Phenylthiazoles with tert-Butyl side chain: Metabolically stable with anti-biofilm activity

Decoding antimicrobial resistance: unraveling molecular mechanisms and targeted strategies

Antimicrobial resistance poses a significant global health threat, necessitating innovative approaches for combatting it. This review explores various mechanisms of antimicrobial resistance observed in various strains of bacteria. We examine various strategies, including antimicrobial peptides (AMPs), novel antimicrobial materials, drug delivery systems, vaccines, antibody therapies, and non-traditional antibiotic treatments. Through a comprehensive literature review, the efficacy and challenges of these strategies are evaluated. Findings reveal the potential of AMPs in combating resistance due to their unique mechanisms and lower propensity for resistance development. Additionally, novel drug delivery systems, such as nanoparticles, show promise in enhancing antibiotic efficacy and overcoming resistance mechanisms. Vaccines and antibody therapies offer preventive measures, although challenges exist in their development. Non-traditional antibiotic treatments, including CRISPR-Cas systems, present alternative approaches to combat resistance. Overall, this review underscores the importance of multifaceted strategies and coordinated global efforts to address antimicrobial resistance effectively.

Current Development of Thiazole-Containing Compounds as Potential Antibacterials against Methicillin-Resistant Staphylococcus aureus

The emergence of multi-drug-resistant bacteria is threatening to human health and life around the world. In particular, methicillin-resistant Staphylococcus aureus (MRSA) causes fatal injuries to human beings and serious economic losses to animal husbandry due to its easy transmission and difficult treatment. Currently, the development of novel, highly effective, and low-toxicity antimicrobials is important to combat MRSA infections. Thiazole-containing compounds with good biological activity are widely used in clinical practice, and appropriate structural modifications make it possible to develop new antimicrobials. Here, we review thiazole-containing compounds and their antibacterial effects against MRSA reported in the past two decades and discuss their structure-activity relationships as well as the corresponding antimicrobial mechanisms. Some thiazole-containing compounds exhibit potent antibacterial efficacy in vitro and in vivo after appropriate structural modifications and could be used as antibacterial candidates. This Review provides insights into the development of thiazole-containing compounds as antimicrobials to combat MRSA infections.

Novel phenylthiazoles with a tert-butyl moiety: promising antimicrobial activity against multidrug-resistant pathogens with enhanced ADME properties

The structure-activity relationship of a new tert-butylphenylthiazole series, with a pyrimidine linker, was investigated. We wished to expand knowledge of this novel class of antibiotics by generating 21 new derivatives bearing ≥2 heteroatoms in their side chains. Their activity was examined against isolates of methicillin-resistant Staphylococcus aureus (MRSA), Clostridium difficile, Escherichia coli, Neisseria gonorrhoeae, and Candida albicans. Two compounds with 1,2-diaminocyclohexane as a nitrogenous side chain showed promising activity against the highly infectious MRSA USA300 strain, with a minimum inhibitory concentration (MIC) of 4 μg mL-1. One of these two compounds demonstrated potent activity against C. difficile, with a MIC of 4 μg mL-1. Moderate activities against a C. difficile strain with a MIC of 8 μg mL-1 were noted. Some new compounds possessed antifungal activity against a wild fluconazole-resistant C. albicans strain, with MIC values of 4-16 μg mL-1. ADME and metabolism-simulation studies were performed for the most promising compound and compared with lead compounds. Our results revealed that one compound possessed greater penetration of bacterial membranes and metabolic resistance, which aided a longer duration of action against MRSA.

2,4-Diacetylphloroglucinol (DAPG) derivatives rapidly eradicate methicillin-resistant staphylococcus aureus without resistance development by disrupting membrane

Methicillin-resistant Staphylococcus aureus (MRSA) causes severe public health challenges throughout the world, and the multi-drug resistance (MDR) of MRSA to antibiotics necessitates the development of more effective antibiotics. Natural 2,4-diacetylphloroglucinol (DAPG), produced by Pseudomonas, displays moderate inhibitory activity against MRSA. A series of DAPG derivatives was synthesized and evaluated for their antibacterial activities, and some showed excellent activities (MRSA MIC = 0.5-2 μg/mL). Among these derivatives, 7g demonstrated strong antibacterial activity without resistance development over two months. Mechanistic studies suggest that 7g asserted its activity by targeting bacterial cell membranes. In addition, 7g exhibited significant synergistic antibacterial effects with oxacillin both in vitro and in vivo, with a tendency to eradicate MRSA biofilms. 7g is a promising lead for the treatment of MRSA.

New Phenylthiazoles: Design, Synthesis, and Biological Evaluation as Antibacterial, Antifungal, and Anti-COVID-19 Candidates

The combination of antibacterial and antiviral agents is becoming a very important aspect of dealing with resistant bacterial and viral infections. The N-phenylthiazole scaffold was found to possess significant anti-MRSA, antifungal, and anti-COVID-19 activities as previously published; hence, a slight refinement was proposed to attach various alkyne lipophilic tails to this promising scaffold, to investigate their effects on the antimicrobial activity of the newly synthesized compounds and to provide a valuable structure-activity relationship. Phenylthiazole 4 m exhibited the most potent anti-MRSA activity with 8 μg/mL MIC value. Compounds 4 k and 4 m demonstrated potent activity against Clostridium difficile with MIC values of 2 μg/mL and moderate activity against Candida albicans with MIC value of 4 μg/mL. When analyzed for their anti-COVID-19 inhibitory effect, compound 4 b emerged with IC50 =1269 nM and the highest selectivity of 138.86 and this was supported by its binding score of -5.21 kcal mol-1 when docked against SARS-CoV-2 M pro . Two H-bonds were formed, one with His164 and the other with Met49 stabilizing phenylthiazole derivative 4 b, inside the binding pocket. Additionally, it created two arene-H bonds with Asn142 and Glu166, through the phenylthiazole scaffold and one arene-H bond with Leu141 via the phenyl ring of the lipophilic tail.

Naphthylthiazoles: a class of broad-spectrum antifungals

Cryptococcal infections remain a major cause of mortality worldwide due to the ability of Cryptococci to pass through the blood-brain barrier (BBB) causing lethal meningitis. The limited number of available therapeutics, which exhibit limited availability, severe toxicity and low tolerability, necessitates the development of new therapeutics. Investigating the antifungal activity of a novel series of naphthylthiazoles provided trans-diaminocyclohexyl derivative 18 with many advantageous attributes as a potential therapeutic for cryptococcal meningitis. Briefly, the antimycotic activity of 18 against cryptococcal strains was highly comparable to that of amphotericin-B and fluconazole with MIC values as low as 1 μg mL-1. Moreover, compound 18 possessed additional advantages over fluconazole; it significantly reduced the intracellular burden of Cryptococci and markedly inhibited cryptococcal biofilm formation. Initial PK assessment of 18 indicated its ability to reach the CNS after oral administration with high permeability, and it maintained therapeutic plasma concentrations for 18 h. Its antifungal activity extended to other clinically relevant strains, such as fluconazole-resistant C. auris.

Substituted salicylic acid analogs offer improved potency against multidrug-resistant Neisseria gonorrhoeae and good selectivity against commensal vaginal bacteria

Drug-resistant Neisseria gonorrhoeae represents a major threat to public health; without new effective antibiotics, untreatable gonococcal infections loom as a real possibility. In a previous drug-repurposing study, we reported that salicylic acid had good potency against azithromycin-resistant N. gonorrhoeae. We now report that the anti-gonococcal activity in this scaffold is easily lost by inopportune substitution, but that select substituted naphthyl analogs (3b, 3o and 3p) have superior activity to salicylic acid itself. Furthermore, these compounds retained potency against multiple ceftriaxone- and azithromycin-resistant strains, exhibited rapid bactericidal activity against N. gonorrhoeae, and showed high tolerability to mammalian cells (CC50 > 128 µg/mL). Promisingly, these compounds also show very weak growth inhibition of commensal vaginal bacteria.

Optimization and Antibacterial Evaluation of Novel 3-(5-Fluoropyridine-3-yl)-2-oxazolidinone Derivatives Containing a Pyrimidine Substituted Piperazine

In this study, a series of novel 3-(5-fluoropyridine-3-yl)-2-oxazolidinone derivatives were designed and synthesized based on compounds previously reported, and their antibacterial activity was investigated. Then their antibacterial activity was investigated for the first time. Preliminary screening results showed that all these compounds exhibited antibacterial activity against gram-positive bacteria, including 7 drug-sensitive strains and 4 drug-resistant strains, among which compound 7j exhibited an 8-fold stronger inhibitory effect than linezolid, with a minimum inhibitory concentration (MIC) value of 0.25 µg/mL. Further molecular docking studies predicted the possible binding mode between active compound 7j and the target. Interestingly, these compounds could not only hamper the formation of biofilms, but also have better safety, as confirmed by cytotoxicity experiments. All these results indicate that these 3-(5-fluoropyridine-3-yl)-2-oxazolidinone derivatives have the potential to be developed into novel candidates for the treatment of gram-positive bacterial infections.

Novel copper-containing ferrite nanoparticles exert lethality to MRSA by disrupting MRSA cell membrane permeability, depleting intracellular iron ions, and upregulating ROS levels

Objective

The widespread use of antibiotics has inevitably led to the emergence of multidrug-resistant bacterial strains, such as methicillin-resistant Staphylococcus aureus (MRSA), making treatment of this infection a serious challenge. This study aimed to explore new treatment strategies for MRSA infection.

Methods

The structure of Fe₃O₄ NPs with limited antibacterial activity was optimized, and the Fe²⁺ ↔ Fe³⁺ electronic coupling was eliminated by replacing 1/2 Fe²⁺ with Cu²⁺. A new type of copper-containing ferrite nanoparticles (hereinafter referred to as Cu@Fe NPs) that fully retained oxidation-reduction activity was synthesized. First, the ultrastructure of Cu@Fe NPs was examined. Then, antibacterial activity was determined by testing the minimum inhibitory concentration (MIC) and safety for use as an antibiotic agent. Next, the mechanisms underlying the antibacterial effects of Cu@Fe NPs were investigated. Finally, mice models of systemic and localized MRSA infections was established for in vivo validation.

Results

It was found that Cu@Fe NPs exhibited excellent antibacterial activity against MRSA with MIC of 1 μg/mL. It effectively inhibited the development of MRSA resistance and disrupted the bacterial biofilms. More importantly, the cell membranes of MRSA exposed to Cu@Fe NPs underwent significant rupture and leakage of the cell contents. Cu@Fe NPs also significantly reduced the iron ions required for bacterial growth and contributed to excessive intracellular accumulation of exogenous reactive oxygen species (ROS). Therefore, these findings may important for its antibacterial effect. Furthermore, Cu@Fe NPs treatment led to a significant reduction in colony forming units within intra-abdominal organs, such as the liver, spleen, kidney, and lung, in mice with systemic MRSA infection, but not for damaged skin in those with localized MRSA infection.

Conclusion

The synthesized nanoparticles has an excellent drug safety profile, confers high resistant to MRSA, and can effectively inhibit the progression of drug resistance. It also has the potential to exert anti-MRSA infection effects systemically in vivo. In addition, our study revealed a unique multifaceted antibacterial mode of Cu@Fe NPs: (1) an increase in cell membrane permeability, (2) depletion of Fe ions in cells, (3) generation of ROS in cells. Overall, Cu@Fe NPs may be potential therapeutic agents for MRSA infections.

SAR investigation and optimization of benzimidazole-based derivatives as antimicrobial agents against Gram-negative bacteria

Antibiotic-resistant bacteria represent a serious threat to modern medicine and human life. Only a minority of antibacterial agents are active against Gram-negative bacteria. Hence, the development of novel antimicrobial agents will always be a vital need. In an effort to discover new therapeutics against Gram-negative bacteria, we previously reported a structure-activity-relationship (SAR) study on 1,2-disubstituted benzimidazole derivatives. Compound III showed a potent activity against tolC-mutant Escherichia coli with an MIC value of 2 μg/mL, representing a promising lead for further optimization. Building upon this study, herein, 49 novel benzimidazole compounds were synthesized to investigate their antibacterial activity against Gram-negative bacteria. Our design focused on three main goals, to address the low permeability of our compounds and improve their cellular accumulation, to expand the SAR study to the unexplored ring C, and to optimize the lead compound (III) by modification of the methanesulfonamide moiety. Compounds (25a-d, 25f-h, 25k, 25l, 25p, 25r, 25s, and 26b) exhibited potent activity against tolC-mutant E. coli with MIC values ranging from 0.125 to 4 μg/mL, with compound 25d displaying the highest potency among the tested compounds with an MIC value of 0.125 μg/mL. As its predecessor, III, compound 25d exhibited an excellent safety profile without any significant cytotoxicity to mammalian cells. Time-kill kinetics assay indicated that 25d exhibited a bacteriostatic activity and significantly reduced E. coli JW55031 burden as compared to DMSO. Additionally, combination of 25d with colistin partially restored its antibacterial activity against Gram-negative bacterial strains (MIC values ranging from 4 to 16 μg/mL against E. coli BW25113, K. pneumoniae, A. baumannii, and P. aeruginosa). Furthermore, formulation of III and 25d as lipidic nanoparticles (nanocapsules) resulted in moderate enhancement of their antibacterial activity against Gram-negative bacterial strains (A. Baumannii, N. gonorrhoeae) and compound 25d demonstrated superior activity to the lead compound III. These findings establish compound 25d as a promising candidate for treatment of Gram-negative bacterial infections and emphasize the potential of nano-formulations in overcoming poor cellular accumulation in Gram-negative bacteria where further optimization and investigation are warranted to improve the potency and broaden the spectrum of our compounds.

The Anti-MRSA Activity of Phenylthiazoles: A Comprehensive Review

Antimicrobial resistance is an aggravating global issue therefore it has been under extensive research in an attempt to reduce the number of antibiotics that are constantly reported as obsolete jeopardizing the lives of millions worldwide. Thiazoles possess a reputation as one of the most diverse biologically active nuclei, and phenylthiazoles are no less exceptional with an assorted array of biological activities such as anthelmintic, insecticidal, antimicrobial, antibacterial, and antifungal activity. Recently phenyl thiazoles came under the spotlight as a scaffold having strong potential as an anti-MRSA lead compound. It is a prominent pharmacophore in designing and synthesizing new compounds with antibacterial activity against multidrug-resistant bacteria such as MRSA, which is categorized as a serious threat pathogen, that exhibited concomitant resistance to most of the first-line antibiotics. MRSA has been associated with soft tissue and skin infections resulting in high death rates, rapid dissemination, and loss of millions of dollars of additional health care costs. In this brief review, we have focused on the advances of phenylthiazole derivatives as potential anti-MRSA from 2014 to 2021. The review encompasses the effect on biological activity due to combining this molecule with various synthetic pharmacophores. The physicochemical aspects were correlated with the pharmacokinetic properties of the reviewed compounds to reach a structure-activity relationship profile. Lead optimization of phenyl thiazole derivatives has additionally been outlined where the lipophilicity of the compounds was balanced with the metabolic stability and oral solubility to aid the researchers in medicinal chemistry, design, and synthesizing effective anti- MRSA phenylthiazoles in the future.

Novel benzothiazole‒urea hybrids: Design, synthesis and biological activity as potent anti-bacterial agents against MRSA

Novel benzothiazole‒urea hybrids were designed, synthesized and evaluated their anti-bacterial activity. They only exhibited anti-bacterial activity against Gram-positive bacteria, including clinical methicillin-resistant S. aureus (MRSA), compounds 5f, 5i, 8e, 8k and 8l exhibited potent activity (MIC = 0.39 and 0.39/0.78 μM against SA and MRSA, respectively). Crystal violet assay showed that compounds 5f, 8e and 8l not only inhibited the formation of biofilms but also eradicated preformed biofilms. Compound 8l had membrane disruption, little propensity to induce resistance, benign safety and in vivo anti-MRSA efficacy in a mouse model of abdominal infection. Therefore, our data demonstrated the potential to advance benzothiazole‒urea hybrids as a new class of antibiotics.

Bacterial virulence factors: a target for heterocyclic compounds to combat bacterial resistance

Antibiotic resistance is one of the most important challenges of the 21st century. However, the growing understanding of bacterial pathogenesis and cell-to-cell communication has revealed many potential strategies for the discovery of drugs that can be used for the treatment of bacterial infections. Interfering with bacterial virulence and/or quorum sensing could be a particularly interesting approach, because it is believed to exert less selective pressure on the bacterial resistance than with traditional strategies, geared toward killing bacteria or preventing their growth. Here, we discuss the mechanism of bacterial virulence, presenting promising strategies and recently synthesized heterocyclic compounds to combat future bacterial infections.

Evaluation of bisphenylthiazoles as a promising class for combating multidrug-resistant fungal infections

To minimize the intrinsic toxicity of the antibacterial agent hydrazinyloxadiazole 1, the hydrazine moiety was replaced with ethylenediamine (compound 7). This replacement generated a potent antifungal agent with no antibacterial activity. Notably, use of a 1,2-diaminocyclohexane moiety, as a conformationally-restricted isostere for ethylenediamine, potentiated the antifungal activity in both the cis and trans forms of N-(5-(2-([1,1’-biphenyl]-4-yl)-4-methylthiazol-5-yl)-1,3,4-oxadiazol-2-yl)cyclohexane-1,2-diamine (compounds 16 and 17). Both compounds 16 and 17 were void of any antibacterial activity; nonetheless, they showed equipotent antifungal activity in vitro to that of the most potent approved antifungal agent, amphotericin B. The promising antifungal effects of compounds 16 and 17 were maintained when assessed against an additional panel of 26 yeast and mold clinical isolates, including the Candida auris and C. krusei. Furthermore, compound 17 showed superior activity to amphotericin B in vitro against Candida glabrata and Cryptococcus gattii. Additionally, neither compound inhibited the normal human microbiota, and both possessed excellent safety profiles and were 16 times more tolerable than amphotericin B.

Synthetic cajaninstilbene acid derivatives eradicate methicillin-resistant Staphylococcus aureus persisters and biofilms

The Staphylococcus aureus can switch to a transient genotype-invariant dormancy, known as a persister, to survive treatment with high doses of antibiotics. This transient persister is an important reason underlying its resistance. There is an urgent need to find new antibacterial agents capable of eradicating methicillin-resistant S. aureus (MRSA) persisters. In this study, 37 new derivatives of cajaninstilbene acid (CSA) were designed and synthesized, and their biological activity against MRSA persisters was evaluated. Most of the newly synthesized derivatives exhibit more potent antimicrobial properties against S. aureus and MRSA than CSA itself, and 23 of the 37 derivatives show a tendency to eradicate MRSA persisters. A representative compound (A6) was demonstrated to target bacterial cell membranes. It eradicated the adherent biofilm of MRSA in a concentration dependent manner, and showed a synergistic antibacterial effect with piperacilin. In a model mouse abscess caused by MRSA persisters, A6 effectively reduced the bacterial load in vivo. These results indicate that A6 is a potential candidate for treatment of MRSA persister infections.

High-throughput screening identifies a novel natural product-inspired scaffold capable of inhibiting Clostridioides difficile in vitro

Clostridioides difficile is an enteric pathogen responsible for causing debilitating diarrhea, mostly in hospitalized patients. The bacterium exploits on microbial dysbiosis induced by the use of antibiotics to establish infection that ranges from mild watery diarrhea to pseudomembranous colitis. The increased prevalence of the disease accompanied by exacerbated comorbidity and the paucity of anticlostridial drugs that can tackle recurrence entails novel therapeutic options. Here, we report new lead molecules with potent anticlostridial activity from the AnalytiCon NATx library featuring natural product-inspired or natural product-derived small molecules. A high-throughput whole-cell-based screening of 5000 synthetic compounds from the AnalytiCon NATx library helped us identify 10 compounds capable of inhibiting the pathogen. Out of these 10 hits, we found 3 compounds with potent activity against C. difficile (MIC = 0.5-2 μg/ml). Interestingly, these compounds had minimal to no effect on the indigenous intestinal microbial species tested, unlike the standard-of-care antibiotics vancomycin and fidaxomicin. Further in vitro investigation revealed that the compounds were nontoxic to Caco-2 cell line. Given their potent anticlostridial activity, natural product-inspired scaffolds may suggest potential avenues that can address the unmet needs in preventing C. difficile mediated disease.

Synthetic accesses to biguanide compounds

Biguanide is a unique chemical function, which has attracted much attention a century ago and is showing resurgent interest in recent years after a long period of dormancy. This class of compounds has found broad applications such as reaction catalysts, organic strong bases, ligands for metal complexation, or versatile starting materials in organic synthesis for the preparation of nitrogen-containing heterocycles. Moreover, biguanides demonstrate a wide range of biological activities and some representatives are worldwide known such as metformin, the first-line treatment against type II diabetes, or chlorhexidine, the gold standard disinfectant and antiseptic. Although scarcely represented, the number of "success stories" with biguanide-containing compounds highlights their value and their unexploited potential as future drugs in various therapeutic fields or as efficient metal ligands. This review provides an extensive and critical overview of the synthetic accesses to biguanide compounds, as well as their comparative advantages and limitations. It also underlines the need of developing new synthetic methodologies to reach a wider variety of biguanides and to overcome the underrepresentation of these compounds.

A New Pyrimidine Schiff Base with Selective Activities against Enterococcus faecalis and Gastric Adenocarcinoma

Enterococcus faecalis is known as a significant nosocomial pathogen due to its natural resistance to many antibacterial drugs. Moreover, it was found that E. faecalis infection causes inflammation, production of reactive oxygen species, and DNA damage to human gastric cancer cells, which can induce cancer. In this study, we synthesized and tested the biological activity of a new Schiff base, 5-[(4-ethoxyphenyl)imino]methyl-N-(4-fluorophenyl)-6-methyl-2-phenylpyrimidin-4-amine (3), and compared its properties with an analogous amine (2). In the biological investigation, 3 was found to have antibacterial activity against E. faecalis 29212 and far better anticancer properties, especially against gastric adenocarcinoma (human Caucasian gastric adenocarcinoma), than 2. In addition, both derivatives were non-toxic to normal cells. It is worth mentioning that 3 could potentially inhibit cancer cell growth by inducing cell apoptosis. The results suggest that the presence of the -C=N- bond in the molecule of 3 increases its activity, indicating that 5-iminomethylpyrimidine could be a potent core for further drug discovery research.

Development of Biphenylthiazoles Exhibiting Improved Pharmacokinetics and Potent Activity Against Intracellular Staphylococcus aureus

Exploring the structure-activity relationship (SAR) at the cationic part of arylthiazole antibiotics revealed hydrazine as an active moiety. The main objective of the study is to overcome the inherited toxicity associated with the free hydrazine. A series of hydrocarbon bridges was inserted in between the groups, to separate the two amino groups. Hence, the aminomethylpiperidine-containing analog 16 was identified as a new promising antibacterial agent with efficient antibacterial and pharmacokinetic profiles. Briefly, compound 16 outperformed vancomycin in terms of the antibacterial spectrum against vancomycin-resistant staphylococcal and enterococcal strains with minimum inhibitory concentrations (MICs) ranging from 2 to 4 μg/mL, which is a faster bactericidal mode of action, completely eradicating the high staphylococcal burden within 6-8 h, and it has a unique ability to completely clear intracellular staphylococci. In addition, the initial pharmacokinetic assessment confirmed the high metabolic stability of compound 16 (biological half-life >4 h); it had a good extravascular distribution and maintained a plasma concentration higher than the average MIC value for over 12 h. Moreover, compound 16 significantly reduced MRSA burden in an in vivo MRSA skin infection mouse experiment. These attributes collectively suggest that compound 16 is a good therapeutic candidate for invasive staphylococcal and enterococcal infections. From a mechanistic point of view, compound 16 inhibited undecaprenyl diphosphate phosphatase (UppP) with an IC₅₀ value of 29 μM.

Ultrapotent Inhibitor of Clostridioides difficile Growth, Which Suppresses Recurrence In Vivo

Clostridioides difficile is the leading cause of healthcare-associated infection in the U.S. and considered an urgent threat by the Centers for Disease Control and Prevention (CDC). Only two antibiotics, vancomycin and fidaxomicin, are FDA-approved for the treatment of C. difficile infection (CDI), but these therapies still suffer from high treatment failure and recurrence. Therefore, new chemical entities to treat CDI are needed. Trifluoromethylthio-containing N-(1,3,4-oxadiazol-2-yl)benzamides displayed very potent activities [sub-μg/mL minimum inhibitory concentration (MIC) values] against Gram-positive bacteria. Here, we report remarkable antibacterial activity enhancement via halogen substitutions, which afforded new anti-C. difficile agents with ultrapotent activities [MICs as low as 0.003 μg/mL (0.007 μM)] that surpassed the activity of vancomycin against C. difficile clinical isolates. The most promising compound in the series, HSGN-218, is nontoxic to mammalian colon cells and is gut-restrictive. In addition, HSGN-218 protected mice from CDI recurrence. Not only does this work provide a potential clinical lead for the development of C. difficile therapeutics but also highlights dramatic drug potency enhancement via halogen substitution.

Optimization of Acetazolamide-Based Scaffold as Potent Inhibitors of Vancomycin-Resistant Enterococcus

Vancomycin-resistant enterococci (VRE) are the second leading cause of hospital-acquired infections (HAIs) attributed to a drug-resistant bacterium in the United States, and resistance to the frontline treatments is well documented. To combat VRE, we have repurposed the FDA-approved carbonic anhydrase drug acetazolamide to design potent antienterococcal agents. Through structure-activity relationship optimization we have arrived at two leads possessing improved potency against clinical VRE strains from MIC = 2 μg/mL (acetazolamide) to MIC = 0.007 μg/mL (22) and 1 μg/mL (26). Physicochemical properties were modified to design leads that have either high oral bioavailability to treat systemic infections or low intestinal permeability to treat VRE infections in the gastrointestinal tract. Our data suggest the intracellular targets for the molecules are putative α-carbonic and γ-carbonic anhydrases, and homology modeling and molecular dynamics simulations were performed. Together, this study presents potential anti-VRE therapeutic options to provide alternatives for problematic VRE infections.

Multidrug-resistant gram-negative organisms: a review of recently approved antibiotics and novel pipeline agents

Background The discovery of antibiotics several decades ago was a defining moment in history. They were used to treat previously incurable diseases and save many lives. However, the use of antibiotics is not benign. Antibiotic resistance occurs due to the natural evolution of bacteria and gene transfer between bacteria via vertical and horizontal routes, resulting in protective mechanisms that render antibacterial agents ineffective. Aim of the review To list and describe current, novel pipeline antibiotics indicated for multidrug-resistant gram-negative bacteria. This review discusses the limited number of novel pipeline drugs available to combat the rapidly increasing number of multidrug-resistant bacteria and the need for initiatives to research and discover more novel antibiotics. Method A search of MEDLINE/PubMed using the search terms antibacterial pipeline OR antibiotic pipeline including publications between 1 January 2018 through 23 January 2020 resulted in 230 items. The results obtained were narrowed by adding the search term AND multi-drug resistant which resulted in 12 items. Then, ClinicalTrials.gov was searched for phase 2-3 "interventional" trials registered between 1 January 2018 and 23 January 2020 with the status "recruiting" or "completed" function and including World Health Organization-defined priority pathogens in the "condition or disease" field. The search process was then completed by introducing the term antibacterial agents in the "other terms" field. The trials search and selection resulted in 13 items. Relevant English-language studies and those conducted in humans were considered. Those drugs belonging to new antibiotic classes or to antibiotic classes already known but with new chemical structure were defined as "novel antibiotics". Results The studies selected and reviewed were those referring to a novel antibiotics. Thus, from MEDLINE/PubMed, we found only 1 item referred to a novel chemical class (Murepavadin n = 1). From ClinicalTrials.gov a total of 4 citations were identified (Ftortiazinon n = 1, Zoliflodacin n = 1, Gepotidacin n = 1, ETX2514 + sulbactam n = 1). Conclusion The antibiotics annually approved by the Food and Drug Administration (FDA) mostly belong to existing classes of antibiotics and have specific indications, limiting their use in many multidrug-resistant infections. There are limited novel drug classes targeting gram-negative infections in the pipeline. Providers must be vigilant with the use of current antibiotics, especially until research and development (R&D) advancements are made.

Evaluation of N-phenyl-2-aminothiazoles for treatment of multi-drug resistant and intracellular Staphylococcus aureus infections

The increasing emergence of antibiotic-resistant bacterial pathogens calls for additional urgency in the development of new antibacterial candidates. N-Phenyl-2-aminothiazoles are promising candidates that possess potent anti-MRSA activity and could potentially replenish the MRSA antibiotic pipeline. The initial screen of a series of compounds in this novel class against several bacterial strains revealed that the aminoguanidine analogues possessed promising activities and superior safety profiles. The determined MICs of these compounds were comparable to, if not better than, those of the control drugs (linezolid and vancomycin). Remarkably, compounds 3a, 3b, and 3e possessed potent activities against multidrug resistant staphylococcal isolates and several clinically important pathogens, such as vancomycin-resistant enterococci (VRE) and Streptococcus pneumoniae. In addition, the compounds were superior to vancomycin in the rapid killing of MRSA and the longer post-antibiotic effects. Furthermore, low concentrations of compounds 3a, 3b, and 3e reduced the intracellular burden of MRSA by greater than 90%. Initial in vitro PK/toxicity assessments revealed that compound 3e was highly tolerable and possessed a low metabolic clearance rate and a highly acceptable half-life.

Repurposing the Antiamoebic Drug Diiodohydroxyquinoline for Treatment of Clostridioides difficile Infections

Clostridioides difficile, the leading cause of nosocomial infections, is an urgent health threat worldwide. The increased incidence and severity of disease, the high recurrence rates, and the dearth of effective anticlostridial drugs have created an urgent need for new therapeutic agents. In an effort to discover new drugs for the treatment of Clostridioides difficile infections (CDIs), we investigated a panel of FDA-approved antiparasitic drugs against C. difficile and identified diiodohydroxyquinoline (DIHQ), an FDA-approved oral antiamoebic drug. DIHQ exhibited potent activity against 39 C. difficile isolates, inhibiting growth of 50% and 90% of these isolates at concentrations of 0.5 μg/ml and 2 μg/ml, respectively. In a time-kill assay, DIHQ was superior to vancomycin and metronidazole, reducing a high bacterial inoculum by 3 log₁₀ within 6 h. Furthermore, DIHQ reacted synergistically with vancomycin and metronidazole against C. difficile in vitro. Moreover, at subinhibitory concentrations, DIHQ was superior to vancomycin and metronidazole in inhibiting two key virulence factors of C. difficile, toxin production and spore formation. Additionally, DIHQ did not inhibit the growth of key species that compose the host intestinal microbiota, such as Bacteroides, Bifidobacterium, and Lactobacillus spp. Collectively, our results indicate that DIHQ is a promising anticlostridial drug that warrants further investigation as a new therapeutic for CDIs.

Oxadiazolylthiazoles as novel and selective antifungal agents

Studying the structure-activity relationships (SAR) of oxadiazolylthiazole antibiotics unexpectedly led us to identify ethylenediamine- and propylenediamine-analogs as potential antimycotic novel lead structures. Replacement of the ethylenediamine moiety for the lead compound 7 with cis-diaminocyclohexyl group (compound 18) significantly enhanced the antifungal activity. In addition to the high safety margin of 18 against mammalian cells, it showed highly selective broad-spectrum activity against fungal cells without inhibiting the human normal microbiota. The antifungal activity of 18 was investigated against 20 drug-resistant clinically important fungi, including Candida species, Cryptococcus, and Aspergillus fumigatus strains. In addition to the low MIC values that mostly ranged between 0.125 and 2.0 μg/mL, compound 18 outperformed fluconazole in disrupting mature Candida biofilm.

Balancing Physicochemical Properties of Phenylthiazole Compounds with Antibacterial Potency by Modifying the Lipophilic Side Chain

Bacterial resistance to antibiotics is presently one of the most pressing healthcare challenges and necessitates the discovery of new antibacterials with unique chemical scaffolds. However, the determination of the optimal balance between structural requirements for pharmacological action and pharmacokinetic properties of novel antibacterial compounds is a significant challenge in drug development. The incorporation of lipophilic moieties within a compound's core structure can enhance biological activity but have a deleterious effect on drug-like properties. In this Article, the lipophilicity of alkynylphenylthiazoles, previously identified as novel antibacterial agents, was reduced by introducing cyclic amines to the lipophilic side chain. In this regard, substitution with methylpiperidine (compounds 14-16) and thiomorpholine (compound 19) substituents significantly enhanced the aqueous solubility profile of the new compounds more than 150-fold compared to the first-generation lead compound 1b. Consequently, the pharmacokinetic profile of compound 15 was significantly enhanced with a notable improvement in both half-life and the time the compound's plasma concentration remained above its minimum inhibitory concentration (MIC) against methicillin-resistant Staphylococcus aureus (MRSA). In addition, compounds 14-16 and 19 were found to exert a bactericidal mode of action against MRSA and were not susceptible to resistance formation after 14 serial passages. Moreover, these compounds (at 2× MIC) were superior to the antibiotic vancomycin in the disruption of the mature MRSA biofilm. The modifications to the alkynylphenylthiazoles reported herein successfully improved the pharmacokinetic profile of this new series while maintaining the compounds' biological activity against MRSA.

Synthesis and antimicrobial evaluation of new halogenated 1,3-Thiazolidin-4-ones

The ongoing prevalence of multidrug-resistant bacterial pathogens requires the development of new effective antibacterial agents. In this study, two series of halogenated 1,3-thiazolidin-4-ones were synthesized and characterized. All the synthesized thiazolidinone derivatives were evaluated for their antimicrobial activity. Biological screening of the tested compounds revealed the antibacterial activity of the chlorinated thiazolidinones 4a, 4b and 4c against Escherichia coli TolC-mutant, with MIC values of 16 µg/mL. A combination of a sub-inhibitory concentration of colistin (0.25 × MIC) with compounds 4a, 4b or 4c showed antibacterial activity against different Gram-negative bacteria (MICs = 4-16 µg/mL). Interestingly, compounds 4a, 4b and 4c were not cytotoxic to murine fibroblasts and Caco-2 cells. The chlorinated thiazolidinone derivative 16d demonstrated a bacteriostatic activity against a panel of pathogenic Gram-positive bacteria, including clinical isolates of methicillin and vancomycin-resistant Staphylococcus aureus, Listeria monocytogenes and multidrug-resistant Staphylococcus epidermidis (MICs = 8 - 64 µg/mL), with no cytotoxicity against both Caco-2 and L929 cells. Compound 16d was superior to vancomycin in disruption of the pre-formed MRSA biofilm. Furthermore, the three fluorinated thiazolidinone derivatives 26c, 30c and 33c showed a hindrance to hemolysin activity, without cytotoxicity against L929 cells.

Identification of a Phenylthiazole Small Molecule with Dual Antifungal and Antibiofilm Activity Against Candida albicans and Candida auris

Candida species are a leading source of healthcare infections globally. The limited number of antifungal drugs combined with the isolation of Candida species, namely C. albicans and C. auris, exhibiting resistance to current antifungals necessitates the development of new therapeutics. The present study tested 85 synthetic phenylthiazole small molecules for antifungal activity against drug-resistant C. albicans. Compound 1 emerged as the most potent molecule, inhibiting growth of C. albicans and C. auris strains at concentrations ranging from 0.25–2 µg/mL. Additionally, compound 1 inhibited growth of other clinically-relevant yeast (Cryptococcus) and molds (Aspergillus) at a concentration as low as 0.50 µg/mL. Compound 1 exhibited rapid fungicidal activity, reducing the burden of C. albicans and C. auris below the limit of detection within 30 minutes. Compound 1 exhibited potent antibiofilm activity, similar to amphotericin B, reducing the metabolic activity of adherent C. albicans and C. auris biofilms by more than 66% and 50%, respectively. Furthermore, compound 1 prolonged survival of Caenorhabditis elegans infected with strains of C. albicans and C. auris, relative to the untreated control. The present study highlights phenylthiazole small molecules, such as compound 1, warrant further investigation as novel antifungal agents for drug-resistant Candida infections.

Inhibitors of Intracellular Gram-Positive Bacterial Growth Synthesized via Povarov-Doebner Reactions

Staphylococcus aureus can survive both inside and outside of phagocytic and nonphagocytic host cells. Once in the intracellular milieu, most antibiotics have reduced ability to kill S. aureus, thus resulting in relapse of infection. Consequently, there is a need for antibacterial agents that can accumulate to lethal concentrations within host cells to clear intracellular infections. We have identified tetrahydrobenzo[a or c]phenanthridine and tetrahydrobenzo[a or c]acridine compounds, synthesized via a one-flask Povarov-Doebner operation from readily available amines, aldehydes, and cyclic ketones, as potent agents against drug-resistant S. aureus. Importantly, the tetrahydrobenzo[a or c]phenanthridine and tetrahydrobenzo[a or c]acridine compounds can accumulate in macrophage cells and reduce the burden of intracellular MRSA better than the drug of choice, vancomycin. We observed that MRSA could not develop resistance (by passage 30) against tetrahydrobenzo[a or c]acridine compound 15. Moreover, tetrahydrobenzo[c]acridine compound 15 and tetrahydrobenzo[c]phenanthridine compound 16 were nontoxic to red blood cells and were nonmutagenic. Preliminary data indicated that compound 16 reduced bacterial load (MRSA USA300) in mice (thigh infection model) to the same degree as vancomycin. These observations suggest that compounds 15 and 16 and analogues thereof could become therapeutic agents for the treatment of chronic MRSA infections.

Development of benzimidazole-based derivatives as antimicrobial agents and their synergistic effect with colistin against gram-negative bacteria

Gram-negative bacteria pose a distinctive risk worldwide, especially with the evolution of major resistance to carbapenems, fluoroquinolones and colistin. Therefore, development of new antibacterial agents to target Gram-negative infections is of utmost importance. Using phenotypic screening, we synthesized and tested thirty-one benzimidazole derivatives against E. coli JW55031 (TolC mutant strain). Compound 6c showed potent activity with MIC value of 2 μg/ml, however, it lacked activity against several Gram-negative microbes with intact efflux systems, including E. coli BW25113 (wild-type strain). Combination of 6c with colistin partially restored its antibacterial activity against wild strains (MIC range, 8-16 μg/ml against E. coli, K. pneumoniae, A. baumannii, and P. aeruginosa). 6c exhibited no cytotoxicity against two mammalian cell lines. Therefore, compound 6c represents a promising lead for further optimization to overcome Gram-negative resistance alone or in combination therapy.

Modifying the lipophilic part of phenylthiazole antibiotics to control their drug-likeness

Compounds with high lipophilic properties are often associated with bad physicochemical properties, triggering many off-targets, and less likely to pass clinical trials. Two metabolically stable phenylthiazole antibiotic scaffolds having notable high lipophilic characters, one with alkoxy side chain and the other one with alkynyl moiety, were derivatized by inserting a cyclic amine at the lipophilic tail with the objective of improving physicochemical properties and the overall pharmacokinetic behavior. Only alkynyl derivatives with 4- or 5-membered rings showed remarkable antibacterial activity. The azetidine-containing compound 8 was the most effective and it revealed a potent antibacterial effect against 15 multi-drug resistant (MDR)-Gram positive pathogens including Staphylococcus aureus, Streptococcus pneumoniae, Staphylococcus epidermidis and enterococci. Compound 8 was also highly effective in clearing 99.7% of the intracellular methicillin-resistant S. aureus (MRSA) harbored inside macrophages. In addition to the remarkable enhancement in aqueous solubility, the in vivo pharmacokinetic study in rats indicated that compound 8 can penetrate gut cells and reach plasma at a therapeutic concentration within 15 min and maintain effective plasma concentration for around 12 h. Interestingly, the main potential metabolite (compound 9) was also active as an antibacterial agent with potent antibiofilm activity.

Peptidoglycan pathways: there are still more!

The discovery of 3rd and 4th generations of currently existing classes of antibiotics has not hindered bacterial resistance, which is escalating at an alarming global level. This review follows WHO recommendations through implementing new criteria for newly discovered antibiotics. These recommendations focus on abandoning old scaffolds and hitting new targets. In light of these recommendations, this review discusses seven bacterial proteins that no commercial antibiotics have targeted yet, alongside their reported chemical scaffolds.

From Phenylthiazoles to Phenylpyrazoles: Broadening the Antibacterial Spectrum toward Carbapenem-Resistant Bacteria

The narrow antibacterial spectrum of phenylthiazole antibiotics was expanded by replacing central thiazole with a pyrazole ring while maintaining its other pharmacophoric features. The most promising derivative, compound 23, was more potent than vancomycin against multidrug-resistant Gram-positive clinical isolates, including vancomycin- and linezolid-resistant methicillin-resistant Staphylococcus aureus (MRSA), with a minimum inhibitory concentration (MIC) value as low as 0.5 μg/mL. Moreover, compound 23 was superior to imipenem and meropenem against highly pathogenic carbapenem-resistant strains, such as Acinetobacter baumannii, Klebsiella pneumoniae, and Escherichia coli. In addition to the notable biofilm inhibition activity, compound 23 outperformed both vancomycin and kanamycin in reducing the intracellular burden of both Gram-positive and Gram-negative pathogenic bacteria. Compound 23 cleared 90% of intracellular MRSA and 98% of Salmonella enteritidis at 2× the MIC. Moreover, preliminary pharmacokinetic investigations indicated that this class of novel antibacterial compounds is highly metabolically stable with a biological half-life of 10.5 h, suggesting a once-daily dosing regimen.

Phenylthiazoles with nitrogenous side chain: An approach to overcome molecular obesity

A novel series of phenylthiazoles bearing cyclic amines at the phenyl-4 position was prepared with the objective of decreasing lipophilicity and improving the overall physicochemical properties and pharmacokinetic profile of the compounds. Briefly, the piperidine ring (compounds 10 and 12) provided the best ring size in terms of antibacterial activity when tested against 16 multidrug-resistant clinical isolates. Both compounds were superior to vancomycin in the ability to eliminate methicillin-resistant Staphylococcus aureus (MRSA), residing within infected macrophages and to disrupt mature MRSA biofilm. Additionally, compounds 10 and 12 exhibited a fast-bactericidal mode of action in vitro. Furthermore, the new derivatives were 160-times more soluble in water than the previous lead compound 1b. Consequently, compound 10 was orally bioavailable with a highly-acceptable pharmacokinetic profile in vivo that exhibited a half-life of 4 h and achieved a maximum plasma concentration that exceeded the minimum inhibitory concentration (MIC) values against all tested bacterial isolates.

Lipophilic efficient phenylthiazoles with potent undecaprenyl pyrophosphatase inhibitory activity

Antibiotic resistance remains a pressing medical challenge for which novel antibacterial agents are urgently needed. The phenylthiazole scaffold represents a promising platform to develop novel antibacterial agents for drug-resistant infections. However, enhancing the physicochemical profile of this class of compounds remains a challenging endeavor to address to successfully translate these molecules into novel antibacterial agents in the clinic. We extended our understanding of the SAR of the phenylthiazoles' lipophilic moiety by exploring its ability to accommodate a hydrophilic group or a smaller sized hetero-ring with the objective of enhancing the physicochemical properties of this class of novel antimicrobials. Overall, the 2-thienyl derivative 20 and the hydroxyl-containing derivative 31 emerged as the most promising antibacterial agents inhibiting growth of drug-resistant Staphylococcus aureus at a concentration as low as 1 μg/mL. Remarkably, compound 20 suppressed bacterial undecaprenyl pyrophosphatase (UppP), the molecular target of the phenylthiazole compounds, in a sub nano-molar concentration range (almost 20,000 times more potent than the lead compounds 1a and 1b). Compound 31 possessed the most balanced antibacterial and physicochemical profile. The compound exhibited rapid bactericidal activity against S. aureus, and successfully cleared intracellular S. aureus within infected macrophages. Furthermore, insertion of the hydroxyl group enhanced the aqueous solubility of 31 by more than 50-fold relative to the first-generation lead 1c.

Second-generation aryl isonitrile compounds targeting multidrug-resistant Staphylococcus aureus

Antibiotic resistance remains a major global public health threat that requires sustained discovery of novel antibacterial agents with unexploited scaffolds. Structure-activity relationship of the first-generation aryl isonitrile compounds we synthesized led to an initial lead molecule that informed the synthesis of a second-generation of aryl isonitriles. From this new series of 20 compounds, three analogues inhibited growth of methicillin-resistant Staphylococcus aureus (MRSA) (from 1 to 4 µM) and were safe to human keratinocytes. Compound 19, with an additional isonitrile group exhibited improved activity against MRSA compared to the first-generation lead compound. This compound emerged as a candidate worthy of further investigation and further reinforced the importance of the isonitrile functionality in the compounds' anti-MRSA activity. In a murine skin wound model, 19 significantly reduced the burden of MRSA, similar to the antibiotic fusidic acid. In summary, 19 was identified as a new lead aryl isonitrile compound effective against MRSA.

tert-Butylphenylthiazoles with an oxadiazole linker: a novel orally bioavailable class of antibiotics exhibiting antibiofilm activity

The structure–activity and structure–kinetic relationships of a new tert-butylphenylthiazole series with oxadiazole linkers were conducted with the objective of obtaining a new orally available antibacterial compounds. Twenty-two new compounds were prepared, purified and identified. Their activity against methicillin-resistant Staphylococcus aureus were examined. Compound 20 with 3-hydroxyazetidine as a nitrogenous side chain showed promising activity against twenty-four clinical isolates, including vancomycin-resistant staphylococcal and enterococcal species with MIC values ranging from 4–8 μg mL⁻¹. Additional advantages of this compound include an ability to eradicate staphylococcal biofilm mass in a dose-dependent manner as well as high metabolic stability after an oral dose of 25 mg kg⁻¹ with a biological half-life that exceeds 5 hours and a plasma concentration (C_max) that exceeds the MIC values.

The structure–activity and structure–kinetic relationships of a new tert-butylphenylthiazole series with oxadiazole linkers were conducted with the objective of obtaining a new orally available antibacterial compounds.

Naphthylthiazoles: Targeting Multidrug-Resistant and Intracellular Staphylococcus aureus with Biofilm Disruption Activity

Thirty-two new naphthylthiazole derivatives were synthesized with the aim of exploring their antimicrobial effect on multidrug-resistant Gram-positive bacteria. Compounds 25 and 32, with ethylenediamine and methylguanidine side chains, represent the most promising derivatives, as their antibacterial spectrum includes activity against multidrug-resistant staphylococcal and enterococcal strains. Moreover, the new derivatives are highly advantageous over the existing frontline therapeutics for the treatment of multidrug-resistant Gram-positive bacteria. In this vein, compound 25 possesses three attributes: no bacterial resistance was developed against it even after 15 passages, it was very efficient in targeting intracellular pathogens, and it exhibited a concentration-dependent ability to disrupt the preformed bacterial biofilm.