Siderophore: A Suitable Candidate for Drug Delivery Using the Trojan Horse Strategy
Behnoush Khasheii
(MSc, PhD Candidate) Department of Pathobiology, Faculty of Veterinary Science, Bu-Ali Sina University, Hamedan, Iran
Pezhman Mahmoodi
(DVM, DVSc) Assistant Professor, Department of Pathobiology, Faculty of Veterinary Science, Bu-Ali Sina University, Hamedan, Iran
Abdolmajid Mohammadzadeh
(DVM, PhD) Associate Professor, Department of Pathobiology, Faculty of Veterinary Science, Bu-Ali Sina University, Hamedan, Iran
Corresponding author: Pezhman Mahmoodi
Tel: +988134227350
Address: Department of Pathobiology, Faculty of Veterinary Science, Bu-Ali Sina University, Hamedan, Iran
Email: mahmoodi_pezhman@basu.ac.ir; mahmoodi_pezhman@yahoo.com
 
ABSTRACT
Increasing antibiotic resistance is a global health problem. In recent years, due to the indiscriminate use of antibacterial compounds, many bacterial pathogens, including staphylococci, members of the Enterobacteriaceae family including Klebsiella pneumoniae and bacteria such as Pseudomonas aeruginosa and Acinetobacter baumannii have become multi-drug resistant. Consequently, it is important to explore alternative approaches for eliminating resistant strains. Bacteria synthesize low-weight molecules called siderophores to chelate iron from the environment as a vital element for their growth and survival. One way to deal with resistant bacterial strains is to utilize siderophore-mediated iron uptake pathways as entrance routes for drug delivery. Therefore, the production of drugs with Trojan horse strategy in the form of conjugated siderophore-antibiotic complexes has recently received much attention for dealing with resistant isolates. In this review, we discuss the efficacy of siderophore-antibiotic conjugates as a Trojan horse strategy for eliminating drug-resistant pathogens.
Keywords: Siderophores, Iron, Drug delivery systems, Drug Carriers, Drug resistance, Anti-Bacterial Agents
 
INTRODUCTION
The increasing rate of resistance in bacterial pathogens has contributed to the growing number of incurable infections (1, 2). According to the Centers for Disease Control and Prevention’s report in 2019, 2.8 million antibiotic-resistant infections occur in the United States each year that result in almost 35,000 deaths (3). Therefore, development of new antibiotics and novel therapeutic and diagnostic approaches seems crucial. Otherwise, common bacterial infections may re-emerge as  potential public health threats (4). Although antibiotic resistance has been discovered in both gram-negative and gram-positive bacteria, it is more common in gram-negative bacteria, such as Acinetobacter, Pseudomonas and members of the Enterobacteriaceae family (5). These bacteria may be resistant to many antibiotics, including carbapenems and third-generation cephalosporins as the drugs of choice for elimination of multidrug-resistant bacteria (6). Bacterial resistance is referred to the ability of bacterial cells to inhibit the bactericidal and bacteriostatic effects of antibiotics (7). Various mechanisms have been described for the development of antibiotic resistance in bacteria, which can be divided into five main categories:
Changes that occur in drug-related receptors and/or molecules that are targeted by specific antibiotics, which can be in complex with enzymes and ribosomes (8). Most of these known types of resistance are in macrolide antibiotics (9). The most well-known example of this resistance is the development of penicillin resistance due to mutations in penicillin-binding proteins in Staphylococcus aureus, Streptococcus pneumoniae, Neisseria meningitides and Enterococcus faecium strains (10).
Enzymatic inactivation of antibiotics: In this case, a cellular enzyme changes the antibiotic structure in a way that it will no longer affect the bacterium. Beta-lactamase that hydrolyzes the most widely used antibiotics such as beta-lactams (penicillins and cephalosporins), aminoglycosidase and modifying enzymes of chloramphenicol and erythromycin are among typical examples (11, 12).
Excretion of the drug by activating the efflux pumps: These proteins can pump out a wide range of compounds from the periplasmic space of bacterial cells and are activated by bacteria to excrete antibiotics as an important resistance mechanism, especially in Pseudomonas aeruginosa and Acinetobacter strains (11). This type of resistance has been observed against tetracycline through which the antibiotic is excreted and cannot accumulate in the bacterial cell with the help of an energy-dependent active pumping system (13).
Decreased absorption by changing inner and outer membrane permeability: These changes in the permeability of the inner and outer membranes reduce drug uptake by the cell (14).
Changing drug targets and alternative metabolic pathways: Such alterations can reduce or eliminate the effectiveness of antibiotic binding to its target in the bacterial cells, limiting the potency of the antibiotic (15). However, bacteria can obtain folic acid from the environment instead of synthesizing it, in this way, they become resistant to sulfonamide and trimethoprim (16).
The use of iron delivery systems is an attractive option to deal with antibiotic resistance (17). Iron is an important element for the maintenance, growth and survival of bacterial cells. The siderophore pathway is a route for iron uptake. Siderophores are small organic chelators with a molecular weight of 200-2000 Da, which are synthesized by bacteria to attract iron in the surrounding environment and transfer it into the bacterial cytoplasm through specific pathways involving outer membrane transporters in gram-negative bacteria as well as siderophore-binding proteins, permease and ATPase in gram-positive bacteria (18-21). To date, many siderophores have been characterized and more than 270 of them have been structurally distinguished (22). Based on the functional groups involved in iron binding (Figure 1), siderophores can be classified into three groups: catecholate, hydroxamate and α-hydroxy carboxylate. In addition, there is a fourth unclassifiable category of siderophores called mixed-type that contain more than one functional group (23).












Lack of intracellular iron induces siderophore biosynthesis in microorganisms (25, 26) through a non-ribosomal peptide synthetases-dependent and -independent pathways (27). Transporter proteins drive siderophores out of the cell by efflux pumps. The three main types of these proteins that participate in this process include major facilitator superfamily, resistance nodulation and cell division superfamily and ABC superfamily (28, 29). Then, the iron-siderophore complex enters the cell via two general routs: 1) Iron is released from the complex and reaches the cell as a single cation e.g. in algae and fungi, and 2) The complex enters the cell, which is common in most bacteria (30). Siderophore-mediated iron uptake requires a specific outer membrane receptors in gram-negative bacteria such as FepA, FecA, and FhuA that bind to the ferric-siderophore complex (31). Antibiotics can form covalent bonds with siderophores (32, 33) and after exploration for iron, the siderophore-antibiotic hybrid is identified by bacterial ferric-siderophore uptake pathways. Thus, antibiotics can use siderophore as a carrier (so-called Trojan horse) to take advantage of ferric-siderophore transporters and penetrate bacteria. In other words, antibiotics will be conveyed into the bacterial cell simultaneously with iron transfer by siderophore (5). This strategy can significantly reduce drug resistance due to impermeability through target selection and is particularly useful in the control of multidrug-resistant pathogens (34). Previous studies have also discussed the chemical structure, linker type and binding of various antibiotics to siderophores (35, 36). In this article, we have briefly describe the mechanism of antibiotic resistance, the properties of siderophore and the importance of producing new antibiotics with a special focus on the use of Trojan horse strategy in the production of siderophore-antibiotic conjugates.
 
The Trojan horse strategy (siderophore-antibiotic conjugate)
It has been shown that bacterial siderophores possess antifungal and antibiotic activity (37, 38). Conjugation with siderophores can be utilized to transfer antibiotics in a manner similar to that of the Trojan horse strategy described by Homer in the Odyssey (Figure 2) (35, 39).
pathogens (32, 39). Drug-siderophore conjugate appears to be a promising approach for the treatment of multidrug-resistant bacterial pathogens belonging to the ESKAPEE group, which includes E. faecium, S. aureus, Klebsiella pneumoniae, Acinetobacter baumannii, P. aeruginosa, Escherichia coli, Enterobacter aerogenes and possibly other bacteria distinguished by the Infectious Diseases Society ofc America (40). Interestingly, this clever strategy for the direct transfer of antibacterial agents into the bacterial cells has not been developed by humans. There are several naturally occurring siderophore-antibiotic conjugates called sideromycin. These complexes are produced by a variety of gram-positive and -negative bacteria, including the genera Streptomyces, Salmonella, Klebsiella and E. coli so that they can fight for survival against rival bacteria (41). Albomycin, which was discovered in 1947, is a naturally occurring sideromycin produced by some Streptomyces strains with antimicrobial activity against several bacteria (42). Other examples include ferrimycin produced by Streptomyces griseoflavus (43) and salmycin generated by Streptomyces violaceus (44). Albomycin is composed of a trihydroxymate siderophore that binds to a thionucleoside moiety by a serine linker to inhibit aminoacyl−tRNA synthetases. Albomycin has access to the bacterial cytoplasm through a specific membrane transfer pathway for siderophore and when it is established in the cytoplasm, a serine protease detaches part of thionucleoside from the siderophore, blocking protein synthesis by inhibiting aminoacyl tRNA synthetases (45). Ferrimycin also consists of a hybrid of ferrioxamine B with an active antibiotic group (44). The structure of salmycin consists of a trihydroxamate siderophore known as danoxamine, which is attached to an aminoglycoside antibiotic by a succinyl linker. Salmycin reaches the cytoplasm through the hydroxamate siderophores membrane transport pathway where the pharmacophore or functional part of the aminoglycoside acts by inhibiting protein synthesis. Salmycin is more effective against gram-positive bacteria such as Staphylococcus and Streptococcus and less effective against many gram-negative bacteria (46, 47). An attractive group of sideromycins is produced by E. coli, K. pneumoniae and Salmonella, which is known as class IIb microcins (47). These compounds are linear antimicrobial polypeptides composed of 60-84 amino acids binding the C-terminal region of salmochelin analog, a glycosylated form of enterobactin (48). Microcin MccE492 15 is transported into the periplasmic space of E. coli by catecholate siderophores such as FepA, Cir, and Fiu that are located in the outer membrane (49). The highest in vitro antibacterial activity among sideromycins has been observed in albomycin, an antibiotic that has also been shown to possess high antibacterial activity in a mouse model of bacterial infections (41). Natural examples of sideromycins demonstrate significant drug-siderophore conjugate capability which increase drug reposition in target cell, which ultimately increases antibacterial effectiveness, especially against gram-negative pathogens. Therefore, researchers were encouraged to design and develop drug-siderophore conjugate structures (32).
The successful development of a drug-siderophore conjugate involves designing of a compound containing a siderophore component capable of identification of and entry into the bacterial cell, a suitable and stable binder in the extracellular environment but unstable in cytoplasm or periplasm, and an effective drug component (mainly from the beta-lactam family) (50). All components of such complex have an essential function, and when the siderophore-drug conjugate enters the cytoplasm, the microorganism may be destroyed by several methods, including drug release, complete activation of antimicrobial agents and blockage of iron uptake pathways (51). Up to now, catecholate and hydroxamate siderophores have mainly been used as vectors of antibacterial agents to overcome the problems of drug penetration into the membrane (52), and studies have shown that the use of strong iron chelators, such as triscatechols, increases the chances for conjugates to compete with the natural siderophores (53). However, siderophores of carboxylate type like staphyloferrin A are considered as appropriate candidates for certain applications since this type of siderophore exhibits iron-chelating properties in acidic environments relative to catecholate and hydroxamate siderophores (52). Hydroxamate siderophores and their analogs are good candidates for development of siderophore-antifungal drug conjugates that use iron uptake systems through hydroxamate siderophores molecules (35). The first siderophore-drug conjugates were synthesized by the Zahner group in 1977 by linking ferricrocin and ferrioxamine B to sulfonamides (54). There have been significant changes in the design of siderophore-drug conjugates so that a siderophore-monosulfactam conjugate like BAL30072 showed favorable results and advanced to phase I clinical trial in 2013 (55). In this type of conjugate, a lactam or similar compound is combined with a small molecule mimicking siderophore. BAL30072 is a combination of a dihydroxypyridone moiety and a monocyclic beta-lactam antibiotic moiety. The oxyiminoacyl side-chain allows easy access to the bacterial cell through the iron uptake system, while the second moiety reduces the sensitivity to inactivation by various beta-lactamases (56). The majority of cases associated with the Trojan horse drug delivery rely on beta-lactamases (50). This facilitates penetration of siderophore-bound beta-lactam antibiotics in the outer membrane and allows specific pathogen targeting by the modified siderophore conjugates. For example, triscatecholate siderophore-aminopenicillin conjugate specifically inhibits the replication of gram-negative bacteria such as P. aeruginosa (57). Beta-lactam antibiotics are useful as the drug moiety in siderophore-drug conjugate for two reasons. Firstly, penicillin-binding proteins are located in the periplasm and the siderophore-drug conjugate only needs to cross the outer membrane to reach them. Secondly, unlike most other antibiotics, the binding site of beta-lactams to target is different from that of the siderophore moiety so that the conjugate can be completely activated without detachment of the siderophore moiety (58).
 
Design of synthetic siderophore-antibiotic conjugate drugs
In 1987, Watanabe et al. synthesized a new siderophore-cephalosporin conjugate called E-0702with antimicrobial activity against gram-negative bacteria such as K. pneumoniae, Salmonella typhimurium, P. aeruginosa and Serratia marcescens. They also revealed that E-0702 had the highest antibacterial activity against iron-starved bacteria but had effect on iron-rich bacteria (59).
Milner et al. prepared a panel of carboxylate conjugates of staphyloferrin A siderophore with methyl ester derivatives of fluoroquinolone, ciprofloxacin and norfloxacin as potential anti-Staphylococcus agents. Staphyloferrin A was chosen as a carrier since unlike catecholates or hydroxylates, carboxylate-type siderophores have higher affinity for chelating iron in mildly acidic environments. Figure 3 shows the conjugated compounds (52).
Kinzel et al. isolated and purified two pyoverdine siderophores produced by P. aeruginosa ATCC 27853 and Pseudomonas fluorescens ATCC 13525, and ligated ampicillin to them by a non-degradable capric acid binder (Figure 4). The resulting 118 and 119 compounds with respective minimum inhibitory concentration (MIC) of 0.39 mM and 0.024 mM showed strong antibacterial activity against P. aeruginosa strains (60). Catechol-cephalosporin conjugate (GR69153) was found to be effective against E. coli and P. aeruginosa with the lowest MIC in the environments containing very low levels of iron (61).
In 2012, Ji et al. synthesized two artificial triscatecholate compounds with ampicillin and amoxicillin that were effective against P. aeruginosa strains. Although these gram-negative bacteria contain periplasmic binding proteins as a target of beta-lactam antibiotics, they are resistant to these antibiotics due to impaired passage and arrival at periplasmic space through the outer membrane pores. The researchers showed that both of these beta-lactam-siderophore compounds are effective against P. aeruginosa and that their antibacterial activity is higher in environments with low iron concentrations (57).
The most commonly used iron hydroxymate chelator for generation of siderophore-antimicrobial agent conjugate is desferrioxamine B (DFOB), a linear trihydroxamate siderophore sold under the brand name of Desferal (35). In this regard, DFO conjugated with lorabid 24, ciprofloxacin 25 or triclosan showed good activity against several pathogenic bacteria in humans, including S. aureus, E. coli, and Mycobacterium vaccae in vitro (34). The DFO-nalidixic acid conjugate can be synthesized through direct binding of nalidixic acid carboxyl group with N-terminal of DFO. This conjugate was highly active against multidrug-resistant strains of Plasmodium falciparum with a MIC of 0.6 µg/ml (62).
In 2013, Duhme-Klair et al. synthesized a staphyloferrin-ciprofloxacin conjugate siderophore with a covalent linkage between fluoroquinolone and carboxylic acid. The prepared conjugates had an average antibacterial activity against a number of gram-positive bacteria and significant antibacterial activity against gram-negative bacteria including P. aeruginosa, Serratia marcescens NCTC 1998 and E. coli NCTC 10418 (52).
In 2014, Sonnet et al. used the Enterobactin analog introduced by Miller et al. to transfer fluoroquinolones, but the resulting conjugate was no effective against P. aeruginosa possibly due to the application of non-degradable ligands, which were previously shown to be ineffective for fluoroquinolone conjugates (63, 64).
Mycobacterium tuberculosis produces a hydroxamate siderophore known as mycobactin T in order to survive under limited iron conditions. Due to the presence of long fatty acid chains, mycobactin can enter the cell via an energy-independent route. In search of new anti-tuberculosis drugs, a conjugate consisting of a mycobactin T analog and an anti-malarial drug called Artemisinin was designed and found to be highly active against multidrug resistant strains of M. tuberculosis (Figure 5) (65).
against multidrug-resistant P. aeruginosa and Enterobacteriaceae producing extended-spectrum beta-lactamases as well as A. baumannii. Nevertheless, further research is needed to overcome the hydrolytic instability of this compound. In a study using a rat model of lung infection, the high affinity of this compound to plasma proteins also limited its effect (66).
Cefiderocol (S-649266) was the first siderophore-antibiotic conjugate to reach phase III clinical trial. This catecholate-cephalosporin conjugate siderophore has the structural properties of 3rd and 4th generation cephalosporins, namely ceftazidime and cefepime, which can bind to iron through a catechol moiety at C3 position to enter the cell through siderophore transporter proteins. Cefiderocol has in vitro and in vivo activity against carbapenem-resistant Enterobacteriaceae, P. aeruginosa, and A. baumannii (67, 68).
In 2015, Chairatana et al. designed and prepared a conjugate of glycosylated enterobactin (GlcEnt)-β-lactam antibiotics "ampicillin and amoxicillin" by a chemoenzymatic method. The results of the study showed that the conjugate had 1000-fold higher antimicrobial activity against uropathogenic E. coli compared to β-lactams antibiotics alone (69).
In 2017, Ghosh et al. synthesized a conjugate consisting of the mix ligand analog of the selective siderophore fimsbactin (A. baumannii siderophore)-daptomycin (abbreviated to conjugate 11) with strong antibacterial activity against multidrug-resistant strains of A. baumannii both in vitro and in vivo. Using the Trojan Horse strategy, their results revealed that conjugation of a drug that are much larger than siderophore facilitates drug uptake and makes it effective against both gram-positive and gram-negative bacteria (4).
In 2018, Ghosh et al. synthesized and tested other conjugates with modified mimetic siderophores-daptomycin (conjugate 5: bis-catechol-daptomycin and conjugate 6: tri-catechol- daptomycin). Conjugate 5 with a MIC of 1.6 μM had stronger activity than conjugate 6 with MIC of 25 μM against A. baumannii producing carbapenemase and cephalosporinase. The compound also maintained its activity at lower doses of daptomycin alone against S. aureus (70).
In 2018, Neumann et al. reported the synthesis of siderophore (enterobactin)-antibiotic (ciprofloxacin) conjugate, wherein enterobactin was attached to ciprofloxacin by an alkyl linker. This conjugate showed antibacterial activity against E. coli strains expressing IroA gene cluster. In addition, it had significant antibacterial activity against uropathogenic E. coli UTI89 and CFT073 compared to the unmodified ciprofloxacin (71). In 2020, Boyce et al. designed a conjugate containing a siderophore moiety, a degradable protease linker and an amine-containing antibiotic targeting E. coli periplasmic proteases. Using this strategy, daptomycin, which is only effective against gram-positive bacteria became effective against gram-negative bacteria such as Acinetobacter species. The results of this study illustrated the usefulness of this platform for the production of protease-activated drugs including Trojan horse antibiotics (72).
 
CONCLUSION
Given the growing number of multidrug-resistant bacteria, the exigency for the development of new drugs and antibiotics is increasingly felt. One way to evaluate the occurrence of antibiotic resistance in bacteria is to use alternative therapeutic strategies, including iron acquisition pathways. Iron chelators such as siderophore molecules can be utilized for the introduction of drugs and antibacterial compounds into bacterial cells, a process known as the Trojan horse strategy. Using this strategy, antibiotics can pass through impenetrable membranes of gram-negative bacteria and destroy them. As discussed in this article, the Trojan horse strategy is a promising and flexible approach for the production of antimicrobials against multi-drug resistant bacteria. Nevertheless, further studies are required to gain a better understanding of the interaction of siderophore and bacterial surface receptors, siderophore uptake mechanisms, dynamic and functional properties of antibiotics and proper linkers.
 
ACKNOWLEDGMENTS
The authors are grateful to the Bu-Ali Sina University of Hamedan.
 
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest.
References
1. Andersson DI, Hughes D. Persistence of antibiotic resistance in bacterial populations. FEMS microbiology reviews. 2011; 35(5): 901-11. [View at Publisher] [DOI:10.1111/j.1574-6976.2011.00289.x] [PubMed] [Google Scholar]
2. Fischbach MA, Walsh CT. Antibiotics for emerging pathogens. Science. 2009; 325(5944): 1089-93. [View at Publisher] [DOI:10.1126/science.1176667] [PubMed] [Google Scholar]
3. Chang C-k, Sue S-C, Yu T-h, Hsieh C-M, Tsai C-K, Chiang Y-C, et al. The dimer interface of the SARS coronavirus nucleocapsid protein adapts a porcine respiratory and reproductive syndrome virus-like structure. FEBS letters. 2005;579(25):5663-8. [View at Publisher] [DOI:10.1016/j.febslet.2005.09.038] [PubMed] [Google Scholar]
4. Ghosh M, Miller PA, Möllmann U, Claypool WD, Schroeder VA, Wolter WR, et al. Targeted antibiotic delivery: selective siderophore conjugation with daptomycin confers potent activity against multidrug resistant Acinetobacter baumannii both in vitro and in vivo. Journal of medicinal chemistry. 2017; 60(11): 4577-83. [DOI:10.1021/acs.jmedchem.7b00102] [PubMed] [Google Scholar]
5. Schalk IJ. Siderophore-antibiotic conjugates: exploiting iron uptake to deliver drugs into bacteria. Clinical microbiology and infection: the official publication of the European Society of Clinical Microbiology and Infectious Diseases. 2018; 24(8):801. [View at Publisher] [DOI:10.1016/j.cmi.2018.03.037] [PubMed] [Google Scholar]
6. Schalk IJ. A trojan-horse strategy including a bacterial suicide action for the efficient use of a specific Gram-positive antibiotic on Gram-negative bacteria. ACS Publications; 2018; 61: 3842−3844. [View at Publisher] [DOI:10.1021/acs.jmedchem.8b00522] [PubMed] [Google Scholar]
7. Munita JM, Arias CA. Mechanisms of antibiotic resistance. Virulence mechanisms of bacterial pathogens. 2016; 4(2): 4-2. [View at Publisher] [DOI:10.1128/microbiolspec.VMBF-0016-2015] [PubMed] [Google Scholar]
8. Prashanth K, Vasanth T, Saranathan R, Makki AR, Pagal S. Antibiotic resistance, biofilms and quorum sensing in Acinetobacter species. Antibiotic resistant bacteria: a continuous challenge in the new millennium. 2012;179-212. [View at Publisher] [DOI:10.5772/28813] [Google Scholar]
9. Shaikh SA, Jain T, Sandhu G, Latha N, Jayaram B. From drug target to leads-sketching a physicochemical pathway for lead molecule design in silico. Current pharmaceutical design. 2007; 13(34): 3454-70. [View at Publisher] [DOI:10.2174/138161207782794220] [PubMed] [Google Scholar]
10. Southon SB, Beres SB, Kachroo P, Saavedra MO, Erlendsdóttir H, Haraldsson G, et al. Population genomic molecular epidemiological study of macrolide-resistant Streptococcus pyogenes in Iceland, 1995 to 2016: identification of a large clonal population with a pbp2x mutation conferring reduced in vitro β-lactam susceptibility. Journal of clinical microbiology. 2020;58(9):e00638-20. [View at Publisher] [DOI:10.1128/JCM.00638-20] [PubMed] [Google Scholar]
11. Blair JM, Webber MA, Baylay AJ, Ogbolu DO, Piddock LJ. Molecular mechanisms of antibiotic resistance. Nature reviews microbiology. 2015; 13(1): 42-51. [View at Publisher] [DOI:10.1038/nrmicro3380] [PubMed] [Google Scholar]
12. Sharkey LK, O'Neill AJ. Molecular Mechanisms of Antibiotic Resistance-Part II. Bacterial Resistance to Antibiotics-From Molecules to Man. 2019:27-50. [View at Publisher] [DOI:10.1002/9781119593522.ch2]
13. Li T, Liu C, Lu J, Gaurav GK, Chen W. Determination of how tetracycline influences nitrogen removal performance, community structure, and functional genes of biofilm systems. Journal of the Taiwan Institute of Chemical Engineers. 2020;106:99-109. [View at Publisher] [DOI:10.1016/j.jtice.2019.10.004] [Google Scholar]
14. Santajit S, Indrawattana N. Mechanisms of antimicrobial resistance in ESKAPE pathogens. BioMed research international. 2016;2016. [View at Publisher] [DOI:10.1155/2016/2475067] [PubMed] [Google Scholar]
15. Clatworthy AE, Pierson E, Hung DT. Targeting virulence: a new paradigm for antimicrobial therapy. Nature chemical biology. 2007;3(9):541-8. [View at Publisher] [DOI:10.1038/nchembio.2007.24] [PubMed] [Google Scholar]
16. Tan J, Tay J, Hedrick J, Yang YY. Synthetic macromolecules as therapeutics that overcome resistance in cancer and microbial infection. Biomaterials. 2020; 252: 120078. [View at Publisher] [DOI:10.1016/j.biomaterials.2020.120078] [PubMed] [Google Scholar]
17. Anderson CP, Shen M, Eisenstein RS, Leibold EA. Mammalian iron metabolism and its control by iron regulatory proteins. Biochimica et Biophysica Acta (BBA)-Molecular Cell Research. 2012;1823(9):1468-83. [View at Publisher] [DOI:10.1016/j.bbamcr.2012.05.010] [PubMed] [Google Scholar]
18. Schalk IJ, Mislin GL, Brillet K. Structure, function and binding selectivity and stereoselectivity of siderophore-iron outer membrane transporters. Current topics in membranes. 69: Elsevier. 2012; 37-66. [View at Publisher] [DOI:10.1016/B978-0-12-394390-3.00002-1] [PubMed] [Google Scholar]
19. Ahmed E, Holmström SJ. Siderophores in environmental research: roles and applications. Microbial biotechnology. 2014;7(3):196-208. [View at Publisher] [DOI:10.1111/1751-7915.12117] [PubMed] [Google Scholar]
20. Saha R, Saha N, Donofrio RS, Bestervelt LL. Microbial siderophores: a mini review. Journal of basic microbiology. 2013;53(4):303-17. [View at Publisher] [DOI:10.1002/jobm.201100552] [PubMed] [Google Scholar]
21. Khasheii B, Mahmoodi P, Mohammadzadeh A. Siderophores: Importance in Bacterial Pathogenesis and Applications in Medicine and Industry. Microbiological Research. 2021:126790. [View at Publisher] [DOI:10.1016/j.micres.2021.126790] [PubMed] [Google Scholar]
22. Hider RC, Kong X. Chemistry and biology of siderophores. Natural product reports. 2010; 27(5): 637-57. [DOI:10.1039/b906679a] [PubMed] [Google Scholar]
23. Khan A, Singh P, Srivastava A. Synthesis, nature and utility of universal iron chelator-Siderophore: A review. Microbiological research. 2018;212:103-11. [View at Publisher] [DOI:10.1016/j.micres.2017.10.012] [PubMed] [Google Scholar]
24. Krewulak KD, Vogel HJ. Structural biology of bacterial iron uptake. Biochimica et Biophysica Acta (BBA)-Biomembranes. 2008; 1778(9): 1781-804. [View at Publisher] [DOI:10.1016/j.bbamem.2007.07.026] [PubMed] [Google Scholar]
25. Andrews SC, Robinson AK, Rodríguez-Quiñones F. Bacterial iron homeostasis. FEMS microbiology reviews. 2003;27(2-3):215-37. [View at Publisher] [DOI:10.1016/S0168-6445(03)00055-X] [PubMed] [Google Scholar]
26. Wandersman C, Delepelaire P. Bacterial iron sources: from siderophores to hemophores. Annu Rev Microbiol. 2004;58:611-47. [View at Publisher] [DOI:10.1146/annurev.micro.58.030603.123811] [PubMed] [Google Scholar]
27. Sah S, Singh R. Siderophore: Structural and functional characterisation-A comprehensive review. Agriculture (Pol'nohospodárstvo). 2015; 61(3): 97-114. [DOI:10.1515/agri-2015-0015] [PubMed] [Google Scholar]
28. Furrer JL, Sanders DN, Hook‐Barnard IG, McIntosh MA. Export of the siderophore enterobactin in Escherichia coli: involvement of a 43 kDa membrane exporter. Molecular microbiology. 2002;44(5):1225-34. [View at Publisher] [DOI:10.1046/j.1365-2958.2002.02885.x] [PubMed] [Google Scholar]
29. Bleuel C, Große C, Taudte N, Scherer J, Wesenberg D, Krauß GJ, et al. TolC is involved in enterobactin efflux across the outer membrane of Escherichia coli. Journal of bacteriology. 2005;187(19):6701-7. [View at Publisher] [DOI:10.1128/JB.187.19.6701-6707.2005] [PubMed] [Google Scholar]
30. Page MG. The role of iron and siderophores in infection, and the development of siderophore antibiotics. Clinical Infectious Diseases. 2019;69(Supplement_7):S529-S37. [DOI:10.1093/cid/ciz825] [PubMed] [Google Scholar]
31. Köster W. ABC transporter-mediated uptake of iron, siderophores, heme and vitamin B12. Research in microbiology. 2001;152(3-4): 291-301. [View at Publisher] [DOI:10.1016/S0923-2508(01)01200-1] [PubMed] [Google Scholar]
32. Mislin GL, Schalk IJ. Siderophore-dependent iron uptake systems as gates for antibiotic Trojan horse strategies against Pseudomonas aeruginosa. Metallomics. 2014;6(3):408-20. [DOI:10.1039/C3MT00359K] [PubMed] [Google Scholar]
33. Klahn P, Brönstrup M. Bifunctional antimicrobial conjugates and hybrid antimicrobials. Natural product reports. 2017;34(7):832-85. [View at Publisher] [DOI:10.1039/C7NP00006E] [PubMed] [Google Scholar]
34. Wencewicz TA, Möllmann U, Long TE, Miller MJ. Is drug release necessary for antimicrobial activity of siderophore-drug conjugates? Syntheses and biological studies of the naturally occurring salmycin "Trojan Horse" antibiotics and synthetic desferridanoxamine-antibiotic conjugates. Biometals. 2009;22(4):633-48. [View at Publisher] [DOI:10.1007/s10534-009-9218-3] [PubMed] [Google Scholar]
35. Skwarecki AS, Milewski S, Schielmann M, Milewska MJ. Antimicrobial molecular nanocarrier-drug conjugates. Nanomedicine: Nanotechnology, Biology and Medicine. 2016;12(8):2215-40. [View at Publisher] [DOI:10.1016/j.nano.2016.06.002] [PubMed] [Google Scholar]
36. Kong H, Cheng W, Wei H, Yuan Y, Yang Z, Zhang X. An overview of recent progress in siderophore-antibiotic conjugates. European journal of medicinal chemistry. 2019;182:111615. [View at Publisher] [DOI:10.1016/j.ejmech.2019.111615] [PubMed] [Google Scholar]
37. Pramanik A, Stroeher UH, Krejci J, Standish AJ, Bohn E, Paton JC, et al. Albomycin is an effective antibiotic, as exemplified with Yersinia enterocolitica and Streptococcus pneumoniae. International Journal of Medical Microbiology. 2007;297(6):459-69. [View at Publisher] [DOI:10.1016/j.ijmm.2007.03.002] [Google Scholar]
38. Sulochana MB, Jayachandra SY, Kumar SKA, Dayanand A. Antifungal attributes of siderophore produced by the Pseudomonas aeruginosa JAS‐25. Journal of Basic Microbiology. 2014;54(5):418-24. [View at Publisher] [DOI:10.1002/jobm.201200770] [PubMed] [Google Scholar]
39. Górska A, Sloderbach A, Marszałł MP. Siderophore-drug complexes: potential medicinal applications of the 'Trojan horse'strategy. Trends in pharmacological sciences. 2014;35(9):442-9. [View at Publisher] [DOI:10.1016/j.tips.2014.06.007] [PubMed] [Google Scholar]
40. Boucher HW, Talbot GH, Bradley JS, Edwards JE, Gilbert D, Rice LB, et al. Bad bugs, no drugs: no ESKAPE! An update from the Infectious Diseases Society of America. Clinical infectious diseases. 2009;48(1):1-12. [View at Publisher] [DOI:10.1086/595011] [PubMed] [Google Scholar]
41. Braun V, Pramanik A, Gwinner T, Köberle M, Bohn E. Sideromycins: tools and antibiotics. Biometals. 2009;22(1):3. [View at Publisher] [DOI:10.1007/s10534-008-9199-7] [PubMed] [Google Scholar]
42. Reynolds DM, Schatz A, Waksman SA. Grisein, a new antibiotic produced by a strain of Streptomyces griseus. Proceedings of the Society for Experimental Biology and Medicine. 1947;64(1):50-4. [View at Publisher] [DOI:10.3181/00379727-64-15695] [PubMed] [Google Scholar]
43. Sackmann W, Reusser P, Neipp L, Kradolfer F, Gross F. Ferrimycin A, a new iron-containing antibiotic. Antibiotics & Chemotherapy. 1962;12(1):34-45. [PubMed] [Google Scholar]
44. Pramanik A, Braun V. Albomycin uptake via a ferric hydroxamate transport system of Streptococcus pneumoniae R6. Journal of bacteriology. 2006;188(11):3878-86. [View at Publisher] [DOI:10.1128/JB.00205-06] [PubMed] [Google Scholar]
45. Ballouche M, Cornelis P, Baysse C. Iron metabolism: a promising target for antibacterial strategies. Recent patents on anti-infective drug discovery. 2009;4(3):190-205. [DOI:10.2174/157489109789318514] [PubMed] [Google Scholar]
46. Page MG. Siderophore conjugates. Annals of the New York Academy of Sciences. 2013;1277(1):115-26. [View at Publisher] [DOI:10.1111/nyas.12024] [PubMed] [Google Scholar]
47. Vassiliadis G, Destoumieux-Garzón D, Lombard C, Rebuffat S, Peduzzi J. Isolation and characterization of two members of the siderophore-microcin family, microcins M and H47. Antimicrobial agents and chemotherapy. 2010; 54(1): 288-97. [View at Publisher] [DOI:10.1128/AAC.00744-09] [PubMed] [Google Scholar]
48. Duquesne S, Destoumieux-Garzón D, Peduzzi J, Rebuffat S. Microcins, gene-encoded antibacterial peptides from enterobacteria. Natural product reports. 2007;24(4):708-34. [DOI:10.1039/b516237h] [PubMed] [Google Scholar]
49. Strahsburger E, Baeza M, Monasterio O, Lagos R. Cooperative uptake of microcin E492 by receptors FepA, Fiu, and Cir and inhibition by the siderophore enterochelin and its dimeric and trimeric hydrolysis products. Antimicrobial agents and chemotherapy. 2005;49(7):3083-6. [View at Publisher] [DOI:10.1128/AAC.49.7.3083-3086.2005] [PubMed] [Google Scholar]
50. de Carvalho CC, Fernandes P. Siderophores as "Trojan Horses": tackling multidrug resistance? Frontiers in microbiology. 2014;5:290. [DOI:10.3389/fmicb.2014.00290] [PubMed] [Google Scholar]
51. Miller MJ, Zhu H, Xu Y, Wu C, Walz AJ, Vergne A, et al. Utilization of microbial iron assimilation processes for the development of new antibiotics and inspiration for the design of new anticancer agents. Biometals. 2009;22(1):61. [View at Publisher] [DOI:10.1007/s10534-008-9185-0] [PubMed] [Google Scholar]
52. Milner SJ, Seve A, Snelling AM, Thomas GH, Kerr KG, Routledge A, et al. Staphyloferrin A as siderophore-component in fluoroquinolone-based Trojan horse antibiotics. Organic & biomolecular chemistry. 2013;11(21):3461-8. [DOI:10.1039/c3ob40162f] [PubMed] [Google Scholar]
53. Gasser V, Baco E, Cunrath O, August PS, Perraud Q, Zill N, et al. Catechol siderophores repress the pyochelin pathway and activate the enterobactin pathway in P seudomonas aeruginosa: an opportunity for siderophore-antibiotic conjugates development. Environmental microbiology. 2016;18(3):819-32. [View at Publisher] [DOI:10.1111/1462-2920.13199] [PubMed] [Google Scholar]
54. Zähner H, Diddens H, Keller-Schierlein W, Nägeli H. Some experiments with semisynthetic sideromycins. The Japanese journal of antibiotics. 1977;30:201. [PubMed] [Google Scholar]
55. Butler MS, Blaskovich MA, Cooper MA. Antibiotics in the clinical pipeline in 2013. The Journal of antibiotics. 2013; 66(10): 571-91. [View at Publisher] [DOI:10.1038/ja.2013.86] [PubMed] [Google Scholar]
56. Hofer B, Dantier C, Gebhardt K, Desarbre E, Schmitt-Hoffmann A, Page MG. Combined effects of the siderophore monosulfactam BAL30072 and carbapenems on multidrug-resistant Gram-negative bacilli. Journal of Antimicrobial Chemotherapy. 2013;68(5):1120-9. [View at Publisher] [DOI:10.1093/jac/dks527] [PubMed] [Google Scholar]
57. Ji C, Miller PA, Miller MJ. Iron transport-mediated drug delivery: practical syntheses and in vitro antibacterial studies of tris-catecholate siderophore-aminopenicillin conjugates reveals selectively potent antipseudomonal activity. Journal of the American Chemical Society. 2012;134(24):9898-901. [View at Publisher] [DOI:10.1021/ja303446w] [PubMed] [Google Scholar]
58. Möllmann U, Heinisch L, Bauernfeind A, Köhler T, Ankel-Fuchs D. Siderophores as drug delivery agents: application of the "Trojan Horse" strategy. Biometals. 2009;22(4):615-24. [View at Publisher] [DOI:10.1007/s10534-009-9219-2] [PubMed] [Google Scholar]
59. Watanabe N, Nagasu T, Katsu K, Kitoh K. E-0702, a new cephalosporin, is incorporated into Escherichia coli cells via the tonB-dependent iron transport system. Antimicrobial agents and chemotherapy. 1987;31(4):497-504. [View at Publisher] [DOI:10.1128/AAC.31.4.497] [PubMed] [Google Scholar]
60. Kinzel O, Tappe R, Gerus I, Budzikiewicz H. The synthesis and antibacterial activity of two pyoverdin-ampicillin conjugates, entering Pseudomonas aeruginosa via the pyoverdin-mediated iron uptake pathway. The Journal of antibiotics. 1998;51(5):499-507. [DOI:10.7164/antibiotics.51.499] [PubMed] [Google Scholar]
61. Silley P, Griffiths JW, Monsey D, Harris AM. Mode of action of GR69153, a novel catechol-substituted cephalosporin, and its interaction with the tonB-dependent iron transport system. Antimicrobial agents and chemotherapy. 1990; 34(9): 1806-8. [DOI:10.1128/AAC.34.9.1806] [PubMed] [Google Scholar]
62. Ghosh M, Lambert LJ, Huber PW, Miller MJ. Synthesis, bioactivity, and DNA-cleaving ability of desferrioxamine B-nalidixic acid and anthraquinone carboxylic acid conjugates. Bioorganic & Medicinal Chemistry Letters. 1995;5(20):2337-40. [View at Publisher] [DOI:10.1016/0960-894X(95)00412-M] [Google Scholar]
63. Fardeau S, Dassonville-Klimpt A, Audic N, Sasaki A, Pillon M, Baudrin E, et al. Synthesis and antibacterial activity of catecholate-ciprofloxacin conjugates. Bioorganic & medicinal chemistry. 2014;22(15):4049-60. [View at Publisher] [DOI:10.1016/j.bmc.2014.05.067] [PubMed] [Google Scholar]
64. Hennard C, Truong QC, Desnottes J-F, Paris J-M, Moreau NJ, Abdallah MA. Synthesis and Activities of Pyoverdin− Quinolone Adducts: A Prospective Approach to a Specific Therapy Against Pseudomonas a eruginosa. Journal of medicinal chemistry. 2001;44(13):2139-51. [View at Publisher] [DOI:10.1021/jm990508g] [PubMed] [Google Scholar]
65. Miller MJ, Walz AJ, Zhu H, Wu C, Moraski G, Möllmann U, et al. Design, synthesis, and study of a mycobactin− artemisinin conjugate that has selective and potent activity against tuberculosis and malaria. Journal of the American chemical society. 2011;133(7):2076-9. [View at Publisher] [DOI:10.1021/ja109665t] [PubMed] [Google Scholar]
66. Flanagan ME, Brickner SJ, Lall M, Casavant J, Deschenes L, Finegan SM, et al. Preparation, Gram-negative antibacterial activity, and hydrolytic stability of novel siderophore-conjugated monocarbam diols. ACS medicinal chemistry letters. 2011;2(5):385-90. [View at Publisher] [DOI:10.1021/ml200012f] [PubMed] [Google Scholar]
67. Hackel MA, Tsuji M, Yamano Y, Echols R, Karlowsky JA, Sahm DF. In vitro activity of the siderophore cephalosporin, cefiderocol, against carbapenem-nonsusceptible and multidrug-resistant isolates of Gram-negative bacilli collected worldwide in 2014 to 2016. Antimicrobial agents and chemotherapy. 2018;62(2):e01968-17. [View at Publisher] [DOI:10.1128/AAC.01968-17] [PubMed] [Google Scholar]
68. Zhanel GG, Golden AR, Zelenitsky S, Wiebe K, Lawrence CK, Adam HJ, et al. Cefiderocol: a siderophore cephalosporin with activity against carbapenem-resistant and multidrug-resistant gram-negative bacilli. Drugs. 2019; 79(3): 271-89. [View at Publisher] [DOI:10.1007/s40265-019-1055-2] [PubMed] [Google Scholar]
69. Chairatana P, Zheng T, Nolan EM. Targeting virulence: salmochelin modification tunes the antibacterial activity spectrum of β-lactams for pathogen-selective killing of Escherichia coli. Chemical science. 2015;6(8):4458-71. [DOI:10.1039/C5SC00962F] [PubMed] [Google Scholar]
70. Ghosh M, Lin Y-M, Miller PA, Möllmann U, Boggess WC, Miller MJ. Siderophore conjugates of daptomycin are potent inhibitors of carbapenem resistant strains of Acinetobacter baumannii. ACS Infectious Diseases. 2018;4(10):1529-35. [View at Publisher] [DOI:10.1021/acsinfecdis.8b00150] [PubMed] [Google Scholar]
71. Neumann W, Sassone-Corsi M, Raffatellu M, Nolan EM. Esterase-catalyzed siderophore hydrolysis activates an enterobactin-ciprofloxacin conjugate and confers targeted antibacterial activity. Journal of the American Chemical Society. 2018;140(15):5193-201. [View at Publisher] [DOI:10.1021/jacs.8b01042] [PubMed] [Google Scholar]
72. Boyce JH, Dang B, Ary B, Edmondson Q, Craik CS, DeGrado WF, et al. Platform to Discover Protease-Activated Antibiotics and Application to Siderophore-Antibiotic Conjugates. Journal of the American Chemical Society. 2020;142(51):21310-21. [View at Publisher] [DOI:10.1021/jacs.0c06987] [PubMed] [Google Scholar]