Clostridium difficile drug pipeline: challenges in discovery and development of new agents

The role of the methoxy group in approved drugs

The methoxy substituent is prevalent in natural products and, consequently, is present in many natural product-derived drugs. It has also been installed in modern drug molecules with no remnant of natural product features because medicinal chemists have been taking advantage of the benefits that this small functional group can bestow on ligand-target binding, physicochemical properties, and ADME parameters. Herein, over 230 methoxy-containing small-molecule drugs, as well as several fluoromethoxy-containing drugs, are presented from the vantage point of the methoxy group. Biochemical mechanisms of action, medicinal chemistry SAR studies, and numerous X-ray cocrystal structures are analyzed to identify the precise role of the methoxy group for many of the drugs and drug classes. Although the methoxy substituent can be considered as the hybridization of a hydroxy and a methyl group, the combination of these functionalities often results in unique effects that can amount to more than the sum of the individual parts.

Antibiotic Resistances of Clostridioides difficile

The rapid evolution of antibiotic resistance in Clostridioides difficile and the consequent effects on prevention and treatment of C. difficile infections (CDIs) are a matter of concern for public health. Antibiotic resistance plays an important role in driving C. difficile epidemiology. Emergence of new types is often associated with the emergence of new resistances, and most of the epidemic C. difficile clinical isolates is currently resistant to multiple antibiotics. In particular, it is to worth to note the recent identification of strains with reduced susceptibility to the first-line antibiotics for CDI treatment and/or for relapsing infections. Antibiotic resistance in C. difficile has a multifactorial nature. Acquisition of genetic elements and alterations of the antibiotic target sites, as well as other factors, such as variations in the metabolic pathways or biofilm production, contribute to the survival of this pathogen in the presence of antibiotics. Different transfer mechanisms facilitate the spread of mobile elements among C. difficile strains and between C. difficile and other species. Furthermore, data indicate that both genetic elements and alterations in the antibiotic targets can be maintained in C. difficile regardless of the burden imposed on fitness, and therefore resistances may persist in C. difficile population in absence of antibiotic selective pressure.

Identification of Micrococcin P2-Derivatives as Antibiotic Candidates against Two Gram-Positive Pathogens

Thiopeptides exhibit potent antimicrobial activity against Gram-positive pathogens by inhibiting bacterial protein synthesis. Micrococcins are among the structurally simpler thiopeptides, but they have not been exploited in detail. This research involved a computational simulation of micrococcin P2 (MP2) docking in parallel with the structure-activity relationship (SAR) studied. The incorporation of particular nitrogen heterocycles in the side chain of MP2 enhances the antimicrobial activity. Micrococcin analogues 6c and 6d thus proved to be more effective against impetigo and C. difficile infection (CDI), respectively, as compared to current first-line treatments. Compound 6c also showed a shorter treatment period than that of a first-line treatment for impetigo. This may be attributed to its ability to downregulate pro-inflammatory cytokines. Compound 6d had no observed recurrence for C. difficile and exerted a minimal impact on the beneficial gut microbiome. Their pharmacokinetic properties and low toxicity profile make these compounds ideal candidates for the treatment of impetigo and CDI and validate their involvement in preclinical development.

A Potent and Narrow-Spectrum Antibacterial against Clostridioides difficile Infection

Clostridioides difficile is an anaerobic Gram-positive bacterium that colonizes the gut of patients treated with broad-spectrum antibiotics. The normal gut microflora prevents C. difficile colonization; however, dysbiosis by treatment with broad-spectrum antibiotics causes recurrent C. difficile infection (CDI) in 25% of patients. There are no fully effective antibiotics for multiple recurrent CDIs. We report herein that oxadiazole antibiotics exhibit bactericidal activity against C. difficile vegetative cells. We screened a library of 75 oxadiazoles against C. difficile ATCC 43255. The findings from this collection served as the basis for the syntheses of an additional 58 analogs, which were tested against the same strain. We report a potent (MIC50 = 0.5 μg/mL and MIC90 = 1 μg/mL values for 101 C. difficile strains) and narrow-spectrum oxadiazole (3-(4-(cyclopentyloxy)phenyl)-5-(4-nitro-1H-imidazol-2-yl)-1,2,4-oxadiazole; compound 57), which is not active against common gut bacteria or other tested organisms. Compound 57 is selectively bactericidal against C. difficile and targets cell-wall synthesis.

Amidinoquinoxaline N-oxides: synthesis and activity against anaerobic bacteria

We present herein an in-depth study on the activity of amidinoquinoxaline N-oxides 1 against Gram-positive and Gram-negative anaerobic bacteria. Based on 5-phenyl-2,3-dihydropyrimidoquinoxaline N-oxide 1a, the selected structural variations included in our study comprise the substituents α- to the N-oxide function, the benzofused ring, substitution and quaternization of the amidine moiety, and the amidine ring size. Compounds 1 showed good to excellent antianaerobic activity, evaluated as the corresponding CIM50 and CIM90 values, and an antimicrobial spectrum similar to metronidazole. Six out of 13 compounds 1 had CIM90 values significantly lower than the reference drug. Among them, imidazoline derivatives 1i-l were the most active structures. Such compounds were synthesized by base-promoted ring closure of the corresponding amidines. The N-oxides under study showed no significant cytotoxicity against RAW 264.7 cells, with high selectivity indexes. Their calculated ADME properties indicate that the compounds are potentially good oral drug candidates. The antianaerobic activity correlated satisfactorily with the electron affinity of the compounds, suggesting that they may undergo bioreductive activation before exerting their antibacterial activity.

Design, Synthesis and Biological Evaluation of Novel PEG-Rakicidin B1 Hybrid as Clostridium difficile (CD) Targeted Anti-Bacterial Agent

Rakicidin B1 was isolated and purified from the culture broth of a marine Streptomyces sp. as a potent anti-cancer agent, and lately the compound and its derivatives have firstly been found to possess anti-Clostridium difficile (CD) activity but with high cytotoxicity. Herein, following our previous discovery on anti-CD activity of Rakicidin B1, structure modification was performed at the OH position of Rakicidin B1 and a new Rakicidin B1-PEG hybrids FIMP2 was facilely designed and synthesized by conjugating the PEG2000 with the scaffolds of Rakicidin B1 via the linkage of carbamate. The cytotoxicity of the FIMP2 was first evaluated against three different cancer cell lines, including HCT-8 cells, PANC-1, and Caco-2, with IC50 values at 0.519 μM, 0.815 μM, and 0.586 μM, respectively. Obviously, as compared with a positive control group treated with Rakicidin B1, the IC50 value of FIMP2 increased by nearly 91-fold, 50-fold, and 67-fold, suggesting that the PEGylation strategy significantly reduced the cytotoxicity of FIMP2. Thus, this preliminary result may be beneficial to increase its safety index (SI) value due to the decreased cytotoxicity of FIMP2. In addition, this decreased cytotoxicity of FIMP2 was further confirmed based on a zebrafish screening model in vivo. Thereafter, the anti-CD activity of FIMP2 was evaluated in vivo, and its efficacy to treat CDI was found to be better than that of vancomycin. The mortality and recurrence rate of FIMP2 is not as low compared with that of vancomycin; these results demonstrated that compound FIMP2 is a new, promising anti-CD agent with significant efficacy against CD recurrence with low cytotoxicity towards bodies.

CDBN-YGXZ, a Novel Small-Molecule Drug, Shows Efficacy against Clostridioides difficile Infection and Recurrence in Mouse and Hamster Infection Models

Clostridioides difficile infection (CDI) causes severe diarrhea and colitis, leading to significant morbidity, mortality, and high medical costs worldwide. Oral vancomycin, a first-line treatment for CDI, is associated with a high risk of recurrence, necessitating novel therapies for primary and recurrent CDI. A novel small-molecule compound, CDBN-YGXZ, was synthesized by modifying the benzene ring of nitazoxanide with lauric acid. The mechanism of action of CDBN-YGXZ was validated using a pyruvate:ferredoxin/flavodoxin oxidoreductase (PFOR) inhibition assay. The efficacy of CDBN-YGXZ was evaluated using the MIC test and CDI infection model in mice and hamsters. Furthermore, metagenomics was used to reveal the underlying reasons for the effective reduction or prevention of CDI after CDBN-YGXZ treatment. The inhibitory activity against PFOR induced by CDBN-YGXZ. MIC tests showed that the in vitro activity of CDBN-YGXZ against C. difficile ranging from 0.1 to 1.5 μg/mL. In the mouse and hamster CDI models, CDBN-YGXZ provided protection during both treatment and relapse, while vancomycin treatment resulted in severe relapse and significant clinical scores. Compared with global effects on the indigenous gut microbiota induced by vancomycin, CDBN-YGXZ treatment had a mild influence on gut microbes, thus resulting in the disappearance or reduction of CDI recurrence. CDBN-YGXZ displayed potent activity against C. difficile in vitro and in vivo, reducing or preventing relapse in infected animals, which could merit further development as a potential drug candidate for treating CDI.

Micrococcin P2 Targets Clostridioides difficile

Clostridioides difficile infection is a global public health threat. Extensive in vitro assays using clinical isolates have identified micrococcin P2 (MP2, 1) as a particularly effective anti-C. difficile agent. MP2 possesses a mode of action that differs from other antibiotics and pharmacokinetic properties that render it especially promising. Its time-kill studies have been investigated using hypervirulent C. difficile ribotype 027. DSS (dextran sulfate sodium)-induced in vivo mouse studies with that strain indicate that 1 is better than vancomycin and fidaxomicin. Thus, micrococcin P2 is a valuable platform to be exploited for the development of new anti-C. difficile antibiotics.

“Omic” Approaches to Bacteria and Antibiotic Resistance Identification

The quick and accurate identification of microorganisms and the study of resistance to antibiotics is crucial in the economic and industrial fields along with medicine. One of the fastest-growing identification methods is the spectrometric approach consisting in the matrix-assisted laser ionization/desorption using a time-of-flight analyzer (MALDI-TOF MS), which has many advantages over conventional methods for the determination of microorganisms presented. Thanks to the use of a multiomic approach in the MALDI-TOF MS analysis, it is possible to obtain a broad spectrum of data allowing the identification of microorganisms, understanding their interactions and the analysis of antibiotic resistance mechanisms. In addition, the literature data indicate the possibility of a significant reduction in the time of the sample preparation and analysis time, which will enable a faster initiation of the treatment of patients. However, it is still necessary to improve the process of identifying and supplementing the existing databases along with creating new ones. This review summarizes the use of “-omics” approaches in the MALDI TOF MS analysis, including in bacterial identification and antibiotic resistance mechanisms analysis.

Risk Factors, Diagnosis, and Management of Clostridioides difficile Infection in Patients with Inflammatory Bowel Disease

Clostridioides difficile infection (CDI) and inflammatory bowel disease (IBD) are two pathologies that share a bidirectional causal nexus, as CDI is known to have an aggravating effect on IBD and IBD is a known risk factor for CDI. The colonic involvement in IBD not only renders the host more prone to an initial CDI development but also to further recurrences. Furthermore, IBD flares, which are predominantly set off by a CDI, not only create a need for therapy escalation but also prolong hospital stay. For these reasons, adequate and comprehensive management of CDI is of paramount importance in patients with IBD. Microbiological diagnosis, correct evaluation of clinical status, and consideration of different treatment options (from antibiotics and fecal microbiota transplantation to monoclonal antibodies) carry pivotal importance. Thus, the aim of this article is to review the risk factors, diagnosis, and management of CDI in patients with IBD.

Nitro-Group-Containing Thiopeptide Derivatives as Promising Agents to Target Clostridioides difficile

The US Centers for Disease Control and Prevention (CDC) lists Clostridioides difficile as an urgent bacterial threat. Yet, only two drugs, vancomycin and fidaxomicin, are approved by the FDA for the treatment of C. difficile infections as of this writing, while the global pipeline of new drugs is sparse at best. Thus, there is a clear and urgent need for new antibiotics against that organism. Herein, we disclose that AJ-024, a nitroimidazole derivative of a 26-membered thiopeptide, is a promising anti-C. difficile lead compound. Despite their unique mode of action, thiopeptides remain largely unexploited as anti-infective agents. AJ-024 combines potent in vitro activity against various strains of C. difficile with a noteworthy safety profile and desirable pharmacokinetic properties. Its time-kill kinetics against a hypervirulent C. difficile ribotype 027 and in vivo (mouse) efficacy compare favorably to vancomycin, and they define AJ-024 as a valuable platform for the development of new anti-C. difficile antibiotics.

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.

Frontiers in antibiotic alternatives for Clostridioides difficile infection

Clostridioides difficile (C. difficile) is a gram-positive, anaerobic spore-forming bacterium and a major cause of antibiotic-associated diarrhea. Humans are naturally resistant to C. difficile infection (CDI) owing to the protection provided by healthy gut microbiota. When the gut microbiota is disturbed, C. difficile can colonize, produce toxins, and manifest clinical symptoms, ranging from asymptomatic diarrhea and colitis to death. Despite the steady-if not rising-prevalence of CDI, it will certainly become more problematic in a world of antibiotic overuse and the post-antibiotic era. C. difficile is naturally resistant to most of the currently used antibiotics as it uses multiple resistance mechanisms. Therefore, current CDI treatment regimens are extremely limited to only a few antibiotics, which include vancomycin, fidaxomicin, and metronidazole. Therefore, one of the main challenges experienced by the scientific community is the development of alternative approaches to control and treat CDI. In this Frontier article, we collectively summarize recent advances in alternative treatment approaches for CDI. Over the past few years, several studies have reported on natural product-derived compounds, drug repurposing, high-throughput library screening, phage therapy, and fecal microbiota transplantation. We also include an update on vaccine development, pre- and pro-biotics for CDI, and toxin antidote approaches. These measures tackle CDI at every stage of disease pathology via multiple mechanisms. We also discuss the gaps and concerns in these developments. The next epidemic of CDI is not a matter of if but a matter of when. Therefore, being well-equipped with a collection of alternative therapeutics is necessary and should be prioritized.

Plague Prevention and Therapy: Perspectives on Current and Future Strategies

Plague, caused by the bacterial pathogen Yersinia pestis, is a vector-borne disease that has caused millions of human deaths over several centuries. Presently, human plague infections continue throughout the world. Transmission from one host to another relies mainly on infected flea bites, which can cause enlarged lymph nodes called buboes, followed by septicemic dissemination of the pathogen. Additionally, droplet inhalation after close contact with infected mammals can result in primary pneumonic plague. Here, we review research advances in the areas of vaccines and therapeutics for plague in context of Y. pestis virulence factors and disease pathogenesis. Plague continues to be both a public health threat and a biodefense concern and we highlight research that is important for infection mitigation and disease treatment.

Cationic Peptidomimetic Amphiphiles Having a N-Aryl- or N-Naphthyl-1,2,3-Triazole Core Structure Targeting Clostridioides (Clostridium) difficile: Synthesis, Antibacterial Evaluation, and an In Vivo C. difficile Infection Model

Clostridioides (also known as Clostridium) difficile is a Gram-positive anaerobic, spore producing bacterial pathogen that causes severe gastrointestinal infection in humans. The current chemotherapeutic options are inadequate, expensive, and limited, and thus inexpensive drug treatments for C. difficile infection (CDI) with improved efficacy and specificity are urgently needed. To improve the solubility of our cationic amphiphilic 1,1′-binaphthylpeptidomimetics developed earlier that showed promise in an in vivo murine CDI model we have synthesized related compounds with an N-arytriazole or N-naphthyltriazole moiety instead of the 1,1′-biphenyl or 1,1′-binaphthyl moiety. This modification was made to increase the polarity and thus water solubility of the overall peptidomimetics, while maintaining the aromatic character. The dicationic N-naphthyltriazole derivative 40 was identified as a C. difficile-selective antibacterial with MIC values of 8 µg/mL against C. difficile strains ATCC 700057 and 132 (both ribotype 027). This compound displayed increased water solubility and reduced hemolytic activity (32 µg/mL) in an in vitro hemolysis assay and reduced cytotoxicity (CC₅₀ 32 µg/mL against HEK293 cells) relative to lead compound 2. Compound 40 exhibited mild efficacy (with 80% survival observed after 24 h compared to the DMSO control of 40%) in an in vivo murine model of C. difficile infection by reducing the severity and slowing the onset of disease.

Pre-formulation and delivery strategies for the development of bacteriocins as next generation antibiotics

Bacteriocins, a class of antimicrobial peptide produced by bacteria, may offer a potential alternative to traditional antibiotics, an important step towards mitigating the ever-increasing antimicrobial resistance crisis. They are active against a range of clinically relevant Gram-positive and Gram-negative bacteria. Bacteriocins have been discussed in the literature for over a century. Although they are used as preservatives in food, no medicine based on their antimicrobial activity exists on the market today. In order to formulate them into clinical antibiotics, pre-formulation studies on their biophysical and physicochemical properties that will influence their activity in vivo and their stability during manufacture must be elucidated. Thermal, pH and enzymatic stability of bacteriocins are commonly studied and regularly reported in the literature. Solubility, permeability and aggregation properties on the other hand are less frequently reported for many bacteriocins, which may contribute to their poor clinical progression. Promising cytotoxicity studies report that bacteriocins exhibit few cytotoxic effects on a variety of mammalian cell lines, at active concentrations. This review highlights the lack of quantitative data and in many cases even qualitative data, on bacteriocins' solubility, stability, aggregation, permeability and cytotoxicity. The formulation strategies that have been explored to date, proposed routes of administration, trends in in vitro/in vivo behaviour and efforts in clinical development are discussed. The future promise of bacteriocins as a new generation of antibiotics may require tailored local delivery strategies to fulfil their potential as a force to combat antimicrobial-resistant bacterial infections.

Evolution of clinical guidelines for antimicrobial management of Clostridioides difficile infection

Clostridioides difficile infection (CDI) has been an epidemic for many years. Our biggest challenge in treating CDI is preventing recurrence, which is seen in approximately 25% of patients with initial infection and in 40-60% of those with subsequent episodes. Given the major disease burden of this infection, appropriate data-driven treatment remains essential. Clinical treatment guidelines provide an unbiased critical analysis of the literature, integrating the quality of the available data to make recommendations. As CDI has been evolving and more research has become available, the frequency of guideline issue from various global societies has increased, as has the detail of the recommendations to fit more relevant clinical scenarios. In this review, we will discuss clinical guideline recommendations over three time periods: The Initial Guidelines 1995-1997, The Second Wave 2009-2013, and The Modern Era 2014-present. We see the changing recommendations from metronidazole or vancomycin for initial infection during earlier times to preferential treatment with fidaxomicin within the Infectious Diseases Society of America (IDSA) and Society of Healthcare Epidemiology of America (SHEA) joint guidelines provisional update in late 2020. The recommended treatments for first recurrence were initially with the same antimicrobial as the first episode but have since changed to having multiple options for one or more recurrences. We have also seen the addition of immune boosting treatments, including fecal microbiota transplantation (FMT)/microbiota restoration therapy (MRT) and bezlotoxumab in the more modern recommendations. As the guidelines are evolving with the times, it remains important to understand the differences among them so we can apply this information clinically and optimize patient outcomes.

Computational modeling of the gut microbiota reveals putative metabolic mechanisms of recurrent Clostridioides difficile infection

Approximately 30% of patients who have Clostridioides difficile infection (CDI) will suffer at least one incident of reinfection. While the underlying causes of CDI recurrence are poorly understood, interactions between C. difficile and commensal gut bacteria are thought to play an important role. In this study, an in silico pipeline was used to process 16S rRNA gene amplicon sequence data of 225 stool samples from 93 CDI patients into sample-specific models of bacterial community metabolism. Clustered metabolite production rates generated from post-diagnosis samples generated a high Enterobacteriaceae abundance cluster containing disproportionately large numbers of recurrent samples and patients. This cluster was predicted to have significantly reduced capabilities for secondary bile acid synthesis but elevated capabilities for aromatic amino acid catabolism. When applied to 16S sequence data of 40 samples from fecal microbiota transplantation (FMT) patients suffering from recurrent CDI and their stool donors, the community modeling method generated a high Enterobacteriaceae abundance cluster with a disproportionate large number of pre-FMT samples. This cluster also was predicted to exhibit reduced secondary bile acid synthesis and elevated aromatic amino acid catabolism. Collectively, these in silico predictions suggest that Enterobacteriaceae may create a gut environment favorable for C. difficile spore germination and/or toxin synthesis.

New perspectives in oral peptide delivery

Owing to their structural diversity, peptides are a unique source of innovative active ingredients. However, their development has been challenging because of their disadvantageous pharmacokinetic (PK) properties. Over the past decade, many attempts have been made to improve the oral bioavailability of peptide drugs. In this review, we highlight the most recent and promising techniques aimed at the improvement of the oral bioavailability of peptides. The most recent findings will influence future approaches of pharmaceutical companies in the development of new, more efficient, and safer orally delivered peptides.

The Clostridioides difficile Cysteine-Rich Exosporium Morphogenetic Protein, CdeC, Exhibits Self-Assembly Properties That Lead to Organized Inclusion Bodies in Escherichia coli

The endospore of Clostridioides difficile is the vehicle for transmission and persistence of the pathogen, and, specifically, the exosporium is the first contact between the host and the spore. The underlying mechanisms that govern exosporium assembly in C. difficile remain understudied, in part due to difficulties in obtaining pure soluble recombinant proteins of the C. difficile exosporium. Understanding the exosporium assembly’s molecular bases may be essential to developing new therapies against C. difficile infection.

Clostridioides difficile is an obligately anaerobic, spore-forming, Gram-positive pathogenic bacterium that is considered the leading cause of nosocomial diarrhea worldwide. Recent studies have attempted to understand the biology of the outermost layer of C. difficile spores, the exosporium, which is believed to contribute to early interactions with the host. The fundamental role of the cysteine-rich proteins CdeC and CdeM has been described. However, the molecular details behind the mechanism of exosporium assembly are missing. The underlying mechanisms that govern exosporium assembly in C. difficile remain poorly studied, in part due to difficulties in obtaining pure soluble recombinant proteins of the C. difficile exosporium. In this work, we observed that CdeC was able to form organized inclusion bodies (IBs) in Escherichia coli filled with lamella-like structures separated by an interspace of 5 to 15 nm; however, CdeC expression in an E. coli strain with a more oxidative environment led to the loss of the lamella-like organization of CdeC IBs. Additionally, dithiothreitol (DTT) treatment of CdeC inclusion bodies released monomeric soluble forms of CdeC. Deletions in different portions of CdeC did not affect CdeC’s ability to aggregate and form oligomers stable under denaturation conditions but affected CdeC’s self-assembly properties. Overall, these observations have important implications in further studies elucidating the role of CdeC in the exosporium assembly of C. difficile spores.

IMPORTANCE The endospore of Clostridioides difficile is the vehicle for transmission and persistence of the pathogen, and, specifically, the exosporium is the first contact between the host and the spore. The underlying mechanisms that govern exosporium assembly in C. difficile remain understudied, in part due to difficulties in obtaining pure soluble recombinant proteins of the C. difficile exosporium. Understanding the exosporium assembly’s molecular bases may be essential to developing new therapies against C. difficile infection.

Translation-Targeting RiPPs and Where to Find Them

Prokaryotic translation is among the major targets of diverse natural products with antibacterial activity including several classes of clinically relevant antibiotics. In this review, we summarize the information about the structure, biosynthesis, and modes of action of translation inhibiting ribosomally synthesized and post-translationally modified peptides (RiPPs). Azol(in)e-containing RiPPs are known to target translation, and several new compounds inhibiting the ribosome have been characterized recently. We performed a systematic search for biosynthetic gene clusters (BGCs) of azol(in)e-containing RiPPs. This search uncovered several groups of clusters that likely direct the synthesis of novel compounds, some of which may be targeting the ribosome.

Aryl-alkyl-lysines: Novel agents for treatment of C. difficile infection

Clostridium difficile infections (CDIs) are a growing health concern worldwide. The recalcitrance of C. difficile spores to currently available treatments and concomitant virulence of vegetative cells has made it imperative to develop newer modalities of treatment. Aryl-alkyl-lysines have been earlier reported to possess antimicrobial activity against pathogenic bacteria, fungi, and parasites. Their broad spectrum of activity is attributed to their ability to infiltrate microbial membranes. Herein, we report the activity of aryl-alkyl-lysines against C. difficile and associated pathogens. The most active compound NCK-10 displayed activity comparable to the clinically-used antibiotic vancomycin. Indeed, against certain C. difficile strains, NCK-10 was more active than vancomycin in vitro. Additionally, NCK-10 exhibited limited permeation across the intestinal tract as assessed via a Caco-2 bidirectional permeability assay. Overall, the findings suggest aryl-alkyl-lysines warrant further investigation as novel agents to treat CDI.

Screening for potent and selective anticlostridial leads among FDA-approved drugs

Clostridium difficile is a leading cause of morbidity and mortality particularly in hospital settings. In addition, treatment is very challenging due to the scarcity of effective therapeutic options. Thus, there remains an unmet need to identify new therapeutic agents capable of treating C. difficile infections. In the current study, we screened two FDA-approved drug libraries against C. difficile. Out of almost 3200 drugs screened, 50 drugs were capable of inhibiting the growth of C. difficile. Remarkably, some of the potent inhibitors have never been reported before and showed activity in a clinically achievable range. Structure-activity relationship analysis of the active hits clustered the potent inhibitors into four chemical groups; nitroimidazoles (MIC₅₀ = 0.06-2.7 μM), salicylanilides (MIC₅₀ = 0.2-0.6 μM), imidazole antifungals (MIC₅₀ = 4.8-11.6 μM), and miscellaneous group (MIC₅₀ = 0.4-22.2 μM). The most potent drugs from the initial screening were further evaluated against additional clinically relevant strains of C. difficile. Moreover, we tested the activity of potent inhibitors against representative strains of human normal gut microbiota to investigate the selectivity of the inhibitors towards C. difficile. Overall, this study provides a platform that could be used for further development of potent and selective anticlostridial antibiotics.

Next-generation strategy for treating drug resistant bacteria: Antibiotic hybrids

Resistance against nearly all antibiotics used clinically have been documented in bacteria. There is an ever-increasing danger caused by multidrug-resistant Gram-negative bacteria in both hospital and community settings. In Gram-negative bacteria, intrinsic resistance to currently available antibiotics is mainly due to overexpressed efflux pumps which are constitutively present and also presence of protective outer membrane. Combination therapy, i.e., use of two or more antibiotics, was thought to be an effective strategy because it took advantage of the additive effects of multiple antimicrobial mechanisms, lower risk of resistance development and lower mortality and improved clinical outcome. However, none of the benefits were seen in in vivo studies. Antibiotic hybrids are being used to challenge the growing drug resistance threat and increase the usefulness of current antibiotic arsenal. Antibiotic hybrids are synthetic constructs of two molecules which are covalently linked. These could be two antibiotics or antibiotic with an adjuvant (efflux pump inhibitor, siderophore, etc.) which increases the access of the antibiotics to the target. The concepts, developments and challenges in the future use of antibiotic hybrids are discussed here. Majority of the studies have been conducted on fluoroquinolones and aminoglycosides molecules. The antibiotic tobramycin has the property to enhance the action of antimicrobial agents against which the multidrug-resistant Gram-negative bacteria were earlier resistant, and thus potentiating the action of legacy antibiotics. Antibiotic hybrids may have a role as the silver bullet in Gram-negative bacteria to overcome drug resistance as well as extend the spectrum of existing antibiotics.

Antibiotics: past, present and future

The first antibiotic, salvarsan, was deployed in 1910. In just over 100 years antibiotics have drastically changed modern medicine and extended the average human lifespan by 23 years. The discovery of penicillin in 1928 started the golden age of natural product antibiotic discovery that peaked in the mid-1950s. Since then, a gradual decline in antibiotic discovery and development and the evolution of drug resistance in many human pathogens has led to the current antimicrobial resistance crisis. Here we give an overview of the history of antibiotic discovery, the major classes of antibiotics and where they come from. We argue that the future of antibiotic discovery looks bright as new technologies such as genome mining and editing are deployed to discover new natural products with diverse bioactivities. We also report on the current state of antibiotic development, with 45 drugs currently going through the clinical trials pipeline, including several new classes with novel modes of action that are in phase 3 clinical trials. Overall, there are promising signs for antibiotic discovery, but changes in financial models are required to translate scientific advances into clinically approved antibiotics.

Chemical Space Exploration around Thieno[3,2-d]pyrimidin-4(3H)-one Scaffold Led to a Novel Class of Highly Active Clostridium difficile Inhibitors

Clostridium difficile infection (CDI) is the leading cause of healthcare-associated infection in the United States. Therefore, development of novel treatments for CDI is a high priority. Toward this goal, we began in vitro screening of a structurally diverse in-house library of 67 compounds against two pathogenic C. difficile strains (ATCC BAA 1870 and ATCC 43255), which yielded a hit compound, 2-methyl-8-nitroquinazolin-4(3H)-one (2) with moderate potency (MIC = 312/156 μM). Optimization of 2 gave lead compound 6a (2-methyl-7-nitrothieno[3,2-d]pyrimidin-4(3H)-one) with improved potency (MIC = 19/38 μM), selectivity over normal gut microflora, CC₅₀s > 606 μM against mammalian cell lines, and acceptable stability in simulated gastric and intestinal fluid. Further optimization of 6a at C2-, N3-, C4-, and C7-positions resulted in a library of >50 compounds with MICs ranging from 3 to 800 μM against clinical isolates of C. difficile. Compound 8f (MIC = 3/6 μM) was identified as a promising lead for further optimization.

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.

Metabolic Modeling of Clostridium difficile Associated Dysbiosis of the Gut Microbiota

Recent in vitro experiments have demonstrated the ability of the pathogen Clostridium difficile and commensal gut bacteria to form biofilms on surfaces, and biofilm development in vivo is likely. Various studies have reported that 3%–15% of healthy adults are asymptomatically colonized with C. difficile, with commensal species providing resistance against C. difficile pathogenic colonization. C. difficile infection (CDI) is observed at a higher rate in immunocompromised patients previously treated with broad spectrum antibiotics that disrupt the commensal microbiota and reduce competition for available nutrients, resulting in imbalance among commensal species and dysbiosis conducive to C. difficile propagation. To investigate the metabolic interactions of C. difficile with commensal species from the three dominant phyla in the human gut, we developed a multispecies biofilm model by combining genome-scale metabolic reconstructions of C. difficile, Bacteroides thetaiotaomicron from the phylum Bacteroidetes, Faecalibacterium prausnitzii from the phylum Firmicutes, and Escherichia coli from the phylum Proteobacteria. The biofilm model was used to identify gut nutrient conditions that resulted in C. difficile-associated dysbiosis characterized by large increases in C. difficile and E. coli abundances and large decreases in F. prausnitzii abundance. We tuned the model to produce species abundances and short-chain fatty acid levels consistent with available data for healthy individuals. The model predicted that experimentally-observed host-microbiota perturbations resulting in decreased carbohydrate/increased amino acid levels and/or increased primary bile acid levels would induce large increases in C. difficile abundance and decreases in F. prausnitzii abundance. By adding the experimentally-observed perturbation of increased host nitrate secretion, the model also was able to predict increased E. coli abundance associated with C. difficile dysbiosis. In addition to rationalizing known connections between nutrient levels and disease progression, the model generated hypotheses for future testing and has the capability to support the development of new treatment strategies for C. difficile gut infections.

The Fatty Acid Synthesis Protein Enoyl-ACP Reductase II (FabK) is a Target for Narrow-Spectrum Antibacterials for Clostridium difficile Infection

Clostridium difficile infection (CDI) is an antibiotic-induced microbiota shift disease of the large bowel. While there is a need for narrow-spectrum CDI antibiotics, it is unclear which cellular proteins are appropriate drug targets to specifically inhibit C. difficile. We evaluated the enoyl-acyl carrier protein (ACP) reductase II (FabK), which catalyzes the final step of bacterial fatty acid biosynthesis. Bioinformatics showed that C. difficile uses FabK as its sole enoyl-ACP reductase, unlike several major microbiota species. The essentiality of fabK for C. difficile growth was confirmed by failure to delete this gene using ClosTron mutagenesis and by growth inhibition upon gene silencing with CRISPR interference antisense to fabK transcription or by blocking protein translation. Inhibition of C. difficile's FASII pathway could not be circumvented by supply of exogenous fatty acids, either during fabK's gene silencing or upon inhibition of the enzyme with a phenylimidazole-derived inhibitor (1). The inability of fatty acids to bypass FASII inhibition is likely due to the function of the transcriptional repressor FapR. Inhibition of FabK also inhibited spore formation, reflecting the enzyme's role in de novo fatty acid biosynthesis for the formation of spore membrane lipids. Compound 1 did not inhibit growth of key microbiota species. These findings suggest that C. difficile FabK is a druggable target for discovering narrow-spectrum anti- C. difficile drugs that treat CDI but avoid collateral damage to the gut microbiota.

Clostridium difficile ClpP Homologues are Capable of Uncoupled Activity and Exhibit Different Levels of Susceptibility to Acyldepsipeptide Modulation

Caseinolytic protease P (ClpP) has emerged as a promising new target for antibacterial development. While ClpPs from single isoform expressing bacteria have been studied in detail, the function and regulation of systems with more than one ClpP homologue are still poorly understood. Herein, we present fundamental studies toward understanding the ClpP system in C. difficile, an anaerobic spore-forming pathogen that contains two chromosomally distant isoforms of ClpP. Examination of proteomic and genomic data suggest that ClpP1 is the primary isoform responsible for normal growth and virulence, but little is known about the function of ClpP2 or the context required for the formation of functional proteases. For the first time in a pathogenic bacterium, we demonstrate that both isoforms are capable of forming operative proteases. Interestingly, ClpP1 is the only homologue that possesses characteristic response to small molecule acyldepsipeptide activation. On the contrary, both ClpP1 and ClpP2 respond to cochaperone activation to degrade an ssrA-tagged substrate. These observations indicate that ClpP2 is less susceptible to acyldepsipeptide activation but retains the ability to interact with a known cochaperone. Homology models reveal no obvious characteristics that would allow one to predict less efficient acyldepsipeptide binding. The reported findings establish the uniqueness of the ClpP system in C. difficile, open new avenues of inquiry, and highlight the importance of more detailed structural, genetic, and biological characterization of the ClpP system in C. difficile.

Increased sporulation underpins adaptation of Clostridium difficile strain 630 to a biologically-relevant faecal environment, with implications for pathogenicity

Clostridium difficile virulence is driven primarily by the processes of toxinogenesis and sporulation, however many in vitro experimental systems for studying C. difficile physiology have arguably limited relevance to the human colonic environment. We therefore created a more physiologically–relevant model of the colonic milieu to study gut pathogen biology, incorporating human faecal water (FW) into growth media and assessing the physiological effects of this on C. difficile strain 630. We identified a novel set of C. difficile–derived metabolites in culture supernatants, including hexanoyl– and pentanoyl–amino acid derivatives by LC-MSⁿ. Growth of C. difficile strain 630 in FW media resulted in increased cell length without altering growth rate and RNA sequencing identified 889 transcripts as differentially expressed (p < 0.001). Significantly, up to 300–fold increases in the expression of sporulation–associated genes were observed in FW media–grown cells, along with reductions in motility and toxin genes’ expression. Moreover, the expression of classical stress–response genes did not change, showing that C. difficile is well–adapted to this faecal milieu. Using our novel approach we have shown that interaction with FW causes fundamental changes in C. difficile biology that will lead to increased disease transmissibility.

Comparison of the in vitro antibacterial activity of Ramizol, fidaxomicin, vancomycin, and metronidazole against 100 clinical isolates of Clostridium difficile by broth microdilution

Antibiotic drug development remains a major challenge with few candidates in clinical development. Ramizol, a first-in-class styrylbenzene antibiotic, is under development for the treatment of Clostridium difficile associated disease. Here, we investigate the in vitro antibacterial activity of Ramizol in comparison to fidaxomicin, vancomycin and metronidazole against 100 clinical isolates of C. difficile by the broth microdilution method. We show there is no apparent impact of ribotype, toxin-production, or resistance to fidaxomicin, vancomycin or metronidazole on the activity of Ramizol. Moreover, we show Ramizol has a narrower MIC range translating to potentially better control over the therapeutic dose. Together, these results support the further development of Ramizol for the treatment of C. difficile associated disease.

Novel FR-900493 Analogues That Inhibit the Outgrowth of Clostridium difficile Spores

The spectrum of antibacterial activity for the nucleoside antibiotic FR-900493 (1) can be extended by chemical modifications. We have generated a small focused library based on the structure of 1 and identified UT-17415 (9), UT-17455 (10), UT-17460 (11), and UT-17465 (12), which exhibit anti-Clostridium difficile growth inhibitory activity. These analogues also inhibit the outgrowth of C. difficile spores at 2× minimum inhibitory concentration. One of these analogues, 11, relative to 1 exhibits over 180-fold and 15-fold greater activity against the enzymes, phospho-MurNAc-pentapeptide translocase (MraY) and polyprenyl phosphate-GlcNAc-1-phosphate transferase (WecA), respectively. The phosphotransferase inhibitor 11 displays antimicrobial activity against several tested bacteria including Bacillus subtilis, Clostridium spp., and Mycobacterium smegmatis, but no growth inhibitory activity is observed against the other Gram-positive and Gram-negative bacteria. The selectivity index (Vero cell cytotoxicity/C. difficileantimicrobial activity) of 11 is approximately 17, and 11 does not induce hemolysis even at a 100 μM concentration.

Clostridium difficile Infections: A Global Overview of Drug Sensitivity and Resistance Mechanisms

Clostridium difficile (C. difficile) is the most prevalent causative pathogen of healthcare-associated diarrhea. Notably, over the past 10 years, the number of Clostridium difficile outbreaks has increased with the rate of morbidity and mortality. The occurrence and spread of C. difficile strains that are resistant to multiple antimicrobial drugs complicate prevention as well as potential treatment options. Most C. difficile isolates are still susceptible to metronidazole and vancomycin. Incidences of C. difficile resistance to other antimicrobial drugs have also been reported. Most of the antibiotics correlated with C. difficile infection (CDI), such as ampicillin, amoxicillin, cephalosporins, clindamycin, and fluoroquinolones, continue to be associated with the highest risk for CDI. Still, the detailed mechanism of resistance to metronidazole or vancomycin is not clear. Alternation in the target sites of the antibiotics is the main mechanism of erythromycin, fluoroquinolone, and rifamycin resistance in C. difficile. In this review, different antimicrobial agents are discussed and C. difficile resistance patterns and their mechanism of survival are summarized.

Post-translational modifications involved in the biosynthesis of thiopeptide antibiotics

Thiopeptide antibiotics are a class of typical ribosomally synthesized and post-translationally modified peptides (RiPPs) with complex chemical structures that are difficult to construct via chemical synthesis. To date, more than 100 thiopeptides have been discovered, and most of these compounds exhibit remarkable biological activities, such as antibacterial, antitumor and immunosuppressive activities. Therefore, studies of the biosynthesis of thiopeptides can contribute to the development of new drug leads and facilitate the understanding of the complex post-translational modifications (PTMs) of peptides and/or proteins. Since the biosynthetic gene clusters of thiopeptides were first discovered in 2009, several research studies regarding the biochemistry and enzymology of thiopeptide biosyntheses have been reported, indicating that their characteristic framework is constructed via a cascade of common PTMs and that additional specific PTMs diversify the molecules. In this review, we primarily summarize recent advances in understanding the biosynthesis of thiopeptide antibiotics and propose some potential applications based on our insights into the biosynthetic logic and machinery.

Identification and initial optimization of inhibitors of Clostridium difficile (C. difficile) toxin B (TcdB)

The discovery, synthesis and preliminary structure-activity relationship (SAR) of a novel class of inhibitors of Clostridium difficile (C. difficile) toxin B (TcdB) is described. A high throughput screening (HTS) campaign resulted in the identification of moderately active screening hits 1-5 the most potent of which was compound 1 (IC₅₀ = 0.77 µM). In silico docking of an early analog offered suggestions for structural modification which resulted in the design and synthesis of highly potent analogs 13j(IC₅₀ = 1 nM) and 13 l(IC₅₀ = 7 nM) which were chosen as leads for further optimization.

Antibiotic Resistances of Clostridium difficile

The rapid evolution of antibiotic resistance in Clostridium difficile and the consequent effects on prevention and treatment of C. difficile infections (CDIs) are matter of concern for public health. Antibiotic resistance plays an important role in driving C. difficile epidemiology. Emergence of new types is often associated with the emergence of new resistances and most of epidemic C. difficile clinical isolates is currently resistant to multiple antibiotics. In particular, it is to worth to note the recent identification of strains with reduced susceptibility to the first-line antibiotics for CDI treatment and/or for relapsing infections. Antibiotic resistance in C. difficile has a multifactorial nature. Acquisition of genetic elements and alterations of the antibiotic target sites, as well as other factors, such as variations in the metabolic pathways and biofilm production, contribute to the survival of this pathogen in the presence of antibiotics. Different transfer mechanisms facilitate the spread of mobile elements among C. difficile strains and between C. difficile and other species. Furthermore, recent data indicate that both genetic elements and alterations in the antibiotic targets can be maintained in C. difficile regardless of the burden imposed on fitness, and therefore resistances may persist in C. difficile population in absence of antibiotic selective pressure.

Nitroimidazoles: Molecular Fireworks That Combat a Broad Spectrum of Infectious Diseases

Infectious diseases claim millions of lives every year, but with the advent of drug resistance, therapeutic options to treat infections are inadequate. There is now an urgent need to develop new and effective treatments. Nitroimidazoles are a class of antimicrobial drugs that have remarkable broad spectrum activity against parasites, mycobacteria, and anaerobic Gram-positive and Gram-negative bacteria. While nitroimidazoles were discovered in the 1950s, there has been renewed interest in their therapeutic potential, particularly for the treatment of parasitic infections and tuberculosis. In this review, we summarize different classes of nitroimidazoles that have been described in the literature in the past five years, from approved drugs and clinical candidates to examples undergoing preclinical or early stage development. The relatively "nonspecific" mode of action and resistance mechanisms of nitromidazoles are discussed, and contemporary strategies to facilitate nitroimidazole drug development are highlighted.

Synthesis and Biological Evaluation of Bile Acid Analogues Inhibitory to Clostridium difficile Spore Germination

Standard antibiotic-based strategies for the treatment of Clostridium difficile infections disrupt indigenous microbiota and commonly fail to eradicate bacterial spores, two key factors that allow recurrence of infection. As an alternative approach to controlling C. difficile infection, a series of bile acid derivatives have been prepared that inhibit taurocholate-induced spore germination. These analogues have been evaluated in a highly virulent NAP1 strain using optical density and phase-contrast microscopy assays. Heterocycle substitutions at C24 were well-tolerated and several tetrazole-containing derivatives were highly potent inhibitors in both assays, with complete inhibition of spore germination observed at 10-25 μM. To limit intestinal absorption, C7-sulfated analogues designed to avoid active and passive transport pathways were prepared. One of these derivatives, compound 21b, was found to be a potent inhibitor of C. difficile spore germination and poorly permeable in a Caco-2 model of intestinal epithelial absorption, suggesting that it is likely to be gut-restricted.

Conventional and alternative treatment approaches for Clostridium difficile infection

Clostridium difficile-associated disease continues to be one of the leading health concerns worldwide. C. difficile is considered as a causative agent of nosocomial diarrhea that causes serious infection, which may result in death. The incidences of C. difficile infection (CDI) in developed countries have become increasingly high which may be attributed to the emergence of newer epidemic strains, extensive use of antibiotics, and limited alternative therapies. The available treatment options against CDI are expensive and promote resistance. Therefore, there is urgent need for new approaches to meet these challenges. This review discusses the current understanding of CDI, the existing clinical treatment strategies and future potential options as antidifficile agents based on the available published works.

Subtyping and antimicrobial susceptibility of Clostridium difficile PCR ribotype 078/126 isolates of human and animal origin

The Clostridium difficile PCR ribotype complex 078/126 (RT078/126) is often involved in human disease and is also frequently isolated from diverse animal species. The high genetic relatedness between human and animal RT078/126 isolates found in different regions has encouraged discussion about the zoonotic potential of this lineage. We compared for the first time the genetic diversity and antimicrobial susceptibility profiles of human and animal C. difficile RT078/126 isolates from Spain. A collection of 96 isolates (50 of human and 46 of animal origin; 63 and 33 of ribotypes 078 and 126, respectively) was subtyped by an improved amplified fragment length polymorphism (AFLP) fingerprinting method and tested for in vitro antimicrobial susceptibility. A total of 67 genotypes were distinguished, three of which grouped together isolates of human and animal origin. Furthermore, two main groups of isolates that mostly correlated with PCR ribotypes could be distinguished in the AFLP dendrogram. Human origin was significantly associated with resistance to ertapenem, erythromycin and moxifloxacin; resistance to clindamycin and erythromycin was associated with RT126 and AFLP group 1. Twenty-nine isolates (30.2% of total) displayed heteroresistance to metronidazole. Substantial differences were observed in the susceptibility profiles of isolates belonging to a same genotype. Altogether, these results provide a valuable baseline for future studies on the epidemiology of C. difficile RT078/126.

Advances in the Microbiome: Applications to Clostridium difficile Infection

Clostridium difficile is a major cause of morbidity and mortality worldwide, causing over 400,000 infections and approximately 29,000 deaths in the United States alone each year. C. difficile is the most common cause of nosocomial diarrhoea in the developed world, and, in recent years, the emergence of hyper-virulent (mainly ribotypes 027 and 078, sometimes characterised by increased toxin production), epidemic strains and an increase in the number of community-acquired infections has caused further concern. Antibiotic therapy with metronidazole, vancomycin or fidaxomicin is the primary treatment for C. difficile infection (CDI). However, CDI is unique, in that, antibiotic use is also a major risk factor for acquiring CDI or recurrent CDI due to disruption of the normal gut microbiota. Therefore, there is an urgent need for alternative, non-antibiotic therapeutics to treat or prevent CDI. Here, we review a number of such potential treatments which have emerged from advances in the field of microbiome research.

Reprofiled anthelmintics abate hypervirulent stationary-phase Clostridium difficile

Prolonged use of broad-spectrum antibiotics disrupts the indigenous gut microbiota, which consequently enables toxigenic Clostridium difficile species to proliferate and cause infection. The burden of C. difficile infections was exacerbated with the outbreak of hypervirulent strains that produce copious amounts of enterotoxins and spores. In recent past, membrane-active agents have generated a surge of interest due to their bactericidal property with a low propensity for resistance. In this study, we capitalized on the antimicrobial property and low oral bioavailability of salicylanilide anthelmintics (closantel, rafoxanide, niclosamide, oxyclozanide) to target the gut pathogen. By broth microdilution techniques, we determined the MIC values of the anthelmintics against 16 C. difficile isolates of defined PCR-ribotype. The anthelmintics broadly inhibited C. difficile growth in vitro via a membrane depolarization mechanism. Interestingly, the salicylanilides were bactericidal against logarithmic- and stationary-phase cultures of the BI/NAP1/027 strain 4118. The salicylanilides were poorly active against select gut commensals (Bacteroides, Bifidobacterium and Lactobacillus species), and were non-hemolytic and non-toxic to mammalian cell lines HepG2 and HEK 293T/17 within the range of their in vitro MICs and MBCs. The salicylanilide anthelmintics exhibit desirable properties for repositioning as anti-C. difficile agents.

Oral delivery of macromolecular drugs: Where we are after almost 100years of attempts

Since the first attempt to administer insulin orally in humans more than 90years ago, the oral delivery of macromolecular drugs (>1000g/mol) has been rather disappointing. Although several clinical pilot studies have demonstrated that the oral absorption of macromolecules is possible, the bioavailability remains generally low and variable. This article reviews the formulations and biopharmaceutical aspects of orally administered biomacromolecules on the market and in clinical development for local and systemic delivery. The most successful approaches for systemic delivery often involve a combination of enteric coating, protease inhibitors and permeation enhancers in relatively high amounts. However, some of these excipients have induced local or systemic adverse reactions in preclinical and clinical studies, and long-term studies are often missing. Therefore, strategies aimed at increasing the oral absorption of macromolecular drugs should carefully take into account the benefit-risk ratio. In the absence of specific uptake pathways, small and potent peptides that are resistant to degradation and that present a large therapeutic window certainly represent the best candidates for systemic absorption. While we acknowledge the need for systemically delivering biomacromolecules, it is our opinion that the oral delivery to local gastrointestinal targets is currently more promising because of their accessibility and the lacking requirement for intestinal permeability enhancement.