Activatable cell-penetrating peptides: 15 years of research

Conditional Cell-Penetrating Peptide Exposure as Selective Nanoparticle Uptake Signal

A major bottleneck diminishing the therapeutic efficacy of various drugs is that only small proportions of the administered dose reach the site of action. One promising approach to increase the drug amount in the target tissue is the delivery via nanoparticles (NPs) modified with ligands of cell surface receptors for the selective identification of target cells. However, since receptor binding can unintentionally trigger intracellular signaling cascades, our objective was to develop a receptor-independent way of NP uptake. Cell-penetrating peptides (CPPs) are an attractive tool since they allow efficient cell membrane crossing. So far, their applicability is severely limited as their uptake-promoting ability is nonspecific. Therefore, we aimed to achieve a conditional CPP-mediated NP internalization exclusively into target cells. We synthesized different CPP candidates and investigated their influence on nanoparticle stability, ζ-potential, and uptake characteristics in a core-shell nanoparticle system consisting of poly(lactid-co-glycolid) (PLGA) and poly(lactic acid)-poly(ethylene glycol) (PLA10kPEG2k) block copolymers with CPPs attached to the PEG part. We identified TAT47-57 (TAT) as the most promising candidate and subsequently combined the TAT-modified PLA10kPEG2k polymer with longer PLA10kPEG5k polymer chains, modified with the potent angiotensin-converting enzyme 2 (ACE2) inhibitor MLN-4760. While MLN-4760 enables selective target cell identification, the additional PEG length hides the CPP during a first unspecific cell contact. Only after the previous selective binding of MLN-4760 to ACE2, the established spatial proximity exposes the CPP, triggering cell uptake. We found an 18-fold uptake improvement in ACE2-positive cells compared to unmodified particles. In summary, our work paves the way for a conditional and thus highly selective receptor-independent nanoparticle uptake, which is beneficial in terms of avoiding side effects.

Covalent conjugation and non-covalent complexation strategies for intracellular delivery of proteins using cell-penetrating peptides

Therapeutic proteins provided new opportunities for patients and high sales volumes. However, they are formulated for extracellular targets. The lipophilic barrier of the plasma membrane renders the vast array of intracellular targets out of reach. Peptide-based delivery systems, namely cell-penetrating peptides (CPPs), have few safety concerns, and low immunogenicity, with control over administered doses. This study investigates CPP-based protein delivery systems by classifying them into CPP-protein "covalent conjugation" and CPP: protein "non-covalent complexation" categories. Covalent conjugates ensure the proximity of the CPP to the cargo, which can improve cellular uptake and endosomal escape. We will discuss various aspects of covalent conjugates through non-cleavable (stable) or cleavable bonds. Non-cleavable CPP-protein conjugates are produced by recombinant DNA technology to express the complete fusion protein in a host cell or by chemical ligation of CPP and protein, which ensures stability during the delivery process. CPP-protein cleavable bonds are classified into pH-sensitive and redox-sensitive bonds, enzyme-cleavable bonds, and physical stimuli cleavable linkers (light radiation, ultrasonic waves, and thermo-responsive). We have highlighted the key characteristics of non-covalent complexes through electrostatic and hydrophobic interactions to preserve the conformational integrity of the CPP and cargo. CPP-mediated protein delivery by non-covalent complexation, such as zippers, CPP adaptor methods, and avidin-biotin technology, are featured. Conclusively, non-covalent complexation methods are appropriate when a high number of CPP or protein samples are to be screened. In contrast, when the high biological activity of the protein is critical in the intracellular compartment, conjugation protocols are preferred.

Conditional Cell Penetration of Masked CPPs by an ADEPT-like Approach

The intracellular delivery of cargos via cell penetrating peptides (CPPs) holds significant promise as a drug delivery vehicle, but a major issue is their lack of cell type specificity, which can lead to detrimental off-target effects. We use an ADEPT-like concept to introduce conditional and selective activation of cellular uptake by using the lysine-rich, cationic, and amphiphilic L17E peptide as a model CPP. By masking the lysine residues of the L17E peptide with enzyme-cleavable acetyl protecting groups, the delivery of the covalently conjugated fluorophore TAMRA to HeLa cells was diminished. Recovery of cellular uptake could be achieved by deacetylation of the masked acetylated L17E peptide using the NAD-dependent sirtuin 2 (SirT2) deacetylase in vitro. Finally, trastuzumab-SirT2 and anti-B7H3-SirT2 antibody-enzyme conjugates were generated for the conditional and selective delivery of a cryptophycin cytotoxin by the L17E peptide. While the masked peptide still demonstrated some cytotoxicity, selective cell killing mediated by the antibody-enzyme conjugates was observed.

Enhanced Ocular Delivery of Beva via Ultra-Small Polymeric Micelles for Noninvasive Anti-VEGF Therapy

Pathological ocular neovascularization resulting from retinal ischemia constitutes a major cause of vision loss. Current anti-VEGF therapies rely on burdensome intravitreal injections of Bevacizumab (Beva). Herein ultrasmall polymeric micelles encapsulating Beva (P@Beva) are developed for noninvasive topical delivery to posterior eye tissues. Beva is efficiently loaded into 11 nm micelles fabricated via self-assembly of hyperbranched amphiphilic copolymers. The neutral, brush-like micelles demonstrate excellent drug encapsulation and colloidal stability. In vitro, P@Beva enhances intracellular delivery of Beva in ocular cells versus free drug. Ex vivo corneal and conjunctival-sclera-choroidal tissues transport after eye drops are improved 23-fold and 7.9-fold, respectively. Anti-angiogenic bioactivity is retained with P@Beva eliciting greater inhibition of endothelial tube formation and choroid sprouting over Beva alone. Remarkably, in an oxygen-induced retinopathy (OIR) model, topical P@Beva matching efficacy of intravitreal Beva injection, is the clinical standard. Comprehensive biocompatibility verifies safety. Overall, this pioneering protein delivery platform holds promise to shift paradigms from invasive intravitreal injections toward simplified, noninvasive administration of biotherapeutics targeting posterior eye diseases.

Antimicrobial peptide A9K as a gene delivery vector in cancer cells

Designed peptides are promising biomaterials for biomedical applications. The amphiphilic cationic antimicrobial peptide (AMP), A9K, can self-assemble into nano-rod structures and has shown cancer cell selectivity and could therefore be a promising candidate for therapeutic delivery into cancer cells. In this paper, we investigate the selectivity of A9K for cancer cell models, examining its effect on two human cancer cell lines, A431 and HCT-116. Little or no activity was observed on the control, human dermal fibroblasts (HDFs). In the cancer cell lines the peptide inhibited cellular growth through changes in mitochondrial morphology and membrane potential while remaining harmless towards HDFs. In addition, the peptide can bind to and protect nucleic acids while transporting them into both 2D cultures and 3D spheroids of cancer cells. A9K showed high efficiency in delivering siRNA molecules into the centre of the spheroids. A9K was also explored in vivo, using a zebrafish (Danio rerio) development toxicity assay, showing that the peptide is safe at low doses. Finally, a high-content imaging screen, using RNA interference (RNAi) targeted towards cellular uptake, in HCT-116 cells was carried out. Our findings suggest that active cellular uptake is involved in peptide internalisation, mediated through clathrin-mediated endocytosis. These new discoveries make A9K attractive for future developments in clinical and biotechnological applications.

Advances in peptide-based drug delivery systems

Drug delivery systems (DDSs) are designed to deliver drugs to their specific targets to minimize their toxic effects and improve their susceptibility to clearance during targeted transport. Peptides have high affinity, low immunogenicity, simple amino acid composition, and adjustable molecular size; therefore, most peptides can be coupled to drugs via linkers to form peptide-drug conjugates (PDCs) and act as active pro-drugs. PDCs are widely thought to be promising DDSs, given their ability to improve drug bio-compatibility and physiological stability. Peptide-based DDSs are often used to deliver therapeutic substances such as anti-cancer drugs and nucleic acid-based drugs, which not only slow the degradation rate of drugs in vivo but also ensure the drug concentration at the targeted site and prolong the half-life of drugs in vivo. This article provides an profile of the advancements and future development in functional peptide-based DDSs both domestically and internationally in recent years, in the expectation of achieving targeted drug delivery incorporating functional peptides and taking full advantage of synergistic effects.

Development of polypeptide-based materials toward messenger RNA delivery

Messenger RNA (mRNA)-based therapeutic agents have demonstrated significant potential in recent times, particularly in the context of the COVID-19 pandemic outbreak. As a promising prophylactic and therapeutic strategy, polypeptide-based mRNA delivery systems attract significant interest because of their low cost, simple preparation, tuneable sizes and morphology, convenient large-scale production, biocompatibility, and biodegradability. In this review, we begin with a brief discussion of the synthesis of polypeptides, followed by a review of commonly used polypeptides in mRNA delivery, including classical polypeptides and cell-penetrating peptides. Then, the challenges against mRNA delivery, including extracellular, intracellular, and clinical barriers, are discussed in detail. Finally, we highlight a range of strategies for polypeptide-based mRNA delivery, offering valuable insights into the advancement of polypeptide-based mRNA carrier development.

Design of pH-responsive antimicrobial peptide melittin analog-camptothecin conjugates for tumor therapy

Melittin, a classical antimicrobial peptide, is a highly potent antitumor agent. However, its significant toxicity seriously hampers its application in tumor therapy. In this study, we developed novel melittin analogs with pH-responsive, cell-penetrating and membrane-lytic activities by replacing arginine and lysine with histidine. After conjugation with camptothecin (CPT), CPT-AAM-1 and CPT-AAM-2 were capable of killing tumor cells by releasing CPT at low concentrations and disrupting cell membranes at high concentrations under acidic conditions. Notably, we found that the C-terminus of the melittin analogs was more suitable for drug conjugation than the N-terminus. CPT-AAM-1 significantly suppressed melanoma growth in vivo with relatively low toxicity. Collectively, the present study demonstrates that the development of antitumor drugs based on pH-responsive antimicrobial peptide-drug conjugates is a promising strategy.

Intracellular Protein Delivery: Approaches, Challenges, and Clinical Applications

Protein biologics are powerful therapeutic agents with diverse inhibitory and enzymatic functions. However, their clinical use has been limited to extracellular applications due to their inability to cross plasma membranes. Overcoming this physiological barrier would unlock the potential of protein drugs for the treatment of many intractable diseases. In this review, we highlight progress made toward achieving cytosolic delivery of recombinant proteins. We start by first considering intracellular protein delivery as a drug modality compared to existing Food and Drug Administration-approved drug modalities. Then, we summarize strategies that have been reported to achieve protein internalization. These techniques can be broadly classified into 3 categories: physical methods, direct protein engineering, and nanocarrier-mediated delivery. Finally, we highlight existing challenges for cytosolic protein delivery and offer an outlook for future advances.

Discovery of reactive peptide inhibitors of human papillomavirus oncoprotein E6

Human papillomavirus (HPV) infections account for nearly all cervical cancer cases, which is the fourth most common cancer in women worldwide. High-risk variants, including HPV16, drive tumorigenesis in part by promoting the degradation of the tumor suppressor p53. This degradation is mediated by the HPV early protein 6 (E6), which recruits the E3 ubiquitin ligase E6AP and redirects its activity towards ubiquitinating p53. Targeting the protein interaction interface between HPV E6 and E6AP is a promising modality to mitigate HPV-mediated degradation of p53. In this study, we designed a covalent peptide inhibitor, termed reactide, that mimics the E6AP LXXLL binding motif by selectively targeting cysteine 58 in HPV16 E6 with quantitative conversion. This reactide provides a starting point in the development of covalent peptidomimetic inhibitors for intervention against HPV-driven cancers.

Programmable Peptides Activated Macropinocytosis for Direct Cytosolic Delivery

Bioactive macromolecules show great promise for the treatment of various diseases. However, the cytosolic delivery of peptide-based drugs remains a challenging task owing to the existence of multiple intracellular barriers and ineffective endosomal escape. To address these issues, herein, programmable self-assembling peptide vectors are reported to amplify cargo internalization into the cytoplasm through receptor-activated macropinocytosis. Programmable self-assembling peptide vector-active human epidermal growth factor receptor-2 (HER2) signaling induces the receptor-activated macropinocytosis pathway, achieving efficient uptake in tumor cells. Shrinking macropinosomes accelerate the process of assembly dynamics and form nanostructures in the cytoplasm to increase peptide-based cargo accumulation and retention. Inductively coupled plasma mass (ICP-MS) spectrometry quantitative analysis indicates that the Gd delivery efficiency in tumor tissue through the macropinocytosis pathway is improved 2.5-fold compared with that through the use of active targeting molecular delivery. Finally, compared with nanoparticles and active targeting delivery, the delivery of bioactive peptide drugs through the self-assembly of peptide vectors maintains high drug activity (the IC50 decreased twofold) in the cytoplasm and achieves effective inhibition of tumor cell growth. Programmable self-assembling peptide vectors represent a promising platform for the intracellular delivery of diverse bioactive drugs, including molecular drugs, peptides, and biologics.

Cell Penetrating Peptides: Classification, Mechanisms, Methods of Study, and Applications

Cell-penetrating peptides (CPPs) encompass a class of peptides that possess the remarkable ability to cross cell membranes and deliver various types of cargoes, including drugs, nucleic acids, and proteins, into cells. For this reason, CPPs are largely investigated in drug delivery applications in the context of many diseases, such as cancer, diabetes, and genetic disorders. While sharing this functionality and some common structural features, such as a high content of positively charged amino acids, CPPs represent an extremely diverse group of elements, which can differentiate under many aspects. In this review, we summarize the most common characteristics of CPPs, introduce their main distinctive features, mechanistic aspects that drive their function, and outline the most widely used techniques for their structural and functional studies. We highlight current gaps and future perspectives in this field, which have the potential to significantly impact the future field of drug delivery and therapeutics.

Cell-Penetrating Peptide-Based Delivery of Macromolecular Drugs: Development, Strategies, and Progress

Cell-penetrating peptides (CPPs), developed for more than 30 years, are still being extensively studied due to their excellent delivery performance. Compared with other delivery vehicles, CPPs hold promise for delivering different types of drugs. Here, we review the development process of CPPs and summarize the composition and classification of the CPP-based delivery systems, cellular uptake mechanisms, influencing factors, and biological barriers. We also summarize the optimization routes of CPP-based macromolecular drug delivery from stability and targeting perspectives. Strategies for enhanced endosomal escape, which prolong its half-life in blood, improved targeting efficiency and stimuli-responsive design are comprehensively summarized for CPP-based macromolecule delivery. Finally, after concluding the clinical trials of CPP-based drug delivery systems, we extracted the necessary conditions for a successful CPP-based delivery system. This review provides the latest framework for the CPP-based delivery of macromolecular drugs and summarizes the optimized strategies to improve delivery efficiency.

Cell-Penetrating Peptides as Valuable Tools for Nose-to-Brain Delivery of Biological Drugs

Due to their high specificity toward the target and their low toxicity, biological drugs have been successfully employed in a wide range of therapeutic areas. It is yet to be mentioned that biologics exhibit unfavorable pharmacokinetic properties, are susceptible to degradation by endogenous enzymes, and cannot penetrate biological barriers such as the blood-brain barrier (i.e., the major impediment to reaching the central nervous system (CNS)). Attempts to overcome these issues have been made by exploiting the intracerebroventricular and intrathecal routes of administration. The invasiveness and impracticality of these procedures has, however, prompted the development of novel drug delivery strategies including the intranasal route of administration. This represents a non-invasive way to achieve the CNS, reducing systemic exposure. Nonetheless, biotherapeutics strive to penetrate the nasal epithelium, raising the possibility that direct delivery to the nervous system may not be straightforward. To maximize the advantages of the intranasal route, new approaches have been proposed including the use of cell-penetrating peptides (CPPs) and CPP-functionalized nanosystems. This review aims at describing the most impactful attempts in using CPPs as carriers for the nose-to-brain delivery of biologics by analyzing their positive and negative aspects.

The Dawn of a New Era: Targeting the "Undruggables" with Antibody-Based Therapeutics

The high selectivity and affinity of antibodies toward their antigens have made them a highly valuable tool in disease therapy, diagnosis, and basic research. A plethora of chemical and genetic approaches have been devised to make antibodies accessible to more "undruggable" targets and equipped with new functions of illustrating or regulating biological processes more precisely. In this Review, in addition to introducing how naked antibodies and various antibody conjugates (such as antibody-drug conjugates, antibody-oligonucleotide conjugates, antibody-enzyme conjugates, etc.) work in therapeutic applications, special attention has been paid to how chemistry tools have helped to optimize the therapeutic outcome (i.e., with enhanced efficacy and reduced side effects) or facilitate the multifunctionalization of antibodies, with a focus on emerging fields such as targeted protein degradation, real-time live-cell imaging, catalytic labeling or decaging with spatiotemporal control as well as the engagement of antibodies inside cells. With advances in modern chemistry and biotechnology, well-designed antibodies and their derivatives via size miniaturization or multifunctionalization together with efficient delivery systems have emerged, which have gradually improved our understanding of important biological processes and paved the way to pursue novel targets for potential treatments of various diseases.

The Recent Advance of Cell-Penetrating and Tumor-Targeting Peptides as Drug Delivery Systems Based on Tumor Microenvironment

Cancer has become the primary reason for industrial countries death. Although first-line treatments have achieved remarkable results in inhibiting tumors, they could have serious side effects because of insufficient selectivity. Therefore, specific localization of tumor cells is currently the main desire for cancer treatment. In recent years, cell-penetrating peptides (CPPs), as a kind of promising delivery vehicle, have attracted much attention because they mediate the high-efficiency import of large quantities of cargos in vivo and vitro. Unfortunately, the poor targeting of CPPs is still a barrier to their clinical application. In order to solve this problem, researchers use the various characteristics of tumor microenvironment and multiple receptors to improve the specificity toward tumors. This review focuses on the characteristics of the tumor microenvironment, and introduces the development of strategies and peptides based on these characteristics as drug delivery system in the tumor-targeted therapy.

A General Method to Edit Histone H3 Modifications on Chromatin Via Sortase-Mediated Metathesis

The post-translational modifications (PTMs) on the tail of histone H3 control chromatin structure and influence epigenetics and gene expression. The current chemical methods including unnatural amino acid incorporation and protein splicing enable preparations of the histone with diverse PTMs in cellular contexts, but they are not applicable to edit native chromatin. The manipulation of histone-modifying enzymes alter the endogenous histone PTMs but the lack of specificity of most histone-modifying enzymes prevents precise control of specific H3 tail PTM patterns. Here we report a new method to edit the N-tail of histone H3 via sortase mediated metathesis (SMM). The sortase can install desired PTM patterns into histone H3 on nucleosomes in vitro and in cellulo. This study expands the application scope of sortase from ligation to metathesis in live cells using cell-penetrating peptides (CPPs). In addition, it offers a strategy to edit PTMs of cellular histone H3 with potential for the development of precise epigenome editing.

Predicting the Assembly of the Transmembrane Domains of Viral Channel Forming Proteins and Peptide Drug Screening Using a Docking Approach

A de novo assembly algorithm is provided to propose the assembly of bitopic transmembrane domains (TMDs) of membrane proteins. The algorithm is probed using, in particular, viral channel forming proteins (VCPs) such as M2 of influenza A virus, E protein of severe acute respiratory syndrome corona virus (SARS-CoV), 6K of Chikungunya virus (CHIKV), SH of human respiratory syncytial virus (hRSV), and Vpu of human immunodeficiency virus type 2 (HIV-2). The generation of the structures is based on screening a 7-dimensional space. Assembly of the TMDs can be achieved either by simultaneously docking the individual TMDs or via a sequential docking. Scoring based on estimated binding energies (EBEs) of the oligomeric structures is obtained by the tilt to decipher the handedness of the bundles. The bundles match especially well for all-atom models of M2 referring to an experimentally reported tetrameric bundle. Docking of helical poly-peptides to experimental structures of M2 and E protein identifies improving EBEs for positively charged (K,R,H) and aromatic amino acids (F,Y,W). Data are improved when using polypeptides for which the coordinates of the amino acids are adapted to the Cα coordinates of the respective experimentally derived structures of the TMDs of the target proteins.

Cell-Penetrating Peptides (CPPs) as Therapeutic and Diagnostic Agents for Cancer

Cell-Penetrating Peptides (CPPs) are short peptides consisting of <30 amino acids. Their ability to translocate through the cell membrane while carrying large cargo biomolecules has been the topic of pre-clinical and clinical trials. The ability to deliver cargo complexes through membranes yields potential for therapeutics and diagnostics for diseases such as cancer. Upon cellular entry, some CPPs have the ability to target specific organelles. CPP-based intracellular targeting strategies hold tremendous potential as they can improve efficacy and reduce toxicities and side effects. Further, recent clinical trials show a significant potential for future CPP-based cancer treatment. In this review, we summarize recent advances in CPPs based on systematic searches in PubMed, Embase, Web of Science, and Scopus databases until 30 September 2022. We highlight targeted delivery and explore the potential uses for CPPs as diagnostics, drug delivery, and intrinsic anti-cancer agents.

Ligand-switchable nanoparticles resembling viral surface for sequential drug delivery and improved oral insulin therapy

Mutual interference between surface ligands on multifunctional nanoparticles remains a significant obstacle to achieving optimal drug-delivery efficacy. Here, we develop ligand-switchable nanoparticles which resemble viral unique surfaces, enabling them to fully display diverse functions. The nanoparticles are modified with a pH-responsive stretchable cell-penetrating peptide (Pep) and a liver-targeting moiety (Gal) (Pep/Gal-PNPs). Once orally administered, the acidic environments trigger the extension of Pep from surface in a virus-like manner, enabling Pep/Gal-PNPs to traverse intestinal barriers efficiently. Subsequently, Gal is exposed by Pep folding at physiological pH, thereby allowing the specific targeting of Pep/Gal-PNPs to the liver. As a proof-of-concept, insulin-loaded Pep/Gal-PNPs are fabricated which exhibit effective intestinal absorption and excellent hepatic deposition of insulin. Crucially, Pep/Gal-PNPs increase hepatic glycogen production by 7.2-fold, contributing to the maintenance of glucose homeostasis for effective diabetes management. Overall, this study provides a promising approach to achieving full potential of diverse ligands on multifunctional nanoparticles.

Cell-penetrating peptide: A powerful delivery tool for DNA-free crop genome editing

Genome editing system based on the CRISPR/Cas (clustered regularly interspaced short palindromic repeats) technology is a milestone for biology. However, public concerns regarding genetically modified organisms (GMOs) and recalcitrance in the crop of choice for regeneration have limited its application. Cell-penetrating peptides (CPPs) are derived from protein transduction domains (PTDs) that can take on various cargoes across the plant wall, and membrane of target cells. Selected CPPs show mild cytotoxicity and are a suitable delivery tool for DNA-free genome editing. Moreover, CPPs may also be applied for the transient delivery of morphogenic transcription factors, also known as developmental regulators (DRs), to overcome the bottleneck of the crop of choice regeneration. In this review, we introduce a brief history of cell-penetrating peptides and discuss the practice of CPP-mediated DNA-free transfection and the prospects of this potential delivery tool for improving crop genome editing.

Novel Pharmaceutical Strategies for Enhancing Skin Penetration of Biomacromolecules

Skin delivery of biomacromolecules holds great advantages in the systemic and local treatment of multiple diseases. However, the densely packed stratum corneum and the tight junctions between keratinocytes stand as formidable skin barriers against the penetration of most drug molecules. The large molecular weight, high hydrophilicity, and lability nature of biomacromolecules pose further challenges to their skin penetration. Recently, novel penetration enhancers, nano vesicles, and microneedles have emerged as efficient strategies to deliver biomacromolecules deep into the skin to exert their therapeutic action. This paper reviews the potential application and mechanisms of novel skin delivery strategies with emphasis on the pharmaceutical formulations.

New Technologies Bloom Together for Bettering Cancer Drug Conjugates

Drug conjugates, including antibody-drug conjugates, are a step toward realizing Paul Ehrlich's idea from over 100 years ago of a "magic bullet" for cancer treatment. Through balancing selective targeting molecules with highly potent payloads, drug conjugates can target specific tumor microenvironments and kill tumor cells. A drug conjugate consists of three parts: a targeting agent, a linker, and a payload. In some conjugates, monoclonal antibodies act as the targeting agent, but new strategies for targeting include antibody derivatives, peptides, and even small molecules. Linkers are responsible for connecting the payload to the targeting agent. Payloads impact vital cellular processes to kill tumor cells. At present, there are 12 antibody-drug conjugates on the market for different types of cancers. Research on drug conjugates is increasing year by year to solve problems encountered in conjugate design, such as tumor heterogeneity, poor circulation, low drug loading, low tumor uptake, and heterogenous expression of target antigens. This review highlights some important preclinical research on drug conjugates in recent years. We focus on three significant areas: improvement of antibody-drug conjugates, identification of new conjugate targets, and development of new types of drug conjugates, including nanotechnology. We close by highlighting the critical barriers to clinical translation and the open questions going forward. SIGNIFICANCE STATEMENT: The development of anticancer drug conjugates is now focused in three broad areas: improvements to existing antibody drug conjugates, identification of new targets, and development of new conjugate forms. This article focuses on the exciting preclinical studies in these three areas and advances in the technology that improves preclinical development.

Cell-penetrating peptide-mediated delivery of therapeutic peptides/proteins to manage the diseases involving oxidative stress, inflammatory response and apoptosis

OBJECTIVES

Peptides and proteins represent great potential for modulating various cellular processes including oxidative stress, inflammatory response, apoptosis and consequently the treatment of related diseases. However, their therapeutic effects are limited by their inability to cross cellular barriers. Cell-penetrating peptides (CPPs), which can transport cargoes into the cell, could resolve this issue, as would be discussed in this review.

KEY FINDINGS

CPPs have been successfully exploited in vitro and in vivo for peptide/protein delivery to treat a wide range of diseases involving oxidative stress, inflammatory processes and apoptosis. Their in vivo applications are still limited due to some fundamental issues of CPPs, including nonspecificity, proteolytic instability, potential toxicity and immunogenicity.

SUMMARY

Totally, CPPs could potentially help to manage the diseases involving oxidative stress, inflammatory response and apoptosis by delivering peptides/proteins that could selectively reach proper intracellular targets. More studies to overcome related CPP limitations and confirm the efficacy and safety of this strategy are needed before their clinical usage.

The role of cell-penetrating peptides in potential anti-cancer therapy

Due to the complex physiological structure, microenvironment and multiple physiological barriers, traditional anti-cancer drugs are severely restricted from reaching the tumour site. Cell-penetrating peptides (CPPs) are typically made up of 5-30 amino acids, and can be utilised as molecular transporters to facilitate the passage of therapeutic drugs across physiological barriers. Up to now, CPPs have widely been used in many anti-cancer treatment strategies, serving as an excellent potential choice for oncology treatment. However, their drawbacks, such as the lack of cell specificity, short duration of action, poor stability in vivo, compatibility problems (i.e. immunogenicity), poor therapeutic efficacy and formation of unwanted metabolites, have limited their further application in cancer treatment. The cellular uptake mechanisms of CPPs involve mainly endocytosis and direct penetration, but still remain highly controversial in academia. The CPPs-based drug delivery strategy could be improved by clever design or chemical modifications to develop the next-generation CPPs with enhanced cell penetration capability, stability and selectivity. In addition, some recent advances in targeted cell penetration that involve CPPs provide some new ideas to optimise CPPs.

Constructing New Acid-Activated Anticancer Peptide by Attaching a Desirable Anionic Binding Partner Peptide

Improving the cell selectivity of anticancer peptides (ACPs) is a major hurdle in their clinical utilization. In this study, a new acid-activated ACP was designed by conjugating a cationic ACP LK to its anionic binding partner peptide (LEH) via a disulfide linker to trigger antitumor activity at acidic pH while masking its killing activity at normal pH. Three anionic binding peptides containing different numbers of glutamic acid (Glu) and histidine were engineered to obtain an efficient acid-activated ACP. The conjugates LK-LEH2 and LK-LEH3 exhibited 6.1 and 8.0-fold higher killing activity at pH 6.0 relative to at pH 7.4, respectively, suggesting their excellent pH-dependent antitumor activity; and their cytotoxicity was 10-fold lower than that of LK. However, LK-LEH4 had no pH-responsive killing effect. Interestingly, increasing the number of Glu from 2 to 4 increased the pH-response of the physical mixture of LK and LEH; conversely, they weakly decreased the cytotoxicity of LK, suggesting that the conjugate connection was required to achieve excellent pH dependence while maintaining minimum toxicity. LK-LEH2 and LK-LEH3 were more enzymatically stable than LK, indicating their potential for in vivo application. Our work provided a basis for designing promising ACPs with good selectivity and low toxicity.

Redesigning of Cell-Penetrating Peptides to Improve Their Efficacy as a Drug Delivery System

Cell-penetrating peptides (CPP) are promising tools for the transport of a broad range of compounds into cells. Since the discovery of the first members of this peptide family, many other peptides have been identified; nowadays, dozens of these peptides are known. These peptides sometimes have very different chemical–physical properties, but they have similar drawbacks; e.g., non-specific internalization, fast elimination from the body, intracellular/vesicular entrapment. Although our knowledge regarding the mechanism and structure–activity relationship of internalization is growing, the prediction and design of the cell-penetrating properties are challenging. In this review, we focus on the different modifications of well-known CPPs to avoid their drawbacks, as well as how these modifications may increase their internalization and/or change the mechanism of penetration.

How Cargo Identity Alters the Uptake of Cell-Penetrating Peptide (CPP)/Cargo Complexes: A Study on the Effect of Net Cargo Charge and Length

Cell-penetrating peptides (CPPs) have emerged as a powerful tool for the delivery of otherwise impermeable cargoes into intact cells. Recent efforts to improve the delivery capability of peptides have mainly focused on the identity of the CPP; however, there is evidence that the identity of the cargo itself affects the uptake. The goal of this work was to investigate how the characteristics of a peptide cargo, including net charge and length, either enhance or diminish the internalization efficiency of the CPP/cargo complex. A small library of CPP/cargo complexes were synthesized consisting of structured and unstructured CPPs with cargoes of net positive, negative, or neutral charge and lengths of 4 or 8 amino acids. Cargoes with a net positive charge were found to enhance the overall uptake of the complexes while net neutral and negatively charged cargoes diminished uptake. Conversely, the net length of the cargo had no significant effect on uptake of the CPP/cargo complexes. Microcopy images confirmed the increased uptake of the positively charged cargoes; however, an increase in punctate regions with the addition of a cargo was also observed. The effects of the net positively charged cargoes were confirmed with both structured and unstructured CPPs, which demonstrated similar trends of an increase in uptake with the addition of positively charged residues. These findings demonstrate that the net charge of cargoes impacts the uptake of the complex, which can be considered in the future when designing peptide-based reporters or therapeutics.

CPP-Based Bioactive Drug Delivery to Penetrate the Blood-Brain Barrier: A Potential Therapy for Glioblastoma Multiforme

BACKGROUND

A large number of studies have been conducted on the treatment of glioblastoma multiforme (GBM). Chemotherapeutic drugs cannot penetrate deeply into the brain parenchyma due to the presence of the blood-brain barrier (BBB). Hence, crossing BBB is the significant obstacle in developing new therapeutic methods for GBM.

OBJECTIVE

Cell penetrating peptides (CPPs) have emerged as new tools that can efficiently deliver various substances across BBB. CPPs beneficial properties, such as BBB penetration capacity, low toxicity, and the ability to achieve active targeting and controllable drug release, have made them worthy candidates for GBM treatment. However, their application is limited by several drawbacks, including lack of selectivity, insufficient transport efficacy, and low stability. In order to overcome the selectivity issue, tumor targeting peptides and sequences that can be activated at the target site have been embedded into the structure of CPPs. To overcome their insufficient transport efficacy into the cells, which is mostly due to endosomal entrapment, various endosomolytic moieties have been incorporated into CPPs. Finally, their instability in blood circulation can be solved through different modifications to their structures. As this field is moving beyond preclinical studies, the discovery of new and more efficient CPPs for GBM treatment has become crucial. Thus, by using display techniques, such as phage display, this encouraging treatment strategy can be developed further.

CONCLUSION

Consequently, despite several challenges in CPPs application, recent progress in studies has shown their potential for the development of the next generation GBM therapeutics.

Activatable Peptides for Rapid and Simple Visualization of Protease Activity Secreted in Living Cells

Activity-based monitoring of cell-secreted proteases has gained significant interest due to the implication of these substances in diverse cellular functions. Here, we demonstrated a cell-based method of monitoring protease activity using fluorescent cell-permeable peptides. The activatable peptide consists of anionic (EEEE), cleavable, and cationic sequences (RRRR) that enable intracellular delivery by matrix metalloproteinase-2 (MMP2), which is secreted by living cancer cells. Compared to HT-29 cells (MMP2-negative), HT-1080 cells (MMP2-positive) showed a strong fluorescence response to the short fluorescent peptide via cell-secreted protease activation. Our approach is expected to find applications for the rapid visualization of protease activity in living cells.

Emerging landscape of cell-penetrating peptide-mediated nucleic acid delivery and their utility in imaging, gene-editing, and RNA-sequencing

The safety issues like immunogenicity and unacceptable cancer risk of viral vectors for DNA/mRNA vaccine delivery necessitate the development of non-viral vectors with no toxicity. Among the non-viral strategies, cell-penetrating peptides (CPPs) have been a topic of interest recently because of their ability to cross plasma membranes and facilitate nucleic acids delivery both in vivo and in vitro. In addition to the application in the field of gene vaccine and gene therapy, CPPs based nucleic acids delivery have been proved by its potential application like gene editing, RNA-sequencing, and imaging. Here, we focus on summarizing the recent applications and progress of CPPs-mediated nucleic acids delivery and discuss the current problems and solutions in this field.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Hydrolysis Mechanism of the Linkers by Matrix Metalloproteinase-9 Using QM/MM Calculations

Activatable cell-penetrating peptides (ACPPs) are known to be able to decrease the cytotoxicity of cell-penetrating peptide (CPP)-based drug delivery systems. Furthermore, they can improve the targeting of CPPs when specifically recognized and hydrolyzed by characteristic proteases. A comprehensive and profound understanding of the recognition and hydrolysis process will provide a better design of the ACPP-based drug delivery system. Previous studies have clearly described how ACPPs are recognized and bound by MMPs. However, the hydrolysis mechanism of ACPPs is still unsolved. This work focuses on a proteinase-sensitive cleavable linker of ACPPs (PLGLAG), the key structure for recognition and hydrolysis, trying to determine the mechanism by which MMP-9 hydrolyzes its substrate PLGLAG. The quantum mechanics/molecular mechanics (QM/MM) calculations herein show that MMP-9 proteolysis is a water-mediated four-step reaction. More specifically, it consists of (i) nucleophilic attack, (ii) hydrogen-bond rearrangement, (iii) proton transfer, and finally (iv) amide bond rupture. Considering the reversibility of multistep reaction, the second step (i.e., hydrogen-bond rearrangement) has the highest barrier and is the rate-limiting step in the hydrolysis of PLGLAG. The possible design and improvement of the key P1 and P1' sites are also explored through mutations. The present results indicate that, while the mutations affect the reaction energy barriers and the rate-limiting steps, all mutants considered could be hydrolyzed by MMP-9. To provide further insights, the hydrolysis mechanism of MMP-2, which has a similar hydrolysis process to that of MMP-9 but with different reaction barriers, is also studied and compared. As a result, this work provides detailed insights into the hydrolysis mechanism of ACPPs by MMP-9 and, thus, also possible insights for the development of new strategies for ACPP-based delivery systems.

Delivery of Various Cargos into Cancer Cells and Tissues via Cell-Penetrating Peptides: A Review of the Last Decade

Cell-penetrating peptides (CPPs), also known as protein transduction domains, are a class of diverse amino acid sequences with the ability to cross cellular membranes. CPPs can deliver several bioactive cargos, including proteins, peptides, nucleic acids and chemotherapeutics, into cells. Ever since their discovery, synthetic and natural CPPs have been utilized in therapeutics delivery, gene editing and cell imaging in fundamental research and clinical experiments. Over the years, CPPs have gained significant attention due to their low cytotoxicity and high transduction efficacy. In the last decade, multiple investigations demonstrated the potential of CPPs as carriers for the delivery of therapeutics to treat various types of cancer. Besides their remarkable efficacy owing to fast and efficient delivery, a crucial benefit of CPP-based cancer treatments is delivering anticancer agents selectively, rather than mediating toxicities toward normal tissues. To obtain a higher therapeutic index and to improve cell and tissue selectivity, CPP-cargo constructions can also be complexed with other agents such as nanocarriers and liposomes to obtain encouraging outcomes. This review summarizes various types of CPPs conjugated to anticancer cargos. Furthermore, we present a brief history of CPP utilization as delivery systems for anticancer agents in the last decade and evaluate several reports on the applications of CPPs in basic research and preclinical studies.

Selective Moonlighting Cell-Penetrating Peptides

Cell penetrating peptides (CPPs) are molecules capable of passing through biological membranes. This capacity has been used to deliver impermeable molecules into cells, such as drugs and DNA probes, among others. However, the internalization of these peptides lacks specificity: CPPs internalize indistinctly on different cell types. Two major approaches have been described to address this problem: (i) targeting, in which a receptor-recognizing sequence is added to a CPP, and (ii) activation, where a non-active form of the CPP is activated once it interacts with cell target components. These strategies result in multifunctional peptides (i.e., penetrate and target recognition) that increase the CPP's length, the cost of synthesis and the likelihood to be degraded or become antigenic. In this work we describe the use of machine-learning methods to design short selective CPP; the reduction in size is accomplished by embedding two or more activities within a single CPP domain, hence we referred to these as moonlighting CPPs. We provide experimental evidence that these designed moonlighting peptides penetrate selectively in targeted cells and discuss areas of opportunity to improve in the design of these peptides.

Design of acid-activated cell-penetrating peptides with nuclear localization capacity for anticancer drug delivery

Camptothecin (CPT), a DNA-toxin drug, exerts anticancer activity by inhibiting topoisomerase I. Targeted delivery of CPT into the cancer cell nucleus is important for enhancing its therapeutic efficiency. In this study, a new type of acid-activated cell-penetrating peptide (CPP) with nuclear localization capacity was constructed by conjugating six histidine residues and a hydrophobic peptide sequence, PFVYLI, to the nuclear localization sequence (NLS). Our results indicated that HNLS-3 displayed significant pH-dependent cellular uptake efficiency, endosomal escape ability, and nuclear localization activity. More importantly, the HNLS-3-CPT conjugate showed obviously enhanced cytotoxicity and selectivity compared with CPT. Taken together, our findings provide an effective approach to develop efficient CPPs with both cancer- and nucleus-targeting properties.

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.