Development and evaluation of [125 I]IPPI for Tau imaging in postmortem human Alzheimer's disease brain

Radioiodinated Tau Imaging Agent III Molecular Modeling, Synthesis, and Evaluation of a New Tau Imaging Agent, [125I]ISAS in Post-Mortem Human Alzheimer's Disease Brain

Using a molecular modeling approach for Tau-binding sites, we modified our previously reported imaging agent, [125I]INFT, for the potential improvement of binding properties to Tau in an Alzheimer's disease (AD) brain. Two new derivatives, namely [125I]ISAS and [125I]NIPZ, were designed, where binding energies at site 1 of Tau were -7.4 and -6.0 kcal/mole, respectively, compared to [125I]INFT (-7.6 kcal/mole). The radiosynthesis of [125I]ISAS and [125I]NIPZ was carried out by using iodine-125 and purified chromatographically to achieve >90% purity. In vitro binding affinities (IC50) for Tau were as follows: INFT = 7.3 × 10-8 M; ISAS = 4.7 × 10-8 M; NIPZ > 10-6 M. The binding of [125I]ISAS to gray matter (GM) correlated with the presence of Tau in the AD brain, confirmed by anti-Tau immunohistochemistry. [125I]NIPZ did not bind to Tau, with similar levels of binding observed in GM and white matter (WM). Four radiotracers were compared and the rank order of binding to Tau was found to be [125I]IPPI > [125I]INFT > [125I]ISAS >>> [125I]NIPZ with GM/WM ratios of [125I]IPPI = 7.74 > [125I]INFT = 4.86 > [125I]ISAS = 3.62 >> [125I]NIPZ = 1.24. The predictive value of Chimera-AutoDock for structurally related compounds binding to the Tau binding sites (measured as binding energy) was good. A binding energy of less than -7 kcal/mole is necessary and less than -8 kcal/mole will be more suitable for developing imaging agents.

[125I]IPC-Lecanemab: Synthesis and Evaluation of Aβ-Plaque-Binding Antibody and Comparison with Small-Molecule [18F]Flotaza and [125I]IBETA in Postmortem Human Alzheimer's Disease

Therapeutic antibodies for reducing Aβ plaque load in Alzheimer's disease (AD) is currently making rapid progress. The diagnostic imaging of Aβ plaque load in AD has been underway and is now used in clinical studies. Here, we report our preliminary findings on imaging a therapeutic antibody, Lecanemab, in a postmortem AD brain anterior cingulate. [125I]5-iodo-3-pyridinecarboxamido-Lecanemab ([125I]IPC-Lecanemab) was prepared by coupling N-succinimidyl-5-([125I]iodo)-3-pyridinecarboxylate with Lecanemab in modest yields. The distinct binding of [125I]IPC-Lecanemab to Aβ-rich regions in postmortem human AD brains was higher in grey matter (GM) containing Aβ plaques compared to white matter (WM) (GM/WM was 1.6). Anti-Aβ immunostaining was correlated with [125I]IPC-Lecanemab regional binding in the postmortem AD human brains. [125I]IPC-Lecanemab binding was consistent with the binding of Aβ small molecules, [18F]flotaza and [125I]IBETA, in the same subjects. [18F]Flotaza and [125I]IBETA, however, exhibited significantly higher GM/WM ratios (>20) compared to [125I]IPC-Lecanemab. Our results suggest that radiolabeled [125I]IPC-Lecanemab retains the ability to bind to Aβ in human AD and may therefore be useful as a PET imaging radiotracer when labeled as [124I]IPC-Lecanemab. The ability to directly visualize in vivo a promising therapeutic antibody for AD may be useful in treatment planning and dosing and could be complimentary to small-molecule diagnostic imaging to assess outcomes of therapeutic interventions.

Glutamate's Effects on the N-Methyl-D-Aspartate (NMDA) Receptor Ion Channel in Alzheimer's Disease Brain: Challenges for PET Radiotracer Development for Imaging the NMDA Ion Channel

In an effort to further understand the challenges facing in vivo imaging probe development for the N-methyl-D-aspartate (NMDA) receptor ion channel, we have evaluated the effect of glutamate on the Alzheimer's disease (AD) brain. Human post-mortem AD brain slices of the frontal cortex and anterior cingulate were incubated with [3H]MK-801 and adjacent sections were tested for Aβ and Tau. The binding of [3H]MK-801 was measured in the absence and presence of glutamate and glycine. Increased [3H]MK-801 binding in AD brains was observed at baseline and in the presence of glutamate, indicating a significant increase (>100%) in glutamate-induced NMDA ion channel activity in AD brains compared to cognitively normal brains. The glycine effect was lower, suggesting a decrease of the co-agonist effect of glutamate and glycine in the AD brain. Our preliminary findings suggest that the targeting of the NMDA ion channel as well as the glutamate site may be appropriate in the diagnosis and treatment of AD. However, the low baseline levels of [3H]MK-801 binding in the frontal cortex and anterior cingulate in the absence of glutamate and glycine indicate significant hurdles for in vivo imaging probe development and validation.

Preclinical Evaluation of Novel PET Probes for Dementia

The development of novel PET imaging agents that selectively bind specific dementia-related targets can contribute significantly to accurate, differential and early diagnosis of dementia causing diseases and support the development of therapeutic agents. Consequently, in recent years there has been a growing body of literature describing the development and evaluation of potential new promising PET tracers for dementia. This review article provides a comprehensive overview of novel dementia PET probes under development, classified by their target, and pinpoints their preclinical evaluation pathway, typically involving in silico, in vitro and ex/in vivo evaluation. Specific target-associated challenges and pitfalls, requiring extensive and well-designed preclinical experimental evaluation assays to enable successful clinical translation and avoid shortcomings observed for previously developed 'well-established' dementia PET tracers are highlighted in this review.

[124I]IBETA: A New Aβ Plaque Positron Emission Tomography Imaging Agent for Alzheimer's Disease

Several fluorine-18-labeled PET β-amyloid (Aβ) plaque radiotracers for Alzheimer’s disease (AD) are in clinical use. However, no radioiodinated imaging agent for Aβ plaques has been successfully moved forward for either single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. Radioiodinated pyridyl benzofuran derivatives for the SPECT imaging of Aβ plaques using iodine-123 and iodine-125 are being pursued. In this study, we assess the iodine-124 radioiodinated pyridyl benzofuran derivative 5-(5-[124I]iodobenzofuran-2-yl)-N,N-dimethylpyridin-2-amine ([124I]IBETA) (Ki = 2.36 nM) for utilization in PET imaging for Aβ plaques. We report our findings on the radioiododestannylation reaction used to prepare [124/125I]IBETA and evaluate its binding to Aβ plaques in a 5 × FAD mouse model and postmortem human AD brain. Both [125I]IBETA and [124I]IBETA are produced in >25% radiochemical yield and >85% radiochemical purity. The in vitro binding of [125I]IBETA and [124I]IBETA in transgenic 5 × FAD mouse model for Aβ plaques was high in the frontal cortex, anterior cingulate, thalamus, and hippocampus, which are regions of high Aβ accumulation, with very little binding in the cerebellum (ratio of brain regions to cerebellum was >5). The in vitro binding of [125I]IBETA and [124I]IBETA in postmortem human AD brains was higher in gray matter containing Aβ plaques compared to white matter (ratio of gray to white matter was >5). Anti-Aβ immunostaining strongly correlated with [124/125I]IBETA regional binding in both the 5 × FAD mouse and postmortem AD human brains. The binding of [124/125I]IBETA in 5 × FAD mouse and postmortem human AD brains was displaced by the known Aβ plaque imaging agent, Flotaza. Preliminary PET/CT studies of [124I]IBETA in the 5 × FAD mouse model suggested [124I]IBETA was relatively stable in vivo with a greater localization of [124I]IBETA in the brain regions with a high concentration of Aβ plaques. Some deiodination was observed at later time points. Therefore, [124I]IBETA may potentially be a useful PET radioligand for Aβ plaques in brain studies.

Development and evaluation of [18F]Flotaza for Aβ plaque imaging in postmortem human Alzheimer's disease brain

Positron emission tomographic (PET) studies of amyloid β (Aβ) accumulation in Alzheimer's disease (AD) have shown clinical utility. The aim of this study was to develop and evaluate the effectiveness of a new fluorine-18 radiotracer [18F]Flotaza (2-{2-[2-[18F]fluoroethoxy]ethoxy}ethoxy)-4'-N,N-dimethylaminoazobenzene), for Aβ plaque imaging. Nucleophilic [18F]fluoride was used in a one-step radiosynthesis for [18F]flotaza. Using post mortem human AD brain tissues consisting of anterior cingulate (AC) and corpus callosum (CC), binding affinity of Flotaza, Ki = 1.68 nM for human Aβ plaques and weak (>10-5 M) for Tau protein. Radiosynthesis of [18F]Flotaza was very efficient in high radiochemical yields (>25%) with specific activities >74 GBq/μmol. Brain slices from all AD subjects were positively immunostained with anti-Aβ. Ratio of [18F]Flotaza in gray matter AC to white matter CC was >100 in all the 6 subjects. Very little white matter binding was seen. [18F]Flotaza binding in AC strongly correlated with anti-Aβ immunostains. [18F]Flotaza is therefore a suitable fluorine-18 PET radiotracer for PET imaging studies of human Aβ plaques.