Application of Human Stem Cell−derived Cardiomyocytes in Safety Pharmacology Requires Caution Beyond hERG

Abstract

Human embryonic stem cell-derived cardiomyocytes (hESC-CM) have been proposed as a new model for safety pharmacology. So far, a thorough description of their basic electrophysiology and extensive testing, and mechanistic explanations, of their overall pro-arrhythmic ability is lacking. Under standardized conditions, we have evaluated the sensitivity of hESC-CM to proarrhythmic provocations by blockade of hERG and other channels. Using voltage patch clamp, some ion current densities (pA/pF) in hESC-CM were comparable to adult CM: IKr (-12.5±6.9), IKs (0.65±0.12), INa,peak (-72±21), INa,late (-1.10±0.36), and ICa,L (-4.3±0.6). If density was larger (-10±1.1) and IK1 not existent or very small (-2.67±0.3). The low IK1 density was corroborated by low KCNJ2 mRNA levels. Effects of pro-arrhythmic compounds on action potential (AP) parameters and provocation of early afterdepolarizations (EADs) revealed that Chromanol293B (100 μmol/l) and Bay K8644 (1 μmol/l) both significantly prolonged APD90. ATX-II (<1 μmol/l ) and BaCl2 (10 μmol/l ) had no effect on APD. The only compound that triggered EADs was hERG blocker Cisapride. Computer simulations and APclamp showed that the immature AP of hESC-CM prevents proper functioning of INa-channels, and result in lower peak/maximal currents of several other channels, compared to the adult situation. Lack of functional IK1 channels and shifted INa channel activation cause a rather immature electrophysiological phenotype in hESC-CM, and thereby limits the potential of this model to respond accurately to pro-arrhythmic triggers other than hERG block. Maturation of the electrical phenotype is a prerequiste for future implementation of the model in arrhythmogenic safety testing.