The impact of uncertainty in hERG binding mechanism on in silico predictions of drug-induced proarrhythmic risk

BACKGROUND AND PURPOSE

Drug-induced reduction of the rapid delayed rectifier potassium current carried by the human Ether-à-go-go-Related Gene (hERG) channel is associated with increased risk of arrhythmias. Recent updates to drug safety regulatory guidelines attempt to capture each drug's hERG binding mechanism by combining in vitro assays with in silico simulations. In this study, we investigate the impact on in silico proarrhythmic risk predictions due to uncertainty in the hERG binding mechanism and physiological hERG current model.

EXPERIMENTAL APPROACH

Possible pharmacological binding models were designed for the hERG channel to account for known and postulated small molecule binding mechanisms. After selecting a subset of plausible binding models for each compound through calibration to available voltage-clamp electrophysiology data, we assessed their effects, and the effects of different physiological models, on proarrhythmic risk predictions.

KEY RESULTS

For some compounds, multiple binding mechanisms can explain the same data produced under the safety testing guidelines, which results in different inferred binding rates. This can result in substantial uncertainty in the predicted torsade risk, which often spans more than one risk category. By comparison, we found that the effect of a different hERG physiological current model on risk classification was subtle.

CONCLUSION AND IMPLICATIONS

The approach developed in this study assesses the impact of uncertainty in hERG binding mechanisms on predictions of drug-induced proarrhythmic risk. For some compounds, these results imply the need for additional binding data to decrease uncertainty in safety-critical applications.

Spreading depression as an innate antiseizure mechanism

Spreading depression (SD) is an intense and prolonged depolarization in the central nervous systems from insect to man. It is implicated in neurological disorders such as migraine and brain injury. Here, using an in vivo mouse model of focal neocortical seizures, we show that SD may be a fundamental defense against seizures. Seizures induced by topical 4-aminopyridine, penicillin or bicuculline, or systemic kainic acid, culminated in SDs at a variable rate. Greater seizure power and area of recruitment predicted SD. Once triggered, SD immediately suppressed the seizure. Optogenetic or KCl-induced SDs had similar antiseizure effect sustained for more than 30 min. Conversely, pharmacologically inhibiting SD occurrence during a focal seizure facilitated seizure generalization. Altogether, our data indicate that seizures trigger SD, which then terminates the seizure and prevents its generalization.

Spontaneous reported cardiotoxicity induced by lopinavir/ritonavir in COVID-19. An alleged past-resolved problem

The antiretroviral drug lopinavir/ritonavir has been recently repurposed for the treatment of COVID-19. Its empirical use has been associated with multiple cardiac adverse reactions pertaining to its ancillary multi-channel blocking properties, vaguely characterized until now. We aimed to characterize qualitatively the cardiotoxicity associated with lopinavir/ritonavir in the setting of COVID-19. Spontaneous notifications of cardiac adverse drug reactions reported to the national Pharmacovigilance Network were collected for 8 weeks since March 1st 2020. The Nice Regional Center of Pharmacovigilance, whose scope of expertise is drug-induced long QT syndrome, analyzed the cases, including the reassessment of all available ECGs. QTc ≥ 500 ms and delta QTc > 60 ms from baseline were deemed serious. Twenty-two cases presented with 28 cardiac adverse reactions associated with the empirical use of lopinavir/ritonavir in a hospital setting. Most adverse reactions reflected lopinavir/ritonavir potency to block voltage-gated potassium channels with 5 ventricular arrhythmias and 17 QTc prolongations. An average QTc augmentation of 97 ± 69 ms was reported. Twelve QTc prolongations were deemed serious. Other cases were likely related to lopinavir/ritonavir potency to block sodium channels: 1 case of bundle branch block and 5 recurrent bradycardias. The incidence of cardiac adverse reactions of lopinavir/ritonavir was estimated between 0.3% and 0.4%. These cardiac adverse drug reactions offer a new insight in its ancillary multi-channel blocking functions. Lopinavir/ritonavir cardiotoxicity may be of concern for its empirical use during the COVID-19 pandemic. Caution should be exerted relative to this risk where lopinavir/ritonavir summary of product characteristics should be implemented accordingly.

Comparison of Two Highly Automated ECG Algorithms for Detection of Drug-Induced Cardiac Ion Channel Block

US Food and Drug Administration (FDA) investigators recently demonstrated in a crossover study that early (J-Tpeak c) and late (Tpeak -Tend ) repolarization duration can differentiate selective potassium block with a high arrhythmia risk from multichannel block with lower risk in subjects receiving dofetilide, verapamil, quinidine, or ranolazine. The purpose of this study was to determine if the findings by the FDA using their published software algorithm could be corroborated using an alternative software algorithm for the same metrics and to determine if methodological differences resulted in clinically meaningful differences in interpretation. Exposure-response relationships computed with linear mixed effects models and mean maximal effects on ECG intervals measured by the two algorithms were similar, corroborating the FDA findings, but with some differences in the modeled slopes and magnitude of changes. The alternative software resulted in an average 25% reduction in the 95% confidence intervals of the mixed effects models with generally lower Akaike Information Criterion values.

The role of KATP channels in cerebral ischemic stroke and diabetes

ATP-sensitive potassium (KATP) channels are ubiquitously expressed on the plasma membrane of cells in multiple organs, including the heart, pancreas and brain. KATP channels play important roles in controlling and regulating cellular functions in response to metabolic state, which are inhibited by ATP and activated by Mg-ADP, allowing the cell to couple cellular metabolic state (ATP/ADP ratio) to electrical activity of the cell membrane. KATP channels mediate insulin secretion in pancreatic islet beta cells, and controlling vascular tone. Under pathophysiological conditions, KATP channels play cytoprotective role in cardiac myocytes and neurons during ischemia and/or hypoxia. KATP channel is a hetero-octameric complex, consisting of four pore-forming Kir6.x and four regulatory sulfonylurea receptor SURx subunits. These subunits are differentially expressed in various cell types, thus determining the sensitivity of the cells to specific channel modifiers. Sulfonylurea class of antidiabetic drugs blocks KATP channels, which are neuroprotective in stroke, can be one of the high stoke risk factors for diabetic patients. In this review, we discussed the potential effects of KATP channel blockers when used under pathological conditions related to diabetics and cerebral ischemic stroke.

[Lead compound optimization strategy(5) – reducing the hERG cardiac toxicity in drug development]

The potassium channel encoded by the human ether-a-go-go related gene(hERG) plays a very important role in the physiological and pathological processes in human. hERG potassium channel determines the outward currents which facilitate the repolarization of the myocardial cells. Some drugs were withdrawn from the market for the serious side effect of long QT interval and arrhythmia due to blockade of hERG channel. The strategies for lead compound optimization are to reduce inhibitory activity of hERG potassium channel and decrease cardiac toxicity. These methods include reduction of lipophilicity and basicity of amines, introduction of hydroxyl and acidic groups, and restricting conformation.

Effect of fampridine on axonal excitability in multiple sclerosis

OBJECTIVE

To investigate the effects of fampridine on nerve excitability, the present study utilized peripheral axonal excitability techniques in 18 MS patients receiving treatment with fampridine.

METHODS

Studies were performed at baseline and repeated 3months after institution of fampridine at standard dosing.

RESULTS

Following treatment with fampridine there were significant changes in axonal excitability for those parameters associated with fast K(+) channels that shifted towards normal control values. Specifically, increases were noted in the peak superexcitability of recovery cycle (fampridine, -25.6±1.6%; baseline -22.8±1.7%; p<0.004), peak depolarizing threshold electrotonus (fampridine, 69.1±1.0%; baseline 67.0±1.4%; p<0.004), and depolarizing threshold electrotonus between 40 and 60ms after onset of depolarization (fampridine, 52.8±1.3%; baseline 49.9±1.4%; p=0.02).

CONCLUSION

The present study has established that fampridine at standard doses exerts effects on peripheral nerve function that may be mediated by reduction of fast K(+) conductances.

SIGNIFICANCE

Modulation of fast K(+) conductances by fampridine may contribute to the improvement observed in MS symptoms including motor fatigue.

Global analysis reveals families of chemical motifs enriched for HERG inhibitors

Promiscuous inhibition of the human ether-à-go-go-related gene (hERG) potassium channel by drugs poses a major risk for life threatening arrhythmia and costly drug withdrawals. Current knowledge of this phenomenon is derived from a limited number of known drugs and tool compounds. However, in a diverse, naïve chemical library, it remains unclear which and to what degree chemical motifs or scaffolds might be enriched for hERG inhibition. Here we report electrophysiology measurements of hERG inhibition and computational analyses of >300,000 diverse small molecules. We identify chemical ‘communities’ with high hERG liability, containing both canonical scaffolds and structurally distinctive molecules. These data enable the development of more effective classifiers to computationally assess hERG risk. The resultant predictive models now accurately classify naïve compound libraries for tendency of hERG inhibition. Together these results provide a more complete reference map of characteristic chemical motifs for hERG liability and advance a systematic approach to rank chemical collections for cardiotoxicity risk.

Immunohistochemistry of cerebellar seizures: mossy fiber afferents play an important role in seizure spread and initiation in the rat

Clinical reports suggest the participation of the cerebellum in epilepsy. Mossy fibers are the main excitatory afferents of the cerebellar cortex; most of them use glutamate and strongly excite granule cells through NMDA and AMPA receptors. The role of the ponto-cerebellar mossy fibers in cerebellar neuronal hyperactivity was investigated in the present study in experimental adult Wistar rats. We detected neuronal hyperactivity through the expression of the glutamate-induced c-fos protein, by means of immunohistochemistry and immunoblotting in the vermis and in the hemispheres. Generalized seizures were induced by means of intraperitoneal 4-aminopyridine injections. Following the 4-aminopyridine seizures, the c-fos expression of cerebellar granule cells was significantly elevated at 1.5h in every lobule. Maximum c-fos expression was seen at 3h. The role of the ponto-cerebellar mossy fiber afferents in the induction of c-fos expression was examined after the transection of the middle cerebellar peduncle on the left side. Immunohistochemical analysis 14 days after the surgery revealed that the synapsin I immunoreactivity was significantly reduced in the cerebellar cortex on the operated side, compared to the sham-operated controls and to the non-operated cerebellar hemisphere of the operated animals, indicating the degeneration of mossy fiber terminals. Transection of the middle cerebellar peduncle suppressed cerebellar c-fos expression in the vermis and in the hemispheres significantly. These findings suggest the strong involvement of the middle cerebellar peduncle and the ponto-cerebellar mossy fibers in the pathophysiology of cerebellar epilepsy.

Natural products as potential human ether-a-go-go-related gene channel inhibitors - outcomes from a screening of widely used herbal medicines and edible plants

Inhibition of the human ether-a-go-go-related gene channel is the single most important risk factor leading to acquired long QT syndrome. Drug-induced QT prolongation can cause severe cardiac complications, including arrhythmia, and is thus a liability in drug development. Considering the importance of the human ether-a-go-go-related gene channel as an antitarget and the daily intake of plant-derived foods and herbal products, surprisingly few natural products have been tested for channel blocking properties. In an assessment of possible human ether-a-go-go-related gene liabilities, a selection of widely used herbal medicines and edible plants (vegetables, fruits, and spices) was screened by means of a functional two-microelectrode voltage-clamp assay with Xenopus oocytes. The human ether-a-go-go-related gene channel blocking activity of selected extracts was investigated with the aid of a high-performance liquid chromatography-based profiling approach, and attributed to tannins and alkaloids. Major European medicinal plants and frequently consumed food plants were found to have a low risk for human ether-a-go-go-related gene toxicity.

[Novel class III antiarrhythmic agent niferidile: review of experimental and clinical studies]

This article reviews experimental and clinical studies of a novel antiarrhythmic agent niferidile. Niferidile, a class III antiarrhythmic agent, blocks potassium outward currents, prolongs repolarization and refractory periods predominantly in atria than in ventricles. Intravenous Niferidile was efficient for interruption of AV-nodal and orthodromic re-entrant tachycardias with rates of 75% to 80%. Niferidile had a conversion rate of up to 87.3% in persistent atrial fibrillation and up to 100% in persistent atrial flutter. Proarrhythmic action of niferidil administration manifested as nonsustained torsade de pointes and monomorphic ventricular tachycardia in 1.2 and 3.7% of cases, respectively. Niferidile can be used for pharmacological cardioversion of persistent atrial fibrillation and flutter as an alternative to electrical cardioversion.

In silico screening of the impact of hERG channel kinetic abnormalities on channel block and susceptibility to acquired long QT syndrome

Accurate diagnosis of predisposition to long QT syndrome is crucial for reducing the risk of cardiac arrhythmias. In recent years, drug-induced provocative tests have proved useful to unmask some latent mutations linked to cardiac arrhythmias. In this study we expanded this concept by developing a prototype for a computational provocative screening test to reveal genetic predisposition to acquired long-QT syndrome (aLQTS). We developed a computational approach to reveal the pharmacological properties of IKr blocking drugs that are most likely to cause aLQTS in the setting of subtle alterations in IKr channel gating that would be expected to result from benign genetic variants. We used the model to predict the most potentially lethal combinations of kinetic anomalies and drug properties. In doing so, we also implicitly predicted ideal inverse therapeutic properties of K channel openers that would be expected to remedy a specific defect. We systematically performed "in silico mutagenesis" by altering discrete kinetic transition rates of the Fink et al. Markov model of human IKr channels, corresponding to activation, inactivation, deactivation and recovery from inactivation of IKr channels. We then screened and identified the properties of IKr blockers that caused acquired long QT and therefore unmasked mutant phenotypes for mild, moderate and severe variants. Mutant IKr channels were incorporated into the O'Hara et al. human ventricular action potential (AP) model and subjected to simulated application of a wide variety of IKr-drug interactions in order to identify the characteristics that selectively exacerbate the AP duration (APD) differences between wild-type and IKr mutated cells. Our results show that drugs with disparate affinities to conformation states of the IKr channel are key to amplify variants underlying susceptibility to acquired long QT syndrome, an effect that is especially pronounced at slow frequencies. Finally, we developed a mathematical formulation of the M54T MiRP1 latent mutation and simulated a provocative test. In this setting, application of dofetilide dramatically amplified the predicted QT interval duration in the M54T hMiRP1 mutation compared to wild-type.

Are users of sulphonylureas at the time of an acute coronary syndrome at risk of poorer outcomes?

AIMS

Adenosine triphosphate sensitive potassium (K(ATP)) channel activity is cardioprotective during ischaemia. One of the purported mechanisms for sulphonylurea adverse effects is through inhibition of these channels. The purpose of this study is to examine whether patients using K(ATP) channel inhibitors at the time of an acute coronary syndrome are at greater risk of death or heart failure (HF) than those not exposed.

METHODS

Using linked administrative databases we identified all adults who had an acute coronary syndrome between April 2002 and October 2006 (n = 21 023).

RESULTS

Within 30 days of acute coronary syndrome, 5.3% of our cohort died and 15.6% were diagnosed with HF. Individuals with diabetes exhibited significantly higher risk of death (adjusted OR: 1.20, 95% CI: 1.03-1.40) and death or HF (aOR: 1.73, 95% CI: 1.59-1.89) than individuals without diabetes. However, there was no significantly increased risk of death (aOR: 1.00, 95% CI: 0.76-1.33) or death/HF (aOR: 1.06, 95% CI: 0.89-1.26) in patients exposed to K(ATP) channel inhibitors versus patients not exposed to K(ATP) channel inhibitors prior to their acute coronary syndrome.

CONCLUSIONS

Diabetes is associated with an increased risk of death or HF within 30 days of an acute coronary syndrome. However, we did not find any excess risk of death or HF associated with use of K(ATP) channel inhibitors at the time of an acute coronary syndrome, raising doubts about the hypothesis that sulphonylureas inhibit the cardioprotective effects of myocardial K(ATP) channels.

Bupropion can close KATP channel and induce insulin secretion

Recently, a case of newborn infant with transient hyperinsulinism has been reported. This infant was reported to be free from typical perinatal risk factors of hyperinsulinism except for the fact that the mother of the baby was receiving the antidepressant bupropion during her pregnancy. However, the mother did not experience hyperinsulinism and, so far, there are no reports about the pharmacological mechanism of bupropion causing hyperinsulinemia. In this study, bupropion was shown to inhibit KATP channel activity in pancreatic β-cell membranes and induce insulin secretion in relatively high concentration. This study shows, for the first time, that bupropion has a direct electrophysiological action on pancreatic β-cells and can cause insulin secretion and also highlights the risk of using bupropion during pregnancy.

Dalfampridine prior authorization program: a cohort study

BACKGROUND

Dalfampridine (Ampyra) is indicated to improve walking in patients with multiple sclerosis (MS) and was found to be effective in 35%-43% of individuals with MS in clinical trials. Dalfampridine may increase seizure risk, particularly in patients with renal impairment. A U.S. managed care expert consensus panel agreed that patient access to dalfampridine is best managed by a prior authorization (PA) in accordance with the FDA-approved labeling. To ensure safe and appropriate dalfampridine use, a health plan developed and implemented a 2-phase point-of-sale PA program.

OBJECTIVES

To evaluate dalfampridine PA review decisions, utilization, and pharmacy expenditures following the implementation of a dalfampridine safety and clinical PA program compared with a group of dalfampridine utilizers unexposed to a PA program.

METHODS

The study utilized retrospective administrative pharmacy claims data from a commercial health plan averaging 1.3 million members per month. The plan implemented a 2-phase dalfampridine safety and effectiveness PA program on August 1, 2010. A comparison group that did not implement the dalfampridine PA program was identified from a commercially insured population with approximately 350,000 members per month. Members in both groups were required to be continuously enrolled from August 1, 2010, through January 31, 2011. A member's earliest paid or rejected claim found from August 1, 2010, through October 31, 2010, was defined as the index claim. Dalfampridine-weighted 30-day supply claims were summed and compared between groups from index date through January 31, 2011. A pharmacy cost avoidance estimate was calculated using the difference in average claims per member from index claim through January 31, 2011, multiplied by dalfampridine wholesale acquisition cost. Overall, dalfampridine utilization was evaluated between the intervention and comparison populations from August 2010 (implementation of PA in intervention group) through December 2011. Linear regression and Poisson models were used to test the trend differences.

RESULTS

The 60 PA-exposed dalfampridine members' average follow-up was 157 days. Phase 1 approval was obtained by 32 (53.3%) members; 4 (6.7%) members received a denial because of renal impairment; 8 (13.3%) members received a denial due to inability to obtain walking time; 1 (1.7%) member with relapse-remitting MS was denied a PA due to no concomitant disease-modifying agent; and 15 (25.0%) members did not initiate the PA process. Phase 2 approval was obtained by 23 (38.3%) of the 60 members. The 60 PA members had a total of 126 claims and an average utilization of 2.1 (SD 1.8) claims per member. The 20 non-PA dalfampridine members' average follow-up was 157 days. The comparison group members had a total of 84 claims and an average utilization of 4.2 (SD 2.0) claims per member. The PA program resulted in an average of 2.1 (P  less than  0.001) fewer claims per member in the PA group. The total dalfampridine cost avoidance estimate was $143,010 or $0.03 per member per month. The overall measure of a monthly claims utilization difference over time was statistically significantly different at P  less than  0.001, using the linear regression slope trend test. The trend line slope was not statistically significantly different, P = 0.841, between the intervention and comparison populations.

CONCLUSIONS

The study indicates that a dalfampridine PA program potentially improved safety and minimized dalfampridine costs. A PA program is effective in selecting appropriate utilizers for initial therapy. Addition of care management may further optimize use by encouraging adherence and tracking patient response.

Dalfampridine: a new agent for symptomatic management of multiple sclerosis

PURPOSE The pharmacology, pharmacokinetics, clinical efficacy, safety, dosage, and place in therapy of dalfampridine are reviewed. SUMMARY Dalfampridine is a novel drug with a unique mechanism for the symptomatic management of multiple sclerosis (MS) among all classifications. Dalfampridine was approved in January 2010 to improve walking for patients with MS. Dalfampridine blocks potassium channels on demyelinated neurons and allows normal electrical conduction, thus improving locomotor difficulty. Dalfampridine is rapidly absorbed after oral administration, reaching its peak plasma concentration in 1.3 hours. Approximately 95.9% of dalfampridine and its metabolites (3-hydroxy-4-aminopyridine and 3- hydroxy-4-aminopyridine sulfate) is excreted in the urine. Dalfampridine is not an inhibitor or inducer of a major cytochrome P-450 isoenzyme; therefore, the potential for drug-drug interactions is minimal. Clinical studies have shown dalfampridine to improve walking speed. The dosage of dalfampridine varied in clinical trials, but the recommended dosage is 10 mg orally twice daily. Dalfampridine is not appropriate for patients with seizures or moderate-to-severe renal impairment. Phase III studies found that extended-release fampridine 10 mg twice daily is well tolerated. The most frequent adverse events reported in dalfampridine clinical trials were insomnia, dizziness, headache, nausea, and weakness. The Food and Drug Administration has required the manufacturer to have a risk evaluation and mitigation strategy for dalfampridine. Ongoing trials will determine the long-term benefit of dalfampridine. CONCLUSION Dalfampridine is a potassium channel blocker that has demonstrated efficacy for improving the symptoms of MS. Several studies have demonstrated increased walking speed in patients, though high doses should be avoided due to the risk of seizures.

[Aminopyridines for symptomatic treatment of multiple sclerosis]

3,4-diaminopyridine (DAP) and 4-aminopyridine (AP) block potassium channels and can improve action potentials in demyelinated nerve fibres. We identified ten randomised placebo-controlled trials investigating AP/DAP as symptomatic treatment in multiple sclerosis. There is evidence that AP and DAP improve muscle strength in the lower extremities and that AP increases walking speed, and it might improve Expanded Disability Status Scale scores, spasticity and fatigue. There is a lack of evidence-based guidelines of treatment and studies investigating the effect on participation/activity and quality of life.

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.

Vernakalant, a mixed sodium and potassium ion channel antagonist that blocks K(v)1.5 channels, for the potential treatment of atrial fibrillation

Despite being the most common arrhythmia currently treated by cardiologists, safe and effective treatments for atrial fibrillation (AF) remain elusive. To address this issue, Astellas Pharma Inc, Merck & Co Inc and Cardiome Pharma Corp are developing vernakalant (RSD-1235), a drug which dose-dependently inhibits sodium channels and several potassium repolarizing currents. Of particular note, vernakalant inhibits I(Kur) (K(v)1.5), a current that is more predominant in atrial than in ventricular tissue. Consistent with this observation, vernakalant produced increases in atrial refractory period with minimal actions on QTc interval or ventricular refractory period in both humans and animals. Intravenous vernakalant terminated recent-onset AF in several animal models, and also in patients with short-duration AF or AF following cardiac surgery enrolled in phase II and III clinical trials. Vernakalant was well tolerated and adverse reactions were transient and mild. Thus, vernakalant holds considerable promise for the treatment of recent-onset AF; however, given its relatively short half-life, continuous dosing may be required in order to maintain sinus rhythm following conversion from AF. The efficacy and safety of vernakalant for the long-term management of AF remains to be determined. Phase III clinical trials with intravenous vernakalant are ongoing, and phase II clinical trials are also being conducted with an oral formulation intended for chronic use.

Non-cardiovascular drugs that inhibit hERG-encoded potassium channels and risk of sudden cardiac death

BACKGROUND

Virtually all QTc-prolonging drugs act by blocking the human ether a go-go-related gene (hERG)-encoded potassium channels (hERG channels), whereas not all QTc-prolonging drugs are associated with an increased risk of serious cardiac arrhythmias. This study assessed whether non-cardiovascular hERG channel blockers are associated with an increased risk of sudden cardiac death (SCD) and whether hERG-channel-inhibiting capacity is an indicator of the risk of SCD.

METHODS AND RESULTS

The risk of SCD was studied in the Integrated Primary Care Information database, a longitudinal general practice research database. A case-control study was performed, matched for age, gender and calendar time. Odds ratios were calculated with conditional logistic regression, multivariably adjusted. In addition, the hERG-channel-inhibiting capacity of the different drugs was compared, defined as the effective free therapeutic plasma concentration (ETCP(unbound)) divided by the concentration that inhibits 50% of the potassium channels (IC50), with the risk of SCD. 1424 cases of SCD and 14 443 controls were identified. Current use of hERG channel blockers was associated with an increased risk of SCD. The risk of SCD was significantly increased in users of antipsychotic drugs. Patients using hERG channel blockers with a high ETCP(unbound)/IC50 ratio (≥ 0.033) had a higher risk of SCD than patients using drugs with a low ETCP(unbound)/IC50 ratio (<0.033).

CONCLUSIONS

The current use of hERG channel blockers was associated with an increased risk of SCD in the general population. In addition, drugs with a high hERG-channel-inhibiting capacity had a higher risk of SCD than drugs with a low hERG-channel-inhibiting capacity.

[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.

An unusual case of 4-aminopyridine toxicity in a multiple sclerosis patient: epileptic disorder or toxic encephalopathy?

We report the case of a 45-year old female multiple sclerosis patient, who accidentally was overdosed with 4-aminopyridine which resulted in dystonic, choreathetoid type abnormal movements in the four limbs, motoric distress, confusion and opisthotonus. There is little known about 4-aminopyridine toxicity. There are only a few reported cases ranging from mild paresthesias to tonic-clonic seizures. 4-aminopyridine enhances neuronal conduction at neuromuscular synapses and is indicated in the treatment of selected neurological disorders including multiple sclerosis (MS) and myasthenia gravis, among others.

The role of ATP-sensitive potassium channel blockers in ischemia-reperfusion-induced renal injury versus their effects on cardiac ischemia reperfusion in rats

In renal ischemia reperfusion (V/R), opening of adenosine-triphosphate (ATP)-sensitive potassium (K(ATP)) channels results in massive influx of neutrophils in both renal and lung tissues. Our study was focused on the role of ATP-dependent potassium channel modulators, glimepiride and glibenclamide on I/R induced renal injury in rats. Additionally we evaluated their effects on normal heart and on ischemic reperfused heart subjected to ischemic preconditioning protection afforded by diazoxide. To test this hypothesis, we used renal I/R and cardiac I/R experiment. Renal ischemia reperfusion induced marked renal dysfunction associated with significant increase in arterial pressure, TNF-alpha levels, superoxide anion production, and myeloperoxidase activity. Treatment with glibenclamide or glimepiride, demonstrated a significant improvement in the reperfusion-induced injury in both kidney and lung. Glimepiride has no effect on superoxide anion production. However glibenclamide induced a significant improvement in these measurements as compared to glimepiride group. Before coronary artery ligation, neither diazoxide nor glimepiride pretreatment influenced significantly the electrocardiographic parameters in comparison with control group. Conversely, glibenclamide supplementation induced a significant elevation in these parameters. After left coronary artery ligation, reperfusion of the ischemic hearts caused a significant elevation in the measured electrocardiographic parameters. These elevations were significantly ameliorated by the pretreatment with diazoxide. In conclusion, the administration of glibenclamide significantly abolished the protective effects of diazoxide, while the pretreatment with glimepiride didn't abolish it. So, glimepiride offers some promise for therapy of renal I/R with minimizing the undesirable cardiac side effects.

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.

Methadone-induced mortality in the treatment of chronic pain: role of QT prolongation

Methadone is increasingly prescribed for chronic pain, yet the associated mortality appears to be rising disproportionately relative to other opioid analgesics. We review the available evidence on methadone-associated mortality, and explore potential pharmacokinetic and pharmacodynamic explanations for its greater apparent lethality. While methadone shares properties of central nervous system and respiratory depression with other opioids, methadone is unique as a potent blocker of the delayed rectifier potassium ion channel (IKr). This results in QT-prolongation and torsade de pointes (TdP) in susceptible individuals. In some individuals with low serum protein binding of methadone, the extent of blockade is roughly comparable to that of sotalol, a potent QT-prolonging drug. Predicting an individual's propensity for methadone-induced TdP is difficult at present given the inherent limitations of the QT interval as a risk-stratifier combined with the multifactorial nature of the arrhythmia. Consensus recommendations have recently been published to mitigate the risk of TdP until further studies better define the arrhythmia risk factors for methadone. Studies are needed to provide insights into the clinical covariates most likely to result in methadone-associated arrhythmia and to assess the feasibility of current risk mitigation strategies.

Tuning out of hERG

A number of drug withdrawals in recent years have been related to cardiovascular toxicity associated with undesirable blockade of the hERG potassium channel. A promiscuous target, hERG has been demonstrated to interact with pharmaceuticals of widely varying structure. Computational and statistical modeling efforts encompassing homology modeling, pharmacophore and quantitative structure-activity relationship models, and also various classification methods, are aimed at defining the molecular features that confer hERG inhibitory activity and understanding the structure-activity relationships that govern hERG-drug interactions. The organization-wide adoption of hERG models is driven by their ability to produce specific and testable structural hypotheses that lead to compounds devoid of hERG liability.

Modulation of spiral wave reentry by K(+) channel blockade

It is well established that spiral wave reentry is the primary mechanism of ventricular tachyarrhythmias (ventricular fibrillation/tachycardia, VF/VT), but information is still limited concerning pharmacological modification of spiral waves by ion channel blockers. In this brief review, the antiarrhythmic and proarrhythmic actions of K(+)-channel blockade (I(Kr) and I (K1)) are discussed in terms of spiral wave dynamics, primarily based on recent experimental findings in ventricular preparations perfused in vitro with the aid of high-resolution optical mapping, as well as their related theoretical studies using computer simulation.

Effects of long-term 4-aminopyridine therapy on glucose tolerance and glucokinetics in patients with spinal cord injury

STUDY OBJECTIVE

To assess the effects of the potassium channel blocker, 4-aminopyridine, on glucose tolerance and glucokinetics in patients with spinal cord injury.

DESIGN

Prospective, dose level-blinded study.

SETTING

University-affiliated, tertiary-level care, Veterans Affairs medical center, and a university-affiliated research center.

PATIENTS

Thirty-one patients with spinal cord injury of more than 1 year's duration.

INTERVENTION

In a fasting state, patients ingested 75 g of glucose and completed a 5-hour oral glucose tolerance test before and after 6 months of treatment with an oral, immediate-release formulation of 4-aminopyridine.

MEASUREMENTS AND MAIN RESULTS

The time course of glucose plasma concentrations during the oral glucose tolerance tests was profiled for each patient, and glucokinetic parameters were estimated. Results were compared at baseline and after 6 months of treatment with 4-aminopyridine. Of the 31 patients, 29 (94%) had impaired glucose tolerance at baseline. After 6 months of treatment with 4-aminopyridine, 12 (41%) of the 29 patients had a 2-hour postprandial glucose level that no longer supported a diagnosis of impaired glucose tolerance. No significant changes or clinically important trends were seen in fasting blood glucose concentrations or in other glucokinetic parameters in these patients.

CONCLUSIONS

The long-term administration of an oral, immediate-release formulation of 4-aminopyridine to patients with longstanding spinal cord injury was associated with readily discernible, potentially clinically significant improvements in glucose tolerance. Because impaired glucose tolerance is a common finding in patients with spinal cord injury, more research, including randomized controlled trials with large study populations, is warranted on this potential treatment.

Gastric emptying effect by 4-aminopyridine in patients with chronic spinal cord injury

BACKGROUND

4-Aminopyridine (4-AP) given to patients with chronic spinal cord injury (SCI) has shown beneficial effects in some somatic and autonomic functions, although patients often develop dyspeptic symptoms. 4-AP is a potassium-channel blocker capable of altering gastro-pyloric functions as demonstrated experimentally. Our objective was to examine the influence of 4-AP treatment on gastric emptying in patients with chronic SCI.

METHODS

Gastric emptying was measured by the acetaminophen absorption test in 18 patients (9 with cervical and 9 with thoracic injury), and 9 healthy volunteers. Patients received increasing oral doses, 5 mg day(-1) of oral 4-AP (5-30 mg day(-1)), for 12 weeks. Patients were studied before and at the end of the last week of 4-AP treatment, whereas healthy volunteers (without 4-AP treatment) were studied only once. Whole blood samples of 2.5 mL were drawn at 0 (before 1 g of oral acetaminophen) and at 15, 30, 45, 60, 75, 90, 105 and 120 min postdose. Acetaminophen concentration in plasma was determined by high-pressure liquid chromatography.

RESULTS

Treatment with 4-AP significantly delayed stomach emptying in patients with chronic SCI, considering the significant decreasing of acetaminophen absorption (t paired test, p <0.05). This effect did not correlate either to the level or ASIA score of the injury (linear regression correlation analysis, r(2) = 0.003 and 0.015, respectively). No significant differences were observed by comparing data of patients before 4-AP treatment with healthy volunteers.

CONCLUSIONS

4-AP intake in patients with chronic spinal cord injury significantly slowed gastric emptying regardless of level and ASIA score of the injury.

Vernakalant: RSD 1235, RSD-1235, RSD1235

Vernakalant is an atrial-selective antiarrhythmic drug discovered by Cardiome Pharma (formerly Nortran Pharmaceuticals). Vernakalant may have potential in the treatment of atrial arrhythmias, including acute atrial fibrillation and atrial flutter. Vernakalant is a mixed sodium/potassium channel blocker and selectively blocks ion channels in the heart that are known to be active during episodes of atrial fibrillation. An IV formulation of vernakalant is awaiting registration in the US for the acute conversion of atrial fibrillation. Also, an oral formulation of the compound is in phase II clinical development as a chronic-use product for the maintenance of normal heart rhythm following termination of atrial fibrillation. Cardiome is seeking co-development partners for intravenous vernakalant in the treatment of atrial arrhythmia, atrial fibrillation and atrial flutter in Europe and Japan. In October 2003, Cardiome Pharma and Fujisawa Healthcare, the US subsidiary of Fujisawa Pharmaceutical Co., Ltd (now Astellas Pharma), executed a $US68 million strategic partnership agreement for the co-development of vernakalant. On 1 April 2005, Fujisawa merged with Yamanouchi to form Astellas Pharma. The partnership grants Astellas Pharma exclusive commercialisation rights for vernakalant. Under the terms of the agreement, Cardiome and Astellas Pharma will co-develop vernakalant as an intravenous formulation for the treatment of atrial fibrillation and atrial flutter for North American markets. Astellas Pharma will be financially responsible for 75% of all future clinical development costs, with Cardiome responsible for the remaining 25% of costs. Astellas Pharma will be responsible for the development plan, NDA application (and NDA re-submission costs) and registration, along with the commercial manufacturing, marketing and sale of vernakalant. Cardiome will manage the phase III trials ACT 1 and ACT 2 and will also be responsible for the continued manufacturing of clinical supplies of vernakalant. Cardiome will receive royalties on end-user sales of vernakalant reflective of Cardiome's 25% share of development costs and other financial considerations. Product rights to the IV formulation of vernakalant for markets outside of North America and world rights to the oral formulation of vernakalant for chronic atrial fibrillation are not included within the scope of this partnership. Cardiome intends to form future additional alliances for these product opportunities or maintain such opportunities for commercialisation on its own. Cardiome and Astellas amended their agreement for vernakalant in relation to the re-submission of the NDA with the US FDA. Under the terms of the new agreement, Astellas agreed to fund 100% of the costs associated with re-submission, including engagement and external consultants. Astellas also agreed to modify the timing of the $US10 million NDA milestone to the date of resubmission. In February 2005, Cardiome Pharma received a $US6 million milestone payment from its co-development partner, Fujisawa Healthcare Inc. This milestone payment was triggered by the successful completion of ACT 1.A pivotal phase II trial demonstrated in September 2002 that vernakalant rapidly and effectively terminated recent onset atrial fibrillation and the study met both primary and secondary study endpoints. Following discussions with the FDA, Cardiome initiated three separate phase III clinical trials in order to enable Cardiome to apply for marketing approval for vernakalant. In August 2003, Cardiome Pharma commenced patient dosing in its first phase III efficacy study of vernakalant for the acute treatment of atrial fibrillation. This initial study, called ACT 1 (Atrial fibrillation Conversion Trial 1), measured the safety and efficacy of vernakalant in 416 patients with atrial arrhythmias. The placebo-controlled study was carried out in 45 centers in the US, Canada and Scandinavia. The ACT 1 study included two substudies of 60 patients with atrial flutter and 119 patients with longer term atrial fibrillation. The primary efficacy endpoint was acute conversion of atrial arrhythmia to normal heart rhythm. Cardiome commenced its second phase III efficacy study in March 2004, known as ACT 2. The ACT 2 study in post-cardiac surgery (coronary artery bypass graft) patients with atrial fibrillation, evaluated the safety and efficacy of vernakalant (IV) in the termination of atrial arrhythmias in patients after cardiac surgery. Around 210 patients from 25 centres in the US, Canada and Europe were enrolled in this study. The primary efficacy endpoint was acute conversion of atrial arrhythmias to normal heart rhythm. The ACT 2 study is ongoing. The third phase III study, known as ACT 3 (Atrial arrhythmia Conversion Trial 3), was initiated by Cardiome Pharma in July 2004. In September 2005, Cardiome and Astellas reported that ACT 3 had been completed, achieving its primary endpoint, with over half of the 170 patients with recent-onset atrial fibrillation (AF) who received vernakalant intravenously converting to normal heart rhythm, compared with only 4% in the placebo group. The study was being conducted by co-development partner Astellas Pharma and measured the safety and efficacy of intravenous vernakalant in recent onset atrial arrhythmia patients. The placebo-controlled study was being carried out in 276 patients in more than 50 centres throughout the world.ACT 4, a phase III safety study evaluating safety of IV vernakalant in approximately 120 AF patients from 30 centres in the US, Canada and Europe, was initiated in October 2005. Results from this trial are expected to supplement trial results from the pivotal ACT 1 and 3 trials. This study is ongoing. Cardiome Pharma successfully completed phase I studies for its controlled-release oral formulation of vernakalant in 2005. The oral, controlled-release formulation of vernakalant is expected to help prevent or slow the recurrence of atrial fibrillation, and will be used as a follow-on therapy to intravenous vernakalant.

Dronedarone: Drondarone, SR 33589, SR 33589B

Dronedarone, a potassium channel antagonist, is chemically related to amio-darone. It is being developed by sanofi-aventis as a class III antiarrhythmic agent for the treatment of atrial fibrillation and atrial flutter in the US and Europe. Dronedarone has a favourable benefit/risk ratio, with the absence of any proarrhythmic effects. Sanofi merged with Synthélabo to form Sanofi-Synthélabo in 1999. In August 2004, Sanofi-Synthelabo merged with Aventis to form sanofi-aventis. The ATHENA trial is a multinational, randomised, double-blind trial evaluating the effects of dronedarone (400mg bid) compared with placebo, over a minimum 12-month follow-up period, in patients with atrial fibrillation or flutter. The trial is investigating the efficacy of dronedarone in preventing cardiovascular hospitalisations or death from any cause. Enrolment was extended to 4300 patients in order to attain the planned rate of adverse events; patient recruitment is ongoing.Previously, sanofi-aventis completed two pivotal phase III trials in atrial fibrillation. The trials, EURIDIS (EURopean trial In atrial fibrillation or flutter patients for the maintenance of Sinus rhythm) and ADONIS (American-Australasian trial with DronedarONe In atrial fibrillation or flutter patients for the maintenance of Sinus rhythm), involved 1237 patients who were in sinus rhythm at the time of randomisation. Results showed dronedarone to have anti-arrhythmic effects and a favourable benefit/risk ratio, with the absence of any proarrhythmic effect.Another trial, ERATO (Efficacy and safety of dronedARone for The cOntrol of ventricular rate), took place in 35 centres across nine European countries assessing dronedarone in 174 patients with permanent atrial fibrillation. Dronedarone was in phase II trials in Japan for the treatment of atrial fibrillation; however, no recent developments have been reported.

Modulatory role of verapamil treatment on the cardiac electrophysiological effects of cisapride

The role of transport proteins in the distribution of drugs in various tissues has obvious implications for drug effects. Recent reports indicate that such transporters are present not only in the liver, intestine, or blood−brain barrier but also in the heart. The objective of our study was to determine whether treatment of animals with verapamil, a well-known L-type calcium channel blocker with modulatory properties of membrane transporters, would alter distribution and cardiac electrophysiological effects of an IKrblocker. Male guinea pigs (n = 72) were treated with either saline or verapamil at various doses (1.5 to 15 mg/kg) and for various durations (1 to 7 d). Animals were sacrified 24 h after the last dose of verapamil (or saline), and their hearts were isolated and retroperfused with cisapride, a gastrokinetic drug with IKrblockade properties. In hearts obtained from animals treated with vehicle, 50 nmol/L cisapride prolonged MAPD90by 15 ± 5 ms vs. 36 ± 8 ms in hearts from animals treated with verapamil 15 mg·kg−1·d−1for 5 d (p < 0.01). Treatment effects were dose- and time-dependent. Cardiac myocytes isolated from animals treated with vehicle or verapamil were incubated for 3 h with 100 ng/mL cisapride. Intracellular concentrations of cisapride in cardiac myocytes from animals treated with verapamil were 1.6-fold higher than those measured in myocytes from animals treated with vehicle (p < 0.01). The increase in intracellular concentrations of cisapride and potentiation of cisapride electrophysiological effects suggest that chronic treatment with drugs such as verapamil may modulate drug effects on the QT interval because of an increased access to intracellular binding sites on IKrchannels.

Isolated perfused and paced guinea pig heart to test for drug-induced changes of the QT interval

INTRODUCTION

One of the biomarkers for assessing the risk of a cardiac adverse event is drug-induced prolongation of the QT interval. A model is needed for evaluating the potential liability of test compounds on QT interval in vitro. Since QT intervals can be generated from paced or spontaneously beating hearts, data so generated can also be used for validating QT(c) correction equations.

METHODS

Isolated guinea pig hearts were perfused in Locke's solution according to the Langendorff method. QT intervals were routinely measured from Lead II ECG waveforms.

RESULTS

Compounds known to inhibit HERG channel, such as dofetilide, prolonged the QT interval in this model. (+/-)Bay K8644, a calcium channel activator, prolonged the QT interval, while verapamil, a calcium channel blocker, shortened it. Procainamide, a sodium channel blocker, also prolonged the QT interval. Many of the compounds, which prolonged the QT interval, also prolonged PR interval, suggesting dual inhibition of the Ikr channel, the rapid component of delayed rectifier potassium channel, and the calcium channel. The QT/RR intervals exhibited a curvilinear relationship, which could be corrected into nearly straight horizontal lines by using correction equations derived from linear, parabolic, and hyperbolic models. However, these correction equations yielded different results on the QT prolongation produced by sotalol, which also slowed down the heart rate. With the data set obtained in this investigation, correction equations derived from linear and parabolic models worked better than the equations derived from the hyperbolic model. The exponential model did not fit at all.

CONCLUSION

QT intervals obtained under paced conditions provide the most direct and reliable QT information for a drug. The isolated perfused and paced guinea pig heart is a convenient model for studying the effect of compounds on QT interval in vitro.

The cardiac hERG/IKr potassium channel as pharmacological target: structure, function, regulation, and clinical applications

Human ether-a-go-go-related gene (hERG) potassium channels conduct the rapid component of the delayed rectifier potassium current, IKr, which is crucial for repolarization of cardiac action potentials. Moderate hERG blockade may produce a beneficial class III antiarrhythmic effect. In contrast, a reduction in hERG currents due to either genetic defects or adverse drug effects can lead to hereditary or acquired long QT syndromes characterized by action potential prolongation, lengthening of the QT interval on the surface ECG, and an increased risk for "torsade de pointes" arrhythmias and sudden death. This undesirable side effect of non-antiarrhythmic compounds has prompted the withdrawal of several blockbuster drugs from the market. Studies on mechanisms of hERG channel inhibition provide significant insights into the molecular factors that determine state-, voltage-, and use-dependency of hERG current block. In addition, crucial properties of the high-affinity drug binding site in hERG and its interaction with drug molecules have been identified, providing the basis for more refined approaches in drug design, safety pharmacology and in silico modeling. Recently, mutations in hERG have been shown to cause current increase and hereditary short QT syndrome with a high risk for life-threatening arrhythmias. Finally, the discovery of adrenergic mechanisms of hERG channel regulation as well as the development of strategies to enhance hERG currents and to modify intracellular hERG protein processing may provide novel antiarrhythmic options in repolarization disorders. In conclusion, the increasing understanding of hERG channel function and molecular mechanisms of hERG current regulation could improve prevention and treatment of hERG-associated cardiac repolarization disorders.

No benefit of 3,4-diaminopyridine in essential tremor: a placebo-controlled crossover study

3,4-Diaminopyridine (3,4-DAP) is a potassium channel blocker that has recently demonstrated an antioscillatory effect in humans by significantly reducing downbeat nystagmus. Based on the presumed role of intrinsic oscillations in the pathophysiology of essential tremor (ET), the authors conducted a double-blind, placebo-controlled crossover study assessing the antitremor effect of a single dose of 3,4-DAP in 19 patients with ET. They did not find any significant change in tremor severity as measured by clinical ratings or accelerometry.

Drug therapy for atrial fibrillation

Although the maintenance of sinus rhythm would be the ideal scenario for patients with atrial fibrillation (AF), recent randomised trials have questioned the value of this approach. A careful interpretation of their results showed the limited efficacy of currently available antiarrhythmic drugs in maintaining sinus rhythm, as well as their potentially serious side effects. Therefore, it is imperative to develop safer and more effective drugs for AF. Based on our improved understanding of the pathophysiology of AF and the mechanism of action of antiarrhythmic drugs, significant efforts are being made to develop new antiarrhythmic agents that would prevent electrophysiological remodelling, would be selective for the atria and, therefore, would not prolong ventricular repolarisation, thus lacking any proarrhythmic effect.

Antidepressants: their effects on cardiac channels, QT prolongation and Torsade de Pointes

The cardiovascular side effects of older antidepressants, such as tricyclic antidepressants, are well established and are known to be linked to their capacity to inhibit cardiac and vascular ion channels. Newer compounds, such as selective serotonin reuptake inhibitors, mirtazapine and venlafaxine, have been reported to have a more benign cardiovascular profile, although they also share antagonistic properties with regard to voltage-dependent ion channels in different tissues. The electrophysiological effects that antidepressants exert on ion channels may affect the cardiac action potential (AP), lengthening both depolarization and repolarization phases, widening the QRS complex, prolonging the QT interval or causing Brugada-like electrocardiogram patterns. Lengthening of the depolarization phase can slow conduction through the His-Purkinje system and myocardium, while slowing repolarization can lead to early after depolarizations and Torsade de Pointes (TdP). In this review, we discuss data from experimental animal models regarding the effects of antidepressants on the cardiac AP, as well as antidepressant-induced QT prolongation in humans and sudden death in patients treated with antidepressants. It appears that although various experimental studies may lead to an understanding of the mechanisms involved in the modulation of cardiac electrical activity, there are significant discrepancies between in vitro data describing the action of antidepressants on the AP, data from clinical trials on QT prolongation by antidepressants and risk of TdP. The role of genetic polymorphisms of potassium-channel-encoding genes in determining the individual risk of cardiac arrhythmias and the limits of QT use as a marker of risk are discussed. Extensive pharmacokinetic and pharmacodynamic studies are required to determine the doses and plasma ranges of each drug that are associated with the greatest risk of arrhythmic complications.

An unusual case of 4-aminopyridine toxicity

4-aminopyridine (4-AP) is an orphan drug in the United States. It enhances neuronal conduction at synapses and is indicated in the treatment of selected neuromuscular disorders, including multiple sclerosis and myasthenia gravis, among others. Its documented toxicity generally has been limited to central nervous system (CNS) hyperexcitation and gastrointestinal upset. In this case, a 56-year-old man accidentally overdosed on an unknown amount of generic 4-AP. This history was unknown by his family and unavailable to initial providers. Approximately 1 h after ingestion, his son found him diaphoretic, vomiting, and having unintelligible speech. In the ensuing 2-3 h, the patient became moderately hypothermic (32.8 degrees C; 91 degrees F), developed atrial fibrillation with a rapid ventricular response, and had neurological changes that were confused with an acute cerebrovascular accident. After a 36-h stay in the intensive care unit that included mechanical ventilation, cardioversion, passive rewarming, and an extensive medical workup, the patient recovered without sequelae. After extubation he stated that he thought he may have ingested too much 4-AP after rubbing a large amount of it against a sore tooth to take advantage of its local analgesic properties. This case of 4-AP overdose resulting in atrial fibrillation with rapid ventricular response, hypothermia, and acute neurological changes mistaken for an acute cerebrovascular accident is an unusual one. This case shows that overdose of 4-AP can cause or mimic several serious medical conditions, and that a detailed history and physical examination are essential for uncovering unusual diagnoses.

Evidences of the gender-related differences in cardiac repolarization and the underlying mechanisms in different animal species and human

Clinical and experimental studies have shown that gender differences exist in cardiac repolarization in various animal species and human, as is evidenced by significantly longer QT, JT intervals and action potential duration in females than in males due to a reduced repolarization reserve in females. The latter is shown by the relatively greater increase in ventricular repolarization and higher incidence of torsades de pointes (TdP) in preparations from females by drugs blocking repolarizing K(+) currents. These results can be modulated by gonadectomy, suggesting that gonadal steroids are important determinants of gender difference in repolarization. In human subjects, QT and JT intervals are longer in women, whereas QT dispersion and Tp-e interval (the interval from the peak to the end of T wave) are longer in men. At slow heart rates greater prolongation in QT and increase in transmural repolarization heterogeneity (i.e. increase in Tp-e) may predispose to TdP tachycardias in women. In healthy postmenopausal women, hormone replacement therapy with estrogen alone usually produced a prolongation of QT interval, while estrogen plus progesterone had no significant effects on QT interval but reduced QT dispersion. Along with these, there are still conflicting data reported. Further work is needed before the elucidation of the basis of gender differences in ventricular repolarization.

Structural determinants of potassium channel blockade and drug-induced arrhythmias

Cardiac K+ channels play an important role in the regulation of the shape and duration of the action potential. They have been recognized as targets for the actions of neurotransmitters, hormones, and anti-arrhythmic drugs that prolong the action potential duration (APD) and increase refractoriness. However, pharmacological therapy, often for the purpose of treating syndromes unrelated to cardiac disease, can also increase the vul- nerability of some patients to life-threatening rhythm disturbances. This may be due to an underlying propensity stemming from inherited mutations or polymorphisms, or structural abnormalities that provide a substrate allowing for the initiation of arrhythmic triggers. A number of pharmacological agents that have proved useful in the treatment of allergic reactions, gastrointestinal disorders, and psychotic disorders, among others, have been shown to reduce repolarizing K+ currents and prolong the Q-T interval on the electrocardiogram. Understanding the structural determinants of K+ channel blockade might provide new insights into the mechanism and rate-dependent effects of drugs on cellular physiology. Drug-induced disruption of cellular repolarization underlies electrocardiographic abnormalities that are diagnostic indicators of arrhythmia susceptibility.

Estimation of potency of HERG channel blockers: impact of voltage protocol and temperature

INTRODUCTION

The HERG channel is widely used for the assessment of proarrhythmic risk for new drugs. HERG channel blockers obstruct channel functions through various mechanisms, which usually show time dependence, voltage dependence, and state dependence. The voltage protocol and temperature may affect the estimation of drug potency, but limited information is available in this regard. The purpose of this study was to evaluate the influence of voltage protocol and temperature on predicting the potency of HERG channel blockers, and to determine electrophysiological approaches for new drugs screening studies.

METHOD

Whole-cell patch-clamp electrophysiology was carried out by utilizing different voltage step protocols to examine the potency of compounds known to preferentially block the channel in the closed (ketoconazole and BeKm-1), open, and/or inactivated states (E-4031, astemizole, and terfenadine) in HEK293 cells transfected with HERG cDNA at room temperature and near-physiological temperature.

RESULTS

Drug potency determined using different voltage protocols varied dependent on the mechanisms of drug actions. For most compounds, the IC(50) values obtained with a long pulse step protocol at room temperature were close to those determined with the voltage protocols designed to disclose their intrinsic potency. Relative to room temperature, the potency of E-4031, terfenadine, and ketoconazole was not changed at approximately 35 degrees C, but potency of astemizole was reduced.

DISCUSSION

The long pulse step protocol with room temperature can be selected for HERG channel safety screening studies. Alternative voltage protocols or temperatures should be considered if HERG study results are not consistent with other cardiac safety assessments.

A new method to calculate the beat-to-beat instability of QT duration in drug-induced long QT in anesthetized dogs

INTRODUCTION

Instability of QT duration is a marker to predict Torsade de Pointes (TdP) associated with both congenital and drug-induced long QT syndrome. We describe a new method for the quantification of instability of repolarization.

METHODS

Female, adult beagle dogs anesthetized with a potent morphinomimetic were treated with either solvent (n=7) or dofetilide (n=7). Poincaré plots with QT(n) versus QT(n+1) were constructed to visualize the beat-to-beat variation in QT intervals from the lead II ECG. Short-term instability (STI), long-term instability (LTI) and total instability (TI) were quantified by calculating the distances of 30 consecutive data-points from the x and y-coordinate to the "centre of gravity" of the data cluster. Dofetilide at 0.0025 to 0.04 mg/kg i.v. (plasma concentrations of 4+/-0.6 to 41+/-2.7 ng/ml), dose-dependently prolonged QT and QTcV (at 0.04 mg/kg i.v.: QT: 280+/-ms versus 236+/-5 ms with solvent; p<0.05 and QTcV: 290+/-9 ms versus 252+/-4 ms with solvent; p<0.05). Concomitantly, the compound induced an increase in the instability parameters in a similar dose-dependent manner (at 0.04 mg/kg i.v.: TI: 6.8+/-0.9 ms versus 1.7+/-0.3 ms; p<0.05, LTI: 3.6+/-0.5 ms versus 1.0+/-0.2 ms; p<0.05 and STI: 4.2+/-0.6 ms versus 1.0+/-0.2 ms; p<0.05). The increases induced by dofetilide were associated with a high incidence of early afterdepolarizations (EADs) in the endocardial monophasic action potential (in 6 out of the 7 compound-treated animals versus 0 out of the 7 solvent animals; p<0.05).

CONCLUSION

Quantification of beat-to-beat QT instability by our method clearly detects changes in short-term, long-term and total instability induced by dofetilide, already at pre-arrhythmic doses. Dofetilide administration to anesthetized dogs prolongs ventricular repolarization, concomitantly increases beat-to-beat QT instability and induces early after depolarizations (EADs). As such, the use of these parameters in this in vivo model shows clear potential for risk identification in cardiovascular safety assessment.