The use of oral mexiletine for the treatment of pain after peripheral nerve injury

Cardiac protection of Bauhinia championii against reperfusion injury

This study aims to investigate the protective effects of the Bauhinia championii (BC) against ischemia/reperfusion (I/R)-induced injury in an isolated heart model. Langendorff-perfused C57BL/6JNarl mice hearts were performed with 30 minutes ischemia and 60 minutes reperfusion by left anterior descending artery ligation. Before reperfusion, boiling water extracts of BC (10 mg/L) was pretreated for 15 minutes. During reperfusion, BC significantly decreased the occurrence of ventricular arrhythmias by lead II electrocardiogram (ECG). Electrophysiological effect of BC was further determined in isolated ventricular myocytes by whole-cell patch clamp technique. The underlying mechanism may result from its Na⁺ channel blocking activity characterized with reduced rise slope of action potential and Na⁺ current density. Moreover, BC dramatically reduced I/R-caused infarct size, which was accessed by 2,3,5-triphenyltetrazolium chloride (TTC) assay. Since BC decreased I/R-induced myoglobin release and oxidation of Ca²⁺ -calmodulin-dependent protein kinase, inhibition of myocardial necroptosis may account for the protective effects of BC on myocytes lose. This study indicated that BC may prevent I/R induced ventricular arrhythmias and myocyte death by blocking Na⁺ channels and decreasing necroptosis, respectively. Since most of the available antiarrhythmic remedies have unwanted adverse actions, BC could be a novel candidate for the treatment of myocardial infarction and ventricular arrhythmia.

A Molecularly Detailed NaV1.5 Model Reveals a New Class I Antiarrhythmic Target

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Antiarrhythmic therapies remain suboptimal due to our inability to predict how drug interactions with ion channels will affect the ability of the tissue to initiate and sustain an arrhythmia.

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We built a computational framework that allows for in silico design of precision-targeted therapeutic agents that simultaneously assesses antiarrhythmic markers of success and failure at multiple spatial and time scales. Using this framework, a novel in silico mexiletine “booster” was designed that may dramatically improve the efficacy of mexiletine in suppression of arrhythmia triggers.

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These results provide a roadmap for the design of novel molecular-based therapy to treat myriad arrhythmia syndromes, including ventricular tachycardia, heart failure arrhythmias, and inherited arrhythmia syndromes.

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In summary, computational modeling approaches to drug discovery represent a novel tool to design and test precision-targeted therapeutic agents. By exploiting nontraditional ion channel drug targets, an entirely new dimension can be added to the wide parameter space of traditional antiarrhythmic drugs to develop more precision-targeted and potent Class I therapeutic agents.

Antiarrhythmic treatment strategies remain suboptimal due to our inability to predict how drug interactions with ion channels will affect the ability of the tissues to initiate and sustain an arrhythmia. We built a multiscale molecular model of the Na⁺ channel domain III (domain III voltage-sensing domain) to highlight the molecular underpinnings responsible for mexiletine drug efficacy. This model predicts that a hyperpolarizing shift in the domain III voltage-sensing domain is critical for drug efficacy and may be leveraged to design more potent Class I molecules. The model was therefore used to design, in silico, a theoretical mexiletine booster that can dramatically rescue a mutant resistant to the potent antiarrhythmic effects of mexiletine. Our framework provides a strategy for in silico design of precision-targeted therapeutic agents that simultaneously assesses antiarrhythmic markers of success and failure at multiple spatial and time scales. This approach provides a roadmap for the design of novel molecular-based therapy to treat myriad arrhythmia syndromes, including ventricular tachycardia, heart failure arrhythmias, and inherited arrhythmia syndromes.

Voltage gated sodium channels as therapeutic targets for chronic pain

Being maladaptive and frequently unresponsive to pharmacotherapy, chronic pain presents a major unmet clinical need. While an intact central nervous system is required for conscious pain perception, nociceptor hyperexcitability induced by nerve injury in the peripheral nervous system (PNS) is sufficient and necessary to initiate and maintain neuropathic pain. The genesis and propagation of action potentials is dependent on voltage-gated sodium channels, in particular, Nav1.7, Nav1.8 and Nav1.9. However, nerve injury triggers changes in their distribution, expression and/or biophysical properties, leading to aberrant excitability. Most existing treatment for pain relief acts through non-selective, state-dependent sodium channel blockage and have narrow therapeutic windows. Natural toxins and developing subtype-specific and molecular-specific sodium channel blockers show promise for treatment of neuropathic pain with minimal side effects. New approaches to analgesia include combination therapy and gene therapy. Here, we review how individual sodium channel subtypes contribute to pain, and the attempts made to develop more effective analgesics for the treatment of chronic pain.

Discovery of N-Aryloxypropylbenzylamines as Voltage-Gated Sodium Channel NaV 1.2-Subtype-Selective Inhibitors

We previously reported that a lipophilic N-(4'-hydroxy-3',5'-di-tert-butylbenzyl) derivative (1) of the voltage-gated sodium channel blocker mexiletine, was a more potent sodium channel blocker in vitro and in vivo. We demonstrate that replacing the chiral methylethylene linker between the amine and di-tert-butylphenol with an achiral 1,3-propylene linker (to give (2)) maintains potency in vitro. We synthesized 25 analogues bearing the 1,3-propylene linker and found that minor structural changes resulted in pronounced changes in state dependence of blocking human Na_V 1.2 and 1.6 channels by high-throughput patch-clamp analysis. Compared to mexiletine, compounds 1 and 2 are highly selective Na_V 1.2 inhibitors and >500 times less potent in inhibiting Na_V 1.6 channels. On the other hand, a derivative (compound 4) bearing 2,6-dimethoxy groups in place of the 2,6-dimethyl groups found in mexiletine was found to be the most potent inhibitor, but is nonselective against both channels in the tonic, frequency-dependent and inactivated states. In a kindled mouse model of refractory epilepsy, compound 2 inhibited seizures induced by 6 Hz 44 mA electrical stimulation with an IC₅₀ value of 49.9±1.6 mg kg^-1 . As established sodium channel blockers do not suppress seizures in this mouse model, this indicates that 2 could be a promising candidate for treating pharmaco-resistant epilepsy.

Mexiletine Block of Voltage-Gated Sodium Channels: Isoform- and State-Dependent Drug-Pore Interactions

Mexiletine is a class Ib antiarrhythmic drug and is also used clinically to reduce or prevent myotonia. In addition, mexiletine has neuroprotective effects in models of brain ischemia. We compared state-dependent affinities of mexiletine for Na_v1.2, Na_v1.4, and Na_v1.5 and examined the effects of mutations of transmembrane segment S6 residues on mexiletine block of Na_v1.5. Three channel isoforms had similar affinities of mexiletine for the rested state, and Na_v1.4 and Na_v1.5 had similar affinities for the open and inactivated states, while Na_v1.2 had lower affinity for these states than Na_v1.4 and Na_v1.5. Mutational studies revealed that the largest affinity change was observed for an Ala substitution of Phe in domain IV S6. In our homology modeling based on the bacterial Na⁺ channel, mexiletine changed its location and orientation in the pore depending on the state of the channel, irrespective of the channel isoform. Mexiletine occurred in the upper part in the pore in the open state and lower in the closed state. High-affinity binding of mexiletine in the open states of Na_v1.4 and Na_v1.5 was caused by a π-π interaction with Phe, whereas mexiletine was located away from Phe in the open state of Na_v1.2. These results provide crucial information on the mechanism of isoform differences in state-dependent block by local anesthetics and related drugs. Mexiletine at upper locations in the open state may effectively cause an electrostatic mechanism of block.

Interactions of Mexiletine with Novel Antiepileptic Drugs in the Maximal Electroshock Test in Mice: An Isobolographic Analysis

The aim of the study was to evaluate precisely the type of interactions between mexiletine (an antiarrhythmic drug) and four new generation antiepileptic drugs: lamotrigine, oxcarbazepine, topiramate and pregabalin in the maximal electroshock test in mice (MES). The isobolographic analysis was used to assess the nature of interactions between the tested drugs. Total brain concentrations of antiepileptics were also measured to detect possible pharmacokinetic interactions. The results obtained indicated that the mixture of mexiletine and pregabalin at the fixed ratios of 1:1 and 3:1 led to supra-additive interaction in terms of seizure suppression, while the proportion of 1:3 occurred additive. Synergism was also demonstrated for the combination of mexiletine and topiramate in all three proportions. Combinations of mexiletine with lamotrigine and mexiletine with oxcarbazepine were found to be additive. Adverse-effect profiles of mexiletine, antiepileptics and drug combinations were evaluated in the chimney test (motor coordination) and step-through passive-avoidance task (long-term memory). Mexiletine and drug combinations did not impair long-term memory. Moreover, all combinations of mexiletine with lamotrigine, oxcarbazepine and topiramate had no significant effect on motor coordination. However, the results from the chimney test indicated that pregabalin, administered alone at its ED₅₀ dose from the MES-test, significantly impaired motor performance. Similar adverse effects were observed when mexiletine was co-administered with pregabalin at the fixed-dose ratio combinations of 1:1 and 1:3. However, reduction of pregabalin dose at the fixed ratio of 3:1 seems to prevent significant motor impairment. The results may indicate that mexiletine can be considered as an adjunctive drug in antiepileptic treatment, particularly in patients with concomitant cardiac arrhythmia.

Beyond the muscular involvement in non-dystrophic myotonias: The emerging role of neuromodulation

BACKGROUND

The central nervous system involvement, in terms of a maladaptive sensory-motor plasticity, is well known in patients with dystrophic myotonias (DMs). To date, there are no data suggesting a central nervous system involvement in non-dystrophic myotonias (NDMs).

OBJECTIVE

To investigate sensory-motor plasticity in patients with Myotonia Congenita (MC) and Paramyotonia Congenita (PMC) with or without mexiletine.

METHODS

Twelve patients with a clinical, genetic, and electromyographic evidence of MC, fifteen with PMC, and 25 healthy controls (HC) were included in the study. TMS on both primary motor cortices (M1) and a rapid paired associative stimulation (rPAS) paradigm were carried out to assess M1 excitability and sensory-motor plasticity.

RESULTS

patients showed a higher cortical excitability and a deterioration of the topographic specificity of rPAS aftereffects, as compared to HCs. There was no correlation among neurophysiological and clinical-demographic characteristics. Noteworthy, the patients who were under mexiletine showed a minor impairment of the topographic specificity of rPAS aftereffects as compared to those who did not take the drug.

CONCLUSION

our findings could suggest the deterioration of cortical sensory-motor plasticity in patients with NDMs as a trait of the disease.

Case Report: Utility of Ketamine, Lidocaine, and Mexiletine as Nonopioid Adjuvants in Complex Cancer-Associated Pain

Ketamine, lidocaine, and mexiletine are potential nonopioid adjuvant medications for the use of refractory cancer-related pain, particularly when opioids are demonstrating limited objective benefit. This is a case report of a single patient admitted to a large academic medical center in the United States. The patient is a 43-year-old woman with a history of Crohn's disease complicated by rectal squamous cell carcinoma and complex, progressive, and intractable pelvic and rectal pain. Over the course of hospitalization, her pain demonstrated limited opioid responsiveness despite marked fluctuations of her oral morphine equivalent doses. She also demonstrates variable responsiveness to ketamine. Lidocaine continuous infusion ultimately proves beneficial, and she is discharged after conversion to oral mexiletine. An overview of the hospital system's protocols for ketamine and lidocaine continuous infusions for pain and considerations for transitioning to mexiletine from lidocaine infusion are included.

Management of Chronic Postthoracotomy Pain

Postthoracotomy pain management is essential imme diately after surgery and crucial for the patient suffering from this problem for a long duration. Understanding the causative and pathophysiological basis for this condition requires frequent assessment and a multidi mensional approach. Inadequate pain control has detri mental effects on the routine activities of patients as well as on their physical and emotional well-being. During the last 2 decades, the understanding of various pain-related issues has been strengthened resulting in early intervention and pain management. Various phar macological modalities including the role of nonopiates, opiates, and adjuvant drugs are helpful in combating postthoracotomy pain. A selective group of patients will benefit from interventional techniques. The treat ment decision requires a clear understanding of the disease and its impact on the patient. There must also be an understanding of risk and benefits as well as cost and benefits for various drugs and techniques before selecting the appropriate therapy.

Conotoxins That Could Provide Analgesia through Voltage Gated Sodium Channel Inhibition

Chronic pain creates a large socio-economic burden around the world. It is physically and mentally debilitating, and many sufferers are unresponsive to current therapeutics. Many drugs that provide pain relief have adverse side effects and addiction liabilities. Therefore, a great need has risen for alternative treatment strategies. One rich source of potential analgesic compounds that has emerged over the past few decades are conotoxins. These toxins are extremely diverse and display selective activity at ion channels. Voltage gated sodium (Na_V) channels are one such group of ion channels that play a significant role in multiple pain pathways. This review will explore the literature around conotoxins that bind Na_V channels and determine their analgesic potential.

Systematic review and meta-analysis of pharmacological therapies for pain associated with postherpetic neuralgia and less common neuropathic conditions

OBJECTIVE

To estimate the relative efficacy of pharmacological therapies for the treatment of postherpetic neuralgia (PHN), multiple sclerosis (MS)-related pain, posttraumatic pain, central poststroke pain (CPSP) and human immunodeficiency virus (HIV)-related neuropathic pain (NeP).

METHODS

This systematic review (through June 2011) identified randomised, controlled trials of treatments for these conditions. Bayesian mixed treatment comparison (MTC) methods were used to determine the relative efficacy and safety among the treatments within each pain condition.

RESULTS

Fifty studies were identified: 33 PHN, 2 MS-related pain, 3 CPSP, 3 posttraumatic pain and 9 HIV-related NeP. Data from 28 PHN studies including 21 interventions and 4317 patients were included into the PHN MTC. Of treatments studied in ≥ 50 patients, opioids had the greatest mean pain reduction of -1.70 vs. placebo on an 11-point numeric rating scale. Pregabalin ≥ 300 mg/day was most effective for ≥ 30% and ≥ 50% pain reduction [relative risk (RR) vs. placebo = 2.44 and 2.13, respectively]. Data identified for MS-related pain, CPSP, posttraumatic pain and HIV-related NeP were sparse; only 7 of 17 studies had ≥ 50 patients. Adverse events (AEs) and discontinuations for most treatments were not significantly greater than placebo except in PHN, where 8 of 12 treatments had higher risks of AEs compared with placebo and tricyclic antidepressants and opioids had higher risk of discontinuation compared with placebo.

CONCLUSIONS

Guideline-recommended treatments for PHN were more effective than placebo on the pain NRS and for ≥ 30% and ≥ 50% pain reduction. Although guidelines exist for the management of less common NeP conditions, little published evidence supports them. These results highlight the need for additional evaluations and more complete reporting of outcomes to help guide physicians' treatment selections.

Discovery and optimization of aminopyrimidinones as potent and state-dependent Nav1.7 antagonists

Clinical genetic data have shown that the product of the SCN9A gene, voltage-gated sodium ion channel Nav1.7, is a key control point for pain perception and a possible target for a next generation of analgesics. Sodium channels, however, historically have been difficult drug targets, and many of the existing structure-activity relationships (SAR) have been defined on pharmacologically modified channels with indirect reporter assays. Herein we describe the discovery, optimization, and SAR of potent aminopyrimidinone Nav1.7 antagonists using electrophysiology-based assays that measure the ligand-receptor interaction directly. Within this series, rapid functionalization at the polysubstituted aminopyrimidinone head group enabled exploration of SAR and of pharmacokinetic properties. Lead optimized N-Me-aminopyrimidinone 9 exhibited improved Nav1.7 potency, minimal off-target hERG liability, and improved rat PK properties.

Intravenous lidocaine and mexiletine in the management of trigeminal autonomic cephalalgias

Lidocaine and mexiletine are class 1B antiarrhythmic drugs that act on sodium channels. Lidocaine is also an important anesthetic and topical agent that is useful in the treatment of multiple pain disorders, and mexiletine is commonly used for neuropathic pain and myotonia. Both intravenous lidocaine and mexiletine are increasingly used to treat pain syndromes and appear to be particularly effective in neuropathic pain. This suggests a role for these agents in patients with headache disorders. This article describes the role of intravenous lidocaine and mexiletine in the management of headache and trigeminal autonomic cephalalgias based on the published literature to date and provides practical guidelines for their use.

A systematic review of pharmacologic treatments of pain after spinal cord injury

OBJECTIVE

To conduct a systematic review of published research on the pharmacologic treatment of pain after spinal cord injury (SCI).

DATA SOURCES

MEDLINE, CINAHL, EMBASE, and PsycINFO databases were searched for articles published 1980 to June 2009 addressing the treatment of pain post SCI. Randomized controlled trials (RCTs) were assessed for methodologic quality using the Physiotherapy Evidence Database (PEDro) assessment scale, whereas non-RCTs were assessed by using the Downs and Black (D&B) evaluation tool. A level of evidence was assigned to each intervention by using a modified Sackett scale.

STUDY SELECTION

The review included RCTs and non-RCTs, which included prospective controlled trials, cohort, case series, case-control, pre-post studies, and post studies. Case studies were included only when there were no other studies found.

DATA EXTRACTION

Data extracted included the PEDro or D&B score, the type of study, a brief summary of intervention outcomes, the type of pain, the type of pain scale, and the study findings.

DATA SYNTHESIS

Articles selected for this particular review evaluated different interventions in the pharmacologic management of pain after SCI. Twenty-eight studies met inclusion criteria; there were 21 randomized controlled trials; of these, 19 had level 1 evidence. Treatments were divided into 5 categories: anticonvulsants, antidepressants, analgesics, cannabinoids, and antispasticity medications.

CONCLUSIONS

Most studies did not specify participants' types of pain, making it difficult to identify the type of pain being targeted by the treatment. Anticonvulsant and analgesic drugs had the highest levels of evidence and were the drugs most often studied. Gabapentin and pregabalin had strong evidence (5 level 1 RCTs) for effectiveness in treating post-SCI neuropathic pain as did intravenous analgesics (lidocaine, ketamine, and morphine), but the latter only had short-term benefits. Tricyclic antidepressants only showed benefit for neuropathic pain in depressed persons. Intrathecal baclofen reduced musculoskeletal pain associated with spasticity; however, there was conflicting evidence for the reduction in neuropathic pain. Studies assessing the effectiveness of opioids were limited and revealed only small benefits. Cannabinoids showed conflicting evidence in improving spasticity-related pain. Clonidine and morphine when given together had a significant synergistic neuropathic pain-relieving effect.

Attenuating effect of mexiletine hydrochloride on herpetic pain in mice infected with herpes simplex virus

The influence of mexiletine hydrochloride on herpes-related pain responses was examined using mice infected with herpes virus. BALB/c mice were inoculated with herpes simplex virus (HSV; 1 times 106 plaque-forming units) on the right hind paw, and the contralateral hind paw was without inoculation. The changes in nociceptive threshold were examined using electric von fray meter. BALB/c mice inoculated with HSV showed a decrease in nociceptive threshold. Intraperitoneal administration of mexiletine prevented the decrease in nociceptive threshold dose-dependently in HSV-inoculated mice, which was firstly observed at a dose of 15.0 mg kg−1, and peaked at doses more than 17.5 mg kg−1. This antinociceptive effect of mexiletine attained peaks at 60–90 min after administration and declined gradually to non-treated levels by 150 min. Intraperitoneal administration of mexiletine at a dose of 17.5 mg kg−1 (but not 10.0 mg kg−1) caused significant increase in β-endorphin levels in the mid brain and hypothalamus of HSV-inoculated mice. However, mexiletine scarcely affected noradrenaline (norepinephrine) levels in the pons and medulla oblongata, even when HSV-inoculated mice were treated with 17.5 mg kg−1 mexiletine. These results strongly suggested that mexiletine exerts antinociceptive effects on herpes-related pain through enhancement of β-endorphin levels in the central nervous system in HSV-inoculated mice. It is also suggested that mexiletine will be a good candidate for an antinociceptive drug in the treatment of acute herpetic pain in man.

Postherpetic neuralgia: from preclinical models to the clinic

Postherpetic neuralgia (PHN), a common complication of herpes zoster, which results from reactivation of varicella zoster virus, is a challenging neuropathic pain syndrome. The incidence and severity of herpes zoster and PHN increases with immune impairment or age and may become a greater burden both in terms of health economics and individual suffering. A clearer understanding of the underlying mechanisms of this disease and translation of preclinical outcomes to the clinic may lead to more efficacious treatment options. Here we give an overview of recent findings from preclinical models and clinical research on PHN.

Sodium channel blockers for the treatment of neuropathic pain

Drugs that block voltage-gated sodium channels are efficacious in the management of neuropathic pain. Accordingly, this class of ion channels has been a major focus of analgesic research both in academia and in the pharmaceutical/biotechnology industry. In this article, we review the history of the use of sodium channel blockers, describe the current status of sodium channel drug discovery, highlight the challenges and hurdles to attain sodium channel subtype selectivity, and review the potential usefulness of selective sodium channel blockers in neuropathic pain.

Dercum's disease (Lipomatosis dolorosa): successful therapy with pregabalin and manual lymphatic drainage and a current overview

Dercum's disease which is also termed lipomatosis dolorosa is a rare and relatively unknown disease. In this entity the upper arms, elbows, stomach wall, buttocks, thighs and knees are predominantly affected showing painful subcutaneous adipose tissue deposits. In addition severe hyperalgesia can be triggered by light pressure and touch. Analgesic and/or nonsteroidal antirheumatic drugs have usually only a minor or no effect. Here, we report a patient with Dercum's disease who was successfully treated with pregabalin and manual lymphatic drainage, and present a current overview of the literature.

Topical analgesics

Historically, analgesics were applied by the topical route of administration. With the advent of oral formulations of drugs, topical application became less popular among physicians, although patients still rated this method of drug delivery as efficacious and practical. We now appreciate that peripheral mechanisms of actions of a variety of preparations rationalizes their topical application and gives further opportunity to target peripheral receptors and neural pathways that previously required systemic administration to achieve therapeutic effect. Therefore, a peripheral effect can be generated by using locally applied drug and, consequently, systemic concentrations of that drug may not reach the level at which systemic side effects can occur.

Topical analgesics

Our knowledge and understanding of the pathophysiology and treatment of pain is increasing; however, we should not lose sight of the simple opportunities that exist for intercepting pain at peripheral targets. Although systemic medication often has peripheral and central modes of action, the appeal for provision of medication close to where these peripheral targets exist should be high. If these sites can be attacked with relatively high concentrations of active drug while keeping systemic levels of that drug below the level at which systemic side effects become apparent, then this should lead to desirable outcomes. Even though the number of true topical agents with an indication for this use is small, a number of other topical agents are available that evidence suggests have the possibility of being effective. Given the increased understanding of pain, the likelihood of further topical agents becoming available is high.

EFNS guidelines on pharmacological treatment of neuropathic pain

Neuropathic pain treatment remains unsatisfactory despite a substantial increase in the number of trials. This EFNS Task Force aimed at evaluating the existing evidence about the pharmacological treatment of neuropathic pain. Studies were identified using first the Cochrane Database then Medline. Trials were classified according to the aetiological condition. All class I and II controlled trials (according to EFNS classification of evidence) were assessed, but lower-class studies were considered in conditions that had no top level studies. Only treatments feasible in an outpatient setting were evaluated. Effects on pain symptoms/signs, quality of life and comorbidities were particularly searched for. Most of the randomized controlled trials included patients with postherpetic neuralgia (PHN) and painful polyneuropathies (PPN) mainly caused by diabetes. These trials provide level A evidence for the efficacy of tricyclic antidepressants, gabapentin, pregabalin and opioids, with a large number of class I trials, followed by topical lidocaine (in PHN) and the newer antidepressants venlafaxine and duloxetine (in PPN). A small number of controlled trials were performed in central pain, trigeminal neuralgia, other peripheral neuropathic pain states and multiple-aetiology neuropathic pains. The main peripheral pain conditions respond similarly well to tricyclic antidepressants, gabapentin, and pregabalin, but some conditions, such as HIV-associated polyneuropathy, are more refractory. There are too few studies on central pain, combination therapy, and head-to-head comparison. For future trials, we recommend to assess quality of life and pain symptoms or signs with standardized tools.

On the mechanism of carbamazepine-induced antinociception in the formalin test

In the present study, the effect of lidocaine (a sodium channel blocker) on carbamazepine-induced antinociception, in formalin test was investigated. Intraperitoneal (i.p.) administration of different doses of carbamazepine (3.5, 7, 15, and 30 mg/kg) induced a dose-dependent antinociception in mice, in the first and second phases of the test. Different doses of lidocaine as a sodium channel blocker (5, 10, and 20 mg/kg, i.p.) also induced antinociception in both phases of the formalin test. It is noted that lidocaine could potentiate the response of carbamazepine in the first, but not in the second, phase of the formalin test. Meanwhile i.p. administration of different doses of Prazosin, alpha1 adrenoceptor antagonist (0.125, 0.25, and 0.5 mg/kg), Yohimbine, alpha2 adrenoceptor antagonist (0.25, 0.5, and 1 mg/kg), Bicuculline, GABAA receptor antagonist (1.5 and 3 mg/kg), and CGP 35348, GABAB receptor antagonist (100 and 200 mg/kg) exert dose-dependent antinociceptive effect in both phases of the formalin test. It should be noted that bicuculline 0.75 mg/kg by itself increased pain score in the second phase of the formalin test, indicating that blockade of GABAA receptor subtype may induce chronic pain. None of the aforementioned drugs could alter the antinociceptive response of carbamazepine in the formalin test. It is concluded that sodium channel mechanisms may be involved partly in the antinociceptive induced by carbamazepine.

Evaluation of the pharmacological activity of the major mexiletine metabolites on skeletal muscle sodium currents

BACKGROUND AND PURPOSE

Mexiletine (Mex), an orally effective antiarrhythmic agent used to treat ventricular arrhythmias, has also been found to be effective for myotonia and neuropathic pain. It is extensively metabolized in humans but little information exists about the pharmacodynamic properties of its metabolites.

EXPERIMENTAL APPROACH

To determine their contribution to the clinical activity of Mex, p-hydroxy-mexiletine (PHM), hydroxy-methyl-mexiletine (HMM), N-hydroxy-mexiletine (NHM) (phase I reaction products) and N-carbonyloxy beta-D-glucuronide (NMG) (phase II reaction product) were tested on sodium currents (I(Na)) of frog skeletal muscle fibres. Sodium currents were elicited with depolarizing pulses from different holding potentials (HP=-140, -100, -70 mV) and stimulation frequencies (0.25, 0.5, 1, 2, 5, 10 Hz) using the vaseline-gap voltage-clamp method.

KEY RESULTS

All the hydroxylated derivatives blocked the sodium channel in a voltage- and use-dependent manner. The PHM, HMM and NHM metabolites were up to 10-fold less effective than the parent compound. However, HMM showed a greater use-dependent behaviour (10 Hz), compared to Mex and the other metabolites. Similar to Mex, these products behaved as inactivating channel blockers. Conjugation with glucuronic acid (NMG) resulted in almost complete abolition of the pharmacological activity of the parent compound.

CONCLUSIONS AND IMPLICATIONS

Thus, although less potent, the phase I metabolites tested demonstrated similar pharmacological behaviour to Mex and might contribute to its clinical profile.

Tolerability of treatments for postherpetic neuralgia

Herpes zoster occurs in up to 20% of people infected with varicella-zoster virus, due to reactivation of the virus from latently infected sensory ganglia. Although pain is a typical feature of acute zoster, pain persisting for more than a month after resolution of the rash is less common and is termed postherpetic neuralgia (PHN). The pain associated with PHN is neuropathic in origin and is notoriously difficult to treat. The incidence of herpes zoster and its associated complications both increase with age, so PHN should be seen more commonly in an aging population. Vaccination with live, attenuated varicella vaccine is safe and efficacious, particularly in children. It decreases the incidence of acute varicella and subsequent herpes zoster. Aciclovir is well tolerated, with renal toxicity only at high intravenous doses. Treatment of acute varicella with aciclovir attenuates acute illness but does not prevent herpes zoster. Treatment of herpes zoster with aciclovir or its derivatives minimises symptoms and may reduce the rate of PHN. Foscarnet is an alternative for an aciclovir-resistant virus but its use is limited by renal and CNS toxicity. Corticosteroids reduce acute pain in herpes zoster but do not affect the incidence of PHN. Their use in some patients may be limited by adverse effects such as gastritis and impaired glucose tolerance. Treatment of established PHN is difficult and may require a holistic approach. Tricyclic antidepressants and gabapentin are the systemic agents with the most proven benefit, although opioids such as oxycodone and NMDA receptor antagonists such as ketamine may be useful in some people. Adverse effects from tricyclic antidepressants are common but usually mild, while gabapentin is generally well tolerated. Although effective, the relatively common adverse effects of opioids and ketamine limit their usefulness in treating PHN. Topical treatment with 5% lidocaine patch or capsaicin is of benefit in some patients and is generally well tolerated. Intrathecal methyl prednisolone may be considered for intractable pain but efficacy and safety have not been confirmed.

Effect of mexiletine on vincristine-induced painful neuropathy in mice

In the present study, we examined the effect of mexiletine on vincristine-induced thermal hyperalgesia in mice. Mice were intraperitoneally treated with vincristine at a dose of 0.05 mg/kg one day after the measurement of the pre-drug latency in the tail-flick test, and then treated with a dose of 0.125 mg/kg twice a week for 6 weeks. In vincristine-treated mice, a significant decrease in tail-flick latency developed at 6 weeks after treatment. Pretreatment with mexiletine, at doses of 3, 10 and 30 mg/kg, i.p., dose-dependently increased the tail-flick latency in vincristine-treated mice. A significant reduction of the tail-flick latency was observed when the tail-flick latency was examined 60 min after i.t. administration of NG-nitro-L-arginine methyl ester (L-NAME, 30 nmol), a nitric oxide synthase (NOS) inhibitor, in naive mice. This L-NAME-induced thermal hyperalgesia was dose-dependently attenuated by pretreatment with mexiletine (10 and 30 mg/kg, i.p.), 10 min before the injection of L-NAME. The duration of nociceptive behavioral response induced by fenvalerate, at a dose of 0.1 microg, i.t., was significantly increased by pretreatment with L-NAME (30 nmol, i.t.). Intrathecal pretreatment with L-arginine (300 pmol) significantly reversed the L-NAME-induced enhancement of fenvalerate-induced nociceptive responses. The present study demonstrates that systemic mexiletine can effectively attenuate vincristine-induced thermal hyperalgesia. Furthermore, these results suggest that blockade of nitric oxide-induced enhancement of nociceptive transmission, in which tetrodotoxin-resistant sodium channels play an important role, may participate in the antinociceptive effect of mexiletine on vincristine-induced thermal hyperalgesia.

Small-fiber neuropathy: answering the burning questions

Small-fiber neuropathy is a peripheral nerve disease that most commonly presents in middle-aged and older people, who develop burning pain in their feet. Although it can be caused by disorders of metabolism such as diabetes, chronic infections (such as with human immunodeficiency virus), genetic abnormalities, toxicity from various drugs, and autoimmune diseases, the cause often remains a mystery because standard electrophysiologic tests for nerve injury do not detect small-fiber function. Inadequate ability to test for and diagnose small-fiber neuropathies has impeded patient care and research, but new tools offer promise. Infrequently, the underlying cause of small-fiber dysfunction is identified and disease-modifying therapy can be instituted. More commonly, the treatments for small-fiber neuropathy involve symptomatic treatment of neuropathic pain.

Ion Channel Targets and Treatment Efficacy in Neuropathic Pain

Chronic neuropathic pain due to injury or dysfunction of the nervous system remains a formidable treatment challenge in spite of a growing range of medication choices. We review current clinical research supporting the use of ion channel modulators for neuropathic pain states. New modes of local drug delivery, novel Ca2+ channel targets, and increased choices for drugs with activity at Na+channels are transforming this longstanding therapeutic strategy. Clinical decision making is increasingly informed by a more nuanced understanding of the role of voltage-gated Na+channels (VGSCs) and Ca2+ channels (VGCCs) in the pathophysiology of nerve injury. Although holding great promise for the future, mechanism-based approaches to treatment will require greater understanding of the analgesic mechanisms of drug action and of the relationships between pathophysiologic mechanisms and clinical presentation.

PERSPECTIVE

Treatment options for neuropathic pain targeting ion channels have grown rapidly in the past decade. An evolving body of clinical research supports the widespread use of this longstanding therapeutic strategy. Improved efficacy of ion channel modulators hinges upon further elucidation of the relationship between signs and symptoms of pain and underlying pathophysiology.

The pharmacotherapy of chronic pain: a review

The past two decades have contributed a large body of preclinical work that has assisted in our understanding of the underlying pathophysiological mechanisms that cause chronic pain. In this context, it has been recognized that effective treatment of pain is a priority and that treatment often involves the use of one or a combination of agents with analgesic action. The current review presents an evidence-based approach to the pharmacotherapy of chronic pain. Medline searches were done for all agents used as conventional treatment in chronic pain. Published papers up to June 2005 were included. The search strategy included randomized, controlled trials, and where available, systematic reviews and meta-analyses. Further references were found in reference sections of papers located using the above search strategy. Agents for which there were no controlled trials supporting efficacy in treatment of chronic pain were not included in the present review, except in cases where preclinical science was compelling, or where initial human work has been positive and where it was thought the reader would be interested in the scientific evidence to date.

Systemic administration of local anesthetics to relieve neuropathic pain: a systematic review and meta-analysis

We reviewed randomized controlled trials to determine the efficacy and safety of systemically administered local anesthetics compared with placebo or active drugs. Of 41 retrieved studies, 27 trials of diverse quality were included in the systematic review. Ten lidocaine and nine mexiletine trials had data suitable for meta-analysis (n = 706 patients total). Lidocaine (most commonly 5 mg/kg IV over 30-60 min) and mexiletine (median dose, 600 mg daily) were superior to placebo (weighted mean difference on a 0-100 mm pain intensity visual analog scale = -10.60; 95% confidence interval: -14.52 to -6.68; P < 0.00001) and equal to morphine, gabapentin, amitriptyline, and amantadine (weighted mean difference = -0.60; 95% confidence interval: -6.96 to 5.75) for neuropathic pain. The therapeutic benefit was more consistent for peripheral pain (trauma, diabetes) and central pain. The most common adverse effects of lidocaine and mexiletine were drowsiness, fatigue, nausea, and dizziness. The adverse event rate for systemically administered local anesthetics was more than for placebo but equivalent to morphine, amitriptyline, or gabapentin (odds ratio: 1.23; 95% confidence interval: 0.22 to 6.90). Lidocaine and mexiletine produced no major adverse events in controlled clinical trials, were superior to placebo to relieve neuropathic pain, and were as effective as other analgesics used for this condition.

Varicella zoster virus induces neuropathic changes in rat dorsal root ganglia and behavioral reflex sensitisation that is attenuated by gabapentin or sodium channel blocking drugs

Reactivation of latent varicella zoster virus (VZV) within sensory trigeminal and dorsal root ganglia (DRG) neurons produces shingles (zoster), often accompanied by a chronic neuropathic pain state, post-herpetic neuralgia (PHN). PHN persists despite latency of the virus within human sensory ganglia and is often unresponsive to current analgesic or antiviral agents. To study the basis of varicella zoster-induced pain, we have utilised a recently developed model of chronic VZV infection in rodents. Immunohistochemical analysis of DRG following VZV infection showed the presence of a viral immediate early gene protein (IE62) co-expressed with markers of A- (neurofilament-200; NF-200) and C- (peripherin) afferent sensory neurons. There was increased expression of neuropeptide Y (NPY) in neurons co-expressing NF-200. In addition, there was an increased expression of alpha2delta1 calcium channel, Na(v)1.3 and Na(v)1.8 sodium channels, the neuropeptide galanin and the nerve injury marker, Activating Transcription Factor-3 (ATF-3) as determined by Western blotting in DRG of VZV-infected rats. VZV infection induced increased behavioral reflex responsiveness to both noxious thermal and mechanical stimuli ipsilateral to injection (lasting up to 10 weeks post-infection) that is mediated by spinal NMDA receptors. These changes were reversed by systemic administration of gabapentin or the sodium channel blockers, mexiletine and lamotrigine, but not by the non-steroidal anti-inflammatory agent, diclofenac. This is the first time that the profile of VZV infection-induced phenotypic changes in DRG has been shown in rodents and reveals that this profile appears to be broadly similar (but not identical) to changes in other neuropathic pain models.

Systemic administration of local anesthetic agents to relieve neuropathic pain

BACKGROUND

Lidocaine, mexiletine, tocainide, and flecainide are local anesthetics which give an analgesic effect when administered orally or parenterally. Early reports described the use of intravenous lidocaine or procaine to relieve cancer and postoperative pain (Keats 1951; Gilbert 1951; De Clive-Lowe 1958; Bartlett 1961). Interest reappeared decades later when patient series and clinical trials reported that parenteral lidocaine and its oral analogs tocainide, mexiletine, and flecainide relieved neuropathic pain in some patients (Boas 1982; Lindblom 1984; Petersen 1986; Dunlop 1988; Bach 1990; Awerbuch 1990). With the recent publication of clinical trials with high quality standards, we have reviewed the use of systemic lidocaine and its oral analogs in neuropathic pain to update our knowledge, to measure their benefit and harm, and to better define their role in therapy.

OBJECTIVES

To evaluate pain relief and adverse effect rates between systemic local anesthetic-type drugs and other control interventions.

SEARCH STRATEGY

We searched MEDLINE (1966 through 15 May 2004), EMBASE (January 1980 to December 2002), Cancer Lit (through 15 December 2002), Cochrane Central Register of Controlled Trials (2nd Quarter, 2004), System for Information on Grey Literature in Europe (SIGLE), and LILACS, from January 1966 through March 2001. We also hand searched conference proceedings, textbooks, original articles and reviews.

SELECTION CRITERIA

We included trials with random allocation, that were double blinded, with a parallel or crossover design. The control intervention was a placebo or an analgesic drug for neuropathic pain from any cause.

DATA COLLECTION AND ANALYSIS

We collected efficacy and safety data from all published and unpublished trials. We calculated combined effect sizes using continuous and binary data for pain relief and adverse effects as primary and secondary outcome measurements, respectively.

MAIN RESULTS

Thirty-two controlled clinical trials met the selection criteria; two were duplicate articles. The treatment drugs were intravenous lidocaine (16 trials), mexiletine (12 trials), lidocaine plus mexiletine sequentially (one trial), and tocainide (one trial). Twenty-one trials were crossover studies, and nine were parallel. Lidocaine and mexiletine were superior to placebo [weighted mean difference (WMD) = -11; 95% CI: -15 to -7; P <0.00001], and limited data showed no difference in efficacy (WMD = -0.6; 95% CI: -7 to 6), or adverse effects versus carbamazepine, amantadine, gabapentin or morphine. In these trials, systemic local anesthetics were safe, with no deaths or life-threatening toxicities. Sensitivity analysis identified data distribution in three trials as a probable source of heterogeneity. There was no publication bias.

AUTHORS' CONCLUSIONS

Lidocaine and oral analogs were safe drugs in controlled clinical trials for neuropathic pain, were better than placebo, and were as effective as other analgesics. Future trials should enroll specific diseases and test novel lidocaine analogs with better toxicity profiles. More emphasis is necessary on outcomes measuring patient satisfaction to assess if statistically significant pain relief is clinically meaningful.

Algorithm for neuropathic pain treatment: an evidence based proposal

New studies of the treatment of neuropathic pain have increased the need for an updated review of randomized, double-blind, placebo-controlled trials to support an evidence based algorithm to treat neuropathic pain conditions. Available studies were identified using a MEDLINE and EMBASE search. One hundred and five studies were included. Numbers needed to treat (NNT) and numbers needed to harm (NNH) were used to compare efficacy and safety of the treatments in different neuropathic pain syndromes. The quality of each trial was assessed. Tricyclic antidepressants and the anticonvulsants gabapentin and pregabalin were the most frequently studied drug classes. In peripheral neuropathic pain, the lowest NNT was for tricyclic antidepressants, followed by opioids and the anticonvulsants gabapentin and pregabalin. For central neuropathic pain there is limited data. NNT and NNH are currently the best way to assess relative efficacy and safety, but the need for dichotomous data, which may have to be estimated retrospectively for old trials, and the methodological complexity of pooling data from small cross-over and large parallel group trials, remain as limitations.

Parenteral lidocaine for severe intractable pain in six hospice patients continued at home

A small number of patients at the end of life suffer from severe or intractable pain that is difficult to manage with opioids. We have observed that lidocaine infusions successfully treat otherwise severe refractory pain syndromes. In general, the lidocaine infusion is continued for a few days and gradually supplanted by oral adjuvant analgesics or by invasive pain management techniques. We report six cases where prolonged lidocaine infusions were successfully continued in the home care setting. The average lidocaine infusion rate was 44 mg/hour (range 10-80 mg/h), serum levels ranged from 1.5-9.3 microg/mL, and duration of therapy ranged from 24-240 days. Mechanisms of action of lidocaine and sodium channel blocking adjuvant analgesics are reviewed. Parenteral lidocaine deserves additional study for its ability to relieve pain in terminally ill patients.

Effect of sodium channel blocker (mexiletine) on pathological ectopic firing pattern in a rat chronic constriction nerve injury model

We studied the efficacy of mexiletine as a sodium channel blocker for neuropathic pain by investigating the effect of mexiletine on the pathological ectopic firing pattern in a chronic constriction nerve injury (CCI) model. The experiment was conducted with 60 male Wistar rats. The CCI model was created by loosely ligating the sciatic nerve. After breeding 7 days, the frequency and pattern of ectopic firing antidromically recorded from the sural nerve and the amplitude of antidromic sensory nerve-evoked potential were analyzed. The CCI rats were given an intravenous injection of normal saline and mexiletine (5 or 15 mg/kg). Mexiletine significantly suppressed spontaneous firing frequency, an on-off firing pattern that consisted of cyclic bursting spikes and ectopic firing generation under the hypoxic condition. Mexiletine did not influence the amplitude of A-delta component in the antidromic sensory nerve-evoked potential. Mexiletine suppressed ectopic firing by blocking activity of the abnormal sodium channel at the nerve-injured site and dorsal root ganglion without blocking nerve conduction. This study suggests that mexiletine is useful for treating neuropathic pain in peripheral neuropathy.

Management of cancer pain

There are four basic approaches to cancer pain control: modify the source of pain, alter central perception of pain, modulate transmission of pain to the central nervous system, and block transmission of pain to the central nervous system. Systemic pharmacologic management aimed at the first three of these approaches is the cornerstone of the treatment of most cancer patients with moderate to severe pain. Optimal pharmacologic management of cancer pain requires selection of the appropriate analgesic drug; prescription of the appropriate dose; administration of the analgesic by the appropriate route; scheduling of the appropriate dosing interval; prevention of persistent pain and relief of breakthrough pain; aggressive titration of the dose of the analgesic; prevention, anticipation, and management of analgesic side effects; consideration of sequential trials of opioid analgesics; and use of appropriate co-analgesic drugs for specific pain syndromes. Most clinicians should be able to control most of the pain in most of their cancer patients. Collaboration with pain and hospice/palliative care experts should help the rest. No cancer patient should live or die with unrelieved pain.

Pharmacological strategies in relieving neuropathic pain

The pharmacological treatment of neuropathic pain relies, to a large extent, on drugs belonging to a small number of defined classes. Opioids, tricyclic antidepressants, antiepileptic drugs and membrane stabilisers form the current basis of treatment. Varying levels of evidence support the use of individual members of these classes and overall show no indication that one class of drug, or individual drug has universal effectiveness. More refined knowledge of the modes of action of these agents used to treat neuropathic pain should lead to a more logical approach to the management of this difficult series of conditions. A number of drugs currently licensed for a different indication have recently had an analgesic effect in neuropathic pain attributed to them. In addition, a number of novel compounds are undergoing investigation and provide hope of dicovering more efficacious treatment options in the future.

[Neuropathic pain due to malignancy: mechanisms, clinical manifestations and therapy]

INTRODUCTION

Neuropathic pain in cancer patients requires a focused clinical evaluation based on knowledge of common neuropathic pain syndromes.

DEFINITION

Neuropathic pain is a non-nociceptive pain or "deafferentation" pain, which suggests abnormal production of impulses by neural tissue that is separated from afferent input. Impulses arise from the peripheral nervous system or central nervous system. CAUSES OF NEUROPATHIC PAIN DUE TO MALIGNANCY: Neuropathic pain is caused directly by cancer-related pathology (compression/infiltration of nerve tissue, combination of compression/infiltration) or by diagnostic and therapeutic procedures (surgical procedures, chemotherapy, radiotherapy).

MECHANISMS

Pathophysiological mechanisms are very complex and still not clear enough. Neuropathic pain is generated by electrical hyperactivity of neurons along the pain pathways. Peripheral mechanisms (primary sensitization of nerve endings, ectopically generated action potentials within damaged nerves, abnormal electrogenesis within sensory ganglia) and central mechanisms (loss of input from peripheral nociceptors into dorsal horn, aberrant sprouting within dorsal horn, central sensitization, loss of inhibitory interneurons, mechanisms at higher centers) are involved.

DIAGNOSIS

The quality of pain presents as spontaneous pain (continuous and paroxysmal), abnormal pain (allodynia, hyperalgesia, hyperpathia), paroxysmal pain.

CLINICAL MANIFESTATIONS

Clinically, neuropathic pain is described as the pain in the peripheral nerve (cranial nerves, other mononeuropathies, radiculopathy, plexopathy, paraneoplastic peripheral neuropathy) and relatively infrequent, central pain syndrome.

THERAPY

Treatment of neuropathic pain remains a challenge for clinicians, because there is no accepted algorithm for analgesic treatment of neuropathic pain. Pharmacotherapy is considered to be the first line therapy. Opioids combined with non-steroidal antiinflammatory drugs are warranted. If patient is relatively unresponsive to an opioid, a trial with adjuvant analgesics might be considered. Tricyclic antidepressants might be selected for patients with continuous dysesthesia, and anticonvulsants might be used if the pain is predominanty lancinating or paroxysmal. The complexity of neuropathic syndromes and underlying etiologic mechanisms warrant clinical trials to determine appropriate treatment.

Voltage-gated sodium channels and hyperalgesia

Physiological and pharmacological evidence both have demonstrated a critical role for voltage-gated sodium channels (VGSCs) in many types of chronic pain syndromes because these channels play a fundamental role in the excitability of neurons in the central and peripheral nervous systems. Alterations in function of these channels appear to be intimately linked to hyperexcitability of neurons. Many types of pain appear to reflect neuronal hyperexcitability, and importantly, use-dependent sodium channel blockers are effective in the treatment of many types of chronic pain. This review focuses on the role of VGSCs in the hyperexcitability of sensory primary afferent neurons and their contribution to the inflammatory or neuropathic pain states. The discrete localization of the tetrodotoxin (TTX)-resistant channels, in particular NaV1.8, in the peripheral nerves may provide a novel opportunity for the development of a drug targeted at these channels to achieve efficacious pain relief with an acceptable safety profile.

Perioperative intravenous lidocaine has preventive effects on postoperative pain and morphine consumption after major abdominal surgery

Sodium channel blockers are approved for IV administration in the treatment of neuropathic pain states. Preclinical studies have suggested antihyperalgesic effects on the peripheral and central nervous system. Our objective in this study was to determine the time course of the analgesic and antihyperalgesic mechanisms of perioperative lidocaine administration. Forty patients undergoing major abdominal surgery participated in this randomized and double-blinded study. Twenty patients received lidocaine 2% (bolus injection of 1.5 mg/kg in 10 min followed by an IV infusion of 1.5 mg. kg(-1). h(-1)), and 20 patients received saline placebo. The infusion started 30 min before skin incision and was stopped 1 h after the end of surgery. Lidocaine blood concentrations were measured. Postoperative pain ratings (numeric rating scale of 0-10) and morphine consumption (patient-controlled analgesia) were assessed up to 72 h after surgery. Mean lidocaine levels during surgery were 1.9 +/- 0.7 microg/mL. Patient-controlled analgesia with morphine produced good postoperative analgesia (numeric rating scale at rest, <or=3; 90%-95%; no group differences). Patients who received lidocaine reported less pain during movement and needed less morphine during the first 72 h after surgery (103.1 +/- 72.0 mg versus 159.0 +/- 73.3 mg; Student's t-test; P < 0.05). Because this opioid-sparing effect was most pronounced on the third postoperative day, IV lidocaine may have a true preventive analgesic activity, most likely by preventing the induction of central hyperalgesia in a clinically relevant manner.

IMPLICATIONS

The perioperative administration of systemic small-dose lidocaine reduces pain during surgery associated with the development of pronounced central hyperalgesia, presumably by affecting mechanoinsensitive nociceptors, because these have been linked to the induction of central sensitization and were shown to be particularly sensitive to small-dose lidocaine.

Pharmacological management of neuropathic pain

In health, the nervous system exists in a balance between inhibitory and excitatory influences. This balance may be upset if neural tissue is damaged or irritated and may give rise to neuropathic pain. Such neuropathic pain does not respond consistently to opioid analgesics or NSAIDs and it may therefore be necessary to utilise other therapeutic agents with known activity on either the excitatory or inhibitory components of the pain pathway. These other agents are traditionally considered with reference to their original uses; we still refer to tricyclic antidepressants (TCAs) and anticonvulsant drugs when a consideration of their modes of action may allow more rational use. For example, carbamazepine is related to the TCAs by virtue of its chemical structure and proposed mode of action and yet is still classified as an anticonvulsant drug. With respect to the opioids, increasing evidence points to an analgesic effect in neuropathic pain, although concerns regarding tolerance and dependence still prevent more widespread use. The anticonvulsants comprise a group of compounds possessing anticonvulsant and analgesic properties, but each possesses differing modes of action and so several members of the class should be tried before a conclusion is reached that they, as a whole, are ineffective. TCAs may also have a role in the treatment of neuropathic pain. As with all drugs, if their use is not associated with pain relief in a defined period of time, their use should be terminated. Topical TCAs may also have a role where the area of neuropathic pain is small. Other options, such as SSRIs, membrane stabilisers, capsaicin, baclofen and clonidine may have potential in treating neuropathic pain. The available evidence regarding the efficacy of currently available agents for the treatment of neuropathic pain is sparse. With the knowledge of achieving analgesia, according to the modes of actions of various agents it is hoped that the treatment of this difficult condition may be more logical and successful.