Brain and whole-body imaging of O-linked-N-acetyl-glucosamine hydrolase enzyme in healthy humans, monkeys, and mice using PET ligand^sup 18^F-OGA1

Introduction: Neurofibrillary tangles (NFTs), neuropil threads, and dystrophic neurites-all of which contain hyper-phosphorylated, insoluble tau protein-are a hallmark of Alzheimer’s disease (AD) and other tauopathies. Inhibiting O-linked N-acetyl-glucosamine hydrolase (O-GlcNAcase)-an enzyme that deglycosylates intracellular proteins including tau-is a key strategy for treating AD and related tauopathies. Here, we report on a novel positron emission tomography (PET) imaging radioligand that binds to O-GlcNAcase: 18F-OGA1 ([18F]LSN3316612). Methods: 18F-OGA1 was synthesized and evaluated in four rhesus monkey brains at baseline and under blocking conditions, pretreatment with thiamet G an O-GlcNAcase specific blocker (10mg/kg). Distribution volume (VT) was measured via an unconstrained two-tissue compartment model using a radiometabolite-corrected arterial input function. In monkeys, non-displaceable VT (VND) was estimated via the Lassen plot using differences in VT between baseline and blocked scans. Brain PET scans were also performed in seven healthy human subjects for three hours with full arterial blood sampling. To indirectly assess whether radiometabolites accumulate in brain during the course of the scan, the stability of VT was calculated over time. Six additional healthy human subjects underwent whole-body PET scan to estimate the radiation dose of 18F-OGA1. Wild type mice and brain O-GlcNAcase knockout mice were also scanned to confirm specificity of the radioligand. Results: In both monkeys and humans, the in vivo distribution of radioactivity was widespread in the brain (Fig. 1). In monkeys, the uptake of radioactivity was high, with highest uptake in striatum standardized uptake value (SUV ~6) and lowest in cerebellum (Fig. 2A). After pretreatment with thiamet-G (Fig. 2B) or cold parent in monkeys, brain uptake decreased > 90%. Brain uptake of 18F-OGA1 in wild type mice was significantly higher than in O-GlcNAcase knockout mice, further supported high specificity of the radioligand. In healthy human subjects, brain uptake peaked around 4-5 SUV followed by moderate washout. Based on the VT, the highest uptake was observed in amygdala followed by insula, cingulate, hippocampus and cerebellum. A two-tissue compartment model fit the curves from all regions (with SE < 10%). One hundred and eighty-minute scans provided stable VT values in all brain regions. Because no brain region lacks O-GlcNAcase expression, we could not apply a reference region