An evaluation of 30 clinical drugs against the comprehensive in vitro proarrhythmia assay (CiPA) proposed ion channel panel
Mar 30, 2016 • By William J. Crumb Jr. , Jose Vicente, Lars Johannesen, David G. Strauss
Abstract
Introduction: The Comprehensive in vitro Proarrhythmia Assay (CiPA) is intended to address the misidentification of drug-associated torsade de pointes risk based solely on hERG and QT data. This newparadigmwill consist of four interrelated components, one of which is a panel consisting of six ion channelswhose currents are important in both depolarization and repolarization of the cardiac action potential. This study examined the effects of 30 clinical drugs on these ion channels. Methods: Ion currentswere evaluated in expression systems using themanualwhole cell patch clamp technique. Currents were elicited using either a ventricular action potential waveform or step-ramp voltage protocols. Results: Of the seven ion currents studied, hERGwas themost often blocked current followed by Nav1.5-late, and Cav1.2. Using a 20% reduction in current amplitude as an arbitrary maker, at a free plasma Cmax concentration, no drug tested blocked Nav1.5-peak, KvLQT1/mink, Kir2.1 and Kv4.3 by that amount. At a 3x free plasma Cmax, every current except Kir2.1 had at least one drug reduce current amplitude by at least 20%. Discussion: This is the first study of its kind to examine the effects of 30 clinical drugs against the seven ion currents currently proposed to makeup the CiPA ion channel panel. The results indicate the importance of drug-induced block of hERG, Nav1.5-late and Cav1.2 at clinically relevant concentrations, with low risk torsade drugs having equal or greater Nav1.5-late or Cav1.2 block compared to hERG block. In addition, the results of this study provide data which can be used to test the ability of various in silico models to predict drug-induced arrhythmias.
1. Introduction
The Comprehensive in vitro Proarrhythmia Assay (CiPA) is intended to address the misidentification of drug-associated torsade de pointes risk based solely on hERG and QT data (Sager, Gintant, Turner, et al., 2014). This new paradigmwill consist of four interrelated components: an ion channel panel, in silico action potential reconstructions of the ion channel panel activity, and verification of results in stem cell derived human cardiomyocytes and in human phase 1 ECGs. To this end, the Safety Pharmacology Society organized the Ion Channel Working Group (ICWG) consisting of representatives from the pharmaceutical industry, regulatory agencies, contract research organizations, and academia. The ICWG was tasked with selecting the ion channels to be tested (Fermini, Hancox, Abi-Gerges, et al., 2016). The ion channel panel decided upon consists of six ion channelswhose currents are important in both depolarization and repolarization of the cardiac action potential (IKr, INa, ICa, IKs, Ito and IK1) (Fermini et al., 2016). These currents contribute to all components of the cardiac action potential (Grant, 2009; Nerbonne & Kass, 2005). Starting from a resting membrane potential, determined largely by the inwardly rectifying potassium current (IK1), the upstroke or Vmax of the cardiac action potential is due to an influx of Na+ ions through the Na channel (INa-peak). This is followed by a rapid phase of repolarization or phase 1 due to the efflux of K+ ions through the transient outward potassium current (Ito). Following this is the plateau of the cardiac action potential carried by an influx of Ca++ ions through the L-type Ca channel (ICa) and to a smaller extent through an influx of Na+ ions through the Na channel. This Na current flowing during the plateau period is commonly referred to as the late Na current (INa-late). Blockade of both the L-type Ca current and INa-late have been associatedwith a reduction inQTc prolongation and torsade even in the presence of hERG block (Belardinelli et al., 2013; Gintant, Su,Martin, & Cox, 2006; Johannesen et al., 2014). The action potential returns to a resting state via an efflux of K+ ions through both the rapid and slow components of the delayed rectifier potassium current (IKr and IKs, respectively). For ease of use across industry and academia, these currents are most often recorded in expression systems and not primary cardiac myocytes. Therefore, IK1, INa (peak and late), Ito, ICa, IKr, and IKs are routinely recorded using cell lines expressing Kir2.1, Nav1.5, Kv4.3, Cav1.2, hERG, and KvLQT1/mink, respectively.