“Smart” Nanocarriers: A New Paradigm for Tumor Targeting Drug Delivery Systems

Cancer, once considered as an incurable disease, today, becomes not that difficult to be treated due to the newly emerging nanotechnology and the rapid development of “smart” drug delivery systems. New cancer therapeutics ought to overcome the defects of conventional drug delivery systems, such as nonspecific bio-distribution or targeting, short circulation time, etc. Tumor targeted drug delivery systems basically includes two strategies: passive and active targeting. Passive targeting is often referred to the enhanced permeability and retention effect (EPR effect) due to the enhanced vascular penetration of tumor capillaries and limited lymphatic drainage of lymphatic capillaries. Active targeting systems are based on local stimuli characteristic of the target pathological zone (such as, increased temperature or lowered pH values, redox potential changes, characteristic of inflamed, the receptor mediated internalization by target cells, etc.). Intriguingly, the combination of the two strategies above is promising and exciting. Herein, we give a brief review of tumor targeted drug delivery systems based on tumor cell itself and its external milieu.