PROJECT SUMMARY/ABSTRACT Cardiomyopathy patients are prone to ventricular arrhythmias (VA) and sudden death. Current therapies to prevent VA include radiofrequency ablation to destroy slowly conducting pathways of viable myocardium which support re-entry. Here we propose to test the reverse concept, namely that boosting local tissue viability in zones of slow conduction might eliminate slow conduction and suppress VA. We seek to do so by local injection, into mapped areas of slow conduction, of exosomes secreted by cardiosphere-derived cells (CDCs), a stromal/progenitor cell type from the human heart. Exosomes are extracellular vesicles laden with bioactive cargo. Those secreted by CDCs (CDCEXO) reduce scar and improve heart function after intramyocardial delivery. In a VA-prone porcine model of ischemic cardiomyopathy, we present preliminary data in which we injected CDCEXO or vehicle into zones of delayed conduction defined by electroanatomic mapping. Up to one month post- injection, CDCEXO, but not vehicle, decreased myocardial scar, suppressed slowly conducting electrical pathways, and inhibited VA induction by programmed stimulation. In silico reconstruction of electrical activity based on magnetic resonance images accurately reproduced the suppression of VA inducibility by CDCEXO. Here we will explore alternative intracoronary delivery methods which would be more readily translatable clinically than the intramyocardial injection approach, which requires non-approved catheters. We will also explore mechanism of the anti-VA effects histologically and by RNA sequencing, as well as in a co-culture assay of cardiomyocytes and fibroblasts. We will also compare directly the efficacy of exosomes versus conventional RF ablation. In conclusion, we seek to establish biological substrate modification by exosome injection as a nondestructive alternative to conventional ablation for the prevention of recurrent ventricular tachyarrhythmias.