Adaptation-based resistance to siderophore-conjugated antibacterial agents by Pseudomonas aeruginosa

Bacterial siderophores: diversity, uptake pathways and applications

Iron is an essential nutrient for the growth, survival and virulence of almost all bacteria. To access iron, many bacteria produce siderophores, molecules with a high affinity for iron. Research has highlighted substantial diversity in the chemical structure of siderophores produced by bacteria, as well as remarkable variety in the molecular mechanisms involved in strategies for acquiring iron through these molecules. The metal-chelating properties of siderophores, characterized by their high affinity for iron and ability to chelate numerous other metals (albeit with lower affinity compared with iron), have also generated interest in diverse fields. Siderophores find applications in the environment, such as in bioremediation and agriculture, in which emerging and innovative strategies are being developed to address pollution and enhance nutrient availability for plants. Moreover, in medicine, siderophores could be used as a tool for novel antimicrobial therapies and medical imaging, as well as in haemochromatosis, thalassemia or cancer treatments. This Review offers insights into the diversity of siderophores, highlighting their potential applications in environmental and medical contexts.

Exploring the Localization of Siderophore-Mediated Cargo Delivery in Gram-Negative Bacteria Using 3-Hydroxypyridin-4(1H)-one-Fluorescein Probes

The design of siderophore-antibiotic conjugates is a promising strategy to overcome drug resistance in negative bacteria. However, accumulating studies have shown that only those antibiotics acting on the cell wall or cell membrane multiply their antibacterial effects when coupled with siderophores, while antibiotics acting on targets in the cytoplasm of bacteria do not show an obvious enhancement of their antibacterial effects when coupled with siderophores. To explore the causes of this phenomenon, we synthesized several conjugate probes using 3-hydroxypyridin-4(1H)-ones as siderophores and replacing the antibiotic cargo with 5-carboxyfluorescein (5-FAM) or malachite green (MG) cargo. By monitoring changes in the fluorescence intensity of FAM conjugate 20 in bacteria, the translocation of the conjugate across the outer membranes of Gram-negative pathogens was confirmed. Further, the use of the fluorogen activating protein(FAP)/MG system revealed that 3-hydroxypyridin-4(1H)-one-MG conjugate 26 was ultimately distributed mainly in the periplasm rather than being translocated into the cytosol of Escherichia coli and Pseudomonas aeruginosa PAO1. Additional mechanistic studies suggested that the uptake of the conjugate involved the siderophore-dependent iron transport pathway and the 3-hydroxypyridin-4(1H)-ones siderophore receptor-dependent mechanism. Meanwhile, we demonstrated that the conjugation of 3-hydroxypyridin-4(1H)-ones to the fluorescein 5-FAM can reduce the possibility of the conjugates crossing the membrane layers of mammalian Vero cells by passive diffusion, and the advantages of the mono-3-hydroxypyridin-4(1H)-ones as a delivery vehicle in the design of conjugates compared to the tri-3-hydroxypyridin-4(1H)-ones. Overall, this work reveals the localization rules of 3-hydroxypyridin-4(1H)-ones as siderophores to deliver the cargo into Gram-negative bacteria. It provides a theoretical basis for the subsequent design of siderophore-antibiotic conjugates, especially based on 3-hydroxypyridin-4(1H)-ones as siderophores.

Siderophore-mediated iron acquisition by Klebsiella pneumoniae

Microbes synthesize and secrete siderophores, that bind and solubilize precipitated or otherwise unavailable iron in their microenvironments. Gram (-) bacterial TonB-dependent outer membrane receptors capture the resulting ferric siderophores to begin the uptake process. From their similarity to fepA, the structural gene for the Escherichia coli ferric enterobactin (FeEnt) receptor, we identified four homologous genes in the human and animal ESKAPE pathogen Klebsiella pneumoniae (strain Kp52.145). One locus encodes IroN (locus 0027 on plasmid pII), and three other loci encode other FepA orthologs/paralogs (chromosomal loci 1658, 2380, and 4984). Based on the crystal structure of E. coli FepA (1FEP), we modeled the tertiary structures of the K. pneumoniae FepA homologs and genetically engineered individual Cys substitutions in their predicted surface loops. We subjected bacteria expressing the Cys mutant proteins to modification with extrinsic fluorescein maleimide (FM) and used the resulting fluorescently labeled cells to spectroscopically monitor the binding and transport of catecholate ferric siderophores by the four different receptors. The FM-modified FepA homologs were nanosensors that defined the ferric catecholate uptake pathways in pathogenic strains of K. pneumoniae. In Kp52.145, loci 1658 and 4984 encoded receptors that primarily recognized and transported FeEnt; locus 0027 produced a receptor that principally bound and transported FeEnt and glucosylated FeEnt (FeGEnt); locus 2380 encoded a protein that bound ferric catecholate compounds but did not detectably transport them. The sensors also characterized the uptake of iron complexes, including FeGEnt, by the hypervirulent, hypermucoviscous K. pneumoniae strain hvKp1.

IMPORTANCE

Both commensal and pathogenic bacteria produce small organic chelators, called siderophores, that avidly bind iron and increase its bioavailability. Klebsiella pneumoniae variably produces four siderophores that antagonize host iron sequestration: enterobactin, glucosylated enterobactin (also termed salmochelin), aerobactin, and yersiniabactin, which promote colonization of different host tissues. Abundant evidence links bacterial iron acquisition to virulence and infectious diseases. The data we report explain the recognition and transport of ferric catecholates and other siderophores, which are crucial to iron acquisition by K. pneumoniae.

Catechol-Siderophore Mimics Convey Nucleic Acid Therapeutics into Bacteria

Antibacterial resistance is a major threat for human health. There is a need for new antibacterials to stay ahead of constantly-evolving resistant bacteria. Nucleic acid therapeutics hold promise as powerful antibiotics, but issues with their delivery hamper their applicability. Here, we exploit the siderophore-mediated iron uptake pathway to efficiently transport antisense oligomers into bacteria. We appended a synthetic siderophore to antisense oligomers targeting the essential acpP gene in Escherichia coli. Siderophore-conjugated PNA and PMO antisense oligomers displayed potent antibacterial properties. Conjugates bearing a minimal siderophore consisting of a mono-catechol group showed equally effective. Targeting the lacZ transcript resulted in dose-dependent decreased β-galactosidase production, demonstrating selective protein downregulation. Applying this concept to Acinetobacter baumannii also showed concentration-dependent growth inhibition. Whole-genome sequencing of resistant mutants and competition experiments with the endogenous siderophore verified selective uptake through the siderophore-mediated iron uptake pathway. Lastly, no toxicity towards mammalian cells was found. Collectively, we demonstrate for the first time that large nucleic acid therapeutics can be efficiently transported into bacteria using synthetic siderophore mimics.

Cajaninstilbene acid derivatives conjugated with siderophores of 3-hydroxypyridin-4(1H)-ones as novel antibacterial agents against Gram-negative bacteria based on the Trojan horse strategy

The low permeability of the outer membrane of Gram-negative bacteria is a serious obstacle to the development of new antibiotics against them. Conjugation of antibiotic with siderophore based on the "Trojan horse strategy" is a promising strategy to overcome the outer membrane obstacle. In this study, series of antibacterial agents were designed and synthesized by conjugating the 3-hydroxypyridin-4(1H)-one based siderophores with cajaninstilbene acid (CSA) derivative 4 which shows good activity against Gram-positive bacteria by targeting their cell membranes but is ineffective against Gram-negative bacteria. Compared to the inactive parent compound 4, the conjugates 45c or 45d exhibits significant improvement in activity against Gram-negative bacteria, including Escherichia coli, Klebsiella pneumoniae and especially P. aeruginosa (minimum inhibitory concentrations, MICs = 7.8-31.25 μM). The antibacterial activity of the conjugates is attributed to the CSA derivative moiety, and the action mechanism is by disruption of bacterial cell membranes. Further studies on the uptake mechanisms showed that the bacterial siderophore-dependent iron transport system was involved in the uptake of the conjugates. In addition, the conjugates 45c and 45d showed a lower cytotoxic effects in vivo and in vitro and a positive therapeutic effect in the treatment of C. elegans infected by P. aeruginosa. Overall, our work describes a new class and a promising 3-hydroxypyridin-4(1H)-one-CSA derivative conjugates for further development as antibacterial agents against Gram-negative bacteria.

Conjugation to Native and Nonnative Triscatecholate Siderophores Enhances Delivery and Antibacterial Activity of a β-Lactam to Gram-Negative Bacterial Pathogens

Developing new antibiotics and delivery strategies is of critical importance for treating infections caused by Gram-negative bacterial pathogens. Hijacking bacterial iron uptake machinery, such as that of the siderophore enterobactin (Ent), represents one promising approach toward these goals. Here, we report a novel Ent-inspired siderophore-antibiotic conjugate (SAC) employing an alternative siderophore moiety as the delivery vector and demonstrate the potency of our SACs harboring the β-lactam antibiotic ampicillin (Amp) against multiple pathogenic Gram-negative bacterial strains. We establish the ability of N,N',N''-(nitrilotris(ethane-2,1-diyl))tris(2,3-dihydroxybenzamide) (TRENCAM, hereafter TC), a synthetic mimic of Ent, to facilitate drug delivery across the outer membrane (OM) of Gram-negative pathogens. Conjugation of Amp to a new monofunctionalized TC scaffold affords TC-Amp, which displays markedly enhanced antibacterial activity against the gastrointestinal pathogen Salmonella enterica serovar Typhimurium (STm) compared with unmodified Amp. Bacterial uptake, antibiotic susceptibility, and microscopy studies with STm show that the TC moiety facilitates TC-Amp uptake by the OM receptors FepA and IroN and that the Amp warhead inhibits penicillin-binding proteins. Moreover, TC-Amp achieves targeted activity, selectively killing STm in the presence of a commensal lactobacillus. Remarkably, we uncover that TC-Amp and its Ent-based predecessor Ent-Amp achieve enhanced antibacterial activity against diverse Gram-negative ESKAPE pathogens that express Ent uptake machinery, including strains that possess intrinsic β-lactam resistance. TC-Amp and Ent-Amp exhibit potency comparable to that of the FDA-approved SAC cefiderocol against Gram-negative pathogens. These results demonstrate the effective application of native and appropriately designed nonnative siderophores as vectors for drug delivery across the OM of multiple Gram-negative bacterial pathogens.

Siderophores promote cooperative interspecies and intraspecies cross-protection against antibiotics in vitro

The antibiotic cefiderocol hijacks iron transporters to facilitate its uptake and resists β-lactamase degradation. While effective, resistance has been detected clinically with unknown mechanisms. Here, using experimental evolution, we identified cefiderocol resistance mutations in Pseudomonas aeruginosa. Resistance was multifactorial in host-mimicking growth media, led to multidrug resistance and paid fitness costs in cefiderocol-free environments. However, kin selection drove some resistant populations to cross-protect susceptible individuals from killing by increasing pyoverdine secretion via a two-component sensor mutation. While pyochelin sensitized P. aeruginosa to cefiderocol killing, pyoverdine and the enterobacteria siderophore enterobactin displaced iron from cefiderocol, preventing uptake by susceptible cells. Among 113 P. aeruginosa intensive care unit clinical isolates, pyoverdine production directly correlated with cefiderocol tolerance, and high pyoverdine producing isolates cross-protected susceptible P. aeruginosa and other Gram-negative bacteria. These in vitro data show that antibiotic cross-protection can occur via degradation-independent mechanisms and siderophores can serve unexpected protective cooperative roles in polymicrobial communities.

Cracking spoilage in jar cream cheese: Introducing, modeling and preventing

This study aimed to investigate the modeling of antimicrobial activity (AA) of nisin and sorbate on Clostridium sporogenes in jar cream cheese (JCC) using the linear regression (LR), multilayer perceptron (MLP) neural network, and reduced error pruning tree (REPTree) methods, in order to prevent the late blowing defect (LBD) in the cheese. Both preservatives used in JCC samples showed AA against C. sporogenes; so that sorbate at all the concentrations used in JCC samples inhibited cracking spoilage during storage period at 35 °C. However, nisin could not inhibit cracking spoilage at concentration of 30 ppm in the samples, and a higher concentration of it was needed. The three models used in this study, followed the similar pattern in both training and validation datasets for nisin and sorbat in JCC. The R2 and root mean square error (RMSE) values of training and validation datasets showed the superiority of the REPTree model compared to the MLP and LR models (conventional methods) in the modeling of AA of nisin and sorbate against C. sporogenes in JCC.

Harnessing Transition Metal Scaffolds for Targeted Antibacterial Therapy

Antimicrobial resistance, caused by persistent adaptation and growing resistance of pathogenic bacteria to overprescribed antibiotics, poses one of the most serious and urgent threats to global public health. The limited pipeline of experimental antibiotics in development further exacerbates this looming crisis and new drugs with alternative modes of action are needed to tackle evolving pathogenic adaptation. Transition metal complexes can replenish this diminishing stockpile of drug candidates by providing compounds with unique properties that are not easily accessible using pure organic scaffolds. We spotlight four emerging strategies to harness these unique properties to develop new targeted antibacterial agents.

Siderophore conjugates to combat antibiotic-resistant bacteria

Antimicrobial resistance (AMR) is a global threat to society due to the increasing emergence of multi-drug resistant bacteria that are not susceptible to our last line of defence antibiotics. Exacerbating this issue is a severe gap in antibiotic development, with no new clinically relevant classes of antibiotics developed in the last two decades. The combination of the rapidly increasing emergence of resistance and scarcity of new antibiotics in the clinical pipeline means there is an urgent need for new efficacious treatment strategies. One promising solution, known as the 'Trojan horse' approach, hijacks the iron transport system of bacteria to deliver antibiotics directly into cells - effectively tricking bacteria into killing themselves. This transport system uses natively produced siderophores, which are small molecules with a high affinity for iron. By linking antibiotics to siderophores, to make siderophore antibiotic conjugates, the activity of existing antibiotics can potentially be reinvigorated. The success of this strategy was recently exemplified with the clinical release of cefiderocol, a cephalosporin-siderophore conjugate with potent antibacterial activity against carbapenem-resistant and multi-drug resistant Gram-negative bacilli. This review discusses the recent advancements in siderophore antibiotic conjugates and the challenges associated with the design of these compounds that need to be overcome to deliver more efficacious therapeutics. Potential strategies have also been suggested for new generations of siderophore-antibiotics with enhanced activity.

3-Hydroxy-pyridin-4(1H)-ones as siderophores mediated delivery of isobavachalcone enhances antibacterial activity against pathogenic Pseudomonas aeruginosa

The natural prenylated chalcone isobavachalcone (IBC) shows good antibacterial activity against Gram-positive bacteria but is ineffective against Gram-negative bacteria, most likely due to the outer membrane barrier of Gram-negative bacteria. The Trojan horse strategy has been shown to be an effective strategy to overcome the reduction in the permeability of the outer membrane of Gram-negative bacteria. In this study, eight different 3-hydroxy-pyridin-4(1H)-one-isobavachalcone conjugates were designed and synthesized based on the siderophore Trojan horse strategy. The conjugates exhibited 8- to 32-fold lower minimum inhibitory concentrations (MICs) and 32- to 177-fold lower half-inhibitory concentrations (IC₅₀s) against Pseudomonas aeruginosa PAO1 as well as clinical multidrug-resistant (MDR) strains compared to the parent IBC under iron limitation. Further studies showed that the antibacterial activity of the conjugates was regulated by the bacterial iron uptake pathway under different iron concentration conditions. Studies on the antibacterial mechanism of conjugate 1b showed that it exerts antibacterial activity by disrupting cytoplasmic membrane integrity and inhibiting cell metabolism. Finally, conjugate 1b showed a lower cytotoxic effects on Vero cells than IBC and a positive therapeutic effect in the treatment of bacterial infections caused by Gram-negative bacteria PAO1. Overall, this work demonstrates that IBC can be delivered to Gram-negative bacteria when combined with 3-hydroxy-pyridin-4(1H)-ones as siderophores and provides a scientific basis for the development of effective antibacterial agents against Gram-negative bacteria.

Metals to combat antimicrobial resistance

Bacteria, similar to most organisms, have a love-hate relationship with metals: a specific metal may be essential for survival yet toxic in certain forms and concentrations. Metal ions have a long history of antimicrobial activity and have received increasing attention in recent years owing to the rise of antimicrobial resistance. The search for antibacterial agents now encompasses metal ions, nanoparticles and metal complexes with antimicrobial activity ('metalloantibiotics'). Although yet to be advanced to the clinic, metalloantibiotics are a vast and underexplored group of compounds that could lead to a much-needed new class of antibiotics. This Review summarizes recent developments in this growing field, focusing on advances in the development of metalloantibiotics, in particular, those for which the mechanism of action has been investigated. We also provide an overview of alternative uses of metal complexes to combat bacterial infections, including antimicrobial photodynamic therapy and radionuclide diagnosis of bacterial infections.

Emerging Target-Directed Approaches for the Treatment and Diagnosis of Microbial Infections

With the rising levels of drug resistance, developing efficient antimicrobial therapies has become a priority. A promising strategy is the conjugation of antibiotics with relevant moieties that can potentiate their activity by target-directing. The conjugation of siderophores with antibiotics allows them to act as Trojan horses by hijacking the microorganisms' highly developed iron transport systems and using them to carry the antibiotic into the cell. Through the analysis of relevant examples of the past decade, this Perspective aims to reveal the potential of siderophore-antibiotic Trojan horses for the treatment of infections and the role of siderophores in diagnostic techniques. Other conjugated molecules will be the subject of discussion, namely those involving vitamin B12, carbohydrates, and amino acids, as well as conjugated compounds targeting protein degradation and β-lactamase activated prodrugs.

Understanding the Potential and Risk of Bacterial Siderophores in Cancer

Siderophores are iron chelating molecules produced by nearly all organisms, most notably by bacteria, to efficiently sequester the limited iron that is available in the environment. Siderophores are an essential component of mammalian iron homeostasis and the ongoing interspecies competition for iron. Bacteria produce a broad repertoire of siderophores with a canonical role in iron chelation and the capacity to perform versatile functions such as interacting with other microbes and the host immune system. Siderophores are a vast area of untapped potential in the field of cancer research because cancer cells demand increased iron concentrations to sustain rapid proliferation. Studies investigating siderophores as therapeutics in cancer generally focused on the role of a few siderophores as iron chelators; however, these studies are limited and some show conflicting results. Moreover, siderophores are biologically conserved, structurally diverse molecules that perform additional functions related to iron chelation. Siderophores also have a role in inflammation due to their iron acquisition and chelation properties. These diverse functions may contribute to both risks and benefits as therapeutic agents in cancer. The potential of siderophore-mediated iron and bacterial modulation to be used in the treatment of cancer warrants further investigation. This review discusses the wide range of bacterial siderophore functions and their utilization in cancer treatment to further expand their functional relevance in cancer detection and treatment.

Assessment of sustained efficacy and resistance emergence under human-simulated exposure of cefiderocol against Acinetobacter baumannii using in vitro chemostat and in vivo murine infection models

Objectives

This study evaluated the sustained kill and potential for resistance development of Acinetobacter baumannii exposed to human-simulated exposure of cefiderocol over 72 h in in vitro and in vivo infection models.

Methods

Seven A. baumannii isolates with cefiderocol MICs of 0.12-2 mg/L were tested. The sustained bactericidal activity compared with the initial inoculum and the resistance appearance over 72 h treatment were evaluated in both an in vitro chemostat and an in vivo murine thigh infection model under the human-simulated exposure of cefiderocol (2 g every 8 h as 3 h infusion).

Results

In the in vitro model, regrowth was observed against all seven tested isolates and resistance emergence (>2 dilution MIC increase) was observed in five test isolates. Conversely, sustained killing over 72 h and no resistance emergence were observed in six of seven tested isolates in vivo. The mechanism of one resistant isolate that appeared only in the in vitro chemostat studies was a mutation in the tonB-exbB-exbD region, which contributes to the energy transduction on the iron transporters. The resistance acquisition mechanisms of other isolates have not been identified.

Conclusions

The discrepancy in the sustained efficacy and resistance emergence between in vitro and in vivo models was observed for A. baumannii. Although the resistance mechanisms in vitro have not been fully identified, sustained efficacy without resistance emergence was observed in vivo for six of seven isolates. These studies reveal the in vivo bactericidal activity and the low potential for development of resistance among A. baumannii evaluated under human-simulated exposures.

In a Pinch: Cefiderocol for CRAB Infections

Carbapenem-resistant Acinetobacter baumannii (CRAB) can cause significant infections with limited treatment options available. Falcone et al. (https://doi.org/10.1128/aac.02142-21) describe a single-center retrospective study comparing clinical outcomes among patients with CRAB infections treated with cefiderocol-containing versus colistin-containing regimens. Patients who received cefiderocol-containing regimens had lower 30-day mortality, though there are several limitations raised here, which make interpretation and applicability difficult.

Mechanisms of Reduced Susceptibility to Cefiderocol Among Isolates from the CREDIBLE-CR and APEKS-NP Clinical Trials

The objective of this study was to characterize isolates with reduced susceptibility to cefiderocol in patients receiving cefiderocol for nosocomial pneumonia or carbapenem-resistant infections in the Phase 3 APEKS-NP and CREDIBLE-CR studies. Susceptibility testing of isolates was conducted at a central laboratory, and post-treatment changes were evaluated according to available breakpoints for cefiderocol. Whole-genome sequencing and multilocus sequence typing were performed for isolates to confirm their origin and identify mutations. Five (APEKS-NP) and nine (CREDIBLE-CR) isolates demonstrated a ≥ 4-fold minimum inhibitory concentration (MIC) increase compared with genetically related baseline isolates; most remained susceptible to cefiderocol despite the ≥4-fold MIC increase. Mutations in β-lactamases or penicillin-binding protein (PBP) were identified in 4/14 isolates: one Enterobacter cloacae (amino acid [AA] substitution [A313P] in ACT-17); two Acinetobacter baumannii (one PBP3 AA substitution [H370Y], one with OXA-23 substitutions [N85I and P225S]); and one Pseudomonas aeruginosa (PDC-30 [4AA deletion "TPMA" position 316-319]). Cloning experiments using isogenic Escherichia coli strains containing wild-type and those mutant cephalosporinase enzymes show that the mutant enzymes may contribute to decreased susceptibility to cefiderocol. Pharmacokinetic data were available for nine patients, for whom cefiderocol exposures exceeded 100% fT > 4 × MIC. No clear pattern between mutations and development or extent of MIC increases was observed. No mutations were identified in genes related to iron transport, including fiu, cirA, piuA/C, and pirA, among recovered Gram-negative isolates. Clinicaltrials.gov: APEKS-NP: NCT03032380; CREDIBLE-CR: NCT02714595.

Fluorescent sensors of siderophores produced by bacterial pathogens

Siderophores are iron-chelating molecules that solubilize Fe3+ for microbial utilization and facilitate colonization or infection of eukaryotes by liberating host iron for bacterial uptake. By fluorescently labeling membrane receptors and binding proteins, we created 20 sensors that detect, discriminate, and quantify apo- and ferric siderophores. The sensor proteins originated from TonB-dependent ligand-gated porins (LGPs) of Escherichia coli (Fiu, FepA, Cir, FhuA, IutA, BtuB), Klebsiella pneumoniae (IroN, FepA, FyuA), Acinetobacter baumannii (PiuA, FepA, PirA, BauA), Pseudomonas aeruginosa (FepA, FpvA), and Caulobacter crescentus (HutA) from a periplasmic E. coli binding protein (FepB) and from a human serum binding protein (siderocalin). They detected ferric catecholates (enterobactin, degraded enterobactin, glucosylated enterobactin, dihydroxybenzoate, dihydroxybenzoyl serine, cefidericol, MB-1), ferric hydroxamates (ferrichromes, aerobactin), mixed iron complexes (yersiniabactin, acinetobactin, pyoverdine), and porphyrins (hemin, vitamin B12). The sensors defined the specificities and corresponding affinities of the LGPs and binding proteins and monitored ferric siderophore and porphyrin transport by microbial pathogens. We also quantified, for the first time, broad recognition of diverse ferric complexes by some LGPs, as well as monospecificity for a single metal chelate by others. In addition to their primary ferric siderophore ligands, most LGPs bound the corresponding aposiderophore with ∼100-fold lower affinity. These sensors provide insights into ferric siderophore biosynthesis and uptake pathways in free-living, commensal, and pathogenic Gram-negative bacteria.

Harnessing microbial iron chelators to develop innovative therapeutic agents

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Microbial iron chelators as a new route to develop inspiring antimicrobials.

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Siderophore-mimicking antibiotics as a pathogen-targeted strategy.

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Effectiveness of iron chelators on antibiotic-resistant Gram-negative bacteria.

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Iron chelators and the treatment of iron overload diseases.

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Iron chelators as powerful tools for cancer therapy.

Background

Aim of Review

Key Scientific Concepts of Review

Antibiotic Conjugates with an Artificial MECAM-Based Siderophore Are Potent Agents against Gram-Positive and Gram-Negative Bacterial Pathogens

The development of novel drugs against Gram-negative bacteria represents an urgent medical need. To overcome their outer cell membrane, we synthesized conjugates of antibiotics and artificial siderophores based on the MECAM core, which are imported by bacterial iron uptake systems. Structures, spin states, and iron binding properties were predicted in silico using density functional theory. The capability of MECAM to function as an effective artificial siderophore in Escherichia coli was proven in microbiological growth recovery and bioanalytical assays. Following a linker optimization focused on transport efficiency, five β-lactam and one daptomycin conjugates were prepared. The most potent conjugate 27 showed growth inhibition of Gram-positive and Gram-negative multidrug-resistant pathogens at nanomolar concentrations. The uptake pathway of MECAMs was deciphered by knockout mutants and highlighted the relevance of FepA, CirA, and Fiu. Resistance against 27 was mediated by a mutation in the gene encoding ExbB, which is involved in siderophore transport.

Antibacterial Activity of LCB10-0200 against Klebsiella pneumoniae

Klebsiella pneumoniae is one of the important clinical organisms that causes various infectious diseases, including urinary tract infections, necrotizing pneumonia, and surgical wound infections. The increase in the incidence of multidrug-resistance K. pneumoniae is a major problem in public healthcare. Therefore, a novel antibacterial agent is needed to treat this pathogen. Here, we studied the in vitro and in vivo activities of a novel antibiotic LCB10-0200, a siderophore-conjugated cephalosporin, against clinical isolates of K. pneumoniae. In vitro susceptibility study found that LCB10-0200 showed potent antibacterial activity against K. pneumoniae, including the beta-lactamase producing strains. The in vivo efficacy of LCB10-0200 was examined in three different mouse infection models, including systemic, thigh, and urinary tract infections. LCB10-0200 showed more potent in vivo activity than ceftazidime in the three in vivo models against the drug-susceptible and drug-resistant K. pneumoniae strains. Taken together, these results show that LCB10-0200 is a potential antibacterial agent to treat infection caused by K. pneumoniae.

Discrepancy in sustained efficacy and resistance emergence under human-simulated exposure of cefiderocol against Stenotrophomonas maltophilia between in vitro chemostat and in vivo murine infection models

OBJECTIVES

The present study evaluated the sustained kill and the potential for resistance development of Stenotrophomonas maltophilia exposed to a human-simulated exposure of cefiderocol over 72 h in in vitro and in vivo infection models.

METHODS

A total of seven S. maltophilia isolates with cefiderocol MICs of 0.03-0.5 mg/L were utilized. The sustained bactericidal activity compared with the initial inoculum and the appearance of resistance after the 72 h treatment were evaluated in both an in vitro chemostat model (four strains) and an in vivo murine thigh infection model (six strains) under the human-simulated exposure of cefiderocol (2 g every 8 h as a 3 h infusion).

RESULTS

In the in vitro model, regrowth was observed for three of four tested isolates and resistance emergence (>2-dilution MIC increase) was observed for all of the four test isolates. Conversely, sustained killing over 72 h and no resistance emergence were observed for all of the six tested isolates in the in vivo models. The mechanism of all resistant isolates that appeared only in the in vitro chemostat studies was a mutation in the tonB-exbB-exbD region, which contributes to the energy transduction on the iron transporters.

CONCLUSIONS

The discrepancy in the sustained efficacy and resistance emergence between in vivo and in vitro models appears to be due to the resistance acquisition mechanism caused by mutation in the tonB-exbB-exbD region developing in the enriched media utilized in vitro. These studies reveal the in vivo bactericidal activity and the low potential for development of resistance among Stenotrophomonas evaluated under human-simulated exposures.

Synthetic Mimics of Native Siderophores Disrupt Iron Trafficking in Acinetobacter baumannii

Many pathogenic bacteria biosynthesize and excrete small molecule metallophores, known as siderophores, that are used to extract ferric iron from host sources to satisfy nutritional need. Native siderophores are often structurally complex multidentate chelators that selectively form high-affinity octahedral ferric iron complexes with defined chirality recognizable by cognate protein receptors displayed on the bacterial cell surface. Simplified achiral analogues can serve as synthetically tractable siderophore mimics with potential utility as chemical probes and therapeutic agents to better understand and treat bacterial infections, respectively. Here, we demonstrate that synthetic spermidine-derived mixed ligand bis-catecholate monohydroxamate siderophores (compounds 1-3) are versatile structural and biomimetic analogues of two native siderophores, acinetobactin and fimsbactin, produced by Acinetobacter baumannii, a multidrug-resistant Gram-negative human pathogen. The metal-free and ferric iron complexes of the synthetic siderophores are growth-promoting agents of A. baumannii, while the Ga(III)-complexes are potent growth inhibitors of A. baumannii with MIC values <1 μM. The synthetic siderophores compete with native siderophores for uptake in A. baumannii and maintain comparable apparent binding affinities for ferric iron (K_Fe) and the siderophore-binding protein BauB (K_d). Our findings provide new insight to guide the structural fine-tuning of these compounds as siderophore-based therapeutics targeting pathogenic strains of A. baumannii.

New Perspectives on Antimicrobial Agents: Cefiderocol

Bacterial resistance to carbapenem agents has reached alarming levels. Accordingly, collaborative efforts between national and international organizations and the pharmaceutical industry have led to an impressive expansion of commercially available β-lactam agents in recent years. No available agent comes close to the broad range of activity afforded by cefiderocol, a novel siderophore-cephalosporin conjugate. The novelty of and need for cefiderocol are clear, but available clinical data are conflicting, leaving infectious diseases specialists puzzled as to when to prescribe this agent in clinical practice. After a brief overview of cefiderocol pharmacokinetics and pharmacodynamics, safety data, cefiderocol susceptibility testing, and putative mechanisms of cefiderocol resistance, this review focuses on determining cefiderocol's role in the management of specific pathogens, including carbapenem-resistant Acinetobacter baumannii complex, carbapenem-resistant Pseudomonas aeruginosa, carbapenem-resistant Enterobacterales, and less commonly identified glucose-nonfermenting organisms such as Stenotrophomonas maltophilia, Burkholderia species, and Achromobacter species. Available preclinical, clinical trial, and postmarketing data are summarized for each organism, and each section concludes with our opinions on where to position cefiderocol as a clinical therapeutic.

Glycosylated Nanotherapeutics with β-Lactamase Reversible Competitive Inhibitory Activity Reinvigorates Antibiotics against Gram-Negative Bacteria

Antibiotics are currently first-line therapy for bacterial infections. However, the curative effect of antibiotic remedies is limited due to increasingly prevalent bacterial resistance. The strategy to reverse intrinsic acquired drug resistance presents a promising option for reinvigorating antibiotic therapy. Here, we developed a β-lactamase-inhibiting macromolecule composed of benzoxaborole and dextran for precise transport of β-lactam antibiotics to strains overexpressing β-lactamase. Benzoxaborole-derived nanotherapeutics enabled specific recognition and rapid internalization, and the nanotherapeutics with a high affinity toward bacteria distinctly inhibited the catalytic activity of bacterially secreted β-lactamase by a reversible competitive mechanism. Thus, the system entrapping cefoxitin harbored a significantly enhanced ability to kill drug-resistant Escherichia coli compared to the ability of the drug by specifically overcoming the membrane barrier and acquired resistance mechanism of β-lactamase overproduction. The reversible competitive nanotherapeutics exhibited a robust therapeutic efficacy in rat wounds infected with drug-resistant bacteria; the efficacy was due to efficient bacterial elimination and collateral benzoxaborole-dependent amelioration of the inflammatory response. The above results offered insights into the facile design of precise macromolecular adjuvants to exclusively reverse the acquired bacterial resistance mechanism and increase the utility of antibiotic therapies against antibiotic-resistant bacterial infections.

Iron Acquisition Systems of Gram-negative Bacterial Pathogens Define TonB-Dependent Pathways to Novel Antibiotics

Iron is an indispensable metabolic cofactor in both pro- and eukaryotes, which engenders a natural competition for the metal between bacterial pathogens and their human or animal hosts. Bacteria secrete siderophores that extract Fe3+ from tissues, fluids, cells, and proteins; the ligand gated porins of the Gram-negative bacterial outer membrane actively acquire the resulting ferric siderophores, as well as other iron-containing molecules like heme. Conversely, eukaryotic hosts combat bacterial iron scavenging by sequestering Fe3+ in binding proteins and ferritin. The variety of iron uptake systems in Gram-negative bacterial pathogens illustrates a range of chemical and biochemical mechanisms that facilitate microbial pathogenesis. This document attempts to summarize and understand these processes, to guide discovery of immunological or chemical interventions that may thwart infectious disease.

Design and Syntheses of New Antibiotics Inspired by Nature's Quest for Iron in an Oxidative Climate

This Account describes fundamental chemistry that promoted the discovery of new antibiotics. Specifically, the NH acidity of simple hydroxamic acid derivatives facilitated the syntheses of novel β-lactams (oxamazins and monobactams), siderophore mimics that limit bacterial iron uptake and bacterially targeted sideromycins (siderophore-antibiotic conjugates). The development of resistance to our current limited set of antibiotic scaffolds has created a dire medical situation. As recently stated, "if you weren't taking antibiotic resistance seriously before, now would be a good time to start." A project commissioned by the British government (https://amr-review.org/) has released estimates of the near-future global toll of antibiotic resistance that are jaw-dropping in their seriousness and scale: 10 million deaths per year and at least $100 trillion in sacrificed gross national product. The 2020 COVID pandemic confirmed that infectious disease problems are no longer localized but worldwide. Many classical antibiotics, especially β-lactams, previously provided economical cures, but the evolution of antibiotic destructive enzymes (i.e., β-lactamases), efflux pumps, and bacterial cell wall permeability barriers has made many types of bacteria, especially Gram-negative strains, resistant. Still, and in contrast to other therapies, the public expectation is that any new antibiotic must be inexpensive. This creates market limitations that have caused most major pharmaceutical companies to abandon antibiotic research. Much needs to be done to address this significant problem.The critical need for bacteria to sequester essential iron provides an Achilles' heel for new antibiotic development. Although ferric iron is extremely insoluble, bacteria need micromolar intracellular concentrations for growth and virulence. To this end, they biosynthesize siderophores (Gr. iron bearer) and excrete them into their environment, where they bind iron with high affinity. The iron complexes are recognized by specific outer-membrane transporters, and once actively internalized, the iron is released for essential processes. To conserve biosynthetic energy, some bacteria recognize and utilize siderophores made by competing strains. As a counter-revolution in the never-ending fight for survival, bacteria have also evolved sideromycins, which are siderophores conjugated to warheads that are lethal to rogue bacteria. While none are now used therapeutically, natural sideromycins called albomycins have been used clinically, and others have been shown to be well tolerated and active in animal infection models. Herein we describe practical methods to synthesize new antibiotics and artificial sideromycins with the generalized structure shown above (siderophore-linker drug). Utilizing the molecular-recognition-based siderophore/sideromycin bacterial assimilation processes, it is possible to design both broad spectrum and exquisitely narrow spectrum (targeted) sideromycins and even repurpose older or more classical antibiotics. Relevant microbiological assays, in vivo animal infection studies, and the recent FDA approval of cefiderocol demonstrate their effectiveness.

Total synthesis of landomycins Q and R and related core structures for exploration of the cytotoxicity and antibacterial properties

Herein, we report the total synthesis of landomycins Q and R as well as the aglycone core, namely anhydrolandomycinone and a related core analogue. The synthesis features an acetate-assisted arylation method for construction of the hindered B-ring in the core component and a one-pot aromatization-deiodination-denbenzylation procedure to streamline the global functional and protecting group manuipulation. Subsequent cytotoxicity and antibacterial studies revealed that the landomycin R is a potential antibacterial agent against methicillin-resistant Staphylococcus aureus.

The Odd Couple(s): An Overview of Beta-Lactam Antibiotics Bearing More Than One Pharmacophoric Group

β-lactam antibiotics are among the most important and widely used antimicrobials worldwide and are comprised of a large family of compounds, obtained by chemical modifications of the common scaffolds. Usually these modifications include the addition of active groups, but less frequently, molecules were synthesized in which either two β-lactam rings were joined to create a single bifunctional compound, or the azetidinone ring was joined to another antibiotic scaffold or another molecule with a different activity, in order to create a molecule bearing two different pharmacophoric functions. In this review, we report some examples of these derivatives, highlighting their biological properties and discussing how this strategy can lead to the development of innovative antibiotics that can represent either novel weapons against the rampant increase of antimicrobial resistance, or molecules with a broader spectrum of action.

Experimental Methods for Evaluating the Bacterial Uptake of Trojan Horse Antibacterials

The field of antibacterial siderophore conjugates, referred to as Trojan Horse antibacterials, has received increasing attention in recent years, driven by the rise of antimicrobial resistance. Trojan Horse antibacterials offer an opportunity to exploit the specific pathways present in bacteria for active iron uptake, potentially allowing the drugs to bypass membrane-associated resistance mechanisms. Hence, the Trojan Horse approach might enable the redesigning of old antibiotics and the development of antibacterials that target specific pathogens. Critical parts of evaluating such Trojan Horse antibacterials and improving their design are the quantification of their bacterial uptake and the identification of the pathways by which this occurs. In this minireview, we highlight a selection of the biological and chemical methods used to study the uptake of Trojan Horse antibacterials, exemplified with case studies, some of which have led to drug candidates in clinical development or approved antibiotics.

Emerging Strategies to Combat β-Lactamase Producing ESKAPE Pathogens

Since the discovery of penicillin by Alexander Fleming in 1929 as a therapeutic agent against staphylococci, β-lactam antibiotics (BLAs) remained the most successful antibiotic classes against the majority of bacterial strains, reaching a percentage of 65% of all medical prescriptions. Unfortunately, the emergence and diversification of β-lactamases pose indefinite health issues, limiting the clinical effectiveness of all current BLAs. One solution is to develop β-lactamase inhibitors (BLIs) capable of restoring the activity of β-lactam drugs. In this review, we will briefly present the older and new BLAs classes, their mechanisms of action, and an update of the BLIs capable of restoring the activity of β-lactam drugs against ESKAPE (Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) pathogens. Subsequently, we will discuss several promising alternative approaches such as bacteriophages, antimicrobial peptides, nanoparticles, CRISPR (clustered regularly interspaced short palindromic repeats) cas technology, or vaccination developed to limit antimicrobial resistance in this endless fight against Gram-negative pathogens.

Siderophore piracy enhances Vibrio cholerae environmental survival and pathogenesis

Vibrio cholerae, the aetiological agent of cholera, possesses multiple iron acquisition systems, including those for the transport of siderophores. How these systems benefit V. cholerae in low-iron, polymicrobial communities in environmental settings or during infection remains poorly understood. Here, we demonstrate that in iron-limiting conditions, co-culture of V. cholerae with a number of individual siderophore-producing microbes significantly promoted V. cholerae growth in vitro. We further show that in the host environment with low iron, V. cholerae colonizes better in adult mice in the presence of the siderophore-producing commensal Escherichia coli. Taken together, our results suggest that in aquatic reservoirs or during infection, V. cholerae may overcome environmental and host iron restriction by hijacking siderophores from other microbes.

Naphthoquinone-derivative as a synthetic compound to overcome the antibiotic resistance of methicillin-resistant S. aureus

The treatment of Staphylococcus aureus (S. aureus) infections has become more difficult due to the emergence of multidrug resistance in the bacteria. Here, we report the synthesis of a lawsone (2-hydroxy-1,4-naphthoquinone)-based compound as an antimicrobial agent against methicillin-resistant S. aureus (MRSA). A series of lawsone-derivative compounds were synthesized by means of tuning the lipophilicity of lawsone and screened for minimum inhibitory concentrations against MRSA to identify a candidate compound that possesses a potent antibacterial activity. The identified lawsone-derivative compound exhibited significantly improved drug resistance profiles against MRSA compared to conventional antibiotics. The therapeutic efficacy of the compound was validated using murine models of wound infection as well as non-lethal systemic infection induced by MRSA. Our study further revealed the multifaceted modes of action of the compound, mediated by three distinctive mechanisms: (1) cell membrane damage, (2) chelation of intracellular iron ions, and (3) generation of intracellular reactive oxygen species.

Ronghui Song et al. demonstrate that a lawsone (2-hydroxy-1,4-naphthoquinone)-based compound decreases the drug resistance of methicillin-resistant Staphylococcus aureus much better than conventional antibiotics. This study provides insights into the design and action mechanism of effective antibiotics that overcome the antibiotic resistance of bacteria.

A Glance on the Role of Bacterial Siderophore from the Perspectives of Medical and Biotechnological Approaches

Iron, which is described as the most basic component found in nature, is hard to be assimilated by microorganisms. It has become increasingly complicated to obtain iron from nature as iron (II) in the presence of oxygen oxidized to press (III) oxide and hydroxide, becoming unsolvable at neutral pH. Microorganisms appeared to produce organic molecules known as siderophores in order to overcome this condition. Siderophore's essential function is to connect with iron (II) and make it dissolvable and enable cell absorption. These siderophores, apart from iron particles, have the ability to chelate various other metal particles that have collocated away to focus the use of siderophores on wound care items. There is a severe clash between the host and the bacterial pathogens during infection. By producing siderophores, small ferric iron-binding molecules, microorganisms obtain iron. In response, host immune cells produce lipocalin 2 to prevent bacterial reuptake of siderophores loaded with iron. Some bacteria are thought to produce lipocalin 2-resistant siderophores to counter this risk. The aim of this article is to discuss the recently described roles and applications of bacterial siderophore.

Cefiderocol for treatment of an empyema due to extensively drug-resistant Pseudomonas aeruginosa: Clinical observations and susceptibility testing considerations

Cefiderocol is a novel siderophore cephalosporin antibacterial with activity against carbapenem-resistant Gram-negative bacteria including Pseudomonas aeruginosa. We report a medically complex patient treated with compassionate use cefiderocol for an empyema caused by extensively drug-resistant P. aeruginosa as well as clinical considerations for cefiderocol use based on our findings. We observed a potential discordance in cefiderocol susceptibility testing results depending if disk diffusion or iron-depleted cation-adjusted Mueller Hinton Broth dilution is used. Furthermore, interpretative criteria differ between the Clinical Laboratory Standards Institute and United States Food and Drug Administration for P. aeruginosa, which makes cefiderocol interpretation potentially challenging for clinicians. We may have also observed selective pressure from prior cefiderocol exposure given the respective increases and decreases in MIC values and zone diameters for P. aeruginosa isolates following cefiderocol treatment. Additional data are needed to further describe cefiderocol use, susceptibility testing, and resistance development as real-world clinical use expands.

The Battle Is on: New Beta-Lactams for the Treatment of Multidrug-Resistant Gram-Negative Organisms

PURPOSE OF REVIEW

Resistant gram-negative infections are becoming increasingly difficult to treat, prompting increased focus on drug development. This review will focus primarily on the new beta-lactam agents developed in the past 5 years that target multidrug-resistant (MDR) gram-negative organisms, including those producing carbapenemases.

RECENT FINDINGS

Four new agents including ceftazidime-avibactam (CAZ-AVI), meropenem-vaborbactam (MER-VAB), imipenem-relebactam (IMI-REL), and cefiderocol have recently been approved for the treatment of resistant gram-negative infections. CAZ-AVI remains an option for blaOXA-48-producing isolates and potentially MDR Pseudomonas aeruginosa, but the concern for resistance arises when using the agent for KPC-producing Enterobacteriales. MER-VAB appears to be more stable than CAZ-AVI in the treatment of KPC-producing Enterobacteriales but less is known about its propensity for the development of resistance and the drug does not reliably expand the coverage of meropenem-resistant P. aeruginosa isolates. IMI-REL expands the spectrum of imipenem-cilastatin to include KPC-producing Enterobacteriales as well as MDR P. aeruginosa but much less is known about its real-world clinical utility. Cefiderocol is the only of the four new agents with efficacy against metallo-beta-lactamases and resistant Acinetobacter species, but comparator studies using best available therapy for carbapenem-resistant gram-negative bacterial infections show higher mortality rates with the new drug, making its role in clinical therapy still to be determined. The new beta-lactams differ in their mechanisms of combatting resistance and thus have unique roles in therapy. Additional evidence is needed regarding the potential for development of resistance in the newer combination agents, as well as for the role of cefiderocol in carbapenem-resistant gram-negative infections.

Development of Neutropenic Murine Models of Iron Overload and Depletion To Study the Efficacy of Siderophore-Antibiotic Conjugates

Siderophore-antibiotic conjugates have increased in vitro activity in low-iron environments where bacteria express siderophores and associated transporters. The host immune hypoferremic response reduces iron availability to bacteria; however, patients with iron overload or deficiency may have altered ability to restrict iron, which may affect the efficacy of siderophore-antibiotic conjugates. In vivo models of infection with iron overload and deficiency are needed to perform this assessment. The standard neutropenic murine thigh infection model was supplemented with iron-altering treatments: iron dextran at 100 mg/kg of body weight daily for 14 days to load iron or deferoxamine at 100 mg/kg daily plus a low-iron diet for up to 30 days to deplete iron. Human-simulated regimens of cefiderocol and meropenem were administered in both models to assess any impact of iron alteration on plasma pharmacokinetics. Median iron in overloaded mice was significantly higher than that of controls in plasma (1,657 versus 336 μg/dl; P < 0.001), liver (2,133 versus 11 μg/g; P < 0.001), and spleen (473 versus 144 μg/g; P < 0.001). At 30 days, depleted mice had significantly lower iron than controls in liver (2.4 versus 6.5 μg/g; P < 0.001) and spleen (72 versus 133 μg/g; P = 0.029) but not plasma (351 versus 324 μg/dl; P = 0.95). Cefiderocol and meropenem plasma concentrations were similar in iron overloaded and control mice but varied in iron-depleted mice. The iron-overloaded murine thigh infection model was established, and human-simulated regimens of cefiderocol and meropenem were validated therein. While deferoxamine successfully reduced liver and splenic iron, this depleting treatment altered the pharmacokinetics of both antimicrobials.

TonB-dependent uptake of β-lactam antibiotics in the opportunistic human pathogen Stenotrophomonas maltophilia

The β-lactam antibiotic ceftazidime is one of the handful of drugs with proven clinical efficacy against the important opportunistic human pathogen Stenotrophomonas maltophilia. Here, we show that mutations in the energy transducer TonB, encoded by smlt0009 in S. maltophilia, confer ceftazidime resistance and smlt0009 mutants have reduced uptake of ceftazidime. This breaks the dogma that β-lactams enter Gram-negative bacteria only by passive diffusion through outer membrane porins. We also show that ceftazidime-resistant TonB mutants are cross-resistant to fluoroquinolone antimicrobials and a siderophore-conjugated lactivicin antibiotic designed to target TonB-dependent uptake. This implies that attempts to improve the penetration of antimicrobials into S. maltophilia by conjugating them with TonB substrates will suffer from the fact that β-lactams and fluoroquinolones coselect resistance to these novel and otherwise promising antimicrobials. Finally, we show that smlt0009 mutants already exist among S. maltophilia clinical isolates and have reduced susceptibility to siderophore-conjugated lactivicin, despite the in vitro growth impairment seen in smlt0009 mutants selected in the laboratory.

Cefiderocol: Discovery, Chemistry, and In Vivo Profiles of a Novel Siderophore Cephalosporin

The emergence of antimicrobial resistance is a significant public health issue worldwide, particularly for healthcare-associated infections caused by carbapenem-resistant gram-negative pathogens. Cefiderocol is a novel siderophore cephalosporin targeting gram-negative bacteria, including strains with carbapenem resistance. The structural characteristics of cefiderocol show similarity to both ceftazidime and cefepime, which enable cefiderocol to withstand hydrolysis by β-lactamases. The unique chemical component is the addition of a catechol moiety on the C-3 side chain, which chelates iron and mimics naturally occurring siderophore molecules. Following the chelation of iron, cefiderocol is actively transported across the outer membrane of the bacterial cell to the periplasmic space via specialized iron transporter channels. Furthermore, cefiderocol has demonstrated structural stability against hydrolysis by both serine- and metallo-β-lactamases, including clinically relevant carbapenemases such as Klebsiella pneumoniae carbapenemase, oxacillin carbapenemase-48, and New Delhi metallo-β-lactamase. Cefiderocol has demonstrated promising in vitro antibacterial and bactericidal activity, which correlates with its in vivo efficacy in several animal models. This article reviews the discovery and chemistry of cefiderocol, as well as some of the key microbiological and in vivo findings on cefiderocol from recently conducted investigations.

The Role of Iron and Siderophores in Infection, and the Development of Siderophore Antibiotics

Iron is an essential nutrient for bacterial growth, replication, and metabolism. Humans store iron bound to various proteins such as hemoglobin, haptoglobin, transferrin, ferritin, and lactoferrin, limiting the availability of free iron for pathogenic bacteria. However, bacteria have developed various mechanisms to sequester or scavenge iron from the host environment. Iron can be taken up by means of active transport systems that consist of bacterial small molecule siderophores, outer membrane siderophore receptors, the TonB-ExbBD energy-transducing proteins coupling the outer and the inner membranes, and inner membrane transporters. Some bacteria also express outer membrane receptors for iron-binding proteins of the host and extract iron directly from these for uptake. Ultimately, iron is acquired and transported into the bacterial cytoplasm. The siderophores are small molecules produced and released by nearly all bacterial species and are classified according to the chemical nature of their iron-chelating group (ie, catechol, hydroxamate, α-hydroxyl-carboxylate, or mixed types). Siderophore-conjugated antibiotics that exploit such iron-transport systems are under development for the treatment of infections caused by gram-negative bacteria. Despite demonstrating high in vitro potency against pathogenic multidrug-resistant bacteria, further development of several candidates had stopped due to apparent adaptive resistance during exposure, lack of consistent in vivo efficacy, or emergence of side effects in the host. However, cefiderocol, with an optimized structure, has advanced and has been investigated in phase 1 to 3 clinical trials. This article discusses the mechanisms implicated in iron uptake and the challenges associated with the design and utilization of siderophore-mimicking antibiotics.

Siderophore-Antibiotic Conjugate Design: New Drugs for Bad Bugs?

Antibiotic resistance is a global health concern and a current threat to modern medicine and society. New strategies for antibiotic drug design and delivery offer a glimmer of hope in a currently limited pipeline of new antibiotics. One strategy involves conjugating iron-chelating microbial siderophores to an antibiotic or antimicrobial agent to enhance uptake and antibacterial potency. Cefiderocol (S-649266) is a promising cephalosporin–catechol conjugate currently in phase III clinical trials that utilizes iron-mediated active transport and demonstrates enhanced potency against multi-drug resistant (MDR) Gram-negative pathogens. Such molecules demonstrate that siderophore–antibiotic conjugates could be important future medicines to add to our antibiotic arsenal. This review is written in the context of the chemical design of siderophore–antibiotic conjugates focusing on the differing siderophore, linker, and antibiotic components that make up conjugates. We selected chemically distinct siderophore–antibiotic conjugates as exemplary conjugates, rather than multiple analogues, to highlight findings to date. The review should offer a general guide to the uninitiated in the molecular design of siderophore–antibiotic conjugates.

In Vivo Pharmacodynamic Study of Cefiderocol, a Novel Parenteral Siderophore Cephalosporin, in Murine Thigh and Lung Infection Models

The pharmacokinetic (PK) and pharmacodynamic (PD) parameters which correlated with the in vivo efficacy of cefiderocol were evaluated using neutropenic murine thigh and lung infection models in which the infections were caused by a variety of Gram-negative bacilli. The dose fractionation study using the thigh infection model in which the infection was caused by Pseudomonas aeruginosa showed that the cumulative percentage of a 24-h period that the free drug concentration in plasma exceeds the MIC (%fT>MIC) rather than the free peak level divided by the MIC (fCmax/MIC) and the area under the free concentration-time curve over 24 h divided by the MIC (fAUC/MIC) was the PK/PD parameter that best correlated with efficacy. The study with multiple carbapenem-resistant strains revealed that the %fT>MIC determined in iron-depleted cation-adjusted Mueller-Hinton broth (ID-CAMHB) better reflected the in vivo efficacy of cefiderocol than the %fT>MIC determined in cation-adjusted Mueller-Hinton broth (CAMHB). The mean %fT>MIC of cefiderocol required for a 1-log10 reduction against 10 strains of Enterobacteriaceae and 3 strains of Pseudomonas aeruginosa in the thigh infection models were 73.3% and 77.2%, respectively. The mean %fT>MIC for Enterobacteriaceae, P. aeruginosa, Acinetobacter baumannii, and Stenotrophomonas maltophilia in the lung infection model were 64.4%, 70.3%, 88.1%, and 53.9%, respectively. These results indicate that cefiderocol has potent efficacy against Gram-negative bacilli, including carbapenem-resistant strains, irrespective of the bacterial species, in neutropenic thigh and lung infection models and that the in vivo efficacy correlated with the in vitro MIC under iron-deficient conditions.

Targeting Mobilization of Ferrous Iron in Pseudomonas aeruginosa Infection with an Iron(II)-Caged LpxC Inhibitor

Iron is essential to all life, and competition for this vital nutrient is central to host-pathogen interactions during infection. The opportunistic Gram-negative pathogen Pseudomonas aeruginosa utilizes a diverse array of iron-acquisition strategies, including those enabling import of extracellular ferrous iron. We hypothesize that soluble and redox-active ferrous iron can be employed to activate caged antibiotics at sites of infection in vivo. Here we describe new chemistry that expands the application of our laboratory's Fe²⁺-activated-prodrug chemistry to cage hydroxamic acids, a class of drugs that present manifold development challenges. We synthesize the caged form of a known LpxC inhibitor and show that it is efficacious in an acute P. aeruginosa mouse-lung infection model, despite showing little activity in cell-culture experiments. Overall, our results are consistent with the Fe²⁺-promoted uncaging of an antibacterial payload at sites of infection in an animal and lend support to recent reports indicating that extracellular pools of ferrous iron can be utilized by bacterial pathogens like P. aeruginosa during infection.

Defining the Role of Novel β-Lactam Agents That Target Carbapenem-Resistant Gram-Negative Organisms

With the current carbapenem-resistant organism crisis, conventional approaches to optimizing pharmacokinetic-pharmacodynamic parameters are frequently inadequate, and traditional salvage agents (eg, colistin, tigecycline, etc) confer high toxicity and/or have low efficacy. However, several β-lactam agents with activity against carbapenem-resistant organisms were approved recently by the US Food and Drug Administration, and more are anticipated to be approved in the near future. The primary goal of this review is to assist infectious disease practitioners with preferentially selecting 1 agent over another when treating patients infected with a carbapenem-resistant organism. However, resistance to some of these antibiotics has already developed. Antibiotic stewardship programs can ensure that they are reserved for situations in which other options are lacking and are paramount for the survival of these agents.

Metabolism, Excretion, and Pharmacokinetics of [14 C]-Cefiderocol (S-649266), a Siderophore Cephalosporin, in Healthy Subjects Following Intravenous Administration

The objectives of this study were to characterize the concentration-time profiles of total radioactivity equivalent and unchanged cefiderocol, the route(s) of elimination and mass balance, and safety of cefiderocol after intravenous administration of a single 1000-mg (100 μCi) dose of [14 C]-cefiderocol as a 1-hour infusion in healthy adult male subjects. Unchanged cefiderocol accounted for the majority of total radioactivity in plasma, and the partitioning of total radioactivity into red blood cells was negligible. The recovery of total radioactivity was complete in all subjects within 120 hours after initiation of the infusion (101.5% of the administered dose). Cefiderocol-related material was primarily excreted into urine, with 98.7% of the administered dose of [14 C]-cefiderocol excreted as total radioactivity into urine and negligible excretion into feces. Based on the results of metabolite profiling, cefiderocol accounted for 92.3% of area under the concentration-time curve of total radioactivity in plasma and accounted for 90.6% of the administered dose excreted into urine. Metabolism was a minor route of elimination for cefiderocol. Cefiderocol was generally safe and well tolerated in healthy adult male subjects. In conclusion, unchanged cefiderocol represents the majority of total radioactivity in plasma. Cefiderocol is primarily excreted as unchanged drug into urine. This study indicates that cefiderocol and drug-related material did not remain in the body.

Reversing resistance to counter antimicrobial resistance in the World Health Organisation's critical priority of most dangerous pathogens

The speed at which bacteria develop antimicrobial resistance far outpace drug discovery and development efforts resulting in untreatable infections. The World Health Organisation recently released a list of pathogens in urgent need for the development of new antimicrobials. The organisms that are listed as the most critical priority are all Gram-negative bacteria resistant to the carbapenem class of antibiotics. Carbapenem resistance in these organisms is typified by intrinsic resistance due to the expression of antibiotic efflux pumps and the permeability barrier presented by the outer membrane, as well as by acquired resistance due to the acquisition of enzymes able to degrade β-lactam antibiotics. In this perspective article we argue the case for reversing resistance by targeting these resistance mechanisms - to increase our arsenal of available antibiotics and drastically reduce antibiotic discovery times - as the most effective way to combat antimicrobial resistance in these high priority pathogens.

In vitro activities of cefiderocol, ceftolozane/tazobactam, ceftazidime/avibactam and other comparative drugs against imipenem-resistant Pseudomonas aeruginosa and Acinetobacter baumannii, and Stenotrophomonas maltophilia, all associated with bloodstream infections in Taiwan

Objectives

We investigated the in vitro activities of cefiderocol, ceftazidime/avibactam, ceftolozane/tazobactam and other related drugs against imipenem-resistant Pseudomonas aeruginosa, imipenem-resistant Acinetobacter baumannii and Stenotrophomonas maltophilia isolates.

Methods

Non-duplicated bacteraemia isolates (n = 300) of imipenem-resistant P. aeruginosa (n = 100), imipenem-resistant A. baumannii (n = 100) and S. maltophilia (n = 100) were evaluated. Imipenem-resistant P. aeruginosa and imipenem-resistant A. baumannii isolates were defined as isolates exhibiting imipenem MIC ≥8 mg/L, as determined using the VITEK 2 system. The MICs of 11 other antimicrobial agents for the isolates were determined by the broth microdilution method. Iron-depleted CAMHB was used to determine the MICs of cefiderocol.

Results

The rates of colistin resistance of imipenem-resistant P. aeruginosa and imipenem-resistant A. baumannii were 5% and 10%, respectively. The MIC90 values of cefiderocol, ceftolozane/tazobactam, ceftazidime/avibactam, tigecycline and colistin were as follows: imipenem-resistant P. aeruginosa: 1, 4, 16, >4 and 2 mg/L; imipenem-resistant A. baumannii: 8, >64, >64, 4 and 2 mg/L; and S. maltophilia: 0.25, >64, >64, 2 and >8 mg/L, respectively. For imipenem-resistant A. baumannii isolates, the MICs of cefiderocol, ceftolozane/tazobactam and ceftazidime/avibactam were ≤4 mg/L for 88%, 8% and 1% of the isolates, respectively. Cefiderocol MICs were ≤4 mg/L for the five colistin-resistant imipenem-resistant P. aeruginosa isolates and 70% of the 10 colistin-resistant imipenem-resistant A. baumannii isolates.

Conclusions

Cefiderocol exhibited more potent in vitro activity than ceftolozane/tazobactam and ceftazidime/avibactam against imipenem-resistant P. aeruginosa, imipenem-resistant A. baumannii and S. maltophilia isolates.

Efficacy of Humanized Cefiderocol Exposures over 72 Hours against a Diverse Group of Gram-Negative Isolates in the Neutropenic Murine Thigh Infection Model

Herein, we evaluated sustainability of humanized exposures of cefiderocol in vivo over 72 h against pathogens with cefiderocol MICs of 0.5 to 16 μg/ml in the neutropenic murine thigh model. In Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae displaying MICs of 0.5 to 8 μg/ml (n = 11), sustained kill was observed at 72 h among 9 isolates. Postexposure MICs revealed a single 2-dilution increase in one animal compared with controls (1/54 samples, 1.8%) at 72 h. Adaptive resistance during therapy was not observed.

Association between Beta-lactam Antibiotic Resistance and Virulence Factors in AmpC Producing Clinical Strains of P. aeruginosa

Objectives

The purpose of this study was to determine the presence of IMP and OXA genes in clinical strains of Pseudomonas aeruginosa (P. aeruginosa) that are carriers of the ampC gene.

Methods

In this study, 105 clinical isolates of P. aeruginosa were collected. Antibiotic resistance patterns were determined using the disk diffusion method. The strains carrying AmpC enzymes were characterized by a combination disk method. Multiplex-PCR was used to identify resistance and virulence genes, chi-square test was used to determine the relationship between variables.

Results

Among 105 isolates of P. aeruginosa, the highest antibiotic resistance was to cefotaxime and aztreonam, and the least resistance was to colictin and ceftazidime. There were 49 isolates (46.66%) that showed an AmpC phenotype. In addition, the frequencies of the resistance genes were; OXA48 gene 85.2%, OXA199, 139 3.8%, OXA23 3.8%, OXA2 66.6%, OXA10 3.8%, OXA51 85.2% and OXA58 3.8%. The IMP27 gene was detected in 9 isolates (8.57%) and the IMP3.34 was detected in 11 isolates (10.47%). Other genes detected included; lasR (17.1%), lasB (18%) and lasA (26.6%). There was a significant relationship between virulence factors and the OX and IMP genes (p ≤ 0.05).

Conclusion

The relationship between antibiotic resistance and virulence factors observed in this study could play an important role in outbreaks associated with P. aeruginosa infections.