Current Delivery Strategies to Improve the Target of Cell Penetrating Peptides Used for Tumor-Related Therapeutics

Cell Penetrating Peptides (CPPs) equipped with a high penetrating ability are used as a promising tool to gain access to the cell interior, cross the cell membrane and deliver bioactive small or macromolecular cargos into the cytoplasm or nucleus. The superiority of wide range of applications, high transport efficiency and low biological toxicity make them particularly desirable in laboratory or clinical studies. Previous studies have shown that their non-selectivity and reaction with proteins in plasma hamper their application for tumor therapy, which might adversely affect the treatment effect and even induce some side effects. However, several recent studies have found that various kinds of modifiers of CPPs can effectively increase the target selectivity, reduce cytotoxicity to normal cells and produce multiple antitumor functions due to the different cleavable bonds which are sensitive to the tumor microenvironment or other novel designs. Apparently, these designs of ‘smart’ CPPs appear to be promising in the field of antitumor drug delivery. Here, we review these current improved approaches which mainly involve strategies of physical, chemical as well as biological pathways and we also explain the possible uptake mechanisms of direct penetration, internalization and escape which have been discussed in some publications with specific attention. In addition, some possible problems needed to be considered in the process of improving CPPs are discussed at the end of this review. This study aims to present an overview of the latest progress of CPPs, and provides a comprehensive theoretical background and reference guidance for future laboratory research and clinical application.