Recent Advances in Amphipathic Peptidomimetics as Antimicrobial Agents to Combat Drug Resistance.

The development of antimicrobial drugs with novel structures and clear mechanisms of action that are active against drug-resistant bacteria has become an urgent need of safeguarding human health due to the rise of bacterial drug resistance. The discovery of AMPs and the development of amphipathic peptidomimetics have lay the foundation for novel antimicrobial agents to combat drug resistance due to their overall strong antimicrobial activities and unique membrane-active mechanisms. To break the limitation of AMPs, researchers have invested in great endeavors through various approaches in the past years. This review summarized the recent advances including the development of antibacterial small molecule peptidomimetics and peptide-mimic cationic oligomers/polymers, as well as mechanism-of-action studies. As this exciting interdisciplinary field is continuously expanding and growing, we hope this review will benefit researchers in the rational design of novel antimicrobial peptidomimetics in the future.

Design and synthesis of broad-spectrum antimicrobial amphiphilic peptidomimetics to combat drug-resistance.

The gradual depletion of antibiotic discovery pipeline makes the antibiotic resistance a difficult clinical problem and a global health emergency. The membrane-active antimicrobial peptides (AMPs) attracted much attention due to a lower tendency to bacterial resistance than traditional antibiotics. However, some immanent drawbacks of AMPs may hamper their application in combating antibiotic resistance in the long run, such as susceptible to enzymatic degradation and low cell permeability. Herein, we report the design and synthesis of a novel series of amphiphilic peptidomimetics, from which we identified compounds that exhibited potent antimicrobial activity against a panel of clinically relevant Gram-positive and Gram-negative bacteria strains. The most potent compound 20 (SD-110-12) is able to kill intracellular bacterial pathogens and prevent the development of bacterial resistance under the tested conditions by targeting cell membranes. Additionally, compound 20 (SD-110-12) obtains good in vivo efficacy that is comparative to vancomycin by eradicating MRSA and suppressing inflammation in a mice infected skin wound model, demonstrating its promising therapeutic potential.

Facilely accessible quinoline derivatives as potent antibacterial agents.

Quinoline compounds have been extensively explored as anti-malaria and anti-cancer agents for decades and show profound functional bioactivities, however, the studies of these compounds in other medicinal fields have lagged dramatically. In this study, we report the development of a series of facilely accessible quinoline derivatives that display potent antibacterial activity against a panel of multidrug-resistant Gram-positive bacterial strains, especially C. difficile. We also demonstrated that these molecules are effective in vivo against C. difficile. These results revealed that these types of quinoline compounds could serve as prototypes for the development of an appealing class of antibiotic agents used to combat Gram-positive drug-resistant bacterial strains, including C. difficile.

Nano-Sized Lipidated Dendrimers as Potent and Broad-Spectrum Antibacterial Agents.

There is considerable interest in the development of antimicrobial polymers including dendrimers due to the ease of synthesis and low manufacturing cost compared to host defense peptides (HDPs). Herein, a new class of nanomaterials-lipidated amphiphilic dendrimers-is presented that mimic the antibacterial mechanism of HDPs by compromising bacterial cell membranes. Unlike conventional dendrimers that are prepared generation by generation symmetrically with molecular weight distribution, these lipidated dendrimers are prepared on the solid phase with a hanging lipid tail and precisely controlled structure. It is shown through rational design that these lipidated dendrimers display potent and selective antimicrobial activity against both Gram-positive and Gram-negative bacteria, including multidrug-resistant strains. In addition to antibacterial activity against planktonic bacteria, these dendrimers are also shown to inhibit bacterial biofilms effectively. This class of dendrimers as a new class of biomaterials may lead to a useful generation of antibiotic agents with practical applications.

Structure and Function of AApeptides.

The intrinsic drawbacks encountered in bioactive peptides in chemical biology and biomedical sciences have diverted research efforts to the development of sequence-specific peptidomimetics that are capable of mimicking the structure and function of peptides and proteins. Modifications in the backbone and/or the side chain of peptides have been explored to develop biomimetic molecular probes or drug leads for biologically important targets. To expand the family of oligomeric peptidomimetics to facilitate their further application, we recently developed a new class of peptidomimetics, AApeptides based on a chiral peptide nucleic acid backbone. AApeptides are resistant to proteolytic degradation and amenable to enormous chemical diversification. Moreover, they could mimic the primary structure of peptides and also fold into discrete secondary structure such as helices and turn-like structures. Furthermore, they have started to show promise in applications in material and biomedical sciences. Herein, we highlight the structural design and some function of AApeptides and present our perspective on their future development.

Polycarbonates with Potent and Selective Antimicrobial Activity toward Gram-Positive Bacteria.

The resistance developed by life-threatening bacteria toward conventional antibiotics has become a major concern in public health. To combat antibiotic resistance, there has been a significant interest in the development of antimicrobial cationic polymers due to the ease of synthesis and low manufacturing cost compared to host-defense peptides (HDPs). Herein, we report the design and synthesis of amphiphilic polycarbonates containing primary amino groups. These polymers exhibit potent antimicrobial activity and excellent selectivity to Gram-positive bacteria, including multidrug resistant pathogens. Fluorescence and TEM studies suggest that these polymers are likely to kill bacteria by disrupting bacterial membranes. These polymers also show low tendency to elicit resistance in bacteria. Their further development may lead to new antimicrobial agents combating drug-resistance.

Helical 1:1 α/Sulfono-γ-AA Heterogeneous Peptides with Antibacterial Activity.

As one of the greatest threats facing the 21st century, antibiotic resistance is now a major public health concern. Host-defense peptides (HDPs) offer an alternative approach to combat emerging multi-drug-resistant bacteria. It is known that helical HDPs such as magainin 2 and its analogs adopt cationic amphipathic conformations upon interaction with bacterial membranes, leading to membrane disruption and subsequent bacterial cell death. We have previously shown that amphipathic sulfono-γ-AApeptides could mimic magainin 2 and exhibit bactericidal activity. In this article, we demonstrate for the first time that amphipathic helical 1:1 α/sulfono-γ-AA heterogeneous peptides, in which regular amino acids and sulfono-γ-AApeptide building blocks are alternatively present in a 1:1 pattern, display potent antibacterial activity against both Gram-positive and Gram-negative bacterial pathogens. Small angle X-ray scattering (SAXS) suggests that the lead sequences adopt defined helical structures. The subsequent studies including fluorescence microscopy and time-kill experiments indicate that these hybrid peptides exert antimicrobial activity by mimicking the mechanism of HDPs. Our findings may lead to the development of HDP-mimicking antimicrobial peptidomimetics that combat drug-resistant bacterial pathogens. In addition, our results also demonstrate the effective design of a new class of helical foldamer, which could be employed to interrogate other important biological targets such as protein-protein interactions in the future.

Role of mouse and human autophagy proteins in IFN-γ-induced cell-autonomous responses against Toxoplasma gondii.

IFN-γ mediates cellular innate immunity against an intracellular parasite, Toxoplasma gondii, by inducing immunity-related GTPases such as p47 IFN-γ-regulated GTPases (IRGs) and p65 guanylate-binding proteins (GBPs), which also participate in antibacterial responses via autophagy. An essential autophagy protein, Atg5, was previously shown to play a critical role in anti-T. gondii cell-autonomous immunity. However, the involvement of other autophagy proteins remains unknown. In this study, we show that essential autophagy proteins differentially participate in anti-T. gondii cellular immunity by recruiting IFN-γ-inducible GTPases. IFN-γ-induced suppression of T. gondii proliferation and recruitment of an IRG Irgb6 and GBPs are profoundly impaired in Atg7- or Atg16L1-deficient cells. In contrast, cells lacking other essential autophagy proteins, Atg9a and Atg14, are capable of mediating the anti-T. gondii response and recruiting Irgb6 and GBPs to the parasites. Although IFN-γ also stimulates anti-T. gondii cellular immunity in humans, whether this response requires GBPs and human autophagy proteins remains to be seen. To analyze the role of human ATG16L1 and GBPs in IFN-γ-mediated anti-T. gondii responses, human cells lacking ATG16L1 or GBPs were generated by the Cas9/CRISPR genome-editing technique. Although both ATG16L1 and GBPs are dispensable for IFN-γ-induced inhibition of T. gondii proliferation in the human cells, human ATG16L1 is also required for the recruitment of GBPs. Taken together, human ATG16L1 and mouse autophagy components Atg7 and Atg16L1, but not Atg9a and Atg14, participate in the IFN-γ-induced recruitment of the immunity-related GTPases to the intracellular pathogen.

Chemical Strategies Toward Prodrugs and Fluorescent Probes for Gasotransmitters.

Three gaseous molecules are widely accepted as important gasotransmitters in mammalian cells, namely NO, CO and H2S. Due to the pharmacological effects observed in preclinical studies, these three gasotransmitters represent promising drug candidates for clinical translation. Fluorescent probes of the gasotransmitters are also in high demand; however, the mechanisms of actions or the roles played by gasotransmitters under both physiological and pathological conditions remain to be answered. In order to bring these challenges to the attention of both chemists and biologists working in this field, we herein summarize the chemical strategies used for the design of both probes and prodrugs of these three gasotransmitters.

Hydantoin derivative dimers as broad-spectrum antimicrobial agents against ESKAPE pathogens with enhanced killing rate and stability.

A new series of hydantoin derivative dimers as potential broad-spectrum antibiotic agents is designed and synthesized to combat ESKAPE pathogens. As membrane-active antimicrobial agents, in addition to cationic charged and hydrophobic groups that mimic AMPs (antimicrobial peptides), hydantoin backbones and aromatic linkers increased the rigidity and lipophilicity of the designed compounds, thus improving the stability and bactericidal killing rate. After whole cell phenotypic screening against eight bacterial strains, including MRSA (methicillin-resistant S. aureus), compound 18 was chosen as the lead compound with overall excellent broad-spectrum antibacterial activity (GM = 7.32 µg mL-1) and good selectivity. Kill-kinetic studies of compound 18 showed that the bacterial growth of both Gram-positive and Gram-negative was completely inhibited within one hour, which demonstrated excellent sterilization efficiency of 18. Furthermore, drug resistance and mechanism studies showed that compound 18 exhibited a steady antibacterial performance during 25 passages and could disrupt bacterial cell membrane integrity and cause cell death. Along with the facile synthesis procedures in solution, this series of hydantoin derivative dimer compounds could be an appealing next generation of antibiotic agents to combat emergent drug resistance.

Copper-Catalyzed, N-Directed Distal C(sp3)-H Functionalization toward Azepanes.

We herein report a copper-catalyzed formal [5 + 2] aza-annulation of N-fluorosulfonamides and 1,3-dienes/1,3-enynes for synthesis of structurally diverse alkene/alkyne-containing azepanes. The reaction features selective functionalization of distal unactivated C(sp3)-H bonds and a broad substrate scope, thus allowing the late-stage modification of pharmaceuticals and natural products. A radical mechanism involving 1,5-hydrogen atom transfer of N-radicals, facile coupling of alkyl radicals with 1,3-dienes/1,3-enynes, and the construction of azepane motifs via C-N bond formation is proposed.

An update on the emerging approaches for histone deacetylase (HDAC) inhibitor drug discovery and future perspectives.

INTRODUCTION: HDACs catalyze the removal of acetyl groups from the ε-N-acetylated lysine residues of various protein substrates including both histone and nonhistone proteins. Different HDACs have distinct biological functions and are recruited to specific regions of the genome. HDAC inhibitors have attracted much attention in recent decades; indeed, there have been more than thirty HDAC inhibitors investigated in clinic trials with five approvals being achieved. AREAS COVERED: This review covers the emerging approaches for HDAC inhibitor drug discovery from the past five years and includes discussion of structure-based rational design, isoform selectivity, and dual mechanism/multi-targeting. Chemical structures in addition to the in vitro and in vivo inhibiting activity of these compounds have also been discussed. EXPERT OPINION: The exact role and biological functions of HDACs is still under investigation with a variety of HDAC inhibitors having been designed and evaluated. HDAC inhibitors have shown promise in treating cancer, AD, metabolic disease, viral infection, and multiple sclerosis, but there is still a lot of room for clinical improvement. In the future, more efforts should be put into (i) HDAC isoform identification (ii) the optimization of selectivity, activity, and pharmacokinetics; and (iii) unconventional approaches for discovering different effective scaffolds and pharmacophores.

Recent Progress on the Discovery of NLRP3 Inhibitors and their Therapeutic Potential.

BACKGROUND: Inflammation is the body's immune system's fast coordinating response to irritants caused by pathogens, external injuries, and chemical or radiation effects. The nucleotidebinding oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome is a critical component of the innate immune system. The dysfunction of NLRP3 inflammasome contributes to various pathogeneses of complex diseases, such as uncontrolled infection, autoimmune diseases, neurodegenerative diseases, and metabolic disorders. This review describes recent progress on the discovery of NLRP3 inflammasome inhibitors and their therapeutic potential. METHODS: Based on the mechanism of NLRP3 activation, several types of NLRP3 inhibitors are described and summarized according to their origins, structures, bioactivity, and mechanism of action. Structure-Activity Relationship (SAR) is also listed for different scaffolds, as well as effective pharmacophore. RESULTS: Over one-hundred papers were included in the review. The development of NLRP3 inhibitors has been described from the earliest glyburide in 2001 to the latest progress in 2019. Several series of inhibitors have been categorized, such as JC-series based on glyburide and BC-series based on 2APB. Many other small molecules such as NLRP3 inhibitors are also listed. SAR, application in related therapeutic models, and five different action mechanisms are described. CONCLUSION: The findings of this review confirmed the importance of developing NLRP3 inflammasome inhibitors. Various NLRP3 inhibitors have been discovered as effective therapeutic treatments for multiple diseases, such as type II diabetes, experimental autoimmune encephalomyelitis, stressrelated mood disorders, etc. The development of a full range of NLRP3 inflammasome inhibitors is still at its foundational phase. We are looking forward to the identification of inhibitory agents that provide the most potent therapeutic strategies and efficiently treat NLRP3 inflammasome-related inflammatory diseases.

γ-AApeptides as a New Class of Peptidomimetics: Design, Synthesis, and Applications.

Peptidomimetics are studied for medicinal application because of their ability to mimic hierarchical structures of peptides and proteins. To break the limitation and expand the peptidomimetics family, a new class of peptidomimetics based on peptide nucleic acids (PNAs) backbone - "γ-AApeptides" was developed. Compared with previous peptidomimetics, γ-AApeptides possess prominent advantages such as resistance to proteolytic degradation, enhanced chemodiversity, good selectivity and outstanding bioactivity. The synthesis of γ-AApeptides is carried out using a ''monomer building block'' strategy which is facile and efficient. γ-AApeptides are able to mimic primary and secondary structures of therapeutic peptides, which make them promising candidates for molecular probes and potential drug leads. In the past decade, several interesting structures and applications of γ-AApeptides have been developed by different approaches such as structure-based design, combinatorial library screening, and peptides selfassembly and folding. By following the mechanism of host-defense peptides (HDPs), antibiotic γ- AApeptides showed broad-spectrum activity. At the same time, γ-AApeptides can be used for combinatorial library screening because of their structural stability and their chemodiversity. Anticancer agents, anti-T2DM (Type 2 diabetes mellitus) agents, anti-HIV (human immuno-deficiency virus) agents and anti-Alzheimer's disease agents were developed by combinatorial screening and rational design. Furthermore, γ-AApeptides as biopolymers, nanomaterials, supramolecular structures and self-assembly architectures were studied due to their unique backbone structures. Therefore, γ-AApeptides may play an important role in the development of peptidomimetics.

PSMA-targeted nanoparticles for specific penetration of blood-brain tumor barrier and combined therapy of brain metastases.

Breast cancer brain metastases (BCBM) represent a major cause of morbidity and mortality among patients with breast cancer. Systemic drug therapy, which is usually effective against peripheral breast cancers, is often ineffective on BCBM due to its poor penetration through the blood-brain tumor barrier (BTB). In this study, prostate-specific membrane antigen (PSMA) with internalization function was found to be specifically up-regulated on BCBM-associated BTB while barely detectable in normal blood-brain barrier (BBB). Here, a nanotechnology approach is reported that can overcome the BTB through ACUPA (A) and cyclic TT1 (cT) co-functionalized nanoparticles (A-NPs-cT). A-NPs-cT selectively target PSMA on BTB for specific BTB crossing and specially bind with p32 for BCBM targeting. We disclosed the effectual synergism of doxorubicin (DOX) and lapatinib (LAP) for BCBM combined therapy. A-NPs-cT exhibited boosted uptake than integrin-targeting RGD-modified NPs in BTB endothelial cells and displayed about 4.57-fold stronger penetration through the BCBM-associated BTB as compared to the normal BBB. In vivo studies showed specific BTB crossing, and remission of BCBM and prolonged survival with DOX and LAP combinatorial regimen. A-NPs-cT based DOX and LAP innovative combined therapy envisioned improved therapeutic intervention for clinical management of BCBM, for which surgery is generally inapplicable and insufficient.

Radical Aza-Cyclization of α-Imino-oxy Acids for Synthesis of Alkene-Containing N-Heterocycles via Dual Cobaloxime and Photoredox Catalysis.

Nitrogen-containing heterocycles are prevalent in both naturally and synthetically bioactive molecules. We report herein an unprecedented protocol for radical aza-cyclization of α-imino-oxy acids with pendant alkenes via synergistic photoredox and cobaloxime catalysis. With or without alkenes as the intermolecular cross-coupling partners, the transformation provides a variety of corresponding alkene-containing dihydropyrrole products in satisfactory yields. In the presence of external alkenes, the tandem reaction generates E-selective coupling products with excellent chemo- and stereoselectivity.

Polymyxin derivatives as broad-spectrum antibiotic agents.

We designed a few polymyxin derivatives which exhibit broad-spectrum antimicrobial activity. Lead compound P1 could disrupt bacterial membranes rapidly without developing resistance, inhibit biofilms formed by E. coli, and exhibit excellent in vivo activity in an MRSA-infected thigh burden mouse model.

The Activity of Small Urea-γ-AApeptides Toward Gram-Positive Bacteria.

Host Defense Peptides (HDPs) have gained considerable interest due to the omnipresent threat of bacterial infection as a serious public health concern. However, development of HDPs is impeded by several drawbacks, such as poor selectivity, susceptibility to proteolytic degradation, low-to-moderate activity and requiring complex syntheses. Herein we report a class of lipo-linear α/urea-γ-AApeptides with a hybrid backbone and low molecular weight. The heterogeneous backbone not only enhances chemodiversity, but also shows effective antimicrobial activity against Gram-positive bacteria and is capable of disrupting bacterial membranes and killing bacteria rapidly. Given their low molecular weight and ease of access via facile synthesis, they could be practical antibiotic agents.

Lipidated α/α-AA heterogeneous peptides as antimicrobial agents.

With an increase of resistance in bacteria there is an urgent need for alternative treatment methods that could complement conventional antibiotics. In the past two decades, focus has been drawn to Host Defense Peptides (HDPs) as potential antibiotic agents. Herein we reported our studies on the development of lipidated α/α-AA heterogeneous peptides as a new class of HDP mimetics. These compounds showed potent antimicrobial activity toward both Gram-positive and Gram-negative bacteria, and they also displayed excellent selectivity as they only exhibited limited hemolytic activity. The fluorescence microscopy suggested that the mechanism of action of these heterogeneous peptides is bacterial membrane disruption, which is believed to be the major reason why it is difficult for bacteria to develop resistance. The subsequent time kill studies suggested that these compounds could rapidly eradicate bacteria. Moreover, this class of compounds could also effectively clear biofilms formed by both Gram-positive and Gram-negative bacteria. These findings suggested that lipidated α/α-AA heterogeneous peptides, as a new class of peptidomimetics, are promising antibiotic agents combating antibiotic resistance.

Supramolecular Kandinsky circles with high antibacterial activity.

Nested concentric structures widely exist in nature and designed systems with circles, polygons, polyhedra, and spheres sharing the same center or axis. It still remains challenging to construct discrete nested architecture at (supra)molecular level. Herein, three generations (G2-G4) of giant nested supramolecules, or Kandinsky circles, have been designed and assembled with molecular weight 17,964, 27,713 and 38,352 Da, respectively. In the ligand preparation, consecutive condensation between precursors with primary amines and pyrylium salts is applied to modularize the synthesis. These discrete nested supramolecules are prone to assemble into tubular nanostructures through hierarchical self-assembly. Furthermore, nested supramolecules display high antimicrobial activity against Gram-positive pathogen methicillin-resistant Staphylococcus aureus (MRSA), and negligible toxicity to eukaryotic cells, while the corresponding ligands do not show potent antimicrobial activity.