Drug-induced block of the cardiac hERG (human Ether-à-go-go-Related Gene) potassium channel delays cardiac repolarization and increases the risk of Torsade de Pointes (TdP), a potentially lethal arrhythmia. A positive hERG assay has been embraced by regulators as a non-clinical predictor of TdP despite a discordance of about 30%. To test whether assaying concomitant block of multiple ion channels (Multiple Ion Channel Effects or MICE) improves predictivity we measured the concentration-responses of hERG, Nav1.5 and Cav1.2 currents for 32 torsadogenic and 23 non-torsadogenic drugs from multiple classes. We used automated gigaseal patch clamp instruments to provide higher throughput along with accuracy and reproducibility. Logistic regression models using the MICE assay showed a significant reduction in false positives (Type 1 errors) and false negatives (Type 2 errors) when compared to the hERG assay. The best MICE model only required a comparison of the blocking potencies between hERG and Cav1.2.

The International Council on Harmonization (ICH) S7B and E14 regulatory guidelines are sensitive but not specific for predicting which drugs are pro-arrhythmic. In response, the Comprehensive In Vitro Proarrhythmia Assay (CiPA) was proposed that integrates multi-ion channel pharmacology data in vitro into a human cardiomyocyte model in silico for proarrhythmia risk assessment. Previously, we reported the model optimization and proarrhythmia metric selection based on CiPA training drugs. In this study, we report the application of the prespecified model and metric to independent CiPA validation drugs. Over two validation datasets, the CiPA model performance meets all pre-specified measures for ranking and classifying validation drugs, and outperforms alternatives, despite some in vitro data differences between the two datasets due to different experimental conditions and quality control procedures. This suggests that the current CiPA model/metric may be fit for regulatory use, and standardization of experimental protocols and quality control criteria could increase the model prediction accuracy even further.

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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We have determined the effects of coexpression of Kv2.1 with electrically silent Kv5.1 or Kv6.1 α-subunits in Xenopus oocytes on channel gating. Kv2.1/5.1 selectively accelerated the rate of inactivation at intermediate potentials (−30 to 0 mV), without affecting the rate at strong depolarization (0 to +40 mV), and markedly accelerated the rate of cumulative inactivation evoked by high-frequency trains of short pulses. Kv5.1 coexpression also slowed deactivation of Kv2.1. In contrast, Kv6.1 was much less effective in speeding inactivation at intermediate potentials, had a slowing effect on inactivation at strong depolarizations, and had no effect on cumulative inactivation. Kv6.1, however, had profound effects on activation, including a negative shift of the steady-state activation curve and marked slowing of deactivation tail currents. Support for the notion that the Kv5.1’s effects stem from coassembly of α-subunits into heteromeric channels was obtained from biochemical evidence of protein-protein interaction and single-channel measurements that showed heterogeneity in unitary conductance. Our results show that Kv5.1 and Kv6.1 function as regulatory α-subunits that when coassembled with Kv2.1 can modulate gating in a physiologically relevant manner.