Screening for Acute IKr Block Is Insufficient to Detect Torsades de Pointes Liability Role of Late Sodium Current

Background— New drugs are routinely screened for I Kr blocking properties thought to predict QT prolonging and arrhythmogenic liability. However, recent data suggest that chronic (hours) drug exposure to phosphoinositide 3-kinase inhibitors used in cancer can prolong QT by inhibiting potassium currents and increasing late sodium current ( I Na-L) in cardiomyocytes. We tested the extent to which I Kr blockers with known QT liability generate arrhythmias through this pathway. Methods and Results— Acute exposure to dofetilide, an I Kr blocker without other recognized electropharmacologic actions, produced no change in ion currents or action potentials in adult mouse cardiomyocytes, which lack I Kr. By contrast, 2 to 48 hours of exposure to the drug generated arrhythmogenic afterdepolarizations and ≥15-fold increases in I Na-L. Including phosphatidylinositol 3,4,5-trisphosphate, a downstream effector for the phosphoinositide 3-kinase pathway, in the pipette inhibited these effects. I Na-L was also increased, and inhibitable by phosphatidylinositol 3,4,5-trisphosphate, with hours of dofetilide exposure in human-induced pluripotent stem cell–derived cardiomyocytes and in Chinese hamster ovary cells transfected with SCN5A , encoding sodium current. Cardiomyocytes from dofetilide-treated mice similarly demonstrated increased I Na-L and afterdepolarizations. Other agents with variable I Kr-blocking potencies and arrhythmia liability produced a range of effects on I Na-L, from marked increases (E-4031, d-sotalol, thioridazine, and erythromycin) to little or no effect (haloperidol, moxifloxacin, and verapamil). Conclusions— Some but not all drugs designated as arrhythmogenic I Kr blockers can generate arrhythmias by augmenting I Na-L through the phosphoinositide 3-kinase pathway. These data identify a potential mechanism for individual susceptibility to proarrhythmia and highlight the need for a new paradigm to screen drugs for QT prolonging and arrhythmogenic liability. # CLINICAL PERSPECTIVE {#article-title-51}