Octadentate Zirconium(IV)-Loaded Macrocycles with Varied Stoichiometry Assembled From Hydroxamic Acid Monomers using Metal-Templated Synthesis

Diagnostic Positron Emission Tomography Imaging with Zirconium-89 Desferrioxamine B Squaramide: From Bench to Bedside

Molecular imaging with antibodies radiolabeled with positron-emitting radionuclides combines the affinity and selectivity of antibodies with the sensitivity of Positron Emission Tomography (PET). PET imaging allows the visualization and quantification of the biodistribution of the injected radiolabeled antibody, which can be used to characterize specific biological interactions in individual patients. This characterization can provide information about the engagement of the antibody with a molecular target such as receptors present in elevated levels in tumors as well as providing insight into the distribution and clearance of the antibody. Potential applications of clinical PET with radiolabeled antibodies include identifying patients for targeted therapies, characterization of heterogeneous disease, and monitoring treatment response.Antibodies often take several days to clear from the blood pool and localize in tumors, so PET imaging with radiolabeled antibodies requires the use of a radionuclide with a similar radioactive half-life. Zirconium-89 is a positron-emitting radionuclide that has a radioactive half-life of 78 h and relatively low positron emission energy that is well suited to radiolabeling antibodies. It is essential that the zirconium-89 radionuclide be attached to the antibody through chemistry that provides an agent that is stable in vivo with respect to the dissociation of the radionuclide without compromising the biological activity of the antibody.This Account focuses on our research using a simple derivative of the bacterial siderophore desferrioxamine (DFO) with a squaramide ester functional group, DFO-squaramide (DFOSq), to link the chelator to antibodies. In our work, we produce conjugates with an average ∼4 chelators per antibody, and this does not compromise the binding of the antibody to the target. The resulting antibody conjugates of DFOSq are stable and can be easily radiolabeled with zirconium-89 in high radiochemical yields and purity. Automated methods for the radiolabeling of DFOSq-antibody conjugates have been developed to support multicenter clinical trials. Evaluation of several DFOSq conjugates with antibodies and low molecular weight targeting agents in tumor mouse models gave PET images with high tumor uptake and low background. The promising preclinical results supported the translation of this chemistry to human clinical trials using two different radiolabeled antibodies. The potential clinical impact of these ongoing clinical trials is discussed.The use of DFOSq to radiolabel relatively low molecular weight targeting molecules, peptides, and peptide mimetics is also presented. Low molecular weight molecules typically clear the blood pool and accumulate in target tissue more rapidly than antibodies, so they are usually radiolabeled with positron-emitting radionuclides with shorter radioactive half-lives such as fluorine-18 (t1/2 ∼ 110 min) or gallium-68 (t1/2 ∼ 68 min). Radiolabeling peptides and peptide mimetics with zirconium-89, with its longer radioactive half-life (t1/2 = 78 h), could facilitate the centralized manufacture and distribution of radiolabeled tracers. In addition, the ability to image patients at later time points with zirconium-89 based agents (e.g. 4-24 h after injection) may also allow the delineation of small or low-uptake disease sites as the delayed imaging results in increased clearance of the tracer from nontarget tissue and lower background signal.

DFO-Km: A Modular Chelator as a New Chemical Tool for the Construction of Zirconium-89-Based Radiopharmaceuticals

Zirconium-89-labeled monoclonal antibodies and other large macromolecules such as nanoparticles hold great promise as positron emission tomography imaging agents. In general, zirconium-89 is an ideal radionuclide for long-circulating vectors such as antibodies or nanoparticles. It is also a promising radionuclide for theranostic radiopharmaceuticals due to its suitable match in half-life with actinium-225, thorium-227, lutetium-177, and others. As such, demand for new and optimized bifunctional chelators for zirconium-89 continues to grow. Herein, we present the modular chelator DFO-Km, which is octadentate and features lysine as a modular amino acid linker. The modular amino acid linker can be changed to other natural or unnatural amino acids to access different bioconjugation chemistries, while the chelating portion is unchanged thus retaining identical metal ion coordination properties to DFO-Km. The epsilon-amine in the DFO-Km linker (lysine) was used to complete synthesis of a bifunctional derivative bearing a p-SCN-Ph moiety. The chelator DFO-Km includes a redesigned hydroxamic acid, which provides more flexibility for metal ion coordination relative to the monomer used in the previously published DFO-Em. Moreover, a set of comprehensive DFT calculations were performed to model and evaluate 16 geometric isomers of Zr-(DFO-Km), which suggested the complex would form the optimum cic-cis-trans-trans octadentate Zr(IV) coordination geometry with no aqua or hydroxide ligands present. The bifunctional derivative p-SCN-Ph-DFO-Km was compared directly with the commercially available p-SCN-Ph-DFO, and both underwent efficient conjugation to a nonspecific human serum antibody (IgG) to yield two model immunoconjugates. The behavior of [89Zr]Zr-DFO-Km-IgG was studied in healthy mice for 2 weeks and compared to an equivalent cohort injected with [89Zr]Zr-DFO-IgG as a clinical "gold standard" control. PET-CT and biodistribution results revealed higher stability of [89Zr]Zr-(DFO-Km)-IgG in vivo over [89Zr]Zr-DFO-IgG, as demonstrated by the significant reduction of zirconium-89 in the whole skeleton as visualized and quantified by PET-CT at 1, 3, 7, and 14 days post-injection. Using CT-gated regions of interest over these PET-CT images, the whole skeleton was selected and uptake values were measured at 14 days post-injection of 3.6 ± 0.9 (DFO) vs 1.9 ± 0.1 (DFO-Km) %ID/g (n = 4, * p = 0.02), which represents a ∼48% reduction in bone uptake with DFO-Km relative to DFO. Biodistribution experiments performed on these same mice following the 14 day imaging time point revealed bone (both tibia) uptake values of 3.7 ± 1.3 (DFO) vs 2.0 ± 0.6 (DFO-Km) %ID/g (n = 6, * p < 0.05), with the tibia uptake values in close agreement with whole-skeleton ROI PET-CT data. These results indicate that DFO-Km is an improved chelator for [89Zr]Zr4+ applications relative to DFO. The bifunctional chelator p-SCN-Ph-DFO-Km shows potential as a new chemical tool for creating bioconjugates using targeting vectors such as antibodies, peptides, and nanoparticles.

Synthesis of Modular Desferrioxamine Analogues and Evaluation of Zwitterionic Derivatives for Zirconium Complexation

The natural siderophore desferrioxamine B (DFOB) has been used for targeted PET imaging with 89 Zr before. However, Zr-DFOB has a limited stability and a number of derivatives have been developed with improved chelation properties for zirconium. We describe the synthesis of pseudopeptidic analogues of DFOB with azido side chains. These are termed AZA-DFO (hexadentate) and AZA-DFO* (octadentate) and are assembled via a modular synthesis from Orn-β-Ala and Lys-β-Ala. Nine different chelators have been conjugated to zwitterionic moieties by copper-catalyzed azide-alkyne cycloaddition (CuAAC). The resulting water-soluble chelators form Zr complexes under mild conditions (room temperature for 90 min). Transchelation assays with 1000-fold excess of EDTA and 300-fold excess of DFOB revealed that a short spacing of hydroxamates in (Orn-β-Ala)3-4 leads to improved complex stability compared to a longer spacing in (Lys-β-Ala)3-4 . We found that the alignment of amide groups in the pseudopeptide backbone and the presence of zwitterionic sidechains did not compromise the stability of the Zr-complexes with our chelators. We believe that the octadentate derivative AZA-DFO* is particularly valuable for the preparation of new Zr-chelators for targeted imaging which combine tunable pharmacokinetic properties with high complex stability and fast Zr-complexation kinetics.

Radiochemical, Computational, and Spectroscopic Evaluation of High-Denticity Desferrioxamine Derivatives DFO2 and DFO2p toward an Ideal Zirconium-89 Chelate Platform

Desferrioxamine (DFO) has long been considered the gold standard chelator for incorporating [⁸⁹Zr]Zr⁴⁺ in radiopharmaceuticals for positron emission tomography (PET) imaging. To improve the stability of DFO with zirconium-89 and to expand its coordination sphere to enable binding of large therapeutic radiometals, we have synthesized the highest denticity DFO derivatives to date: dodecadentate DFO2 and DFO2p. In this study, we describe the synthesis and characterization of a novel DFO-based chelator, DFO2p, which is comprised of two DFO strands connected by an p-NO₂-phenyl linker and therefore contains double the chelating moieties of DFO (potential coordination number up to 12 vs 6). The chelator DFO2p offers an optimized synthesis comprised of only a single reaction step and improves water solubility relative to DFO2, but the shorter linker reduces molecular flexibility. Both DFO2 and DFO2p, each with 6 potential hydroxamate ligands, are able to reach a more energetically favorable 8-coordinate environment for Zr(IV) than DFO. The zirconium(IV) coordination environment of these complexes were evaluated by a combination of density functional theory (DFT) calculations and synchrotron spectroscopy (extended X-ray absorption fine structure), which suggest the inner-coordination sphere of zirconium(IV) to be comprised of the outermost four hydroxamate ligands. These results also confirm a single Zr(IV) in each chelator, and the hydroxide ligands which complete the coordination sphere of Zr(IV)-DFO are absent from Zr(IV)-DFO2 and Zr(IV)-DFO2p. Radiochemical stability studies with zirconium-89 revealed the order of real-world stability to be DFO2 > DFO2p ≫ DFO. The zirconium-89 complexes of these new high-denticity chelators were found to be far more stable than DFO, and the decreased molecular flexibility of DFO2p, relative to DFO2, could explain its decreased stability, relative to DFO2.

Two-Dimensional Zr/Hf-Hydroxamate Metal-Organic Frameworks

Novel two-dimensional kagome metal-organic frameworks with mononuclear Zr4+/Hf4+ nodes chelated by benzene-1,4-dihydroxamate linkers were synthesized. The MOFs, namely SUM-1, are chemically robust and kinetically favorable, as confirmed by theoretical and...

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.

Directing macrocyclic architecture using iron(III)-, gallium(III)-, or zirconium(IV)-assisted ring closure of linear dimeric endo-hydroxamic acid ligands

Dimeric hydroxamic acid macrocycles are a subclass of bacterial siderophores produced for iron acquisition. Limited yields from natural sources provides the impetus to develop synthetic routes to improve access to these compounds, which have potential utility in metal ion binding applications in the environment and medicine. This work has examined the role of metal ions in forming pre-complexes with linear endo-hydroxamic acid (endo-HXA) ligands bearing terminal amine and carboxylic acid groups optimally configured for in situ ring closure reactions. The 1:1 reaction between Fe(III) and the dimeric endo-HXA ligand 5-((5-(5-((5-aminopentyl)(hydroxy)amino)-5-oxopentanamido)pentyl)(hydroxy)amino)-5-oxopentanoic acid (PPH-PPH) (1) formed the pre-complex (PC) [Fe(PP-PP)-PC]⁺ with in situ amide coupling generating the macrocycle (MC) [Fe(PP)₂-MC]⁺ and, following Fe(III) removal, the apo-macrocycle 1,13-dihydroxy-1,7,13,19-tetraazacyclotetracosane-2,6,14,18-tetraone (PPH)₂-MC (2). The 1:2 reaction system between Fe(III) and the monomeric endo-HXA ligand 5-((5-aminopentyl)(hydroxy)amino)-5-oxopentanoic acid (PPH) gave significantly less [Fe(PP)₂-MC]⁺ than the former system, due to the requirement to form two rather than one amide bond(s). The 1:1 Ga(III):1 system yielded [Ga(PP-PP)-PC]⁺ and [Ga(PP)₂-MC]⁺. Neither [Zr(PP-PP)-PC]²⁺ nor [Zr(PP)₂-MC]²⁺ was detected in the 1:1 Zr(IV):1 system. Instead, the Zr(IV) system showed the formation of a 1:2 Zr(IV):1 pre-complex [Zr(PP-PP)₂-PC], which following in situ amide bond forming chemistry, generated two Zr(IV) macrocyclic complexes with distinct architectures: a dimer-of-dimers complex [Zr((PP)₂)₂-MC] and an end-to-end macrocycle [Zr(PP)₄-MC]. The formation of [Fe(PP)₂-MC]⁺, [Ga(PP)₂-MC]⁺ or [Zr((PP)₂)₂-MC] was confirmed from reconstitution experiments with 2. The work has shown that the choice of metal ion in metal-assisted ring closure reactions directs the assembly of macrocyclic complexes with distinct architectures.

Structural Characterization of the Solution Chemistry of Zirconium(IV) Desferrioxamine: A Coordination Sphere Completed by Hydroxides

Positron emission tomography (PET) using radiolabeled, monoclonal antibodies has become an effective, noninvasive method for tumor detection and is a critical component of targeted radionuclide therapy. Metal ion chelator and bacterial siderophore desferrioxamine (DFO) is the gold standard compound for incorporation of zirconium-89 in radiotracers for PET imaging because it is thought to form a stable chelate with [⁸⁹Zr]Zr⁴⁺. However, DFO may not bind zirconium-89 tightly in vivo, with free zirconium-89 reportedly liberated into the bones of experimental mouse models. Although high bone uptake has not been observed to date in humans, this potential instability has been proposed to be related to the unsaturated coordination sphere of [⁸⁹Zr]Zr-DFO, which is thought to consist of the 3 hydroxamate groups of DFO and 1 or 2 water molecules. In this study, we have used a combination of X-ray absorption spectroscopy and density functional theory (DFT) geometry optimization calculations to further probe the coordination chemistry of this complex in solution. We find the extended X-ray absorption fine structure (EXAFS) curve fitting of an aqueous solution of Zr(IV)-DFO to be consistent with an 8-coordinate Zr with oxygen ligands. DFT calculations suggest that the most energetically favorable Zr(IV) coordination environment in DFO likely consists of the 3 hydroxamate ligands from DFO, each with bidentate coordination, and 2 hydroxide ligands. Further EXAFS curve fitting provides additional support for this model. Therefore, we propose that the coordination sphere of Zr(IV)-DFO is most likely completed by 2 hydroxide ligands rather than 2 water molecules, forming Zr(DFO)(OH)₂.

Reply to the 'Comment on "Investigation of Zr(iv) and 89Zr(iv) complexation with hydroxamates: progress towards designing a better chelator than desferrioxamine B for immuno-PET imaging"' by A. Bianchi and M. Savastano, Chem. Commun., 2020, 56, D0CC01189D

The alternative analysis of A. Bianchi and M. Savastano is a valuable contribution to the understanding of the complex systems at stake in the complexation chemistry of Zr⁴⁺ by considering polynuclear species. Placed in the context of nuclear medicine where such aggregates are unlikely and considering recent literature data, this however points out that no clear agreement exists to describe such complex formation.

A High-Denticity Chelator Based on Desferrioxamine for Enhanced Coordination of Zirconium-89

Herein we report a new high-denticity chelator based on the iron siderophore desferrioxamine (DFO). Our new chelator-DFO2-is acyclic and was designed and synthesized with the purpose of improving the coordination chemistry and radiolabeling performance with radioactive zirconium-89. The radionuclide zirconium-89 ([⁸⁹Zr]Zr⁴⁺) has found wide use for positron emission tomography (PET) imaging when it is coupled with proteins, antibodies, and nanoparticles. DFO2 has a potential coordination number of 12, which uniquely positions this chelator for binding large, high-valent, and oxophilic metal ions. Following synthesis of the DFO2 chelator and the [^natZr]Zr-(DFO2) complex we performed density functional theory calculations to study its coordination sphere, followed by zirconium-89 radiolabeling experiments for comparisons with the "gold standard" chelator DFO. DFO (CN 6) can coordinate with zirconium in a hexadentate fashion, leaving two open coordination sites where water is thought to coordinate (total CN 8). DFO2 (potential CN 12, dodecadentate) can saturate the coordination sphere of zirconium with four hydroxamate groups (CN 8), with no room left for water to directly coordinate, and only binds a single atom of zirconium per chelate. Following quantitative radiolabeling with zirconium-89, the preformed [⁸⁹Zr]Zr-(DFO) and [⁸⁹Zr]Zr-(DFO2) radiometal-chelate complexes were subjected to a battery of in vitro stability challenges, including human blood serum, apo-transferrin, serum albumin, iron, hydroxyapatite, and EDTA. One objective of these stability challenges was to determine if the increased denticity of DFO2 over that of DFO imparted improved complex stability, and another was to determine which of these assays is most relevant to perform with future chelators. In all of the assays DFO2 showed superior stability with zirconium-89, except for the iron challenge, where both DFO2 and DFO were identical. Substantial differences in stability were observed for human blood serum using a precipitation method of analysis, apo-transferrin, hydroxyapatite, and EDTA challenges. These results suggest that DFO2 is a promising next-generation scaffold for zirconium-89 chelators and holds promise for radiochemistry with even larger radionuclides, which we anticipate will expand the utility of DFO2 into theranostic applications.

Radiolabelling of the octadentate chelators DFO* and oxoDFO* with zirconium-89 and gallium-68

In recent years, clinical imaging with zirconium-89 (⁸⁹Zr)-labelled monoclonal antibodies (Ab) by positron emission tomography (immunoPET) has been gaining significant importance in nuclear medicine for the diagnosis of different types of cancer. For complexation of the radiometal ⁸⁹Zr and its attachment to the Ab, chelating agents are required. To date, only the hexadentate chelator desferrioxamine (DFO) is applied in the clinic for this purpose. However, there is increasing preclinical evidence that the [⁸⁹Zr]Zr-DFO complex is not sufficiently stable and partly releases the radiometal in vivo due to the incomplete coordination sphere of the metal. This leads to unfavourable unspecific uptake of the osteophilic radiometal in bones, hence decreasing the signal-to-noise-ratio and leading to an increased dose to the patient. In the past, several new chelators with denticities > 6 have been published, notably the octadentate DFO derivative DFO*. DFO*, however, shows limited water solubility, wherefore an oxygen containing analogue, termed oxoDFO*, was developed in 2017. However, no data on the suitability of oxoDFO* for radiolabelling with ⁸⁹Zr has yet been reported. In this proof-of-concept study, we present the first radiolabelling results of the octadentate, water-soluble chelator oxoDFO*, as well as the in vitro stability of the resulting complex [⁸⁹Zr]Zr-oxoDFO* in comparison to the analogous octadentate, but less water-soluble derivative DFO* and the current "standard" chelator DFO. In addition, the suitability of DFO* and oxoDFO* for radiolabeling with the short-lived PET metal gallium-68 is discussed. The water-soluble, octadentate chelator oxoDFO* provides stable complexes with the positron emitter Zirconium-89. The radiolabelling can be performed at room temperature and neutral pH and thus, oxoDFO* represents a promising chelator for applications in immunoPET.

Snapshots of Dynamic Adaptation: Two-Dimensional Molecular Architectonics with Linear Bis-Hydroxamic Acid Modules

Linear modules equipped with two terminal hydroxamic acid groups act as the building block of diverse two-dimensional supramolecular motifs and patterns with room-temperature stability on the close-packed single-crystal surfaces of silver and gold, revealing a complex self-assembly scenario. By combining multiple investigation techniques (scanning tunneling microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations), we analyze the characteristics of the ordered assemblies which range from close-packed structures to polyporous networks featuring an exceptionally extended primitive unit cell with a side length exceeding 7 nm. The polyporous network shows potential for hosting and promoting the formation of chiral supramolecules, whereas a transition from 1D chiral randomness to an ordered racemate is discovered in a different porous phase. We correlate the observed structural changes to the adaptivity of the building block and surface-induced changes in the chemical state of the hydroxamic acid functional group.

The chemical biology and coordination chemistry of putrebactin, avaroferrin, bisucaberin, and alcaligin

Dihydroxamic acid macrocyclic siderophores comprise four members: putrebactin (putH₂), avaroferrin (avaH₂), bisucaberin (bisH₂), and alcaligin (alcH₂). This mini-review collates studies of the chemical biology and coordination chemistry of these macrocycles, with an emphasis on putH₂. These Fe(III)-binding macrocycles are produced by selected bacteria to acquire insoluble Fe(III) from the local environment. The macrocycles are optimally pre-configured for Fe(III) binding, as established from the X-ray crystal structure of dinuclear [Fe₂(alc)₃] at neutral pH. The dimeric macrocycles are biosynthetic products of two endo-hydroxamic acid ligands flanked by one amine group and one carboxylic acid group, which are assembled from 1,4-diaminobutane and/or 1,5-diaminopentane as initial substrates. The biosynthesis of alcH₂ includes an additional diamine C-hydroxylation step. Knowledge of putH₂ biosynthesis supported the use of precursor-directed biosynthesis to generate unsaturated putH₂ analogues by culturing Shewanella putrefaciens in medium supplemented with unsaturated diamine substrates. The X-ray crystal structures of putH₂, avaH₂ and alcH₂ show differences in the relative orientations of the amide and hydroxamic acid functional groups that could prescribe differences in solvation and other biological properties. Functional differences have been borne out in biological studies. Although evolved for Fe(III) acquisition, solution coordination complexes have been characterised between putH₂ and oxido-V(IV/V), Mo(VI), or Cr(V). Retrosynthetic analysis of 1:1 complexes of [Fe(put)]⁺, [Fe(ava)]⁺, and [Fe(bis)]⁺ that dominate at pH < 5 led to a forward metal-templated synthesis approach to generate the Fe(III)-loaded macrocycles, with apo-macrocycles furnished upon incubation with EDTA. This mini-review aims to capture the rich chemistry and chemical biology of these seemingly simple compounds.

Recent applications of a single quadrupole mass spectrometer in 11C, 18F and radiometal chemistry

Mass spectrometry (MS) has longstanding applications in radiochemistry laboratories, stemming from carbon-dating. However, research on the development of radiotracers for molecular imaging with either positron emission tomography (PET) or single photon emission computed tomography has yet to take full advantage of MS. This inertia has been attributed to the relatively low concentrations of radiopharmaceutical formulations and lack of access to the required MS equipment due to the high costs for purchase and maintenance of specialized MS systems. To date, single quadrupole (SQ)-MS coupled to liquid chromatography (LC) systems is the main form of MS that has been used in radiochemistry laboratories. These LC/MS systems are primarily used for assessing the chemical purity of radiolabeling precursor or standard molecules but also have applications in the determination of metabolites. Herein, we highlight personal experiences using a compact SQ-MS in our PET radiochemistry laboratories, to monitor the small amounts of carrier observed in most radiotracer preparations, even at high molar activities. The use of a SQ-MS in the observation of the low mass associated with non-radioactive species which are formed along with the radiotracer from the trace amounts of carrier found is demonstrated. Herein, we describe a pre-concentration system to detect dilute radiopharmaceutical formulations and metabolite analyses by SQ-MS. Selected examples where SQ-MS was critical for optimization of radiochemical reactions and for unequivocal characterization of radiotracers are showcased. We also illustrate examples where SQ-MS can be applied in identification of radiometal complexes and development of a new purification methodology for Pd-catalyzed radiofluorination reactions, shedding light on the identity of metal complexes present in the labelling solution.

The chemistry of PET imaging with zirconium-89

This Tutorial Review aims to provide an overview of the use of zirconium-89 complexes in biomedical imaging. Over the past decade there have been many new papers in this field, ranging from chemistry through to preclinical and clinical applications. Here we attempt to summarise the main developments that have occurred in this period. The primary focus is on coordination chemistry but other aspects such as isotope production, isotope properties, handling and radiochemical techniques and characterisation of cold and labelled complexes are included. Selected results from animal and human clinical studies are presented in the context of the stabilities and properties of the labelled bioconjugates.

Multifunctional Desferrichrome Analogues as Versatile 89Zr(IV) Chelators for ImmunoPET Probe Development

New bifunctional hexa- and octadentate analogues of the hydroxamate-containing siderophore desferrichrome (DFC) have been synthesized and evaluated as ⁸⁹Zr-chelating agents for immunoPET applications. The in vitro and in vivo inertness of these new ligands, Orn3-hx (hexadentate) and Orn-4hx derivatives (octadentate), was compared to the gold standard hexadentate, hydroxamate-containing chelator for ⁸⁹Zr desferrioxamine (DFO). Density functional theory was employed to model the geometries of the resulting Zr(IV) complexes and to predict their relative stabilities as follows: Zr(Orn4-hx) > Zr(DFC) > Zr(Orn3-hx). Transchelation challenge experiments of the corresponding radiochemical complexes with excess ethylenediaminetetraacetate (EDTA) indicated complex stability in accordance with DFT calculations. Radiolabeling of these ligands with ⁸⁹Zr was quantitative (0.25 μmol of ligand, pH 7.4, room temperature, 20 min). For antibody conjugation, the isothiocyanate (NCS) functional group was introduced to the N terminus of Orn3-hx and Orn-4hx. An additional trifunctional derivative that bears a silicon-rhodamine fluorophore on the C-terminus and NCS on the N terminus was also furnished. As proof of concept, all NCS derivatives were conjugated to the HER2-targeting antibody, trastuzumab. Radiolabeling of immunoconjugates with ⁸⁹Zr was accomplished with radiochemical yields of 16 ± 2% to 95 ± 2%. These constructs were administered to naive mice (male, C57BL/6J, n = 4) to assess in vivo inertness, which is inversely correlated with uptake of ⁸⁹Zr in bone, after 96 h circulation time. We found bone uptake to range from 7.0 ± 2.2 to 10.7 ± 1.3% ID/g, values that compare well to the corresponding DFO conjugate (7.1 ± 0.8% ID/g). In conclusion, we have rationally designed linear, bifunctional and trifunctional desferrichrome analogues suitable for the mild and inert radiolabeling of antibodies with the radionuclide ⁸⁹Zr.

Structural Basis for Xenosiderophore Utilization by the Human Pathogen Staphylococcus aureus

Staphylococcus aureus produces a cocktail of metallophores (staphylopine, staphyloferrin A, and staphyloferrin B) to scavenge transition metals during infection of a host. In addition, S. aureus displays the extracellular surface lipoproteins FhuD1 and FhuD2 along with the ABC transporter complex FhuCBG to facilitate the use of hydroxamate xenosiderophores such as desferrioxamine B (DFOB) for iron acquisition. DFOB is used as a chelation therapy to treat human iron overload diseases and has been linked to an increased risk of S. aureus infections. We used a panel of synthetic DFOB analogs and a FhuD2-selective trihydroxamate sideromycin to probe xenosiderophore utilization in S. aureus and establish structure-activity relationships for Fe(III) binding, FhuD2 binding, S. aureus growth promotion, and competition for S. aureus cell entry. Fe(III) binding assays and FhuD2 intrinsic fluorescence quenching experiments revealed that diverse chemical modifications of the terminal ends of linear ferrioxamine siderophores influences Fe(III) affinity but not FhuD2 binding. Siderophore-sideromycin competition assays and xenosiderophore growth promotion assays revealed that S. aureus SG511 and ATCC 11632 can distinguish between competing siderophores based exclusively on net charge of the siderophore-Fe(III) complex. Our work provides a roadmap for tuning hydroxamate xenosiderophore scaffolds to suppress (net negative charge) or enhance (net positive or neutral charge) uptake by S. aureus for applications in metal chelation therapy and siderophore-mediated antibiotic delivery, respectively.

Exploiting the biosynthetic machinery of Streptomyces pilosus to engineer a water-soluble zirconium(iv) chelator

A combined microbiology-chemistry approach has been used to generate a water-soluble chain-extended octadentate hydroxamic acid designed as a high affinity and selective Zr(iv) ligand.

A solid phase-assisted approach for the facile synthesis of a highly water-soluble zirconium-89 chelator for radiopharmaceutical development

The solid-phase synthesis of a water-soluble bifunctional chelator for ⁸⁹Zr is reported.