A Langendorff-heart discovery pipeline demonstrates cardiomyocyte targeting by extracellular vesicles functionalized with beta-blockers using click-chemistry.

Extracellular vesicles (EVs) are widely explored as vehicles for delivering therapeutic or experimental cargo to cardiomyocytes. Efforts to improve EV bioavailability in the heart, and reduce their off-target actions, require screening methods that can replicate the physiological and anatomical barriers present in the myocardium. Additionally, discovery pipelines must exercise control over EV dosage and timing, and provide a means of assessing cargo incorporation into cardiomyocytes specifically. These criteria are not generally met by experiments on cultured cells or animals. Here, we present a Langendorff-heart discovery pipeline that combines the strengths of in vivo and in vitro approaches. Langendorff-mode perfusion enables controlled exposure of beating hearts to re-circulated EVs. Following perfusion, cardiomyocytes can be isolated enzymatically for analysis, such as imaging. We tested this discovery pipeline by functionalizing EVs with beta-blockers (atenolol, metoprolol) using click-chemistry and incorporating the fluorescent protein NeonGreen2 to track the fate of EV cargo. Fluorescence in cardiomyocytes, including their nuclear regions, increased after Langendorff-treatment with beta-blocker decorated EVs, but only if these contained NeonGreen2, implicating the fluorescent cargo as the source of signal. Superior binding efficacy of beta-blockers was confirmed by referencing to the substantially lower signals obtained using wild-type EVs or EVs presenting myomaker or myomixer proteins, motifs that modestly enrich cardiac EV uptake in mice. Our findings demonstrate successful cardiomyocyte targeting using EVs decorated with beta-receptor binders. We propose the Langendorff-perfused heart as an intermediate step - nested between in vitro characterisation and animal testing - in discovery pipelines for seeking improved cardiac-specific EV designs.