Thorough QT/QTc not so thorough: removes torsadogenic predictors from the T-wave, incriminates safe drugs, and misses profibrillatory drugs

Characteristics of electromechanical window in anesthetized rabbit models of short QT and long QT syndromes

The current regulatory guidelines recommend the use of QT interval to assess the risk of arrhythmogenic potential of new chemical entities. Recently, the electromechanical window (EMW), the difference in duration between electrical and mechanical systole, has been proposed as markers for drug-induced torsades de pointes (TdP); however, data of EMW in short QT model are not available. This study aimed to characterize the EMW as a marker for drug-induced ventricular arrhythmias in anesthetized rabbit model of long QT syndrome type 2 (LQT2) and short QT syndrome (SQTS) infused with reference compounds known to lengthen or shorten QT intervals. After rabbits were anesthetized with isoflurane, body surface electrocardiograms and left ventricular pressure were recorded. The LQT2 was produced by intravenous infusion with dofetilide (n = 6), quinidine (n = 6) and sotalol (n = 6) whereas the SQTS was induced by intravenous escalating concentrations of nicorandil (n = 7), pinacidil (n = 5) and cromakalim (n = 5). The EMW in anesthetized rabbits ranged from 1.3 to 53.3 msec. All three drugs known to lengthen QT intervals prolonged QT and QTcF interval while the EMW was markedly decreased to negative values. Pinacidil significantly produced QT and QTcF shortening and significantly abbreviated the EMW (p < 0.05). This study demonstrated that the EMW is associated with QT intervals (p < 0.001). It is negative in the presence of QT-prolonging drugs while it is more positive in the presence of QT-shortening drugs. The results suggest that the EMW in anesthetized rabbits can be used in drug safety evaluation in addition to the QT interval.

Have the Findings from Clinical Risk Prediction and Trials Any Key Messages for Safety Pharmacology?

Anti-arrhythmic drugs are a mainstay in the management of symptoms related to arrhythmias, and are adjuncts in prevention and treatment of life-threatening ventricular arrhythmias. However, they also have the potential for pro-arrhythmia and thus the prediction of arrhythmia predisposition and drug response are critical issues. Clinical trials are the latter stages in the safety testing and efficacy process prior to market release, and as such serve as a critical safeguard. In this review, we look at some of the lessons to be learned from approaches to arrhythmia prediction in patients, clinical trials of drugs used in the treatment of arrhythmias, and the implications for the design of pre-clinical safety pharmacology testing.

Automated T-wave analysis can differentiate acquired QT prolongation from congenital long QT syndrome

BACKGROUND

Prolongation of the QT on the surface electrocardiogram can be due to either genetic or acquired causes. Distinguishing congenital long QT syndrome (LQTS) from acquired QT prolongation has important prognostic and management implications. We aimed to investigate if quantitative T-wave analysis could provide a tool for the physician to differentiate between congenital and acquired QT prolongation.

METHODS

Patients were identified through an institution-wide computer-based QT screening system which alerts the physician if the QTc ≥ 500 ms. ECGs were retrospectively analyzed with an automated T-wave analysis program. Congenital LQTS was compared in a 1:3 ratio to those with an identified acquired etiology for QT prolongation (electrolyte abnormality and/or prescription of known QT prolongation medications). Linear discriminant analysis was performed using 10-fold cross-validation to statistically test the selected features.

RESULTS

The 12-lead ECG of 38 patients with congenital LQTS and 114 patients with drug-induced and/or electrolyte-mediated QT prolongation were analyzed. In lead V₅ , patients with acquired QT prolongation had a shallower T wave right slope (-2,322 vs. -3,593 mV/s), greater T-peak-Tend interval (109 vs. 92 ms), and smaller T wave center of gravity on the x axis (290 ms vs. 310 ms; p < .001). These features could distinguish congenital from acquired causes in 77% of cases (sensitivity 90%, specificity 58%).

CONCLUSION

T-wave morphological analysis on lead V₅ of the surface ECG could successfully differentiate congenital from acquired causes of QT prolongation.

No Apparent Cardiac Conduction Effects of Acute Treatment with Risperidone in Children with Autism Spectrum Disorder

OBJECTIVES

Risperidone is approved for the treatment of serious behavioral problems in children with autism spectrum disorder (ASD). This study examined the effects of risperidone on cardiac conduction in children with ASD.

METHODS

Data were collected from an 8-week, five-site trial conducted by the Research Units on Pediatric Psychopharmacology Autism Network. Children (age 5-17 years) were randomly assigned to risperidone (n = 49) or placebo (n = 52) under double-blind conditions. Risperidone was superior to placebo in reducing serious behavioral problems. A standard 12-lead, electrocardiogram (ECG) was obtained in most subjects at screening and week 8. A pediatric electrophysiologist blind to treatment assignment reviewed all available ECGs for readability, abnormalities, and cardiac conduction parameters, including QTc. The electrophysiologist measurements were compared to machine readings. A second blinded electrophysiologist examined all available ECGs for abnormalities and a 20% random sample for QTc.

RESULTS

Of the 101 randomized subjects in the trial, complete pretreatment and week 8 data were available on 65 subjects (placebo n = 30; risperidone n = 35). The electrophysiologist did not identify any cardiac conduction adverse effects of risperidone and there was no difference in mean change on the QTc compared to placebo. The Bland-Altman plot showed a systematic bias in QTc measurements by the electrophysiologist and machine. Machine readings produced higher values than the electrophysiologist for shorter QTc intervals and machine scoring was lower than electrophysiologist readings for longer QTc values (p = 0.001). Two electrophysiologists had overall percent agreements of 82.9% (95% CI: 76.3 to 89.6) on qualitative assessment and 88.6% (95% CI: 79.3 to 98.0) on QTc interval.

CONCLUSION

Using conventional doses during acute treatment in children with ASD and serious behavioral problems, there was no difference in the mean change in QTc between risperidone and placebo. Compared to the electrophysiologist, the machine readings may miss elevated QTc measurements.

Electrocardiographic Predictors of Torsadogenic Risk During Dofetilide or Sotalol Initiation: Utility of a Novel T Wave Analysis Program

INTRODUCTION

Initiation of class III anti-arrhythmic medications requires telemetric monitoring for ventricular arrhythmias and QT prolongation to reduce the risk of torsades de pointes (TdP). Heart rate-corrected QT interval (QTc) is an indicator of risk, however it is imperfect, and subtle abnormalities of repolarization have been linked with arrhythmogenesis.

PURPOSE

Identification of electrocardiographic predictors of torsadogenic risk through the application of a novel T wave analysis tool.

METHODS

Among all patients admitted to Mayo Clinic for initiation of dofetilide or sotalol, we identified 13 cases who developed drug-induced TdP and 26 age and sex matched controls that did not develop TdP. The immediate pre-TdP ECG of those with TdP was compared to the last ECG performed prior to hospital discharge in controls using a novel T wave program that quantified subtle changes in T wave morphology.

RESULTS

The QTc and 12 T wave parameters successfully distinguished TdP cases from controls. The top performing parameters were the QTc in lead V3 (mean case vs control 480 vs 420 msec, p < 0.001, r = 0.72) and T wave right slope in lead I (mean case vs control -840.29 vs -1668.71 mV/s, p = 0.002, r = 0.45). The addition of T wave right slope to QTc improved prediction accuracy from 79 to 88 %.

CONCLUSION

Our data demonstrate that, in addition to QTc, the T wave right slope is correlated strongly with TdP risk. This suggests that a computer-based repolarization measurement tool that integrates additional data beyond the QTc may identify patients with the greatest torsadogenic potential.

A computational model predicts adjunctive pharmacotherapy for cardiac safety via selective inhibition of the late cardiac Na current

Background

The QT interval is a phase of the cardiac cycle that corresponds to action potential duration (APD) including cellular repolarization (T-wave). In both clinical and experimental settings, prolongation of the QT interval of the electrocardiogram (ECG) and related proarrhythmia have been so strongly associated that a prolonged QT interval is largely accepted as surrogate marker for proarrhythmia. Accordingly, drugs that prolong the QT interval are not considered for further preclinical development resulting in removal of many promising drugs from development. While reduction of drug interactions with hERG is an important goal, there are promising means to mitigate hERG block. Here, we examine one possibility and test the hypothesis that selective inhibition of the cardiac late Na current (I_NaL) by the novel compound GS-458967 can suppress proarrhythmic markers.

Methods and results

New experimental data has been used to calibrate I_NaL in the Soltis-Saucerman computationally based model of the rabbit ventricular action potential to study effects of GS-458967 on I_NaL during the rabbit ventricular AP. We have also carried out systematic in silico tests to determine if targeted block of I_NaL would suppress proarrhythmia markers in ventricular myocytes described by TRIaD: Triangulation, Reverse use dependence, beat-to-beat Instability of action potential duration, and temporal and spatial action potential duration Dispersion.

Conclusions

Our computer modeling approach based on experimental data, yields results that suggest that selective inhibition of I_NaL modifies all TRIaD related parameters arising from acquired Long-QT Syndrome, and thereby reduced arrhythmia risk. This study reveals the potential for adjunctive pharmacotherapy via targeted block of I_NaL to mitigate proarrhythmia risk for drugs with significant but unintended off-target hERG blocking effects.

Comparison of electrophysiological effects of calcium channel blockers on cardiac repolarization

Dihydropyridine (DHP) calcium channel blockers (CCBs) have been widely used to treat of several cardiovascular diseases. An excessive shortening of action potential duration (APD) due to the reduction of Ca²⁺ channel current (I_Ca) might increase the risk of arrhythmia. In this study we investigated the electrophysiological effects of nicardipine (NIC), isradipine (ISR), and amlodipine (AML) on the cardiac APD in rabbit Purkinje fibers, voltage-gated K⁺ channel currents (I_Kr, I_Ks) and voltage-gated Na⁺ channel current (I_Na). The concentration-dependent inhibition of Ca²⁺ channel currents (I_Ca) was examined in rat cardiomyocytes; these CCBs have similar potency on I_Ca channel blocking with IC₅₀ (the half-maximum inhibiting concentration) values of 0.142, 0.229, and 0.227 nM on NIC, ISR, and AML, respectively. However, ISR shortened both APD₅₀ and APD₉₀ already at 1 µM whereas NIC and AML shortened APD₅₀ but not APD₉₀ up to 30 µM. According to ion channel studies, NIC and AML concentration-dependently inhibited I_Kr and I_Ks while ISR had only partial inhibitory effects (<50% at 30 µM). Inhibition of I_Na was similarly observed in the three CCBs. Since the I_Kr and I_Ks mainly contribute to cardiac repolarization, their inhibition by NIC and AML could compensate for the AP shortening effects due to the block of I_Ca.

A computational modelling approach combined with cellular electrophysiology data provides insights into the therapeutic benefit of targeting the late Na+ current

KEY POINTS

The ventricular action potential plateau is a phase of high resistance, which makes ventricular myocytes vulnerable to small electrical perturbations. We developed a computationally based model of GS-458967 interaction with the cardiac Na+ channel, informed by experimental data recorded from guinea pig isolated single ventricular myocytes. The model predicts that the therapeutic potential of GS-458967 derives largely from the designed property of significant potent selectivity for INaL.

ABSTRACT

Selective inhibition of the slowly inactivating or late Na(+) current (INaL) in patients with inherited or acquired arrhythmia syndrome may confer therapeutic benefit by reducing the incidence of triggers for arrhythmia and suppressing one component of arrhythmia-promoting cardiac substrates (e.g. prolonged refractoriness and spatiotemporal dispersion of action potential duration). Recently, a novel compound that preferentially and potently reduces INaL, GS-458967 (IC50 for block of INaL = 130 nM) has been studied. Experimental measurements of the effects of GS-458967 on endogenous INaL in guinea pig ventricular myocytes demonstrate a robust concentration-dependent reduction in action potential duration (APD). Using experimental data to calibrate INaL and the rapidly activating delayed rectifier K(+) current, IKr, in the Faber-Rudy computationally based model of the guinea pig ventricular action potential, we simulated effects of GS-458967 on guinea pig ventricular APD. GS-458967 (0.1 μM) caused a 28.67% block of INaL and 12.57% APD shortening in experiments, while the model predicted 10.06% APD shortening with 29.33% block of INaL. An additional effect of INaL block is to reduce the time during which the membrane potential is in a high resistance state (i.e. the action potential plateau). To test the hypothesis that targeted block of INaL would make ventricular myocytes less susceptible to small electrical perturbations, we used the computational model to test the degree of APD prolongation induced by small electrical perturbations in normal cells and in cells with simulated long QT syndrome. The model predicted a substantial dose-dependent reduction in sensitivity to small electrical perturbations as evidenced by action potential duration at 90% repolarization variability in the presence of GS-458967-induced INaL block. This effect was especially potent in the 'disease setting' of inherited long QT syndrome. Using a combined experimental and theoretical approach, our results suggest that INaL block is a potent therapeutic strategy. This is because reduction of INaL stabilizes the action potential waveform by reducing depolarizing current during the plateau phase of the action potential. This reduces the most vulnerable phase of the action potential with high membrane resistance. In summary, by reducing the sensitivity of the myocardial substrate to small electrical perturbations that promote arrhythmia triggers, agents such as GS-458967 may constitute an effective antiarrhythmic pharmacological strategy.

Predictive Power of f99 Repolarization Index for the Occurrence of Ventricular Arrhythmias

BACKGROUND

Defects of cardiac repolarization, noninvasively identifiable by analyzing the electrocardiographic (ECG) ST segment and T wave, are among the major causes of sudden cardiac death. Still, no repolarization-based index has so far shown sufficient sensitivity and specificity to justify preventive treatments. Thus, the aim of this work was to evaluate the predictive power of our recently proposed f99 index for the occurrence of ventricular arrhythmias.

METHODS

Our study populations included 170 patients with implanted cardiac defibrillator (ICD), 44 of which developed ventricular tachycardia and/or fibrillation during the 4-year follow-up (ICD_Cases) and 126 did not (ICD_Controls). The f99 index, defined as the frequency at which the repolarization normalized cumulative energy reaches 99%, was computed in each of the 15 (I to III, aVl, aVr, aVf, V1 -V6 , X, Y, Z) available ECG leads independently, and then maximized over the 6 precordial leads (f99_MaxV1 -V6 ), 12 standard leads (f99_Max12STD) and three orthogonal leads (f99_MaxXYZ) to avoid dispersion-related issues. Each index predictive power was quantified as the area under the receiving operating characteristic curve (AUC).

RESULTS

Median f99_MaxV1 -V6 , f99_Max12STD and f99_MaxXYZ values were significantly higher in the ICD_Cases than in the ICD_Controls (48 Hz vs. 35 Hz, P<0.05; 51 Hz vs. 43 Hz, P<0.05; 45 Hz vs. 31 Hz, P<10(-3) ; respectively), indicating a more fragmented repolarization in the former group. The AUC values were 0.62, 0.63 and 0.68, respectively.

CONCLUSIONS

The f99 represents a promising risk index for the occurrence of ventricular arrhythmias, especially when maximized over the three orthogonal leads.

A phenotypic in vitro model for the main determinants of human whole heart function

This article details the construction and testing of a phenotypic assay system that models in vivo cardiac function in a parallel in vitro environment with human stem cell derived cardiomyocytes. The major determinants of human whole-heart function were experimentally modeled by integrating separate 2D cellular systems with BioMicroelectromechanical Systems (BioMEMS) constructs. The model features a serum-free defined medium to enable both acute and chronic evaluation of drugs and toxins. The integration of data from both systems produced biologically relevant predictions of cardiac function in response to varying concentrations of selected drugs. Sotalol, norepinephrine and verapamil were shown to affect the measured parameters according to their specific mechanism of action, in agreement with clinical data. This system is applicable for cardiac side effect assessment, general toxicology, efficacy studies, and evaluation of in vitro cellular disease models in body-on-a-chip systems.

In silico assessment of drug safety in human heart applied to late sodium current blockers

Drug-induced action potential (AP) prolongation leading to Torsade de Pointes is a major concern for the development of anti-arrhythmic drugs. Nevertheless the development of improved anti-arrhythmic agents, some of which may block different channels, remains an important opportunity. Partial block of the late sodium current (I(NaL)) has emerged as a novel anti-arrhythmic mechanism. It can be effective in the settings of free radical challenge or hypoxia. In addition, this approach can attenuate pro-arrhythmic effects of blocking the rapid delayed rectifying K(+) current (I(Kr)). The main goal of our computational work was to develop an in-silico tool for preclinical anti-arrhythmic drug safety assessment, by illustrating the impact of I(Kr)/I(NaL) ratio of steady-state block of drug candidates on "torsadogenic" biomarkers. The O'Hara et al. AP model for human ventricular myocytes was used. Biomarkers for arrhythmic risk, i.e., AP duration, triangulation, reverse rate-dependence, transmural dispersion of repolarization and electrocardiogram QT intervals, were calculated using single myocyte and one-dimensional strand simulations. Predetermined amounts of block of I(NaL) and I(Kr) were evaluated. "Safety plots" were developed to illustrate the value of the specific biomarker for selected combinations of IC(50)s for I(Kr) and I(NaL) of potential drugs. The reference biomarkers at baseline changed depending on the "drug" specificity for these two ion channel targets. Ranolazine and GS967 (a novel potent inhibitor of I(NaL)) yielded a biomarker data set that is considered safe by standard regulatory criteria. This novel in-silico approach is useful for evaluating pro-arrhythmic potential of drugs and drug candidates in the human ventricle.

Looking for virtuous promiscuity: electrocardiographic evidence of multichannel drug block

The finding of QTc prolongation often sounds the death knell for a new molecule, but investigators have long suspected that QTc prolongation alone may be an indifferent predictor of risk. Premature or inappropriate rejection of promising molecules deprives clinicians of new therapies and depletes industry resources. Could it be that the conventional electrocardiogram contains information that might prevent us from relegating "virtuous" compounds to a fate they do not deserve?

Drug-induced long QT syndrome and fatal arrhythmias in the intensive care unit

Long QT syndrome (LQTS) is a genetic or acquired condition characterised by a prolonged QT interval on the surface electrocardiogram (ECG) and is associated with a high risk of sudden cardiac death because of polymorph ventricular tachyarrhythmia called Torsade de Pointes arrhythmia. Drug-induced LQTS can occur as a side effect of commonly used cardiac and non-cardiac drugs in predisposed patients, often with baseline QT prolongation lengthened by medication and/or electrolyte disturbances. Hospitalised patients often have several risk factors for proarrhythmic response, such as advanced age and structural heart disease. Patients in the intensive care unit (ICU) are particularly prone to develop drug induced LQTS because they receive several different intravenous medications. Additionally, they might have impaired drug elimination because of reduced kidney and/or liver function, and also drug-drug-interactions. The clinical symptoms and signs of LQTS range from asymptomatic patients to sudden death because of malignant arrhythmias, and it is therefore important to recognise the clinical characteristics and typical ECG changes. Treatment of acquired LQTS is mainly awareness, identification and discontinuation of QT prolonging drugs, in addition to eventually supplement of magnesium and potassium. Overdrive cardiac pacing is highly effective in preventing recurrences, and antiarrhythmic drugs should be avoided. Recent data suggest that QT prolongation is quite common in ICU patients and adversely affects patient mortality. Thus, high-risk patients should be sufficiently monitored, and the use of medications known to cause drug-induced LQTS might have to be restricted.

Assessment of drug-induced torsade de pointes risk for hospitalized high-risk patients receiving QT-prolonging agents

BACKGROUND

Although risk factors for torsade de pointes (TdP) are known, identifying hospitalized patients at greatest risk for QTcP who should receive cardiac monitoring is poorly defined.

OBJECTIVES

Describe the prevalence of risk for TdP in patients and associations between risk factors and QTc prolongation (QTcP) at a tertiary teaching hospital.

METHODS

This retrospective analysis assessed physiological and pharmacological risk factors for TdP of adult patients receiving ≥1 QTc-prolonging medications (QTcMed) during hospitalization. The QTcMeds were stratified by risk for causing TdP (probable, possible, and conditional). Baseline electrocardiograms (ECGs) were assessed for QTcP associated with risk for TdP.

RESULTS

During a 6-month period, 12,401 (51%) hospitalizations received ≥1 QTcMed. A baseline ECG was obtained for 2381 (19%) patients. A total of 386 (16%) patients with a baseline ECG were found to have QTcP. Significant associations for QTcP were found with the following physiological risk factors: female (P = .021), left-ventricular ejection fraction <40% (P < .0001), cardiac arrest (P < .0001), and cardioversion (P = .007). Significantly more patients with QTcP (n = 209, 54%) received probable-risk QTcMeds than those without QTcP (n = 542, 27%; P < .0001). Probable-risk QTcMeds administered alone or concomitantly with other QTcMeds were more frequently associated with QTcP. No documented cases of TdP were identified.

CONCLUSIONS

Of the population receiving QTcMeds, only a small portion had a baseline ECG, identifying a large population at risk of QTcP without appropriate monitoring. Patients with cardiac disease receiving probable-risk QTcMeds were associated with the highest risk of QTcP and should be monitored closely.

QT variability during initial exposure to sotalol: experience based on a large electronic medical record

AIMS

A prolonged QT interval is associated with increased risk of Torsades de pointes (TdP) and may be fatal. We sought to investigate the extent to which clinical covariates affect the change in QT interval among 'real-world' patients treated with sotalol and followed in an electronic medical record (EMR) system.

METHODS AND RESULTS

We used clinical alerts in our EMR system to identify all patients in whom a new prescription for sotalol was written (2001-11). Rate-corrected QT (QTc) was calculated by Bazett's formula. Correlates of sotalol-induced change in the QTc interval and sotalol discontinuation were examined using linear and logistic regression, respectively. Overall, 541 sotalol-exposed patients were identified (n = 200 women, 37%). The mean first sotalol dose was 86 ± 39 mg, age 64 ± 13 years, and BMI 30 ± 7 kg/m(2). Atrial fibrillation/flutter was the predominant indication (92.2%). After initial exposure, the change in the QTc interval from baseline was highly variable: ΔQTc after 2 h = 3 ± 42 ms (P = 0.17) and 11 ± 37 ms after ≥48 h (P < 0.001). Multivariable linear regression analysis identified female gender and age, reduced left ventricular ejection fraction, high sotalol dose, hypertrophic cardiomyopathy, and loop diuretic co-administration as correlates of increased ΔQTc at ≥48 h (P < 0.05 for all). Within 3 days of initiation, 12% discontinued sotalol of which 31% were because of exaggerated QTc prolongation. One percent developed TdP.

CONCLUSION

In this EMR-based cohort, the increase in QTc with sotalol initiation was highly variable, and multiple clinical factors contributed. These data represent an important step in ongoing work to identify real-world patients likely to tolerate long-term therapy and reinforces the utility of EMR-based cohorts as research tools.

Improvement of cardiac efficacy and safety models in drug discovery by the use of stem cell-derived cardiomyocytes

BACKGROUND

The pharmaceutical industry suffers from high attrition rates during late phases of drug development. Improved models for early evaluation of drug efficacy and safety are needed to address this problem. Recent developments have illustrated that human stem cell-derived cardiomyocytes are attractive for using as a model system for different cardiac diseases and as a model for screening, safety pharmacology and toxicology.

OBJECTIVE

In this review, we discuss contemporary drug discovery models and their characteristics for cardiac efficacy testing and safety assessment. Additionally, we evaluate various sources of stem cells and how these cells could potentially improve early screening and safety models.

CONCLUSION

We conclude that human stem cells offer a source of physiologically relevant cells that show great potential as a future tool in cardiac drug discovery. However, some technical challenges related to cell differentiation and production and also to validation of improved platforms remain and must be overcome before successful application can become a reality.

A thorough QTc study of 3 doses of iloperidone including metabolic inhibition via CYP2D6 and/or CYP3A4 and a comparison to quetiapine and ziprasidone

The potential for iloperidone, a D2/5-HT2A antipsychotic, to affect the heart rate-corrected QT interval (QTc) was assessed in the absence and presence of metabolic inhibitors in a randomized, open-label, multicenter study. QT interval prolongation by medications, including both conventional and atypical antipsychotic drugs, can predispose patients to cardiac arrhythmias and result in sudden death. Adults with schizophrenia or schizoaffective disorder and normal electrocardiograms at baseline (N = 188) were randomized 1:1:1:1:1 to iloperidone, 8 mg twice daily (BID), 12 mg BID, 24 mg once daily (QD); quetiapine, 375 mg BID; or ziprasidone, 80 mg BID during period 1 (no metabolic inhibitors present). Iloperidone BID produced mean changes in QTc Fridericia correction (QTcF) interval (8.5-9.0 milliseconds [ms]) similar to those produced by ziprasidone (9.6 ms) and higher than those produced by quetiapine (1.3 ms). Iloperidone, 24 mg QD, produced a mean QTcF change of 15.4 ms. Coadministration of metabolic inhibitors with iloperidone during periods 2 (paroxetine) and 3 (paroxetine and ketoconazole) resulted in greater increases in the QTc interval. Increased QTc was observed in individuals with specific cytochrome P450 2D6 polymorphisms. Up to 10% of patients on iloperidone experienced QTc intervals of 60 ms or longer in the presence of metabolic inhibition and QD dosing. However, no patients experienced QTc changes of clinical concern (QTc ≥ 500 ms). The most common adverse events with iloperidone were headache, anxiety, and dyspepsia. The only cardiovascular adverse events with iloperidone were non-concentration-dependent tachycardia that was mild in most patients and did not lead to further sequelae. Pharmacogenetics and recommendations are discussed.

The electro-mechanical window in anaesthetized guinea pigs: a new marker in screening for Torsade de Pointes risk

BACKGROUND AND PURPOSE

QT prolongation is commonly used as a surrogate marker for Torsade de Pointes (TdP) risk of non-cardiovascular drugs. However, use of this indirect marker often leads to misinterpretation of the realistic TdP risk, as tested compounds may cause QT prolongation without evoking TdP in humans. A negative electro-mechanical (E-M) window has recently been proposed as an alternative risk marker for TdP in a canine LQT1 model. Here, we evaluated the E-M window in anaesthetized guinea pigs as a screening marker for TdP in humans.

EXPERIMENTAL APPROACH

The effects of various reference drugs and changes in body temperature on the E-M window were assessed in instrumented guinea pigs. The E-M window was defined as the delay between the duration of the electrical (QT interval) and mechanical (QLVP(end) ) systole.

KEY RESULTS

Drugs with known TdP liability (quinidine, haloperidol, domperidone, terfenadine, thioridazine and dofetilide), but not those with no TdP risk in humans (salbutamol and diltiazem) consistently decreased the E-M window. Interestingly, drugs with known clinical QT prolongation, but with low risk for TdP (amiodarone, moxifloxacin and ciprofloxacin) did not decrease the E-M window. Furthermore, the E-M window was minimally affected by changes in heart rate or body temperature.

CONCLUSIONS AND IMPLICATIONS

A decreased E-M window was consistently observed with drugs already known to have high TdP risk, but not with drugs with low or no TdP risk. These results suggest that the E-M window in anaesthetized guinea pigs is a risk marker for TdP in humans.

Assessment of QT-prolonging drugs in the isolated normal and failing rabbit hearts

Lengthening of QTc is the usual signal to indicate torsadogenic potential of a therapeutic agent. The ICH S7B guideline recommends that new chemical entities should be assessed for potential of delayed ventricular repolarization in animal models. The aim of this study was to determine a feasibility of using isolated failing heart rabbit to assess the QT-lengthening drugs in comparison with their effects on isolated normal heart rabbits. Heart failure was induced by ligation of the left anterior descending and descending branch of left circumflex coronary arteries. One month after ligation, all rabbits were anesthetized and the hearts were removed quickly, and they were perfused with the oxygenated Krebs-Henseleit solution to which escalating concentrations of QT-lengthening compounds were added. RR, QT, and QTc(F) were not significantly different, at rest, between failing and normal hearts. During baseline, dP/dt<inf>max</inf> was lower and dP/dt<inf>min</inf> was higher for failing hearts than for normals. In responses to all three QT-lengthening compounds, RR, QT and QTc(F) lengthened similarly in a dose-response manner in both the failing and normal hearts. Neither the failing nor the normal hearts developed fatal arrhythmias, torsades de pointes. Langendorff preparations of failing hearts are as good as normal isolated hearts and can be use to assess the potential of delayed ventricular repolarization of test articles.

Keeping the rhythm: hERG and beyond in cardiovascular safety pharmacology

Following its involvement in life-threatening cardiac arrhythmias, the catchword 'hERG' has become infamous in the drug discovery community. The blockade of the ion channel coded by the human ether-á-go-go-related gene (hERG) has been correlated to a prolongation of the QT interval in the ECG, which again is correlated to a potential risk of a life-threatening polymorphic ventricular tachycardia - torsades de pointes (TdP). Therefore, in vitro investigations for blockade of this ion channel have become a standard, starting early in most drug discovery projects and often accompanying the whole project; at some stage, scientists in many medicinal chemistry programs have to deal with hERG channel liabilities. Data for the compound effects on hERG channel activity are generally part of the safety pharmacology risk assessment in regulatory submissions and, at this stage, are ideally conducted in compliance with good laboratory practice. With the withdrawal of clobutinol from the market, owing to its perceived risk of introducing TdP, the importance of the hERG channel has very recently been reconfirmed. Despite being of such importance for drug discovery, the relevance and impact of hERG data are sometimes misinterpreted, as there are drugs that block the hERG-coded ion channel but do not cause TdP, and drugs that cause TdP but do not block the hERG channel. This review aims to provide an overview of TdP, including the cardiac action potential and the ion channels involved in it, as well as on the relevance and interpretation of in vitro hERG channel data and their impact for drug discovery projects. Finally, novel cardiac safety test systems beyond in vitro hERG channel screening are discussed.

Automated electrophysiology makes the pace for cardiac ion channel safety screening

The field of automated patch-clamp electrophysiology has emerged from the tension between the pharmaceutical industry’s need for high-throughput compound screening versus its need to be conservative due to regulatory requirements. On the one hand, hERG channel screening was increasingly requested for new chemical entities, as the correlation between blockade of the ion channel coded by hERG and torsades de pointes cardiac arrhythmia gained increasing attention. On the other hand, manual patch-clamping, typically quoted as the “gold-standard” for understanding ion channel function and modulation, was far too slow (and, consequently, too expensive) for keeping pace with the numbers of compounds submitted for hERG channel investigations from pharmaceutical R&D departments. In consequence it became more common for some pharmaceutical companies to outsource safety pharmacological investigations, with a focus on hERG channel interactions. This outsourcing has allowed those pharmaceutical companies to build up operational flexibility and greater independence from internal resources, and allowed them to obtain access to the latest technological developments that emerged in automated patch-clamp electrophysiology – much of which arose in specialized biotech companies. Assays for nearly all major cardiac ion channels are now available by automated patch-clamping using heterologous expression systems, and recently, automated action potential recordings from stem-cell derived cardiomyocytes have been demonstrated. Today, most of the large pharmaceutical companies have acquired automated electrophysiology robots and have established various automated cardiac ion channel safety screening assays on these, in addition to outsourcing parts of their needs for safety screening.

A new ECG biomarker for drug toxicity: a combined signal processing and computational modeling study

QT prolongation is the only clinically proven, yet insufficient, electrocardiogram (ECG) biomarker for drug-induced cardiac toxicity. The goal of this study is to evaluate whether JT area, i.e., total area of the T-wave, can serve as an ECG biomarker for drug-induced cardiac toxicity using both signal processing and computational modeling approaches. An ECG dataset that contained recordings from patients under control and sotalol condition was analyzed. In order to relate sotalol-induced ECG changes to its effect on ion channel level, i.e., blockade of the rapid component of the delayed rectifier potassium channel (I(Kr)), varied degrees of I(Kr) blockade were simulated in a slab of ventricular tissue. The mean JT area increased by 36.5% following the administration of sotalol in patients. Simulations in the slab tissue showed that sotalol increased action potential duration preferentially in the midmyocardium, which led to increased transmural dispersion of repolarization and JT area. In conclusion, JT area reflects the transmural dispersion of repolarization and may be a potentially useful surrogate/supplemental ECG biomarker to assess drug safety.

Drug-induced long QT syndrome

The drug-induced long QT syndrome is a distinct clinical entity that has evolved from an electrophysiologic curiosity to a centerpiece in drug regulation and development. This evolution reflects an increasing recognition that a rare adverse drug effect can profoundly upset the balance between benefit and risk that goes into the prescription of a drug by an individual practitioner as well as the approval of a new drug entity by a regulatory agency. This review will outline how defining the central mechanism, block of the cardiac delayed-rectifier potassium current I(Kr), has contributed to defining risk in patients and in populations. Models for studying risk, and understanding the way in which clinical risk factors modulate cardiac repolarization at the molecular level are discussed. Finally, the role of genetic variants in modulating risk is described.

Drug safety evaluation of ziprasidone

INTRODUCTION

Ziprasidone is a second-generation antipsychotic approved for the treatment of schizophrenia and bipolar disorder. The purpose of this review is to assess the overall safety profile of ziprasidone, including its risk for prolonging the electrocardiogram (ECG) QT interval.

AREAS COVERED

This paper is a review of product labeling and English language reports located through PubMed and information available on regulatory agency websites, with a focus on the safety and tolerability of ziprasidone.

EXPERT OPINION

Although ziprasidone can prolong the ECG QT interval, this has not resulted in increases in sudden death or cardiac sudden death as noted in a large, simple trial and supported by almost a decade of real-world use in the US. Ziprasidone's principal advantage over some other second-generation antipsychotics has been its overall favorable weight and metabolic profile. Similar to most second-generation antipsychotics, ziprasidone has a lower propensity for extrapyramidal side effects and hyperprolactinemia compared to first-generation antipsychotics.

Oscillations of cardiac wave length and proarrhythmia

Drug-induced action potential duration (APD) prolongation was first proposed to be antiarrhythmic, but is now widely presumed to be torsadogenic. To elucidate this paradox, we tested the effect of APD upon liability for torsade de pointes. In addition, torsadogenicity is commonly associated with disturbances of repolarization, but at least in theory, it could also result from disturbances of conduction. These possibilities were tested in female rabbit hearts. Dofetilide, ATX II, and sodium channel blockers that did not prolong the action potential duration were used to modulate the APD and induce disturbances of conduction and disturbances of repolarization. Torsadogenicity could be induced by dofetilide and ATX II starting at normal APD (210 ms), reaching a peak incidence around a doubling of APD (400 to 450 ms), to then sharply decline with further APD prolongation, until torsade de pointes disappeared above 725 ms. Early afterdepolarizations (EAD) were regular triggers for torsade de pointes; while most of the EADs occurred in the plateau range, their incidence declined with repolarization but their potential for torsadogenicity increased. Sodium channel blockers that shorten the APD, even when devoid of hERG blocking properties, can yield torsade de pointes. Torsade de pointes can occur at normal, prolonged, and shortened APD, so that QT prolongation is an incomplete predictor of torsadogenicity. Torsade de pointes can result not only from disturbances of repolarization but also from disturbances of conduction.

Arrhythmic risk biomarkers for the assessment of drug cardiotoxicity: from experiments to computer simulations

In this paper, we illustrate how advanced computational modelling and simulation can be used to investigate drug-induced effects on cardiac electrophysiology and on specific biomarkers of pro-arrhythmic risk. To do so, we first perform a thorough literature review of proposed arrhythmic risk biomarkers from the ionic to the electrocardiogram levels. The review highlights the variety of proposed biomarkers, the complexity of the mechanisms of drug-induced pro-arrhythmia and the existence of significant animal species differences in drug-induced effects on cardiac electrophysiology. Predicting drug-induced pro-arrhythmic risk solely using experiments is challenging both preclinically and clinically, as attested by the rise in the cost of releasing new compounds to the market. Computational modelling and simulation has significantly contributed to the understanding of cardiac electrophysiology and arrhythmias over the last 40 years. In the second part of this paper, we illustrate how state-of-the-art open source computational modelling and simulation tools can be used to simulate multi-scale effects of drug-induced ion channel block in ventricular electrophysiology at the cellular, tissue and whole ventricular levels for different animal species. We believe that the use of computational modelling and simulation in combination with experimental techniques could be a powerful tool for the assessment of drug safety pharmacology.

hERG-related drug toxicity and models for predicting hERG liability and QT prolongation

BACKGROUND

hERG K(+) channels have been recognized as a primary antitarget in safety pharmacology. Their blockade, caused by several drugs with different therapeutic indications, may lead to QT prolongation and, eventually, to potentially fatal arrhythmia, namely torsade de pointes. Therefore, a number of preclinical models have been developed to predict hERG liability early in the drug development process.

OBJECTIVE

The aim of this review is to outline the present state of the art on drug-induced hERG blockade, providing insights on the predictive value of in vitro and in silico models for hERG liability.

METHODS

On the basis of latest reports, high-throughput preclinical models have been discussed outlining advantages and limitations.

CONCLUSION

Although no single model has an absolute value, an integrated risk assessment is recommended to predict the pro-arrhythmic risk of a given drug. This prediction requires expertise from different areas and should encompass emerging issues such as interference with hERG trafficking and QT shortening.

Iatrogenic QT Abnormalities and Fatal Arrhythmias: Mechanisms and Clinical Significance

Severe and occasionally fatal arrhythmias, commonly presenting as Torsade de Pointes [TdP] have been reported with Class III-antiarrhythmics, but also with non-antiarrhythmic drugs. Most cases result from an action on K(+) channels encoded by the HERG gene responsible for the IKr repolarizing current, leading to a long QT and repolarization abnormalities. The hydrophobic central cavity of the HERG-K+ channels, allows a large number of structurally unrelated drugs to bind and cause direct channel inhibition. Some examples are dofetilide, quinidine, sotalol, erythromycin, grepafloxacin, cisapride, dolasetron, thioridazine, haloperidol, droperidol and pimozide. Other drugs achieve channel inhibition indirectly by impairing channel traffic from the endoplasmic reticulum to the cell membrane, decreasing channel membrane density (pentamidine, geldalamicin, arsenic trioxide, digoxin, and probucol). Whereas, ketoconazole, fluoxetine and norfluoxetine induce both direct channel inhibition and impaired channel trafficking. Congenital long QT syndrome, subclinical ion-channel mutations, subjects and relatives of subjects with previous history of drug-induced long QT or TdP, dual drug effects on cardiac repolarization [long QT plus increased QT dispersion], increased transmural dispersion of repolarization and T wave abnormalities, use of high doses, metabolism inhibitors and/or combinations of QT prolonging drugs, hypokalemia, structural cardiac disease, sympathomimetics, bradycardia, women and older age, have been shown to increase the risk for developing drug-induced TdP. Because most of these reactions are preventable, careful evaluation of risk factors and increased knowledge of drugs use associated with repolarization abnormalities is strongly recommended. Future genetic testing and development of practical and simple provocation tests are in route to prevent iatrogenic TdP.

Slow delayed rectifier K+ current block by HMR 1556 increases dispersion of repolarization and promotes Torsades de Pointes in rabbit ventricles

BACKGROUND AND PURPOSE

The slow delayed rectifier K(+) current (I(Ks)) contributes to ventricular repolarization when the action potential (AP) is prolonged. I(Ks) block during drug-induced AP prolongation may promote Torsades de Pointes (TdP), but whether this is due to additional AP prolongation is uncertain.

EXPERIMENTAL APPROACH

In bradycardic perfused rabbit ventricles, the incidence of spontaneous TdP, monophasic AP duration at 90% repolarization (MAPD(90)) and ECG interval between the peak and the end of T wave (T(peak-end)) (index of dispersion of repolarization) were measured after the administration of veratridine (125 nM, slows Na(+) channel inactivation), dofetilide (7.5 or 10 nM, a rapid delayed rectifier blocker) and HMR 1556 (HMR, 100 nM, an I(Ks) blocker), alone or in combinations (n=6 each).

KEY RESULTS

HMR did not prolong MAPD(90), whereas veratridine or 7.5 nM dofetilide prolonged MAPD(90) (P<0.01) without inducing TdP. Veratridine+7.5 nM dofetilide additively prolonged MAPD(90) (P<0.05), induced 4+/-6 TdP per heart and prolonged T(peak-end) by 12+/-10 ms. Subsequent addition of HMR did not further prolonged MAPD(90), but increased the number of TdP to 22+/-18 per heart and increased T(peak-end) by 39+/-21 ms (P<0.05). Increasing dofetilide concentration from 7.5 to 10 nM (added to veratridine) produced a longer MAPD(90), but fewer TdP (5+/-5 per heart) and less T(peak-end) prolongation (17+/-8 ms) compared to the veratridine+7.5 nM dofetilide+HMR group (P<0.05).

CONCLUSIONS AND IMPLICATIONS

Adding I(Ks) block markedly increases TdP incidence in hearts predisposed to TdP development by increasing the dispersion of repolarization, but without additional AP prolongation.

Ultrafast sodium channel block by dietary fish oil prevents dofetilide-induced ventricular arrhythmias in rabbit hearts

Several epidemiologic and clinical studies show that following myocardial infarction, dietary supplements of ω-3 polyunsaturated fatty acids (ω3FA) reduce sudden death. Animal data show that ω3FA have antiarrhythmic properties, but their mechanisms of action require further elucidation. The effects of ω3FA supplementation were studied in female rabbits to analyze whether their antiarrhythmic effects are due to a reduction of triangulation, reverse use-dependence, instability, and dispersion (TRIaD) of the cardiac action potential (TRIaD as a measure of proarrhythmic effects). In Langendorff-perfused hearts challenged by a selective rapidly activating delayed rectifier potassium current inhibitor that has been shown to exhibit proarrhythmic effects (dofetilide; 1 to 100 nM), ω3FA pretreatment (30 days; n = 6) prolonged the plateau phase of the monophasic action potential; did not slow the terminal fast repolarization; reduced the dofetilide-induced prolongation of the action potential duration; reduced dofetilide-induced triangulation; and reduced dofetilide-induced reverse use-dependence, instability of repolarization, and dispersion. Dofetilide reduced excitability in ω3FA-pretreated hearts but not in control hearts. Whereas torsades de pointes (TdP) were observed in five out of six in control hearts, none were observed in ω3FA-pretreated hearts. Docosahexaenoic acid (DHA) inhibited the sodium current with ultrafast kinetics. Dietary ω3FA supplementation markedly reduced dofetilide-induced TRIaD and abolished dofetilide-induced TdP. Ultrafast sodium channel block by DHA may account for the antiarrhythmic protection of the dietary supplements of ω3FA against dofetilide-induced proarrhythmia observed in this animal model.

In silico risk assessment for drug-induction of cardiac arrhythmia

The main components of repolarization reserve for the ventricular action potential (AP) are the rapid (I(Kr)) and slow (I(Ks)) delayed outward K(+) currents. While many drugs block I(Kr) and cause life-threatening arrhythmias including torsades de pointes, the frequency of arrhythmias varies between different I(Kr)-blockers. Different types of block of I(Kr) cause distinct phenotypes of prolongation of action potential duration (APD), increase in transmural dispersion of repolarization (TDR) and, accordingly, occurrence of torsades de pointes. Therefore the assessment of a drug's proarrhythmic risk requires a method that provides quantitative and comprehensive comparison of the effects of different forms of I(Kr)-blockade upon APDs and TDR. However, most currently available methods are not adapted to such an extensive comparison. Here, we introduce I(Kr)-I(Ks) two-dimensional maps of APD and TDR as a novel risk-assessment method. Taking the kinetics of I(Kr)-blockade into account, APDs can be calculated upon a ventricular AP model which systematically alters the magnitudes of I(Kr) and I(Ks). The calculated APDs are then plotted on a map where the x axis represents the conductance of I(Kr) while the y axis represents that of I(Ks). TDR is simulated with models corresponding to APs in epicardial, midcardial and endocardial myocardium. These two-dimensional maps of APD and TDR successfully account for differences in the risk resulting from three distinct types of I(Kr)-blockade which correspond to the effects of dofetilide, quinidine and vesnarinone. This method may be of use to assess the arrhythmogenic risk of various I(Kr)-blockers.

Predicting drug-induced changes in QT interval and arrhythmias: QT-shortening drugs point to gaps in the ICHS7B Guidelines

Background and purpose:

The regulatory guidelines (ICHS7B) recommending inhibition of the delayed rectifier K⁺ current (I_Kr), carried by human ether-a-go-go-related gene (hERG) channels in cardiac cells (the hERG test), as a ‘first line' test for identifying compounds inducing QT prolongation, have limitations, some of which are outlined here.

Experimental approach:

hERG current was measured in HEK293 cells, stably transfected with hERG channels; action potential duration (APD) and arrhythmogenic effects were measured in isolated Purkinje fibres and perfused hearts from rabbits.

Key results:

576 compounds were screened in the hERG test: 58% were identified as hERG inhibitors, 39% had no effect and 3% were classified as stimulators. Of the hERG inhibitors, 92 were tested in the APD assay: 55.4% of these prolonged APD, 28.3% had no effect and 16.3% shortened APD. Of the 70 compounds without effect on hERG channels, 54.3% did not affect APD, 25.7% prolonged, while 20% significantly shortened APD. Dofetilide (hERG inhibitor; IC₅₀, 29 nM) prolonged QT and elicited early after-depolarizations and/or torsade de pointes (TdP) in isolated hearts. Mallotoxin and NS1643 (hERG current stimulators at 3 μM), levcromakalim and nicorandil (no effect on hERG current), all significantly shortened APD and QT, and elicited ventricular fibrillation (VF) in isolated hearts.

Conclusion and implications:

The hERG assay alone did not adequately identify drugs inducing QT prolongation. It is also important to detect drug-induced QT shortening, as this effect is associated with a potential risk for ventricular tachycardia and VF, the latter being invariably fatal, whereas TdP has an ∼15–25% incidence of death.

Importance of vagally mediated bradycardia for the induction of torsade de pointes in an in vivo model

BACKGROUND AND PURPOSE

Bradycardia is a risk factor for the development of torsade de pointes (TdP). The aim of this work was to compare the importance of changes in heart rate and arterial blood pressure in the development of drug-induced TdP and to investigate the role of vagal influences.

EXPERIMENTAL APPROACH

Experiments were performed in open-chest, pentobarbital-anaesthetized, male rabbits which were given clofilium (20, 60 and 200 nmol kg(-1) min(-1)) with rising doses of either phenylephrine (75, 150, 225 and 300 nmol kg(-1) min(-1)), angiotensin II (0.25, 0.5, 0.75 and 1 nmol kg(-1) min(-1)) or saline. A fourth group received phenylephrine and cloflium after bilateral vagotomy. ECGs, haemodynamics and epicardial monophasic action potentials were recorded.

KEY RESULTS

TdP occurred in 57% of rabbits given phenylephrine and clofilium. Replacement of phenylephrine with saline or angiotensin II reduced the incidence of TdP to 0 and 17%, respectively. Vagotomy prevented TdP in rabbits given phenylephrine and clofilium. Increases in blood pressure induced by phenylephrine and angiotensin II were similar. Bradycardia only occurred with phenylephrine and was reduced but not abolished by vagotomy. Neither short-term variability of repolarization nor action potential triangulation could predict TdP.

CONCLUSIONS AND IMPLICATIONS

These results indicate that reflex activation of vagal nerve activity is essential for the induction of drug-induced TdP in alpha1-adrenoceptor-stimulated anaesthetized rabbits. This implies that alterations in vagal activity may also precipitate episodes of drug-induced TdP in man and that this should be considered in selecting models used in drug development.

Relevance of in vitro SCREENIT results for drug-induced QT interval prolongation in vivo: a database review and analysis

INTRODUCTION

The use of an isolated rabbit heart model (SCREENIT) to predict drug-induced QTc prolongation in animals was assessed using hERG and telemetry data.

PURPOSE

We compiled data from (i) hERG assay (IC50s), (ii) SCREENIT assay (APD60) and (iii) in vivo non-rodent telemetry studies (QTc interval) and evaluated the reliability of APD60 to fit with IC50s and QTc prolongation using the ratio to free plasma level (FPL). Eighty-two compounds were separated into three classes based on hERG IC50s (class I: IC50s< or =1 microM, n=7; class II: IC50s>1 microM to < or =10 microM, n=15; class III: IC50s>10 microM, n=60).

RESULTS

Three class I compounds did not prolong QTc at the FPL equivalent to their IC50s (43% hERG false positives). There were no false positives in SCREENIT. Six class II compounds prolonged the QTc interval. Results showed 40% hERG false negatives and no SCREENIT false negatives. Nine compounds had no effect on QTc, and two prolonged APD60 at an equivalent concentration/FPL (13% false positives). Three class III compounds prolonged QTc at an FPL lower than maximum SCREENIT concentrations (5% false negatives). Four other compounds generated SCREENIT false positive results (7%).

CONCLUSION

SCREENIT increased the predictability of preclinical results for QTc prolongation without generating any false positive results in class I (13% in class II). Making decisions without isolated heart data increases the risk for eliminating efficient drugs displaying hERG inhibition.

Short patent lives jeopardize drug and patient safety

Exposure of patients to new medications carries potential safety hazards during widespread clinical practice. These are often detected only after a substantial period of use. Thus, there is considerable need for measures reducing drug-related morbidity and mortality, such as adequate, active postmarketing drug-safety surveillance systems with obligatory follow-up studies of suspected safety problems, or even an additional "Phase IV" safety study before marketing. However, drug development processes erode substantially into the useful patent life of a new drug. Therefore, we suggest that the potential benefits of patent life prolongation should be considered, under certain conditions for the sake of patient safety.

The hERG K+ channel: target and antitarget strategies in drug development

The human ether-à-go-go related gene (hERG) K+ channel is of great interest for both basic researchers and clinicians because its blockade by drugs can lead to QT prolongation, which is a risk factor for torsades de pointes, a potentially life-threatening arrhythmia. A growing list of agents with "QT liability" have been withdrawn from the market or restricted in their use, whereas others did not even receive regulatory approval for this reason. Thus, hERG K+ channels have become a primary antitarget (i.e. an unwanted target) in drug development because their blockade causes potentially serious side effects. On the other hand, the recent identification and functional characterization of hERG K+ channels not only in the heart, but also in several other tissues (e.g. neurons, smooth muscle and cancer cells) may have far reaching implications for drug development for a possible exploitation of hERG as a target, especially in oncology and cardiology.

Electrophysiological profile of propiverine--relationship to cardiac risk

Drugs that prolong the QT interval by blocking human ether-a-go-go (HERG) channels may enhance the risk of ventricular arrhythmia. The spasmolytic drug propiverine is widely used for the therapy of overactive bladder (OAB). Here, we have investigated the effects of propiverine on cardiac ion channels and action potentials as well as on contractile properties of cardiac tissue, in order to estimate its cardiac safety profile, because other drugs used in this indication had to be withdrawn due to safety reasons. Whole-cell patch clamp technique was used to record the following cardiac ion currents: rapidly and slowly activating delayed rectifier K+ current (I(Kr), I(Ks)), ultra rapidly activating delayed rectifier K+ current (I(Kur)), inwardly rectifying K+ current I(K1), transient outward K+ current (I(to)), and L-type Ca2+ current (I(Ca,L)). Action potentials in cardiac tissue biopsies were recorded with conventional microelectrodes. The torsade de pointes screening assay (TDPScreen) was used for drug scoring. Propiverine blocked in a concentration-dependent manner HERG channels expressed in HEK293 cells, as well as native I(Kr) current in ventricular myocytes of guinea pig (IC50 values: 10 microM and 1.8 microM respectively). At high concentrations (100 microM), propiverine suppressed I(Ks). I(K1) and the transient outward current I(to) and I(Kur) were not affected. In guinea-pig ventricular and human atrial myocytes, propiverine also blocked I(Ca,L) (IC50 values: 34.7 microM and 41.7 microM, respectively) and reduced force of contraction. Despite block of I(Kr), action potential duration was not prolonged in guinea-pig and human ventricular tissue, but decreased progressively until excitation failed altogether. Similar effects were observed in dog Purkinje fibers. Propiverine obtained a low score in the TDPScreen. In conclusion, in vitro and in vivo studies of propiverine do not provide evidence for an enhanced cardiovascular safety risk. We propose that lack of torsadogenic risk of propiverine is related to enhancement of repolarization reserve by block of I(Ca,L).