Electrophysiological mechanisms of vandetanib-induced cardiotoxicity: Comparison of action potentials in rabbit Purkinje fibers and pluripotent stem cell-derived cardiomyocytes

Vandetanib, a multi-kinase inhibitor used for the treatment of various cancers, has been reported to induce several adverse cardiac effects. However, the underlying mechanisms of vandetanib-induced cardiotoxicity are unclear. This study aimed to investigate the mechanism of vandetanib-induced cardiotoxicity using intracellular electrophysiological recordings on human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), rabbit Purkinje fibers, and HEK293 cells transiently expressing human ether-a-go-go-related gene (hERG; the rapidly activating delayed rectifier K⁺ channel, I_Kr), KCNQ1/KCNE1 (the slowly activating delayed rectifier K⁺ current, I_Ks), KCNJ2 (the inwardly rectifying K⁺ current, I_K1) or SCN5A (the inward Na⁺ current, I_Na). Purkinje fiber assays and ion channel studies showed that vandetanib at concentrations of 1 and 3 μM inhibited the hERG currents and prolonged the action potential duration. Alanine scanning and in silico hERG docking studies demonstrated that Y652 and F656 in the hERG S6 domain play critical roles in vandetanib binding. In hiPSC-CMs, vandetanib markedly reduced the maximum rate of depolarization during the AP upstroke. Ion channel studies revealed that hiPSC-CMs were more sensitive to inhibition of the I_Na by vandetanib than in a heterogeneously expressed HEK293 cell model, consistent with the changes in the AP parameters of hiPSC-CMs. The subclasses of Class I antiarrhythmic drugs inhibited I_Na currents in a dose-dependent manner in hiPSC-CMs and SCN5A-encoded HEK293 cells. The inhibitory potency of vandetanib for I_Na was much higher in hiPSC-CMs (IC₅₀: 2.72 μM) than in HEK293 cells (IC₅₀: 36.63 μM). These data suggest that AP and I_Na assays using hiPSC-CMs are useful electrophysiological models for prediction of drug-induced cardiotoxicity.

Nonclinical cardiovascular safety of pitolisant: comparing International Conference on Harmonization S7B and Comprehensive in vitro Pro-arrhythmia Assay initiative studies

BACKGROUND AND PURPOSE

We evaluated the concordance of results from two sets of nonclinical cardiovascular safety studies on pitolisant.

EXPERIMENTAL APPROACH

Nonclinical studies envisaged both in the International Conference on Harmonization (ICH) S7B guideline and Comprehensive in vitro Pro-arrhythmia Assay (CiPA) initiative were undertaken. The CiPA initiative included in vitro ion channels, stem cell-derived human ventricular myocytes, and in silico modelling to simulate human ventricular electrophysiology. ICH S7B-recommended assays included in vitro hERG (KV 11.1) channels, in vivo dog studies with follow-up investigations in rabbit Purkinje fibres and the in vivo Carlsson rabbit pro-arrhythmia model.

KEY RESULTS

Both sets of nonclinical data consistently excluded pitolisant from having clinically relevant QT-liability or pro-arrhythmic potential. CiPA studies revealed pitolisant to have modest calcium channel blocking and late INa reducing activities at high concentrations, which resulted in pitolisant reducing dofetilide-induced early after-depolarizations (EADs) in the ICH S7B studies. Studies in stem cell-derived human cardiomyocytes with dofetilide or E-4031 given alone and in combination with pitolisant confirmed these properties. In silico modelling confirmed that the ion channel effects measured are consistent with results from both the stem cell-derived cardiomyocytes and rabbit Purkinje fibres and categorized pitolisant as a drug with low torsadogenic potential. Results from the two sets of nonclinical studies correlated well with those from two clinical QT studies.

CONCLUSIONS AND IMPLICATIONS

Our findings support the CiPA initiative but suggest that sponsors should consider investigating drug effects on EADs and the use of pro-arrhythmia models when the results from CiPA studies are ambiguous.

Mechanism of action of antidysrhythmic drugs on ventricular conduction as studied with intramural electrodes

Using the Dürrer electrode needle to record the intramural electrogram and a standard peripheral electrocardiogram (ECG) lead as reference, the authors studied the action of 9 different drugs in the conduction system of dogs. The authors concluded that diphenylhydantoin and lignocaine would be useful in dysrhythmias related to increased excitability and perhaps increased automatism. Ajmaline and quinidine may be some of some interest in dysrhythmias due to conduction disturbances; and, finally, ajmaline, diphenylhydantoin, di-isopyramide and quinidine may be useful in dysrhythmias due to focal re-entry.

Diclofenac prolongs repolarization in ventricular muscle with impaired repolarization reserve

BACKGROUND

The aim of the present work was to characterize the electrophysiological effects of the non-steroidal anti-inflammatory drug diclofenac and to study the possible proarrhythmic potency of the drug in ventricular muscle.

METHODS

Ion currents were recorded using voltage clamp technique in canine single ventricular cells and action potentials were obtained from canine ventricular preparations using microelectrodes. The proarrhythmic potency of the drug was investigated in an anaesthetized rabbit proarrhythmia model.

RESULTS

Action potentials were slightly lengthened in ventricular muscle but were shortened in Purkinje fibers by diclofenac (20 µM). The maximum upstroke velocity was decreased in both preparations. Larger repolarization prolongation was observed when repolarization reserve was impaired by previous BaCl(2) application. Diclofenac (3 mg/kg) did not prolong while dofetilide (25 µg/kg) significantly lengthened the QT(c) interval in anaesthetized rabbits. The addition of diclofenac following reduction of repolarization reserve by dofetilide further prolonged QT(c). Diclofenac alone did not induce Torsades de Pointes ventricular tachycardia (TdP) while TdP incidence following dofetilide was 20%. However, the combination of diclofenac and dofetilide significantly increased TdP incidence (62%). In single ventricular cells diclofenac (30 µM) decreased the amplitude of rapid (I(Kr)) and slow (I(Ks)) delayed rectifier currents thereby attenuating repolarization reserve. L-type calcium current (I(Ca)) was slightly diminished, but the transient outward (I(to)) and inward rectifier (I(K1)) potassium currents were not influenced.

CONCLUSIONS

Diclofenac at therapeutic concentrations and even at high dose does not prolong repolarization markedly and does not increase the risk of arrhythmia in normal heart. However, high dose diclofenac treatment may lengthen repolarization and enhance proarrhythmic risk in hearts with reduced repolarization reserve.

Field and action potential recordings in heart slices: correlation with established in vitro and in vivo models

BACKGROUND AND PURPOSE

Action potential (AP) recordings in ex vivo heart preparations constitute an important component of the preclinical cardiac safety assessment according to the ICH S7B guideline. Most AP measurement models are sensitive, predictive and informative but suffer from a low throughput. Here, effects of selected anti-arrhythmics (flecainide, quinidine, atenolol, sotalol, dofetilide, nifedipine, verapamil) on field/action potentials (FP/AP) of guinea pig and rabbit ventricular slices are presented and compared with data from established in vitro and in vivo models.

EXPERIMENTAL APPROACH

Data from measurements of membrane currents (hERG, I(Na) ), AP/FP (guinea pig and rabbit ventricular slices), AP (rabbit Purkinje fibre), haemodynamic/ECG parameters (conscious, telemetered dog) were collected, compared and correlated to complementary published data (focused literature search).

KEY RESULTS

The selected anti-arrhythmics, flecainide, quinidine, atenolol, sotalol, dofetilide, nifedipine and verapamil, influenced the shape of AP/FP of guinea pig and rabbit ventricular slices in a manner similar to that observed for rabbit PF. The findings obtained from slice preparations are in line with measurements of membrane currents in vitro, papillary muscle AP in vitro and haemodynamic/ECG parameters from conscious dogs in vivo, and were also corroborated by published data.

CONCLUSION AND IMPLICATIONS

FP and AP recordings from heart slices correlated well with established in vitro and in vivo models in terms of pharmacology and predictability. Heart slice preparations yield similar results as papillary muscle but offer enhanced throughput for mechanistic investigations and may substantially reduce the use of laboratory animals.

Direct effects of arsenic trioxide on action potentials in isolated cardiac tissues: importance of the choice of species, type of cardiac tissue and perfusion time

INTRODUCTION

The aim of the present study was to evaluate direct/acute effects of arsenic trioxide on action potentials (APs) in isolated cardiac tissues, and to investigate if the choice of species and tissue and the duration of the perfusion play a role in arsenic-induced acute/direct prolongation of AP/QT.

METHODS AND RESULTS

Direct electrophysiological effects of arsenic trioxide were measured in cardiac tissues isolated from four different species using micro-electrode recording. Arsenic (after 30 to 95 min perfusion at 10 μM) significantly prolonged APD(90), increased triangulation of the AP and elicited early afterdepolarizations (EADs) only in isolated guinea-pig and dog Purkinje fibers but not in rabbit and porcine (minipig) Purkinje fibers. Arsenic induced a prolongation of the APD(90) and increases in triangulation and the occurrence of EADs was not observed in papillary muscles of guinea-pigs and rabbits. Arsenic at 4 increasing concentrations from 0.1 μM to 10 μM at the standard perfusion-time of 15 min per concentration, and after a continuous 90-min perfusion at 1 μM and 1 Hz did not induce these direct effects on APD(90), triangulation and EADs in isolated guinea-pig Purkinje fibers, but it at 1 µM elicited EADs in 2 out of 7 preparations after 90 min at 0.2 Hz.

DISCUSSION

The present study demonstrates that the choice of species and cardiac tissue as well as perfusion-time play important roles in arsenic-induced direct/acute effects on APD(90) and induction of EADs in vitro.

Effects of mixed herbal extracts from parched Puerariae radix, gingered Magnoliae cortex, Glycyrrhizae radix and Euphorbiae radix (KIOM-79) on cardiac ion channels and action potentials

KIOM-79, a mixture of ethanol extracts from four herbs (parched Puerariae radix, gingered Magnoliae cortex, Glycyrrhizae radix and Euphorbiae radix), has been developed for the potential therapeutic application to diabetic symptoms. Because screening of unexpected cardiac arrhythmia is compulsory for the new drug development, we investigated the effects of KIOM-79 on the action potential (AP) and various ion channel currents in cardiac myocytes. KIOM-79 decreased the upstroke velocity (V(max)) and plateau potential while slightly increased the duration of action potential (APD). Consistent with the decreased V(max) and plateau potential, the peak amplitude of Na+ current (I(Na)) and Ca2+ current (I(Ca,L)) were decreased by KIOM-79. KIOM-79 showed dual effects on hERG K+ current; increase of depolarization phase current (I(depol)) and decreased tail current at repolarization phase (I(tail)). The increase of APD was suspected due to the decreased I(tail). In computer simulation, the change of cardiac action potential could be well simulated based on the effects of KIOM-79 on various membrane currents. As a whole, the influence of KIOM-79 on cardiac ion channels are minor at concentrations effective for the diabetic models (0.1-10 microg/mL). The results suggest safety in terms of the risk of cardiac arrhythmia. Also, our study demonstrates the usefulness of the cardiac computer simulation in screening drug-induced long-QT syndrome.

Electrophysiological Safety of Novel Fluoroquinolone Antibiotic Agents Gemifloxacin and Balofloxacin

Some fluoroquinolones have been reported to induce QT interval prolongation associated with the onset of torsades de pointes (TdP), resulting in a life-threatening ventricular arrhythmia. We investigated the cardiac electrophysiological effects of two new fluoroquinolones, gemifloxacin and balofloxacin, by using conventional microelectrode recording techniques in isolated rabbit Purkinje fiber and whole-cell patch-clamp techniques in human ether-á-go-go related gene (hERG)-transient transfected CHO cells. Gemifloxacin had no significant effects on the resting membrane potential, total amplitude, action potential, and Vmax of phase 0 depolarization at concentrations up to 30 microM, but gemifloxacin at 100 microM significantly decreased total amplitude (p < 0.01). These values of gemifloxacin (30 and 100 microM) were approximately 25- and 83-fold more than the free plasma concentration of 1.2 microM in a single therapeutic injection in humans. For I(hERG), the IC(50) value was about 300 microM. Balofloxacin had also no significant effects on the resting membrane potential, total amplitude, action potential duration, and Vmax of phase 0 depolarization at concentrations up to 30 microM, but balofloxacin at 100 microM significantly (p < 0.01) prolonged action potentials at both 50% repolarization (APD(50)) and 90% repolarization (APD(90)). These values of balofloxacin (30 and 100 microM) were approximately 6.8- and 23-fold more than the free plasma concentration of 4.4 microM in a single therapeutic injection in humans. For I(hERG), the IC(50) value was 214 +/- 14 microM. Therefore, our data suggested that in the electrophysiological aspect, gemifloxacin and balofloxacin may have no torsadogenic potenties up to 30 microM.

Alpha-2 adrenergic antagonism enhances risk of ventricular tachycardia during acute ischemia

OBJECTIVE

In this study we tested the hypothesis that alpha-2 adrenergic antagonism could facilitate induction of previously non-inducible ventricular tachycardia (VT) during acute ischemia. Previous reports suggest that VT during ischemia may be modulated by (alpha-2 adrenergic agonists.

DESIGN

The left anterior descending artery was occluded after instrumentation of the ischemic risk zone with 21 multipolar plunge needles, each recording 6 bipolar electrograms. Three dimensional mapping characterized the mechanism of VT induced with extrastimuli.

RESULTS

Of 16 non-inducible dogs included, eight which were given the alpha-2 adrenergic antagonist yohimbine all had inducible VT, while all eight in the control group remained non-inducible (p < 0.05). Six of the VTs were of focal Purkinje origin. The cycle length of the VTwas 119 +/- 4 ms. Mean arterial pressure (81+/- 8 to 82 +/- 8 mmHg, p = ns), ventricular effective refractory period (146 +/- 6 to 144 +/- 5 ms, p = ns) and ischemic zone size (55 +/-6% vs. 61 +/- 4%, p = 0.45) were not altered by yohimbine indicating minimal central or pre-junctional effects of the drug.

CONCLUSIONS

Yohimbine facilitates induction of VT, especially those with focal Purkinje fiber origin, suggestive of an effect mediated through antagonism of post-junctional alpha-2 adrenoceptors on Purkinje fibers.

Nitroglycerin on contractile force in canine cardiac Purkinje fibers

The objective of this study was to investigate the effects of nitroglycerin (NG) on the contractile force in order to elucidate regulatory roles of cyclic guanosine monophosphate (cGMP) in the bundle of canine cardiac Purkinje fibers. NG suppressed the developed tension in canine Purkinje fibers immersed in normal Tyrode's solution. The contraction was also elicited in the depolarizing solution containing 30 mM K+ by adding isoproterenol (Iso) or tetraethylammonium (TEA). The suppression caused by NG was more marked in fibers in the depolarizing solution treated with TEA than in the depolarizing solution treated with Iso. 8-Bromo-cGMP and sodium nitroprusside had a slight inhibition on the contraction in the fibers. Methylene blue did not affect the decrease in contractile force induced by NG. The weaker inhibition caused by NG on the contraction in the presence of Iso might result from a possible increase in intracellular cAMP levels in the fibers.

Atrium-selective sodium channel block as a strategy for suppression of atrial fibrillation: differences in sodium channel inactivation between atria and ventricles and the role of ranolazine

BACKGROUND

The development of selective atrial antiarrhythmic agents is a current strategy for suppression of atrial fibrillation (AF).

METHODS AND RESULTS

Whole-cell patch clamp techniques were used to evaluate inactivation of peak sodium channel current (I(Na)) in myocytes isolated from canine atria and ventricles. The electrophysiological effects of therapeutic concentrations of ranolazine (1 to 10 micromol/L) and lidocaine (2.1 to 21 micromol/L) were evaluated in canine isolated coronary-perfused atrial and ventricular preparations. Half-inactivation voltage of I(Na) was approximately 15 mV more negative in atrial versus ventricular cells under control conditions; this difference increased after exposure to ranolazine. Ranolazine produced a marked use-dependent depression of sodium channel parameters, including the maximum rate of rise of the action potential upstroke, conduction velocity, and diastolic threshold of excitation, and induced postrepolarization refractoriness in atria but not in ventricles. Lidocaine also preferentially suppressed these parameters in atria versus ventricles, but to a much lesser extent than ranolazine. Ranolazine produced a prolongation of action potential duration (APD90) in atria, no effect on APD90 in ventricular myocardium, and an abbreviation of APD90 in Purkinje fibers. Lidocaine abbreviated both atrial and ventricular APD90. Ranolazine was more effective than lidocaine in terminating persistent AF and in preventing the induction of AF.

CONCLUSIONS

Our study demonstrates important differences in the inactivation characteristics of atrial versus ventricular sodium channels and a striking atrial selectivity for the action of ranolazine to produce use-dependent block of sodium channels, leading to suppression of AF. Our results point to atrium-selective sodium channel block as a novel strategy for the management of AF.

Towards automation of a valuable preclinical cardiac safety pharmacology assay: Evaluation of the effects of cardiac ion channel blockers on cardiac repolarisation in vitro

INTRODUCTION

Purkinje fibre repolarisation assays are valuable tools for identifying compounds which affect cardiac ion channels. The throughput of compound testing in this assay is low therefore we designed a novel recording system to improve screening and animal tissue usage efficiencies.

METHODS

The system was used to evaluate compounds using standard sharp microelectrode techniques. Animal tissue usage efficiencies were quantified by adding up the total number of Purkinje fibres from which recordings were attempted and dividing this by the number of experimental data sets generated, to arrive at a 'fibres per data set' ratio. Test compounds were dofetilide (3 x 10(-10) to 10(-8) M), cisapride (10(-8) to 3 x 10(-7) M), terodiline (10(-6) to 3 x 10(-5) M) and verapamil (3 x 10(-7) to 10(-5) M).

RESULTS

Using the novel modified system, 21 data sets were generated from 29 fibres, compared to 24 data sets from 41 fibres using the conventional manual recording system, demonstrating a 24% improvement in the efficiency of animal tissue usage. Comparing data from the manual and modified systems revealed differences in absolute values for all parameters including APD90 (308.73 +/- 9.97 ms, n = 24, compared to 275.27 +/- 8.25 ms, n = 21, respectively; P < 0.05). Differences in the magnitude of changes in action potential parameters between the systems were also evident for all compounds including terodiline (1 x 10(-5) M) which caused a -27.1 +/- 16.5% reduction in APD50 in the manual system, compared to a - 55.2 +/- 2.2% reduction in the modified system.

DISCUSSION

Although the value of the present study is limited by the small sample sizes, it has demonstrated utility of the modified system in improving efficiency of animal tissue usage. It offers potential utility in a higher throughput screening environment for examining the electrophysiological properties of novel compounds in native cardiac tissues, particularly where functional patch clamp data are limited.

Comparison of guinea-pig ventricular myocytes and dog Purkinje fibres for in vitro assessment of drug-induced delayed repolarization

INTRODUCTION

QT interval prolongation and Torsade de Pointes (TdP) arrhythmias are recognised as a potential risk with many drugs, most of which delay cardiac repolarization by inhibiting the rapidly activating K(+) current (I(Kr)). The objective of this study was to compare the effects of compounds on cardiac action potentials recorded from guinea-pig ventricular myocytes and dog Purkinje fibres.

METHODS AND RESULTS

Effects of dofetilide, sotalol, cisapride, terfenadine, haloperidol and sparfloxacin, compounds known to cause QT prolongation (positive controls), and nifedipine and verapamil, not associated with QT prolongation (negative controls) were studied on intracellular action potentials recorded from guinea-pig isolated ventricular myocytes (VM) and dog isolated Purkinje fibres (PF). Prolongation of action potential duration (APD) by sotalol, dofetilide and sparfloxacin was concentration-dependent and of greater magnitude in dog PF compared to guinea-pig VM. The maximum prolongation of APD in guinea-pig VM at 0.5 and 1 Hz was approximately 25% and this was associated with complete inhibition of I(Kr) by dofetilide. Effects on APD of cisapride and haloperidol in both preparations, and terfenadine in guinea-pig VM, were biphasic, consistent with inhibition of multiple ion channels. There was no effect of terfenadine on APD in dog PF. Haloperidol increased APD by more than 25% in guinea-pig VM, consistent with effects on additional repolarizing currents. The negative controls shortened APD to a greater extent in guinea-pig VM compared to dog PF. In general, the positive control drugs increased action potential triangulation (APD(40-90)) to a greater extent than APD(90).

CONCLUSION

Guinea-pig isolated VM may be more sensitive for detecting APD prolongation with compounds inhibiting multiple ion channels and action potential triangulation (APD(40-90)). Effects on repolarizing currents other than I(Kr) were also distinguished in guinea-pig VM.

Suitability of commonly used excipients for electrophysiological in-vitro safety pharmacology assessment of effects on hERG potassium current and on rabbit Purkinje fiber action potential

INTRODUCTION

Regulatory guidelines require investigation of the liability for delayed ventricular repolarization by new chemical entities within a broad concentration range in-vitro. However, investigation can be limited by poor drug aqueous solubility, and by solvent physicochemical attributes that disrupt cell membrane integrity. Although excipients or solubilizing agents may aid to achieve the necessary high concentrations, no comprehensive overview on the suitability of solvents for in-vitro electrophysiological safety studies exists.

METHODS

Excipients were tested for potential interference with the hERG (human ether-a-go-go-related gene) K(+) current (whole-cell voltage-clamp, 23+/-2 degrees C), and the shape of rabbit Purkinje fiber action potentials (conventional glass microelectrode technique, 37+/-1 degrees C).

RESULTS AND DISCUSSION

Water-soluble complexation builders/carriers had little effect on hERG K(+) current at up to 50 mg/ml (BSA, bovine serum albumin) and 11 mg/ml (HP-beta-CD, hydroxypropyl-beta-cyclodextrin; IC(20), concentration of 20% inhibition). Water-soluble organic (co)solvents inhibited hERG K(+) currents (IC(20), %/mM): 0.7/152, ethanol; 0.9/67, Transcutol; 1.2/154, DMSO (dimethylsulfoxide); 1.6/389, acetonitrile; 1.9/48, polyethylene glycol 400; 2.1/660, methanol. Part of their inhibitory effect is attributed to the osmolality of extracellular solutions, because hERG IC(20) and extrapolated osmolality at the hERG IC(20) strongly correlate. Water-soluble non-ionic solubilizers/surfactants are potent inhibitors of hERG K(+) current with IC(20) concentrations of 0.07% (Cremophor EL) or lower (Tween 20, Tween 80: approximately 0.001%). Part of this inhibitory effect is attributed to their interaction with lipid membranes, because hERG inhibition occurs close to critical micelle concentrations (Cremophor, approximately 0.009%; Tween 20, approximately 0.007%). Purkinje fiber action potentials are little affected by HP-beta-CD at up to 2 mg/ml, while DMSO tends to shorten the action potential duration at 1%.

CONCLUSION

When conducting electrophysiological in-vitro assessments of drug effects, solubilizers/surfactants (Cremophor EL, Tween 20, Tween 80) should be avoided. Instead, water-soluble organic (co)solvents (methanol, acetonitrile, DMSO) or complexation builders/carriers (HP-beta-CD, BSA) appear to be more favorable.

Electrophysiological effects of haloperidol on isolated rabbit Purkinje fibers and guinea pigs papillary muscles under normal and simulated ischemia

AIM

Overdoses of haloperidol are associated with major ventricular arrhythmias, cardiac conduction block, and sudden death. The aim of this experiment was to study the effect of haloperidol on the action potentials in cardiac Purkinje fibers and papillary muscles under normal and simulated ischemia conditions in rabbits and guinea pigs.

METHODS

Using the standard intracellular microelectrode technique, we examined the effects of haloperidol on the action potential parameters [action potential amplitude (APA), phase 0 maximum upstroke velocity (V(max)), action potential amplitude at 90% of repolarization (APD(90)), and effective refractory period (ERP)] in rabbit cardiac Purkinje fibers and guinea pig cardiac papillary cells, in which both tissues were under simulated ischemic conditions.

RESULTS

Under ischemic conditions, different concentrations of haloperidol depressed APA and prolonged APD(90) in a concentration-dependent manner in rabbit Purkinje fibers. Haloperidol (3 micromol/L) significantly depressed APA and prolonged APD(90), and from 1 micromol/L, haloperidol showed significant depression on V(max); ERP was not significantly affected. In guinea pig cardiac papillary muscles, the thresholds of significant reduction in APA, V(max), EPR, and APD(90) were 10, 0.3, 1, and 1 mumol/L, respectively, for haloperidol.

CONCLUSION

Compared with cardiac conductive tissues, papillary muscles were more sensitive to ischemic conditions. Under ischemia, haloperidol prolonged ERP and APD(90) in a concentration-dependent manner and precipitated the decrease in V(max) induced by ischemia. The shortening of ERP and APD(90) in papillary muscle action potentials may be inhibited by haloperidol.

Syntheses of Potent, Selective, and Orally Bioavailable Indazole-Pyridine Series of Protein Kinase B/Akt Inhibitors with Reduced Hypotension

Compound 7 was identified as a potent (IC50 = 14 nM), selective, and orally bioavailable (F = 70% in mouse) inhibitor of protein kinase B/Akt. While promising efficacy was observed in vivo, this compound showed effects on depolarization of Purkinje fibers in an in vitro assay and CV hypotension in vivo. Guided by an X-ray structure of 7 bound to protein kinase A, which has 80% homology with Akt in the kinase domain, our efforts have focused on structure-activity relationship (SAR) studies of the phenyl moiety, in an attempt to address the cardiovascular liability and further improve the Akt potency. A novel and efficient synthetic route toward diversely substituted phenyl derivatives of 7 was developed utilizing a copper-mediated aziridine ring-opening reaction as the key step. To improve the selectivity of these Akt inhibitors over other protein kinases, a nitrogen atom was incorporated into selected phenyl analogues of 7 at the C-6 position of the methyl indazole scaffold. These modifications resulted in the discovery of inhibitor 37c with greater potency (IC50 = 0.6 nM vs Akt), selectivity, and improved cardiovascular safety profile. The SARs, pharmacokinetic profile, and CV safety of selected Akt inhibitors will be discussed.

Action potential experiments complete hERG assay and QT-interval measurements in cardiac preclinical studies

INTRODUCTION

The ICHS7B guideline focused on hERG and QT assays, although other factors have also been linked with the induction of severe arrhythmias. Thus, the aim of the present study was to demonstrate that two in vitro action potential recordings constitute convincing models of predictive drug-induced Torsades de pointes (TdP) and re-entry arrhythmias.

METHODS

The effects of D,L-sotalol, flecainide and quinidine were investigated on potassium (hERG) and sodium (Na(V)1.5) currents transfected in HEK-293 cells to determine the repercussion of the blockade of these currents on rabbit Purkinje fibre (PF) and atrial action potentials. Atrial conduction velocity was also investigated as a model of re-entry arrhythmias.

RESULTS

hERG channels were blocked by D,L-sotalol, quinidine and flecainide (IC(50): 69, 0.33 and 0.74 micromol/L, respectively). D,L-sotalol (30 micromol/L) induced reverse-use dependent increases in action potential duration (APD(90): +31.7% and +81.2% at 1 and 0.2 Hz) and triangulation (APD(90-40): +34.7% and +73.6% at 1 and 0.2 Hz) in PF but not in atria. Quinidine (10 micromol/L) also increased APD(90) (+14.5% and +68.5% at 1 and 0.2 Hz) and APD(90-40) (+73.3% and +152.1% at 1 and 0.2 Hz) in PF. Flecainide (10 micromol/L) shortened APD(90) in PF (-26.0% and - 22.2% at 1 and 0.2 Hz). Quinidine and flecainide blocked Na(V)1.5 channels by 32.3% and 73.1%, respectively, and produced decreases in dV/dt(max) which were more marked in atria (-20.4% and -31.9%) compared to PF (-12.8% and 22.4%) at 1 Hz. Finally, quinidine and flecainide decreased atrial conduction speed by 14.6% and 30.8%, respectively.

CONCLUSION

Results obtained with flecainide demonstrate that use of the hERG channel alone should not be considered as a useful single assay. Rabbit Purkinje fiber action potentials can be considered as a comparable model for detection of reverse-use dependent APD prolongation and triangulation whereas the rabbit atria can be considered as a useful model for detection of sodium channel blockade associated with decreases in dV/dt(max) and conduction velocity.

Effect of clebopride, antidopaminergic gastrointestinal prokinetics, on cardiac repolarization

The inhibition of the potassium current I(Kr) and QT prolongation has been known to be associated with drug-induced torsades de pointes arrhythmias (TdP) and sudden cardiac death. In this study, the authors investigated the cardiac electrophysiological effects of clebopride, a class of antidopaminergic gastrointestinal prokinetic, that has been reported to prolong the QT interval by using the conventional microelectrode recording techniques in isolated rabbit Purkinje fiber and whole-cell patch clamp techniques in human ether-à-go-go-related gene (hERG)-stably transfected Chinese hamster ovarian (CHO) cells. Clebopride at 10 microM significantly decreased the Vmax of phase 0 depolarization (p < .05) and significantly prolonged the action potential duration at 90% repolarization (APD90) (p < .01), whereas the action potential duration at 50% repolarization (APD50) was not prolonged. For hERG potassium channel currents, the IC50 value was 0.62 +/- 0.30 microM. Clebopride was found to have no effect on sodium channel currents. When these results were compared with Cmax (1.02 nM) of clinical dosage (1 mg, [p.o.]), it can be suggested that clebopride is safe at the clinical dosage of 1 mg from the electrophysiological aspect. These findings indicate that clebopride, an antidopaminergic gastrointestinal prokinetic drug, may provide a sufficient "safety factor" in terms of the electrophysiological threshold concentration. But, in a supratherapeutic concentration that might possibly be encountered during overdose or impaired metabolism, clebopride may have torsadogenic potency.

σ2-Receptor Ligand-Mediated Inhibition of Inwardly Rectifying K+ Channels in the Heart

The sigma(2)-receptor agonist, ifenprodil, was suggested as an inhibitor of G protein-coupled inwardly rectifying potassium channels. Nevertheless, an analysis of the role of sigma(2) receptors in cardiac electrophysiology has never been done. This work aims i) to identify the roles of cardiac sigma(2) receptors in the regulation of cardiac K(+) channel conductances and ii) to check whether sigma(2)-receptor agonists exhibit class III antiarrhythmic properties. The sigma(2)-receptor agonists ifenprodil, threo-ifenprodil, LNP250A [threo-8-[1-(4-hydroxyphenyl)-1-hydroxy-propan-2-yl]-1-phenyl-1,3,8-triazaspiro[4,5]decane-4-one] (a derivative of ifenprodil devoid of alpha(1)-adrenergic and N-methyl-d-aspartate glutamate receptor-blocking properties), and 1,3-di(2-tolyl)guanidine were used to discriminate the effects linked to sigma(2) receptors from those of the sigma(1) subtype, induced by (+/-)-N-allylnormetazocine (SKF-10,047). The sigma(2)-receptor antagonist 3-alpha-tropanyl-2(pCl-phenoxy)butyrate (SM-21) was employed to characterize sigma(2)-mediated effects in patch-clamp experiments. In rabbits, all sigma(2)-receptor agonists reduced phenylephrine-induced cardiac arrhythmias. They prolonged action potential duration in rabbit Purkinje fibers and reduced human ether-a-go-go-related gene (HERG) K(+) currents. (+)-SKF-10,047 was completely inactive in the last two tests. The effects of threo-ifenprodil were not antagonized by SM-21. In HERG-transfected COS-7 cells, SM-21 potentiated the ifenprodil-induced blockade of the HERG current. These data suggest that sigma(2)-receptor ligands block I(Kr) and that this effect could explain part of the antiarrhythmic properties of this ligands family. Nevertheless, an interaction with HERG channels not involving sigma(2) receptors seems to share this pharmacological property. This work shows for the first time that particular caution has to be taken toward ligands with affinity for sigma(2) receptors. The repolarization prolongation and the early-afterdepolarization can be responsible for "torsades de pointe" and sudden cardiac death.

Discovery of 3-methyl-N-(1-oxy-3',4',5',6'-tetrahydro-2'H-[2,4'-bipyridine]-1'-ylmethyl)benzamide (ABT-670), an orally bioavailable dopamine D4 agonist for the treatment of erectile dysfunction

The goal of this study was to identify a structurally distinct D(4)-selective agonist with superior oral bioavailability to our first-generation clinical candidate 1a (ABT-724) for the potential treatment of erectile dysfunction. Arylpiperazines such as (heteroarylmethyl)piperazine 1a, benzamide 2, and acetamides such as 3a,b exhibit poor oral bioavailability. Structure-activity relationship (SAR) studies with the arylpiperidine template provided potent partial agonists such as 4d and 5k that demonstrated no improvement in oral bioavailability. Further optimization with the (N-oxy-2-pyridinyl)piperidine template led to the discovery of compound 6b (ABT-670), which exhibited excellent oral bioavailability in rat, dog, and monkey (68%, 85%, and 91%, respectively) with comparable efficacy, safety, and tolerability to 1a. The N-oxy-2-pyridinyl moiety not only provided the structural motif required for agonist function but also reduced metabolism rates. The SAR study leading to the discovery of 6b is described herein.

Analysis of arrhythmogenic profile in a canine model of chronic atrioventricular block by comparing in vitro effects of the class III antiarrhythmic drug nifekalant on the ventricular action potential indices between normal heart and atrioventricular block heart

The chronic atrioventricular block dog is a useful model for predicting the future onset of drug-induced long QT syndrome in clinical practice. To better understand the arrhythmogenic profile of this model, we recorded the action potentials of the isolated ventricular tissues in the presence and absence of the class III antiarrhythmic drug nifekalant. The action potential durations of the Purkinje fiber and free wall of the right ventricle were longer in the chronic atrioventricular block dogs than in the dogs with normal sinus rhythm. Nifekalant in concentrations of 1 and 10 microM prolonged the action potential durations of Purkinje fiber and the free wall in a concentration-dependent manner. The extent of prolongation was greater in the chronic atrioventricular block dogs than in the normal dogs. However, increase of temporal dispersion of ventricular repolarization including early afterdepolarization was not detected by nifekalant in either group of dogs, indicating lack of potential to trigger arrhythmias in vitro. These results suggest that the ventricular repolarization delay in the chronic atrioventricular block model by nifekalant may largely depend on the decreased myocardial repolarization reserve, whereas the trigger for lethal arrhythmia was not generated in the in vitro condition in contrast to the in vivo experiment.

Functional effects of the late sodium current inhibition by AZD7009 and lidocaine in rabbit isolated atrial and ventricular tissue and Purkinje fibre

AZD7009 (tert-Butyl-2-(7-[(2S)-3-(4-cyanophenoxy)-2-hydroxypropyl]-9-oxa-3,7-diazabicyclo[3.3.1]non-3-yl)ethylcarbamate) is an antiarrhythmic agent that increases atrial refractoriness, shows high antiarrhythmic efficacy and has low proarrhythmic potential. This study was primarily undertaken to determine the effects of AZD7009 on the late sodium current and to examine the impact of late sodium current inhibition on action potential duration in various myocardial cells. AZD7009 inhibited the late sodium current in Chinese Hamster Ovary K1 (CHO K1) cells expressing hNa(v)1.5 with an IC(50) of 11+/-2 microM. The late sodium current in isolated rabbit atrial and ventricular myocytes was also concentration dependently inhibited by AZD7009. Action potentials were recorded during exposure to 5 microM E-4031 (1-[2-(6-methyl-2pyridyl)ethyl]-4-(4-methylsulfonyl aminobenzoyl)piperidine), a compound that selectively inhibits the rapid delayed rectifier potassium current (I(Kr)), and to E-4031 in combination with AZD7009 or lidocaine in rabbit atrial and ventricular tissue and Purkinje fibres. In Purkinje fibres, but not in ventricular tissue, AZD7009 and lidocaine attenuated the E-4031-induced action potential duration prolongation. In atrial cells, AZD7009, but not lidocaine, further prolonged the E-4031-induced action potential duration. E-4031 induced early afterdepolarisations (EADs) in Purkinje fibres, EADs that were totally suppressed by AZD7009 or lidocaine. In conclusion, excessive action potential duration prolongation induced by E-4031 was attenuated by AZD7009 and lidocaine in rabbit Purkinje fibre, but not in atrial or ventricular tissue, most likely by inhibiting the late sodium current. Furthermore, the opposite effect by AZD7009 on action potential duration in atrial tissue suggests that AZD7009, in addition to inhibiting I(Kr), also inhibits other repolarising currents in the atria.

In-vitro experimental models for the risk assessment of antibiotic-induced QT prolongation

The prolongation of the ventricular repolarization and proarrhythmic effects (Torsade de Pointes: TdP) of five reference antibiotics were compared in four in-vitro models. 1. Using the patch clamp technique on the human ether-a-gogo-related gene (HERG) current, the rank order for blockade of the HERG-current (IC(50)) was: sparfloxacin (44 microM)>telithromycin=moxifloxacin=erythromycin (+/-100 microM). 2. Assessing their effects on action potential duration (APD(90)) and incidence of early afterdepolarizations in isolated rabbit Purkinje fibers, the rank order was: sparfloxacin>moxifloxacin>telithromycin>erythromycin (prolongation of APD(90) at 100 microM: 83%, 48%, 33% and 17% from baseline compared to +5% with solvent, P<0.05, respectively). 3. Assessing the drug effects on the APD(60), triangulation, reverse use-dependency, and instability in isolated Langendorff-perfused rabbit hearts, the rank order was: moxifloxacin>erythromycin>sparfloxacin>telithromycin. 4. Assessing their torsadogenic potentials (scores of effects on QT-interval, peak of the T wave to end of T wave: T(p-e), T(p-e)/QT ratio, R wave on T wave (R on T) and TdP in isolated rabbit left ventricular wedge preparations, the rank order for their TdP risk score was: sparfloxacin>erythromycin>moxifloxacin>telithromycin. Additional experiments with grepafloxacin indicate that the rank order to detect grepafloxacin-induced long QT was the wedge preparation>the Purkinje fiber>HERG>the isolated heart, where the isolated heart was unable to detect grepafloxacin-induced APD prolongation. The present study demonstrates that the first three in-vitro models can be used to assess the ability of antibiotic compounds to delay ventricular repolarization. However, with respect to their known clinical effects on QT and TdP incidence, the wedge preparation appears to be more predictive and suitable for detecting torsadogenic action of antibiotics.

The D3-dopaminergic agonist 7-hydroxy-dipropylaminotetralin (7-OH-DPAT) increases cardiac action potential duration and blocks human ether-a-go-go-related gene K+ channel

The D3-dopaminergic agonist (+/-) 7-hydroxy-dipropylaminotetralin (7-OH-DPAT) prolonged cycle length and action potential duration, depolarized maximum diastolic potential, and reduced the upstroke velocity of the action potential of rabbit sinoatrial node cells. These effects were not mediated by D3-dopaminergic receptors. In cat Purkinje fibers, the drug increased action potential duration. In voltage-clamped cat ventricular myocytes, 7-OH-DPAT blocked the rapid component of the delayed rectifier potassium current, IKr. This effect was corroborated in experiments studying the effect of the drug on human Ether-a-go-go-related Gene channels expressed in Xenopus oocytes and in HEK293 cells. We conclude that the direct electrophysiologic effects of 7-OH-DPAT on cardiac tissues are caused by the blockade of the rapid component of the delayed rectifier potassium current, IKr.

The role of late I and antiarrhythmic drugs in EAD formation and termination in Purkinje fibers

Multiple components of cardiac Na current play a role in determining electrical excitation in the heart. Recently, the role of nonequilibrium components in controlling cardiac action potential plateau duration, and their importance in regulating the occurrence of afterdepolarizations and arrhythmias have garnered more attention. In particular, late Na current (late I(Na)) has been shown to be important in LQT2 and LQT3 arrhythmias. Class III agents like dofetilide, clofilium, and sotalol, which can all cause a drug-induced form of LQT2, significantly lengthen action potential duration at 50% and 90% repolarization in isolated rabbit Purkinje fibers, and can initiate the formation of early afterdepolarizations, and extra beats. These actions can lead to the development of a serious ventricular tachycardia, torsades de pointes, in animal models and patients. However, pretreatment with agents that block late I(Na), like lidocaine, mexiletine, and RSD1235, a novel mixed ion channel blocker for the rapid pharmacologic conversion of atrial fibrillation, significantly attenuates the prolonging effects of Class III agents or those induced by ATX-II, a specific toxin that delays Na channel inactivation and amplifies late I(Na) greatly, mimicking LQT3. The Na channel block caused by lidocaine and RSD1235 can be through the open or inactivated states of the channel, but both equivalently inhibit a late component of Na current (I(Na)), recorded at 22 degrees C using whole-cell patch clamp of Nav 1.5 expressed in HEK cells. These protective actions of lidocaine, mexiletine, and RSD1235 may result, at least in part, from their ability to inhibit late I(Na) during action potential repolarization, and inhibition of the inward currents contributing to EAD and arrhythmia formation.

Location and clinical implications of high-degree atrioventricular block during dipyridamole infusion: a case report

We describe a patient with bifascicular block, who developed transient high-degree atrioventricular block during dipyridamole infusion. This patient was subsequently found to have significant His-Purkinje disease at electrophysiology study, and underwent permanent pacemaker implantation. Spontaneous atrioventricular block was documented during follow-up. This case report raises the issue of dipyridamole safety in patients with intraventricular conduction defects, and contributes an additional mechanism to the possible explanation of dipyridamole-induced atrioventricular block.

A slowly inactivating sodium current (INa2) in the plateau range in canine cardiac Purkinje single cells

The action potential of Purkinje fibres is markedly shortened by tetrodotoxin, suggesting the possibility that a slowly inactivating sodium current might flow during the plateau. The aim of the present experiments was to investigate, in canine cardiac Purkinje single cells by means of a whole cell patch clamp technique, whether a sodium current slowly inactivates at less negative potentials and (if so) some of its distinctive characteristics. The results showed that a 500 ms depolarizing step from a holding potential of -90 mV to -50 mV induced the fast inward current I(Na) (labelled here I(Na1)). With steps to -40 mV or less negative values, a slowly decaying component (tentatively labelled here I(Na2)) appeared, which peaked at -30 to -20 mV and decayed slowly and incompletely during the 500 ms steps. The I(Na2) was present also during steps to -10 mV, but then the transient outward current (I(to)) appeared. When the holding potential (V(h)) was decreased to -60 to -50 mV, I(Na2) disappeared even if a small I(Na1) might still be present. Tetrodotoxin (30 mum), lignocaine (100 mum) and cadmium (0.2 mm; but not manganese, 1 mm) blocked I(Na2). During fast depolarizing ramps, the rapid inactivation of I(Na1) was followed by a negative slope region. During repolarizing ramps, a region of positive slope was present, whereas I(Na1) was absent. At less negative values of V(h), the amplitude of the negative and positive slopes became gradually smaller. Gradually faster ramps increased the magnitude of the negative slope, and tetrodotoxin (30 mum) reduced or abolished it. Thus, Purkinje cells have a slowly decaying inward current owing to Na(+) entry (I(Na2)) that is different in several ways from the fast I(Na1) and that appears important for the duration of the plateau.

Calmodulin antagonist W-7 prevents sparfloxacin-induced early afterdepolarizations (EADs) in isolated rabbit purkinje fibers: importance of beat-to-beat instability of the repolarization

INTRODUCTION

The occurrence of early afterdepolarizations (EADs) has been related to the incidence of torsades de pointes in drug-induced long QT (LQT). The generation of EADs may be facilitated by Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase).

METHODS AND RESULTS

In the present study, we investigated a possible involvement of Ca(2+)/Calmodulin dependent protein kinase in the generation of sparfloxacin-induced EADs in isolated rabbit Purkinje fibers by means of a calmodulin antagonist W-7. EADs were evident in 8 of the 10 preparations perfused with sparfloxacin at 1 x 10(-4) M and stimulated at 0.2 Hz. The induction of EADs by sparfloxacin was associated with a large prolongation of the duration of the action potential (APD), an increase in the triangulation, and the short-term instability of the repolarization. CaM kinase blockade with the calmodulin antagonist W-7 inhibited sparfloxacin-induced EADs in a concentration-dependent manner (EADs were induced in 3 of 10, 1 of 10, and 0 of 8 preparations in the presence of W-7 at 5 x 10(-7) M, 5 x 10(-6) M, and 5 x 10(-5) M, respectively; P < 0.01 at 5 x 10(-6) M and 5 x 10(-5) M). The inhibition of sparfloxacin-induced EADs by W-7 at 5 x 10(-7) M and 5 x 10(-6) M was associated with a significant decrease in the beat-to-beat instability but not associated with a significant shortening of the APD and reduction of V(max).

CONCLUSION

The present findings support the hypothesis that CaM kinase may be a proarrhythmic signaling molecule and demonstrate that CaM kinase may be involved in the generation of EADs in drug-induced LQT and enhanced beat-to-beat instability of repolarization is essential for the genesis of EADs in rabbit in vitro.

ILSI-HESI cardiovascular safety subcommittee initiative: Evaluation of three non-clinical models of QT prolongation

INTRODUCTION

Drugs that delay cardiac repolarization pose potential safety risks to patients and cause serious regulatory concern because of the link between QT interval prolongation and the potentially fatal arrhythmia torsades de pointes (TdP). Predicting which drugs will cause TdP is an inexact and difficult science. The utility of non-clinical assays was not well understood due in part to variability in methods, species, and consistency in the assays reported in the literature. The Health and Environmental Sciences Institute of the International Life Sciences Institute (ILSI/HESI) outlined a set of studies to determine how well selected commonly used non-clinical assays identified compounds known to cause TdP and prolong QT interval in humans.

METHODS

Compounds known to prolong ventricular repolarization and compounds considered safe by years of clinical use were tested in three assays: HERG ionic current, Purkinje fiber repolarization, and in vivo QT studies in conscious telemeterized dogs.

RESULTS

The data from each of these assays demonstrate that compounds that may pose a proarrhythmia risk for patients can be distinguished from those that are considered safe.

DISCUSSION

Taken collectively, the in-vitro and in-vivo preclinical results can be integrated to develop an accurate preclinical risk assessment to support clinical safety.

Preclinical cardiac safety assessment of pharmaceutical compounds using an integrated systems-based computer model of the heart

Blockade of the delayed rectifier potassium channel current, I(Kr), has been associated with drug-induced QT prolongation in the electrocardiogram and life-threatening cardiac arrhythmias. However, it is increasingly clear that compound-induced interactions with multiple cardiac ion channels may significantly affect QT prolongation that would result from inhibition of only I(Kr) [Redfern, W.S., Carlsson, L., et al., 2003. Relationships between preclinical cardiac electrophysiology, clinical QT interval prolongation and torsade de pointes for a broad range of drugs: evidence for a provisional safety margin in drug development. Cardiovasc. Res. 58(1), 32-45]. Such an assessment may not be feasible in vitro, due to multi-factorial processes that are also time-dependent and highly non-linear. Limited preclinical data, I(Kr) hERG assay and canine Purkinje fiber (PF) action potentials (APs) [Gintant, G.A., Limberis, J.T., McDermott, J.S., Wegner, C.D., Cox, B.F., 2001. The canine Purkinje fiber: an in vitro model system for acquired long QT syndrome and drug-induced arrhythmogenesis. J. Cardiovasc. Pharmacol. 37(5), 607-618], were used for two test compounds in a systems-based modeling platform of cardiac electrophysiology [Muzikant, A.L., Penland, R.C., 2002. Models for profiling the potential QT prolongation risk of drugs. Curr. Opin. Drug. Discov. Dev. 5(1), 127-35] to: (i) convert a canine myocyte model to a PF model by training functional current parameters to the AP data; (ii) reverse engineer the compounds' effects on five channel currents other than I(Kr), predicting significant IC(50) values for I(Na+), sustained and I(Ca2+), L-type , which were subsequently experimentally validated; (iii) use the predicted (I(Na+), sustained and I(Ca2+), L-type) and measured (I(Kr)) IC(50) values to simulate dose-dependent effects of the compounds on APs in endocardial, mid-myocardial, and epicardiac ventricular cells; and (iv) integrate the three types of cellular responses into a tissue-level spatial model, which quantifiably predicted no potential for the test compounds to induce either QT prolongation or increased transmural dispersion of repolarization in a dose-dependent and reverse rate-dependent fashion, despite their inhibition of I(Kr) in vitro.

Gender differences in the slow delayed (IKs) but not in inward (IK1) rectifier K+ currents of canine Purkinje fibre cardiac action potential: key roles for IKs, beta-adrenoceptor stimulation, pacing rate and gender

As the beagle dog is a commonly used preclinical species to test the effects of new drugs on cardiac repolarisation and Purkinje fibres have become an established in vitro preparation to assess the effects of these new drugs on action potential duration (APD), the main aim of this study was therefore to evaluate the relative contribution of the inward (I(K1)) and slow delayed (I(Ks)) rectifier cardiac K(+) currents to action potential repolarisation in beagle Purkinje fibres under three different experimental conditions: (i) selective block of I(K1) with BaCl(2), (ii) selective block of I(Ks) with (-) chromanol 293B under basal conditions and (iii) selective block of I(Ks) during beta-adrenoceptor stimulation. Furthermore, the dependence of this contribution on gender and pacing rate was investigated. Microelectrode techniques were employed to measure APD in Purkinje fibres from adult female and male dogs. At stimulation rates of 3.33, 1.0 and 0.2 Hz, the degree of prolongation of APD evoked by BaCl(2) (10 microM) was comparable in fibres from female and male dogs. At the same stimulation rates, 10 microM (-) chromanol 293B did not change the APD in fibres from female and male dogs. During beta-adrenoceptor stimulation with 0.1 microM isoproterenol, an APD prolonging effect of (-) chromanol 293B was detected. In the presence of isoproterenol, action potentials in fibres from male dogs get shorter when changing the stimulation rate from 1.0 to 0.2 Hz, while the opposite is seen in fibres from female dogs. This alteration was completely reversed by (-) chromanol 293B. In conclusion, our findings confirm that beta-adrenoceptor stimulation is one condition where there may be an increased role of I(Ks) in action potential repolarisation. Gender differences in the autonomic modulation of I(Ks) could be a contributing factor to the reported increased susceptibility of female hearts to arrhythmias.

RSD1235 blocks late INa and suppresses early afterdepolarizations and torsades de pointes induced by class III agents

OBJECTIVE

RSD1235 is a novel antiarrhythmic drug with atria-selective electrophysiological actions on Na(+) and K(+) currents. The mechanism for its protection of ventricular repolarization was assessed by its action on Purkinje fibers, and by block of late sodium current active during repolarization. Further, RSD1235's ability to reverse the pro-arrhythmic actions of the class III agents dofetilide and clofilium was assessed in isolated Purkinje fibers and an in vivo model of torsades de pointes (TdP).

METHODS

Action potential and early after-depolarization (EAD) recordings were made from in situ and isolated rabbit Purkinje fibers at 37 degrees C using floating sharp microelectrodes; late I(Na) was recorded using a whole-cell patch clamp technique of Nav1.5 expressed in HEK cells at 22 degrees C; In vivo, anesthetized methoxamine-sensitized rabbits were used to test the ability of RSD1235 to suppress clofilium-induced TdP.

RESULTS

RSD1235 (0.5-30 microM) had minor dose-dependent effects on action potential duration (APD) at 50% and 90% repolarization in Purkinje fibers, but pre-treatment significantly attenuated the APD-prolonging effects of dofetilide (300 nM). EADs induced by 300 nM dofetilide were terminated by 30 microM RSD1235 in all experiments (n=7). RSD1235 blocked a late component of Na current (I(Na)), which can produce inward currents contributing to EAD formation. RSD1235 pre-treatment (1 micromol/kg/min) or acute infusions prevented/terminated TdP induced by clofilium in 8 of 9 rabbits, and reduced the duration of TdP episodes from 71 +/- 23 s in control to 17 +/- 7 and 14 +/- 14 s at infusion rates of 0.3 and 1.0 micromol/kg/min, respectively (n = 9, p < 0.001).

CONCLUSION

RSD1235 itself has minor actions on repolarization in Purkinje fibers, but can reverse the AP-prolonging actions of class III agents and terminate arrhythmias in a model of TdP. We suggest that these protective actions of RSD1235 may result, at least in part, from its ability to inhibit late I(Na) during action potential repolarization.

Evaluation of the Rabbit Purkinje Fibre Assay as an in vitro Tool for Assessing the Risk of Drug-Induced Torsades de Pointes in Humans

BACKGROUND

The issue of drug-induced QT interval prolongation and torsades de pointes represents a major concern for pharmaceutical development. In this investigation, we examined the value of the isolated rabbit Purkinje fibre as an in vitro action potential (AP) assay to predict the potential of drugs to induce these undesirable adverse effects.

METHODS

First, we categorised the proarrhythmic risk of 26 medicinal products based on proportional reporting ratios for these two adverse events recorded in a US FDA database (Spontaneous Reporting System/Adverse Event Reporting System). Second, we measured drug effects on AP in rabbit Purkinje fibres. Finally, the results of the two analyses were compared to evaluate the predictive value of the in vitro assay.

RESULTS

Analysis of the clinical data classified the drugs into 14 positive, 7 negative and 5 questionable for proarrhythmic risk. Based on in vitro electrophysiological profiles, the drugs were grouped into four categories: (i) profile 1 drugs prolong repolarisation without slowing depolarisation; (ii) profile 2 drugs prolong repolarisation and also slow depolarisation; (iii) profile 3 drugs shorten repolarisation; and (iv) profile 4 drugs are without effects. All 14 clinical-positive drugs fell into profiles 1 or 2 (prolongers) with low safety margins (except probucol, which showed no effect, probably because of its low solubility). Clinical-negative drugs belonged mostly to profiles 3 or 4 (non-prolongers) [except clemastine and amlodipine, which were prolongers but had large safety margins]. Clinical-questionable drugs either did not prolong or prolonged slightly but produced additional electrophysiological effects opposing prolongation.

CONCLUSION

The rabbit Purkinje fibre is a valuable assay for evaluating the proarrhythmic liability of pharmaceuticals as it can reveal complex electrophysiological profiles that modulate repolarisation delay.

Additive effects of ziprasidone and d,l-sotalol on the action potential in rabbit Purkinje fibres and on the hERG potassium current

INTRODUCTION

Ziprasidone, an atypical antipsychotic has been shown to be devoid of cardiac adverse effects in spite of its propensity to prolong the QT-interval via a hERG current inhibition. However, the effects of ziprasidone on the action potential (AP) parameters have not been published yet. Moreover, very little information is available concerning pharmacodynamic interactions between ziprasidone and other hERG channel blockers. Thus, we investigated the putative interaction between ziprasidone and D,L-sotalol on the hERG channels at therapeutic concentrations and their consequences on the action potential prolongation.

METHODS

AP were recorded at 1 and 0.2 Hz. Increasing concentrations of ziprasidone (0.01-10 micromol/L) were successively superfused for 30 min alone or in D,L-sotalol 10 micromol/L pre-treated fibres. Moreover, the effects of ziprasidone, alone or in association with d,l-sotalol, were investigated on the hERG current.

RESULTS

Ziprasidone (1-10 microM) induced a concentration and reverse frequency-dependent increase in APD(90) (APD(90): +27% and +36%, respectively at 1 Hz and +50% and +70%, respectively at 0.2 Hz) due to a hERG current blockade (IC50: 0.24 micromol/L). A pre-treatment with D,L-sotalol 10 micromol/L led to an increase in APD(90) of +23% at 1 Hz, stable at 66+/-4 min. In these pre-treated fibres, ziprasidone (1 and 10 micromol/L) induced an additional AP prolongation (APD(90): +16% and +18%, respectively at 1 Hz) as compared to D,L-sotalol pre-treatment. Moreover, D,L-sotalol did not interact with the pharmacological profile of ziprasidone on the hERG channel.

CONCLUSION

The present study demonstrates that ziprasidone induces an AP prolongation due to its propensity to block the hERG channel. Moreover, ziprasidone and d,l-sotalol, superfused concomitantly exhibit additive effects on the AP duration since they do not interact as competitors for the hERG channel.

Prediction of the risk of Torsade de Pointes using the model of isolated canine Purkinje fibres

Torsade de Pointes (TdP) is a well-described major risk associated with various kinds of drugs. However, prediction of this risk is still uncertain both in preclinical and clinical trials. We tested 45 reference compounds on the model of isolated canine Purkinje fibres. Of them, 22 are clearly associated and/or labelled with a risk of TdP, and 13 others are drugs with published clinical evidence of QT prolongation, with only one or two exceptional cases of TdP. The 10 remaining drugs are without reports of TdP and QT prolongation. The relevance of different indicators such as APD(90) increase, reverse use dependency, action potential triangulation or effect on V(max) was evaluated by comparison with available clinical data. Finally, a complex algorithm called TDPscreen and based on two subalgorithms corresponding to particular electrophysiological patterns was defined. This latter algorithm enabled a clear separation of drugs into three groups: (A) drugs with numerous or several reports (>2 cases) of TdP, (B) drugs causing QT prolongation and/or TdP only, the latter at a very low frequency (< or =2 cases), (C) drugs without reports of TdP or QT prolongation. The use of such an algorithm combined with a database accrued from reference compounds with available clinical data is suggested as a basis for testing new candidate drugs in the early stages of development for proarrhythmic risk prediction.

Regional differences of superoxide dismutase activity enhance the superoxide–induced electrical heterogeneity in rabbit hearts

During myocardial ischemia and the subsequent reperfusion, free radicals are important intermediates of the cellular damage and rhythm disturbances. We examined the effects of superoxide radicals or hydrogen peroxide (H(2)O(2)) on the action potentials in isolated rabbit Purkinje fibers, atrial muscle and ventricular muscle. Reactive oxygen species (ROS) donors such as adriamycin, xanthine/xanthine oxidase and menadione induced prolongation of APD(90) in Purkinje fibers. Menadione (30 microM), the most specific superoxide radical donor, prolonged the action potential duration at 90% repolarization (APD(90)) by 17% in Purkinje fibers, whereas it shortened the APD by 57% in ventricular muscle, and it did not affect the atrial APD. All these menadione-induced effects were completely blocked by 2,2,6,6-tetramethyl- 1-peperadinyloxy, a superoxide radical scavenger. Superoxide dismutase (SOD) activity was lowest in Purkinje fibers, it was moderate in atrial muscle and highest in ventricular muscle. H(2)O(2) shortened the APDs of all three cardiac tissues in a concentration-dependent manner. These results suggest that the different electrical responses to O(2) ([Symbol: see text]-) in different cardiac regions may result from the regional differences in the SOD activity, thereby enhancing the regional electrical heterogeneity.

Electromechanical properties of Purkinje fiber strands isolated from human ventricular endocardium

BACKGROUND

Abnormalities in the regulation of intracellular Ca2+ were observed in cardiac cells obtained from failing human hearts. However, the electromechanical properties and pharmacologic responses of human ventricular Purkinje fibers have not been well characterized.

METHODS

Strands of free-running Purkinje fibers and/or trabecular muscle fibers with a diameter of around 1.5 mm were removed from the endocardial surface of ventricles obtained from 16 transplant recipient hearts. Action potential (AP) was detected by conventional microelectrode techniques and twitch force by a force-displacement transducer.

RESULTS

The human Purkinje fiber strands as revealed by histologic examination were composed of Purkinje cells and the surrounding ventricular muscle cells. In well-polarized Purkinje fibers (mean +/- SE of maximum diastolic potential [MDP] = -85 +/- 1 mV) showing fast-response AP (Phase 0 Vmax >100 V/sec), the cardiotonic agents isoproterenol and strophanthidin (1 to 2 micromol/liter) accelerated the slope of diastolic depolarization and induced delayed afterdepolarization but not spontaneous APs. Steady-state contraction and the post-rest potentiation of contraction (PRPC) were similar in both Purkinje fibers and ventricular muscles, but inotropic agents induced tachyarrhythmia only in Purkinje fibers. In partially depolarized Purkinje fibers (MDP <-70 mV) with slow-response AP, isoproterenol and/or strophanthidin readily induced automatic and triggered rhythms.

CONCLUSIONS

Accumulation of excessive cytosolic Ca2+ in the presence of cardiotonic agents could lead to tachyarrhythmias in Purkinje fibers, but rarely in ventricular muscles of failing human hearts.

Cellular electrophysiological effect of terikalant in the dog heart

The cellular mechanism of action of terikalant, an investigational antiarrhythmic agent known to block the inward rectifier and other potassium currents, has not yet been fully clarified. The aim of the present study was therefore to analyse the in vitro electrophysiological effects of terikalant in canine isolated ventricular muscle and Purkinje fibers by applying the standard microelectrode technique. The effects of terikalant on the duration of action potential at a stimulation cycle length of 1000 ms and on the maximum upstroke velocity of the action potential in right ventricular papillary muscle were examined at 1, 2.5, 10, and 20 microM concentrations. Terikalant significantly prolonged the action potential duration measured both at 50% and 90% of repolarization in concentration-dependent manner. The maximum upstroke velocity of the action potential was unaffected at 1 and 2.5 microM concentrations. However, this parameter was significantly reduced at 10 and 20 microM concentrations of terikalant. In Purkinje fibers terikalant (2.5 microM) also produced a marked action potential lengthening effect. Frequency dependence (cycle length of 300-5000 ms) of the action potential lengthening effect of terikalant was studied at a concentration of 2.5 microM. Prolongation of the duration of action potential occurred in a reverse frequency-dependent manner both in papillary muscle and Purkinje fibers, with a more pronounced frequency-dependence observed in Purkinje fibers. The onset kinetics of the terikalant (10 microM) induced block of the maximum upstroke velocity of the action potential was rapid (0.6+/-0.1 beat(-1), n=6) like that of Class I/B antiarrhythmics, and the offset (recovery) kinetics of the drug (2956+/-696 ms, n=6) best resembled that of Class I/A antiarrhythmic drugs. It was concluded that terikalant, unlike pure Class III antiarrhythmic drugs, has combined mode of action by lengthening repolarization and blocking the inward sodium current in a use-dependent manner.

Effects of seven drugs of abuse on action potential repolarisation in sheep cardiac Purkinje fibres

Seven drugs of abuse have been examined for effects on the action potential in sheep isolated cardiac Purkinje fibres. Phencyclidine (5 microM) induced a significant increase (30.7%) in action potential duration at 90% repolarisation (APD(90)). Similarly, 10 microM 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy') induced a significant increase in APD(90) of 12.1%. Although Delta(9)-tetrahydrocannabinol (0.1 microM) induced a small, but statistically significant, 4.8% increase in APD(90), no effects were observed at 0.01 or 1 microM. Cocaethylene (10 microM) induced a significant shortening of APD(90) (-23.8%). Cocaine (up to 1 microM), (+)-methamphetamine ('Speed'; up to 5 microM), and the heroin metabolite, morphine (up to 5 microM), had no statistically significant effects. The possible significance of these findings is discussed in the context of other recognised cardiac effects of the tested drugs.

Age-dependent changes in contraction and regional myocardial myosin heavy chain isoform expression in rats

The goals of this study were to measure the relative levels of the alpha- and beta-isoforms of myosin heavy chain (MHC-alpha and MHC-beta, respectively) in multiple, specific regions of the adult rat heart and to determine whether age-dependent changes in isoform levels in different regions are uniform. Relative amounts of MHC-alpha and MHC-beta were determined in right and left atria and left ventricular (LV) Purkinje fibers (PF), papillary muscles, trabeculae, and endo-, mid-, and epicardial regions at 2, 5, 10, 16, and 21 mo. PFs contained substantial amounts of myosin and were striated and capable of generating force and shortening on activation. Levels of MHC-beta increased in all LV compartments with age, especially between 2 and 5 mo. There was more MHC-beta in PFs than other LV sites. There were regional differences in the level of MHC-beta throughout the LV at all ages, and the rates of change within regions differed. Ca(2+)-activated tension in PFs and trabeculae was compared at 2 and 22 mo. PF tension was less than trabecula tension, and this difference may be explained by differences in MHC content. V(max) and tension-generating ability in PFs decreased with age. Maximal tension generated by trabeculae did not change during aging. A large proportion of the increase in the level of MHC-beta that is normally associated with aging occurs at a relatively early age in rat LV. PFs, with their small diameters and short diffusion distance, should be considered for skinned multicellular myocardial studies.

Lack of action potential-prolonging effect of terfenadine on rabbit myocardial tissue preparations

The effects of terfenadine, an antiallergic drug also known for its QT-prolonging and arrhythmogenic activities, on the action potential of isolated myocardial tissue preparations from rabbits were examined with microelectrode techniques. In the Purkinje fibers and atrium, terfenadine concentration dependently decreased the maximum rate of rise (+.V(max)) without affecting other action potential parameters. In the ventricle, terfenadine had little effect on action potential configuration. In the sinoatrial node, terfenadine 20 microM prolonged cycle length mainly through inhibition of +.V(max). Terfenadine 1 microM completely inhibited the human ether a go-go-related gene (HERG) channel current expressed in HEK293 cells in the same experimental solution as in microelectrode experiments. The lack of terfenadine effect on the action potential duration suggests that there are drugs for which the HERG channel inhibitory action underlying in vivo QT prolongation cannot be evaluated based on their action potential-prolonging activity in isolated myocardial tissue preparations.

Evidence for gender differences in electrophysiological properties of canine Purkinje fibres

Women are more prone to develop torsades de pointes, a rare life-threatening polymorphic ventricular tachycardia, than are men during administration of medicines that have the potential to block I(Kr) (rapid delayed rectifier cardiac K(+) current) and to prolong the QT interval. Blockade of I(Kr), hypokalaemia and extreme bradycardia were used to evaluate whether there are gender differences in cardiac repolarisation in canine Purkinje fibres (PFs). Microelectrode techniques were employed to measure action potential (AP) parameters in PFs from adult female and male dogs. Under control conditions, fibres from female animals in normal or low K(+) conditions exhibited significantly longer AP durations at 50% (APD(50)) and 90% (APD(90)) of repolarisation as compared with APDs of fibres from male animals. Gender-related difference to rate adaptation was also present in APD(90) of fibres from female animals compared to males. At a stimulation rate of 0.2 Hz, but not at 1.0 Hz, dofetilide elicited a significantly higher increase in APD(90), incidence of early afterdepolarisations, triggered and sustained-triggered activities (TAs) in fibres from female animals compared to males in either normal or low K(+) conditions. The sustained TAs were reversed by raising the concentration of [K(+)](0) in Purkinje preparations from both male (one out of one) and female (12 out of 12) dogs. In conclusion, our data provide experimental evidence pointing to gender differences in canine AP repolarisation. PFs from female dogs can be used in safety pharmacology studies as a sensitive model for evaluating the potential proarrhythmic events in vitro of a new medicinal product.

Use-dependent blockade of cardiac pacemaker current (If) by cilobradine and zatebradine

The action of the bradycardiac agents, cilobradine (DK-AH269) and zatebradine (UL-FS49), on the cardiac pacemaker current (If) was investigated on short Purkinje fibres from sheep hearts, using the two-microelectrode voltage-clamp technique, and on isolated rabbit sino-atrial cells with the patch clamp technique. These drugs reduce dose dependently the amplitude of the If, without modifying either the voltage dependence or the kinetics of channel activation. When voltage-clamp pulse trains were applied, cilobradine induced a use-dependent blockade of If that was stronger and faster than that with zatebradine. Recovery from blockade during prolonged hyperpolarization was significantly faster with zatebradine. Presumably, both drugs block the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel by gaining access to a binding site within the open channel pore, and are removed from the blocking site by strong hyperpolarization with large inward If through the open channel. Cilobradine, compared to zatebradine blocks If more effectively and faster in both preparations. Consequently cilobradine strongly reduces the pacemaker diastolic depolarization rate and the cell's firing frequency.

The utility of hERG and repolarization assays in evaluating delayed cardiac repolarization: influence of multi-channel block

Drug-induced delayed cardiac repolarization is a recognized risk factor for proarrhythmia and is associated with block of IKr (the potassium current encoded by the human ether-a- go-go-related gene [hERG]). To evaluate the utility of 2 in vitro assays widely used to assess delayed repolarization, we compared the effects of haloperidol and 9 structurally diverse drugs in a hERG and repolarization (canine Purkinje fiber action potential duration [APD]) assay over wide concentrations. Despite potent hERG current block (IC50 = 0.174 microM), haloperidol elicited a bell-shaped concentration-response relationship for APD prolongation, with lesser prolongation (and reduced plateau height) observed with concentrations eliciting maximal hERG block, consistent with multi-channel block at higher concentrations. Consistent with this hypothesis, APD prolongation with the specific IKr blocker dofetilide was a) reduced by concomitant administration of nifedipine (calcium current block) and b) reversed by lidocaine (late sodium current block). Additional studies demonstrated prominent (>50%) hERG inhibition with most (9/10) drugs despite wide APD changes (158% prolongation - 16% shortening), consistent with multi-channel block. The poor correlation between hERG and repolarization assays suggests that the hERG assay oversimplifies drug effects on the complex repolarization process for drugs demonstrating multi-channel block and that neither assay alone adequately predicts proarrhythmic risk.

Dopamine receptor agonists differ in their actions on cardiac ion channels

Four dopamine receptor agonists used for the treatment of Parkinson's disease (apomorphine, pergolide, ropinirole and sumanirole) were evaluated for the ability to block human ether-a-go-go related gene (hERG) K(+) channels and to modify the duration of canine Purkinje fiber action potentials. Apomorphine, pergolide and ropinirole blocked the hERG-mediated currents with IC(50) values of 2.4, 0.12 and 1.2 microM, respectively. When evaluated in an action potential duration assay, pergolide significantly shortened action potential duration at 90% repolarization (APD(90)) whereas apomorphine and ropinirole significantly prolonged repolarization. Sumanirole only partially blocked hERG K(+) channels at the highest tested concentration (10 microM) and did not modify action potential duration over the tested concentration range (0.65-65 microM). Taken together, these data provide evidence that dopamine receptor agonists developed for the treatment of Parkinson's disease differentially influence hERG K(+) channel function and cardiac action potential duration.

Contrasting roles of a novel K+ channel blocker and a K+ channel opener on electro-mechanical activity in canine heart tissue

We tested the effects of a potassium channel opener diazoxide on the action potential duration (APD) and contractile force changes in canine Purkinje tissue induced by a novel class III anti-arrhythmic agent (C3A), KCB-328 (0.5 microM) with 3,4-dimethoxyphenethyl ring structure (0.5 microM). KCB-328 shortened APD(25) by 8.3+/-2.1%, prolonged APD(50) and APD(90) by 31.2+/-5.3 and 50.0+/-7.1%, respectively. Diazoxide (0.1 mM) shortened APD at all levels by 58.3+/-8.1, 54.1+/-6.1, and 42.8+/-5.8%, respectively. In the presence of diazoxide, KCB-328 still prolonged APD(50) and APD(90) (12.5+/-3.8 and 26.8+/-5.9%, respectively). KCB-328 increased force of contraction in a dose-dependent manner. KCB-328 increased force less in the presence of diazoxide. Administration of diazoxide only, reduced force of contraction. We conclude that APD prolongation by KCB-328 may occur even in the presence of diazoxide. It is not sufficient for the restoration of already diminished contractile force and that such an APD prolongation may be unrelated to the restoration of force of contraction even though both are most often seen to occur simultaneously.