Activity of RBx 11760, a novel biaryl oxazolidinone, against Clostridium difficile

Oxazolidinones as versatile scaffolds in medicinal chemistry

Oxazolidinone is a five-member heterocyclic ring with several biological applications in medicinal chemistry. Among the three possible isomers, 2-oxazolidinone is the most investigated in drug discovery. Linezolid was pioneered as the first approved drug containing an oxazolidinone ring as the pharmacophore group. Numerous analogues have been developed since its arrival on the market in 2000. Some have succeeded in reaching the advanced stages of clinical studies. However, most oxazolidinone derivatives reported in recent decades have not reached the initial stages of drug development, despite their promising pharmacological applications in a variety of therapeutic areas, including antibacterial, antituberculosis, anticancer, anti-inflammatory, neurologic, and metabolic diseases, among other areas. Therefore, this review article aims to compile the efforts of medicinal chemists who have explored this scaffold over the past decades and highlight the potential of the class for medicinal chemistry.

A dual-action antibiotic that kills Clostridioides difficile vegetative cells and inhibits spore germination

Clostridioides difficile infection (CDI) is the most lethal of the five CDC urgent public health treats, resulting in 12,800 annual deaths in the United States alone [Antibiotic Resistance Threats in the United States, 2019 (2019), www.cdc.gov/DrugResistance/Biggest-Threats.html]. The high recurrence rate and the inability of antibiotics to treat such infections mandate discovery of new therapeutics. A major challenge with CDI is the production of spores, leading to multiple recurrences of infection in 25% of patients [C. P. Kelly, J. T. LaMont, N. Engl. J. Med. 359, 1932-1940 (2008)], with potentially lethal consequence. Herein, we describe the discovery of an oxadiazole as a bactericidal anti-C. difficile agent that inhibits both cell-wall peptidoglycan biosynthesis and spore germination. We document that the oxadiazole binds to the lytic transglycosylase SleC and the pseudoprotease CspC for prevention of spore germination. SleC degrades the cortex peptidoglycan, a critical step in the initiation of spore germination. CspC senses germinants and cogerminants. Binding to SleC is with higher affinity than that to CspC. Prevention of spore germination breaks the nefarious cycles of CDI recurrence in the face of the antibiotic challenge, which is a primary cause of therapeutic failure. The oxadiazole exhibits efficacy in a mouse model of recurrent CDI and holds promise in clinical treatment of CDI.

Novel FtsZ inhibitor with potent activity against Staphylococcus aureus

OBJECTIVES

FtsZ is an essential bacterial protein and an unexplored target for the development of antibacterial drugs. The development of a novel inhibitor targeting FtsZ offers a potential opportunity to combat drug resistance. DS01750413, a new derivative of PC190723, is a novel FtsZ inhibitor with improved in vitro and in vivo activity. The objective of this study was to investigate the efficacy of DS01750413 against Staphylococcus spp., including MRSA, in in vitro and in vivo models.

METHODS

In vitro activities of DS01750413 and standard-of-care antibiotics were evaluated against clinical isolates of Gram-positive pathogens. The in vivo efficacy was evaluated in a murine systemic infection model caused by MRSA.

RESULTS

DS01750413 showed potent in vitro activity against MRSA clinical isolates with MIC ranges of 0.5-1 mg/L and also demonstrated concentration-dependent bactericidal killing. In the murine bacteraemia infection model of MRSA, treatment with DS01750413 resulted in prolonged survival of animals compared with placebo-treated animals and exhibited a significant reduction in the bacterial load in liver, spleen, lungs and kidneys.

CONCLUSIONS

DS01750413 showed encouraging in vitro and in vivo activity against MRSA. As a novel chemical class, DS01750413 has the potential to become clinically viable antibiotics to address the drug resistance problem by its unique novel targeting mechanism of action.

In Vitro and In Vivo Activities of DS-2969b, a Novel GyrB Inhibitor, against Clostridium difficile

DS-2969b is a novel GyrB inhibitor that is currently under clinical development for the treatment of Clostridium difficile infection (CDI). In this study, the in vitro and in vivo activities of DS-2969b were evaluated. DS-2969b inhibited the supercoiling activity of C. difficile DNA gyrase. DS-2969b showed potent in vitro activity against C. difficile clinical isolates with a MIC90 of 0.06 μg/ml, which was 2-, 32-, and 16-fold lower than the MIC90s of fidaxomicin, vancomycin, and metronidazole, respectively. DS-2969b did not select spontaneously resistant mutants of various C. difficile strains at 4× MIC, and the frequency of resistance development was less than 4.8 × 10-9 In a hamster CDI model, 5-day oral administration of DS-2969b conferred complete protection from recurrence and mortality at 0.3 mg/kg of body weight once a day, in contrast to a 50% survival rate with fidaxomicin at 3 mg/kg once a day and 0% with vancomycin at a 50-mg/kg/dose twice a day. Even a single oral administration of 1 mg/kg of DS-2969b in the CDI model exhibited 100% animal survival without recurrence. DS-2969b was also efficacious by 5-day subcutaneous administration in the CDI model. DS-2969b showed similar levels of fecal excretion after intravenous and oral administrations in rats. These data support further development of DS-2969b as a drug for oral and intravenous treatment of CDI.

Clostridioides (Clostridium) difficile infection: current and alternative therapeutic strategies

Clostridioides difficile (C. difficile) has become a pathogen of worldwide importance considering that epidemic strains are disseminated in hospitals of several countries, where community-acquired infections act as a constant source of new C. difficile strains into hospitals. Despite the advances in the treatment of infections, more effective therapies against C. difficile are needed but, at the same time, these therapies should be less harmful to the resident gastrointestinal microbiota. The purpose of this review is to present a description of issues associated to C. difficile infection, a summary of current therapies and those in developmental stage, and a discussion of potential combinations that may lead to an increased efficacy of C. difficile infection treatment.

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

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

In Vitro and In Vivo Activities of a Bi-Aryl Oxazolidinone, RBx 11760, against Gram-Positive Bacteria

RBx 11760, a bi-aryl oxazolidinone, was investigated for antibacterial activity against Gram-positive bacteria. The MIC₉₀s of RBx 11760 and linezolid against Staphylococcus aureus were 2 and 4 mg/liter, against Staphylococcus epidermidis were 0.5 and 2 mg/liter, and against Enterococcus were 1 and 4 mg/liter, respectively. Similarly, against Streptococcus pneumoniae the MIC₉₀s of RBx 11760 and linezolid were 0.5 and 2 mg/liter, respectively. In time-kill studies, RBx 11760, tedizolid, and linezolid exhibited bacteriostatic effect against all tested strains except S. pneumoniae RBx 11760 showed 2-log₁₀ kill at 4× MIC while tedizolid and linezolid showed 2-log₁₀ and 1.4-log₁₀ kill at 16× MIC, respectively, against methicillin-resistant S. aureus (MRSA) H-29. Against S. pneumoniae 5051, RBx 11760 showed bactericidal activity, with 4.6-log₁₀ kill at 4× MIC compared to 2.42-log₁₀ and 1.95-log₁₀ kill for tedizolid and linezolid, respectively, at 16× MIC. RBx 11760 showed postantibiotic effects (PAE) at 3 h at 4 mg/liter against MRSA H-29, and linezolid showed the same effect at 16 mg/liter. RBx 11760 inhibited biofilm production against methicillin-resistant S. epidermidis (MRSE) ATCC 35984 in a concentration-dependent manner. In a foreign-body model, linezolid and rifampin resulted in no advantage over stasis, while the same dose of RBx 11760 demonstrated a significant killing compared to the initial control against S. aureus (P < 0.05) and MRSE (P < 0.01). The difference in killing was statistically significant for the lower dose of RBx 11760 (P < 0.05) versus the higher dose of linezolid (P > 0.05 [not significant]) in a groin abscess model. In neutropenic mouse thigh infection, RBx 11760 showed stasis at 20 mg/kg of body weight, whereas tedizolid showed the same effect at 40 mg/kg. These data support RBx 11760 as a promising investigational candidate.

In vitro evaluation of the effect of linezolid and levofloxacin on Bacillus anthracis toxin production, spore formation and cell growth

BACKGROUND

Owing to its ability to form spores and toxins, Bacillus anthracis is considered a bioterror agent. Although current therapeutic strategies can be effective, treatment does not prevent sporulation and toxin production.

OBJECTIVES

To quantify the combined effect of a protein synthesis inhibitor and a bactericidal agent on B. anthracis toxin production, sporulation and cell growth.

METHODS

Susceptibility and synergy titrations were conducted on B. anthracis Sterne and 03-0191 strains using linezolid and levofloxacin. The effect of antibiotic exposure on cell viability was evaluated using a continuous medium replacement model. In vitro static models were used to study the effect of linezolid and levofloxacin on sporulation and toxin production. Spores were quantified using the heat shock method. Toxin was quantified via commercial ELISA.

RESULTS

Synergy titrations indicated that the combination was synergistic or indifferent; however, in all models antagonism was observed. In the spore model, linezolid resulted in the lowest sporulation rates, while combination therapy resulted in the highest. In the toxin model, linezolid prevented toxin production altogether.

CONCLUSIONS

This study advances our understanding of the effects of combination therapy on B. anthracis infection. Used alone, linezolid therapy abolishes toxin production and reduces sporulation. These results suggest that studies using a step-wise approach using linezolid initially to stop sporulation and toxin production followed by levofloxacin to rapidly kill vegetative B. anthracis can be recommended.

Subinhibitory concentrations of LFF571 reduce toxin production by Clostridium difficile

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

Structure of bacterial transcription factor SpoIIID and evidence for a novel mode of DNA binding

SpoIIID is evolutionarily conserved in endospore-forming bacteria, and it activates or represses many genes during sporulation of Bacillus subtilis. An SpoIIID monomer binds DNA with high affinity and moderate sequence specificity. In addition to a predicted helix-turn-helix motif, SpoIIID has a C-terminal basic region that contributes to DNA binding. The nuclear magnetic resonance (NMR) solution structure of SpoIIID in complex with DNA revealed that SpoIIID does indeed have a helix-turn-helix domain and that it has a novel C-terminal helical extension. Residues in both of these regions interact with DNA, based on the NMR data and on the effects on DNA binding in vitro of SpoIIID with single-alanine substitutions. These data, as well as sequence conservation in SpoIIID binding sites, were used for information-driven docking to model the SpoIIID-DNA complex. The modeling resulted in a single cluster of models in which the recognition helix of the helix-turn-helix domain interacts with the major groove of DNA, as expected. Interestingly, the C-terminal extension, which includes two helices connected by a kink, interacts with the adjacent minor groove of DNA in the models. This predicted novel mode of binding is proposed to explain how a monomer of SpoIIID achieves high-affinity DNA binding. Since SpoIIID is conserved only in endospore-forming bacteria, which include important pathogenic Bacilli and Clostridia, whose ability to sporulate contributes to their environmental persistence, the interaction of the C-terminal extension of SpoIIID with DNA is a potential target for development of sporulation inhibitors.

Prevention of Clostridium difficile spore formation by sub-inhibitory concentrations of tigecycline and piperacillin/tazobactam

BACKGROUND

Sporulation of Clostridium difficile during infection and persistence of spores within the gut could partly explain treatment failures and recurrence. However, the influence of antibiotics on sporulation is unclear. The objective of our study was to evaluate the impact of ciprofloxacin, metronidazole, piperacillin/tazobactam, tigecycline, and vancomycin on C. difficile sporulation in vitro.

METHODS

The reference strains ATCC 9689, 630, VPI 10463, and seven other clinical isolates of C. difficile were used, including three epidemic NAP1/027 isolates. Minimum inhibitory concentrations (MIC) were determined and sporulation was assessed after growth in the absence or presence of ≤0.5x MIC concentrations of each antibiotic.

RESULTS

All strains were sensitive to the antibiotics tested, except ribotype 027 isolates that were resistant to ciprofloxacin (MIC = 128 mg/L). Metronidazole and vancomycin generally did not significantly affect spore production in C. difficile, although vancomycin slightly affected sporulation of a few isolates. Ciprofloxacin inhibited sporulation of ribotype 027 isolates mainly. Interestingly, sub-MIC concentrations of piperacillin/tazobactam reduced spore formation in several isolates. However, the most striking observation was made with tigecycline, with an important reduction of spore formation in most isolates.

CONCLUSIONS

The capacity of C. difficile to sporulate can be significantly affected by certain antibiotics. The reduced sporulation observed with tigecycline and piperacillin/tazobactam might explain why these antibiotics are generally associated with lower risk of C. difficile infections. In addition, the inhibition of sporulation might partly explain the apparent efficacy of tigecycline for treatment of patients with recurrent infection.

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

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

Investigations of the mode of action and resistance development of cadazolid, a new antibiotic for treatment of Clostridium difficile infections

Cadazolid is a new oxazolidinone-type antibiotic currently in clinical development for the treatment of Clostridium difficile-associated diarrhea. Here, we report investigations on the mode of action and the propensity for spontaneous resistance development in C. difficile strains. Macromolecular labeling experiments indicated that cadazolid acts as a potent inhibitor of protein synthesis, while inhibition of DNA synthesis was also observed, albeit only at substantially higher concentrations of the drug. Strong inhibition of protein synthesis was also obtained in strains resistant to linezolid, in agreement with low MICs against such strains. Inhibition of protein synthesis was confirmed in coupled transcription/translation assays using extracts from different C. difficile strains, including strains resistant to linezolid, while inhibitory effects in DNA topoisomerase assays were weak or not detectable under the assay conditions. Spontaneous resistance frequencies of cadazolid were low in all strains tested (generally <10(-10) at 2× to 4× the MIC), and in multiple-passage experiments (up to 13 passages) MICs did not significantly increase. Furthermore, no cross-resistance was observed, as cadazolid retained potent activity against strains resistant or nonsusceptible to linezolid, fluoroquinolones, and the new antibiotic fidaxomicin. In conclusion, the data presented here indicate that cadazolid acts primarily by inhibition of protein synthesis, with weak inhibition of DNA synthesis as a potential second mode of action, and suggest a low potential for spontaneous resistance development.

In vitro and in vivo antibacterial evaluation of cadazolid, a new antibiotic for treatment of Clostridium difficile infections

Clostridium difficile is a leading cause of health care-associated diarrhea with significant morbidity and mortality, and new options for the treatment of C. difficile-associated diarrhea (CDAD) are needed. Cadazolid is a new oxazolidinone-type antibiotic that is currently in clinical development for treatment of CDAD. Here, we report the in vitro and in vivo antibacterial evaluation of cadazolid against C. difficile. Cadazolid showed potent in vitro activity against C. difficile with a MIC range of 0.125 to 0.5 μg/ml, including strains resistant to linezolid and fluoroquinolones. In time-kill kinetics experiments, cadazolid showed a bactericidal effect against C. difficile isolates, with >99.9% killing in 24 h, and was more bactericidal than vancomycin. In contrast to metronidazole and vancomycin, cadazolid strongly inhibited de novo toxin A and B formation in stationary-phase cultures of toxigenic C. difficile. Cadazolid also inhibited C. difficile spore formation substantially at growth-inhibitory concentrations. In the hamster and mouse models for CDAD, cadazolid was active, conferring full protection from diarrhea and death with a potency similar to that of vancomycin. These findings support further investigations of cadazolid for the treatment of CDAD.

In vitro and in vivo activities of the novel Ketolide RBx 14255 against Clostridium difficile

The MIC(90) of RBx 14255, a novel ketolide, against Clostridium difficile was 4 μg/ml (MIC range, 0.125 to 8 μg/ml), and this drug was found to be more potent than comparator drugs. An in vitro time-kill kinetics study of RBx 14255 showed time-dependent bacterial killing for C. difficile. Furthermore, in the hamster model of C. difficile infection, RBx 14255 demonstrated greater efficacy than metronidazole and vancomycin, making it a promising candidate for C. difficile treatment.

Efficacy of LFF571 in a hamster model of Clostridium difficile infection

LFF571 is a novel semisynthetic thiopeptide antibiotic with potent activity against a variety of Gram-positive pathogens, including Clostridium difficile. In vivo efficacy of LFF571 was compared to vancomycin in a hamster model of C. difficile infection (CDI). Infection was induced in Golden Syrian hamsters using a toxigenic strain of C. difficile. Treatment started 24 h postinfection and consisted of saline, vancomycin, or LFF571. Cox regression was used to analyze survival data from a cohort of animals evaluated across seven serial experimental groups treated with vancomycin at 20 mg/kg, LFF571 at 5 mg/kg, or vehicle alone. Survival was right censored; animals were not observed beyond day 21. At death or end of study, cecal contents were tested for C. difficile toxins A and B. In summary, the data showed that 5 mg/kg LFF571 decreased the risk of death by 79% (P < 0.0001) and 69% (P = 0.0022) compared with saline and 20 mg/kg vancomycin, respectively. Further analysis of the pooled data indicated that the survival benefit of LFF571 treatment at 5 mg/kg compared to vancomycin at 20 mg/kg was due primarily to a decrease in the risk of recurrence after end of treatment. Animals successfully treated with LFF571 or vancomycin had no detectable C. difficile toxin. Overall, LFF571 was more efficacious at the end of the study, at a lower dose, and with fewer recurrences, than vancomycin in the hamster model of CDI. LFF571 is being assessed in humans for safety and efficacy in the treatment of C. difficile infections.

Treatment of Clostridium difficile-associated diarrhea

Clostridium difficile-associated diarrhea (CDAD) is a serious subtype of antibiotic-associated diarrhea. In recent years, the incidence and severity of CDAD have significantly increased worldwide. Clostridium difficile is a Gram-positive enteric pathogen and can produce toxin A and toxin B, which induce cytopathic changes and lead to a series of clinical manifestations of infection. Although the use of broad spectrum antibiotics is the most common cause for the development of CDAD, age, chronic disease, long-term hospitalization and other factors are also possible risk factors. Patients with confirmed or highly suspected CDAD should immediately discontinue antibiotics and switch to other drugs which do not tend to cause CDAD. Treatment with metronidazole or vancomycin is recommended. Recent studies show that teicoplanin, probiotics, monoclonal antibody and Chinese medicine are also effective for this disease. In this paper, we discuss the prevention and treatment of CDAD.

Current updates on oxazolidinone and its significance

Oxazolidinone is a five-member heterocyclic ring exhibiting potential medicinal properties with preferential antibacterial activity. Scientists reported various synthetic procedures for this heterocyclic structure. Current review articles tried to cover each and every potential aspect of oxazolidinone like synthetic routes, pharmacological mechanism of action, medicinal properties, and current research activities.