Probing cardiac repolarization reserve in drug safety assessment

Excessive prolongation of cardiac repolarization, manifested as QT prolongation on ECG, is common unwanted side effect of many drugs and drug candidates. Prolongation of QT interval may lead to life threatening cardiac arrhythmia – Torsade de Point (TdP). Number of drugs was withdrawn from the market and development of many new drug candidates is terminated because of this side effect. Block of the hERG channel is believed to be the principal reason of QT prolongation and thus current safety testing assays are based on determining the hERG block potency of new drug candidates. Here we demonstrate that focus on hERG block is counterproductive. Drugs affecting other repolarizing currents might be proarrhythmic as well. In the same time, potent hERG blocker with effect on depolarizing currents is not always proarrhythmic. We also show, that TdP can occur after chronic treatment due to effect on protein trafficking. This chronic effect is not tested in cardiac safety assessment at all. We have designed and synthesized 40 analogues of potent hERG blocker dofetilide to analyze molecular substructures required for hERG block. This approach may help design new drugs with low affinity to hERG channel. We identify pentamidine as a direct blocker of another important repolarizing current - IK1. The role of this current in cardiac arrhythmias is poorly studied, mainly because of absence of a specific IK1 blocker. We have tested 7 pentamidine analogues to find a specific and effective IK1 blocker applicable in vivo. Based on our findings, to improve current cardiac safety assessment, we employed five test assays and compare their sensitivity and specificity using hERG blockers with known proarrhythmic potential. We demonstrate that isolated cardiomyocytes lack sufficient specificity to correctly identify safe hERG blocker moxifloxacin. On the other hand, hESC-CM seems to be a promising alternative to current assays.