(R-X-R)4 -Motif Peptides Containing Conformationally Constrained Cyclohexane-Derived Spacers: Effect on Cellular Uptake

Assessment of the safety of the cationic arginine-rich peptides (CARPs) poly-arginine-18 (R18 and R18D) in ex vivo models of mast cell degranulation and red blood cell hemolysis

Our laboratory focuses on the development of novel neuroprotective cationic peptides, such poly-arginine-18 (R18: 18-mer of l-arginine; net charge +18) and its d-enantiomer R18D in stroke and other brain injuries. In the clinical development of R18/R18D, their cationic property raises potential safety concerns on their non-specific effects to induce mast cell degranulation and hemolysis. To address this, we first utilised primary human cultured mast cells (HCMCs) to examine anaphylactoid effects. We also included as controls, the well-characterised neuroprotective TAT-NR2B9c peptide and the widely used heparin reversal peptide, protamine. Degranulation assay based on β-hexosaminidase release demonstrated that R18 and R18D did not induce significant mast cell degranulation in both untreated (naïve) and IgE-sensitised HCMCs in a dose-response study to a maximum peptide concentration of 16 μM. Similarly, TAT-NR2B9c and protamine did not induce significant mast cell degranulation. To examine hemolytic effects, red blood cells (RBCs), were incubated with the peptides at a concentration range of 1–16 μM in the absence or presence of 2% plasma. Measurement of hemoglobin absorbance revealed that only R18 induced a modest, but significant degree of hemolysis at the 16 μM concentration, and only in the absence of plasma. This study addressed the potential safety concern of the application of the cationic neuroprotective peptides, especially, R18D, on anaphylactoid responses and hemolysis. The findings indicate that R18, R18D, TAT-NR2B9c and protamine are unlikely to induce histamine mediated anaphylactoid reactions or RBC hemolysis when administered intravenously to patients.

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Cationic peptides R18, R18D, TAT-NR2B9c and protamine do not stimulate the degranulation of ex vivo untreated (naïve) and IgE-sensitised primary human cultured mast cells (HCMCs).

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R18, R18D, TAT-NR2B9c and protamine do not induce significant hemolysis of human red blood cells (RBCs).

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R18, R18D, TAT-NR2B9c and protamine are unlikely to induce histamine mediated anaphylactoid reactions or hemolysis when administered intravenously to patients.

Studies of cell-penetrating peptides by biophysical methods

Biophysical studies have a very high impact on the understanding of internalization, molecular mechanisms, interactions, and localization of CPPs and CPP/cargo conjugates in live cells or in vivo. Biophysical studies are often first carried out in test-tube set-ups or in vitro, leading to the complicated in vivo systems. This review describes recent studies of CPP internalization, mechanisms, and localization. The multiple methods in these studies reveal different novel and important aspects and define the rules for CPP mechanisms, hopefully leading to their improved applicability to novel and safe therapies.

Cell-penetrating peptides in protein mimicry and cancer therapeutics

Extensive research has been undertaken in the pursuit of anticancer therapeutics. Many anticancer drugs require specificity of delivery to cancer cells, whilst sparing healthy tissue. Cell-penetrating peptides (CPPs), now well established as facilitators of intracellular delivery, have in recent years advanced to incorporate target specificity and thus possess great potential for the targeted delivery of anticancer cargoes. Though none have yet been approved for clinical use, this novel technology has already entered clinical trials. In this review we present CPPs, discuss their classification, mechanisms of cargo internalization and highlight strategies for conjugation to anticancer moieties including their incorporation into therapeutic proteins. As the mainstay of this review, strategies to build specificity into tumor targeting CPP constructs through exploitation of the tumor microenvironment and the use of tumor homing peptides are discussed, whilst acknowledging the extensive contribution made by CPP constructs to target specific protein-protein interactions integral to intracellular signaling pathways associated with tumor cell survival and progression. Finally, antibody/antigen CPP conjugates and their potential roles in cancer immunotherapy and diagnostics are considered. In summary, this review aims to harness the potential of CPP-aided drug delivery for future cancer therapies and diagnostics whilst highlighting some of the most recent achievements in selective delivery of anticancer drugs, including cytostatic drugs, to a range of tumor cells both in vitro and in vivo.

Cell-Penetrating Peptides

In this introductory chapter, we first define cell-penetrating peptides (CPPs), give short overview of CPP history and discuss several aspects of CPP classification. Next section is devoted to the mechanism of CPP penetration into the cells, where direct and endocytic internalization of CPP is explained. Kinetics of internalization is discussed more extensively, since this topic is not discussed in other chapters of this book. At the end of this section some features of the thermodynamics of CPP interaction with the membrane is also presented. Finally, we present different cargoes that can be transferred into the cells by CPPs and briefly discuss the effect of cargo on the rate and efficiency of penetration into the cells.

1988-2018: Thirty years of drug smuggling at the nano scale. Challenges and opportunities of cell-penetrating peptides in biomedical research

In 1988, two unrelated papers reported the discovery of peptide vectors with innate cell translocation properties, setting the ground for a new area of research that over the years has grown into considerable therapeutic potential. The vectors, named cell-penetrating peptides (CPPs), constitute a now large and diversified family, sharing the extraordinary ability to diffuse unaltered across cell membranes while ferrying diverse associated cargos. Such properties have made CPPs ideal tools for delivery of nucleic acids, proteins and other therapeutic/diagnostic molecules to cells and tissues via covalent conjugation or complexation. This year 2018 marks the 30^th anniversary of a peptide research landmark opening new perspectives in drug delivery. Given its vastness, exhaustive coverage of the main features and accomplishments in the CPP field is virtually impossible. Hence this manuscript, after saluting the above 30^th jubilee, focuses by necessity on the most recent contributions, providing a comprehensive list of recognized CPPs and their latest-reported applications over the last two years. In addition, it thoroughly reviews three areas of peptide vector research of particular interest to us, namely (i) efficient transport of low-bioavailability drugs into the brain; (ii) CPP-delivered disruptors of G protein-coupled receptor (GPCRs) heteromers related to several disorders, and (iii) CPP-mediated delivery of useful but poorly internalized drugs into parasites.