A (68)Ga complex based on benzofuran scaffold for the detection of β-amyloid plaques

68Ga-Labeled Benzothiazole Derivatives for Imaging Aβ Plaques in Cerebral Amyloid Angiopathy

Timely diagnostic imaging plays a crucial role in managing cerebral amyloid angiopathy (CAA)-the condition in which amyloid β is deposited on blood vessels. To selectively map these amyloid plaques, we have designed amyloid-targeting ligands that can effectively complex with ⁶⁸Ga³⁺ while maintaining good affinity for amyloid β. In this study, we introduced novel 1,4,7-triazacyclononane-based bifunctional chelators (BFCs) that incorporate a benzothiazole moiety as the Aβ-binding fragment and form charged and neutral species with ⁶⁸Ga³⁺. In vitro autoradiography using 5xFAD and WT mouse brain sections (11-month-old) suggested strong and specific binding of the ⁶⁸Ga complexes to amyloid β. Biodistribution studies in CD-1 mice revealed a low brain uptake of 0.10-0.33% ID/g, thus suggesting ⁶⁸Ga-labeled novel BFCs as promising candidates for detecting CAA.

GdIII and GaIII complexes with a new tris-3,4-HOPO ligand as new imaging probes: complex stability, magnetic properties and biodistribution

The development of metal-based multimodal imaging probes is a highly challenging field in coordination chemistry. In this context, we have developed a bifunctional hexadentate tripodal ligand (H₃L2) with three 3,4-HOPO moieties attached to a flexible tetrahedral carbon bearing a functionalizable nitro group. Complexes formed with different metal ions have potential interest for diagnostic applications, namely magnetic resonance imaging (MRI) and positron emission tomography (PET). The capacity of the ligand to coordinate Gd^III and Ga^III was studied and the thermodynamic stability constants of the respective complexes were determined by potentiometry and spectrophotometry. The ligand forms stable 1 : 1 ML complexes though with considerably higher affinity for Ga^III than for Gd^III (pGa = 26.2 and pGd = 14.3 at pH 7). The molecular dynamics simulations of the Gd^III complex indicate that two water molecules can coordinate the metal ion, thus providing efficient paramagnetic enhancement of water proton relaxation. The relaxation and the water exchange properties of the Gd^III chelate, assessed by a combined ¹⁷O NMR and ¹H NMRD study, showed associative activated water exchange with a relatively low rate constant, k298ex = (0.82 ± 0.11) × 10⁷ s^-1, and some aggregation tendency. Biodistribution studies of the ⁶⁷Ga-L2 complex suggested good in vivo stability and quick renal clearance. Further anchoring of this ligand with specific biotargeting moieties might open future prospectives for applications of labelled conjugates in both MRI and ⁶⁸Ga-PET diagnostic imaging.

Metals in Imaging of Alzheimer's Disease

One of the hallmarks of Alzheimer's disease (AD) is the deposition of amyloid plaques in the brain parenchyma, which occurs 7-15 years before the onset of cognitive symptoms of the pathology. Timely diagnostics of amyloid formations allows identifying AD at an early stage and initiating inhibitor therapy, delaying the progression of the disease. However, clinically used radiopharmaceuticals based on 11C and 18F are synchrotron-dependent and short-lived. The design of new metal-containing radiopharmaceuticals for AD visualization is of interest. The development of coordination compounds capable of effectively crossing the blood-brain barrier (BBB) requires careful selection of a ligand moiety, a metal chelating scaffold, and a metal cation, defining the method of supposed Aβ visualization. In this review, we have summarized metal-containing drugs for positron emission tomography (PET), magnetic resonance imaging (MRI), and single-photon emission computed tomography (SPECT) imaging of Alzheimer's disease. The obtained data allow assessing the structure-ability to cross the BBB ratio.

Photophysical studies on lanthanide(III) chelates conjugated to Pittsburgh compound B as luminescent probes targeted to Aβ amyloid aggregates

The photophysical properties of Eu3+ and Tb3+ complexes of DOTAGA and DO3A-monoamide conjugates of the Pittsburgh compound B (PiB) chromophore, prepared using linkers of different lengths and flexibilities, and which form stable negatively charged (LnL1), and uncharged (LnL2) complexes, respectively, were studied as potential probes for optical detection of amyloid aggregates. The phenylbenzothiazole (PiB) moiety absorbs light at wavelengths longer than 330 nm with a high molar absorption coefficient in both probes, and acts as an antenna in these systems. The presence of the luminescent Ln3+ ion quenches the excited states of PiB through an energy transfer process from the triplet state of PiB to the metal centre, and structured emission is seen from Eu3+ and Tb3+. The luminescence study indicates the presence of a 5D4 → T1 back transfer process in the Tb3+ complexes. It also provides insights on structural properties of the Eu3+ complexes, such as the high symmetry environment of the Eu3+ ion in a single macrocyclic conformation and the presence of one water molecule in its inner coordination sphere. The overall quantum yield of luminescence of EuL1 is higher than for EuL2. However, their low values reflect the low overall sensitization efficiency of the energy transfer process, which is a consequence of the large distances between the metal center and the antenna, especially in the EuL2 complex. DFT calculations confirmed that the most stable conformation of the Eu3+ complexes involves a combination of a square antiprismatic (SAP) geometry of the chelate and an extended conformation of the linker. The large calculated average distances between the metal center and the antenna point to the predominance of the Förster energy transfer mechanism, especially for EuL2. This study provides insights into the behavior of amyloid-targeted Ln3+ complexes as optical probes, and contributes towards their rational design.

Metal-based imaging agents: progress towards interrogating neurodegenerative disease

Transition metals and lanthanide ions display unique properties that enable the development of non-invasive diagnostic tools for imaging. In this review, we highlight various metal-based imaging strategies used to interrogate neurodegeneration.

99mTc-labeled Small Molecules for Diagnosis of Alzheimer’s Disease: Past, Recent and Future Perspectives

Background:

Alzheimer’s disease (AD) is an age-related progressive neurodegenerative disease. Its prominent hallmarks are extracellular deposition of β-amyloids (amyloid plaques), intracellular neurofibrillary tangles (NTFs), neurodegeneration and finally loss of cognitive function. Hence, AD diagnosis in the early stage and monitoring of the disease are of great importance.

Methods:

In this review article, we have reviewed recent efforts for design, synthesis and evaluation of 99mTc labeled small molecule for AD imaging purposes.

Results:

These small molecules include derivatives of Congo red, benzothiazole, benzofuran, benzoxazole, naphthalene, biphenyl, chalcone, flavone, aurone, stilbene, curcumin, dibenzylideneacetone, quinoxaline, etc. The different aspects of 99mTc-labeled small molecules including chemical structure, their affinity toward amyloid plaques, BBB permeation and in vivo/vitro stability will be discussed.

Conclusion:

The findings of this review confirm the importance of 99mTc-labeled small molecules for AD imaging. Future studies based on the pharmacophore of these designed compounds are needed for improvement of these molecules for clinical application.

Amyloid β-targeted metal complexes for potential applications in Alzheimer's disease

Alzheimer's disease (AD) is currently an incurable neurodegenerative disorder that affects millions of people around the world. The aggregation of amyloid-β peptides (Aβ), one of the primary pathological hallmarks of AD, plays a key role in the AD pathogenesis. In this regard, Aβ aggregates have been considered as both biomarkers and drug targets for the diagnosis and therapy of AD. Various Aβ-targeted metal complexes have exhibited promising potential as anti-AD agents due to their fascinating physicochemical properties over the past two decades. This review classifies the complexes into three groups based on their potential applications in AD including therapy, diagnosis and theranosis. The recent representative examples are highlighted in terms of design rationale, working mechanism and potential applications.

Prospective of 68Ga Radionuclide Contribution to the Development of Imaging Agents for Infection and Inflammation

During the last decade, the utilization of ⁶⁸Ga for the development of imaging agents has increased considerably with the leading position in the oncology. The imaging of infection and inflammation is lagging despite strong unmet medical needs. This review presents the potential routes for the development of ⁶⁸Ga-based agents for the imaging and quantification of infection and inflammation in various diseases and connection of the diagnosis to the treatment for the individualized patient management.

Hydrazone–Pd-catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates

The hydrazone–palladium catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates gave the corresponding 2-substituted-3-allylbenzofuran derivatives at room temperature.

Gallium-68 Complexes Conjugated to Pittsburgh Compound B: Radiolabeling and Biological Evaluation

PURPOSE

The aim of this work is to develop an efficient and fully automated radiosynthesis of three derivatives of the Pittsburgh compound B labeled with gallium-68 for the detection of amyloid plaques.

PROCEDURES

The radiolabeling of the precursors and purification of the radiolabeled agents by high pressure liquid chromatography has been studied prior to their in vitro and in vivo evaluations.

RESULTS

The complete process led, in 50 min, to pure Ga-68 products in a 12-38 % yield and with appreciable specific radioactivity (SRA, 85-168 GBq/μmol) which enabled us to demonstrate a considerable in vivo stability of the products. Unfortunately, this result was associated with a poor blood-brain barrier (BBB) permeability and a limited uptake of our compounds by amyloid deposits was observed by in vitro autoradiography.

CONCLUSION

Although we have not yet identified a compound able to significantly mark cerebral amyloidosis, this present investigation will likely contribute to the development of more successful Ga-68 radiotracers.

Self-assembling peptide and protein amyloids: from structure to tailored function in nanotechnology

Self-assembled peptide and protein amyloid nanostructures have traditionally been considered only as pathological aggregates implicated in human neurodegenerative diseases. In more recent times, these nanostructures have found interesting applications as advanced materials in biomedicine, tissue engineering, renewable energy, environmental science, nanotechnology and material science, to name only a few fields. In all these applications, the final function depends on: (i) the specific mechanisms of protein aggregation, (ii) the hierarchical structure of the protein and peptide amyloids from the atomistic to mesoscopic length scales and (iii) the physical properties of the amyloids in the context of their surrounding environment (biological or artificial). In this review, we will discuss recent progress made in the field of functional and artificial amyloids and highlight connections between protein/peptide folding, unfolding and aggregation mechanisms, with the resulting amyloid structure and functionality. We also highlight current advances in the design and synthesis of amyloid-based biological and functional materials and identify new potential fields in which amyloid-based structures promise new breakthroughs.

Recent progress in the development of metal complexes as β-amyloid imaging probes in the brain

In this review, we have focused on the recent progress in metal complexes that are able to bind to β-amyloid (Aβ) species. We have discussed various radioactive complexes of 99mTc, 68Ga, 64Cu, 89Zr, and 111In, which were designed as Aβ imaging agents for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, non-radioactive Re and Ru complexes as Aβ sensors using luminescence methods, and Gd3+ complexes as contrast agents for magnetic resonance imaging (MRI).

Gallium-68 PET: A Powerful Generator-based Alternative to Infection and Inflammation Imaging

The process of inflammation (with or without infection) forms part of essentially every major debilitating disease. Early detection and accurate distinction of inflammation from infection are important to optimize and individualize therapy. Nuclear medicine is ideally suited for the detection of pathologic changes early on and is able to target a magnitude of role players involved in the aforementioned processes. Hybrid modalities such as PET/CT and PET/MRI offer high spatial resolution that combines morphologic and pathophysiological changes and add various quantification possibilities that are preferable in these settings. It follows then that the development of PET radiopharmaceuticals is imperative to make use of these latest advances. Gallium-68 (Ga-68)-based tracers are exceptionally well suited to these indications, considering the year-round availability from a single generator, the relative cost-effectiveness, and relative ease of labeling. Over the past few years, the development of Ga-68-based tracers has understandably exploded with a recent growing interest in infection and inflammation imaging. This review aims to highlight some of the most important and interesting advances made with Ga-68-based PET/CT in the field of infection and inflammation imaging.

Metal complexes for multimodal imaging of misfolded protein-related diseases

Aggregation of misfolded proteins and progressive polymerization of otherwise soluble proteins is a common hallmark of several highly debilitating and increasingly prevalent diseases, including amyotrophic lateral sclerosis, cerebral amyloid angiopathy, type II diabetes and Parkinson's, Huntington's and Alzheimer's diseases.

Synthesis and evaluation of copper-64 labeled benzofuran derivatives targeting β-amyloid aggregates

In vivo imaging of β-amyloid (Aβ) aggregates consisting of Aβ(1-40) and Aβ(1-42) peptides by positron emission tomography (PET) contributes to the diagnosis and therapy for Alzheimer's disease (AD). Because (64)Cu (t1/2=12.7h) is a radionuclide for PET with a longer physical half-life than (11)C (t1/2=20min) and (18)F (t1/2=110min), it is an attractive radionuclide for the development of Aβ imaging probes that are suitable for routine use. In the present study, we designed and synthesized two novel (64)Cu labeled benzofuran derivatives and evaluated their utility as PET imaging probes for Aβ aggregates. In an in vitro binding assay, 6 and 8 showed binding affinity for Aβ(1-42) aggregates with a Ki value of 33 and 243nM, respectively. In addition, these probes bound to Aβ plaques deposited in the brain of an AD model mouse in vitro. In a biodistribution experiment using normal mice, these probes showed low brain uptake (0.33% and 0.36% ID/g) at 2min post-injection. Although refinement to enhance brain uptake is needed, [(64)Cu]6 and [(64)Cu]8 demonstrated the feasibility of developing novel PET probes for imaging Aβ aggregates.

A gallium(III) Schiff base-curcumin complex that binds to amyloid-β plaques

Gallium-68 is a positron-emitting isotope that can be used in positron-emission tomography imaging agents. Alzheimer's disease is associated with the formation of plaques in the brain primarily comprised of aggregates of a 42 amino acid protein called amyloid-β. With the goal of synthesising charge neutral, low molecular weight, lipophilic gallium complexes with the potential to cross the blood-brain barrier and bind to Aβ plaques we have used an ancillary tetradentate N₂O₂ Schiff base ligand and the β-diketone curcumin as a bidentate ligand to give a six-coordinate Ga³⁺ complex. The tetradentate Schiff base ligand adopts the cis-β configuration with deprotonated curcumin acting as a bidentate ligand. The complex binds to amyloid-β plaques in human brain tissue and it is possible that extension of this chemistry to positron-emitting gallium-68 could provide useful imaging agents for Alzheimer's disease.

Hydrazone–palladium catalyzed annulation of 1-cinnamyloxy-2-ethynylbenzene derivatives

The annulation of 1-cinnamyloxy-2-ethynylbenzene derivatives using a hydrazone–palladium catalyst system proceeded smoothly and gave the corresponding 2-substituted-3-cinnamylbenzofurans in good-to-excellent yields.