Figure 1. The enhanced permeability and retention (EPR) effect.

During the initial stages of tumour formation, there is a rapid pro-angiogenic neovascularisation to supply the tumour with all nutritional requirements to survive. As a result of the rapid pace of formation, blood vessels are often defective, with large apertures left between endothelial cells resulting in a leaky vasculature. Nanoparticles have the capacity to exploit this defect in vessels, allowing them to pass from the blood vessels into the tumour site. Additionally, retention of the nanoparticles occurs within the tumour site as a result of poor lymphatic drainage. Image created with the use of Smart Servier Medical Art.