Discussion
Cardiac liabilities remain one of the main causes for drug attrition, not only during the drug development process, but also post-marketing. The early detection of compounds affecting the spontaneous beating of cardiomyocytes, either by interference with electrophysiology or by exerting a cardiomyocyte-specific cytotoxicity, is therefore a desirable capability within the drug discovery process.
Cardiomyocytes (CMs) derived from induced pluripotent stem cells (iPSCs) both of human and mouse origin, provide a primary-like phenotype with added benefit of purity, consistency and ease of use. Applying these CM in the RTCA Cardio xCELLigence (ACEA Biosciences) allows for label-free online measurement of spontaneous beating and cell viability over days and even weeks, as well as detection of changes in these parameters due to compound effects.
After optimizing assay protocols for seeding densities, preculture times, and media composition, both mouse and human iPS-derived CM were used to assess compounds exerting their effects via various mechanisms (e.g. ion channels, metabolism, etc). Baseline beating frequency of mouse CM (80 - 150 bpm) was markedly higher than that of human CM (40 - 90 bpm), reflecting species differences. Interestingly, the beating amplitude of mouse CM as a measure of force of contraction was approx. 10x higher than that of human CM.
Using a set of 20 compounds affecting cardiac elextrophysiology we could demonstrate that both mouse and human CM were capable to clearly identify cardiotoxic compounds. This was demonstrated by effects on beating frequency, amplitude, beat duration, and rhythmicity. Of the twenty compounds, 19 were accurately picked up in this set-up. For 10 of the compounds tested the sensitivity of human CM to the respective effect was markedly greater than that of mouse CM (lowest effect concentration). Besides compounds directly affecting ion channels within minutes or hours after application, the assay is also capable to pick up compounds exerting their cardiotoxic effect only after prolonged treatment, e.g. by inhibiting ion channel traficking (e.g. Pentamidine). In contrast to assays relying on electrophysiological measurements (e.g. Patch Clamp), the RTCA Cardio assay is able to detect also effects on cell viability and metabolism.
Our data suggest that the use of stemcell-derived cardiomyocytes in the RTCA Cardio instrument provide a predictive tool for the early detection of compounds affecting cardiac beating, regardless of the underlying mechanism (e.g. electrophysiology, cytotoxicity, ion channel traficking). In combination, researchers gain cost advantages using mouse CMs in primary screening, and translational advantages using human CMs in candidate selection.
