Flupirtine: A Review of Its Neuroprotective and Behavioral Properties

Effects of a potassium channel opener on brain injury and neurologic outcomes in an animal model of neonatal hypoxic-ischemic injury

BACKGROUND

Hypoxia-ischemia (HI) is the most common cause of brain injury in newborns and the survivors often develop cognitive and sensorimotor disabilities that undermine the quality of life. In the current study, we examined the effectiveness of flupirtine, a potassium channel opener, shown previously in an animal model to have strong anti-neonatal-seizure efficacy, to provide neuroprotection and alleviate later-life disabilities caused by neonatal hypoxic-ischemic injury.

METHODS

The rats were treated with a single dose of flupirtine for 4 days following HI induction in 7-day-old rats. The first dose of flupirtine was given after the induction of HI and during the reperfusion period. The effect of treatment was examined on acute and chronic brain injury, motor functions, and cognitive abilities.

RESULTS

Flupirtine treatment significantly reduced HI-induced hippocampal and cortical tissue loss at acute time point. Furthermore, at chronic time point, flupirtine reduced contralateral hippocampal volume loss and partially reversed learning and memory impairments but failed to improve motor deficits.

CONCLUSION

The flupirtine treatment regimen used in the current study significantly reduced brain injury at acute time point in an animal model of neonatal hypoxic-ischemic encephalopathy. However, these neuroprotective effects were not persistent and only modest improvement in functional outcomes were observed at chronic time points.

Pharmacology and clinical applications of flupirtine: Current and future options

Flupirtine is the first representative in a class of triaminopyridines that exhibits pharmacological properties leading to the suppression of over-excitability of neuronal and non-neuronal cells. Consequently, this drug has been used as a centrally acting analgesic in patients with a range of acute and persistent pain conditions without the adverse effects characteristic of opioids and non-steroidal anti-inflammatory drug and is well tolerated. The pharmacological profile exhibited involves actions on several cellular targets, including Kv7 channels, G-protein-regulated inwardly rectifying K channels and γ-aminobutyric acid type A receptors, but also there is evidence of additional as yet unidentified mechanisms of action involved in the effects of flupirtine. Flupirtine has exhibited effects in a range of cells and tissues related to the locations of these targets. In additional to analgesia, flupirtine has demonstrated pharmacological properties consistent with use as an anticonvulsant, a neuroprotectant, skeletal and smooth muscle relaxant, in treatment of auditory and visual disorders, and treatment of memory and cognitive impairment. Flupirtine is providing important information and clues regarding novel mechanistic approaches to the treatment of a range of clinical conditions involving hyper-excitability of cells. Identification of molecules exhibiting specificity for the pharmacological targets (e.g., Kv7 isoforms) involved in the actions of flupirtine will provide further insight into clinical applications. Whether the broad-spectrum pharmacology of flupirtine or target-specific actions is preferential to gain benefit, especially in complex clinical conditions, requires further investigation. This review will consider recent advancement in understanding of the pharmacological profile and related clinical applications of flupirtine.

In vitro approach to elucidate the relevance of carboxylesterase 2 and N-acetyltransferase 2 to flupirtine-induced liver injury

The use of flupirtine, an analgesic, has been restricted in European countries because it causes liver injury in rare cases. Flupirtine is primarily metabolized to D-13223, an acetylamino form. In the process of D-13223 formation, it has been hypothesized that a reactive metabolite is formed which may be involved in flupirtine hepatotoxicity. The purpose of this study was to identify the potential reactive metabolite and the responsible enzymes in the human liver to get a clue to the mechanism of hepatotoxicity. Using recombinant enzymes, we found that D-13223 was formed from flupirtine via hydrolysis by carboxylesterase 2 (CES2) and subsequent acetylation by N-acetyltransferase (NAT) 2. A conjugate of N-acetyl-l-cysteine (NAC), a nucleophile, was detected by incubation of flupirtine with CES2, and the conjugate formation in human liver microsomes was inhibited by CES2 inhibitors, indicating that a reactive metabolite, which may be a quinone diimine, was produced in the process of CES2-mediated hydrolysis of flupirtine. The formation of the NAC conjugate in liver S9 samples from NAT2 slow acetylators was significantly higher than that from NAT2 rapid/intermediate acetylators, indicating that NAT2 could function as a detoxification enzyme for flupirtine. CES2-overexpressing HepG2 cells showed remarkable lactate dehydrogenase leakage under flupirtine treatment, while no cytotoxicity was observed in control cells, suggesting that the reactive metabolite formed by CES2-mediated hydrolysis of flupirtine would be a trigger of hepatotoxicity. NAT2 slow acetylators with high CES2 activity could be highly susceptible to flupirtine-induced liver injury.

Discovery of Aromatic Carbamates that Confer Neuroprotective Activity by Enhancing Autophagy and Inducing the Anti-Apoptotic Protein B-Cell Lymphoma 2 (Bcl-2)

Neurodegenerative diseases share certain pathophysiological hallmarks that represent common targets for drug discovery. In particular, dysfunction of proteostasis and the resultant apoptotic death of neurons represent common pathways for pharmacological intervention. A library of aromatic carbamate derivatives based on the clinically available drug flupirtine was synthesized to determine a structure-activity relationship for neuroprotective activity. Several derivatives were identified that possess greater protective effect in human induced pluripotent stem cell-derived neurons, protecting up to 80% of neurons against etoposide-induced apoptosis at concentrations as low as 100 nM. The developed aromatic carbamates possess physicochemical properties desirable for CNS therapeutics. The primary known mechanisms of action of the parent scaffold are not responsible for the observed neuroprotective activity. Herein, we demonstrate that neuroprotective aromatic carbamates function to increase the Bcl-2/Bax ratio to an antiapoptotic state and activate autophagy through induction of beclin 1.

Anticonvulsant effect of flupirtine in an animal model of neonatal hypoxic-ischemic encephalopathy

Research studies suggest that neonatal seizures, which are most commonly associated with hypoxic-ischemic injury, may contribute to brain injury and adverse neurologic outcome. Unfortunately, neonatal seizures are often resistant to treatment with current anticonvulsants. In the present study, we evaluated the efficacy of flupirtine, administered at clinically relevant time-points, for the treatment of neonatal seizures in an animal model of hypoxic-ischemic injury that closely replicates features of the human syndrome. We also compared the efficacy of flupirtine to that of phenobarbital, the current first-line drug for neonatal seizures. Flupirtine is a KCNQ potassium channel opener. KCNQ channels play an important role in controlling brain excitability during early development. In this study, hypoxic-ischemic injury was induced in neonatal rats, and synchronized video-EEG records were acquired at various time-points during the experiment to identify seizures. The results revealed that flupirtine, administered either 5 min after the first electroclinical seizure, or following completion of 2 h of hypoxia, i.e., during the immediate reperfusion period, reduced the number of rats with electroclinical seizures, and also the frequency and total duration of electroclinical seizures. Further, daily dosing of flupirtine decreased the seizure burden over 3 days following HI-induction, and modified the natural evolution of acute seizures. Moreover, compared to a therapeutic dose of phenobarbital, which was modestly effective against electroclinical seizures, flupirtine showed greater efficacy. Our results indicate that flupirtine is an extremely effective treatment for neonatal seizures in rats and provide evidence for a trial of this medication in newborn humans.

Quantitative LC-MS/MS determination of flupirtine, its N-acetylated and two mercapturic acid derivatives in man

The non-opiate analgesic drug flupirtine was shown in vitro to undergo hydrolysis followed by N-acetylation to form D13223, glucuronidation and conjugation with glutathione to form the stable mercapturic acid derivatives M-424 and M-466. To quantify flupirtine and its metabolites in samples obtained in a clinical study in healthy subjects selected on their genotype of NAT2, UGT1A1 and GSTP1, two LC-MS/MS methods were developed. The validation range for flupirtine and D-13223 in serum was 0.5-500 ng/ml. For urine and feces, the validation ranges for flupirtine and D-13223 were 20-5000 ng/ml and 5.0-5000 ng/ml, respectively. M-424 and M-466 could be quantified in urine between 5.0 and 5000 ng/ml. Free flupirtine and D-13223 were separated from serum, urine and feces with liquid-liquid extraction. For flupirtine and D-13223, the chromatography was performed on a XTerra C18 column isocratically with a mobile phase consisting of ammonium formate buffer (pH 3.5mM) and acetonitrile (50:50; v/v), for M-466 and M-424 a Synergi(®) Fusion-RP column was used and a linear gradient method with water/HCOOH (pH 3) and acetonitrile. The mass spectrometer operated both with electro spray ionization in positive multiple reaction monitoring mode. The developed methods fulfilled the current FDA criteria on bioanalytical method validation for accuracy (error: -16.9 to 11.2%), precision (1.2-13.4%), recovery, stability and matrix effects over the observed analytical range. Thus, the methods were suitable to quantify flupirtine absorption and metabolic disposition in man after single intravenous and oral dosing (100mg) and repeated oral administration (400mg once daily).

Retigabine, a K(V)7 (KCNQ) potassium channel opener, attenuates L-DOPA-induced dyskinesias in 6-OHDA-lesioned rats

L-DOPA-induced dyskinesias (LID) represent a severe complication of long-time pharmacotherapy in Parkinson's disease that necessitates novel therapeutics. The acute and chronic effects of K(V)7.2-7.5 channel openers (retigabine, flupirtine) on the severity of LID and parkinsonian signs were examined in comparison to the glutamate receptor antagonist amantadine (positive control) in a rat model of LID. Acute treatment with retigabine (2.5, 5 mg/kg i.p.) and flupirtine (5, 10 mg/kg i.p.) significantly reduced the severity of abnormal involuntary movements (AIM) to a comparable extent as amantadine (20, 40 mg/kg s.c.), but flupirtine delayed the disappearance of AIM. Chronic treatment with retigabine (daily 5 mg/kg i.p. over 19 days combined with l-DOPA 10 mg i.p.) did not prevent or delay the development of LID, but reduced the severity of AIM, while antidyskinetic effects of amantadine (40 mg/kg i.p.) were restricted to the first day of treatment. Retigabine caused sedation and ataxia which declined during the chronic treatment, but did not reduce the antiparkinsonian effects of l-DOPA in these experiments. Acute co-injections of retigabine (5 mg) together with l-DOPA (10 mg/kg) neither reduced the motor performance in the rotarod test nor exerted negative effects on the antiparkinsonian efficacy of l-DOPA in the block and stepping test. Nevertheless, the sedative effects of retigabine may limit its therapeutic potential for the treatment of LID. The present data indicate that K(V)7 channels deserve attention in the research of the pathophysiology of dyskinesias. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Flupirtine in pain management: pharmacological properties and clinical use

Flupirtine is a centrally acting, non-opioid analgesic that is available in a number of European countries for the treatment of a variety of pain states. The therapeutic benefits seen with flupirtine relate to its unique pharmacological properties. Flupirtine displays indirect NDMA receptor antagonism via activation of potassium channels and is the first representative of a pharmacological class denoted the 'selective neuronal potassium channel openers'. The generation of the M-current is facilitated by flupirtine via the opening of neuronal Kv7 potassium channels. The opening of these channels inhibits exaggerated neuronal action potential generation and controls neuronal excitability. Neuronal hyperexcitability is a physiological component of many pain states such as chronic pain, migraine and neurogenic pain. Although large-scale clinical trials are lacking, the clinical trial database available to date from smaller-scale studies, together with extensive clinical experience, indicate that flupirtine effectively reduces chronic musculoskeletal pain, migraine and neuralgias, amongst other types of pain. In addition, flupirtine produces, at recommended clinical doses, muscle-relaxing effects in the presence of abnormally increased muscle tension. Its analgesic and muscle-relaxant properties were comparable to tramadol and chlormezanone, respectively, in two prospective trials in patients with lower back pain. Cytoprotective, anti-apoptotic and antioxidant properties have also been associated with flupirtine use in a small number of studies to date. When provided as combination therapy with morphine, flupirtine increases the antinociceptive activity of morphine 4-fold. Flupirtine displays superior tolerability when compared with tramadol and pentazocine. The most common adverse effects associated with flupirtine use are drowsiness, dizziness, heartburn, dry mouth, fatigue and nausea. With respect to its molecular structure, mechanism of action and adverse event profile, flupirtine is a unique drug. Flupirtine is an analgesic with many potential therapeutic benefits that may prove useful in the treatment of many disease states.

Investigation of the in vitro metabolism of the analgesic flupirtine

The in vitro metabolism of flupirtine, ethyl-N-[2-amino-6-(4-fluorophenylmethyl-amino)pyridine-3-yl]carbamate, a centrally acting analgesic with muscle tone-reducing activity, was studied. Two flupirtine metabolites were already known: the N-acetylated analog D13223 and 4-fluorohippuric acid. The structure of flupirtine suggested that redox chemistry may play a role in metabolism, and cyclic voltammetry studies showed that the drug undergoes facile and irreversible redox reactions. Thus, oxidative metabolism was investigated first. With CYP3A1-induced rat liver microsomes an 18% turnover of flupirtine and a 20 to 25% turnover of D13223 took place over 30 min, but less than 5% turnover of flupirtine was observed with all human liver microsomal preparations tested, evidence that cytochrome P450 does not contribute appreciably to the metabolism in humans. Likewise, no involvement of human monoamine oxidase (isoforms A and B) was found for either flupirtine or D13223. In contrast, flupirtine was an excellent substrate for both human myeloperoxidase and horse radish peroxidase (HRP). These enzymes produced detectable amounts of oxidation products. Incubations of flupirtine with HRP produced an oxidation product that could be trapped with glutathione, the resulting glutathione conjugate was characterized by mass spectrometry and NMR. Metabolism of D13223 by both peroxidases was also observed but to a much lesser extent. Porcine liver esterases cleave the carbamate group of flupirtine, and both human N-acetyltransferases 1 and 2 acetylated the hydrolysis product, presumably descarboethoxyflupirtine, with nearly equal efficiencies to yield D13223. Incubations of human liver microsomes with flupirtine or the metabolite D13223 together with UDP-glucuronic acid gave two isomeric N-glucuronides in both cases.

The neuroprotectant properties of glutamate antagonists and antiglutamatergic drugs

In the slowly progressive neurodegenerative disorders like Parkinson's disease and Alzheimer's disease very different neuronal populations undergo degenerative processes, although the cascades of cellular events leading to death are supposed to be similar. We suggest that the complex pattern of degeneration in Parkinson's disease depends on two processes, a 'primary neurodegeneration' that takes place in the striato-nigral dopamine neurons and a 'secondary degeneration', occurring in distant structures of the basal ganglia network. For the purpose of explaining the regionally different expression of 'primary neurodegeneration' in different diseases, we postulate that the origin of neurodegeneration is associated with the local release of a neurotransmitter. For Parkinson's disease this would mean that the metabolism of dopamine in the striatum, nucleus accumbens and presumably the pedunculopontine tegmental nucleus, together with one or more pathological factors contribute to the initial neurodegeneration. There are recent studies indicating that a transneuronal retrograde degeneration of the substantia nigra pars compacta neurons might be induced by a loss of function of dopaminergic synapses in the striatum. We have recently established an animal model of retrograde striato-nigral degeneration, where the assessment of markers for cellular stress is possible. In Parkinson's disease, several structures distal from the substantia nigra pars compacta undergo neuropathological changes, characterizing the 'secondary neurodegeneration. Our recent studies provide experimental evidence for a chronic cellular stress in these structures because of a relative or absolute glutamatergic overactivity due to the initial loss of dopaminergic innervation. Thus, a loss of dopamine transforms the basal ganglia to a 'destructive network'. Both processes, the 'primary' and 'secondary neurodegeneration', affecting each other, characterize the progress of chronic neurodegeneration. From this point of view, we would further like to develop strategies for symptomatic amendment. Excitatory amino acids seem to be involved not only in the secondary processes of neurodegeneration, but also in initiation of the 'primary degeneration' of the substantia nigra pars compacta. Therefore, a reduction of glutamatergic overactivity constitutes a promising neuroprotective strategy. Especially the new antagonists of the NMDA-receptors with high affinity to the NR2B subunit of the receptor are in focus of our interest, since they reveal a favourable profile of side effects, therefore providing a promising tool for neuroprotection.