Investigational antimicrobial agents of 2013

Recent advances in the exploration of oxazolidinone scaffolds from compound development to antibacterial agents and other bioactivities

Bacterial infections cause a variety of life-threatening diseases, and the continuous evolution of drug-resistant bacteria poses an increasing threat to current antimicrobial regimens. Gram-positive bacteria (GPB) have a wide range of genetic capabilities that allow them to adapt to and develop resistance to practically all existing antibiotics. Oxazolidinones, a class of potent bacterial protein synthesis inhibitors with a unique mechanism of action involving inhibition of bacterial ribosomal translation, has emerged as the antibiotics of choice for the treatment of drug-resistant GPB infections. In this review, we discussed the oxazolidinone antibiotics that are currently on the market and in clinical development, as well as an updated synopsis of current advances on their analogues, with an emphasis on innovative strategies for structural optimization of linezolid, structure-activity relationship (SAR), and safety properties. We also discussed recent efforts aimed at extending the activity of oxazolidinones to gram-negative bacteria (GNB), antitumor, and coagulation factor Xa. Oxazolidinone antibiotics can accumulate in GNB by a conjugation to siderophore-mediated β-lactamase-triggered release, making them effective against GNB.

Treatment of MRSA Infection: Where are We?

Staphylococcus aureus is a leading cause of septicemia, endocarditis, pneumonia, skin and soft tissue infections, bone and joint infections, and hospital-acquired infections. In particular, methicillin-resistant Staphylococcus aureus (MRSA) is associated with high morbidity and mortality, and continues to be a major public health problem. The emergence of multidrug-resistant MRSA strains along with the wide consumption of antibiotics has made anti-MRSA treatment a huge challenge. Novel treatment strategies (e.g., novel antimicrobials and new administrations) against MRSA are urgently needed. In the past decade, pharmaceutical companies have invested more in the research and development (R&D) of new antimicrobials and strategies, spurred by favorable policies. All research articles were collected from authentic online databases, including Google Scholar, PubMed, Scopus, and Web of Science, by using different combinations of keywords, including 'anti-MRSA', 'antibiotic', 'antimicrobial', 'clinical trial', 'clinical phase', clinical studies', and 'pipeline'. The information extracted from articles was compared to information provided on the drug manufacturer's website and Clinical Trials.gov (https://clinicaltrials.gov/) to confirm the latest development phase of anti-MRSA agents. The present review focuses on the current development status of new anti-MRSA strategies concerning chemistry, pharmacological target(s), indications, route of administration, efficacy and safety, pharmacokinetics, and pharmacodynamics, and aims to discuss the challenges and opportunities in developing drugs for anti-MRSA infections.

Emerging Target-Directed Approaches for the Treatment and Diagnosis of Microbial Infections

With the rising levels of drug resistance, developing efficient antimicrobial therapies has become a priority. A promising strategy is the conjugation of antibiotics with relevant moieties that can potentiate their activity by target-directing. The conjugation of siderophores with antibiotics allows them to act as Trojan horses by hijacking the microorganisms' highly developed iron transport systems and using them to carry the antibiotic into the cell. Through the analysis of relevant examples of the past decade, this Perspective aims to reveal the potential of siderophore-antibiotic Trojan horses for the treatment of infections and the role of siderophores in diagnostic techniques. Other conjugated molecules will be the subject of discussion, namely those involving vitamin B12, carbohydrates, and amino acids, as well as conjugated compounds targeting protein degradation and β-lactamase activated prodrugs.

The primary pharmacology of ceftazidime/avibactam: in vitro translational biology

Previous reviews of ceftazidime/avibactam have focused on in vitro molecular enzymology and microbiology or the clinically associated properties of the combination. Here we take a different approach. We initiate a series of linked reviews that analyse research on the combination that built the primary pharmacology data required to support the clinical and business risk decisions to perform randomized controlled Phase 3 clinical trials, and the additional microbiological research that was added to the above, and the safety and chemical manufacturing and controls data, that constituted successful regulatory licensing applications for ceftazidime/avibactam in multiple countries, including the USA and the EU. The aim of the series is to provide both a source of reference for clinicians and microbiologists to be able to use ceftazidime/avibactam to its best advantage for patients, but also a case study of bringing a novel β-lactamase inhibitor (in combination with an established β-lactam) through the microbiological aspects of clinical development and regulatory applications, updated finally with a review of resistance occurring in patients under treatment. This first article reviews the biochemistry, structural biology and basic microbiology of the combination, showing that avibactam inhibits the great majority of serine-dependent β-lactamases in Enterobacterales and Pseudomonas aeruginosa to restore the in vitro antibacterial activity of ceftazidime. Translation to efficacy against infections in vivo is reviewed in the second co-published article, Nichols et al. (J Antimicrob Chemother 2022; dkac172).

Investigational and Experimental Drugs for Community-Acquired Pneumonia: the Current Evidence

Community-acquired pneumonia (CAP) is a common infection with a constantly evolving etiological spectrum. This changing etiology conditions the adequate selection of optimal therapeutic regimens, both in empirical and definitive treatments. In recent years, new antimicrobials have been approved by regulatory authorities for use in CAP, although it is necessary to continue incorporating new antimicrobial agents that improve the activity profile in relation to the appearance of bacterial resistance in certain pathogens, such as pneumococcus, Staphylococcus aureus or Pseudomonas aeruginosa. Delafloxacin, omadacycline and lefamulin are the most recently approved antibiotics for CAP. These three antibiotics have shown non-inferiority to their comparators for the treatment of CAP with an excellent safety profile. However, in the 2019 ATS/IDSA guidelines, it has been considered that more information is needed to incorporate these new drugs into community-based treatment. New antimicrobials, such as solithromycin and nemonoxacin, are currently being studied in Phase III clinical trials. Both drugs have shown non-inferiority against the comparators and an acceptable safety profile; however, they have not yet been approved by the regulatory authorities. Several drugs are being tested in Phase I and II clinical trials. These include zabofloxacin, aravofloxacin, nafithromycin, TP-271, gepotidacin, radezolid, delpazolid, and CAL02. The preliminary results of these clinical trials allow us to assure that most of these drugs may play a role in the future treatment of CAP.

Strategic Moves of "Superbugs" Against Available Chemical Scaffolds: Signaling, Regulation, and Challenges

Superbugs' resistivity against available natural products has become an alarming global threat, causing a rapid deterioration in public health and claiming tens of thousands of lives yearly. Although the rapid discovery of small molecules from plant and microbial origin with enhanced bioactivity has provided us with some hope, a rapid hike in the resistivity of superbugs has proven to be the biggest therapeutic hurdle of all times. Moreover, several distinct mechanisms endowed by these notorious superbugs make them immune to these antibiotics subsequently causing our antibiotic wardrobe to be obsolete. In this unfortunate situation, though the time frame for discovering novel "hit molecules" down the line remains largely unknown, our small hope and untiring efforts injected in hunting novel chemical scaffolds with unique molecular targets using high-throughput technologies may safeguard us against these life-threatening challenges to some extent. Amid this crisis, the current comprehensive review highlights the present status of knowledge, our search for bacteria Achilles' heel, distinct molecular signaling that an opportunistic pathogen bestows to trespass the toxicity of antibiotics, and facile strategies and appealing therapeutic targets of novel drugs. Herein, we also discuss multidimensional strategies to combat antimicrobial resistance.

Antimicrobial Stewardship Program in Critical Care-Need of the Hour

Maximum antibiotic usage within hospitals occurs in critical care areas. Reasons for this usage are the moribund state of patients, invasive devices, and protocol based necessity for empiric antibiotic initiation in most critical conditions. Although unavoidable, prudent use of antibiotics (empiric and therapeutic) should be tailored based on national or if available, unit-based hospital antibiogram. This forms the footstool of every antibiotic policy formulated at tertiary care hospitals. Strict adherence to antibiotic policy formulated based on hospital antibiogram largely benefits patients and hospital-wide antimicrobial stewardship is ensured. The necessity, benefits, key targets, and usefulness of antimicrobial stewardship program (AMSP) in critical care has been elaborated in this review. How to cite this article: Vadala R, Princess I. Antimicrobial Stewardship Program in Critical Care-Need of the Hour. Indian J Crit Care Med 2020;24(9):847-854.

Transferable Mechanisms of Quinolone Resistance from 1998 Onward

While the description of resistance to quinolones is almost as old as these antimicrobial agents themselves, transferable mechanisms of quinolone resistance (TMQR) remained absent from the scenario for more than 36 years, appearing first as sporadic events and afterward as epidemics. In 1998, the first TMQR was soundly described, that is, QnrA. The presence of QnrA was almost anecdotal for years, but in the middle of the first decade of the 21st century, there was an explosion of TMQR descriptions, which definitively changed the epidemiology of quinolone resistance. Currently, 3 different clinically relevant mechanisms of quinolone resistance are encoded within mobile elements: (i) target protection, which is mediated by 7 different families of Qnr (QnrA, QnrB, QnrC, QnrD, QnrE, QnrS, and QnrVC), which overall account for more than 100 recognized alleles; (ii) antibiotic efflux, which is mediated by 2 main transferable efflux pumps (QepA and OqxAB), which together account for more than 30 alleles, and a series of other efflux pumps (e.g., QacBIII), which at present have been sporadically described; and (iii) antibiotic modification, which is mediated by the enzymes AAC(6')Ib-cr, from which different alleles have been claimed, as well as CrpP, a newly described phosphorylase.

Low concentrations of acetic and formic acids enhance the inactivation of Staphylococcus aureus and Pseudomonas aeruginosa with pulsed electric fields

Background

Skin infections, particularly caused by drug-resistant pathogens, represent a clinical challenge due to being a frequent cause of morbidity and mortality. The objectives of this study were to examine if low concentrations of acetic and formic acids can increase sensitivity of Staphylococcus aureus and Pseudomonas aeruginosa to pulsed electric field (PEF) and thus, promote a fast and efficient treatment methodology for wound treatment.

Results

We have shown that the combination of PEF (10–30 kV/cm) with organic acids (0.1% formic and acetic acids) increased the bactericidal properties of treatment. The effect was apparent for both acids. The proposed methodology allowed to reduce the energy of electrical pulses and the inhibitory concentrations of acids, while still maintain high efficiency of bacteria eradication.

Conclusions

Application of weak organic acids as bactericidal agents has many advantages over antibiotics because they do not trigger development of drug-resistance in bacteria. The combination with PEF can make the treatment effective even against biofilms. The results of this study are particularly useful for the development of new methodologies for the treatment of extreme cases of wound infections when the chemical treatment is no longer effective or hinders wound healing.

Identification and characterization of novel isothiazolones with potent bactericidal activity against multi-drug resistant Acinetobacter baumannii clinical isolates

Acinetobacter baumannii has emerged as a globally important nosocomial pathogen characterized by an increased multi-drug resistance (MDR), leaving limited options for treating its infection. To identify novel antibacterial compounds with activity against clinical isolates of A. baumannii, we performed high-throughput screening against a chemical library of 42,944 compounds using nonpathogenic Escherichia coli MG1655 and identified 55 hit compounds. The antibacterial activities of 30 pure compounds were determined against MDR clinical isolates of A. baumannii obtained from Los Angeles County hospitals. Two isothiazolones identified, 5-chloro-2-(4-chloro-3-methylphenyl)-4-methyl-3(2H)-isothiazolone (Compound 6) and 5-chloro-2-(4-chlorophenyl)-4-methyl-3(2H)-isothiazolone (Compound 7), possess novel structure and exhibited consistent, potent and cidal activity against all 46 MDR A. baumannii clinical isolates and reference strains. Additionally, structure-activity relationship analysis involving several additional isothiazolones supports the link between a chloro-group on the heterocyclic ring or a fused benzene ring and the cidal activity. Attempts to obtain isothiazolone resistant mutants failed, consistent with the rapid cidal action and indicative of a complex mechanism of action. While cytotoxicity was observed with Compound 7, it had a therapeutic index value of 28 suggesting future therapeutic potential. Our results indicate that high-throughput screening of compound libraries followed by in vitro biological evaluations is a viable approach for the discovery of novel antibacterial agents to contribute in the fight against MDR bacterial pathogens.

Combating a Master Manipulator: Staphylococcus aureus Immunomodulatory Molecules as Targets for Combinatorial Drug Discovery

Staphylococcus aureus is a bacterial pathogen that can cause significant disease burden and mortality by counteracting host defenses through producing virulence factors to survive the immune responses evoked by infection. This emerging drug-resistant pathogen has led to a decline in the efficacy of traditional antimicrobial therapy. To combat these threats, precision antimicrobial therapeutics have been created to target key virulence determinants of specific pathogens. Here we review the benefits of, progresses in, and roadblocks to the development of precision antimicrobial therapeutics using combinatorial chemistry.

Antimicrobial Activity of Murepavadin Tested against Clinical Isolates of Pseudomonas aeruginosa from the United States, Europe, and China

Murepavadin (formerly POL7080), a 14-amino-acid cyclic peptide, and comparators were tested by the broth microdilution method against 1,219 Pseudomonas aeruginosa isolates from 112 medical centers. Murepavadin (MIC_50/90, 0.12/0.12 mg/liter) was 4- to 8-fold more active than colistin (MIC_50/90, 1/1 mg/liter) and polymyxin B (MIC_50/90, 0.5/1 mg/liter) and inhibited 99.1% of isolates at ≤0.5 mg/liter. Only 4 isolates (0.3%) exhibited murepavadin MICs of >2 mg/liter. Murepavadin was equally active against isolates from Europe, the United States, and China.

The Macromolecular Machines that Duplicate the Escherichia coli Chromosome as Targets for Drug Discovery

DNA replication is an essential process. Although the fundamental strategies to duplicate chromosomes are similar in all free-living organisms, the enzymes of the three domains of life that perform similar functions in DNA replication differ in amino acid sequence and their three-dimensional structures. Moreover, the respective proteins generally utilize different enzymatic mechanisms. Hence, the replication proteins that are highly conserved among bacterial species are attractive targets to develop novel antibiotics as the compounds are unlikely to demonstrate off-target effects. For those proteins that differ among bacteria, compounds that are species-specific may be found. Escherichia coli has been developed as a model system to study DNA replication, serving as a benchmark for comparison. This review summarizes the functions of individual E. coli proteins, and the compounds that inhibit them.

[Antibiotic transport and membrane permeability: new insights to fight bacterial resistance]

A main challenge in medicinal chemistry is to determine the parameters modulating the in cellulo drug concentration needed for a therapeutic action. In Gram-negative antibacterial research, the concern is to evaluate the antibiotic permeation across the outer and inner membranes, that delineate the periplasm surrounding the bacterial cytoplasm. Passing through the membrane barrier to reach the inhibitory concentration inside the bacterium is the first pivotal step for antibiotics. The research and the development of new antimicrobials mostly rely on their capacity to reach critical concentrations in the vicinity of their intracellular target. Despite several decades of studies focused on antibiotic/drug activity against bacterial cells using different approaches, no consensus regarding the analysis of the kinetics and accumulation in individual bacterium and in bacterial populations is available to understand the drug translocation into living bacteria as a first step of drug action. Our TRANSLOCATION consortium supports the development of reliable and robust methods to quantify penetration and efflux processes in Gram-negative bacteria and recently we have developed a reliable and efficient method to determine the intra-bacterial concentration of fluorescent antibiotics. By using these powerful approaches, new concepts, "Resident Time Concentration Close to Target" (RTC2T) and "Structure Intracellular Concentration Activity Relationship" (SICAR), have been proposed in order to link chemical and structural aspects with the bacterial membrane and transport aspects. Using RTC2T and SICAR indexes, a new dissection of antibiotic translocation-transport can be obtained to better understand and improve the antibiotic pharmacophoric groups that are related to permeation and efflux.

Investigational drugs in phase I and phase II clinical trials for the treatment of community-acquired pneumonia

INTRODUCTION

Community acquired pneumonia is one of the main infections, remaining as a global cause of considerable morbidity and mortality. Successful treatment hinges on expedient delivery of appropriate antibiotic therapy tailored to both the likely pathogens and the severity of disease. Although antibiotic resistance is increasing and pharmaceutical companies continue to debate the profitability of introducing new antibacterial agents, an encouraging number of new molecules have recently been unveiled which target multidrug-resistant bacteria. Areas covered: Herein, the authors summarize the actual situation of novel antibiotics for CAP in phase I & II of development. For each set of compounds, the medical significance and possible clinical placement are discussed. Current treatment options from the most important international guidelines are also reviewed. Expert opinion: Our review shows that the new antibiotics in the pipeline belong to existing antibiotic classes as β-lactams, macrolides, quinolones, oxazolidinones, tetracyclines, lipoglycopeptides, and cyclic lipopeptides and a few with a narrow spectrum of activity are novel compounds directed against novel targets. With rising outpatient antibiotic resistance in pneumonia, some of the compounds discussed are being considered for more rapid advancement in the pipeline, helping to increase the number of agents in later stages of development.

Design, Synthesis, and Antibacterial Evaluation of Oxazolidinones with Fused Heterocyclic C-Ring Substructure

A series of novel oxazolidinone antibacterials with diverse fused heteroaryl C-rings bearing hydrogen bond donor and hydrogen bond acceptor functionalities were designed and synthesized. The compound with benzoxazinone C-ring substructure (8c) exhibited superior activity compared to linezolid against a panel of Gram-positive and Gram-negative bacteria. Structural modifications at C5-side chain of 8c resulted in identification of several potent compounds (12a, 12b, 12g, and 12h). Selected compounds 8c and 12a showed very good microsomal stability and no CYP₄₅₀ liability, thus clearing preliminary safety hurdles. A docking model of 12a binding to 23S rRNA suggested that the increased potency of 12a is due to additional ligand-receptor interaction.

Activity of LCB01-0371, a Novel Oxazolidinone, against Mycobacterium abscessus

Mycobacterium abscessus is a highly pathogenic drug-resistant rapidly growing mycobacterium. In this study, we evaluated the in vitro, intracellular, and in vivo activities of LCB01-0371, a novel and safe oxazolidinone derivative, for the treatment of M. abscessus infection and compared its resistance to that of other oxazolidinone drugs. LCB01-0371 was effective against several M. abscessus strains in vitro and in a macrophage model of infection. In the murine model, a similar efficacy to linezolid was achieved, especially in the lungs. We induced laboratory-generated resistance to LCB01-0371; sequencing analysis revealed mutations in rplC of T424C and G419A and a nucleotide insertion at the 503 position. Furthermore, LCB01-0371 inhibited the growth of amikacin-, cefoxitin-, and clarithromycin-resistant strains. Collectively, our data indicate that LCB01-0371 might represent a promising new class of oxazolidinones with improved safety, which may replace linezolid for the treatment of M. abscessus.

New Modalities for Challenging Targets in Drug Discovery

Our ever-increasing understanding of biological systems is providing a range of exciting novel biological targets, whose modulation may enable novel therapeutic options for many diseases. These targets include protein-protein and protein-nucleic acid interactions, which are, however, often refractory to classical small-molecule approaches. Other types of molecules, or modalities, are therefore required to address these targets, which has led several academic research groups and pharmaceutical companies to increasingly use the concept of so-called "new modalities". This Review defines for the first time the scope of this term, which includes novel peptidic scaffolds, oligonucleotides, hybrids, molecular conjugates, as well as new uses of classical small molecules. We provide the most representative examples of these modalities to target large binding surface areas such as those found in protein-protein interactions and for biological processes at the center of cell regulation.

Microspectrofluorimetry to dissect the permeation of ceftazidime in Gram-negative bacteria

A main challenge in chemotherapy is to determine the in cellulo parameters modulating the drug concentration required for therapeutic action. It is absolutely urgent to understand membrane permeation and intracellular concentration of antibiotics in clinical isolates: passing the membrane barrier to reach the threshold concentration inside the bacterial periplasm or cytoplasm is the pivotal step of antibacterial activity. Ceftazidime (CAZ) is a key molecule of the combination therapy for treating resistant bacteria. We designed and synthesized different fluorescent CAZ derivatives (CAZ*, CAZ**) to dissect the early step of translocation-accumulation across bacterial membrane. Their activities were determined on E. coli strains and on selected clinical isolates overexpressing ß-lactamases. The accumulation of CAZ* and CAZ** were determined by microspectrofluorimetry and epifluorimetry. The derivatives were properly translocated to the periplasmic space when we permeabilize the outer membrane barrier. The periplasmic location of CAZ** was related to a significant antibacterial activity and with the outer membrane permeability. This study demonstrated the correlation between periplasmic accumulation and antibiotic activity. We also validated the method for approaching ß-lactam permeation relative to membrane permeability and paved the way for an original matrix for determining "Structure Intracellular Accumulation Activity Relationship" for the development of new therapeutic candidates.

Reducing the Bottleneck in Discovery of Novel Antibiotics

Most antibiotics were discovered by screening soil actinomycetes, but the efficiency of the discovery platform collapsed in the 1960s. By now, more than 3000 antibiotics have been described and most of the current discovery effort is focused on the rediscovery of known compounds, making the approach impractical. The last marketed broad-spectrum antibiotics discovered were daptomycin, linezolid, and fidaxomicin. The current state of the art in the development of new anti-infectives is a non-existent pipeline in the absence of a discovery platform. This is particularly troubling given the emergence of pan-resistant pathogens. The current practice in dealing with the problem of the background of known compounds is to use chemical dereplication of extracts to assess the relative novelty of a compound it contains. Dereplication typically requires scale-up, extraction, and often fractionation before an accurate mass and structure can be produced by MS analysis in combination with 2D NMR. Here, we describe a transcriptome analysis approach using RNA sequencing (RNASeq) to identify promising novel antimicrobial compounds from microbial extracts. Our pipeline permits identification of antimicrobial compounds that produce distinct transcription profiles using unfractionated cell extracts. This efficient pipeline will eliminate the requirement for purification and structure determination of compounds from extracts and will facilitate high-throughput screen of cell extracts for identification of novel compounds.

TarO-specific inhibitors of wall teichoic acid biosynthesis restore β-lactam efficacy against methicillin-resistant staphylococci

The widespread emergence of methicillin-resistant Staphylococcus aureus (MRSA) has dramatically eroded the efficacy of current β-lactam antibiotics and created an urgent need for new treatment options. We report an S. aureus phenotypic screening strategy involving chemical suppression of the growth inhibitory consequences of depleting late-stage wall teichoic acid biosynthesis. This enabled us to identify early-stage pathway-specific inhibitors of wall teichoic acid biosynthesis predicted to be chemically synergistic with β-lactams. We demonstrated by genetic and biochemical means that each of the new chemical series discovered, herein named tarocin A and tarocin B, inhibited the first step in wall teichoic acid biosynthesis (TarO). Tarocins do not have intrinsic bioactivity but rather demonstrated potent bactericidal synergy in combination with broad-spectrum β-lactam antibiotics against diverse clinical isolates of methicillin-resistant staphylococci as well as robust efficacy in a murine infection model of MRSA. Tarocins and other inhibitors of wall teichoic acid biosynthesis may provide a rational strategy to develop Gram-positive bactericidal β-lactam combination agents active against methicillin-resistant staphylococci.

The rapid spread of carbapenem-resistant Enterobacteriaceae

Carbapenems, our one-time silver bullet for multidrug resistant bacterial infections, are now threatened by widespread dissemination of carbapenem-resistant Enterobacteriaceae (CRE). Successful expansion of Enterobacteriaceae clonal groups and frequent horizontal gene transfer of carbapenemase expressing plasmids are causing increasing carbapenem resistance. Recent advances in genetic and phenotypic detection facilitate global surveillance of CRE diversity and prevalence. In particular, whole genome sequencing enabled efficient tracking, annotation, and study of genetic elements colocalized with carbapenemase genes on chromosomes and on plasmids. Improved characterization helps detail the co-occurrence of other antibiotic resistance genes in CRE isolates and helps identify pan-drug resistance mechanisms. The novel β-lactamase inhibitor, avibactam, combined with ceftazidime or aztreonam, is a promising CRE treatment compared to current colistin or tigecycline regimens. To halt increasing CRE-associated morbidity and mortality, we must continue quality, cooperative monitoring and urgently investigate novel treatments.

Macrolones Are a Novel Class of Macrolide Antibiotics Active against Key Resistant Respiratory Pathogens In Vitro and In Vivo

As we face an alarming increase in bacterial resistance to current antibacterial chemotherapeutics, expanding the available therapeutic arsenal in the fight against resistant bacterial pathogens causing respiratory tract infections is of high importance. The antibacterial potency of macrolones, a novel class of macrolide antibiotics, against key respiratory pathogens was evaluated in vitro and in vivo MIC values against Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, and Haemophilus influenzae strains sensitive to macrolide antibiotics and with defined macrolide resistance mechanisms were determined. The propensity of macrolones to induce the expression of inducible erm genes was tested by the triple-disk method and incubation in the presence of subinhibitory concentrations of compounds. In vivo efficacy was assessed in a murine model of S. pneumoniae-induced pneumonia, and pharmacokinetic (PK) profiles in mice were determined. The in vitro antibacterial profiles of macrolones were superior to those of marketed macrolide antibiotics, including the ketolide telithromycin, and the compounds did not induce the expression of inducible erm genes. They acted as typical protein synthesis inhibitors in an Escherichia coli transcription/translation assay. Macrolones were characterized by low to moderate systemic clearance, a large volume of distribution, a long half-life, and low oral bioavailability. They were highly efficacious in a murine model of pneumonia after intraperitoneal application even against an S. pneumoniae strain with constitutive resistance to macrolide-lincosamide-streptogramin B antibiotics. Macrolones are the class of macrolide antibiotics with an outstanding antibacterial profile and reasonable PK parameters resulting in good in vivo efficacy.

Therapeutic Potential of Foldamers: From Chemical Biology Tools To Drug Candidates?

Over the past decade, foldamers have progressively emerged as useful architectures to mimic secondary structures of proteins. Peptidic foldamers, consisting of various amino acid based backbones, have been the most studied from a therapeutic perspective, while polyaromatic foldamers have barely evolved from their nascency and remain perplexing for medicinal chemists due to their poor drug-like nature. Despite these limitations, this compound class may still offer opportunities to study challenging targets or provide chemical biology tools. The potential of foldamer drug candidates reaching the clinic is still a stretch. Nevertheless, advances in the field have demonstrated their potential for the discovery of next generation therapeutics. In this perspective, the current knowledge of foldamers is reviewed in a drug discovery context. Recent advances in the early phases of drug discovery including hit finding, target validation, and optimization and molecular modeling are discussed. In addition, challenges and focus areas are debated and gaps highlighted.

Harmonisation in study design and outcomes in paediatric antibiotic clinical trials: a systematic review

There is no global consensus on the conduct of clinical trials in children and neonates with complicated clinical infection syndromes. No comprehensive regulatory guidance exists for the design of antibiotic clinical trials in neonates and children. We did a systematic review of antibiotic clinical trials in complicated clinical infection syndromes (including bloodstream infections and community-acquired pneumonia) in children and neonates (0-18 years) to assess whether standardised European Medicines Agency (EMA) and US Food and Drug Administration (FDA) guidance for adults was used in paediatrics, and whether paediatric clinical trials applied consistent definitions for eligibility and outcomes. We searched MEDLINE, Cochrane CENTRAL databases, and ClinicalTrials.gov between Jan 1, 2000, and Nov 18, 2015. 82 individual studies met our inclusion criteria. The published studies reported on an average of 66% of CONSORT items. Study design, inclusion and exclusion criteria, and endpoints varied substantially across included studies. The comparison between paediatric clinical trials and adult EMA and FDA guidance highlighted that regulatory definitions are only variably applicable and used at present. Absence of consensus for paediatric antibiotic clinical trials is a major barrier to harmonisation in research and translation into clinical practice. To improve comparison of therapies and strategies, international collaboration among all relevant stakeholders leading to harmonised case definitions and outcome measures is needed.

Efflux Pump Blockers in Gram-Negative Bacteria: The New Generation of Hydantoin Based-Modulators to Improve Antibiotic Activity

Multidrug resistant (MDR) bacteria are an increasing health problem with the shortage of new active antibiotic agents. Among effective mechanisms that contribute to the spread of MDR Gram-negative bacteria are drug efflux pumps that expel clinically important antibiotic classes out of the cell. Drug pumps are attractive targets to restore the susceptibility toward the expelled antibiotics by impairing their efflux activity. Arylhydantoin derivatives were investigated for their potentiation of activities of selected antibiotics described as efflux substrates in Enterobacter aerogenes expressing or not AcrAB pump. Several compounds increased the bacterial susceptibility toward nalidixic acid, chloramphenicol and sparfloxacin and were further pharmacomodulated to obtain a better activity against the AcrAB producing bacteria.

Research and development of antibiotics: insights from patents and citation network

INTRODUCTION

Bacterial resistance to antibiotics develops at an alarming rate and leads to the increasing morbidity and health-care costs in recent years. However, the global research and development (R&D) of antibiotics has fallen behind the emergence and spread of bacterial resistance and the world is heading towards a 'post-antibiotic era'. In this context, systematic understanding of the technology landscape and evolving process of antibiotic R&D may help to provide insights for discovering future antibiotics more rationally.

AREAS COVERED

Patents and patent citations are broadly believed to be powerful tools in representing the technology advances and capturing technology flows. In all, 707 U.S. patents related to antibiotic R&D are collected and analyzed. Furthermore, patent citations are visualized by a network-based approach, while the inter-relationship between patented technologies on antibiotics is further revealed.

EXPERT OPINION

The current dry pipeline of antibiotic development requires substantial awareness and political support. It is essential to build an attractive and supportive environment for investment. Thus, a new antibiotic business model is needed to chase the balance between the market-oriented investment and public health goals. Additionally, drug development targeting Gram-negative bacteria, especially resistant Gram-negative bacteria, demands attentions from stakeholders because of their unmet medical needs.

Role of the Gram-Negative Envelope Stress Response in the Presence of Antimicrobial Agents

Bacterial survival necessitates endurance of many types of antimicrobial compound. Many Gram-negative envelope stress responses, which must contend with an outer membrane and a dense periplasm containing the cell wall, have been associated with the status of protein folding, membrane homeostasis, and physiological functions such as efflux and the proton motive force (PMF). In this review, we discuss evidence that indicates an emerging role for Gram-negative envelope stress responses in enduring exposure to diverse antimicrobial substances, focusing on recent studies of the γ-proteobacterial Cpx envelope stress response.

New Structural Templates for Clinically Validated and Novel Targets in Antimicrobial Drug Research and Development

The development of bacterial resistance against current antibiotic drugs necessitates a continuous renewal of the arsenal of efficacious drugs. This imperative has not been met by the output of antibiotic research and development of the past decades for various reasons, including the declining efforts of large pharma companies in this area. Moreover, the majority of novel antibiotics are chemical derivatives of existing structures that represent mostly step innovations, implying that the available chemical space may be exhausted. This review negates this impression by showcasing recent achievements in lead finding and optimization of antibiotics that have novel or unexplored chemical structures. Not surprisingly, many of the novel structural templates like teixobactins, lysocin, griselimycin, or the albicidin/cystobactamid pair were discovered from natural sources. Additional compounds were obtained from the screening of synthetic libraries and chemical synthesis, including the gyrase-inhibiting NTBI's and spiropyrimidinetrione, the tarocin and targocil inhibitors of wall teichoic acid synthesis, or the boronates and diazabicyclo[3.2.1]octane as novel β-lactamase inhibitors. A motif that is common to most clinically validated antibiotics is that they address hotspots in complex biosynthetic machineries, whose functioning is essential for the bacterial cell. Therefore, an introduction to the biological targets-cell wall synthesis, topoisomerases, the DNA sliding clamp, and membrane-bound electron transport-is given for each of the leads presented here.

Investigational Agents for the Treatment of Gram-Negative Bacterial Infections: A Reality Check

Antibiotic-resistant Gram-negative bacteria are, arguably, the most difficult organisms to treat, with a limited number of new antibiotics in the development pipeline. Currently 24 new agents in phase 1, phase 2, or phase 3 clinical development were identified for the potential treatment of infections caused by Gram-negative bacteria. Of these agents, most are improved iterations of known antibiotic classes, including new aminoglycosides, β-lactams, β-lactamase inhibitors, quinolones, and tetracyclines with greater potency or a broader spectrum of activity. However, novel structures also appear, with host defense peptide mimetics, boronic acid, and bridged diazabicyclooctane β-lactamase inhibitors and unique bacterial topoisomerase inhibitors. Most of the new agents have received a Qualified Infectious Disease Product (QIDP) designation that may help to accelerate FDA drug approvals. Because resistance will inevitably arise to any antibacterial agent, it will be necessary to continue to identify additional new agents in the future.

Investigational drugs to treat methicillin-resistant Staphylococcus aureus

INTRODUCTION

Staphylococcus aureus remains one of the leading causes of morbidity and mortality worldwide. This is to a large extent due to antibiotic-resistant strains, in particular methicillin-resistant S. aureus (MRSA). While the toll of invasive MRSA infections appears to decrease in U.S. hospitals, the rate of community-associated MRSA infections remains constant and there is a surge of MRSA in many other countries, a situation that calls for continuing if not increased efforts to find novel strategies to combat MRSA infections.

AREAS COVERED

This review provides an overview of current investigational drugs and therapeutic antibodies against S. aureus in early clinical development (up to phase II clinical development). It includes a short description of the mechanism of action and a presentation of microbiological and clinical data.

EXPERT OPINION

Increased recent antibiotic development efforts and results from pathogenesis research have led to several new antibiotics and therapies, such as anti-virulence drugs, as well as a more informed selection of targets for vaccination efforts against MRSA. This developing portfolio of novel anti-staphylococcal drugs will hopefully provide us with additional and more efficient ways to combat MRSA infections in the near future and prevent us from running out of treatment options, even if new resistances arise.

Pharmacokinetics/Pharmacodynamics of Peptide Deformylase Inhibitor GSK1322322 against Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus in Rodent Models of Infection

GSK1322322 is a novel inhibitor of peptide deformylase (PDF) with good in vitro activity against bacteria associated with community-acquired pneumonia and skin infections. We have characterized the in vivo pharmacodynamics (PD) of GSK1322322 in immunocompetent animal models of infection with Streptococcus pneumoniae and Haemophilus influenzae (mouse lung model) and with Staphylococcus aureus (rat abscess model) and determined the pharmacokinetic (PK)/PD index that best correlates with efficacy and its magnitude. Oral PK studies with both models showed slightly higher-than-dose-proportional exposure, with 3-fold increases in area under the concentration-time curve (AUC) with doubling doses. GSK1322322 exhibited dose-dependent in vivo efficacy against multiple isolates of S. pneumoniae, H. influenzae, and S. aureus. Dose fractionation studies with two S. pneumoniae and S. aureus isolates showed that therapeutic outcome correlated best with the free AUC/MIC (fAUC/MIC) index in S. pneumoniae (R(2), 0.83), whereas fAUC/MIC and free maximum drug concentration (fCmax)/MIC were the best efficacy predictors for S. aureus (R(2), 0.9 and 0.91, respectively). Median daily fAUC/MIC values required for stasis and for a 1-log10 reduction in bacterial burden were 8.1 and 14.4 for 11 S. pneumoniae isolates (R(2), 0.62) and 7.2 and 13.0 for five H. influenzae isolates (R(2), 0.93). The data showed that for eight S. aureus isolates, fAUC correlated better with efficacy than fAUC/MIC (R(2), 0.91 and 0.76, respectively), as efficacious AUCs were similar for all isolates, independent of their GSK1322322 MIC (range, 0.5 to 4 μg/ml). Median fAUCs of 2.1 and 6.3 μg · h/ml were associated with stasis and 1-log10 reductions, respectively, for S. aureus.

A systematic review and critical assessment of incentive strategies for discovery and development of novel antibiotics

Despite the growing threat of antimicrobial resistance, pharmaceutical and biotechnology firms are reluctant to develop novel antibiotics because of a host of market failures. This problem is complicated by public health goals that demand antibiotic conservation and equitable patient access. Thus, an innovative incentive strategy is needed to encourage sustainable investment in antibiotics. This systematic review consolidates, classifies and critically assesses a total of 47 proposed incentives. Given the large number of possible strategies, a decision framework is presented to assist with the selection of incentives. This framework focuses on addressing market failures that result in limited investment, public health priorities regarding antibiotic stewardship and patient access, and implementation constraints and operational realities. The flexible nature of this framework allows policy makers to tailor an antibiotic incentive package that suits a country's health system structure and needs.

Elucidation of DnaE as the Antibacterial Target of the Natural Product, Nargenicin

Resistance to existing classes of antibiotics drives the need for discovery of novel compounds with unique mechanisms of action. Nargenicin A1, a natural product with limited antibacterial spectrum, was rediscovered in a whole-cell antisense assay. Macromolecular labeling in both Staphylococcus aureus and an Escherichia coli tolC efflux mutant revealed selective inhibition of DNA replication not due to gyrase or topoisomerase IV inhibition. S. aureus nargenicin-resistant mutants were selected at a frequency of ∼1 × 10(-9), and whole-genome resequencing found a single base-pair change in the dnaE gene, a homolog of the E. coli holoenzyme α subunit. A DnaE single-enzyme assay was exquisitely sensitive to inhibition by nargenicin, and other in vitro characterization studies corroborated DnaE as the target. Medicinal chemistry efforts may expand the spectrum of this novel mechanism antibiotic.

Permeability Barrier of Gram-Negative Cell Envelopes and Approaches To Bypass It

Gram-negative bacteria are intrinsically resistant to many antibiotics. Species that have acquired multidrug resistance and cause infections that are effectively untreatable present a serious threat to public health. The problem is broadly recognized and tackled at both the fundamental and applied levels. This paper summarizes current advances in understanding the molecular bases of the low permeability barrier of Gram-negative pathogens, which is the major obstacle in discovery and development of antibiotics effective against such pathogens. Gaps in knowledge and specific strategies to break this barrier and to achieve potent activities against difficult Gram-negative bacteria are also discussed.

Antibiotic adjuvants: diverse strategies for controlling drug-resistant pathogens

The growing number of bacterial pathogens that are resistant to numerous antibiotics is a cause for concern around the globe. There have been no new broad-spectrum antibiotics developed in the last 40 years, and the drugs we have currently are quickly becoming ineffective. In this article, we explore a range of therapeutic strategies that could be employed in conjunction with antibiotics and may help to prolong the life span of these life-saving drugs. Discussed topics include antiresistance drugs, which are administered to potentiate the effects of current antimicrobials in bacteria where they are no longer (or never were) effective; antivirulence drugs, which are directed against bacterial virulence factors; host-directed therapies, which modulate the host's immune system to facilitate infection clearance; and alternative treatments, which include such therapies as oral rehydration for diarrhea, phage therapy, and probiotics. All of these avenues show promise for the treatment of bacterial infections and should be further investigated to explore their full potential in the face of a postantibiotic era.

Discovery of a Teraryl Oxazolidinone Compound (S)-N-((3-(3-Fluoro-4-(4-(pyridin-2-yl)-1H-pyrazol-1-yl)phenyl)-2-oxooxazolidin-5-yl)methyl)acetamide Phosphate as a Novel Antimicrobial Agent with Enhanced Safety Profile and Efficacies

A series of novel teraryl oxazolidinone compounds was designed, synthesized, and evaluated for their antimicrobial activity and toxicities. The compounds with aromatic N-heterocyclic substituents at the 4-position of pyrazolyl ring showed better antibacterial activity against the tested bacteria than other compounds with different patterns of substitution. Among all potent compounds, 10f exhibited promising safety profile in MTT assays and in hERG K(+) channel inhibition test. Furthermore, its phosphate was found to be highly soluble in water (47.1 mg/mL), which is beneficial for the subsequent in vivo test. In MRSA systemic infection mice models, 10f phosphate exerted significantly improved survival protection compared with linezolid. The compound also demonstrated high oral bioavailability (F = 99.1%). Moreover, from the results of in vivo toxicology experiments, 10f phosphate would be predicted to have less bone marrow suppression.

Retargeting pre-existing human antibodies to a bacterial pathogen with an alpha-Gal conjugated aptamer

The ever-increasing threat of multi-drug resistant bacterial infections has spurred renewed interest in alternative approaches to classical antibiotic therapy. In contrast to other mammals, humans do not express the galactose-α-1,3-galactosyl-β-1,4-N-acetyl-glucosamine (α-Gal) epitope. As a result of exposure of humans to α-Gal in the environment, a large proportion of circulating antibodies are specific for the trisaccharide. In this study, we examine whether these anti-Gal antibodies can be recruited and redirected to exert anti-bacterial activity. We show that a specific DNA aptamer conjugated to an α-Gal epitope at its 5' end, herein termed an alphamer, can bind to group A Streptococcus (GAS) bacteria by recognition of a conserved region of the surface-anchored M protein. The anti-GAS alphamer was shown to recruit anti-Gal antibodies to the streptococcal surface in an α-Gal-specific manner, elicit uptake and killing of the bacteria by human phagocytes, and slow growth of invasive GAS in human whole blood. These studies provide a first in vitro proof of concept that alphamers have the potential to redirect pre-existing antibodies to bacteria in a specific manner and trigger an immediate antibacterial immune response. Further validation of this novel therapeutic approach of applying α-Gal technology in in vivo models of bacterial infection is warranted.

KEY MESSAGES

. α-Gal-tagged aptamers lead to GAS opsonization with anti-Gal antibodies. . α-Gal-tagged aptamers confer phagocytosis and killing of GAS cells by human phagocytes. . α-Gal-tagged aptamers reduces replication of GAS in human blood. . α-Gal-tagged aptamers may have the potential to be used as novel passive immunization drugs.