FimH antagonists: structure-activity and structure-property relationships for biphenyl α-D-mannopyranosides

Mannose-Presenting "Glyco-Colicins" Convert the Bacterial Cell Surface into a Multivalent Adsorption Site for Adherent Bacteria

Biofilm formation is integral to the pathogenesis of numerous adherent bacteria and contributes to antimicrobial resistance (AMR). The rising threat of AMR means the need to develop novel nonbactericidal antiadhesion approaches against such bacteria is more urgent than ever. Both adherent-invasive Escherichia coli (AIEC, implicated in inflammatory bowel disease) and uropathogenic E. coli (UPEC, responsible for ∼80% of urinary tract infections) adhere to terminal mannose sugars on epithelial glycoproteins through the FimH adhesin on their type 1 pilus. Although mannose-based inhibitors have previously been explored to inhibit binding of adherent bacteria to epithelial cells, this approach has been limited by monovalent carbohydrate-protein interactions. Herein, we pioneer a novel approach to this problem through the preparation of colicin E9 bioconjugates that bind to the abundant BtuB receptor in the outer membrane of bacteria, which enables multivalent presentation of functional motifs on the cell surface. We show these bioconjugates label the surface of live E. coli and furthermore demonstrate that mannose-presenting "glyco-colicins" induce E. coli aggregation, thereby using the bacteria, itself, as a multivalent platform for mannose display, which triggers binding to adjacent FimH-presenting bacteria.

Design, synthesis, biological evaluation and docking study of some new aryl and heteroaryl thiomannosides as FimH antagonists

FimH is a mannose-recognizing lectin that is expressed by Escherichia coli guiding its ability to adhere and infect cells. It is involved in pathogenesis of urinary tract infections and Chron's disease. Several X-ray structure-guided ligand design studies were extensively utilized in the discovery and optimization of small molecule aryl mannoside FimH antagonists. These antagonists retain key specific interactions of the mannose scaffolds with the FimH carbohydrate recognition domains. Thiomannosides are attractive and stable scaffolds, and this work reports the synthesis of some of their new aryl and heteroaryl derivatives as FimH antagonists. FimH-competitive binding assays as well as biofilm inhibition of the new compounds (24-32) were determined in comparison with the reference n-heptyl α-d-mannopyranoside (HM). The affinity among these compounds was found to be governed by the structure of the aryl and heteroarylf aglycones. Two compounds 31 and 32 revealed higher activity than HM. Molecular docking and total hydrophobic to topological polar surface area ratio calculations attributed to explain the obtained biological results. Finally, the SAR study suggested that introducing an aryl or heteroaryl aglycone of sufficient hydrophobicity and of proper orientation within the tyrosine binding site considerably enhance binding affinity. The potent and synthetically feasible FimH antagonists described herein hold potential as leads for the development of sensors for detection of E. coli and treatment of its diseases.

Does size matter? - Comparing pyranoses with septanoses as ligands of the bacterial lectin FimH

The pharmacological modulation of disease-relevant carbohydrate-protein interactions represents an underexplored area of medicinal chemistry. One particular challenge in the design of glycomimetic compounds is the inherent instability of the glycosidic bond toward enzymatic cleavage. This problem has traditionally been approached by employing S-, N-, or C-glycosides with reduced susceptibility toward glycosidases. The application of ring-extended glycomimetics is an innovative approach to circumvent this issue. On the example of the bacterial adhesin FimH, it was explored how design principles from pyranose glycomimetics transfer to analogous septanose structures. A series of ring-extended FimH antagonists exhibiting the well-proven pharmacophore necessary for targeting the tyrosine-gate of FimH was synthesized. The resulting septanoses were evaluated for their affinity to the conformationally rigid isolated lectin domain of FimH (FimHLD), as well as a structurally flexible full-length FimH (FimHFL) construct. Some elements of potent mannoside-based FimH antagonists could be successfully transferred to septanose-based ligands, ultimately resulting in a 32-fold increase in binding affinity. Interestingly, the canonical ca. 100-fold loss of binding affinity between FimHLD and FimHFL is partly mitigated by the more flexible septanose antagonists, hinting at potentially differing interaction features of the flexible glycomimetics with intermediately populated states during the conformational transition of FimHFL.

Discovery of Orally Bioavailable FmlH Lectin Antagonists as Treatment for Urinary Tract Infections

FmlH, a bacterial adhesin of uropathogenic Escherichia coli (UPEC), has been shown to provide a fitness advantage in colonizing the bladder during chronic urinary tract infections (UTIs). Previously reported ortho-biphenyl glycosides based on βGal and βGalNAc have excellent binding affinity to FmlH and potently block binding to its natural carbohydrate receptor, but they lack oral bioavailability. In this paper, we outline studies where we have optimized compounds for improved pharmacokinetics, leading to the discovery of novel analogues with good oral bioavailability. We synthesized galactosides with the anomeric O-linker replaced with more stable S- and C-linked linkers. We also investigated modifications to the GalNAc sugar and modifications to the biphenyl aglycone. We identified GalNAc 69 with an IC50 of 0.19 μM against FmlH and 53% oral bioavailability in mice. We also obtained a FimlH-bound X-ray structure of lead compound 69 (AM4085) which has potential as a new antivirulence therapeutic for UTIs.

Click chemistry inspired facile one-pot synthesis of mannosyl triazoles and their hemagglutination inhibitory properties: The effect of alkyl chain spacer length between the triazole and phthalimide moieties in the aglycone backbone

An efficient one-pot synthesis of a new series of mannosyl triazoles has been achieved through CuAAC reaction where the alkyl chain spacer between the phthalimide moiety and the triazole ring in the aglycone backbone is varied from one methylene to six methylene units. The target compounds were evaluated in terms of their inhibitory potency against FimH using hemagglutination inhibition (HAI) assay. It was found that the length of four methylene units was the optimum for the fitting/binding of the compound to FimH as exemplified by compound 11 (HAI = 1.9 μM), which was approximately 200 times more potent than the reference ligand 1(HAI = 385 μM). The successful implementation of one-pot protocol with building blocks 1-7 and the architecture of ligand 11 will be the subject of our future work for developing more potent FimH inhibitors.

Nature of the Interaction of Alpha-D-Mannose and Escherichia coli Bacteria, and Implications for its Regulatory Classification. A Delphi Panel European Consensus Based on Chemistry and Legal Evidence

The nature of alpha-D-mannose-natural aldohexose sugar, C-2 glucose epimer, whose intended use is for preventing urinary tract infections-in the interaction with E. coli is addressed in order to drive the issue of its regulatory classification as a medicinal product or medical device. PRISMA systematic review approach was applied; Delphi Panel method was used to target consensus on statements retrieved from evidence. Based on regulatory definitions and research evidence, the mechanism of D-mannose does not involve a metabolic or immunological action while there is uncertainty regarding the pharmacological action. Specific interaction between the product and the bacteria within the body occurs, but its nature is inert: it does not induce a direct response activating or inhibiting body processes. Moreover, the action of D-mannose takes place, even if inside the bladder, outside the epithelium on bacteria that have not yet invaded the urothelial tissue. Therefore, its mechanism of action is not directed to host structures but to structures (bacteria) external to the host's tissues. On the basis of current regulation, the uncertainty as regard a pharmacological action of alpha-D-mannose makes possible its medical device classification: new regulations and legal judgments can add further considerations. From a pharmacological perspective, research is driven versus synthetic mannosides: no further considerations are expected on alpha-D-mannose.

Insightful Improvement in the Design of Potent Uropathogenic E. coli FimH Antagonists

Selective antiadhesion antagonists of Uropathogenic Escherichia coli (UPEC) type-1 Fimbrial adhesin (FimH) are attractive alternatives for antibiotic therapies and prophylaxes against acute or recurrent urinary tract infections (UTIs) caused by UPECs. A rational small library of FimH antagonists based on previously described C-linked allyl α-D-mannopyranoside was synthesized using Heck cross-coupling reaction using a series of iodoaryl derivatives. This work reports two new members of FimH antagonist amongst the above family with sub nanomolar affinity. The resulting hydrophobic aglycones, including constrained alkene and aryl groups, were designed to provide additional favorable binding interactions with the so-called FimH "tyrosine gate". The newly synthesized C-linked glycomimetic antagonists, having a hydrolytically stable anomeric linkage, exhibited improved binding when compared to previously published analogs, as demonstrated by affinity measurement through interactions by FimH lectin. The crystal structure of FimH co-crystallized with one of the nanomolar antagonists revealed the binding mode of this inhibitor into the active site of the tyrosine gate. In addition, selected mannopyranoside constructs neither affected bacterial growth or cell viability nor interfered with antibiotic activity. C-linked mannoside antagonists were effective in decreasing bacterial adhesion to human bladder epithelial cells (HTB-9). Therefore, these molecules constituted additional therapeutic candidates' worth further development in the search for potent anti-adhesive drugs against infections caused by UPEC.

A Remote Secondary Binding Pocket Promotes Heteromultivalent Targeting of DC-SIGN

Dendritic cells (DC) are antigen-presenting cells coordinating the interplay of the innate and the adaptive immune response. The endocytic C-type lectin receptors DC-SIGN and Langerin display expression profiles restricted to distinct DC subtypes and have emerged as prime targets for next-generation immunotherapies and anti-infectives. Using heteromultivalent liposomes copresenting mannosides bearing aromatic aglycones with natural glycan ligands, we serendipitously discovered striking cooperativity effects for DC-SIGN⁺ but not for Langerin⁺ cell lines. Mechanistic investigations combining NMR spectroscopy with molecular docking and molecular dynamics simulations led to the identification of a secondary binding pocket for the glycomimetics. This pocket, located remotely of DC-SIGN’s carbohydrate bindings site, can be leveraged by heteromultivalent avidity enhancement. We further present preliminary evidence that the aglycone allosterically activates glycan recognition and thereby contributes to DC-SIGN-specific cell targeting. Our findings have important implications for both translational and basic glycoscience, showcasing heteromultivalent targeting of DCs to improve specificity and supporting potential allosteric regulation of DC-SIGN and CLRs in general.

α-d-Mannoside ligands with a valency ranging from one to three: Synthesis and hemagglutination inhibitory properties

Six mono-, di-, and trivalent α-d-mannopyranosyl conjugates built on aromatic scaffolds were synthesized in excellent yields by Cu(I) catalyzed azide-alkyne cycloaddition reaction (CuAAC). These conjugates were designed to have unique, flexible tails that combine a mid-tail triazole ring, to interact with the tyrosine gate, with a terminal phenyl group armed with benzylic hydroxyl groups to avoid solubility problems as well as to provide options to connect to other supports. Biological evaluation of the prepared conjugates in hemagglutination inhibition (HAI) assay revealed that potency increases with valency and the trivalent ligand 6d (HAI = 0.005 mM) is approximately sevenfold better than the best meta-oriented monovalent analogues 2d and 4d (HAI ≈ 0.033 mM) and so may serve as a good starting point to find new lead ligands.

Recent development in the design of small 'drug-like' and nanoscale glycomimetics against Escherichia coli infections

Glycoconjugates are involved in several pathological processes. Glycomimetics that can favorably emulate complex carbohydrate structures, while competing with natural ligands as inhibitors, are gaining considerable attention owing to their improved hydrolytic stability, binding affinity, and pharmacokinetic (PK) properties. Of particular interest are the families of α-d-mannopyranoside analogs, which can be used as inhibitors against adherent invasive Escherichia coli infections. Bacterial resistance to modern antibiotics triggers the search for new alternative antibacterial strategies that are less susceptible to acquiring resistance. In this review, we highlight recent progress in the chemical syntheses of this family of compounds, one of which having reached clinical trials against Crohn's disease (CD).

Developments in Mannose-Based Treatments for Uropathogenic Escherichia coli-Induced Urinary Tract Infections

During their lifetime almost half of women will experience a symptomatic urinary tract infection (UTI) with a further half experiencing a relapse within six months. Currently UTIs are treated with antibiotics, but increasing antibiotic resistance rates highlight the need for new treatments. Uropathogenic Escherichia coli (UPEC) is responsible for the majority of symptomatic UTI cases and thus has become a key pathological target. Adhesion of type one pilus subunit FimH at the surface of UPEC strains to mannose-saturated oligosaccharides located on the urothelium is critical to pathogenesis. Since the identification of FimH as a therapeutic target in the late 1980s, a substantial body of research has been generated focusing on the development of FimH-targeting mannose-based anti-adhesion therapies. In this review we will discuss the design of different classes of these mannose-based compounds and their utility and potential as UPEC therapeutics.

Novel approaches to glycomimetic design: development of small molecular weight lectin antagonists

Introduction: The direct binding of carbohydrates or those presented on glycoproteins or glycolipids to proteins is the primary effector of many biological responses. One class of carbohydrate-binding proteins, lectins are important in all forms of life. Their functions in animals include regulating cell adhesion, glycoprotein synthesis, metabolism, and mediating immune system response while in bacteria and viruses a lectin-mediated carbohydrate-protein interaction between host cells and the pathogen initiates pathogenesis of the infection.Areas covered: In this review, the authors outline the structural and functional pathogenesis of lectins from bacteria, amoeba, and humans. Mimics of a carbohydrate are referred to as glycomimetics, which are much smaller in molecular weight and are devised to mimic the key binding interactions of the carbohydrate while also allowing additional contacts with the lectin. This article emphasizes the various approaches used over the past 10-15 years in the rational design of glycomimetic ligands.Expert opinion: Medicinal chemistry efforts enabled by X-ray structural biology have identified small-molecule glycomimetic lectin antagonists that have entered or are nearing clinical trials. A common theme in these strategies is the use of biaryl ring systems to emulate the carbohydrate interactions with the lectin.

FimH and Anti-Adhesive Therapeutics: A Disarming Strategy Against Uropathogens

Chaperone-usher fimbrial adhesins are powerful weapons against the uropathogens that allow the establishment of urinary tract infections (UTIs). As the antibiotic therapeutic strategy has become less effective in the treatment of uropathogen-related UTIs, the anti-adhesive molecules active against fimbrial adhesins, key determinants of urovirulence, are attractive alternatives. The best-characterized bacterial adhesin is FimH, produced by uropathogenic Escherichia coli (UPEC). Hence, a number of high-affinity mono- and polyvalent mannose-based FimH antagonists, characterized by different bioavailabilities, have been reported. Given that antagonist affinities are firmly associated with the functional heterogeneities of different FimH variants, several FimH inhibitors have been developed using ligand-drug discovery strategies to generate high-affinity molecules for successful anti-adhesion therapy. As clinical trials have shown d-mannose's efficacy in UTIs prevention, it is supposed that mannosides could be a first-in-class strategy not only for UTIs, but also to combat other Gram-negative bacterial infections. Therefore, the current review discusses valuable and effective FimH anti-adhesive molecules active against UTIs, from design and synthesis to in vitro and in vivo evaluations.

Improvement of Aglycone π-Stacking Yields Nanomolar to Sub-nanomolar FimH Antagonists

Antimicrobial resistance has become a serious concern for the treatment of urinary tract infections. In this context, an anti-adhesive approach targeting FimH, a bacterial lectin enabling the attachment of E. coli to host cells, has attracted considerable interest. FimH can adopt a low/medium-affinity state in the absence and a high-affinity state in the presence of shear forces. Until recently, mostly the high-affinity state has been investigated, despite the fact that a therapeutic antagonist should bind predominantly to the low-affinity state. In this communication, we demonstrate that fluorination of biphenyl α-d-mannosides leads to compounds with perfect π-π stacking interactions with the tyrosine gate of FimH, yielding low nanomolar to sub-nanomolar K_D values for the low- and high-affinity states, respectively. The face-to-face alignment of the perfluorinated biphenyl group of FimH ligands and Tyr48 was confirmed by crystal structures as well as ¹ H,¹⁵ N-HSQC NMR analysis. Finally, fluorination improves pharmacokinetic parameters predictive for oral availability.

A Novel Integrated Way for Deciphering the Glycan Code for the FimH Lectin

The fimbrial lectin FimH from uro- and enteropathogenic Escherichia coli binds with nanomolar affinity to oligomannose glycans exposing Manα1,3Man dimannosides at their non-reducing end, but only with micromolar affinities to Manα1,2Man dimannosides. These two dimannoses play a significantly distinct role in infection by E. coli. Manα1,2Man has been described early on as shielding the (Manα1,3Man) glycan that is more relevant to strong bacterial adhesion and invasion. We quantified the binding of the two dimannoses (Manα1,2Man and Manα1,3Man to FimH using ELLSA and isothermal microcalorimetry and calculated probabilities of binding modes using molecular dynamics simulations. Our experimentally and computationally determined binding energies confirm a higher affinity of FimH towards the dimannose Manα1,3Man. Manα1,2Man displays a much lower binding enthalpy combined with a high entropic gain. Most remarkably, our molecular dynamics simulations indicate that Manα1,2Man cannot easily take its major conformer from water into the FimH binding site and that FimH is interacting with two very different conformers of Manα1,2Man that occupy 42% and 28% respectively of conformational space. The finding that Manα1,2Man binding to FimH is unstable agrees with the earlier suggestion that E. coli may use the Manα1,2Man epitope for transient tethering along cell surfaces in order to enhance dispersion of the infection.

KinITC-One Method Supports both Thermodynamic and Kinetic SARs as Exemplified on FimH Antagonists

Affinity data, such as dissociation constants (K_D ) or inhibitory concentrations (IC₅₀ ), are widely used in drug discovery. However, these parameters describe an equilibrium state, which is often not established in vivo due to pharmacokinetic effects and they are therefore not necessarily sufficient for evaluating drug efficacy. More accurate indicators for pharmacological activity are the kinetics of binding processes, as they shed light on the rate of formation of protein-ligand complexes and their half-life. Nonetheless, although highly desirable for medicinal chemistry programs, studies on structure-kinetic relationships (SKR) are still rare. With the recently introduced analytical tool kinITC this situation may change, since not only thermodynamic but also kinetic information of the binding process can be deduced from isothermal titration calorimetry (ITC) experiments. Using kinITC, ITC data of 29 mannosides binding to the bacterial adhesin FimH were re-analyzed to make their binding kinetics accessible. To validate these kinetic data, surface plasmon resonance (SPR) experiments were conducted. The kinetic analysis by kinITC revealed that the nanomolar affinities of the FimH antagonists arise from both (i) an optimized interaction between protein and ligand in the bound state (reduced off-rate constant k_off ) and (ii) a stabilization of the transition state or a destabilization of the unbound state (increased on-rate constant k_on ). Based on congeneric ligand modifications and structural input from co-crystal structures, a strong relationship between the formed hydrogen-bond network and k_off could be concluded, whereas electrostatic interactions and conformational restrictions upon binding were found to have mainly an impact on k_on .

Targeting Dynamical Binding Processes in the Design of Non-Antibiotic Anti-Adhesives by Molecular Simulation-The Example of FimH

Located at the tip of type I fimbria of Escherichia coli, the bacterial adhesin FimH is responsible for the attachment of the bacteria to the (human) host by specifically binding to highly-mannosylated glycoproteins located on the exterior of the host cell wall. Adhesion represents a necessary early step in bacterial infection and specific inhibition of this process represents a valuable alternative pathway to antibiotic treatments, as such anti-adhesive drugs are non-intrusive and are therefore unlikely to induce bacterial resistance. The currently available anti-adhesives with the highest affinities for FimH still feature affinities in the nanomolar range. A prerequisite to develop higher-affinity FimH inhibitors is a molecular understanding of the FimH-inhibitor complex formation. The latest insights in the formation process are achieved by combining several molecular simulation and traditional experimental techniques. This review summarizes how molecular simulation contributed to the current knowledge of the molecular function of FimH and the importance of dynamics in the inhibitor binding process, and highlights the importance of the incorporation of dynamical aspects in (future) drug-design studies.

Conformational switch of the bacterial adhesin FimH in the absence of the regulatory domain: Engineering a minimalistic allosteric system

For many biological processes such as ligand binding, enzymatic catalysis, or protein folding, allosteric regulation of protein conformation and dynamics is fundamentally important. One example is the bacterial adhesin FimH, where the C-terminal pilin domain exerts negative allosteric control over binding of the N-terminal lectin domain to mannosylated ligands on host cells. When the lectin and pilin domains are separated under shear stress, the FimH-ligand interaction switches in a so-called catch-bond mechanism from the low- to high-affinity state. So far, it has been assumed that the pilin domain is essential for the allosteric propagation within the lectin domain that would otherwise be conformationally rigid. To test this hypothesis, we generated mutants of the isolated FimH lectin domain and characterized their thermodynamic, kinetic, and structural properties using isothermal titration calorimetry, surface plasmon resonance, nuclear magnetic resonance, and X-ray techniques. Intriguingly, some of the mutants mimicked the conformational and kinetic behaviors of the full-length protein and, even in absence of the pilin domain, conducted the cross-talk between allosteric sites and the mannoside-binding pocket. Thus, these mutants represent a minimalistic allosteric system of FimH, useful for further mechanistic studies and antagonist design.

Bifunctional mannoside–glucosinolate glycoconjugates as enzymatically triggered isothiocyanates and FimH ligands

The synthesis of glucosinolate–mannoside glycoconjugates combining both the structural features of a myrosinase substrate and a FimH ligand is described.

The price of flexibility - a case study on septanoses as pyranose mimetics

Seven-membered ring mimetics of mannose were studied as ligands for the mannose-specific bacterial lectin FimH, which plays an essential role in the first step of urinary tract infections (UTI). A competitive binding assay and isothermal titration calorimetry (ITC) experiments indicated an approximately ten-fold lower affinity for the seven-membered ring mannose mimetic 2-O-n-heptyl-1,6-anhydro-d-glycero-d-galactitol (7) compared to n-heptyl α-d-mannopyranoside (2), resulting exclusively from a loss of conformational entropy. Investigations by solution NMR, X-ray crystallography, and molecular modeling revealed that 7 establishes a superimposable H-bond network compared to mannoside 2, but at the price of a high entropic penalty due to the loss of its pronounced conformational flexibility. These results underscore the importance of having access to the complete thermodynamic profile of a molecular interaction to "rescue" ligands from entropic penalties with an otherwise perfect fit to the protein binding site.

Target-directed Dynamic Combinatorial Chemistry: A Study on Potentials and Pitfalls as Exemplified on a Bacterial Target

Target-directed dynamic combinatorial chemistry (DCC) is an emerging technique for the efficient identification of inhibitors of pharmacologically relevant targets. In this contribution, we present an application for a bacterial target, the lectin FimH, a crucial virulence factor of uropathogenic E. coli being the main cause of urinary tract infections. A small dynamic library of acylhydrazones was formed from aldehydes and hydrazides and equilibrated at neutral pH in presence of aniline as nucleophilic catalyst. The major success factors turned out to be an accordingly adjusted ratio of scaffolds and fragments, an adequate sample preparation prior to HPLC analysis, and the data processing. Only then did the ranking of the dynamic library constituents correlate well with affinity data. Furthermore, as a support of DCC applications especially to larger libraries, a new protocol for improved hit identification was established.

Innovative Solutions to Sticky Situations: Antiadhesive Strategies for Treating Bacterial Infections

Bacterial adherence to host tissue is an essential process in pathogenesis, necessary for invasion and colonization and often required for the efficient delivery of toxins and other bacterial effectors. As existing treatment options for common bacterial infections dwindle, we find ourselves rapidly approaching a tipping point in our confrontation with antibiotic-resistant strains and in desperate need of new treatment options. Bacterial strains defective in adherence are typically avirulent and unable to cause infection in animal models. The importance of this initial binding event in the pathogenic cascade highlights its potential as a novel therapeutic target. This article seeks to highlight a variety of strategies being employed to treat and prevent infection by targeting the mechanisms of bacterial adhesion. Advancements in this area include the development of novel antivirulence therapies using small molecules, vaccines, and peptides to target a variety of bacterial infections. These therapies target bacterial adhesion through a number of mechanisms, including inhibition of pathogen receptor biogenesis, competition-based strategies with receptor and adhesin analogs, and the inhibition of binding through neutralizing antibodies. While this article is not an exhaustive description of every advancement in the field, we hope it will highlight several promising examples of the therapeutic potential of antiadhesive strategies.

Sites for Dynamic Protein-Carbohydrate Interactions of O- and C-Linked Mannosides on the E. coli FimH Adhesin

Antagonists of the Escherichia coli type-1 fimbrial adhesin FimH are recognized as attractive alternatives for antibiotic therapies and prophylaxes against acute and recurrent bacterial infections. In this study α-d-mannopyranosides O- or C-linked with an alkyl, alkene, alkyne, thioalkyl, amide, or sulfonamide were investigated to fit a hydrophobic substituent with up to two aryl groups within the tyrosine gate emerging from the mannose-binding pocket of FimH. The results were summarized into a set of structure-activity relationships to be used in FimH-targeted inhibitor design: alkene linkers gave an improved affinity and inhibitory potential, because of their relative flexibility combined with a favourable interaction with isoleucine-52 located in the middle of the tyrosine gate. Of particular interest is a C-linked mannoside, alkene-linked to an ortho-substituted biphenyl that has an affinity similar to its O-mannosidic analog but superior to its para-substituted analog. Docking of its high-resolution NMR solution structure to the FimH adhesin indicated that its ultimate, ortho-placed phenyl ring is able to interact with isoleucine-13, located in the clamp loop that undergoes conformational changes under shear force exerted on the bacteria. Molecular dynamics simulations confirmed that a subpopulation of the C-mannoside conformers is able to interact in this secondary binding site of FimH.

Rational design strategies for FimH antagonists: new drugs on the horizon for urinary tract infection and Crohn's disease

INTRODUCTION

The bacterial adhesin FimH is a virulence factor and an attractive therapeutic target for urinary tract infection (UTI) and Crohn's Disease (CD). Located on type 1 pili of uropathogenic E. coli (UPEC), the FimH adhesin plays an integral role in the pathogenesis of UPEC. Recent efforts have culminated in the development of small-molecule mannoside FimH antagonists that target the mannose-binding lectin domain of FimH, inhibiting its function and preventing UPEC from binding mannosylated host cells in the bladder, thereby circumventing infection. Areas covered: The authors describe the structure-guided design of mannoside ligands, and review the structural biology of the FimH lectin domain. Additionally, they discuss the lead optimization of mannosides for therapeutic application in UTI and CD, and describe various assays used to measure mannoside potency in vitro and mouse models used to determine efficacy in vivo. Expert opinion: To date, mannoside optimization has led to a diverse set of small-molecule FimH antagonists with oral bioavailability. With clinical trials already initiated in CD and on the horizon for UTI, it is the authors, opinion that mannosides will be a 'first-in-class' treatment strategy for UTI and CD, and will pave the way for treatment of other Gram-negative bacterial infections.

Urinary Tract Infection: Which Conformation of the Bacterial Lectin FimH Is Therapeutically Relevant?

Frequent antibiotic treatment of urinary tract infections has resulted in the emergence of antimicrobial resistance, necessitating alternative treatment options. One such approach centers around FimH antagonists that block the bacterial adhesin FimH, which would otherwise mediate binding of uropathogenic Escherichia coli to the host urothelium to trigger the infection. Although the FimH lectin can adopt three distinct conformations, the evaluation of FimH antagonists has mainly been performed with a truncated construct of FimH locked in one particular conformation. For a successful therapeutic application, however, FimH antagonists should be efficacious against all physiologically relevant conformations. Therefore, FimH constructs with the capacity to adopt various conformations were applied. By examining the binding properties of a series of FimH antagonists in terms of binding affinity and thermodynamics, we demonstrate that depending on the FimH construct, affinities may be overestimated by a constant factor of 2 orders of magnitude. In addition, we report several antagonists with excellent affinities for all FimH conformations.

Assembly and inhibitory activity of monovalent mannosides terminated with aromatic methyl esters: The effect of naphthyl groups

A series of monovalent α-D-mannoside ligands terminated with aromatic methyl esters have been synthesized in excellent yields using the Cu(I) catalyzed azide-alkyne 1,3-dipolar cycloaddition ("click chemistry"). These mannosides were designed to have a unique aglycone moiety (tail) that combines a triazole ring attached to aromatic methyl esters via a six carbon alkyl chain. The mannose unit of these ligands was linked at the ortho, meta, and para positions of substituted methyl benzoates and 1-, 3-, and 6-substituted methyl 2-napthaoates. In hemagglutination assays, ligands (32A-38A) showed better inhibitory activities than the standard inhibitor, methyl α-D-mannopyranoside. Overall, the naphthyl-based mannoside ligand (37A) showed the best activity and therefore merits further development.

Mutation of Tyr137 of the universal Escherichia coli fimbrial adhesin FimH relaxes the tyrosine gate prior to mannose binding

The most prevalent diseases manifested by Escherichia coli are acute and recurrent bladder infections and chronic inflammatory bowel diseases such as Crohn's disease. E. coli clinical isolates express the FimH adhesin, which consists of a mannose-specific lectin domain connected via a pilin domain to the tip of type 1 pili. Although the isolated FimH lectin domain has affinities in the nanomolar range for all high-mannosidic glycans, differentiation between these glycans is based on their capacity to form predominantly hydrophobic interactions within the tyrosine gate at the entrance to the binding pocket. In this study, novel crystal structures of tyrosine-gate mutants of FimH, ligand-free or in complex with heptyl α-d-O-mannopyranoside or 4-biphenyl α-d-O-mannopyranoside, are combined with quantum-mechanical calculations and molecular-dynamics simulations. In the Y48A FimH crystal structure, a large increase in the dynamics of the alkyl chain of heptyl α-d-O-mannopyranoside attempts to compensate for the absence of the aromatic ring; however, the highly energetic and stringent mannose-binding pocket of wild-type FimH is largely maintained. The Y137A mutation, on the other hand, is the most detrimental to FimH affinity and specificity: (i) in the absence of ligand the FimH C-terminal residue Thr158 intrudes into the mannose-binding pocket and (ii) ethylenediaminetetraacetic acid interacts strongly with Glu50, Thr53 and Asn136, in spite of multiple dialysis and purification steps. Upon mutation, pre-ligand-binding relaxation of the backbone dihedral angles at position 137 in the tyrosine gate and their coupling to Tyr48 via the interiorly located Ile52 form the basis of the loss of affinity of the FimH adhesin in the Y137A mutant.

Prodruggability of carbohydrates — oral FimH antagonists

The bacterial lectin FimH is a promising therapeutic target for the nonantibiotic prevention and treatment of urinary tract infections. In this communication, an ester prodrug approach is described to achieve oral bioavailability for FimH antagonists. By introducing short-chain acyl promoieties at the C-6 position of a biphenyl α-d-mannopyranoside, prodrugs with an excellent absorption potential were obtained. The human carboxylesterase 2 was identified as a main enzyme mediating rapid bioconversion to the active principle. Despite their propensity to hydrolysis within the enterocytes during absorption, these ester prodrugs present a considerable progress in the development of orally available FimH antagonists.

Antivirulence C-Mannosides as Antibiotic-Sparing, Oral Therapeutics for Urinary Tract Infections

Gram-negative uropathogenic Escherichia coli (UPEC) bacteria are a causative pathogen of urinary tract infections (UTIs). Previously developed antivirulence inhibitors of the type 1 pilus adhesin, FimH, demonstrated oral activity in animal models of UTI but were found to have limited compound exposure due to the metabolic instability of the O-glycosidic bond (O-mannosides). Herein, we disclose that compounds having the O-glycosidic bond replaced with carbon linkages had improved stability and inhibitory activity against FimH. We report on the design, synthesis, and in vivo evaluation of this promising new class of carbon-linked C-mannosides that show improved pharmacokinetic (PK) properties relative to O-mannosides. Interestingly, we found that FimH binding is stereospecifically modulated by hydroxyl substitution on the methylene linker, where the R-hydroxy isomer has a 60-fold increase in potency. This new class of C-mannoside antagonists have significantly increased compound exposure and, as a result, enhanced efficacy in mouse models of acute and chronic UTI.

(19)F NMR-Guided Design of Glycomimetic Langerin Ligands

C-type lectin receptors (CLRs) play a pivotal role in pathogen defense and immune homeostasis. Langerin, a CLR predominantly expressed on Langerhans cells, represents a potential target receptor for the development of anti-infectives or immunomodulatory therapies. As mammalian carbohydrate binding sites typically display high solvent exposure and hydrophilicity, the recognition of natural monosaccharide ligands is characterized by low affinities. Consequently, glycomimetic ligand design poses challenges that extend to the development of suitable assays. Here, we report the first application of (19)F R2-filtered NMR to address these challenges for a CLR, i.e., Langerin. The homogeneous, monovalent assay was essential to evaluating the in silico design of 2-deoxy-2-carboxamido-α-mannoside analogs and enabled the implementation of a fragment screening against the carbohydrate binding site. With the identification of both potent monosaccharide analogs and fragment hits, this study represents an important advancement toward the design of glycomimetic Langerin ligands and highlights the importance of assay development for other CLRs.

The Conformational Variability of FimH: Which Conformation Represents the Therapeutic Target?

FimH is a bacterial lectin found at the tips of type 1 pili of uropathogenic Escherichia coli (UPEC). It mediates shear-enhanced adhesion to mannosylated surfaces. Binding of UPEC to urothelial cells initiates the infection cycle leading to urinary tract infections (UTIs). Antiadhesive glycomimetics based on α-d-mannopyranose offer an attractive alternative to the conventional antibiotic treatment because they do not induce a selection pressure and are therefore expected to have a reduced resistance potential. Genetic variation of the fimH gene in clinically isolated UPEC has been associated with distinct mannose binding phenotypes. For this reason, we investigated the mannose binding characteristics of four FimH variants with mannose-based ligands under static and hydrodynamic conditions. The selected FimH variants showed individually different binding behavior under both sets of conditions as a result of the conformational variability of FimH. Clinically relevant FimH variants typically exist in a dynamic conformational equilibrium. Additionally, we evaluated inhibitory potencies of four FimH antagonists representing different structural classes. Inhibitory potencies of three of the tested antagonists were dependent on the binding phenotype and hence on the conformational equilibrium of the FimH variant. However, the squarate derivative was the notable exception and inhibited FimH variants irrespective of their binding phenotype. Information on antagonist affinities towards various FimH variants has remained largely unconsidered despite being essential for successful antiadhesion therapy.

FimH Antagonists: Phosphate Prodrugs Improve Oral Bioavailability

The widespread occurrence of urinary tract infections has resulted in frequent antibiotic treatment, contributing to the emergence of antimicrobial resistance. Alternative approaches are therefore required. In the initial step of colonization, FimH, a lectin located at the tip of bacterial type 1 pili, interacts with mannosylated glycoproteins on the urothelial mucosa. This initial pathogen/host interaction is efficiently antagonized by biaryl α-d-mannopyranosides. However, their poor physicochemical properties, primarily resulting from low aqueous solubility, limit their suitability as oral treatment option. Herein, we report the syntheses and pharmacokinetic evaluation of phosphate prodrugs, which show an improved aqueous solubility of up to 140-fold. In a Caco-2 cell model, supersaturated solutions of the active principle were generated through hydrolysis of the phosphate esters by brush border-associated enzymes, leading to a high concentration gradient across the cell monolayer. As a result, the in vivo application of phosphate prodrugs led to a substantially increased Cmax and prolonged availability of FimH antagonists in urine.

Antivirulence Isoquinolone Mannosides: Optimization of the Biaryl Aglycone for FimH Lectin Binding Affinity and Efficacy in the Treatment of Chronic UTI

Uropathogenic E. coli (UPEC) employ the mannose-binding adhesin FimH to colonize the bladder epithelium during urinary tract infection (UTI). Previously reported FimH antagonists exhibit good potency and efficacy, but low bioavailability and a short half-life in vivo. In a rational design strategy, we obtained an X-ray structure of lead mannosides and then designed mannosides with improved drug-like properties. We show that cyclizing the carboxamide onto the biphenyl B-ring aglycone of biphenyl mannosides into a fused heterocyclic ring, generates new biaryl mannosides such as isoquinolone 22 (2-methyl-4-(1-oxo-1,2-dihydroisoquinolin-7-yl)phenyl α-d-mannopyranoside) with enhanced potency and in vivo efficacy resulting from increased oral bioavailability. N-Substitution of the isoquinolone aglycone with various functionalities produced a new potent subseries of FimH antagonists. All analogues of the subseries have higher FimH binding affinity than unsubstituted lead 22, as determined by thermal shift differential scanning fluorimetry assay. Mannosides with pyridyl substitution on the isoquinolone group inhibit bacteria-mediated hemagglutination and prevent biofilm formation by UPEC with single-digit nanomolar potency, which is unprecedented for any FimH antagonists or any other antivirulence compounds reported to date.

Unique tetrameric and hexameric mannoside clusters prepared by click chemistry

The synthesis of novel tetrameric and hexameric mannoside clusters bearing 1,2,3-trizole linkages via Cu(I)-catalyzed azide-alkyne cycloaddition reaction ("click chemistry") is described. An attractive feature of these multiarmed mannoside clusters as potential inhibitors of uropathogenic Escherichia coli is the use of an aglycone whose length is designed to fit in the tyrosine gate. The acetylated mannosides were deprotected and the corresponding de-O-acetylated mannosides were found to exhibit good water solubility.

The tyrosine gate of the bacterial lectin FimH: a conformational analysis by NMR spectroscopy and X-ray crystallography

Urinary tract infections caused by uropathogenic E. coli are among the most prevalent infectious diseases. The mannose-specific lectin FimH mediates the adhesion of the bacteria to the urothelium, thus enabling host cell invasion and recurrent infections. An attractive alternative to antibiotic treatment is the development of FimH antagonists that mimic the physiological ligand. A large variety of candidate drugs have been developed and characterized by means of in vitro studies and animal models. Here we present the X-ray co-crystal structures of FimH with members of four antagonist classes. In three of these cases no structural data had previously been available. We used NMR spectroscopy to characterize FimH-antagonist interactions further by chemical shift perturbation. The analysis allowed a clear determination of the conformation of the tyrosine gate motif that is crucial for the interaction with aglycone moieties and was not obvious from X-ray structural data alone. Finally, ITC experiments provided insight into the thermodynamics of antagonist binding. In conjunction with the structural information from X-ray and NMR experiments the results provide a mechanism for the often-observed enthalpy-entropy compensation of FimH antagonists that plays a role in fine-tuning of the interaction.

FimH antagonists: bioisosteres to improve the in vitro and in vivo PK/PD profile

Urinary tract infections (UTIs), predominantly caused by uropathogenic Escherichia coli (UPEC), belong to the most prevalent infectious diseases worldwide. The attachment of UPEC to host cells is mediated by FimH, a mannose-binding adhesin at the tip of bacterial type 1 pili. To date, UTIs are mainly treated with antibiotics, leading to the ubiquitous problem of increasing resistance against most of the currently available antimicrobials. Therefore, new treatment strategies are urgently needed. Here, we describe the development of an orally available FimH antagonist. Starting from the carboxylate substituted biphenyl α-d-mannoside 9, affinity and the relevant pharmacokinetic parameters (solubility, permeability, renal excretion) were substantially improved by a bioisosteric approach. With 3'-chloro-4'-(α-d-mannopyranosyloxy)biphenyl-4-carbonitrile (10j) a FimH antagonist with an optimal in vitro PK/PD profile was identified. Orally applied, 10j was effective in a mouse model of UTI by reducing the bacterial load in the bladder by about 1000-fold.

Synthesis of novel types of polyester glycodendrimers as potential inhibitors of urinary tract infections

Syntheses of highly mannosylated polyester dendrimers with 2, 4, 8, and 16 α-d-mannopyranose residues on their peripheries connected by different linker arms are presented.

Study of the structural and dynamic effects in the FimH adhesin upon α-d-heptyl mannose binding

Uropathogenic Escherichia coli cause urinary tract infections by adhering to mannosylated receptors on the human urothelium via the carbohydrate-binding domain of the FimH adhesin (FimHL). Numerous α-d-mannopyranosides, including α-d-heptyl mannose (HM), inhibit this process by interacting with FimHL. To establish the molecular basis of the high-affinity HM binding, we solved the solution structure of the apo form and the crystal structure of the FimHL-HM complex. NMR relaxation analysis revealed that protein dynamics were not affected by the sugar binding, yet HM addition promoted protein dimerization, which was further confirmed by small-angle X-ray scattering. Finally, to address the role of Y48, part of the "tyrosine gate" believed to govern the affinity and specificity of mannoside binding, we characterized the FimHL Y48A mutant, whose conformational, dynamical, and HM binding properties were found to be very similar to those of the wild-type protein.

Exploring the meaning of sugar configuration in a supramolecular environment: comparison of six octyl glycoside micelles by ITC and NMR spectroscopy

To promote understanding of sugar configuration in a supramolecular context, glycomicelles were compared and a “trinity projection” of glycosides proposed.

Towards bacterial adhesion-based therapeutics and detection methods

Bacterial adhesion is an important first step towards bacterial infection and plays a role in colonization, invasion and biofilm formation.