Recent progress of cell-penetrating peptides as new carriers for intracellular cargo delivery.

The plasma membrane as a selectively permeable barrier of living cells is essential to cell survival and function. In many cases, however, the efficient passage of exogenous bioactive molecules through the plasma membrane remains a major hurdle for intracellular delivery of cargoes. During the last two decades, the potential of peptides for drug delivery into cells has been highlighted by the discovery of numerous cell-penetrating peptides (CPPs). CPPs serving as carriers can successfully intracellular transport cargoes such as siRNA, nucleic acids, proteins, small molecule therapeutic agents, quantum dots and MRI contrast agents. This review mainly introduces recent advances of CPPs as new carriers for the development of cellular imaging, nuclear localization, pH-sensitive and thermally targeted delivery systems. In particular, we highlight the exploiting of the synergistic effects of targeting ligands and CPPs. What's more, the classification and cellular uptake mechanisms of CPPs are briefly discussed as well.