Involvement of voltage-gated sodium channels blockade in the analgesic effects of orphenadrine

Muscle Cramps in Outpatients with Liver Diseases in Tokyo, Japan

Background and Objectives: Muscle cramps are often observed in patients with liver diseases, especially advanced liver fibrosis. The exact prevalence of muscle cramps in outpatients with liver diseases in Japan is unknown. Patients and Methods: This study examined the prevalence of, and therapies for, muscle cramps in outpatients with liver diseases in Tokyo, Japan. A total of 238 outpatients with liver diseases were retrospectively examined. We investigated whether they had muscle cramps using a visual analog scale (VAS) (from 0, none, to 10, strongest), and also investigated their therapies. Results: Muscle cramps were observed in 34 outpatients with liver diseases (14.3%); their mean VAS score was 5.53. A multivariate analysis demonstrated that older age (equal to or older than 66 years) was the only significant factor as-sociated with muscle cramps. The prevalence of muscle cramps among patients with liver diseases seemed not to be higher. The problem was that only 11 (32.4%) of 34 outpatients received therapy for their muscle cramps. Conclusions: Only age is related to muscle cramps, which is rather weak, and it is possible that this common symptom may not be limited to liver disease patients.

Understanding the physiological role of NaV1.9: Challenges and opportunities for pain modulation

Voltage-activated Na⁺ (Na_V) channels are crucial contributors to rapid electrical signaling in the human body. As such, they are among the most targeted membrane proteins by clinical therapeutics and natural toxins. Several of the nine mammalian Na_V channel subtypes play a documented role in pain or other sensory processes such as itch, touch, and smell. While causal relationships between these subtypes and biological function have been extensively described, the physiological role of Na_V1.9 is less understood. Yet, mutations in Na_V1.9 can cause striking disease phenotypes related to sensory perception such as loss or gain of pain and chronic itch. Here, we explore our current knowledge of the mechanisms by which Na_V1.9 may contribute to pain and elaborate on the challenges associated with establishing links between experimental conditions and human disease. This review also discusses the lack of comprehensive insights into Na_V1.9-specific pharmacology, an unfortunate situation since modulatory compounds may have tremendous potential in the clinic to treat pain or as precision tools to examine the extent of Na_V1.9 participation in sensory perception processes.

Inhibition of the Human Neuronal Sodium Channel Nav1.9 by Arachidonyl-2-Chloroethylamide, An Analogue of Anandamide in a hNav1.9/rNav1.4 Chimera, An Experimental and in Silico Study

Cannabinoids regulate analgesia, which has aroused much interest in identifying new pharmacological therapies in the management of refractory pain. Voltage-gated Na+ channels (Na_vs) play an important role in inflammatory and neuropathic pain. In particular, Na_v1.9 is involved in nociception and the understanding of its pharmacology has lagged behind because it is difficult to express in heterologous systems. Here, we utilized the chimeric channel hNa_v1.9_C4, that comprises the extracellular and transmembrane domains of hNa_v1.9, co-expressed with the ß1 subunit on CHO-K1 cells to characterize the electrophysiological effects of ACEA, a synthetic surrogate of the endogenous cannabinoid anandamide. ACEA induced a tonic block, decelerated the fast inactivation, markedly shifted steady-state inactivation in the hyperpolarized direction, decreasing the window current and showed use-dependent block, with a high affinity for the inactivated state (k_i = 0.84 µM). Thus, we argue that ACEA possess a local anaesthetic-like profile. To provide a mechanistic understanding of its mode of action at the molecular level, we combined induced fit docking with Monte Carlo simulations and electrostatic complementarity. In agreement with the experimental evidence, our computer simulations revealed that ACEA binds Tyr1599 of the local anaesthetics binding site of the hNa_v1.9, contacting residues that bind cannabinol (CBD) in the Na_vMs channel. ACEA adopted a conformation remarkably similar to the crystallographic conformation of anandamide on a non-homologous protein, obstructing the Na+ permeation pathway below the selectivity filter to occupy a highly conserved binding pocket at the intracellular side. These results describe a mechanism of action, possibly involved in cannabinoid analgesia.

[The use of a fixed combination of diclofenac and orphenadrine in the treatment of acute pain syndrome in patients with discogenic lumbosacral radiculopathy and lumboischialgia]

Acute pain syndromes caused by discogenic lumbosacral radiculopathy and lumboischialgia are not uncommon in clinical practice and characterized by a high risk of becoming chronic. The pathogenetic aspects, features of the clinical picture, existing approaches to conservative treatment of these conditions are analyzed in this paper. Data on the efficacy and safety of a fixed combination of diclofenac and orphenadrine (Neodolpasse) use in the treatment of vertebrogenic pain syndromes based on the NEODOLEX study results are presented, and the authors' own clinical observations are given. Possible reasons for the high efficacy of Neodolpasse in patients with discogenic radiculopathies and nonspecific back and neck pain are discussed.

Ambroxol for neuropathic pain: hiding in plain sight?

ABSTRACT

Ambroxol is a multifaceted drug with primarily mucoactive and secretolytic actions, along with anti-inflammatory, antioxidant, and local anaesthetic properties. It has a long history of use in the treatment of respiratory tract diseases and has shown to be efficacious in relieving sore throat. In more recent years, ambroxol has gained interest for its potential usefulness in treating neuropathic pain. Research into this area has been slow, despite clear preclinical evidence to support its primary analgesic mechanism of action-blockade of voltage-gated sodium (Na v ) channels in sensory neurons. Ambroxol is a commercially available inhibitor of Na v 1.8, a crucial player in the pathophysiology of neuropathic pain, and Na v 1.7, a particularly exciting target for the treatment of chronic pain. In this review, we discuss the analgesic mechanisms of action of ambroxol, as well as proposed synergistic properties, followed by the preclinical and clinical results of its use in the treatment of persistent pain and neuropathic pain symptoms, including trigeminal neuralgia, fibromyalgia, and complex regional pain syndrome. With its well-established safety profile, extensive preclinical and clinical drug data, and early evidence of clinical effectiveness, ambroxol is an old drug worthy of further investigation for repurposing. As a patent-expired drug, a push is needed to progress the drug to clinical trials for neuropathic pain. We encourage the pharmaceutical industry to look at patented drug formulations and take an active role in bringing an optimized version for neuropathic pain to market.

Intravenous diclofenac and orphenadrine for the treatment of postoperative pain after remifentanil-based anesthesia : A double-blinded, randomized, placebo-controlled study

BACKGROUND

Postoperative intravenous diclofenac reduces patient opioid demand and is commonly used in surgical units. Orphenadrine is mainly used in combination with diclofenac for musculoskeletal injuries and postoperative pain control. The objective of this study was to compare the analgesic efficacy of diclofenac-orphenadrine, diclofenac alone and saline.

METHODS

We performed a double-blind, randomized, placebo-controlled, parallel-group, single-center clinical study investigating the opioid-sparing effect of a combination of diclofenac and orphenadrine versus diclofenac alone versus isotonic saline solution. Initially 72 patients were included and received total intravenous anesthesia during cruciate ligament surgery. All patients were postoperatively treated with a patient-controlled analgesia (PCA) device containing hydromorphone. Pharmacological safety was assessed by laboratory parameters, vital signs, and delirium detection scores.

RESULTS

There was no significant difference between the groups in cumulative dose of PCA analgesics required after 24 h postsurgery, with 5.90 mg (SD ± 2.90 mg) in the placebo group, 5.73 mg (SD ± 4.75 mg) in the diclofenac group, and 4.13 mg (SD ± 2.57 mg) in the diclofenac-orphenadrine group. Furthermore, there was no significant difference between the groups in cumulative dose of PCA analgesics required 2 h postsurgery (n = 65). Mean dose of hydromorphone required after 2 h was 1.54 mg (SD ± 0.57 mg) in the placebo group, 1.56 mg (SD ± 1.19 mg) in the diclofenac-only group, and 1.37 mg (SD ± 0.78 mg) in the diclofenac-orphenadrine group. However, when comparing the diclofenac-orphenadrine group and the diclofenac group combined to placebo there was a significant reduction in PCA usage in the first 24 h postsurgery. In total, there were 25 adverse events reported, none of which were rated as severe.

CONCLUSION

Orphenadrine-diclofenac failed to significantly reduce postoperative opioid requirements. However, in an exploratory post hoc analysis the diclofenac-orphenadrine and the diclofenac group combined versus placebo showed a tendency to reduce opioid demand in postoperative pain control. Further research is required to determine the value of orphenadrine as an adjuvant in a multimodal approach for postoperative pain management.

Treatment of Muscle Cramps in Patients With Cirrhosis of Liver: A Systematic Review

BACKGROUND

Muscle cramps are witnessed in 22-88% of patients with cirrhosis of liver and frequently lead to sleep disturbance with an appalling impact on quality of life. Despite such a high prevalence, there is lack of evidence-based management protocol due to scarcity of trials on treatment options in the literature. This study aimed to review systematically the available therapeutic options for muscle cramps in patients with cirrhosis of liver.

METHODS

A systematic review of the relevant databases (PubMed, Scopus, Embase, and Web of Science) to identify treatments for muscle cramps in patients with hepatic cirrhosis was performed. Studies meeting the selection criteria were reviewed and assessed for risk of bias and analyzed.

RESULTS

Twenty-four publications were identified as eligible for inclusion in this systematic review. Seven randomized controlled trials (RCTs) and 17 prospective studies were included. Taurine, methocarbamol, baclofen, and orphenadrine are relatively safer and effective treatment option for muscle cramps in cirrhosis on the basis of recently conducted RCTs. Moreover, l-carnitine, branched-chain amino acids (BCAAs), pregabalin, zinc, and vitamin D are also safe and showed beneficial effects on muscle cramps. However, studies on vitamin E revealed contradictory results.

CONCLUSION

Taurine, BCAAs, orphenadrine, and baclofen are safe and well-tolerated treatment options for muscle cramps in cirrhosis. However, well-designed randomized controlled clinical trials are the need of the hour to determine the most suitable treatment options for skeletal muscle cramps in patients with cirrhosis of liver.

[New treatment options for piriformis syndrome]

This paper discusses modern approaches to the diagnosis and management of patients with piriformis syndrome. Epidemiological data on the prevalence of this syndrome in neurological practice are presented. The anatomical features of this region, leading to the formation of piriformis syndrome, are described in detail. The authors provide diagnostic criteria based on neurological examination and manual muscle testing and discuss the differential diagnosis of piriformis syndrome. New possibilities of treating this syndrome using a fixed combination of diclofenac and orphenadrine registered in the Russian Federation as a drug for intravenous infusion neodolpasse are discussed in detail. The authors cite materials from their own work, a clinical case of managing a patient with piriformis syndrome using neodolpasse is analyzed.

[Clinical experience with the use of a fixed combination of diclofenac and orphenadrine in the treatment of acute pain syndrome]

Nonsteroidal anti-inflammatory drugs (NSAIDs) and muscle relaxants (MR) are successfully used to relieve pain, both in monotherapy and in combinations. The use of fixed drug combinations not only greatly facilitates daily clinical practice and increases patient adherence, but due to the potentiation of pharmacological effects, it allows to achieve better treatment results. This paper presents 3 clinical cases of successful inpatient use of a fixed combination of diclofenac 75 mg and orphenadrine 30 mg in the form of an infusion solution (NEODOLPASSE) for relief of acute back musculoskeletal pain syndrome.

[The efficacy of the combination of diclofenac and orphenadrine in the treatment of dorsalgia]

The high prevalence of dorsalgia and dorsopathy among the adult population makes a significant contribution to the structure of the financial burden of health care systems. The use of non-steroidal anti-inflammatory drugs (NSAIDs) as the basis for the pharmacotherapy of dorsopathy is recommended by most international clinical guidelines. The pharmacodynamic effects of NSAIDs underlie the clinical efficacy of this group of drugs in patients with pain of musculoskeletal origin, while monotherapy is not always accompanied by the rapid development of a persistent analgesic effect. An urgent direction in the therapy of dorsopathies may include combination of NSAIDs with analgesic drugs of other pharmacological groups capable of additive action. The fixed combination of diclofenac, 75 mg, and orphenadrine, 50 mg, allows achieving an effective analgesic effect in patients with lower back pain of various etiologies. It was demonstrated in a series of clinical cases that included 4 patients with dorsopathy who were treated at the City Clinical Hospital No. 24, Moscow in 2020.

Synthesis and Evaluation of Voltage-Gated Sodium Channel Blocking Pyrroline Derivatives Endowed with Both Antiarrhythmic and Antioxidant Activities

Under the hypothesis that cardioprotective agents might benefit from synergism between antiarrhythmic activity and antioxidant properties, a small series of mexiletine analogues were coupled with the 2,2,5,5-tetramethylpyrroline moiety, known for its antioxidant effect, in order to obtain dual-acting drugs potentially useful in the protection of the heart against post-ischemic reperfusion injury. The pyrroline derivatives reported herein were found to be more potent as antiarrhythmic agents than mexiletine and displayed antioxidant activity. The most interesting tetramethylpyrroline congener, a tert-butyl-substituted analogue, was at least 100 times more active as an antiarrhythmic than mexiletine.

Privileged scaffold-based design to identify a novel drug-like 5-HT7 receptor-preferring agonist to target Fragile X syndrome

Recent preclinical studies have shown that activation of the serotonin 5-HT₇ receptor has the potential to treat neurodevelopmental disorders such as Fragile X syndrome, a rare disease characterized by autistic features. With the aim to provide the scientific community with diversified drug-like 5-HT₇ receptor-preferring agonists, we designed a set of new long-chain arylpiperazines by exploiting structural fragments present in clinically approved drugs or in preclinical candidates (privileged scaffolds). The new compounds were synthesized, tested for their affinity at 5-HT₇ and 5-HT_1A receptors, and screened for their in vitro stability to microsomal degradation and toxicity. Selected compounds were characterized as 5-HT₇ receptor-preferring ligands, endowed with high metabolic stability and low toxicity. Compound 7g emerged as a drug-like 5-HT₇ receptor-preferring agonist capable to rescue synaptic plasticity and attenuate stereotyped behavior in a mouse model of Fragile X syndrome.

Safinamide's potential in treating nondystrophic myotonias: Inhibition of skeletal muscle voltage-gated sodium channels and skeletal muscle hyperexcitability in vitro and in vivo

The antiarrhythmic sodium-channel blocker mexiletine is used to treat patients with myotonia. However, around 30% of patients do not benefit from mexiletine due to poor tolerability or suboptimal response. Safinamide is an add-on therapy to levodopa for Parkinson's disease. In addition to MAOB inhibition, safinamide inhibits neuronal sodium channels, conferring anticonvulsant activity in models of epilepsy. Here, we investigated the effects of safinamide on skeletal muscle hNa_v1.4 sodium channels and in models of myotonia, in-vitro and in-vivo. Using patch-clamp, we showed that safinamide reversibly inhibited sodium currents in HEK293T cells transfected with hNav1.4. At the holding potential (hp) of -120 mV, the half-maximum inhibitory concentrations (IC₅₀) were 160 and 33 μM at stimulation frequencies of 0.1 and 10 Hz, respectively. The calculated affinity constants of safinamide were dependent on channel state: 420 μM for closed channels and 9 μM for fast-inactivated channels. The p.F1586C mutation in hNav1.4 greatly impaired safinamide inhibition, suggesting that the drug binds to the local anesthetic receptor site in the channel pore. In a condition mimicking myotonia, i.e. hp. of -90 mV and 50-Hz stimulation, safinamide inhibited I_Na with an IC₅₀ of 6 μM, being two-fold more potent than mexiletine. Using the two-intracellular microelectrodes current-clamp method, action potential firing was recorded in vitro in rat skeletal muscle fibers in presence of the chloride channel blocker, 9-anthracene carboxylic acid (9-AC), to increase excitability. Safinamide counteracted muscle fiber hyperexcitability with an IC₅₀ of 13 μM. In vivo, oral safinamide was tested in the rat model of myotonia. In this model, intraperitoneal injection of 9-AC greatly increased the time of righting reflex (TRR) due to development of muscle stiffness. Safinamide counteracted 9-AC induced TRR increase with an ED₅₀ of 1.2 mg/kg, which is 7 times lower than that previously determined for mexiletine. In conclusion, safinamide is a potent voltage and frequency dependent blocker of skeletal muscle sodium channels. Accordingly, the drug was able to counteract abnormal muscle hyperexcitability induced by 9-AC, both in vitro and in vivo. Thus, this study suggests that safinamide may have potential in treating myotonia and warrants further preclinical and human studies to fully evaluate this possibility.

Effects of Benzothiazolamines on Voltage-Gated Sodium Channels

Benzothiazole is a versatile fused heterocycle that aroused much interest in drug discovery as anticonvulsant, neuroprotective, analgesic, anti-inflammatory, antimicrobial, and anticancer. Two benzothiazolamines, riluzole and lubeluzole, are known blockers of voltage-gated sodium (Na_v) channels. Riluzole is clinically used as a neuroprotectant in amyotrophic lateral sclerosis. Inhibition of Na_v channels by riluzole is voltage-dependent due to preferential binding to inactivated sodium channels. Yet the drug exerts little use-dependent block, probably because it lacks protonable amine. One important property is riluzole ability to inhibit persistent Na⁺ currents, which likely contributes to its neuroprotective activity. Lubeluzole showed promising neuroprotective effects in animal stroke models, but failed to show benefits in acute ischemic stroke in humans. One important concern is its propensity to prolong the cardiac QT interval, due to hERG K⁺ channel block. Lubeluzole very potently inhibits Na_v channels in a voltage- and use-dependent manner, due to its great preferential affinity for inactivated channels and the presence of a protonable amine group. Patch-clamp experiments suggest that the binding sites of both drugs overlap the local anesthetic receptor within the ion-conducting pathway. Riluzole and lubeluzole displayed very potent antimyotonic activity in a rat model of myotonia, a pathological skeletal muscle condition characterized by high-frequency runs of action potentials. Such results well support the repurposing of riluzole as an antimyotonic drug, allowing the launch of a pilot study in myotonic patients. Riluzole, lubeluzole, and new Na_v channel blockers built on the benzothiazolamine scaffold will certainly continue to be investigated for possible clinical applications.

Pilot study of orphenadrine as a novel treatment for muscle cramps in patients with liver cirrhosis

Background and aims

Muscle cramps markedly affect the quality of life in cirrhotic patients with no available highly effective treatment. The aim of this study was to assess the safety and efficacy of orphenadrine in the treatment of muscle cramps in cirrhotic patients.

Methods

The study enrolled 30 liver cirrhosis patients complaining of frequent muscle cramps (≥3 per week), who were randomized to receive either orphenadrine 100 mg or calcium carbonate 500 mg twice daily as a control for one month. Severity, frequency, and duration of the muscle cramps were assessed before and after treatment as well as recurrence after washout of the drug for two weeks. Side effects were recorded.

Results

One month after treatment with orphenadrine; the frequency of muscle cramps decreased significantly to 0.6 ± 0.74 per week compared to 12.53 ± 6.01 at baseline (p < 0.001), the duration of muscle cramps decreased from 1 min to 0.1 min after treatment (p < 0.001). The pain score improved significantly from a score of 8/10 to 0/10 (p < 0.001). The side effects were few, such as dry mouth, drowsiness, and nausea, with no significant difference between their occurrences in the two groups.

Conclusion

Orphenadrine is safe and effective in treatment of muscle cramps in patients with liver cirrhosis.

Increased sodium channel use-dependent inhibition by a new potent analogue of tocainide greatly enhances in vivo antimyotonic activity

Although the sodium channel blocker, mexiletine, is the first choice drug in myotonia, some myotonic patients remain unsatisfied due to contraindications, lack of tolerability, or incomplete response. More therapeutic options are thus needed for myotonic patients, which require clinical trials based on solid preclinical data. In previous structure-activity relationship studies, we identified two newly-synthesized derivatives of tocainide, To040 and To042, with greatly enhanced potency and use-dependent behavior in inhibiting sodium currents in frog skeletal muscle fibers. The current study was performed to verify their potential as antimyotonic agents. Patch-clamp experiments show that both compounds, especially To042, are greatly more potent and use-dependent blockers of human skeletal muscle hNav1.4 channels compared to tocainide and mexiletine. Reduced effects on F1586C hNav1.4 mutant suggest that the compounds bind to the local anesthetic receptor, but that the increased hindrance and lipophilia of the N-substituent may further strengthen drug-receptor interaction and use-dependence. Compared to mexiletine, To042 was 120 times more potent to block hNav1.4 channels in a myotonia-like cellular condition and 100 times more potent to improve muscle stiffness in vivo in a previously-validated rat model of myotonia. To explore toxicological profile, To042 was tested on hERG potassium currents, motor coordination using rotarod, and C2C12 cell line for cytotoxicity. All these experiments suggest a satisfactory therapeutic index for To042. This study shows that, owing to a huge use-dependent block of sodium channels, To042 is a promising candidate drug for myotonia and possibly other membrane excitability disorders, warranting further preclinical and human studies.

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To040 and To042 are potent use-dependent hNav1.4 sodium channel blockers.

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The compounds strengthen the molecular interaction at the local anesthetic receptor.

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To042 is 120-fold more potent than mexiletine in vitro in myotonia-like conditions.

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To042 is 100-fold more potent than mexiletine in vivo in a rat model of myotonia.

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To042 is a promising antimyotonic drug deserving further investigation.

Inhibition of voltage-gated sodium channels by sumatriptan bioisosteres

Voltage-gated sodium channels are known to play a pivotal role in perception and transmission of pain sensations. Gain-of-function mutations in the genes encoding the peripheral neuronal sodium channels, hNav1.7–1.9, cause human painful diseases. Thus while treatment of chronic pain remains an unmet clinical need, sodium channel blockers are considered as promising druggable targets. In a previous study, we evaluated the analgesic activity of sumatriptan, an agonist of serotonin 5HT_1B/D receptors, and some new chiral bioisosteres, using the hot plate test in the mouse. Interestingly, we observed that the analgesic effectiveness was not necessarily correlated to serotonin agonism. In this study, we evaluated whether sumatriptan and its congeners may inhibit heterologously expressed hNav1.7 sodium channels using the patch-clamp method. We show that sumatriptan blocks hNav1.7 channels only at very high, supratherapeutic concentrations. In contrast, its three analogs, namely 20b, (R)-31b, and (S)-22b, exert a dose and use-dependent sodium channel block. At 0.1 and 10 Hz stimulation frequencies, the most potent compound, (S)-22b, was 4.4 and 1.7 fold more potent than the well-known sodium channel blocker mexiletine. The compound induces a negative shift of voltage dependence of fast inactivation, suggesting higher affinity to the inactivated channel. Accordingly, we show that (S)-22b likely binds the conserved local anesthetic receptor within voltage-gated sodium channels. Combining these results with the previous ones, we hypothesize that use-dependent sodium channel blockade contributes to the analgesic activity of (R)-31b and (S)-22b. These later compounds represent promising lead compounds for the development of efficient analgesics, the mechanism of action of which may include a dual action on sodium channels and 5HT_1D receptors.

Novel insight into antimicrobial resistance and sensitivity phenotypes associated to qac and norA genotypes in Staphylococcus aureus

Staphylococcus aureus strains harboring QacA, QacB, QacC, QacG transporters and norA promoter up-regulating mutations were characterized by phenotype microarray (PM), standard methods for susceptibility testing, and ethidium bromide efflux assays, in order to increase knowledge on phenotypes associated to efflux pumps and their substrates. PM data and standard susceptibility testing lead to the identification of new potential efflux targets, such as guanidine hydrochloride or 8-hydroxyquinoline for QacA and QacC pumps, respectively. The identification of compounds to which the presence of efflux pumps induced increased susceptibility opens new perspectives for potential adjunct anti-resistance treatment (i.e. strains bearing QacB transporters showed increased susceptibility to thioridazine, amitriptyline and orphenadrine). Although the tested isolates were characterized by high degree of heterogeneity, a hallmark of clinical isolates, direct ethidium bromide efflux assays were effective in highlighting differences in efflux efficiency among strains. These data add to characterization of substrate specificity in the different classes of staphylococcal multidrug efflux systems conferring specific substrate profiles and efflux features to each of them.

Orphenadrine-induced convulsive status epilepticus in rats responds to the NMDA antagonist dizocilpine

BACKGROUND

Identification of new molecular targets as well as the new models recapitulating different aspects of pathophysiology of status epilepticus (SE) in humans might prove essential for the breakthrough in the efforts against pharmacoresistance in epilepsy. Recently, we described a new model of generalized convulsive SE induced with orphenadrine (ORPH) in rats with unique characteristics [5]. The current study was aimed at assessing the efficacy of a new generation antiepileptic drugs (AEDs) and some of the experimental agents in suppressing ORPH-evoked seizures in rats.

METHODS

ORPH was administered intraperitoneally (ip) in the dose of 80 mg/kg in male Wistar rats. The latency to first seizure, the number of seizure episodes and the duration of overt SE, as well as the incidence of deaths was scored with simultaneous electroencephalographic (EEG) recordings.

RESULTS

ORPH induced seizures in 100% of animals at a dose of 80 mg/kg, associated with low mortality and good behavioural outcome. Among new generation AEDs: felbamate, levetiracetam, topiramate, lamotrigine and progabide did not affect the seizure incidence. Among the experimental drugs, only dizocilpine, the non-competitive NMDA antagonist, dose-dependently affected the occurrence of the SE (p<0.001). However, CGP-39551 competitive NMDA antagonist, the same as scopolamine and mecamylamine (muscarinic and nicotinic receptors antagonists, respectively) showed no effect.

CONCLUSIONS

Based on the above findings, one may speculate that NMDA activation is partly involved in the proconvulsant activity of orphenadrine but may not be the primary pathomechanism. ORPH-induced seizures may provide an interesting option for studying novel targets for pharmacological interventions in status epilepticus.

Preclinical evaluation of marketed sodium channel blockers in a rat model of myotonia discloses promising antimyotonic drugs

Although the sodium channel blocker mexiletine is considered the first-line drug in myotonia, some patients experiment adverse effects, while others do not gain any benefit. Other antimyotonic drugs are thus needed to offer mexiletine alternatives. In the present study, we used a previously-validated rat model of myotonia congenita to compare six marketed sodium channel blockers to mexiletine. Myotonia was induced in the rat by injection of anthracen-9-carboxylic acid, a muscle chloride channel blocker. The drugs were given orally and myotonia was evaluated by measuring the time of righting reflex. The drugs were also tested on sodium currents recorded in a cell line transfected with the human skeletal muscle sodium channel hNav1.4 using patch-clamp technique. In vivo, carbamazepine and propafenone showed antimyotonic activity at doses similar to mexiletine (ED50 close to 5mg/kg); flecainide and orphenadrine showed greater potency (ED50 near 1mg/kg); lubeluzole and riluzole were the more potent (ED50 near 0.1mg/kg). The antimyotonic activity of drugs in vivo was linearly correlated with their potency in blocking hNav1.4 channels in vitro. Deviation was observed for propafenone and carbamazepine, likely due to pharmacokinetics and multiple targets. The comparison of the antimyotonic dose calculated in rats with the current clinical dose in humans strongly suggests that all the tested drugs may be used safely for the treatment of human myotonia. Considering the limits of mexiletine tolerability and the occurrence of non-responders, this study proposes an arsenal of alternative drugs, which may prove useful to increase the quality of life of individuals suffering from non-dystrophic myotonia. Further clinical trials are warranted to confirm these results.

Combined modifications of mexiletine pharmacophores for new lead blockers of Na(v)1.4 channels

Previously identified potent and/or use-dependent mexiletine (Mex) analogs were used as template for the rational design of new Na(v)-channel blockers. The effects of the novel analogs were tested on sodium currents of native myofibers. Data and molecular modeling show that increasing basicity and optimal alkyl chain length enhance use-dependent block. This was demonstrated by replacing the amino group with a more basic guanidine one while maintaining a proper distance between positive charge and aromatic ring (Me13) or with homologs having the chirality center nearby the amino group or the aromatic ring. Accordingly, a phenyl group on the asymmetric center in the homologated alkyl chain (Me12), leads to a further increase of use-dependent behavior versus the phenyl Mex derivative Me4. A fluorine atom in paraposition and one ortho-methyl group on the xylyloxy ring (Me15) increase potency and stereoselectivity versus Me4. Charge delocalization and greater flexibility of Me15 may increase its affinity for Tyr residues influencing steric drug interaction with the primary Phe residue of the binding site. Me12 and Me15 show limited selectivity against Na(v)-isoforms, possibly due to the highly conserved binding site on Na(v). To our knowledge, the new compounds are the most potent Mex-like Na(v) blockers obtained to date and deserve further investigation.

Vibrational spectra and density functional theoretical calculations on the anti-neurodegenerative drug: Orphenadrine hydrochloride

Vibrational spectral analysis and quantum chemical computations based on density functional theory have been performed on the anti-neuro-degenerative drug Orphenadrine hydrochloride. The geometry, intermolecular hydrogen bond, and harmonic vibrational frequencies of the title molecule have been investigated with the help of B3LYP method. The calculated molecular geometry has been compared with the experimental data. The various intramolecular interactions have been exposed by natural bond orbital analysis. The distribution of Mulliken atomic charges and bending of natural hybrid orbitals also reflect the presence of intramolecular hydrogen bonding. The analysis of the electron density of HOMO and LUMO gives an idea of the delocalization and low value of energy gap indicates electron transport in the molecule and thereby bioactivity. Effective docking of the drug molecule with NMDA receptor subunit 3A also enhances its bioactive nature.

Molecular Insights into the Local Anesthetic Receptor within Voltage-Gated Sodium Channels Using Hydroxylated Analogs of Mexiletine

We previously showed that the β-adrenoceptor modulators, clenbuterol and propranolol, directly blocked voltage-gated sodium channels, whereas salbutamol and nadolol did not (Desaphy et al., 2003), suggesting the presence of two hydroxyl groups on the aromatic moiety of the drugs as a molecular requisite for impeding sodium channel block. To verify such an hypothesis, we synthesized five new mexiletine analogs by adding one or two hydroxyl groups to the aryloxy moiety of the sodium channel blocker and tested these compounds on hNav1.4 channels expressed in HEK293 cells. Concentration–response relationships were constructed using 25-ms-long depolarizing pulses at −30 mV applied from an holding potential of −120 mV at 0.1 Hz (tonic block) and 10 Hz (use-dependent block) stimulation frequencies. The half-maximum inhibitory concentrations (IC₅₀) were linearly correlated to drug lipophilicity: the less lipophilic the drug, minor was the block. The same compounds were also tested on F1586C and Y1593C hNav1.4 channel mutants, to gain further information on the molecular interactions of mexiletine with its receptor within the sodium channel pore. In particular, replacement of Phe1586 and Tyr1593 by non-aromatic cysteine residues may help in the understanding of the role of π–π or π–cation interactions in mexiletine binding. Alteration of tonic block suggests that the aryloxy moiety of mexiletine may interact either directly or indirectly with Phe1586 in the closed sodium channel to produce low-affinity binding block, and that this interaction depends on the electrostatic potential of the drug aromatic tail. Alteration of use-dependent block suggests that addition of hydroxyl groups to the aryloxy moiety may modify high-affinity binding of the drug amine terminal to Phe1586 through cooperativity between the two pharmacophores, this effect being mainly related to drug lipophilicity. Mutation of Tyr1593 further impaired such cooperativity. In conclusion, these results confirm our former hypothesis by showing that the presence of hydroxyl groups to the aryloxy moiety of mexiletine greatly reduced sodium channel block, and provide molecular insights into the intimate interaction of local anesthetics with their receptor.

Orphenadrine induces secondarily generalized convulsive status epilepticus in rats

The current study was aimed to assess the convulsant potency of orphenadrine (ORPH) in rats together with a screen of different conventional antiepileptic drugs (AEDs) on their efficacy to suppress it. ORPH was administered intraperitoneally (i.p.) in doses of 50-80 mg/kg in male Wistar rats. The latency to first seizure, the number of seizure episodes and the duration of overt status epilepticus (SE) as well as the incidence of deaths was scored with simultaneous electroencephalographic (EEG) recordings. Subsequently, the effects of conventional AEDs on ORPH-evoked (80 mg/kg) seizure incidence were studied. ORPH dose-dependently induced seizures in increasing number of animals, reaching 100% at a dose of 80 mg/kg, associated with low mortality and no drug-related neurotoxicity. Epileptic attacks started as complex partial fits consisting of stereotyped behavior, limb movements, head shaking and myoclonic twitches of the body. Subsequently, an overt generalized convulsive SE appeared, lasting for approximately 2h. Among conventional AEDs: carbamazepine, ethosuximide and phenytoin had no effect while valproate (p<0.001), diazepam (p<0.01), and phenobarbital (p<0.001) dose-dependently suppressed seizure activity. All the above characteristics make the new model, a useful, easy to perform experimental tool to study the pathophysiology of SE as well as the effects of new AEDs.

Molecular determinants of state-dependent block of voltage-gated sodium channels by pilsicainide

BACKGROUND AND PURPOSE

Pilsicainide, an anti-arrhythmic drug used in Japan, is described as a pure sodium channel blocker. We examined the mechanisms by which it is able to block open channels, because these properties may be especially useful to reduce hyperexcitability in pathologies characterized by abnormal sodium channel opening.

EXPERIMENTAL APPROACH

The effects of pilsicainide on various heterologously expressed human sodium channel subtypes and mutants were investigated using the patch clamp technique.

KEY RESULTS

Pilsicainide exhibited tonic and use-dependent effects comparable to those of mexiletine and flecainide on hNav1.4 channels. These use-dependent effects were abolished in the mutations F1586C and Y1593C within segment 6 of domain IV, suggesting that the interaction of pilsicainide with these residues is critical for its local anaesthetic action. Its affinity constants for closed channels (K(R)) and channels inactivated from the closed state (K(I)) were high, suggesting that its use-dependent block (UDB) requires the channel to be open for it to reach a high-affinity blocking site. Accordingly, basic pH, which slightly increased the proportion of neutral drug, dramatically decreased K(R) and K(I) values. Effects of pilsicainide were similar on skeletal muscle hNav1.4, brain hNav1.1 and heart hNav1.5 channels. The myotonic R1448C and G1306E hNav1.4 mutants were more and less sensitive to pilsicainide, respectively, due to mutation-induced gating modifications.

CONCLUSIONS AND IMPLICATIONS

Although therapeutic concentrations of pilsicainide may have little effect on resting and closed-state inactivated channels, it induces a strong UDB due to channel opening, rendering the drug ideally suited for inhibition of high-frequency action potential firing.

Drug-induced angioedema without urticaria: prevalence and clinical features

BACKGROUND

Angioedema without urticaria can be caused by drugs. The purpose of our study was to assess the prevalence and clinical features of patients with drug-induced angioedema without urticaria.

METHODS

This study retrospectively reviewed case records at Siriraj Hospital, between January 2007 and December 2008. Patients aged at least 15 years were included.

RESULTS

The prevalence of drug-induced angioedema without urticaria among patients with adverse drug reactions was 2.3%/year. Non-steroidal anti-inflammatory drugs (NSAID) were the most common cause (50%), followed by antibiotics (20%). The commonest NSAID which induced angioedema were ibuprofen and diclofenac. The common sites were periorbital area (67.3%) and lips (27.6%). The median duration of suspected drug therapy before the development of angioedema was 1 day with the range of 10 min to 23 days.

CONCLUSIONS

Non-steroidal anti-inflammatory drugs and antibiotics were the most common drugs causing angioedema without urticaria. The duration of onset ranged from minutes to days. After stopping the suspected drugs, symptoms disappeared within 2-5 days in most patients.