Messenger RNAs (mRNAs) were previously shown to have great potential for preventive vaccination against infectious diseases and therapeutic applications in the treatment of cancers and genetic diseases. Delivery systems for mRNAs, including lipid- and polymer-based carriers, are being developed for improving mRNA bioavailability. Among these systems, cell-penetrating peptides (CPPs) of 4–40 amino acids have emerged as powerful tools for mRNA delivery, which were originally developed to deliver membrane-impermeable drugs, peptides, proteins, and nucleic acids to cells and tissues. Various functionalities can be integrated into CPPs by tuning the composition and sequence of natural and non-natural amino acids for mRNA delivery. With the employment of CPPs, improved endosomal escape efficiencies, selective targeting of dendritic cells (DCs), modulation of endosomal pathways for efficient antigen presentation by DCs, and effective mRNA delivery to the lungs by dry powder inhalation have been reported; additionally, they have been found to prolong protein expression by intracellular stabilization of mRNA. This review highlights the distinctive features of CPP-based mRNA delivery systems.

Magainin 2 (Mag2), which was isolated from the skin of the African clawed frog, is a representative antimicrobial peptide (AMP) that exerts antimicrobial activity via microbial membrane disruption. It has been reported that the helicity and amphipathicity of Mag2 play important roles in its antimicrobial activity. We investigated and recently reported that 17 amino acid residues of Mag2 are required for its antimicrobial activity, and accordingly developed antimicrobial foldamers containing α,α-disubstituted amino acid residues. In this study, we further designed and synthesized a set of Mag2 derivatives bearing the hydrocarbon stapling side chain for helix stabilization. The preferred secondary structures, antimicrobial activities, and cell-membrane disruption activities of the synthesized peptides were evaluated. Our analyses revealed that hydrocarbon stapling strongly stabilized the helical structure of the peptides and enhanced their antimicrobial activity. Moreover, peptide 2 stapling between the first and fifth position from the N-terminus showed higher antimicrobial activity than that of Mag2 against both gram-positive and gram-negative bacteria without exerting significant hemolytic activity. To investigate the modes of action of tested peptides 2 and 8 in antimicrobial and hemolytic activity, electrophysiological measurements were performed.

Proteolysis-targeting chimeras (PROTACs) have emerged as a novel therapeutic modality for the degradation of target proteins via the ubiquitin-proteasome system. Currently, more than a dozen PROTAC...

Peptide-based target protein degradation inducers called PROTACs/SNIPERs have low cell penetrability and poor intracellular stability as drawbacks. These shortcomings can be overcome by easily modifying these peptides by conjugation with cell penetrating peptides and side-chain stapling. In this study, we succeeded in developing the stapled peptide stPERML-R7, which is based on the estrogen receptor alpha (ERα)-binding peptide PERML and composed of natural amino acids. stPERML-R7, which includes a hepta-arginine motif and a hydrocarbon stapling moiety, showed increased α-helicity and similar binding affinity toward ERα when compared with those of the parent peptide PERML. Furthermore, we used stPERML-R7 to develop a peptide-based degrader LCL-stPERML-R7 targeting ERα by conjugating stPERML-R7 with a small molecule LCL161 (LCL) that recruits the E3 ligase IAPs to induce proteasomal degradation via ubiquitylation. The chimeric peptide LCL-stPERML-R7 induced sustained degradation of ERα and potently inhibited ERα-mediated transcription more effectively than the unstapled chimera LCL-PERML-R7. These results suggest that a stapled structure is effective in maintaining the intracellular activity of peptide-based degraders.

Peptide foldamers play a critical role in pharmaceutical research and biomedical applications. This review highlights recent (post-2020) advancements in novel foldamers, synthetic techniques, and their applications in pharmaceutical research. The authors summarize the structures and applications of peptide foldamers such as α, β, γ-peptides, hydrocarbon-stapled peptides, urea-type foldamers, sulfonic-γ-amino acid foldamers, aromatic foldamers, and peptoids, which tackle the challenges of traditional peptide drugs. Regarding antimicrobial use, foldamers have shown progress in their potential against drug-resistant bacteria. In drug development, peptide foldamers have been used as drug delivery systems (DDS) and protein-protein interaction (PPI) inhibitors. These structures exhibit resistance to enzymatic degradation, are promising for therapeutic delivery, and disrupt crucial PPIs associated with diseases such as cancer with specificity, versatility, and stability, which are useful therapeutic properties. However, the complexity and cost of their synthesis, along with the necessity for thorough safety and efficacy assessments, necessitate extensive research and cross-sector collaboration. Advances in synthesis methods, computational modeling, and targeted delivery systems are essential for fully realizing the therapeutic potential of foldamers and integrating them into mainstream medical treatments.

Cell-penetrating peptides (CPPs) are widely used for the intracellular delivery of a variety of cargo molecules, including small molecules, peptides, nucleic acids, and proteins. Many cationic and amphiphilic CPPs have been developed; however, there have been few reports regarding hydrophobic CPPs. Herein, we have developed stapled hydrophobic CPPs based on the hydrophobic CPP, TP10, by introducing an aliphatic carbon side chain on the hydrophobic face of TP10. This side chain maintained the hydrophobicity of TP10 and enhanced the helicity and cell penetrating efficiency. We evaluated the preferred secondary structures, and the ability to deliver 5(6)-carboxyfluorescein (CF) as a model small molecule and plasmid DNA (pDNA) as a model nucleotide. The stapled peptide F-3 with CF, in which the stapling structure was introduced at Gly residues, formed a stable α-helical structure and the highest cell-membrane permeability via an endocytosis process. Meanwhile, peptide F-4 demonstrated remarkable stability when forming a complex with pDNA, making it the optimal choice for the efficient intracellular delivery of pDNA. The results showed that stapled hydrophobic CPPs were able to deliver small molecules and pDNA into cells, and that different stapling positions in hydrophobic CPPs can control the efficiency of the cargo delivery.