For the past decade, cardiac safety screening to evaluate the propensity of drugs to produce QT interval prolongation and Torsades de Pointes (TdP) arrhythmia has been conducted according to ICH S7B and ICH E14 guidelines. Central to the existing approach are hERG channel assays and in vivo QT measurements. Although effective, the present paradigm carries a risk of unnecessary compound attrition and high cost, especially when considering costly thorough QT (TQT) studies conducted later in drug development. The Comprehensive In Vitro Proarrhythmia Assay (CiPA) initiative is a publicprivate collaboration with the aim of updating the existing cardiac safety testing paradigm to better evaluate arrhythmia risk and remove the need for TQT studies. It is hoped that CiPA will produce a standardized ion channel assay approach, incorporating defined tests against major cardiac ion channels, the results of which then inform evaluation of proarrhythmic actions in silico, using human ventricular action potential reconstructions. Results are then to be confirmed using human (stem cell-derived) cardiomyocytes. This perspective article reviews the rationale, progress of, and challenges for the CiPA initiative, if this new paradigm is to replace existing practice and, in time, lead to improved and widely accepted cardiac safety testing guidelines.

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Many drugs inhibit the human ether-a-go-go-related gene (HERG) cardiac Kϩ channel. This leads to action potential prolongation on the cellular level, a prolongation of the QT interval on the electrocardiogram, and sometimes cardiac arrhythmia. To date, no activators of this channel have been reported. Here, we describe the in vitro electrophysiological effects of (3R,4R)-4-[3-(6-methoxyquinolin-4-yl)-3-oxo-propyl]-1-[3-(2,3,5-trifluoro-phenyl)-prop-2-ynyl]-piperidine-3-carboxylic acid (RPR260243), a novel activator of HERG. Using patch-clamp electrophysiology, we found that RPR260243 dramatically slowed current deactivation when applied to cells stably expressing HERG. The effects of RPR260243 on HERG channel deactivation were temperature-and voltagedependent and occurred over the concentration range of 1 to 30 M. RPR260243-modified HERG currents were inhibited by dofetilide (IC 50 ϭ 58 nM). RPR260243 had little effect on HERG current amplitude and no significant effects on steady-state activation parameters or on channel inactivation processes. RPR260243 displayed no activator-like effects on other voltagedependent ion channels, including the closely related erg3 K ϩ channel. RPR260243 enhanced the delayed rectifier current in guinea pig myocytes but, when administered alone, had little effect on action potential parameters in these cells. However, RPR260243 completely reversed the action potential-prolonging effects of dofetilide in this preparation. Using the Langendorff heart method, we found that 5 M RPR260243 increased T-wave amplitude, prolonged the PR interval, and shortened the QT interval. We believe RPR260243 represents the first known HERG channel activator and that the drug works primarily by inhibiting channel closure, leading to a persistent HERG channel current upon repolarization. Compounds like RPR260243 will be useful for studying the physiological role of HERG and may one day find use in treating cardiac disease.

The diamidine pentamidine is used to treat leishmaniasis, trypanosomiasis, and Pneumocystis carinii pneumonia. Treatment may be accompanied by prolongation of the QT interval of the electrocardiogram and torsades de pointes tachycardias. Up to now, it has been thought that therapeutic compounds causing QT prolongation are associated with direct block of the cardiac potassium channel human ether a-go-go-related gene (hERG), which encodes the ␣ subunit of cardiac I Kr currents. We show that pentamidine has no acute effects on currents produced by hERG, KvLQT1/mink, Kv4.3, or SCNA5. Cardiac calcium currents and the guinea pig cardiac action potential were also not affected. After overnight exposure, however, pentamidine reduced hERG currents and inhibited trafficking and maturation of hERG with IC 50 values of 5 to 8 M similar to therapeutic concentrations. Surface expression determined in a chemiluminescence assay was reduced on exposure to 10, 30, and 100 M pentamidine by about 30, 40, and 70%, respectively. These effects were specific for hERG since expression of hKv1.5, KvLQT1/minK, and Kv4.3 was not altered. In isolated guinea pig ventricular myocytes, 10 M pentamidine prolonged action potential duration APD 90 from 374.3 Ϯ 57.1 to 893.9 Ϯ 86.2 ms on overnight incubation. I Kr tail current density was reduced from 0.61 Ϯ 0.09 to 0.39 Ϯ 0.04 pA/pF. We conclude that pentamidine prolongs the cardiac action potential by block of hERG trafficking and reduction of the number of functional hERG channels at the cell surface. We propose that pentamidine, like arsenic trioxide, produces QT prolongation and torsades de pointes in patients by inhibition of hERG trafficking.

Administration of certain fluoroquinolone antibacterials has been associated with prolongation of the QT interval on the electrocardiogram and, on rare occasions, ventricular arrhythmia. Blockade of the human cardiac K+ channel HERG often underlies such clinical findings. Therefore, we examined a series of seven fluoroquinolones for their ability to interact with this channel. Using patch-clamp electrophysiology, we found that all of the drugs tested inhibited HERG channel currents, but with widely differing potencies. Sparfloxacin was the most potent compound, displaying an IC50 value of 18 microM, whereas ofloxacin was the least potent compound, with an IC50 value of 1420 microM. Other IC50 values were as follows: grepafloxacin, 50 microM; moxifloxacin, 129 microM; gatifloxacin, 130 microM; levofloxacin, 915 microM; and ciprofloxacin, 966 microM. Block of HERG by sparfloxacin displayed a positive voltage dependence. In contrast to HERG, the KvLQT1/minK K+ channel was not a target for block by the fluoroquinolones. These results provide a mechanism for the QT prolongation observed clinically with administration of sparfloxacin and certain other fluoroquinolones because free plasma levels of these drugs after therapeutic doses approximate those concentrations that inhibit HERG channel current. In the cases of levofloxacin, ciprofloxacin, and ofloxacin, inhibition of HERG occurs at concentrations much greater than those observed clinically. The data indicate that clinically relevant HERG channel inhibition is not a class effect of the fluoroquinolone antibacterials but is highly dependent upon specific substitutions within this series of compounds. HERG channel affinity should be an important criterion for the development of newer fluoroquinolones.

Cisapride (Propulsid) is a gastrointestinal prokinetic agent commonly used to treat nocturnal heartburn as well as a variety of other gastrointestinal disorders. The use of cisapride has been associated with acquired long QT syndrome and ventricular arrhythmias such as torsades de pointes which produces sudden cardiac death. These cardiotoxic effects can be due to blockade of one or more types of K + channel currents in the human heart. For this reason we compared the effects of cisapride on two cloned human cardiac K + channels, Kvl.5 and the human ether-a-go-go-related gene (HERG) stably transfected into mammalian cells. Using patch clamp electrophysiology, we found that cisapride was a potent inhibitor of HERG displaying an IC50 value of 44.5 nmol/1 when tail currents at -40 mV were measured following a 2 s test depolarization to +20 mV. When HERG currents were measured at the end of prolonged (20 s) depolarizing steps to +20 mV, the apparent affinity of cisapride was increased and measured 6.70 nmol/1. The main effect of cisapride was to enhance the rate of HERG current decay thereby reducing current at the end of the voltage clamp pulse. Furthermore, the potency of cisapride for the HERG channel was similar to that observed for the class III antiarrhythmic agent dofetilide (IC50 = 15.3 nmol/1) and the nonsedating antihistamine terfenadine (IC 50 = 56.0 nmol/1). In contrast to its effects on HERG, cisapride inhibited Kvl.5 channel currents weakly displaying an IC50 value of 21.2 umol/l. It is concluded that cisapride displays specific, high affinity block of the human cardiac K + channel HERG. It is likely that this interaction underlies the proarrhythmic effects of the drug observed under certain clinical settings.

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