Comparison of HERG channel blocking effects of various beta-blockers-- implication for clinical strategy

Differential Effects of the Betablockers Carvedilol, Metoprolol and Bisoprolol on Cardiac Kv4.3 (Ito) Channel Isoforms

Cardiac Kv4.3 channels contribute to the transient outward K+ current, Ito, during early repolarization of the cardiac action potential. Two different isoforms of Kv4.3 are present in the human ventricle and exhibit differential remodeling in heart failure (HF). Cardioselective betablockers are a cornerstone of HF with reduced ejection fraction therapy as well as ventricular arrhythmia treatment. In this study we examined pharmacological effects of betablockers on both Kv4.3 isoforms to explore their potential for isoform-specific therapy. Kv4.3 isoforms were expressed in Xenopus laevis oocytes and incubated with the respective betablockers. Dose-dependency and biophysical characteristics were examined. HEK 293T-cells were transfected with the two Kv4.3 isoforms and analyzed with Western blots. Carvedilol (100 µM) blocked Kv4.3 L by 77 ± 2% and Kv4.3 S by 67 ± 6%, respectively. Metoprolol (100 µM) was less effective with inhibition of 37 ± 3% (Kv4.3 L) and 35 ± 4% (Kv4.3 S). Bisoprolol showed no inhibitory effect. Current reduction was not caused by changes in Kv4.3 protein expression. Carvedilol inhibited Kv4.3 channels at physiologically relevant concentrations, affecting both isoforms. Metoprolol showed a weaker blocking effect and bisoprolol did not exert an effect on Kv4.3. Blockade of repolarizing Kv4.3 channels by carvedilol and metoprolol extend their pharmacological mechanism of action, potentially contributing beneficial antiarrhythmic effects in normal and failing hearts.

The Potential Mechanisms behind Loperamide-Induced Cardiac Arrhythmias Associated with Human Abuse and Extreme Overdose

Loperamide has been a safe and effective treatment for diarrhea for many years. However, many cases of cardiotoxicity with intentional abuse of loperamide ingestion have recently been reported. We evaluated loperamide in in vitro and in vivo cardiac safety models to understand the mechanisms for this cardiotoxicity. Loperamide slowed conduction (QRS-duration) starting at 0.3 µM [~1200-fold (×) its human Free Therapeutic Plasma Concentration; FTPC] and reduced the QT-interval and caused cardiac arrhythmias starting at 3 µM (~12,000× FTPC) in an isolated rabbit ventricular-wedge model. Loperamide also slowed conduction and elicited Type II/III A-V block in anesthetized guinea pigs at overdose exposures of 879× and 3802× FTPC. In ion-channel studies, loperamide inhibited hERG (IKr), INa, and ICa currents with IC50 values of 0.390 µM, 0.526 µM, and 4.091 µM, respectively (i.e., >1560× FTPC). Additionally, in silico trials in human ventricular action potential models based on these IC50s confirmed that loperamide has large safety margins at therapeutic exposures (≤600× FTPC) and confirmed repolarization abnormalities in the case of extreme doses of loperamide. The studies confirmed the large safety margin for the therapeutic use of loperamide but revealed that at the extreme exposure levels observed in human overdose, loperamide can cause a combination of conduction slowing and alterations in repolarization time, resulting in cardiac proarrhythmia. Loperamide's inhibition of the INa channel and hERG-mediated IKr are the most likely basis for this cardiac electrophysiological toxicity at overdose exposures. The cardiac toxic effects of loperamide at the overdoses could be aggravated by co-medication with other drug(s) causing ion channel inhibition.

hERG agonists pose challenges to web-based machine learning methods for prediction of drug-hERG channel interaction

Pharmacological blockade of the IKr channel (hERG) by diverse drugs in clinical use is associated with the Long QT Syndrome that can lead to life threatening arrhythmia. Various computational tools including machine learning models (MLM) for the prediction of hERG inhibition have been developed to facilitate the throughput screening of drugs in development and optimise thus the prediction of hERG liabilities. The use of MLM relies on large libraries of training compounds for the quantitative structure-activity relationship (QSAR) modelling of hERG inhibition. The focus on inhibition omits potential effects of hERG channel agonist molecules and their associated QT shortening risk. It is instructive, therefore, to consider how known hERG agonists are handled by MLM. Here, two highly developed online computational tools for the prediction of hERG liability, Pred-hERG and HergSPred were probed for their ability to detect hERG activator drug molecules as hERG interactors. In total, 73 hERG blockers were tested with both computational tools giving overall good predictions for hERG blockers with reported IC50s below Pred-hERG and HergSPred cut-off threshold for hERG inhibition. However, for compounds with reported IC50s above this threshold such as disopyramide or sotalol discrepancies were observed. HergSPred identified all 20 hERG agonists selected as interacting with the hERG channel. Further studies are warranted to improve online MLM prediction of hERG related cardiotoxicity, by explicitly taking into account channel agonism as well as inhibition.

Multitargeting in cardioprotection: An example of biaromatic compounds

A multitarget drug design approach is actively developing in modern medicinal chemistry and pharmacology, especially with regard to multifactorial diseases such as cardiovascular diseases, cancer, and neurodegenerative diseases. A detailed study of many well-known drugs developed within the single-target approach also often reveals additional mechanisms of their real pharmacological action. One of the multitarget drug design approaches can be the identification of the basic pharmacophore models corresponding to a wide range of the required target ligands. Among such models in the group of cardioprotectors is the linked biaromatic system. This review develops the concept of a "basic pharmacophore" using the biaromatic pharmacophore of cardioprotectors as an example. It presents an analysis of possible biological targets for compounds corresponding to the biaromatic pharmacophore and an analysis of the spectrum of biological targets for the five most known and most studied cardioprotective drugs corresponding to this model, and their involvement in the biological effects of these drugs.

Calculations of the binding free energies of the Comprehensive in vitro Proarrhythmia Assay (CiPA) reference drugs to cardiac ion channels

The evaluation of the inhibitory activities of drugs on multiple cardiac ion channels is required for the accurate assessment of proarrhythmic risks. Moreover, the in silico prediction of such inhibitory activities of drugs on cardiac channels can improve the efficiency of the drug-development process. Here, we performed molecular docking simulations to predict the complex structures of 25 reference drugs that were proposed by the Comprehensive in vitro Proarrhythmia Assay consortium using two cardiac ion channels, the human ether-a-go-go-related gene (hERG) potassium channel and human NaV1.5 (hNaV1.5) sodium channel, with experimentally available structures. The absolute binding free energy (ΔGbind) values of the predicted structures were calculated by a molecular dynamics-based method and compared with the experimental half-maximal inhibitory concentration (IC50) data. Furthermore, the regression analysis between the calculated values and negative of the common logarithm of the experimental IC50 values (pIC50) revealed that the calculated values of four and ten drugs deviated significantly from the regression lines of the hERG and hNaV1.5 channels, respectively. We reconsidered the docking poses and protonation states of the drugs based on the experimental data and recalculated their ΔGbind values. Finally, the calculated ΔGbind values of 24 and 19 drugs correlated with their experimental pIC50 values (coefficients of determination=0.791 and 0.613 for the hERG and hNaV1.5 channels, respectively). Thus, the regression analysis between the calculated ΔGbind and experimental IC50 data ensured the realization of an increased number of reliable complex structures.

Beneficial effects of carvedilol modulating potassium channels on the control of glucose

The increased prevalence of hypertensive patients with type 2 diabetes mellitus (T2DM) is evident worldwide, leading to a higher risk of cardiovascular disease onset, which is substantially associated with disabilities and mortality in the clinic. In order to achieve the satisfyingly clinical outcomes and prognosis, the comprehensive therapies have been conducted with a beneficial effect on both blood pressure and glucose homeostasis, and clinical trials reveal that some kind of antihypertensive drugs such as angiotensin converting enzyme inhibitors (ACE-I) may, at least in part, meet the dual requirement during the disease management. As a nonselective β-blocker, carvedilol is employed for treating many cardiovascular diseases in clinical practice, including hypertension, angina pectoris and heart failure, and also exhibit the effectiveness for glycemic control and insulin resistance. Apart from alleviating sympathetic nervous system activity, several causes, such as lowering oxygen reactive species, may contribute to the effects of carvedilol on controlling plasma glucose levels, suggesting a feature of this drug having multiple targets. Interestingly, numerous distinct K⁺ channels expressed in pancreatic β-cells and peripheral insulin-sensitive tissues, which play a sentential role in glucose metabolism, are subjected to extensive modulation of carvdilol, establishing a linkage between K⁺ channels and drug's effects on the control of glucose. A variety of evidence shows that the impact of carvedilol on different K⁺ channels, including Kv, K_Ach, K_ATP and K_2 P, can lead to positive influences for glucose homeostasis, contributing to its clinical beneficial effectiveness in treatment of hypertensive patients with T2DM. This review focus on the control of plasma glucose conferred by carvedilol modulation on K⁺ channels, providing the novel mechanistic explanation for drug's actions.

A Carvedilol Analogue, VK-II-86, Prevents Hypokalaemia-induced Ventricular Arrhythmia through Novel multi-Channel Effects

BACKGROUND AND PURPOSE

QT prolongation and intracellular Ca²⁺ loading with diastolic Ca²⁺ release via ryanodine receptors (RyR2) are the predominant mechanisms underlying hypokalaemia-induced ventricular arrhythmia. We investigated the antiarrhythmic actions of two RyR2 inhibitors: dantrolene and VK-II-86, a carvedilol analogue with no β-blocking activity, in hypokalaemia.

EXPERIMENTAL APPROACH

Surface ECG and ventricular action potentials (APs) were recorded from whole-heart murine Langendorff preparations. Ventricular arrhythmia incidence was compared in hearts perfused with low [K⁺ ], and those pre-treated with dantrolene or VK-II-86. Whole-cell patch clamping was used in murine and canine ventricular cardiomyocytes to study the effects of dantrolene and VK-II-86 on AP parameters in low [K⁺ ] and the effects of VK-II-86 on the inward rectifier current (I_K1 ), late sodium current (I_{Na_L} ) and the L-type Ca²⁺ current (I_Ca ). Effects of VK-II-86 on I_Kr were investigated in transfected HEK-293 cells. A fluorogenic probe quantified the effects of VK-II-86 on oxidative stress in hypokalaemia.

KEY RESULTS

Dantrolene reduced the incidence of ventricular arrhythmias induced by low [K⁺ ] in explanted murine hearts by 94%, whereas VK-II-86 prevented all arrhythmias. VK-II-86 prevented hypokalaemia-induced AP prolongation and depolarization, but did not alter AP parameters in normokalaemia. Hypokalaemia was associated with a significant reduction of I_K1 and I_Kr , and increase in I_Na-L , and I_Ca . VK-II-86 prevented all hypokalaemia-induced changes in ion channel activity and oxidative stress.

CONCLUSIONS AND IMPLICATIONS

VK-II-86 prevents hypokalaemia-induced arrhythmogenesis by normalising calcium homeostasis and repolarization reserve. VK-II-86 may provide an exciting treatment in hypokalaemia and other arrhythmias caused by delayed repolarization or Ca²⁺ overload.

Flavonoids and hERG channels: Friends or foes?

The cardiac action potential is regulated by several ion channels. Drugs capable to block these channels, in particular the human ether-à-go-go-related gene (hERG) channel, also known as K_V11.1 channel, may lead to a potentially lethal ventricular tachyarrhythmia called "Torsades de Pointes". Thus, evaluation of the hERG channel off-target activity of novel chemical entities is nowadays required to safeguard patients as well as to avoid attrition in drug development. Flavonoids, a large class of natural compounds abundantly present in food, beverages, herbal medicines, and dietary food supplements, generally escape this assessment, though consumed in consistent amounts. Continuously growing evidence indicates that these compounds may interact with the hERG channel and block it. The present review, by examining numerous studies, summarizes the state-of-the-art in this field, describing the most significant examples of direct and indirect inhibition of the hERG channel current operated by flavonoids. A description of the molecular interactions between a few of these natural molecules and the Rattus norvegicus channel protein, achieved by an in silico approach, is also presented.

Intrauterine Growth Retardation in Pregnant Women with Long QT Syndrome Treated with Beta-Receptor Blockers

Pregnant women with inherited long QT syndrome (iLQTS) are at an increased risk for preterm delivery and intrauterine growth retardation (IUGR) due to their underlying disease. Additionally, they are at a risk of arrhythmogenic events, particularly during the postpartum period because of physiological changes and increased emotional/physical stress. β-receptor blockers can effectively prevent life-threatening Torsades de Pointes ventricular tachycardia and they are the treatment of choice in iLQTS. Use of β-receptor blockers in pregnancy is recommended, although IUGR is commonly reported for prenatally exposed infants. IUGR, particularly in preterm infants, can result in adverse neonatal outcomes. This review was performed to support clinicians in their selection of β-receptor blocker treatment for their pregnant iLQTS women by (i) summarizing the available literature addressing the impact of different β-receptor blockers on IUGR and (ii) reporting additional aspects which might influence the β-receptor blocker selection. In general, experts recommend to use nonselective β-receptor blockers, such as nadolol and propranolol, for iLQTS management as these drugs seem to be superior in effectiveness. However, β-1-selective receptor blockers, such as bisoprolol or metoprolol, seem to affect less likely uterine contraction, peripheral vasodilation, and are associated with lower IUGR rates and fetal hypoglycemia. They are therefore recommended, except atenolol, as first-line therapy for pregnant women. Additionally, maternal factors such as iLQTS genotype, other underlying comorbidities (e.g., diabetes mellitus type 1, asthma bronchiale), and uteroplacental dysfunction or fetal factors have to be taken into account. Therefore, each woman with iLQTS who wants to become pregnant should be well-advised for a personalized β-receptor blocker therapy according to the individual risk-benefit evaluation by a multidisciplinary team of cardiologists, gynecologists, pediatric cardiologists, neonatologists, and clinical pharmacologists. During pregnancy, a close monitoring of IUGR and, after birth, monitoring of bradycardia, hypoglycemia, and respiratory depression in the neonate is mandatory. This review summarizes available data on β-receptor blocker-related risk for IUGR in prenatally exposed infants and illustrates which factors might influence β-receptor blocker selection with the aim to support clinicians in their pharmacological management of their pregnant iLQTS patients.

Usefulness of Bnet, a Simple Linear Metric in Discerning Torsades De Pointes Risks in 28 CiPA Drugs

The Comprehensive in vitro Proarrhythmia Assay (CiPA) project suggested the torsade metric score (TMS) which requires substantial computing resources as a useful biomarker to predict proarrhythmic risk from human ether-à-go-go-related gene (hERG) and a few other ion channel block data. The TMS was useful to predict low TdP risks of drugs blocking Na+ (ranolazine) and Ca2+ (verapamil) channels as well as the hERG channel. However, Mistry asserted that the simple linear metric, Bnet reflecting net blockade of a few influential ion channels has similar predictive power. Here we compared the predictability of Bnet and TMS for the 12 training and 16 validation CiPA drugs which were pre-classified into three categories according to the known TdP risks (low, intermediate, and high risk) by CiPA. Bnet at 5×Cmax (Bnet5×Cmax) was calculated using the ion-channel IC50 and Hill coefficients of CiPA drugs collected from previous reports by the CiPA team and others. The receiver operating characteristic curve area under curve (ROC AUC) values for TMS and Bnet5×Cmax as performance metrics in discerning low versus intermediate/high risk categories for the 28 CiPA drugs were similar. However, Bnet5×Cmax was much inferior to TMS at discerning between intermediate- and high-risk drugs. Dynamic Bnet, which used in silico hERG dynamic parameters unlike conventional Bnet, improved the misspecification. Thus, we propose that Bnet5×Cmax is used for quick screening of TdP risks of drug candidates and if the "intermediate/high" risk is predicted by Bnet5×Cmax, in silico approaches, such as dynamic Bnet or TMS, may be further considered.

Carvedilol blockage of delayed rectifier Kv2.1 channels and its molecular basis

Previous studies indicated that one of the action targets of carvedilol is the voltage-gated potassium (Kv) channel, which has a fundamental role in the control of electrical properties in excitable cells. It is not clear whether this compound exerts any actions specifically on delayed rectifier Kv2.1 channels. The present study is conducted on Kv2.1 currents heterologously expressed in HEK293 cells to characterize the block by carvedilol in detail, identifying molecular determinants and providing biophysical insights of the block. Macroscopic Kv2.1 currents obtained by whole-cell recording were substantially inhibited after addition of carvedilol with an IC₅₀ value of 5.1 μM. This drug also led to a largely hyperpolarizing shift (30 mV) of the inactivation curve, which may contribute to the blocking action due to more inactivation of Kv2.1 currents occurred in depolarization potentials. Mutations at Y380 (a putative TEA binding site) and K356 (a K⁺ binding site) in the outer vestibule of Kv2.1 channels significantly eliminated the inhibitory effects of carvedilol and prevented the leftward shift of inactivation. Moreover, mutations at above positions modulated the effects of carvedilol on the deactivation but not activation kinetics of Kv2.1 channels. Collected data indicate that carvedilol can exert a blocking effect on the closed-state of Kv2.1 channels, and specific residues within the S5-P and P-S6 linkers in the outer vestibule may play crucial roles in carvedilol-induced blocking for Kv2.1 channels.

Effects of Calcium, Magnesium, and Potassium Concentrations on Ventricular Repolarization in Unselected Individuals

BACKGROUND

Subclinical changes on the electrocardiogram are risk factors for cardiovascular mortality. Recognition and knowledge of electrolyte associations in cardiac electrophysiology are based on only in vitro models and observations in patients with severe medical conditions.

OBJECTIVES

This study sought to investigate associations between serum electrolyte concentrations and changes in cardiac electrophysiology in the general population.

METHODS

Summary results collected from 153,014 individuals (54.4% women; mean age 55.1 ± 12.1 years) from 33 studies (of 5 ancestries) were meta-analyzed. Linear regression analyses examining associations between electrolyte concentrations (mmol/l of calcium, potassium, sodium, and magnesium), and electrocardiographic intervals (RR, QT, QRS, JT, and PR intervals) were performed. The study adjusted for potential confounders and also stratified by ancestry, sex, and use of antihypertensive drugs.

RESULTS

Lower calcium was associated with longer QT intervals (-11.5 ms; 99.75% confidence interval [CI]: -13.7 to -9.3) and JT duration, with sex-specific effects. In contrast, higher magnesium was associated with longer QT intervals (7.2 ms; 99.75% CI: 1.3 to 13.1) and JT. Lower potassium was associated with longer QT intervals (-2.8 ms; 99.75% CI: -3.5 to -2.0), JT, QRS, and PR durations, but all potassium associations were driven by use of antihypertensive drugs. No physiologically relevant associations were observed for sodium or RR intervals.

CONCLUSIONS

The study identified physiologically relevant associations between electrolytes and electrocardiographic intervals in a large-scale analysis combining cohorts from different settings. The results provide insights for further cardiac electrophysiology research and could potentially influence clinical practice, especially the association between calcium and QT duration, by which calcium levels at the bottom 2% of the population distribution led to clinically relevant QT prolongation by >5 ms.

Differential Free Intracellular Calcium Release by Class II Antiarrhythmics in Cancer Cell Lines

Class II antiarrhythmics or β-blockers are antisympathetic nervous system agents that act by blocking β-adrenoceptors. Despite their common clinical use, little is known about the effects of β-blockers on free intracellular calcium (Ca²⁺ _i), an important cytosolic second messenger and a key regulator of cell function. We investigated the role of four chemical analogs, commonly prescribed β-blockers (atenolol, metoprolol, propranolol, and sotalol), on Ca²⁺ _i release and whole-cell currents in mammalian cancer cells (PC3 prostate cancer and MCF7 breast cancer cell lines). We discovered that only propranolol activated free Ca²⁺ _i release with distinct kinetics, whereas atenolol, metoprolol, and sotalol did not. The propranolol-induced Ca²⁺ _i release was significantly inhibited by the chelation of extracellular calcium with ethylene glycol tetraacetic acid (EGTA) and by dantrolene, an inhibitor of the endoplasmic reticulum (ER) ryanodine receptor channels, and it was completely abolished by 2-aminoethoxydiphenyl borate, an inhibitor of the ER inositol-1,4,5-trisphosphate (IP₃) receptor channels. Exhaustion of ER stores with 4-chloro-m-cresol, a ryanodine receptor activator, or thapsigargin, a sarco/ER Ca²⁺ ATPase inhibitor, precluded the propranolol-induced Ca²⁺ _i release. Finally, preincubation of cells with sotalol or timolol, nonselective blockers of β-adrenoceptors, also reduced the Ca²⁺ _i release activated by propranolol. Our results show that different β-blockers have differential effects on whole-cell currents and free Ca²⁺ _i release and that propranolol activates store-operated Ca²⁺ _i release via a mechanism that involves calcium-induced calcium release and putative downstream transducers such as IP₃ The differential action of class II antiarrhythmics on Ca²⁺ _i release may have implications on the pharmacology of these drugs.

International Multisite Study of Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes for Drug Proarrhythmic Potential Assessment

To assess the utility of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) as an in vitro proarrhythmia model, we evaluated the concentration dependence and sources of variability of electrophysiologic responses to 28 drugs linked to low, intermediate, and high torsades de pointes (TdP) risk categories using two commercial cell lines and standardized protocols in a blinded multisite study using multielectrode array or voltage-sensing optical approaches. Logistical and ordinal linear regression models were constructed using drug responses as predictors and TdP risk categories as outcomes. Three of seven predictors (drug-induced arrhythmia-like events and prolongation of repolarization at either maximum tested or maximal clinical exposures) categorized drugs with reasonable accuracy (area under the curve values of receiver operator curves ∼0.8). hiPSC-CM line, test site, and platform had minimal influence on drug categorization. These results demonstrate the utility of hiPSC-CMs to detect drug-induced proarrhythmic effects as part of the evolving Comprehensive In Vitro Proarrhythmia Assay paradigm.

Characterization and molecular basis for the block of Kv1.3 channels induced by carvedilol in HEK293 cells

Carvedilol is a non-selective β-adrenoreceptor antagonist and exhibits a wide range of biological activities. The voltage-gated K⁺ (Kv) channel is one of the target ion channels of this compound. The rapidly activating Kv1.3 channel is expressed in several different tissues and plays an important role in the regulation of physiological functions, including cell proliferation and apoptosis. However, little is known about the possible action of carvedilol on Kv1.3 currents. Using the whole-cell configuration of the patch-clamp technique, we have revealed that exposure to carvedilol produced a concentration-dependent blocking of Kv1.3 channels heterologously expressed in HEK293 cells, with an IC₅₀ value of 9.7 μM. This chemical decelerated the deactivation tail current of Kv1.3 currents, resulting in a tail crossover phenomenon. In addition, carvedilol generated a markedly hyperpolarizing shift (20 mV) of the inactivation curve, but failed to affect the activation curve. Mutagenesis experiments of Kv1.3 channels identified G427 and H451, two related sites of TEA block, as important residues for carvedilol-mediated blocking. The present results suggest that carvedilol acts directly on Kv1.3 currents by inducing closed- and open-channel block and helps to elucidate the mechanisms of action of this compound on Kv channels.

Inhibition of rapid delayed rectifier potassium current (IKr) by ischemia/reperfusion and its recovery by vitamin E in ventricular myocytes

Ischemia/reperfusion (I/R) induces prolongation of QT interval and action potential duration (APD), which is a major cardiac electrical disorder in patients with arrhythmias. However, the mechanism of QT interval prolongation induced by I/R remains unclear. In the present study, we hypothesized that the rapid component of delayed rectifier potassium (I_Kr) channel plays an important role in I/R-induced QT interval prolongation. We observed a marked attenuation of I_Kr and a significant prolongation of action potential duration (APD) in a simulated I/R system with sodium dithionite (Na₂S₂O₄) in ventricular myocytes of guinea pigs. The I_Kr current density was inhibited by 64% and APD increased by 87% respectively. Moreover, the inhibition of I_Kr is primarily ascribed to overproduction of reactive oxygen species (ROS) by I/R, which can be partly reversed by antioxidant vitamin E (100μmol/L). The value of I_Kr tail current density increased from 0.516±0.040 pA/pF in I/R to 0.939±0.091 pA/pF when treated with vitamin E. Moreover, we also demonstrated that QTc interval was increased by I/R and reversed by Vitamin E in isolated guinea pig hearts. In conclusion, the inhibition of I_Kr is one of the underlying mechanisms of prolongation of QT interval and APD in I/R. Vitamin E might have a benefit in coronary reperfusion therapy.

T-Wave Morphology Analysis in Congenital Long QT Syndrome Discriminates Patients From Healthy Individuals

OBJECTIVES

This study aims to assess the capability of T-wave analysis to: 1) identify genotype-positive long QT syndrome (LQTS) patients; 2) identify LQTS patients with borderline or normal QTc interval (≤460 ms); and 3) classify LQTS subtype.

BACKGROUND

LQTS often presents with a nondiagnostic electrocardiogram (ECG). T-wave abnormalities may be the only marker of this potentially lethal arrhythmia syndrome.

METHODS

ECGs taken at rest in 108 patients (43 with LQTS1, 20 with LQTS2, and 45 control subjects) were evaluated for T-wave flatness, asymmetry, and notching, which produces a morphology combination score (MCS) of the 3 features (MCS = 1.6 × flatness + asymmetry + notch) using QT Guard Plus Software (GE Healthcare, Milwaukee, Wisconsin). To assess for heterogeneity of repolarization, the principal component analysis ratio 2 (PCA-2) was calculated.

RESULTS

Mean QTc intervals were 486 ± 50 ms (LQTS1), 479 ± 36 ms (LQTS2), and 418 ± 24 ms (control subjects) (p < 0.05). MCS and PCA-2 differed between LQTS patients and control subjects (MCS: 117.8 ± 57.4 vs. 71.9 ± 16.2; p < 0.001; PCA-2: 20.2 ± 10.4% vs. 14.6 ± 5.5%; p < 0.001), LQTS1 and LQTS2 patients (MCS: 96.3 ± 28.7 vs. 164 ± 75.2; p < 0.001; PCA-2: 17.8 ± 8.3% vs. 25 ± 12.6%; p < 0.001), and between LQTS patients with borderline or normal QTc intervals (n = 17) and control subjects (MCS: 105.7 ± 49.9 vs. 71.9 ± 16.2; p < 0.001; PCA-2: 18.1 ± 7.2% vs. 14.6 ± 5.5%; p < 0.001). T-wave metrics were consistent across multiple ECGs from individual patients based on the average intraclass correlation coefficient (MCS: 0.96; PCA-2: 0.86).

CONCLUSIONS

Automated T-wave morphology analysis accurately discriminates patients with pathogenic LQTS mutations from control subjects and between the 2 most common LQTS subtypes. Mutation carriers without baseline QTc prolongation were also identified. This may be a useful tool for screening families of LQTS patients, particularly when the QTc interval is subthreshold and genetic testing is unavailable.

Filamin C: a novel component of the KCNE2 interactome during hypoxia

Aim

KCNE2 encodes for the potassium voltage-gated channel, KCNE2. Mutations in KCNE2 have been associated with long-QT syndrome (LQTS). While KCNE2 has been extensively studied, the functions of its C-terminal domain remain inadequately described. Here, we aimed to elucidate the functions of this domain by identifying its protein interactors using yeast two-hybrid analysis.

Methods

The C-terminal domain of KCNE2 was used as bait to screen a human cardiac cDNA library for putative interacting proteins. Co-localisation and co-immunoprecipitation analyses were used for verification.

Results

Filamin C (FLNC) was identified as a putative interactor with KCNE2. FLNC and KCNE2 co-localised within the cell, however, a physical interaction was only observed under hypoxic conditions.

Conclusion

The identification of FLNC as a novel KCNE2 ligand not only enhances current understanding of ion channel function and regulation, but also provides valuable information about possible pathways likely to be involved in LQTS pathogenesis.

Control of the heart rate of rat embryos during the organogenic period

The aim of this study was to gain insight into whether the first trimester embryo could control its own heart rate (HR) in response to hypoxia. The gestational day 13 rat embryo is a good model for the human embryo at 5–6 weeks gestation, as the heart is comparable in development and, like the human embryo, has no functional autonomic nerve supply at this stage. Utilizing a whole-embryo culture technique, we examined the effects of different pharmacological agents on HR under normoxic (95% oxygen) and hypoxic (20% oxygen) conditions. Oxygen concentrations ≤60% caused a concentration-dependent decrease in HR from normal levels of ~210 bpm. An adenosine agonist, AMP-activated protein kinase (AMPK) activator and K_ATP channel opener all caused bradycardia in normoxic conditions; however, putative antagonists for these systems failed to prevent or ameliorate hypoxia-induced bradycardia. This suggests that the activation of one or more of these systems is not the primary cause of the observed hypoxia-induced bradycardia. Inhibition of oxidative phosphorylation also decreased HR in normoxic conditions, highlighting the importance of ATP levels. The β-blocker metoprolol caused a concentration-dependent reduction in HR supporting reports that β₁-adrenergic receptors are present in the early rat embryonic heart. The cAMP inducer colforsin induced a positive chronotropic effect in both normoxic and hypoxic conditions. Overall, the embryonic HR at this stage of development is responsive to the level of oxygenation, probably as a consequence of its influence on ATP production.

Genetics of inherited primary arrhythmia disorders

A sudden unexplained death is felt to be due to a primary arrhythmic disorder when no structural heart disease is found on autopsy, and there is no preceding documentation of heart disease. In these cases, death is presumed to be secondary to a lethal and potentially heritable abnormality of cardiac ion channel function. These channelopathies include congenital long QT syndrome, catecholaminergic polymorphic ventricular tachycardia, Brugada syndrome, and short QT syndrome. In certain cases, genetic testing may have an important role in supporting a diagnosis of a primary arrhythmia disorder, and can also provide prognostic information, but by far the greatest strength of genetic testing lies in the screening of family members, who may be at risk. The purpose of this review is to describe the basic genetic and molecular pathophysiology of the primary inherited arrhythmia disorders, and to outline a rational approach to genetic testing, management, and family screening.

Modulation of K2P 2.1 and K2P 10.1 K(+) channel sensitivity to carvedilol by alternative mRNA translation initiation

BACKGROUND AND PURPOSE

The β-receptor antagonist carvedilol blocks a range of ion channels. K2P 2.1 (TREK1) and K2P 10.1 (TREK2) channels are expressed in the heart and regulated by alternative translation initiation (ATI) of their mRNA, producing functionally distinct channel variants. The first objective was to investigate acute effects of carvedilol on human K2P 2.1 and K2P 10.1 channels. Second, we sought to study ATI-dependent modulation of K2P K(+) current sensitivity to carvedilol.

EXPERIMENTAL APPROACH

Using standard electrophysiological techniques, we recorded currents from wild-type and mutant K2P 2.1 and K2P 10.1 channels in Xenopus oocytes and HEK 293 cells.

KEY RESULTS

Carvedilol concentration-dependently inhibited K2P 2.1 channels (IC50 ,oocytes = 20.3 μM; IC50 , HEK = 1.6 μM) and this inhibition was frequency-independent. When K2P 2.1 isoforms generated by ATI were studied separately in oocytes, the IC50 value for carvedilol inhibition of full-length channels (16.5 μM) was almost 5-fold less than that for the truncated channel variant (IC50 = 79.0 μM). Similarly, the related K2P 10.1 channels were blocked by carvedilol (IC50 ,oocytes = 24.0 μM; IC50 , HEK = 7.6 μM) and subject to ATI-dependent modulation of drug sensitivity.

CONCLUSIONS AND IMPLICATIONS

Carvedilol targets K2P 2.1 and K2P 10.1 K(+) channels. This previously unrecognized mechanism supports a general role of cardiac K2P channels as antiarrhythmic drug targets. Furthermore, the work reveals that the sensitivity of the cardiac ion channels K2P 2.1 and K2P 10.1 to block was modulated by alternative mRNA translation initiation.

Functional cross-talk between the α1- and β1-adrenergic receptors modulates the rapidly activating delayed rectifier potassium current in guinea pig ventricular myocytes

The rapidly activating delayed rectifier potassium current (I_Kr) plays a critical role in cardiac repolarization. Although I_Kr is known to be regulated by both α₁- and β₁-adrenergic receptors (ARs), the cross-talk and feedback mechanisms that dictate its response to α₁- and β₁-AR activation are not known. In the present study, I_Kr was recorded using the whole-cell patch-clamp technique. I_Kr amplitude was measured before and after the sequential application of selective adrenergic agonists targeting α₁- and β₁-ARs. Stimulation of either receptor alone (α₁-ARs using 1 μM phenylephrine (PE) or β₁-ARs using 10 μM xamoterol (Xamo)) reduced I_Kr by 0.22 ± 0.03 and 0.28 ± 0.01, respectively. The voltage-dependent activation curve of I_Kr shifted in the negative direction. The half-maximal activation voltage (V_0.5) was altered by −6.35 ± 1.53 and −1.95 ± 2.22 mV, respectively, with no major change in the slope factor (k). When myocytes were pretreated with Xamo, PE-induced reduction in I_Kr was markedly blunted and the corresponding change in V_0.5 was significantly altered. Similarly, when cells were pretreated with PE, Xamo-induced reduction of I_Kr was significantly attenuated. The present results demonstrate that functional cross-talk between α₁- and β₁-AR signaling regulates I_Kr. Such non-linear regulation may form a protective mechanism under excessive adrenergic stimulation.

Differential effects of the β-adrenoceptor blockers carvedilol and metoprolol on SQT1- and SQT2-mutant channels

BACKGROUND

N588K-KCNH2 and V307L-KCNQ1 mutations lead to a gain-of-function of IKr and IKs thus causing short-QT syndromes (SQT1, SQT2). Combined pharmacotherapies using K(+) -channel-blockers and β-blockers are effective in SQTS. Since β-blockers can block IKr and IKs , we aimed at determining carvedilol's and metoprolol's electrophysiological effects on N588K-KCNH2 and V307L-KCNQ1 channels.

METHODS

Wild-type (WT)-KCNH2, WT-KCNQ1 and mutant N588K-KCNH2 and V307L-KCNQ1 channels were expressed in CHO-K1 or HEK-293T cells and IKs and IKr were recorded at baseline and during β-blocker exposure.

RESULTS

Carvedilol (10 μM) reduced IKs tail in WT- and V307L-KCNQ1 by 36.5 ± 5% and 18.6 ± 9% (P < 0.05). IC50 values were 16.3 μM (WT) and 46.1 μM (V307L), indicating a 2.8-fold decrease in carvedilol's IKs -blocking potency in V307L-KCNQ1. Carvedilol's (1 μM) inhibition of the IKr tail was attenuated in N588K-KCNH2 (4.5 ± 3% vs 50.3 ± 4%, WT, P < 0.001) with IC50 values of 2.8 μM (WT) and 25.4 μM (N588K). Carvedilol's IKr end-pulse inhibition, however, was increased in N588K-KCNH2 (10 μM, 60.7 ± 6% vs 36.5 ± 5%, WT, P < 0.01). Metoprolol (100 μM) reduced IKr end-pulse by 0.23 ± 3% (WT) and 74.1 ± 7% (N588K, P < 0.05), IKr tail by 32.9 ± 10% (WT) and 68.8 ± 7% (N588K, P < 0.05), and reduced IKs end-pulse by 18.3 ± 5% (WT) and 57.1 ± 11% (V307L, P < 0.05) and IKs tail by 3.3 ± 1% (WT) and 45.1 ± 13 % (V307L, P < 0.05), indicating an increased sensitivity to metoprolol in SQT mutated channels.

CONCLUSIONS

N588K-KCNH2 and V307L-KCNQ1 mutations decrease carvedilol's inhibition of the IKs or IKr tail but increase carvedilol's IKr end-pulse inhibition and metoprolol's inhibition of tail and end-pulse currents. These different effects on SQT1 and SQT2 mutated channels should be considered when using β-blocker therapy in SQTS patients.

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.

Adrenoceptor-mediated enhancement of interleukin-33 production by dendritic cells

While noradrenaline and adrenaline suppress some aspects of immune functions, the immune enhancement via these catecholamines is not well understood. Interleukin (IL)-33, a novel member of the IL-1 family, promotes T helper type 2 (T(h)2)-associated inflammations and plays a role in allergic diseases. However, the precise immune cell source and the stimulating factors for IL-33 production are less well characterized. In the present study, we examined the effects of noradrenaline and adrenaline, stress-related catecholamines, on IL-33 production by dendritic cells (DCs). Murine bone marrow-derived DCs were stimulated with lipopolysaccharide (LPS) in the presence or absence of these catecholamines. LPS alone slightly increased IL-33 production by DCs. Noradrenaline or adrenaline dramatically enhanced IL-33 mRNA expression and its protein synthesis by DCs upon LPS stimulation. The noradrenaline-induced enhancement of IL-33 production was completely blocked by β(2)-adrenoceptor specific antagonist ICI 118,551, while β(2)-adrenoceptor specific agonist salmeterol enhanced DC production of IL-33. Protein kinase A (PKA) specific inhibitor H89 blocked the noradrenaline-induced IL-33 production. Cyclic adenosine monophosphate (cAMP) and its analogue enhanced DC production of IL-33 upon LPS stimulation. Thus, β(2)-adrenoceptor-mediated cAMP-PKA pathway appears to enhance DC production of IL-33. The adrenoceptor-mediated enhancement of IL-33 production by DCs might be associated with the stress-related progression of T(h)2-associated disorders.

Carvedilol targets human K2P 3.1 (TASK1) K+ leak channels

BACKGROUND AND PURPOSE

Human K(2P) 3.1 (TASK1) channels represent potential targets for pharmacological management of atrial fibrillation. K(2P) channels control excitability by stabilizing membrane potential and by expediting repolarization. In the heart, inhibition of K(2P) currents by class III antiarrhythmic drugs results in action potential prolongation and suppression of electrical automaticity. Carvedilol exerts antiarrhythmic activity and suppresses atrial fibrillation following cardiac surgery or cardioversion. The objective of this study was to investigate acute effects of carvedilol on human K(2P) 3.1 (hK(2P) 3.1) channels.

EXPERIMENTAL APPROACH

Two-electrode voltage clamp and whole-cell patch clamp electrophysiology was used to record hK(2P) 3.1 currents from Xenopus oocytes, Chinese hamster ovary (CHO) cells and human pulmonary artery smooth muscle cells (hPASMC).

KEY RESULTS

Carvedilol concentration-dependently inhibited hK(2P) 3.1 currents in Xenopus oocytes (IC(50) = 3.8 µM) and in mammalian CHO cells (IC(50) = 0.83 µM). In addition, carvedilol sensitivity of native I(K2P3.1) was demonstrated in hPASMC. Channels were blocked in open and closed states in frequency-dependent fashion, resulting in resting membrane potential depolarization by 7.7 mV. Carvedilol shifted the current-voltage (I-V) relationship by -6.9 mV towards hyperpolarized potentials. Open rectification, characteristic of K(2P) currents, was not affected.

CONCLUSIONS AND IMPLICATIONS

The antiarrhythmic drug carvedilol targets hK(2P) 3.1 background channels. We propose that cardiac hK(2P) 3.1 current blockade may suppress electrical automaticity, prolong atrial refractoriness and contribute to the class III antiarrhythmic action in patients treated with the drug.

Development of small-molecule endotoxin sequestering agents

Sepsis, otherwise referred to as "blood poisoning" is a serious clinical problem, the incidence of which continues to rise in the US and worldwide despite advances in antimicrobial chemotherapy. The primary trigger in Gram-negative sepsis is endotoxin, a lipopolysaccharide (LPS) constituent of the outer membrane of all Gram-negative bacteria. The structurally highly conserved glycolipid called lipid A is the active moiety of LPS. Lipid A is composed of a hydrophilic, bis-phosphorylated di-glucosamine backbone, and a hydrophobic polyacyl domain. The bis-anionic, amphiphilic nature of lipid A enables it to interact with a variety of cationic hydrophobic ligands, including polymyxin B, a toxic peptide antibiotic which binds to lipid A and neutralizes endotoxicity. Having determined the structural basis of the interaction of polymyxin B with lipid A, our long-term goal has been to rationally design non-peptidic, nontoxic, small-molecule LPS-sequestrants. Our efforts began with defining the central pharmacophore that determined LPS-recognition and -neutralization properties in small molecules, which led to the discovery of a novel lipopolyamine lead, DS-96. DS-96 is an effective LPS-neutralizer, rivaling polymyxin B in a panel of vitro assays, as well as in protecting animals against endotoxicosis. Structure-activity relationships in our effort to rationally design endotoxin sequestering agents, preclinical assessment of hits and leads, and approaches to overcoming issues with toxicity are described in this chapter.

Pharmacological effects of carvedilol on T-type calcium current in murine HL-1 cells

Carvedilol is widely used in the treatment of cardiovascular diseases including atrial fibrillation. T-type Ca(2+) channels have been recognized recently in the mechanisms underlying atrial arrhythmias. However, it is unclear whether carvedilol may affect the T-type Ca(2+) channel. The present study evaluated the pharmacological effects of carvedilol on T-type calcium current (I(Ca,T)) in the murine HL-1 cell line. I(Ca)(,T) was recorded by the whole-cell patch-clamp technique. Calcium transient was monitored by the fluorescent dye Fluo-4/AM and confocal laser scanning microscopy. Carvedilol reversibly inhibited I(Ca)(,T) in a concentration-dependent manner, with an IC50 of 2.1 microM. 3 microM carvedilol was found to decrease the peak I(Ca)(,T) amplitude at -20 mV from 20.1+/-1.8pA/pF to 10.9+/-2.1pA/pF. Carvedilol significantly shifted the steady-state inactivation curve of I(Ca)(,T) towards more negative potential by 12.8 mV, while the activation curve was not significantly altered. Carvedilol delayed recovery from inactivation of I(Ca)(,T), time constant (tau) was 112.4+/-3.5 ms in control and 270.1+/-4.7 ms in carvedilol. Carvedilol-induced inhibition rate in I(Ca)(,T) was enhanced with the increase in stimuli frequency, the inhibitory rate was 23.2+/-4.1% at 0.2 Hz and 47.2+/-0.6% at 2 Hz. Carvedilol still produced the significant decrease in the amplitude of I(Ca)(,T) in the presence of H-89, PKA inhibitor. Carvedilol significantly inhibited the amplitude of the calcium transient in a concentration-dependent manner. These findings indicate that carvedilol inhibits I(Ca)(,T) in atrial cells by mechanisms involving preferential interaction with the inactivated state of T-type Ca(2+) channel.

Collation, assessment and analysis of literature in vitro data on hERG receptor blocking potency for subsequent modeling of drugs' cardiotoxic properties

The assessment of the torsadogenic potency of a new chemical entity is a crucial issue during lead optimization and the drug development process. It is required by the regulatory agencies during the registration process. In recent years, there has been a considerable interest in developing in silico models, which allow prediction of drug-hERG channel interaction at the early stage of a drug development process. The main mechanism underlying an acquired QT syndrome and a potentially fatal arrhythmia called torsades de pointes is the inhibition of potassium channel encoded by hERG (the human ether-a-go-go-related gene). The concentration producing half-maximal block of the hERG potassium current (IC(50)) is a surrogate marker for proarrhythmic properties of compounds and is considered a test for cardiac safety of drugs or drug candidates. The IC(50) values, obtained from data collected during electrophysiological studies, are highly dependent on experimental conditions (i.e. model, temperature, voltage protocol). For the in silico models' quality and performance, the data quality and consistency is a crucial issue. Therefore the main objective of our work was to collect and assess the hERG IC(50) data available in accessible scientific literature to provide a high-quality data set for further studies.

Therapeutic effect of {beta}-adrenoceptor blockers using a mouse model of dilated cardiomyopathy with a troponin mutation

AIMS

Extensive clinical studies have demonstrated that beta-adrenoceptor blocking agents (beta-blockers) are beneficial in the treatment of chronic heart failure, which is due to various aetiologies, including idiopathic dilated cardiomyopathy (DCM) and ischaemic heart disease. However, little is known about the therapeutic efficacy of beta-blockers in the treatment of the inherited form of DCM, of which causative mutations have recently been identified in various genes, including those encoding cardiac sarcomeric proteins. Using a mouse model of inherited DCM with a troponin mutation, we aim to study the treatment benefits of beta-blockers.

METHODS AND RESULTS

Three different types of beta-blockers, carvedilol, metoprolol, and atenolol, were orally administered to a knock-in mouse model of inherited DCM with a deletion mutation DeltaK210 in the cardiac troponin T gene (TNNT2). Therapeutic effects were examined on the basis of survival and myocardial remodelling. The lipophilic beta(1)-selective beta-blocker metoprolol was found to prevent cardiac dysfunction and remodelling and extend the survival of knock-in mice. Conversely, both the non-selective beta-blocker carvedilol and the hydrophilic beta(1)-selective beta-blocker atenolol had no beneficial effects on survival and myocardial remodelling in this mouse model of inherited DCM.

CONCLUSION

The highly lipophilic beta(1)-selective beta-blocker metoprolol, known to prevent ventricular fibrillation via central nervous system-mediated vagal activation, may be especially beneficial to DCM patients showing a family history of frequent sudden cardiac death, such as those with a deletion mutation DeltaK210 in the TNNT2 gene.

Rapid component I(Kr) of cardiac delayed rectifier potassium currents in guinea-pig is inhibited by alpha(1)-adrenoreceptor activation via protein kinase A and protein kinase C-dependent pathways

Ventricular tachyarrhythmias are often precipitated by physical or emotional stress, indicating a link between increased adrenergic stimulation and cardiac ion channel activity. Human ether-a-go-go related gene (hERG) potassium channels conduct the rapid component of delayed rectifier potassium current, I(kr), a crucial component for action potential repolarization. To evaluate the correlation between increased alpha(1)-adrenergic activity and the rapid component of cardiac I(kr), whole-cell patch-clamp recording was performed in isolated guinea-pig ventricular myocytes. Stimulation of alpha(1)-adrenoceptors using phenylephrine (0.1 nM-100 microM) reduced I(kr) current in a dose-dependent manner at 37 degrees C. Phenylephrine (0.1 microM) reduced I(kr) current to 66.83+/-3.16%. Chelerythrine (1 microM), a specific inhibitor of protein kinase C (PKC) completely inhibited the changes in I(kr) trigged by 0.1 microM phenylephrine. KT5720 (2.5 microM), a specific inhibitor of protein kinase A (PKA) partially inhibited the current decrease induced by 0.1 microM phenylephrine. Both chelerythrine and KT5720 drastically reduced the phenylephrine-induced effects, indicating possible involvement of PKC and PKA in the alpha(1)-adrenergic inhibition of I(kr). Our data suggest a link between I(kr) and the alpha(1)-adrenoceptor, involving activation of PKC and PKA in arrhythmogenesis.

Prediction of drug-induced QT interval prolongation in telemetered common marmosets

Drug-induced QT interval prolongation is a critical issue in development of new chemical entities, so the pharmaceutical industry needs to evaluate risk as early as possible. Common marmosets have been in the limelight in early-stage development due to their small size, which requires only a small amount of test drug. The purpose of this study was to determine the utility of telemetered common marmosets for predicting drug-induced QT interval prolongation. Telemetry transmitters were implanted in common marmosets (male and female), and QT and RR intervals were measured. The QT interval was corrected for the RR interval by applying Bazett's and Fridericia's correction formulas and individual rate correction. Individual correction showed the least slope for the linear regression of corrected QT (QTc) intervals against RR intervals, indicating that it dissociated changes in heart rate most effectively. With the individual correction method, the QT-prolonging drugs (astemizole, dl-sotalol) showed QTc interval prolongations and the non-QT-prolonging drugs (dl-propranolol, nifedipine) did not show QTc interval prolongations. The plasma concentrations of astemizole and dl-sotalol associated with QTc interval prolongations in common marmosets were similar to those in humans, suggesting that the sensitivity of common marmosets would be appropriate for evaluating risk of drug-induced QT interval prolongation. In conclusion, telemetry studies in common marmosets are useful for predicting clinical QT prolonging potential of drugs in early stage development and require only a small amount of test drug.

A composite model for HERG blockade

hERG blockade is one of the major toxicological problems in lead structure optimization. Reliable ligand-based in silico models for predicting hERG blockade therefore have considerable potential for saving time and money, as patch-clamp measurements are very expensive and no crystal structures of the hERG-encoded channel are available. Herein we present a predictive QSAR model for hERG blockade that differentiates between specific and nonspecific binding. Specific binders are identified by preliminary pharmacophore scanning. In addition to descriptor-based models for the compounds selected as hitting one of two different pharmacophores, we also use a model for nonspecific binding that reproduces blocking properties of molecules that do not fit either of the two pharmacophores. PLS and SVR models based on interpretable quantum mechanically derived descriptors on a literature dataset of 113 molecules reach overall R(2) values between 0.60 and 0.70 for independent validation sets and R(2) values between 0.39 and 0.76 after partitioning according to the pharmacophore search for the test sets. Our findings suggest that hERG blockade may occur through different types of binding, so that several different models may be necessary to assess hERG toxicity.

Effects of carvedilol on transient outward and ultra-rapid delayed rectifier potassium currents in human atrial myocytes

Carvedilol is a beta- and alpha(1)-adrenoceptor antagonist. It is widely used in the treatment of cardiovascular diseases including atrial arrhythmias. However, it is unclear whether carvedilol may affect the repolarization currents, transient outward K(+) current (I(to)) and ultra-rapid delayed rectifier K(+) current (I(Kur)) in the human atrium. The present study evaluated effects of carvedilol on I(to) and I(Kur) in isolated human atrial myocytes by whole-cell patch-clamp recording technique. We found that carvedilol reversibly inhibited I(to) and I(Kur) in a concentration-dependent manner. Carvedilol (0.3 microM) suppressed I(to) from 9.2+/-0.5 pA/pF to 4.8+/-0.5 pA/pF (P<0.01) and I(Kur) from 3.6+/-0.5 pA/pF to 1.9+/-0.3 pA/pF (P<0.01) at +50 mV. I(to) was inhibited in a voltage-dependent manner, being significantly attenuated at test potentials from +10 to +50 mV, whereas the inhibition of I(Kur) was independent. The concentration giving a 50% inhibition was 0.50 microM for I(to) and 0.39 microM for I(Kur). Voltage-dependence of activation, inactivation and time-dependent recovery from inactivation of I(to) were not altered by carvedilol. However, time to peak and time-dependent inactivation of I(to) were significantly accelerated, indicating an open channel blocking action. The findings indicate that carvedilol significantly inhibits the major repolarization K(+) currents I(to) and I(Kur) in human atrial myocytes.