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

Trained immunity is regulated by T cell-induced CD40-TRAF6 signaling

Trained immunity is characterized by histone modifications and metabolic changes in innate immune cells following exposure to inflammatory signals, leading to heightened responsiveness to secondary stimuli. Although our understanding of the molecular regulation of trained immunity has increased, the role of adaptive immune cells herein remains largely unknown. Here, we show that T cells modulate trained immunity via cluster of differentiation 40-tissue necrosis factor receptor-associated factor 6 (CD40-TRAF6) signaling. CD40-TRAF6 inhibition modulates functional, transcriptomic, and metabolic reprogramming and modifies histone 3 lysine 4 trimethylation associated with trained immunity. Besides in vitro studies, we reveal that single-nucleotide polymorphisms in the proximity of CD40 are linked to trained immunity responses in vivo and that combining CD40-TRAF6 inhibition with cytotoxic T lymphocyte antigen 4-immunoglobulin (CTLA4-Ig)-mediated co-stimulatory blockade induces long-term graft acceptance in a murine heart transplantation model. Combined, our results reveal that trained immunity is modulated by CD40-TRAF6 signaling between myeloid and adaptive immune cells and that this can be leveraged for therapeutic purposes.

Construction of [89Zr]Zr-Labeled HuL13 for ImmunoPET Imaging of LAG-3 Checkpoint Expression on Tumor-Infiltrating T Cells

Lymphocyte activation gene 3 (LAG-3) has attracted much attention as a potentially valuable immune checkpoint. Individual identification of LAG-3 expression at screening and during treatment could improve the successful implementation of anti-LAG-3 therapies. HuL13 is a human IgG1 monoclonal antibody that binds to the LAG-3 receptor in T cells. Here, we used [89Zr]Zr-labeled HuL13 to delineate LAG-3+ T-cell infiltration into tumors via positron emission tomography (PET) imaging. A549/LAG-3 cells, which stably express LAG-3, were generated by infection with lentivirus. The uptake of [89Zr]Zr-DFO-HuL13 in A549/LAG-3 cells was greater than that in the negative control (A549/NC) cells at each time point. The equilibrium dissociation constant (Kd) of [89Zr]Zr-DFO-HuL13 for the LAG-3 receptor was 8.22 nM. PET imaging revealed significant uptake in the tumor areas of A549/LAG-3 tumor-bearing mice from 24 h after injection (SUVmax = 2.43 ± 0.06 at 24 h). As a proof of concept, PET imaging of the [89Zr]Zr-DFO-HuL13 tracer was further investigated in an MC38 tumor-bearing humanized LAG-3 mouse model. PET imaging revealed that the [89Zr]Zr-DFO-HuL13 tracer specifically targets human LAG-3 expressed on tumor-infiltrating lymphocytes (TILs). In addition to the tumors, the spleen was also noticeably visible. Tumor uptake of the [89Zr]Zr-DFO-HuL13 tracer was lower than its uptake in the spleen, but high uptake in the spleen could be reduced by coinjection of unlabeled antibodies. Coinjection of unlabeled antibodies increases tracer activity in the blood pool, thereby improving tumor uptake. Dosimetry evaluation of the healthy mouse models revealed that the highest absorbed radiation dose was in the spleen, followed by the liver and heart wall. In summary, these studies demonstrate the feasibility of using the [89Zr]Zr-DFO-HuL13 tracer for the detection of LAG-3 expression on TILs. Further clinical evaluation of the [89Zr]Zr-DFO-HuL13 tracer may be of significant help in the stratification and management of patients suitable for anti-LAG-3 therapy.

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.

Zirconium Coordination Chemistry and Its Role in Optimizing Hydroxymate Chelation: Insights from Molecular Dynamics

In the past decade, there has been a growth in using Zirconium-89 (89Zr) as a radionuclide in nuclear medicine for cancer diagnostic imaging and drug discovery processes. Although one of the most popular chelators for 89Zr, desferrioxamine (DFO) is typically presented as a hexadentate ligand, our work suggests a different scenario. The coordination structure of the Zr4+-DFO complex has primarily been informed by DFT-based calculations, which typically ignore temperature and therefore entropic and dynamic solvent effects. In this work, free energy calculations using molecular dynamics simulations, where the conformational fluctuations of both the ligand and the solvent are explicitly included, are used to compare the binding of Zr4+ cations with two different chelators, DFO and 4HMS, the latter of which is an octadentate ligand that has been recently proposed as a better chelator due to the presence of four hydroxymate groups. We find that thermally induced disorder leads to an open hexadentate chelate structure of the Zr4+-DFO complex, leaving the Zr4+ metal exposed to the solvent. A stable coordination of Zr4+ with 4HMS, however, is formed by involving both hydroxamate groups and water molecules in a more closely packed structure.

Evaluation of coumarin-tagged deferoxamine as a Zr(IV)-based PET/fluorescence dual imaging probe

Desferoxamine (DFO) is currently the golden standard chelator for 89Zr4+, a promising nuclide for positron emission tomography imaging (PET). The natural siderophore DFO had previously been conjugated with fluorophores to obtain Fe(III) sensing molecules. In this study, a fluorescent coumarin derivative of DFO (DFOC) has been prepared and characterized (potentiometry, UV-Vis spectroscopy) for what concerns its protonation and metal coordination properties towards PET-relevant ions (Cu(II), Zr(IV)), evidencing strong similarity with pristine DFO. Retention of DFOC fluorescence emission upon metal binding has been checked (fluorescence spectrophotometry), as it would - and does - allow for optical (fluorescent) imaging, thus unlocking bimodal (PET/fluorescence) imaging for 89Zr(IV) tracers. Crystal violet and MTT assays on NIH-3 T3 fibroblasts and MDA-MB 231 mammary adenocarcinoma cell lines demonstrated, respectively, no cytotoxicity nor metabolic impairment at usual radiodiagnostic concentrations of ZrDFOC. Clonogenic colony-forming assay performed on X-irradiated MDA-MB 231 cells showed no interference of ZrDFOC with radiosensitivity. Morphological biodistribution (confocal fluorescence, transmission electron microscopy) assays on the same cells suggested internalization of the complex through endocytosis. Overall, these results support fluorophore-tagged DFO as a suitable option to achieve dual imaging (PET/fluorescence) probes based on 89Zr.

Metal Coordination Properties of a Chromophoric Desferrioxamine (DFO) Derivative: Insight on the Coordination Stoichiometry and Thermodynamic Stability of Zr4+ Complexes

Desferrioxamine (DFO) is the current "gold standard" chelator for ⁸⁹Zr⁴⁺, which is used to label monoclonal antibodies for applications in immunopositron emission tomography. Recently, controversial data have been reported regarding the speciation and the stability of the complexes formed by DFO with Zr⁴⁺ in solution. To shed some light on this point, we studied the coordination properties in solution ofa chromophoric DFO derivative bearing a substituted pyrimidine residue (DFO-Pm) toward several metal ions (Zr⁴⁺, Cu²⁺, Zn²⁺, Mg²⁺, Ca²⁺, Na⁺, K⁺). Potentiometric titrations showed that DFO-Pm and pristine DFO form complexes with very similar stoichiometry and stability. DFO-Pm, which can consequently be taken as a model system for DFO, provides a photochemical response to metal coordination that can be used to further define the complexes formed. In the critical case of Zr⁴⁺, spectrophotometric measurements allowed the verification of the formation of 1:1 and 2:3 complexes that, together with 2:2 complexes form the coordination model that was obtained through the use of our potentiometric measurements. Additionally, mass spectrometry measurements verified the formation of 1:1 and 2:3 complexes and showed that 1:2 species can be easily generated through the fragmentation of the 2:3 species. In conclusion, the results obtained with DFO-Pm validate the complexation model of Zr⁴⁺/DFO composed of 1:1, 2:2, and 2:3 metal-to-ligand complexes. Convergences and conflicts with other works are addressed.