Stealth nanoparticles: high density but sheddable PEG is a key for tumor targeting.

There are three major requirements for a nanoparticle to deliver its payload to the tumor. First, the particles need to be stable in the circulation without releasing the drug prematurely. Second, the nanoparticles accumulate in the tumor efficiently. Third, the drug is released locally in the tumor tissue or inside the tumor cells. A variety of approaches have been developed to fulfill these requirements. In this perspective, we will discuss them together with some examples.