The influence of the combination of carboxylate and phosphinate pendant arms in 1,4,7-triazacyclononane-based chelators on their 68Ga labelling properties

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.

Divalent 2-(4-Hydroxyphenyl)benzothiazole Bifunctional Chelators for 64Cu Positron Emission Tomography Imaging in Alzheimer's Disease

Herein, we report a new series of divalent 2-(4-hydroxyphenyl)benzothiazole bifunctional chelators (BFCs) with high affinity for amyloid β aggregates and favorable lipophilicity for blood-brain barrier penetration. The addition of an alkyl carboxylate ester pendant arm offers high binding affinity toward Cu(II). The novel BFCs form stable 64Cu-radiolabeled complexes and exhibit promising partition coefficient (logD) values of 1.05-1.85. Among the five compounds tested, the 64Cu-YW-15 complex exhibits significant staining of amyloid β plaques in ex vivo autoradiography studies. In addition, biodistribution studies show that 64Cu-YW-15-Me exhibits moderate brain uptake (0.69 ± 0.08 %ID/g) in wild type mice.

PIDAZTA: Structurally Constrained Chelators for the Efficient Formation of Stable Gallium-68 Complexes at Physiological pH

Two structurally constrained chelators based on a fused bicyclic scaffold, 4-amino-4-methylperhydro-pyrido[1,2-a][1,4]diazepin-N,N',N'-triacetic acids [(4R*,10aS*)-PIDAZTA (L1) and (4R*,10aR*)-PIDAZTA (L2)], were designed for the preparation of Ga^III -based radiopharmaceuticals. The stereochemistry of the ligand scaffold has a deep impact on the properties of the complexes, with unexpected [Ga(L2)OH] species being superior in terms of both thermodynamic stability and inertness. This peculiar behavior was rationalized on the basis of molecular modeling and appears to be related to a better fit in size of Ga^III into the cavity of L2. Fast and efficient formation of the Ga^III chelates at room temperature was observed at pH values between 7 and 8, which enables ⁶⁸ Ga radiolabeling under truly physiological conditions (pH 7.4).

Site-Specific Immuno-PET Tracer to Image PD-L1

The rapid ascension of immune checkpoint blockade treatments has placed an emphasis on the need for viable, robust, and noninvasive imaging methods for immune checkpoint proteins, which could be of diagnostic value. Immunoconjugate-based positron emission tomography (immuno-PET) allows for sensitive and quantitative imaging of target levels and has promising potential for the noninvasive evaluation of immune checkpoint proteins. However, the advancement of immuno-PET is currently limited by available imaging tools, which heavily rely on full-length IgGs with Fc-mediated effects and are heterogeneous mixtures upon random conjugation with chelators for imaging. Herein, we have developed a site-specific αPD-L1 Fab conjugate with the chelator 1,4,7-triazacyclononane- N, N', N″-triacetic acid (NOTA), enabling radiolabeling for PET imaging, using the amber suppression-mediated genetic incorporation of unnatural amino acid (UAA), p-azidophenylalanine. This Fab conjugate is homogeneous and demonstrated tight binding toward the PD-L1 antigen in vitro. The radiolabeled version, ⁶⁴Cu-NOTA-αPD-L1, has been employed in PET imaging to allow for effective visualization and mapping of the biodistribution of PD-L1 in two normal mouse models, including the capturing of different PD-L1 expression levels in the spleens of the different mouse types. Follow-up in vivo blocking studies and ex vivo fluorescent staining further validated specific tissue uptakes of the imaging agent. This approach illustrates the utility of UAA-based site-specific Fab conjugation as a general strategy for making sensitive PET imaging probes, which could facilitate the elucidation of the roles of a wide variety of immune checkpoint proteins in immunotherapy.

Recent developments in the synthesis and applications of phosphinic peptide analogs

Synthetic pseudopeptides that fit well with the active site architecture allow the most effective binding to enzymes, similar to native substrates in high-energy transition states. Phosphinic acid peptide analogs that comprise the tetrahedral phosphorus moiety introduced to replace an internal amide bond exert such an isosteric or isoelectronic resemblance, combined with providing other advantageous features, for example, metal complexing properties. Accordingly, they are capable of inhibiting metal-dependent enzymes involved in biological functions in eukaryotic and prokaryotic cells. These enzymes are associated with notorious human diseases, such as cancer, e.g., matrix metalloproteinases, or are etiological factors of protozoal and bacterial infections, e.g., metalloaminopeptidases. The affinity and selectivity of these compounds can be conveniently adjusted, either by structural modification of dedicated side chains or by backbone elongation to enhance specific interactions with the corresponding binding pockets. Recent approaches to the synthesis of these compounds are illustrated by examples of the preparation of rationally designed structures of inhibitors of particular enzymes. Activity against appealing enzymatic targets is presented, along with the molecular mechanisms of action and therapeutic implications. Innovative aspects of phosphinic peptide application, e.g., as activity-based probes, and ligands of complexes of radioisotopes for nuclear medicine are also outlined.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Synthesis and Evaluation of New Bifunctional Chelators with Phosphonic Acid Arms for Gallium-68 Based PET Imaging in Melanoma

Due to the increasing use of generator-produced radiometal Gallium-68 (⁶⁸Ga) in positron-emission tomography/computed tomography (PET/CT), reliable bifunctional chelators that can efficiently incorporate ⁶⁸Ga³⁺ into biomolecules are highly desirable. In this study, we synthesized two new bifunctional chelators bearing one or two phosphonic acid functional groups, named p-SCN-PhPr-NE2A1P and p-SCN-PhPr-NE2P1A, with the aim of enabling facile production of ⁶⁸Ga-based radiopharmaceuticals. Both chelators were successfully conjugated to LLP2A-PEG₄, a very late antigen-4 (VLA-4) targeting peptidomimetic ligand, to evaluate their application in ⁶⁸Ga-based PET imaging. NE2P1A-PEG₄-LLP2A exhibited the highest ⁶⁸Ga³⁺ binding ability with molar activity of 37 MBq/nmol under mild temperature and neutral pH. Excellent serum stability of ⁶⁸Ga-NE2P1A-PEG₄-LLP2A was observed, which was consistent with the result obtained from density functional theory calculation. The in vitro cell study showed that ⁶⁸Ga-NE2P1A-PEG₄-LLP2A had significantly longer retention in B16F10 cells comparing to the reported retention of ⁶⁴Cu-NE3TA-PEG₄-LLP2A, although the uptake was relatively lower. In the biodistribution and micro-PET/CT imaging studies, high tumor uptake and low background were observed after ⁶⁸Ga-NE2P1A-PEG₄-LLP2A was injected into mice bearing B16F10 tumor xenografts, making it a highly promising radiotracer for noninvasive imaging of VLA-4 receptors overexpressed in melanoma.

Density Functional Theory Study on the Complexation of NOTA as a Bifunctional Chelator with Radiometal Ions

1,4,7-Triazacyclononane-1,4,7-triacetic acid (NOTA) is a key bifunctional chelator utilized for the complexation of metal ions in radiopharmaceutical applications; the ability of these chelators depends on the strength of their binding with ions. The focus of the present work is to evaluate the complexation of Cu²⁺, Ga³⁺, Sc³⁺, and In³⁺ radiometal ions with NOTA using density functional theory (B3LYP functional) and 6-311+G(2d,2p)/DGDZVP basis sets. The significant role of ion-water interactions in the chelation interaction energies in solution reflects the competition between ion-water and NOTA-ion interaction in the chelation process. There is reasonable agreement between experimental and theoretical binding constants, geometries, and ¹H NMR chemical shifts. Chelation interaction energies, Gibbs free energies, and entropies in solution show that the NOTA-Ga³⁺ and NOTA-Cu²⁺ are the most and least stable complexes, respectively. The natural atomic charges and second order perturbation analysis reveal charge transfer between NOTA and radiometal ions. The theoretical ¹H NMR chemical shifts of NOTA are in good agreement with experiment; these values are influenced by the presence of the ions, which have a deshielding effect on the protons of NOTA. Global scalar properties such as E_HOMO/E_LUMO, ΔE_LUMO-HOMO, and chemical hardness/softness confirm that the NOTA-Cu²⁺ complex, which has a singly occupied molecular orbital, has the lowest ΔE_LUMO-HOMO value, the least chemical hardness, and the highest chemical softness. The significant variation of the hardness and ΔE_LUMO-HOMO values of the complexes can be attributed to the different positions of the metal ions on the periodic table. This study affirms that, among the radiometal ions, Ga³⁺ can be used to effectively radiolabel NOTA chelator for radiopharmaceutical usage as it binds most stably with NOTA.