The positive frequency-dependent electrophysiological effects of the IKur inhibitor XEN-D0103 are desirable for the treatment of atrial fibrillation.

BACKGROUND: Selective inhibitors of Kv1.5 channels are being developed for the treatment of atrial fibrillation (AF). OBJECTIVES: The purpose of this study was to investigate the effects of the highly selective Kv1.5 inhibitor XEN-D0103 on human atrial action potentials (APs) at high excitation rates and to assess safety. METHODS: Intracellular APs (stimulation rates 1-5 Hz) were measured in right atrial trabeculae from patients in sinus rhythm (SR), chronic AF (cAF; AF of >6 months duration), and paroxysmal AF (pAF). The safety and tolerability of XEN-D0103 were tested in a double-blind, randomized, placebo-controlled phase 1 study. RESULTS: Depending on its concentration, XEN-D0103 elevated the plateau potential. At 1 Hz, XEN-D0103 (3 µM) shortened action potential duration at 90% repolarization (APD90) and effective refractory period (ERP) in SR preparations, but prolonged these parameters in cAF preparations. In SR and pAF preparations, the shortening effects on APD90 and ERP turned into prolongation at high rates. In cAF trabeculae, XEN-D0103 prolonged APD90 and ERP at 2 and 3 Hz. At high rates, more SR and pAF preparations failed to capture excitation in the presence of the drug than in its absence. XEN-D0103 (10 µM) did not significantly affect human ventricular APs. Even with plasma concentrations reaching 7000 ng/mL, XEN-D0103 did not increase ∆∆QTcF (QT interval corrected by the Fridericia formula) in the analysis of electrocardiograms of healthy volunteers, and no subjects receiving an active treatment had a QT or QTcF interval >450 ms, or increase in QTcF from baseline >30 ms. CONCLUSION: APD prolongation and suppression of APs by XEN-D0103 at high stimulation rates in SR and pAF tissue, but not cAF, could be of therapeutic benefit for reducing AF burden. This concept needs to be confirmed in clinical trials.

Crude oil impairs cardiac excitation-contraction coupling in fish.

Crude oil is known to disrupt cardiac function in fish embryos. Large oil spills, such as the Deepwater Horizon (DWH) disaster that occurred in 2010 in the Gulf of Mexico, could severely affect fish at impacted spawning sites. The physiological mechanisms underlying such potential cardiotoxic effects remain unclear. Here, we show that crude oil samples collected from the DWH spill prolonged the action potential of isolated cardiomyocytes from juvenile bluefin and yellowfin tunas, through the blocking of the delayed rectifier potassium current (I(Kr)). Crude oil exposure also decreased calcium current (I(Ca)) and calcium cycling, which disrupted excitation-contraction coupling in cardiomyocytes. Our findings demonstrate a cardiotoxic mechanism by which crude oil affects the regulation of cellular excitability, with implications for life-threatening arrhythmias in vertebrates.

Developmental changes in action potential prolongation by K+-channel blockers in chick myocardium.

The effects of K(+)-channel blockers on the action potential duration of the myocardium were examined in isolated right ventricles from the 7 - 10-day-old, 11 - 13-day-old, and 14 - 20-day-old embryo and 1 - 7-day-old hatched chicks. E-4031 significantly prolonged action potential duration at all developmental stages examined; the prolongation was largest in the 11 - 13-day-old embryo and was accompanied by early after-depolarizations. Chromanol 293B showed smaller prolongation at all stages examined. Terfenadine prolonged action potential duration in the 11 - 13-day-old embryo, but not in other stages. Thus, the chick ventricular myocardium changes its repolarization properties during development.

Quantitative analysis of T-wave morphology increases confidence in drug-induced cardiac repolarization abnormalities: evidence from the investigational IKr inhibitor Lu 35-138.

This study investigates repolarization changes induced by a new candidate drug to determine whether a composite electrocardiographic (ECG) measure of T-wave morphology could be used as a reliable marker to support the evidence of abnormal repolarization, which is indicated by QT interval prolongation. Seventy-nine healthy subjects were included in this parallel study. After a baseline day during which no drug was given, 40 subjects received an I(Kr)-blocking antipsychotic compound (Lu 35-138) on 7 consecutive days while 39 subjects received placebo. Resting ECGs were recorded and used to determine a combined measure of repolarization morphology (morphology combination score [MCS]), based on asymmetry, flatness, and notching. Replicate measurements were used to determine reliable change and study power for both measures. Lu 35-138 increased the QTc interval with corresponding changes in T-wave morphology as determined by MCS. For subjects taking Lu 35-138, T-wave morphology was a more reliable indicator of I(Kr) inhibition than QTcF (chi(2) = 20.3, P = .001). At 80% study power for identifying a 5-millisecond placebo-adjusted change from baseline for QTcF, the corresponding study power for MCS was 93%. As a covariate to the assessment of QT interval liability, MCS offered important additive information to the effect of Lu 35-138 on cardiac repolarization.

Inhibitory effects of isoliquiritigenin and licorice extract on voltage-dependent K(+) currents in H9c2 cells.

The effect of isoliquiritigenin (ISL), a component of licorice, on the voltage-dependent, ultra-rapidly activating delayed-rectifier K(+) current (IKur) was examined in H9c2 cells, a cell-line derived from rat cardiac myoblasts. IKur was recorded using the whole-cell patch clamp method with a pipette solution containing 140 mM K(+). Depolarizing voltage pulses of 200-ms duration were given with 10-mV steps every 10 s from -40 mV holding potential. ISL inhibited IKur in a concentration-dependent manner. The median inhibitory concentration (IC(50)) of ISL was approximately 0.11 microM and the Hill coefficient was 0.71. Using CHO cells expressing Kv1.5 IKur channels, ISL also inhibited Kv1.5 IKur, but less potently than the IKur current in H9c2 cells. Furthermore, in H9c2 cells, the licorice extract itself inhibited IKur in a manner similar to ISL. We conclude that ISL, one component of licorice, is a potent inhibitor of K(+) channels, which specifically in H9c2 cells could be Kv2.1, and that this inhibition may be involved in various pharmacological effects of licorice.

The hERG potassium channel and hERG screening for drug-induced torsades de pointes.

Drug-induced torsades de pointes (TdP) arrhythmia is a major safety concern in the process of drug design and development. The incidence of TdP tends to be low, so early pre-clinical screens rely on surrogate markers of TdP to highlight potential problems with new drugs. hERG (human ether-à-go-go-related gene, alternative nomenclature KCNH2) is responsible for channels mediating the 'rapid' delayed rectifier K+ current (IKr) which plays an important role in ventricular repolarization. Pharmacological inhibition of native IKr and of recombinant hERG channels is a shared feature of diverse drugs associated with TdP. In vitro hERG assays therefore form a key element of an integrated assessment of TdP liability, with patch-clamp electrophysiology offering a 'gold standard'. However, whilst clearly necessary, hERG assays cannot be assumed automatically to provide sufficient information, when considered in isolation, to differentiate 'safe' from 'dangerous' drugs. Other relevant factors include therapeutic plasma concentration, drug metabolism and active metabolites, severity of target condition and drug effects on other cardiac ion channels that may mitigate or exacerbate effects of hERG blockade. Increased understanding of the nature of drug-hERG channel interactions may ultimately help eliminate potential hERG blockade early in the design and development process. Currently, for promising drug candidates integration of data from hERG assays with information from other pre-clinical safety screens remains essential.

Discovery of talatisamine as a novel specific blocker for the delayed rectifier K+ channels in rat hippocampal neurons.

Blocking specific K+ channels has been proposed as a promising strategy for the treatment of neurodegenerative diseases. Using a computational virtual screening approach and electrophysiological testing, we found four Aconitum alkaloids are potent blockers of the delayed rectifier K+ channel in rat hippocampal neurons. In the present study, we first tested the action of the four alkaloids on the voltage-gated K+, Na+ and Ca2+ currents in rat hippocampal neurons, and then identified that talatisamine is a specific blocker for the delayed rectifier K+ channel. External application of talatisamine reversibly inhibited the delayed rectifier K+ current (IK) with an IC50 value of 146.0+/-5.8 microM in a voltage-dependent manner, but exhibited very slight blocking effect on the voltage-gated Na+ and Ca2+ currents even at the high concentration of 1-3 mM. Moreover, talatisamine exerted a significant hyperpolarizing shift of the steady-state activation, but did not influence the steady state inactivation of IK and its recovery from inactivation, suggesting that talatisamine had no allosteric action on IK channel and was a pure blocker binding to the external pore entry of the channel. Our present study made the first discovery of potent and specific IK channel blocker from Aconitum alkaloids. It has been argued that suppressing K+ efflux by blocking IK channel may be favorable for Alzheimer's disease therapy. Talatisamine can therefore be considered as a leading compound worthy of further investigations.

Comparison of four rate-correction algorithms for the ventricular repolarization period in assessing net effects of IKr blockers in dogs.

The utility of corrected and uncorrected QT interval changes for assessing net repolarization delay by I(Kr) (a rapid component of delayed rectifier K(+) currents) blockers was assessed in halothane-anesthetized dogs using the electrocardiogram and monophasic action potential (MAP) recordings with electrical ventricular pacing. Intravenous administration of dl-sotalol (0.2 - 2 mg/kg) prolonged the MAP duration and RR interval, while terfenadine (3 mg/kg) increased the MAP duration but transiently shortened RR interval. The order of correlation coefficient between the MAP duration at a pacing cycle length of 400 ms and MAP duration itself or that with arithmetical correction was uncorrected > Van de Water = Matsunaga > Fridericia > Bazett. These results suggest that Matsunaga's and Van de Water's formulae would better predict the net repolarization delay in the in vivo canine model. Also, the risk of drug candidates that may prolong the QT interval should be judged by change in uncorrected QT interval as well as corrected QT interval.

Halothane sensitizes the guinea-pig heart to pharmacological IKr blockade: comparison with urethane anesthesia.

Potential utility of halothane-anesthetized guinea pigs for detecting drug-induced repolarization delay was analyzed in comparison with urethane-anesthesia (n = 4 for both groups). Basal QT interval was significantly greater under halothane-anesthesia than urethane-anesthesia (192 +/- 7 vs 132 +/- 5 ms, respectively), whereas the reverse was true for the heart rate (190 +/- 7 vs 248 +/- 11 beats/min, respectively). The typical I(Kr)-blocker dl-sotalol (0.1 to 3 mg/kg, i.v.) induced dose-related bradycardia and QT interval prolongation under each anesthesia. The extent of maximum prolongation in the QT interval was greater under halothane-anesthesia than urethane-anesthesia (+101 +/- 15 vs +49 +/- 3 ms, respectively), whereas that of peak change in the heart rate was smaller under the former than the latter (-49 +/- 8 vs -63 +/- 5 beats/min, respectively). Pretreatment of the animals under urethane-anesthesia with the selective I(Ks) blocker chromanol 293B (n = 6) increased the extent of the dl-sotalol-induced QT interval prolongation to +57 +/- 8 ms, which was only 0.56 times of that under the halothane-anesthesia, whereas the pretreatment increased the peak change in the heart rate to -76 +/- 12 ms. These results indicate that the halothane-anesthesia may effectively sensitize the guinea-pig heart to pharmacological I(Kr) blockade.