enhanced permeability and retention (EPR) effect

Think of a tumour as a hastily built suburb where the water pipes were laid carelessly and the drains were never finished. Water that leaks out of the loose pipes has nowhere to drain, so it pools. The EPR effect is the same idea for nanocarriers in tumours: they leak out of the faulty blood vessels and then stay, because the tissue cannot flush them away.

Rapidly growing tumours sprout new blood vessels quickly and chaotically, leaving gaps between the cells that line them. Particles of roughly nanoparticle size can slip through these gaps into the tumour, something they cannot do in normal tissue where vessel walls are tight. Tumours also have poor or absent lymphatic drainage, so once a carrier is inside it tends to be retained rather than carried away. EPR is the main mechanism behind passive targeting of tumours.

EPR is the theoretical justification for many anticancer nanomedicines, but its real-world contribution is debated. The effect is highly variable between tumour types, between patients, and over time, and it is generally weaker in humans than in the small-animal models where it was discovered. Many nanomedicines that relied on EPR have disappointed in the clinic, so it should be treated as a real but limited and unreliable phenomenon.