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

Synthesis and evaluation of bifunctional DFO2K: a modular chelator with ideal properties for zirconium-89 chelation

The synthesis and evaluation of the newest generation of our DFO2 chelator family-DFO2K-is described. DFO2K was designed with a simple synthetic route to access different bifunctional derivatives, with each derivative having similar metal ion coordination spheres and high denticity (up to 12 coordinate) to ensure stable coordination of zirconium-89. The high denticity could potentially enhance stability with other large oxophilic radiometals. Zirconium-89 is the most popular radionuclide to pair with large macromolecules such as antibodies (immunoPET) for positron emission tomography applications. Although clinically successful, the stability of the "gold standard" chelator desferrioxamine B (DFO) can be improved as significant bone uptake is observed in animal models, despite no obvious stability issues in humans. Following the synthesis of DFO2K we assessed its radiolabeling efficiency with zirconium-89 and compared with DFO, which revealed rapid and nearly identical radiolabeling kinetics to DFO. The resultant [89Zr]Zr-DFO2K complex showed improved stability over [89Zr]Zr-DFO in different in vitro stability assays such as hydroxyapatite and 1000-fold molar excess EDTA challenges. Furthermore, biodistribution studies of the non-bifunctional chelators in healthy mice showed that [89Zr]Zr-DFO2K had a similar distribution profile and clearance to [89Zr]Zr-DFO. The bifunctional derivative p-SCN-Ph-DFO2K was conjugated to a non-specific human IgG antibody and evaluated after 2 weeks circulating in healthy female CD1 mice. Mice administered [89Zr]Zr-DFO2K-IgG showed substantially lower bone uptake in PET-CT images than [89Zr]Zr-DFO-IgG, with PET ROI data and ex vivo biodistribution revealing a statistically significantly lower bone uptake for DFO2K. Overall, owing to its high denticity, ease of synthesis, improved solubility over DFO2 and DFO2p, and stable chelation of zirconium-89, DFO2K appears to be an improved alternative chelator to DFO for zirconium-89 chelation.

Combining Desferriferrioxamine B and 1-Hydroxy-2-Piperidone ((PIPO)H) to Chelate Zirconium. Solution Structure of a Model Complex of the [89Zr]Zr-DFOcyclo*-mAb Radioimmunoconjugate

89Zr-immunoPET is a hot topic as 89Zr cumulates the advantages of 64Cu and 124I without their drawbacks. We report the synthesis of a model ligand of a chiral bioconjugable tetrahydroxamic chelator combining the desferriferrioxamine B siderophore and 1-hydroxy-2-piperidone ((PIPO)H), a chiral cyclic hydroxamic acid derivative, and the study by NMR spectroscopy of its zirconium complex. Nuclear Overhauser effect measurements (ROESY) indicated that the complex exists in the form of two diastereomers, in 77 : 23 ratio, resulting from the combination of the central chiralities at the 3-C of the (PIPO)H component and at the Zr4+ cation. The 44 lowest energy structures out of more than 1000 configurations/conformations returned by calculations based on density functional theory were examined. Comparison of the ROESY data and the calculated interatomic H⋅⋅⋅H distances allowed us to select the most probable configuration and conformations of the major complex.

Preparation of a Zirconium-89 Labeled Clickable DOTA Complex and Its Antibody Conjugate

Desferrioxamine B (DFO) is the clinical standard chelator for preparing zirconium-89 labeled antibodies. In the current study, the stabilities of a zirconium-89 labeled panitumumab (PAN; Vectibix®) with three different chelators (DFO, DFO*, and DOTA) were compared. PAN is an anti-HER1/EGFR monoclonal antibody approved by the FDA for the treatment of HER1-expressing colorectal cancers and was used as the model antibody for this study. DFO/DFO* conjugates of PAN were directly radiolabeled with zirconium-89 at room temperature to produce [89Zr]Zr-DFO/DFO*-PAN conjugates following a well-established procedure. A zirconium-89 labeled DOTA-PAN conjugate was prepared by an indirect radiolabeling method. A cyclooctyne-linked DOTA chelator (BCN-DOTA-GA) was first radiolabeled with zirconium-89 at 90 °C under a two-step basic pH adjustment method followed by conjugation with PAN-tetrazene at 37 °C to produce a labeled conjugate, BCN-[89Zr]Zr-DOTA-GA-PAN. High reproducibility of the radiolabeling was observed via this two-step basic pH adjustment. The overall radiochemical yield was 40-50% (n = 12, decay uncorrected) with a radiochemical purity of >95% in 2 h synthesis time. All three conjugates were stable in whole human serum for up to 7 days at 37 °C. The kinetic inertness of the conjugates was assessed against the EDTA challenge. BCN-[89Zr]Zr-DOTA-GA-PAN exhibited excellent inertness followed by [89Zr]Zr-DFO*-PAN. [89Zr]Zr-DFO-PAN displayed the lowest level of inertness.

Transition and Post-Transition Radiometals for PET Imaging and Radiotherapy

Radiometals are an exciting class of radionuclides because of the large number of metallic elements available that have medically useful isotopes. To properly harness radiometals, they must be securely bound by chelators, which must be carefully matched to the radiometal ion to maximize radiolabeling performance and the stability of the resulting complex. This chapter focuses on practical aspects of radiometallation chemistry including chelator selection, radiolabeling procedures and conditions, radiolysis prevention, purification, quality control, requisite equipment and reagents, and useful tips.

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.

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.

Design, Synthesis, and Evaluation of DFO-Em: A Modular Chelator with Octadentate Chelation for Optimal Zirconium-89 Radiochemistry

Zirconium-89 has quickly become a favorite radionuclide among academics and clinicians for nuclear imaging. This radiometal has a relatively long half-life, which matches the biological half-life of most antibodies, suitable decay properties for positron emission tomography (PET), and efficient and affordable cyclotron production and purification. The "gold standard" chelator for [⁸⁹Zr]Zr⁴⁺ is desferrioxamine B (DFO), and although it has been used both preclinically and clinically for immunoPET with great success, it has revealed its suboptimal stability in vivo. DFO can only bind to [⁸⁹Zr]Zr⁴⁺ through its six available coordination sites made up by three hydroxamic acid (HA) moieties, which is not sufficient to saturate the coordination sphere (CN 7-8). In this study, we have designed, synthesized, and characterized a new octadentate chelator we have called DFO-Em, which is an improved derivative of our previously published dodecadentate chelator DFO2. This octadentate DFO-Em chelator is smaller than DFO2 but still satisfies the coordination sphere of zirconium-89 and forms a highly stable radiometal-chelator complex. DFO-Em was synthesized by tethering a hydroxamic acid monomer to commercially available DFO using glutamic acid as a linker, providing an octadentate chelator built on a modular amino acid-based synthesis platform. Radiolabeling performance and radiochemical stability of DFO-Em were assessed in vitro by serum stability, ethylenediamine tetraacetic acid (EDTA), and hydroxyapatite challenges. Furthermore, [⁸⁹Zr]Zr-(DFO-Em) and [⁸⁹Zr]Zr-DFO were injected in healthy mice and measured in vivo by PET/CT imaging and ex vivo biodistribution. Additionally, the coordination of DFO-Em with Zr(IV) and its isomers was studied using density functional theory (DFT) calculations. The radiolabeling studies revealed that DFO-Em has a comparable radiolabeling profile to the gold standard chelator DFO. The in vitro stability evaluation showed that [⁸⁹Zr]Zr-(DFO-Em) was significantly more stable than [⁸⁹Zr]Zr-DFO, and in vivo both had similar clearance in healthy mice with a small decrease in tissue retention for [⁸⁹Zr]Zr-(DFO-Em) at 24 h post injection. The DFT calculations also confirmed that Zr-(DFO-Em) can adopt highly stable 8-coordinate geometries, which along with NMR characterization suggest no fluxional behavior and the presence of a single isomer. The modular design of DFO-Em means that any natural or unnatural amino acid can be utilized as a linker to gain access to different chemistries (e.g., thiol, amine, carboxylic acid, azide) while retaining an identical coordination sphere to DFO-Em.

Heptadentate chelates for 89Zr-radiolabelling of monoclonal antibodies

Zirconium complexation chemistry is an important area of research in the context of developing radiolabelled proteins for applications in diagnostic positron emission tomography (PET) imaging. Herein, we report the synthesis...

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.

Re-Evaluations of Zr-DFO Complex Coordination Chemistry for the Estimation of Radiochemical Yields and Chelator-to-Antibody Ratios of 89Zr Immune-PET Tracers

(1) Background: Deferoxamine B (DFO) is the most widely used chelator for labeling of zirconium-89 (89Zr) to monoclonal antibody (mAb). Despite the remarkable developments of the clinical 89Zr-immuno-PET, chemical species and stability constants of the Zr-DFO complexes remain controversial. The aim of this study was to re-evaluate their stability constants by identifying species of Zr-DFO complexes and demonstrate that the stability constants can estimate radiochemical yield (RCY) and chelator-to-antibody ratio (CAR). (2) Methods: Zr-DFO species were determined by UV and ESI-MS spectroscopy. Stability constants and speciation of the Zr-DFO complex were redetermined by potentiometric titration. Complexation inhibition of Zr-DFO by residual impurities was investigated by competition titration. (3) Results: Unknown species, ZrHqDFO2, were successfully detected by nano-ESI-Q-MS analysis. We revealed that a dominant specie under radiolabeling condition (pH 7) was ZrHDFO, and its stability constant (logβ111) was 49.1 ± 0.3. Competition titration revealed that residual oxalate inhibits Zr-DFO complex formation. RCYs in different oxalate concentration (0.1 and 0.04 mol/L) were estimated to be 86% and >99%, which was in good agreement with reported results (87%, 97%). (4) Conclusion: This study succeeded in obtaining accurate stability constants of Zr-DFO complexes and estimating RCY and CAR from accurate stability constants established in this study.