Glycogen Synthase Kinase-3 Maleimide Inhibitors As Potential PET-Tracers for Imaging Alzheimer's Disease: 11C-Synthesis and In Vivo Proof of Concept

Identifying Potential Ligand Binding Sites on Glycogen Synthase Kinase 3 Using Atomistic Cosolvent Simulations

Glycogen synthase kinase 3 β (GSK3β) is a serine/threonine kinase that phosphorylates several protein substrates in crucial cell signaling pathways. Owing to its therapeutic importance, there is a need to develop GSK3β inhibitors with high specificity and potency. One approach is to find small molecules that can allosterically bind to the GSK3β protein surface. We have employed fully atomistic mixed-solvent molecular dynamics (MixMD) simulations to identify three plausible allosteric sites on GSK3β that can facilitate the search for allosteric inhibitors. Our MixMD simulations narrow down the allosteric sites to precise regions on the GSK3β surface, thereby improving upon the previous predictions of the locations of these sites.

Development of [18F]Thiazolylacylaminopyridine-Based Glycogen Synthase Kinase-3β Ligands for Positron Emission Tomography Imaging

Glycogen synthase kinase-3β (GSK-3β) regulates numerous of CNS-specific signaling pathways, and is particularly implicated in various pathogenetic mechanisms of Alzheimer's disease (AD). A noninvasive method for detecting GSK-3β in AD brains via positron emission tomography (PET) imaging could enhance the understanding of AD pathogenesis and aid in the development of AD therapeutic drugs. In this study, an array of fluorinated thiazolyl acylaminopyridines (FTAAP) targeting GSK-3β were designed and synthesized. These compounds showed moderate to high affinities (IC₅₀ = 6.0 - 426 nM) for GSK-3β in vitro. A potential GSK-3β tracer, [¹⁸F]8, was successfully radiolabeled. [¹⁸F]8 had unsatisfactory initial brain uptake despite its suitable lipophilicity, molecular size and good stability. Further structural refinement of the lead compound is needed to develop promising [¹⁸F]-labeled radiotracers for the detection of GSK-3β in AD brains.

In Vivo Imaging and Kinetic Modeling of Novel Glycogen Synthase Kinase-3 Radiotracers [11C]OCM-44 and [18F]OCM-50 in Non-Human Primates

Glycogen synthase kinase 3 (GSK-3) is a potential therapeutic target for a range of neurodegenerative and psychiatric disorders. The goal of this work was to evaluate two leading GSK-3 positron emission tomography (PET) radioligands, [11C]OCM-44 and [18F]OCM-50, in non-human primates to assess their potential for clinical translation. A total of nine PET scans were performed with the two radiotracers using arterial blood sampling in adult rhesus macaques. Brain regional time-activity curves were extracted and fitted with one- and two-tissue compartment models using metabolite-corrected arterial input functions. Target selectivity was assessed after pre-administration of the GSK-3 inhibitor PF-04802367 (PF-367, 0.03-0.25 mg/kg). Both radiotracers showed good brain uptake and distribution throughout grey matter. [11C]OCM-44 had a free fraction in the plasma of 3% at baseline and was metabolized quickly. The [11C]OCM-44 volume of distribution (VT) values in the brain increased with time; VT values from models fitted to truncated 60-min scan data were 1.4-2.9 mL/cm3 across brain regions. The plasma free fraction was 0.6% for [18F]OCM-50 and VT values (120-min) were 0.39-0.87 mL/cm3 in grey matter regions. After correcting for plasma free fraction increases during blocking scans, reductions in regional VT indicated >80% target occupancy by 0.1 mg/kg of PF-367 for both radiotracers, supporting target selectivity in vivo. [11C]OCM-44 and [18F]OCM-50 warrant further evaluation as radioligands for imaging GSK-3 in the brain, though radio-metabolite accumulation may confound image analysis.

Organocatalytic Enantioselective Michael Reaction of Aminomaleimides with Nitroolefins Catalyzed by Takemoto's Catalyst

Known as electrophiles, maleimides are often used as acceptors in Michael additions to produce succinimides. However, reactions with maleimides as nucleophiles for enantioselective functionalization are only rarely performed. In this paper, a series of bifunctional Takemoto's catalysts were used to organocatalyze the enantioselective Michael reaction of aminomaleimides with nitroolefins. The resulting products were obtained in good yields (76-86%) with up to 94% enantiomer excess (ee). The catalyst type and the substrate scope were broadened using this methodology.

Positron Emission Tomography in Animal Models of Alzheimer's Disease Amyloidosis: Translational Implications

Animal models of Alzheimer's disease amyloidosis that recapitulate cerebral amyloid-beta pathology have been widely used in preclinical research and have greatly enabled the mechanistic understanding of Alzheimer's disease and the development of therapeutics. Comprehensive deep phenotyping of the pathophysiological and biochemical features in these animal models is essential. Recent advances in positron emission tomography have allowed the non-invasive visualization of the alterations in the brain of animal models and in patients with Alzheimer's disease. These tools have facilitated our understanding of disease mechanisms and provided longitudinal monitoring of treatment effects in animal models of Alzheimer's disease amyloidosis. In this review, we focus on recent positron emission tomography studies of cerebral amyloid-beta accumulation, hypoglucose metabolism, synaptic and neurotransmitter receptor deficits (cholinergic and glutamatergic system), blood-brain barrier impairment, and neuroinflammation (microgliosis and astrocytosis) in animal models of Alzheimer's disease amyloidosis. We further propose the emerging targets and tracers for reflecting the pathophysiological changes and discuss outstanding challenges in disease animal models and future outlook in the on-chip characterization of imaging biomarkers towards clinical translation.