The exit of nanoparticles from solid tumours.

Nanoparticles enter tumours through endothelial cells, gaps or other mechanisms, but how they exit is unclear. The current paradigm states that collapsed tumour lymphatic vessels impair the exit of nanoparticles and lead to enhanced retention. Here we show that nanoparticles exit the tumour through the lymphatic vessels within or surrounding the tumour. The dominant lymphatic exit mechanism depends on the nanoparticle size. Nanoparticles that exit the tumour through the lymphatics are returned to the blood system, allowing them to recirculate and interact with the tumour in another pass. Our results enable us to define a mechanism of nanoparticle delivery to solid tumours alternative to the enhanced permeability and retention effect. We call this mechanism the active transport and retention principle. This delivery principle provides a new framework to engineer nanomedicines for cancer treatment and detection.