Catechol-Based Functionalizable Ligands for Gallium-68 Positron Emission Tomography Imaging

"Off-Label Use" of the Siderophore Enterobactin Enables Targeted Imaging of Cancer with Radioactive Ti(IV)

The development of inert, biocompatible chelation methods is required to harness the emerging positron emitting radionuclide 45Ti for radiopharmaceutical applications. Herein, we evaluate the Ti(IV)-coordination chemistry of four catechol-based, hexacoordinate chelators using synthetic, structural, computational, and radiochemical approaches. The siderophore enterobactin (Ent) and its synthetic mimic TREN-CAM readily form mononuclear Ti(IV) species in aqueous solution at neutral pH. Radiolabeling studies reveal that Ent and TREN-CAM form mononuclear complexes with the short-lived, positron-emitting radionuclide 45Ti(IV), and do not transchelate to plasma proteins in vitro and exhibit rapid renal clearance in naïve mice. These features guide efforts to target the 45Ti isotope to prostate cancer tissue through the design, synthesis, and evaluation of Ent-DUPA, a small molecule conjugate composed of a prostate specific membrane antigen (PSMA) targeting peptide and a monofunctionalized Ent scaffold. The [45Ti][Ti(Ent-DUPA)]2- complex forms readily at room temperature. In a tumor xenograft model in mice, selective tumor tissue accumulation (8±5 %, n=5), and low off-target uptake in other organs is observed. Overall, this work demonstrates targeted imaging with 45Ti(IV), provides a foundation for advancing the application of 45Ti in nuclear medicine, and reveals that Ent can be repurposed as a 45Ti-complexing cargo for targeted nuclear imaging applications.

A novel radiolabelled salmochelin derivative for bacteria-specific PET imaging: synthesis, radiolabelling and evaluation

For specific imaging of bacterial infections we aimed at targeting the exclusive bacterial iron transport system via siderophore-based radiotracers. De novo synthesis and radiolabeling yielded the salmochelin-based PET radiotracer [68Ga]Ga-RMA693, which showed a favourable biodistribution and a bacteria-specific uptake in an animal model of Escherichia coli infection.

Evaluation of a novel hexadentate 1,2-hydroxypyridinone-based acyclic chelate, HOPO-O6-C4, for 43Sc/47Sc, 68Ga, and 45Ti radiopharmaceuticals

INTRODUCTION

Chelators play a crucial role in the development of metal-based radiopharmaceuticals, and with the continued interest in 68Ga and increasing availability of new radiometals such as 43Sc/47Sc and 45Ti, there is a growing demand for tailored chelators that can form stable complexes with these metals. This work reports the synthesis and characterization of a hexadentate tris-1,2-hydroxypyridonone chelator HOPO-O6-C4 and its in vitro and in vivo evaluation with the above mentioned radiometals.

METHODS

To investigate the affinity of HOPO-O6-C4, macroscopic studies were performed with Sc3+, and Ga3+ followed by DFT structural optimization of the Sc3+, Ga3+ and Ti4+ complexes. Further tracer studies with 43Sc (and 47Sc), 45Ti, and 68Ga were performed to determine the potential for positron emission tomography (PET) imaging with these complexes. In vitro stability studies followed by in vivo imaging and biodistribution studies were performed to understand the kinetic stability of the resultant radiometal-complexes of HOPO-O6-C4.

RESULTS

Promising radiolabeling results with HOPO-O6-C4 were obtained with 43Sc, 47Sc, 45Ti, and 68Ga radionuclides; rapid radiolabeling was observed at 37 °C and pH 7 in under 30-min. Apparent molar activity measurements were performed for radiolabeling of HOPO-O6-C4 with 43Sc (4.9 ± 0.26 GBq/μmol), 47Sc (1.58 ± 0.01 GBq/μmol), 45Ti (11.5 ± 1.6 GBq/μmol) and 68Ga (5.74 ± 0.7 GBq/μmol), respectively. Preclinical in vivo imaging studies resulted in promising results with [68Ga]Ga-HOPO-O6-C4 indicating a rapid clearance through hepatic excretion route and no decomplexation whereas [43Sc]Sc-HOPO-O6-C4, [47Sc]Sc-HOPO-O6-C4 and [45Ti]Ti-HOPO-O6-C4 showed modest and significant evidence of decomplexation, respectively.

CONCLUSIONS

The tris-1,2-HOPO chelator HOPO-O6-C4 is a promising scaffold for elaboration into a 68Ga- based radiopharmaceutical.

Modern Developments in Bifunctional Chelator Design for Gallium Radiopharmaceuticals

The positron-emitting radionuclide gallium-68 has become increasingly utilised in both preclinical and clinical settings with positron emission tomography (PET). The synthesis of radiochemically pure gallium-68 radiopharmaceuticals relies on careful consideration of the coordination chemistry. The short half-life of 68 min necessitates rapid quantitative radiolabelling (≤10 min). Desirable radiolabelling conditions include near-neutral pH, ambient temperatures, and low chelator concentrations to achieve the desired apparent molar activity. This review presents a broad overview of the requirements of an efficient bifunctional chelator in relation to the aqueous coordination chemistry of gallium. Developments in bifunctional chelator design and application are then presented and grouped according to eight categories of bifunctional chelator: the macrocyclic chelators DOTA and TACN; the acyclic HBED, pyridinecarboxylates, siderophores, tris(hydroxypyridinones), and DTPA; and the mesocyclic diazepines.

A General Design Strategy Enabling the Synthesis of Hydrolysis-Resistant, Water-Stable Titanium(IV) Complexes

Despite its prevalence in the environment, the chemistry of the Ti⁴⁺ ion has long been relegated to organic solutions or hydrolyzed TiO₂ polymorphs. A knowledge gap in stabilizing molecular Ti⁴⁺ species in aqueous environments has prevented the use of this ion for various applications such as radioimaging, design of water-compatible metal-organic frameworks (MOFs), and aqueous-phase catalysis applications. Herein, we show a thorough thermodynamic screening of bidentate chelators with Ti⁴⁺ in aqueous solution, as well as computational and structural analyses of key compounds. In addition, the hexadentate analogues of catechol (benzene-1,2-diol) and deferiprone (3-hydroxy-1,2-dimethyl-4(1H)-pyridone), TREN-CAM and THP^Me respectively, were assessed for chelation of the ⁴⁵ Ti isotope (t_1/2 =3.08 h, β⁺ =85 %, E_β+ =439 keV) towards positron emission tomography (PET) imaging applications. Both were found to have excellent capacity for kit-formulation, and [⁴⁵ Ti]Ti-TREN-CAM was found to have remarkable stability in vivo.

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.

Intracellular Synthesis of Hybrid Gallium-68 Nanoparticle Enhances MicroPET Tumor Imaging

Positron-emission tomography (PET) imaging enables cancer diagnosis at an early stage and to determine its pathological degree. However, tumor uptake efficiency of traditional PET radiotracers is usually low. Herein, we rationally designed a precursor CBT-NODA, the cold analogue CBT-NODA-Ga, and its corresponding radiotracer CBT-NODA-⁶⁸Ga. Using these three compounds, we verified that coinjection of CBT-NODA-⁶⁸Ga with CBT-NODA or CBT-NODA-Ga could lead to the synthesis of hybrid gallium-68 nanoparticles in furin-overexpressing cancer cells and enhance microPET tumor imaging in mice. In vivo experiments showed that coinjection of CBT-NODA-⁶⁸Ga with CBT-NODA-Ga had the most prolonged retention of the radiotracer in blood, the highest radioactivity in tumor regions, and the most enhanced microPET tumor imaging in mice. We anticipate that, by combining the coinjection strategy with our CBT-Cys click condensation reaction, more radiotracers are developed for microPET imaging of more tumors in clinical settings in the future.

Exclusive formation of a meridional complex of a tripodand and perfect suppression of guest recognition

Tripodal ligands have been utilized for complexation-induced structural change, but all the tripodal complexes reported so far are facial isomers, which do not completely reduce the recognition ability by closing the binding pocket. We now report the first example of the selective synthesis of a meridional tripodal complex. The tripodal ligand with a 1,3,5-triethyl-2,4,6-tris(methylene)benzene pivot possessing 2,2'-bipyridine on each arm exclusively formed a mononuclear complex with the mer-[Fe(bpy)]2+ unit. The meridional tripodal complex has a unique structure in which one bipyridine unit is self-penetrated. As a result of cavity blockage, the ion recognition property of the tripodand has been successfully suppressed.