Research and development of antibacterial agents

Targeting mycobacterial membranes and membrane proteins: Progress and limitations

Among the various bacterial infections, tuberculosis continues to hold center stage. Its causative agent, Mycobacterium tuberculosis, possesses robust defense mechanisms against most front-line antibiotic drugs and host responses due to their complex cell membranes with unique lipid molecules. It is now well-established that bacteria change their membrane composition to optimize their environment to survive and elude drug action. Thus targeting membrane or membrane components is a promising avenue for exploiting the chemical space focussed on developing novel membrane-centric anti-bacterial small molecules. These approaches are more effective, non-toxic, and can attenuate resistance phenotype. We present the relevance of targeting the mycobacterial membrane as a practical therapeutic approach. The review highlights the direct and indirect targeting of membrane structure and function. Direct membrane targeting agents cause perturbation in the membrane potential and can cause leakage of the cytoplasmic contents. In contrast, indirect membrane targeting agents disrupt the function of membrane-associated proteins involved in cell wall biosynthesis or energy production. We discuss the chronological chemical improvements in various scaffolds targeting specific membrane-associated protein targets, their clinical evaluation, and up-to-date account of their ''mechanisms of action, potency, selectivity'' and limitations. The sources of anti-TB drugs/inhibitors discussed in this work have emerged from target-based identification, cell-based phenotypic screening, drug repurposing, and natural products. We believe this review will inspire the exploration of uncharted chemical space for informing the development of new scaffolds that can inhibit novel mycobacterial membrane targets.

Mg(OH)2 nanoparticles enhance the antibacterial activities of macrophages by activating the reactive oxygen species

Infection often causes disastrous consequences in all fields of clinical medicine, especially orthopedics. Hence, critical efforts are being made to engineer novel nanomaterials for the treatment of orthopedic infections due to the high biocompatibility and antibacterial properties they possess. The purpose of this study was to investigate the antibacterial effects of magnesium hydroxide (Mg(OH)₂ ) nanoparticles (NPs) in vitro and determine their possible mechanisms of action. In this study, Escherichia coli was selected as the pathogenic bacteria and it was found that Mg(OH)₂ NPs significantly inhibited the growth of E. coli by promoting nucleic acid leakage, inhibiting protein synthesis, and suppressing the metabolic activity. The minimum inhibitory concentration for these bacteria was determined to be 4.4 μg/ml. In vitro flow cytometry and immunofluorescence tests indicated that Mg(OH)₂ NPs induced the macrophages to generate reactive oxygen species to kill the bacteria. To understand the mechanisms involved in this process, western blotting was performed and it was found that Mg(OH)₂ NPs activated the phosphatidylinositol-3-kinase/serine-threonine kinase (PI3K/Akt) signaling pathway of macrophages to enhance their phagocytosis with no obvious cytotoxicity. Thus, Mg(OH)₂ NPs are a suitable choice to develop promising agents or coating materials for the treatment of clinically widespread infections in view of their safety, biocompatibility, and powerful antibacterial properties.

Targeting the Bacterial Cytoskeleton of the Burkholderia cepacia Complex for Antimicrobial Development: A Cautionary Tale

Burkholderia cepacia complex (BCC) bacteria are a group of opportunistic pathogens that cause severe lung infections in cystic fibrosis (CF). Treatment of BCC infections is difficult, due to the inherent and acquired multidrug resistance of BCC. There is a pressing need to find new bacterial targets for antimicrobials. Here, we demonstrate that the novel compound Q22, which is related to the bacterial cytoskeleton destabilising compound A22, can reduce the growth rate and inhibit growth of BCC bacteria. We further analysed the phenotypic effects of Q22 treatment on BCC virulence traits, to assess its feasibility as an antimicrobial. BCC bacteria were grown in the presence of Q22 with a broad phenotypic analysis, including resistance to H₂O₂-induced oxidative stress, changes in the inflammatory potential of cell surface components, and in-vivo drug toxicity studies. The influence of the Q22 treatment on inflammatory potential was measured by monitoring the cytokine responses of BCC whole cell lysates, purified lipopolysaccharide, and purified peptidoglycan extracted from bacterial cultures grown in the presence or absence of Q22 in differentiated THP-1 cells. BCC bacteria grown in the presence of Q22 displayed varying levels of resistance to H₂O₂-induced oxidative stress, with some strains showing increased resistance after treatment. There was strain-to-strain variation in the pro-inflammatory ability of bacterial lysates to elicit TNFα and IL-1β from human myeloid cells. Despite minimal toxicity previously shown in vitro with primary CF cell lines, in-vivo studies demonstrated Q22 toxicity in both zebrafish and mouse infection models. In summary, destabilisation of the bacterial cytoskeleton in BCC, using compounds such as Q22, led to increased virulence-related traits in vitro. These changes appear to vary depending on strain and BCC species. Future development of antimicrobials targeting the BCC bacterial cytoskeleton may be hampered if such effects translate into the in-vivo environment of the CF infection.

Structure-based ligand design of novel bacterial RNA polymerase inhibitors

Bacterial RNA polymerase (RNAP) is essential for transcription and is an antibacterial target for small molecule inhibitors. The binding region of myxopyronin B (MyxB), a bacterial RNAP inhibitor, offers the possibility of new inhibitor design. The molecular design program SPROUT has been used in conjunction with the X-ray cocrystal structure of Thermus thermophilus RNAP with MyxB to design novel inhibitors based on a substituted pyridyl-benzamide scaffold. A series of molecules, with molecular masses <350 Da, have been prepared using a simple synthetic approach. A number of these compounds inhibited Escherichia coli RNAP.

Study of antibacterial activity of ZnZnbisporphyrin complexes and its free components on Staphylococcus aureus by microcalorimetry

The antibacterial effect of Zn(II), tetraphenyl porphyrin (TPP), propdioxyl bridged tetraphenyl bisporphyrin 1, and its metallobisporphyrin complexes (ZnMnbisporphyrin 2 and ZnZnbisporphyrin 3) towards Staphylococcus aureus growth was investigated by microcalorimetry at 37 degrees C. Differences in their capacities to inhibit the growth metabolism of S. aureus were observed. By analyzing the power-time curves, crucial parameters such as the rate constant of bacterial growth (k), inhibitory ratio (I), and generation time (t (G)) were determined. The growth rate constant (k) of S. aureus (in the log phase) in the presence of the drugs decreased linearly with increasing concentrations of the complexes. The sequence of the antibacterial activities of these compounds tested was 3 > 2 > 1 > Zn(II) > TPP. ZnZnbisporphyrin 3 is proposed to benefit from the synergetic effects of Zn(II) and 1.

In vitro antibacterial effects of antilipopolysaccharide DNA aptamer-C1qrs complexes

DNA aptamers were developed against lipopolysaccharide (LPS) from E. coli O111:B4 and shown to bind both LPS and E. coli by a colorimetric enzyme-based microplate assay. The polyclonal aptamers were coupled to human C1qrs protein either directly using a bifunctional linker or indirectly using biotinylated aptamers and a streptavidin-C1qrs complex. Both systems significantly reduced colony counts when applied to E. coli O111:B4 and K12 strains across a series of 10x dilutions of the bacteria in the presence of human serum; it was diluted 1: 10(3) in order to avoid significant bacterial lysis by the competing alternate pathway of complement activation. A number of candidate DNA aptamer sequences were cloned and sequenced from the anti-LPS aptamer library for future screening of antibacterial or "antibiotic" potential and to aid in eventual development of an alternative therapy for antibiotic-resistant bacterial infections.

Treatment and control of severe infections caused by multiresistant Pseudomonas aeruginosa

Pseudomonas aeruginosa is one of the leading causes of nosocomial infections. Severe infections, such as pneumonia or bacteraemia, are associated with high mortality rates and are often difficult to treat, as the repertoire of useful anti-pseudomonal agents is limited (some beta-lactams, fluoroquinolones and aminoglycosides, and the polymyxins as last-resort drugs); moreover, P. aeruginosa exhibits remarkable ability to acquire resistance to these agents. Acquired resistance arises by mutation or acquisition of exogenous resistance determinants and can be mediated by several mechanisms (degrading enzymes, reduced permeability, active efflux and target modification). Overall, resistance rates are on the increase, and may be different in different settings, so that surveillance of P. aeruginosa susceptibility is essential for the definition of empirical regimens. Multidrug resistance is frequent, and clinical isolates resistant to virtually all anti-pseudomonal agents are increasingly being reported. Monotherapy is usually recommended for uncomplicated urinary tract infections, while combination therapy is normally recommended for severe infections, such as bacteraemia and pneumonia, although, at least in some cases, the advantage of combination therapy remains a matter of debate. Antimicrobial use is a risk factor for P. aeruginosa resistance, especially with some agents (fluoroquinolones and carbapenems), and interventions based on antimicrobial rotation and restriction of certain agents can be useful to control the spread of resistance. Similar measures, together with the prudent use of antibiotics and compliance with infection control measures, are essential to preserve the efficacy of the currently available anti-pseudomonal agents, in view of the dearth, in the near future, of new options against multidrug-resistant P. aeruginosa strains.

Antibiotic resistance in Staphylococcus aureus: concerns, causes and cures

Methicillin-resistant Staphylococcus aureus (MRSA) is currently a predominant and dangerous nosocomial pathogen. Unfortunately, infections caused by this organism are becoming more difficult to treat as further evolution of drug resistance occurs within the pathogen. Vancomycin has become the drug of choice for treating MRSA infections. However, treatment failures, adverse side effects and the emergence of vancomycin-resistant MRSA are leading to urgent requirements for alternative antiMRSA therapies. Linezolid is a new agent recently developed for Gram-positive infections, including MRSA. However, resistance to this drug is already developing. The need for superior antiMRSA drugs is therefore highly evident. This article explores some of the opportunities for the development of new treatments for MRSA. These arise principally from extensions of existing drug classes and the discovery of novel agents by genome-driven technologies. Other possibilities concern the re-evaluation of earlier pharmacophores that have not so far been developed as antiMRSA agents.

Optimal use of antibiotic resistance surveillance systems

Increasing concern about the emergence of resistance in clinically important pathogens has led to the establishment of a number of surveillance programmes to monitor the true extent of resistance at the local, regional and national levels. Although some programmes have been operating for several years, their true usefulness is only now being realised. This review describes some of the major surveillance initiatives and the way in which the data have been used in a number of different settings. In the hospital, surveillance data have been used to monitor local antibiograms and determine infection control strategies and antibiotic usage policies. In the community, surveillance data have been used to monitor public health threats, such as infectious disease outbreaks involving resistant pathogens and the effects of bioterrorism countermeasures, by following the effects of prophylactic use of different antibiotics on resistance. Initially, the pharmaceutical industry sponsored surveillance programmes to monitor the susceptibility of clinical isolates to marketed products. However, in the era of burgeoning resistance, many developers of antimicrobial agents find surveillance data useful for defining new drug discovery and development strategies, in that they assist with the identification of new medical needs, allow modelling of future resistance trends, and identify high-profile isolates for screening the activity of new agents. Many companies now conduct pre-launch surveillance of new products to benchmark activity so that changes in resistance can be monitored following clinical use. Surveillance data also represent an integral component of regulatory submissions for new agents and, together with clinical trial data, are used to determine breakpoints. It is clear that antibiotic resistance surveillance systems will continue to provide valuable data to health care providers, university researchers, pharmaceutical companies, and government and regulatory agencies.

Harnessing the potential of chemical defenses from antimicrobial activities

Resistance to the drugs used in the treatment of many infectious diseases is increasing, while microbial infections are being found to be responsible for more life-threatening diseases than previously thought. Despite a large investment in the invention and application of high-throughput screening techniques involving miniaturization and automation, and a diverse array of strategies for designing and constructing various chemical libraries, relatively few new drugs have resulted. Natural products, however, have been a major source of drugs for centuries. Since some of them are produced by organisms as a result of selection in favour of improved defense against competing deleterious microorganisms, in principle they would be less likely to incur resistance. Furthermore, the production of those defensive secondary metabolites is inducible because their original function is a response to environmental challenges. Moreover, symbioses, co-habitation associations between two or more different species of organisms, are universal in nature, and the production of secondary metabolites by symbiotic microbes may be an important adaptation allowing microbes to affect their hosts. Therefore, co-culture strategies, using combinations of plant cell-pathogenic microbes, plant cell-endophytes (or symbionts), and symbiont-pathogenic microbes, based on the principles of chemical defense and the known mechanisms of organism interactions, may be an efficient general approach in the search for new anti-microbial drugs.

MexAB-OprM-Specific efflux pump inhibitors in Pseudomonas aeruginosa. Part 1: Discovery and early strategies for lead optimization

The identification of a series of compounds that specifically inhibit efflux by the MexAB-OprM pump system in Pseudomonas aeruginosa is described. Synthesis and in vitro structure-activity relationships (SARs) are outlined. Early leads lacked activity in animal models, and efforts to improve solubility and reduce serum protein binding by the introduction of polar groups are discussed.

New developments in tetracycline antibiotics: glycylcyclines and tetracycline efflux pump inhibitors

The tetracyclines, discovered in the 1940s, are a well-established class of antibiotics that still have a role in treating microbial infections in man. However, the widespread emergence of bacterial resistance due to efflux and ribosomal protection mechanisms has severely limited their effectiveness. A new generation of tetracyclines, the glycylcyclines, has been developed to overcome resistance to earlier tetracyclines. One of the new glycylcyclines, 9-t-butylglyclamido-minocycline (GAR-936, tigecycline) is currently undergoing clinical trials. This review considers the current status of glycylcyclines and the possibility that resistance to these agents might arise in the future. Other approaches are also being taken to address the emergence of resistance to tetracyclines. Recently, a number of tetracycline efflux pump inhibitors have been discovered that might be used in combination with earlier tetracyclines to restore their activity against resistant organisms. However, the development of tetracycline efflux pump inhibitors is complicated by the occurrence of several efflux pump sub-families and by the presence of both efflux and ribosomal protection mechanisms in the same organism, especially in naturally occurring, Gram-positive clinical isolates.

Therapies in development for community-acquired pneumonia

The current use of antimicrobials has become more complex due to the extensive emergence of antibiotic resistance. The single most important approach in resistance control is probably the judicious use of chemotherapeutic agents. New agents that may be of use in the treatment of community-acquired pneumonia are currently in development. Antimicrobials can be grouped according to their mechanism of action. These include protein synthesis inhibitors (ketolides, oxazolidinones, streptogramins and glycylcyclines), nucleic acid synthesis inhibitors (fluoroquinolones), peptidoglycan synthesis inhibitors (beta-lactams and glycopeptides) and agents interfering with membrane function (cationic peptides and lipopeptides). Among those agents under development, only the oxazolidinones, the cationic peptides and the lipopeptide antibiotics can be truly regarded as structurally novel inhibitors as the other agents are analogues of existing compounds which have been in clinical use for many years.

Advances in the research and development of chemotherapeutic agents for respiratory tract bacterial infections

The activity of existing antibiotics is diminishing due to the increasing number of resistant strains and by the increase of infections with naturally resistant microorganisms. New agents are urgently needed to meet this challenge and the molecular strategies adopted for the discovery of these compounds must focus on minimizing the emergence of future resistance to them. Novel compounds can be grouped on the basis of their mechanism of action: inhibitors of nucleic acid synthesis (fluoroquinolones), inhibitors of protein synthesis (ketolides, oxazolidinones, streptogramins, and glycylcyclines), inhibitors of peptidoglycan synthesis (beta-lactams and glycopeptides), and agents interfering with membrane function (cationic peptides, and lipopeptides). Regarding the agents that are already in the research and development pipeline, only the oxazolidinones, the cationic peptides and the lipopeptide antibiotics can be truly considered as structurally novel inhibitors because the other agents are analogues of existing compounds that have been in use for many years.

High therapeutic index of factor C Sushi peptides: potent antimicrobials against Pseudomonas aeruginosa

Factor C protein isolated from the horseshoe crab, Carcinoscorpius rotundicauda, has endotoxin binding capability. Synthetic peptides of 34 amino acids based on the sequence of two regions of factor C (Sushi 1 and Sushi 3) as well as their corresponding mutants exhibited activities against 30 clinical isolates of Pseudomonas aeruginosa. Collectively, all four peptides demonstrated exceptionally effective bactericidal activity against P. aeruginosa with 90% minimal bactericidal concentrations (MBC(90)s) in the range of 0.06 to 0.25 microg/ml (16 to 63 nM). Viable bacteria were reduced by 90% after 7 min and were totally eradicated within 40 to 50 min. These peptides are minimally hemolytic against both rabbit and human erythrocytes even at concentrations up to 1,600-fold their MBC(90)s. Both in vitro and in vivo studies indicate that cytotoxic effects are small even at 1,000-fold their MBC(90)s. Furthermore, the Sushi peptides are tolerant of high-salt and adverse pH conditions. These findings demonstrate the promising therapeutic potential of the Sushi peptides.

The 2-pyridone antibacterial agents: bacterial topoisomerase inhibitors

Many attempts have been made to prepare analogs of 4-quinolone antibacterial agents bearing novel ring systems, which might retain the favorable properties of these widely used antibacterial agents and at the same time increase activity against multidrug-resistant bacteria, streptococci, and anaerobic microorganisms. One such attempt involved bioisosteric exchange of the 1-N atom and 4a-C atom of naphthyridones, quinolones, and benzoxazines to produce a family of highly active pyridopyrimidines, quinolizines, and ofloxacin bioisosteres. These new antibacterial agents have been named collectively as the 2-pyridones. Many hundreds of 2-pyridones have been synthesized and evaluated in vitro and in vivo, and selected members are advancing toward human clinical trials. Preparation of these bioisosteres required the development of enabling chemistry, as previous methods were unsuccessful in producing the needed core structures. This review compares the structure-activity relationships of these agents with known trends among 4-quinolones, from which it is seen that there are many parallels, but also some significant departures as well. Generally, 2-pyridones are more highly active in vitro and in vivo and more water soluble than comparable 4-quinolones. These properties are posited to arise from electronic and conformational alternations in these new substances. Selected members show excellent pharmacodynamic properties, justifying the view that this is a very promising new class of totally synthetic antibacterial agents.