The Anthelmintic Drug Niclosamide Synergizes with Colistin and Reverses Colistin Resistance in Gram-Negative Bacilli

Diclofenac Sodium Restores the Sensitivity of Colistin-Resistant Gram-Negative Bacteria to Colistin

The continuous rise of multidrug-resistant (MDR) Gram-negative bacteria poses a severe threat to public health worldwide. Colistin(COL), employed as the last-line antibiotic against MDR pathogens, is now at risk due to the emergence of colistin-resistant (COL-R) bacteria, potentially leading to adverse patient outcomes. In this study, synergistic activity was observed when colistin and diclofenac sodium (DS) were combined and used against clinical COL-R strains of Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumoniae), Acinetobacter baumannii (A. baumannii), and Pseudomonas aeruginosa (P. aeruginosa) both in vitro and in vivo. The checkerboard method and time-killing assay showed that DS, when combined with COL, exhibited enhanced antibacterial activity compared to DS and COL monotherapies. Crystal violet staining and scanning electron microscopy showed that COL-DS inhibited biofilm formation compared with monotherapy. The in vivo experiment showed that the combination of DS and COL reduced bacterial loads in infected mouse thighs. Synergistic activity was observed when COL and DS were use in combination against clinical COL-R strains of E. coli, K. pneumoniae, A. baumannii and P. aeruginosa both in vitro and in vivo. The synergistic antibacterial effect of the COL-DS combination has been confirmed by performing various in vitro and in vivo experiments, which provides a new treatment strategy for infections caused by MDR bacteria.

Nitazoxanide synergizes polymyxin B against Escherichia coli by depleting cellular energy

The rapid expansion of antibiotic-resistant bacterial diseases is a global burden on public health. It makes sense to repurpose and reposition already-approved medications for use as supplementary agents in synergistic combinations with existing antibiotics. Here, we demonstrate that the anthelmintic drug nitazoxanide (NTZ) synergistically enhances the effectiveness of the lipopeptide antibiotic polymyxin B in inhibiting gram-negative bacteria, including those resistant to polymyxin B. Mechanistic investigations revealed that nitazoxanide inhibited calcium influx and cell membrane depolarization, enhanced the affinity between polymyxin B and the extracellular membrane, and promoted intracellular ATP depletion and an increase in reactive oxygen species (ROS), thus enhancing the penetration and disruption of the Escherichia coli cell membrane by polymyxin B. The transcriptomic analysis revealed that the combination resulted in energy depletion by inhibiting both aerobic and anaerobic respiration patterns in bacterial cells. The increased bactericidal effect of polymyxin B on the E. coli ∆nuoC strain further indicates that NuoC could be a promising target for nitazoxanide. Furthermore, the combination of nitazoxanide and polymyxin B showed promising therapeutic effects in a mouse infection model infected with E. coli. Taken together, these results demonstrate the potential of nitazoxanide as a novel adjuvant to polymyxin B, to overcome antibiotic resistance and improve therapeutic outcomes in refractory infections.IMPORTANCEThe rapid spread of antibiotic-resistant bacteria poses a serious threat to public health. The search for potential compounds that can increase the antibacterial activity of existing antibiotics is a promising strategy for addressing this issue. Here, the synergistic activity of the FDA-approved agent nitazoxanide (NTZ) combined with polymyxin B was investigated in vitro using checkerboard assays and time-kill curves. The synergistic mechanisms of the combination of nitazoxanide and polymyxin B were explored by fluorescent dye, transmission electron microscopy (TEM), and transcriptomic analysis. The synergistic efficacy was evaluated in vivo by the Escherichia coli and mouse sepsis models. These results suggested that nitazoxanide, as a promising antibiotic adjuvant, can effectively enhance polymyxin B activity, providing a potential strategy for treating multidrug-resistant bacteria.

Sulfonamide Bioisosteres of Niclosamide Enhance Antibacterial Activity of Colistin and Bacitracin

Multicomponent therapy combining antibiotics with enhancer molecules known as adjuvants is an emerging strategy to combat antimicrobial resistance. Niclosamide is a clinically relevant anthelmintic drug with potential to be repurposed for its inherent antibacterial activity against Gram-positive bacteria and its ability to potentiate the antibacterial activity of colistin against susceptible and resistant Gram-negative bacteria. Herein, sulfonamide analogs of niclosamide were prepared and found to enhance colistin activity against Gram-negative bacteria. The ability of niclosamide and the new sulfonamide analogs to synergize with bacitracin against vancomycin-resistant Enterococcus faecium was also discovered.

Discovery of Salifungin as a Repurposed Antibiotic against Methicillin-Resistant Staphylococcus aureus with Limited Resistance Development

Exploring novel antimicrobial drugs and strategies has become essential to the fight MRSA-associated infections. Herein, we found that membrane-disrupted repurposed antibiotic salifungin had excellent bactericidal activity against MRSA, with limited development of drug resistance. Furthermore, adding salifungin effectively decreased the minimum inhibitory concentrations of clinical antibiotics against Staphylococcus aureus. Evaluations of the mechanism demonstrated that salifungin disrupted the level of H+ and K+ ions using hydrophilic and lipophilic groups to interact with bacterial membranes, causing the disruption of bacterial proton motive force followed by impacting on bacterial the function of the respiratory chain and adenosine 5'-triphosphate, thereby inhibiting phosphatidic acid biosynthesis. Moreover, salifungin also significantly inhibited the formation of bacterial biofilms and eliminated established bacterial biofilms by interfering with bacterial membrane potential and inhibiting biofilm-associated gene expression, which was even better than clinical antibiotics. Finally, salifungin exhibited efficacy comparable to or even better than that of vancomycin in the MRSA-infected animal models. In conclusion, these results indicate that salifungin can be a potential drug for treating MRSA-associated infections.

Azomycin Orchestrate Colistin-Resistant Enterobacter cloacae Complex's Colistin Resistance Reversal In Vitro and In Vivo

The Enterobacter cloacae complex (ECC) is a group of nosocomial pathogens that pose a challenge in clinical treatment due to its intrinsic resistance and the ability to rapidly acquire resistance. Colistin was reconsidered as a last-resort antibiotic for combating multidrug-resistant ECC. However, the persistent emergence of colistin-resistant (COL-R) pathogens impedes its clinical efficacy, and novel treatment options are urgently needed. We propose that azomycin, in combination with colistin, restores the susceptibility of COL-R ECC to colistin in vivo and in vitro. Results from the checkerboard susceptibility, time-killing, and live/dead bacterial cell viability tests showed strong synergistic antibacterial activity in vitro. Animal infection models suggested that azomycin-colistin enhanced the survival rate of infected Galleria mellonella and reduced the bacterial load in the thighs of infected mice, highlighting its superior in vivo synergistic antibacterial activity. Crystal violet staining and scanning electron microscopy unveiled the in vitro synergistic antibiofilm effects of azomycin-colistin. The safety of azomycin and azomycin-colistin at experimental concentrations was confirmed through cytotoxicity tests and an erythrocyte hemolysis test. Azomycin-colistin stimulated the production of reactive oxygen species in COL-R ECC and inhibited the PhoPQ two-component system to combat bacterial growth. Thus, azomycin is feasible as a colistin adjuvant against COL-R ECC infection.

New Antimicrobial Strategies to Treat Multi-Drug Resistant Infections Caused by Gram-Negatives in Cystic Fibrosis

People with cystic fibrosis (CF) suffer from recurrent bacterial infections which induce inflammation, lung tissue damage and failure of the respiratory system. Prolonged exposure to combinatorial antibiotic therapies triggers the appearance of multi-drug resistant (MDR) bacteria. The development of alternative antimicrobial strategies may provide a way to mitigate antimicrobial resistance. Here we discuss different alternative approaches to the use of classic antibiotics: anti-virulence and anti-biofilm compounds which exert a low selective pressure; phage therapies that represent an alternative strategy with a high therapeutic potential; new methods helping antibiotics activity such as adjuvants; and antimicrobial peptides and nanoparticle formulations. Their mechanisms and in vitro and in vivo efficacy are described, in order to figure out a complete landscape of new alternative approaches to fight MDR Gram-negative CF pathogens.

Exploring Structure-Activity Relationships of Niclosamide-Based Colistin Potentiators in Colistin-Resistant Gram-Negative Bacteria

Colistin is primarily used as a last resort antibiotic against highly resistant Gram-negative bacteria (GNB). Rising rates of colistin resistance, however, may limit future use of this agent. The anthelmintic drug niclosamide has been shown to enhance colistin activity in combination therapy, but a detailed structure-activity relationship (SAR) for niclosamide against GNB has yet to be studied. A series of niclosamide analogs were synthesized to perform an SAR, leading to the discovery of a lead compound that displayed comparable colistin-potentiating activity to niclosamide with reduced cytotoxicity. Overall, this work provides important insights into synthetic strategies for the future development of new niclosamide derivatives and demonstrates that toxicity to mammalian cells can be reduced while maintaining colistin potentiation.

Inhibiting fatty acid synthesis overcomes colistin resistance

Treating multidrug-resistant infections has increasingly relied on last-resort antibiotics, including polymyxins, for example colistin. As polymyxins are given routinely, the prevalence of their resistance is on the rise and increases mortality rates of sepsis patients. The global dissemination of plasmid-borne colistin resistance, driven by the emergence of mcr-1, threatens to diminish the therapeutic utility of polymyxins from an already shrinking antibiotic arsenal. Restoring sensitivity to polymyxins using combination therapy with sensitizing drugs is a promising approach to reviving its clinical utility. Here we describe the ability of the biotin biosynthesis inhibitor, MAC13772, to synergize with colistin exclusively against colistin-resistant bacteria. MAC13772 indirectly disrupts fatty acid synthesis (FAS) and restores sensitivity to the last-resort antibiotic, colistin. Accordingly, we found that combinations of colistin and other FAS inhibitors, cerulenin, triclosan and Debio1452-NH₃, had broad potential against both chromosomal and plasmid-mediated colistin resistance in chequerboard and lysis assays. Furthermore, combination therapy with colistin and the clinically relevant FabI inhibitor, Debio1452-NH₃, showed efficacy against mcr-1 positive Klebsiella pneumoniae and colistin-resistant Escherichia coli systemic infections in mice. Using chemical genomics, lipidomics and transcriptomics, we explored the mechanism of the interaction. We propose that inhibiting FAS restores colistin sensitivity by depleting lipid synthesis, leading to changes in phospholipid composition. In all, this work reveals a surprising link between FAS and colistin resistance.

Synergistic Activity of Colistin in Combination with Clofoctol against Colistin Resistant Gram-Negative Pathogens

Colistin is a bactericidal antibiotic identified decades ago which is active against a number of Gram-negative pathogens. After early elimination from clinical use due to toxicity issues, colistin has been reintroduced as a last-resort treatment for antibiotic-resistant Gram-negative infections lacking other therapeutic options. Inevitably, colistin resistance has emerged among clinical isolates, making the development of colistin adjuvants extremely beneficial. Clofoctol is a synthetic antibiotic active against Gram-positive bacteria, with low toxicity and high tropism for the airways. Interestingly, clofoctol has been found to have multiple biological activities and has been proposed for the treatment of several obstructive lung diseases, including asthma, lung cancer, and SARS-CoV-2 infection. In this study, the activity of clofoctol as a colistin adjuvant was investigated in Gram-negative lung pathogens that are critical for the high prevalence of multidrug-resistant isolates, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii. Clofoctol potentiated the bactericidal effect of colistin in all tested strains and reduced colistin MICs below the susceptibility breakpoint in nearly all colistin-resistant strains. Overall, this observation supports the development of inhaled clofoctol-colistin formulations for the treatment of difficult-to-treat airway infections caused by Gram-negative pathogens. IMPORTANCE Colistin is used as a last-resort antibiotic against extensively drug-resistant Gram-negative pathogens. However, colistin resistance is on the rise. Clofoctol is an antibiotic used against Gram-positive bacteria, with low toxicity and high penetration and storage in the airways. Here, a strong synergistic activity of the colistin-clofoctol combination against colistin-resistant Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii isolates is reported, supporting the development of clofoctol-colistin formulations for the therapy of difficult-to-treat airways infections caused by these Gram-negative pathogens.

Mechanism of Action of Isopropoxy Benzene Guanidine against Multidrug-Resistant Pathogens

The increasing emergence of antibiotic resistance is an urgent threat to global health care; thus, there is a need for new therapeutics. Guanidine is the preferred functional group for antimicrobial design and development. Herein, the potential antibacterial activity of the guanidine derivative isopropoxy benzene guanidine (IBG) against multidrug-resistant (MDR) bacteria was discovered. The synergistic antibacterial activity of IBG and colistin was determined by checkerboard assay, time-killing curve, and mouse experiments. The antibacterial mechanism of IBG was verified in fluorescent probe experiments, intracellular oxidative phosphorylation assays, and transcriptome analysis. The results showed that IBG displays efficient antibacterial activity against Gram-positive pathogens and Gram-negative pathogens with permeabilized outer membranes. Further mechanistic studies showed that IBG triggers cytoplasmic membrane damage by binding to phosphatidylglycerol and cardiolipin, leading to the dissipation of proton motive force and accumulation of intracellular ATP. IBG combined with low levels of colistin enhances bacterial outer membrane permeability and increases the accumulation of reactive oxygen species, as further evidenced by transcriptome analysis. Furthermore, the efficacy of IBG with colistin against MDR Escherichia coli in three infection models was demonstrated. Together, these results suggest that IBG is a promising adjuvant of colistin, providing an alternative approach to address the prevalent infections caused by MDR Gram-negative pathogens. IMPORTANCE As antibiotic discovery stagnates, the world is facing a growing menace from the emergence of bacteria that are resistant to almost all available antibiotics. The key to winning this race is to explore distinctive mechanisms of antibiotics. Thus, novel efficient antibacterial agents and alternative strategies are urgently required to fill the void in antibiotic development. Compared with the large amount of money and time required to develop new agents, the antibiotic adjuvant strategy is a promising approach to inhibit bacterial resistance and increase killing of bacteria. In this study, we found that the guanidine derivatives IBG not only displayed efficient antibacterial activities against Gram-positive bacteria but also restored colistin susceptibility of Gram-negative pathogens as an antibiotic adjuvant. More in-depth study showed that IBG is a potential lead to overcome antibiotic resistance, providing new insight into future antibiotic discovery and development.

Antimicrobial Peptides Therapy: An Emerging Alternative for Treating Drug-Resistant Bacteria

Microbial resistance to antibiotics is an ancient and dynamic issue that has brought a situation reminiscent of the pre-antibiotic era to the limelight. Currently, antibiotic resistance and the associated infections are widespread and pose significant global health and economic burden. Thus, the misuse of antibiotics, which has increased resistance, has necessitated the search for alternative therapeutic agents for combating resistant pathogens. Antimicrobial peptides (AMPs) hold promise as a viable therapeutic approach against drug-resistant pathogens. AMPs are oligopeptides with low molecular weight. They have broad-spectrum antimicrobial activities against pathogenic microorganisms. AMPs are nonspecific and target components of microbes that facilitate immune response by acting as the first-line defense mechanisms against invading pathogenic microbes. The diversity and potency of AMPs make them good candidates for alternative use. They could be used alone or in combination with several other biomaterials for improved therapeutic activity. They can also be employed in vaccine production targeting drug-resistant pathogens. This review covers the opportunities and advances in AMP discovery and development targeting antimicrobial resistance (AMR) bacteria. Briefly, it presents an overview of the global burden of the antimicrobial resistance crisis, portraying the global magnitude, challenges, and consequences. After that, it critically and comprehensively evaluates the potential roles of AMPs in addressing the AMR crisis, highlighting the major potentials and prospects.

Colistin Resistance and Management of Drug Resistant Infections

Colistin resistance is a globalized sensible issue because it has been considered a drug of the last-line resort to treat drug-resistant bacterial infections. The product of the mobilized colistin resistance (mcr) gene and its variants are the significant causes of colistin resistance, which is emerging due to the frequent colistin use in veterinary, and these genes circulate among the bacterial community. Apart from mcr genes, some other intrinsic genes and proteins are also involved in colistin resistance. Researchers focus on the most advanced genomics (whole genome sequencing), proteomics, and bioinformatics approaches to explore the question of colistin resistance. To combat colistin resistance, researchers developed various strategies such as the development of newer drugs, the repurposing of existing drugs, combinatorial treatment by colistin with other drugs, a nano-based approach, photodynamic therapy, a CRISPRi-based strategy, and a phage-based strategy. In this timeline review, we have discussed the development of colistin resistance and its management in developing countries.

Niclosamide as a repurposing drug against Gram-positive bacterial infections

OBJECTIVES

Niclosamide is commonly used as an antiparasitic drug in veterinary clinics. The objectives of this study were to evaluate the efficacy of niclosamide against resistant Gram-positive bacteria in vitro and in an in vivo experimental model of topical bacterial infection. Moreover, to study the antibacterial mechanism of niclosamide to Staphylococcus aureus.

METHODS

A mouse topical infection model was established to detect the antibacterial activity of niclosamide in vivo. The antimicrobial mechanism was probed by visualizing the bacterial morphologies using scanning electron microscopy and transmission electron microscopy. Moreover, the haemolytic assay and western blotting analysis were performed to evaluate whether niclosamide could inhibit the secretion of alpha-haemolysin (α-HL) from S. aureus.

RESULTS

The MICs of niclosamide were below 0.5 mg/L for Gram-positive bacteria, showing excellent antibacterial activity in vitro. The in vivo antibacterial activity results indicated that niclosamide treatment at 10 mg/kg of body weight caused a significant reduction in the abscess area and the number of S. aureus cells. Moreover, the antibacterial mechanism of niclosamide showed that the surface morphology of S. aureus displayed noticeable shrinkage, with an increasing number of small vacuole-like structures observed as the drug concentration increased. Intracellular ATP levels were found to decrease in a niclosamide dose-dependent manner. Haemolysis and western blotting analyses revealed that niclosamide inhibited the haemolytic activity of S. aureus by inhibiting α-HL expression under subinhibitory concentration conditions.

CONCLUSIONS

Niclosamide has significant potential for development into drugs that prevent and treat diseases caused by Gram-positive bacteria such as Staphylococcus and Streptococcus.

A Niclosamide-releasing hot-melt extruded catheter prevents Staphylococcus aureus experimental biomaterial-associated infection

Biomaterial-associated infections are a major healthcare challenge as they are responsible for high disease burden in critically ill patients. In this study, we have developed drug-eluting antibacterial catheters to prevent catheter-related infections. Niclosamide (NIC), originally an antiparasitic drug, was incorporated into the polymeric matrix of thermoplastic polyurethane (TPU) via solvent casting, and catheters were fabricated using hot-melt extrusion technology. The mechanical and physicochemical properties of TPU polymers loaded with NIC were studied. NIC was released in a sustained manner from the catheters and exhibited in vitro antibacterial activity against Staphylococcus aureus and Staphylococcus epidermidis. Moreover, the antibacterial efficacy of NIC-loaded catheters was validated in an in vivo biomaterial-associated infection model using a methicillin-susceptible and methicillin-resistant strain of S. aureus. The released NIC from the produced catheters reduced bacterial colonization of the catheter as well as of the surrounding tissue. In summary, the NIC-releasing hot-melt extruded catheters prevented implant colonization and reduced the bacterial colonization of peri-catheter tissue by methicillin sensitive as well as resistant S. aureus in a biomaterial-associated infection mouse model and has good prospects for preclinical development.

Structure-Activity Relationship Studies of [1,2,5]Oxadiazolo[3,4-b]pyrazine-Containing Polymyxin-Selective Resistance-Modifying Agents

Multidrug-resistant (MDR) Gram-negative bacteria are an urgent and rapidly spreading threat to human health with limited treatment options. Previously, we discovered a novel [1,2,5]oxadiazolo[3,4-b]pyrazine-containing compound (1) that selectively re-sensitized a variety of MDR Gram-negative bacteria to colistin, one of the last-resort antibiotic. Herein, we report the structure-activity relationship studies of compound 1 that led to the discovery of several more potent and/or less toxic resistance-modifying agents (RMAs). Further evaluation of these RMAs showed that they were effective in a wide range of MDR bacteria. These results demonstrated these compounds as a novel class of RMAs and may be further developed as therapeutic agents.

Targeting the Holy Triangle of Quorum Sensing, Biofilm Formation, and Antibiotic Resistance in Pathogenic Bacteria

Chronic and recurrent bacterial infections are frequently associated with the formation of biofilms on biotic or abiotic materials that are composed of mono- or multi-species cultures of bacteria/fungi embedded in an extracellular matrix produced by the microorganisms. Biofilm formation is, among others, regulated by quorum sensing (QS) which is an interbacterial communication system usually composed of two-component systems (TCSs) of secreted autoinducer compounds that activate signal transduction pathways through interaction with their respective receptors. Embedded in the biofilms, the bacteria are protected from environmental stress stimuli, and they often show reduced responses to antibiotics, making it difficult to eradicate the bacterial infection. Besides reduced penetration of antibiotics through the intricate structure of the biofilms, the sessile biofilm-embedded bacteria show reduced metabolic activity making them intrinsically less sensitive to antibiotics. Moreover, they frequently express elevated levels of efflux pumps that extrude antibiotics, thereby reducing their intracellular levels. Some efflux pumps are involved in the secretion of QS compounds and biofilm-related materials, besides being important for removing toxic substances from the bacteria. Some efflux pump inhibitors (EPIs) have been shown to both prevent biofilm formation and sensitize the bacteria to antibiotics, suggesting a relationship between these processes. Additionally, QS inhibitors or quenchers may affect antibiotic susceptibility. Thus, targeting elements that regulate QS and biofilm formation might be a promising approach to combat antibiotic-resistant biofilm-related bacterial infections.

Niclosamide as a Repurposing Drug against Corynebacterium striatum Multidrug-Resistant Infections

Corynebacterium striatum (C. striatum) is an emerging multidrug-resistant (MDR) pathogen associated with nosocomial infections. In this scenario, we screened the antimicrobial activity of the anthelmintic drugs doramectin, moxidectin, selamectin and niclosamide against 20 C. striatum MDR clinical isolates. Among these, niclosamide was the best performing drug against C. striatum. Niclosamide cytotoxicity was evaluated by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay on immortalized human keratinocyte cells (HaCaT). After 20 h of treatment, the recorded 50% cytotoxic concentration (CC₅₀) was 2.56 μg/mL. The antibacterial efficacy was determined via disc diffusion, broth microdilution method and time-killing. Against C. striatum, niclosamide induced a growth inhibitory area of 22 mm and the minimum inhibitory concentration that inhibits 90% of bacteria (MIC₉₀) was 0.39 μg/mL, exhibiting bactericidal action. The biofilm biomass eradicating action was investigated through crystal violet (CV), MTT and confocal laser scanning microscopy (CLSM). Niclosamide affected the biofilm viability in a dose-dependent manner and degraded biomass by 55 and 49% at 0.39 μg/mL and 0.19 μg/mL. CLSM images confirmed the biofilm biomass degradation, showing a drastic reduction in cell viability. This study could promote the drug-repurposing of the anthelmintic FDA-approved niclosamide as a therapeutic agent to counteract the C. striatum MDR infections.

Effects of Lysine N-ζ-Methylation in Ultrashort Tetrabasic Lipopeptides (UTBLPs) on the Potentiation of Rifampicin, Novobiocin, and Niclosamide in Gram-Negative Bacteria

Outer membrane (OM) drug impermeability typically associated with a molecular weight above 600 Da and high hydrophobicity prevents accumulation of many antibiotics in Gram-negative bacteria (GNB). Previous studies have shown that ultrashort tetrabasic lipopeptides (UTBLPs) containing multiple lysine residues potentiate Gram-positive bacteria (GPB)-selective antibiotics in GNB by enhancing OM permeability. However, there is no available information on how N-substitution at the ζ-position of lysine in UTBLPs affects antibiotic potentiation in GNB. To study these effects, we prepared a series of branched and linear UTBLPs that differ in the degree of N-ζ-methylation and studied their potentiating effects with GPB-selective antibiotics including rifampicin, novobiocin, niclosamide, and chloramphenicol against wild-type and multidrug-resistant GNB isolates. Our results show that increasing N-ζ-methylation reduces or abolishes the potentiating effects of UTBLPs with rifampicin, novobiocin, and niclosamide against GNB. No trend was observed with chloramphenicol that is largely affected by efflux. We were unable to observe a correlation between the strength of the antibiotic potentiating effect to the increase in fluorescence in the 1-N-phenylnaphthylamine (NPN) OM permeability assay suggesting that other factors besides OM permeability of NPN play a role in antibiotic potentiation. In conclusion, our study has elucidated crucial structure–activity relationships for the optimization of polybasic antibiotic potentiators in GNB.

In Vitro Screening of a 1280 FDA-Approved Drugs Library against Multidrug-Resistant and Extensively Drug-Resistant Bacteria

Alternative strategies against multidrug-resistant (MDR) bacterial infections are suggested to clinicians, such as drug repurposing, which uses rapidly available and marketed drugs. We gathered a collection of MDR bacteria from our hospital and performed a phenotypic high-throughput screening with a 1280 FDA-approved drug library. We used two Gram positive (Enterococcus faecium P5014 and Staphylococcus aureus P1943) and six Gram negative (Acinetobacter baumannii P1887, Klebsiella pneumoniae P9495, Pseudomonas aeruginosa P6540, Burkholderia multivorans P6539, Pandoraea nosoerga P8103, and Escherichia coli DSM105182 as the reference and control strain). The selected MDR strain panel carried resistance genes or displayed phenotypic resistance to last-line therapies such as carbapenems, vancomycin, or colistin. A total of 107 compounds from nine therapeutic classes inhibited >90% of the growth of the selected Gram negative and Gram positive bacteria at a drug concentration set at 10 µmol/L, and 7.5% were anticancer drugs. The common hit was the antiseptic chlorhexidine. The activity of niclosamide, carmofur, and auranofin was found against the selected methicillin-resistant S. aureus. Zidovudine was effective against colistin-resistant E. coli and carbapenem-resistant K. pneumoniae. Trifluridine, an antiviral, was effective against E. faecium. Deferoxamine mesylate inhibited the growth of XDR P. nosoerga. Drug repurposing by an in vitro screening of a drug library is a promising approach to identify effective drugs for specific bacteria.

Dioctanoyl Ultrashort Tetrabasic β-Peptides Sensitize Multidrug-Resistant Gram-Negative Bacteria to Novobiocin and Rifampicin

Recently reported peptidomimetics with increased resistance to trypsin were shown to sensitize priority multidrug-resistant (MDR) Gram-negative bacteria to novobiocin and rifampicin. To further optimize proteolytic stability, β-amino acid-containing derivatives of these compounds were prepared, resulting in three dioctanoyl ultrashort tetrabasic β-peptides (dUSTBβPs). The nonhemolytic dUSTBβP 3, comprised of three β³-homoarginine residues and two fatty acyl tails eight carbons long, enhanced the antibacterial activity of various antibiotics from different classes. Notably, compound 3 retained the ability to potentiate novobiocin and rifampicin in wild-type Gram-negative bacteria against MDR clinical isolates of Pseudomonas aeruginosa, Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae. dUSTBβP 3 reduced the minimum inhibitory concentration of novobiocin and rifampicin below their interpretative susceptibility breakpoints. Furthermore, compound 3 exhibited improved in vitro stability (86.8 ± 3.7% remaining) relative to its α-amino acid-based counterpart (39.5 ± 7.4% remaining) after a 2 h incubation in human plasma.

Benzimidazole Isosteres of Salicylanilides Are Highly Active Colistin Adjuvants

Multidrug-resistant bacterial infections have become a global threat. We recently disclosed that the known IKK-β inhibitor IMD-0354 and subsequent analogues abrogate colistin resistance in several Gram-negative strains. Herein, we report the activity of a second-generation library of IMD-0354 analogues incorporating a benzimidazole moiety as an amide isostere. We identified several analogues that show increased colistin potentiation activity against Gram-negative bacteria.

A niclosamide-tobramycin hybrid adjuvant potentiates cefiderocol against P. aeruginosa

There is an urgent need for new therapies to overcome antimicrobial resistance (AMR) especially against Gram-negative bacilli (GNB). Multicomponent therapy combining antibiotics with enhancer molecules known as adjuvants is an emerging strategy to combat AMR. We have previously reported tobramycin-based adjuvants which are able to potentiate various antibiotics. In order to expand the repertoire of tobramycin hybrid adjuvants, a new hybrid containing niclosamide, an FDA approved anthelmintic which has recently demonstrated a variety of interesting biological effects, was synthesized. It was found that this conjugate can potentiate several antibiotics against multidrug-resistant GNB, including the recently approved siderophore cephalosporin cefiderocol. 8 μg ml^-1 of the niclosamide-tobramycin hybrid in combination therapy against a pandrug-resistant strain of P. aeruginosa was able to lower the cefiderocol MIC 32-fold, from 8 μg ml^-1 to 0.25 μg ml^-1 in iron-rich media where siderophore uptake is reduced. These results indicate that a niclosamide-tobramycin hybrid adjuvant can serve to potentiate a newly approved antibiotic.

Benzydamine Reverses TMexCD-TOprJ-Mediated High-Level Tigecycline Resistance in Gram-Negative Bacteria

Recently, a novel efflux pump gene cluster called tmexCD1-toprJ1 and its variants have been identified, which undermine the antibacterial activity of tigecycline, one of the last remaining options effective against multidrug-resistant (MDR) Gram-negative bacteria. Herein, we report the potent synergistic effect of the non-steroidal anti-inflammatory drug benzydamine in combination with tigecycline at sub-inhibitory concentrations against various temxCD-toprJ-positive Gram-negative pathogens. The combination of benzydamine and tigecycline killed all drug-resistant pathogens during 24 h of incubation. In addition, the evolution of tigecycline resistance was significantly suppressed in the presence of benzydamine. Studies on the mechanisms of synergism showed that benzydamine disrupted the bacterial proton motive force and the functionality of this kind of novel plasmid-encoded resistance-nodulation-division efflux pump, thereby promoting the intracellular accumulation of tigecycline. Most importantly, the combination therapy of benzydamine and tigecycline effectively improved the survival of Galleria mellonella larvae compared to tigecycline monotherapy. Our findings provide a promising drug combination therapeutic strategy for combating superbugs carrying the tmexCD-toprJ gene.

Current Update on Intrinsic and Acquired Colistin Resistance Mechanisms in Bacteria

Colistin regained global interest as a consequence of the rising prevalence of multidrug-resistant Gram-negative Enterobacteriaceae. In parallel, colistin-resistant bacteria emerged in response to the unregulated use of this antibiotic. However, some Gram-negative species are intrinsically resistant to colistin activity, such as Neisseria meningitides, Burkholderia species, and Proteus mirabilis. Most identified colistin resistance usually involves modulation of lipid A that decreases or removes early charge-based interaction with colistin through up-regulation of multistep capsular polysaccharide expression. The membrane modifications occur by the addition of cationic phosphoethanolamine (pEtN) or 4-amino-l-arabinose on lipid A that results in decrease in the negative charge on the bacterial surface. Therefore, electrostatic interaction between polycationic colistin and lipopolysaccharide (LPS) is halted. It has been reported that these modifications on the bacterial surface occur due to overexpression of chromosomally mediated two-component system genes (PmrAB and PhoPQ) and mutation in lipid A biosynthesis genes that result in loss of the ability to produce lipid A and consequently LPS chain, thereafter recently identified variants of plasmid-borne genes (mcr-1 to mcr-10). It was hypothesized that mcr genes derived from intrinsically resistant environmental bacteria that carried chromosomal pmrC gene, a part of the pmrCAB operon, code three proteins viz. pEtN response regulator PmrA, sensor kinase protein PmrAB, and phosphotransferase PmrC. These plasmid-borne mcr genes become a serious concern as they assist in the dissemination of colistin resistance to other pathogenic bacteria. This review presents the progress of multiple strategies of colistin resistance mechanisms in bacteria, mainly focusing on surface changes of the outer membrane LPS structure and other resistance genetic determinants. New handier and versatile methods have been discussed for rapid detection of colistin resistance determinants and the latest approaches to revert colistin resistance that include the use of new drugs, drug combinations and inhibitors. Indeed, more investigations are required to identify the exact role of different colistin resistance determinants that will aid in developing new less toxic and potent drugs to treat bacterial infections. Therefore, colistin resistance should be considered a severe medical issue requiring multisectoral research with proper surveillance and suitable monitoring systems to report the dissemination rate of these resistant genes.

Aspirin, sodium benzoate and sodium salicylate reverse resistance to colistin in Enterobacteriaceae and Pseudomonas aeruginosa

BACKGROUND

MDR bacterial infections are currently a serious problem for clinicians worldwide. Klebsiella pneumoniae and Enterobacter spp., among Enterobacteriaceae, and Pseudomonas aeruginosa, are part of the group of ESCAPE pathogens or bacteria that 'escape' from common antibacterial treatments. The lack of effectiveness of the first common line of antibiotics has led to the search for new therapies based on older antibiotics, such as colistin.

OBJECTIVES

We searched for new enhancers of the action of colistin against MDR Gram-negative bacteria that can be easily applicable to clinical treatments.

METHODS

Colistin MICs were determined alone and with the protonophores CCCP, sodium benzoate, sodium salicylate and aspirin using the broth microdilution method and FIC indexes were calculated to assess synergy between colistin and each chemical. Time-kill assays of colistin with and without protonophores were performed to determine the bactericidal action of combinations of colistin with protonophores. Likewise, the effect of sucrose, l-arginine and l-glutamic acid on the MICs of colistin alone and combined with each protonophore was assessed.

RESULTS

It was found that sodium benzoate, sodium salicylate and aspirin, at concentrations allowed for human and animal use, partially or totally reversed resistance to colistin in P. aeruginosa and highly resistant enterobacterial strains. The mechanism of action could be related to their negative charge at a physiological pH along with their lipid-soluble character.

CONCLUSIONS

Sodium benzoate, sodium salicylate and aspirin are good enhancers to use in antibiotic therapies that include colistin.

A novel colistin adjuvant identified by virtual screening for ArnT inhibitors

Background

Colistin is a last-resort treatment option for many MDR Gram-negative bacteria. The covalent addition of l-aminoarabinose to the lipid A moiety of LPS is the main colistin resistance mechanism in the human pathogen Pseudomonas aeruginosa.

Objectives

Identification (by in silico screening of a chemical library) of potential inhibitors of ArnT, which catalyses the last committed step of lipid A aminoarabinosylation, and their validation in vitro as colistin adjuvants.

Methods

The available ArnT crystal structure was used for a docking-based virtual screening of an in-house library of natural products. The resulting putative ArnT inhibitors were tested in growth inhibition assays using a reference colistin-resistant P. aeruginosa strain. The most promising compound was further characterized for its range of activity, specificity and cytotoxicity. Additionally, the effect of the compound on lipid A aminoarabinosylation was verified by MS analyses of lipid A.

Results

A putative ArnT inhibitor (BBN149) was discovered by molecular docking and demonstrated to specifically potentiate colistin activity in colistin-resistant P. aeruginosa isolates, without relevant effect on colistin-susceptible strains. BBN149 also showed adjuvant activity against colistin-resistant Klebsiella pneumoniae and low toxicity to bronchial epithelial cells. Lipid A aminoarabinosylation was reduced in BBN149-treated cells, although only partially.

Conclusions

This study demonstrates that in silico screening targeting ArnT can successfully identify inhibitors of colistin resistance and provides a promising lead compound for the development of colistin adjuvants for the treatment of MDR bacterial infections.

Repositioning rafoxanide to treat Gram-negative bacilli infections

OBJECTIVES

Repurposing drugs provides a new approach to the fight against MDR Gram-negative bacilli (MDR-GNB). Rafoxanide, a veterinary antihelminthic drug, has shown antibacterial activity in vitro against Gram-positive bacteria. We aimed to analyse the in vitro and in vivo efficacy of rafoxanide in combination with colistin against colistin-susceptible (Col-S) and colistin-resistant (Col-R) GNB.

METHODS

A collection of Col-S and Col-R Acinetobacter baumannii, Pseudomonas aeruginosa and Klebsiella pneumoniae were used. Chequerboard and time-kill curve analyses were performed to determine the synergy between rafoxanide and colistin. Changes in membrane structure and permeability were analysed using transmission electron microscopy and fluorescence assays. A murine peritoneal sepsis model using Col-R strains of these pathogens was performed to study the efficacy of rafoxanide (10 mg/kg/24 h, IV), colistimethate sodium (CMS) (20 mg/kg/8 h, intraperitoneally) and rafoxanide (10 mg/kg/24 h, IV) plus CMS (20 mg/kg/8 h, intraperitoneally) for 72 h.

RESULTS

Rafoxanide showed MICs ≥256 mg/L for all Col-S and Col-R strains. Chequerboard and time-kill curve analyses showed that rafoxanide (1 mg/L) is more synergistic with colistin against Col-R than Col-S strains. Col-R, but not Col-S, strains treated with rafoxanide demonstrated higher membrane permeabilization. Transmission electron microscopy visualization confirmed that Col-R strains suffer morphological changes. In the murine peritoneal sepsis model with Col-R strains, rafoxanide plus CMS, compared with CMS alone, increased mouse survival to 53.8% and 73.3%, and reduced bacterial loads in tissues and blood between 2.34 and 4.99 log10 cfu/g or mL, respectively.

CONCLUSIONS

Rafoxanide repurposing, as monotherapy and in combination with CMS, may address the urgent need for new treatments for infections caused by MDR-GNB.

Synergistic Activity and Biofilm Formation Effect of Colistin Combined with PFK-158 Against Colistin-Resistant Gram-Negative Bacteria

Purpose

The emergence of colistin resistance among Gram-negative bacteria (GNB) poses a serious public health threat. Therefore, it is necessary to enhance the antibacterial activity of colistin through the combination with other drugs. In this study, we demonstrated the synergistic activity and the possible synergy mechanism of colistin with PFK-158 against colistin-resistant GNB, including non-fermenting bacteria and Enterobacteriaceae.

Patients and Methods

Thirty-one colistin-resistant GNB, including Pseudomonas aeruginosa (n = 9), Acinetobacter baumannii (n = 5), Escherichia coli (n = 8) and Klebsiella pneumoniae (n = 9), were collected as the experimental strains and the minimum inhibitory concentrations (MICs) of colistin, other routine antimicrobial agents and PFK-158 against all strains were determined by the broth microdilution method. The synergistic activity of colistin with PFK-158 was assessed by the checkerboard assay and time-kill assay. The biofilm formation assay and scanning electron microscopy were used to demonstrate the biofilm formation effect of colistin with PFK-158 against colistin-resistant GNB.

Results

The results of the checkerboard assay showed that when colistin was used in combination with PFK-158, synergistic activity was observed against the 31 colistin-resistant GNB. The time-kill assay presented a significant killing activity of colistin with PFK-158 against the 9 colistin-resistant GNB selected randomly, including Pseudomonas aeruginosa (n = 6), Acinetobacter baumannii (n = 1), Escherichia coli (n = 1), and Klebsiella pneumoniae (n = 1). The biofilm formation assay and scanning electron microscopjihy showed that colistin with PFK-158 can effectively suppress the formation of biofilm and reduce the cell arrangement density of biofilm against most experimental strains.

Conclusion

The results of the performed experiments suggest that the combination of colistin and PFK-158 may be a potential new choice as a new antibiofilm group for the treatment of infections caused by the colistin-resistant GNB.

Antibiotic Adjuvant Activity Revealed in a Photoaffinity Approach to Determine the Molecular Target of Antipyocyanin Compounds

Infections with Pseudomonas aeruginosa are a looming threat to public health. New treatment strategies are needed to combat this pathogen, for example, by blocking the production of virulence factors like pyocyanin. A photoaffinity analogue of an antipyocyanin compound was developed to interrogate the inhibitor's molecular mechanism of action. While we sought to develop antivirulence inhibitors, the proteomics results suggested that the compounds had antibiotic adjuvant activity. Unexpectedly, we found that these compounds amplify the bactericidal activity of colistin, a well-characterized antibiotic, suggesting they may represent a first-in-class antibiotic adjuvant therapy. Analogues have the potential not only to widen the therapeutic index of cationic antimicrobial peptides like colistin, but also to be effective against colistin-resistant strains, strengthening our arsenal to combat P. aeruginosa infections.

Drug repurposing for next-generation combination therapies against multidrug-resistant bacteria

Antimicrobial resistance has been a global health challenge that threatens our ability to control and treat life-threatening bacterial infections. Despite ongoing efforts to identify new drugs or alternatives to antibiotics, no new classes of antibiotic or their alternatives have been clinically approved in the last three decades. A combination of antibiotics and non-antibiotic compounds that could inhibit bacterial resistance determinants or enhance antibiotic activity offers a sustainable and effective strategy to confront multidrug-resistant bacteria. In this review, we provide a brief overview of the co-evolution of antibiotic discovery and the development of bacterial resistance. We summarize drug-drug interactions and uncover the art of repurposing non-antibiotic drugs as potential antibiotic adjuvants, including discussing classification and mechanisms of action, as well as reporting novel screening platforms. A pathogen-by-pathogen approach is then proposed to highlight the critical value of drug repurposing and its therapeutic potential. Finally, general advantages, challenges and development trends of drug combination strategy are discussed.

Development and evaluation of inhalable composite niclosamide-lysozyme particles: A broad-spectrum, patient-adaptable treatment for coronavirus infections and sequalae

Niclosamide (NIC) has demonstrated promising in vitro antiviral efficacy against SARS-CoV-2, the causative agent of the COVID-19 pandemic. Though NIC is already FDA-approved, administration of the currently available oral formulation results in systemic drug levels that are too low for the inhibition of SARS-CoV-2. We hypothesized that the co-formulation of NIC with an endogenous protein, human lysozyme (hLYS), could enable the direct aerosol delivery of the drug to the respiratory tract as an alternative to oral delivery, thereby effectively treating COVID-19 by targeting the primary site of SARS-CoV-2 acquisition and spread. To test this hypothesis, we engineered and optimized composite particles containing NIC and hLYS suitable for delivery to the upper and lower airways via dry powder inhaler, nebulizer, and nasal spray. The novel formulation demonstrates potent in vitro and in vivo activity against two coronavirus strains, MERS-CoV and SARS-CoV-2, and may offer protection against methicillin-resistance staphylococcus aureus pneumonia and inflammatory lung damage occurring secondary to SARS-CoV-2 infections. The suitability of the formulation for all stages of the disease and low-cost development approach will ensure rapid clinical development and wide-spread utilization.

Mechanistic Understanding Enables the Rational Design of Salicylanilide Combination Therapies for Gram-Negative Infections

There is a critical need for more-effective treatments to combat multidrug-resistant Gram-negative infections. Combination therapies are a promising strategy, especially when these enable existing clinical drugs to be repurposed as antibiotics. We examined the mechanisms of action and basis of innate Gram-negative resistance for the anthelmintic drug niclosamide and subsequently exploited this information to demonstrate that niclosamide and analogs kill Gram-negative bacteria when combined with antibiotics that inhibit drug efflux or permeabilize membranes. We confirm the synergistic potential of niclosamide in vitro against a diverse range of recalcitrant Gram-negative clinical isolates and in vivo in a mouse abscess model. We also demonstrate that nitroreductases can confer resistance to niclosamide but show that evolution of these enzymes for enhanced niclosamide resistance confers a collateral sensitivity to other clinical antibiotics. Our results highlight how detailed mechanistic understanding can accelerate the evaluation and implementation of new combination therapies.

One avenue to combat multidrug-resistant Gram-negative bacteria is the coadministration of multiple drugs (combination therapy), which can be particularly promising if drugs synergize. The identification of synergistic drug combinations, however, is challenging. Detailed understanding of antibiotic mechanisms can address this issue by facilitating the rational design of improved combination therapies. Here, using diverse biochemical and genetic assays, we examine the molecular mechanisms of niclosamide, a clinically approved salicylanilide compound, and demonstrate its potential for Gram-negative combination therapies. We discovered that Gram-negative bacteria possess two innate resistance mechanisms that reduce their niclosamide susceptibility: a primary mechanism mediated by multidrug efflux pumps and a secondary mechanism of nitroreduction. When efflux was compromised, niclosamide became a potent antibiotic, dissipating the proton motive force (PMF), increasing oxidative stress, and reducing ATP production to cause cell death. These insights guided the identification of diverse compounds that synergized with salicylanilides when coadministered (efflux inhibitors, membrane permeabilizers, and antibiotics that are expelled by PMF-dependent efflux), thus suggesting that salicylanilide compounds may have broad utility in combination therapies. We validate these findings in vivo using a murine abscess model, where we show that niclosamide synergizes with the membrane permeabilizing antibiotic colistin against high-density infections of multidrug-resistant Gram-negative clinical isolates. We further demonstrate that enhanced nitroreductase activity is a potential route to adaptive niclosamide resistance but show that this causes collateral susceptibility to clinical nitro-prodrug antibiotics. Thus, we highlight how mechanistic understanding of mode of action, innate/adaptive resistance, and synergy can rationally guide the discovery, development, and stewardship of novel combination therapies.

Small Molecule Adjuvants Potentiate Colistin Activity and Attenuate Resistance Development in Escherichia coli by Affecting pmrAB System

Background

Colistin is one of the last-resort antibiotics to treat multi-drug resistant (MDR) Gram-negative bacterial infections in humans. Further, colistin has been also used to prevent and treat Enterobacteriaceae infections in food animals. However, chromosomal mutations and mobile colistin resistance (mcr) genes, which confer resistance to colistin, have been detected in bacterial isolates from food animals and humans worldwide; thus, limiting the use of colistin. Therefore, strategies that could aid in ameliorating colistin resistance are critically needed.

Objective

Investigate the adjuvant potential of novel small molecules (SMs) on colistin.

Materials and Methods

Previously, we identified 11 membrane-affecting SMs with bactericidal activity against avian pathogenic Escherichia coli (APEC). Here, we investigated the potentiation effect of those SMs on colistin using checkerboard assays and wax moth (Galleria mellonella) larval model. The impact of the SM combination on colistin resistance evolution was also investigated by analyzing whole genome sequences of APEC isolates passaged with colistin alone or in combination with SMs followed by quantitating pmrCAB and pmrH expression in those isolates.

Results

The SM combination synergistically reduced the minimum bactericidal concentration of colistin by at least 10-fold. In larvae, the SM combination increased the efficacy of colistin by two-fold with enhanced (>50%) survival and reduced (>4 logs) APEC load. Further, the SM combination decreased the frequency (5/6 to 1/6) of colistin resistance evolution and downregulated the pmrCAB and pmrH expression. Previously unknown mutations in pmrB (L14Q, T92P) and pmrA (A80V), which were predicted deleterious, were identified in the colistin-resistant (Col^R) APEC isolates when passaged with colistin alone but not in combination with SMs. Our study also identified mutations in hypothetical and several phage-related proteins in Col^R APEC isolates in concurrent with pmrAB mutations.

Conclusion

Our study identified two SMs (SM2 and SM3) that potentiated the colistin activity and attenuated the development of colistin resistance in APEC. These SMs can be developed as anti-evolution drugs that can slow down colistin resistance development.

Dilipid Ultrashort Tetrabasic Peptidomimetics Potentiate Novobiocin and Rifampicin Against Multidrug-Resistant Gram-Negative Bacteria

The development of new antibacterial agents and therapeutic approaches is of high importance to address the global problem of antibiotic resistance. Although antimicrobial peptides are known to synergize with certain antibiotics, their clinical application is limited by their systemic toxicity, protease instability, and high production cost. To overcome these problems, nine dilipid ultrashort tetrabasic peptidomimetics (dUSTBPs) were prepared consisting of three basic amino acids separated by a molecular scaffold, bis(3-aminopropyl)glycine, and were ligated to two fatty acids. Several nonhemolytic dUSTBPs were shown to enhance the activity of several antibiotics against pathogenic Gram-negative bacteria. More importantly, dUSTBP 8, consisting of three l-arginine units and a dilipid of 8 carbons long, potentiated novobiocin and rifampicin consistently against multidrug-resistant (MDR) clinical isolates of Pseudomonas aeruginosa, Acinetobacter baumannii, and Enterobacteriaceae. Preliminary studies suggested that dUSTBPs were likely to potentiate antibiotics through outer membrane permeabilization and/or disruption of active efflux and that dUSTBP 8 exhibited enhanced resistance to trypsin in comparison to the previously described di-C₉-KKKK-NH₂ antibiotic potentiator. The antibacterial activity of rifampicin and novobiocin was enhanced by dUSTBP 8 comparable to other known outer membrane permeabilizing potentiators including the gold standard polymyxin B nonapeptide. Our results indicate that ultrashort tetrabasic peptidomimetics are potent adjuvants that repurpose novobiocin and rifampicin as potent agents against priority MDR Gram-negative pathogens.

Polymyxin Delivery Systems: Recent Advances and Challenges

Polymyxins are vital antibiotics for the treatment of multiresistant Gram-negative ESKAPE pathogen infections. However, their clinical value is limited by their high nephrotoxicity and neurotoxicity, as well as their poor permeability and absorption in the gastrointestinal tract. This review focuses on various polymyxin delivery systems that improve polymyxin bioavailability and reduce drug toxicity through targeted and controlled release. Currently, the most suitable systems for improving oral, inhalation, and parenteral polymyxin delivery are polymer particles, liposomes, and conjugates, while gels, polymer fibers, and membranes are attractive materials for topical administration of polymyxin for the treatment of infected wounds and burns. In general, the application of these systems protects polymyxin molecules from the negative effects of both physiological and pathological factors while achieving higher concentrations at the target site and reducing dosage and toxicity. Improving the properties of polymyxin will be of great interest to researchers who are focused on developing antimicrobial drugs that show increased efficacy and safety.

In Vitro and In Vivo Effects of the Polymyxin-Vorinostat Combination Therapy Against Multidrug-Resistant Gram-Negative Pathogens

With the stagnancy of antibiotics development, polymyxins have become the last defense for treatment of multidrug-resistant (MDR) Gram-negative bacteria, whereas the effect of polymyxin monotherapy is limited by resistance. The objective of this study was to evaluate the effects of polymyxin B (PMNB)-vorinostat (SAHA) combination therapy against Gram-negative pathogens in vitro and in vivo. The antibacterial activities of PMNB and SAHA were evaluated by susceptibility testing. The synergistic effect was assessed by checkerboard tests and time-killing kinetics experiments. Cellular morphology studies and reactive oxygen species (ROS) assay were conducted to explore potential mechanisms. Also, Galleria mellonella models were made to evaluate the antibacterial effects in vivo. PMNB-SAHA had the synergistic effect against all tested isolates, reducing >2 log₁₀ colony-forming units (CFU)/mL at 40 minutes, and showed more powerful antibacterial effects than PMNB alone in the 24-hour window. Cellular morphology study showed the change of membrane and disruption of integrity. ROS assay showed more oxidative stress in combination than PMNB or SAHA monotherapy. In animal models, PMNB-SAHA showed a higher survival rate than that of monotherapy. This study is the first to report the synergistic antibacterial effect of PMNB-SAHA therapy against MDR Gram-negative bacteria. Further clinical research is needed to confirm the results.

Antibiotics in the clinical pipeline in October 2019

The development of new and effective antibacterial drugs to treat multi-drug resistant (MDR) bacteria, especially Gram-negative (G−ve) pathogens, is acknowledged as one of the world’s most pressing health issues; however, the discovery and development of new, nontoxic antibacterials is not a straightforward scientific task, which is compounded by a challenging economic model. This review lists the antibacterials, β-lactamase/β-lactam inhibitor (BLI) combinations, and monoclonal antibodies (mAbs) first launched around the world since 2009 and details the seven new antibiotics and two new β-lactam/BLI combinations launched since 2016. The development status, mode of action, spectra of activity, lead source, and administration route for the 44 small molecule antibacterials, eight β-lactamase/BLI combinations, and one antibody drug conjugate (ADC) being evaluated in worldwide clinical trials at the end of October 2019 are described. Compounds discontinued from clinical development since 2016 and new antibacterial pharmacophores are also reviewed. There has been an increase in the number of early stage clinical candidates, which has been fueled by antibiotic-focused funding agencies; however, there is still a significant gap in the pipeline for the development of new antibacterials with activity against β-metallolactamases, orally administered with broad spectrum G−ve activity, and new treatments for MDR Acinetobacter and gonorrhea.

A Whole-Cell Screen Identifies Small Bioactives That Synergize with Polymyxin and Exhibit Antimicrobial Activities against Multidrug-Resistant Bacteria

The threat of diminished antibiotic discovery has global health care in crisis. In the United States, it is estimated each year that over 2 million bacterial infections are resistant to first-line antibiotic treatments and cost in excess of 20 billion dollars. Many of these cases result from infection with the ESKAPE pathogens ( Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species), which are multidrug-resistant bacteria that often cause community- and hospital-acquired infections in both healthy and immunocompromised patients. Physicians have turned to last-resort antibiotics like polymyxins to tackle these pathogens, and as a consequence, polymyxin resistance has emerged and is spreading. Barring the discovery of new antibiotics, another route to successfully mitigate polymyxin resistance is to identify compounds that can complement the existing arsenal of antibiotics. We recently designed and performed a large-scale robotic screen to identify 43 bioactive compounds that act synergistically with polymyxin B to inhibit the growth of polymyxin-resistant Escherichia coli Of these 43 compounds, 5 lead compounds were identified and characterized using various Gram-negative bacterial organisms to better assess their synergistic activity with polymyxin. Several of these compounds reduce polymyxin to an MIC of <2 μg/ml against polymyxin-resistant and polymyxin-heteroresistant Gram-negative pathogens. Likewise, four of these compounds exhibit antimicrobial activity against Gram-positive bacteria, one of which rapidly eradicated methicillin-resistant Staphylococcus aureus We present multiple first-generation (i.e., not yet optimized) compounds that warrant further investigation and optimization, since they can act both synergistically with polymyxin and also as lone antimicrobials for combating ESKAPE pathogens.

Recent progress on elucidating the molecular mechanism of plasmid-mediated colistin resistance and drug design

Antibiotic resistance is a growing global challenge to public health. Polymyxin is considered to be the last-resort antibiotic against most gram-negative bacteria. Recently, discoveries of a plasmid-mediated, transferable mobilized polymyxin resistance gene (mcr-1) in many countries have heralded the increased threat of the imminent emergence of pan-drug-resistant super bacteria. MCR-1 is an inner membrane protein that enables bacteria to develop resistance to polymyxin by transferring phosphoethanolamine to lipid A. However, the mechanism associated with polymyxin resistance has yet to be elucidated, and few drugs exist to address this issue. Here, we review our current understanding regarding MCR-1 and small molecule inhibitors to provide a detailed enzymatic mechanism of MCR-1 and the associated implications for drug design.

Repurposing Eukaryotic Kinase Inhibitors as Colistin Adjuvants in Gram-Negative Bacteria

Kinase inhibitors comprise a diverse cohort of chemical scaffolds that are active in multiple biological systems. Currently, thousands of eukaryotic kinase inhibitors are commercially available, have well-characterized targets, and often carry pharmaceutically favorable toxicity profiles. Recently, our group disclosed that derivatives of the natural product meridianin D, a known inhibitor of eukaryotic kinases, modulated behaviors of both Gram-positive and Gram-negative bacteria. Herein, we expand our exploration of kinase inhibitors in Gram-negative bacilli utilizing three commercially available kinase inhibitor libraries and, ultimately, identify two chemical structures that potentiate colistin (polymyxin E) in multiple strains. We report IMD-0354, an inhibitor of IKK-β, as a markedly effective adjuvant in colistin-resistant bacteria and also describe AR-12 (OSU-03012), an inhibitor of pyruvate dehydrogenase kinase-1 (PDK-1), as a potentiator in colistin-sensitive strains. This report comprises the first description of the novel cross-reactivity of these molecules.

Maternal-to-zygotic transition as a potential target for niclosamide during early embryogenesis

Niclosamide is an antihelminthic drug used worldwide for the treatment of tapeworm infections. Recent drug repurposing screens have highlighted the broad bioactivity of niclosamide across diverse mechanisms of action. As a result, niclosamide is being evaluated for a range of alternative drug-repurposing applications, including the treatment of cancer, bacterial infections, and Zika virus. As new applications of niclosamide will require non-oral delivery routes that may lead to exposure in utero, it is important to understand the mechanism of niclosamide toxicity during early stages of embryonic development. Previously, we showed that niclosamide induces a concentration-dependent delay in epiboly progression in the absence of effects on oxidative phosphorylation - a well-established target for niclosamide. Therefore, the overall objective of this study was to further examine the mechanism of niclosamide-induced epiboly delay during zebrafish embryogenesis. Based on this study, we found that (1) niclosamide exposure during early zebrafish embryogenesis resulted in a decrease in yolk sac integrity with a concomitant decrease in the presence of yolk sac actin networks and increase in cell size; (2) within whole embryos, niclosamide exposure did not alter non-polar metabolites and lipids, but significantly altered amino acids specific to aminoacyl-tRNA biosynthesis; (3) niclosamide significantly altered transcripts related to translation, transcription, and mRNA processing pathways; and (4) niclosamide did not significantly alter levels of rRNA and tRNA. Overall, our findings suggest that niclosamide may be causing a systemic delay in embryonic development by disrupting the translation of maternally-supplied mRNAs, an effect that may be mediated through disruption of aminoacyl-tRNA biosynthesis.

Maternal-to-zygotic transition as a potential target for niclosamide during early embryogenesis

Niclosamide is an antihelminthic drug used worldwide for the treatment of tapeworm infections. Recent drug repurposing screens have highlighted the broad bioactivity of niclosamide across diverse mechanisms of action. As a result, niclosamide is being evaluated for a range of alternative drug-repurposing applications, including the treatment of cancer, bacterial infections, and Zika virus. As new applications of niclosamide will require non-oral delivery routes that may lead to exposure in utero, it is important to understand the mechanism of niclosamide toxicity during early stages of embryonic development. Previously, we showed that niclosamide induces a concentration-dependent delay in epiboly progression in the absence of effects on oxidative phosphorylation - a well-established target for niclosamide. Therefore, the overall objective of this study was to further examine the mechanism of niclosamide-induced epiboly delay during zebrafish embryogenesis. Based on this study, we found that (1) niclosamide exposure during early zebrafish embryogenesis resulted in a decrease in yolk sac integrity with a concomitant decrease in the presence of yolk sac actin networks and increase in cell size; (2) within whole embryos, niclosamide exposure did not alter non-polar metabolites and lipids, but significantly altered amino acids specific to aminoacyl-tRNA biosynthesis; (3) niclosamide significantly altered transcripts related to translation, transcription, and mRNA processing pathways; and (4) niclosamide did not significantly alter levels of rRNA and tRNA. Overall, our findings suggest that niclosamide may be causing a systemic delay in embryonic development by disrupting the translation of maternally-supplied mRNAs, an effect that may be mediated through disruption of aminoacyl-tRNA biosynthesis.

Repurposed Antimicrobial Combination Therapy: Tobramycin-Ciprofloxacin Hybrid Augments Activity of the Anticancer Drug Mitomycin C Against Multidrug-Resistant Gram-Negative Bacteria

The lack of therapeutic options to treat infections caused by multidrug-resistant (MDR) pathogens, especially Gram-negative bacteria, is apparent. Therefore, it is imperative to develop new strategies to address the problem of antimicrobial resistance. Repurposing non-antibiotic commercial drugs for antimicrobial therapy presents a viable option. We screened six anticancer drugs for their potential use in antimicrobial therapy. Here, we provide in vitro evidence that suggests feasibility to repurpose the anticancer drug mitomycin C against MDR Gram-negative bacteria. We also demonstrated that mitomycin C, etoposide and doxorubicin were affected by drug efflux in Pseudomonas aeruginosa. In combination with a tobramycin-ciprofloxacin antibiotic hybrid (TOB-CIP), the antibacterial activity of mitomycin C was enhanced against MDR clinical isolates of P. aeruginosa, Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, and Enterobacter cloacae. In fact, 4 μg/mL (3 μM) TOB-CIP reduced the minimum inhibitory concentration of mitomycin C to ≤1 μg/mL against MDR Gram-negative bacteria, except A. baumannii. We showed that synergy was inherent to TOB-CIP and that neither tobramycin nor ciprofloxacin individually synergized with mitomycin C. Our finding supports identifying adjuvant partners for mitomycin C, such as TOB-CIP, to enhance suitability for antimicrobial therapy.

The anthelmintic oxyclozanide restores the activity of colistin against colistin-resistant Gram-negative bacilli

Due to the significant increase in antimicrobial resistance in Gram-negative bacilli (GNB), development of non-antimicrobial therapeutic alternatives, which can be used together with the few and non-optimal available antimicrobial agents such as colistin, has become an urgent need. In this context, dysregulation of the bacterial cell wall could be a therapeutic adjuvant to the activity of colistin. The aim of this study was to analyse the activity of oxyclozanide, an anthelmintic drug, in combination with colistin against colistin-susceptible (Col-S) and colistin-resistant (Col-R) GNB. Three Col-S reference strains and 13 clinical isolates (1 Col-S, 12 Col-R) of Acinetobacter baumannii, Pseudomonas aeruginosa and Klebsiella pneumoniae were studied. Microdilution assays and time-kill curves were performed to examine the activity of oxyclozanide in combination with colistin. The outer membrane protein (OMP) profile, membrane permeability and cell wall structure of Col-S and Col-R A. baumannii, P. aeruginosa and K. pneumoniae in the presence of oxyclozanide were assessed by SDS-PAGE, fluorescence microscopy and transmission electron microscopy, respectively. Oxyclozanide in combination with colistin increased the activity of colistin against Col-S and Col-R A. baumannii, P. aeruginosa and K. pneumoniae. Time-kill curves showed synergistic activity between oxyclozanide and colistin against these bacterial isolates. Moreover, Col-R A. baumannii, P. aeruginosa and K. pneumoniae in the presence of oxyclozanide presented greater permeability and disruption of their cell wall than Col-S strains, without modification of their OMP profile. These data suggest that combination of oxyclozanide and colistin may be a new alternative for the treatment of Col-R GNB infections.

Synergistic combinations of anthelmintic salicylanilides oxyclozanide, rafoxanide, and closantel with colistin eradicates multidrug-resistant colistin-resistant Gram-negative bacilli

Repurposing nonantibiotic drugs for antimicrobial therapy presents a viable approach to drug discovery. Development of therapeutic strategies that overcome existing resistance mechanisms is important especially against those bacterial infections in which treatment options are limited, such as against multidrug-resistant Gram-negative bacilli. Herein, we provide in vitro data that suggest the addition of anthelmintic salicylanilides, including oxyclozanide, rafoxanide, and closantel, in colistin therapy to treat multidrug-resistant colistin-susceptible but more importantly colistin-resistant Gram-negative bacilli. As a stand-alone agent, the three salicylanilides suffered from limited outer membrane permeation in Pseudomonas aeruginosa, with oxyclozanide also susceptible to efflux. Synergy was apparent for the combinations against multidrug-resistant clinical isolates of P. aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Escherichia coli, and Enterobacter cloacae. Susceptibility breakpoints for colistin, but also with polymyxin B, were reached upon addition of 1 µg ml^-1 of the corresponding salicylanilide against colistin-resistant Gram-negative bacilli. Furthermore, enhanced bacterial killing was observed in all combinations. Our data corroborate the repositioning of the three salicylanilides as adjuvants to counter resistance to the antibiotic of last resort colistin. Our findings are timely and relevant since the global dissemination of plasmid-mediated colistin resistance had been realized.