scarlet1967 wrote: This article published in August 2021 gives a good detailed overview of cell penetrating peptides and their advantages as a targeted delivery technology. They touch based on peptides lipophilicity (ability to dissolve in fat), solubility, and small molecular sizes etc. which contribute to internalization of them into cancer cells. Cancer can remodel the cell membrane resulting in less anticancer efficacy of chemotherapeutic agent however CPPs are positive charged are biodegradable and often possess low immunogenicity and low toxicity and favor the negatively charged tumor membranes over those of healthy cells which are electrically neutral. The targeted drug delivery technology can bypass the multi drug resistance due to better selectivity, better bioaccessibility ( the fraction of the total amount of a substance that is potentially available for absorption) to the tumor cells, enhanced permeability and retention and “utilization of a homing device such as an antibody or a targeting ligand that will enable to attach to tumor cells through antigens or receptors on their surface, usually overexpressed in certain tumor types” among others. The article goes on and discuss the pros and cons of covalently or noncovalently bounds with anticancer agents and additional beneficial feature of coupling a chemotherapeutic with a CPP.
“The ability to permeabilize in the membrane and gain access to the cytosolic compartment is largely dependent on the physicochemical qualities of the therapeutic entities including the most relevant such as lipophilicity (ability to dissolve in fat…), polarity (positive and negative charges at the ends of the molecule) and molecular size. Among them, a fairly but not excessively hydrophobic (water insoluble) character is most conducive to the drug transport; very large, polar hydrophilic molecules are being poorly internalized similarly as extremely hydrophobic (the former ones will fail to enter the membrane, the latter will enter readily but may have difficulty in leaving it). At the cellular level, most anticancer drugs act on membrane surface receptors or have intracellular targets. In the latter case, a drug must cross the cellular membrane that constitutes an additional barrier for it. A further complication for drug traversing the cellular membranes is their remodeling induced by cancer itself, which brings about complex changes in the membrane structure and composition. They lead to a drop in anticancer drug efficacy manifested by simultaneous cancer cell proliferation, avoidance of apoptosis and development of cellular resistance.
Generally, anticancer therapy bears a high incidence of toxicity due to a low therapeutic index of the drugs and the simultaneous necessity for the usage of high doses in order to produce the clinical effect. An explanation for these unfavorable parameters of the anticancer therapy is lack of drug selectivity (they damage not only cancerous cells but also healthy rapidly growing ones) and poor bioaccessibility to the tumor cells. Another shortcoming of this therapy is multidrug resistance (MDR), which constitutes one of the dominant reasons for its failure.
An attractive application of the EPR (enhanced permeability and retention) effect in tumor chemotherapy comprises therapeutics in the form of liposomes, polymers or micelles. This effect has also been utilized by experimental technologies based on cell-penetrating peptides coupled with diverse types of cargoes (low molecular weight anticancer drugs).
CPPs are a class of diverse short sequence peptides (usually <30 amino acids) of natural (protein-derived or chimeric) or synthetic origin.
Internalization mechanisms of CPPs According to the available data, CPPs are internalized by two possible main pathways, i.e. endocytosis (energy dependent) and/or direct translocation (energy independent).
The role of CPPs in cancer treatment (CPPs as vectors for anticancer drug delivery). The interaction of a CPP with cancerous cells should be considered in light of the current concept of cancer-induced modifications within the cell membrane itself. They concern both its heteropolysaccharide (They are compounds that are made up of two or more different types of monomer. Made up of the same repeating unit. As a rule, they are overexpressed in cancerous membranes) and lipid components (Also, the lipid component contributes to the anionic character of the cancerous membrane due to the exposition of the negatively charged phosphatidylserine in its outer leaflet which correlates with a more acidic pH of their external media as well), ensuring the uncontrolled growth, progression and invasiveness of the tumor cells.
Generally, the CPPs possessing mostly cationic nature due to arginine and lysine residues favor the negatively charged tumor membranes over those of healthy cells, which are electrically neutral.
Improvement of anticancer selectivity of CPPs
The most characteristic feature of them is that they are able not only to distinguish between cancer and normal cells but also target specific tumor cell types.
Another method of increasing the tumor cell selectivity of CPPs is utilization of a homing device such as an antibody or a targeting ligand that will enable to attach to tumor cells through antigens or receptors on their surface, usually overexpressed in certain tumor types.
Conjugation of CPPs with a cargo – two different strategies The association of the CPPs with their cargo may be divided into covalently or noncovalently bound. Both formulation approaches have pros and cons. The advantage of the covalent one (chemical cross-linking between the CPP and cargo) is obtaining a final product with a well-defined chemical structure and reproducibility of the procedure.
On the other hand, noncovalent interaction, i.e. physical complexation involves a simple bulk-mixing procedure of the compounds (CPP and cargo with or without a linker). This strategy, in comparison to the above-described one, is more simple and much easier to perform. Furthermore, it enables the usage of versatile cargos (with the preservation of their functionality) and low concentrations for induction of the biological response. However, the difficulty to control the final ratio and orientation of the peptide and cargo remains an important limitation of this method.
The anticancer activity improvement of the standard chemotherapeutics by CPPs Recently, it has been found that the pharmacological properties of the chemotherapeutics may be improved by conjugating them with the CPPs, and this results in an increased efficacy of the transported drugs. Thanks to the presence of the CPP in the conjugate, the drug internalization is promoted with the consequence of high concentrations within the tumorous cells.
It is noteworthy that the improved pharmacokinetics is not the only factor that impacts the activity profile of the conjugate. There is also evidence for possible pharmacodynamic interactions between its constituents since certain CPPs possess anticancer action. An additional beneficial feature of coupling a chemotherapeutic with a CPP is overcoming MDR, which frequently occurs after repeated exposure of tumor cells to the same drug.
Conclusions Concerning CPPs
- As potential vectors for cellular delivery of small molecule drugs, they indicate a certain level of selectivity to cancer cells, called passive selectivity.
- Besides drug transporting activity, certain peptides possess anticancer activity per se, accomplished mainly by membrane lytic effect, proapoptotic activity, or both actions.
- Among the anticancer peptides, the most promising are p28 and LTX-315. Of great interest is particularly the latter one, the first locally acting oncolytic agent that reorganizes the tumor microenvironment. It entered several settings of phase I and II clinical trials.”
https://pubmed.ncbi.nlm.nih.gov/34402743/