Phenotypic Assays for Characterizing Compound Effects on Induced Pluripotent Stem Cell-Derived Cardiac Spheroids

Development of more complex, biologically relevant, and predictive cell-based assays for compound screening is a major challenge in drug discovery. The focus of this study was to establish high-throughput compatible three-dimensional (3D) cardiotoxicity assays using human induced pluripotent stem cell-derived cardiomyocytes. Using both high-content imaging and fast kinetic fluorescence imaging, the impact of various compounds on the beating rates and patterns of cardiac spheroids was monitored by changes in intracellular Ca²⁺ levels with calcium-sensitive dyes. Advanced image analysis methods were implemented to provide multiparametric characterization of the Ca²⁺ oscillation patterns. In addition, we used confocal imaging and 3D analysis methods to characterize compound effects on the morphology of 3D spheroids. This phenotypic assay allows for the characterization of parameters such as beating frequency, amplitude, peak width, rise and decay times, as well as cell viability and morphological characteristics. A set of 22 compounds, including a number of known cardioactive and cardiotoxic drugs, was assayed at different time points, and the calculated EC₅₀ values for compound effects were compared between 3D and two-dimensional (2D) model systems. A significant concordance in the phenotypes was observed for compound effects between the two models, but essential differences in the concentration responses and time dependencies of the compound-induced effects were observed. Together, these results indicate that 3D cardiac spheroids constitute a functionally distinct biological model system from traditional flat 2D cultures. In conclusion, we have demonstrated that phenotypic assays using 3D model systems are enabled for screening and suitable for cardiotoxicity assessment in vitro.