Pyrazolo[4,3-c][1,2]benzothiazines 5,5-Dioxide: A Promising New Class of Staphylococcus aureus NorA Efflux Pump Inhibitors

Enantioselective synthesis of C3-functionalized 2,1-benzothiazine 2,2-dioxides by N-heterocyclic carbene catalysis

We present herein an approach for the enantioselective C3-functionalization of 2,1-benzothiazine 2,2-dioxides using N-heterocyclic carbene (NHC) catalysis. Our method involves a sequence of [3+3] cycloaddition and ring-opening reactions with different N- and O-nucleophiles, followed by silylation. Overcoming the challenge of selectivity targeting the C3 position, this protocol demonstrates a broad substrate scope and high enantioselectivity. This marks a significant advancement in the field of NHC-catalyzed transformations.

Endophytes: a uniquely tailored source of potential antibiotic adjuvants

Multidrug microbial resistance is risking an annual loss of more than 10 million people' lives by 2050. Solutions include the rational use of antibiotics and the use of drugs that reduce resistance or completely obliterate them. Here endophytes come to play due to their high-yield production and inherent nature to produce antimicrobial molecules. Around 40%, 45% and 17% of antibacterial agents were obtained from fungi, actinomycetes, and bacteria, respectively, whose secondary metabolites revealed effectiveness against resistant microbes such as MRSA, MRSE, and Shigella flexneri. Endophyte's role was not confined to bactericidal effect but extended to other mechanisms against MDR microbes, among which was the adjuvant role or the "magic bullets". Scarce focus was given to antibiotic adjuvants, and many laboratories today just screen for the antimicrobial activity without considering combinations with traditional antibiotics, which means real loss of promising resistance combating molecules. While some examples of synthetic adjuvants were introduced in the last decade, the number is still far from covering the disused antibiotics and restoring them back to clinical use. The data compiled in this article demonstrated the significance of quorum sensing as a foreseen mechanism for adjuvants from endophytes secondary metabolites, which call for urgent in-depth studies of their molecular mechanisms. This review, comprehensively and for the first time, sheds light on the significance of endophytes secondary metabolites in solving AMR problem as AB adjuvants.

Unraveling the ligand specificity and promiscuity of the Staphylococcus aureus NorA efflux pump: a computational study

Staphylococcus aureus, a gram-positive bacterial pathogen, develops antibiotic resistance partly through enhanced activity of transmembrane multi-drug efflux pump proteins like NorA. Being a prominent member of the Major Facilitator Superfamily (MFS), NorA transports various small molecules including hydrophilic fluoroquinolone antibiotics across the cell membrane. Intriguingly, NorA is inhibited by a structurally diverse set of small molecule inhibitors as well, indicating a highly promiscuous ligand/inhibitor recognition. Our study aims to elucidate the structural facets of this promiscuity. Known NorA inhibitors were grouped into five clusters based on chemical class and docked into ligand binding pockets on NorA conformations generated via molecular dynamics simulations. We discovered that several key residues, such as I23, E222, and F303, are involved in inhibitor binding. Additionally, residues I244, T223, F303, and F140 were identified as prominent in interactions with specific ligand clusters. Our findings suggest that NorA's substrate binding site, encompassing residues aiding ligand recognition based on chemical nature, facilitates the recognition of chemically diverse ligands. This insight into NorA's structural promiscuity in ligand recognition not only enhances understanding of antibiotic resistance mechanisms in S. aureus but also sets the stage for the development of more effective efflux pump inhibitors, vital for combating multidrug resistance.Communicated by Ramaswamy H. Sarma.

Bibliometric insights into the most influential papers on antibiotic adjuvants: a comprehensive analysis

Background: The utilization of antibiotic adjuvants presents a promising strategy for addressing bacterial resistance. Recently, the development of antibiotic adjuvants has attracted considerable attention from researchers in academia and industry. This study aimed to identify the most influential publications on antibiotic adjuvants and elucidate the hotspots and research trends in this field. Method: Original articles and reviews related to antibiotic adjuvants were retrieved from the Web of Science Core Collection database. The top 100 highly cited publications were selected and the visual analyses of publication outputs, countries, institutions, authors, journals, and keywords were conducted using Excel, VOSviewer, or CtieSpace software tools. Results: The top 100 cited publications concerning antibiotic adjuvants spanned the years 1977-2020, with citation counts ranging from 174 to 2,735. These publications encompassed 49 original articles and 51 reviews. The journal "Antimicrobial Agents and Chemotherapy" accounted for the highest number of publications (12%). The top 100 cited publications emanated from 39 countries, with the United States leading in production. Institutions in Canada and the United States exhibited the most substantial contributions to these highly cited publications. A total of 526 authors participated in these studies, with Robert E.W. Hancock, Laura J. V. Piddock, Xian-Zhi Li, Hiroshi Nikaido, and Olga Lomovskaya emerging as the most frequently nominated authors. The most common keywords included "E. coli", "P. aeruginosa", "S. aureus", "in-vitro activity", "antimicrobial peptide", "efflux pump inhibitor" "efflux pump", "MexAB-OprM" and "mechanism". These keywords underscored the hotspots of bacterial resistance mechanisms and the development of novel antibiotic adjuvants. Conclusion: Through the bibliometric analysis, this study identified the top 100 highly cited publications on antibiotic adjuvants. Moreover, the findings offered a comprehensive understanding of the characteristics and frontiers in this field.

Inhibition of Staphylococcus pseudintermedius Efflux Pumps by Using Staphylococcus aureus NorA Efflux Pump Inhibitors

One promising approach in treating antibiotic-resistant bacteria is to "break" resistances connected with antibacterial efflux by co-administering efflux pump inhibitors (EPIs) with antibiotics. Here, ten compounds, previously optimized to restore the susceptibility to ciprofloxacin (CIP) of norA-overexpressing Staphylococcus aureus, were evaluated for their ability to inhibit norA-mediated efflux in Staphylococcus pseudintermedius and synergize with CIP, ethidium bromide (EtBr), gentamycin (GEN), and chlorhexidine digluconate (CHX). We focused efforts on S. pseudintermedius as a pathogenic bacterium of concern within veterinary and human medicine. By combining data from checkerboard assays and EtBr efflux inhibition experiments, the hits 2-arylquinoline 1, dihydropyridine 6, and 2-phenyl-4-carboxy-quinoline 8 were considered the best EPIs for S. pseudintermedius. Overall, most of the compounds, except for 2-arylquinoline compound 2, were able to fully restore the susceptibility of S. pseudintermedius to CIP and synergize with GEN as well, while the synergistic effect with CHX was less significant and often did not show a dose-dependent effect. These are valuable data for medicinal chemistry optimization of EPIs for S. pseudintermedius and lay the foundation for further studies on successful EPIs to treat staphylococcal infections.

Antibiotic Adjuvants: A Versatile Approach to Combat Antibiotic Resistance

The problem of antibiotic resistance is on the rise, with multidrug-resistant strains emerging even to the last resort antibiotics. The drug discovery process is often stalled by stringent cut-offs required for effective drug design. In such a scenario, it is prudent to delve into the varying mechanisms of resistance to existing antibiotics and target them to improve antibiotic efficacy. Nonantibiotic compounds called antibiotic adjuvants which target bacterial resistance can be used in combination with obsolete drugs for an improved therapeutic regime. The field of "antibiotic adjuvants" has gained significant traction in recent years where mechanisms other than β-lactamase inhibition have been explored. This review discusses the multitude of acquired and inherent resistance mechanisms employed by bacteria to resist antibiotic action. The major focus of this review is how to target these resistance mechanisms by the use of antibiotic adjuvants. Different types of direct acting and indirect resistance breakers are discussed including enzyme inhibitors, efflux pump inhibitors, inhibitors of teichoic acid synthesis, and other cellular processes. The multifaceted class of membrane-targeting compounds with poly pharmacological effects and the potential of host immune-modulating compounds have also been reviewed. We conclude with providing insights about the existing challenges preventing clinical translation of different classes of adjuvants, especially membrane-perturbing compounds, and a framework about the possible directions which can be pursued to fill this gap. Antibiotic-adjuvant combinatorial therapy indeed has immense potential to be used as an upcoming orthogonal strategy to conventional antibiotic discovery.

Medicinal Chemistry of Inhibitors Targeting Resistant Bacteria

The discovery of antibiotics was a revolutionary feat that provided countless health benefits. The identification of penicillin by Alexander Fleming initiated the era of antibiotics, represented by constant discoveries that enabled effective treatments for the different classes of diseases caused by bacteria. However, the indiscriminate use of these drugs allowed the emergence of resistance mechanisms of these microorganisms against the available drugs. In addition, the constant discoveries in the 20th century generated a shortage of new molecules, worrying health agencies and professionals about the appearance of multidrug-resistant strains against available drugs. In this context, the advances of recent years in molecular biology and microbiology have allowed new perspectives in drug design and development, using the findings related to the mechanisms of bacterial resistance to generate new drugs that are not affected by such mechanisms and supply new molecules to be used to treat resistant bacterial infections. Besides, a promising strategy against bacterial resistance is the combination of drugs through adjuvants, providing new expectations in designing new antibiotics and new antimicrobial therapies. Thus, this manuscript will address the main mechanisms of bacterial resistance under the understanding of medicinal chemistry, showing the main active compounds against efflux mechanisms, and also the application of the use of drug delivery systems, and finally, the main potential natural products as adjuvants or with promising activity against resistant strains.

Urea-Based Ligand as an Efflux Pump Inhibitor: Warhead to Counter Ciprofloxacin Resistance and Inhibit Collagen Adhesion by MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is a frontline human pathogen in which efflux pump activity confers high levels of antibiotic-resistance and poses a therapeutic challenge in the clinics. The present study illustrates the potential of urea-based ligand as an efflux pump inhibitor (EPI) in order to restore the efficacy of ciprofloxacin (CPX) against MRSA. Among eight structurally varying urea-based ligands, the ligand C8 could significantly inhibit efflux pump activity in the clinical MRSA strain S. aureus 4s and was superior to the known EPI reserpine. In combinatorial treatment, C8 enhanced cellular accumulation of CPX, rendered a 16× decrease in the MIC of CPX, and restored the susceptibility of S. aureus 4s to CPX. Notably, C8 downregulated the expression of norA gene coding for the efflux pump in MRSA and treatment with 10 μM C8 and 2.0 μM CPX prevented emergence of the CPX resistance trait and suppressed MRSA cell growth till 120 generations. For potential anti-MRSA therapy, C8-loaded poly(d,l-lactide-co-glycolide) nanocarrier (C8-PNC) was generated, which facilitated facile release of C8 in physiologically relevant fluid. C8-PNC (loaded with 50 μM C8) rendered efflux pump inhibition and eliminated MRSA in combination with only 2.0 μM CPX. Treatment with the non-toxic C8-PNC (loaded with 50 μM C8) and CPX (2.0 μM) also hindered MRSA adhesion on collagen manifold higher as compared to cells treated with 32 μM CPX and significantly downregulated norA gene expression in non-adhered MRSA cells. The urea-based ligand presented herein is a promising biocompatible therapeutic material for effective mitigation of MRSA infections.

Efflux-Mediated bile Resistance in Gram-Positive Pathogens

Gram-positive pathogens are causing many serious infections that affect humans and result in mild to severe diseases worldwide. In order to survive and initiate infection, enteric pathogens must resist the physiochemical defence factors in the human intestinal tract. One of these defence factors is bile, a potent antibacterial like compound in the intestine. Efflux pumps are the important mechanism by which bacteria resist antibacterial agents such as bile. Efflux of antimicrobial substances outside the bacterial cell is considered as a key factor for intestinal colonization and virulence of enteric pathogens. This paper will review the research conducted on efflux–mediated bile resistance in Staphylococcus aureus, Listeria monocytogenes, Enterococcus faecalis and Clostridium perfringens. These bacteria colonize in the human & animal gastrointestinal tract and they have a multiple mechanism to resist the innate defences in the gut and antibacterial activity of bile. However, bile resistance in these bacteria is not fully understood. The evidence from this review suggests that Gram-positive pathogens have the ability to active transport of bile. Further research is needed to know how these pathogens sense bile and how bile regulates its virulence factor. In general, therefore, it seems that understanding the specific mechanism of bile resistance in enteric bacteria including gram-positive pathogens may involve in the development of novel strategies to control and treatment of gastrointestinal infections.

Efflux Pump Mediated Antimicrobial Resistance by Staphylococci in Health-Related Environments: Challenges and the Quest for Inhibition

The increasing emergence of antimicrobial resistance in staphylococcal bacteria is a major health threat worldwide due to significant morbidity and mortality resulting from their associated hospital- or community-acquired infections. Dramatic decrease in the discovery of new antibiotics from the pharmaceutical industry coupled with increased use of sanitisers and disinfectants due to the ongoing COVID-19 pandemic can further aggravate the problem of antimicrobial resistance. Staphylococci utilise multiple mechanisms to circumvent the effects of antimicrobials. One of these resistance mechanisms is the export of antimicrobial agents through the activity of membrane-embedded multidrug efflux pump proteins. The use of efflux pump inhibitors in combination with currently approved antimicrobials is a promising strategy to potentiate their clinical efficacy against resistant strains of staphylococci, and simultaneously reduce the selection of resistant mutants. This review presents an overview of the current knowledge of staphylococcal efflux pumps, discusses their clinical impact, and summarises compounds found in the last decade from plant and synthetic origin that have the potential to be used as adjuvants to antibiotic therapy against multidrug resistant staphylococci. Critically, future high-resolution structures of staphylococcal efflux pumps could aid in design and development of safer, more target-specific and highly potent efflux pump inhibitors to progress into clinical use.

Stress impact of liposomes loaded with ciprofloxacin on the expression level of MepA and NorB efflux pumps of methicillin-resistant Staphylococcus aureus

One mechanism of ciprofloxacin resistance is attributed to chromosomal DNA-encoded efflux pumps such as the MepA and NorB proteins. The goal of this research is to find a way to bypass Staphylococcus aureus' efflux pumps. Because of its high membrane permeability and low association with NorB and MepA efflux proteins, a liposome-encapsulating antibiotic is one of the promising, cost-effective drug carriers and coating mechanisms for overcoming active transport of methicillin-resistant S. aureus (MRSA) multidrug-resistant efflux protein . The calculated "Log Perm RRCK" membrane permeability values of 1,2-distearoyl-sn-glycerol-3-phosphocholine (DSPC) ciprofloxacin liposome-encapsulated (CFL) showed a lower negative value of - 4,652 cm/s and greater membrane permeability than ciprofloxacin free (CPF). The results of RT-qPCR showed that cationic liposomes containing ciprofloxacin in liposome-encapsulated form (CFL) improved CPF antibacterial activity and affinity for negatively charged bacterial cell surface membrane in comparison to free drug and liposome, as it overcame several resistance mechanisms and reduced the expression of efflux pumps. Ciprofloxacin liposome-encapsulated (CFL) is therefore more effective than ciprofloxacin alone. Liposomes can be combined with a variety of drugs that interact with bacterial cell efflux pumps to maintain high sustained levels of antibiotics in bacterial cells.

Development of an Antibiotic Resistance Breaker to Resensitize Drug-Resistant Staphylococcus aureus: In Silico and In Vitro Approach

Efflux pumps are one of the predominant microbial resistant mechanisms leading to the development of multidrug resistance. In Staphylococcus aureus, overexpression of NorA protein enables the efflux of antibiotics belonging to the class of fluoroquinolones and, thus, makes S. aureus resistant. Hence, NorA efflux pumps are being extensively exploited as the potential drug target to evade bacterial resistance and resensitize bacteria to the existing antibiotics. Although several molecules are reported to inhibit NorA efflux pump effectively, boronic acid derivatives were shown to have promising NorA efflux pump inhibition. In this regard, the current study exploits 6-(3-phenylpropoxy)pyridine-3-boronic acid to further improve the activity and reduce cytotoxicity using the bioisostere approach, a classical medicinal chemistry concept. Using the SWISS-Bioisostere online tool, from the parent compound, 42 compounds were obtained upon the replacement of the boronic acid. The 42 compounds were docked with modeled NorA protein, and key molecular interactions of the prominent compounds were assessed. The top hit compounds were further analyzed for their drug-like properties using ADMET studies. The identified potent lead, 5-nitro-2-(3-phenylpropoxy)pyridine (5-NPPP), was synthesized, and in vitro efficacy studies have been proven to show enhanced efflux inhibition, thus acting as a potent antibiotic breaker to resensitize S. aureus without elucidating any cytotoxic effect to the host Hep-G2 cell lines.

An Update on Staphylococcus aureus NorA Efflux Pump Inhibitors

BACKGROUND

Methicillin-resistant and vancomycin-resistant Staphylococcus aureus are pathogens causing severe infectious diseases that pose real public health threats problems worldwide. In S. aureus, the most efficient multidrug-resistant system is the NorA efflux pump. For this reason, it is critical to identify efflux pump inhibitors.

OBJECTIVE

In this paper, we present an update of the new natural and synthetic compounds that act as modulators of antibiotic resistance through the inhibition of the S. aureus NorA efflux pump.

RESULTS

Several classes of compounds capable of restoring the antibiotic activity have been identified against resistant-S. aureus strains, acting as NorA efflux pump inhibitors. The most promising classes of compounds were quinolines, indoles, pyridines, phenols, and sulfur-containing heterocycles. However, the substantial degree structural diversity of these compounds makes it difficult to establish good structure- activity correlations that allow the design of compounds with more promising activities and properties.

CONCLUSION

Despite substantial efforts put forth in the search for new antibiotic adjuvants that act as efflux pump inhibitors, and despite several promising results, there are currently no efflux pump inhibitors authorized for human or veterinary use, or in clinical trials. Unfortunately, it appears that infection control strategies have remained the same since the discovery of penicillin, and that most efforts remain focused on discovering new classes of antibiotics, rather than trying to prolong the life of available antibiotics, and simultaneously fighting mechanisms of bacterial resistance.

Antihypertensive potential of selected pyrimidine derivatives: Explanation of underlying mechanistic pathways

This study was designed to determine the effectiveness of 5-(3-Hydroxybenzylidene)-2, 4, 6(1H, 3H, 5H)-pyrimidinetrione (SR-5), 5-(4-Hydroxybenzylidene)-2, 4, 6(1H, 3H, 5H)-pyrimidinetrione (SR-8), 5-(3-Chlorobenzylidene)-2, 4, 6(1H, 3H, 5H)-pyrimidinetrione (SR-9) and 5-(4-Chlorobenzylidene)-2, 4, 6(1H, 3H, 5H)-pyrimidinetrione (SR-10) against hypertension. In deoxycorticosterone acetate-salt rats, SR-5, SR-8, SR-9, and SR-10 reduced blood pressure and normalized renal functions. In isolated rat aortic rings, SR-5, SR-8, SR-9, and SR-10 relaxed phenylephrine (PE) and K⁺-induced contractions. The vasodilator effect was endothelium-independent. Test compounds caused a rightward shift of Ca⁺⁺ and PE concentration-response curves with a reduction of maximum response. SR-5, SR-8, SR-9, and SR-10 inhibited PE peak contractions in a Ca⁺⁺ free medium. In guinea-pig atria, SR5, SR-8, SR-9, and SR-10 caused a mild-to-moderate inhibition of force and rate of contractions. In the aorta and heart tissues, the test compounds enhanced glutathione-s-transferase, reduced glutathione and catalase levels, improved cellular architecture, and decreased lipid peroxidation and expression of inflammatory markers: cyclooxygenase 2, tumor necrosis factor alpha, phosphorylated c-Jun N-terminal kinase, and phosphorylated-nuclear factor kappa B, evidenced in the immunohistochemistry, enzyme-linked immunosorbent assay, western blot molecular investigations and a decreased mRNA expression of calcium channel in RT-PCR analysis. SR-5, SR-8, SR-9, and SR-10 increased the urinary output in rats and inhibited the human platelet aggregation. This study revealed that SR-5, SR-8, SR-9, and SR-10 possess BP lowering, reno-protective, vasodilatory (mediated via Ca⁺⁺ antagonist, antioxidant and anti-inflammatory pathways), partial cardio-suppressant, diuretic, and antiplatelet effects, demonstrating their therapeutic potential in hypertension management.

Chemotherapeutic Strategies for Combating Staphylococcus aureus Infections

Staphylococcus aureus is a prominent human pathogen that causes nosocomial and community acquired infections. The accelerating emergence and prevalence of staphylococcal infections have grotesque health consequences which are mostly due to its anomalous capability to acquire drug resistance and scarcity of novel classes of antibacterials. Many combating therapies are centered on primary targets of S. aureus which are cell envelope, ribosomes and nucleic acids. This review describes various chemotherapeutic strategies for combating S. aureus infections which includes monotherapy, combination drug therapy, phage endolysin therapy, lysostaphins and antibacterial drones. Monotherapy has dwindled in due course of time but combination therapy, endolysin therapy, lysostaphin and antibacterial drones are emerging alternatives which efficiently conquer the shortcomings of monotherapy. Combinations of more than one antibiotic agents or combination of adjuvant with antibiotics provide a synergistic approach to combat infections causing pathogenic strains. Phage endolysin therapy and lysostaphin are also presents as possible alternatives to conventional antibiotic therapies. Antibacterial Drones goes a step further by specifically targeting the virulence genes in bacteria giving them a certain advantage over existing antibacterial strategies. But the challenge remains on the better understanding of these strategies for executing and implementing them in health sector. In this day and age, most of the S. aureus strains are resistant to ample number of antibiotics, so there is an urgent need to overcome such multidrug resistant strains for the welfare of our community.

Synthesis and biological evaluation as well as in silico studies of arylpiperazine-1,2-benzothiazine derivatives as novel anti-inflammatory agents

Novel arylpiperazine-1,2-benzothiazine derivatives have been designed and synthesized as potential anti-inflammatory agents. Their structure and properties have been studied using spectroscopic techniques (¹H NMR, ¹³C NMR, FT-IR), MS, elemental analyses, and single-crystal X-ray diffraction (SCXRD, for compound 7b). This study aimed to evaluate the inhibitory activity of new derivatives against both cyclooxygenase isoforms COX-1 and COX-2 due to the similarity of new compounds to oxicams drugs from the NSAIDs group. All new compounds were divided into two series - A and B - with a different linker between thiazine and piperazines nitrogens. Series A included the three-carbon aliphatic linker and series B - two-carbon with a carbonyl group. According to in vitro and molecular docking studies all new compounds exhibited cyclooxygenase inhibitory activity. The series of A compounds included COX-1 inhibitors only. In contrast, the B series showed inhibition of both COX-1 and COX-2, which suggested the importance of the acetoxy linker for COX-2 inhibition. Moreover, the most selective compound 7b, towards COX-2, was non-toxic for the normal human cell line (in concentration of 10 µM) comparable to reference drug meloxicam. Additionally, investigation of influence on model membranes confirmed the ability of the compound 7b to penetrate lipid bilayers which seemed to be important to the influence with membrane protein-cyclooxygenase.

Supramolecular self-assembled drug delivery system (SADDs) of vancomycin and tocopherol succinate as an antibacterial agent: in vitro, in silico and in vivo evaluations

In this study self-assembled drug delivery system (SADDs) composed of a hydrophobic d-α-tocopherol succinate (TS) and a hydrophilic vancomycin (VCM) were formulated, and its potential for enhancing the antibacterial activity of VCM against Staphylococcus aureus (SA) and Methicillin-resistant Staphylococcus aureus (MRSA) were explored. The SADDs were synthesized via supramolecular complexation, then characterized for in silico, in vitro and in vivo studies. In silico studies confirmed the self-assembly of VCM/TS into NPs. The size, surface charge and drug loading of the SADDs was ˂100 nm, -27 mV and 68%, respectively. The SADDs were non-hemolytic and biosafe. A sustained release of VCM from SADDs was noted, with 52.2% release after 48 hr. The in vitro antibacterial test showed a twofold decrease in Minimum inhibitory concentration (MIC) against SA and MRSA, and a significantly higher reduction in MRSA biofilms compared to bare VCM. Further, in silico studies confirmed strong and stable binding of TS to MRSA efflux pumps. The in vivo study using mice skin infection models showed a 9.5-fold reduction in bacterial load after treatment with SADDs, in comparison with bare VCM. These findings affirmed that VCM/TS NPs as a promising novel nano-delivery for treating bacterial infections.

Functional and Structural Roles of the Major Facilitator Superfamily Bacterial Multidrug Efflux Pumps

Pathogenic microorganisms that are multidrug-resistant can pose severe clinical and public health concerns. In particular, bacterial multidrug efflux transporters of the major facilitator superfamily constitute a notable group of drug resistance mechanisms primarily because multidrug-resistant pathogens can become refractory to antimicrobial agents, thus resulting in potentially untreatable bacterial infections. The major facilitator superfamily is composed of thousands of solute transporters that are related in terms of their phylogenetic relationships, primary amino acid sequences, two- and three-dimensional structures, modes of energization (passive and secondary active), and in their mechanisms of solute and ion translocation across the membrane. The major facilitator superfamily is also composed of numerous families and sub-families of homologous transporters that are conserved across all living taxa, from bacteria to humans. Members of this superfamily share several classes of highly conserved amino acid sequence motifs that play essential mechanistic roles during transport. The structural and functional importance of multidrug efflux pumps that belong to the major facilitator family and that are harbored by Gram-negative and -positive bacterial pathogens are considered here.

Assembly of Benzothiazine and Triazole in a Single Molecular Entity: Synthesis of -Oxicam Derived Novel Molecules as Potential Antibacterial/ Anti-cancer Agents

BACKGROUND

Benzothiazine derivatives, because of their various biological activities have attracted particular attention in Med Chem and drug discovery efforts. The synthetic modifications of 1,2-benzothiazine 1,1-dioxides have been undertaken in order to explore and identify novel compounds or new analogues possessing promising biological activities. In our effort we have designed -oxicam derived bezothiazine-1,2,3-triazole derivatives as potential antibacterial agents.

METHODS

These compounds were synthesized via a multi-step sequence involving the Cu catalyzed azide- alkyne cycloaddition (CuAAC) as a key step. The CuAAC proceeded at room temperature in DMF to afford 26 novel molecules in good (70-90%) yields.

RESULTS

All these compounds were tested for their antibacterial properties against four strains of bacterial microorganisms and subsequently cytotoxic properties against lung and colon cancer cell lines. The compound 4e showed activities against majority of the bacterial species used (nearly comparable to amoxicillin, ciprofloxacin and ofloxacin against P. vulgaris) whereas 4d and 4f showed cytotoxicities selective towards cancer cells.

CONCLUSION

The present bezothiazine-1,2,3-triazole framework represents a new template for the identification of novel and potent antibacterial/anticancer agents.

Celecoxib potentiates antibiotic uptake by altering membrane potential and permeability in Staphylococcus aureus

Background

We have shown previously that celecoxib enhances the antibacterial effect of antibiotics and has sensitized drug-resistant bacteria to antibiotics at low concentrations using in vitro and in vivo model systems and also using clinically isolated ESKAPE pathogens.

Objectives

To identify the mechanism of action of celecoxib in potentiating the effect of antibiotics on bacteria.

Methods

Toxicogenomic expression analysis of Staphylococcus aureus in the presence or absence of ampicillin, celecoxib or both was carried out by microarray followed by validation of microarray results by flow cytometry and real-time PCR analysis, cocrystal development and analysis.

Results

The RNA expression map clearly indicated a change in the global transcriptome of S. aureus in the presence of cells treated with ampicillin alone, which was similar to that of celecoxib-treated cells in co-treated cells. Several essential, non-essential and virulence genes such as α-haemolysin (HLA), enterotoxins and β-lactamase were differentially regulated in co-treated cells. Further detailed analysis of the expression data indicated that the ion transporters and enzymes of the lipid biosynthesis pathway were down-regulated in co-treated cells leading to decreased membrane permeability and membrane potential. Cocrystal studies using Powder-X-Ray Diffraction (PXRD) and differential scanning calorimetry (DSC) indicated interactions between celecoxib and ampicillin, which might help in the entry of antibiotics.

Conclusions

Although further studies are warranted, here we report that celecoxib alters membrane potential and permeability, specifically by affecting the Na+/K+ ion transporter, and thereby increases the uptake of ampicillin by S. aureus.

Promising antibacterial agents against multidrug resistant Staphylococcus aureus

Rapid emergence of multidrug resistant Staphylococcus aureus infections has created a critical health menace universally. Resistance to all the available chemotherapeutics has been on rise which led to WHO to stratify Staphylococcus aureus as high tier priorty II pathogen. Hence, discovery and development of new antibacterial agents with new mode of action is crucial to address the multidrug resistant Staphylococcus aureus infections. The egressing understanding of new antibacterials on their biological target provides opportunities for new therapeutic agents. This review underlines on various aspects of drug design, structure activity relationships (SARs) and mechanism of action of various new antibacterial agents and also covers the recent reports on new antibacterial agents with potent activity against multidrug resistant Staphylococcus aureus. This review provides attention on in vitro and in vivo pharmacological activities of new antibacterial agents in the point of view of drug discovery and development.

Efflux pump inhibitors of clinically relevant multidrug resistant bacteria

Bacterial infections are an increasingly serious issue worldwide. The inability of existing therapies to treat multidrug-resistant pathogens has been recognized as an important challenge of the 21st century. Efflux pumps are important in both intrinsic and acquired bacterial resistance and identification of small molecule efflux pump inhibitors (EPIs), capable of restoring the effectiveness of available antibiotics, is an active research field. In the last two decades, much effort has been made to identify novel EPIs. However, none of them has so far been approved for therapeutic use. In this article, we explore different structural families of currently known EPIs for multidrug resistance efflux systems in the most extensively studied pathogens (NorA in Staphylococcus aureus, AcrAB-TolC in Escherichia coli, and MexAB-OprM in Pseudomonas aeruginosa). Both synthetic and natural compounds are described, with structure-activity relationship studies and optimization processes presented systematically for each family individually. In vitro activities against selected test strains are presented in a unifying manner for all the EPIs described, together with the most important toxicity, pharmacokinetic and in vivo efficacy data. A critical evaluation of lead-likeness characteristics and the potential for clinical development of the most promising inhibitors of the three efflux systems is described. This overview of EPIs is a good starting point for the identification of novel effective antibacterial drugs.

Synthesis, X-ray crystal and monoamine oxidase inhibitory activity of 4,6-dihydrobenzo[c]pyrano[2,3-e][1,2]thiazine 5,5-dioxides: In vitro studies and docking analysis

We report the synthesis and biological evaluation of two new series of 2-amino-6-benzyl-4-phenyl-4,6-dihydrobenzo[c]pyrano[2,3-e][1,2]thiazine-3‑carbonitrile 5,5-dioxides and 2-amino-6-methyl-4-phenyl-4,6-dihydrobenzo[c]pyrano[2,3-e][1,2]thiazine-3‑carbonitrile 5,5-dioxides. The synthetic methodology involves a multistep reaction starting with methyl anthranilate which was coupled with methane sulfonyl chloride. The product of the reaction was subjected to N-benzylation and N-methylation reactions followed by ring closure with sodium hydride resulting in the formation of respective 2,1-benzothiazine 2,2-dioxides. These 2,1-benzothiazine precursors were subjected to multicomponent reaction with malononitrile and substituted benzaldehydes for the synthesis of two new series of pyranobenzothiazines (6a-r and 7a-r). The synthesized compounds were screened as selective inhibitors of monoamine oxidase A and monoamine oxidase B. The in vitro results suggested that compound 6d and 7q are the selective inhibitors of monoamine oxidase A, however, the selective and potent inhibitors of monoamine oxidase B included compounds 6h and 7r. Moreover, some dual inhibitors were noticed like 7l having more inhibitory activity towards both the isozymes. Moreover, the binding modes of the selective and potent inhibitors of monoamine oxidase A and B were investigated by molecular docking analysis. The results suggested that the synthetic derivatives may be potential towards the monoamine oxidase isozymes.

On water: iodine-mediated direct construction of 1,3-benzothiazines from ortho-alkynylanilines by regioselective 6-exo-dig cyclization

Herein, we report the 6-exo-dig ring closure of ortho-alkynylanilines with readily available aroyl isothiocyanate. An environmentally benign, metal- and base-free, iodine promoted cascade synthesis of highly functionalized (benzo[1,3]thiazin-2-yl)benzimidic acids has been accomplished via in situ generated ortho-alkynylthiourea. The established methodology employs the abundant chemical feedstock of ortho-alkynylanilines and aroyl isothiocyanates and could be applied in the late-stage synthesis of pharmaceutically active 1,3-benzothiazine containing molecules. Furthermore, the discovered protocol exclusively delivers bis (benzo[1,3]thiazin-2-yl)dibenzimidic acid products and preserves the iodo-olefin substitution pattern which can be exploited by further derivatization.

Synthesis of amides from (E)-3-(1-chloro-3,4-dihydronaphthalen-2-yl)acrylic acid and substituted amino acid esters as NorA efflux pump inhibitors of Staphylococcus aureus

Inhibitors for NorA efflux pump of Staphylococcus aureus have attracted the attention of many researchers towards the discovery and development of novel efflux pump inhibitors (EPIs). In an attempt to find specific potent inhibitors of NorA efflux pump of S. aureus, a total of 15 amino acid conjugates of 3-(1-chloro-3,4-dihydronaphthalen-2-yl)acrylic acid (4-18) were synthesized using a simple convenient synthetic approach and bioevaluated against NorA efflux pump. Two compounds 7 and 8 (each having MEC of 1.56 µg/mL) were found to restore the activity of ciprofloxacin through reduction of the MIC elucidated by comparing the ethidium bromide efflux in dose dependent manner in addition to ethidium bromide efflux inhibition and accumulation study using NorA overexpressing strain SA-1199B. Most potent compounds among these were able to restore the antibacterial activity of ciprofloxacin completely against SA-1199B. Structure activity relationship (SAR) studies and docking study of potent compounds 7 and 8 could elucidate the structural requirements necessary for interaction with the NorA efflux pumps. On the whole, compounds 7 and 8 have ability to reverse the NorA efflux mediated resistance and could be further optimized for development of potent efflux pump inhibitors.

Studies on 2-phenylquinoline Staphylococcus aureus NorA efflux pump inhibitors: New insights on the C-6 position

The alarming and rapid spread of antimicrobial resistance among bacteria represents a high risk for global health. Targeting factors involved in resistance to restore the activity of failing antibiotics is a promising strategy to overcome this urgent medical need. Efflux pump inhibitors are able to increase antibiotic concentrations in bacteria, thus they can be considered true antimicrobial resistance breakers. In this work, continuing our studies on inhibitors of the Staphylococcus aureus NorA pump, we designed, synthesized and biologically evaluated novel 2-phenylquinoline derivatives starting from our hits 1 and 2. Two of the synthesized compounds (6 and 7) bearing a C-6 benzyloxy group showed the best NorA inhibition activity, thereby providing an excellent starting point to direct future chemical optimizations.

Prostaglandin E2 attenuates synergistic bactericidal effects between COX inhibitors and antibiotics on Staphylococcus aureus

PGE2 is found to attenuate the bactericidal effects of kanamycin or ampicillin in Staphylococcus aureus, as well as the methicillin-resistant S. aureus (MRSA). Co-treatment with cyclooxygenase (COX) inhibitors (celecoxib, aspirin or naproxen) synergistically enhances kanamycin or ampicillin-induced cell death of S. aureus and MRSA. COX inhibitors repressed bacterial multidrug resistance through down-regulating efflux pump activity in antibiotics-treated S. aureus and MRSA. However, this synergistic bactericidal effects are reduced by the treatment with PGE2. PGE2 restores the efflux pump activity as well as increases biofilm formation in S. aureus and MRSA. Collectively, the enhancement of efflux pump activity and biofilm formation with PGE2 might partially explain the resistance to synergistic bactericidal effects between COX inhibitors and antibiotics in PGE2-treated S. aureus.

Novel 1,2,3‐Triazole‐Functionalized 1,2‐Benzothiazine 1,1‐Dioxide Derivatives: Regioselective Synthesis, Biological Evaluation and Docking Studies

A series of novel 1,2‐benzothiazine‐1,1‐dioxide derivatives (Z)‐3‐hydroxy‐1‐(4‐hydroxy‐2‐methyl‐1,1‐dioxido‐2H‐benzo[e][1, 2]thiazin‐3‐yl)‐3‐phenyl substituted prop‐2‐en‐1‐one (6 a‐d) were synthesized starting from sodium salt of saccharin 1 in series of steps via 3‐acetyl‐2‐methyl‐1,1‐dioxido‐2H‐benzo[e][1, 2]thiazin‐4‐yl substituted benzoates (5 a‐d). Compound 6 e was obtained alternately from 1‐(4‐Hydroxy‐2‐methyl‐1,1‐dioxo‐1,2‐dihydro‐1λ6‐benzo[e][1, 2]thiazin‐3‐yl)‐ethanone (4). Compounds 6 a‐e were further reacted with aromatic azides to form (4‐hydroxy‐2‐methyl‐1,1‐dioxido‐2H‐benzo[e][1, 2]thiazin‐3‐yl)(1‐substitutedphenyl)‐5‐ substituted pheny or methyl‐1H‐1,2,3‐triazol‐4‐yl)methanone derivatives (7 a‐o) by regioselective cyclization. All the compounds were evaluated for anti‐inflammatory and anti‐cancer activities. Compounds 5 a‐b, 6 a‐b, 7 a, 7 c‐d, 7 i and 7 k‐l which showed significant anti‐inflammatory activity at micro molar concentration have been identified. Also screened for cytotoxic activity against four human cancer cells and one normal cell such as prostate cancer (PC‐3), breast adenocarcinoma (MDA‐MB‐231), liver hepatocellular carcinoma (Hep G2), cervical cancer (HeLa) and normal umbilical vein endothelial cell (HUVEC). Compounds 6 a, 7 g, 7 h and 7 k have been identified as promising candidates. Further, anti‐inflammatory activity is also validated by docking studies and compounds 5 a, 5 b and 7 d found to show good interactions when docked with IL‐1β signaling complex.

Small-Molecule Potentiators for Conventional Antibiotics against Staphylococcus aureus

Antimicrobial resistance constitutes a global health problem, while the discovery and development of novel antibiotics is stagnating. Methicillin-resistant Staphylococcus aureus, responsible for the establishment of recalcitrant, biofilm-related infections, is a well-known and notorious example of a highly resistant micro-organism. Since resistance development is unavoidable with conventional antibiotics that target bacterial viability, it is vital to develop alternative treatment options on top. Strategies aimed at more subtle manipulation of bacterial behavior have recently attracted attention. Here, we provide a literature overview of several small-molecule potentiators for antibiotics, identified for the treatment of Staphylococcus aureus infection. Typically, these potentiators are not bactericidal by themselves and function by reversing resistance mechanisms, by attenuating Staphylococcus aureus virulence, and/or by interfering with quorum sensing.

Natural isoflavone biochanin A as a template for the design of new and potent 3-phenylquinolone efflux inhibitors against Mycobacterium avium

Mycobacterium avium is a difficult-to-treat pathogen able to quickly develop drug resistance. Like for other microbial species, overexpression of efflux pumps is one of the main mechanisms in developing multidrug resistance. Although the use of efflux pumps inhibitors (EPIs) represents a promising strategy to reverse resistance, to date few M. avium EPIs are known. Recently, we showed that in-house 2-phenylquinoline S. aureus NorA EPIs exhibited also a good activity against M. avium efflux pumps. Herein, we report a series of 3-phenylquinolones designed by modifying the isoflavone biochanin A, a natural EPI of the related M. smegmatis, taking into account some important SAR information obtained around the 2-phenylquinoline NorA EPIs. The 3-phenylquinolones inhibited M. avium efflux pumps with derivatives 1e and 1g that displayed the highest synergistic activity against all the strains considered in the study, bringing down (from 4- to 128-fold reduction) the MIC values of macrolides and fluoroquinolones.

Searching for Novel Inhibitors of the S. aureus NorA Efflux Pump: Synthesis and Biological Evaluation of the 3-Phenyl-1,4-benzothiazine Analogues

Bacterial resistance to antimicrobial agents has become an increasingly serious health problem in recent years. Among the strategies by which resistance can be achieved, overexpression of efflux pumps such as NorA of Staphylococcus aureus leads to a sub-lethal concentration of the antibacterial agent at the active site that in turn may predispose the organism to the development of high-level target-based resistance. With an aim to improve both the chemical stability and potency of our previously reported 3-phenyl-1,4-benzothiazine NorA inhibitors, we replaced the benzothiazine core with different nuclei. None of the new synthesized compounds showed any appreciable intrinsic antibacterial activity, and, in particular, 2-(3,4-dimethoxyphenyl)quinoline (6 c) was able to decrease, in a concentration-dependent manner, the ciprofloxacin MIC against the norA-overexpressing strains S. aureus SA-K2378 (norA++) and SA-1199B (norA+/A116E GrlA).

Pharmaceutical Approaches to Target Antibiotic Resistance Mechanisms

There is urgent need for new therapeutic strategies to fight the global threat of antibiotic resistance. The focus of this Perspective is on chemical agents that target the most common mechanisms of antibiotic resistance such as enzymatic inactivation of antibiotics, changes in cell permeability, and induction/activation of efflux pumps. Here we assess the current landscape and challenges in the treatment of antibiotic resistance mechanisms at both bacterial cell and community levels. We also discuss the potential clinical application of chemical inhibitors of antibiotic resistance mechanisms as add-on treatments for serious drug-resistant infections. Enzymatic inhibitors, such as the derivatives of the β-lactamase inhibitor avibactam, are closer to the clinic than other molecules. For example, MK-7655, in combination with imipenem, is in clinical development for the treatment of infections caused by carbapenem-resistant Enterobacteriaceae and Pseudomonas aeruginosa, which are difficult to treat. In addition, other molecules targeting multidrug-resistance mechanisms, such as efflux pumps, are under development and hold promise for the treatment of multidrug resistant infections.

Pharmacophore-Based Repositioning of Approved Drugs as Novel Staphylococcus aureus NorA Efflux Pump Inhibitors

An intriguing opportunity to address antimicrobial resistance is represented by the inhibition of efflux pumps. Focusing on NorA, the most important efflux pump of Staphylococcus aureus, an efflux pump inhibitors (EPIs) library was used for ligand-based pharmacophore modeling studies. By exploitation of the obtained models, an in silico drug repositioning approach allowed for the identification of novel and potent NorA EPIs.

Investigation on the effect of known potent S. aureus NorA efflux pump inhibitors on the staphylococcal biofilm formation

The emergence of multidrug resistant microorganisms has triggered the impending need of developing effective antibacterial strategies.

QSAR Studies on Bacterial Efflux Pump Inhibitors

Antimicrobial drug resistance occurs when bacteria undergo certain modifications to eliminate the effectiveness of drugs, chemicals, or other agents designed to cure infections. To date, the burden of resistance has remained one of the major clinical concerns as it renders prolonged and complicated treatments, thereby increasing the medical costs with lengthier hospital stays. Of complex causes for bacterial resistance, there has been increasing evidence that proved the significant role of efflux pumps in antibiotic resistance. Coadministration of Efflux Pump Inhibitors (EPIs) with antibiotics has been considered one of the promising ways not only to improve the efficacy but also to extend the clinical utility of existing antibiotics. This chapter begins with outlining current knowledge about bacterial efflux pumps and drug designs applied in identification of their modulating compounds. Following, the chapter addresses and provides a discussion on Quantitative Structure-Activity Relationship (QSAR) analyses in search of novel and potent efflux pump inhibitors.

Efflux as a mechanism of antimicrobial drug resistance in clinical relevant microorganisms: the role of efflux inhibitors

INTRODUCTION

Microbial resistance against antibiotics is a serious threat to the effective treatment of infectious diseases. Several mechanisms exist through which microorganisms can develop resistance against antimicrobial drugs, of which the overexpression of genes to produce efflux pumps is a major concern. Several efflux transporters have been identified in microorganisms, which infer resistance against specific antibiotics and even multidrug resistance. Areas covered: This paper focuses on microbial resistance against antibiotics by means of the mechanism of efflux and gives a critical overview of studies conducted to overcome this problem by combining efflux pump inhibitors with antibiotics. Information was obtained from a literature search done with MEDLINE, Pubmed, Scopus, ScienceDirect, OneSearch and EBSCO host. Expert opinion: Efflux as a mechanism of multidrug resistance has presented a platform for improved efficacy against resistant microorganisms by co-administration of efflux pump inhibitors with antimicrobial agents. Although proof of concept has been shown for this approach with in vitro experiments, further research is needed to develop more potent inhibitors with low toxicity which is clinically effective.