Normal microvessels with thick endothelial cells and tumour microvessels with leaky endothelial gaps due to overexpression of VEGFR (a) [33]. NPs are trapped in the subendothelial void by basement membrane surrounding vessels, forming a perivascular NPs pool that hinders their extravasation (b). Collagen hydrolase-mediated basement membrane destruction is used to increase NP extravasation (c) [34].