Investigating the impact of thiol reactivity and disulfide formation on cellular uptake of cell-permeable peptides

Cell-penetrating peptides (CPPs) have been explored as versatile tools to transport various molecules into cells. The uptake mechanism of CPPs is still not clearly understood and most probably depends on several factors like the nature of the CPP itself, the attached cargo, the investigated cell system, and other experimental conditions, such as temperature and concentration. One of the first steps of internalization involves the interaction of CPPs with negatively charged molecules present at the outer layer of the cell membrane. Recently, thiol-mediated uptake has been found to support the effective translocation of sulfhydryl-bearing substances that would actually not be cell-permeable. Within this work, we aimed to understand the relevance of thiol reactivity for the uptake mechanism of cysteine-containing CPPs that we have developed previously in our group. Therefore, we compared the two peptides, sC18-Cys and CaaX-1, in their single reduced and dimeric disulfide versions. Cytotoxicity, intracellular accumulation, and impact on the internalization process of the disulfides were investigated in HeLa cells. Both disulfide CPPs demonstrated significantly stronger cytotoxic effects and membrane activity compared with their reduced counterparts. Notably, thiol-mediated uptake could be excluded as a main driver for translocation, showing that peptides like CaaX-1 are most likely taken up by other mechanisms.

Triazole-Bridged Peptides with Enhanced Antimicrobial Activity and Potency against Pathogenic Bacteria

There are still no linear antimicrobial peptides (AMPs) available as a treatment option against bacterial infections. This is caused by several drawbacks that come with AMPs such as limited proteolytic stability and low selectivity against human cells. In this work, we screened a small library of rationally designed new peptides based on the cell-penetrating peptide sC18* toward their antimicrobial activity. We identified several effective novel AMPs and chose one out of this group to further increase its potency. Therefore, we introduced a triazole bridge at different positions to provide a preformed helical structure, assuming that this modification would improve (i) proteolytic stability and (ii) membrane activity. Indeed, placing the triazole bridge within the hydrophilic part of the linear analogue highly increased membrane activity as well as stability against enzymatic digestion. The new peptides, 8A and 8B, demonstrated high activity against several bacterial species tested including pathogenic N. gonorrhoeae and methicillin-resistant S. aureus. Since they exhibited significantly good tolerability against human fibroblast and blood cells, these novel peptides offer true alternatives for future clinical applications and are worth studying in more detail.

Hydrophobicity: The door to drug delivery

The engineering of intracellular delivery systems with the goal of achieving personalized medicine has been encouraged by advances in nanomaterial science as well as a greater understanding of diseases and of the biochemical pathways implicated in many disorders. The development of vectors able to transport the drug to a target location and release it only on demand is undoubtedly the primary issue. From a molecular perspective, the topography of drug carrier surfaces is directly related to the design of an effective drug carrier because it provides a physical hint to modifying its interactions with biological systems. For instance, the initial ratio of hydrophilic to hydrophobic surfaces and the changes brought about by external factors enable the release or encapsulation of a therapeutic molecule and the ability of the nanosystem to cross biological barriers and reach its target without causing systemic toxicity. The first step in creating new materials with enhanced functionality is to comprehend and characterize the interplay between hydrophilic and hydrophobic molecules at the molecular level. Therefore, the focus of this review is on the function of hydrophobicity, which is essential for matching the complexity of biological environments with the intended functionality.

Potential of oligonucleotide- and protein/peptide-based therapeutics in the management of toxicant/stressor-induced diseases

Exposure to toxicants/stressors has been linked to the development of many human diseases. They could affect various cellular components, such as DNA, proteins, lipids, and non-coding RNAs (ncRNA), thereby triggering various cellular pathways, particularly oxidative stress, inflammatory responses, and apoptosis, which can contribute to pathophysiological states. Accordingly, modulation of these pathways has been the focus of numerous investigations for managing related diseases. The involvement of various ncRNAs, such as small interfering RNA (siRNA), microRNAs (miRNA), and long non-coding RNAs (lncRNA), as well as various proteins and peptides in mediating these pathways, provides many target sites for pharmaceutical intervention. In this regard, various oligonucleotide- and protein/peptide-based therapies have been developed to treat toxicity-induced diseases, which have shown promising results in vitro and in vivo. This comprehensive review provides information about various aspects of toxicity-related diseases including their causing factors, main underlying mechanisms and intermediates, and their roles in pathophysiological states. Particularly, it highlights the principles and mechanisms of oligonucleotide- and protein/peptide-based therapies in the treatment of toxicity-related diseases. Furthermore, various issues of oligonucleotides and proteins/peptides for clinical usage and potential solutions are discussed.

Cell-permeable peptide-based delivery vehicles useful for subcellular targeting and beyond

Personal medicine aims to provide tailor-made diagnostics and treatments and has been emerged as a promising but challenging strategy during the last years. This includes the active delivery and localization of a therapeutic compound to a targeted site of action within a cell. An example being targeting the interference of a distinct protein-protein interaction (PPI) within the cell nucleus, mitochondria or other subcellular location. Therefore, not only the cell membrane has to be overcome but also the final intracellular destination has to be reached. One approach which fulfills both requirements is to use short peptide sequences that are able to translocate into cells as targeting and delivery vehicles. In fact, recent progress in this field demonstrates how these tools can modulate the pharmacological parameters of a drug without compromising its biological activity. Beside classical targets that are addressed by various small molecule drugs such as receptors, enzymes, or ion channels, PPIs have received increasing attention as potential therapeutic targets. Within this review, we will provide a recent update on cell-permeable peptides targeting subcellular destinations. We include chimeric peptide probes that combine cell-penetrating peptides (CPPs) and a targeting sequence, as well peptides having intrinsic cell-permeability and which are often used to target PPIs.

Cyclic Citrullinated Peptide Aptamer Treatment Attenuates Collagen-Induced Arthritis

We describe the study of a novel aptamer-based candidate for treatment of seropositive rheumatoid arthritis. The candidate is a nanoparticle-formulated cyclic citrullinated peptide aptamer, which targets autoantibodies and/or the immune reactions leading to antibody production. Due to its specificity, the peptide aptamer nanoparticles might not interfere with normal immune functions as seen with other disease-modifying antirheumatic drugs. Over a 3-week course of treatment, joint swelling and arthritis score in collagen-induced rats were significantly decreased compared with animals treated with phosphate-buffered saline, unloaded nanoparticles, or nanoparticles with a noncitrullinated control peptide. The reduction in joint swelling was associated with decreased anticitrullinated peptide autoantibody levels in the blood. Treatment with aptamer nanoparticles also increased interleukin-10 levels. The effect seen with the proposed treatment candidate could be mediated by upregulation of anti-inflammatory mediators and decreased levels of anticitrullinated peptide antibodies.

Discovery of a Cyclic Cell-Penetrating Peptide with Improved Endosomal Escape and Cytosolic Delivery Efficiency

Cyclic cell-penetrating peptide 12 (CPP12) is highly efficient for the cytosolic delivery of a variety of cargo molecules into mammalian cells in vitro and in vivo. However, its cytosolic entry efficiency is substantially reduced at lower concentrations or in the presence of serum proteins. In this study, CPP12 analogs were prepared by replacing its hydrophobic residues with amino acids of varying hydrophobicity and evaluated for cellular entry. Substitution of l-3-benzothienylalanine (Bta) for l-2-naphthylalanine (Nal) resulted in CPP12-2, which exhibits up to 3.8-fold higher cytosolic entry efficiency than CPP12, especially at low CPP concentrations; thanks to improved endosomal escape efficiency. CPP12-2 is well suited for the cytosolic delivery of highly potent cargos to achieve biological activity at low concentrations.

Genetic and Covalent Protein Modification Strategies to Facilitate Intracellular Delivery

Protein-based therapeutics represent a rapidly growing segment of approved disease treatments. Successful intracellular delivery of proteins is an important precondition for expanded in vivo and in vitro applications of protein therapeutics. Direct modification of proteins and peptides for improved cytosolic translocation are a promising method of increasing delivery efficiency and expanding the viability of intracellular protein therapeutics. In this Review, we present recent advances in both synthetic and genetic protein modifications for intracellular delivery. Active endocytosis-based and passive internalization pathways are discussed, followed by a review of modification methods for improved cytosolic delivery. After establishing how proteins can be modified, general strategies for facilitating intracellular delivery, such as chemical supercharging or inclusion of cell-penetrating motifs, are covered. We then outline protein modifications that promote endosomal escape. We finally examine the delivery of two potential classes of therapeutic proteins, antibodies and associated antibody fragments, and gene editing proteins, such as cas9.

Different Approaches to Cyclize a Cell-Penetrating Peptide and to Tether Bioactive Payloads

Cell-penetrating peptides (CPPs) are versatile tools to deliver various molecules into different cell types. The majority of CPPs are usually represented by linear structures, but numerous recent studies demonstrated cyclization to be an effective strategy leading to favorable biological activities. Here we describe two different methods for the side chain and backbone cyclization of CPPs . Furthermore, we highlight straightforward procedures for the covalent coupling of fluorophores or cytotoxic payloads.

TAT for Enzyme/Protein Delivery to Restore or Destroy Cell Activity in Human Diseases

Much effort has been dedicated in the recent decades to find novel protein/enzyme-based therapies for human diseases, the major challenge of such therapies being the intracellular delivery and reaching sub-cellular organelles. One promising approach is the use of cell-penetrating peptides (CPPs) for delivering enzymes/proteins into cells. In this review, we describe the potential therapeutic usages of CPPs (mainly trans-activator of transcription protein, TAT) in enabling the uptake of biologically active proteins/enzymes needed in cases of protein/enzyme deficiency, concentrating on mitochondrial diseases and on the import of enzymes or peptides in order to destroy pathogenic cells, focusing on cancer cells.

Macrocyclization strategies for cyclic peptides and peptidomimetics

Peptides are a growing therapeutic class due to their unique spatial characteristics that can target traditionally "undruggable" protein-protein interactions and surfaces. Despite their advantages, peptides must overcome several key shortcomings to be considered as drug leads, including their high conformational flexibility and susceptibility to proteolytic cleavage. As a general approach for overcoming these challenges, macrocyclization of a linear peptide can usually improve these characteristics. Their synthetic accessibility makes peptide macrocycles very attractive, though traditional synthetic methods for macrocyclization can be challenging for peptides, especially for head-to-tail cyclization. This review provides an updated summary of the available macrocyclization chemistries, such as traditional lactam formation, azide-alkyne cycloadditions, ring-closing metathesis as well as unconventional cyclization reactions, and it is structured according to the obtained functional groups. Keeping peptide chemistry and screening in mind, the focus is given to reactions applicable in solution, on solid supports, and compatible with contemporary screening methods.

Recent Advances and Trends in Chemical CPP-Drug Conjugation Techniques

This review summarizes recent developments in conjugation techniques for the synthesis of cell-penetrating peptide (CPP)–drug conjugates targeting cancer cells. We will focus on small organic molecules as well as metal complexes that were used as cytostatic payloads. Moreover, two principle ways of coupling chemistry will be discussed direct conjugation as well as the use of bifunctional linkers. While direct conjugation of the drug to the CPP is still popular, the use of bifunctional linkers seems to gain increasing attention as it offers more advantages related to the linker chemistry. Thus, three main categories of linkers will be highlighted, forming either disulfide acid-sensitive or stimuli-sensitive bonds. All techniques will be thoroughly discussed by their pros and cons with the aim to help the reader in the choice of the optimal conjugation technique that might be used for the synthesis of a given CPP–drug conjugate

Challenge to overcome current limitations of cell-penetrating peptides

The penetration of biological membranes is a prime obstacle for the delivery of pharmaceutical drugs. Cell-penetrating peptide (CPP) is an efficient vehicle that can deliver various cargos across the biological membranes. Since the discovery, CPPs have been rigorously studied to unveil the underlying penetrating mechanism as well as to exploit CPPs for various biomedical applications. This review will focus on the various strategies to overcome current limitations regarding stability, selectivity, and efficacy of CPPs.

Cell-penetrating peptides containing 2,5-diketopiperazine (DKP) scaffolds as shuttles for anti-cancer drugs: conformational studies and biological activity

A series of linear and cyclic peptidomimetics composed of a cell-penetrating peptide and a non-natural, bifunctional 2,5-diketopiperazine scaffold is reported. Conformational studies revealed well-defined helical structures in micellar medium for linear structures, while cyclic peptidomimetics were more flexible. Biological investigations showed higher membrane-activity of cyclic derivatives allowing their use as shuttles for anti-cancer drugs.

Cell penetrating peptides: A versatile vector for co-delivery of drug and genes in cancer

Biological barriers hamper the efficient delivery of drugs and genes to targeted sites. Cell penetrating peptides (CPP) have the ability to rapidly internalize across biological membranes. CPP have been effective for delivery of various chemotherapeutic agents used to combat cancer. CPP can enhance delivery of drugs to a targeted site when combined with tumor targeting peptides. CPP can be linked with various cargos like nanoparticles, micelles and liposomes to deliver drugs and genes to the cancer cell. Here, we focus on CPP mediated delivery of drugs to the tumor sites, delivery of genes (siRNA,pDNA) and co-delivery of drugs and genes to combat drug resistance.

Cell-permeable CaaX-peptides affect K-Ras downstream signaling and promote cell death in cancer cells

Cysteine prenylation is a post-translational modification that is used by nature to control crucial biological functions of proteins, such as membrane trafficking, signal transduction, and apoptosis. It mainly occurs in eukaryotic proteins at a C-terminal CaaX box and is mediated by prenyltransferases. Since the discovery of prenylated proteins, various tools have been developed to study the mechanisms of prenyltransferases, as well as to visualize and to identify prenylated proteins. Herein, we introduce cell-permeable peptides bearing a C-terminal CaaX motif based on Ras sequences. We demonstrate that intracellular accumulation of those peptides in different cells is controlled by the presence of their CaaX motif and that they specifically interact with intracellular prenyltransferases. As proof of concept, we further highlight their utilization to alter downstream signaling of Ras proteins, particularly of K-Ras-4B, in pancreatic cancer cells. Application of this strategy holds great promise to better understand and regulate post-translational cysteine prenylation.

Advances in cell penetrating peptides and their functionalization of polymeric nanoplatforms for drug delivery

Cell penetrating peptides (CPPs), known as protein translocation domains, have emerged as efficient molecular transporters to overcome biological barriers and deliver cell-impermeable cargoes into cells. The conjugation of CPPs to polymeric nanoplatforms enhances the drug delivery efficiency thus increasing their therapeutic efficacy. However, conventional CPPs are generally lack of cell specificity and could be easily degraded in vivo. These limitations lead to the development of new CPPs with superior properties. To address the issue of cell specificity, activatable CPPs have been designed to be activated at desired site through different stimuli. On the other hand, macrocyclization has been used to constrain linear CPPs into their cyclic forms. This chemical optimization of peptides endows CPPs with enhanced stability and cell permeability. This brief review will cover recent advances in terms of different types of CPPs for enhanced cell penetration. In addition, the modification chemistry used to functionalize polymeric nanoplatforms with CPPs and their recent applications for drug delivery will also be discussed. This article is categorized under: Nanotechnology Approaches to Biology > Cells at the Nanoscale Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease.

Cyclic Peptide-Gadolinium Nanoparticles for Enhanced Intracellular Delivery

A cyclic peptide containing one cysteine and five alternating tryptophan and arginine amino acids [(WR)₅C] was synthesized using Fmoc/tBu solid-phase methodology. The ability of the synthesized cyclic peptide to produce gadolinium nanoparticles through an in situ one-pot mixing of an aqueous solution of GdCl₃ with [(WR)₅C] peptide solution was evaluated. Transmission electron microscopy showed the formed peptide-Gd nanoparticles in star-shape morphology with a size of ~250 nm. Flow cytometry investigation showed that the cellular uptake of a cell-impermeable fluorescence-labeled phosphopeptide (F′-GpYEEI, where F′ = fluorescein) was approximately six times higher in the presence of [(WR)₅C]-Gd nanoparticles than those of F′-GpYEEI alone in human leukemia adenocarcinoma (CCRF-CEM) cells after 2 h incubation. The antiproliferative activities of cisplatin and carboplatin (5 µM) were increased in the presence of [(WR)₅C]-GdNPs (50 μM) by 41% and 18%, respectively, after 72-h incubation in CCRF-CEM cells. The intracellular release of epirubicin, an anticancer drug, from the complex showed that 15% and 60% of the drug was released intracellularly within 12 and 48 h, respectively. This report provides insight about using a non-toxic MRI agent, gadolinium nanoparticles, for the delivery of various types of molecular cargos.

Development of a Cell-Permeable Cyclic Peptidyl Inhibitor against the Keap1-Nrf2 Interaction

Macrocyclic peptides have proven to be highly effective inhibitors of protein-protein interactions but generally lack cell permeability to access intracellular targets. We show herein that macrocyclic peptides may be rendered highly cell-permeable and biologically active by conjugating them with a cyclic cell-penetrating peptide (CPP). A previously reported cyclic peptidyl inhibitor against the Kelch-like ECH-associated protein 1 (Keap1)-nuclear factor erythroid-2 (Nrf2) interaction (K_D = 18 nM) was covalently attached to a cyclic CPP through a flexible linker. The resulting bicyclic peptide retained the Keap1-binding activity, resisted proteolytic degradation, readily entered mammalian cells, and activated the transcriptional activity of Nrf2 at nanomolar to low micromolar concentrations in cell culture. The inhibitor provides a useful tool for investigating the biological function of Keap1-Nrf2 and a potential lead for further development into a novel class of anti-inflammatory and anticancer agents. Our data suggest that other membrane-impermeable cyclic peptides may be similarly rendered cell-permeable by conjugation with a cyclic CPP.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Understanding Cell Penetration of Cyclic Peptides

Approximately 75% of all disease-relevant human proteins, including those involved in intracellular protein-protein interactions (PPIs), are undruggable with the current drug modalities (i.e., small molecules and biologics). Macrocyclic peptides provide a potential solution to these undruggable targets because their larger sizes (relative to conventional small molecules) endow them the capability of binding to flat PPI interfaces with antibody-like affinity and specificity. Powerful combinatorial library technologies have been developed to routinely identify cyclic peptides as potent, specific inhibitors against proteins including PPI targets. However, with the exception of a very small set of sequences, the vast majority of cyclic peptides are impermeable to the cell membrane, preventing their application against intracellular targets. This Review examines common structural features that render most cyclic peptides membrane impermeable, as well as the unique features that allow the minority of sequences to enter the cell interior by passive diffusion, endocytosis/endosomal escape, or other mechanisms. We also present the current state of knowledge about the molecular mechanisms of cell penetration, the various strategies for designing cell-permeable, biologically active cyclic peptides against intracellular targets, and the assay methods available to quantify their cell-permeability.

Cyclic Cell-Penetrating Peptides as Efficient Intracellular Drug Delivery Tools

Cyclic cell-penetrating peptides are relatively a newer class of peptides that have a huge potential for the intracellular delivery of therapeutic agents aimed at treating challenging ailments like multidrug-resistant bacterial diseases, cancer, and HIV infection. Cell-penetrating peptides (CPPs) have been extensively explored as intracellular delivery vehicles; however, they have some inherent limitations like poor stability, endosomal entrapment, toxicity, and suboptimal cell penetration. Owing to their favorable properties that avoid these limitations, cyclic CPPs can provide a good alternative to linear CPPs. Several Reviews have been published in the past decade that cover CPPs and cyclic peptides independently. To the best of our knowledge, this is one of the first Reviews that covers cyclic CPPs comprehensively in the light of studies published so far. In this Review, we have detailed examples of cyclic CPPs, their structures, and cyclization strategies followed by a detailed account of their advantages over their linear counterparts. A hot area in cyclic CPPs is the exploration of cell-penetration mechanisms; this Review highlights this topic in detail. Finally, we will review the applications of cyclic CPPs, followed by conclusions and future prospects.

Cyclic Cell-Penetrating Peptides with Single Hydrophobic Groups

A new family of cyclic cell-penetrating peptides (CPPs) has been discovered; they differ from previously reported cyclic CPPs by containing only a single hydrophobic residue. The optimal CPP structure consists of four arginine residues and a hydrophobic residue with a long alkyl chain (e.g., a decyl group) in a cyclohexapeptide ring. The most active member of this family, CPP 17, has an intrinsic cellular entry efficiency similar to that of cyclic CPP12, the most active CPP reported to date. However, CPP 17 is 2.8 times more active than CPP12 under high serum protein concentrations, presumably because of the lower protein binding. CPP 17 enters the cell primarily by direct translocation at a relatively low concentration (≥5 μm).

Kiss and Run: Promoting Effective and Targeted Cellular Uptake of a Drug Delivery Vehicle Composed of an Integrin-Targeting Diketopiperazine Peptidomimetic and a Cell-Penetrating Peptide

Cell-penetrating peptides (CPPs) have emerged as powerful tools in terms of drug delivery. Those short, often cationic peptides are characterized by their usually low toxicity and their ability to transport diverse cargos inside almost any kinds of cells. Still, one major drawback is their nonselective uptake making their application in targeted cancer therapies questionable. In this work, we aimed to combine the power of a CPP (sC18) with an integrin-targeting unit (c[DKP-f3-RGD]). The latter is composed of the Arg-Gly-Asp peptide sequence cyclized via a diketopiperazine scaffold and is characterized by its high selectivity toward integrin α_vβ₃. The two parts were linked via copper-catalyzed alkyne-azide click reaction (CuAAC), while the CPP was additionally functionalized with either a fluorescent dye or the anticancer drug daunorubicin. Both functionalities allowed a careful biological evaluation of these novel peptide-conjugates regarding their cellular uptake mechanism, as well as cytotoxicity in α_vβ₃ integrin receptor expressing cells versus cells that do not express α_vβ₃. Our results show that the uptake follows a "kiss-and-run"-like model, in which the conjugates first target and recognize the receptor, but translocate mainly by CPP mediation. Thereby, we observed significantly more pronounced toxic effects in α_vβ₃ expressing U87 cells compared to HT-29 and MCF-7 cells, when the cells were exposed to the substances with only very short contact times (15 min). All in all, we present new concepts for the design of cancer selective peptide-drug conjugates.

Head to tail cyclisation of cell-penetrating peptides: impact on GAG-dependent internalisation and direct translocation

A series of cyclic lipidated oligo-Arg cell penetrating peptides were synthesised with varied macrocycle size and lipid chain anchoring site. The study of their cellular uptake revealed different structural requirements to promote efficient glycosaminoglycan-dependent endocytosis and direct translocation.

Targeted Subcellular Protein Delivery Using Cleavable Cyclic Cell-Penetrating Peptides

The delivery of entire functional proteins into living cells is a long-sought goal in science. Cyclic cell-penetrating peptides (cCPPs) have proven themselves to be potent delivery vehicles to carry proteins upon conjugation into the cytosol of living cells with immediate bioavailability via a non-endosomal uptake pathway. With this strategy, we pursue the cytosolic delivery of mCherry, a medium-sized fluorescent protein. Afterward, we achieve subcellular delivery of mCherry to different intracellular loci by genetic fusion of targeting peptides to the protein sequence. We show efficient transport into a membrane-bound compartment, the nucleus, as well as targeting of the actin cytoskeleton, marking one of the first ways to label actin fluorescently in genetically unmodified living cells. Furthermore, we demonstrate that only by conjugation of cCPPs via a disulfide bond, is flawless localization to the target area achieved. This finding underlines the importance of using a cCPP-based delivery vehicle that is cleaved inside cells, for the precise intracellular localization of a protein of interest.

Antimicrobial and Cell-Penetrating Peptides: How to Understand Two Distinct Functions Despite Similar Physicochemical Properties

Antimicrobial and cell-penetrating peptides are both classes of membrane-active peptides sharing similar physicochemical properties. Both kinds of peptides have attracted much attention owing to their specific features. AMPs disrupt cell membranes of bacteria and display urgently needed antibiotic substances with alternative modes of action. Since the multidrug resistance of bacterial pathogens is a more and more raising concern, AMPs have gained much interest during the past years. On the other side, CPPs enter eukaryotic cells without substantially affecting the plasma membrane. They can be used as drug delivery platforms and have proven their usefulness in various applications. However, although both groups of peptides are quite similar, their intrinsic activity is often different, and responsible factors are still in discussion. The aim of this chapter is to summarize and shed light on recent findings and concepts dealing with differences and similarities of AMPs and CPPs and to understand these different functions.

Design of a novel cell-permeable chimeric peptide to promote wound healing

Biological membranes are impermeable to almost all compounds having a molecular weight greater than 500 Da. Recently, cell penetrating peptides (CPPs) as delivery vehicles have attracted great interest in the medical sector for the development of novel therapeutic agents or cosmetic products. Herein, a wound healing promoting sequence, namely Tylotoin, was covalently coupled with a cell penetrating peptide to improve the delivery of Tylotoin across cellular membranes. Indeed, internalization studies indicated that the cellular uptake of these novel peptide conjugates into keratinocytes was significantly improved accompanied by good tolerability. In a scratch wound closure assay used to investigate the wound healing capability, the most promising novel peptide chimera (Tylotoin-sC18*) was found to promote the migration of keratinocytes indicating that the fusion to Tylotoin did not cause any loss in its activity. Even more, proliferative effects on keratinocytes were observed, an important step during the wound healing process. Still more encouraging is the capability of Tylotoin-sC18* to exhibit strong antimicrobial activities since the process of wound healing is often affected by bacterial infections. Owing to their multiple functions, the novel peptide chimera may have potential as future agents for the treatment of infected wounds.

A platform for discovery of functional cell-penetrating peptides for efficient multi-cargo intracellular delivery

Cell penetrating peptides (CPPs) offer great potential to deliver therapeutic molecules to previously inaccessible intracellular targets. However, many CPPs are inefficient and often leave their attached cargo stranded in the cell’s endosome. We report a versatile platform for the isolation of peptides delivering a wide range of cargos into the cytoplasm of cells. We used this screening platform to identify multiple “Phylomer” CPPs, derived from bacterial and viral genomes. These peptides are amenable to conventional sequence optimization and engineering approaches for cell targeting and half-life extension. We demonstrate potent, functional delivery of protein, peptide, and nucleic acid analog cargos into cells using Phylomer CPPs. We validate in vivo activity in the cytoplasm, through successful transport of an oligonucleotide therapeutic fused to a Phylomer CPP in a disease model for Duchenne’s muscular dystrophy. This report thus establishes a discovery platform for identifying novel, functional CPPs to expand the delivery landscape of druggable intracellular targets for biological therapeutics.

Cell penetrating peptides in ocular drug delivery: State of the art

Despite the increasing number of effective therapeutics for eye diseases, their treatment is still challenging due to the presence of effective barriers protecting eye tissues. Cell Penetrating Peptides (CPPs), synthetic and natural short amino acid sequences able to cross cellular membrane thanks to a transduction domain, have been proposed as possible enhancing strategies for ophthalmic delivery. In this review, a general description of CPPs classes, design approaches and proposed cellular uptake mechanisms will be provided to the reader as an introduction to ocular CPPs application, together with an overview of the main problems related to ocular administration. The results obtained with CPPs for the treatment of anterior and posterior segment eye diseases will be then introduced, with a focus on non-invasive or minimally invasive administration, shifting from CPPs capability to obtain intracellular delivery to their ability to cross biological barriers. The problems related to in vitro, ex vivo and in vivo models used to investigate CPPs mediated ocular delivery will be also addressed together with potential ocular toxicity issues.

Design and biological characterization of novel cell-penetrating peptides preferentially targeting cell nuclei and subnuclear regions

Within this study, we report about the design and biological characterization of novel cell-penetrating peptides (CPPs) with selective suborganelle-targeting properties. The nuclear localization sequence N50, as well as the nucleoli-targeting sequence NrTP, respectively, were fused to a shortened version of the cell-penetrating peptide sC18. We examined cellular uptake, subcellular fate and cytotoxicity of these novel peptides, N50-sC18* and NrTP-sC18*, and found that they are nontoxic up to a concentration of 50 or 100 µM depending on the cell lines used. Moreover, detailed cellular uptake studies revealed that both peptides enter cells via energy-independent uptake, although endocytotic processes cannot completely excluded. However, initial drug delivery studies demonstrated the high versatility of these new peptides as efficient transport vectors targeting specifically nuclei and nucleoli. In future, they could be further explored as parts of newly created peptide-drug conjugates.

Tumor-targeting delivery of herb-based drugs with cell-penetrating/tumor-targeting peptide-modified nanocarriers

Cancer has become one of the leading causes of mortality globally. The major challenges of conventional cancer therapy are the failure of most chemotherapeutic agents to accumulate selectively in tumor cells and their severe systemic side effects. In the past three decades, a number of drug delivery approaches have been discovered to overwhelm the obstacles. Among these, nanocarriers have gained much attention for their excellent and efficient drug delivery systems to improve specific tissue/organ/cell targeting. In order to enhance targeting efficiency further and reduce limitations of nanocarriers, nanoparticle surfaces are functionalized with different ligands. Several kinds of ligand-modified nanomedicines have been reported. Cell-penetrating peptides (CPPs) are promising ligands, attracting the attention of researchers due to their efficiency to transport bioactive molecules intracellularly. However, their lack of specificity and in vivo degradation led to the development of newer types of CPP. Currently, activable CPP and tumor-targeting peptide (TTP)-modified nanocarriers have shown dramatically superior cellular specific uptake, cytotoxicity, and tumor growth inhibition. In this review, we discuss recent advances in tumor-targeting strategies using CPPs and their limitations in tumor delivery systems. Special emphasis is given to activable CPPs and TTPs. Finally, we address the application of CPPs and/or TTPs in the delivery of plant-derived chemotherapeutic agents.

Constrained cell penetrating peptides

In this review we provide an overview of recent developments in the field of cell penetrating peptides (CPPs) on research that aims to achieve better control over their transduction properties - one of the big challenges - by means of restraining them. Three different constraining strategies are presented: triggerable activation, backbone rigidification and macrocyclization. Each of these methods have their opportunities in gaining control over CPP activity and selectivity.

Cell-Penetrating Peptides: Design Strategies beyond Primary Structure and Amphipathicity

Efficient intracellular drug delivery and target specificity are often hampered by the presence of biological barriers. Thus, compounds that efficiently cross cell membranes are the key to improving the therapeutic value and on-target specificity of non-permeable drugs. The discovery of cell-penetrating peptides (CPPs) and the early design approaches through mimicking the natural penetration domains used by viruses have led to greater efficiency of intracellular delivery. Following these nature-inspired examples, a number of rationally designed CPPs has been developed. In this review, a variety of CPP designs will be described, including linear and flexible, positively charged and often amphipathic CPPs, and more rigid versions comprising cyclic, stapled, or dimeric and/or multivalent, self-assembled peptides or peptido-mimetics. The application of distinct design strategies to known physico-chemical properties of CPPs offers the opportunity to improve their penetration efficiency and/or internalization kinetics. This led to increased design complexity of new CPPs that does not always result in greater CPP activity. Therefore, the transition of CPPs to a clinical setting remains a challenge also due to the concomitant involvement of various internalization routes and heterogeneity of cells used in the in vitro studies.

Peptide-Membrane Interaction between Targeting and Lysis

Certain cationic peptides interact with biological membranes. These often-complex interactions can result in peptide targeting to the membrane, or in membrane permeation, rupture, and cell lysis. We investigated the relationship between the structural features of membrane-active peptides and these effects, to better understand these processes. To this end, we employed a computational method for morphing a membranolytic antimicrobial peptide into a nonmembranolytic mitochondrial targeting peptide by "directed simulated evolution." The results obtained demonstrate that superficially subtle sequence modifications can strongly affect the peptides' membranolytic and membrane-targeting abilities. Spectroscopic and computational analyses suggest that N- and C-terminal structural flexibility plays a crucial role in determining the mode of peptide-membrane interaction.

The Current Role of Cell-Penetrating Peptides in Cancer Therapy

Cell-penetrating peptides (CPPs) are a heterogeneous class of peptides with the ability to translocate across the plasma membrane and to carry attached cargos inside the cell. Two main entry pathways are discussed, as direct translocation and endocytosis , whereas the latter is often favored when bulky cargos are added to the CPP. Attachment to the CPP can be achieved by means of covalent coupling or non-covalent complex formation, depending on the chemical nature of the cargo. Owing to their striking abilities the further development and application of CPP-based delivery strategies has steadily emerged during the past years. However, one main pitfall when using CPPs is their non-selective uptake in nearly all types of cells. Thus, one particular interest lies in the design of targeting strategies that help to circumvent this drawback but still benefit from the potent delivery abilities of CPPs. The following review aims to summarize some of these very recent concepts and to highlight the current role of CPPs in cancer therapy.

Cyclization of a cell-penetrating peptide via click-chemistry increases proteolytic resistance and improves drug delivery

In this work we report synthesis and biological evaluation of a cell-penetrating peptide (CPP), that is partly cyclized via a triazole bridge. Recently, beneficious properties have been reported for cyclized peptides concerning their metabolic stability and intracellular uptake. A CPP based on human calcitonin was used in this study, and side chain cyclization was achieved via copper catalyzed alkyne-azide click reaction. Cell viability studies in several cell-lines revealed no cytotoxic effects. Furthermore, efficient uptake in breast cancer MCF-7 cells could be determined. Moreover, preliminary studies using this novel peptide as drug transporter for daunorubicin were performed. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Intracellular Delivery of Proteins with Cell-Penetrating Peptides for Therapeutic Uses in Human Disease

Protein therapy exhibits several advantages over small molecule drugs and is increasingly being developed for the treatment of disorders ranging from single enzyme deficiencies to cancer. Cell-penetrating peptides (CPPs), a group of small peptides capable of promoting transport of molecular cargo across the plasma membrane, have become important tools in promoting the cellular uptake of exogenously delivered proteins. Although the molecular mechanisms of uptake are not firmly established, CPPs have been empirically shown to promote uptake of various molecules, including large proteins over 100 kiloDaltons (kDa). Recombinant proteins that include a CPP tag to promote intracellular delivery show promise as therapeutic agents with encouraging success rates in both animal and human trials. This review highlights recent advances in protein-CPP therapy and discusses optimization strategies and potential detrimental effects.

Bioactive clusters promoting cell penetration and nucleic acid complexation for drug and gene delivery applications: from designed to self-assembled and responsive systems

Recent developments in the (self-)assembly of cationic clusters promoting nucleic acids complexation and cell penetration open the door to applications in drug and gene delivery.