Cy3‐Conjugated Biocompatible Polymer Nanoparticles for Long‐Term Mitochondrial Imaging

The cyanine dye Cy3 has recently gained attention as a membrane potential–responsive, small‐molecule mitochondrial imaging probe. However, the efficiency of Cy3 for mitochondrial imaging is relatively low, as diffusion of Cy3 leads to staining of the entire cell over time. This problem could be due to the passive diffusion‐based, concentration‐dependent uptake into cells and molecular diffusion inherent to small‐molecule compounds. Therefore, we hypothesized that polymer chemistry approach could be applied to overcome these limitations. In this study, we designed modified dye in which biocompatible polymers were covalently attached to Cy3. This enabled control of intracellular dynamics that could not be achieved using Cy3 alone. Toward this objective, we synthesized a Cy3‐modified amphiphilic dextran with phenylalanine ethyl ether side chains. The amphiphilic polymers self‐assembled into nanoparticles. We then characterized the synthesized polymer in terms of intracellular uptake and the mitochondria translocation capacity of the nanoparticles using human skeletal muscle satellite cells. Our approach changed the cellular internalization process, resulting in increased cellular uptake efficiency, enhanced mitochondrial imaging capability, and improved mitochondrial retention over the long term. These nanoparticles, which can stain mitochondria regardless of cell type, have the potential to become a new type of mitochondrial imaging reagent.