Enhanced permeability and retention effect

[3][4][5] The general explanation that is given for this phenomenon is that, in order for tumor cells to grow quickly, they must stimulate the production of blood vessels.

They are poorly aligned defective endothelial cells with wide fenestrations, lacking a smooth muscle layer, or innervation with a wider lumen, and impaired functional receptors for angiotensin II.

This phenomenon is referred to as the "enhanced permeability and retention (EPR) effect" of macromolecules and lipids in solid tumors.

One factor that leads to the increased retention is the lack of lymphatics around the tumor region which would filter out such particles under normal conditions.

The EPR effect is usually employed to describe nanoparticle and liposome delivery to cancer tissue.