Efficacy of LFF571 in a hamster model of Clostridium difficile infection

In vivo evaluation of Clostridioides difficile enoyl-ACP reductase II (FabK) inhibition by phenylimidazole unveils a promising narrow-spectrum antimicrobial strategy

Clostridioides difficile infection (CDI) is a leading cause of hospital-acquired diarrhea, which often stems from disruption of the gut microbiota by broad-spectrum antibiotics. The increasing prevalence of antibiotic-resistant C. difficile strains, combined with disappointing clinical trial results for recent antibiotic candidates, underscores the urgent need for novel CDI antibiotics. To this end, we investigated C. difficile enoyl ACP reductase (CdFabK), a crucial enzyme in de novo fatty acid synthesis, as a drug target for microbiome-sparing antibiotics. To test this concept, we evaluated the efficacy and in vivo spectrum of activity of the phenylimidazole analog 296, which is validated to inhibit intracellular CdFabK. Against major CDI-associated ribotypes 296 had an Minimum inhibitory concentration (MIC90) of 2 µg/mL, which was comparable to vancomycin (1 µg/mL), a standard of care antibiotic. In addition, 296 achieved high colonic concentrations and displayed dosed-dependent efficacy in mice with colitis CDI. Mice that were given 296 retained colonization resistance to C. difficile and had microbiomes that resembled the untreated mice. Conversely, both vancomycin and fidaxomicin induced significant changes to mice microbiomes, in a manner consistent with prior reports. CdFabK, therefore, represents a potential target for microbiome-sparing CDI antibiotics, with phenylimidazoles providing a good chemical starting point for designing such agents.

In vivo evaluation of Clostridioides difficile enoyl-ACP reductase II (FabK) Inhibition by phenylimidazole unveils a promising narrow-spectrum antimicrobial strategy

Clostridioides difficile infection (CDI) is a leading cause of hospital-acquired diarrhea, which often stem from disruption of the gut microbiota by broad-spectrum antibiotics. The increasing prevalence of antibiotic-resistant C. difficile strains, combined with disappointing clinical trials results for recent antibiotic candidates, underscore the urgent need for novel CDI antibiotics. To this end, we investigated C. difficile enoyl ACP reductase (CdFabK), a crucial enzyme in de novo fatty acid synthesis, as a drug target for microbiome-sparing antibiotics. To test this concept, we evaluated the efficacy and in vivo spectrum of activity of the phenylimidazole analog 296, which is validated to inhibit intracellular CdFabK. Against major CDI-associated ribotypes 296 had an MIC90 of 2 μg/ml, which was comparable to vancomycin (1 μg/ml), a standard of care antibiotic. In addition, 296 achieved high colonic concentrations and displayed dosed-dependent efficacy in mice with colitis CDI. Mice that were given 296 retained colonization resistance to C. difficile and had microbiomes that resembled the untreated mice. Conversely, both vancomycin and fidaxomicin induced significant changes to mice microbiomes, in a manner consistent with prior reports. CdFabK therefore represents a potential target for microbiome-sparing CDI antibiotics, with phenylimidazoles providing a good chemical starting point for designing such agents.

Towards Development of a Non-Toxigenic Clostridioides difficile Oral Spore Vaccine against Toxigenic C. difficile

Clostridioides difficile is an opportunistic gut pathogen which causes severe colitis, leading to significant morbidity and mortality due to its toxins, TcdA and TcdB. Two intra-muscular toxoid vaccines entered Phase III trials and strongly induced toxin-neutralising antibodies systemically but failed to provide local protection in the colon from primary C. difficile infection (CDI). Alternatively, by immunising orally, the ileum (main immune inductive site) can be directly targeted to confer protection in the large intestine. The gut commensal, non-toxigenic C. difficile (NTCD) was previously tested in animal models as an oral vaccine for natural delivery of an engineered toxin chimera to the small intestine and successfully induced toxin-neutralising antibodies. We investigated whether NTCD could be further exploited to induce antibodies that block the adherence of C. difficile to epithelial cells to target the first stage of pathogenesis. In NTCD strain T7, the colonisation factor, CD0873, and a domain of TcdB were overexpressed. Following oral immunisation of hamsters with spores of recombinant strain, T7-0873 or T7-TcdB, intestinal and systemic responses were investigated. Vaccination with T7-0873 successfully induced intestinal antibodies that significantly reduced adhesion of toxigenic C. difficile to Caco-2 cells, and these responses were mirrored in sera. Additional engineering of NTCD is now warranted to further develop this vaccine.

Nocathiacin, Thiazomycin, and Polar Analogs Are Highly Effective Agents against Toxigenic Clostridioides difficile

Clostridioides difficile is a commensal Gram-positive gut bacterium that causes C. difficile-associated diarrhea. Currently available antibacterial therapeutic treatment options are effective except for the repeated recurrences significantly burdening the health care system and causing mortality. The development of new therapeutic modalities including new effective antibiotics with a low rate of recurrence has been unpredictive and exceedingly challenging, requiring continued profiling of many new classes of antibiotics. Nocathiacins and thiazomycins are a class of thiazolyl peptides exhibiting potent and selective broad-spectrum Gram-positive activity including activity against the anaerobe C. difficile. These compounds showed MIC values of 0.015-0.06 μg/mL against C. difficile with more than 100-200-fold selectivity versus commensurate Gram-negative Bacteroides fragilis. Nocathiacin I and one of its analogs exhibited potent in vivo efficacy in the gold-standard hamster model of C. difficile infection, providing 100% protection in this lethal model at 6.25 mg/kg orally twice daily. The efficacy was corroborated by robust reduction of cecum C. difficile burden and proportionate exposure of the compounds in the cecum contents without any systemic absorption. In this paper, details of the results of in vitro, in vivo, pharmacodynamics, and pharmacokinetic studies have been described.

In Vitro and In Vivo Anti-Clostridioides difficile Effect of a Probiotic Bacillus amyloliquefaciens Strain

Clostridioides difficile infection (CDI) is a significant cause of hospital-acquired and antibiotic-mediated intestinal diseases and is a growing global public health concern. Overuse of antibiotics and their effect on normal intestinal flora has increased the incidence and severity of infections. Thus, the development of new, effective, and safe treatment options is a high priority. Here, we report a new probiotic strain, Bacillus amyloliquefaciens (BA PMC-80), and its in vitro/in vivo anti-C. difficile effect as a prospective novel candidate for replacing conventional antibiotics. BA PMC-80 showed a significant anti-C. difficile effect in coculture assay, and its cell-free supernatant (CFS) also exhibited a considerable anti-C. difficile effect with an 89.06 μg/ml 50% minimal inhibitory concentration (MIC) in broth microdilution assay. The CFS was stable and equally functional under different pHs, heat, and proteinase treatments. It also exhibited a high sensitivity against current antibiotics and no toxicity in subchronic toxicity testing in hamsters. Finally, BA PMC-80 showed a moderate effect in a hamster CDI model with reduced infection severity and delayed death. However, further studies are required to optimize the treatment condition of the hamster CDI model for better efficacy and identify the antimicrobial compound produced by BA PMC-80.

Current Molecular Therapeutic Agents and Drug Candidates for Mycobacterium abscessus

Mycobacterium abscessus has been recognised as a dreadful respiratory pathogen among the non-tuberculous mycobacteria (NTM) because of misdiagnosis, prolonged therapy with poor treatment outcomes and a high cost. This pathogen also shows extremely high antimicrobial resistance against current antibiotics, including the anti-tuberculosis agents. Therefore, current chemotherapies require a long curative period and the clinical outcomes are not satisfactory. Thus, there is an urgent need for discovering and developing novel, more effective anti-M. abscessus drugs. In this review, we sum the effectiveness of the current anti-M. abscessus drugs and drug candidates. Furthermore, we describe the shortcomings and difficulties associated with M. abscessus drug discovery and development.

Alteration of Intestinal Microbiome of Clostridioides difficile-Infected Hamsters during the Treatment with Specific Cow Antibodies

Clostridioides difficile infection (CDI) often develops after pretreatment with antibiotics, which can lead to damage of the intestinal microbiome. The approach of this study was to use specific polyclonal antibodies isolated from the milk of immunized cows to treat CDI, in contrast to the standard application of nonspecific antibiotics. To gain a deeper understanding of the role of the microbiome in the treatment of CDI with bovine antibodies, stool and intestinal fluid samples of hamsters were collected in large quantities from various treatments (>400 samples). The results show that the regeneration of the microbiome instantly begins with the start of the antibody treatment, in contrast to the Vancomycin-treated group where the diversity decreased significantly during the treatment duration. All antibody-treated hamsters that survived the initial phase also survived the entire study period. The results also show that the regeneration of the microbiome was not an antibody-induced regeneration, but a natural regeneration that occurred because no microbiota-inactivating substances were administered. In conclusion, the treatment with bovine antibodies is a functional therapy for both the acute treatment and the prevention of recurrence in hamsters and could meet the urgent need for CDI treatment alternatives in humans.

In vivo efficacy of auranofin in a hamster model of Clostridioides difficile infection

Clostridioides difficile infections (CDIs) are an urgent public health threat worldwide and are a leading cause of morbidity and mortality in healthcare settings. The increasing incidence and severity of infections combined with the scarcity of effective anti-CDI agents has made treatment of CDI very challenging. Therefore, development of new, effective anticlostridial agents remains a high priority. The current study investigated the in vivo efficacy of auranofin in a CDI hamster model. All hamsters treated with auranofin (5 mg/kg) survived a lethal challenge with C. difficile. Furthermore, auranofin (5 mg/kg) was as effective as vancomycin, the drug of choice for treatment of CDIs, against relapsing CDI. Furthermore, auranofin (5 mg/kg) generated a 3.15-log₁₀ reduction (99.97%) in C. difficile count in the cecal contents of hamsters. These results indicate that auranofin warrants further investigation as a new agent to replenish the pipeline of anti-CDI therapeutics.

The adaptive immune response to Clostridioides difficile: A tricky balance between immunoprotection and immunopathogenesis

Clostridioides difficile (C. difficile) is the major cause of hospital-acquired gastrointestinal infections in individuals following antibiotics treatment. The pathogenesis of C. difficile infection (CDI) is mediated mainly by the production of toxins that induce tissue damage and host inflammatory responses. While innate immunity is well characterized in human and animal models of CDI, adaptive immune responses remain poorly understood. In this review, the current understanding of adaptive immunity is summarized and its influence on pathogenesis and disease outcome is discussed. The perspectives on what we believe to be the main pending questions and the focus of future research are also provided. There is no doubt that the innate immune response provides a first line of defense to CDI. But, is the adaptive immune response a friend or a foe? Probably it depends on the course of the disease. Adaptive immunity is essential for pathogen eradication, but may also trigger uncontrolled or pathological inflammation. Most of the understanding of the role of T cells is based on findings from experimental models. While they are a very valuable tool for research studies, more studies in human are needed to translate these findings into human disease. Another main challenge is to unravel the role of the different T cell populations on protection or induction of immunopathogenesis.

Phenomic and genomic approaches to studying the inhibition of multiresistant Salmonella enterica by microcin J25

In livestock production, antibiotics are used to promote animal growth, control infections and thereby increase profitability. This practice has led to the emergence of multiresistant bacteria such as Salmonella, of which some serovars are disseminated in the environment. The objective of this study is to evaluate microcin J25 as an inhibitor of Salmonella enterica serovars of various origins including human, livestock and food. Among the 116 isolates tested, 37 (31.8%) were found resistant to at least one antibiotic, and 28 were multiresistant with 19 expressing the penta-resistant phenotype ACSSuT. Microcin J25 inhibited all isolates, with minimal inhibitory concentration values ranging from 0.06 μg/ml (28.4 nM) to 400 μg/ml (189 μM). Interestingly, no cross-resistance was found between microcin J25 and antibiotics. Multiple sequence alignments of genes encoding for the different proteins involved in the recognition and transport of microcin J25 showed that only ferric-hydroxamate uptake is an essential determinant for susceptibility of S. enterica to microcin J25. Examination of Salmonella strains exposed to microcin J25 by transmission electronic microscopy showed for the first-time involvement of a pore formation mechanism. Microcin J25 was a strong inhibitor of several multiresistant isolates of Salmonella and may have a great potential as an alternative to antibiotics.

Bacteriocins From LAB and Other Alternative Approaches for the Control of Clostridium and Clostridiodes Related Gastrointestinal Colitis

The gut microbiome is considered as a promising target for future non-conventional therapeutic treatment of inflammatory and infectious diseases. The search for appropriate safe and beneficial (lactic acid bacterial and other) putative probiotic strains and/or their antimicrobial metabolites represents a challenging approach for combating several problematic and emerging infections. The process of selecting suitable strains, especially of lactic acid bacteria (LAB) with superior properties, has been accelerated and intensified during the past two decades, also thanks to recent developments in lab techniques. Currently, special focus is on the potential of antimicrobial metabolites produced by some LAB strains and their application as active therapeutic agents. The vision is to develop a scientific basis for 'biotherapeutics' as alternative to conventional approaches in both human and veterinary medicine. Consequently, innovative and promising applications of LAB to the therapeutic practice are presently emerging. An overview of the existing literature indicates that some antimicrobial metabolites such as bacteriocins, widely produced by different bacterial species including LAB, are promising biotherapeutic agents for controlling infections caused by potential pathogens, such as Clostridium and Clostridiodes. Non-conventional, safe and well designed therapeutic treatments may contribute to the improvement of gut dysbiotic conditions. Thereby gut homeostasis can be restored and inflammatory conditions such as gastrointestinal colitis ameliorated. Combining the knowledge on the production, characterization and application of bacteriocins from probiotic LAB, together with their antibacterial properties, appears to be a promising and novel approach in biotherapy. In this overview, different scenarios for the control of Clostridium spp. by application of bacteriocins as therapeutic agents, also in synergistic combination with antibiotics, will be discussed.

Thiostrepton: A Novel Therapeutic Drug Candidate for Mycobacterium abscessus Infection

Mycobacterium abscessus is a rapid-growing, multidrug-resistant, non-tuberculous mycobacterial species responsible for a variety of human infections, such as cutaneous and pulmonary infections. M. abscessus infections are very difficult to eradicate due to the natural and acquired multidrug resistance profiles of M. abscessus. Thus, there is an urgent need for the development of effective drugs or regimens against M. abscessus infections. Here, we report the activity of a US Food and Drug Administration approved drug, thiostrepton, against M. abscessus. We found that thiostrepton significantly inhibited the growth of M. abscessus wild-type strains, subspecies, clinical isolates, and drug-resistant mutants in vitro and in macrophages. In addition, treatment of macrophages with thiostrepton significantly decreased proinflammatory cytokine production in a dose-dependent manner, suggesting an inhibitory effect of thiostrepton on inflammation induced during M. abscessus infection. We further showed that thiostrepton exhibits antimicrobial effects in vivo using a zebrafish model of M. abscessus infection.

Highly efficient and selective antimicrobial isonicotinylhydrazide-coated polyoxometalate-functionalized silver nanoparticles

With the rapidly approaching post-antibiotic era, new and effective combinations of antibiotics are imperative to combat multiple drug resistance (MDR). We have synthesized multimodal antimicrobials that integrate the antibiotic isonicotinylhydrazide (INH), silver nanoparticles (AgNPs^INH), and two different polyoxometalates (POMs) namely, phosphotungstic acid (PTA) and phosphomolybdic acid (PMA) to prepare AgNPs^INH@PTA and AgNPs^INH@PMA, respectively. AgNPs^INH have peroxidase-like (nanozyme) activity and very high antibacterial potential toward S. aureus, which was further enhanced upon modification with POMs. The selectivity of these functional nanozymes was evaluated with m5S mouse fibroblasts using WST-8, LDH viability, in vitro reactive oxygen species (ROS) generation assays, and crystal violet morphological studies. These investigations showed very low cytotoxicity for the nanoparticles compared to free metal ions (Ag⁺), pristine POMs and INH, demonstrating the ability of multifunctional materials to provide efficient and selective antimicrobials.

Syrian Hamster as an Animal Model for the Study on Infectious Diseases

Infectious diseases still remain one of the biggest challenges for human health. In order to gain a better understanding of the pathogenesis of infectious diseases and develop effective diagnostic tools, therapeutic agents, and preventive vaccines, a suitable animal model which can represent the characteristics of infectious is required. The Syrian hamster immune responses to infectious pathogens are similar to humans and as such, this model is advantageous for studying pathogenesis of infection including post-bacterial, viral and parasitic pathogens, along with assessing the efficacy and interactions of medications and vaccines for those pathogens. This review summarizes the current status of Syrian hamster models and their use for understanding the underlying mechanisms of pathogen infection, in addition to their use as a drug discovery platform and provides a strong rationale for the selection of Syrian hamster as animal models in biomedical research. The challenges of using Syrian hamster as an alternative animal model for the research of infectious diseases are also addressed.

Heteroaryl Rings in Peptide Macrocycles

This Review is devoted to the chemistry of macrocyclic peptides having heterocyclic fragments in their structure. These motifs are present in many natural products and synthetic macrocycles designed against a particular biochemical target. Thiazole and oxazole are particularly common constituents of naturally occurring macrocyclic peptide molecules. This frequency of occurrence is because the thiazole and oxazole rings originate from cysteine, serine, and threonine residues. Whereas other heteroaryl groups are found less frequently, they offer many insightful lessons that range from conformational control to receptor/ligand interactions. Many options to develop new and improved technologies to prepare natural products have appeared in recent years, and the synthetic community has been pursuing synthetic macrocycles that have no precedent in nature. This Review attempts to summarize progress in this area.

Cationic biaryl 1,2,3-triazolyl peptidomimetic amphiphiles targeting Clostridioides (Clostridium) difficile: Synthesis, antibacterial evaluation and an in vivo C. difficile infection model

Clostridioides (formerly Clostridium) difficile is a Gram-positive anaerobic bacterial pathogen that causes severe gastrointestinal infection in humans. The current chemotherapeutic options are vastly inadequate, expensive and limited; this results in an exorbitant medical and financial burden. New, inexpensive chemotherapeutic treatments for C. difficile infection with improved efficacy are urgently needed. A streamlined synthetic pathway was developed to allow access to 38 novel mono- and di-cationic biaryl 1,2,3-triazolyl peptidomimetics with increased synthetic efficiency, aqueous solubility and enhanced antibacterial efficacy. The monocationic arginine derivative 28 was identified as a potent, Gram-positive selective antibacterial with MIC values of 4 μg/mL against methicillin-resistant Staphylococcus aureus and 8 μg/mL against C. difficile. Furthermore, the dicationic bis-triazole analogue 50 was found to exhibit broad-spectrum activity with substantial Gram-negative efficacy against Acinetobacter baumannii (8 μg/mL), Pseudomonas aeruginosa (8 μg/mL) and Klebsiella pneumoniae (16 μg/mL); additionally, compound 50 displayed reduced haemolytic activity (<13%) in an in vitro haemolysis assay. Membrane-disruption assays were conducted on selected derivatives to confirm the membrane-active mechanism of action inherent to the synthesized amphiphilic compounds. A comparative solubility assay was developed and utilized to optimize the aqueous solubility of the compounds for in vivo studies. The biaryl peptidomimetics 28 and 67 were found to exhibit significant efficacy in an in vivo murine model of C. difficile infection by reducing the severity and slowing the onset of disease.

Management of adult Clostridium difficile digestive contaminations: a literature review

Clostridium difficile infections (CDI) dramatically increased during the last decade and cause a major public health problem. Current treatments are limited by the high disease recurrence rate, severity of clinical forms, disruption of the gut microbiota, and colonization by vancomycin-resistant enterococci (VRE). In this review, we resumed current treatment options from official recommendation to promising alternatives available in the management of adult CDI, with regard to severity and recurring or non-recurring character of the infection. Vancomycin remains the first-line antibiotic in the management of mild to severe CDI. The use of metronidazole is discussed following the latest US recommendations that replaced it by fidaxomicin as first-line treatment of an initial episode of non-severe CDI. Fidaxomicin, the most recent antibiotic approved for CDI in adults, has several advantages compared to vancomycin and metronidazole, but its efficacy seems limited in cases of multiple recurrences. Innovative therapies such as fecal microbiota transplantation (FMT) and antitoxin antibodies were developed to limit the occurrence of recurrence of CDI. Research is therefore very active, and new antibiotics are being studied as surotomycin, cadazolid, and rinidazole.

Novel Antimicrobials for the Treatment of Clostridium difficile Infection

The current picture of Clostridium difficile infection (CDI) is alarming with a mortality rate ranging between 3% and 15% and a CDI recurrence rate ranging from 12% to 40%. Despite the great efforts made over the past 10 years to face the CDI burden, there are still gray areas in our knowledge on CDI management. The traditional anti-CDI antimicrobials are not always adequate in addressing the current needs in CDI management. The aim of our review is to give an update on novel antimicrobials for the treatment of CDI, considering the currently available evidences on their efficacy, safety, molecular mechanism of action, and their probability to be successfully introduced into the clinical practice in the near future. We identified, through a PubMed search, 16 novel antimicrobial molecules under study for CDI treatment: cadazolid, surotomycin, ridinilazole, LFF571, ramoplanin, CRS3123, fusidic acid, nitazoxanide, rifampin, rifaximin, tigecycline, auranofin, NVB302, thuricin CD, lacticin 3147, and acyldepsipeptide antimicrobials. In comparison with the traditional anti-CDI antimicrobial treatment, some of the novel antimicrobials reviewed in this study offer several advantages, i.e., the favorable pharmacokinetic and pharmacodynamic profile, the narrow-spectrum activity against CD that implicates a low impact on the gut microbiota composition, the inhibitory activity on CD sporulation and toxins production. Among these novel antimicrobials, the most active compounds in reducing spore production are cadazolid, ridinilazole, CRS3123, ramoplanin and, potentially, the acyldepsipeptide antimicrobials. These antimicrobials may potentially reduce CD environment spread and persistence, thus reducing CDI healthcare-associated acquisition. However, some of them, i.e., surotomycin, fusidic acid, etc., will not be available due to lack of superiority versus standard of treatment. The most CD narrow-spectrum novel antimicrobials that allow to preserve microbiota integrity are cadazolid, ridinilazole, auranofin, and thuricin CD. In conclusion, the novel antimicrobial molecules under development for CDI have promising key features and advancements in comparison to the traditional anti-CDI antimicrobials. In the near future, some of these new molecules might be effective alternatives to fight CDI.

Clostridium difficile - From Colonization to Infection

Clostridium difficile is the most frequent cause of nosocomial antibiotic-associated diarrhea. The incidence of C. difficile infection (CDI) has been rising worldwide with subsequent increases in morbidity, mortality, and health care costs. Asymptomatic colonization with C. difficile is common and a high prevalence has been found in specific cohorts, e.g., hospitalized patients, adults in nursing homes and in infants. However, the risk of infection with C. difficile differs significantly between these cohorts. While CDI is a clear indication for therapy, colonization with C. difficile is not believed to be a direct precursor for CDI and therefore does not require treatment. Antibiotic therapy causes alterations of the intestinal microbial composition, enabling C. difficile colonization and consecutive toxin production leading to disruption of the colonic epithelial cells. Clinical symptoms of CDI range from mild diarrhea to potentially life-threatening conditions like pseudomembranous colitis or toxic megacolon. While antibiotics are still the treatment of choice for CDI, new therapies have emerged in recent years such as antibodies against C. difficile toxin B and fecal microbial transfer (FMT). This specific therapy for CDI underscores the role of the indigenous bacterial composition in the prevention of the disease in healthy individuals and its role in the pathogenesis after alteration by antibiotic treatment. In addition to the pathogenesis of CDI, this review focuses on the colonization of C. difficile in the human gut and factors promoting CDI.

Advances in the diagnosis and treatment of Clostridium difficile infections

Clostridium difficile is a leading cause of antibiotic-associated diarrhea worldwide. The diagnosis of C. difficile infection (CDI) requires both clinical manifestations and a positive laboratory test for C. difficile and/or its toxins. While antibiotic therapy is the treatment of choice for CDI, there are relatively few classes of effective antibiotics currently available. Therefore, the development of novel antibiotics and/or alternative treatment strategies for CDI has received a great deal of attention in recent years. A number of emerging agents such as cadazolid, surotomycin, ridinilazole, and bezlotoxumab have demonstrated activity against C. difficile; some of these have been approved for limited clinical use and some are in clinical trials. In addition, other approaches such as early and accurate diagnosis of CDI as well as disease prevention are important for clinical management. While the toxigenic culture and the cell cytotoxicity neutralization assay are still recognized as the gold standard for the diagnosis of CDI, new diagnostic approaches such as nucleic acid amplification methods have become available. In this review, we will discuss both current and emerging diagnostic and therapeutic modalities for CDI.

Clostridium difficile - From Colonization to Infection

Clostridium difficile is the most frequent cause of nosocomial antibiotic-associated diarrhea. The incidence of C. difficile infection (CDI) has been rising worldwide with subsequent increases in morbidity, mortality, and health care costs. Asymptomatic colonization with C. difficile is common and a high prevalence has been found in specific cohorts, e.g., hospitalized patients, adults in nursing homes and in infants. However, the risk of infection with C. difficile differs significantly between these cohorts. While CDI is a clear indication for therapy, colonization with C. difficile is not believed to be a direct precursor for CDI and therefore does not require treatment. Antibiotic therapy causes alterations of the intestinal microbial composition, enabling C. difficile colonization and consecutive toxin production leading to disruption of the colonic epithelial cells. Clinical symptoms of CDI range from mild diarrhea to potentially life-threatening conditions like pseudomembranous colitis or toxic megacolon. While antibiotics are still the treatment of choice for CDI, new therapies have emerged in recent years such as antibodies against C. difficile toxin B and fecal microbial transfer (FMT). This specific therapy for CDI underscores the role of the indigenous bacterial composition in the prevention of the disease in healthy individuals and its role in the pathogenesis after alteration by antibiotic treatment. In addition to the pathogenesis of CDI, this review focuses on the colonization of C. difficile in the human gut and factors promoting CDI.

Existing and investigational therapies for the treatment of Clostridium difficile infection: A focus on narrow spectrum, microbiota-sparing agents

Despite intense international attention and efforts to reduce its incidence, Clostridium difficile infection (CDI) remains a significant concern for patients, clinicians, and healthcare organizations. It is costly for payers and disabling for patients. Furthermore, recurrent CDI is particularly difficult to manage, resulting in excess mortality, hospital length of stay, and other healthcare resource use. A greater understanding of the role of the gut microbiome has emphasized the importance of this diverse community in providing colonization resistance against CDI. The introduction of fidaxomicin, which has limited effect on the microflora has improved clinical outcomes in relation to disease recurrence. There are a number of other new agents in development, which appear to have a narrow spectrum of activity whilst exerting minimal effect on the microflora. Whilst the role of these emerging agents in the treatment of CDI is presently unclear, they appear to be promising candidates.

Muscarine, imidazole, oxazole and thiazole alkaloids

Covering: July 2012 to June 2015. Previous review: Nat. Prod. Rep., 2013, 30, 869-915The structurally diverse imidazole-, oxazole-, and thiazole-containing secondary metabolites are widely distributed in terrestrial and marine environments, and exhibit extensive pharmacological activities. In this review the latest progress involving the isolation, biological activities, and chemical and biogenetic synthesis studies on these natural products has been summarized.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

A Review of Experimental and Off-Label Therapies for Clostridium difficile Infection

In spite of increased awareness and the efforts taken to optimize Clostridium difficile infection (CDI) management, with the limited number of currently available antibiotics for C. difficile the halt of this increasing epidemic remains out of reach. There are, however, close to 80 alternative treatment methods with controversial anti-clostridial efficacy or in experimental phase today. Indeed, some of these therapies are expected to become acknowledged members of the recommended anti-CDI arsenal within the next few years. None of these alternative treatment methods can respond in itself to all the major challenges of CDI management, which are primary prophylaxis in the susceptible population, clinical cure of severe cases, prevention of recurrences, and forestallment of asymptomatic C. difficile carriage and in-hospital spread. Yet, the greater the variety of treatment choices on hand, the better combination strategies can be developed to reach these goals in the future. The aim of this article is to provide a comprehensive summary of these experimental and currently off-label therapeutic options.

Novel antibiotics in development to treat Clostridium difficile infection

PURPOSE OF REVIEW

Clostridium difficile infections (CDI) remain a challenge to treat clinically due primarily to limited number of antibiotics available and unacceptably high recurrence rates. Because of this, there has been significant demand for creating innovative therapeutics, which has resulted in the development of several novel antibiotics.

RECENT FINDINGS

This review updates seven different antibiotics that are currently in development to treat CDI including fidaxomicin, surotomycin, ridinilazole, ramoplanin, cadazolid, LFF571, and CRS3123. Available preclinical and clinical data are compared between these antibiotics.

SUMMARY

Many of these new antibiotics display almost ideal properties for antibiotics directed against CDI. Despite these properties, not all clinical development of these compounds has been successful. These studies have provided key insights into the pathogenesis of CDI and will continue to inform future drug development. Successful phase III clinical trials should result in several new and novel antibiotics to treat CDI.

Clostridium difficile colitis: pathogenesis and host defence

Clostridium difficile is a major cause of intestinal infection and diarrhoea in individuals following antibiotic treatment. Recent studies have begun to elucidate the mechanisms that induce spore formation and germination and have determined the roles of C. difficile toxins in disease pathogenesis. Exciting progress has also been made in defining the role of the microbiome, specific commensal bacterial species and host immunity in defence against infection with C. difficile. This Review will summarize the recent discoveries and developments in our understanding of C. difficile infection and pathogenesis.

New antibiotics in clinical trials for Clostridium difficile

INTRODUCTION

There are limited number of approved therapies for C. difficile infections (CDIs) and new treatments are needed to decrease recurrence rates. Over the past 5 years, four novel antibiotics have been evaluated in clinical trials that offer distinct advantages over existing therapies for the treatment of CDI.

AREAS COVERED

This article reviews the preclinical and clinical studies of cadazolid, LFF571, ridinilazole, and surotomycin. The advantages that these antibiotics may have in the treatment of CDI is compared with current therapies metronidazole, vancomycin, and fidaxomicin. Expert commentary: The antibiotics examined have the potential to improve rates of CDI treatment without recurrence. We anticipate that one or more of these medications will be approved within five years.

Recurrent Clostridium difficile infection: From colonization to cure

Clostridium difficile infection (CDI) is increasingly prevalent, dangerous and challenging to prevent and manage. Despite intense national and international attention the incidence of primary and of recurrent CDI (PCDI and RCDI, respectively) have risen rapidly throughout the past decade. Of major concern is the increase in cases of RCDI resulting in substantial morbidity, morality and economic burden. RCDI management remains challenging as there is no uniformly effective therapy, no firm consensus on optimal treatment, and reliable data regarding RCDI-specific treatment options is scant. Novel therapeutic strategies are critically needed to rapidly, accurately, and effectively identify and treat patients with, or at-risk for, RCDI. In this review we consider the factors implicated in the epidemiology, pathogenesis and clinical presentation of RCDI, evaluate current management options for RCDI and explore novel and emerging therapies.

Toxigenic Clostridium difficile PCR ribotypes in edible marine bivalve molluscs in Italy

Even though food of animal sources and different foodstuffs are well known to be potentially carrier of Clostridium difficile, few data are available on the occurrence of C. difficile in seafood. This work investigated the occurrence of C. difficile in edible bivalve molluscs in southern Italy. Out of the 925 investigated samples, 3.9% contained C. difficile. Eighteen strains harboured both genes for toxins A and B whereas 1 only had toxin B gene. Binary toxin genes were found in 22.2% of the isolates. The most frequently ribotypes found were 078/126 (22.2%), 010 (19.4%), and 001 (8.3%). All isolates were susceptible to metronidazole, vancomycin, fidaxomicin, and to the new semisynthetic thiopeptide antibiotic LFF571, whereas 19.4% of them were resistant to moxifloxacin, 30.5% to clindamycin, 38.8% to erythromycin, and 100% to ciprofloxacin. This study points out that edible molluscs could be a potential source of toxigenic C. difficile ribotypes and a potential risk for human health.

Multicenter, randomized clinical trial to compare the safety and efficacy of LFF571 and vancomycin for Clostridium difficile infections

Clostridium difficile infection causes serious diarrheal disease. Although several drugs are available for treatment, including vancomycin, recurrences remain a problem. LFF571 is a semisynthetic thiopeptide with potency against C. difficile in vitro. In this phase 2 exploratory study, we compared the safety and efficacy (based on a noninferiority analysis) of LFF571 to those of vancomycin used in adults with primary episodes or first recurrences of moderate C. difficile infection. Patients were randomized to receive 200 mg of LFF571 or 125 mg of vancomycin four times daily for 10 days. The primary endpoint was the proportion of clinical cures at the end of therapy in the per-protocol population. Secondary endpoints included clinical cures at the end of therapy in the modified intent-to-treat (mITT) population, the time to diarrhea resolution, and the recurrence rate. Seventy-two patients were randomized, with 46 assigned to receive LFF571. Based on the protocol-specified definition, the rate of clinical cure for LFF571 (90.6%) was noninferior to that of vancomycin (78.3%). The 30-day sustained cure rates for LFF571 and vancomycin were 56.7% and 65.0%, respectively, in the per-protocol population and 58.7% and 60.0%, respectively, in the modified intent-to-treat population. Using toxin-confirmed cases only, the recurrence rates were lower for LFF571 (19% versus 25% for vancomycin in the per-protocol population). LFF571 was generally safe and well tolerated. The incidence of adverse events (AEs) was higher for LFF571 (76.1% versus 69.2% for vancomycin), although more AEs in the vancomycin group were suspected to be related to the study drug (38.5% versus 32.6% for LFF571). One patient receiving LFF571 discontinued the study due to an AE. (This study has been registered at ClinicalTrials.gov under registration no. NCT01232595.).

Pharmacokinetics of LFF571 and vancomycin in patients with moderate Clostridium difficile infections

Clostridium difficile infection causes diarrheal disease with potentially fatal complications. Although treatments are available, including vancomycin, metronidazole, and fidaxomicin, the recurrence of disease after therapy remains a problem. LFF571 is a novel thiopeptide antibacterial that shows in vitro potency against C. difficile that is comparable to or greater than that of other clinically used antibiotics. Here, we compare the pharmacokinetics (PK) of LFF571 and vancomycin in patients with C. difficile infection as part of an early efficacy study. This multicenter, randomized, evaluator-blind, and active-controlled study evaluated the safety, efficacy, and pharmacokinetics of LFF571 in adults with primary episodes or first relapses of moderate C. difficile infections. Patients were randomized to receive 200 mg of LFF571 or 125 mg of vancomycin four times daily for 10 days. The PK parameters were calculated from drug concentrations measured in serum and fecal samples. The systemic exposure following oral administration of 200 mg of LFF571 four times per day for 10 days in patients with C. difficile infection was limited. The highest LFF571 serum concentration observed was 41.7 ng/ml, whereas the levels in feces at the end of treatment were between 107 and 12,900 μg/g. In comparison, the peak vancomycin level observed in serum was considerably higher, at 2.73 μg/ml; the levels of vancomycin in feces were not measured. Similar to healthy volunteers, patients with C. difficile infections exhibited high fecal concentrations and low serum levels of LFF571. These results are consistent with the retention of LFF571 in the lumen of the gastrointestinal tract. (This study has been registered at ClinicalTrials.gov under registration no. NCT01232595.).

Subinhibitory concentrations of LFF571 reduce toxin production by Clostridium difficile

LFF571 is a novel semisynthetic thiopeptide antibacterial that is undergoing investigation for safety and efficacy in patients with moderate Clostridium difficile infections. LFF571 inhibits bacterial protein synthesis by interacting with elongation factor Tu (EF-Tu) and interrupting complex formation between EF-Tu and aminoacyl-tRNA. Given this mechanism of action, we hypothesized that concentrations of LFF571 below those necessary to inhibit bacterial growth would reduce steady-state toxin levels in C. difficile cultures. We investigated C. difficile growth and toxin A and B levels in the presence of LFF571, fidaxomicin, vancomycin, and metronidazole. LFF571 led to strain-dependent effects on toxin production, including decreased toxin levels after treatment with subinhibitory concentrations, and more rapid declines in toxin production than in inhibition of colony formation. Fidaxomicin, which is an RNA synthesis inhibitor, conferred a similar pattern to LFF571 with respect to toxin levels versus viable cell counts. The incubation of two toxigenic C. difficile strains with subinhibitory concentrations of vancomycin, a cell wall synthesis inhibitor, increased toxin levels in the supernatant over those of untreated cultures. A similar phenomenon was observed with one metronidazole-treated strain of C. difficile. These studies indicate that LFF571 and fidaxomicin generally result in decreased C. difficile toxin levels in culture supernatants, whereas treatment of some strains with vancomycin or metronidazole had the potential to increase toxin levels. Although the relevance of these findings remains to be studied in patients, reducing toxin levels with sub-growth-inhibitory concentrations of an antibiotic is hypothesized to be beneficial in alleviating symptoms.

The potential for emerging therapeutic options for Clostridium difficile infection

Clostridium difficile is mainly a nosocomial pathogen and is a significant cause of antibiotic-associated diarrhea. It is also implicated in the majority of cases of pseudomembranous colitis. Recently, advancements in next generation sequencing technology (NGS) have highlighted the extent of damage to the gut microbiota caused by broad-spectrum antibiotics, often resulting in C. difficile infection (CDI). Currently the treatment of choice for CDI involves the use of metronidazole and vancomycin. However, recurrence and relapse of CDI, even after rounds of metronidazole/vancomycin administration is a problem that must be addressed. The efficacy of alternative antibiotics such as fidaxomicin, rifaximin, nitazoxanide, ramoplanin and tigecycline, as well as faecal microbiota transplantation has been assessed and some have yielded positive outcomes against C. difficile. Some bacteriocins have also shown promising effects against C. difficile in recent years. In light of this, the potential for emerging treatment options and efficacy of anti-C. difficile vaccines are discussed in this review.

Characterization of a novel plasmid-borne thiopeptide gene cluster in Staphylococcus epidermidis strain 115

Thiopeptides are small (12- to 17-amino-acid), heavily modified peptides of bacterial origin. This antibiotic family, with more than 100 known members, is characterized by the presence of sulfur-containing heterocyclic rings and dehydrated residues within a macrocyclic peptide structure. Thiopeptides, including micrococcin P1, have garnered significant attention in recent years for their potent antimicrobial activity against bacteria, fungi, and even protozoa. Micrococcin P1 is known to target the ribosome; however, like those of other thiopeptides, its biosynthesis and mechanisms of self-immunity are poorly characterized. We have discovered an isolate of Staphylococcus epidermidis harboring the genes for thiopeptide production and self-protection on a 24-kb plasmid. Here we report the characterization of this plasmid, identify the antimicrobial peptide that it encodes, and provide evidence of a target replacement-mediated mechanism of self-immunity.

Thiopeptide engineering: a multidisciplinary effort towards future drugs

The recent development of thiopeptide analogues of antibiotics has allowed some of the limitations inherent to these naturally occurring substances to be overcome. Chemical synthesis, semisynthetic derivatization, and engineering of the biosynthetic pathway have independently led to complementary modifications of various thiopeptides. Some of the new substances have displayed improved profiles, not only as antibiotics, but also as antiplasmodial and anticancer drugs. The design of novel molecules based on the thiopeptide scaffold appears to be the only strategy to exploit the high potential they have shown in vitro. Herein we present the most relevant achievements in the production of thiopeptide analogues and also discuss the way the different approaches might be combined in a multidisciplinary strategy to produce more sophisticated structures.

New developments in chemotherapeutic options for Clostridium difficile colitis

PURPOSE OF REVIEW

Clostridium difficile infections (CDIs) are of increasing concern in healthcare due to increasing incidence as well as suboptimal response to standard therapies. This review focuses on current updates in chemotherapeutic treatment options for primary CDI as well as for relapse.

RECENT FINDINGS

Metronidazole and vancomycin remain the standard therapy for mild and severe CDI, respectively. Fidaxomicin was approved for use in CDI by the US Food and Drug Administration in 2011 and new studies have shown a decreased rate of recurrence as compared with vancomycin as well as potential promise for use as a chaser. Rifaximin may be useful in salvage therapy for recurrent CDI as well as for a chaser. Tigecycline, teicoplanin, doxycycline, linezolid, nitazoxanide, amixicile, LFF571, and CB-183 315 have in-vitro activity and are under different stages of study. Monoclonal antitoxin antibodies for prevention of relapse of CDI are currently under evaluation in a phase 3 clinical trial.

SUMMARY

A variety of promising new treatment options for Clostridium difficile are under development, although further studies are necessary to determine the efficacy of these newer treatments for cure and preventing disease relapse.

Thiopeptide antibiotics: retrospective and recent advances

Thiopeptides, or thiazolyl peptides, are a relatively new family of antibiotics that already counts with more than one hundred different entities. Although they are mainly isolated from soil bacteria, during the last decade, new members have been isolated from marine samples. Far from being limited to their innate antibacterial activity, thiopeptides have been found to possess a wide range of biological properties, including anticancer, antiplasmodial, immunosuppressive, etc. In spite of their ribosomal origin, these highly posttranslationally processed peptides have posed a fascinating synthetic challenge, prompting the development of various methodologies and strategies. Regardless of their limited solubility, intensive investigations are bringing thiopeptide derivatives closer to the clinic, where they are likely to show their veritable therapeutic potential.

Progress in the discovery of treatments for C. difficile infection: A clinical and medicinal chemistry review

Clostridium difficile is an anaerobic, Gram-positive pathogen that causes C. difficile infection, which results in significant morbidity and mortality. The incidence of C. difficile infection in developed countries has become increasingly high due to the emergence of newer epidemic strains, a growing elderly population, extensive use of broad spectrum antibiotics, and limited therapies for this diarrheal disease. Because treatment options currently available for C. difficile infection have some drawbacks, including cost, promotion of resistance, and selectivity problems, new agents are urgently needed to address these challenges. This review article focuses on two parts: the first part summarizes current clinical treatment strategies and agents under clinical development for C. difficile infection; the second part reviews newly reported anti-difficile agents that have been evaluated or reevaluated in the last five years and are in the early stages of drug discovery and development. Antibiotics are divided into natural product inspired and synthetic small molecule compounds that may have the potential to be more efficacious than currently approved treatments. This includes potency, selectivity, reduced cytotoxicity, and novel modes of action to prevent resistance.

Therapy of Clostridium difficile infection: perspectives on a changing paradigm

INTRODUCTION

Clostridium difficile disease (CDI) have increased in frequency and severity over the past decade and are a leading cause of hospital acquired infections, contributing to increased hospital length of stay and costs, as well as associated increased mortality, especially amongst the elderly. Standard therapy has been associated with 20 - 30% relapse rates. Consequently, new CDI therapeutic approaches have emerged.

AREAS COVERED

The role of metronidazole, vancomycin, fidaxomicin, rifaximin, nitizoxanide, tigecycline, fusidic acid, LFF-571, cardazolid, SMT 19969, CamSA and surotomycin were reviewed.

EXPERT OPINION

New IDSA/SHEA guidelines are expected within the next year and may impact selection of primary therapy for CDI. Until then, metronidazole will likely remain as first line therapy because of low cost and despite its inferiority compared to vancomycin. Vancomycin will likely see increasing use, especially as generics become available. Fidaxomicin will emerge as an important therapy for relapse patients and perhaps as initial therapy for patients at greatest risk for relapse, with concomitant antibiotics, multiple comorbidities and renal insufficiency, advanced age and hypoalbuminemia. Biotherapeutics such as fecal microbiota transplantation and non-toxogenic C. difficile prevention will emerge as the preferred therapy in multiple relapse patients and the development of an oral formulation will occur within five years.

Carbohydrate-based Clostridium difficile vaccines

Clostridium difficile is responsible for thousands of deaths each year and a vaccine would be welcomed, especially one that would disrupt bacterial maintenance, colonization and persistence in carriers and convalescent patients. Structural explorations at the University of Guelph (ON, Canada) discovered that C. difficile may express three phosphorylated polysaccharides, named PSI, PSII and PSIII; this review captures our recent efforts to create vaccines based on these glycans, especially PSII, the common antigen that has precipitated immediate attention. The authors describe the design and immunogenicity of vaccines composed of raw polysaccharides and conjugates thereof. So far, it has been observed that anti-PSII antibodies can be raised in farm animals, mice and hamster models; humans and horses carry anti-PSII IgA and IgG antibodies from natural exposure to C. difficile, respectively; phosphate is an indispensable immunogenic epitope and vaccine-induced PSII antibodies recognize PSII on C. difficile outer surface.

Antimicrobial activity of LFF571 and three treatment agents against Clostridium difficile isolates collected for a pan-European survey in 2008: clinical and therapeutic implications

OBJECTIVES

In November 2008, a study was performed with support from the European Centre for Disease Prevention and Control (ECDC) to obtain an overview of Clostridium difficile infections (CDIs) in European hospitals. A collection of 398 C. difficile isolates obtained from this hospital-based survey was utilized to identify antimicrobial susceptibility patterns of common C. difficile PCR ribotypes across Europe.

METHODS

The MICs of three approved therapeutic agents (vancomycin, metronidazole and fidaxomicin) and LFF571 (a novel semi-synthetic thiopeptide antibiotic) were determined by the agar dilution method.

RESULTS

MICs of fidaxomicin and LFF571 were in general 2-4-fold lower than those of vancomycin and metronidazole. Isolates belonging to clade 2, including the hypervirulent ribotype 027, had one-dilution higher MIC50 and MIC90 values for fidaxomicin and metronidazole, whereas similar MIC values were observed for vancomycin and LFF571. Isolates belonging to C. difficile PCR ribotype 001 were more susceptible to fidaxomicin than other frequently found PCR ribotypes 014/020 and 078. Six isolates from three different countries had a metronidazole MIC of 2 mg/L. Four of the six isolates were characterized as PCR ribotype 001.

CONCLUSIONS

There was no evidence of in vitro resistance of C. difficile to any of the four agents tested. However, the results suggest type-specific differences in susceptibility for the treatment agents we investigated. Continuous surveillance of C. difficile isolates in Europe is needed to determine the possible clinical implications of ribotype-specific changes in susceptibility to therapeutic agents.

Developing new antibacterials through natural product research

INTRODUCTION

Natural products have long been instrumental for discovering antibiotics, but many pharmaceutical companies abandoned this field and new antibiotics declined. In contrast, microbial resistance to current antibiotics has approached critical levels.

AREAS COVERED

This article gives historical perspectives by providing background about present-day economic realities and medical needs for antibiotic research, whose pipeline is mostly focused toward older known agents and newer semi-synthetic derivatives. Future research trends and projected technological developments open many innovative opportunities to discover novel antibacterials and find ways to control pathogenic bacteria without conventional antibiotics that provoke resistance.

EXPERT OPINION

The successful registration of daptomycin, retapamulin and fidaxomicin indicate the re-emergence of natural products has already begun. Semi-synthetic derivatives from other under-explored classes are progressing. More effort is being put into approaches such as total synthesis, discovery of new structural scaffolds for synthesis, alterations of biosynthetic pathways, combinatorial biosynthesis, new screening targets and new resources from which to isolate natural products. A return to successful screening of actinomycetes depends on solving the rate-limiting dereplication obstacle. Long-term solutions need to come from greater exploration of the massive numbers of uncultured microbes. An ultimate solution to the antibiotic-promoted microbial resistance cycle may lie in finding ways to control bacteria by non-lethal means.