Synthesis, Structure-Activity Relationship Study, Bioactivity, and Nephrotoxicity Evaluation of the Proposed Structure of the Cyclic Lipodepsipeptide Brevicidine B

The brevicidines represent a novel class of nonribosomal antimicrobial peptides that possess remarkable potency and selectivity toward highly problematic and resistant Gram-negative pathogenic bacteria. A recently discovered member of the brevicidine family, coined brevicidine B (2), comprises a single amino acid substitution (from d-Tyr2 to d-Phe2) in the amino acid sequence of the linear moiety of brevicidine (1) and was reported to exhibit broader antimicrobial activity against both Gram-negative (MIC = 2-4 μgmL-1) and Gram-positive (MIC = 2-8 μgmL-1) pathogens. Encouraged by this, we herein report the first total synthesis of the proposed structure of brevicidine B (2), building on our previously reported synthetic strategy to access brevicidine (1). In agreement with the original isolation paper, pleasingly, synthetic 2 demonstrated antimicrobial activity toward Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae (MIC = 4-8 μgmL-1). Interestingly, however, synthetic 2 was inactive toward all of the tested Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus strains. Substitution of d-Phe2 with its enantiomer, and other hydrophobic residues, yields analogues that were either inactive or only exhibited activity toward Gram-negative strains. The striking difference in the biological activity of our synthetic 2 compared to the reported natural compound warrants the re-evaluation of the original natural product for purity or possible differences in relative configuration. Finally, the evaluation of synthetic 1 and 2 in a human kidney organoid model of nephrotoxicity revealed substantial toxicity of both compounds, although 1 was less toxic than 2 and polymyxin B. These results indicate that modification to position 2 may afford a strategy to mitigate the nephrotoxicity of brevicidine.

Creating New Antifoulants Using the Tools and Tactics of Medicinal Chemistry

ConspectusThe unwanted accumulation of marine micro- and macroorganisms such as algae and barnacles on submerged man-made structures and vessel hulls is a major challenge for any marine operation. Known as biofouling, this problem leads to reduced hydrodynamic efficiency, significantly increased fuel usage, microbially induced corrosion, and, if not managed appropriately, eventual loss of both performance and structural integrity. Ship hull biofouling in the international maritime transport network conservatively accounts for 0.6% of global carbon emissions, highlighting the global scale and the importance of this problem. Improved antifouling strategies to limit surface colonization are paramount for essential activities such as shipping, aquaculture, desalination, and the marine renewable energy sector, representing both a multibillion dollar cost and a substantial practical challenge. From an ecological perspective, biofouling is a primary contributor to the global spread of invasive marine species, which has extensive implications for the marine environment.Historically, heavy metal-based toxic biocides have been used to control biofouling. However, their unwanted collateral ecological damage on nontarget species and bioaccumulation has led to recent global bans. With expanding human activities within aquaculture and offshore energy, it is both urgent and apparent that environmentally friendly surface protection remains key for maintaining the function of both moving and stationary marine structures. Biofouling communities are typically a highly complex network of both micro- and macroorganisms, representing a broad section of life from bacteria to macrophytes and animals. Given this diversity, it is unrealistic to expect that a single antifouling "silver bullet" will prevent colonization with the exception of generally toxic biocides. For that reason, modern and future antifouling solutions are anticipated to rely on novel coating technologies and "combination therapies" where mixtures of narrow-spectrum bioactive components are used to provide coverage across fouling species. In contrast to the existing cohort of outdated, toxic antifouling strategies, such as copper- and tributyltin-releasing paints, modern drug discovery techniques are increasingly being employed for the rational design of effective yet safe alternatives. The challenge for a medicinal chemistry approach is to effectively account for the large taxonomic diversity among fouling organisms combined with a lack of well-defined conserved molecular targets within most taxa.The current Account summarizes our work employing the tools of modern medicinal chemistry to discover, modify, and develop optimized and scalable antifouling solutions based on naturally occurring antifouling and repelling compounds from both marine and terrestrial sources. Inspiration for rational design comes from targeted studies on allelopathic natural products, natural repelling peptides, and secondary metabolites from sessile marine organisms with clean exteriors, which has yielded several efficient and promising antifouling leads.

Harnessing the power of a photoinitiated thiol-ene "click" reaction for the efficient synthesis of S-lipidated collagen model peptide amphiphiles

A photoinitiated thiol-ene "click" reaction was used to synthesize S-lipidated collagen model peptide amphiphiles. Use of 2-iminothiolane provided an epimerization-free thiol handle required for thiol-ene based incorporation of lipid moieties onto collagen-based peptide sequences. This approach not only led to improvements in the triple helical characteristics of the resulting collagen model peptides but also increased the aqueous solubility of the peptide amphiphiles. As a result, this methodology holds significant potential for the design and advancement of functional peptide amphiphiles, offering enhanced capabilities across a wide range of applications.

The Leptospermum scoparium (Mānuka)-Specific Nectar and Honey Compound 3,6,7-Trimethyllumazine (LepteridineTM) That Inhibits Matrix Metalloproteinase 9 (MMP-9) Activity

3,6,7-trimethyllumazine (Lepteridine™) is a newly discovered natural pteridine derivative unique to Mānuka (Leptospermum scoparium) nectar and honey, with no previously reported biological activity. Pteridine derivative-based medicines, such as methotrexate, are used to treat auto-immune and inflammatory diseases, and Mānuka honey reportedly possesses anti-inflammatory properties and is used topically as a wound dressing. MMP-9 is a potential candidate protein target as it is upregulated in recalcitrant wounds and intestinal inflammation. Using gelatin zymography, 40 μg/mL LepteridineTM inhibited the gelatinase activities of both pro- (22%, p < 0.0001) and activated (59%, p < 0.01) MMP-9 forms. By comparison, LepteridineTM exerted modest (~10%) inhibition against a chromogenic peptide substrate and no effect against a fluorogenic peptide substrate. These findings suggest that LepteridineTM may not interact within the catalytic domain of MMP-9 and exerts a negligible effect on the active site hydrolysis of small soluble peptide substrates. Instead, the findings implicate fibronectin II domain interactions by LepteridineTM which impair gelatinase activity, possibly through perturbed tethering of MMP-9 to the gelatin matrix. Molecular modelling analyses were equivocal over interactions at the S1' pocket versus the fibronectin II domain, while molecular dynamic calculations indicated rapid exchange kinetics. No significant degradation of synthetic or natural LepteridineTM in Mānuka honey occurred during simulated gastrointestinal digestion. MMP-9 regulates skin and gastrointestinal inflammatory responses and extracellular matrix remodelling. These results potentially implicate LepteridineTM bioactivity in Mānuka honey's reported beneficial effects on wound healing via topical application and anti-inflammatory actions in gastrointestinal disorder models via oral consumption.

Crystal Structure of Inhibitor-Bound GII.4 Sydney 2012 Norovirus 3C-Like Protease

Norovirus is the leading cause of viral gastroenteritis worldwide, and there are no approved vaccines or therapeutic treatments for chronic or severe norovirus infections. The structural characterisation of the norovirus protease and drug development has predominantly focused upon GI.1 noroviruses, despite most global outbreaks being caused by GII.4 noroviruses. Here, we determined the crystal structures of the GII.4 Sydney 2012 ligand-free norovirus protease at 2.79 Å and at 1.83 Å with a covalently bound high-affinity (IC50 = 0.37 µM) protease inhibitor (NV-004). We show that the active sites of the ligand-free protease structure are present in both open and closed conformations, as determined by their Arg112 side chain orientation. A comparative analysis of the ligand-free and ligand-bound protease structures reveals significant structural differences in the active site cleft and substrate-binding pockets when an inhibitor is covalently bound. We also report a second molecule of NV-004 non-covalently bound within the S4 substrate binding pocket via hydrophobic contacts and a water-mediated hydrogen bond. These new insights can guide structure-aided drug design against the GII.4 genogroup of noroviruses.

NMR Shows Why a Small Chemical Change Almost Abolishes the Antimicrobial Activity of Glycocin F

Glycocin F (GccF), a ribosomally synthesized, post-translationally modified peptide secreted by Lactobacillus plantarum KW30, rapidly inhibits the growth of susceptible bacteria at nanomolar concentrations. Previous studies have highlighted structural features important for its activity and have shown the absolute requirement for the Ser18 O-linked GlcNAc on the eight-residue loop linking the two short helices of the (C-X6-C)2 structure. Here, we show that an ostensibly very small chemical modification to Ser18, the substitution of the Cα proton with a methyl group, reduces the antimicrobial activity of GccF 1000-fold (IC50 1.5 μM cf. 1.5 nM). A comparison of the GccFα-methylSer18 NMR structure (PDB 8DFZ) with that of the native protein (PDB 2KUY) showed a marked difference in the orientation and mobility of the loop, as well as a markedly different positioning of the GlcNAc, suggesting that loop conformation, dynamics, and glycan presentation play an important role in the interaction of GccF with as yet unknown but essential physiological target molecules.

On-Resin Conjugation of the Ruthenium Anticancer Agent Plecstatin-1 to Peptide Vectors

Ruthenium piano-stool complexes have been explored for their anticancer activity and some promising compounds have been reported. Herein, we conjugated a derivative of plecstatin-1 to peptides in order to increase their cancer cell targeting ability. For this purpose, plecstatin-1 was modified at the arene ligand to introduce a functional amine handle (3), which resulted in a compound that showed similar activity in an in vitro anticancer activity assay. The cell-penetrating peptide TAT48-60, tumor-targeting neurotensin8-13, and plectin-targeting peptide were functionalized with succinyl or β-Ala-succinyl linkers under standard solid-phase peptide synthesis (SPPS) conditions to spatially separate the peptide backbones from the bioactive metal complexes. These modifications allowed for conjugating precursor 3 to the peptides on resin yielding the desired metal-peptide conjugates (MPCs), as confirmed by high-performance liquid chromatography (HPLC), NMR spectroscopy, and mass spectrometry (MS). The MPCs were studied for their behavior in aqueous solution and under acidic conditions and resembled that of the parent compound plecstatin-1. In in vitro anticancer activity studies in a small panel of cancer cell lines, the TAT-based MPCs showed the highest activity, while the other MPCs were virtually inactive. However, the MPCs were significantly less active than the small molecules plecstatin-1 and 3, which can be explained by the reduced cell uptake as determined by inductively coupled plasma MS (ICP-MS). Although the MPCs did not display potent anticancer activities, the developed conjugation strategy can be extended toward other metal complexes, which may be able to utilize the targeting properties of peptides.

P1 Glutamine isosteres in the design of inhibitors of 3C/3CL protease of human viruses of the Pisoniviricetes class

Viral infections are one of the leading causes of acute morbidity in humans and much endeavour has been made by the synthetic community for the development of drugs to treat associated diseases. Peptide-based enzyme inhibitors, usually short sequences of three or four residues, are one of the classes of compounds currently under development for enhancement of their activity and pharmaceutical properties. This review reports the advances made in the design of inhibitors targeting the family of highly conserved viral proteases 3C/3CLpro, which play a key role in viral replication and present minimal homology with mammalian proteases. Particular focus is put on the reported development of P1 glutamine isosteres to generate potent inhibitors mimicking the natural substrate sequence at the site of recognition.'

Expedient synthesis of imino-C-nucleoside fleximers featuring a one-pot procedure to prepare aryl triazoles

Nucleoside analogues such as the antiviral agents galidesivir and ribavirin are of synthetic interest. This work reports a "one-pot" preparation of similar fleximers using a bifunctional copper catalyst that generates the aryl azide in situ, which is captured by a terminal alkyne to effect triazole formation.

Recent developments in the cleavage, functionalization, and conjugation of proteins and peptides at tyrosine residues

Peptide and protein selective modification at tyrosine residues has become an exploding field of research as tyrosine constitutes a robust alternative to lysine and cysteine-targeted traditional peptide/protein modification protocols. This review offers a comprehensive summary of the latest advances in tyrosine-selective cleavage, functionalization, and conjugation of peptides and proteins from the past three years. This updated overview complements the extensive body of work on site-selective modification of peptides and proteins, which holds significant relevance across various disciplines, including chemical, biological, medical, and material sciences.

Synthesis of the spiroimine fragment of portimines A and B

Portimines A and B are spirocyclic imine natural products, which display remarkable anticancer, anti-HIV, and antifouling activities. Herein, we report a facile synthesis of the spirocyclic core of portimines A and B. Our strategy utilized a scalable Diels-Alder addition of a 2-bromo-1,3-butadiene to a symmetrical malonate dienophile, coupled with a diastereoselective lactonization of the resulting malonate that enabled differentiation of the two carbonyl groups. This approach overcame issues encountered in previous studies focused on the use of exo selective Diels-Alder reactions, by programming formation of the key stereodiad of the spiroimine fragment into the diastereoselective lactonization event, rather than the cycloaddition step. Elaboration of the key lactone intermediate afforded a functionalized spirolactam fragment as a useful intermediate en route to the portimines. Importantly, a key alcohol intermediate could be resolved by enzymatic resolution, thereby providing an asymmetric route to the spiroimine fragment of portimines A and B.

Antimicrobial Peptides Incorporating Halogenated Marine-Derived Amino Acid Substituents

Small synthetic mimics of cationic antimicrobial peptides represent a promising class of compounds with leads in clinical development for the treatment of persistent microbial infections. The activity and selectivity of these compounds rely on a balance between hydrophobic and cationic components, and here, we explore the activity of 19 linear cationic tripeptides against five different pathogenic bacteria and fungi, including clinical isolates. The compounds incorporated modified hydrophobic amino acids inspired by motifs often found in bioactive marine secondary metabolites in combination with different cationic residues to probe the possibility of generating active compounds with improved safety profiles. Several of the compounds displayed high activity (low μM concentrations), comparable with the positive controls AMC-109, amoxicillin, and amphotericin B. A higher activity was observed against the fungal strains, and a low in vitro off-target toxicity was observed against erythrocytes and HeLa cells, thereby illustrating effective means for tuning the activity and selectivity of short antimicrobial peptides.

Nature-Inspired Peptide Antifouling Biocide: Coating Compatibility, Field Validation, and Environmental Stability

This study reports the development of a class of eco-friendly antifouling biocides based on a cyclic dipeptide scaffold, 2,5-diketopiperazine (2,5-DKP). The lead compound cyclo(N-Bip-l-Arg-N-Bip-l-Arg) (1) was synthesized in gram amounts and used to assess the compatibility with an ablation/hydration coating, efficacy against biofouling, and biodegradation. Leaching of 1 from the coating into seawater was assessed via a rotating drum method, revealing relatively stable and predictable leaching rates under dynamic shear stress conditions (36.1 ± 19.7 to 25.2 ± 9.1 ng^-1 cm^-2 day^-1) but low or no leaching under static conditions. The coatings were further analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS), with 1 seen to localize at the surface of the coating in a surfactant-like fashion. When coatings were deployed in the ocean, detectable reductions in biofouling development were measured for up to 11 weeks. After this time, biofouling overwhelmed the performance of the coating, consistent with leaching kinetics. Biodegradation of 1 in seawater was assessed using theoretical oxygen demand and analytical quantification. Masking effects were observed at higher concentrations of 1 due to antimicrobial properties, but half-lives were calculated ranging from 13.4 to 16.2 days. The results can rationally inform future development toward commercial antifouling products.

Azide-Enolate Cycloaddition-Rearrangement Enables Direct α-Amination of Amides and Enelactam Synthesis from Esters

Invited for the cover of this issue are Dan Furkert, Joe Bell-Tyrer and co-workers at the University of Auckland and Victoria University of Wellington. The image depicts a tandem cycloaddition-rearrangement process delivering a diverse range of molecular frameworks from simple precursors. Read the full text of the article at 10.1002/chem.202300261.

Total Synthesis of Spiroketal Alkaloids Lycibarbarines A-C

Lycibarbarines A-C are spirocyclic alkaloids with a unique tetracyclic framework, consisting of tetrahydroquinoline and spiro-fused oxazine-sugar spiroketal subunits. The first total syntheses of lycibarbarines A-C were achieved over 10 steps (longest linear sequence) each. Through this work, it was discovered that the spiroketal unit of lycibarbarines A-C exhibits unusually high resistance to acid-mediated isomerization and epimerization, likely due to the basic nitrogen atom. As such, the lycibarbarines present an interesting case study in preventing the interconversion of spiroketal isomers, which may prove to be instructive in efforts to obtain nonthermodynamic spiroketal frameworks.

Factors influencing on-resin depsipeptide bond formation: case studies on daptomycin- and brevicidine-derived sequences

Depsipeptides are an important class of bioactive natural products, where a growing number of genome-mined structures that display anti-microbial activity are macrocyclic depsipeptides. Chemically, peptide ester (depsipeptide) bond formation often displays low yields, and thereby hampers efforts to access these structures for structure-activity studies. Herein, we present a systematic study of the variables that influence depsipeptide bond formation on-resin, using simplified sequences derived from antibiotic peptides, daptomycin and brevicidine, prepared via Fmoc-based solid-phase synthesis. Our study highlights reaction solvent as the key determinant, where switching the solvent from DMF to DCM in almost all cases increased the amount of depsipeptide product. Limiting the number of amino-acids N-terminal to the reactive alcohol was also noted to significantly improve the acylation efficiency. The impact of different N-terminal and side-chain protecting groups, as well as stereochemistry, was also investigated. Additives to the reaction, such as inclusion of surfactants for esterification of long hydrophobic sequences, did not improve conversion. 6-ClHOBt, often added to improve acylation efficiency, notably decreased the amount of depsipeptide observed. Lastly, no significant difference between polystyrene and Tentagel® (PEG-decorated) resins were found for these sequences.

Thia-Michael addition: the route to promising opportunities for fast and cysteine-specific modification

Over the last few decades, design and discovery of chemical reactions that enable modification of proteins at pre-determined sites have been the focus of synthetic organic chemists. As an invaluable tool, the site-and chemoselective functionalization of peptides and proteins offers an exciting opportunity for creating high-value multicomponent conjugates with diverse applications in life sciences and pharmacology. In recent years, multiple strategies have emerged that target natural amino acids directly or convert them into other reactive species for further ligations. However, reactivity and selectivity are still key issues in the current state of chemical modification methodologies. Cysteine is one of the least abundant amino acids and exhibits unique chemistry of the thiol or thiolate group which makes it susceptible to a series of post-translational modifications. The thia-Michael "click" addition reactions, which can proceed under facile conditions provide a promising way for thiol-selective modification of cysteine-containing proteins. In this review, we summarize various reactions for cysteine-selective peptide and protein modification, focus on thia-Michael "click" addition reactions, elaborate on their historical perspective and mechanism, and highlight their applications in modifying biomolecules in a site-specific way.

A randomised controlled trial of long NY-ESO-1 peptide-pulsed autologous dendritic cells with or without alpha-galactosylceramide in high-risk melanoma

AIM

We have previously reported that polyfunctional T cell responses can be induced to the cancer testis antigen NY-ESO-1 in melanoma patients injected with mature autologous monocyte-derived dendritic cells (DCs) loaded with long NY-ESO-1-derived peptides together with α-galactosylceramide (α-GalCer), an agonist for type 1 Natural Killer T (NKT) cells.

OBJECTIVE

To assess whether inclusion of α-GalCer in autologous NY-ESO-1 long peptide-pulsed DC vaccines (DCV + α-GalCer) improves T cell responses when compared to peptide-pulsed DC vaccines without α-GalCer (DCV).

DESIGN, SETTING AND PARTICIPANTS

Single-centre blinded randomised controlled trial in patients ≥ 18 years old with histologically confirmed, fully resected stage II-IV malignant cutaneous melanoma, conducted between July 2015 and June 2018 at the Wellington Blood and Cancer Centre of the Capital and Coast District Health Board.

INTERVENTIONS

Stage I. Patients were randomised to two cycles of DCV or DCV + α-GalCer (intravenous dose of 10 × 10⁶ cells, interval of 28 days). Stage II. Patients assigned to DCV + α-GalCer were randomised to two further cycles of DCV + α-GalCer or observation, while patients initially assigned to DCV crossed over to two cycles of DCV + α-GalCer.

OUTCOME MEASURES

Primary: Area under the curve (AUC) of mean NY-ESO-1-specific T cell count detected by ex vivo IFN-γ ELISpot in pre- and post-treatment blood samples, compared between treatment arms at Stage I. Secondary: Proportion of responders in each arm at Stage I; NKT cell count in each arm at Stage I; serum cytokine levels at Stage I; adverse events Stage I; T cell count for DCV + α-GalCer versus observation at Stage II, T cell count before versus after cross-over.

RESULTS

Thirty-eight patients gave written informed consent; 5 were excluded before randomisation due to progressive disease or incomplete leukapheresis, 17 were assigned to DCV, and 16 to DCV + α-GalCer. The vaccines were well tolerated and associated with increases in mean total T cell count, predominantly CD4⁺ T cells, but the difference between the treatment arms was not statistically significant (difference - 6.85, 95% confidence interval, - 21.65 to 7.92; P = 0.36). No significant improvements in T cell response were associated with DCV + α-GalCer with increased dosing, or in the cross-over. However, the NKT cell response to α-GalCer-loaded vaccines was limited compared to previous studies, with mean circulating NKT cell levels not significantly increased in the DCV + α-GalCer arm and no significant differences in cytokine response between the treatment arms.

CONCLUSIONS

A high population coverage of NY-ESO-1-specific T cell responses was achieved with a good safety profile, but we failed to demonstrate that loading with α-GalCer provided an additional advantage to the T cell response with this cellular vaccine design.

CLINICAL TRIAL REGISTRATION

ACTRN12612001101875. Funded by the Health Research Council of New Zealand.

Azide-Enolate Cycloaddition-Rearrangement Enables Direct α-Amination of Amides and Enelactam Synthesis from Esters

Azide-enolate cycloaddition-rearrangements offer potential for rapid access to diverse molecular frameworks, from simple precursors. We report here that investigations into the cycloadditions of ester or amide enolates with vinyl azides led to the identification of two reaction processes - direct α-amination of amides and lactams, and synthesis of ene-γ-lactams from esters. These reaction outcomes depended on the fate of key vinyl triazoline intermediates generated in the initial cycloaddition step. Isolation of reaction intermediates in the ene-g-lactam synthesis revealed the unexpected addition of two enolate equivalents, one of which is later eliminated. Computational studies further suggested an unusual reaction pathway involving direct addition of an enolate to the terminal carbon of the N-vinyl triazoline. In contrast, the α-amination of amides and lactams proceeded by rearrangement of the intermediate triazoline to give an imine, hydrolysis or reduction of which gave access to primary or secondary α-amino amides or lactams.

Stereoselective synthesis of the spirocyclic core of 13-desmethyl spirolide C using an aza-Claisen rearrangement and an exo-selective Diels-Alder cycloaddition

13-Desmethyl spirolide C is a marine natural product of the cyclic imine class that demonstrates remarkable bioactivity against several biomarkers of Alzheimer's Disease, which renders its [7,6]-spirocyclic imine pharmacophore of significant synthetic interest. This work describes a facile and efficient synthesis of the [7,6]-spirocyclic core of 13-desmethyl spirolide C from inexpensive starting materials, featuring an aza-Claisen rearrangement to simultaneously set both stereocentres of the dimethyl moiety with complete atom economy, and a highly exo-selective Diels-Alder cycloaddition to construct the challenging contiguous tertiary and quaternary stereocentres of the spirocyclic core of 13-desmethyl spirolide C. A comprehensive study of the key Diels-Alder reaction was also performed to evaluate the stereoselectivity and reactivity of various functionalised dienes and protected lactam dienophiles, wherein the first successful exo-selective Diels-Alder cycloaddition to construct spirocyclic structures using a bromodiene and α-exo-methylene dienophiles is reported. This strategy not only establishes a more efficient stereoselective access to the spirocyclic core that can be used for the total synthesis of 13-desmethyl spirolide C, but also serves as a sound platform for convenient preparations of a range of spirocyclic analogues required for a comprehensive biological evaluation of this desirable pharmacophore.

Synthesis, Antibacterial Activity, and Nephrotoxicity of Polymyxin B Analogues Modified at Leu-7, d-Phe-6, and the N-Terminus Enabled by S-Lipidation

With the post-antibiotic era rapidly approaching, many have turned their attention to developing new treatments, often by structural modification of existing antibiotics. Polymyxins, a family of lipopeptide antibiotics that are used as a last line of defense in the clinic, have recently developed resistance and exhibit significant nephrotoxicity issues. Using thiol-ene chemistry, the facile preparation of six unique S-lipidated building blocks was demonstrated and used to generate lipopeptide mimetics upon incorporation into solid-phase peptide synthesis (SPPS). We then designed and synthesized 38 polymyxin analogues, incorporating these unique building blocks at the N-terminus, or to replace hydrophobic residues at positions 6 and 7 of the native lipopeptides. Several polymyxin analogues bearing one or more S-linked lipids were found to be equipotent to polymyxin, showed minimal kidney nephrotoxicity, and demonstrated activity against several World Health Organisation (WHO) priority pathogens. The S-lipidation strategy has demonstrated potential as a novel approach to prepare innovative new lipopeptide antibiotics.

Novel Fluorescently Labeled PACAP and VIP Highlight Differences between Peptide Internalization and Receptor Pharmacology

The related peptides pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) have diverse biological functions in peripheral tissues and the central nervous system. Therefore, these peptides and their three receptors represent potential drug targets for several conditions, including neurological and pain-related disorders. However, very little is known about how these peptides regulate their receptors through processes such as internalization. Therefore, we developed tools to study receptor regulation through the synthesis of fluorescently labeled analogues of PACAP-38, PACAP-27, and VIP using copper-mediated 1,3-dipolar cycloaddition of the Cy5 fluorophore. The functionality of Cy5-labeled peptides at their receptors was confirmed in cAMP accumulation assays. Internalization of the Cy5-labeled peptides was then examined and quantified at two distinct PAC₁ receptor splice variants, VPAC₁ and VPAC₂ receptors in transfected cells. All labeled peptides were functional, exhibiting comparable cAMP pharmacology to their unlabeled counterparts and underwent internalization in a time-dependent manner. Temporal differences in the internalization profiles were observed between Cy5-labeled peptides at the PAC_1n, PAC_1s, VPAC₁, and VPAC₂ receptors. Interestingly, the pattern of Cy5-labeled peptide activity differed for cAMP accumulation and internalization, indicating that these peptides differentially stimulate cAMP accumulation and internalization and therefore display biased agonism. This novel insight into PACAP-responsive receptor signaling and internalization may provide a unique avenue for future therapeutic development. The fluorescently labeled PACAP and VIP peptides described herein, which we validated as tools to study receptor internalization, will have utility across a broad range of applications and provide greater insight into this receptor family.

Effect of regio- and stereoisomerism on antifouling 2,5-diketopiperazines

Marine biofouling is a problem that plagues all maritime industries at vast economic and environmental cost. Previous and current methods to prevent biofouling have employed the use of heavy metals and other toxic or highly persistent chemicals, and these methods are now coming under immense regulatory pressure. Recent studies have illustrated the potential of nature-inspired tetrasubstituted 2,5-diketopiperazines (2,5-DKPs) as eco-friendly marine biocides for biofouling control. These highly active symmetrically substituted 2,5-DKPs can be generated by combining structural motifs from cationic innate defence peptides and natural marine antifoulants. A balance between a threshold hydrophobic contribution and sufficient cationic charge has been established as key for bioactivity, and our current study further increases understanding of the antifouling mechanism by investigating the effect of both regio- and stereochemistry. Novel synthetic routes for the generation of unsymmetrical 2,5-DKPs were developed and a library of nine compounds was prepared. The compounds were screened against a series of four model macrofouling organisms (Ciona savignyi, Mytilus galloprovincialis, Spirobranchus cariniferus, and Undaria pinnatifida). Several of the evaluated compounds displayed inhibitory activity at sub-micromolar concentrations. The structural contributions to antifouling bioactivity were studied using NMR spectroscopy and molecular modelling, revealing a strong dependence on a stable amphiphilic solution structure regardless of substitution pattern.

PAC1, VPAC1, and VPAC2 Receptor Expression in Rat and Human Trigeminal Ganglia: Characterization of PACAP-Responsive Receptor Antibodies

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuropeptide expressed in the trigeminal ganglia (TG). The TG conducts nociceptive signals in the head and may play roles in migraine. PACAP infusion provokes headaches in healthy individuals and migraine-like attacks in patients; however, it is not clear whether targeting this system could be therapeutically efficacious. To effectively target the PACAP system, an understanding of PACAP receptor distribution is required. Therefore, this study aimed to characterize commercially available antibodies and use these to detect PACAP-responsive receptors in the TG. Antibodies were initially validated in receptor transfected cell models and then used to explore receptor expression in rat and human TG. Antibodies were identified that could detect PACAP-responsive receptors, including the first antibody to differentiate between the PAC_1n and PAC_1s receptor splice variants. PAC₁, VPAC₁, and VPAC₂ receptor-like immunoreactivity were observed in subpopulations of both neuronal and glial-like cells in the TG. In this study, PAC₁, VPAC₁, and VPAC₂ receptors were detected in the TG, suggesting they are all potential targets to treat migraine. These antibodies may be useful tools to help elucidate PACAP-responsive receptor expression in tissues. However, most antibodies exhibited limitations, requiring the use of multiple methodologies and the careful inclusion of controls.

Synthesis and Biological Evaluation of Termini-Modified and Cyclic Variants of the Connexin43 Inhibitor Peptide5

Peptide5 is a 12–amino acid mimetic peptide that corresponds to a region of the extracellular loop 2 (EL2) of connexin43. Peptide5 regulates both cellular communication with the cytoplasm (hemichannels) and cell-to-cell communication (gap junctions), and both processes are implicated in neurological pathologies. To address the poor in vivo stability of native peptide5 and to improve its activity, twenty-five novel peptide5 mimetics were designed and synthesized. All the analogues underwent biological evaluation as a hemichannel blocker and as a gap junction disruptor, and several were assessed for stability in human serum. From this study, it was established that several acylations on the N-terminus were tolerated in the hemichannel assay. However, the replacement of the L-Lys with an N-methylated L-Lys to give H-VDCFLSRPTE-N-MeKT-OH showed good hemichannel and gap junction activity and was more stable in human serum. The cyclic peptide variants generally were not tolerated in either the hemichannel and gap junction assay although several possessed outstanding stability in human serum.

N-Vinyl Acrylamides: Versatile Heterobifunctional Electrophiles for Thiol-Thiol Bioconjugations

Herein we report the first examples of thiol-selective heterobifunctional electrophiles, N-vinyl acrylamides, that enable efficient highly selective thiol-thiol bioconjugations and cysteine modification of peptides. We demonstrate that these new classes of thiol-selective scaffolds can readily undergo a thia-Michael addition and an orthogonal radical induced thiol-ene "click" reaction under biocompatible conditions. Furthermore, the formation of an unexpected Markovnikov N,S-acetal hydrothiolation was explained using computational studies. We also reveal that N-methylation of the N-vinyl acrylamide scaffold changes the regioselectivity of the reaction. We demonstrate that use of N-vinyl acrylamides shows promise as an efficient, mild, and exquisite cysteine-selective protocol for facile construction of fluorophore-labeled peptides and proteins and that the resultant conjugates are resistant to degradation and thiol exchange, thus significantly improving their biophysical properties.

Glycolipid-peptide conjugate vaccines elicit CD8+ T-cell responses and prevent breast cancer metastasis

Objectives

Metastasis is the principal cause of breast cancer mortality. Vaccines targeting breast cancer antigens have yet to demonstrate clinical efficacy, and there remains an unmet need for safe and effective treatment to reduce the risk of metastasis, particularly for people with triple-negative breast cancer (TNBC). Certain glycolipids can act as vaccine adjuvants by specifically stimulating natural killer T (NKT) cells to provide a universal form of T-cell help.

Methods

We designed and made a series of conjugate vaccines comprising a prodrug of the NKT cell-activating glycolipid α-galactosylceramide covalently linked to tumor-expressed peptides, and assessed these using E0771- and 4T1-based breast cancer models in vivo. We employed peptides from the model antigen ovalbumin and from clinically relevant breast cancer antigens HER2 and NY-ESO-1.

Results

Glycolipid-peptide conjugate vaccines that activate NKT cells led to antigen-presenting cell activation, induced inflammatory cytokines, and, compared with peptide alone or admixed peptide and α-galactosylceramide, specifically enhanced CD8⁺ T-cell responses against tumor-associated peptides. Primary tumor growth was delayed by vaccination in all tumor models. Using 4T1-based cell lines expressing HER2 or NY-ESO-1, a single administration of the relevant conjugate vaccine prevented tumor colonisation of the lung following intravenous inoculation of tumor cells or spontaneous metastasis from breast, respectively.

Conclusion

Glycolipid-peptide conjugate vaccines that activate NKT cells prevent lung metastasis in breast cancer models and warrant investigation as adjuvant therapies for high-risk breast cancer.

Synthesis and Systematic Study on the Effect of Different PEG Units on Stability of PEGylated, Integrin-αvβ6-Specific A20FMDV2 Analogues in Rat Serum and Human Plasma

A20FMDV2 is a 20-mer peptide that exhibits high selectivity and affinity for the tumour-related αvβ6 integrin that can compete with extracellular ligands for the crucial RGD binding site, playing a role as a promising αvβ6-specific inhibitor for anti-cancer therapies. Unfortunately, the clinical value of A20FMDV2 is limited by its poor half-life in blood caused by rapid renal excretion and its reported high susceptibility to serum proteases. The incorporation of poly (ethylene glycol) chains, coined PEGylation, is a well-established approach to improve the pharmacokinetic properties of drug molecules. Here, we report a systematic study on the incorporation of a varying number of ethylene glycol units (1-20) into the A20FMDV2 peptide to establish the effects of PEGylation size on the peptide stability in both rat serum and human plasma. In addition, the effect of acetyl and propionyl PEGylation handles on peptide stability is also described. Selected peptide analogues were assessed for integrin-αvβ6-targeted binding, showing good specificity and activity in vitro. Stability studies in rat serum established that all of the PEGylated peptides displayed good stability, and an A20FMDV2 peptide containing twenty ethylene glycol units (PEG20) was the most stable. Surprisingly, the stability testing in human plasma identified shorter PEGs (PEG2 and PEG5) as more resistant to degradation than longer PEGs, a trend which was also observed with affinity binding to integrin αvβ6.

Stereochemical Effects on the Antimicrobial Properties of Tetrasubstituted 2,5-Diketopiperazines

Antimicrobial drug resistance is a looming health crisis facing us in the modern era, and new drugs are urgently needed to combat this growing problem. Synthetic mimics of antimicrobial peptides have recently emerged as a promising class of compounds for the treatment of persistent microbial infections. In the current study, we investigate five cyclic N-alkylated amphiphilic 2,5-diketopiperazines against 15 different strains of bacteria and fungi, including drug-resistant clinical isolates. Several of the 2,5-diketopiperazines displayed activities similar or superior to antibiotics currently in clinical use, with activities coupled to both the cationic and hydrophobic substituents. All possible stereoisomers of the lead peptide were prepared, and the effects of stereochemistry and amphiphilicity were investigated via 1D and 2D NMR spectroscopy, solution dynamics, and membrane interaction modeling. Clear differences in solution structures and membrane interaction potentials explain the differences seen in the bioactivity and physicochemical properties of each stereoisomer.

Lipidated Calcitonin Gene-Related Peptide (CGRP) Peptide Antagonists Retain CGRP Receptor Activity and Attenuate CGRP Action In Vivo

Signaling through calcitonin gene-related peptide (CGRP) receptors is associated with pain, migraine, and energy expenditure. Small molecule and monoclonal antibody CGRP receptor antagonists that block endogenous CGRP action are in clinical use as anti-migraine therapies. By comparison, the potential utility of peptide antagonists has received less attention due to suboptimal pharmacokinetic properties. Lipidation is an established strategy to increase peptide half-life in vivo. This study aimed to explore the feasibility of developing lipidated CGRP peptide antagonists that retain receptor antagonist activity in vitro and attenuate endogenous CGRP action in vivo. CGRP peptide analogues based on the archetypal CGRP receptor antagonist, CGRP_8-37, were palmitoylated at the N-terminus, position 24, and near the C-terminus at position 35. The antagonist activities of the lipidated peptide analogues were tested in vitro using transfected Cos-7 cells expressing either the human or mouse CGRP receptor, amylin subtype 1 (AMY₁) receptor, adrenomedullin (AM) receptors, or calcitonin receptor. Antagonist activities were also evaluated in SK-N-MC cells that endogenously express the human CGRP receptor. Lipidated peptides were then tested for their ability to antagonize endogenous CGRP action in vivo using a capsaicin-induced dermal vasodilation (CIDV) model in C57/BL6J mice. All lipidated peptides except for the C-terminally modified analogue retained potent antagonist activity compared to CGRP_8-37 towards the CGRP receptor. The lipidated peptides also retained, and sometimes gained, antagonist activities at AMY₁, AM₁ and AM₂ receptors. Several lipidated peptides produced robust inhibition of CIDV in mice. This study demonstrates that selected lipidated peptide antagonists based on αCGRP_8-37 retain potent antagonist activity at the CGRP receptor and are capable of inhibition of endogenous CGRP action in vivo. These findings suggest that lipidation can be applied to peptide antagonists, such as αCGRP_8-37 and are a potential strategy for antagonizing CGRP action.

Towards eco-friendly marine antifouling biocides - Nature inspired tetrasubstituted 2,5-diketopiperazines

Marine biofouling plagues all maritime industries at vast economic and environmental cost. Previous and most current methods to control biofouling have employed highly persistent toxins and heavy metals, including tin, copper, and zinc. These toxic methods are resulting in unacceptable environmental harm and are coming under immense regulatory pressure. Eco-friendly alternatives are urgently required to effectively mitigate the negative consequence of biofouling without causing collateral harm. Amphiphilic micropeptides have recently been shown to exhibit excellent broad-spectrum antifouling activity, with a non-toxic mode of action and innate biodegradability. The present work focused on incorporating the pharmacophore derived from amphiphilic micropeptides into a 2,5-diketopiperazine (DKP) scaffold. This privileged structure is present in a vast number of natural products, including marine natural product antifoulants, and provides advantages of synthetic accessibility and adaptability. A novel route to symmetrical tetrasubstituted DKPs was developed and a library of amphiphilic 2,5-DKPs were subsequently synthesised. These biodegradable compounds were demonstrated to be potent marine antifoulants displaying broad-spectrum activity in the low micromolar range against a range of common marine fouling organisms. The outcome of planned coating and field trials will dictate the future development of the lead compounds.

A novel tyrosine hyperoxidation enables selective peptide cleavage

A novel tyrosine hyperoxidation enabling selective peptide cleavage is reported. The scission of the N-terminal amide bond of tyrosine was achieved with Dess–Martin periodinane under mild conditions, generating a C-terminal peptide fragment bearing the unprecedented hyperoxidized tyrosine motif, 4,5,6,7-tetraoxo-1H-indole-2-carboxamide, along with an intact N-terminal peptide fragment. This reaction proceeds with high site-selectivity for tyrosine and exhibits broad substrate scope for various peptides, including those containing post-translational modifications. More importantly, this oxidative cleavage was successfully applied to enable sequencing of three naturally occurring cyclic peptides, including one depsipeptide and one lipopeptide. The linearized peptides generated from the cleavage reaction significantly simplify cyclic peptide sequencing by MS/MS, thus providing a robust tool to facilitate rapid sequence determination of diverse cyclic peptides containing tyrosine. Furthermore, the highly electrophilic nature of the hyperoxidized tyrosine unit disclosed in this work renders it an important electrophilic target for the selective bioconjugation or synthetic manipulation of peptides containing this unit.

A Tyr-selective peptide cleavage was reported using Dess–Martin periodinane. The cleavage generates an unprecedented hyperoxidized tyrosine motif in the C-terminal fragment and showed excellent site-specificity and broad scope for various peptides.

Pharmacological characterisation of mouse calcitonin and calcitonin receptor-like receptors reveals differences compared with human receptors

BACKGROUND AND PURPOSE

The calcitonin (CT) receptor family is complex, comprising two receptors (the CT receptor [CTR] and the CTR-like receptor [CLR]), three accessory proteins (RAMPs) and multiple endogenous peptides. This family contains several important drug targets, including CGRP, which is targeted by migraine therapeutics. The pharmacology of this receptor family is poorly characterised in species other than rats and humans. To facilitate understanding of translational and preclinical data, we need to know the receptor pharmacology of this family in mice.

EXPERIMENTAL APPROACH

Plasmids encoding mouse CLR/CTR and RAMPs were transiently transfected into Cos-7 cells. cAMP production was measured in response to agonists in the absence or presence of antagonists.

KEY RESULTS

We report the first synthesis and characterisation of mouse adrenomedullin, adrenomedullin 2 and βCGRP and of mouse CTR without or with mouse RAMPs. Receptors containing m-CTR had subtly different pharmacology than human receptors; they were promiscuous in their pharmacology, both with and without RAMPs. Several peptides, including mouse αCGRP and mouse adrenomedullin 2, were potent agonists of the m-CTR:m-RAMP3 complex. Pharmacological profiles of receptors comprising m-CLR:m-RAMPs were generally similar to those of their human counterparts, albeit with reduced specificity.

CONCLUSION AND IMPLICATIONS

Mouse receptor pharmacology differed from that in humans, with mouse receptors displaying reduced discrimination between ligands. This creates challenges for interpreting which receptor may underlie an effect in preclinical models and thus translation of findings from mice to humans. It also highlights the need for new ligands to differentiate between these complexes.

LINKED ARTICLES

This article is part of a themed issue on Advances in Migraine and Headache Therapy (BJP 75th Anniversary).. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.3/issuetoc.

Synthesis and development of seven-membered constrained cyclic urea based PSMA inhibitors via RCM

Intramolecular ring-closing metathesis on an N,N-diallyl Glu-urea-Gly substrate affords 7-membered cyclic ureas as inhibitors of prostrate specific membrane antigen (PMSA).

Vesiculin derived from IGF-II drives increased islet cell mass in a mouse model of pre-diabetes

Pancreatic islet-cell function and volume are both key determinants of the maintenance of metabolic health. Insulin resistance and islet-cell dysfunction often occur in the earlier stages of type 2 diabetes (T2D) progression. The ability of the islet cells to respond to insulin resistance by increasing hormone output accompanied by increased islet-cell volume is key to maintaining blood glucose control and preventing further disease progression. Eventual β-cell loss is the main driver of full-blown T2D and insulin-dependency. Researchers are targeting T2D with approaches that include those aimed at enhancing the function of the patient's existing β-cell population, or replacing islet β-cells. Another approach is to look for agents that enhance the natural capacity of the β-cell population to expand. Here we aimed to study the effects of a new putative β-cell growth factor on a mouse model of pre-diabetes. We asked whether: 1) 4-week's treatment with vesiculin, a two-chain peptide derived by processing from IGF-II, had any measurable effect on pre-diabetic mice vs vehicle; and 2) whether the effects were the same in non-diabetic littermate controls. Although treatment with vesiculin did not alter blood glucose levels over this time period, there was a doubling of the Proliferating Cell Nuclear Antigen (PCNA) detectable in the islets of treated pre-diabetic but not control mice and this was accompanied by increased insulin- and glucagon-positive stained areas in the pancreatic islets.

A solid support-based synthetic strategy for the site-selective functionalization of peptides with organometallic half-sandwich moieties

The number of donor atoms available on peptides that can competitively coordinate to metal centers renders the site-selective generation of advanced metal-peptide conjugates in high purity a challenging venture. Herein, we present a transmetalation-based synthetic approach on solid support in which an imidazolium proligand can be used to selectively anchor a range of transition metal half-sandwich complexes onto peptides in the presence of multiple coordinative motifs. Amenable to solid support, a range of N-terminus and/or lysine conjugated metal-peptide conjugates were obtained in high purity after cleavage from the resin. The metalated peptides were evaluated for their anticancer properties against human cancer cell lines. While no cytotoxic activity was observed, this platform has the potential to i) provide a pathway to site-selective peptide labelling, ii) be explored as a biorthogonal handle and/or iii) generate a new strategy for ligand design in transition metal catalysts.

Fourth-Generation Analogues of the Anticancer Peptaibol Culicinin D: Probing the Effects of Hydrophobicity and Halogenation on Cytotoxicity

Preliminary results of the effect of hydrophobicity and halogenation on the cytotoxicity of the anticancer peptaibol culicinin D are reported. Building on previous work, the synthetically challenging (2S,4S,6R)-2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid and (2S,4R)-2-amino-4-methyldecanoic acid building blocks were replaced with derivatives of l-phenylalanine and 2-aminodecanoic acid, respectively. Substitution of (2S,4S,6R)-2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid with l-4,4′-biphenylalanine yielded an analogue that was tenfold more potent than the natural product and was also the most hydrophobic analogue, as judged by an antiproliferative IC50 assay and logD calculations; these results suggest that the potency of culicinin D may be governed by its hydrophobicity. However, the introduction of halogenated moieties into the peptide sequence generated analogues that were similarly potent, although not necessarily hydrophobic. Thus, the parameters regulating the cytotoxicity of culicinin D, and by extension other peptaibols, are multimodal and include both halogenation and hydrophobicity.

Aminovinyl Cysteine Containing Peptides: A Unique Motif That Imparts Key Biological Activity

Natural products that contain distinctive chemical functionality can serve as useful starting points to develop Nature's compounds into viable therapeutics. Peptide natural products, an under-represented class of medicines, such as ribosomally synthesized and post-translationally modified peptides (RiPPs), often contain noncanonical amino acids and structural motifs that give rise to potent biological activity. However, these motifs can be difficult to obtain synthetically, thereby limiting the transition of RiPPs to the clinic. Aminovinyl cysteine containing peptides, which display potent antimicrobial or anticancer activity, possess an intricate C-terminal ring that is critical for bioactivity. To date, successful methods for the total chemical synthesis of such peptides are yet to be realized, although several advancements have been achieved. In this perspective, we review this burgeoning class of aminovinyl cysteine peptides and critically evaluate the chemical strategies to install the distinct aminovinyl cysteine motif.

Characterisation of agonist signalling profiles and agonist-dependent antagonism at PACAP-responsive receptors: Implications for drug discovery

BACKGROUND AND PURPOSE

The pituitary adenylate cyclase-activating peptide (PACAP) family is of clinical interest for the treatment of migraine. These peptides activate three different PACAP-responsive class B G protein-coupled receptors: the PAC₁ , VPAC₁ and VPAC₂ receptors. The PAC₁ receptor may be alternatively spliced, generating variants that can differ in their pharmacological or signalling profiles. To inform drug discovery efforts targeting migraine, we need to better understand how the different PACAP-responsive receptors signal and how effectively these responses can be blocked by antagonists.

EXPERIMENTAL APPROACH

The signalling profiles of the human PAC_1n , PAC_1s , VPAC₁ and VPAC₂ receptors were examined in transfected Cos7 cells for cAMP, IP₁ , pAkt, pERK and pCREB. Biased signalling was then quantified. The ability of antagonists to block PACAP-38, PACAP-27 or VIP stimulated cAMP accumulation at PACAP-responsive receptors was also determined.

KEY RESULTS

PACAP-responsive receptors exhibited varied pharmacological profiles but activated signalling in a similar manner. The PAC_1n and PAC_1s receptors displayed distinct pharmacology. At the PAC_1s receptor, VIP and PHM were more potent than at the PAC_1n receptor. PACAP-responsive receptors displayed agonist-dependent antagonism where PACAP-38 was less effectively antagonised compared to PACAP-27 and VIP.

CONCLUSIONS AND IMPLICATIONS

The distinct pharmacological profile displayed by the PAC_1s receptor suggests that it can act as a dual receptor for VIP and PACAP. Furthermore, the effectiveness of blocking a signalling pathway can be influenced by which endogenous PACAP family agonist is present. These effects have potential implications for the development and effectiveness of drugs targeting the PACAP system.

The tryptophan connection: cyclic peptide natural products linked via the tryptophan side chain

Covering: from 1938 up to March 2021The electron-rich indole side chain of tryptophan is a versatile substrate for peptide modification. Upon the action of various cyclases, the tryptophan side chain may be linked to a nearby amino acid residue, opening the door to a diverse range of cyclic peptide natural products. These compounds exhibit a wide array of biological activity and possess fascinating molecular architectures, which have made them popular targets for total synthesis studies. This review examines the isolation and bioactivity of tryptophan-linked cyclic peptide natural products, along with a discussion of their first total synthesis, and biosynthesis where this has been studied.

Synthesis of the C4-C16 Polyketide Fragment of Portimines A and B

Stereoselective synthesis of the C4-C16 polyketide fragment of portimines A and B is reported, enabled by our previously established method for the stereoselective synthesis of syn-α,α'-dihydroxyketones. The preparation of this advanced fragment provides insights useful for the total synthesis of portimines A and B. An asymmetric Evans aldol reaction was used to install the C10-C11 adjacent stereogenic centers before incorporation of indantrione, followed by epoxidation and epoxide opening to forge the challenging syn-α,α'-dihydroxyketone functionality.

The Total Chemical Synthesis and Biological Evaluation of the Cationic Antimicrobial Peptides, Laterocidine and Brevicidine

Antimicrobial resistance is a significant threat to public health systems worldwide, prompting immediate attention to develop new therapeutic agents with novel mechanisms of action. Recently, two new cationic non-ribosomal peptides (CNRPs), laterocidine and brevicidine, were discovered from Brevibacillus laterosporus through a global genome-mining approach. Both laterocidine and brevicidine exhibit potent antimicrobial activity toward Gram-negative bacteria, including difficult-to-treat Pseudonomas aeruginosa and colistin-resistant Escherichia coli, and a low risk of resistance development. Herein, we report the first total syntheses of laterocidine and brevicidine via an efficient and high-yielding combination of solid-phase synthesis and solution-phase macrolactamization. The crucial depsipeptide bond of the macrolactone rings of laterocidine and brevicidine was established on-resin between the side-chain hydroxy group of Thr⁹ with Alloc-Gly-OH or Alloc-Ser(tBu)-OH, respectively. A conserved glycine residue within the lactone macrocycle is exploited for the initial immobilization onto the hyper acid-labile 2-chlorotrityl chloride resin, subsequently enabling an efficient solution-phase macrocyclization to yield laterocidine and brevicidine in 36% and 10% overall yields, respectively (with respect to resin loading). A biological evaluation against both Gram-positive and Gram-negative bacteria demonstrated that synthetic laterocidine and brevicidine possessed a potent and selective antimicrobial activity toward Gram-negative bacteria, in accordance with the isolated compounds.

Scalable Biomimetic Syntheses of Paeciloketal B, 1-epi-Paeciloketal B, and Bysspectin A

The first total synthesis of the benzannulated 5,5-spiroketal natural products paeciloketal B and 1-epi-paeciloketal B has been achieved in 10 linear steps employing a biomimetic spiroketalization. This approach also furnished the related natural product bysspectin A from the same putative biosynthetic precursor as the paeciloketals. Alternatively, bysspectin A could be accessed in only six steps using an improved route. This scalable and efficient synthesis affords insight into the biosynthesis of these natural products in nature.

Disruption of Metallostasis in the Anaerobic Human Pathogen Fusobacterium nucleatum by the Zinc Ionophore PBT2

The Gram-negative anaerobe Fusobacterium nucleatum is an opportunistic human pathogen, most frequently associated with periodontal disease through dental biofilm formation and, increasingly, with colorectal cancer development and progression. F. nucleatum infections are routinely treated by broad-spectrum β-lactam antibiotics and metronidazole. However, these antibiotics can negatively impact the normal microflora. Therefore, the development of novel narrow-spectrum antimicrobials active against anaerobic pathogens is of great interest. Here, we examined the antimicrobial Zn ionophore PBT2, an 8-hydroxyquinoline analogue with metal chelating properties, against a single type isolate F. nucleatum ATCC 25586. PBT2-Zn was a potent inhibitor of growth and exhibited synergistic bactericidal (>3-log₁₀ killing) activity at 5× MIC in planktonic cells, and at the MIC in biofilms grown in vitro. Physiological and transcriptional analyses uncovered a strong cellular response relating to Zn and Fe homeostasis in PBT2-Zn treated cells across subinhibitory and inhibitory concentrations. At 1× MIC, PBT2 alone induced a 3.75-fold increase in intracellular Zn, whereas PBT2-Zn challenge induced a 19-fold accumulation of intracellular Zn after 2 h. A corresponding 2.1-fold loss of Fe was observed at 1× MIC. Transcriptional analyses after subinhibitory PBT2-Zn challenge (0.125 μg/mL and 200 μM ZnSO₄) revealed significant differential expression of 15 genes at 0.5 h, and 12 genes at 1 h. Upregulated genes included those with roles in Zn homeostasis (e.g., a Zn-transporting ATPase and the Zn-sensing transcriptional regulator, smtB) and hemin transport (hmuTUV) to re-establish Fe homeostasis. A concentration-dependent protective effect was observed for cells pretreated with hemin (50 μg/mL) prior to PBT2-Zn challenge. The data presented here supports our proposal that targeting the disruption of metallostasis by Zn-translocating ionophores is a strategy worth investigating further for the treatment of Gram-negative anaerobic pathogens.