Quantum dots (Qdots) have become ubiquitous in biomedical research due to their excellent
brightness, photostability, monodispersity, and fluorescent yield. Furthermore, they have
become increasingly useful as imaging agents which are valuable for answering molecular
questions in living subjects. However, little is currently known about how nanoparticles such
as Qdots interact at the microscale within the vasculature and tumor microenvironments in
living subjects. In order to further our understanding of the dynamic processes involved in
Qdot targeting in the intact tumor, we developed an in vivo binding assay to visualize and fully
elucidate this approach using a variety of animal models and tumor types. We employed
argine-glycine-aspartic acid (RGD) peptides to specifically target the αvβ3 integrins which are
expressed on the surface of endothelial cells comprising newly formed or forming blood
vessels; RGD peptides were conjugated to the Qdot surface. Exploiting intravital microscopy
with subcellular-level resolution, we directly observed and recorded the binding of
nanoparticle conjugates in two different murine models, using three different tumor cell lines.
Using this generalizable approach, we learned that RGD-qdots unexpectedly bind to tumor
blood vessels in all models tested only as aggregates rather than individually. Understanding
these issues on the microscale using such techniques will provide a platform for the rational
design of molecularly-targeted nanoparticles including Qdots. This is critical for
nanoparticles to become a valuable research tool with the potential to become clinically
valuable imaging and therapeutic agents, particularly for ensuring regulatory approval of
such nanoparticles.
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