance Ca2+-activated K-channel (BK). We have previously described that the racemate (+/-)BMS-204352 reversibly modulates KCNQ4 voltage dependency. Here we show that (+/-)BMS-204352 also induces a voltage-independent KCNQ4 current. The channels were stably expressed in human embryonic kidney cells (HEK293), and investigated by use of the whole-cell mode of the patch-clamp technique. (+/-)BMS-204352 was applied extracellularly (10 microM) in order to precipitate the robust appearance of the voltage-independent current. The voltage-independent KCNQ4 currents were recorded as instantaneous increases in currents upon hyperpolarizing or depolarizing voltage steps elicited from holding potentials of -90 or -110 mV. The voltage-independent current reversed at the equilibrium potential for potassium ( E(K)), hence was carried by a K+ conductance, and was blocked by the selective KCNQ channel blockers XE991 and linopirdine. Similar results were obtained with KCNQ4 channels transiently transfected into Chinese hamster ovary cells (CHO). When (+/-)BMS-204352 was applied to stably expressed BK channels, only the voltage dependency was modulated. Retigabine, the classic activator of KCNQ channels, did not induce voltage-independent currents. Our data indicate that KCNQ4 channels may conduct voltage-dependent and voltage-independent currents in the presence of (+/-)BMS-204352.

Stoichiometry of expressed KCNQ2/KCNQ3 potassium channels and subunit composition of native ganglionic M channels deduced from block by tetraethylammonium

KCNQ2 and KCNQ3 potassium-channel subunits can form both homomeric and heteromeric channels; the latter are thought to constitute native ganglionic M channels. We have tried to deduce the stoichiometric contributions of KCNQ2 and KCNQ3 subunits to currents generated by the coexpression of KCNQ2 and KCNQ3 cDNA plasmids in Chinese hamster ovary (CHO) cells, and to native M currents in dissociated rat superior cervical ganglion (SCG) neurons, by comparing the block of these currents produced by tetraethylammonium (TEA) with the block of currents generated by a tandem KCNQ3/2 construct. TEA concentration-inhibition curves against coexpressed KCNQ2 plus KCNQ3 currents, and against native M currents in SCG neurons from 6-week-old [postnatal day 45 (P45)] rats, were indistinguishable from those for the expressed tandem construct, and fully accorded with a 1:1 stoichiometry. Inhibition curves in neurons from younger (P17) rats could be better fitted assuming an additional small proportion of current carried by KCNQ2 homomultimers. Single-cell PCR yielded signals for KCNQ2, KCNQ3, and KCNQ5 mRNAs in all SCG neurons tested from both P17 and P45 rats. Quantitative PCR of whole-ganglion mRNA revealed stable levels of KCNQ2 and KCNQ5 mRNA between P7 and P45, but excess and incrementing levels of KCNQ3 mRNA. Increasing levels of KCNQ3 protein between P17 and P45 were confirmed by immunocytochemistry. We conclude that coexpressed KCNQ2 plus KCNQ3 cDNAs generate channels with 1:1 (KCNQ2:KCNQ3) stoichiometry in CHO cells and that native M channels in SCG neurons adopt the same conformation during development, assisted by the increased expression of KCNQ3 mRNA and protein.

Pharmacokinetic interaction between retigabine and lamotrigine in healthy subjects

PURPOSE

The antiepileptic drugs (AEDs) retigabine (RGB) and lamotrigine (LTG) undergo predominantly N-glucuronidation and renal excretion. This study was performed to evaluate potential pharmacokinetic interactions between both AEDs.

METHODS

Twenty-nine healthy male subjects participated in the study. Group A ( n=14) received single oral 200-mg RGB doses on day 1 and day 7, and 25 mg o.i.d. LTG on days 3-8. Group B ( n=15) received single oral 200-mg LTG doses on day 1 and day 17, and was up-titrated to 300 mg RGB b.i.d. on days 6-20. Blood samples were collected to compare the pharmacokinetics of both AEDs and the N-acetyl metabolite of RGB (AWD21-360) after single and concomitant treatments.

RESULTS

RGB was rapidly absorbed and eliminated with a mean half-life (t(1/2)) of 6.3+/-1.1 h and an apparent clearance (CL/F) of 0.69+/-1.4 l/h/kg. Under co-administration of LTG, mean RGB t(1/2) and area under the plasma concentration-time curve (AUC) were increased by 7.5% ( P=0.045) and 15% ( P=0.006), respectively, while CL/F was decreased by 13% ( P=0.06). Consistent results were obtained for AWD21-360. LTG was moderately rapidly absorbed, eliminated with a mean t(1/2) of 37+/-10.4 h and a CL/F of 0.028+/-0.007 l/h/kg. Under co-administration of RGB, mean LTG t(1/2) and AUC decreased by 15% and 18%, respectively, while CL/F increased by 22% (all parameters, P=0.001).

CONCLUSIONS

RGB and LTG exhibit a modest pharmacokinetic interaction on each other. The slight decline in RGB clearance due to LTG is believed to result from competition for renal elimination rather than competition for glucuronidation. The induction of LTG clearance due to retigabine was unexpected since RGB did not show enzyme induction in various other drug-drug interaction studies. Further studies in patients are needed to assess the clinical relevance of these findings for concomitant treatment with both drugs in the upper recommended dose range.

The anticonvulsant retigabine attenuates nociceptive behaviours in rat models of persistent and neuropathic pain

We have tested for anti-nociceptive effects of the anticonvulsant KCNQ channel opener, N-(2-amino-4-(4-fluorobenzylamino)-phenyl)carbamic acid ethyl ester (retigabine), in rat models of experimental pain. In the chronic constriction injury and spared nerve models of neuropathic pain, injection of retigabine (5 and 20 mg/kg, p.o.) significantly attenuated (P<0.05) mechanical hypersensitivity in response to pin prick stimulation of the injured hindpaw. In contrast, retigabine had no effect on mechanical hypersensitivity to von Frey stimulation of the injured hindpaw in either model. Cold sensitivity in response to ethyl chloride was only attenuated (P<0.05) in the chronic constriction injury model. In the formalin test, retigabine (20 mg/kg, p.o.) attenuated flinching behaviour in the second phase compared with vehicle (P<0.05), and this effect was completely reversed by the KCNQ channel blocker 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone (XE-991; 3 mg/kg, i.p.). Neither retigabine nor XE-991 administration affected the latency to respond to noxious thermal stimulation of the tail in control animals. These results suggest that retigabine may prove to be effective in the treatment of neuropathic pain.

Molecular correlates of the M-current in cultured rat hippocampal neurons

M-type K(+) currents (I(K(M))) play a key role in regulating neuronal excitability. In sympathetic neurons, M-channels are thought to be composed of a heteromeric assembly of KCNQ2 and KCNQ3 K(+) channel subunits. Here, we have tried to identify the KCNQ subunits that are involved in the generation of I(K(M)) in hippocampal pyramidal neurons cultured from 5- to 7-day-old rats. RT-PCR of either CA1 or CA3 regions revealed the presence of KCNQ2, KCNQ3, KCNQ4 and KCNQ5 subunits. Single-cell PCR of dissociated hippocampal pyramidal neurons gave detectable signals for only KCNQ2, KCNQ3 and KCNQ5; where tested, most also expressed mRNA for the vesicular glutamate transporter VGLUT1. Staining for KCNQ2 and KCNQ5 protein showed punctate fluorescence on both the somata and dendrites of hippocampal neurons. Staining for KCNQ3 was diffusely distributed whereas KCNQ4 was undetectable. In perforated patch recordings, linopirdine, a specific M-channel blocker, fully inhibited I(K(M)) with an IC(50) of 3.6 +/- 1.5 microM. In 70 % of these cells, TEA fully suppressed I(K(M)) with an IC(50) of 0.7 +/- 0.1 mM. In the remaining cells, TEA maximally reduced I(K(M)) by only 59.7 +/- 5.2 % with an IC(50) of 1.4 +/- 0.3 mM; residual I(K(M)) was abolished by linopirdine. Our data suggest that KCNQ2, KCNQ3 and KCNQ5 subunits contribute to I(K(M)) in these neurons and that the variations in TEA sensitivity may reflect differential expression of KCNQ2, KCNQ3 and KCNQ5 subunits.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

Activation of KCNQ5 channels stably expressed in HEK293 cells by BMS-204352

The novel anti-ischemic compound, BMS-204352 ((3S)-(+)-(5-chloro-2-methoxyphenyl)-1,3-dihydro-3-fluoro-6-(trifluoromethyl)-2H-indol-2-one)), strongly activates the voltage-gated K+ channel KCNQ5 in a concentration-dependent manner with an EC50 of 2.4 microM. At 10 microM, BMS-204352 increased the steady state current at -30 mV by 12-fold, in contrast to the 2-fold increase observed for the other KCNQ channels [Schrøder et al., 2001]. Retigabine ((D-23129; N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester) induced a smaller, yet qualitatively similar effect on KCNQ5. Furthermore, BMS-204352 (10 microM) did not significantly shift the KCNQ5 activation curves (threshold and potential for half-activation, V1/2), as observed for the other KCNQ channels. In the presence of BMS-204352, the activation and deactivation kinetics of the KCNQ5 currents were slowed as the slow activation time constant increased up to 10-fold. The M-current blockers, linopirdine (DuP 996; 3,3-bis(4-pyridinylmethyl)-1-phenylindolin-2-one) and XE991 (10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone), inhibited the activation of the KCNQ5 channel induced by the BMS-204352. Thus, BMS-204352 appears to be an efficacious KCNQ channels activator, and the pharmacological properties of the compound on the KCNQ5 channel seems to be different from what has been obtained on the other KCNQ channels.

Multiple-dose, linear, dose-proportional pharmacokinetics of retigabine in healthy volunteers

Retigabine, a first-in-class selective M-current potassium channel opener, is a novel antiepileptic compound currently in clinical development. The purpose of this randomized placebo-controlled study was to assess retigabine oral safety and pharmacokinetics in healthy male volunteers (N = 45). Subjects received one dose on day 1 and doses every 12 hours for the next 14 days. Fixed doses were given to the first four groups (200, 400, 500, and 600 mg per day). Titrated doses were given to group 5 in 100 mg increases every 4 days, achieving 700 mg per day on day 15. Serial blood samples were collected on days 1 and 15. Pharmacokinetic parameters were compared between days and among dose groups. After administration of a single dose, retigabine was rapidly absorbed, with maximum concentrations of 387 ng/ml (normalized to a 100 mg dose) occurring within 1.5 hours. Retigabine was eliminated with a mean terminal half-life of 8.0 hours and an apparent oral clearance of 0.70 L/h/kg in white subjects. In black subjects, retigabine clearance and volume of distribution were 25% and 30% lower, respectively, after normalizing by body weight, leading to higher exposure in this population. Retigabine's pharmocokinetics was linearly dose proportional. Steady-state pharmacokinetics was in agreement with single-dose pharmacokinetics, and the accumulation ratio was about 1.5. Retigabine and AWD21-360 trough evening concentrations were significantly lower (about 30% to 35%) than morning values. The titration regimen allowed for higher doses to be tolerated compared to the fixed-dose regimen. In conclusion, the pharmacokinetics of retigabine is linearly dose proportional for daily doses of 100 to 700 mg and is not modified on multiple administrations.

Potassium channels: gene family, therapeutic relevance, high-throughput screening technologies and drug discovery

Existing drugs that modulate ion channels represent a key class of pharmaceutical agents across many therapeutic areas and there is considerable further potential for potassium channel drug discovery. Potassium channels represent the largest and most diverse sub-group of ion channels and they play a central role in regulating the membrane potential of cells. Recent advances in genomics have greatly added to the number of these potential drug targets, but selecting a suitable potassium channel for drug discovery research is a key step. In particular, the potential therapeutic relevance of a potassium channel should be taken into account when selecting a target for screening. Potassium channel drug discovery is being driven by a need to identify lead compounds that can provide tractable starting points for medicinal chemistry. Furthermore, advances in laboratory automation have brought significant opportunities to increase screening throughput for potassium channel assays, but careful assay configuration to model drug-target interactions in a physiological manner is an essential consideration. Several potassium channel screening platforms are described in this review in order to provide some insight into the variety of formats available for screening, together with some of their inherent advantages and limitations. Particular emphasis is placed on the mechanistic basis of drug-target interaction and those aspects of structure/function that are of prime importance in potassium channel drug discovery.

Activation of expressed KCNQ potassium currents and native neuronal M-type potassium currents by the anti-convulsant drug retigabine

Retigabine [D-23129; N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester] is a novel anticonvulsant compound that is now in clinical phase II development. It has previously been shown to enhance currents generated by KCNQ2/3 K(+) channels when expressed in Chinese hamster ovary (CHO) cells (Main et al., 2000; Wickenden et al., 2000). In the present study, we have compared the actions of retigabine on KCNQ2/3 currents with those on currents generated by other members of the KCNQ family (homomeric KCNQ1, KCNQ2, KCNQ3, and KCNQ4 channels) expressed in CHO cells and on the native M current in rat sympathetic neurons [thought to be generated by KCNQ2/3 channels (Wang et al., 1998)]. Retigabine produced a hyperpolarizing shift of the activation curves for KCNQ2/3, KCNQ2, KCNQ3, and KCNQ4 currents with differential potencies in the following order: KCNQ3 > KCNQ2/3 > KCNQ2 > KCNQ4, as measured either by the maximum hyperpolarizing shift in the activation curves or by the EC(50) values. In contrast, retigabine did not enhance cardiac KCNQ1 currents. Retigabine also produced a hyperpolarizing shift in the activation curve for native M channels in rat sympathetic neurons. The retigabine-induced current was inhibited by muscarinic receptor stimulation, with similar agonist potency but 25% reduced maximum effect. In unclamped neurons, retigabine produced a hyperpolarization and reduced the number of action potentials produced by depolarizing current injections, without change in action potential configuration.

Activation of expressed KCNQ potassium currents and native neuronal M-type potassium currents by the anti-convulsant drug retigabine

Retigabine [D-23129; N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester] is a novel anticonvulsant compound that is now in clinical phase II development. It has previously been shown to enhance currents generated by KCNQ2/3 K(+) channels when expressed in Chinese hamster ovary (CHO) cells (Main et al., 2000; Wickenden et al., 2000). In the present study, we have compared the actions of retigabine on KCNQ2/3 currents with those on currents generated by other members of the KCNQ family (homomeric KCNQ1, KCNQ2, KCNQ3, and KCNQ4 channels) expressed in CHO cells and on the native M current in rat sympathetic neurons [thought to be generated by KCNQ2/3 channels (Wang et al., 1998)]. Retigabine produced a hyperpolarizing shift of the activation curves for KCNQ2/3, KCNQ2, KCNQ3, and KCNQ4 currents with differential potencies in the following order: KCNQ3 > KCNQ2/3 > KCNQ2 > KCNQ4, as measured either by the maximum hyperpolarizing shift in the activation curves or by the EC(50) values. In contrast, retigabine did not enhance cardiac KCNQ1 currents. Retigabine also produced a hyperpolarizing shift in the activation curve for native M channels in rat sympathetic neurons. The retigabine-induced current was inhibited by muscarinic receptor stimulation, with similar agonist potency but 25% reduced maximum effect. In unclamped neurons, retigabine produced a hyperpolarization and reduced the number of action potentials produced by depolarizing current injections, without change in action potential configuration.

Properties of single M-type KCNQ2/KCNQ3 potassium channels expressed in mammalian cells

1. The single channel properties of KCNQ2/KCNQ3 channels underlying neuronal voltage-dependent M-type potassium currents were studied in cell-attached patches from transfected Chinese hamster ovary (CHO) cells. Macroscopic currents produced by homo- and heteromeric KCNQ2/KCNQ3 channels were measured using the perforated-patch whole-cell technique. 2. Compared with heteromeric KCNQ2 + KCNQ3 channels, homomeric KCNQ2 channels had lower slope conductance (9.0 +/- 0.3 and 5.8 +/- 0.3 pS, respectively) and open probability at 0 mV (0.30 +/- 0.07 and 0.15 +/- 0.03, respectively), consistent with their 3.8-fold smaller macroscopic currents. By contrast, homomeric KCNQ3 channels had the same slope conductance (9.0 +/- 1.1 pS) as KCNQ2 + KCNQ3 channels, and higher open probability (0.59 +/- 0.11), inconsistent with their 12.7-fold smaller macroscopic currents. Thus, KCNQ2 and KCNQ3 subunits may play different roles in the expression of M-type currents, with KCNQ2 ensuring surface expression of underlying channels and KCNQ3 modifying their function. 3. Both in homo- and heteromeric KCNQ2/KCNQ3 channels the shut time distributions were fitted with three, and the open time distributions with two, exponential components. By measuring these and other parameters (e.g. conductance and open probability) KCNQ2/ KCNQ3 channels can be shown to resemble previously characterised neuronal M-type channels.