Antimicrobial activity of doubly-stapled alanine/lysine-based peptides

Dual Antimicrobial and Anticancer Activity of Membrane-Active Peptide BP52

The linear undecapeptide BP52 was previously reported to have antibacterial activity against phytopathogenic bacteria species. Due to the structural similarities to naturally occurring cationic helical antimicrobial peptides, it was speculated that this peptide could potentially target microbial pathogens and cancer cells found in mammals. Consequently, this study aims to further investigate the structural and biological properties of this peptide. Our findings indicate that BP52 exhibits strong antimicrobial and anticancer activity while displaying relatively low levels of hemolytic activity. Hence, this study suggests that BP52 could be a potential lead compound for drug discovery against infectious diseases and cancer. Besides, new insights into the relationships between the structure and the multifunctional properties of antimicrobial peptides were also explored.

Systematic alanine and stapling mutational analysis of antimicrobial peptide Chem-KVL

Chem-KVL is a tandem repeating peptide, with 14 amino acids that was modified based on a short peptide from a fragment of the human host defense protein chemerin. Chem-KVL increases cationicity and hydrophobicity and shows broad-spectrum antibacterial activity. To determine the molecular determinants of Chem-KVL and whether staple-modified Chem-KVL would improve antibacterial activity and protease stability or decrease cytotoxicity, we combined alanine and stapling scanning, and designed a series of alanine and staple-derived Chem-KVL peptides, termed Chem-A1 to Chem-A14 and SCL-1 to SCL-7. We next examined their antibacterial activity against several gram-positive and gram-negative bacteria, their proteolytic stability, and their cytotoxicity. Ala scanning of Chem-KVL suggested that both the positively charged residues (Lys and Arg) and the hydrophobic residues (Lue and Val) were critical for the antibacterial activities of Chem-KVL peptide. Of note, Chem-A4 was able to remarkably inhibit the growth of gram-positive and gram-negative bacteria when compared to the original peptide. And the antibacterial activities of stapled SCL-4 and SCL-7 were several times higher than those of the linear peptide against gram-positive and gram-negative bacteria. Stapling modification of peptides resulted in increased helicity and protein stability when compared with the linear peptide. These stapled peptides, especially SCL-4 and SCL-7, may serve as the leading compounds for further optimization and antimicrobial therapy.

The amphipathic design in helical antimicrobial peptides

Amphipathicity is a critical characteristic of helical antimicrobial peptides (AMPs). The hydrophilic region, primarily composed of cationic residues, plays a pivotal role in the initial binding to negatively charged components on bacterial membranes through electrostatic interactions. Subsequently, the hydrophobic region interacts with hydrophobic components, inducing membrane perturbation, ultimately leading to cell death, or inhibiting intracellular function. Due to the extensive diversity of natural and synthetic AMPs with regard to the design of amphipathicity, it is complicated to study the structure-activity relationships. Therefore, this work aims to categorize the common amphipathic design and investigate their impact on the biological properties of AMPs. Besides, the connection between current structural modification approaches and amphipathic styles was also discussed.

Impact of chemical modifications on the antimicrobial and hemolytic activity of helical amphipathic peptide Lasioglossin LL-III

Insect venom is abundant in potential antimicrobial peptides (AMPs), which can serve as novel alternatives to conventional antibiotics. Among them, Lasioglossin III LL-III) is a promising candidate with a broad spectrum against many fungi strains and both types of bacteria, whereas almost non-toxic to red blood cells. Many chemical approaches have been recently applied to improve its pharmacological properties and provide useful information regarding structure-activity relationships. Hence, this review focused on highlighting the lesson learned from each modification and supporting the future design of potent, selective, and metabolically stable AMPs.

Peptide Stapling Applied to Antimicrobial Peptides

Antimicrobial peptides (AMPs) are considered a promising therapeutic approach against multi-drug resistant microorganisms. Besides their advantages, there are limitations to be overcome so that these molecules can become market competitive. One of the biggest limitations is proteolytic susceptibility, which could be overcome by structural modifications such as cyclization, especially for helix-constraining strategies. Over the years, many helix stabilization techniques have arisen, such as lactam-bridging, triazole-based, N-alkylation and all-hydrocarbon stapling. All-hydrocarbon stapling takes advantage of modified amino acid residues and olefinic cross-linking to constrain peptide helices. Despite being a well-established strategy and presenting efficient stability results, there are different limitations especially related to toxicity. In this review, recent studies on stapled AMPs for antimicrobial usage are explored with the aim of understanding the future of these molecules as putative antimicrobial agents.

Structure-Activity Relationship Study of Helix-Stabilized Antimicrobial Peptides Containing Nonproteinogenic Amino Acids

Helical amphipathic peptides containing cationic and hydrophobic amino acid residues can possess potent antimicrobial activity against both Gram-positive and Gram-negative bacteria. In this study, several amphipathic peptides with enhanced helical structures containing nonproteinogenic amino acids were designed, and the relationships between the antimicrobial activity, hemolytic activity, and cytotoxicity were evaluated. In particular, the effect on the antimicrobial activity and cytotoxicity of the number and position of stapling structures introduced into the sequence was investigated. Peptide stp1 containing α,α-disubstituted amino acids showed potent antimicrobial activity against multidrug-resistant bacteria (MDRP, SP45, and Staphylococcus aureus) without causing appreciable hemolytic activity or cytotoxicity. The cytotoxicity was found to be somewhat correlated to the hydrophobicity of the peptides.

Design, synthesis and antitumor activity of Ascaphin-8 derived stapled peptides based on halogen-sulfhydryl click chemical reactions

Ascaphin-8 (GFKDLLKGAAKALVKTVLF-NH₂), isolated from the norepinephrine-stimulated skin secretion of the North American-tailed frog Ascaphus truei, is a C-terminal α-helical antimicrobial peptide with potential antitumor activity. However, linear peptides are difficult to be applied directly as drugs because of their inherent defects, such as low hydrolytic enzyme tolerance and poor structural stability. In this study, we designed and synthesized a series of stapled peptides based on Ascaphin-8 via thiol-halogen click chemistry. Most of the stapled peptide derivatives showed enhanced antitumor activity. Among them, A8-2-o and A8-4-Dp had the most improved structural stability, stronger hydrolytic enzyme tolerance and highest biological activity. This research may provide a reference for the stapled modification of other similar natural antimicrobial peptides.

Rational design of stapled antimicrobial peptides

The global increase in antimicrobial drug resistance has dramatically reduced the effectiveness of traditional antibiotics. Structurally diverse antibiotics are urgently needed to combat multiple-resistant bacterial infections. As part of innate immunity, antimicrobial peptides have been recognized as the most promising candidates because they comprise diverse sequences and mechanisms of action and have a relatively low induction rate of resistance. However, because of their low chemical stability, susceptibility to proteases, and high hemolytic effect, their usage is subject to many restrictions. Chemical modifications such as D-amino acid substitution, cyclization, and unnatural amino acid modification have been used to improve the stability of antimicrobial peptides for decades. Among them, a side-chain covalent bridge modification, the so-called stapled peptide, has attracted much attention. The stapled side-chain bridge stabilizes the secondary structure, induces protease resistance, and increases cell penetration and biological activity. Recent progress in computer-aided drug design and artificial intelligence methods has also been used in the design of stapled antimicrobial peptides and has led to the successful discovery of many prospective peptides. This article reviews the possible structure-activity relationships of stapled antimicrobial peptides, the physicochemical properties that influence their activity (such as net charge, hydrophobicity, helicity, and dipole moment), and computer-aided methods of stapled peptide design. Antimicrobial peptides under clinical trial: Pexiganan (NCT01594762, 2012-05-07). Omiganan (NCT02576847, 2015-10-13).

Lysine-Tethered Stable Bicyclic Cationic Antimicrobial Peptide Combats Bacterial Infection in Vivo

Antimicrobial peptides (AMPs) have attracted great attention as next generation antibiotics for the treatment of multidrug-resistant (MDR) bacterial infections. Poor proteolytic stability has however undermined clinical applications of AMPs. A novel peptide cyclization approach is described to enhance the in vivo antibacterial activity of AMPs. Bicyclic antimicrobial peptides were synthesized by cross-linking the ε-amino groups of three lysine residues with a 1,3,5-trimethylene benzene spacer. In a proof of principal study, four bicyclic peptides were synthesized from the cationic AMP OH-CM6. One bicyclic peptide retained strong antimicrobial activity and low toxicity but exhibited a prolonged half-life in serum. Antibacterial activity was consequently improved in vivo without renal or hepato-toxicity. The novel peptide cyclization approach represents an important tool for enhancing AMP proteolytic stability for improved treatment of bacterial infection.

Design, Synthesis, and Antitumor Activity Study of All-Hydrocarbon-Stapled B1-Leu Peptides

B1-Leu peptide is a structural optimization compound derived from the lysine- and phenylalanine-rich antimicrobial peptide Cathelicidin-BF. It has shown promising antibacterial and antitumor biological activity. However, linear peptides are not the best choice for novel drug development due to their poor pharmacokinetic properties. In this study, various all-hydrocarbon stapled B1-Leu derivatives were designed and synthesized. Their secondary structure, protease stability, and antitumor and hemolytic activities were also investigated to evaluate their clinical value for cancer therapy. Among them, B1-L-3 and B1-L-6 showed both damaging the tumor cell membrane stability and antitumor activity, showing that they are promising lead compounds for the development of novel cancer therapeutics.

Application of the All-Hydrocarbon Stapling Technique in the Design of Membrane-Active Peptides

The threats of drug resistance and new emerging pathogens have led to an urgent need to develop alternative treatment therapies. Recently, considerable research efforts have focused on membrane-active peptides (MAPs), a category of peptides in drug discovery with antimicrobial, anticancer, and cell penetration activities that have demonstrated their potential to be multifunctional agents. Nonetheless, natural MAPs have encountered various disadvantages, which mainly include poor bioavailability, the lack of a secondary structure in short peptides, and high production costs for long peptide sequences. Hence, an "all-hydrocarbon stapling system" has been applied to these peptides and proven to effectively stabilize the helical conformations, improving proteolytic resistance and increasing both the potency and the cell permeability. In this review, we summarized and categorized the advances made using this powerful technique in the development of stapled MAPs. Furthermore, outstanding issues and suggestions for future design within each subcategory were thoroughly discussed.

Lysine Stapling Screening Provides Stable and Low Toxic Cationic Antimicrobial Peptides Combating Multidrug-Resistant Bacteria In Vitro and In Vivo

Cationic antimicrobial peptides (CAMPs) are promising for treatment of multidrug-resistant (MDR) bacteria-caused infections. However, clinical application of CAMPs has been hampered mostly due to their poor proteolytic stability and hemolytic toxicity. Recently, lysine-stapled CAMPs developed by us had been proved to increase peptide stability in vitro without induction of hemolysis. Herein, the applicability of the lysine stapling strategy was further explored by using five natural or artificial CAMPs as model peptides. Lysine stapling screening was implemented to provide 13 cyclic analogues in total. Biological screening of these cyclic analogues showed that CAMPs with a better amphiphilic structure were inclined to exhibit improved antimicrobial activity, protease stability, and biocompatibility after lysine-stapling. One of the stapled analogues of BF15-a1 was found to have extended half-life in plasma, enhanced antimicrobial activity against clinically isolated MDR ESKAPE pathogens, and remained highly effective in combating MRSA infection in a mouse model.

Current synthetic chemistry towards cyclic antimicrobial peptides

Antimicrobial peptides (AMPs) have great potentials for developing novel antibiotics against multi-drug resistant (MDR) bacteria. However, the clinical application of AMPs is limited due to their poor protease stability and high hemolytic toxicity. Various strategies have been widely explored to improve the pharmacological properties of natural or artificial antimicrobial peptides, including D- or non-natural amino acid residue replacement, backbone modification, cyclization, PEGlytion, and lipidation. Among others, peptide cyclization, which has been widely applied to enhance the biostability and target selectivity of bioactive peptide, is a very appealing and promising strategy for developing novel antibiotics based on AMPs. Herein, we summarize the current strategies for synthesizing cyclic antimicrobial peptides and the resulting influence of peptide cyclization on the biological activities.

Advances on chemically modified antimicrobial peptides for generating peptide antibiotics

Antimicrobial peptides (AMPs) are pinpointed as promising molecules against antibiotic-resistant bacterial infections. Nevertheless, there is a discrepancy between the AMP sequences generated and the tangible outcomes in clinical trials. AMPs' limitations include enzymatic degradation, chemical/physical instability and toxicity toward healthy human cells. These factors compromise AMPs' bioavailability, resulting in limited therapeutic potential. To overcome such obstacles, peptidomimetic approaches, including glycosylation, PEGylation, lipidation, cyclization, grafting, D-amino acid insertion, stapling and dendrimers are promising strategies to fine-tune AMPs. Here we focused on chemical modifications applied for AMP optimization and how they have helped these peptide-based antibiotic candidates' design and translational potential.

Rational Design of a Dual-Targeting Natural Toxin-Like Bicyclic Peptide for Selective Bioenergetic Blockage in Cancer Cells

Stapled α-helical peptides emerge as one of the attractive peptidomimetics which can efficiently penetrate the cell membrane to access intracellular targets. However, the incorporation of a highly lipophilic cross-link may lead to nonspecific membrane toxicity in certain cases. Here, we report a new class of thioether-tethered bicyclic α-helical peptide to mimic the highly constrained loop-helix structure of natural toxins with the dual-targeting ability for both cell-surface receptors and intracellular targets. The thioether cross-links are introduced to replace the redox-sensitive disulfide bonds in natural toxins via a photoinduced thiol-yne reaction followed by macrolactamization. As a proof of concept, αVβ3 integrin targeting ligand was grafted into one of the macrocycles in the bicyclic scaffold, while a mitochondria-targeting proapoptotic motif was introduced into the other macrocycle stabilized by an i, i + 7 alkyl thioether cross-link to recapitulate its α-helical conformation. The obtained dual-targeting bicyclic α-helical BIRK peptides showed highly stable α-helical conformation in the presence of denaturants or under high temperature. Notably, BIRK peptides could induce selective cell death in αVβ3 integrin-positive B16F10 cells by interfering with the bioenergetic functions of mitochondria. This work provides a new avenue to design and stabilize α-helical peptides in a highly constrained bicyclic loop-helix scaffold with dual functionality.

DRAMP 3.0: an enhanced comprehensive data repository of antimicrobial peptides

Stapled antimicrobial peptides are an emerging class of artificial cyclic peptide molecules which have antimicrobial activity and potent structure stability. We previously published the Data Repository of Antimicrobial Peptides (DRAMP) as a manually annotated and open-access database of antimicrobial peptides (AMPs). In the update of version 3.0, special emphasis was placed on the new development of stapled AMPs, and a subclass of specific AMPs was added to store information on these special chemically modified AMPs. To help design low toxicity AMPs, we also added the cytotoxicity property of AMPs, as well as the expansion of newly discovered AMP data. At present, DRAMP has been expanded and contains 22259 entries (2360 newly added), consisting of 5891 general entries, 16110 patent entries, 77 clinical entries and 181 stapled AMPs. A total of 263 entries have predicted structures, and more than 300 general entries have links to experimentally determined structures in the Protein Data Bank. The update also covers new annotations, statistics, categories, functions and download links. DRAMP is available online at http://dramp.cpu-bioinfor.org/.

The multifaceted nature of antimicrobial peptides: current synthetic chemistry approaches and future directions

Bacterial infections caused by 'superbugs' are increasing globally, and conventional antibiotics are becoming less effective against these bacteria, such that we risk entering a post-antibiotic era. In recent years, antimicrobial peptides (AMPs) have gained significant attention for their clinical potential as a new class of antibiotics to combat antimicrobial resistance. In this review, we discuss several facets of AMPs including their diversity, physicochemical properties, mechanisms of action, and effects of environmental factors on these features. This review outlines various chemical synthetic strategies that have been applied to develop novel AMPs, including chemical modifications of existing peptides, semi-synthesis, and computer-aided design. We will also highlight novel AMP structures, including hybrids, antimicrobial dendrimers and polypeptides, peptidomimetics, and AMP-drug conjugates and consider recent developments in their chemical synthesis.

Antimicrobial Peptides and Proteins: From Nature's Reservoir to the Laboratory and Beyond

Rapid rise of antimicrobial resistance against conventional antimicrobials, resurgence of multidrug resistant microbes and the slowdown in the development of new classes of antimicrobials, necessitates the urgent development of alternate classes of therapeutic molecules. Antimicrobial peptides (AMPs) are small proteins present in different lifeforms in nature that provide defense against microbial infections. They have been effective components of the host defense system for a very long time. The fact that the development of resistance by the microbes against the AMPs is relatively slower or delayed compared to that against the conventional antibiotics, makes them prospective alternative therapeutics of the future. Several thousands of AMPs have been isolated from various natural sources like microorganisms, plants, insects, crustaceans, animals, humans, etc. to date. However, only a few of them have been translated commercially to the market so far. This is because of some inherent drawbacks of the naturally obtained AMPs like 1) short half-life owing to the susceptibility to protease degradation, 2) inactivity at physiological salt concentrations, 3) cytotoxicity to host cells, 4) lack of appropriate strategies for sustained and targeted delivery of the AMPs. This has led to a surge of interest in the development of synthetic AMPs which would retain or improve the antimicrobial potency along with circumventing the disadvantages of the natural analogs. The development of synthetic AMPs is inspired by natural designs and sequences and strengthened by the fusion with various synthetic elements. Generation of the synthetic designs are based on various strategies like sequence truncation, mutation, cyclization and introduction of unnatural amino acids and synthons. In this review, we have described some of the AMPs isolated from the vast repertoire of natural sources, and subsequently described the various synthetic designs that have been developed based on the templates of natural AMPs or from de novo design to make commercially viable therapeutics of the future. This review entails the journey of the AMPs from their natural sources to the laboratory.

Identification of brevinin-1EMa-derived stapled peptides as broad-spectrum virus entry blockers

Based on the previously reported 13-residue antibacterial peptide analog, brevinin-1EMa (FLGWLFKVASKVL, peptide B), we attempted to design a novel class of antiviral peptides. For this goal, we synthesized three peptides with different stapling positions (B-2S, B-8S, and B-5S). The most active antiviral peptide with the specific stapling position (B-5S) was further modified in combination with either cysteine (B-5S3C, B-5S7C, and B-5S10C) or hydrophilic amino acid substitution (Bsub and Bsub-5S). Overall, B, B-5S, and Bsub-5S peptides showed superior antiviral activities against enveloped viruses such as retrovirus, lentivirus, hepatitis C virus, and herpes simplex virus with EC₅₀ values of 1-5 μM. Murine norovirus, a non-enveloped virus, was not susceptible to the virucidal actions of these peptides, suggesting the virus membrane disruption as their main antiviral mechanisms of action. We believe that these three novel peptides could serve as promising candidates for further development of membrane-targeting antiviral drugs in the future.

Chemical approaches in the development of natural nontoxic peptide Polybia-MP1 as a potential dual antimicrobial and antitumor agent

Polybia-MP1 is a well-known natural antimicrobial peptide that has been intensively studied recently due to its therapeutic potential. MP1 exhibited not only potent antibacterial activity but also antifungal and anticancer properties. More importantly, MP1 shows relatively low hemolytic activity compared to other antimicrobial peptides having a similar origin. Thus, besides investigating possible mechanisms of action, great efforts have been invested to develop this peptide to become more "druggable". In this review, we summarized all the chemical approaches, both success and failure, that using MP1 as a lead compound to create modified analogs with better pharmacological properties. As there have been thousands of natural AMPs found and deposited in numerous databases, such useful information in both the success and failure will provide insight into the research and development of antimicrobial peptides and guiding for the next steps.

Helical Antimicrobial Peptide Foldamers Containing Non-proteinogenic Amino Acids

Antimicrobial peptides (AMPs) are potential novel therapeutic drugs against microbial infections. Most AMPs function by disrupting microbial membranes because of their amphipathic properties and ordered secondary structures. In this minireview, we describe recent efforts to develop helical AMP foldamers containing non-proteinogenic amino acids, such as α,α-disubstituted α-amino acids, β-amino acids, γ-amino acids, side-chain stapling and N-alkyl glycines.

Antimicrobial peptides - Advances in development of therapeutic applications

The severe infection is becoming a significant health problem which threaten the lives of patients and the safety and economy of society. In the way of finding new strategy, antimicrobial peptides (AMPs) - an important part of host defense family, emerged with tremendous potential. Up to date, huge numbers of AMPs has been investigated from both natural and synthetic sources showing not only the ability to kill microbial pathogens but also propose other benefits such as wound healing, anti-tumor, immune modulation. In this review, we describe the involvements of AMPs in biological systems and discuss the opportunity in developing AMPs for clinical applications. In the detail, their properties in antibacterial activity is followed by their application in some infection diseases and cancer. The key discussions are the approaches to improve biological activities of AMPs either by modifying chemical structure or incorporating into delivery systems. The new applications and perspectives for the future of AMPs would open the new era of their development.

Cytotoxic and antitumor peptides as novel chemotherapeutics

Covering: up to 2020Treatment resistance and drug-induced refractory malignancies pose significant challenges for current chemotherapy drugs. There have been increasing research efforts aimed at developing novel chemotherapeutics, especially from natural products and related derivatives. Natural cytotoxic peptides, an emerging source of chemotherapeutics, have exhibited the advantage of overcoming drug resistance and displayed broad-spectrum antitumor activities in the clinic. This highlight examines the increasingly popular cytotoxic peptides from isolated natural products. In-depth review of several peptides provides examples for how this novel strategy can lead to the improved anti-tumor effects. The mechanisms and current application of representative natural cytotoxic peptides (NCPs) have also been discussed, with a particular focus on future directions for interdisciplinary research.

Novel Stapling by Lysine Tethering Provides Stable and Low Hemolytic Cationic Antimicrobial Peptides

Cationic antimicrobial peptides (CAMPs) are potent therapeutics for drug-resistant bacterial infections. However, the clinical application of CAMPs is hampered by its poor proteolytic stability and hemolytic activity toward eukaryotic cells. Great efforts have been made to design and generate derivatives of CAMPs with improved pharmacological properties. Here, we report a novel stapling protocol, which tethers two ε-amino groups of the lysine residue by the N-alkylation reaction on the hydrophilic face of amphiphilic antimicrobial peptides. A series of lysine-tethered stapled CAMPs were synthesized, employing the antimicrobial peptide OH-CM6 as a model. Biological screening of the stapled CAMPs provided an analogue with strong antimicrobial activity, high proteolytic stability, and low hemolytic activity. This novel stapling approach offers an important chemical tool for developing CAMP-based antibiotics.

Design of stapled antimicrobial peptides that are stable, nontoxic and kill antibiotic-resistant bacteria in mice

The clinical translation of cationic α-helical antimicrobial peptides (AMPs) has been hindered by structural instability, proteolytic degradation and in vivo toxicity from nonspecific membrane lysis. Although analyses of hydrophobic content and charge distribution have informed the design of synthetic AMPs with increased potency and reduced in vitro hemolysis, nonspecific membrane toxicity in vivo continues to impede AMP drug development. Here, we analyzed a 58-member library of stapled AMPs (StAMPs) based on magainin II and applied the insights from structure-function-toxicity measurements to devise an algorithm for the design of stable, protease-resistant, potent and nontoxic StAMP prototypes. We show that a lead double-stapled StAMP named Mag(i+4)1,15(A9K,B21A,N22K,S23K) can kill multidrug-resistant Gram-negative pathogens, such as colistin-resistant Acinetobacter baumannii in a mouse peritonitis-sepsis model, without observed hemolysis or renal injury in murine toxicity studies. Inputting the amino acid sequences alone, we further generated membrane-selective StAMPs of pleurocidin, CAP18 and esculentin, highlighting the generalizability of our design platform.

Functional insights into the Streptococcus pneumoniae HicBA toxin-antitoxin system based on a structural study

Streptococcus pneumonia has attracted increasing attention due to its resistance to existing antibiotics. TA systems are essential for bacterial persistence under stressful conditions such as nutrient deprivation, antibiotic treatment, and immune system attacks. In particular, S. pneumoniae expresses the HicBA TA gene, which encodes the stable HicA toxin and the labile HicB antitoxin. These proteins interact to form a non-toxic TA complex under normal conditions, but the toxin is activated by release from the antitoxin in response to unfavorable growth conditions. Here, we present the first crystal structure showing the complete conformation of the HicBA complex from S. pneumonia. The structure reveals that the HicA toxin contains a double-stranded RNA-binding domain that is essential for RNA recognition and that the C-terminus of the HicB antitoxin folds into a ribbon-helix-helix DNA-binding motif. The active site of HicA is sterically blocked by the N-terminal region of HicB. RNase activity assays show that His36 is essential for the ribonuclease activity of HicA, and nuclear magnetic resonance (NMR) spectra show that several residues of HicB participate in binding to the promoter DNA of the HicBA operon. A toxin-mimicking peptide that inhibits TA complex formation and thereby increases toxin activity was designed, providing a novel approach to the development of new antibiotics.

Hydrocarbon Stapled Antimicrobial Peptides

Antimicrobial peptides are promising candidates for anti-infective pharmaceuticals. Unfortunately, because of their low proteolytic and chemical stability, their usage is generally narrowed down to topical formulations. Until now, numerous approaches to increase peptide stability have been proposed. One of them, peptide hydrocarbon stapling, a modification based on stabilizing peptide secondary structure with a side-chain covalent hydrocarbon bridge, have been successfully applied to many peptides. Moreover, constraining secondary structure of peptides have also been proven to increase their biological activity. This review article describes studies on hydrocarbon stapled antimicrobial peptides with respect to improved drug-like properties.

Stapling strategy enables improvement of antitumor activity and proteolytic stability of host-defense peptide hymenochirin-1B

Hymenochirin-1B is a cationic, amphipathic, α-helical host-defense peptide with 29 residues, which was isolated from skin secretions of the Congo clawed frog and showed potent cytotoxic activities against a range of tumor cell lines. However, the application of hymenochirin-1B as a drug is limited due to its conformational flexibility and poor proteolytic stability. In this research, a series of hydrocarbon-stapled analogs of hymenochirin-1B were designed, synthesized, and tested. Some analogs showed remarkable improvement not only in α-helicity, but also in antitumor activity and protease resistance when compared to the parent peptide. The results indicated that most stapled peptide analogues possessed improved activities against a series of tumor cells; in particular, the bicyclic stapled peptide H-10 showed promising prospects for novel anti-tumor drug development. Our data demonstrated the important impacts of the all-hydrocarbon crosslink stapling strategy on the biological activity, proteolytic stability and helicity of hymenochirin-1B.

A series of stapled peptide analogs of hymenochirin-1B were efficiently prepared by an Fmoc-SPPS procedure. The peptide stapling strategy can improve the helicity, proteolytic stability and tumor cell-killing activity of linear peptide hymenochirin-1B.

Development of helix-stabilized antimicrobial peptides composed of lysine and hydrophobic α,α-disubstituted α-amino acid residues

Lysine-based amphipathic nonapeptides, including homochiral peptides [Ac-(l-Lys-l-Lys-Xaa)₃-NH₂ (Xaa=Gly, Ala, Aib, Ac₅c, or Ac₆c) and Ac-(d-Lys-d-Lys-Aib)₃-NH₂], a heterochiral peptide [Ac-(l-Lys-d-Lys-Aib)₃-NH₂], and a racemic mixture of diastereomeric peptides [Ac-(rac-Lys-rac-Lys-Aib)₃-NH₂] were designed and synthesized to investigate the relationship between their preferred secondary structures and their antimicrobial activity. Peptide 5, [Ac-(l-Lys-l-Lys-Ac₆c)₃-NH₂] formed a stable α-helical structure and exhibited strong activity against Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa).

Design, synthesis, and antiproliferative activities of stapled melittin peptides

Melittin is a 26-residue, amphipathic, cell-penetrating, α-helical anti-hepatoma peptide isolated from bee venom.

Protease-Resistant and Cell-Permeable Double-Stapled Peptides Targeting the Rab8a GTPase

Small GTPases comprise a family of highly relevant targets in chemical biology and medicinal chemistry research and have been considered "undruggable" due to the persisting lack of effective synthetic modulators and suitable binding pockets. As molecular switches, small GTPases control a multitude of pivotal cellular functions, and their dysregulation is associated with many human diseases such as various forms of cancer. Rab-GTPases represent the largest subfamily of small GTPases and are master regulators of vesicular transport interacting with various proteins via flat and extensive protein-protein interactions (PPIs). The only reported synthetic inhibitor of a PPI involving an activated Rab GTPase is the hydrocarbon stapled peptide StRIP3. However, this macrocyclic peptide shows low proteolytic stability and cell permeability. Here, we report the design of a bioavailable StRIP3 analogue that harbors two hydrophobic cross-links and exhibits increased binding affinity, combined with robust cellular uptake and extremely high proteolytic stability. Localization experiments reveal that this double-stapled peptide and its target protein Rab8a accumulate in the same cellular compartments. The reported approach offers a strategy for the implementation of biostability into conformationally constrained peptides while supporting cellular uptake and target affinity, thereby conveying drug-like properties.

Stapling of unprotected helical peptides via photo-induced intramolecular thiol-yne hydrothiolation

Peptide stapling emerged as a versatile strategy to recapitulate the bioactive helical conformation of unstructured short peptides in water to improve their therapeutic properties in targeting intracellular "undruggable" targets. Here, we describe the development of photo-induced intramolecular thiol-yne macrocyclization for rapid access to short stapled peptides with enhanced biophysical properties. This new peptide stapling technique provides rapid access to conformationally constrained helices with satisfying functional group tolerance. Notably, the vinyl sulfide linkage shows distinct lipophilicity with reduced membrane toxicity compared to the corresponding all-hydrocarbon analogue. As a proof of principle, we constructed stabilized helices modulating intracellular estrogen receptor (ER)-coactivator interactions with a nanomolar binding affinity, enhanced serum stability, a diffuse cellular distribution and selective cytotoxicity towards ER-positive MCF-7 cells.

Computational scrutiny of the effect of N-terminal proline and residue stereochemistry in the nucleation of α-helix fold

N-Terminal l- to d-residue mutation nucleate helical fold in Ac–^DAla–^LAla₃–NHMe (Ib, m2), Ac–^DPro–^LAla₃–NHMe (IIb, m1), and Ac–^DPro–^LPro–^LAla₂–NHMe (IIIb, m2) peptides.