Carlos Obejero-Paz

MICE Models: Superior to the HERG Model in Predicting Torsade de Pointes

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.

A New Perspective in the Field of Cardiac Safety Testing through the Comprehensive In Vitro Proarrhythmia Assay Paradigm

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 public-private 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.

Quantitative Profiling of the Effects of Vanoxerine on Human Cardiac Ion Channels and its Application to Cardiac Risk.

Vanoxerine has been in clinical trials for Parkinsonism, depression and cocaine addiction but lacked efficacy. Although a potent blocker of hERG, it produced no serious adverse events. We attributed the unexpected result to offsetting Multiple Ion Channel Effects (MICE). Vanoxerine’s effects were strongly frequency-dependent and we repositioned it for treatment of atrial fibrillation and flutter. Vanoxerine terminated AF/AFL in an animal model and a dose-ranging clinical trial. Reversion to normal rhythm was associated with QT prolongation yet absent proarrhythmia markers for Torsade de Pointes (TdP). To understand the QT/TdP discordance, we used quantitative profiling and compared vanoxerine with dofetilide, a selective hERG-blocking torsadogen used for intractable AF, verapamil, a non-torsadogenic MICE comparator and bepridil, a torsadogenic MICE comparator. At clinically relevant concentrations, verapamil blocked hCav1.2 and hERG, as did vanoxerine and bepridil both of which also blocked hNav1.5. In acute experiments and simulations, dofetilide produced early after depolarizations (EADs) and arrhythmias, whereas verapamil, vanoxerine and bepridil produced no proarrhythmia markers. Of the MICE drugs only bepridil inhibited hERG trafficking following overnight exposure. The results are consistent with the emphasis on MICE of the CiPA assay. Additionally we propose that trafficking inhibition of hERG be added to CiPA.

Functional Characterization of Human Stem Cell–Derived Cardiomyocytes

Cardiac toxicity is a leading contributor to late stage attrition in the drug discovery process and to withdrawal of approved drugs from the market. In vitro assays that enable earlier and more accurate testing for cardiac risk provide early stage predictive indicators that aid in mitigating risk. Human cardiomyocytes, the most relevant subjects for early stage testing, are severely limited in supply, but human stem cell–derived cardiomyocytes (SC‐hCM) are readily available from commercial sources and are increasingly used in academic research, drug discovery, and safety pharmacology. As a result, SC‐hCM electrophysiology has become a valuable tool for assessing cardiac risk associated with drug administration. Described in this unit are techniques for recording individual sodium, calcium, and potassium currents, as well as single‐cell action potentials and impedance recordings from contracting syncytia of thousands of interconnected cells. Curr. Protoc. Pharmacol. 64:11.12.1‐11.12.26.

UNIT 11.12 Functional Characterization of Human Stem Cell–Derived Cardiomyocytes

Cardiac toxicity is a leading contributor to late stage attrition in the drug discovery process and to withdrawal of approved drugs from the market. In vitro assays that enable earlier and more accurate testing for cardiac risk provide early stage predictive indicators that aid in mitigating risk. Human cardiomyocytes, the most relevant subjects for early stage testing, are severely limited in supply, but human stem cell–derived cardiomyocytes (SC‐hCM) are readily available from commercial sources and are increasingly used in academic research, drug discovery, and safety pharmacology. As a result, SC‐hCM electrophysiology has become a valuable tool for assessing cardiac risk associated with drug administration. Described in this unit are techniques for recording individual sodium, calcium, and potassium currents, as well as single‐cell action potentials and impedance recordings from contracting syncytia of thousands of interconnected cells. Curr. Protoc. Pharmacol. 64:11.12.1‐11.12.26.

Functional Characterization of Human Stem Cell-Derived Cardiomyocytes: Functional Characterization of SC-Derived Cardiomyocytes

Cardiac toxicity is a leading contributor to late stage attrition in the drug discovery process and to withdrawal of approved drugs from the market. In vitro assays that enable earlier and more accurate testing for cardiac risk provide early stage predictive indicators that aid in mitigating risk. Human cardiomyocytes, the most relevant subjects for early stage testing, are severely limited in supply, but human stem cell–derived cardiomyocytes (SC‐hCM) are readily available from commercial sources and are increasingly used in academic research, drug discovery, and safety pharmacology. As a result, SC‐hCM electrophysiology has become a valuable tool for assessing cardiac risk associated with drug administration. Described in this unit are techniques for recording individual sodium, calcium, and potassium currents, as well as single‐cell action potentials and impedance recordings from contracting syncytia of thousands of interconnected cells. Curr. Protoc. Pharmacol. 64:11.12.1‐11.12.26.