Peptides as targeting elements and tissue penetration devices for nanoparticles.

The use of nanoparticles in medicine (nanomedicine) has recently become an intensely studied field. Nanoparticles carrying drugs and imaging agents have already reached the clinic, but they are essentially passive delivery vehicles, not what are referred to as "smart" nanoparticles. An important function to add to make nanoparticles smarter is active homing to the target tissue. It makes nanoparticles accumulate in the target tissue at higher concentrations than would be the case without this feature, increasing therapeutic efficacy and reducing side effects. This review discusses the recent developments in the nanoparticle targeting field with emphasis on peptides that home to vascular "zip codes" in target tissues and provide a tissue- and cell-penetrating function.