Biomimetic Analogues of the Desferrioxamine E Siderophore for PET Imaging of Invasive Aspergillosis: Targeting Properties and Species Specificity

The pathogenic fungus Aspergillus fumigatus utilizes a cyclic ferrioxamine E (FOXE) siderophore to acquire iron from the host. Biomimetic FOXE analogues were labeled with gallium-68 for molecular imaging with PET. [68Ga]Ga(III)-FOXE analogues were internalized in A. fumigatus cells via Sit1. Uptake of [68Ga]Ga(III)-FOX 2-5, the most structurally alike analogue to FOXE, was high by both A. fumigatus and bacterial Staphylococcus aureus. However, altering the ring size provoked species-specific uptake between these two microbes: ring size shortening by one methylene unit (FOX 2-4) increased uptake by A. fumigatus compared to that by S. aureus, whereas lengthening the ring (FOX 2-6 and 3-5) had the opposite effect. These results were consistent both in vitro and in vivo, including PET imaging in infection models. Overall, this study provided valuable structural insights into the specificity of siderophore uptake and, for the first time, opened up ways for selective targeting and imaging of microbial pathogens by siderophore derivatization.

Iron(III) Complexes with Hybrid-Type Artificial Siderophores Containing Catecholate and Hydroxamate Sites

Two hybrid-type artificial siderophore ligands containing both catecholate and hydroxamate groups as iron-capturing sites, bis(2,3-dihydroxybenzamidepropyl)mono[2-propyl]aminomethane (H5LC2H1) and mono(2,3-dihydroxybenzamide-propyl)bis[2-propyl]aminomethane (H4LC1H2), were designed and synthesized. Iron(III) complexes, K2[FeIIILC2H1] and K[FeIIILC1H2], were prepared and characterized spectroscopically, potentiometrically, and electrochemically. The results were compared with those previously reported for iron complexes with non-hybridized siderophores containing either catecholate or hydroxamate groups, K3[FeIIILC3] and [FeIIILH3]. Both K2[FeIIILC2H1] and K[FeIIILC1H2] formed six-coordinate octahedral iron(III) complexes. Evaluation of the thermodynamic properties of the complexes in an aqueous solution indicated high log β values of 37.3 and 32.3 for K2[FeIIILC2H1] and K[FeIIILC1H2], respectively, which were intermediate between those of K3[FeIIILC3] (44.2) and [FeIIILH3] (31). Evaluation of the ultraviolet-visible and Fourier transform infrared spectra of the two hybrid siderophore-iron complexes under different pH or pD (potential of dueterium) conditions showed that the protonation of K2[FeIIILC2H1] and K[FeIIILC1H2] generated the corresponding protonated species, [FeIIIHnLC2H1](2-n)- and [FeIIIHnLC1H2](1-n)-, accompanied by a significant change in the coordination mode. The protonated hybrid-type siderophore-iron complexes showed high reduction potentials, which were well within the range of those of biological reductants. The results suggest that the hybrid-type siderophore easily releases an iron(III) ion at low pH. The biological activity of the four artificial siderophore-iron complexes against Microbacterium flavescens and Escherichia coli clearly depends on the structural differences between the complexes. This finding demonstrates that the changes in the coordination sites of the siderophores enable close control of the interactions between the siderophores and receptors in the cell membrane.

Advances in the Synthesis of Enterobactin, Artificial Analogues, and Enterobactin-Derived Antimicrobial Drug Conjugates and Imaging Tools for Infection Diagnosis

Iron is an essential growth factor for bacteria, but although highly abundant in nature, its bioavailability during infection in the human host or the environment is limited. Therefore, bacteria produce and secrete siderophores to ensure their supply of iron. The triscatecholate siderophore enterobactin and its glycosylated derivatives, the salmochelins, play a crucial role for iron acquisition in several bacteria. As these compounds can serve as carrier molecules for the design of antimicrobial siderophore drug conjugates as well as siderophore-derived tool compounds for the detection of infections with bacteria, their synthesis and the design of artificial analogues is of interest. In this review, we give an overview on the synthesis of enterobactin, biomimetic as well as totally artificial analogues, and related drug-conjugates covering up to 12/2021.

1 Introduction

2 Antibiotic Crisis and Sideromycins as Natural Templates for New Antimicrobial Drugs

3 Biosynthesis of Enterobactin, Salmochelins, and Microcins

4 Total Synthesis of Enterobactin and Salmochelins

5 Chemoenzymatic Semi-synthesis of Salmochelins and Microcin E492m Derivatives

6 Synthesis of Biomimetic Enterobactin Derivatives with Natural Tris-lactone Backbone

7 Synthesis of Artificial Enterobactin Derivatives without Tris-lactone Backbone

8 Conclusions

Cyclic Analogs of Desferrioxamine E Siderophore for 68Ga Nuclear Imaging: Coordination Chemistry and Biological Activity in Staphylococcus aureus

As multidrug-resistant bacteria are an emerging problem and threat to humanity, novel strategies for treatment and diagnostics are actively sought. We aim to utilize siderophores, iron-specific strong chelating agents produced by microbes, as gallium ion carriers for diagnosis, applying that Fe(III) can be successfully replaced by Ga(III) without losing biological properties of the investigated complex, which allows molecular imaging by positron emission tomography (PET). Here, we report synthesis, full solution chemistry, thermodynamic characterization, and the preliminary biological evaluation of biomimetic derivatives (FOX) of desferrioxamine E (FOXE) siderophore, radiolabeled with ⁶⁸Ga for possible applications in PET imaging of S. aureus. From a series of six biomimetic analogs, which differ from FOXE with cycle length and position of hydroxamic and amide groups, the highest Fe(III) and Ga(III) stability was determined for the most FOXE alike compounds–FOX 2-4 and FOX 2-5; we have also established the stability constant of the Ga-FOXE complex. For this purpose, spectroscopic and potentiometric titrations, together with the Fe(III)–Ga(III) competition method, were used. [⁶⁸Ga]Ga-FOXE derivatives uptake and microbial growth promotion studies conducted on S. aureus were efficient for compounds with a larger cavity, i.e., FOX 2-5, 2-6, and 3-5. Even though showing low uptake values, Fe-FOX 2-4 seems to be also a good Fe-source to support the growth of S. aureus. Overall, proposed derivatives may hold potential as inert and stable carrier agents for radioactive Ga(III) ions for diagnostic medical applications or interesting starting compounds for further modifications.

In this work, the authors have investigated a set of novel ferrioxamine E analogs as potential Ga-68 chelators and tools for infection imaging.

Thermodynamic Stability and Speciation of Ga(III) and Zr(IV) Complexes with High-Denticity Hydroxamate Chelators

Increasing attention has been recently devoted to ⁸⁹Zr(IV) and ⁶⁸Ga(III) radionuclides, due to their favorable decay characteristics for positron emission tomography (PET). In the present paper, a deep investigation is presented on Ga(III) and Zr(IV) complexes with a series of tri-(H₃L1, H₃L3, H₃L4 and desferrioxamine E, DFOE) and tetrahydroxamate (H₄L2) ligands. Herein, we describe the rational design and synthesis of two cyclic complexing agents (H₃L1 and H₄L2) bearing three and four hydroxamate chelating groups, respectively. The ligand structures allow us to take advantage of the macrocyclic effect; the H₄L2 chelator contains an additional side amino group available for a possible further conjugation with a biomolecule. The thermodynamic stability of Ga(III) and Zr(IV) complexes in solution has been measured using a combination of potentiometric and pH-dependent UV–vis titrations, on the basis of metal–metal competition. The Zr(IV)-H₄L2 complex is characterized by one of the highest formation constants reported to date for a tetrahydroxamate zirconium chelate (log β = 45.9, pZr = 37.0), although the complex-stability increase derived from the introduction of the fourth hydroxamate binding unit is lower than that predicted by theoretical calculations. Solution studies on Ga(III) complexes revealed that H₃L1 and H₄L2 are stronger chelators in comparison to DFOB. The complex stability obtained with the new ligands is also compared with that previously reported for other hydroxamate ligands. In addition to increasing the library of the thermodynamic stability data of Ga(III) and Zr(IV) complexes, the present work allows new insights into Ga(III) and Zr(IV) coordination chemistry and thermodynamics and broadens the selection of available chelators for ⁶⁸Ga(III) and ⁸⁹Zr(IV).

Solution equilibria studies on Ga(III) and Zr(IV) complexes with a series of tri- and tetrahydroxamate ligands are presented. For this purpose, the rational design and synthesis of two cyclic complexing agents bearing three and four hydroxamate chelating groups was performed. The thermodynamic and speciation studies allow a discussion of the structure−complex stability dependence. The Zr(IV)-tetrahydroxamate complex is characterized by one of the highest formation constants reported to date for a hydroxamate zirconium chelator.

Siderophores for medical applications: Imaging, sensors, and therapeutics

Siderophores are low-molecular-weight chelators produced by microorganisms to scavenge iron from the environment and deliver it to cells via specific receptors. Tremendous researches on the molecular basis of siderophore regulation, synthesis, secretion, and uptake have inspired their diverse applications in the medical field. Replacing iron with radionuclides in siderophores, such as the most prominent Ga-68 for positron emission tomography (PET), carves out ways for targeted imaging of infectious diseases and cancers. Additionally, the high affinity of siderophores for metal ions or microorganisms makes them a potent detecting moiety in sensors that can be used for diagnosis. As for therapeutics, the notable Trojan horse-inspired siderophore-antibiotic conjugates demonstrate enhanced toxicity against multi-drug resistant (MDR) pathogens. Besides, siderophores can tackle iron overload diseases and, when combined with moieties such as hydrogels and nanoparticles, a wide spectrum of iron-induced diseases and even cancers. In this review, we briefly outline the related mechanisms, before summarizing the siderophore-based applications in imaging, sensors, and therapeutics.

Siderophore-Based Molecular Imaging of Fungal and Bacterial Infections-Current Status and Future Perspectives

Invasive fungal infections such as aspergillosis are life-threatening diseases mainly affecting immuno-compromised patients. The diagnosis of fungal infections is difficult, lacking specificity and sensitivity. This review covers findings on the preclinical use of siderophores for the molecular imaging of infections. Siderophores are low molecular mass chelators produced by bacteria and fungi to scavenge the essential metal iron. Replacing iron in siderophores by radionuclides such as gallium-68 allowed the targeted imaging of infection by positron emission tomography (PET). The proof of principle was the imaging of pulmonary Aspergillus fumigatus infection using [68Ga]Ga-triacetylfusarinine C. Recently, this approach was expanded to imaging of bacterial infections, i.e., with Pseudomonas aeruginosa. Moreover, the conjugation of siderophores and fluorescent dyes enabled the generation of hybrid imaging compounds, allowing the combination of PET and optical imaging. Nevertheless, the high potential of these imaging probes still awaits translation into clinics.

Hybrid Imaging of Aspergillus fumigatus Pulmonary Infection with Fluorescent, 68Ga-Labelled Siderophores

Aspergillus fumigatus (A. fumigatus) is a human pathogen causing severe invasive fungal infections, lacking sensitive and selective diagnostic tools. A. fumigatus secretes the siderophore desferri-triacetylfusarinine C (TAFC) to acquire iron from the human host. TAFC can be labelled with gallium-68 to perform positron emission tomography (PET/CT) scans. Here, we aimed to chemically modify TAFC with fluorescent dyes to combine PET/CT with optical imaging for hybrid imaging applications. Starting from ferric diacetylfusarinine C ([Fe]DAFC), different fluorescent dyes were conjugated (Cy5, SulfoCy5, SulfoCy7, IRDye 800CW, ATTO700) and labelled with gallium-68 for in vitro and in vivo characterisation. Uptake assays, growth assays and live-cell imaging as well as biodistribution, PET/CT and ex vivo optical imaging in an infection model was performed. Novel fluorophore conjugates were recognized by the fungal TAFC transporter MirB and could be utilized as iron source. Fluorescence microscopy showed partial accumulation into hyphae. µPET/CT scans of an invasive pulmonary aspergillosis (IPA) rat model revealed diverse biodistribution patterns for each fluorophore. [68Ga]Ga-DAFC-Cy5/SufloCy7 and -IRDye 800CW lead to a visualization of the infected region of the lung. Optical imaging of ex vivo lungs corresponded to PET images with high contrast of infection versus non-infected areas. Although fluorophores had a decisive influence on targeting and pharmacokinetics, these siderophores have potential as a hybrid imaging compounds combining PET/CT with optical imaging applications.