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

Abstract

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 (pKa 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.

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

“…Surprisingly, the brain uptake of 64 Cu- L 0 , albeit low, was slightly higher than that of 64 Cu- L 1 , likely due to the formation of a neutral Cu complex for L 0 vs a monocationic Cu complex for L 1 . 55 Among all BFCs tested, 64 Cu- L 2 showed the highest brain uptake of 1.33 ± 0.27% ID/g at 2 min post-injection, which dropped to 0.27 ± 0.03% ID/g at 60 min. 64 Cu- L 4 and 64 Cu- L 5 also showed good brain uptake of 0.61 ± 0.14 and 0.75 ± 0.16% ID/g at 2 min post-injection, respectively ( Figure 7 ).…”

“… 44 , 53 For example, the BFCs L 1 – L 5 were generated using a convergent synthetic route based on a Mannich reaction between 2-(4-hydroxy-3-methoxyphenyl-benzothiazole and strong metal chelators such as 2,4-dimethyl-1,4,7-triazacyclononane (for L 2 ) and 2,11-diaza[3.3](2,6)-pyridinophane derivatives (for L 3 – L 5 ) in the presence of paraformaldehyde ( Figure 1 ). 54 , 55 The 4-hydroxyphenyl-benzothiazole molecular structure is derived from Thioflavin T, a well-known amyloid-binding fluorescent dye, and o -vanilin, a compound shown to have affinity for Aβ oligomers. 56 , 57 The previously reported metal chelator N , N ′-diacetate-2,11-diaza[3.3](2,6)-pyridinophane ( L 0 or N4DA ) was employed as a control compound that does not contain an amyloid-binding fragment.…”

“… 64 − 66 In that regard, we have first synthesized and fully characterized the cold Cu complexes of these BFCs. 55 Spectrophotometric titrations reveal that L 1 – L 5 are extremely strong chelators for Cu 2+ with log K stability constants of 27–32 for the corresponding Cu complexes, 55 which are tighter than the common metal scavengers EDTA and DTPA. 67 …”

Fully Automated Production and Characterization of 64Cu and Proof‐of‐Principle Small‐Animal PET Imaging Using 64Cu‐Labelled CA XII Targeting 6A10 Fab

“…[30] Chelator conjugating is even used to radiolabel nanoparticles [31] andl iposomes for breast cancer diagnosis. [32] Copper-labelled complexing agents such as ATSM or some bifunctional chelators have been furtheri nvestigated for use as diagnostics against hypoxia [33,34] and Alzheimer'sd isease, [35,36] respectively.…”

Successful Design of a Multivalent Bifunctional Chelator for Diagnostic 64Cu PET Imaging in Alzheimer’s Disease

“…Previously we have reported Aβ-targeting bifunctional chelators (BFCs) for potential therapeutic and PET imaging applications. [32][33][34][35][36][37] Herein, we report novel compounds containing an Aβ-interacting fragment and a metal-chelating group designed based on a dual multivalent ligand and variable-length spacer strategy. Multivalent ligands commonly bind to receptors with improved affinity vs the monovalent derivatives.…”