Cardiotoxic response is the leading cause of attrition in drug development. Much emphasis has been duly placed on functional impairments caused by electrophysiological disruptions such as pro-arrhythmia, but there are structural components and toxicities that can play critical roles. These structural toxicities can target the cardiomyocytes directly or alternatively act upon the non-myocytes such as the cardiac endothelial and fibroblast cells. For example damage to the myocardial endothelium is a primary mechanism of toxicity for kinase inhibitors, a class of drugs often used in cancer treatment. This is especially noteworthy because current testing strategies focus almost exclusively on pro-arrhythmic toxicity markers. Furthermore, oncology treatments are the highest grossing therapeutic area by a wide margin, with $38 billion in sales in 2014 (data from The Top 25 Best Selling Drugs of 2014 – GEN).
Cyprotex have developed in vitro three dimensional (3D) human cardiac microtissue models for assessing structural cardiotoxicity. By implementing a tri-culture method which combines iPSC-derived cardiomyocytes, cardiac endothelial cells and cardiac fibroblasts, we have developed a spontaneously beating 3D cardiac microtissue model that recapitulates the in vivo cellular physiology of the myocardium more closely than 2D culture or monoculture 3D microtissue models, and provides improved sensitivity and specificity for detecting structural cardiovascular toxicity.
These results were demonstrated in a validation experiment in which four cultures (tri-cultured iPSC-derived cardiac microtissues, tri-cultured H9c2 microtissues, iPSC-derived cardiomyocyte monolayers and H9c2 monolayers) were exposed to 10 compounds, eight of which were known structural toxicants. Although all cultures performed well, only the spontaneously beating tri-cultured cardiac model correctly identified all structural cardiotoxins and distinguished these drugs from those reported not to be cardiotoxic.
Furthermore, an iPSC-derived cardiomyocyte monoculture microtissue model was evaluated for its ability to predict hypertrophy. Utilisation of a mono-culture model allows the detection of cardiomyocyte mass increase which was assessed by measuring microtissue area. Although based on limited data, the model provided a robust prediction of cardiac hypertrophy prior to cardiomyocyte death.
In summary, whereas 2D models can provide excellent understanding of the electrophysiological implications and risks carried by a given drug, 3D models provide more relevant insight into cellular and tissue structure toxicities that may arise. A High Content Screening (HCS) approach can be used in conjunction with 3D microtissues to assess structural toxicity markers like calcium homeostasis, mitochondrial membrane potential, cellular ATP content and microtissue area in order to provide valuable tools for the early stage detection of structural toxicity to the heart.
For more information on 3D microtissues and their applicability to detecting cardiotoxicity, download our poster presented at Eurotox 2015.