Ramoplanin: a lipoglycodepsipeptide antibiotic

Clostridioides difficile infection: history, epidemiology, risk factors, prevention, clinical manifestations, treatment, and future options

SUMMARYClostridioides difficile infection (CDI) is one of the major issues in nosocomial infections. This bacterium is constantly evolving and poses complex challenges for clinicians, often encountered in real-life scenarios. In the face of CDI, we are increasingly equipped with new therapeutic strategies, such as monoclonal antibodies and live biotherapeutic products, which need to be thoroughly understood to fully harness their benefits. Moreover, interesting options are currently under study for the future, including bacteriophages, vaccines, and antibiotic inhibitors. Surveillance and prevention strategies continue to play a pivotal role in limiting the spread of the infection. In this review, we aim to provide the reader with a comprehensive overview of epidemiological aspects, predisposing factors, clinical manifestations, diagnostic tools, and current and future prophylactic and therapeutic options for C. difficile infection.

Synthetic ramoplanin analogues are accessible by effective incorporation of arylglycines in solid-phase peptide synthesis

The threat of antimicrobial resistance to antibiotics requires a continual effort to develop alternative treatments. Arylglycines (or phenylglycines) are one of the signature amino acids found in many natural peptide antibiotics, but their propensity for epimerization in solid-phase peptide synthesis (SPPS) has prevented their use in long peptide sequences. We have now identified an optimized protocol that allows the synthesis of challenging non-ribosomal peptides including precursors of the glycopeptide antibiotics and an analogue of feglymycin (1 analogue, 20%). We have exploited this protocol to synthesize analogues of the peptide antibiotic ramoplanin using native chemical ligation/desulfurization (1 analogue, 6.5%) and head-to-tail macrocyclization in excellent yield (6 analogues, 3-9%), with these compounds extensively characterized by NMR (U-shaped structure) and antimicrobial activity assays (two clinical isolates). This method significantly reduces synthesis time (6-9 days) when compared with total syntheses (2-3 months) and enables drug discovery programs to include arylglycines in structure-activity relationship studies and drug development.

Insights into the central role of N-acetyl-glucosamine-1-phosphate uridyltransferase (GlmU) in peptidoglycan metabolism and its potential as a therapeutic target

Several decades after the discovery of the first antibiotic (penicillin) microbes have evolved novel mechanisms of resistance; endangering not only our abilities to combat future bacterial pandemics but many other clinical challenges such as acquired infections during surgeries. Antimicrobial resistance (AMR) is attributed to the mismanagement and overuse of these medications and is complicated by a slower rate of the discovery of novel drugs and targets. Bacterial peptidoglycan (PG), a three-dimensional mesh of glycan units, is the foundation of the cell wall that protects bacteria against environmental insults. A significant percentage of drugs target PG, however, these have been rendered ineffective due to growing drug resistance. Identifying novel druggable targets is, therefore, imperative. Uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) is one of the key building blocks in PG production, biosynthesized by the bifunctional enzyme N-acetyl-glucosamine-1-phosphate uridyltransferase (GlmU). UDP-GlcNAc metabolism has been studied in many organisms, but it holds some distinctive features in bacteria, especially regarding the bacterial GlmU enzyme. In this review, we provide an overview of different steps in PG biogenesis, discuss the biochemistry of GlmU, and summarize the characteristic structural elements of bacterial GlmU vital to its catalytic function. Finally, we will discuss various studies on the development of GlmU inhibitors and their significance in aiding future drug discoveries.

Role of membrane proteins in bacterial resistance to antimicrobial peptides

Several natural antimicrobial peptides (AMPs), including the novel semisynthetic lipoglycopeptide antibiotics telavancin, dalbavancin, and oritavancin, have been approved for clinical use to address the growing problem of multiple antibiotic-resistant Gram-positive bacterial infections. Nevertheless, the efficacy of these antibiotics has already been compromised. The SARS-CoV-2 pandemic led to the increased clinical use of all antibiotics, further promoting the development of bacterial resistance. Therefore, it is critical to gain a deeper understanding of the role of resistance mechanisms to minimize the consequential risks of long-term antibiotic use and misuse. Here, we summarize for the first time the current knowledge of resistance mechanisms that have been shown to cause resistance to clinically used AMPs, with particular focus on membrane proteins that have been reported to interfere with the activity of AMPs by affecting the binding of AMPs to bacteria.

Opportunities for Nanomedicine in Clostridioides difficile Infection

Clostridioides difficile, a spore-forming bacterium, is a nosocomial infectious pathogen which can be found in animals as well. Although various antibiotics and disinfectants were developed, C. difficile infection (CDI) remains a serious health problem. C. difficile spores have complex structures and dormant characteristics that contribute to their resistance to harsh environments, successful transmission and recurrence. C. difficile spores can germinate quickly after being exposed to bile acid and co-germinant in a suitable environment. The vegetative cells produce endospores, and the mature spores are released from the hosts for dissemination of the pathogen. Therefore, concurrent elimination of C. difficile vegetative cells and inhibition of spore germination is essential for effective control of CDI. This review focused on the molecular pathogenesis of CDI and new trends in targeting both spores and vegetative cells of this pathogen, as well as the potential contribution of nanotechnologies for the effective management of CDI.

Investigational Treatment Agents for Recurrent Clostridioides difficile Infection (rCDI)

Clostridioides difficile infection (CDI) is a major cause of nosocomial diarrhea that is deemed a global health threat. C. difficile strain BI/NAP1/027 has contributed to the increase in the mortality, severity of CDI outbreaks and recurrence rates (rCDI). Updated CDI treatment guidelines suggest vancomycin and fidaxomicin as initial first-line therapies that have initial clinical cure rates of over 80%. Unacceptably high recurrence rates of 15–30% in patients for the first episode and 40% for the second recurrent episode are reported. Alternative treatments for rCDI include fecal microbiota transplant and a human monoclonal antibody, bezlotoxumab, that can be used in patients with high risk of rCDI. Various emerging potential therapies with narrow spectrum of activity and little systemic absorption that are in development include 1) Ibezapolstat (formerly ACX-362E), MGB-BP-3, and DS-2969b-targeting bacterial DNA replication, 2) CRS3213 (REP3123)-inhibiting toxin production and spore formation, 3) ramizol and ramoplanin-affecting bacterial cell wall, 4) LFF-571-blocking protein synthesis, 5) Alanyl-L-Glutamine (alanylglutamine)-inhibiting damage caused by C. difficile by protecting intestinal mucosa, and 6) DNV3837 (MCB3681)-prodrug consisting of an oxazolidinone–quinolone combination that converts to the active form DNV3681 that has activity in vitro against C. difficile. This review article provides an overview of these developing drugs that can have potential role in the treatment of rCDI and in lowering recurrence rates.

Novel therapies and preventative strategies for primary and recurrent Clostridium difficile infections

Clostridium difficile is the leading infectious cause of antibiotic-associated diarrhea and colitis. C. difficile infection (CDI) places a heavy burden on the healthcare system, with nearly half a million infections yearly and an approximate 20% recurrence risk after successful initial therapy. The high incidence has driven new research on improved prevention such as the emerging use of probiotics, intestinal microbiome manipulation during antibiotic therapies, vaccinations, and newer antibiotics that reduce the disruption of the intestinal microbiome. While the treatment of acute C. difficile is effective in most patients, it can be further optimized by adjuvant therapies that improve the initial treatment success and decrease the risk of subsequent recurrence. Finally, the high risk of recurrence has led to multiple emerging therapies that target toxin activity, recovery of the intestinal microbial community, and elimination of latent C. difficile in the intestine. In summary, CDIs illustrate the complex interaction among host physiology, microbial community, and pathogen that requires specific therapies to address each of the factors leading to primary infection and recurrence.

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.

Biosynthesis and biotechnological application of non-canonical amino acids: Complex and unclear

Compared with the better-studied canonical amino acids, the distribution, metabolism and functions of natural non-canonical amino acids remain relatively obscure. Natural non-canonical amino acids have been mainly discovered in plants as secondary metabolites that perform diversified physiological functions. Due to their specific characteristics, a broader range of natural and artificial non-canonical amino acids have recently been applied in the development of functional materials and pharmaceutical products. With the rapid development of advanced methods in biotechnology, non-canonical amino acids can be incorporated into peptides, proteins and enzymes to improve the function and performance relative to their natural counterparts. Therefore, biotechnological application of non-canonical amino acids in artificial bio-macromolecules follows the central goal of synthetic biology to: create novel life forms and functions. However, many of the non-canonical amino acids are synthesized via chemo- or semi-synthetic methods, and few non-canonical amino acids can be synthesized using natural in vivo pathways. Therefore, further research is needed to clarify the metabolic pathways and key enzymes of the non-canonical amino acids. This will lead to the discovery of more candidate non-canonical amino acids, especially for those that are derived from microorganisms and are naturally bio-compatible with chassis strains for in vivo biosynthesis. In this review, we summarize representative natural and artificial non-canonical amino acids, their known information regarding associated metabolic pathways, their characteristics and their practical applications. Moreover, this review summarizes current barriers in developing in vivo pathways for the synthesis of non-canonical amino acids, as well as other considerations, future trends and potential applications of non-canonical amino acids in advanced biotechnology.

Teicoplanin - Tigecycline Combination Shows Synergy Against Mycobacterium abscessus

Lung disease caused by non-tuberculous mycobacteria (NTM), relatives of Mycobacterium tuberculosis, is increasing. M. abscessus is the most prevalent rapid growing NTM. This environmental pathogen is intrinsically resistant to most commonly used antibiotics, including anti-tuberculosis drugs. Current therapies take years to achieve cure, if cure if achieved. Thus, there is an urgent medical need to identify new, more efficacious treatments. Here, we explore the possibility of repurposing antibiotics developed for other indications. We asked whether novel two-drug combinations of clinically used antibiotics can be identified that show synergistic activity against this mycobacterium. An in vitro checkerboard titration assay was employed to test 180 dual combinations of 41 drugs against the clinical isolate M. abscessus Bamboo. The most attractive novel combination was further profiled against reference strains representing three sub-species (M. abscessus subsp. abscessus, massiliense and bolletii) and a collection of clinical isolates. This resulted in the identification of a novel synergistic antibiotic pair active against the M. abscessus complex: the glycopeptide teicoplanin with the glycylcycline tigecycline showed inhibitory activity at 2–3 μM (teicoplanin) and 1–2 μM (tigecycline). This novel combination can now be tested in M. abscessus animal models of infection and/or patients.

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.

Production of Ramoplanin and Ramoplanin Analogs by Actinomycetes

Ramoplanin is a glycolipodepsipeptide antibiotic obtained from fermentation of Actinoplanes sp. ATCC 33076 that exhibits activity against clinically important multi-drug-resistant, Gram-positive pathogens including vancomycin-resistant Enterococcus (VRE), methicillin-resistant Staphylococcus aureus (MRSA), and vancomycin-intermediate resistant Clostridium difficile. It disrupts bacterial cell wall through a unique mechanism of action by sequestering the peptidoglycan intermediate Lipid II and therefore does not show cross-resistance with other antibiotics. However, while demonstrating excellent antimicrobial activity in systemic use in animal models of infection, ramoplanin presents low local tolerability when injected intravenously. As a consequence of this limitation, new derivatives are desirable to overcome this issue. During a natural product screening program developed to discover compounds that disrupt bacterial cell wall synthesis by inhibiting peptidoglycan transglycosylation through binding to the intermediate Lipid II, 49 actinomycete strains were identified by HR-LCMS as producers of ramoplanin-related compounds. The producing strains were isolated from environmental samples collected worldwide comprising both tropical and temperate areas. To assess the diversity of this microbial population, the 49 isolates were initially identified to the genus level on the basis of their micromorphology, and 16S sequencing confirmed the initial identification of the strains. These analyses resulted in the identification of members of genus Streptomyces, as well as representatives of the families Micromonosporaceae, Nocardiaceae, Thermomonosporaceae, and Pseudonocardiaceae, suggesting that the production of ramoplanins is relatively widespread among Actinomycetes. In addition, all of these isolates were tested against a panel of Gram-positive and Gram-negative bacteria, filamentous fungi, and yeast in order to further characterize their antimicrobial properties. This work describes the diversity of actinomycete strains that produced ramoplanin-related compounds, and the analysis of the antimicrobial activity exhibited by these isolates. Our results strongly suggest the presence of new ramoplanin-analogs among these actinomycete producers.

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.

Replacement of Soybean Meal with Animal Origin Protein Meals Improved Ramoplanin A2 Production by Actinoplanes sp. ATCC 33076

Ramoplanin A2 is the last resort antibiotic for treatment of many high morbidity- and mortality-rated hospital infections, and it is expected to be marketed in the forthcoming years. Therefore, high-yield production of ramoplanin A2 gains importance. In this study, meat-bone meal, poultry meal, and fish meal were used instead of soybean meal for ramoplanin A2 production by Actinoplanes sp. ATCC 33076. All animal origin nitrogen sources stimulated specific productivity. Ramoplanin A2 levels were determined as 406.805 mg L(-1) in fish meal medium and 374.218 mg L(-1) in poultry meal medium. These levels were 4.25- and 4.09-fold of basal medium, respectively. However, the total yield of poultry meal was higher than that of fish meal, which is also low-priced. In addition, the variations in pH levels, protein levels, reducing sugar levels, extracellular protease, amylase and lipase activities, and intracellular free amino acid levels were monitored during the incubation period. The correlations between ramoplanin production and these variables with respect to the incubation period were determined. The intracellular levels of L-Phe, D-Orn, and L-Leu were found critical for ramoplanin A2 production. The strategy of using animal origin nitrogen sources can be applied for large-scale ramoplanin A2 production.

Clostridium difficile colitis: review of the therapeutic approach

Clostridium difficile infection (CDI) is the leading cause of antibiotic-associated and nosocomial infectious diarrhea. Presenting as clostridium difficile colitis, it is a significant cause of morbidity and mortality. Metronidazole is regarded as the agent of choice for CDl therapy and also for the first recurrence in most patients with mild to moderate CDI. Vancomycin is recommended as an initial therapy for patients with severe CDI. With recent Food and Drug Administration-approval fidaxomicin is available for clinical use and is as effective as vancomycin with lower relapse rates. Rifaximin and fecal bacteriotherapy are alternative approaches in patients with severe or refractory CDI, before surgical intervention. Antibiotic research is ongoing to add potential new drugs such as teicoplanin, ramoplanin, fusidic acid, nitazoxanide, rifampin, bacitracin to our armamentarium. Role of toxin-binding agents is still questionable. Monoclonal antibody and intravenous immunoglobulin are still investigational therapies that could be promising options. The ongoing challenges in the treatment of CDI include management of recurrence and presence of resistance strains such as NAP1/BI/027, but early recognition of surgical candidates can potentially decrease mortality in CDI.

Ambush of Clostridium difficile spores by ramoplanin: activity in an in vitro model

Clostridium difficile infection (CDI) is a gastrointestinal disease caused by C. difficile, a spore-forming bacterium that in its spore form is tolerant to standard antimicrobials. Ramoplanin is a glycolipodepsipeptide antibiotic that is active against C. difficile with MICs ranging from 0.25 to 0.50 μg/ml. The activity of ramoplanin against the spores of C. difficile has not been well characterized; such activity, however, may hold promise, since posttreatment residual intraluminal spores are likely elements of disease relapse, which can impact more than 20% of patients who are successfully treated. C. difficile spores were found to be stable in deionized water for 6 days. In vitro spore counts were consistently below the level of detection for 28 days after even brief (30-min) exposure to ramoplanin at concentrations found in feces (300 μg/ml). In contrast, suppression of spore counts was not observed for metronidazole or vancomycin at human fecal concentrations during treatment (10 μg/ml and 500 μg/ml, respectively). Removal of the C. difficile exosporium resulted in an increase in spore counts after exposure to 300 μg/ml of ramoplanin. Therefore, we propose that rather than being directly sporicidal, ramoplanin adheres to the exosporium for a prolonged period, during which time it is available to attack germinating cells. This action, in conjunction with its already established bactericidal activity against vegetative C. difficile forms, supports further evaluation of ramoplanin for the prevention of relapse after C. difficile infection in patients.

Structural aspects of phenylglycines, their biosynthesis and occurrence in peptide natural products

Phenylglycine-type amino acids occur in a wide variety of peptide natural products. Herein structures and properties of these peptides as well as the biosynthetic origin and incorporation of phenylglycines are discussed.

Ramoplanin at bactericidal concentrations induces bacterial membrane depolarization in Staphylococcus aureus

Ramoplanin is an actinomycetes-derived antibiotic with broad-spectrum activity against Gram-positive bacteria that has been evaluated in clinical trials for the treatment of gastrointestinal vancomycin-resistant enterococci (VRE) and Clostridium difficile infections. Recent studies have proposed that ramoplanin binds to bacterial membranes as a C2 symmetrical dimer that can sequester Lipid II, which causes inhibition of cell wall peptidoglycan biosynthesis and cell death. In this study, ramoplanin was shown to bind to anionic and zwitterionic membrane mimetics with a higher affinity for anionic membranes and to induce membrane depolarization of methicillin-susceptible Staphylococcus aureus (MSSA) ATCC 25923 at concentrations at or above the minimal bactericidal concentration (MBC). The ultrastructural effects of ramoplanin on S. aureus were also examined by transmission electron microscopy (TEM), and this showed dramatic changes to bacterial cell morphology. The correlation observed between membrane depolarization and bacterial cell viability suggests that this mechanism may contribute to the bactericidal activity of ramoplanin.

Inactivation of the phosphoglucomutase gene pgm in Paenibacillus polymyxa leads to overproduction of fusaricidin

Fusaricidin, a lipodepsipeptide isolated from Paenibacillus polymyxa, has high antimicrobial activity against fungi and Gram-positive bacteria. Through mutagenesis, we obtained two mutant strains, N1U7 and N17U7, which produce 6.2- to 7.9-fold more fusaricidin than their parent strain. Causal mutations were identified by whole-genome sequencing, and the two strains each contained at least eleven point mutations, including four common mutations. A mutation in the PPE04441 gene (pgm), encoding an α-phosphoglucomutase, was found to be an important factor in fusaricidin overproduction by complementation experiments. Null mutation of pgm in the parental strain increased fusaricidin production by 5.2-fold. Increased growth and cell viability in stationary phase, reduced exopolysaccharide production, and increased fusA expression were observed in the pgm mutant strains, which might be related to fusaricidin overproduction. This is the first report revealing that PGM deficiency leads to an overproduction of fusaricidin.

Ramoplanin: a topical lipoglycodepsipeptide antibacterial agent

Ramoplanin, a novel antibiotic with activity against aerobic and anaerobic gram-positive bacteria, acts to prevent cell wall peptidoglycan formation by binding to a key intermediate moiety, lipid II. It has been fast-tracked by the US FDA for the prevention of enterococcal infections and the treatment of Clostridium difficile. The minimum inhibitory concentration(90s) have been < or = 1.0 microg/ml against gram-positive organisms examined. In carriers of vancomycin-resistant enterococci, a double-blind, placebo-controlled Phase II trial of two doses of ramoplanin versus placebo showed proof of concept. A second Phase II trial also demonstrated the equivalence of ramoplanin compared with vancomycin for the treatment of C. difficile colitis. The clinical value and place in therapy of ramoplanin is dependent upon the results of Phase III trials addressing its utility in suppressing carriage of target organisms in the gastrointestinal tract or in the nares.

Biodiversity in production of antibiotics and other bioactive compounds

Microbes continue to play a highly considerable role in the drug discovery and development process. Nevertheless, the number of new chemical entities (NCEs) of microbial origin that has been approved by the Food and Drug Administration (FDA) has been reduced in the past decade. This scarcity can be partly attributed to the redundancy in the discovered molecules from microbial isolates, which are isolated from common terrestrial ecological units. However, this situation can be partly overcome by exploring rarely exploited ecological niches as the source of microbes, which reduces the chances of isolating compounds similar to existing ones. The use of modern and advanced isolation techniques, modification of the existing fermentation methods, genetic modifications to induce expression of silent genes, analytical tools for the detection and identification of new chemical entities, use of polymers in fermentation to enhance yield of fermented compounds, and so on, have all aided in enhancing the frequency of acquiring novel compounds. These compounds are representative of numerous classes of diverse compounds. Thus, compounds of microbial origin and their analogues undergoing clinical trials continue to demonstrate the importance of compounds from microbial sources in modern drug discovery.

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.

Treatment of Clostridium difficile infection: recent trial results

Clostridium difficile is a major cause of infection worldwide and is associated with increasing morbidity and mortality in vulnerable patient populations. Metronidazole and oral vancomycin are the currently recommended therapies for the treatment of C. difficile infection (CDI) but are associated with unacceptably high rates of disease recurrence. Novel therapies for the treatment of CDI and prevention of recurrent CDI are urgently needed. Important developments in the treatment of CDI are currently underway and include: novel antibacterial agents with narrower antimicrobial spectra of activity, manipulation of the gut microbiota and enhancement of the host antibody immune response.

Nonproteinogenic amino acid building blocks for nonribosomal peptide and hybrid polyketide scaffolds

Freestanding nonproteinogenic amino acids have long been recognized for their antimetabolite properties and tendency to be uncovered to reactive functionalities by the catalytic action of target enzymes. By installing them regiospecifically into biogenic peptides and proteins, it may be possible to usher a new era at the interface between small molecule and large molecule medicinal chemistry. Site-selective protein functionalization offers uniquely attractive strategies for posttranslational modification of proteins. Last, but not least, many of the amino acids not selected by nature for protein incorporation offer rich architectural possibilities in the context of ribosomally derived polypeptides. This Review summarizes the biosynthetic routes to and metabolic logic for the major classes of the noncanonical amino acid building blocks that end up in both nonribosomal peptide frameworks and in hybrid nonribosomal peptide-polyketide scaffolds.

Cyclic lipodepsipeptides: a new class of antibacterial agents in the battle against resistant bacteria

In order to provide effective treatment options for infections caused by multidrug-resistant bacteria, innovative antibiotics are necessary, preferably with novel modes of action and/or belonging to novel classes of drugs. Naturally occurring cyclic lipodepsipeptides, which contain one or more ester bonds along with the amide bonds, have emerged as promising candidates for the development of new antibiotics. Some of these natural products are either already marketed or in advanced stages of clinical development. However, despite the progress in the development of new antibacterial agents, it is inevitable that resistant strains of bacteria will emerge in response to the widespread use of a particular antibiotic and limit its lifetime. Therefore, development of new antibiotics remains our most efficient way to counteract bacterial resistance.

Emerging therapies for Clostridium difficile infection - focus on fidaxomicin

The epidemiology of Clostridium difficile infections (CDI) has evolved during the last decades, with an increase in the reported incidence, severity of cases, and rate of mortality and relapses. These increases have primarily affected some special populations including the elderly, patients requiring concomitant antibiotic therapy, patients with renal failure, and patients with cancer. Until recently, the treatment of CDI was limited to either metronidazole or vancomycin. New therapeutic options have emerged to address the shortcomings of current antibiotic therapy. Fidaxomicin stands out as the first-in-class oral macrocyclic antibiotic with targeted activity against C. difficile and minimal collateral damage on the normal colonic flora. Fidaxomicin has demonstrated performance not inferior to what is considered the "gold standard" available therapy for CDI, vancomycin, in two separate Phase III clinical trials, but with significant advantages, including fewer recurrences and higher rates of sustained clinical cures. Fidaxomicin constitutes an important development in targeted antibiotic therapy for CDI and must be considered as a first-line agent for patients with risk factors known to portend relapse and severe infection.

New advances in the treatment of Clostridium difficile infection (CDI)

Clostridium difficile infections (CDI) have increased in frequency throughout the world. In addition to an increase in frequency, recent CDI epidemics have been linked to a hypervirulent C. difficile strain resulting in greater severity of disease. Although most mild to moderate cases of CDI continue to respond to metronidazole or vancomycin, refractory and recurrent cases of CDI may require alternative therapies. This review provides a brief overview of CDI and summarizes studies involving alternative antibiotics, toxin binders, probiotics, and immunological therapies that can be considered for treatment of acute and recurrent CDI in severe and refractory situations.

Toxin-specific antibodies for the treatment of Clostridium difficile: current status and future perspectives

Therapeutic agents targeting bacterial virulence factors are gaining interest as non-antibiotic alternatives for the treatment of infectious diseases. Clostridium difficile is a Gram-positive pathogen that produces two primary virulence factors, enterotoxins A and B (TcdA and TcdB), which are responsible for Clostridium difficile-associated disease (CDAD) and are targets for CDAD therapy. Antibodies specific for TcdA and TcdB have been shown to effectively treat CDAD and prevent disease relapse in animal models and in humans. This review summarizes the various toxin-specific antibody formats and strategies under development, and discusses future directions for CDAD immunotherapy, including the use of engineered antibody fragments with robust biophysical properties for systemic and oral delivery.

Vancomycin resistance: are there better glycopeptides coming?

The appearance and dissemination of vancomycin resistance among clinically important Gram-positive bacteria was an important watershed in antimicrobial resistance trends that drastically narrows therapeutic options, particularly among the enterococci. Clinical resistance despite apparent susceptibility has also become an increasingly recognized issue with vancomycin treatment of methicillin-resistant Staphylococcus aureus pneumonia and endocarditis, which may be, in part, due to vancomycin-heteroresistant strains. The newly developed glycopeptides telavancin, dalbavancin and oritavancin have superior in vitro activity, enhanced bactericidality and unique pharmacokinetic properties compared with vancomycin and teicoplanin. Current clinical trial data show noninferiority to vancomycin or standard-of-care antistaphylococcal therapy for complicated skin-skin structure infections, and acceptable safety profiles. Although promising, whether or not these new compounds are clinically efficacious for the true therapeutic deficits created by in vitro and clinical vancomycin resistance is yet to be determined.

New agents for Clostridium difficile-associated disease

BACKGROUND

Clostridia-derived diseases, in particular C. difficile-associated disease (CDAD), have been increasing in incidence, severity, and morbidity. The mainstay of treatment options has relied upon metronidazole and vancomycin, but these treatments routinely result in high relapse rates (20%) and, in the case of metronidazole, decreasing efficacy.

OBJECTIVE

Evaluate and compare the current clinical and preclinical therapies of CDAD.

METHODS

RESULTS/CONCLUSION

The new antibiotics in development and preclinical development reflect next-generation versions of older drugs or two new mechanism-of-action class drugs (OPT-80, REP3123). Based on the current preclinical and clinical data, the next-generation drugs impart only a subtle difference from the intrinsic weaknesses of their genre. In contrast, OPT-80 and REP3123 seem to be differentiated.

Multi-targeting by monotherapeutic antibacterials

Antibacterial discovery research has been driven, medically, commercially and intellectually, by the need for new therapeutics that are not subject to the resistance mechanisms that have evolved to combat previous generations of antibacterial agents. This need has often been equated with the identification and exploitation of novel targets. But efforts towards discovery and development of inhibitors of novel targets have proved frustrating. It might be that the 'good old targets' are qualitatively different from the crop of all possible novel targets. What has been learned from existing targets that can be applied to the quest for new antibacterials?

Antimicrobial therapy of Clostridium difficile-associated diarrhea

Clostridium difficile-associated diarrhea (CDAD) is the most common etiologically-defined cause of hospital-acquired diarrhea. Caused by the toxins of certain strains of C difficile, CDAD represents a growing concern, with epidemic outbreaks in some hospitals where very aggressive and difficult-to-treat strains have recently been found. Incidence of CDAD varies ordinarily between 1 to 10 in every 1,000 admissions. Evidence shows that CDAD increases morbidity, length of stay, and costs. This article described the clinical manifestations of CDAD, related risk factors, considerations for confirming CDAD, antimicrobial and non-antimicrobial treatment of CDAD, and issues related to relapses. The article concludes with a discussion of recent epidemic outbreaks involving CDAD.

Mechanisms of daptomycin resistance in Staphylococcus aureus

Daptomycin resistance in Staphylococcus aureus emerged during therapy of tricuspid endocarditis. Susceptibility to daptomycin of the parent strain (SA-675), other daptomycin-susceptible strains and the non-susceptible mutant (SA-684) was heterogeneous; however, subpopulations growing at concentrations above the minimum inhibitory concentration (MIC) were not stably resistant. Stable resistance was produced only by serial passage on daptomycin-containing media. Daptomycin dissipated the membrane potential of SA-675 but not SA-684, which also lost an 81 kDa membrane protein. Whole cells and membranes of SA-684 bound a reduced amount of daptomycin. Reduced drug binding in SA-684 correlates with daptomycin resistance, possibly as a result of the loss of a membrane protein 'chaperone' with which daptomycin interacts. Heterogeneity of daptomycin MICs in susceptible strains may be an important factor in the development of stable, clinically relevant resistance.

Lipid II as a target for antibiotics

Lipid II is a membrane-anchored cell-wall precursor that is essential for bacterial cell-wall biosynthesis. The effectiveness of targeting Lipid II as an antibacterial strategy is highlighted by the fact that it is the target for at least four different classes of antibiotic, including the clinically important glycopeptide antibiotic vancomycin. However, the growing problem of bacterial resistance to many current drugs, including vancomycin, has led to increasing interest in the therapeutic potential of other classes of compound that target Lipid II. Here, we review progress in understanding of the antibacterial activities of these compounds, which include lantibiotics, mannopeptimycins and ramoplanin, and consider factors that will be important in exploiting their potential as new treatments for bacterial infections.

Antibacterial drug discovery—Then, now and the genomics future

Drug discovery research in the area of infectious diseases, in particular that dealing with antibacterial/antibiotic susceptibility and resistance, is in a process of continuing evolution. Steeped in the history of the highly successful intervention with chemotherapeutic agents to treat human infections, the emergence of drug-resistant pathogens worldwide presents a serious unmet medical need, if not a pending catastrophe. Research in both academia and industry over the past 30 years using molecular biology, genetics and more recently--bacterial genomics--has assembled key enabling technologies to increase productivity and success rates in the discovery and development of novel antibacterial agents. However genomics is not limited only to antibacterial target selection but provides the opportunity to further understand key interactions in the use of antibacterial compounds as therapeutic agents (such as resistance emergence, susceptibility, efflux, interactions between compound and pathogen, etc.). Genomics also offers the potential for insights into: bacterial niche adaptation, host susceptibility, treatment regimens, antibiotic resistance, pharmacokinetics (e.g., host metabolism differences), safety and the microbial genesis of chronic diseases (e.g., gastric ulceration).

Natural products--the future scaffolds for novel antibiotics?

Natural products have played a pivotal role in antibiotic drug discovery with most antibacterial drugs being derived from a natural product or natural product lead. However, the rapid onset of resistance to most antibacterial drugs diminishes their effectiveness considerably and necessitates a constant supply of new antibiotics for effective treatment of infections. The natural product templates of actinonin, pleuromutilin, ramoplanin and tiacumicin B, which are compounds undergoing clinical evaluation, represent templates not found in currently marketed antibacterial drugs. In addition, the new templates present in the recently discovered lead antibacterials arylomycin, GE23077, mannopeptimycin, muraymycin/caprazamycin, nocathiacin and ECO-0501, are discussed. Despite extensive efforts to identify antibiotic leads from molecular targets, only the peptide deformylase inhibitor LBM-415 is currently in clinical trials. It is proposed that new antibacterial assays which combine cell-based screening with molecular targets could offer better prospects for lead discovery.