Drug-induced arrhythmia has been a major cause of drug development failure and the market withdrawal of novel compounds. Of particular concern is the block of the ion channel I_Kr (hERG) by drugs which can result in torsades de pointes (TdP), a dangerous ventricular fibrillation.  Unanticipated toxic effects on contractility such as with the tyrosine kinase inhibitors and anthracyclines are also of increasing concern.  Current methods to assess drug-induced toxicity during drug development heavily rely on animal models such as the Langendorff Heart preparation or overexpressed hERG channel cell models.  Despite proving effective methods of identifying potential cardiotoxic liabilities during drug development these still create concern over their direct relevance to human physiology and toxicology.  Human iPSC-derived cardiomyocytes (hiPSC-CMs) offer the opportunity to screen drugs in vitro using a more physiologically relevant model that expresses multiple ion channels and spontaneously contracts.

Axol Bioscience Ltd have shown their human iPSC-derived Ventricular Cardiomyocytes (ax2508) express all significant ventricular cardiomyocyte markers through RNAseq (two replicates) and immunocytochemistry (34 replicates) and have performed extensive electrophysiological characterisation using Multi-Electrode Array (MEA) demonstrating the presence of all key ion channels (min n=4) and a typical ventricular cardiomyocyte-like waveform (10 replicates).   

In addition, innoVitro GmBH reproduced the correct contractility responses to isoprenaline, S-Bay K8644, 4-AP and the atrial-specific Carbachol, in ax2508, on their FLEXcyte 96 platform (n=4 per compound and concentration, assessed using the Wilcoxon rank-sum test). 

Clyde Biosciences tested the CiPA28 acute cardiotoxic reference compounds on the CellOPTIQ™ platform against Axol’s ventricular cardiomyocytes. The cardiomyocytes were grown for 6 days in multiwell format and then transitioned to a serum-free media and loaded with a voltage sensitive dye. Optical measurement of voltage changes allowed the assessment of the effect of each of the 28 compounds (min. n=5), which have varying levels of known TdP risk. hERG block was detected in Axol cardiomyocytes with a range of compounds, as evidenced by action potential duration at 90% repolarisation (APD₉₀) prolongation, APD triangulation and early afterdepolarisations (EADs). For example, clear hERG block was detected at even the lowest conc. of the classic hERG blocker dofetilide (0.3nM) through QT-prolongation (500ms to 550ms) and increased triangulation and the Ca²⁺-channel blocker nifedipine produced a shortened FPD (500ms to 300ms) while the low TdP risk anti-histamine loratidine only had minimal effects.  In addition, the predictive power of Axol cardiomyocytes was reinforced when those compounds with multiple ion channel effects and varying risk profiles, such as verapamil, azimilide and ranolazine, modes of action and relative risks were correctly identified so that verapamil’s calcium block correctly counteracted its hERG block, ranolazine increased FPD, by 100ms, but only at intermediate concentrations and azimilide’s pro-arrhythmic actions were only apparent at concentrations above 1μM.  Drug effects were compared to vehicle control within the same well and were assessed using paired t-tests with P-values adjusted for multiple comparisons using the Benjamini-Hochberg procedure.

Therefore, Axol’s human iPSC-derived Ventricular Cardiomyocytes can provide a reliable, physiological-relevant model to perform cardiotoxicity studies at scale and within a short time-frame on a range of different platforms.