Proarrhythmia - the triggering of arrhythmias following drug therapy - is a rare, but potentially lethal side-effect of various drugs, and therefore, a major safety concern during drug development. Most often proarrhythmia is caused by the drugs’ potential to interact with various K+-channels in the heart, leading to a prolongation of cardiac repolarization that is usually observed on the ECG as prolonged QT interval (drug-induced acquired long QT syndrome; aLQTS). Although drug-induced long-QT-related proarrhythmia is most frequently found in patients with impaired cardiac repolarization due to disease-induced structural and/or electrophysiological remodelling of the heart; most cellular, tissue and whole animal model systems used for drug safety screening are based on normal, healthy models. This approach has serious limitations; therefore, novel animal models that mimic the pathophysiological conditions under which drugs display the highest proarrhythmic risk - such as models with impaired cardiac repolarization - would be desirable for proarrhythmia safety testing. The aims of the present study: Drug-induced (HMR-1556 to block IKs) acquired LQTS, and various transgenic (congenital) LQTS rabbit models with impaired cardiac repolarization due to cardio-selective overexpression of loss-of-function mutations of human KCNH2 (HERG-G628S, α-subunit of IKr, loss of IKr, LQT2), KCNE1 (KCNE1-G52R, β-subunit of IKs, decreased IKs, LQT5)[1] or both KCNQ1 and KCNE1 transgenes (LQT2-5) were used to investigate: - the proarrhythmic potential of SZV-270, a novel antiarrhythmic drug candidate with combined Class I/B and Class III effects (acquired LQTS model). - the electrophysiological characteristics of a newly generated, double-transgenic LQT2-5 rabbit model - the utility of transgenic LQT2, LQT5 and LQT2-5 rabbit models for more reliable prediction of drug-induced ventricular arrhythmias Main findings: The acquired LQTS rabbit proarrhythmia model with pharmacologically reduced repolarization reserve (by the IKs inhibitor HMR-1556) was able to predict the known torsadogenic potential of the IKr blocker dofetilide, while indicated no SZV-270-induced proarrhythmia risk. This advantageous electrophysiological effect of the SZV-270 - prolongation of ventricular repolarization without increased arrhythmia risk - is assumed to be attributed to its combined IKr (Class III) and INa (Class I/B) blocking characteristics. Transgenic LQTS rabbit models reflected patients with clinically ‘silent’ - normal QT interval (LQT5) - or 'manifest' - prolonged QT interval (LQT2 and LQT2-5) - impairment in cardiac repolarization reserve capacity due to different pathomechanisms. The LQTS animals were more sensitive in detecting IKr - (LQT5) or IK1/IKs - (LQT2 and LQT2-5) blocking properties of drugs compared to healthy wild type (WT) animals. Impaired QT-shortening capacity at fast heart rates was observed due to disturbed IKs function in LQT5 and LQT2-5. Importantly, the transgenic LQTS models did not only show more pronounced changes in different proarrhythmia markers in response to potassium channel blockers but also exhibited higher incidence, longer duration and more malignant type of ex vivo arrhythmias than WT. Conclusions: Drug-induced and transgenic LQTS rabbit models reflect human pathophysiological settings - patients with reduced repolarization reserve - that favour drug-induced arrhythmia formation. As they demonstrate increased sensitivity to different specific ion-channel blockers (IKr-blockade in LQT5 or in HMR-1556 induced acquired LQTS model, IK1 - and IKs - blockade in LQT2 and LQT2-5), their combined use could provide more reliable, and more thorough prediction of (multi-channel-based) pro-arrhythmic potential of novel drug candidates especially in the setting of impaired cardiac repolarization reserve.
Ionic mechanisms limiting cardiac repolarization reserve in humans compared to dogs