New noncovalent inhibitors of penicillin-binding proteins from penicillin-resistant bacteria

Drug Discovery in the Field of β-Lactams: An Academic Perspective

β-Lactams are the most widely prescribed class of antibiotics that inhibit penicillin-binding proteins (PBPs), particularly transpeptidases that function in peptidoglycan synthesis. A major mechanism of antibiotic resistance is the production of β-lactamase enzymes, which are capable of hydrolyzing β-lactam antibiotics. There have been many efforts to counter increasing bacterial resistance against β-lactams. These studies have mainly focused on three areas: discovering novel inhibitors against β-lactamases, developing new β-lactams less susceptible to existing resistance mechanisms, and identifying non-β-lactam inhibitors against cell wall transpeptidases. Drug discovery in the β-lactam field has afforded a range of research opportunities for academia. In this review, we summarize the recent new findings on both β-lactamases and cell wall transpeptidases because these two groups of enzymes are evolutionarily and functionally connected. Many efforts to develop new β-lactams have aimed to inhibit both transpeptidases and β-lactamases, while several promising novel β-lactamase inhibitors have shown the potential to be further developed into transpeptidase inhibitors. In addition, the drug discovery progress against each group of enzymes is presented in three aspects: understanding the targets, screening methodology, and new inhibitor chemotypes. This is to offer insights into not only the advancement in this field but also the challenges, opportunities, and resources for future research. In particular, cyclic boronate compounds are now capable of inhibiting all classes of β-lactamases, while the diazabicyclooctane (DBO) series of small molecules has led to not only new β-lactamase inhibitors but potentially a new class of antibiotics by directly targeting PBPs. With the cautiously optimistic successes of a number of new β-lactamase inhibitor chemotypes and many questions remaining to be answered about the structure and function of cell wall transpeptidases, non-β-lactam transpeptidase inhibitors may usher in the next exciting phase of drug discovery in this field.

Unraveling the mechanism of ceftaroline-induced allosteric regulation in penicillin-binding protein 2a: insights for novel antibiotic development against methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) acquires high-level resistance against β-lactam antibiotics by expressing penicillin-binding protein 2a (PBP2a). PBP2a is a cell wall-synthesizing protein whose closed active site exhibits a reduced binding affinity toward β-lactam antibiotics. Ceftaroline (CFT), a fifth-generation cephalosporin, can effectively inhibit the PBP2a activity by binding to an allosteric site to trigger the active site opening, allowing a second CFT to access the active site. However, the essential mechanism behind the allosteric behavior of PBP2a remains unclear. Herein, computational simulations are employed to elucidate how CFT allosterically regulates the conformation and dynamics of the active site of PBP2a. While CFT stabilizes the allosteric domain surrounding it, it simultaneously enhances the dynamics of the catalytic domain. Specifically, the study successfully captured the opening process of the active pocket in the allosteric CFT-bound systems and discovered that CFT alters the potential signal-propagating pathways from the allosteric site to the active site. These findings reveal the implied mechanism of the CFT-mediated allostery in PBP2a and provide new insights into dual-site drug design or combination therapy against MRSA targeting PBP2a.

Innovative Phospholipid Carriers: A Viable Strategy to Counteract Antimicrobial Resistance

The overuse and misuse of antibiotics have led to the emergence and spread of multidrug-resistant (MDR), extensively drug-resistant (XDR), and pan-drug-resistant (PDR) bacteria strains, usually associated with poorer patient outcomes and higher costs. In order to preserve the usefulness of these life-saving drugs, it is crucial to use them appropriately, as also recommended by the WHO. Moreover, innovative, safe, and more effective approaches are being investigated, aiming to revise drug treatments to improve their pharmacokinetics and distribution and to reduce the onset of drug resistance. Globally, to reduce the burden of antimicrobial resistance (AMR), guidelines and indications have been developed over time, aimed at narrowing the use and diminishing the environmental spread of these life-saving molecules by optimizing prescriptions, dosage, and times of use, as well as investing resources into obtaining innovative formulations with better pharmacokinetics, pharmacodynamics, and therapeutic results. This has led to the development of new nano-formulations as drug delivery vehicles, characterized by unique structural properties, biocompatible natures, and targeted activities such as state-of-the-art phospholipid particles generally grouped as liposomes, virosomes, and functionalized exosomes, which represent an attractive and innovative delivery approach. Liposomes and virosomes are chemically synthesized carriers that utilize phospholipids whose nature is predetermined based on their use, with a long track record as drug delivery systems. Exosomes are vesicles naturally released by cells, which utilize the lipids present in their cellular membranes only, and therefore, are highly biocompatible, with investigations as a delivery system having a more recent origin. This review will summarize the state of the art on microvesicle research, liposomes, virosomes, and exosomes, as useful and effective tools to tackle the threat of antibiotic resistance.

Drug Repurposing Approach in Developing New Furosemide Analogs as Antimicrobial Candidates and Anti-PBP: Design, Synthesis, and Molecular Docking

Multidrug-resistant microorganisms have become a global health problem, prompting research into new antimicrobials. Drug repurposing is a new technique in drug discovery used to improve drug development success. As a well-studied medication with a sulfonamide moiety, furosemide was chosen to study its antimicrobial effect on different microbial strains. In addition, a new family of furosemide analogs was investigated for their antimicrobial efficacy. According to the obtained results, the majority of the examined molecules exhibited potential antimicrobial activity. Compounds 3b and 4a had the best anti-MRSA results, with an MIC = 7.81 µg/mL. They also demonstrated potent anti-gram-negative activity against E. coli (MIC = 1.95 µg/mL and 3.91 µg/mL, respectively). A time-killing kinetics study against E. coli and MRSA showed bactericidal actions of 3b and 4a within 120-150 min. Moreover, an anti-PBP activity and an in vitro cytotoxicity evaluation were performed. Furosemide decreased the PBP2a levels in MRSA by 21.5% compared to the control. However, the furosemide analogs 3b and 4a demonstrated superior anti-PBP activity (55.9 and 57.1 % reduction in the expression of PBP2a, respectively). In addition, compound 4a was nearly nontoxic to normal WI-38 cells (IC₅₀ = 248.60 μg /mL) indicating its high safety profile. Finally, the ability of furosemide and compounds 3b and 4a to bind to the target PBP2a enzyme has also been supported by molecular docking research.

DFT calculations, molecular docking, in vitro antimicrobial and antidiabetic studies of green synthesized Schiff bases: as Covid-19 inhibitor

In this investigation, we synthesized Schiff bases 2-(2-methoxyphenoxy)-N-(4-methylbenzylidene)ethanamine, N-(4-methoxybenzylidene)-2-(2-methoxyphenoxy)ethanamine and 2-(2-methoxyphenoxy)-N-(4-nitrobenzylidene)ethanamine from 2-(2-methoxyphenoxy)ethanamine and various aromatic aldehydes by the environmentally friendly sonication method. The B3LYP method with a 6-311++G (d, p) basis set was used in the DFT calculation to obtain the optimized structure of the Schiff base MPEA-NIT. The compounds were tested in vitro for inhibition of bacterial growth (disc well method) and inhibition of α-amylase (starch-iodine method). The compounds tested showed inhibitory activities. In addition, they were subjected to PASS analysis, drug likeness, and bioactivity score predictions using online software. To confirm the experimental findings, molecular docking analyses of synthesized compounds on α-amylase (PDB ID: 1SMD), tRNA threonylcarbamoyladenosine (PDB ID: 5MVR), glycosyl transferase (PDB ID: 6D9T), and peptididoglycan D,D-transpeptidase (PDB ID: 6HZQ) were performed. The emergence of a new coronavirus epidemic necessitates the development of antiviral medications (SARS-CoV-2). Docking active site interactions were investigated to predict compounds' activity against COVID-19 by binding with the SARS-CoV-2 (PDB ID: 6Y84).Communicated by Ramaswamy H. Sarma.

Discovery of Pyrrolidine-2,3-diones as Novel Inhibitors of P. aeruginosa PBP3

The alarming threat of the spread of multidrug resistant bacteria currently leaves clinicians with very limited options to combat infections, especially those from Gram-negative bacteria. Hence, innovative strategies to deliver the next generation of antibacterials are urgently needed. Penicillin binding proteins (PBPs) are proven targets inhibited by β-lactam antibiotics. To discover novel, non-β-lactam inhibitors against PBP3 of Pseudomonas aeruginosa, we optimised a fluorescence assay based on a well-known thioester artificial substrate and performed a target screening using a focused protease-targeted library of 2455 compounds, which led to the identification of pyrrolidine-2,3-dione as a potential scaffold to inhibit the PBP3 target. Further chemical optimisation using a one-pot three-component reaction protocol delivered compounds with excellent target inhibition, initial antibacterial activities against P. aeruginosa and no apparent cytotoxicity. Our investigation revealed the key structural features; for instance, 3-hydroxyl group (R²) and a heteroaryl group (R¹) appended to the N-pyrroldine-2,3-dione via methylene linker required for target inhibition. Overall, the discovery of the pyrrolidine-2,3-dione class of inhibitors of PBP3 brings opportunities to target multidrug-resistant bacterial strains and calls for further optimisation to improve antibacterial activity against P. aeruginosa.

In vitro antibacterial and in silico docking studies of two Schiff bases on Staphylococcus aureus and its target proteins

Background

Schiff base compounds have extensive applications in various fields such as analytical, inorganic, organic, and biological fields. They have excellent pharmacology application prospects in the modern era and are widely used in the pharmaceutical industry. In the present work in vitro antibacterial and in silico docking studies of two Schiff base compounds 2,2’-(5,5-dimethylcyclohexane-1,3-diylidene)bis(azan-1-yl-1-ylidene)diphenol (DmChDp) and N,N’-(5,5-dimethylcyclohexane-1,3-diylidene)dianiline (DmChDa) were carried out against the bacterial strain Staphylococcus aureus and its target proteins.

Results

The tests proved that the ligands have potential antibacterial activity. In the computational analysis, the drug-like properties of the compounds were first pre-filtered using the Lipinski rule of five. Then, molecular docking study was conducted using the AutoDock 4.2 program, to establish the mechanism by which the molecules inhibit the growth of S. aureus. For this purpose, 6 different target proteins (PDB ID: 1T2P, 3U2D, 2W9S, 1N67, 2ZCO, and 4H8E) of S. aureus were selected. Both the Schiff bases showed a good binding affinity with the target protein dihydrofolate reductase enzyme (PDB ID: 2W9S) but in different sites. Maximum binding energies of about − 10.3 and − 10.2 kcal/mol were observed when DmChDp and DmChDa were docked with 2W9S.

Conclusion

Schiff base compounds DmChDp and DmChDa have appreciable growth-inhibitory power against S. aureus, which can be attributed to the deactivation of the enzyme, dihydrofolate reductase.

The search for novel treatment strategies for Streptococcus pneumoniae infections

This review provides an overview of the most important novel treatment strategies against Streptococcus pneumoniae infections published over the past 10 years. The pneumococcus causes the majority of community-acquired bacterial pneumonia cases, and it is one of the prime pathogens in bacterial meningitis. Over the last 10 years, extensive research has been conducted to prevent severe pneumococcal infections, with a major focus on (i) boosting the host immune system and (ii) discovering novel antibacterials. Boosting the immune system can be done in two ways, either by actively modulating host immunity, mostly through administration of selective antibodies, or by interfering with pneumococcal virulence factors, thereby supporting the host immune system to effectively overcome an infection. While several of such experimental therapies are promising, few have evolved to clinical trials. The discovery of novel antibacterials is hampered by the high research and development costs versus the relatively low revenues for the pharmaceutical industry. Nevertheless, novel enzymatic assays and target-based drug design, allow the identification of targets and the development of novel molecules to effectively treat this life-threatening pathogen.

Penicillin binding protein 2a: An overview and a medicinal chemistry perspective

Antimicrobial resistance is an imminent threat worldwide. Methicillin-resistant Staphylococcus aureus (MRSA) is one of the "superbug" family, manifesting resistance through the production of a penicillin binding protein, PBP2a, an enzyme that provides its transpeptidase activity to allow cell wall biosynthesis. PBP2a's low affinity to most β-lactams, confers resistance to MRSA against numerous members of this class of antibiotics. An Achilles' heel of MRSA, PBP2a represents a substantial target to design novel antibiotics to tackle MRSA threat via inhibition of the bacterial cell wall biosynthesis. In this review we bring into focus the PBP2a enzyme and examine the various aspects related to its role in conferring resistance to MRSA strains. Moreover, we discuss several antibiotics and antimicrobial agents designed to target PBP2a and their therapeutic potential to meet such a grave threat. In conclusion, we consider future perspectives for targeting MRSA infections.

Synthesis and Penicillin-binding Protein Inhibitory Assessment of Dipeptidic 4-Phenyl-β-lactams from α-Amino Acid-derived Imines

Monocyclic β-lactams revive the research field on antibiotics, which are threatened by the emergence of resistant bacteria. A six-step synthetic route was developed, providing easy access to new 3-amino-1-carboxymethyl-4-phenyl-β-lactams, of which the penicillin-binding protein (PBP) inhibitory potency was demonstrated biochemically.

In Silico Design and Enantioselective Synthesis of Functionalized Monocyclic 3-Amino-1-carboxymethyl-β-lactams as Inhibitors of Penicillin-Binding Proteins of Resistant Bacteria

As a complement to the renowned bicyclic β-lactam antibiotics, monocyclic analogues provide a breath of fresh air in the battle against resistant bacteria. In that framework, the present study discloses the in silico design and unprecedented ten-step synthesis of eleven nocardicin-like enantiomerically pure 2-{3-[2-(2-aminothiazol-4-yl)-2-(methoxyimino)acetamido]-2-oxoazetidin-1-yl}acetic acids starting from serine as a readily accessible precursor. The capability of this novel class of monocyclic 3-amino-β-lactams to inhibit penicillin-binding proteins (PBPs) of various (resistant) bacteria was assessed, revealing the potential of α-benzylidenecarboxylates as interesting leads in the pursuit of novel PBP inhibitors. No deactivation by representative enzymes belonging to the four β-lactamase classes was observed, while weak inhibition of class C β-lactamase P99 was demonstrated.

Synergistic antibacterial activity between penicillenols and antibiotics against methicillin-resistant Staphylococcus aureus

Penicillenol A2 (isolated from deep-sea fungus Penicillium biourgeianum DFFSCS023) has good antibacterial activity against methicillin-sensitive Staphylococcus aureus and in combination with beta-lactam antibiotics it could significantly decrease methicillin-resistant Staphylococcus aureus (MRSA) survival, which provides a novel treatment consideration for MRSA-caused infections.

New strategies for targeting and treatment of multi-drug resistant Staphylococcus aureus

Staphylococcus aureus is a major cause of bacterial infection in humans, and has been notoriously able to acquire resistance to a variety of antibiotics. An example is methicillin-resistant S. aureus (MRSA), which despite having been initially associated with clinical settings, now is one of the key causative agents of community-acquired infections. Antibiotic resistance in S. aureus involves mechanisms ranging from drug efflux to increased expression or mutation of target proteins, and this has required innovative approaches to develop novel treatment methodologies. This review provides an overview of the major mechanisms of antibiotic resistance developed by S. aureus, and describes the emerging alternatives being sought to circumvent infection and proliferation, including new generations of classic antibiotics, synergistic approaches, antibodies, and targeting of virulence factors.

Synergistic antibacterial effects of herbal extracts and antibiotics on methicillin-resistant Staphylococcus aureus: A computational and experimental study

Antibiotic resistance has become a serious global concern, and the discovery of antimicrobial herbal constituents may provide valuable solutions to overcome the problem. In this study, the effects of therapies combining antibiotics and four medicinal herbs on methicillin-resistant Staphylococcus aureus (MRSA) were investigated. Specifically, the synergistic effects of Magnolia officinalis, Verbena officinalis, Momordica charantia, and Daphne genkwa in combination with oxacillin or gentamicin against methicillin-resistant (ATCC43300) and methicillin-susceptible (ATCC25923) S. aureus were examined. In vitro susceptibility and synergistic testing were performed to measure the minimum inhibitory concentration and fractional inhibitory concentration (FIC) index of the antibiotics and medicinal herbs against MRSA and methicillin-susceptible S. aureus. To identify the active constituents in producing these synergistic effects, in silico molecular docking was used to investigate the binding affinities of 139 constituents of the four herbs to the two common MRSA inhibitory targets, penicillin binding proteins 2a (PBP2a) and 4 (PBP4). The physicochemical and absorption, distribution, metabolism, and excretion properties and drug safety profiles of these compounds were also analyzed. D. genkwa extract potentiated the antibacterial effects of oxacillin against MRSA, as indicated by an FIC index value of 0.375. M. officinalis and V. officinalis produced partial synergistic effects when combined with oxacillin, whereas M. charantia was found to have no beneficial effects in inhibiting MRSA. Overall, tiliroside, pinoresinol, magnatriol B, and momorcharaside B were predicted to be PBP2a or PBP4 inhibitors with good drug-like properties. This study identifies compounds that deserve further investigation with the aim of developing therapeutic agents to modulate the effect of antibiotics on MRSA. Impact statement Antibiotic resistant is a well-known threat to global health and methicillin-resistant Staphylococcus aureus is one of the most significant ones. These resistant bacteria kill thousands of people every year and therefore a new effective antimicrobial treatment is necessary. This study identified the herbs and their associated bioactive ingredients that can potential the effects of current antibiotics. These herbs have long history of human usage in China and have well-defined monograph in the Chinese Pharmacopeia. These indicate their relatively high clinical safety and may have a quicker drug development process than that of a new novel antibiotic. Based on the results of this study, the authors will perform further in vitro and animal studies, aiming to accumulate significant data for the application of clinical trial.

Mechanisms of β-lactam antimicrobial resistance and epidemiology of major community- and healthcare-associated multidrug-resistant bacteria

Alexander Fleming's discovery of penicillin heralded an age of antibiotic development and healthcare advances that are premised on the ability to prevent and treat bacterial infections both safely and effectively. The resultant evolution of antimicrobial resistant mechanisms and spread of bacteria bearing these genetic determinants of resistance are acknowledged to be one of the major public health challenges globally, and threatens to unravel the gains of the past decades. We describe the major mechanisms of resistance to β-lactam antibiotics - the most widely used and effective antibiotics currently - in both Gram-positive and Gram-negative bacteria, and also briefly detail the existing and emergent pharmacological strategies to overcome such resistance. The global epidemiology of the four major types of bacteria that are responsible for the bulk of antimicrobial-resistant infections in the healthcare setting - methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, Enterobactericeae, and Acinetobacter baumannii - are also briefly described.

In silico study on Penicillin derivatives and Cephalosporins for upper respiratory tract bacterial pathogens

Upper respiratory tract infection (URTI) is an acute infection which involves the upper respiratory tract: nose, sinuses, tonsils and pharynx. URT infections are caused mainly by pathogenic bacteria like Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus. Conventionally, β-lactam antibiotics are used to treat URT infections. Penicillin binding proteins (PBPs) catalyze the cell wall synthesis in bacteria. β-Lactam antibiotics like Penicillin, Cephalosporins, Carbapenems and Monobactams inhibit bacterial cell wall synthesis by binding with PBPs. Pathogenic bacteria have efficiently evolved to resist these β-lactam antibiotics. New generation antibiotics are capable of inhibiting the action of PBP due to its new and peculiar structure. New generation antibiotics and Penicillin derivatives are selected in this study and virtually compared on the basis of interaction studies. 3-Dimensional (3D) interaction studies between Lactivicin, Cefuroxime, Cefadroxil, Ceftaroline, Ceftobiprole and Penicillin derivatives with PBPs of the above-mentioned bacteria are carried out. The aim of this study was to suggest a potent new generation molecule for further modification to increase the efficacy of the drug for the URTI.

Bacterial proteases: targets for diagnostics and therapy

Proteases are essential for the proliferation and growth of bacteria, and are also known to contribute to bacterial virulence. This makes them interesting candidates as diagnostic and therapeutic targets for infectious diseases. In this review, the authors discuss the most recent developments and potential applications for bacterial proteases in the diagnosis and treatment of bacterial infections. Current and future bacterial protease targets are described and their limitations outlined.

Crystal structures of bifunctional penicillin-binding protein 4 from Listeria monocytogenes

Penicillin-binding proteins (PBPs), which catalyze the biosynthesis of the peptidoglycan chain of the bacterial cell wall, are the major molecular target of bacterial antibiotics. Here, we present the crystal structures of the bifunctional peptidoglycan glycosyltransferase (GT)/transpeptidase (TP) PBP4 from Listeria monocytogenes in the apo-form and covalently linked to two β-lactam antibiotics, ampicillin and carbenicillin. The orientation of the TP domain with respect to the GT domain is distinct from that observed in the previously reported structures of bifunctional PBPs, suggesting interdomain flexibility. In this structure, the active site of the GT domain is occluded by the close apposition of the linker domain, which supports the hypothesis that interdomain flexibility is related to the regulation of GT activity. The acylated structures reveal the mode of action of β-lactam antibiotics toward the class A PBP4 from the human pathogen L. monocytogenes. Ampicillin and carbenicillin can access the active site and be acylated without requiring a structural rearrangement. In addition, the active site of the TP domain in the apo-form is occupied by the tartrate molecule via extensive hydrogen bond interactions with the catalytically important residues; thus, derivatives of the tartrate molecule may be useful in the search for new antibiotics to inhibit PBPs.

Inhibition of DD-peptidases by a specific trifluoroketone: crystal structure of a complex with the Actinomadura R39 DD-peptidase

Inhibitors of bacterial DD-peptidases represent potential antibiotics. In the search for alternatives to β-lactams, we have investigated a series of compounds designed to generate transition state analogue structures upon reaction with DD-peptidases. The compounds contain a combination of a peptidoglycan-mimetic specificity handle and a warhead capable of delivering a tetrahedral anion to the enzyme active site. The latter includes a boronic acid, two alcohols, an aldehyde, and a trifluoroketone. The compounds were tested against two low-molecular mass class C DD-peptidases. As expected from previous observations, the boronic acid was a potent inhibitor, but rather unexpectedly from precedent, the trifluoroketone [D-α-aminopimelyl(1,1,1-trifluoro-3-amino)butan-2-one] was also very effective. Taking into account competing hydration, we found the trifluoroketone was the strongest inhibitor of the Actinomadura R39 DD-peptidase, with a subnanomolar (free ketone) inhibition constant. A crystal structure of the complex between the trifluoroketone and the R39 enzyme showed that a tetrahedral adduct had indeed formed with the active site serine nucleophile. The trifluoroketone moiety, therefore, should be considered along with boronic acids and phosphonates as a warhead that can be incorporated into new and effective DD-peptidase inhibitors and therefore, perhaps, antibiotics.

Inhibition of WTA synthesis blocks the cooperative action of PBPs and sensitizes MRSA to β-lactams

Rising drug resistance is limiting treatment options for infections by methicillin-resistant Staphylococcus aureus (MRSA). Herein we provide new evidence that wall teichoic acid (WTA) biogenesis is a remarkable antibacterial target with the capacity to destabilize the cooperative action of penicillin-binding proteins (PBPs) that underlie β-lactam resistance in MRSA. Deletion of gene tarO, encoding the first step of WTA synthesis, resulted in the restoration of sensitivity of MRSA to a unique profile of β-lactam antibiotics with a known selectivity for penicillin binding protein 2 (PBP2). Of these, cefuroxime was used as a probe to screen for previously approved drugs with a cryptic capacity to potentiate its activity against MRSA. Ticlopidine, the antiplatelet drug Ticlid, strongly potentiated cefuroxime, and this synergy was abolished in strains lacking tarO. The combination was also effective in a Galleria mellonella model of infection. Using both genetic and biochemical strategies, we determined the molecular target of ticlopidine as the N-acetylglucosamine-1-phosphate transferase encoded in gene tarO and provide evidence that WTA biogenesis represents an Achilles heel supporting the cooperative function of PBP2 and PBP4 in creating highly cross-linked muropeptides in the peptidoglycan of S. aureus. This approach represents a new paradigm to tackle MRSA infection.

Viable screening targets related to the bacterial cell wall

The synthesis of the bacterial peptidoglycan has been recognized for over 50 years as fertile ground for antibacterial discovery. Initially, empirical screening of natural products for inhibition of bacterial growth detected many chemical classes of antibiotics whose specific mechanisms of action were eventually dissected and defined. Of the nontoxic antibiotics discovered, most were found to be inhibitors of either protein synthesis or cell wall synthesis, which led to more directed screening for inhibitors of these pathways. Directed screening and design programs for cell wall inhibitors have been undertaken since the 1960s. In that time it has become clear that, while certain steps and intermediates have yielded selective inhibitors and are established targets, other potential targets have not yielded inhibitors whose antibacterial activity is proven to be solely due to that inhibition. Why has this search been so problematic? Are the established targets still worth pursuing? This review will attempt to answer these and other questions and evaluate the viability of targets related to peptidoglycan synthesis.

Multitarget ligands in antibacterial research: progress and opportunities

INTRODUCTION

Resistance to current antibacterial therapies is an inevitability that represents a significant global health concern. Bacteria have the capacity to render all current drug treatments ineffective, which places a demand on the drug discovery community to constantly develop new antibacterial agents. Compounds that inhibit multiple biological targets, often referred to as multitarget ligands, are an inviting prospect in antibacterial research because, although they will not solve the issue of resistance, they might help to delay the onset.

AREAS COVERED

This review covers some of the recent progress in identifying new ligands that deliberately interact with more than one essential biological target in bacteria. The two principal areas covered are inhibitors of DNA replication and cell wall biosynthesis.

EXPERT OPINION

Antibacterial programs for the design of multitarget ligands present an important opportunity for production of antibacterial agents. Their longevity, due to slow development of resistance, is comparable to that seen with other successful agents - but is much improved over single-targeted agents for which resistance can appear in vitro overnight. The preclinical development of these agents will have to overcome the standard problems of antibacterial discovery. Such problems include optimization of characteristics favoring cell entry and particularly the demonstration of selectivity of inhibition of the desired multiple targets without inhibition of other bacterial or any mammalian functions.

Development of new drugs for an old target: the penicillin binding proteins

The widespread use of β-lactam antibiotics has led to the worldwide appearance of drug-resistant strains. Bacteria have developed resistance to β-lactams by two main mechanisms: the production of β-lactamases, sometimes accompanied by a decrease of outer membrane permeability, and the production of low-affinity, drug resistant Penicillin Binding Proteins (PBPs). PBPs remain attractive targets for developing new antibiotic agents because they catalyse the last steps of the biosynthesis of peptidoglycan, which is unique to bacteria, and lies outside the cytoplasmic membrane. Here we summarize the “current state of the art” of non-β-lactam inhibitors of PBPs, which have being developed in an attempt to counter the emergence of β-lactam resistance. These molecules are not susceptible to hydrolysis by β-lactamases and thus present a real alternative to β-lactams. We present transition state analogs such as boronic acids, which can covalently bind to the active serine residue in the catalytic site. Molecules containing ring structures different from the β-lactam-ring like lactivicin are able to acylate the active serine residue. High throughput screening methods, in combination with virtual screening methods and structure based design, have allowed the development of new molecules. Some of these novel inhibitors are active against major pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and thus open avenues new for the discovery of novel antibiotics.

High-throughput screening for novel inhibitors of Neisseria gonorrhoeae penicillin-binding protein 2

The increasing prevalence of N. gonorrhoeae strains exhibiting decreased susceptibility to third-generation cephalosporins and the recent isolation of two distinct strains with high-level resistance to cefixime or ceftriaxone heralds the possible demise of β-lactam antibiotics as effective treatments for gonorrhea. To identify new compounds that inhibit penicillin-binding proteins (PBPs), which are proven targets for β-lactam antibiotics, we developed a high-throughput assay that uses fluorescence polarization (FP) to distinguish the fluorescent penicillin, Bocillin-FL, in free or PBP-bound form. This assay was used to screen a 50,000 compound library for potential inhibitors of N. gonorrhoeae PBP 2, and 32 compounds were identified that exhibited >50% inhibition of Bocillin-FL binding to PBP 2. These included a cephalosporin that provided validation of the assay. After elimination of compounds that failed to exhibit concentration-dependent inhibition, the antimicrobial activity of the remaining 24 was tested. Of these, 7 showed antimicrobial activity against susceptible and penicillin- or cephalosporin-resistant strains of N. gonorrhoeae. In molecular docking simulations using the crystal structure of PBP 2, two of these inhibitors docked into the active site of the enzyme and each mediate interactions with the active site serine nucleophile. This study demonstrates the validity of a FP-based assay to find novel inhibitors of PBPs and paves the way for more comprehensive high-throughput screening against highly resistant strains of N. gonorrhoeae. It also provides a set of lead compounds for optimization of anti-gonococcal agents.

4-quinolones as noncovalent inhibitors of high molecular mass penicillin-binding proteins

Penicillin-binding proteins (PBPs) are important bacterial enzymes that carry out the final steps of bacterial cell wall assembly. Their DD-transpeptidase activity accomplishes the essential peptide cross-linking step of the cell wall. To date, all attempts to discover effective inhibitors of PBPs, apart from β-lactams, have not led to new antibiotics. Therefore, the need for new classes of efficient inhibitors of these enzymes remains. Guided by a computational fragment-based docking procedure, carried out on Escherichia coli PBP5, we have designed and synthesized a series of 4-quinolones as potential inhibitors of PBPs. We describe their binding to the PBPs of E. coli and Bacillus subtilis. Notably, these compounds bind quite tightly to the essential high molecular mass PBPs.

Synthesis and evaluation of boronic acids as inhibitors of Penicillin Binding Proteins of classes A, B and C

In response to the widespread use of β-lactam antibiotics bacteria have evolved drug resistance mechanisms that include the production of resistant Penicillin Binding Proteins (PBPs). Boronic acids are potent β-lactamase inhibitors and have been shown to display some specificity for soluble transpeptidases and PBPs, but their potential as inhibitors of the latter enzymes is yet to be widely explored. Recently, a (2,6-dimethoxybenzamido)methylboronic acid was identified as being a potent inhibitor of Actinomadura sp. R39 transpeptidase (IC(50): 1.3 μM). In this work, we synthesized and studied the potential of a number of acylaminomethylboronic acids as inhibitors of PBPs from different classes. Several derivatives inhibited PBPs of classes A, B and C from penicillin sensitive strains. The (2-nitrobenzamido)methylboronic acid was identified as a good inhibitor of a class A PBP (PBP1b from Streptococcus pneumoniae, IC(50) = 26 μM), a class B PBP (PBP2xR6 from Streptococcus pneumoniae, IC(50) = 138 μM) and a class C PBP (R39 from Actinomadura sp., IC(50) = 0.6 μM). This work opens new avenues towards the development of molecules that inhibit PBPs, and eventually display bactericidal effects, on distinct bacterial species.