4-Aminopyridine in multiple sclerosis: prolonged administration

Targeting Ion Channels and Purkinje Neuron Intrinsic Membrane Excitability as a Therapeutic Strategy for Cerebellar Ataxia

In degenerative neurological disorders such as Parkinson's disease, a convergence of widely varying insults results in a loss of dopaminergic neurons and, thus, the motor symptoms of the disease. Dopamine replacement therapy with agents such as levodopa is a mainstay of therapy. Cerebellar ataxias, a heterogeneous group of currently untreatable conditions, have not been identified to have a shared physiology that is a target of therapy. In this review, we propose that perturbations in cerebellar Purkinje neuron intrinsic membrane excitability, a result of ion channel dysregulation, is a common pathophysiologic mechanism that drives motor impairment and vulnerability to degeneration in cerebellar ataxias of widely differing genetic etiologies. We further propose that treatments aimed at restoring Purkinje neuron intrinsic membrane excitability have the potential to be a shared therapy in cerebellar ataxia akin to levodopa for Parkinson's disease.

Phase IIB Randomized Trial on the Use of 4-Aminopyridine in Guillain-Barré Syndrome

OBJECTIVE

To determine the safety and efficacy of orally delivered 4-aminopyridine (4-AP) in persons with Guillain-Barré Syndrome (GBS) >6 months from initial diagnosis.

DESIGN

A randomized, double-blind, placebo-controlled, crossover study.

SETTING

Tertiary care clinical outpatient program.

PARTICIPANTS

Nineteen participants enrolled (14 male, 5 female; N=19), neurologic impairment secondary to GBS and functional loss on the FIM motor score (stable for ≥12mo) and >3.0 but <5.0 on the American Spinal Injury motor scale. Twelve participants (mean age, 59y; range, 23-77y) completed the study.

INTERVENTIONS

A 4-AP dose-escalation study with 8 weeks in each period with a 3-week washout period, followed by 3 months open-label follow-up.

MAIN OUTCOME MEASURES

FIM motor score was the primary outcome measure; also evaluated were the American Spinal Injury motor strength score (all limbs), handheld dynamometer, 6-minute walk test, Medical Outcomes Study 12-Item Short Form, Center for Epidemiological Studies Depression scale, Positive and Negative Affect Schedule, pain, GBS disability scale, Jepsen-Taylor Hand Function Test, Minnesota Manual Dexterity Test and Minnesota Rate of Manipulation Test, Get Up and Go Test, McGill Pain Inventory, Craig Handicap Assessment and Reporting Technique, and participant self-evaluation.

RESULTS

Seven participants discontinued the study prematurely: 3 because of adverse events, 3 because of travel difficulties or relocation, and 1 because of pretreatment laboratory abnormalities. After removing 3 participants with maximum FIM scores, 4-AP arm trended superior to placebo (P=.065). Patients subjectively could always tell when they were on the active agent usually by tingling sensations or a sense of wellness. No statistically significant differences were found for other outcome measures although there were strong trends.

CONCLUSIONS

This study demonstrates the safety of 4-AP in the patient population with GBS as the predominate goal of the study. A trend toward improved function after treatment was noted with most patients electing to stay on the medication after the trial.

Neuroprotective Properties of 4-Aminopyridine

As an antagonist of voltage-gated potassium (Kv) channels, 4-aminopyridine (4-AP) is used as symptomatic therapy in several neurologic disorders. The improvement of visual function and motor skills and relieve of fatigue in patients with MS have been attributed to 4-AP. Its prolonged release formulation (fampridine) has been approved for the symptomatic treatment of walking disability in MS. The beneficial effects were explained by the blockade of axonal Kv channels, thereby enhancing conduction along demyelinated axons. However, an increasing body of evidence suggests that 4-AP may have additional properties beyond the symptomatic mode of action. In this review, we summarize preclinical and clinical data on possible neuroprotective features of 4-AP.

Aminopiridines in the treatment of multiple sclerosis and other neurological disorders

Symptomatic treatment has a great relevance for the management of patients with neurologic diseases, since it reduces disease burden and improves quality of life. Aminopyridines (APs) are a group of potassium (K+) channel blocking agents that exert their activity both at central nervous system level and on neuromuscular junction. This review describes the use of APs for the symptomatic treatment of neurological conditions. We will describe trials leading to the approval of the extended-release 4-aminopyridine for MS and evidence in support of the use in other neurological diseases.

Efficacy and safety of fampridine for walking disability in multiple sclerosis

Fampridine is the only drug approved for the treatment of walking impairment in multiple sclerosis. Around a third of the patients on treatment obtained an improvement in walking speed during the development phase. The effects are clinically significant, appear soon after the start of the treatment and are long-lasting, but disappear soon after the drug is withdrawn. In the real-world setting, the number of patients with a significant response to the treatment seems to be higher (around 70%). The tolerance is good, with mild to moderate, and transient adverse events. The most commonly reported are insomnia, headache, fatigue, back pain, dizziness, nausea and balance disorders. The main contraindications are a history of seizures, renal impairment and concomitant treatment with OCT2 inhibitors.

Cognitive Fatigability Interventions in Neurological Conditions: A Systematic Review

INTRODUCTION

Although fatigue is a well-studied concept in neurological disease, cognitive fatigability (CF) is less understood. While most studies measure fatigue using subjective self-report, fewer have measured CF objectively. Given the negative impact of CF on quality-of-life, there is a need for targeted interventions. The objective of this review was to determine which procedural, behavioural and pharmacological treatments for objectively measured CF are available to people living with neurological conditions.

METHODS

In accordance with the PRISMA guidelines, systematic searches for randomized control trials (RCTs), case-controlled studies and case reports/series were conducted across the Ovid Medline, PsycInfo, EMBASE and Cochrane Library databases. English-language articles published between 1980 and February 2019 were considered for eligibility. Included were those that objectively measured CF in individuals with neurological disease/disorder/dysfunction between the ages of 18 and 65 years. Studies were reviewed using a modified Cochrane Data Extraction Template. Risk of bias was assessed using the Cochrane Risk of Bias tool. The review process was facilitated using Covidence software (www.covidence.org). Two authors reviewed articles independently, with a third resolving conflicts regarding article inclusion.

RESULTS

The search identified 450 records. After duplicates were removed and remaining titles/abstracts were screened for eligibility, 28 full-text articles were assessed, and two studies were included in the qualitative synthesis. Studies were a priori divided into those with pharmacological, procedural or behavioural interventions. Two studies met eligibility criteria; both of these included participants with multiple sclerosis. One study utilized a procedural intervention (i.e. transcranial direct current stimulation), while the other utilized a pharmacological intervention (i.e. fampridine-SR). Studies were evaluated for risk of bias, and evidence from both eligible studies was discussed.

CONCLUSION

Despite the positive results of the procedural intervention, the paucity of eligible studies and the nascent nature of the field suggests that more studies are required before firm conclusions can be drawn regarding the amenability of CF to treatment.

TRIAL REGISTRATION

The review was registered with PROSPERO (CRD42019118706).

Restoring Axonal Function with 4-Aminopyridine: Clinical Efficacy in Multiple Sclerosis and Beyond

The oral potassium channel blocker 4-aminopyridine has been used in various neurological conditions for decades. Numerous case reports and studies have supported its clinical efficacy in ameliorating the clinical presentation of certain neurological disorders. However, its short half-life, erratic drug levels, and safety-related dose restrictions limited its use as a self-compounded drug in clinical practice. This changed with the introduction of a prolonged-release formulation, which was successfully tested in patients with multiple sclerosis. It was fully approved by the US FDA in January 2010 but initially received only conditional approval from the European Medicines Agency (EMA) in July 2011. After additional clinical studies, this conditional approval was changed to unrestricted approval in August 2017. This article reviews and discusses these recent studies and places aminopyridines and their clinical utility into the context of a broader spectrum of neurological disorders, where clinical efficacy has been suggested. In 2010, prolonged-release 4-aminopyridine became the first drug specifically licensed to improve walking in patients with multiple sclerosis. About one-third of patients across disease courses benefit from this treatment. In addition, various reports indicate clinical efficacy beyond multiple sclerosis, which may broaden its use in clinical practice.

Suppression of Inflammatory Demyelinaton and Axon Degeneration through Inhibiting Kv3 Channels

The development of neuroprotective and repair strategies for treating progressive multiple sclerosis (MS) requires new insights into axonal injury. 4-aminopyridine (4-AP), a blocker of voltage-gated K⁺ (Kv) channels, is used in symptomatic treatment of progressive MS, but the underlying mechanism remains unclear. Here we report that deleting Kv3.1—the channel with the highest 4-AP sensitivity—reduces clinical signs in experimental autoimmune encephalomyelitis (EAE), a mouse model for MS. In Kv3.1 knockout (KO) mice, EAE lesions in sensory and motor tracts of spinal cord were markedly reduced, and radial astroglia were activated with increased expression of brain derived neurotrophic factor (BDNF). Kv3.3/Kv3.1 and activated BDNF receptors were upregulated in demyelinating axons in EAE and MS lesions. In spinal cord myelin coculture, BDNF treatment promoted myelination, and neuronal firing via altering channel expression. Therefore, suppressing Kv3.1 alters neural circuit activity, which may enhance BNDF signaling and hence protect axons from inflammatory insults.

The effect of Fampridine-SR on cognitive fatigue in a randomized double-blind crossover trial in patients with MS

BACKGROUND

Cognitive fatigue (CF) is a common complaint in persons with MS (PwMS). Fampridine-SR improves ambulation, fatigue and endurance, due to enhancing action potential formation by blocking potassium channels in demyelinated axons. Thus, through this same mechanism, it is hypothesized that Fampridine-SR could improve CF.

OBJECTIVE

To determine if Fampridine-SR objectively improves CF in PwMS.

METHODS

Sixty PwMS of any type with CF, defined as 3 or less correct responses when comparing the last third to the first third on the Paced Auditory Serial Addition Test (PASAT), were recruited from a tertiary care MS clinic in London (ON) Canada. Subjects also had to be between 18 and 64 years of age, inclusive, not had a relapse in the last 60 days or corticosteroids in the last 30 days, EDSS 0.0-7.0, and no other diagnosis that could cause cognitive impairment. A randomized double blind crossover design was used: subjects were randomized to either placebo or Fampridine-SR for 4 weeks, then after at least a one week washout, received the opposite treatment. Subjects were assessed before and after each treatment block. The primary outcome was the PASAT CF score after treatment with Fampridine-SR compared to placebo. T-tests and chi-square were used to compare demographics between the two groups (placebo-Fampridine-SR vs. Fampridine SR-placebo). Treatment effects were assessed using factorial ANOVA, with treatment (Fampridine-SR vs. placebo) and time (before and after treatment) as within-subject variables.

RESULTS

Of the 60 subjects randomized, 48 completed the study; three were removed due to an adverse event while in the treatment arm (one due to relapse while on placebo, one due to urinary retention and one due to dizziness and headache while on Fampridine-SR). The subjects had a mean age of 46.5±10.0 years, education of 13.6±1.9 years, and were diagnosed with MS 10.6±9.6 years ago. The majority were female (46, 76.7%), had relapsing remitting MS (41, 68.3%) with median EDSS of 3.5 (range 1.0-7.0). There were no significant demographic differences between the two groups. The treatment x time interaction within the factorial ANOVA on PASAT CF scores was statistically significant, F(1, 45)=8.28, p=0.006, suggesting there is a difference between the treatments (placebo vs. Fampridine-SR), over the course of the study. An evaluation of the mean scores suggests, however, that subjects saw a greater improvement when they were given the placebo, than when they were given the active medication. Similarly, individuals showed a greater increase in their information processing speed (as measured by the PASAT) over the course of treatment when they were given the placebo, as compared with the active medication F(1,45)=4.17, p=0.047.

CONCLUSION

Although this small pilot study does not suggest that Fampridine-SR results in a statistically significant improvement of CF in MS patients, as compared to placebo, individuals demonstrated an improvement in both information processing speed and CF, suggesting further studies are warranted.

Fatigue in multiple sclerosis: multidimensional assessment and response to symptomatic treatment

Sixty relapsing-remitting multiple sclerosis (MS) patients were selected on the basis of their score on the Fatigue Severity Scale (FSS) and formed two groups: 40 patients (fatigued MS; MSf) scored above the 75th percentile of a previously assessed representative MS sample (100 patients), and 20 age- and sex-matched patients (nonfatigued MS patients; MSnf) scored below the 25th percentile. The patients underwent clinical evaluation (Expanded Disability Status Scale (EDSS)), further assessment of fatigue (Fatigue Impact Scale), scales evaluating depression (Hamilton Depression Rating Scale (HDRS) and Beck’s Depression Inventory (BDI)) and neuropsychological tests. All patients were evaluated for muscle fatigability and central activation by means of a biomechanical test of sustained contraction; they also underwent somatosensory evoked potentials (SSEPs) and transcranial magnetic stimulation (TMS). The patients of the MSf subgroup were then randomized to one of the following two treatments: 4-aminopyridine (4-AP) 24 mg/day and fluoxetine (FLX) 20 mg/day. After a one-week titration this treatment proceeded for 8 weeks. At the end of the treatment, EDSS, fatigue and depression scores were further evaluated. At baseline, fatigue test scores consistently correlated with depression and cognitive test scores, but not with the fatigability test. Fatigue scores decreased in both treatment groups in a similar way. Due to the design of the study, this cannot be disjoined from a placebo effect. The changes of fatigue scores could not be predicted in the FLX group, whereas in the 4-AP group higher basal fatigability test scores were associated with greater reduction in fatigue scores.

Development of dalfampridine, a novel pharmacologic approach for treating walking impairment in multiple sclerosis

Walking impairment is a clinical hallmark of multiple sclerosis (MS). Dalfampridine-ER, an extended-release formulation of dalfampridine (also known by its chemical name, 4-aminopyridine, and its international nonproprietary name, fampridine), was developed to maintain drug plasma levels within a narrow therapeutic window, and assessed for its ability to improve walking in MS. The putative mechanism of action of dalfampridine-ER is restoration of axonal conduction via blockade of the potassium channels that become exposed during axonal demyelination. Two pivotal phase III clinical trials demonstrated that dalfampridine-ER 10-mg tablets administered twice daily improved walking speed and patient-reported perceptions of walking in some patients. Dalfampridine-ER was generally well tolerated, and, at the approved dose, risk of seizure was neither elevated relative to placebo nor higher than the rate in the MS population. Dalfampridine-ER (AMPYRA®) was approved in the United States for the treatment of walking in patients with MS as demonstrated by an increase in walking speed. The use of the dalfampridine-ER is contraindicated in patients with a history of seizure. It is the first pharmacologic therapy for this indication and has been incorporated into clinical management of MS.

Changes in cognition, arm function and lower body function after slow-release Fampridine treatment

OBJECTIVE

We aimed to evaluate the effect of slow-release (SR) Fampridine on multiple outcome measures reflecting different domains, and to compare the responsiveness of the Six Spot Step Test (SSST) and the Timed 25 Foot Walk (T25FW).

METHODS

For this study 108 participants were included. On day 0 they were tested with the T25FW, the SSST, the 9-Hole Peg Test (9-HPT), the 5 Times Sit-To-Stand test (5-STS) and the Symbol Digit Modalities Test (SDMT). Four weeks of treatment with SR Fampridine 10 mg BID was commenced. Participants were tested again after 26-28 days of treatment.

RESULTS

Mean changes observed were: SSST -3.4±6.4 s (p<0.001), T25FW -1.2±3.7 s (p<0.001), 9-HPT -1.2±6.0 s (p<0.001), 5- STS -3.4±7.2 s (p<0.001) and SDMT 1.4±4.8 a.u. (p=0.003). Change on the SSST differed significantly from T25FW (SSST 17.0±19.6% vs. T25FW 11.2±17.1%, p=0.0013). Some 48.6% were found to have a meaningful change on the SSST compared with 25.7% on the T25FW. The response to treatment with SR Fampridine did not correlate with age, sex, Expanded Disability Status Scale and disease duration.

CONCLUSION

SR Fampridine treatment has significant effects on different domains including upper and lower body and cognition. Furthermore, the SSST is more responsive to the effect of SR Fampridine than is the T25FW. ClinicalTrials.gov identifier: NCT01656148.

4-Aminopyridine for symptomatic treatment of multiple sclerosis: a systematic review

This systematic review summarizes the existing evidence on the effect of 4-aminopyridine (4-AP) as a symptomatic treatment of decreased walking capacity in patients with multiple sclerosis (MS) when administered as an immediate release compound and a slow release compound. It summarizes existing evidence on the basic mechanisms of 4-AP from experimental studies and evidence on the clinical use of the compound. A systematic literature search was conducted of the following databases: PubMed and EMBASE. Thirty-five studies were included in the review divided into 16 experimental studies, two clinical studies with paraclinical endpoints and 17 clinical studies with clinical endpoints. Animal studies show that 4-AP can improve impulse conduction through demyelinated lesions. In patients with MS this translates into improved walking speed and muscle strength of the lower extremities in a subset of patients at a level that is often of clinical relevance. Phase III trials demonstrate approximately 25% increase in walking speed in roughly 40% and improved muscle strength in the lower extremities. Furthermore, 4-AP might have an effect on other domains such as cognition, upper extremity function and bowel and bladder, but this warrants further investigation. Side effects are mainly mild to moderate, consisting primarily of paraesthesia, dizziness, nausea/vomiting, falls/balance disorders, insomnia, urinary tract infections and asthenia. Side effects are worse when administered intravenously and when administered as an immediate release compound. Serious adverse events are rarely seen in the marketed clinical dosages. In conclusion, 4-AP is easy and safe to use. Slow release 4-AP shows more robust clinical effects and a more beneficial side-effect profile than immediate release 4-AP.

Introducing treatment strategy for cerebellar ataxia in mutant med mice: combination of acetazolamide and 4-aminopyridine

Purkinje neurons are the sole output neuron of the cerebellar cortex, and they generate high-frequency action potentials. Electrophysiological dysfunction of Purkinje neurons causes cerebellar ataxia. Mutant med mice have the lack of expression of the Scn8a gene. This gene encodes the NaV1.6 protein. In med Purkinje neurons, regular high-frequency firing is slowed, and med mice are ataxic. The aim of this study was to propose the neuroprotective drugs which could be useful for ataxia treatment in med mice, and to investigate the neuroprotective effects of these drugs by simulation. Simulation results showed that Kv4 channel inhibitors and BK channel activators restored the normal electrophysiological properties of the med Purkinje neurons. 4-Aminopyridine (4-AP) and acetazolamide (ACTZ) were proposed as neuroprotective drugs for Kv4 channel inhibitor and BK channel activator, respectively.

Enhancing neural transmission in multiple sclerosis (4-aminopyridine therapy)

Enhancing neural transmission by improving axonal conduction and synaptic neurotransmitter release is a novel strategy to improve symptoms in multiple sclerosis. Dalfampridine (4-aminopyridine extended-release) is a first-in-class medication that targets the damaged nervous system through blockage of voltage-gated potassium channels. Through a series of clinical trials, dalfampridine (dosed at 10 mg twice daily) has been found to improve walking speed by approximately 25 % on average in one third of individuals with multiple sclerosis regardless of disease stage. Furthermore, it significantly improves patients' perception of their ambulatory disability and may improve lower extremity strength. Given the mechanism of action, the most serious adverse effect is its pro-convulsant property, which occurs more frequently at high serum concentrations. The most common adverse events include increased falls, urinary tract infections, dizziness, insomnia, and headaches. Despite these potential side-effects, the vast majority of individuals who derive benefit continue on the treatment. The exact mechanism of action is uncertain, as is the reason for response variability. The medication serves as proof-of-concept that targeting axonal transmission can improve disability in multiple sclerosis.

In vitro and intrathecal siRNA mediated K(V)1.1 knock-down in primary sensory neurons

K(V)1.1 is a Shaker homologue K(+) channel that contributes to the juxta-paranodal membrane conductance in myelinated axons, and is blocked by fampridine (4-aminopyridine), used to treat the symptoms of multiple sclerosis. The present experiments investigate K(V)1.1 function in primary sensory neurons and A-fibres, and help define its characteristics as a drug-target using sequence specific small-interfering RNAs (siRNAs). siRNA (71nM) was used to knock-down functional expression of K(V)1.1 in sensory neurons (>25μm in apparent diameter) in culture, and was also delivered intrathecally in vivo (9.3μg). K(+) channel knock-down in sensory neurons was found to make the voltage-threshold for action potential generation significantly more negative than in control (p=0.02), led to the breakdown of accommodation and promoted spontaneous action potential firing. Exposure to dendrotoxin-K (DTX-K, 10-100nM) also selectively abolished K(+) currents at negative potentials and made voltage-threshold more negative, consistent with K(V)1.1 controlling excitability close to the nominal resting potential of the neuron cell body, near -60mV. Introduction of one working siRNA sequence into the intrathecal space in vivo was associated with a small increase in the amplitude of the depolarising after-potential in sacral spinal roots (p<0.02), suggesting a reduction in the number of working K(+) channels in internodal axon membrane. Our study provides evidence that K(V)1.1 contributes to the control of peripheral sensory nerve excitability, and suggests that its characteristics as a putative drug target can be assessed by siRNA transfection in primary sensory neurons in vitro and in vivo.

Treatment of walking impairment in multiple sclerosis with dalfampridine

Potassium channel blockade has long been considered a potential therapeutic strategy for treatment of multiple sclerosis (MS) based on the pathophysiology of demyelinated axons. Dalfampridine, which is also known as fampridine or 4-aminopyridine (4-AP), is the potassium channel blocker that has been studied most extensively in MS and other demyelinating neurologic disorders. An extended-release formulation of dalfampridine was recently approved by the US Food and Drug Administration to improve walking in patients with MS. In randomized, double-blind, placebo-controlled trials, with dalfampridine extended release tablets 10 mg taken twice daily, about 12 h apart, walking speed was improved in approximately one-third of treated patients; in these patients, average walking speed on therapy was about 25% above baseline. This improvement was clinically meaningful as assessed by concurrent measurement of patient-reported severity of walking-related disability. Dalfampridine extended release tablets were generally well tolerated, with a range of adverse effects that appear to be related to increases in central nervous system excitation. There is a dose-dependent increase in the occurrence of seizures at doses higher than the recommended 10 mg twice daily.

Dalfampridine: a brief review of its mechanism of action and efficacy as a treatment to improve walking in patients with multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) can cause progressive walking impairment that contributes to disability, loss of independence, and reduced quality of life. Dalfampridine (4-aminopyridine), a voltage-dependent potassium channel blocker, has been shown to improve walking in patients with MS, as demonstrated by an increase in walking speed.

OBJECTIVE

To summarize knowledge about the mechanism of action of dalfampridine in the context of clinical evidence of walking improvement in MS patients.

METHODS

Although this was not a systematic review, which is the primary limitation of this study, searches of PubMed were performed using relevant search terms to identify studies that examined the mechanism of action related to MS and its effects in patients with MS in clinical trials.

RESULTS

Voltage-gated potassium channels represent a family of related proteins that span cell membranes, open and close in response to changes in the transmembrane potential, and help regulate ionic potassium currents. Action potential conduction deficits in demyelinated axons result in part from the exposure after demyelination of the paranodal and internodal potassium channels that are distributed in the axonal membrane. This exposure leads to abnormal currents across the axonal membrane that can slow action potential conduction, result in conduction failure, or affect the axon's capacity for repetitive discharge. While dalfampridine is a broad-spectrum blocker of voltage-dependent potassium channels at millimolar concentrations, studies have shown improvement in action potential conduction in demyelinated axons at concentrations as low as 1 μM, and therapeutic plasma concentrations (associated with improved walking) are in the range of 0.25 µM. However, no specific potassium channel subtype has yet been characterized with significant sensitivity to dalfampridine in this range, and the effects of the drug at this low concentration appear to be quite selective. Improved conduction translates into clinical benefit as measured by objectively and subjectively assessed walking relative to placebo. Such improvements were observed in approximately one third of patients treated with an extended-release formulation of dalfampridine in clinical trials. These patients who responded to dalfampridine had an average increase in walking speed of approximately 25%, and greater improvements than nonresponders on a self-reported subjective measure of walking.

CONCLUSIONS

The extended-release formulation of dalfampridine has been shown in clinical trials to improve walking speed in approximately one third of MS patients with ambulatory impairment. The putative mechanism of action of dalfampridine is restoration of action potential conduction via blockade of an as yet uncharacterized subset of potassium channels in demyelinated axons.

Pharmacological interventions for spinal cord injury: where do we stand? How might we step forward?

Despite numerous studies reporting some measures of efficacy in the animal literature, there are currently no effective therapies for the treatment of traumatic spinal cord injuries (SCI) in humans. The purpose of this review is to delineate key pathophysiological processes that contribute to neurological deficits after SCI, as well as to describe examples of pharmacological approaches that are currently being tested in clinical trials, or nearing clinical translation, for the therapeutic management of SCI. In particular, we will describe the mechanistic rationale to promote neuroprotection and/or functional recovery based on theoretical, yet targeted pathological events. Finally, we will consider the clinical relevancy for emerging evidence that pharmacologically targeting mitochondrial dysfunction following injury may hold the greatest potential for increasing tissue sparing and, consequently, the extent of functional recovery following traumatic SCI.

Potassium channel blockers as an effective treatment to restore impulse conduction in injured axons

Most axons in the vertebral central nervous system are myelinated by oligodendrocytes. Myelin protects and insulates neuronal processes, enabling the fast, saltatory conduction unique to myelinated axons. Myelin disruption resulting from trauma and biochemical reaction is a common pathological event in spinal cord injury and chronic neurodegenerative diseases. Myelin damage-induced axonal conduction block is considered to be a significant contributor to the devastating neurological deficits resulting from trauma and illness. Potassium channels are believed to play an important role in axonal conduction failure in spinal cord injury and multiple sclerosis. Myelin damage has been shown to unmask potassium channels, creating aberrant potassium currents that inhibit conduction. Potassium channel blockade reduces this ionic leakage and improves conduction. The present review was mainly focused on the development of this technique of restoring axonal conduction and neurological function of demyelinated axons. The drug 4-aminopyridine has recently shown clinical success in treating multiple sclerosis symptoms. Further translational research has also identified several novel potassium channel blockers that may prove effective in restoring axonal conduction.

Impact of extended-release dalfampridine on walking ability in patients with multiple sclerosis

Dalfampridine extended release (ER) 10 mg is an oral tablet form of the potassium (K(+)) channel-blocking compounded dalfampridine, also known as fampridine, and chemically 4-aminopyridine or 4-AP, which received regulatory approval in the United States for the treatment of walking in patients with multiple sclerosis (MS) in January 2010. Two pivotal Phase 3 clinical trials demonstrated significant improvements in walking in patients with the four primary forms of MS following administration of dalfampridine ER tablets 10 mg twice daily. The drug is thought to act by restoring conduction in focally demyelinated axons and by enhancing neurotransmission, thereby leading to improved neurological function. This review describes how dalfampridine represents a new pharmacotherapeutic approach to the clinical management of mobility impairment. It describes the mechanism of action and chemistry of dalfampridine ER, its pharmacokinetics, tolerability, and side effects, and the outcomes of multicenter trials showing its efficacy in improving walking speed. Clinician and patient global assessments, as well as patient self-assessment of the impact of MS on their gait disability, confirm clinically relevant benefit from the therapy. Patients tolerate the drug well and their improvement in terms of household and community ambulation, inferred from analysis of pooled data from several studies, is likely to translate into benefits in the performance of instrumental activities of daily living and a reduction in the neuropsychiatric burden of disease.

Walking impairment in patients with multiple sclerosis: exercise training as a treatment option

Multiple sclerosis (MS) is a chronic disease of the central nervous system that culminates in the progression of physical and cognitive disability over time. Walking impairment is a ubiquitous feature of MS and a sentinel characteristic of the later or advanced stages of the disease. This paper presents a conceptual rationale along with empirical evidence for exercise training as a rehabilitation approach for managing walking impairment and improving walking function in persons with MS. Conceptually, MS is associated with a decrease in physical activity, which, in turn, can result in deconditioning across multiple domains of physiological functioning. The resulting deconditioning feeds back and further drives physical inactivity until a threshold is reached that likely initiates the progression of walking impairment in MS. Empirically, physical activity and exercise training have been associated with beneficial effects on walking function in persons with MS. This is based on cross-sectional, longitudinal, and experimental research that included diversity in the breadth of measures of walking, persons with MS, and exercise/physical activity characteristics. Of particular importance, future researchers might consider examining the combinatory effects of exercise training plus pharmacological agents on walking mobility in MS. Collectively, exercise training and physical activity might hold significant potential for the management of progressive mobility disability in MS.

Dalfampridine for the treatment of ambulatory impairment in multiple sclerosis

Dalfampridine is a potassium-channel blocker that is approved by the US FDA as 10 mg extended-release tablets to improve walking in patients with multiple sclerosis. Approval is currently pending in Europe. This is the first pharmacological symptomatic treatment approved for multiple sclerosis patients of any type with walking difficulties. Relative to an immediate-release formulation, the extended-release formulation of dalfampridine lowers peak serum concentrations that contribute to toxicity while maintaining a comparable amount of total drug exposure. Several studies show the efficacy and tolerability of dalfampridine. The pivotal published clinical trial demonstrated a treatment–responder rate of 35% compared with an 8% placebo–responder rate (p < 0.0001). The subjects who responded to treatment had an average improvement in their 25-foot walking test time of 25.2% (95% CI: 21.5–28.8) compared with an average improvement of 4.7% (95% CI: 1.0–8.4) in subjects who responded to placebo. Seizures are the most serious adverse effect of dalfampridine with a probable dose-dependent likelihood of occurrence.

[4-Aminopyridine (Fampridine). A new attempt for the symptomatic treatment of multiple sclerosis]

Mobility limitation is a frequent clinical symptom of multiple sclerosis (MS) that poses a therapeutic challenge. For years results of animal experiments and clinical experience have indicated that the potassium channel blocker 4-aminopyridine improves axonal excitatory circuits and thus muscular strength in demyelinating diseases. A recently conducted randomized, placebo-controlled, multicenter phase 3 clinical trial in MS patients was able to show that an oral sustained-release formulation of 4-aminopyridine (Fampridine-SR) represents a suitable agent for treatment of walking disability in MS patients.This overview presents the study data and discusses the value of 4-aminopyridine for the symptomatic treatment of MS as a neurofunctional modifier of this disabling disease.

Sustained-release oral fampridine in multiple sclerosis: a randomised, double-blind, controlled trial

BACKGROUND

Clinical studies suggested that fampridine (4-aminopyridine) improves motor function in people with multiple sclerosis. This phase III study assessed efficacy and safety of oral, sustained-release fampridine in people with ambulatory deficits due to multiple sclerosis.

METHODS

We undertook a randomised, multicentre, double-blind, controlled phase III trial. We randomly assigned 301 patients with any type of multiple sclerosis to 14 weeks of treatment with either fampridine (10 mg twice daily; n=229) or placebo (n=72), using a computer-generated sequence stratified by centre. We used consistent improvement on timed 25-foot walk to define response, with proportion of timed walk responders in each treatment group as the primary outcome. We used the 12-item multiple sclerosis walking scale to validate the clinical significance of the response criterion. Efficacy analyses were based on a modified intention-to-treat population (n=296), which included all patients with any post-treatment efficacy data. The study is registered with ClinicalTrials.gov, number NCT00127530.

FINDINGS

The proportion of timed walk responders was higher in the fampridine group (78/224 or 35%) than in the placebo group (6/72 or 8%; p<0.0001). Improvement in walking speed in fampridine-treated timed walk responders, which was maintained throughout the treatment period, was 25.2% (95% CI 21.5% to 28.8%) and 4.7% (1.0% to 8.4%) in the placebo group. Timed walk responders showed greater improvement in 12-item multiple sclerosis walking scale scores (-6.84, 95% CI -9.65 to -4.02) than timed walk non-responders (0.05, -1.48 to 1.57; p=0.0002). Safety data were consistent with previous studies.

INTERPRETATION

Fampridine improved walking ability in some people with multiple sclerosis. This improvement was associated with a reduction of patients' reported ambulatory disability, and is a clinically meaningful therapeutic benefit.

Fampridine-SR for multiple sclerosis and spinal cord injury

Fampridine-SR is a sustained-release tablet form of the K(+) channel-blocking compound 4-aminopyridine that has been shown to restore conduction in focally demyelinated axons, to enhance synaptic transmission in many types of neurons and to potentiate muscle contraction. The present review describes the mechanism of action and chemistry of Fampridine-SR, its pharmacokinetics and safety, and the outcomes of clinical trials of its safety and efficacy for enhancing neuromuscular function in patients with multiple sclerosis or spinal cord injury. Randomized clinical trials completed to date indicate that this form of K(+) channel blockade may be useful for the improvement of walking ability in patients with multiple sclerosis.

Fampridine-SR in multiple sclerosis: a randomized, double-blind, placebo-controlled, dose-ranging study

Objective To determine the safety of sustained-release 4-aminopyridine in subjects with mutiple sclerosis (MS) and to examine dose-related efficacy up to 40 mg twice daily.

Method Multicenter, randomized, double-blind, placebo-controlled, study. Following a 4-week baseline peroid, subjects were randomly assigned to receive Fampridine-SR (n=25, doses from 10 to 40 mg twice daily, increasing in 5 mg increments weekly) or placebo (n=11). A battery of assessments was performed weekly, including the MS Functional Composite (MSFC), fatigue questionnaires, and lower extremity manual muscle testing.

Results The most common adverse events were dizziness, insomnia, paresthesia, asthenia, nausea, headache, and tremor. Five subjects were discontinued from Fampridine-SR because of adverse events at doses greater than 25 mg, and these included convulsions in two subjects at doses of 30 and 35 mg twice daily. Improvement were seen in lower extremity muscle strength (prospective analysis) and walking speed (post-hoc analysis) in the Fampridine-SR group compared to placebo (unadjusted p-values of 0.01 and 0.03, respectively). There were no significant differences in other MSFC measure or fatigue scores.

Conclusions Future studies should employ doses up to 20 mg twice daily with lower extremity strength and walking speed as potential outcome measures. Multiple Sclerosis 2007; 13: 357-368. http://msj.sagepub.com

Potassium channel blockers in multiple sclerosis: Neuronal Kv channels and effects of symptomatic treatment

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) characterized by demyelination, with a relative sparing of axons. In MS patients, many neurologic signs and symptoms have been attributed to the underlying conduction deficits. The idea that neurologic function might be improved if conduction could be restored in CNS demyelinated axons led to the testing of potassium (K(+)) channel blockers as a symptomatic treatment. To date, only 2 broad-spectrum K(+) channel blockers, 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP), have been tested in MS patients. Although both 4-AP and 3,4-DAP produce clear neurologic benefits, their use has been limited by toxicity. Here we review the current status of basic science and clinical research related to the therapeutic targeting of voltage-gated K(+) channels (K(v)) in MS. By bringing together 3 distinct but interrelated disciplines, we aim to provide perspective on a vast body of work highlighting the lengthy and ongoing process entailed in translating fundamental K(v) channel knowledge into new clinical treatments for patients with MS and other demyelinating diseases. Covered are (1) K(v) channel nomenclature, structure, function, and pharmacology; (2) classic and current experimental morphology and neurophysiology studies of demyelination and conduction deficits; and (3) a comprehensive overview of clinical trials utilizing 4-AP and 3,4-DAP in MS patients.

Phase 2 trial of sustained-release fampridine in chronic spinal cord injury

STUDY DESIGN

Double-blind, randomized, placebo-controlled, parallel-group clinical trial.

OBJECTIVE

Assess safety and efficacy of sustained-release fampridine in subjects with chronic spinal cord injury.

SETTING

A total of 11 academic rehabilitation research centers in the United States.

METHODS

A total of 91 subjects with motor-incomplete spinal cord injury (SCI), randomized to three arms: fampridine, sustained release, 25 mg b.i.d. (Group I), 40 mg b.i.d. (Group II), and placebo (Group III) for 8 weeks.

OUTCOME MEASURES

Patient diary questionnaire, Ashworth score, American Spinal Cord Injury Association International Standards, International Index of Erectile Function, bladder and bowel management questionnaires, and Clinician and Subject Global Impressions (Clinician Global Impression of change, Subject Global Impression (SGI)). Safety was evaluated from adverse events, physical examinations, vital signs, electrocardiograms, and laboratory tests.

RESULTS

In total, 78% of the subjects completed the study. More (13/30) discontinued from Group II than Group I (4/30) and Group III (3/31). The most frequent adverse events across groups were hypertonia, generalized spasm, insomnia, dizziness, asthenia, pain, constipation, and headache. One subject in Group II experienced a seizure. SGI changed significantly in favor of Group I (P=0.02). Subgroup analysis of subjects with baseline Ashworth scores >1 showed significant improvement in spasticity in Group I versus III (P=0.02).

CONCLUSIONS

Group I showed significant improvement in SGI, and potential benefit on spasticity. The drug was well tolerated. Group II showed more adverse events and discontinuations.

Dose responses of three 4-aminopyridine derivatives on axonal conduction in spinal cord trauma

To explore novel treatments for enhancing conduction through traumatically injured spinal cord we have synthesized structurally distinct pyridine based compounds; N-(4-pyridyl) methyl carbamate, N-(4-pyridyl) ethyl carbamate, and N-(4-pyridyl) t-butyl carbamate. With the use of a double sucrose gap-recording chamber we perform a dose-response assay to examine the effects of these compounds on axonal conduction following an in vitro stretch injury. The tested compounds significantly enhanced axonal conduction to the stretch injured cord at 1 microM, a dose that coincides with the clinically relevant dose of potassium channel blocker 4-aminopyridine (4-AP). Methyl carbamate enhanced conduction maximally at 100 microM. This is also the most effective concentration of 4-AP in vitro. The other compounds ethyl carbamate and t-butyl carbamate enhanced conduction maximally at lower concentrations of 10 and 1 microM. At higher concentrations each of these compounds continued to increased CAP amplitude, however not significantly. Additionally, two of the compounds ethyl and t-butyl carbamate appear to have negative effects on CAP amplitude when administered at or beyond 100 microM. These compounds demonstrate the possibility that derivatives of 4-AP can retain the ability to increase axonal conduction in the injured spinal cord.

Potassium channels, memory T cells, and multiple sclerosis

Multiple sclerosis is a chronic inflammatory autoimmune disease of the central nervous system characterized by demyelination and axonal damage that result in disabling neurological deficits. Here the authors explain the rationale for the use of inhibitors of the Kv1.3 K+ channel in immune cells as a therapy for multiple sclerosis and other autoimmune disorders.

The use of 4-aminopyridine (fampridine) in demyelinating disorders

4-Aminopyridine (4-AP or fampridine) is a potassium channel-blocking agent that has been shown to restore conduction in focally demyelinated axons. A sustained-release matrix tablet form of 4-AP (fampridine-SR) is currently undergoing multicenter clinical trials in patients with multiple sclerosis or chronic spinal cord injury. This review describes the pharmacology and mechanisms of action of 4-AP, its pharmacokinetics in human subjects, and the outcomes of clinical trials employing either immediate-release or sustained-release formulations of the drug. The randomized clinical trials that have been completed to date indicate that K+ channel blockade may prove to be a useful strategy for ameliorating central conduction deficits due to demyelination. Diverse neurological gains have been reported for both motor and sensory domains. At the present time, however, the clinical trials have not provided sufficiently robust or definitive evidence of efficacy to gain regulatory approval for the symptomatic management of patients with either multiple sclerosis or spinal cord injury.

Pharmacokinetics of an immediate-release oral formulation of Fampridine (4-aminopyridine) in normal subjects and patients with spinal cord injury

Plasma concentration profiles of the K+ channel-blocking compound Fampridine were obtained from (1) control subjects (n = 6) following oral administration of doses of 10, 15, 20, and 25 mg and (2) patients with spinal cord injury (SCI) (n = 11) following a single oral dose of 10 mg of an immediate-release formulation. Plasma concentrations were determined using a reversed-phase ion-pair high-performance liquid chromatography (HPLC) assay with ultraviolet light detection employing liquid extraction. The drug was rapidly absorbed with a tmax approximately 1 hour for both groups; tmax was independent of dose. Cmax and AUC0-infinity were linearly related to dose, and t 1/2 was 3 to 4 hours for both groups. There were no obvious differences in the (10-mg) plasma concentration profiles between control subjects and SCI patients. The drug was well tolerated, with only mild and transient side effects of light-headedness, dysesthesias, and dizziness.

Opisthotonic posturing with neuromuscular irritability attributable to 4-aminopyridine ingestion by a healthy pediatric patient

INTRODUCTION

4-Aminopyridine (4-AP) is a potassium channel blocker used to increase muscle strength in the treatment of demyelinating diseases such as multiple sclerosis. We describe a case of ingestion by an 8-month-old child that resulted in severe but transient symptoms.

CASE REPORT

An 8-month-old boy was found with greenish saliva, and a capsule with green 4-AP powder was missing. On arrival to an emergency department, he was jittery, tachycardic, and tachypneic. Activated charcoal, a cathartic, and midazolam (0.5 mg/kg) were administered before transfer to a tertiary pediatric hospital. On arrival, the infant remained tachycardic and tachypneic. His eyes deviated upward and he was noted to have 3+ deep tendon reflexes bilaterally. He was administered 0.9% normal saline (20 mL/kg) for a wide pulse pressure with low diastolic blood pressure. The patient developed dramatic opisthotonic posturing and vermiform tongue fasciculations. The symptoms responded well to repeated intravenous doses of benzodiazepines. In this case, we used 2 doses of lorazepam (0.05 mg/kg each). During opisthotonic posturing, an electroencephalogram performed in the intensive care unit revealed no evidence of seizure activity. Within 20 hours after admission, the patient became asymptomatic.

CONCLUSION

This case is, to our knowledge, the first documented pediatric 4-AP ingestion. Clinical signs and symptoms are described as well as the response to therapy with benzodiazepines. The electroencephalogram performed while the patient was symptomatic was negative for seizures.

Aminopyridines for symptomatic treatment in multiple sclerosis

BACKGROUND

Because of their ability to increase nerve conduction in demyelinated nerve fibers, potassium channel blockers 4-aminopyridine (AP) and 3,4-diaminopyridine (DAP) have been proposed as a symptomatic therapy for people with multiple sclerosis (MS).

OBJECTIVES

To determine the efficacy and safety of aminopyridines in improving neurological deficits in people with MS.

SEARCH STRATEGY

Computerised general (MEDLINE, EMBASE) and specialised databases (Cochrane MS Group's trials register, CCTR). Hand search of bibliographic references from retrieved studies and recent MS symposia reports. Contact with principal investigators of known studies.

SELECTION CRITERIA

Trials were included if they fulfilled all following criteria: randomised controlled trials (RCTs); adults with MS, out of exacerbation; AP or DAP treatment versus placebo; clinical endpoints.

DATA COLLECTION AND ANALYSIS

We identified 26 potentially pertinent studies. Three reviewers independently extracted data and assessed trial quality from the 16 studies available as full papers.

MAIN RESULTS

Five studies (six publications) and 144 participants were considered in this review. Two more abstracts are awaiting assessment. All five studies were single-centre, double-blind, crossover trials. Four studies assessed the efficacy of AP versus placebo, one compared DAP with active placebo. The duration of treatment ranged from hours to three months. The median quality score of the studies was 3 (range 2-5). The heterogeneity of outcome assessment and the absence of information on individual study periods, allowed quantitative pooling of results for few categorical variables. Of the 144 treated patients, there were six major side effects: one acute encephalopathy, three episodes of confusion, and two seizures. Manual muscle testing was assessed in three studies (54 patients), with 29 patients (54%) improving in at least one muscular district during study treatment versus four patients (7%) during placebo (odds ratio [OR] 14.5, 95% confidence interval [CI] 4.7-43.7). Ambulation was assessed in three studies (54 patients): 9 patients (17%) improved during study treatment versus none during placebo (p<0.001). An improvement in EDSS score was found in 13 of the 144 participants during study treatment (9%) versus none during placebo (p<0.001). No improvement in neuropsychological tests was found in the two trials that evaluated cognitive function. Finally, 47 of 136 people with MS (35%) felt improved when receiving the study drug, against 7(5%) on placebo (OR 9.7, 95% CI 4.3-22.0).

REVIEWER'S CONCLUSIONS

Based on currently available information, no unbiased statement can be made about the safety or efficacy of aminopyridines for treating MS symptoms. Furthermore, we could not obtain any data on three unpublished RCTs involving more than 300 participants. We conclude that publication bias remains a pervasive problem in this area, and that until the results of these unpublished studies are available to the scientific community, no confident estimate of effectiveness of aminopyridines in the management of MS symptoms is possible.

Absorption characteristics of sustained-release 4-aminopyridine (fampridine SR) in patients with chronic spinal cord injury

Fampridine SR (4-aminopyridine) is a potassium channel-blocking drug currently being investigated for its therapeutic efficacy in ameliorating central conduction deficits due to demyelination in patients with spinal cord injury (SCI). The present open-label pharmacokinetic trial examined the absorption characteristics of a sustained-release form of the drug in 25 SCI subjects with chronic incomplete injuries. The overall group mean Cmax of 27.7 +/- 6.2 ng/mL occurred at a tmax of 3.4 +/- 1.4 hours. AUC0-12 was 210.5 +/- 49.5 ng/mL.h. For paraplegics, AUCtmax was 76.02 +/- 33.28 and for tetraplegics was significantly less at 51.25 +/- 20.36 (p = 0.037). A statistically significant difference in the initial rate and extent of absorption, but not in total 4-AP bioavailability over the 12-hour study period, was evident between tetraplegic patients, 0.60 +/- 0.23, and paraplegic patients, 0.39 +/- 0.14 (p = 0.02). There was a linear correlation (p < 0.05) between the neurological level of injury and Cmax/AUCtmax. These results confirm and extend previous observations of different rates of drug absorption among SCI patients with lesions above and below the sympathetic outflow (T6) and provide evidence of the absorption characteristics of this sustained-release form of 4-aminopyridine, which is helpful for optimal dosing.

Safety and efficacy of 4-aminopyridine in humans with spinal cord injury: a long-term, controlled trial

STUDY OBJECTIVE

To determine the effects of the long-term administration of 4-aminopyridine (4-AP) on sensorimotor function in humans with long-standing spinal cord injury (SCI).

DESIGN

Randomized, open-label, active-treatment control, dosage-blinded study.

SETTING

University-affiliated, tertiary-level care, Department of Veterans Affairs Medical Center.

PATIENTS

Twenty-one healthy men and women outpatients suffering from traumatic SCI (14 tetraplegic, 7 paraplegic) for 2 years or more.

INTERVENTIONS

Dosages of an immediate-release formulation of 4-AP were titrated. At 3 months, 16 subjects were receiving 4-AP 30 mg/day (high dose); 5 subjects were receiving 4-AP 6 mg/day (low dose) and served as an active-treatment control group.

MEASUREMENTS AND MAIN RESULTS

Composite motor and sensory scores had statistically significant increases at 3 months. Maximal expiratory pressure, maximal inspiratory pressure, forced vital capacity, and forced expiratory volume in 1 second showed clinically meaningful and/or statistically significant increases among patients receiving 4-AP 30 mg/day. These subjects also had significant decreases in spasticity (modified Ashworth Scale). Serial biochemical profiles and electroencephalographs were unchanged from baseline, and no clinically significant drug toxicity was encountered.

CONCLUSIONS

Long-term oral administration of immediate-release 4-AP was associated with improvement in and recovery of sensory and motor function, enhanced pulmonary function, and diminished spasticity in patients with long-standing SCI. 4-Aminopyridine appears to be safe and relatively free from toxicity when administered orally over 3 months. Each patient who received immediate-release 4-AP 30 mg/day showed a response in one or more of the outcome measures.

Randomized double-blind crossover trial of fampridine-SR (sustained release 4-aminopyridine) in patients with incomplete spinal cord injury

A randomized double-blind dose-titration crossover trial of the safety and efficacy of oral fampridine-SR (sustained release 4-aminopyridine) was conducted on spinal cord injured (SCI) patients at two centers. Twenty-six patients (n = 26) with incomplete lesions completed the trial. These patients all had chronic (>2 years) and stable neurological deficits. They received fampridine-SR 12.5 and 17.5 mg b.i.d. over a 2-week treatment period, followed by a 1-week washout and 2 weeks of placebo, or vice versa. Patients reported significant benefit of fampridine-SR over placebo on patient satisfaction (McNemar's test, p2 < 0.05) and quality of life scores (p2 < 0.01). Sensory scores (p1 < 0.01), including both pin prick (p1 = 0.059) and light touch (p1 = 0.058), and motor scores (adjusted to reflect only paretic segments) (p1 < 0.01) all yielded evidence of benefit of fampridine-SR over placebo. The Ashworth scale of spasticity was significantly (p2 < 0.05) reduced when patients received fampridine-SR. There were no statistically significant benefits of the drug on measures of pain or bowel, bladder and sexual function, or functional independence. Side effects of lightheadedness and nausea were transient and trivial relative to efficacy, and approximately 30% of patients reported a wish to continue to use fampridine-SR. The clinical benefits most likely derive from the K+ channel blocking action of the drug. Potassium channel blockade enhances axonal conduction across demyelinated internodes and enhances neuroneuronal and neuromuscular transmission in preserved axons. These results provide the first evidence of therapeutic benefit of fampridine-SR in SCI patients.

Effects of 4-aminopyridine on motor evoked potentials in patients with spinal cord injury

The potassium (K+) channel-blocking agent 4-aminopyridine (4-AP) is currently being investigated for its potential therapeutic value in patients with spinal cord injury (SCI). The present study was designed to test the hypothesis that 4-AP ameliorates central motor conduction deficits in individuals with SCI. Oral 4-AP (10 mg) was administered to 19 (n = 19) SCI subjects with stable neurological deficits. Their response to the drug was monitored using motor evoked potentials (MEPs) following transcranial magnetic stimulation of motor cortex and various measures of segmental or peripheral reflex activity (F-waves, H-reflex, and M-response) recorded from lower limb muscles. The mean MEP amplitude in the extensor digitorum brevis muscle (left) was significantly (p < .05) increased from x = .25 +/- .42 mV to x = .59 +/- 1.04 mV at 2 h after drug administration, and the cortical stimulation threshold was reduced (p < .05) by 5.8%. Similar results were obtained in all subjects exhibiting MEPs (n = 13) and in all muscles (n = 6) studied. These changes were maintained at 4 h postdrug. MEP latencies were reduced in all subjects who initially exhibited abnormally prolonged MEP latencies relative to control group (n = 13) values. F-wave, H-reflex, and M-response values (latency and amplitude) were not systematically altered by 4-AP, leading to the conclusion that it was central motor conduction that was enhanced. This interpretation was supported by observed reductions in central motor conduction time (CMCT) in the majority of SCI subjects from whom CMCT measurements were obtained, two of whom anecdotally reported improved motor control after 4-AP, and by increased MEP:M-wave amplitude ratios. The MEP:M-wave ratios indicated that the magnitude of the effect of 4-AP on motoneuron recruitment was not large, in absolute terms (<4% motoneuron pool), but was appreciable relative to the initial level of motoneuron recruitment. These results provide the first statistically significant, objective evidence of improved functioning of the neuromuscular system in chronically injured SCI subjects receiving 4-AP and suggest that the improvements are mediated through enhanced central conduction. The results further support the emerging view that pharmaceutical management of central conduction deficits may prove to be a useful therapeutic strategy for some patients with long-standing SCI.

Differential effects of low and high concentrations of 4-aminopyridine on axonal conduction in normal and injured spinal cord

Blockade of potassium channels with the drug 4-aminopyridine has been shown to effect recovery of action potential conduction in myelinated axons under a variety of pathological conditions, but the mechanism and significance of this phenomenon are not completely understood. This study examined the effects of a range of 4-aminopyridine concentrations on conduction in an experimental model of chronic spinal cord injury in guinea-pigs, using sucrose-gap recording from isolated spinal cord strips. The amplitude of the compound action potential increased in response to bath application of 4-aminopyridine, with a threshold between 0.5 and 1 microM and the peak response between 10 and 100 microns. Conduction was suppressed at concentrations of 1 and 10 mM. Uninjured white matter showed no effect on the compound potential of 4-aminopyridine below 1 mM, but there was a similar suppression at concentrations above 1 mM, accompanied by marked membrane depolarization. Peripheral nerve showed only slight action potential suppression and depolarization in the presence of 10 mM 4-aminopyridine. The sensitivity of injured axons to 1 microM 4-aminopyridine is consistent with the hypothesis that some beneficial effects of the drug seen in patients with spinal cord injury are related to improved conduction in myelinated axons, since cerebrospinal fluid levels of 4-aminopyridine should approach this concentration following clinical systemic doses, although it remains likely that synaptic effects also play a role. The blockade of action potential conduction produced by much higher levels of 4-aminopyridine in vitro is possibly a consequence of interference with the resting potential mechanism of the axon membrane, which appears to differ between central and peripheral nerve fibers.

Atypical presentation of 4-aminopyridine overdose

4-Aminopyridine (4-AP) is an investigational drug for the treatment of neurologic disorders including multiple sclerosis (MS). Until recently, relatively little was known about 4-AP toxicity in overdose; the only recorded cases involved neurologic symptoms ranging from mild parasthesias to tonic-clonic seizures. We report a case of accidental 4-AP overdose that resulted in continuous, dystonic, choreoathetoid-type movements that responded to treatment with standard anticonvulsant dosages of benzodiazepines.

The effects of anticonvulsants on 4-aminopyridine-induced bursting: in vitro studies on rat peripheral nerve and dorsal roots

1. Aminopyridines have been used as beneficial symptomatic treatments in a variety of neurological conditions including multiple sclerosis but have been associated with considerable toxicity in the form of abdominal pain, paraesthesias and (rarely) convulsions. 2. Extracellular and intracellular recording was used to characterize action potentials in rat sciatic nerves and dorsal roots and the effects of 4-aminopyridine (4-AP). 3. In sciatic nerve trunks, 1 mM 4-AP produced pronounced after potentials at room temperature secondary to regenerative firing in affected axons (5-10 spikes per stimulus). At physiological temperatures, after potentials (2-3 spikes) were greatly attenuated in peripheral axons. 4. 4-AP evoked more pronounced and prolonged after discharges in isolated dorsal roots at 37 degrees C (3-5.5 mV and 80-100 ms succeeded by a smaller inhibitory/depolarizing voltage shift) which were used to assess the effects of anticonvulsants. 5. Phenytoin, carbamazepine and lamotrigine dose-dependently reduced the area of 4-AP-induced after potentials at 100 and 320 microM but the amplitude of compound action potentials (evoked at 0.5 Hz) was depressed in parallel. 6. The tonic block of sensory action potentials by all three drugs (at 320 microM) was enhanced by high frequency stimulation (5-500 Hz). 7. The lack of selectivity of these frequency-dependent Na+ channel blockers for burst firing compared to low-frequency spikes, is discussed in contrast to their effects on 4-AP-induced seizures and paroxysmal activity in CNS tissue (which is associated with large and sustained depolarizing plateau potentials). 8. In conclusion, these in vitro results confirm the marked sensitivity of sensory axons to 4-AP (the presumptive basis for paraesthesias). Burst firing was not preferentially impaired at relatively high concentrations suggesting that anticonvulsants will not overcome the toxic peripheral actions of 4-AP in neurological patients.