Teixobactin "Swapmers" with l Tail Stereochemistry Retain Antibiotic Activity

The unusual d-l-l-d-d-l-l pattern of stereochemistry in residues 1-7 of the peptide antibiotic teixobactin is critical to its extraordinary antibiotic activity, creating an unusual amphiphilic β-sheetlike structure that is essential to its mechanism of action. The current study sought to replace the three d-amino acids in the tail with l-amino acids while maintaining amphiphilicity. We find that swapping residues d-Gln4 and d-allo-Ile5 in O-acyl isopeptide prodrugs of teixobactin permits the introduction of l-stereochemistry with retention of antibiotic activity. Nevertheless, modifying the N-terminal stereochemistry results in a loss of antibiotic activity.

Unearthing naturally-occurring cyclic antibacterial peptides and their structural optimization strategies

Natural products with antibacterial activity are highly desired globally to combat against multidrug-resistant (MDR) bacteria. Antibacterial peptide (ABP), especially cyclic ABP (CABP), is one of the abundant classes. Most of them were isolated from microbes, demonstrating excellent bactericidal effects. With the improved proteolytic stability, CABPs are normally considered to have better druggability than linear peptides. However, most clinically-used CABP-based antibiotics, such as colistin, also face the challenges of drug resistance soon after they reached the market, urgently requiring the development of next-generation succedaneums. We present here a detail review on the novel naturally-occurring CABPs discovered in the past decade and some of them are under clinical trials, exhibiting anticipated application potential. According to their chemical structures, they were broadly classified into five groups, including (i) lactam/lactone-based CABPs, (ii) cyclic lipopeptides, (iii) glycopeptides, (iv) cyclic sulfur-rich peptides and (v) multiple-modified CABPs. Their chemical structures, antibacterial spectrums and proposed mechanisms are discussed. Moreover, engineered analogs of these novel CABPs are also summarized to preliminarily analyze their structure-activity relationship. This review aims to provide a global perspective on research and development of novel CABPs to highlight the effectiveness of derivatives design in identifying promising antibacterial agents. Further research efforts in this area are believed to play important roles in fighting against the multidrug-resistance crisis.

Strategies of Pharmacological Repositioning for the Treatment of Medically Relevant Mycoses

Fungal infections represent a worldwide concern for public health, due to their prevalence and significant increase in cases each year. Among the most frequent mycoses are those caused by members of the genera Candida, Cryptococcus, Aspergillus, Histoplasma, Pneumocystis, Mucor, and Sporothrix, which have been treated for years with conventional antifungal drugs, such as flucytosine, azoles, polyenes, and echinocandins. However, these microorganisms have acquired the ability to evade the mechanisms of action of these drugs, thus hindering their treatment. Among the most common evasion mechanisms are alterations in sterol biosynthesis, modifications of drug transport through the cell wall and membrane, alterations of drug targets, phenotypic plasticity, horizontal gene transfer, and chromosomal aneuploidies. Taking into account these problems, some research groups have sought new therapeutic alternatives based on drug repositioning. Through repositioning, it is possible to use existing pharmacological compounds for which their mechanism of action is already established for other diseases, and thus exploit their potential antifungal activity. The advantage offered by these drugs is that they may be less prone to resistance. In this article, a comprehensive review was carried out to highlight the most relevant repositioning drugs to treat fungal infections. These include antibiotics, antivirals, anthelmintics, statins, and anti-inflammatory drugs.

New Teixobactin Analogues with a Total Lactam Ring

Teixobactin is a new antibiotic peptide with strong efficacy against several Gram-positive resistant bacteria, the structure of which is extremely difficult to obtain in the laboratory via multistep conventional synthesis. To face the increasing antibiotic resistant bacteria, it is fundamental to introduce new types of antibiotics with innovative mechanisms of action without resistance; thus, many scientists are studying and developing new methods to synthesize teixobactin analogues. In this work, seven Arg10-teixobactin analogues with a total lactam ring have been prepared via solid phase peptide synthesis. In order to obtain the total lactam ring, d-Thr8 was replaced by (2R,3S)-diamino-propionic acid. To verify their antimicrobial activity and efficacy, each analogue was tested with MIC against different resistant pathogens, showing an interesting activity for Nle11 containing compounds.

Development of teixobactin analogues containing hydrophobic, non-proteogenic amino acids that are highly potent against multidrug-resistant bacteria and biofilms

Teixobactin is a cyclic undecadepsipeptide that has shown excellent potency against multidrug-resistant pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococci (VRE). In this article, we present the design, synthesis, and antibacterial evaluations of 16 different teixobactin analogues. These simplified analogues contain commercially available hydrophobic, non-proteogenic amino acid residues instead of synthetically challenging expensive L-allo-enduracididine amino acid residue at position 10 together with different combinations of arginines at positions 3, 4 and 9. The new teixobactin analogues showed potent antibacterial activity against a broad panel of Gram-positive bacteria, including MRSA and VRE strains. Our work also presents the first demonstration of the potent antibiofilm activity of teixobactin analogoues against Staphylococcus species associated with serious chronic infections. Our results suggest that the use of hydrophobic, non-proteogenic amino acids at position 10 in combination with arginine at positions 3, 4 and 9 holds the key to synthesising a new generation of highly potent teixobactin analogues to tackle resistant bacterial infections and biofilms.

Isobactins: O-acyl isopeptide prodrugs of teixobactin and teixobactin derivatives

The antibiotic teixobactin is a promising drug candidate against drug-resistant pathogens, such as MRSA and VRE, but forms insoluble gels that may limit intravenous administration. O-Acyl isopeptide prodrug analogues of teixobactin circumvent the problem of gel formation while retaining antibiotic activity. The teixobactin prodrug analogues contain ester linkages between Ile6 and Ser7, Ile2 and Ser3, or between both Ile6 and Ser7 and Ile2 and Ser3. Upon exposure to physiological pH, the prodrug analogues undergo clean conversion to the corresponding amides, with half-lives between 13 and 115 min. Prodrug analogues containing lysine, arginine, or leucine at position 10 exhibit good antibiotic activity against a variety of Gram-positive bacteria while exhibiting little or no cytotoxicity or hemolytic activity. Because O-acyl isopeptide prodrug analogues of teixobactin exhibit clean conversion to the corresponding teixobactin analogues with reduced propensity to form gels, it is anticipated that teixobactin prodrugs will be superior to teixobactin as drug candidates.

Escaping mechanisms of ESKAPE pathogens from antibiotics and their targeting by natural compounds

The microorganisms that have developed resistance to available therapeutic agents are threatening the globe and multidrug resistance among the bacterial pathogens is becoming a major concern of public health worldwide. Bacteria develop protective mechanisms to counteract the deleterious effects of antibiotics, which may eventually result in loss of growth-inhibitory potential of antibiotics. ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) pathogens display multidrug resistance and virulence through various mechanisms and it is the need of the hour to discover or design new antibiotics against ESKAPE pathogens. In this article, we have discussed the mechanisms acquired by ESKAPE pathogens to counteract the effect of antibiotics and elaborated on recently discovered secondary metabolites derived from bacteria and plant sources that are endowed with good antibacterial activity towards pathogenic bacteria in general, ESKAPE organisms in particular. Abyssomicin C, allicin, anthracimycin, berberine, biochanin A, caffeic acid, daptomycin, kibdelomycin, piperine, platensimycin, plazomicin, taxifolin, teixobactin, and thymol are the major metabolites whose antibacterial potential have been discussed in this article.

Discovery, synthesis, and optimization of teixobactin, a novel antibiotic without detectable bacterial resistance

Discovering new antibiotics with novel chemical scaffolds and antibacterial mechanisms presents a challenge for medicinal scientists worldwide as the ever-increasing bacterial resistance poses a serious threat to human health. A new cyclic peptide-based antibiotic termed teixobactin was discovered from a screen of uncultured soil bacteria through iChip technology in 2015. Teixobactin exhibits excellent antibacterial activity against all the tested gram-positive pathogens and Mycobacterium tuberculosis, including drug-resistant strains. Given that teixobactin targets the highly conserved lipid II and lipid III, which induces the simultaneous inhibition of both peptidoglycan and teichoic acid synthesis, the emergence of resistance is considered to be rather difficult. The novel structure, potent antibacterial activity, and highly conservative targets make teixobactin a promising lead compound for further antibiotic development. This review provides a comprehensive treatise on the advances of teixobactin in the areas of discovery processes, antibacterial activity, mechanisms of action, chemical synthesis, and structural optimizations. The synthetic methods for the key building block l-allo-End, natural teixobactin, representative teixobactin analogues, as well as the structure-activity relationship studies will be highlighted and discussed in details. Finally, some insights into new trends for the generation of novel teixobactin analogues and tips for future work and directions will be commented.

Perfluoro-tert-butanol for selective on-resin detritylation: a mild alternative to traditionally used methods

Solid-phase synthesis of cyclic, branched or side-chain-modified peptides typically involves introduction of a residue carrying a temporary side-chain protecting group that undergoes selective on-resin removal. In particular, Nα-Fmoc-Nε-(4-methyltriphenylmethyl) (Mtt)-protected lysine and its shorter analogues are commercially available and extensively used in this context. Nevertheless, rapid reliable methods for on-resin removal of Mtt groups in the presence of tert-butyloxycarbonyl (Boc) groups are needed. Current commonly used conditions involve low concentrations (1-3%) of trifluoroacetic acid (TFA) in dichloromethane, albeit adjustment to each specific application is required to avoid premature removal of Boc groups or cleavage from the linker. Hence, a head-to-head comparison of several deprotection conditions was performed. The selected acids represent a wide range of acidity from TFA to trifluoroethanol. Also, on-resin removal of the N-(4-methoxytriphenylmethyl) (Mmt) and O-trityl groups (on serine) was investigated under similar conditions. The mildest conditions identified for Mtt deprotection involve successive treatments with 30% hexafluoroisopropanol (HFIP) or 30% perfluoro-tert-butanol [(CF3)3COH] in dichloromethane (3 × 5 or 3 × 15 min, respectively), while 30% HFIP, 30% (CF3)3COH, or 10% AcOH-20% trifluoroethanol (TFE) in CH2Cl2 (3 × 5 min) as well as 5% trichloroacetic acid in CH2Cl2 (3 × 2 min) enabled Mmt removal. Treatment with 1% TFA with/without 2% triisopropylsilane added (3 × 5 min), but also prolonged treatment with 30% (CF3)3COH (5 × 15 min), led to selective deprotection of an O-Trt group on a serine residue. In all cases, the sequences also contained N-Boc or O-tBu protecting groups, which were not affected by 30% HFIP or 30% (CF3)3COH even after a prolonged reaction time of 4 h. Finally, the optimized conditions involving HFIP or (CF3)3COH proved applicable also for selective deprotection of a longer resin-bound peptide [i.e., Ac-Gly-Leu-Leu-Lys(Mtt)-Arg(Pbf)-Ile-Lys(Boc)-Ser(tBu)-Leu-Leu-RAM-PS] as well as allowed for an almost complete deprotection of a Dab(Mtt) residue.

Discovery, Synthesis, and Optimization of Peptide-Based Antibiotics

The rise of multidrug resistant bacteria has significantly compromised our supply of antibiotics and poses an alarming medical and economic threat to society. To combat this problem, it is imperative that new antibiotics and treatment modalities be developed, especially those toward which bacteria are less capable of developing resistance. Peptide natural products stand as promising candidates to meet this need as bacterial resistance is typically slow in response to their unique modes of action. They also have additional benefits including favorable modulation of host immune responses and often possess broad-spectrum activity against notoriously treatment resistant bacterial biofilms. Moreover, nature has provided a wealth of peptide-based natural products from a range of sources, including bacteria and fungi, which can be hijacked in order to combat more dangerous clinically relevant infections.This Account highlights recent advances in the total synthesis and development of a range of peptide-based natural product antibiotics and details the medicinal chemistry approaches used to optimize their activity.In the context of antibiotics with potential to treat Gram-positive bacterial infections, this Account covers the synthesis and optimization of the natural products daptomycin, glycocin F, and alamethicin. In particular, the reported synthesis of daptomycin highlights the utility of on-resin ozonolysis for accessing a key kynurenine residue from the canonical amino acid tryptophan. Furthermore, the investigation into glycocin F analogues uncovered a potent lead compound against Lactobacillus plantarum that bears a non-native thioacetal linkage to a N-acetyl-d-glucosamine (GlcNAc) sugar, which is otherwise O-linked in its native form.For mycobacterial infections, this Account covers the synthesis and optimization of teixobactin, callyaerin A, lassomycin, and trichoderin A. The synthesis of callyaerin A, in particular, highlighted the importance of a (Z)-2,3-diaminoacrylamide motif for antimicrobial activity against Mycobacterium tuberculosis, while the synthesis of trichoderin A highlighted the importance of (R)-stereoconfiguration in a key 2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid (AHMOD) residue.Lastly, this Account covers lipopeptide antibiotics bearing activity toward Gram-negative bacterial infections, namely, battacin and paenipeptin C. In both cases, optimization of the N-terminal lipid tails led to the identification of analogues with potent activity toward Escherichia coli and Pseudomonas aeruginosa.

Emerging peptide antibiotics with therapeutic potential

This review covers some of the recent progress in the field of peptide antibiotics with a focus on compounds with novel or established mode of action and with demonstrated efficacy in animal infection models. Novel drug discovery approaches, linear and macrocyclic peptide antibiotics, lipopeptides like the polymyxins as well as peptides addressing targets located in the plasma membrane or in the outer membrane of bacterial cells are discussed.

Combining Colistin and Fluconazole Synergistically Increases Fungal Membrane Permeability and Antifungal Cidality

The increasing emergence of drug-resistant fungal pathogens, together with the limited number of available antifungal drugs, presents serious clinical challenges to treating systemic, life-threatening infections. Repurposing existing drugs to augment the antifungal activity of well-tolerated antifungals is a promising antifungal strategy with the potential to be implemented rapidly. Here, we explored the mechanism by which colistin, a positively charged lipopeptide antibiotic, enhances the antifungal activity of fluconazole, the most widely used orally available antifungal. In a range of susceptible and drug-resistant isolates and species, colistin was primarily effective at reducing fluconazole tolerance, a property of subpopulations of cells that grow slowly in the presence of a drug and may promote the emergence of persistent infections and resistance. Clinically relevant concentrations of colistin synergized with fluconazole, reducing fluconazole minimum inhibitory concentration 4-fold. Combining fluconazole and colistin also increased survival in a C. albicans Galleria mellonella infection, especially for a highly fluconazole-tolerant isolate. Mechanistically, colistin increased permeability to fluorescent antifungal azole probes and to intracellular dyes, accompanied by an increase in cell death that was dependent upon pharmacological or genetic inhibition of the ergosterol biosynthesis pathway. The positive charge of colistin is critical to its antifungal, and antibacterial, activity: colistin directly binds to several eukaryotic membrane lipids (i.e., l-α-phosphatidylinositol, l-α-phosphatidyl-l-serine, and l-α-phosphatidylethanolamine) that are enriched in the membranes of ergosterol-depleted cells. These results support the idea that colistin binds to fungal membrane lipids and permeabilizes fungal cells in a manner that depends upon the degree of ergosterol depletion.

Photo-induced radical thiol-ene chemistry: a versatile toolbox for peptide-based drug design

While the global market for peptide/protein-based therapeutics is witnessing significant growth, the development of peptide drugs remains challenging due to their low oral bioavailability, poor membrane permeability, and reduced metabolic stability. However, a toolbox of chemical approaches has been explored for peptide modification to overcome these obstacles. In recent years, there has been a revival of interest in photoinduced radical thiol-ene chemistry as a powerful tool for the construction of therapeutic peptides.

De Novo Resistance to Arg10-Teixobactin Occurs Slowly and Is Costly

Bacterial pathogens are rapidly evolving resistance to all clinically available antibiotics. One part of the solution to this complex issue is to better understand the resistance mechanisms to new and existing antibiotics. Here, we focus on two antibiotics. Teixobactin is a recently discovered promising antibiotic that is claimed to "kill pathogens without detectable resistance" (L. L. Ling, T. Schneider, A. J. Peoples, A. L. Spoering, et al., Nature 517:455-459, 2015, https://doi.org/10.1038/nature14098). Moenomycin A has been extensively used in animal husbandry for over 50 years with no meaningful antibiotic resistance arising. However, the nature, mechanisms, and consequences of the evolution of resistance to these "resistance-proof" compounds have not been investigated. Through a fusion of experimental evolution, whole-genome sequencing, and structural biology, we show that Staphylococcus aureus can develop significant resistance to both antibiotics in clinically meaningful timescales. The magnitude of evolved resistance to Arg₁₀-teixobactin is 300-fold less than to moenomycin A over 45 days, and these are 2,500-fold and 8-fold less than evolved resistance to rifampicin (control), respectively. We have identified a core suite of key mutations, which correlate with the evolution of resistance, that are in genes involved in cell wall modulation, lipid synthesis, and energy metabolism. We show the evolution of resistance to these antimicrobials translates into significant cross-resistance against other clinically relevant antibiotics for moenomycin A but not Arg₁₀-teixobactin. Lastly, we show that resistance is rapidly lost in the absence of antibiotic selection, especially for Arg₁₀-teixobactin. These findings indicate that teixobactin is worth pursuing for clinical applications and provide evidence to inform strategies for future compound development and clinical management.

Thiol-ene Enabled Chemical Synthesis of Truncated S-Lipidated Teixobactin Analogs

Herein is described the introduction of lipid moieties onto a simplified teixobactin pharmacophore using a modified Cysteine Lipidation on a Peptide or Amino acid (CLipPA) technique, whereby cysteine was substituted for 3-mercaptopropionic acid (3-MPA). A truncated teixobactin analog was prepared with the requisite thiol handle, thus enabling an array of vinyl esters to be conveniently conjugated onto the simplified teixobactin pharmacophore to yield S-lipidated cyclic lipopeptides.

Teixobactin: A Paving Stone toward a New Class of Antibiotics?

Antimicrobial resistance is a serious threat to human health worldwide, prompting research efforts on a massive scale in search of novel antibiotics to fill an urgent need for a remedy. Teixobactin, a macrocyclic depsipeptide natural product, isolated from uncultured bacteria (Eleftheria terrae), displayed potent activity against several Gram-positive pathogenic bacteria. The distinct pharmacological profile and interesting structural features of teixobactin with nonstandard amino acid (three d-amino acids and l-allo-enduracididine) residues attracted several research groups to work on this target molecule in search of novel antibiotics with new mechanism. Herein, we present a comprehensive and critical perspective on immense possibilities offered by teixobactin in the domain of drug discovery. Efforts made by various research groups since its isolation are discussed, highlighting the molecule's considerable potential with special emphasis on replacement of amino acids. Critical analysis of synthetic efforts, SAR studies, and the way forward are provided hereunder.

Recent Highlights in Anti-infective Medicinal Chemistry from South Africa

Global advancements in biological technologies have vastly increased the variety of and accessibility to bioassay platforms, while simultaneously improving our understanding of druggable chemical space. In the South African context, this has resulted in a rapid expansion in the number of medicinal chemistry programmes currently operating, particularly on university campuses. Furthermore, the modern medicinal chemist has the advantage of being able to incorporate data from numerous related disciplines into the medicinal chemistry process, allowing for informed molecular design to play a far greater role than previously possible. Accordingly, this review focusses on recent highlights in drug-discovery programmes, in which South African medicinal chemistry groups have played a substantive role in the design and optimisation of biologically active compounds which contribute to the search for promising agents for infectious disease.

Synthesis and structure-activity relationships of teixobactin

The discovery of antibiotics has led to the effective treatment of bacterial infections that were otherwise fatal and has had a transformative effect on modern medicine. Teixobactin is an unusual depsipeptide natural product that was recently discovered from a previously unculturable soil bacterium and found to possess potent antibacterial activity against several Gram positive pathogens, including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococci. One of the key features of teixobactin as an antibiotic lead is that resistance could not be generated in a laboratory setting. This is proposed to be a result of a mechanism of action that involves binding to essential cell wall synthesis building blocks, lipid II and lipid III. Since the initial isolation report in 2015, significant efforts have been made to understand its unique mechanism of action, develop efficient synthetic routes for its production, and thus enable the generation of analogues for structure-activity relationship studies and optimization of its pharmacological properties. Our review provides a comprehensive treatise on the progress in understanding teixobactin chemistry, structure-activity relationships, and mechanisms of antibacterial activity. Teixobactin represents an exciting starting point for the development of new antibiotics that can be used to combat multidrug-resistant bacterial ("superbug") infections.

The impact of backbone N-methylation on the structure-activity relationship of Leu10 -teixobactin

Antimicrobial resistance is a serious threat to global human health; therefore, new anti-infective therapeutics are required. The cyclic depsi-peptide teixobactin exhibits potent antimicrobial activity against several Gram-positive pathogens. To study the natural product's mechanism of action and improve its pharmacological properties, efficient chemical methods for preparing teixobactin analogues are required to expedite structure-activity relationship studies. Described herein is a synthetic route that enables rapid access to analogues. Furthermore, our new N-methylated analogues highlight that hydrogen bonding along the N-terminal tail is likely to be important for antimicrobial activity.

Gram-scale total synthesis of teixobactin promoting binding mode study and discovery of more potent antibiotics

Teixobactin represents a new class of antibiotics with novel structure and excellent activity against Gram-positive pathogens and Mycobacterium tuberculosis. Herein, we report a one-pot reaction to conveniently construct the key building block L-allo-Enduracidine in 30-gram scale in just one hour  and a convergent strategy (3 + 2 + 6) to accomplish a gram-scale total synthesis of teixobactin. Several analogs are described, with 20 and 26 identified as the most efficacious analogs with 3~8-fold and 2~4-fold greater potency against vancomycin resistant Enterococcus faecalis and methicillin-resistant Staphylococcus aureus respectively in comparison with teixobactin. In addition, they show high efficiency in Streptococcus pneumoniae septicemia mouse model and neutropenic mouse thigh infection model using methicillin-resistant Staphylococcus aureus. We also propose that the antiparallel β-sheet of teixobactin is important for its bioactivity and an antiparallel dimer of teixobactin is the minimal binding unit for lipid II via key amino acids variations and molecular docking.

Recent Progress in Natural-Product-Inspired Programs Aimed To Address Antibiotic Resistance and Tolerance

Bacteria utilize multiple mechanisms that enable them to gain or acquire resistance to antibiotic therapies during the treatment of infections. In addition, bacteria form biofilms which are surface-attached communities of enriched populations containing persister cells encased within a protective extracellular matrix of biomolecules, leading to chronic and recurring antibiotic-tolerant infections. Antibiotic resistance and tolerance are major global problems that require innovative therapeutic strategies to address the challenges associated with pathogenic bacteria. Historically, natural products have played a critical role in bringing new therapies to the clinic to treat life-threatening bacterial infections. This Perspective provides an overview of antibiotic resistance and tolerance and highlights recent advances (chemistry, biology, drug discovery, and development) from various research programs involved in the discovery of new antibacterial agents inspired by a diverse series of natural product antibiotics.

The Necessity of d-Thr in the New Antibiotic Teixobactin: A Molecular Dynamics Study

Ever since the discovery of the new antibiotic teixobactin, studies of its structure-activity relationships have never ceased. Here we focus on the chirality of the threonine (Thr) residue, which belongs to the ring motif of teixobactin and plays an important role in the binding with its target, lipid II molecule. We study the structural propensity of the open and closed ring motifs with different chiral Thr residues as well as the teixobactin-lipid II complex with the help of molecular dynamics simulations. Our results suggest that different chiralities lead to different NH orientations of Thr with respect to the ring plane. Only in the closed ring motif with d-Thr is a favored binding cavity achievable with all four NH groups facing the same side of the ring plane. This study develops a deeper understanding of the binding mechanism of teixobactin and lipid II and is expected to be beneficial to new teixobactin-based drug design.

Using chemical synthesis to optimise antimicrobial peptides in the fight against antimicrobial resistance

The emergence of multidrug-resistant bacteria has necessitated the urgent need for novel antibacterial agents. Antimicrobial peptides (AMPs), the host-defence molecules of most living organisms, have shown great promise as potential antibiotic candidates due to their multiple mechanisms of action which result in very low or negligible induction of resistance. However, the development of AMPs for clinical use has been limited by their potential toxicity to animal cells, low metabolic stability and high manufacturing cost. Extensive efforts have therefore been directed towards the development of enhanced variants of natural AMPs to overcome these aforementioned limitations. In this review, we present our efforts focused on development of efficient strategies to prepare several recently discovered AMPs including antitubercular peptides. The design and synthesis of more potent and stable AMP analogues with synthetic modifications made to the natural peptides containing glycosylated residues or disulfide bridges are described.

Cysteines and Disulfide-Bridged Macrocyclic Mimics of Teixobactin Analogues and Their Antibacterial Activity Evaluation against Methicillin-Resistant Staphylococcus Aureus (MRSA)

Teixobactin is a highly potent cyclic depsipeptide which kills a broad range of multi-drug resistant, Gram-positive bacteria, such as Methicillin-resistant Staphylococcus aureus (MRSA) without detectable resistance. In this work, we describe the design and rapid synthesis of novel teixobactin analogues containing two cysteine moieties, and the corresponding disulfide-bridged cyclic analogues. These analogues differ from previously reported analogues, such as an Arg₁₀-teixobactin, in terms of their macrocyclic ring size, and feature a disulfide bridge instead of an ester linkage. The new teixobactin analogues were screened against Methicillin-resistant Staphylococcus aureus and Methicillin-sensitive Staphylococcus aureus. Interestingly, one teixobactin analogue containing all l-amino acid building blocks showed antibacterial activity against MRSA for the first time. Our data indicates that macrocyclisation of teixobactin analogues with disulfide bridging is important for improved antibacterial activity. In our work, we have demonstrated the unprecedented use of a disulfide bridge in constructing the macrocyclic ring of teixobactin analogues.

Probing key elements of teixobactin-lipid II interactions in membranes

Two binding poses of the teixobactin–lipid II complex were captured with MD simulations at the membrane surface.

Teixobactin (Txb) is a recently discovered antibiotic against Gram-positive bacteria that induces no detectable resistance. The bactericidal mechanism is believed to be the inhibition of cell wall biosynthesis by Txb binding to lipid II and lipid III. Txb binding specificity likely arises from targeting of the shared lipid component, the pyrophosphate moiety. Despite synthesis and functional assessment of numerous chemical analogs of Txb, and consequent identification of the Txb pharmacophore, the detailed structural information of Txb–substrate binding is still lacking. Here, we use molecular modeling and microsecond-scale molecular dynamics simulations to capture the formation of Txb–lipid II complexes at a membrane surface. Two dominant binding conformations were observed, both showing characteristic lipid II phosphate binding by the Txb backbone amides near the C-terminal cyclodepsipeptide (d-Thr8–Ile11) ring. Additionally, binding by Txb also involved the side chain hydroxyl group of Ser7, as well as a secondary phosphate binding provided by the side chain of l-allo-enduracididine. Interestingly, those conformations differ by swapping two groups of hydrogen bond donors that coordinate the two phosphate moieties of lipid II, resulting in opposite orientations of lipid II binding. In addition, residues d-allo-Ile5 and Ile6 serve as the membrane anchors in both Txb conformations, regardless of the detailed phosphate binding interactions near the cyclodepsipeptide ring. The role of hydrophobic residues in Txb activity is primarily for its membrane insertion, and subsidiarily to provide non-polar interactions with the lipid II tail. Based on the Txb–lipid II interactions captured in their complexes, as well as their partitioning depths into the membrane, we propose that the bactericidal mechanism of Txb is to arrest cell wall synthesis by selectively inhibiting the transglycosylation of peptidoglycan, while possibly leaving the transpeptidation step unaffected. The observed “pyrophosphate caging” mechanism of lipid II inhibition appears to be similar to some lantibiotics, but different from that of vancomycin or bacitracin.

In Vitro Antibacterial Activity of Teixobactin Derivatives on Clinically Relevant Bacterial Isolates

Methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococcus (VRE) are included on the WHO high priority list of pathogens that require urgent intervention. Hence emphasis needs to be placed on developing novel class of molecules to tackle these pathogens. Teixobactin is a new class of antibiotic that has demonstrated antimicrobial activity against common bacteria. Here we examined the antimicrobial properties of three Teixobactin derivatives against clinically relevant bacterial isolates taken from South African patients. The minimum inhibitory concentration (MIC), the minimal bactericidal concentration (MBC), the effect of serum on MICs and the time-kill kinetics studies of our synthesized Teixobactin derivatives (3, 4, and 5) were ascertained following the CLSI 2017 guidelines and using the broth microdilution method. Haemolysis on red blood cells (RBCs) and cytotoxicity on peripheral blood mononuclear cells (PBMCs) were performed to determine the safety of these compounds. The MICs of 3, 4, and 5 against reference strains were 4–64 μg/ml, 2–64 μg/ml, and 0.5–64 μg/ml, respectively. The MICs observed for MRSA were (3) 32 μg/ml, (4) 2–4 μg/ml and (5) 2–4 μg/ml whilst those for VRE were (3) 8–16 μg/ml, (4) 4 μg/ml and (5) 2–16 μg/ml, respectively. In the presence of 50% human serum, there was no significant effect on the MICs. The compounds did not exhibit any effect on cell viability at their effective concentrations. Teixobactin derivatives (3, 4, and 5) inhibited bacterial growth in drug-resistant bacteria and hence emerge as potential antimicrobial agents. Molecular dynamic simulations suggested that the most dominant binding mode of Lys10-teixobactin (4) to lipid II is through the amide protons of the cycle, which is identical to data described in the literature for the natural teixobactin hence predicting the possibility of a similar mechanism of action.

Developing Equipotent Teixobactin Analogues against Drug-Resistant Bacteria and Discovering a Hydrophobic Interaction between Lipid II and Teixobactin

Teixobactin, targeting lipid II, represents a new class of antibiotics with novel structures and has excellent activity against Gram-positive pathogens. We developed a new convergent method to synthesize a series of teixobactin analogues and explored structure-activity relationships. We obtained equipotent and simplified teixobactin analogues, replacing the l- allo-enduracididine with lysine, substituting oxygen to nitrogen on threonine, and adding a phenyl group on the d-phenylalanine. On the basis of the antibacterial activities that resulted from corresponding modifications of the d-phenylalanine, we propose a hydrophobic interaction between lipid II and the N-terminal of teixobactin analogues, which we map out with our analogue 35. Finally, a representative analogue from our series showed high efficiency in a mouse model of Streptococcus pneumoniae septicemia.

Lipopeptidomimetics derived from teixobactin have potent antibacterial activity against Staphylococcus aureus

A series of lipopeptidomimetics derived from teixobactin have been prepared that probe the role of residues (1-6) as a membrane anchor and the function of enduracididine. The most active compounds, with a farnesyl tail and End10 to Lys10 or Orn10 substitution have potent activity (MIC 8 μg mL^-1) against S. aureus. These results pave the way for the synthesis of simple, cost-effective yet potent lipopeptidomimetic antimicrobials.

X-ray crystallographic structure of a teixobactin analogue reveals key interactions of the teixobactin pharmacophore

The X-ray crystallographic structure of a truncated teixobactin analogue reveals hydrogen-bonding and hydrophobic interactions and a cavity that binds a chloride anion. Minimum inhibitory concentration (MIC) assays against Gram-positive bacteria correlate the observed structure with antibiotic activity.

A Thiolactone Strategy for Straightforward Synthesis of Disulfide-Linked Side-Chain-to-Tail Cyclic Peptides Featuring an N-Terminal Modification Handle

The development of straightforward and versatile peptide cyclisation methods is highly desired to meet the demand for more stable peptide-based drugs. Herein, a new method for the synthesis of side-chain-to-tail cyclic peptides with the simultaneous introduction of an N-terminal handle, based on the introduction of an N-terminal thiolactone building block, is described. A primary amine liberates a homocysteine analogue from the thiolactone building block, which further enables cyclisation of the peptide through disulfide-bond formation with a C-terminal cysteamine. Postcyclisation modification can be achieved by using small bifunctional amines. Alternatively, the synthesis of lipopeptides is demonstrated through direct thiolactone opening with long-chain alkyl amines.

Design and Syntheses of Highly Potent Teixobactin Analogues against Staphylococcus aureus, Methicillin-Resistant Staphylococcus aureus (MRSA), and Vancomycin-Resistant Enterococci (VRE) in Vitro and in Vivo

The cyclic depsipeptide, teixobactin, kills a number of Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), and Mycobacterium tuberculosis without detectable resistance. To date, teixobactin is the only molecule in its class that has shown in vivo antibacterial efficacy. In this work, we designed and synthesized 10 new in vivo ready teixobactin analogues. These analogues showed highly potent antibacterial activities against Staphylococcus aureus, MRSA, and vancomycin-resistant enterococci (VRE) in vitro. One analogue, d-Arg₄-Leu₁₀-teixobactin, 2, was found to be noncytotoxic in vitro and in vivo. Moreover, topical instillation of peptide 2 in a mouse model of S. aureus keratitis decreased the bacterial bioburden (>99.0% reduction) and corneal edema significantly as compared to untreated mouse corneas. Collectively, our results have established the high therapeutic potential of a teixobactin analogue in attenuating bacterial infections and associated severities in vivo.

Total synthesis of teixobactin and its stereoisomers

The total syntheses of teixobactin and a series of its stereoisomers at positions 2, 5, 6, 10 and 11 were achieved via a combined strategy of solution and solid phase peptide synthesis.

Chemistry and Biology of Teixobactin

Bacterial resistance to existing drugs is becoming a serious public health issue, urging extensive search for new antibiotics. Teixobactin, a cyclic depsipeptide discovered in a screen of uncultured bacteria, shows potent activity against all the tested Gram-positive bacteria. Remarkably, no teixobactin-resistant bacterial strain has been obtained despite extensive efforts, highlighting the great potential of teixobactin as a lead compound in the fight against antimicrobial resistance (AMR). This review summarizes recent progresses in the understanding of many aspects of teixobactin, including chemical structure, biological activity, biosynthetic pathway, and mode of action. We also discuss the different synthetic strategies in producing teixobactin and its analogues, and the structure-activity relationship (SAR) studies.

Teixobactin and Its Analogues: A New Hope in Antibiotic Discovery

Increasing bacterial resistance against current antibiotics and lack of new molecules to combat bacterial resistance are key challenges to global health. There is, therefore, a continuing need to develop new antibiotics. Teixobactin, a cyclic undecapeptide, displays excellent antibacterial activities against a range of pathogenic bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA) and Mycobacterium tuberculosis. Interestingly, it operates by multiple modes of actions and is bactericidal toward S. aureus without detectable resistance. This unique combination of wide Gram-positive activity coupled with its inability to elicit resistance make teixobactin a very attractive molecule for antimicrobial therapeutic development. This Viewpoint discusses teixobactin, its analogues, and the challenges and opportunities associated with their future development.

Alanine scan reveals modifiable residues in teixobactin

An alanine scan of Lys10-teixobactin reveals that a cationic residue at position 10 is not necessary for antibiotic activity and that position 3 tolerates substitution without loss of activity. An unexpected correlation between poor aqueous solubility and better antibiotic activity of the teixobactin analogues is observed.

Teixobactin analogues reveal enduracididine to be non-essential for highly potent antibacterial activity and lipid II binding

Leu₁₀-teixobactin and Ile₁₀-teixobactin have shown comparable activity to natural teixobactin.

Teixobactin is a highly promising antibacterial depsipeptide consisting of four d-amino acids and a rare l-allo-enduracididine amino acid. l-allo-Enduracididine is reported to be important for the highly potent antibacterial activity of teixobactin. However, it is also a key limiting factor in the development of potent teixobactin analogues due to several synthetic challenges such as it is not commercially available, requires a multistep synthesis, long and repetitive couplings (16–30 hours). Due to all these challenges, the total synthesis of teixobactin is laborious and low yielding (3.3%). In this work, we have identified a unique design and developed a rapid synthesis (10 min μwave assisted coupling per amino acid, 30 min cyclisation) of several highly potent analogues of teixobactin with yields of 10–24% by replacing the l-allo-enduracididine with commercially available non-polar residues such as leucine and isoleucine. Most importantly, the Leu₁₀-teixobactin and Ile₁₀-teixobactin analogues have shown highly potent antibacterial activity against a broader panel of MRSA and Enterococcus faecalis (VRE). Furthermore, these synthetic analogues displayed identical antibacterial activity to natural teixobactin (MIC 0.25 μg mL^–1) against MRSA ATCC 33591 despite their simpler design and ease of synthesis. We have confirmed lipid II binding and measured the binding affinities of individual amino acid residues of Ala₁₀-teixobactin towards geranyl pyrophosphate by NMR to understand the nature and strength of binding interactions. Contrary to current understanding, we have shown that a cationic amino acid at position 10 is not essential for target (lipid II) binding and potent antibacterial activity of teixobactin. We thus provide strong evidence contrary to the many assumptions made about the mechanism of action of this exciting new antibiotic. Introduction of a non-cationic residue at position 10 allows for tremendous diversification in the design and synthesis of highly potent teixobactin analogues and lays the foundations for the development of teixobactin analogues as new drug-like molecules to target MRSA and Mycobacterium tuberculosis.

Teixobactin as a scaffold for unlimited new antimicrobial peptides: SAR study

It looks that a new era of antimicrobial peptides (AMPs) started with the discovery of teixobactin, which is a "head to side-chain" cyclodepsipeptide. It was isolated from a soil gram-negative b-proteobacteria by means of a revolutionary technique. Since there, several groups have developed synthetic strategies for efficient synthesis of this peptide and its analogues as well. Herein, all chemistries reported as well as the biological activity of the analogues are analyzed. Finally, some inputs regarding new trends for the next generation of analogues are discussed.

Investigation of the N-Terminus Amino Function of Arg10-Teixobactin

Teixobactin is a recently described antimicrobial peptide that shows high activity against gram-positive bacteria as well as mycobacterium tuberculosis. Due to both its structure as a head-to-side chain cyclodepsipeptide and its activity, it has attracted the attention of several research groups. In this regard, a large number of analogs with substitutions in both the cycle and the tail has been described. Here, we report the contribution of the N-terminus residue, N-Me-d-Phe, to the activity of Arg₁₀-teixobactin. On the basis of our findings, we conclude that the N-terminus accepts minimum changes but not the presence of long alkyl chains. The presence of a positive charge is a requirement for the activity of the peptide. Furthermore, acylation of the N-terminus leads to total loss of activity.