CPPS

Cell-penetrating peptides (CPPs) are short peptides that enable the intracellular delivery of diverse cargoes. Usually under 30 amino acids in length, they are often rich in basic residues and can cross cellular membranes by multiple pathways, depending on their sequence and the attached cargo. CPPs have been used to transport small molecules, nucleic acids, proteins, and nanoparticles for research and therapeutic purposes.

The terminology and concept emerged in the 1990s with the discovery of protein transduction domains. Notable CPPs include HIV-1 TAT-derived domains, penetratin from the Antennapedia homeodomain, and various poly-arginine sequences. Uptake mechanisms are diverse and can involve direct translocation across the membrane as well as energy-dependent endocytosis, including clathrin- and caveolin-mediated pathways and macropinocytosis. The preferred route depends on the peptide, cargo, concentration, and cell type. CPPs can be linked to cargo covalently or associated non-covalently, and the design influences delivery efficiency and intracellular fate.

Applications of CPPs span therapeutic and diagnostic areas. They enable delivery of siRNA, plasmid DNA, proteins, enzymes, and imaging probes. To enhance intracellular release and endosomal escape, CPP designs often incorporate features such as fusogenic elements or pH-responsive linkers. CPPs are also used to functionalize nanoparticles and liposomes to improve stability, pharmacokinetics, and targeting.

Challenges remain, including variable uptake across cell types, potential off-target delivery, cytotoxicity, immunogenicity, and endosomal entrapment. Ongoing research aims to improve selectivity, safety, and efficiency through sequence optimization, chemical modification, and integration with targeting strategies.