Designed Multifunctional Peptides for Intracellular Targets

Nature’s way for bioactive peptides is to provide them with several related functions and the ability to cooperate in performing their job. Natural cell-penetrating peptides (CPP), such as penetratins, inspired the design of multifunctional constructs with CPP ability. This review focuses on known and novel peptides that can easily reach intracellular targets with little or no toxicity to mammalian cells. All peptide candidates were evaluated and ranked according to the predictions of low toxicity to mammalian cells and broad-spectrum activity. The final set of the 20 best peptide candidates contains the peptides optimized for cell-penetrating, antimicrobial, anticancer, antiviral, antifungal, and anti-inflammatory activity. Their predicted features are intrinsic disorder and the ability to acquire an amphipathic structure upon contact with membranes or nucleic acids. In conclusion, the review argues for exploring wide-spectrum multifunctionality for novel nontoxic hybrids with cell-penetrating peptides.

The role of cell-penetrating peptides in potential anti-cancer therapy

Due to the complex physiological structure, microenvironment and multiple physiological barriers, traditional anti-cancer drugs are severely restricted from reaching the tumour site. Cell-penetrating peptides (CPPs) are typically made up of 5-30 amino acids, and can be utilised as molecular transporters to facilitate the passage of therapeutic drugs across physiological barriers. Up to now, CPPs have widely been used in many anti-cancer treatment strategies, serving as an excellent potential choice for oncology treatment. However, their drawbacks, such as the lack of cell specificity, short duration of action, poor stability in vivo, compatibility problems (i.e. immunogenicity), poor therapeutic efficacy and formation of unwanted metabolites, have limited their further application in cancer treatment. The cellular uptake mechanisms of CPPs involve mainly endocytosis and direct penetration, but still remain highly controversial in academia. The CPPs-based drug delivery strategy could be improved by clever design or chemical modifications to develop the next-generation CPPs with enhanced cell penetration capability, stability and selectivity. In addition, some recent advances in targeted cell penetration that involve CPPs provide some new ideas to optimise CPPs.

Insight of Synergistic Effect between CPP and Cargo on the Facilitation Mechanisms of R7-PTX Translocation: Experiments and Molecular Simulations

In our previous study, a novel cell penetrating peptide (CPP) R7 (Arg-Arg-Arg-Arg-Arg-Trp-Trp, RRRRRWW) has been developed to help cellular internalization of paclitaxel (PTX) through the non-covalent interaction with CPP. However, the facilitation mechanism of R7 mediated PTX translocation is not clear. Here the uptake pathways of R7 and R7-PTX were investigated by in vitro test and molecular simulations. In vitro experiments reveal that both R7 and R7-PTX complex translocate through the direct translocation and clathrin mediated endocytosis and associate with the macropinocytosis pathway at high CPP concentration. The translocation of R7(0.1 mM)-PTX complex further involves the lipid raft/caveolae mediated endocytosis. The simulation results show that the synergistic effect between R7 and PTX not only changes the penetration energy barrier but also activates the macropinocytosis and lipid raft/caveolae mediated pathway, resulting in the improvement in the translocation. The presence of heparin also improves the R7 and R7-PTX translocation. These studies provide a theoretical basis for understanding PTX delivery facilitated by the synergistic effect between CPP and cargo and paves a way for CPP design.

Intracellular Paclitaxel Delivery Facilitated by a Dual-Functional CPP with a Hydrophobic Hairpin Tail

In our pervious study, a dual-functional peptide R7 was developed to form a complex with paclitaxel (PTX) for enhancement of PTX translocation. However, because of the unstable noncovalent bond between R7 and PTX, PTX redistributed after the introduction of heparin, leading to R7-PTX complex dissociation, further causing less PTX penetration than expected. Thus, a novel positive CPP carrier of P9 was developed to improve CPP-PTX affinity via a double-proline (Pro, P) hairpin tail and enhance PTX translocation through the reduction of translocation energy barrier, confirmed by the MM-PBSA analysis and umbrella sampling simulation. Cellular uptake study reveals that P9 can quickly translocate into the HeLa cells within 1 min and exhibits no noticeable cytotoxicity. Compared to R7, P9 is able to help PTX translocation, leading to a remarkable increase in the intracellular concentration of PTX, eventually resulting in a significant loss in tumor cell viability. In vivo experiments demonstrate that a vein injection of P9-PTX complex dramatically inhibits tumor growth. Our study provides a novel perspective for designing CPP-facilitated drug carrier to enhance antitumor efficiency.