Non-Carbohydrate Glycomimetics as Inhibitors of Calcium(II)-Binding Lectins

Photochemical Strategies toward Precision Targeting against Multidrug-Resistant Bacterial Infections

Infectious diseases pose a serious threat and a substantial economic burden on global human and public health security, especially with the frequent emergence of multidrug-resistant (MDR) bacteria in clinical settings. In response to this urgent need, various photobased anti-infectious therapies have been reported lately. This Review explores and discusses several photochemical targeted antibacterial therapeutic strategies for addressing bacterial infections regardless of their antibiotic susceptibility. In contrast to conventional photobased therapies, these approaches facilitate precise targeting of pathogenic bacteria and/or infectious microenvironments, effectively minimizing toxicity to mammalian cells and surrounding healthy tissues. The highlighted therapies include photodynamic therapy, photocatalytic therapy, photothermal therapy, endogenous pigments-based photobleaching therapy, and polyphenols-based photo-oxidation therapy. This comprehensive exploration aims to offer updated information to facilitate the development of effective, convenient, safe, and alternative strategies to counter the growing threat of MDR bacteria in the future.

Glycomimetics for the inhibition and modulation of lectins

Carbohydrates are essential mediators of many processes in health and disease. They regulate self-/non-self- discrimination, are key elements of cellular communication, cancer, infection and inflammation, and determine protein folding, function and life-times. Moreover, they are integral to the cellular envelope for microorganisms and participate in biofilm formation. These diverse functions of carbohydrates are mediated by carbohydrate-binding proteins, lectins, and the more the knowledge about the biology of these proteins is advancing, the more interfering with carbohydrate recognition becomes a viable option for the development of novel therapeutics. In this respect, small molecules mimicking this recognition process become more and more available either as tools for fostering our basic understanding of glycobiology or as therapeutics. In this review, we outline the general design principles of glycomimetic inhibitors (Section 2). This section is then followed by highlighting three approaches to interfere with lectin function, i.e. with carbohydrate-derived glycomimetics (Section 3.1), novel glycomimetic scaffolds (Section 3.2) and allosteric modulators (Section 3.3). We summarize recent advances in design and application of glycomimetics for various classes of lectins of mammalian, viral and bacterial origin. Besides highlighting design principles in general, we showcase defined cases in which glycomimetics have been advanced to clinical trials or marketed. Additionally, emerging applications of glycomimetics for targeted protein degradation and targeted delivery purposes are reviewed in Section 4.

Neutralizing the Impact of the Virulence Factor LecA from Pseudomonas aeruginosa on Human Cells with New Glycomimetic Inhibitors

Bacterial adhesion, biofilm formation and host cell invasion of the ESKAPE pathogen Pseudomonas aeruginosa require the tetravalent lectins LecA and LecB, which are therefore drug targets to fight these infections. Recently, we have reported highly potent divalent galactosides as specific LecA inhibitors. However, they suffered from very low solubility and an intrinsic chemical instability due to two acylhydrazone motifs, which precluded further biological evaluation. Here, we isosterically substituted the acylhydrazones and systematically varied linker identity and length between the two galactosides necessary for LecA binding. The optimized divalent LecA ligands showed improved stability and were up to 1000-fold more soluble. Importantly, these properties now enabled their biological characterization. The lead compound L2 potently inhibited LecA binding to lung epithelial cells, restored wound closure in a scratch assay and reduced the invasiveness of P. aeruginosa into host cells.

Discovery of potent 1,1-diarylthiogalactoside glycomimetic inhibitors of Pseudomonas aeruginosa LecA with antibiofilm properties

In this work, β-thiogalactoside mimetics bearing 1,1-diarylmethylene or benzophenone aglycons have been prepared and assayed for their affinity towards LecA, a lectin and virulence factor from Pseudomonas aeruginosa involved in bacterial adhesion and biofilm formation. The hit compound presents higher efficiency than previously described monovalent inhibitors and the crystal structure confirmed the occurrence of additional contacts between the aglycone and the protein surface. The highest affinity (160 nM) was obtained for a divalent ligand containing two galactosides. The monovalent high affinity compound (K_d = 1 μM) obtained through structure-activity relationship (SAR) showed efficient antibiofilm activity with no associated bactericidal activity.

Involvement of sialoglycans in SARS-COV-2 infection: Opportunities and challenges for glyco-based inhibitors

Viral infections have been the causes of global pandemics, including the ongoing coronavirus disease 2019, which prompted the investigation into the infection mechanisms to find treatment and aid the vaccine design. Betacoronaviruses use spike glycoprotein on their surface to bind to host receptors, aiding their host attachment and cell fusion. Protein-glycan interaction has been implicated in the viral entry mechanism of many viruses and has recently been shown in SARS-CoV-2. Here, we reviewed the current knowledge on protein-glycan interactions that facilitate SARS-CoV-2 host entry, with special interest in sialoglycans present on both the virions and host cell surfaces. We also analyze how such information provides opportunities and challenges in glyco-based inhibitors.

Discovery of N-β-l-Fucosyl Amides as High-Affinity Ligands for the Pseudomonas aeruginosa Lectin LecB

The Gram-negative pathogen Pseudomonas aeruginosa causes severe infections mainly in immunocompromised or cystic fibrosis patients and is able to resist antimicrobial treatments. The extracellular lectin LecB plays a key role in bacterial adhesion to the host and biofilm formation. For the inhibition of LecB, we designed and synthesized a set of fucosyl amides, sulfonamides, and thiourea derivatives. Then, we analyzed their binding to LecB in competitive and direct binding assays. We identified β-fucosyl amides as unprecedented high-affinity ligands in the two-digit nanomolar range. X-ray crystallography of one α- and one β-anomer of N-fucosyl amides in complex with LecB revealed the interactions responsible for the high affinity of the β-anomer at atomic level. Further, the molecules showed good stability in murine and human blood plasma and hepatic metabolism, providing a basis for future development into antibacterial drugs.

Lectin-Targeted Prodrugs Activated by Pseudomonas aeruginosa for Self-Destructive Antibiotic Release

Chronic Pseudomonas aeruginosa infections are characterized by biofilm formation, a major virulence factor of P. aeruginosa and cause of extensive drug resistance. Fluoroquinolones are effective antibiotics but are linked to severe side effects. The two extracellular P. aeruginosa-specific lectins LecA and LecB are key structural biofilm components and can be exploited for targeted drug delivery. In this work, several fluoroquinolones were conjugated to lectin probes by cleavable peptide linkers to yield lectin-targeted prodrugs. Mechanistically, these conjugates therefore remain non-toxic in the systemic distribution and will be activated to kill only once they have accumulated at the infection site. The synthesized prodrugs proved stable in the presence of host blood plasma and liver metabolism but rapidly released the antibiotic cargo in the presence of P. aeruginosa in a self-destructive manner in vitro. Furthermore, the prodrugs showed good absorption, distribution, metabolism, and elimination (ADME) properties and reduced toxicity in vitro, thus establishing the first lectin-targeted antibiotic prodrugs against P. aeruginosa.

Multicomponent reaction derived small di- and tri-carbohydrate-based glycomimetics as tools for probing lectin specificity

Lectins, carbohydrate-binding proteins, play important functions in all forms of life from bacteria and viruses to plants, animals, and humans, participating in cell-cell communication and pathogen binding. In an attempt to modify lectin functions, artificial lectin ligands were made usually as big dendrimeric or cluster multivalent glycomimetic structures. Here we synthesized a novel set of glycomimetic ligands through protection/deprotection multicomponent reactions (MCR) approach. Multivalent di-and tri-carbohydrate glycomimetics containing D-fructose, D-galactose, and D-allose moieties were prepared in 63-96% yield. MCR glycomimetics demonstrated different binding abilities for plant lectins Con A and UEA I, and human galectin-3. Information gained about the influence of molecule structure, multivalency and optical purity on the lectin binding ability can be used in lectin detection and sensitivity measurements to further facilitate understanding of carbohydrate recognition process.

Targeting undruggable carbohydrate recognition sites through focused fragment library design

Carbohydrate-protein interactions are key for cell-cell and host-pathogen recognition and thus, emerged as viable therapeutic targets. However, their hydrophilic nature poses major limitations to the conventional development of drug-like inhibitors. To address this shortcoming, four fragment libraries were screened to identify metal-binding pharmacophores (MBPs) as novel scaffolds for inhibition of Ca²⁺-dependent carbohydrate-protein interactions. Here, we show the effect of MBPs on the clinically relevant lectins DC-SIGN, Langerin, LecA and LecB. Detailed structural and biochemical investigations revealed the specificity of MBPs for different Ca²⁺-dependent lectins. Exploring the structure-activity relationships of several fragments uncovered the functional groups in the MBPs suitable for modification to further improve lectin binding and selectivity. Selected inhibitors bound efficiently to DC-SIGN-expressing cells. Altogether, the discovery of MBPs as a promising class of Ca²⁺-dependent lectin inhibitors creates a foundation for fragment-based ligand design for future drug discovery campaigns.

Smart Multifunctional Polymer Systems as Alternatives or Supplements of Antibiotics To Overcome Bacterial Resistance

In recent years, infectious diseases have again become a critical threat to global public health largely due to the challenges posed by antimicrobial resistance. Conventional antibiotics have played a crucial role in combating bacterial infections; however, their efficacy is significantly impaired by widespread drug resistance. Natural antimicrobial peptides (AMPs) and their polymeric mimics demonstrate great potential for killing bacteria with low propensity of resistance as they target the microbial membrane rather than a specific molecular target, but they are also toxic to the host eukaryotic cells. To minimize antibiotics systemic spread and the required dose that promote resistance and to advocate practical realization of the promising activity of AMPs and polymers, smart systems to target bacteria are highly sought after. This review presents bacterial recognition by various specific targeting molecules and the delivery systems of active components in supramolecules. Bacteria-induced activations of antimicrobial-based nanoformulations are also included. Recent advances in the bacteria targeting and delivery of synthetic antimicrobial agents may assist in developing new classes of highly selective antimicrobial systems which can improve bactericidal efficacy and greatly minimize the spread of bacterial resistance.

Structural considerations for building synthetic glycoconjugates as inhibitors for Pseudomonas aeruginosa lectins

Pseudomonas aeruginosa is a pathogenic bacterium, responsible for a large portion of nosocomial infections globally and designated as critical priority by the World Health Organisation. Its characteristic carbohydrate‐binding proteins LecA and LecB, which play a role in biofilm‐formation and lung‐infection, can be targeted by glycoconjugates. Here we review the wide range of inhibitors for these proteins (136 references), highlighting structural features and which impact binding affinity and/or therapeutic effects, including carbohydrate selection; linker length and rigidity; and scaffold topology, particularly for multivalent candidates. We also discuss emerging therapeutic strategies, which build on targeting of LecA and LecB, such as anti‐biofilm activity, anti‐adhesion and drug‐delivery, with promising prospects for medicinal chemistry.

Targeting extracellular lectins of Pseudomonas aeruginosa with glycomimetic liposomes

The antimicrobial resistance crisis requires novel approaches for the therapy of infections especially with Gram-negative pathogens. Pseudomonas aeruginosa is defined as priority 1 pathogen by the WHO and thus of particular interest. Its drug resistance is primarily associated with biofilm formation and essential constituents of its extracellular biofilm matrix are the two lectins, LecA and LecB. Here, we report microbial lectin-specific targeted nanovehicles based on liposomes. LecA- and LecB-targeted phospholipids were synthesized and used for the preparation of liposomes. These liposomes with varying surface ligand density were then analyzed for their competitive and direct lectin binding activity. We have further developed a microfluidic device that allowed the optical detection of the targeting process to the bacterial lectins. Our data showed that the targeted liposomes are specifically binding to their respective lectin and remain firmly attached to surfaces containing these lectins. This synthetic and biophysical study provides the basis for future application in targeted antibiotic delivery to overcome antimicrobial resistance.

Medicinal chemistry of the myeloid C-type lectin receptors Mincle, Langerin, and DC-SIGN

In their role as pattern-recognition receptors on cells of the innate immune system, myeloid C-type lectin receptors (CLRs) assume important biological functions related to immunity, homeostasis, and cancer. As such, this family of receptors represents an appealing target for therapeutic interventions for modulating the outcome of many pathological processes, in particular related to infectious diseases. This review summarizes the current state of research into glycomimetic or drug-like small molecule ligands for the CLRs Mincle, Langerin, and DC-SIGN, which have potential therapeutic applications in vaccine research and anti-infective therapy.

Targeting the Central Pocket of the Pseudomonas aeruginosa Lectin LecA

Pseudomonas aeruginosa is an opportunistic ESKAPE pathogen that produces two lectins, LecA and LecB, as part of its large arsenal of virulence factors. Both carbohydrate‐binding proteins are central to the initial and later persistent infection processes, i. e. bacterial adhesion and biofilm formation. The biofilm matrix is a major resistance determinant and protects the bacteria against external threats such as the host immune system or antibiotic treatment. Therefore, the development of drugs against the P. aeruginosa biofilm is of particular interest to restore efficacy of antimicrobials. Carbohydrate‐based inhibitors for LecA and LecB were previously shown to efficiently reduce biofilm formations. Here, we report a new approach for inhibiting LecA with synthetic molecules bridging the established carbohydrate‐binding site and a central cavity located between two LecA protomers of the lectin tetramer. Inspired by in silico design, we synthesized various galactosidic LecA inhibitors with aromatic moieties targeting this central pocket. These compounds reached low micromolar affinities, validated in different biophysical assays. Finally, X‐ray diffraction analysis revealed the interactions of this compound class with LecA. This new mode of action paves the way to a novel route towards inhibition of P. aeruginosa biofilms.

Pan-selectin inhibitors as potential therapeutics for COVID-19 treatment: in silico screening study

Coronavirus disease 2019 (COVID-19) has spread rapidly throughout the globe. The spectrum of disease is broad but among hospitalized patients with COVID-19, respiratory failure from acute respiratory distress syndrome is the leading cause of mortality. There is an urgent need for an effective treatment. The current focus has been developing novel therapeutics, including antivirals, protease inhibitors, vaccines and targeting the overactive cytokine response with anti-cytokine therapy. The overproduction of early response proinflammatory cytokines results in what has been described as a "cytokine storm" is leading eventually to death when the cells fail to terminate the inflammatory response. Accumulating evidence shows that inflammatory cytokines induce selectin ligands that play a crucial role in the pathogenesis of inflammatory diseases by mediating leukocyte migration from the blood into the tissue. Thus, the selectins and selectin ligands represent a promising therapeutic target for the treatment of COVID-19. In this paper, potential pan-selectin inhibitors were identified employing a virtual screening using a docking procedure. For this purpose, the Asinex and ZINC databases of ligands, including approved drugs, biogenic compounds and glycomimetics, altogether 923,602 compounds, were screened against the P-, L- and E-selectin. At first, the experimentally confirmed inhibitors were docked into all three selectins' carbohydrate recognition domains to assess the suitability of the screening procedure. Finally, based on the evaluation of ligands binding, we propose 10 purchasable pan-selectin inhibitors to develop COVID-19 therapeutics.

Discovering mycobacterial lectins as potential drug targets and vaccine candidates for tuberculosis treatment: a theoretical approach

M. tuberculosis proliferates within the macrophages during infection and they are bounded by carbohydrates in the cell wall, called lectins. Despite their surface localization, the studies on exact functions of lectins are unexplored. Hence, in our study, using insilico approaches, 11 potential lectins of Mtb was explored as potential drug targets and vaccine candidates. Initially, a gene interaction network was constructed for the 11 potential lectins and identified its functional partners. A gene ontology analysis was also performed for the 11 mycobacterial lectins along with its functional partners and found most of the proteins are present in the extracellular region of the bacterium and belongs to the PE/PPE family of proteins. Further, molecular docking studies were performed for two of the potential lectins (Rv2075c and Rv1917c). A novel series of quinoxalinone and fucoidan derivatives have been made to dock against these selected lectins. Molecular docking study reveals that quinoxalinone derivatives showed better affinity against Rv2075c, whereas fucoidan derivatives have good binding affinity against Rv1917c. Moreover, the mycobacterial lectins can interact with the host and they are considered as potential vaccine candidates. Hence, immunoinformatics study was carried out for all the 11 potential lectins. B-cell and T-cell binding epitopes were predicted using insilico tools. Further, an immunodominant epitope ¹⁰⁶²SIPAIPLSVEV¹⁰⁷² of Rv1917c was identified, which was predicted to bind B-cell and most of the MHC alleles. Thus, the study has explored that mycobacterial lectins could be potentially used as drug targets and vaccine candidates for tuberculosis treatment.

Non-Carbohydrate Glycomimetics as Inhibitors of Calcium(II)-Binding Lectins

Because of the antimicrobial resistance crisis, lectins are considered novel drug targets. Pseudomonas aeruginosa utilizes LecA and LecB in the infection process. Inhibition of both lectins with carbohydrate-derived molecules can reduce biofilm formation to restore antimicrobial susceptibility. Here, we focused on non-carbohydrate inhibitors for LecA to explore new avenues for lectin inhibition. From a screening cascade we obtained one experimentally confirmed hit, a catechol, belonging to the well-known PAINS compounds. Rigorous analyses validated electron-deficient catechols as millimolar LecA inhibitors. The first co-crystal structure of a non-carbohydrate inhibitor in complex with a bacterial lectin clearly demonstrates the catechol mimicking the binding of natural glycosides with LecA. Importantly, catechol 3 is the first non-carbohydrate lectin ligand that binds bacterial and mammalian calcium(II)-binding lectins, giving rise to this fundamentally new class of glycomimetics.