In Vitro and In Vivo Evaluation of GSK-3 Radioligands in Alzheimer's Disease: Preliminary Evidence of Sex Differences

Synthesis and preliminary evaluation of novel PET probes for GSK-3 imaging

Non-invasive imaging of GSK-3 expression in the brain will help to understand the role of GSK-3 in disease pathology and progression. Herein, we report the radiosynthesis and evaluation of two novel isonicotinamide based 18F labeled PET probes, [18F]2 and [18F]6 for noninvasive imaging of GSK3. Among the developed PET probes, the in vitro blood-brain permeability coefficient of 2 (38 ± 20 × 10-6 cm/s, n = 3) was found to be better than 6 (8.75 ± 3.90 × 10-6 cm/s, n = 5). The reference compounds 2 and 6 showed nanomolar affinity towards GSK-3α and GSK-3β. PET probe [18F]2 showed higher stability (100%) in mouse and human serums compared to [18F]6 (67.01 ± 4.93%, n = 3) in mouse serum and 66.20 ± 6.38%, n = 3) in human serum at 120 min post incubation. The in vivo imaging and blocking studies were performed in wild-type mice only with [18F]2 due to its observed stability. [18F]2 showed a SUV of 0.92 ± 0.28 (n = 6) in mice brain as early as 5 min post-injection followed by gradual clearance over time.

A computational model of Alzheimer's disease at the nano, micro, and macroscales

Introduction

Mathematical models play a crucial role in investigating complex biological systems, enabling a comprehensive understanding of interactions among various components and facilitating in silico testing of intervention strategies. Alzheimer's disease (AD) is characterized by multifactorial causes and intricate interactions among biological entities, necessitating a personalized approach due to the lack of effective treatments. Therefore, mathematical models offer promise as indispensable tools in combating AD. However, existing models in this emerging field often suffer from limitations such as inadequate validation or a narrow focus on single proteins or pathways.

Methods

In this paper, we present a multiscale mathematical model that describes the progression of AD through a system of 19 ordinary differential equations. The equations describe the evolution of proteins (nanoscale), cell populations (microscale), and organ-level structures (macroscale) over a 50-year lifespan, as they relate to amyloid and tau accumulation, inflammation, and neuronal death.

Results

Distinguishing our model is a robust foundation in biological principles, ensuring improved justification for the included equations, and rigorous parameter justification derived from published experimental literature.

Conclusion

This model represents an essential initial step toward constructing a predictive framework, which holds significant potential for identifying effective therapeutic targets in the fight against AD.

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.

PET imaging of glycogen synthase kinase-3 in pancreatic cancer xenograft mouse models

Glycogen synthase kinase-3 (GSK-3) contributes to tumorigenesis in pancreatic cancer by modulating cell proliferation and survival. This study evaluated the lead GSK-3 targeted PET radiotracers for neuro-PET imaging, [¹¹C]PF-367 and [¹¹C]OCM-44, in pancreatic cancer xenograft mice. Immunohistochemistry showed that GSK-3α and GSK-3β were overexpressed in PANC-1 xenografts. In autoradiography studies, higher specific binding was observed for [³H]PF-367 compared to [³H]OCM-44 when co-incubated with unlabeled PF-367 (59.2±1.8% vs 22.6±3.75%, respectively). Co-incubation of [¹¹C]OCM-44 with OCM-44 did not improve the specific binding (25.5±30.2%). In dynamic PET imaging of PANC-1 xenograft mouse models, tumors were not visualized with [¹¹C]PF-367 but were well visualized with [¹¹C]OCM-44. Time-activity curves revealed no difference in accumulation in PANC-1 tumor tissue compared to muscle tissue in [¹¹C]PF-367 baseline studies, while a significant difference was observed for [¹¹C]OCM-44 with a tumor-to-muscle ratio of 1.6. Tumor radioactivity accumulation following injection with [¹¹C]OCM-44 was not displaced by pre-treatment with unlabeled PF-367. Radiometabolite analysis showed that intact [¹¹C]PF-367 accounted for 7.5% of tumor radioactivity, with >30% in plasma, at 40 min post-injection of the radiotracer, and that intact [¹¹C]OCM-44 accounted for 20% of tumor radioactivity, with >60% in plasma. [¹¹C]OCM-44 is superior to [¹¹C]PF-367 for detecting lesions in preclinical mouse models of pancreatic cancer, however, both radiotracers undergo rapid metabolism in vivo. GSK-3 PET radiotracers with improved in vivo stability are needed for clinical translation. To our knowledge this work represents the first PET imaging study of GSK-3 in oncology.

Positron Emission Tomography in Animal Models of Tauopathies

The microtubule-associated protein tau (MAPT) plays an important role in Alzheimer's disease and primary tauopathy diseases. The abnormal accumulation of tau contributes to the development of neurotoxicity, inflammation, neurodegeneration, and cognitive deficits in tauopathy diseases. Tau synergically interacts with amyloid-beta in Alzheimer's disease leading to detrimental consequence. Thus, tau has been an important target for therapeutics development for Alzheimer's disease and primary tauopathy diseases. Tauopathy animal models recapitulating the tauopathy such as transgenic, knock-in mouse and rat models have been developed and greatly facilitated the understanding of disease mechanisms. The advance in PET and imaging tracers have enabled non-invasive detection of the accumulation and spread of tau, the associated microglia activation, metabolic, and neurotransmitter receptor alterations in disease animal models. In vivo microPET studies on mouse or rat models of tauopathy have provided significant insights into the phenotypes and time course of pathophysiology of these models and allowed the monitoring of treatment targeting at tau. In this study, we discuss the utilities of PET and recently developed tracers for evaluating the pathophysiology in tauopathy animal models. We point out the outstanding challenges and propose future outlook in visualizing tau-related pathophysiological changes in brain of tauopathy disease animal models.

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.