Remarkable Inertness of Copper(II) Chelates of Cyclen-Based Macrobicycles with Two trans-N-Acetate Arms

2-(4-Hydroxyphenyl)benzothiazole dicarboxylate ester TACN chelators for 64Cu PET imaging in Alzheimer's disease

Herein we report a new series of bifunctional chelators (BFCs) with high affinity for amyloid β aggregates, strong binding affinity towards Cu(II), and favorable lipophilicity for potential blood-brain barrier (BBB) penetration. The alkyl carboxylate ester pendant arms show high binding affinity towards Cu(II). The BFCs form stable ⁶⁴Cu-radiolabeled complexes and exhibit favorable partition coefficient (log D) values of 0.75-0.95. Among the five compounds tested, 64Cu-YW-1 and 64Cu-YW-13 complexes exhibit significant staining of amyloid plaques in ex vivo autoradiography studies.

Complexation of C-Functionalized Cyclams with Copper(II) and Zinc(II): Similarities and Changes When Compared to Parent Cyclam Analogues

Herein, we report a comprehensive coordination study of the previously reported ligands cyclam, CB-cyclam, TMC, DMC, and CB-DMC and of their C-functional analogues, cyclam-E, CB-cyclam-E, TMC-E, DMC-E, and CB-DMC-E. This group of ligands includes cyclam, cross-bridged cyclams, their di- or tetramethylated derivatives, and the analogues bearing an additional hydroxyethyl group on one β-N position of the ring. The Cu(II) and Zn(II) complexes of these macrocycles have been highlighted previously for the biological interest, but the details of their structures in the solid state and in solution remained largely unexplored. In particular, we analyzed the impact that adding noncoordinating N-methyl and C-hydroxyethyl functionalities has in the structures of the complexes. All the Cu(II) and Zn(II) complexes were synthesized and investigated using single crystal X-ray diffraction and NMR, electronic absorption, and EPR spectroscopies, along with DFT studies. Dissociation kinetics experiments in acidic conditions and electrochemical studies were also performed. Special attention was paid to analyze the different configurations present in solution and in the solid state, as well as the impact of the C-appended hydroxyethyl group on the coordination behavior. Various ratios of the trans-I, trans-III, and cis-V configurations have been observed depending on the degree of N-methylation and the presence of the ethylene cross-bridge.

Cyclam-Based Chelators Bearing Phosphonated Pyridine Pendants for 64Cu-PET Imaging: Synthesis, Physicochemical Studies, Radiolabeling, and Bioimaging

Herein we present the preparation of two novel cyclam-based macrocycles (te1pyp and cb-te1pyp), bearing phosphonate-appended pyridine side arms for the coordination of copper(II) ions in the context of ⁶⁴Cu PET imaging. The two ligands have been prepared through conventional protection-alkylation sequences on cyclam, and their coordination properties have been thoroughly investigated. The corresponding copper complexes have been fully characterized in the solid state (X-ray diffraction analysis) and in solution (EPR and UV-vis spectroscopies). Potentiometric studies combined with spectrometry have also allowed us to determine their thermodynamic stability constants, confirming their high affinity for copper(II) cations. The kinetic inertness of the complexes has been verified by acid-assisted dissociation experiments, enabling their use in ⁶⁴Cu-PET imaging in mice for the first time. Indeed, the two ligands could be quantitatively radiolabeled under mild conditions, and the resulting ⁶⁴Cu complexes have demonstrated excellent stability in serum. PET imaging demonstrated a set of features emerging from the combination of picolinates and phosphonate units: high stability in vivo, fast clearance from the body via renal elimination, and most interestingly, very low fixation in the liver. This is in contrast with what was observed for monopicolinate cyclam (te1pa), which had a non-negligible accumulation in the liver, owing probably to its different charge and lipophilicity. These results thus pave the way for the use of such phosphonated pyridine chelators for in vivo ⁶⁴Cu-PET imaging.

Computationally-Guided Development of a Chelated NHC-P Iridium(I) Complex for the Directed Hydrogen Isotope Exchange of Aryl Sulfones

Herein, we report the rational, computationally-guided design of an iridium(I) catalyst system capable of enabling directed hydrogen isotope exchange (HIE) with the challenging sulfone directing group. Substrate binding energy was used as a parameter to guide rational ligand design via an in silico catalyst screen, resulting in a lead series of chelated iridium(I) NHC-phosphine complexes. Subsequent preparative studies show that the optimal catalyst system displays high levels of activity in HIE, and we demonstrate the labeling of a broad scope of substituted aryl sulfones. We also show that the activity of the catalyst is maintained at low pressures of deuterium gas and apply these conditions to tritium radiolabeling, including the expedient synthesis of a tritium-labeled drug molecule.

Thioether sulfur-bound [Cu2] complexes showing catechol oxidase activity and DNA cleaving behaviour

Ligand backbone alteration leads to different mechanisms for catecholase activity and order of interaction with DNA molecules.

Ditopic receptors containing urea groups for solvent extraction of Cu(ii) salts

The ditopic receptor L3 [1-(2-((7-(4-(tert-butyl)benzyl)-1,4,7,10-tetraazacyclododecan-1-yl)methyl)phenyl)-3-(3-nitrophenyl)urea] containing a macrocyclic cyclen unit for Cu(ii)-coordination and a urea moiety for anion binding was designed for recognition of metal salts. The X-ray structure of [CuL3(SO₄)] shows that the sulfate anion is involved in cooperative binding via coordination to the metal ion and hydrogen-bonding to the urea unit. This behaviour is similar to that observed for the related receptor L1 [1-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenyl)-3-(3-nitrophenyl)urea], which forms a dimeric [CuL1(μ-SO₄)]₂ structure in the solid state. In contrast, the single crystal X-ray structure of [ZnL3(NO₃)₂] contains a 1 : 2 complex (metal : anion) where one anion coordinates to the metal and the other is hydrogen-bonded to the urea group. Spectrophotometric titrations performed for the [CuL3(OSMe₂)]²⁺ complex indicate that this system is able to bind a wide range of anions with an affinity sequence: MeCO₂^- > Cl^- > H₂PO₄^- > Br^- > NO₂^- > HSO₄^- > NO₃^-. Lipophilic analogues of L1 and L3 extract CuSO₄ and CuCl₂ from water into chloroform with high selectivity over the corresponding Co(ii), Ni(ii) and Zn(ii) salts.

Current advances in ligand design for inorganic positron emission tomography tracers 68Ga, 64Cu, 89Zr and 44Sc

A key part of the development of metal based Positron Emission Tomography probes is the chelation of the radiometal. In this review the recent developments in the chelation of four positron emitting radiometals, ⁶⁸Ga, ⁶⁴Cu, ⁸⁹Zr and ⁴⁴Sc, are explored. The factors that effect the chelation of each radio metal and the ideal ligand system will be discussed with regards to high in vivo stability, complexation conditions, conjugation to targeting motifs and complexation kinetics. A series of cyclic, cross-bridged and acyclic ligands will be discussed, such as CP256 which forms stable complexes with ⁶⁸Ga under mild conditions and PCB-TE2A which has been shown to form a highly stable complex with ⁶⁴Cu. ⁸⁹Zr and ⁴⁴Sc have seen significant development in recent years with a number of chelates being applied to each metal - eight coordinate di-macrocyclic terephthalamide ligands were found to rapidly produce more stable complexes with ⁸⁹Zr than the widely used DFO.

A family of [Cu2], [Cu4] and [Cu5] aggregates: alteration of reaction conditions, ancillary bridges and capping anions

Enforced coordination by NO₃⁻, ClO₄⁻ and CF₃COO⁻ groups resulted in the formation of [Cu₂] (1), [Cu₄] (2) and [Cu₅] (3) complexes using H₅L1.

Coordination Chemistry of Bifunctional Chemical Agents Designed for Applications in 64Cu PET Imaging for Alzheimer's Disease

Positron emission tomography (PET) is emerging as one of the most important diagnostic tools for brain imaging, yet the most commonly used radioisotopes in PET imaging, ¹¹C and ¹⁸F, have short half-lives, and their usage is thus somewhat limited. By comparison, the ⁶⁴Cu radionuclide has a half-life of 12.7 h, which is ideal for administering and imaging purposes. In spite of appreciable research efforts, high-affinity copper chelators suitable for brain imaging applications are still lacking. Herein, we present the synthesis and characterization of a series of bifunctional compounds (BFCs) based on macrocyclic 1,4,7-triazacyclononane and 2,11-diaza[3.3](2,6)pyridinophane ligand frameworks that exhibit a high affinity for Cu²⁺ ions. In addition, these BFCs contain a 2-phenylbenzothiazole fragment that is known to interact tightly with amyloid β fibrillar aggregates. Determination of the protonation constants (pK_a values) and stability constants (log β values) of these BFCs, as well as characterization of the isolated copper complexes using X-ray crystallography, electron paramagnetic resonance spectroscopy, and electrochemical studies, suggests that these BFCs exhibit desirable properties for the development of novel ⁶⁴Cu PET imaging agents for Alzheimer’s disease.

Novel bifunctional chelators (BFCs) containing 1,4,7-triazacyclononane or pyridinophane macrocycles and amyloid-binding 2-phenylbenzothiazole fragments have been synthesized, and their copper coordination properties have been characterized in detail. These BFCs are attractive candidates for the development of novel ⁶⁴Cu-labeled PET imaging agents for Alzheimer’s disease.

Evaluation of 64Cu-Based Radiopharmaceuticals that Target Aβ Peptide Aggregates as Diagnostic Tools for Alzheimer's Disease

Positron emission tomography (PET) imaging agents that detect amyloid plaques containing amyloid beta (Aβ) peptide aggregates in the brain of Alzheimer's disease (AD) patients have been successfully developed and recently approved by the FDA for clinical use. However, the short half-lives of the currently used radionuclides 11C (20.4 min) and 18F (109.8 min) may limit the widespread use of these imaging agents. Therefore, we have begun to evaluate novel AD diagnostic agents that can be radiolabeled with 64Cu, a radionuclide with a half-life of 12.7 h, ideal for PET imaging. Described herein are a series of bifunctional chelators (BFCs), L1-L5, that were designed to tightly bind 64Cu and shown to interact with Aβ aggregates both in vitro and in transgenic AD mouse brain sections. Importantly, biodistribution studies show that these compounds exhibit promising brain uptake and rapid clearance in wild-type mice, and initial microPET imaging studies of transgenic AD mice suggest that these compounds could serve as lead compounds for the development of improved diagnostic agents for AD.

Evaluation of H2CHXdedpa, H2dedpa- and H2CHXdedpa-N,N′-propyl-2-NI ligands for 64Cu(ii) radiopharmaceuticals

Hexadentate acyclic chelate H₂CHXdedpa and related N,N′-alkylated ligands are radiolabeled with radioactive ⁶⁴Cu(ii) under mild conditions forming kinetically inert complexes.

Phosphonate Pendant Armed Propylene Cross-Bridged Cyclam: Synthesis and Evaluation as a Chelator for Cu-64

A propylene cross-bridged macrocyclic chelator with two phosphonate pendant arms (PCB-TE2P) was synthesized from cyclam. Various properties of the synthesized chelator, including Cu-complexation, Cu-complex stability, (64)Cu-radiolabeling, and in vivo behavior, were studied and compared with those of a previously reported propylene cross-bridged chelator (PCB-TE2A).

Radiometals: towards a new success story in nuclear imaging?

The use of radiometal isotopes in positron emission tomography: a new success story in nuclear imaging?

Pycup--a bifunctional, cage-like ligand for (64)Cu radiolabeling

In developing targeted probes for positron emission tomography (PET) based on (64)Cu, stable complexation of the radiometal is key, and a flexible handle for bioconjugation is highly advantageous. Here, we present the synthesis and characterization of the chelator pycup and four derivatives. Pycup is a cross-bridged cyclam derivative with a pyridyl donor atom integrated into the cross-bridge resulting in a pentadentate ligand. The pycup platform provides kinetic inertness toward (64)Cu dechelation and offers versatile bioconjugation chemistry. We varied the number and type of additional donor atoms by alkylation of the remaining two secondary amines, providing three model ligands, pycup2A, pycup1A1Bn, and pycup2Bn, in 3-4 synthetic steps from cyclam. All model copper complexes displayed very slow decomplexation in 5 M HCl and 90 °C (t1/2: 1.5 h for pycup1A1Bn, 2.7 h for pycup2A, 20.3 h for pycup2Bn). The single crystal crystal X-ray structure of the [Cu(pycup2Bn)](2+) complex showed that the copper was coordinated in a trigonal, bipyramidal manner. The corresponding radiochemical complexes were at least 94% stable in rat plasma after 24 h. Biodistribution studies conducted in Balb/c mice at 2 h postinjection of (64)Cu labeled pycup2A revealed low residual activity in kidney, liver, and blood pool with predominantly renal clearance observed. Pycup2A was readily conjugated to a fibrin-targeted peptide and labeled with (64)Cu for successful PET imaging of arterial thrombosis in a rat model, demonstrating the utility of our new chelator in vivo.