Synthesis of Oxorhenium(V) and Oxotechnetium(V) Complexes That Bind to Amyloid-β Plaques

Chemoselective Attachment of the Water-Soluble Dark Quencher Hydrodabcyl to Amino Groups in Peptides and Preservation of Its Spectroscopic Properties over a Wide pH Range

The water-soluble quencher hydrodabcyl can be activated as an N-succinimidyl ester that is readily accessible from crude hydrodabcyl and storable for a long time. With primary and secondary amines, it reacts swiftly and chemoselectively, even in the presence of other competing nucleophiles such as those typically present in natural peptides. One of the three phenolic OH groups of hydrodabcyl is amenable to selective mono-Boc protection resulting in reduced polarity, advantageous to its further use in organic synthesis. The advantages of hydrodabcyl over dabcyl in spectrometric applications are exemplified by the pH dependence of its absorbance spectra.

Oxorhenium(V) and Oxotechnetium(V) Complexes of N3S Tetradentate Ligands with a Styrylpyridyl Functional Group: Toward Imaging Agents to Assist in the Diagnosis of Alzheimer's Disease

Alzheimer's disease (AD) is associated with the presence of amyloid plaques in the brain mainly comprised of aggregated forms of amyloid-β (Aβ). Molecules radiolabeled with technetium-99m that cross the blood-brain barrier (BBB) and selectively bind to Aβ plaques have the potential to assist in the diagnosis of AD using single-photon emission computed tomography imaging. In this work, three new tetradentate ligands of pyridyl, amide, amine and thiol donors, featuring a styrylpyridyl group that is known to interact with amyloid plaques, were prepared. The new ligands formed charge-neutral and lipophilic complexes with the [Tc═O]3+ and [Re═O]3+ motifs, and two rhenium complexes were characterized by X-ray crystallography. The rhenium(V) complexes interact with synthetic Aβ1-40 and amyloid plaques on human brain tissue. Two of the new ligands were radiolabeled with 99mTc using a kit-based approach, and their biodistribution in wild-type mice was evaluated. The presence of amide donors in the tetradentate ligand increased the stability of the respective [Tc═O]3+ complexes but reduced brain uptake. While the complexes were able to cross the BBB, the degree of uptake in the brain was not sufficient to justify further investigation of these complexes.

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.

Poly(2-ethyl-2-oxazoline) Conjugates with Salicylic Acid via Degradable Modular Ester Linkages

Conjugation of drugs to polymers is a widely used approach to gain control over the release of therapeutics. In this contribution, salicylic acid, a multipurpose model drug, is conjugated to the biocompatible poly(2-ethyl-2-oxazoline) (PEtOx). The drug is attached to the side chains of a polymer carrier through a hydrolytically cleavable ester linker, via a sequential postpolymerization modification. The chemical modulation of this ester, i.e., by primary or secondary alcohols, is demonstrated to greatly influence the ester hydrolysis rate. This crucial parameter allows us to tune the in vitro kinetics of the sustained drug release for periods exceeding a month in phosphate-buffered saline (PBS). The synthetic accessibility of the cleavable linker, together with the modularity of the drug release rate offered by this approach, highlights the utility of this class of polymers in the field of long-lasting drug delivery systems for persistent and chronic disease treatment.

A Copper Complex of a Thiosemicarbazone-Pyridylhydrazone Ligand Containing a Vinylpyridine Functional Group as a Potential Imaging Agent for Amyloid-β Plaques

Complexes containing positron-emitting radionuclides of copper have the potential to be of use for diagnostic imaging with positron emission tomography. Alzheimer’s disease is characterised by the presence of amyloid-β plaques in the brain. A new thiosemicarbazone-pyridyl hydrazone tetradentate ligand with a pyridyl-4-vinylpyridine functional group was prepared with the aim of making a copper complex that binds to amyloid-β plaques to assist in the diagnosis of Alzheimer’s disease. The ligand forms a charge neutral complex with copper(ii) that was characterised by X-ray crystallography and the electrochemical behaviour of the complex was investigated by cyclic voltammetry. The new ligand can be radiolabelled with positron-emitting copper-64 at room temperature in excellent radiochemical yields. The new complex interacts with synthetic amyloid-β fibrils and binds amyloid-β plaques present in post-mortem Alzheimer’s disease brain tissue.

Rhenium and technetium complexes of thioamide derivatives of pyridylhydrazine that bind to amyloid-β plaques

Age-associated deposition of amyloid-β in cerebral blood vessels, a condition referred to as cerebral amyloid angiopathy, can contribute to stroke and dementia. This research aimed to design new radioactive technetium-99 m complexes that bind to amyloid-β plaques that have the potential to assist in diagnosis of cerebral amyloid angiopathy using single-photon-emitted computed tomography (SPECT) imaging. Six new pyridylthiosemicarbazide ligands containing either benzofuran or styrylpyridyl functional groups that are known to selectively bind to amyloid plaques were prepared. Non-radioactive isotopes of technetium are not available so rhenium was used as a surrogate for exploratory chemistry. The new ligands were used to prepare well-defined [Re-oxo]³⁺ complexes where two pyridylthiosemicarbazide ligands were coordinated to a single metal ion to give bivalent complexes with two amyloid-β targeting functional groups. The interaction of the [Re-oxo]³⁺ complexes with synthetic amyloid-β_1-42 and with amyloid plaques in human brain tissue was investigated. Two ligands were selected to develop methods to prepare their [^99mTc-oxo]³⁺ complexes at the tracer level. These technetium-99 m complexes are likely to be isostructural to their rhenium-oxo analogues.

Ferrocene-based anilides: synthesis, structural characterization and inhibition of butyrylcholinesterase

Twenty-three compounds in two series of ferrocene-based anilides, with the general formula C₅H₅-Fe-C₅H₄-C₆H₄-NH-CO-C₆H₄-R (where R = H, F, Cl, CH₃ and OCH₃), have been synthesized and found to inhibit butyrylcholinesterase.

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).