New generation of cell-penetrating peptides: Functionality and potential clinical application

Cell-penetrating peptides (CPPs) can transport various cargoes through membranes of live cells. Since the first generations of CPPs suffered from insufficient cell and tissue selectivity, stability against proteases, and escape from endosomes, a new generation of peptides, with optimized properties, was developed. These are either derived from natural sources or created through the combination of multivalent structures. The second method allows achieving high internalization efficiency, high cell and tissue selectivity, and release from endosomes via hybrid structures, combining sequences for endosomal release, homing sequences, and sequences for activation at the target tissue and for local delivery of cargoes. CPPs with innate tumor selectivity include azurin, crotamine, maurocalcine, lycosin-I, buffalo cathelicidin, and peptide CB5005. Some of them can penetrate the membranes of live cells and influence intracellular signaling pathways, thereby exerting cytotoxic effects against tumor cells. To obtain multilayer penetration and stabilization against proteolytic degradation, as well as for better handling, CPPs are often conjugated to nanoparticles. A special problem for tumor treatment is the efficiency of drug transport through three-dimensional cell cultures. Therefore, the capability of CPPs to deliver the drug even to the innermost tissues is of crucial importance. Notably, the ability of certain CPPs to penetrate barriers such as skin, the blood-brain barrier (BBB), and cornea or conjunctiva of eyes enabled the replacement of dangerous and painful injections with soothing sprays, creams, and drops. However, it is difficult to rank the efficacy of CPPs because transport efficiency and tissue selectivity depend not only on the CPP itself but also on the target tissue or organ, as well as on the cargo and method of CPP-cargo coupling. Therefore, the present review describes some examples of new-generation CPPs and aims to provide advice on how to find or create the right CPP for a given task.