Optimization of Artificial Siderophores as 68Ga-Complexed PET Tracers for In Vivo Imaging of Bacterial Infections

The diagnosis of bacterial infections at deep body sites benefits from noninvasive imaging of molecular probes that can be traced by positron emission tomography (PET). We specifically labeled bacteria by targeting their iron transport system with artificial siderophores. The cyclen-based probes contain different binding sites for iron and the PET nuclide gallium-68. A panel of 11 siderophores with different iron coordination numbers and geometries was synthesized in up to 8 steps, and candidates with the best siderophore potential were selected by a growth recovery assay. The probes [68Ga]7 and [68Ga]15 were found to be suitable for PET imaging based on their radiochemical yield, radiochemical purity, and complex stability in vitro and in vivo. Both showed significant uptake in mice infected with Escherichia coli and were able to discern infection from lipopolysaccharide-triggered, sterile inflammation. The study qualifies cyclen-based artificial siderophores as readily accessible scaffolds for the in vivo imaging of bacteria.

A Unified Approach to Phytosiderophore Natural Products

This work reports on the concise total synthesis of eight natural products of the mugineic acid and avenic acid families (phytosiderophores). An innovative "east-to-west" assembly of the trimeric products resulted in a high degree of divergence enabling the formation of the final products in just 10 or 11 steps each with a minimum of overall synthetic effort. Chiral pool starting materials (l-malic acid, threonines) were employed for the outer building blocks while the middle building blocks were accessed by diastereo- and enantioselective methods. A highlight of this work consists in the straightforward preparation of epimeric hydroxyazetidine amino acids, useful building blocks on their own, enabling the first synthesis of 3''-hydroxymugineic acid and 3''-hydroxy-2'-deoxymugineic acid.

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.

Design, solid-phase synthesis and evaluation of enterobactin analogs for iron delivery into the human pathogen Campylobacter jejuni

The human enteropathogen Campylobacter jejuni, like many bacteria, employs siderophores such as enterobactin for cellular uptake of ferric iron. This transport process has been shown to be essential for virulence and presents an attractive opportunity for further study of the permissiveness of this pathway to small-molecule intervention and as inspiration for the development of synthetic carriers that may effectively transport cargo into Gram-negative bacteria. In this work, we have developed a facile and robust microscale assay to measure growth recovery of C. jejuni NCTC 11168 in liquid culture as a result of ferric iron uptake. In parallel, we have established the solid-phase synthesis of catecholamide compounds modeled on enterobactin fragments. Applying these methodological developments, we show that small synthetic iron chelators of minimal dimensions provide ferric iron to C. jejuni with equal or greater efficiency than enterobactin.

Total (bio)synthesis: strategies of nature and of chemists

The biosynthetic pathways to a number of natural products have been reconstituted in vitro using purified enzymes. Many of these molecules have also been synthesized by organic chemists. Here we compare the strategies used by nature and by chemists to reveal the underlying logic and success of each total synthetic approach for some exemplary molecules with diverse biosynthetic origins.

Synthesis and biological activity of saccharide based lipophilic siderophore mimetics as potential growth promoters for mycobacteria

Siderophores based on sugar backbones substituted at the 2,3,4- or 2,3,6 positions with hydroxamic or retro-hydroxamic acid chelating units were synthesized and characterized. The alkyl terminus of the iron-coordinating side chain units facilitate lipophilic interactions. Iron coordination properties and complex stability were investigated by ESI-MS and the CAS-Test. The results were correlated to structure activity relationships determined by microbial growth promotion studies under iron limited conditions using wild type strains and iron transport mutants of Mycobacterium smegmatis.

Tren-based analogues of bacillibactin: structure and stability

Synthetic analogues were designed to highlight the effect of the glycine moiety of bacillibactin on the overall stability of the ferric complex as compared to synthetic analogues of enterobactin. Insertion of a variety of amino acids to catecholamide analogues based on a Tren (tris(2-aminoethyl)amine) backbone increased the overall acidity of the ligands, causing an enhancement of the stability of the resulting ferric complex as compared to TRENCAM. Solution thermodynamic behavior of these siderophores and their synthetic analogues was investigated through potentiometric and spectrophotometric titrations. X-ray crystallography, circular dichroism, and molecular modeling were used to determine the chirality and geometry of the ferric complexes of bacillibactin and its analogues. In contrast to the Tren scaffold, addition of a glycine to the catechol chelating arms causes an inversion of the trilactone backbone, resulting in opposite chiralities of the two siderophores and a destabilization of the ferric complex of bacillibactin compared to ferric enterobactin.

Solid-Phase Synthesis of Biologically Interesting Compounds Containing Hydroxamic Acid Moiety

Chemical strategies developed for the solid-phase synthesis of hydroxamates are divided into four groups: (i) the traditional synthesis of hydroxamates via cleavage of resin-bound esters by hydroxylamine and its derivatives, (ii) introduction of hydroxamic acid moiety on the resin-bound precursor, (iii) transformation of polymer-supported hydroxylamine, attached to a solid supported linker either by oxygen (O-linking strategy) or by nitrogen (N-linking strategy), and (iv) synthesis of N-alkyl hydroxamates. The scope and limitation of individual approaches are discussed.

Synthesis, structure, and molecular dynamics of gallium complexes of schizokinen and the amphiphilic siderophore acinetoferrin

A new general synthesis of the citrate-based siderophores acinetoferrin (Af) and schizokinen (Sz) and their analogues is described. The molecular structure of gallium schizokinen, GaSz, was determined by combined (1)H NMR, Hartree-Fock ab initio calculations, DFT, and empirical modeling of vicinal proton NMR spin-spin couplings. The metal-coordination geometry of GaSz was determined from NOE contacts to be cis-cis with respect to the two chelating hydroxamates. One diaminopropane adopts a single chairlike conformation while another is a mixture of two ring pucker arrangements. Both amide hydrogens are internally hydrogen bonded to metal-ligating oxygen atoms. The acyl methyl groups are directed away from each other with an average planar angle of ca. 130 degrees. The kinetics of GaSz racemization were followed by selective, double spin-echo inversion-recovery (1)H NMR spectroscopy over the temperature range of 10-45 degrees C. The racemization proceeds by a multistep mechanism that is proton independent between pD 5 and 12 (k(0) = 1.47 (0.15 s(-1))) and acid catalyzed below pD 4 (k(1) = 2.25 (0.15) x 10(4) M(-1) s(-1)). The activation parameters found for the two sequential steps of the proton independent pathway were DeltaH(++) = 25 +/- 3 kcal M(-1), DeltaS(++) = 25 +/- 7 cal M(-1) K(-1) and DeltaH(++) = 17.1 +/- 0.2 kcal M(-1), DeltaS(++) = 0.3 +/- 2.7 cal M(-1) K(-1). The first step of the proton-independent mechanism was assigned to the dissociation of the carboxyl group. The second step was assigned to complex racemization. The proton-assisted step was assigned to a complete dissociation of the alpha-hydroxy carboxyl group at pD < 4. The ab initio modeling of gallium acinetoferrin, GaAf, and analogues derived from the structure of GaSz has shown that the pendant trans-octenoyl fragments are oriented in opposite directions with the average planar angle of ca. 130 degrees. This arrangement prevents GaAf from adopting a phospholipid-like structural motif. Significantly, iron siderophore complex FeAf was found to be disruptive to phospholipid vesicles and is considerably more hydrophilic than Af, with an eight-fold smaller partition coefficient.

Desketoneoenactin-siderophore conjugates for Candida: evidence of iron transport-dependent species selectivity

We investigated the inhibitory activity of synthetic isocyanurate-based as well as linear mono- and trihydroxamate siderophore-drug conjugates against Candida spp. The conjugated drug was 13C-desketoneoenactin (DE). The MICs of siderophore-drug conjugates were determined in the absence and presence of 2,2'-dipyridyl to restrict iron availability. The ability of various siderophore types to promote growth in an iron-restricted medium was also assayed. Addition of a siderophore portion to the drug strongly impaired the inhibitory activity of DE. However, the activity of the drug conjugates was increased by up to 16-fold in iron-depleted medium for species having their growth strongly promoted by most hydroxamate-type siderophores (C. albicans, C. stellatoidea, and C. pseudotropicalis). The uptake of (55)Fe from ferrichrome and from two siderophore-drug conjugates was improved when C. albicans cells were grown in a low-iron medium. In the same assay, unlabeled ferrichrome was able to compete with the uptake of (55)Fe from both conjugates, indicating a common mechanism of uptake. A C. albicans strain lacking the siderophore transporter CaSit1/CaArn1 was not able to use ferrichrome or the synthetic ornithine-based trihydroxamate siderophore for growth promotion and was much less susceptible to the siderophore-drug conjugates than its isogenic parent strain. In summary, the ability of some Candida spp. to use ferrichrome-like siderophores for growth promotion explains the selective activity of hydroxamate-drug conjugates, and this activity seems to be related to the presence, in C. albicans, of the siderophore transporter CaSit1/CaArn1. New conjugate designs are necessary to fully restore or improve the initial DE activity.

Synthesis and properties of different metal complexes of the siderophore desferriferricrocin

Desferriferricrocin is a cyclic hexa-peptide siderophore with three hydroxamates as primary coordination groups. It forms metal complexes with Fe(III), Cr(III), Al(III), Ga(III), Cu(II), and Zn(II). These complexes were prepared and characterized using UV-vis, circular dichroism spectroscopy (CD), nuclear magnetic resonance spectroscopy (NMR), and electrospray ionization mass spectroscopy (ESI-MS). The mononuclear trivalent metal complexes of desferriferricrocin were stable in aqueous solutions, and their coordination centers primarily adopted the lambda configuration. The formation of multinuclear complexes of desferriferricrocin was determined by ESI-MS. Desferriferricrocin was able to bind up to three Cu(II) and two Zn(II) respectively. Heteronuclear complexes containing one trivalent and one divalent were also determined. In these complexes, amide nitrogens were utilized as alternative binding groups of desferriferricrocin in addition to the primary binding groups, the hydroxamates.

An artificial siderophore for the detection of iron(iii)

An artificial siderophore in the form of a squaraine dye (1) has been synthesized. The bidentate ligand chelates to Fe(III) between the deprotonated hydroxyl group on the ortho position of the ring adjacent to the carbonyl group of the cyclobutadiene ring. The optical response is due to a subtle geometry change of 1 on chelation to Fe(III). This artificial siderophore forms a 2 ratio 1 ligand ratio metal complex, as indicated by a sigmoidal isotherm (K(a)= 10(7) M(-1)). The optical response on the addition of Fe(III) is observed at low concentrations in comparison to other metal salts. The X-ray crystal structure and calculated structures of dye (1) are also included, and will be discussed.

Total synthesis of petrobactin and its homologues as potential growth stimuli for Marinobacter hydrocarbonoclasticus, an oil-degrading bacteria

A modular synthesis was developed to access petrobactin, a catechol-containing siderophore isolated from Marinobacter hydrocarbonoclasticus. A range of petrobactin homologues with differing dihydroxybenzamide motifs and in one case an increased number of carbons in the polyamine backbone were also synthesized. As such, these systems represent new isomeric probes to study iron transport properties in M. hydrocarbonoclasticus. The synthesis of petrobactin and its homologues and the first biological study of how these agents influence the growth of Mycobacterhydrocarbonoclasticus are reported. New synthetic methods were developed to overcome issues (imide formation) encountered in earlier syntheses. Both the (1)H and (13)C NMR of petrobactin were consistent with the recently revised structure showing that petrobactin in fact contains a 3,4-dihydroxybenzene motif rather than a 2,3-dihydroxybenzene motif. The preliminary biological studies suggested that using the native petrobactin 1b for M. hydrocarbonoclasticus-specific growth stimulation may be a poor strategy for oil-spill cleanup.

Catecholates and mixed catecholate hydroxamates as artificial siderophores for mycobacteria

Different mono-, bis- or triscatecholates and mixed mono- or biscatecholate hydroxamates were synthesized as potential siderophores for mycobacteria. SiderOphore activity was tested by growth promotion assays using wild type strains and iron transport mutants of mycobacteria as well as Gram-negative bacteria. Some triscatecholates and biscatecholate hydroxamates were active in mutants of Mycobacterium smegmatis deficient in mycobactin and exochelin biosynthesis or exochelin permease, respectively, indicating an uptake route independent of the exochelin/mycobactin pathway. Structure activity relationships were studied. Ampicillin conjugates of some of these compounds were inactive against mycobacteria but active against Gram-negative bacteria.

Trishydroxamates and triscatecholates based on monosaccharides and myo-inositol as artificial siderophores

New trishydroxamates and triscatecholates based on methyl alpha-D-glucopyranoside, methyl alpha-D-galactopyranoside, methyl alpha-D-ribopyranoside and methyl alpha-D-xylopyranoside as well as on 1,3,5-tri-O-benzyl-myo-inositol were synthesized. N-Methylsuccinohydroxamate, N-methylglutarohydroxamate and their O-benzoyl derivatives were used as hydroxamate moieties. 2,3-Dihydroxybenzoyl derivatives and acylated compounds as well as 2,3- and 3,4-dihydroxybenzylidenehydrazino derivatives, partly with spacer groups, were utilized as catecholate components. The siderophore activity of the prepared siderophore analogues was examined by a growth promotion assay with various Gram-negative bacteria and mycobacteria and by the CAS-assay. Some trishydroxamates and triscatecholates showed siderophore activity on Gram-negative bacteria and triscatecholates on mycobacteria. Iron complexes of the trishydroxamates act as siderophores for all types of iron transport mutants. The recognition and uptake specificity of these compounds was studied by E. coli siderophore receptor and iron transport mutants. Structure activity correlations are discussed.

Formation of the chromophore of the pyoverdine siderophores by an oxidative cascade

The pyoverdine chromophore is formed in a reaction involving a two-electron oxidation, a conjugate addition, and a second two-electron oxidation. This oxidative cascade can be carried out with polyphenol oxidase (PPO), MnO(2), and cell-free extracts from Pseudomonas aeruginosa. [reaction: see text]

Synthesis and biological activity of tris- and tetrakiscatecholate siderophores based on poly-aza alkanoic acids or alkylbenzoic acids and their conjugates with beta-lactam antibiotics

New linear and tripodal tri-aza- and tetra-aza alkanoic acids or alkylbenzoic acids were prepared as basic structures for siderophore mimetics from polyamines and oxocarbonic acids or formylbenzoic acids by catalytic hydrogenation. From these acids acetylated tris- and tetrakiscatecholates or 8-acyloxy-2,4-dioxo-benzoxazine derivatives as well as compounds with spacer groups were synthesized. These derivatives were coupled with ampicillin, amoxicillin, bacampicillin or cefaclor to new siderophore antibiotic conjugates. Most of the catecholate derivatives showed high siderophore activities in strains of Pseudomonas aeruginosa and Escherichia coli in a growth promotion assay under iron limitation conditions. The beta-lactam conjugates were highly active in vitro against Gram-negative bacteria correlating to the siderophore activity of the catecholate moiety and depending on the beta-lactam part. One ampicillin conjugate based on 5-(aminoethyl)-2,5,8-triazaalkylbenzoic acid was highly active against Gram-negative and Gram-positive bacteria. It was shown that conjugates with enhanced activity against Gram-negative bacteria use active iron uptake routes to penetrate the bacterial outer membrane barrier. Correlations between structure and biological activity were studied.

Bacterial siderophores: synthesis and biological activities of novel pyochelin analogues

The synthesis and biological activities of four pyochelin analogues substituted in different parts of the molecule are reported: 5-NHBoc-pyochelin, 3"N-Boc-pyochelin, 3"-nor-NH-pyochelin and neopyochelin II, the enantiomer of natural pyochelin. All these compounds complex iron(III) and transport it at different rates into the cells of Pseudomonas aeruginosa.

The preparation of a fully differentiated "multiwarhead" siderophore precursor

A protected, fully differentiated siderophore analogue has been prepared so that "Trojan Horse"-like siderophore drug conjugates with different drugs can be synthesized. The key steps in the synthesis include controlled preparation of an unsymmetrical urea and its conversion to a fully differentiated isocyanurate by reaction with chloro(carbonyl) isocyanate.

Total synthesis of desferrisalmycin B

The first total synthesis of a naturally occurring siderophore antibiotic, desferrisalmycin B, is described, and the configuration of the unknown stereocenter is assigned. The synthesis features a synthetic strategy of constructing the novel amino-heptopyranoside component by stereoselective dihydroxylation followed by a Bose-modified Mitsunobu reaction. Through this convergent approach, other members of salmycins should also be synthetically accessible.

New synthetic siderophores and their beta-lactam conjugates based on diamino acids and dipeptides

Linking of siderophores to antibiotics improves the penetration and therefore increases the antibacterial activity of the antibiotics. We synthesized the acylated catecholates and hydroxamates as siderophore components for antibiotic conjugates to reduce side effects of unprotected catecholate and hydroxamate moieties. In this paper, we report on bis- and tris-catecholates and mixed catecholate hydroxamates based on diamino acids or dipeptides. These compounds were active as siderophores in a growth promotion assay under iron limitation. Most of the conjugates with beta-lactams showed high in vitro activity against Gram-negative bacteria especially Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Serratia marcescens and Stenotrophomonas maltophilia. The compounds with enhanced antibacterial activity use active iron uptake routes to penetrate the bacterial outer membrane barrier, demonstrated by assays with mutants deficient in components of the iron transport system. Correlation between chemical structure and biological activity was studied.

New synthetic catecholate-type siderophores with triamine backbone

New analogues of triscatecholate siderophores based on linear or tripodal triamines with or without spacer groups or lipophilic and hydrophilic substituents were synthesized. The catecholate moieties were prepared in OH-forms, as acetylated compounds or masked as 8-methoxycarbonyloxy-2,4-dioxo-1,3-benzoxazine derivatives. Some of the new compounds were active as siderophores tested by growth promotion assays using various gram-negative bacteria and mycobacteria under iron limitation and by CAS-assay. Structure-activity-correlations have been studied.

Effects of synthetic siderophores on proliferation of Plasmodium falciparum in infected human erythrocytes

Because iron is essential for Plasmodium falciparum, we investigated the in vitro potential of various synthetic siderophores to kill P. falciparum in infected human erythrocytes. The substances with the most promising profile, i.e., low 50% lethal doses for plasmodia and minimum toxicity towards mammalian cells, were siderophores with an acylated monocatecholate or a triscatecholate as substituent.

Synthesis and biological evaluation of new citrate-based siderophores as potential probes for the mechanism of iron uptake in mycobacteria

Several iron chelators containing alpha,beta-unsaturated hydroxamic acid motifs appended to a citric acid platform were synthesized. Mycobacterium paratuberculosis was then challenged to grow in the presence of a panel of siderophores (mycobactin J, deferrioxamine B, acinetoferrin, and nannochelin A) and the new synthetic agents. Of the structures tested, those containing the trans 2-octenoyl motif were preferred over those with trans cinnamoyl groups. In addition, derivatives containing longer tether lengths between the iron binding ligands (C5) were more efficacious and led to higher growth index values. Perhaps most remarkable was the finding that at 2.4 microM a trans 2-octenoylated, citrate-containing imide 6 was nearly 5-fold more effective in stimulating growth than the native chelator, mycobactin J. In this regard, new structural elements were identified (e.g., an imide motif or 2-octenoyl side chain), whose presence stimulated mycobacterial growth.

Significance of asymmetric sites in choosing siderophores as deferration agents

The syntheses of the microbial iron chelators L-fluviabactin, its unnatural enantiomer, D-fluviabactin, L-homofluviabactin, and L-agrobactin, are described. The key steps involve the selective bis-acylation of the terminal nitrogens of norspermidine, spermidine, or homospermidine with 2,3-bis(benzyloxy)benzoic acid in the presence of 1,1-carbonyldiimidazole, followed by coupling of the N-hydroxysuccinimide ester of CBZ-protected L- or D-threonine with the central nitrogen. The effectiveness of each of these ligands in supporting the growth of Paracoccus denitrificans in a low-iron environment and the ability of these compounds to promote iron uptake are evaluated. The stereochemical configuration of the oxazoline ring is shown to be the major structural factor controlling both microbial growth stimulation and iron uptake. L-Fluviabactin, L-homofluviabactin, and L-agrobactin all promoted growth and iron uptake; D-fluviabactin was only marginally active. As with the microorganism's native siderophore, L-parabactin, all three ligands in the L-configuration investigated exhibited biphasic, i.e., both high-affinity and low-affinity, kinetics. The high-affinity system (iron concentration < 1 microM) yielded K(m) values between 0.11 and 0.23 microM and V(max) values from 157 to 129 pg-atoms Fe min(-1) (mg of protein)(-1), whereas the low-affinity scheme (iron concentration > 1 microM) gave K(m) values from 0.53 to 3.5 microM and V(max) values between 96 and 413 pg-atoms Fe min(-1) (mg of protein)(-1). Both L- and D-fluviabactin are very effective at clearing iron from the bile duct-cannulated rodent; when given subcutaneously at a dose of 150 micromol/kg, both ligands had iron clearing efficiencies of >13%, which is much greater than that of desferrioxamine in this model. Thus, by altering the stereochemistry of certain microbial siderophores, it is possible to generate deferration agents that are still effective at clearing iron from animals, yet do not promote microbial growth.

Computer-aided design of novel siderophores: pyridinochelin

Pyridinochelin, a novel tetradentate catecholate-type siderophore, has been designed on the basis of the active analog enterobactin and was then synthesized. Growth promotion tests indicate that this synthetic siderophore feeds various pathogenic bacteria most effectively with iron even though it lacks one catecholate group compared to enterobactin. The superposition of the mentioned siderophore structures suggests that the structure of the skeleton connecting the catecholate groups might be an important factor for the iron transport.

New artificial siderophores based on a monosaccharide scaffold

New artificial catecholate siderophores with methyl alpha-D-glucopyranoside as scaffold were synthesized. The dihydroxy- or di(acetoxy)benzoyl moieties were attached either directly or via aminopropyl spacer groups, to the carbohydrate scaffold. The siderophore activity of the prepared siderophore analogs was examined by a growth promotion assay using various Gram-negative bacteria and mycobacteria and by the CAS-assay.

The remarkable hydrophobic effect of a fatty acid side chain on the microbial growth promoting activity of a synthetic siderophore

The ability of synthetic derivatives of the siderophore tripeptide of N5-hydroxy-N5-acetyl-L-ornithine to promote the growth of various strains of mycobacteria and Gram negative bacteria was found to depend significantly on the hydrophobic nature of the derivative. Although the tripeptide of N5-hydroxy-N5-acetyl-L-ornithine is not normally utilized by mycobacteria, an N-terminal palmitoyl derivative mimicked natural mycobactin J in all studies.

New synthetic catecholate-type siderophores based on amino acids and dipeptides

New analogs of bacterial siderophores with one, two or three catecholate moieties were synthesized using various mono- and diamino acid and dipetide scaffolds, respectively. In addition to 2,3-dihydroxybenzoyl siderophore analogs and their acylated derivatives, 3,4-dihydroxybenzoyl derivatives were prepared. Furthermore, the synthesis of a new triscatecholate serving as an intimate model for enterobactin is reported. Most of the new compounds gave a positive CAS-test and were active as siderophores tested by growth promotion assays with a set of siderophore indicator mutants under iron limitation. Structure-activity-correlations have also been studied.

Total synthesis of the siderophore danoxamine

The total synthesis of the linear trihydroxamate siderophore, Danoxamine, is described. Danoxamine is a siderophore component of the naturally occurring siderophore-drug conjugates Salmycin A-D. The synthesis of Danoxamine features a series of coupling reactions involving N-(5-benzyloxypentyl)-O-benzylhydroxylamine being linked by a succinoyl linker to N-(benzyloxy)-1,5-pentanediamine. Two more succinoyl linkers and another N-(benzyloxy)-1,5-pentanediamine were used in coupling reactions to afford the fully protected siderophore. The linear tetrabenzyl-protected trihydroxamate was deprotected to afford the natural product Danoxamine.

Studies and syntheses of siderophores, microbial iron chelators, and analogs as potential drug delivery agents

Siderophores (microbial iron chelators) play an extremely important role in microbial pathogenicity. Microbial uptake of siderophore-iron complexes through active transport systems allow microbes to survive and proliferate even under iron deficient environments during invasion of a host. Due to their structural complexity, unique iron (III) chelation, acquisition properties, and their therapeutic potential, siderophores have attracted much attention in a broad range of disciplines. Tremendous progress has been made in siderophore syntheses, in determination of the structures and functions of outer membrane receptors (e.g. FhuA and FepA), and in the mechanistic insight into siderophore-iron-mediated active transport processes. One of the important practical applications of this active transport system is development of species-selective active drug transport (the Trojan Horse approach) to potentially treat infections due to drug resistant strains of microbes. Siderophore-drug conjugates have shown great potential in active drug delivery to target pathogenic microbes.

Syntheses and studies of multiwarhead siderophore-5-fluorouridine conjugates

Siderophores are microbial iron chelating agents that sequester physiologically essential iron for microbes. Conjugation of drugs to siderophores allows use of active iron transport for microbially directed drug delivery. Syntheses and biological studies are described of the first multidrug isocyanurate-based siderophore analogues separately containing one, two, and three 5-fluorouridine (5-FU) derivatives as the drug component. The results indicate that a single siderophore can be used to deliver multiple drugs to target pathogenic microorganisms.

Synthesis and biological evaluation of a carbocyclic azanoraristeromycin siderophore conjugate

Synthesis and biological evaluation of a carbocyclic azanoraristeromycin siderophore conjugate 22 is reported. Coupling of previously prepared L-alanyl-4'-azanoraristeromycin 19 with protected tripeptide trihydroxamate 20, followed by hydrogenolytic removal of all protecting groups, provided the first carbocylic azanoraristeromycin siderophore conjugate (22, 8 with iron). Compounds 19 and 22 showed inhibitory activity against tumor cells, and conjugate 22, in particular, displayed significant activity against those viruses (i.e. reo, parainfluenza, vaccinia, cytomegalo) that are known to be inhibited by S-adenosylhomocysteine hydrolase inhibitors.

Novel catecholate-type siderophore analogs based on a myo-inositol scaffold

A novel 1,3,5-triamino-myo-inositol derivative is presented as a readily available scaffold for the design of tripodal siderophore mimetics. Based on this scaffold, various hexadentate catecholate-type siderophore analogs were synthesized by attaching the catechols to the inositol scaffold via spacer units of different structure and length. The potential to tune the polarity of the inositol containing siderophore analogs has also been demonstrated by varying the protection group strategy. The siderophore activity of the prepared siderophore analogs was examined by cross-feeding tests with various Gram-negative bacteria and mycobacteria.

Peptide siderophores

Siderophores are low molecular weight iron chelators, produced by virtually all bacteria, fungi and some plants. They serve to deliver the essential element iron, barely soluble under aerobic conditions, into microbial cells. Siderophores are therefore important secondary metabolites which are very often based on amino acids and their derivatives. Biosynthesis, transport, regulation and chemical synthesis of natural siderophores and their analogues is of considerable interest for the protein and peptide chemist. This review gives an overview of the structural classes of peptidic siderophores, along with data on their biosynthesis. On a number of representative examples, strategies and schemes of their chemical synthesis are described.

The synthesis and antibacterial activity of two pyoverdin-ampicillin conjugates, entering Pseudomonas aeruginosa via the pyoverdin-mediated iron uptake pathway

Two pyoverdin-ampicillin conjugates were synthesized and their structures were confirmed by mass spectrometry and NMR spectroscopy. In contrast to ampicillin, the conjugates exhibited high antibacterial activity against Pseudomonas aeruginosa ATCC 15692 and ATCC 27853, effective only against the strain which is using the parent pyoverdin for iron uptake. This suggests that the conjugates enter the bacterial cell via the ferripyoverdin uptake pathway. Growth stimulation studies with conjugates hydrolysed at the beta-lactam ring of the ampicillin moiety supported this view.

Synthesis and ESR study of new dihydroxamic acid siderophores S as scavengers of hydroxyl radicals

Five new dihydroxamic acid ligands (L) (8, 10a, 10b, 10c and 13) have been synthesised and characterised as potential chelating agents for iron (Fe3+). The log stability constants of Fe2L3 and FeL+ from Fe3+ and L2- have been estimated to be log beta = 61.96 and log beta 1 = 22.8 respectively. The ability of these compounds to scavenge hydroxyl radicals (oOH) responsible for cell damage have been studied by esr spectroscopy.

Iron transport-mediated drug delivery using mixed-ligand siderophore-beta-lactam conjugates

BACKGROUND

Assimilation of iron is essential for microbial growth. Most microbes synthesize and excrete low molecular weight iron chelators called siderophores to sequester and deliver iron by active transport processes. Specific outer membrane proteins recognize, bind and initiate transport of species-selective ferric siderophore complexes. Organisms most often have specific receptors for multiple types of siderophores, presumably to ensure adequate acquisition of the iron that is essential for their growth. Conjugation of drugs to synthetic hydroxamate or catechol siderophore components can facilitate active iron-transport-mediated drug delivery. While resistance to the siderophore-drug conjugates frequently occurs by selection of mutants deficient in the corresponding siderophore-selective outer membrane receptor, the mutants are less able to survive under iron-deficient conditions and in vivo. We anticipated that synthesis of mixed ligand siderophore-drug conjugates would allow active drug delivery by multiple iron receptor recognition and transport processes, further reducing the likelihood that resistant mutants would be viable.

RESULTS

Mixed ligand siderophore-drug conjugates were synthesized by combining hydroxamate and catechol components in a single compound that could chelate iron, and that also contained a covalent linkage to carbacephalosporins, as representative drugs. The new conjugates appear to be assimilated by multiple active iron-transport processes both in wild type microbes and in selected mutants that are deficient in some outer membrane iron-transport receptors.

CONCLUSIONS

The concept of active iron-transport-mediated drug delivery can now be extended to drug conjugates that can enter the cell through multiple outer membrane receptors. Mutants that are resistant to such conjugates should be severely impaired in iron uptake, and therefore particularly prone to iron starvation.

Specific photoaffinity labelling of a ferripyoverdin outer membrane receptor of Pseudomonas aeruginosa

In order to identify and characterize the receptors involved in pyoverdin-mediated iron transport in Pseudomonas aeruginosa ATCC 15692, a photoactivatable siderophore has been synthesized. In the dark, this probe is stable and is able to promote iron transport at the same rate as the native pyoverdin. Under irradiation at 312 nm, the molecule is photodecomposed and a clear inhibition of the iron transport is observed. With the radioactive form of this photoactivatable probe, we were able to visualize on a SDS-PAGE gel a labelled protein of approximately 90 kDa molecular mass, which is very likely the FpvA receptor or a yet unknown pyoverdin receptor.

Synthesis of optically pure chrysobactin and immunoassay development

Chrysobactin (alpha-N-(2,3-dihydroxybenzoyl)-D-lysyl-L-serine), a siderophore that is essential for systemic virulence by plant pathogenic Erwinia chrysanthemi, was synthesized with high diastereomeric purity. Chrysobactin was prepared by coupling the N-hydroxysuccinimide ester of alpha-N-(2,3-dibenzyloxybenzoyl)-epsilon-N-Cbz-D-lysine with L-serine benzyl ester followed by deprotection via hydrogenolysis. Optically pure chrysobactin was obtained with 98% overall yield. A monoclonal antibody to ferric chrysobactin was developed and characterized as IgM. The antibody reacts with chrysobactin, ferric chrysobactin and less strongly with ferric dihydroxybenzoic acid. The antibody reacts weakly with the siderophores ferrichrome, A, ferric pseudobactin and ferric rhodotorulic acid. This antibody was used in a competitive immunoassay to detect ferric chrysobactin at 10(-8) to 10(-10) mol. This immunoassay may provide a useful method for the detection of chrysobactin in plant samples.

Chemical Determinants of antimalarial activity of reversed siderophores

Reversed siderophores (RSFs) are artificial hydroxamate-based iron chelators designed after the natural siderophore ferrichrome. The modular molecular design of RSF derivatives allowed the synthesis of various congeners with controlled iron-binding capacities and partition coefficients. These two physicochemical properties were assessed by a novel fluorescent method and were found to be the major determinants of RSF permeation across erythrocyte membranes and scavenging of compartmentalized iron. The partition coefficient apparently conferred upon RSFs two major features: (i) the ability to rapidly access iron pools of in vitro-grown Plasmodium falciparum at all developmental stages and to mobilize intracellular iron and transfer it to the medium and (ii) the ability to suppress parasite growth at all developmental stages. These features of RSFs were assessed by quantitative determination of the structure-activity relationships of the biological activities and partition coefficients spanning a wide range of values. The most effective RSF containing the aromatic group of phenylalanine (RSFm2phe) showed 50% inhibitory concentration of 0.60 +/- 0.03 nmol/ml in a 48-h test and a 2-h onset of inhibition of ring development at 5 nmol/ml. The lipophilic compound RSFm2phe and the lipophilic and esterase-cleavable compound RSFm2pee inhibited parasite growth at all developmental stages whether inhibition was assessed in a continuous mode or after discontinuing drug administration. The antimalarial effects of RSFm2phe and cleavable RSFm2pee were potentiated in the presence of desferrioxamine (DFO) at concentrations at which DFO alone had no effect on parasite growth. These studies provide experimental evidence indicating that the effective and persistent antimalarial actions of RSFs are associated with drug access to infected cells and scavenging of iron from intracellular parasites. Moreover, the optimal antimalarial actions of RSFs are apparently also determined by improved accessibility to critical iron pools or by specific interactions with critical parasite targets.

Synthesis of fragments of the peptide component of pseudobactin

Pseudobactin is a structurally complex and physiologically important siderophore (microbial iron chelator] from Pseudomonas putida-fluorescens. Various fragments of the unusual peptide component of pseudobactin listed below were prepared by solution-phase peptide synthesis. L-Lys.D-threo-beta-OH Asp.L-Ala.D-allo-Thr.L-Ala L-Lys.D-threo-beta OH Asp.L-Ala.D-allo-Thr D-threo-beta-OH Asp.L-Ala.D-allo-Thr.L-Ala.D-N-OH-cycloOrn D-threo-beta-OH-Asp.L-Ala.D-allo-Thr.L-Ala L-Ala.D-allo-Thr.L-Ala.D-N-OH-cycloOrn A class of related peptides named pseudomycins have shown promising antifungal activity. To examine if these peptide fragments above would elicit similar activity, the fragments were tested and found to have no antifungal activity in limited bioassays.

Synthesis and in vitro antibacterial activity of spermidine-based mixed catechol- and hydroxamate-containing siderophore--vancomycin conjugates

The first antibiotic conjugates of vancomycin (1) and siderophore analogues containing spermidine-based catechol ligands (conjugate 11) as well as mixed catechol and hydroxamate ligands (conjugate 13) are described. The design of the conjugates was based on the earlier observation that conjugation of siderophore components to beta-lactam antibiotics induced active iron transport-mediated drug delivery. The novel conjugates (11 and 13) were synthesized by selective acylation of the primary amino group of 1. Preliminary biological studies indicated that siderophore modified vancomycins lost some activity (4- to 16-fold) against Gram-positive bacteria relative to vancomycin itself, and were generally similar to vancomycin in activity against Gram-negative bacteria under iron-sufficient conditions. However, under iron-depleted conditions which mimic human serum, conjugate 11 displayed enhanced antibacterial activity against an antibiotic hypersensitive strain of Pseudomonas aeruginosa.

An investigation of desferrithiocin metabolism

The hydrolyses of (S)-desferrithiocin (DFT, 1), (R)-desmethyl-DFT (2), and (R)-desazadesmethyl-DFT (3) were studied at pH 2.5 and 7.2 in order to access the stability of the thiazolines at the pH of the stomach and the serum. At 37 degrees C and pH 2.5, DFT (1) (t1/2 = 18.6 h), desmethyl-DFT (2) (t1/2 = 8.74 h), and desazadesmethyl-DFT (3) (t1/2 = 31.7 h) were shown to open principally to the thiol amides with trace amounts of the corresponding thioesters, < or = 2%. The thiazolines were resistant to hydrolysis at pH 7.2. Iron(III) stabilized significantly the thiazolines in the complexes 16a/b of 3 in regard to hydrolysis at pH 2.5 (t1/2 > 20 days). The iron(III) complexes 16a/b were shown to be stable at pH 7.2. While the thiol amides 13 and 14 of 1 and 2 were isolated from the hydrolysis of the parent desferrithiocins, the thioester 4 and the thiol amide 5 of 3 were synthesized and their stability in aqueous solution, iron-clearance properties, and toxicity were evaluated. Thioester 4 was shown to rearrange to thiol amide 5 at pH 2.5 and 37 degrees C with a half-life of 4.18 h and instantaneously at pH 7.2. Thiol amide 5 is in equilibrium with 4 (5/4 = 49:1) at pH 2.5 and was shown to be stable at pH 7.2. Thioester 4 and thiol amide 5 demonstrated neither iron-clearance activity in iron-overloaded rats nor toxic side effects in mice. Hydrolysis products of the drug, which might be generated in the stomach, seem unlikely to be the source of the drug's toxicity or iron-clearing properties.