Original articleValidation of quantitative measure of repolarization reserve as a novel marker of drug induced proarrhythmia
Graphical abstract
Introduction
Cardiac repolarization abnormalities can be caused by a large number of cardiac specific or non-specific pharmaceutical compounds, as well as by genetic mutations [1], pathologies (e.g. heart failure [2]) or dietary interventions [3]. Action potential (AP) prolongation can lead to life-threatening Torsade de Pointes (TdP) ventricular arrhythmia [4], whereas regional AP prolongation can contribute to the occurrence of unidirectional block and reentrant arrhythmia. Both mechanisms may deteriorate into ventricular fibrillation (VF) which is lethal.
The ability to maintain repolarization despite the inhibition of one or more repolarizing currents has been termed ‘repolarization reserve’. This concept of repolarization reserve has been applied to understand the mechanisms that can lead to repolarization failure (RF). According to this concept, impairment of one particular repolarizing current does not necessarily result in RF due to the presence of other repolarizing currents. Drug effects on a specific repolarizing current (such as the rapid delayed rectifier K+ current (IKr)) can result in arrhythmic AP prolongation abnormalities in hearts, especially when repolarization reserve is already compromised due to pharmacological, pathological, or preexisting congenital conditions (the Long QT Syndromes). However, no metric exists to quantify repolarization reserve.
In this paper, we demonstrate that repolarization is an all-or-none phenomenon which can be used to quantify repolarization reserve. Based on single cell computer simulations of various cell types, we show that repolarization has a clear threshold. We reproduce the phenomenon in several biological cell types, and introduce a novel, simple, fast and unambiguous, quantitative definition of repolarization reserve, called the repolarization reserve current (RRC). Finally, we show the utility of the metric by demonstrating how it identifies torsardegenic risk for several compounds and how it compares to the established but experimentally less easily applicable risk assessment metric qNet.
Section snippets
Repolarization reserve current
We propose a definition of Repolarization Reserve Current (RRC) as the minimum constant current injected into a single cell 4 ms after the upstroke that produced a repolarization abnormality. A repolarization abnormality was defined as one of the two following conditions occurring before the end of the constant current pulse: 1. A failure to complete repolarization (RF), or 2. An early afterdepolarization (EAD). The latter was defined as a polarizing inflection of membrane voltage with
Species differences of repolarization reserve current. simulations
The protocol for the determination of RRC is shown in Fig. 1A. In this example using ORd with a CL of 2000 ms, an inward current greater than 0.7 pA/pF causes an EAD. This current was defined as the repolarization reserve current (RRC) for that particular cell and CL. The normal AP at CL = 2000 ms is indicated in green.
To test the generality of the concept of determination of RRC, we applied the technique of measurement of RRC to several ionic cell models. In all these models of different
Discussion
This study shows that RRC is a stable, fast, simple and reproducible metric that can be obtained from a single cell that has significant heterogeneity between cells. It is a straightforwardly obtained metric to quantify repolarization reserve, and it can identify drug effects in simulation studies that may lead to TdP (Fig. 5). We show that RRC is not necessarily correlated to APD prior to current injection within a cell population. The RRC was also measured in biological models (hiPSC-CM) of
Conclusion
Repolarization is an all-or-none phenomenon. RRC constitutes a straightforward and easy to implement method to quantify repolarization reserve. The method can be used in high throughput applications.
Declaration of Competing Interest
Edward J. Vigmond is an owner of CardioSolv LLC.
Acknowledgements
This study received financial support from the French Government as part of the “Investments of the Future” program managed by the National Research Agency (ANR), Grant reference ANR-10-IAHU-04.
This project has received funding from the Fondation Leducq (Research Grant number 16 CVD 02). D. Christini received support from NIH U01HL136297.
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