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

Biophysical library screening using a Thermo-FMN assay to identify and characterize Clostridioides difficile FabK inhibitors

Clostridioides difficile, a gram-positive anaerobic bacterium, is one of the most frequent causes of nosocomial infections. C. difficile infection (CDI) results in almost a half a million infections and approximately 30,000 deaths in the U.S. each year. Broad-spectrum antibacterial use is a strong risk factor for development of recurring CDI. There is a critical need for narrow-spectrum antibacterials with activity limited to C. difficile. The C. difficile enoyl-acyl carrier protein (ACP) reductase II enzyme (CdFabK), an essential and rate-limiting enzyme in the organism's fatty acid biosynthesis pathway (FAS-2), is an attractive target for narrow-spectrum CDI therapeutics as it is not present in many of the non-pathogenic gut organisms. We have previously characterized inhibitors of the CdFabK enzyme with narrow-spectrum anti-difficile activity and favorable in vivo efficacy, ADME, and low dysbiosis. To expand our knowledge of the structural requirements for CdFabK inhibition, we seek to identify new inhibitors with novel chemical scaffolds. Herein we present the optimization of a thermo-FMN biophysical assay based on the principles of differential scanning fluorimetry, or thermal shift, which leverages the fluorescence signal of the FabK enzyme's FMN prosthetic group. The optimized assay was validated by pilot testing a 10K diversity-based chemical library and novel scaffold hit compounds were identified and biochemically characterized. Additionally, we show that the thermo-FMN assay can be used to determine the thermodynamic dissociation constant, Kd, of CdFabK inhibitors.

Design of a chimeric glycosyltransferase OleD for the site-specific O-monoglycosylation of 3-hydroxypyridine in nosiheptide

To identify the potential role of the 3-hydroxyl group of the pyridine ring in nosiheptide (NOS) for its antibacterial activity against Gram-positive pathogens, enzymatic glycosylation was utilized to regio-selectively create a monoglycosyl NOS derivative, NOS-G. For this purpose, we selected OleD, a UDP glycosyltransferase from Streptomyces antibioticus that has a low productivity for NOS-G. Activity of the enzyme was increased by swapping domains derived from OleI, both single and in combination. Activity enhancement was best in mutant OleD-10 that contained four OleI domains. This chimer was engineered by site-directed mutagenesis (single and in combination) to increase its activity further, whereby variants were screened using a newly-established colorimetric assay. OleD-10 with I117F and T118G substitutions (FG) had an increased NOS-G productivity of 56%, approximately 70 times higher than that of wild-type OleD. The reason for improved activity of FG towards NOS was structurally attributed to a closer distance (<3 Å) between NOS/sugar donor and the catalytic amino acid H25. The engineered enzyme allowed sufficient activity to demonstrate that the produced NOS-G had enhanced stability and aqueous solubility compared to NOS. Using a murine MRSA infection model, it was established that NOS-G resulted in partial protection within 20 h of administration and delayed the death of infected mice. We conclude that 3-hydroxypyridine is a promising site for structural modification of NOS, which may pave the way for producing nosiheptide derivatives as a potential antibiotic for application in clinical treatment.

Synthesis and evaluation of phenylimidazole FabK inhibitors as new Anti-C. Difficile agents

Previously, 1-((4-(4-bromophenyl)-1H-imidazol-2-yl)methyl)-3-(5-(pyridin-2-ylthio)thiazol-2-yl)urea bearing a p-bromine substitution was shown to possess selective inhibitory activity against the Clostridioides difficile enoyl-acyl carrier protein (ACP) reductase II enzyme, FabK. Inhibition of CdFabK by this compound translated to promising antibacterial activity in the low micromolar range. In these studies, we sought to expand our knowledge of the SAR of the phenylimidazole CdFabK inhibitor series while improving the potency of the compounds. Three main series of compounds were synthesized and evaluated based on: 1) pyridine head group modifications including the replacement with a benzothiazole moiety, 2) linker explorations, and 3) phenylimidazole tail group modifications. Overall, improvement in the CdFabK inhibition was achieved, while maintaining the whole cell antibacterial activity. Specifically, compounds 1-((4-(4-bromophenyl)-1H-imidazol-2-yl)methyl)-3-(5-((3-(trifluoromethyl)pyridin-2-yl)thio)thiazol-2-yl)urea, 1-((4-(4-bromophenyl)-1H-imidazol-2-yl)methyl)-3-(6-(trifluoromethyl)benzo[d]thiazol-2-yl)urea, and 1-((4-(4-bromophenyl)-1H-imidazol-2-yl)methyl)-3-(6-chlorobenzo[d]thiazol-2-yl)urea showed CdFabK inhibition (IC50 = 0.10 to 0.24 μM), a 5 to 10-fold improvement in biochemical activity relative to 1-((4-(4-bromophenyl)-1H-imidazol-2-yl)methyl)-3-(5-(pyridin-2-ylthio)thiazol-2-yl)urea, with anti-C. difficile activity ranging from 1.56 to 6.25 μg/mL. Detailed analysis of the expanded SAR, supported by computational analysis, is presented.

Computational medicinal chemistry role in clinical pharmacy education: Ingavirin for coronavirus disease 2019 (COVID-19) discovery model

Objective

Given the major shift to patient-directed education, novel coronavirus (nCoV) provides a live example on how medicinal chemistry could be a key science to teach pharmacy students. In this paper, students and clinical pharmacy practitioners will find a stepwise primer on identifying new potential nCoV treatments mechanistically modulated through angiotensin-converting enzyme 2 (ACE2).

Methods

First, we identified the maximum common pharmacophore between carnosine and melatonin as background ACE2 inhibitors. Second, we performed a similarity search to spot out structures containing the pharmacophore. Third, molinspiration bioactivity scoring enabled us to promote one of the newly identified molecules as the best next candidate for nCoV. Preliminary docking in SwissDock and visualization through University of California San Francisco (UCSF) chimera made it possible to qualify one of them for further detailed docking and experimental validation.

Results

Ingavirin had the best docking results with full fitness of -3347.15 kcal/mol and estimated ΔG of -8.53 kcal/mol compared with melatonin (-6.57 kcal/mol) and carnosine (-6.29 kcal/mol). UCSF chimera showed viral spike protein elements binding to ACE2 retained in the best ingavirin pose in SwissDock at 1.75 Angstroms.

Conclusion

Ingavirin has a promising inhibitory potential to host (ACE2 and nCoV spike protein) recognition, and hence could offer the next best mitigating effect against the current coronavirus disease (COVID-19) pandemic.

Alimentary and Pharmaceutical Approach to Natural Antimicrobials against Clostridioides difficile Gastrointestinal Infection

Incidence of Clostridioides difficile infection (CDI) has been increasing in recent decades due to different factors, namely (i) extended use of broad-spectrum antibiotics, (ii) transmission within asymptomatic and susceptible patients, and (iii) unbalanced gastrointestinal microbiome and collateral diseases that favor C. difficile gastrointestinal domination and toxin production. Although antibiotic therapies have resulted in successful control of CDI in the last 20 years, the development of novel strategies is urged in order to combat the capability of C. difficile to generate and acquire resistance to conventional treatments and its consequent proliferation. In this regard, vegetable and marine bioactives have emerged as alternative and effective molecules to fight against this concerning pathogen. The present review examines the effectiveness of natural antimicrobials from vegetable and algae origin that have been used experimentally in in vitro and in vivo settings to prevent and combat CDI. The aim of the present work is to contribute to accurately describe the prospective use of emerging antimicrobials as future nutraceuticals and preventive therapies, namely (i) as dietary supplement to prevent CDI and reduce CDI recurrence by means of microbiota modulation and (ii) administering them complementarily to other treatments requiring antibiotics to prevent C. difficile gut invasion and infection progression.

Macrocycle-Antibiotic Hybrids: A Path to Clinical Candidates

The tale of abate in antibiotics continued defense mechanisms that chaperone the rise of drug-defying superbugs—on the other hand, the astray in antibacterial drug discovery and development. Our salvation lies in circumventing the genesis of resistance. Considering the competitive advantages of antibacterial chemotherapeutic agents equipped with multiple warheads against resistance, the development of hybrids has rejuvenated. The adoption of antibiotic hybrid paradigm to macrocycles has advanced novel chemical entities to clinical trials. The multi-targeted TD-1792, for instance, retained potent antibacterial activities against multiple strains that are resistant to its constituent, vancomycin. Moreover, the antibiotic conjugation of rifamycins has provided hybrid clinical candidates with desirable efficacy and safety profiles. In 2020, the U.S. FDA has granted an orphan drug designation to TNP-2092, a conjugate of rifamycin and fluoroquinolone, for the treatment of prosthetic joint infections. DSTA4637S is a pioneer antibacterial agent under clinical development and represents a novel class of bacterial therapy, that is, antibody–antibiotic conjugates. DSTA4637S is effective against the notorious persistent S. aureus bacteremia, a revelation of the abracadabra potential of antibiotic hybrid approaches.

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.

Small-Molecule Inhibition of the C. difficile FAS-II Enzyme, FabK, Results in Selective Activity

Clostridioides difficile infection (CDI) is a leading cause of significant morbidity, mortality, and healthcare-related costs in the United States. After standard therapy, recurrence rates remain high, and multiple recurrences are not uncommon. Causes include treatments employing broad-spectrum agents that disrupt the normal host microbiota, as well as treatment-resistant spore formation by C. difficile. Thus, novel druggable anti-C. difficile targets that promote narrow-spectrum eradication and inhibition of sporulation are desired. As a critical rate-limiting step within the FAS-II bacterial fatty acid synthesis pathway, which supplies precursory component phospholipids found in bacterial cytoplasmic and spore-mediated membranes, enoyl-acyl carrier protein (ACP) reductase II (FabK) represents such a target. FabK is essential in C. difficile (CdFabK) and is structurally and mechanistically distinct from other isozymes found in gut microbiota species, making CdFabK an attractive narrow-spectrum target. We report here the kinetic evaluation of CdFabK, the biochemical activity of a series of phenylimidazole analogues, and microbiological data suggesting these compounds' selective antibacterial activity against C. difficile versus several other prominent gut organisms. The compounds display promising, selective, low micromolar CdFabK inhibitory activity without significantly affecting the growth of other gut organisms, and the series prototype (1b) is shown to be competitive for the CdFabK cofactor and uncompetitive for the substrate. A series analogue (1g) shows maintained inhibitory activity while also possessing increased solubility. These findings represent the basis for future drug discovery efforts by characterizing the CdFabK enzyme while demonstrating its druggability and potential role as a narrow-spectrum antidifficile target.

Recent advances in the treatment of C. difficile using biotherapeutic agents

Clostridium difficile (C. difficile) is rapidly becoming one of the most prevalent health care–associated bacterial infections in the developed world. The emergence of new, more virulent strains has led to greater morbidity and resistance to standard therapies. The bacterium is readily transmitted between people where it can asymptomatically colonize the gut environment, and clinical manifestations ranging from frequent watery diarrhea to toxic megacolon can arise depending on the age of the individual or their state of gut dysbiosis. Several inexpensive approaches are shown to be effective against virulent C. difficile in research settings such as probiotics, fecal microbiota transfer and immunotherapies. This review aims to highlight the current advantages and limitations of the aforementioned approaches with an emphasis on recent studies.

Bis-Cyclic Guanidines as a Novel Class of Compounds Potent against Clostridium difficile

Clostridium difficile infection (CDI) symptoms range from diarrhea to severe toxic megacolon and even death. Due to its rapid acquisition of resistance, C. difficile is listed as an urgent antibiotic-resistant threat, and has surpassed methicillin-resistant Staphylococcus aureus (MRSA) as the most common hospital-acquired infection in the USA. To combat this pathogen, a new structural class of pseudo-peptides that exhibit antimicrobial activities could play an important role. Herein we report a set of bis-cyclic guanidine compounds that show potent antibacterial activity against C. difficile with decent selectivity. Eight compounds showed high in vitro potency against C. difficile UK6 with MIC values of 1.0 μg mL^-1 , and cytotoxic selectivity index (SI) values up to 37. Moreover, the most selective compound is also effective in the treatment of C. difficile-induced disease in a mouse model of CDI, and appears to be a very promising new candidate for the treatment of CDI.

Solid-Phase Synthesis and Antibacterial Activity of Cyclohexapeptide Wollamide B Analogs

Herein we report the antibacterial structure-activity relationships of cyclic hexapeptide wollamide analogs derived from solid-phase library synthesis. Wollamide B, a cyclic hexapeptide natural product, has been previously found to have activity against Mycobacterium bovis. To further evaluate its antimycobacterial/antibacterial potential, 27 peptides including wollamides A/B, and desotamide B, were synthesized and subsequently tested against a panel of clinically significant bacterial pathogens. Biological evaluation revealed that the cyclic scaffold, amide functionality in position I, tryptophan residue in position V, and the original stereochemistry pattern of the core scaffold were key for antituberculosis and/or antibacterial activity. In addition, against M. tuberculosis and Gram-positive bacteria, residues in position II and/or VI greatly impacted antibacterial activity and selectivity. Wollamides A (3) and B (2) along with their corresponding II (l-Leu) analog 10 retained the most promising antituberculosis activity, with the lowest minimum inhibitory concentration (MIC) against virulent M. tuberculosis H37Rv (MIC = 1.56 μg/mL), as well as desirable selectivity indices (>100). Importantly, the antimicrobial activities of wollamides A and B do not result from disruption of the bacterial membrane, warranting further investigation into their mechanism of action.

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.

Repurposing niclosamide for intestinal decolonization of vancomycin-resistant enterococci

Enterococci are commensal micro-organisms present in the gastrointestinal tract of humans. Although normally innocuous to the host, strains of enterococcus exhibiting resistance to vancomycin (VRE) have been associated with high rates of infection and mortality in immunocompromised patients. Decolonization of VRE represents a key strategy to curb infection in highly-susceptible patients. However, there is a dearth of decolonizing agents available clinically that are effective against VRE. The present study found that niclosamide, an anthelmintic drug, has potent antibacterial activity against clinical isolates of vancomycin-resistant Enterococcus faecium (minimum inhibitory concentration 1-8 µg/mL). E. faecium mutants exhibiting resistance to niclosamide could not be isolated even after multiple (10) serial passages. Based upon these promising in-vitro results and the limited permeability of niclosamide across the gastrointestinal tract (when administered orally), niclosamide was evaluated in a VRE colonization-reduction murine model. Remarkably, niclosamide outperformed linezolid, an antibiotic used clinically to treat VRE infections. Niclosamide was as effective as ramoplanin in reducing the burden of vancomycin-resistant E. faecium in the faeces, caecal content and ileal content of infected mice after only 8 days of treatment. Linezolid, in contrast, was unable to decrease the burden of VRE in the gastrointestinal tract of mice. The results obtained indicate that niclosamide warrants further evaluation as a novel decolonizing agent to suppress VRE infections.

Clostridium difficile infection in the elderly: an update on management

The burden of Clostridium difficile infection (CDI) is profound and growing. CDI now represents a common cause of health care–associated diarrhea, and is associated with significant morbidity, mortality, and health care costs. CDI disproportionally affects the elderly, possibly explained by the following risk factors: age-related impairment of the immune system, increasing antibiotic utilization, and frequent health care exposure. In the USA, recent epidemiological studies estimate that two out of every three health care–associated CDIs occur in patients 65 years or older. Additionally, the elderly are at higher risk for recurrent CDI. Existing therapeutic options include metronidazole, oral vancomycin, and fidaxomicin. Choice of agent depends on disease severity, history of recurrence, and, increasingly, the drug cost. Bezlotoxumab, a recently approved monoclonal antibody targeting C. difficile toxin B, offers an exciting advancement into immunologic therapies. Similarly, fecal microbiota transplantation is gaining popularity as an effective option mainly for recurrent CDI. The challenge of decreasing CDI burden in the elderly involves adopting preventative strategies, optimizing initial treatment, and decreasing the risk of recurrence. Expanded strategies are certainly needed to improve outcomes in this high-risk population. This review considers available data from prospective and retrospective studies as well as case reports to illustrate the merits and gaps in care related to the management of CDI in the elderly.

Update on Antimicrobial Resistance in Clostridium difficile: Resistance Mechanisms and Antimicrobial Susceptibility Testing

Oral antibiotics such as metronidazole, vancomycin and fidaxomicin are therapies of choice for Clostridium difficile infection. Several important mechanisms for C. difficile antibiotic resistance have been described, including the acquisition of antibiotic resistance genes via the transfer of mobile genetic elements, selective pressure in vivo resulting in gene mutations, altered expression of redox-active proteins, iron metabolism, and DNA repair, as well as via biofilm formation. This update summarizes new information published since 2010 on phenotypic and genotypic resistance mechanisms in C. difficile and addresses susceptibility test methods and other strategies to counter antibiotic resistance of C. difficile.

Production, Crystallization and Structure Determination of C. difficile PPEP-1 via Microseeding and Zinc-SAD

New therapies are needed to treat Clostridium difficile infections that are a major threat to human health. The C. difficile metalloprotease PPEP-1 is a target for future development of inhibitors to decrease the virulence of the pathogen. To perform biophysical and structural characterization as well as inhibitor screening, large amounts of pure and active protein will be needed. We have developed a protocol for efficient production and purification of PPEP-1 by the use of E. coli as the expression host yielding sufficient amounts and purity of protein for crystallization and structure determination. Additionally, using microseeding, highly intergrown crystals of PPEP-1 can be grown to well-ordered crystals suitable for X-ray diffraction analysis. The methods could also be used to produce other recombinant proteins and to study the structures of other proteins producing intergrown crystals.

Hybrid antibiotics - clinical progress and novel designs

INTRODUCTION

There is a growing need for new antibacterial agents, but success in development of antibiotics in recent years has been limited. This has led researchers to investigate novel approaches to finding compounds that are effective against multi-drug resistant bacteria, and that delay onset of resistance. One such strategy has been to link antibiotics to produce hybrids designed to overcome resistance mechanisms.

AREAS COVERED

The concept of dual-acting hybrid antibiotics was introduced and reviewed in this journal in 2010. In the present review the authors sought to discover how clinical candidates described had progressed, and to examine how the field has developed. In three sections the authors cover the clinical progress of hybrid antibiotics, novel agents produced from hybridisation of two or more small-molecule antibiotics, and novel agents produced from hybridisation of antibiotics with small-molecules that have complementary activity.

EXPERT OPINION

Many key questions regarding dual-acting hybrid antibiotics remain to be answered, and the proposed benefits of this approach are yet to be demonstrated. While Cadazolid in particular continues to progress in the clinic, suggesting that there is promise in hybridisation through covalent linkage, it may be that properties other than antibacterial activity are key when choosing a partner molecule.

Antibacterials Developed to Target a Single Organism: Mechanisms and Frequencies of Reduced Susceptibility to the Novel Anti-Clostridium difficile Compounds Fidaxomicin and LFF571

Clostridium difficile is the most common cause of antibacterial-associated diarrhea. Clear clinical presentation and rapid diagnostics enable targeted therapy for C. difficile infection (CDI) to start quickly. CDI treatment includes metronidazole and vancomycin (VAN). Despite decades of use for CDI, no clinically meaningful resistance to either agent has emerged. Fidaxomicin (FDX), an RNA polymerase inhibitor, is also approved to treat CDI. Mutants with reduced susceptibility to FDX have been selected in vitro by single and multistep methods. Strains with elevated FDX minimum inhibitory concentrations (MICs) were also identified from FDX-treated patients in clinical trials. LFF571 is an exploratory agent that inhibits EF-Tu. In a proof-of-concept study, LFF571 was safe and effective for treating CDI. Spontaneous mutants with reduced susceptibility to LFF571 were selected in vitro in a single step, but not via serial passage. Although there are several agents in development for treatment of CDI, this review summarizes the frequencies and mechanisms of C. difficile mutants displaying reduced susceptibility to FDX or LFF71.

Antibacterial and Synergistic Activity of Pentacyclic Triterpenoids Isolated from Alstonia scholaris

(1) Background: Alstonia scholaris (Apocynaceae) is an important medicinal plant that has been historically used in “Dai” ethnopharmacy to treat infectious diseases in China. Although various pharmacological activities have been reported, the antimicrobial constitutes of A. scholaris have not yet been identified. The objective of this study is to evaluate the antibacterial constitutes from the leaf extract of A. scholaris and to assess the synergistic effects of isolated compounds with antibiotics against bacterial pathogens.; (2) Methods: The chemical constitutes isolated from the leaf extract of A. scholaris were structurally identified by NMR. The antibacterial and synergistic effect of compounds was assessed by calculating the minimal inhibitory concentration (MIC), checkerboard dilution test, and time-kill assay.; (3) Results: Six pentacyclic triterpenoids were structurally identified as (1) lupeol, (2) betulin, (3) 3-hydroxy-11-ursen-28,13-olide, (4) betulinic acid, (5) oleanolic acid and (6) ursolic acid. Both oleanolic and ursolic acid showed antibacterial activity but were limited to Gram-positive bacteria. Ursolic acid showed a synergistic effect with ampicillin and tetracycline against both Bacillus cereus and S. aureus.; (4) Conclusion: These findings reflect that pentacyclic triterpenoids are the antibacterial chemicals in A. scholaris. The ability of ursolic acid to enhance the activity of antibiotics can constitute a valuable group of therapeutic agents in the future.

Novel therapeutic strategies for Clostridium difficile infections

INTRODUCTION

In recent years, Clostridium difficile has become the primary cause of antibiotic-associated diarrhea and pseudomembranous colitis, resulting in long and complicated hospital stays that represent a serious burden for patients as well as health care systems. Currently, conservative treatment of C. difficile infection (CDI) relies on the antibiotics vancomycin, metronidazole or fidaxomicin, or in case of multiple recurrences, fecal microbiota transplantation (FMT).

AREAS COVERED

The fast-spreading, epidemic nature of this pathogen urgently necessitates the search for alternative treatment strategies as well as antibiotic targets. Accordingly, in this review, we highlight the recent findings regarding virulence associated traits of C. difficile, evaluate their potential as alternative drug targets, and present current efforts in designing inhibitory compounds, with the aim of pointing out possibilities for future treatment strategies.

EXPERT OPINION

Increased attention on systematic analysis of the virulence mechanisms of C. difficile has already led to the identification of several alternative drug targets. In the future, applying state of the art 'omics' and the development of novel infection models that mimic the human gut, a highly complex ecological niche, will unveil the genomic and metabolic plasticity of this pathogen and will certainly help dealing with future challenges.

Gastrointestinal localization of metronidazole by a lactobacilli-inspired tetramic acid motif improves treatment outcomes in the hamster model of Clostridium difficile infection

OBJECTIVES

Metronidazole, a mainstay treatment for Clostridium difficile infection (CDI), is often ineffective for severe CDI. Whilst this is thought to arise from suboptimal levels of metronidazole in the colon due to rapid absorption, empirical validation is lacking. In contrast, reutericyclin, an antibacterial tetramic acid from Lactobacillus reuteri, concentrates in the gastrointestinal tract. In this study, we modified metronidazole with reutericyclin's tetramic acid motif to obtain non-absorbed compounds, enabling assessment of the impact of pharmacokinetics on treatment outcomes.

METHODS

A series of metronidazole-bearing tetramic acid substituents were synthesized and evaluated in terms of anti-C. difficile activities, gastric permeability, in vivo pharmacokinetics, efficacy in the hamster model of CDI and mode of action.

RESULTS

Most compounds were absorbed less than metronidazole in cell-based Caco-2 permeability assays. In hamsters, lead compounds compartmentalized in the colon rather than the bloodstream with negligible levels detected in the blood, in direct contrast with metronidazole, which was rapidly absorbed into the blood and was undetectable in caecum. Accordingly, four leads were more efficacious (P < 0.05) than metronidazole in C. difficile-infected animals. Improved efficacy was not due to an alternative mode of action, as the leads retained the mode of action of metronidazole.

CONCLUSIONS

This study provides the clearest empirical evidence that the high absorption of metronidazole lowers treatment outcomes for CDI and suggests a role for the tetramic acid motif for colon-specific drug delivery. This approach also has the potential to lower systemic toxicity and drug interactions of nitroheterocyclic drugs for treating gastrointestine-specific diseases.

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

In the past decade Clostridium difficile has become a bacterial pathogen of global significance. Epidemic strains have spread throughout hospitals, while community acquired infections and other sources ensure a constant inoculation of spores into hospitals. In response to the increasing medical burden, a new C. difficile antibiotic, fidaxomicin, was approved in 2011 for the treatment of C. difficile-associated diarrhea. Rudimentary fecal transplants are also being trialed as effective treatments. Despite these advances, therapies that are more effective against C. difficile spores and less damaging to the resident gastrointestinal microbiome and that reduce recurrent disease are still desperately needed. However, bringing a new treatment for C. difficile infection to market involves particular challenges. This review covers the current drug discovery pipeline, including both small molecule and biologic therapies, and highlights the challenges associated with in vitro and in vivo models of C. difficile infection for drug screening and lead optimization.

Resistance-resistant antibiotics

New antibiotics are needed because drug resistance is increasing while the introduction of new antibiotics is decreasing. We discuss here six possible approaches to develop 'resistance-resistant' antibiotics. First, multitarget inhibitors in which a single compound inhibits more than one target may be easier to develop than conventional combination therapies with two new drugs. Second, inhibiting multiple targets in the same metabolic pathway is expected to be an effective strategy owing to synergy. Third, discovering multiple-target inhibitors should be possible by using sequential virtual screening. Fourth, repurposing existing drugs can lead to combinations of multitarget therapeutics. Fifth, targets need not be proteins. Sixth, inhibiting virulence factor formation and boosting innate immunity may also lead to decreased susceptibility to resistance. Although it is not possible to eliminate resistance, the approaches reviewed here offer several possibilities for reducing the effects of mutations and, in some cases, suggest that sensitivity to existing antibiotics may be restored in otherwise drug-resistant organisms.

Fecal microbiota transplantation in the treatment of refractory Clostridium difficile infection in children: an update

PURPOSE OF REVIEW

The use of transplanted fecal material for the treatment of diarrheal illness dates back to the fourth-century China. While fecal microbiota transplant has gained increasing popularity over the past 50 years for the treatment of refractory Clostridium difficile infections (RCDIs) in adults, it has only been recently utilized in children. The purpose of this article is to review the use of fecal microbiota transplant (FMT) in the treatment of pediatric RCDIs.

RECENT FINDINGS

Minimal pediatric data, including few case reports and series, document the successful use of FMT for treatment of RCDI in the past 2 years. Patients in these reports included otherwise healthy children, those with inflammatory bowel disease as well as significantly immunocompromised children. Donor fecal infusion via nasogastric tube, gastroscope or colonoscope in children aged 16 months and older demonstrated a high rate of symptom resolution and organism eradication. No complications to date have been reported in children who have undergone FMT.

SUMMARY

FMT is emerging as a well-tolerated and effective treatment for RCDI in not only adults but also children.