Total syntheses and initial evaluation of [Ψ[C(═S)NH]Tpg⁴]vancomycin, [Ψ[C(═NH)NH]Tpg⁴]vancomycin, [Ψ[CH₂NH]Tpg⁴]vancomycin, and their (4-chlorobiphenyl)methyl derivatives: synergistic binding pocket and peripheral modifications for the glycopeptide antibiotics

Tetrachlorovancomycin: Total Synthesis of a Designed Glycopeptide Antibiotic of Reduced Synthetic Complexity

A technically straightforward total synthesis of a new class of vancomycin analogues of reduced synthetic complexity was developed that provided tetrachlorovancomycin (1, LLS = 15 steps, 15% overall yield) and its precursor aglycon 29 (nearly 20% overall yield). The class retains all the intricate vancomycin structural features that contribute to its target binding affinity and selectivity, maintains the antimicrobial activity of vancomycin, and achieves the simplification by an unusual addition, not removal, of benign substituents to the core structure. The modification, accomplished by addition of two aryl chloride substituents to provide 1, permitted a streamlined total synthesis of the new glycopeptide antibiotic class by removing the challenges associated with CD and DE ring system atropisomer stereochemical control. This also enabled their simultaneous and further-activated SNAr macrocyclizations that establish the tricyclic skeleton of 1. Key elements of the approach include catalyst-controlled diastereoselective formation of the AB biaryl axis of chirality (>30:1 dr), an essentially instantaneous macrolactamization of the AB ring system free of competitive epimerization (>30:1 dr), racemization free coupling of the E ring tetrapeptide, room temperature simultaneous CD and DE ring system cyclizations, a highly refined 4-step conversion of the cyclization product to the aglycon, and a protecting-group-free one-pot enzymatic glycosylation for disaccharide introduction. In addition to the antimicrobial evaluation of tetrachlorovancomycin (1), the preparation of key peripherally modified derivatives, which introduce independent and synergistic mechanisms of action, revealed their exceptional antimicrobial potency and provide the foundation for future use of this new class of synthetic glycopeptide analogues.

Site-Selective Modification of Secondary Amine Moieties on Native Peptides, Proteins, and Natural Products with Ynones

Site-selective modification of biologically relevant secondary amines in peptides, proteins, and natural products has been challenging due to the similar reactivity between primary and secondary amines. Even for the secondary amines, their reactivities are significantly influenced by their structures and environment. Herein, we report a ynone Michael bioconjugation method for selective modification of secondary amines in unprotected peptides and proteins and complex natural products. We show that fine tuning the electronic effect of the ynones enables controlling the Michael acceptor reactivity for the selective reaction with the structurally different secondary amines in densely functionalized complex structures and complicated biological environment.

Single Modification at the N-Terminus of Norvancomycin to Combat Drug-Resistant Gram-Positive Bacteria

In the arsenal of glycopeptide antibiotics, norvancomycin, which differs from vancomycin by a single methyl group, has received much less attention. Facing the risks of serious antibiotic resistance and even the collapse of last-line defenses, we designed and synthesized 40 novel norvancomycin derivatives to combat the threat. 32 compounds are single N-terminally modified derivatives generated through simple and efficient methods. Diversity at the N-terminus was greatly enriched, mainly by lipophilic attachment and strategies for the introduction of lipo-sulfonium moieties for extensive structure-activity relationship analysis. The first incorporation of a sulfonium moiety into the norvancomycin structure gave rise to compounds that exhibited 4- to 2048-fold higher activity against vancomycin-resistant bacteria VISA and VRE. This N-terminal modification for norvancomycin provides an alternatively useful and promising strategy to restore the antibacterial activity of glycopeptide antibiotics against resistant bacteria, highlighting the same importance of the N-terminal site as well as the vancosamine position, which is worth further study and development.

Improved preparative enzymatic glycosylation of vancomycin aglycon and analogues

Modifications to the enzymatic glycosylation of vancomycin and its residue 4 thioamide analogue are detailed that significantly reduce the enzyme loading and amount of glycosyl donor needed for each glycosylation reaction, provide a streamlined synthesis and replacement for the synthetic UDP-vancosamine glycosyl donor to improve both access and storage stability, and permit a single-pot, two-step conversion of the aglycons to the fully glycosylated synthetic glycopeptides now conducted at higher concentrations. The improvements are exemplified with the two-step, one-pot glycosylation of [Ψ[C(=S)NH]Tpg4]vancomycin aglycon (92%) conducted on a 400 mg scale (2 mg to 1 g scales) and vancomycin aglycon itself (5 mg scale, 84%).

Synthesis of a tricyclic hexapeptide -via two consecutive ruthenium-catalyzed macrocyclization steps- with a constrained topology to mimic vancomycin's binding properties toward D-Ala-D-Ala dipeptide

A ring-closing metathesis (RCM) - peptide coupling - ruthenium-catalyzed azide alkyne cycloaddition (RuAAC) strategy was developed to synthesize a tricyclic hexapeptide in which the side chain to side chain connectivity pattern resulted in a mimic with a topology that effectively mimics the bioactivity of vancomycin as a potent binder of the bacterial cell wall D-Ala-D-Ala dipeptide sequence and more importantly being an effective inhibitor of bacterial growth.

Natural and engineered precision antibiotics in the context of resistance

Antibiotics are essential weapons in our fight against infectious disease, yet the consequences of broad-spectrum antibiotic use on microbiome stability and pathogen resistance are prompting investigations into more selective alternatives. Echoing the advent of precision medicine in oncology, precision antibiotics with focused activities are emerging as a means of addressing infections without damaging microbiomes or incentivizing resistance. Historically, antibiotic design principles have been gleaned from Nature, and reinvestigation of overlooked antibacterials is now providing scaffolds and targets for the design of pathogen-specific drugs. In this perspective, we summarize the biosynthetic and antibacterial mechanisms used to access these activities, and discuss how such strategies may be co-opted through engineering approaches to afford precision antibiotics.

Maxamycins: Durable Antibiotics Derived by Rational Redesign of Vancomycin

Since its discovery, vancomycin has been used in the clinic for >60 years. Because of their durability, vancomycin and related glycopeptides serve as the antibiotics of last resort for the treatment of protracted bacterial infections of resistant Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and multidrug-resistant (MDR) Streptococcus pneumoniae. After 30 years of use, vancomycin resistance was first observed and is now widespread in enterococci and more recently in S. aureus. The widespread prevalence of vancomycin-resistant enterococci (VRE) and the emergence of vancomycin-resistant S. aureus (VRSA) represent a call to focus on the challenge of resistance, highlight the need for new therapeutics, and provide the inspiration for the design of more durable antibiotics less prone to bacterial resistance than even vancomycin.Herein we summarize progress on efforts to overcome vancomycin resistance, first addressing recovery of its original durable mechanism of action and then introducing additional independent mechanisms of action intended to increase the potency and durability beyond that of vancomycin itself. The knowledge of the origin of vancomycin resistance and an understanding of the molecular basis of the loss of binding affinity between vancomycin and the altered target ligand d-Ala-d-Lac provided the basis for the subtle and rational redesign of the vancomycin binding pocket to remove the destabilizing lone-pair repulsion or reintroduce a lost H-bond while not impeding binding to the unaltered ligand d-Ala-d-Ala. Preparation of the modified glycopeptide core structure was conducted by total synthesis, providing binding pocket-modified vancomycin aglycons with dual d-Ala-d-Ala/d-Lac binding properties that directly address the intrinsic mechanism of resistance to vancomycin. Fully glycosylated pocket-modified vancomycin analogues were generated through a subsequent two-step enzymatic glycosylation, providing a starting point for peripheral modifications used to introduce additional mechanisms of action. A well-established vancosamine N-(4-chlorobiphenyl)methyl (CBP) modification as well as newly discovered C-terminal trimethylammonium cation (C1) or guanidine modifications were introduced, providing two additional synergistic mechanisms of action independent of d-Ala-d-Ala/d-Lac binding. The CBP modification provides an additional stage for inhibition of cell wall synthesis that results from direct competitive inhibition of transglycosylase, whereas the C1/guanidine modification induces bacteria cell permeablization. The synergistic behavior of the three independent mechanisms of action combined in a single molecule provides ultrapotent antibiotics (MIC = 0.01-0.005 μg/mL against VanA VRE). Beyond the remarkable antimicrobial activity, the multiple mechanisms of action suppress the rate at which resistance may be selected, where any single mechanism of action is protected by the action of others. The results detailed herein show that rational targeting of durable vancomycin-derived antibiotics has generated compounds with a "resistance against resistance", provided new candidate antibiotics, and may serve as a generalizable strategy to combat antibacterial resistance.

The quest for supernatural products: the impact of total synthesis in complex natural products medicinal chemistry

Covering: 2000 up to 2020This review presents select recent advances in the medicinal chemistry of complex natural products that are prepared by total synthesis. The underlying studies highlight enabling divergent synthetic strategies and methods that permit the systematic medicinal chemistry studies of key analogues bearing deep-seated structural changes not readily accessible by semisynthetic or biosynthetic means. Select and recent examples are detailed where the key structural changes are designed to improve defined properties or to overcome an intrinsic limitation of the natural product itself. In the examples presented, the synthetic efforts provided supernatural products, a term first introduced by our colleague Ryan Shenvi (Synlett, 2016, 27, 1145-1164), with properties superseding the parent natural product. The design principles and approaches for creating the supernatural products are highlighted with an emphasis on the properties addressed that include those that improve activity or potency, increase selectivity, enhance durability, broaden the spectrum of activity, improve chemical or metabolic stability, overcome limiting physical properties, add mechanisms of action, enhance PK properties, overcome drug resistance, and/or improve in vivo efficacy. Some such improvements may be regarded by some as iterative enhancements whereas others, we believe, truly live up to their characterization as supernatural products. Most such efforts are also accompanied by advances in synthetic organic chemistry, inspiring the development of new synthetic methodology and providing supernatural products with improved synthetic accessibility.

Next-Generation Total Synthesis of Vancomycin

A next-generation total synthesis of vancomycin aglycon is detailed that was achieved in 17 steps (longest linear sequence, LLS) from the constituent amino acid subunits with kinetically controlled diastereoselective introduction of all three elements of atropisomerism. In addition to new syntheses of three of the seven amino acid subunits, highlights of the approach include a ligand-controlled atroposelective one-pot Miyaura borylation-Suzuki coupling sequence for introduction of the AB biaryl axis of chirality (>20:1 dr), an essentially instantaneous and scalable macrolactamization of the AB ring system nearly free of competitive epimerization (>30:1 dr), and two room-temperature atroposelective intramolecular S_NAr cyclizations for sequential CD (8:1 dr) and DE ring closures (14:1 dr) that benefit from both preorganization by the preformed AB ring system and subtle substituent effects. Combined with a protecting group free two-step enzymatic glycosylation of vancomycin aglycon, this provides a 19-step total synthesis of vancomycin. The approach paves the way for large-scale synthetic preparation of pocket-modified vancomycin analogues that directly address the underlying mechanism of resistance to vancomycin.

Vancomycin C-Terminus Guanidine Modifications and Further Insights into an Added Mechanism of Action Imparted by a Peripheral Structural Modification

A series of vancomycin C-terminus guanidine modifications is disclosed that improves antimicrobial activity, enhances the durability of antimicrobial action against selection or induction of resistance, and introduces a synergistic mechanism of action independent of d-Ala-d-Ala binding and inhibition of cell wall biosynthesis. The added mechanism of action results in induced bacterial cell permeability, which we show may involve interaction with cell envelope teichoic acid. Significantly, the compounds examined that contain two combined peripheral modifications, a (4-chlorobiphenyl)methyl (CBP) and C-terminus guanidinium modification, offer opportunities for new treatments against not only vancomycin-sensitive but especially vancomycin-resistant bacteria where they act by two synergistic and now durable mechanisms of action independent of d-Ala-d-Ala/d-Lac binding and display superb antimicrobial potencies (MIC 0.6-0.15 μg/mL, VanA VRE). For the first time, we demonstrate that the synergistic behavior of the peripheral modifications examined requires the presence of both the CBP and guanidine modifications in a single molecule versus their combined use as an equimolar mixture of singly modified compounds. Finally, we show that a prototypical member of the series, G3-CBP-vancomycin (15), exhibits no hemolytic activity, displays no mammalian cell growth inhibition, possesses improved and especially attractive in vivo pharmacokinetic (PK) properties, and displays excellent in vivo efficacy and potency against an especially challenging multidrug-resistant (MRSA) and VanA vancomycin-resistant (VRSA) Staphylococcus aureus bacterial strain.

Treating a Global Health Crisis with a Dose of Synthetic Chemistry

The SARS-CoV-2 pandemic has prompted scientists from many disciplines to work collaboratively toward an effective response. As academic synthetic chemists, we examine how best to contribute to this ongoing effort.

Instructive Advances in Chemical Microbiology Inspired by Nature's Diverse Inventory of Molecules

Natural product antibiotics have played an essential role in the treatment of bacterial infection in addition to serving as useful tools to explore the intricate biology of bacteria. Our current arsenal of antibiotics operate through the inhibition of well-defined bacterial targets critical for replication and growth. Pathogenic bacteria effectively utilize a diversity of mechanisms that lead to acquired resistance and/or innate tolerance toward antibiotic therapies, which can result in devastating consequences to human life. Several research groups have established innovative programs that work at the chemistry-biology interface to develop new molecules that aim to define and address concerns related to antibiotic resistance and tolerance. In this Review, we present recent progress by select research groups that highlight a diversity of integrated chemical biology and medicinal chemistry approaches aimed at the development and utilization of chemical tools that have led to promising new microbiological insights that may lead to significant clinical advances regarding the treatment of pathogenic bacteria.

Structure-Activity Relationship of Peptide-Conjugated Chloramphenicol for Inhibiting Escherichia coli

Intravenous administration of a prodrug, chloramphenicol succinate (CLsu), is ineffective. Recently, we have shown that conjugation of diglycine of CLsu (CLsuGG) not only increases the antibiotic efficacy against Escherichia coli but also reduces adverse drug effects against bone marrow stromal cells. Here, we report the synthesis of structural analogues of CLsuGG and their activities against E. coli. These analogues reveal several trends: (i) except the water-insoluble analogues, the attachment of peptides to CLsu enhances the efficacy of the prodrugs; (ii) negative charges, high steric hindrance in the side chains, or a rigid diester decreases the activities of prodrugs in comparison to CLsuGG; (iii) dipeptides apparently increase the efficacy of the prodrugs most effectively; and so forth. This work suggests that conjugating peptides to CLsu effectively modulates the properties of prodrugs. The structure-activity relationship of these new conjugates may provide useful insights for expanding the pool of antibiotics.

C1-CBP-vancomycin: Impact of a Vancomycin C-Terminus Trimethylammonium Cation on Pharmacological Properties and Insights into Its Newly Introduced Mechanism of Action

C1-CBP-vancomycin (3) was examined alongside CBP-vancomycin for susceptibility to acquired resistance upon serial exposure against two vancomycin-resistant enterococci strains where its activity proved more durable and remarkably better than many current therapies. Combined with earlier studies, this observation confirmed an added mechanism of action was introduced by incorporation of the trimethylammonium cation and that C1-CBP-vancomycin exhibits activity against vancomycin-resistant organisms through two synergistic mechanisms of action, both independent of d-Ala-d-Ala/d-Lac binding. New insights into this added mechanism of action, induced cell membrane permeabilization, can be inferred from studies that show added exogenous lipoteichoic acid reduces antimicrobial activity, rescues bacteria cell growth inhibition, and blocks induced cell permeabilization properties of C1-CBP-vancomycin, suggesting a direct binding interaction with embedded teichoic acid is responsible for the added mechanism of action and enhanced antimicrobial activity. Further studies indicate that the trimethylammonium cation does not introduce new liabilities in common pharmacological properties of the analogue and established that 3 is well tolerated in mice, displays substantial PK improvements over both vancomycin and CBP-vancomycin, and exhibits in vivo efficacy against a challenging multidrug-resistant and vancomycin-resistant S. aureus strain that is representative of the resistant pathogens all fear will emerge in the general population.

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.

Teicoplanin Reprogrammed with the N-Acyl-Glucosamine Pharmacophore at the Penultimate Residue of Aglycone Acquires Broad-Spectrum Antimicrobial Activities Effectively Killing Gram-Positive and -Negative Pathogens

Lipoglycopeptide antibiotics, for example, teicoplanin (Tei) and A40926, are more potent than vancomycin against Gram-positive (Gram-(+)) drug-resistant pathogens, for example, methicillin-resistant Staphylococcus aureus (MRSA). To extend their therapeutic effectiveness on vancomycin-resistant S. aureus (VRSA), the biosynthetic pathway of the N-acyl glucosamine (Glc) pharmacophore at residue 4 (r4) of teicoplanin pseudoaglycone redirection to residue 6 (r6) was attempted. On the basis of crystal structures, two regioselective biocatalysts Orf2*T (a triple-mutation mutant S98A/V121A/F193Y) and Orf11*S (a single-mutation mutant W163A) were engineered, allowing them to act on GlcNAc at r6. New analogs thereby made show marked antimicrobial activity against MRSA and VRSA by 2-3 orders of magnitude better than teicoplanin and vancomycin. The lipid side chain of the Tei-analogs armed with a terminal mono- or diguanidino group extends the antimicrobial specificity from Gram-(+) to Gram-negative (Gram-(-)), comparable to that of kanamycin. In addition to low cytotoxicity and high safety, the Tei analogs exhibit new modes of action as a result of resensitization of VRSA and Acinetobacter baumannii. The redirection of the biosynthetic pathway for the N-acyl-Glc pharmacophore from r4 to r6 bodes well for large-scale production of selected r6,Tei congeners in an environmentally friendly synthetic biology approach.

Biosynthesis and Chemical Applications of Thioamides

Thioamidation as a posttranslational modification is exceptionally rare, with only a few reported natural products and exactly one known protein example (methyl-coenzyme M reductase from methane-metabolizing archaea). Recently, there has been significant progress in elucidating the biosynthesis and function of several thioamide-containing natural compounds. Separate developments in the chemical installation of thioamides into peptides and proteins have enabled cell biology and biophysical studies to advance the current understanding of natural thioamides. This review highlights the various strategies used by Nature to install thioamides in peptidic scaffolds and the potential functions of this rare but important modification. We also discuss synthetic methods used for the site-selective incorporation of thioamides into polypeptides with a brief discussion of the physicochemical implications. This account will serve as a foundation for the further study of thioamides in natural products and their various applications.

Exploration of the site-specific nature and generalizability of a trimethylammonium salt modification on vancomycin: A-ring derivatives

Vancomycin analogues bearing an A-ring trimethylammonium salt modification were synthesized and their antimicrobial activity against vancomycin-resistant Enterococci (VRE) was evaluated. The modification increased antimicrobial potency and provided the capability to induce bacteria cell membrane permeabilization, but both properties were weaker than that found with our earlier reported similar C-terminus modification. The results provide further insights on the additive effect and generalizability of the structural and site-specific nature of a peripheral quaternary trimethylammonium salt modification of vancomycin.

Addressing Antimicrobial Resistance through New Medicinal and Synthetic Chemistry Strategies

Over the past century, a multitude of derivatives of structural scaffolds with established antimicrobial potential have been prepared and tested, and a variety of new scaffolds have emerged. The effectiveness of antibiotics, however, is in sharp decline because of the emergence of drug-resistant microorganisms. The prevalence of drug resistance, both in clinical and community settings, is a consequence of bacterial ingenuity in altering pathways and/or cell morphology, making it a persistent threat to human health. The fundamental ability of pathogens to survive in a multitude of habitats can be triggered by recognition of chemical signals that warn organisms of exposure to a potentially harmful environment. Host immune defenses, including reactive oxygen intermediates and antibacterial substances, are among the multitude of chemical signals that can subsequently trigger expression of phenotypes better adapted for survival in that hostile environment. Thus, resistance development appears to be unavoidable, which leads to the conclusion that developing an alternative perspective for treatment options is vital. This review will discuss emerging medicinal chemistry approaches for addressing the global multidrug resistance in the 21st century.

The race for antimicrobials in the multidrug resistance era

The appearance of multidrug-resistant pathogens is a major threat to human health with the reemergence of fatal and untreatable diseases. In addition to a rational use of the well-known and available antibiotics, two complementary ways to overcome this public health issue are (i) the discovery of new antimicrobials and (ii) the chemical modification of pre-existing potent antibiotics. In this article, we highlight some of the strategies to generate new and promising antimicrobials for use in the management of these so-called 'superbugs'.

N-Terminus Alkylation of Vancomycin: Ligand Binding Affinity, Antimicrobial Activity, and Site-Specific Nature of Quaternary Trimethylammonium Salt Modification

A series of vancomycin derivatives alkylated at the N-terminus amine were synthesized, including those that contain quaternary trimethylammonium salts either directly at the terminal amine site or with an intervening three-carbon spacer. The examination of their properties provides important comparisons with a C-terminus trimethylammonium salt modification that we recently found to improve the antimicrobial potency of vancomycin analogues through an added mechanism of action. The N-terminus modifications disclosed herein were well-tolerated, minimally altering model ligand binding affinities (d-Ala-d-Ala) and antimicrobial activity, but did not induce membrane permeabilization that was observed with a similar C-terminus modification. The results indicate that our earlier observations with the C-terminus modification are sensitive to the site as well as structure of the trimethylammonium salt modification and are not simply the result of nonspecific effects derived from introduction of a cationic charge.

Developments in Glycopeptide Antibiotics

Glycopeptide antibiotics (GPAs) are a key weapon in the fight against drug resistant bacteria, with vancomycin still a mainstream therapy against serious Gram-positive infections more than 50 years after it was first introduced. New, more potent semisynthetic derivatives that have entered the clinic, such as dalbavancin and oritavancin, have superior pharmacokinetic and target engagement profiles that enable successful treatment of vancomycin-resistant infections. In the face of resistance development, with multidrug resistant (MDR) S. pneumoniae and methicillin-resistant Staphylococcus aureus (MRSA) together causing 20-fold more infections than all MDR Gram-negative infections combined, further improvements are desirable to ensure the Gram-positive armamentarium is adequately maintained for future generations. A range of modified glycopeptides has been generated in the past decade via total syntheses, semisynthetic modifications of natural products, or biological engineering. Several of these have undergone extensive characterization with demonstrated in vivo efficacy, good PK/PD profiles, and no reported preclinical toxicity; some may be suitable for formal preclinical development. The natural product monobactam, cephalosporin, and β-lactam antibiotics all spawned multiple generations of commercially and clinically successful semisynthetic derivatives. Similarly, next-generation glycopeptides are now technically well positioned to advance to the clinic, if sufficient funding and market support returns to antibiotic development.

Old and new glycopeptide antibiotics: From product to gene and back in the post-genomic era

Glycopeptide antibiotics are drugs of last resort for treating severe infections caused by multi-drug resistant Gram-positive pathogens. First-generation glycopeptides (vancomycin and teicoplanin) are produced by soil-dwelling actinomycetes. Second-generation glycopeptides (dalbavancin, oritavancin, and telavancin) are semi-synthetic derivatives of the progenitor natural products. Herein, we cover past and present biotechnological approaches for searching for and producing old and new glycopeptide antibiotics. We review the strategies adopted to increase microbial production (from classical strain improvement to rational genetic engineering), and the recent progress in genome mining, chemoenzymatic derivatization, and combinatorial biosynthesis for expanding glycopeptide chemical diversity and tackling the never-ceasing evolution of antibiotic resistance.

Novel Modifications of Glycopeptide Antibiotics via Total Synthesis

Although vancomycin has been in clinical use since the late 1950s, resistance due to alteration in the target microbe's peptidoglycan can vary significantly, reducing its activity. Total synthesis of derivatives has now led to a molecule with very significant activity against resistant strains.

The Difference a Single Atom Can Make: Synthesis and Design at the Chemistry-Biology Interface

A Perspective of work in our laboratory on the examination of biologically active compounds, especially natural products, is presented. In the context of individual programs and along with a summary of our work, selected cases are presented that illustrate the impact single atom changes can have on the biological properties of the compounds. The examples were chosen to highlight single heavy atom changes that improve activity, rather than those that involve informative alterations that reduce or abolish activity. The examples were also chosen to illustrate that the impact of such single-atom changes can originate from steric, electronic, conformational, or H-bonding effects, from changes in functional reactivity, from fundamental intermolecular interactions with a biological target, from introduction of a new or altered functionalization site, or from features as simple as improvements in stability or physical properties. Nearly all the examples highlighted represent not only unusual instances of productive deep-seated natural product modifications and were introduced through total synthesis but are also remarkable in that they are derived from only a single heavy atom change in the structure.

Natural Products as Platforms To Overcome Antibiotic Resistance

Natural products have served as powerful therapeutics against pathogenic bacteria since the golden age of antibiotics of the mid-20th century. However, the increasing frequency of antibiotic-resistant infections clearly demonstrates that new antibiotics are critical for modern medicine. Because combinatorial approaches have not yielded effective drugs, we propose that the development of new antibiotics around proven natural scaffolds is the best short-term solution to the rising crisis of antibiotic resistance. We analyze herein synthetic approaches aiming to reengineer natural products into potent antibiotics. Furthermore, we discuss approaches in modulating quorum sensing and biofilm formation as a nonlethal method, as well as narrow-spectrum pathogen-specific antibiotics, which are of interest given new insights into the implications of disrupting the microbiome.

In Vitro Reconstitution of OxyA Enzymatic Activity Clarifies Late Steps in Vancomycin Biosynthesis

Studies on the biosynthesis of glycopeptide antibiotics have provided many insights into the strategies that Nature employs to build architecturally strained molecules. A key structural feature of vancomycin, the founding member of this class, is a set of three aromatic cross-links that are introduced via yet unknown mechanisms. Previous reports have identified three cytochrome P450 enzymes involved in this process and demonstrated enzymatic activity for OxyB, which installs the first aromatic cross-link. However, the activities of the remaining two P450 enzymes have not been recapitulated. Herein, we show that OxyA generates the second bis-aryl ether bond in vancomycin and that it exhibits strict substrate specificity toward the chlorinated, OxyB-cross-linked product. No OxyA product is detected with the unchlorinated substrate. Together with previous results, these data suggest that chlorination occurs after OxyB- but before OxyA-catalyzed cross-link formation. Our results have important implications for the chemo-enzymatic synthesis of vancomycin and its analogs.

Peripheral modifications of [Ψ[CH2NH]Tpg4]vancomycin with added synergistic mechanisms of action provide durable and potent antibiotics

Subsequent to binding pocket modifications designed to provide dual d-Ala-d-Ala/d-Ala-d-Lac binding that directly overcome the molecular basis of vancomycin resistance, peripheral structural changes have been explored to improve antimicrobial potency and provide additional synergistic mechanisms of action. A C-terminal peripheral modification, introducing a quaternary ammonium salt, is reported and was found to provide a binding pocket-modified vancomycin analog with a second mechanism of action that is independent of d-Ala-d-Ala/d-Ala-d-Lac binding. This modification, which induces cell wall permeability and is complementary to the glycopeptide inhibition of cell wall synthesis, was found to provide improvements in antimicrobial potency (200-fold) against vancomycin-resistant Enterococci (VRE). Furthermore, it is shown that this type of C-terminal modification may be combined with a second peripheral (4-chlorobiphenyl)methyl (CBP) addition to the vancomycin disaccharide to provide even more potent antimicrobial agents [VRE minimum inhibitory concentration (MIC) = 0.01-0.005 μg/mL] with activity that can be attributed to three independent and synergistic mechanisms of action, only one of which requires d-Ala-d-Ala/d-Ala-d-Lac binding. Finally, it is shown that such peripherally and binding pocket-modified vancomycin analogs display little propensity for acquired resistance by VRE and that their durability against such challenges as well as their antimicrobial potency follow now predictable trends (three > two > one mechanisms of action). Such antibiotics are expected to display durable antimicrobial activity not prone to rapidly acquired clinical resistance.

Total Syntheses of Vancomycin-Related Glycopeptide Antibiotics and Key Analogues

A review of efforts that have provided total syntheses of vancomycin and related glycopeptide antibiotics, their agylcons, and key analogues is provided. It is a tribute to developments in organic chemistry and the field of organic synthesis that not only can molecules of this complexity be prepared today by total synthesis but such efforts can be extended to the preparation of previously inaccessible key analogues that contain deep-seated structural changes. With the increasing prevalence of acquired bacterial resistance to existing classes of antibiotics and with the emergence of vancomycin-resistant pathogens (VRSA and VRE), the studies pave the way for the examination of synthetic analogues rationally designed to not only overcome vancomycin resistance but provide the foundation for the development of even more powerful and durable antibiotics.

Reengineering Antibiotics to Combat Bacterial Resistance: Click Chemistry [1,2,3]-Triazole Vancomycin Dimers with Potent Activity against MRSA and VRE

Vancomycin has long been considered a drug of last resort. Its efficiency in treating multiple drug-resistant bacterial infections, particularly methicillin-resistant Staphylococcus aureus (MRSA), has had a profound effect on the treatment of life-threatening infections. However, the emergence of resistance to vancomycin is a cause for significant worldwide concern, prompting the urgent development of new effective treatments for antibiotic resistant bacterial infections. Harnessing the benefits of multivalency and cooperativity against vancomycin-resistant strains, we report a Click Chemistry approach towards reengineered vancomycin derivatives and the synthesis of a number of dimers with increased potency against MRSA and vancomycin resistant Enterococci (VRE; VanB). These semi-synthetic dimeric ligands were linked together with great efficiency using the powerful CuAAC reaction, demonstrating high levels of selectivity and purity.

History of antimicrobial drug discovery: Major classes and health impact

The introduction of antibiotics into clinical practice revolutionized the treatment and management of infectious diseases. Before the introduction of antibiotics, these diseases were the leading cause of morbidity and mortality in human populations. This review presents a brief history of discovery of the main antimicrobial classes (arsphenamines, β-lactams, sulphonamides, polypeptides, aminoglycosides, tetracyclines, amphenicols, lipopeptides, macrolides, oxazolidinones, glycopeptides, streptogramins, ansamycins, quinolones, and lincosamides) that have changed the landscape of contemporary medicine. Given within a historical timeline context, the review discusses how the introduction of certain antimicrobial classes affected the morbidity and mortality rates due to bacterial infectious diseases in human populations. Problems of resistance to antibiotics of different classes are also extensively discussed.

Naturally Inspired Peptide Leads: Alanine Scanning Reveals an Actin-Targeting Thiazole Analogue of Bisebromoamide

Systematic alanine scanning of the linear peptide bisebromoamide (BBA), isolated from a marine cyanobacterium, was enabled by solid-phase peptide synthesis of thiazole analogues. The analogues have comparable cytotoxicity (nanomolar) to that of BBA, and cellular morphology assays indicated that they target the actin cytoskeleton. Pathway inhibition in human colon tumour (HCT116) cells was explored by reverse phase protein array (RPPA) analysis, which showed a dose-dependent response in IRS-1 expression. Alanine scanning reveals a structural dependence to the cytotoxicity, actin targeting and pathway inhibition, and allows a new readily synthesised lead to be proposed.

Targeting Antibiotic Resistance

Finding strategies against the development of antibiotic resistance is a major global challenge for the life sciences community and for public health. The past decades have seen a dramatic worldwide increase in human-pathogenic bacteria that are resistant to one or multiple antibiotics. More and more infections caused by resistant microorganisms fail to respond to conventional treatment, and in some cases, even last-resort antibiotics have lost their power. In addition, industry pipelines for the development of novel antibiotics have run dry over the past decades. A recent world health day by the World Health Organization titled "Combat drug resistance: no action today means no cure tomorrow" triggered an increase in research activity, and several promising strategies have been developed to restore treatment options against infections by resistant bacterial pathogens.

Evolving medicinal chemistry strategies in antibiotic discovery

Chemical modification of synthetic or natural product antibiotic scaffolds to expand potency and spectrum and to bypass mechanisms of resistance has dominated antibiotic drug discovery and proven immensely successful. However, the inexorable evolution of drug resistance coupled with a drought in innovation in antibiotic discovery contribute to a dearth of new drugs entering to market. Better understanding of the physicochemical properties of antibiotic chemical space is required to inform new antibiotic discovery. Innovations such as the development of antibiotic adjuvants to preserve efficacy of existing drugs together with expanding antibiotic chemical diversity through synthetic biology or new techniques to mine antibiotic producing organisms, are required to bridge the growing gap between the need for new drugs and their discovery.

Endless Resistance. Endless Antibiotics?

The practice of medicine was profoundly transformed by the introduction of the antibiotics (compounds isolated from Nature) and the antibacterials (compounds prepared by synthesis) for the control of bacterial infection. As a result of the extraordinary success of these compounds over decades of time, a timeless biological activity for these compounds has been presumed. This presumption is no longer. The inexorable acquisition of resistance mechanisms by bacteria is retransforming medical practice. Credible answers to this dilemma are far better recognized than they are being implemented. In this perspective we examine (and in key respects, reiterate) the chemical and biological strategies being used to address the challenge of bacterial resistance.