Recent advances in biological strategies for targeted drug delivery.

Targeted drug delivery system, consisting of a guidance component, a transport vehicle and the drug, is aiming at distributing the drugs to a specific organ or cell type in a sufficient concentration to achieve the high efficiency and alleviate the side-effects of the drugs. During the past decade, besides physical and chemical methods, biological strategies have been developing at an amazing speed improving the drug targeting system. These include: i) searching for guidance components, which include integrin ligands, extracellular matrix proteins, carbohydrates, vitamins, antibodies, nucleic acid aptamers and peptides, leading the whole system to the targeting sites; ii) development of the transport vehicles, such as polymers and plasmid/virus vectors that play the role in carrying drugs and protecting them on their way to the destination; and iii) the design or selection of an effective drug, the "final bullet" that functions on the targeting site, to test the efficiency of the system. Here, we briefly review the recent advances on the development of the most essential targeted drug delivery systems and strategies, and particularly focusing on the progress in discovery of the ligand components and construction of carrying vehicles for drug targeted delivery systems. In addition, we briefly discuss the prospect of this field.