In vivo biodistribution and small animal PET of (64)Cu-labeled antimicrobial peptoids

Antiviral effect of peptoids on hepatitis B virus infection in cell culture

Although antimicrobial peptides have been shown to inactivate viruses through disruption of their viral envelopes, clinical use of such peptides has been hampered by a number of factors, especially their enzymatically unstable structures. To overcome the shortcomings of antimicrobial peptides, peptoids (sequence-specific N-substituted glycine oligomers) mimicking antimicrobial peptides have been developed. We aimed to demonstrate the antiviral effects of antimicrobial peptoids against hepatitis B virus (HBV) in cell culture. The anti-HBV activity of antimicrobial peptoids was screened and evaluated in an infection system involving the HBV reporter virus and HepG2.2.15-derived HBV. By screening with the HBV reporter virus infection system, three (TM1, TM4, and TM19) of 12 peptoids were identified as reducing the infectivity of HBV, though they did not alter the production levels of HBs antigen in cell culture. These peptoids were not cytotoxic at the evaluated concentrations. Among these peptoids, TM19 was confirmed to reduce HBV infection most potently in a HepG2.2.15-derived HBV infection system that closely demonstrates authentic HBV infection. In cell culture, the most effective administration of TM19 was virus treatment at the infection step, but the reduction in HBV infectivity by pre-treatment or post-treatment of cells with TM19 was minimal. The disrupting effect of TM19 targeting infectious viral particles was clarified in iodixanol density gradient analysis. In conclusion, the peptoid TM19 was identified as a potent inhibitor of HBV. This peptoid prevents HBV infection by disrupting viral particles and is a candidate for a new class of anti-HBV reagents.

Going Beyond Host Defence Peptides: Horizons of Chemically Engineered Peptides for Multidrug-Resistant Bacteria

Multidrug-resistant (MDR) bacteria are considered a health threat worldwide, and this problem is set to increase over the decades. The ESKAPE, a group of six pathogens including Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp. is the major source of concern due to their high death incidence and nosocomial acquired infection. Host defence peptides (HDPs) are a class of ribosomally synthesised peptides that have shown promising results in combating MDR, including the ESKAPE group, in- and outside bacterial biofilms. However, their poor pharmacokinetics in physiological mediums may impede HDPs from becoming viable clinical candidates. To circumvent this problem, chemical engineering of HDPs has been seen as an emergent approach to not only improve their pharmacokinetics but also their efficacy against pathogens. In this review, we explore several chemical modifications of HDPs that have shown promising results, especially against ESKAPE pathogens, and provide an overview of the current findings with respect to each modification.

Real-Time Monitoring of Multitarget Antimicrobial Mechanisms of Peptoids Using Label-Free Imaging with Optical Diffraction Tomography

Antimicrobial peptides (AMPs) are promising therapeutics in the fight against multidrug-resistant bacteria. As a mimic of AMPs, peptoids with N-substituted glycine backbone have been utilized for antimicrobials with resistance against proteolytic degradation. Antimicrobial peptoids are known to kill bacteria by membrane disruption; however, the nonspecific aggregation of intracellular contents is also suggested as an important bactericidal mechanism. Here,structure-activity relationship (SAR) of a library of indole side chain-containing peptoids resulting in peptoid 29 as a hit compound is investigated. Then, quantitative morphological analyses of live bacteria treated with AMPs and peptoid 29 in a label-free manner using optical diffraction tomography (ODT) are performed. It is unambiguously demonstrated that both membrane disruption and intracellular biomass flocculation are primary mechanisms of bacterial killing by monitoring real-time morphological changes of bacteria. These multitarget mechanisms and rapid action can be a merit for the discovery of a resistance-breaking novel antibiotic drug.

Protein-mimetic peptoid nanoarchitectures for pathogen recognition and neutralization

Recent outbreaks of both new and existing infectious pathogens have threatened healthcare systems around the world. Therefore, it is vital to detect and neutralize pathogens to prevent their spread and treat infected patients. This consideration has led to the development of biosensors and antibiotics inspired by the structure and function of antibodies and antimicrobial peptides (AMPs), which constitute adaptive and innate immunity, efficiently protecting the human body against invading pathogens. Herein, we provide an overview of recent advances in the detection and neutralization of pathogens using protein-mimetic peptoid nanoarchitectures. Peptoids are bio-inspired and sequence-defined polymers composed of repeating N-substituted glycine units. They can spontaneously fold into well-defined three-dimensional nanostructures that encode chemical information depending on their sequences. Loop-functionalized peptoid nanosheets have been constructed by mimicking antibodies containing chemically variable loops as binding motifs for their respective target pathogen. Furthermore, by mimicking the cationic amphipathic features of natural AMPs, helical peptoids and their assemblies have been developed to achieve selective anti-infective activity owing to their intrinsic ability to interact with bacterial membranes and viral envelopes. We believe that this mini-review furnishes in-depth insight into how to construct protein-like nanostructures via the self-assembly of peptoids for application in the detection of pathogens and the treatment of infectious diseases for future healthcare applications.

Potential of Nuclear Imaging Techniques to Study the Oral Delivery of Peptides

Peptides are small biomolecules known to stimulate or inhibit important functions in the human body. The clinical use of peptides by oral delivery, however, is very limited due to their sensitive structure and physiological barriers present in the gastrointestinal tract. These barriers can be overcome with chemical and mechanical approaches protease inhibitors, permeation enhancers, and polymeric encapsulation. Studying the success of these approaches pre-clinically with imaging techniques such as fluorescence imaging (IVIS) and optical microscopy is difficult due to the lack of in-depth penetration. In comparison, nuclear imaging provides a better platform to observe the gastrointestinal transit and quantitative distribution of radiolabeled peptides. This review provides a brief background on the oral delivery of peptides and states examples from the literature on how nuclear imaging can help to observe and analyze the gastrointestinal transit of oral peptides. The review connects the fields of peptide delivery and nuclear medicine in an interdisciplinary way to potentially overcome the challenges faced during the study of oral peptide formulations.

Conditional Antimicrobial Peptide Therapeutics

Antimicrobial peptides (AMPs) constitute a promising class of alternatives to antibiotics to curb antimicrobial resistance. Nonetheless, their utility as a systemic agent is hampered by short circulation time and toxicity. Infection sites, analogous to tumors, harbor an aberrant microenvironment that has the potential to be exploited to develop conditionally activated therapeutics with an improved therapeutic index. In particular, we identified strategies to prolong systemic circulation of small, cationic AMPs in a mouse model of bacterial pneumonia. Specifically, we report an albumin-binding domain (ABD)-AMP conjugate as a long-circulating conditional AMP therapeutic with a masked activity that can be liberated by proteases in the infected tissue microenvironment. Our systemically administered conjugate enhanced the pulmonary delivery of active AMP while also reducing AMP exposure to other off-target organs. Importantly, this reduction in off-target exposure improved the safety profile of the AMP. The framework we present can be generalized to quantify and optimize the performance of this emerging class of conditional therapeutics.

Antibacterial Copolypeptoids with Potent Activity against Drug Resistant Bacteria and Biofilms, Excellent Stability, and Recycling Property

The preparation of a type of innovative cationic copolypeptoid antimicrobials containing various hydrophobic moieties that resemble both structure and membrane-lytic antibacterial mechanism of natural antimicrobial peptides (AMPs) is reported. By finely tuning the hydrophilic/hydrophobic balance, the polypeptoids exhibit a wide spectrum of antibacterial activity against both Gram-positive bacteria and Gram-negative bacteria with the lowest minimum inhibitory concentration (MIC) at only 2 µg mL^-1 , whereas they also show low haemolytic properties. In particular, high selectivity (>128) is achieved from the polymers with butyl moieties. Moreover, the polypeptoids can readily inhibit the formation of biofilms and effectively eradicate the bacteria embedded in the mature biofilms, which is superior to many natural AMPs and vancomycin. Unlike conventional antibiotics, the polypeptoids possess potent activity against drug-resistant bacteria without visible resistance development after repeated usage. Notably, the polypeptoid antimicrobials not only have inherently fast bactericidal properties and excellent stability against incubation with human plasma, but also show excellent in vivo antibacterial effect. The prepared antimicrobials, coated onto magnetic nanospheres show recycling properties and enhanced antibacterial activity as combined with near-infrared (NIR)-induced photothermal antibacterial therapy.

Development of an Anti-Biofilm Screening Technique Leads to the Discovery of a Peptoid with Efficacy against Candida albicans

Bacteria and fungi can secrete and reside within a complex polysaccharide matrix, forming a biofilm that protects these pathogens from the immune response and conventional antibiotics. Because many microbial pathogens grow within biofilms in clinical settings, there is a need for antimicrobial agents effective against biofilm-protected infections. We report the adaptation of a phenotypic high-throughput assay for discovering antimicrobial peptoids toward the screening of combinatorial libraries against established biofilms. This method, termed the Inverted Peptoid Library Agar Diffusion (iPLAD) assay, required optimization of growth media, reducing reagent, and fungal viability reporter. Once optimized, iPLAD was used to screen a combinatorial peptoid library against Candida albicans, a biofilm-forming fungal pathogen responsible for most hospital-acquired infections. This screening resulted in a lipopeptoid termed RMG9-11 with excellent activity against several species of Candida, including drug-resistant strains of C. albicans and the emerging and dangerous C. auris. Additionally, the cytotoxicity of RMG9-11 against several mammalian cell lines was minimal. This work provides a new method for the identification of compounds effective against biofilm-protected pathogens and demonstrates its utility by identifying a promising anti-Candida peptoid.

Helicity Modulation Improves the Selectivity of Antimicrobial Peptoids

The modulation of conformational flexibility in antimicrobial peptides (AMPs) has been investigated as a strategy to improve their efficacy against bacterial pathogens while reducing their toxicity. Here, we synthesized a library of helicity-modulated antimicrobial peptoids by the position-specific incorporation of helix-inducing monomers. The peptoids displayed minimal variations in hydrophobicity, which permitted the specific assessment of the effect of conformational differences on antimicrobial activity and selectivity. Among the moderately helical peptoids, the most dramatic increase in selectivity was observed in peptoid 17, providing more than a 20-fold increase compared to fully helical peptoid 1. Peptoid 17 had potent broad-spectrum antimicrobial activity that included clinically isolated multi-drug-resistant pathogens. Compared to pexiganan AMP, 17 showed superior metabolic stability, which could potentially reduce the dosage needed, alleviating toxicity. Dye-uptake assays and high-resolution imaging revealed that the antimicrobial activity of 17 was, as with many AMPs, mainly due to membrane disruption. However, the high selectivity of 17 reflected its unique conformational characteristics, with differential interactions between bacterial and erythrocyte membranes. Our results suggest a way to distinguish different membrane compositions solely by helicity modulation, thereby improving the selectivity toward bacterial cells with the maintenance of potent and broad-spectrum activity.

The Best Peptidomimetic Strategies to Undercover Antibacterial Peptides

Health-care systems that develop rapidly and efficiently may increase the lifespan of humans. Nevertheless, the older population is more fragile, and is at an increased risk of disease development. A concurrently growing number of surgeries and transplantations have caused antibiotics to be used much more frequently, and for much longer periods of time, which in turn increases microbial resistance. In 1945, Fleming warned against the abuse of antibiotics in his Nobel lecture: "The time may come when penicillin can be bought by anyone in the shops. Then there is the danger that the ignorant man may easily underdose himself and by exposing his microbes to non-lethal quantities of the drug make them resistant". After 70 years, we are witnessing the fulfilment of Fleming's prophecy, as more than 700,000 people die each year due to drug-resistant diseases. Naturally occurring antimicrobial peptides protect all living matter against bacteria, and now different peptidomimetic strategies to engineer innovative antibiotics are being developed to defend humans against bacterial infections.

Surface Design for Immobilization of an Antimicrobial Peptide Mimic for Efficient Anti-Biofouling

Microbial surface attachment negatively impacts a wide range of devices from water purification membranes to biomedical implants. Mimics of antimicrobial peptides (AMPs) constituted from poly(N-substituted glycine) "peptoids" are of great interest as they resist proteolysis and can inhibit a wide spectrum of microbes. We investigate how terminal modification of a peptoid AMP-mimic and its surface immobilization affect antimicrobial activity. We also demonstrate a convenient surface modification strategy for enabling alkyne-azide "click" coupling on amino-functionalized surfaces. Our results verified that the N- and C-terminal peptoid structures are not required for antimicrobial activity. Moreover, our peptoid immobilization density and choice of PEG tether resulted in a "volumetric" spatial separation between AMPs that, compared to past studies, enabled the highest AMP surface activity relative to bacterial attachment. Our analysis suggests the importance of spatial flexibility for membrane activity and that AMP separation may be a controlling parameter for optimizing surface anti-biofouling.

Peptide science: A "rule model" for new generations of peptidomimetics

Peptides have been heavily investigated for their biocompatible and bioactive properties. Though a wide array of functionalities can be introduced by varying the amino acid sequence or by structural constraints, properties such as proteolytic stability, catalytic activity, and phase behavior in solution are difficult or impossible to impart upon naturally occurring α-L-peptides. To this end, sequence-controlled peptidomimetics exhibit new folds, morphologies, and chemical modifications that create new structures and functions. The study of these new classes of polymers, especially α-peptoids, has been highly influenced by the analysis, computational, and design techniques developed for peptides. This review examines techniques to determine primary, secondary, and tertiary structure of peptides, and how they have been adapted to investigate peptoid structure. Computational models developed for peptides have been modified to predict the morphologies of peptoids and have increased in accuracy in recent years. The combination of in vitro and in silico techniques have led to secondary and tertiary structure design principles that mirror those for peptides. We then examine several important developments in peptoid applications inspired by peptides such as pharmaceuticals, catalysis, and protein-binding. A brief survey of alternative backbone structures and research investigating these peptidomimetics shows how the advancement of peptide and peptoid science has influenced the growth of numerous fields of study. As peptide, peptoid, and other peptidomimetic studies continue to advance, we will expect to see higher throughput structural analyses, greater computational accuracy and functionality, and wider application space that can improve human health, solve environmental challenges, and meet industrial needs. STATEMENT OF SIGNIFICANCE: Many historical, chemical, and functional relations draw a thread connecting peptides to their recent cognates, the "peptidomimetics". This review presents a comprehensive survey of this field by highlighting the width and relevance of these familial connections. In the first section, we examine the experimental and computational techniques originally developed for peptides and their morphing into a broader analytical and predictive toolbox. The second section presents an excursus of the structures and properties of prominent peptidomimetics, and how the expansion of the chemical and structural diversity has returned new exciting properties. The third section presents an overview of technological applications and new families of peptidomimetics. As the field grows, new compounds emerge with clear potential in medicine and advanced manufacturing.

Recent advances in the development of anti-infective peptoids

This feature article highlights the progress that has been made towards the development of novel anti-infective peptoids and the key areas for future development within this field.

Toward a clinical antifungal peptoid: Investigations into the therapeutic potential of AEC5

Cryptococcus neoformans is a fungal pathogen that causes cryptococcal meningitis in immunocompromised individuals. Existing antifungal treatment plans have high mammalian toxicity and increasing drug resistance, demonstrating the dire need for new, nontoxic therapeutics. Antimicrobial peptoids are one alternative to combat this issue. Our lab has recently identified a tripeptoid, AEC5, with promising efficacy and selectivity against C. neoformans. Here, we report studies into the broad-spectrum efficacy, killing kinetics, mechanism of action, in vivo half-life, and subchronic toxicity of this compound. Most notably, these studies have demonstrated that AEC5 rapidly reduces fungal burden, killing all viable fungi within 3 hours. Additionally, AEC5 has an in vivo half-life of 20+ hours and no observable in vivo toxicity following 28 days of daily injections. This research represents an important step in the characterization of AEC5 as a practical treatment option against C. neoformans infections.

Peptoids as tools and sensors

A review of molecular tools and sensors assembled on N-substituted glycine, or α-peptoid, oligomers between 2013 and November 2018 with the following sections: (a) Peptoids as crystal growth modifiers, (b) Peptoids as catalysts, (c) Ion and molecule sequestration and transport, (d) Peptoid sensors, (e) Macromolecule recognition, (f) Cellular transporters, (g) Medical imaging, (h) Future direction and (i) Summary and outlook. Peptoids are a promising class of peptide mimic making them an excellent platform for functional molecule preparation. Attributes of peptoid oligomers include: (a) the ease of precise sequence definition and mono-dispersity; (b) access to a vast chemical space within simple and repeating chemical preparative steps and (c) thermal, chemical and biological stability all lending support for their application in a number of areas, with some that have been realised to date. The peptoid tool and sensor examples selected have realised practical utility. They serve to illustrate the rapidity of new insight that can generate in many disparate areas of science and technology, enabling the quick assembly of design criteria for efficient peptoid molecular tools and sensors.

Helical side chain chemistry of a peptoid-based SP-C analogue: Balancing structural rigidity and biomimicry

Surfactant protein C (SP-C) is an important constituent of lung surfactant (LS) and, along with SP-B, is included in exogenous surfactant replacement therapies for treating respiratory distress syndrome (RDS). SP-C's biophysical activity depends upon the presence of a rigid C-terminal helix, of which the secondary structure is more crucial to functionality than precise side-chain chemistry. SP-C is highly sequence-conserved, suggesting that the β-branched, aliphatic side chains of the helix are also important. Nonnatural mimics of SP-C were created using a poly-N-substituted glycine, or "peptoid," backbone. The mimics included varying amounts of α-chiral, aliphatic side chains and α-chiral, aromatic side chains in the helical region, imparting either biomimicry or structural rigidity. Biophysical studies confirmed that the peptoids mimicked SP-C's secondary structure and replicated many of its surface-active characteristics. Surface activity was optimized by incorporating both structurally rigid and biomimetic side chain chemistries in the helical region indicating that both characteristics are important for activity. By balancing these features in one mimic, a novel analogue was created that emulates SP-C's in vitro surface activity while overcoming many of the challenges related to natural SP-C. Peptoid-based analogues hold great potential for use in a synthetic, biomimetic LS formulation for treating RDS.

Controlled synthesis of highly-branched plasmonic gold nanoparticles through peptoid engineering

In nature, specific biomolecules interacting with mineral precursors are responsible for the precise production of nanostructured inorganic materials that exhibit complex morphologies and superior performance. Despite advances in developing biomimetic approaches, the design rules for creating sequence-defined molecules that lead to the synthesis of inorganic nanomaterials with predictable complex morphologies are unknown. Herein we report the design of sequence-defined peptoids for controlled synthesis of highly branched plasmonic gold particles. By engineering peptoid sequences and investigating the resulting particle formation mechanisms, we develop a rule of thumb for designing peptoids that predictively enabled the morphological evolution from spherical to coral-shaped nanoparticles. Through a combination of hyperspectral UV-Vis extinction microscopy and three-photon photoemission electron microscopy, we demonstrate that the individual coral-shaped gold nanoparticles exhibit a plasmonic enhancement as high as 10⁵-fold. This research significantly advances our ultimate vision of predictive bio-inspired materials synthesis using sequence-defined synthetic molecules that mimic proteins and peptides.

Peptoids are promising crystallization agents, as they offer the molecular recognition capabilities of proteins and peptides but with higher stability and synthetic tunability. Here, the authors show that sequence-defined peptoids can controllably template the formation and shape evolution of gold nanostructures with defined morphologies.

Effective in vivo treatment of acute lung injury with helical, amphipathic peptoid mimics of pulmonary surfactant proteins

Acute lung injury (ALI) leads to progressive loss of breathing capacity and hypoxemia, as well as pulmonary surfactant dysfunction. ALI's pathogenesis and management are complex, and it is a significant cause of morbidity and mortality worldwide. Exogenous surfactant therapy, even for research purposes, is impractical for adults because of the high cost of current surfactant preparations. Prior in vitro work has shown that poly-N-substituted glycines (peptoids), in a biomimetic lipid mixture, emulate key biophysical activities of lung surfactant proteins B and C at the air-water interface. Here we report good in vivo efficacy of a peptoid-based surfactant, compared with extracted animal surfactant and a synthetic lipid formulation, in a rat model of lavage-induced ALI. Adult rats were subjected to whole-lung lavage followed by administration of surfactant formulations and monitoring of outcomes. Treatment with a surfactant protein C mimic formulation improved blood oxygenation, blood pH, shunt fraction, and peak inspiratory pressure to a greater degree than surfactant protein B mimic or combined formulations. All peptoid-enhanced treatment groups showed improved outcomes compared to synthetic lipids alone, and some formulations improved outcomes to a similar extent as animal-derived surfactant. Robust biophysical mimics of natural surfactant proteins may enable new medical research in ALI treatment.

In Vitro ADME Properties of Two Novel Antimicrobial Peptoid-Based Compounds as Potential Agents against Canine Pyoderma

Antimicrobial peptides (AMPs) hold promise as the next generation of antimicrobial agents, but often suffer from rapid degradation in vivo. Modifying AMPs with non-proteinogenic residues such as peptoids (oligomers of N-alkylglycines) provides the potential to improve stability. We have identified two novel peptoid-based compounds, B1 and D2, which are effective against the canine skin pathogen Staphylococcus pseudintermedius, the main cause of antibiotic use in companion animals. We report on their potential to treat infections topically by characterizing their release from formulation and in vitro ADME properties. In vitro ADME assays included skin penetration profiles, stability to proteases and liver microsomes, and plasma protein binding. Both B1 and D2 were resistant to proteases and >98% bound to plasma proteins. While half-lives in liver microsomes for both were >2 h, peptoid D2 showed higher stability to plasma proteases than the peptide-peptoid hybrid B1 (>2 versus 0.5 h). Both compounds were suitable for administration in an oil-in-water cream formulation (50% release in 8 h), and displayed no skin permeation, in the absence or presence of skin permeability modifiers. Our results indicate that these peptoid-based drugs may be suitable as antimicrobials for local treatment of canine superficial pyoderma and that they can overcome the inherent limitations of stability encountered in peptides.

Linear peptidomimetics as potent antagonists of Staphylococcus aureus agr quorum sensing

Staphylococcus aureus is an important pathogen causing infections in humans and animals. Increasing problems with antimicrobial resistance has prompted the development of alternative treatment strategies, including antivirulence approaches targeting virulence regulation such as the agr quorum sensing system. agr is naturally induced by cyclic auto-inducing peptides (AIPs) binding to the AgrC receptor and cyclic peptide inhibitors have been identified competing with AIP binding to AgrC. Here, we disclose that small, linear peptidomimetics can act as specific and potent inhibitors of the S. aureus agr system via intercepting AIP-AgrC signal interaction at low micromolar concentrations. The corresponding linear peptide did not have this ability. This is the first report of a linear peptide-like molecule that interferes with agr activation by competitive binding to AgrC. Prospectively, these peptidomimetics may be valuable starting scaffolds for the development of new inhibitors of staphylococcal quorum sensing and virulence gene expression.

Ugi multicomponent reaction to prepare peptide–peptoid hybrid structures with diverse chemical functionalities

Sequence defined peptide–peptoid hybrids create new opportunities for self-assembled nano-structures.

Using biomimetic polymers in place of noncollagenous proteins to achieve functional remineralization of dentin tissues

In calcified tissues such as bones and teeth, mineralization is regulated by an extracellular matrix, which includes non-collagenous proteins (NCP). This natural process has been adapted or mimicked to restore tissues following physical damage or demineralization by using polyanionic acids in place of NCPs, but the remineralized tissues fail to fully recover their mechanical properties. Here we show that pre-treatment with certain amphiphilic peptoids, a class of peptide-like polymers consisting of N-substituted glycines that have defined monomer sequences, enhances ordering and mineralization of collagen and induces functional remineralization of dentin lesions in vitro. In the vicinity of dentin tubules, the newly formed apatite nano-crystals are co-aligned with the c-axis parallel to the tubular periphery and recovery of tissue ultrastructure is accompanied by development of high mechanical strength. The observed effects are highly sequence-dependent with alternating polar and non-polar groups leading to positive outcomes while diblock sequences have no effect. The observations suggest aromatic groups interact with the collagen while the hydrophilic side chains bind the mineralizing constituents and highlight the potential of synthetic sequence-defined biomimetic polymers to serve as NCP mimics in tissue remineralization.

Advances in Development of Antimicrobial Peptidomimetics as Potential Drugs

The rapid emergence of multidrug-resistant pathogens has evolved into a global health problem as current treatment options are failing for infections caused by pan-resistant bacteria. Hence, novel antibiotics are in high demand, and for this reason antimicrobial peptides (AMPs) have attracted considerable interest, since they often show broad-spectrum activity, fast killing and high cell selectivity. However, the therapeutic potential of natural AMPs is limited by their short plasma half-life. Antimicrobial peptidomimetics mimic the structure and biological activity of AMPs, but display extended stability in the presence of biological matrices. In the present review, focus is on the developments reported in the last decade with respect to their design, synthesis, antimicrobial activity, cytotoxic side effects as well as their potential applications as anti-infective agents. Specifically, only peptidomimetics with a modular structure of residues connected via amide linkages will be discussed. These comprise the classes of α-peptoids (N-alkylated glycine oligomers), β-peptoids (N-alkylated β-alanine oligomers), β³-peptides, α/β³-peptides, α-peptide/β-peptoid hybrids, α/γ N-acylated N-aminoethylpeptides (AApeptides), and oligoacyllysines (OAKs). Such peptidomimetics are of particular interest due to their potent antimicrobial activity, versatile design, and convenient optimization via assembly by standard solid-phase procedures.

Foldamers in Medicinal Chemistry

Bio-inspired synthetic backbones leading to foldamers can provide effective biopolymer mimics with new and improved properties in a physiological environment, and in turn could serve as useful tools to study biology and lead to practical applications in the areas of diagnostics or therapeutics. Remarkable progress has been accomplished over the past 20 years with the discovery of many potent bioactive foldamers originating from diverse backbones and targeting a whole spectrum of bio(macro)molecules such as membranes, protein surfaces, and nucleic acids. These current achievements, future opportunities, and key challenges that remain are discussed in this article.

Cyclization Improves Membrane Permeation by Antimicrobial Peptoids

The peptidomimetic approach has emerged as a powerful tool for overcoming the inherent limitations of natural antimicrobial peptides, where the therapeutic potential can be improved by increasing the selectivity and bioavailability. Restraining the conformational flexibility of a molecule may reduce the entropy loss upon its binding to the membrane. Experimental findings demonstrate that the cyclization of linear antimicrobial peptoids increases their bactericidal activity against Staphylococcus aureus while maintaining high hemolytic concentrations. Surface X-ray scattering shows that macrocyclic peptoids intercalate into Langmuir monolayers of anionic lipids with greater efficacy than for their linear analogues. It is suggested that cyclization may increase peptoid activity by allowing the macrocycle to better penetrate the bacterial cell membrane.

Peptoid Efficacy against Polymicrobial Biofilms Determined by Using Propidium Monoazide-Modified Quantitative PCR

Biofilms containing Candida albicans are responsible for a wide variety of clinical infections. The protective effects of the biofilm matrix, the low metabolic activity of microorganisms within a biofilm and their high mutation rate, significantly enhance the resistance of biofilms to conventional antimicrobial treatments. Peptoids are peptide‐mimics that share many features of host defence antimicrobial peptides but have increased resistance to proteases and therefore have better stability in vivo. The activity of a library of peptoids was tested against monospecies and polymicrobial bacterial/fungal biofilms. Selected peptoids showed significant bactericidal and fungicidal activity against the polymicrobial biofilms. This coupled with low cytotoxicity suggests that peptoids could offer a new option for the treatment of clinically relevant polymicrobial infections.

Melanin nanoparticles as an endogenous agent for efficient iron overload therapy

Melanin was an efficient endogenous agent for iron-overload therapy in living mice due to its native biocompatibility, biodegradability and high iron-capturing volume.

Rapid Hepatobiliary Excretion of Micelle-Encapsulated/Radiolabeled Upconverting Nanoparticles as an Integrated Form

In the field of nanomedicine, long term accumulation of nanoparticles (NPs) in the mononuclear phagocyte system (MPS) such as liver is the major hurdle in clinical translation. On the other hand, NPs could be excreted via hepatobiliary excretion pathway without overt tissue toxicity. Therefore, it is critical to develop NPs that show favorable excretion property. Herein, we demonstrated that micelle encapsulated ⁶⁴Cu-labeled upconverting nanoparticles (micelle encapsulated ⁶⁴Cu-NOTA-UCNPs) showed substantial hepatobiliary excretion by in vivo positron emission tomography (PET) and also upconversion luminescence imaging (ULI). Ex vivo biodistribution study reinforced the imaging results by showing clearance of 84% of initial hepatic uptake in 72 hours. Hepatobiliary excretion of the UCNPs was also verified by transmission electron microscopy (TEM) examination. Micelle encapsulated ⁶⁴Cu-NOTA-UCNPs could be an optimal bimodal imaging agent owing to quantifiability of ⁶⁴Cu, ability of in vivo/ex vivo ULI and good hepatobiliary excretion property.

An overview of peptide and peptoid foldamers in medicinal chemistry

INTRODUCTION

Foldamers are artificial self-organizing systems with various critical properties: i) a stable and designable secondary structure; ii) a larger molecular surface as compared with ordinary organic drug molecules; iii) appropriate control of the orientation of the side-chain functional groups; iv) resistance against proteolytic degradation, which leads to potentially increased oral bioavailability and a longer serum half-life relative to ordinary α-peptides; and v) the lower conformational freedom may result in increased receptor binding in comparison with the natural analogs.

AREAS COVERED

This article covers the general properties and types of foldamers. This includes highlighted examples of medicinal chemical applications, including antibacterial and cargo molecules, anti-Alzheimer compounds and protein-protein interaction modifiers.

EXPERT OPINION

Various new foldamers have been created with a range of structures and biological applications. Membrane-acting antibacterial foldamers have been introduced. A general property of these structures is their amphiphilic nature. The amphiphilicity can be stationary or induced by the membrane binding. Cell-penetrating foldamers have been described which serve as cargo molecules, and foldamers have been used as autophagy inducers. Anti-Alzheimer compounds too have been created and the greatest breakthrough was attained via the modification of protein-protein interactions. This can serve as the chemical and pharmaceutical basis for the relevance of foldamers in the future.

Development of Peptoid-Based Ligands for the Removal of Cadmium from Biological Media

To address the lack of current therapeutic strategies for cadmium poisoning, peptoid-based ligands are identified using combinatorial chemistry that can selectively coordinate cadmium in a complex biological sample matrix.

Cadmium poisoning poses a serious health concern due to cadmium's increasing industrial use, yet there is currently no recommended treatment. The selective coordination of cadmium in a biological environment—i.e. in the presence of serum ions, small molecules, and proteins—is a difficult task. To address this challenge, a combinatorial library of peptoid-based ligands has been evaluated to identify structures that selectively bind to cadmium in human serum with minimal chelation of essential metal ions. Eighteen unique ligands were identified in this screening procedure, and the binding affinity of each was measured using metal titrations monitored by UV-vis spectroscopy. To evaluate the significance of each chelating moiety, sequence rearrangements and substitutions were examined. Analysis of a metal–ligand complex by NMR spectroscopy highlighted the importance of particular residues. Depletion experiments were performed in serum mimetics and human serum with exogenously added cadmium. These depletion experiments were used to compare and demonstrate the ability of these peptoids to remove cadmium from blood-like mixtures. In one of these depletion experiments, the peptoid sequence was able to deplete the cadmium to a level comparable to the reported acute toxicity limit. Evaluation of the metal selectivity in buffered solution and in human serum was performed to verify minimal off-target binding. These studies highlight a screening platform for the identification of metal–ligands that are capable of binding in a complex environment. They additionally demonstrate the potential utility of biologically-compatible ligands for the treatment of heavy metal poisoning.

Transferring biomarker into molecular probe: melanin nanoparticle as a naturally active platform for multimodality imaging

Developing multifunctional and easily prepared nanoplatforms with integrated different modalities is highly challenging for molecular imaging. Here, we report the successful transfer of an important molecular target, melanin, into a novel multimodality imaging nanoplatform. Melanin is abundantly expressed in melanotic melanomas and thus has been actively studied as a target for melanoma imaging. In our work, the multifunctional biopolymer nanoplatform based on ultrasmall (<10 nm) water-soluble melanin nanoparticle (MNP) was developed and showed unique photoacoustic property and natural binding ability with metal ions (for example, (64)Cu(2+), Fe(3+)). Therefore, MNP can serve not only as a photoacoustic contrast agent, but also as a nanoplatform for positron emission tomography (PET) and magnetic resonance imaging (MRI). Traditional passive nanoplatforms require complicated and time-consuming processes for prebuilding reporting moieties or chemical modifications using active groups to integrate different contrast properties into one entity. In comparison, utilizing functional biomarker melanin can greatly simplify the building process. We further conjugated αvβ3 integrins, cyclic c(RGDfC) peptide, to MNPs to allow for U87MG tumor accumulation due to its targeting property combined with the enhanced permeability and retention (EPR) effect. The multimodal properties of MNPs demonstrate the high potential of endogenous materials with multifunctions as nanoplatforms for molecular theranostics and clinical translation.

Cell-penetrating peptoids: introduction of novel cationic side chains

During the last decade peptoid-based molecular transporters have been broadly applied. They are highly valued for their easy synthesis and their superior stability against enzymatic degradation. The special structure of peptoids generally allows introducing a variety of different side chains. Yet, the cationic side chains of cell-penetrating peptoids displayed solely lysine- or arginine-like structures. Thus this report is intended to extend the spectrum of cationic peptoid side chains. Herein, we present novel functional groups, like polyamines, aza-crown ethers, or triphenylphosphonium ions that are introduced into peptoids for the first time. In addition, the obtained peptoids were tested for their cell-penetrating properties.

Solvent effects on the conformational preferences of model peptoids. MP2 study

The influence of aqueous environment on the main-chain conformation (ω0 , ϕ, and ψ dihedral angles) of two model peptoids: N-acetyl-N-methylglycine N'-methylamide (Ac-N(Me)-Gly-NHMe) (1) and N-acetyl-N-methylglycine N',N'-dimethylamide (Ac-N(Me)-Gly-NMe₂) (2) was investigated by MP2/6-311++G(d,p) method. The Ramachandran maps of both studied molecules with cis and trans configuration of the N-terminal amide bond in the gas phase and in water environment were obtained and all energy minima localized. The polarizable continuum model was applied to estimate the solvation effect on conformation. Energy minima of the Ac-N(Me)-Gly-NHMe and Ac-N(Me)-Gly-NMe₂ have been analyzed in terms of the possible hydrogen bonds and C = O dipole attraction. To validate the theoretical results obtained, conformations of the similar structures gathered in the Cambridge Crystallographic Data Centre were analyzed. Obtained results indicate that aqueous environment in model peptoids 1 and 2 favors the conformation F (ϕ and ψ = -70º, 180º), and additionally significantly increases the percentage of structures with cis configuration of N-terminal amide bond in studied compounds.

Recent development of small antimicrobial peptidomimetics

Antimicrobial peptides (AMPs) hold promise to circumvent the emergence of drug resistance occurring in the treatment of bacteria using many conventional antibiotics. Antimicrobial peptidomimetics, which mimic bactericidal mechanisms of AMPs, may overcome the disadvantages of AMPs and become the new generation of antibiotic therapeutics. In this review, some recent examples in the development of antimicrobial peptidomimetics are highlighted. The potential of antimicrobial agents has been demonstrated for therapeutic uses. Meanwhile, perspectives on their further development and applications are also presented.

Peptoids and polyamines going sweet: Modular synthesis of glycosylated peptoids and polyamines using click chemistry

Sugar moieties are present in a wide range of bioactive molecules. Thus, having versatile and fast methods for the decoration of biomimetic molecules with sugars is of fundamental importance. The glycosylation of peptoids and polyamines as examples of such biomimetic molecules is reported here. The method uses Cu-catalyzed azide alkyne cycloaddition to promote the reaction of azidosugars with either polyamines or peptoids. In addition, functionalized nucleic acids were attached to polyamines via the same route. Based on a modular solid-phase synthesis of peralkynylated peptoids with up to six alkyne groups, the latter were modified with azidosugar building blocks by using copper-catalyzed azide alkyne cycloadditions. In addition, the up-scaling of some particular azide-modified sugars is described.

Cell Penetrating Peptoids (CPPos): Synthesis of a Small Combinatorial Library by Using IRORI MiniKans

Cell penetrating peptoids (CPPos) are potent mimics of the corresponding cell penetrating peptides (CPPs). The synthesis of diverse oligomeric libraries that display a variety of backbone scaffolds and side-chain appendages are a very promising source of novel CPPos, which can be used to either target different cellular organelles or even different tissues and organs. In this study we established the submonomer-based solid phase synthesis of a “proof of principle” peptoid library in IRORI MiniKans to expand the amount for phenotypic high throughput screens of CPPos. The library consisting of tetrameric peptoids [oligo(N-alkylglycines)] was established on Rink amide resin in a split and mix approach with hydrophilic and hydrophobic peptoid side chains. All CPPos of the presented library were labeled with rhodamine B to allow for the monitoring of cellular uptake by fluorescent confocal microscopy. Eventually, all the purified peptoids were subjected to live cell imaging to screen for CPPos with organelle specificity. While highly charged CPPos enter the cells by endocytosis with subsequent endosomal release, critical levels of lipophilicity allow other CPPos to specifically localize to mitochondria once a certain lipophilicity threshold is reached.