The pathways for nanoparticle transport across tumour endothelium.

The active transport and retention principle is an alternative mechanism to the enhanced permeability and retention effect for explaining nanoparticle tumour delivery. It postulates that nanoparticles actively transport across tumour endothelial cells instead of passively moving through gaps between these cells. How nanoparticles transport across tumour endothelial cells remains unknown. Here we show that nanoparticles cross tumour endothelial cells predominantly using the non-receptor-based macropinocytosis pathway. We discovered that tumour endothelial cell membrane ruffles capture circulating nanoparticles, internalize them in intracellular vesicles and release them into the tumour interstitium. Tumour endothelial cells have a higher membrane ruffle density than healthy endothelium, which may partially explain the elevated nanoparticle tumour accumulation. Receptor-based endocytosis pathways such as clathrin-mediated endocytosis contribute to nanoparticle transport to a lesser extent. Nanoparticle size determines the degree of contribution for each pathway. Elucidating the nanoparticle transport mechanism is crucial for developing strategies to control nanoparticle tumour delivery.