Structural Basis for Achieving GSK-3β Inhibition with High Potency, Selectivity, and Brain Exposure for Positron Emission Tomography Imaging and Drug Discovery

Using structure-guided design, several
cell based assays, and microdosed
positron emission tomography (PET) imaging, we identified a series
of highly potent, selective, and brain-penetrant oxazole-4-carboxamide-based
inhibitors of glycogen synthase kinase-3 (GSK-3). An isotopologue
of our first-generation lead, [3H]­PF-367, demonstrates
selective and specific target engagement in vitro, irrespective of
the activation state. We discovered substantial ubiquitous GSK-3-specific
radioligand binding in Tg2576 Alzheimer’s disease (AD), suggesting
application for these compounds in AD diagnosis and identified [11C]­OCM-44 as our lead GSK-3 radiotracer, with optimized brain
uptake by PET imaging in nonhuman primates. GSK-3β-isozyme selectivity
was assessed to reveal OCM-51, the most potent (IC50 =
0.030 nM) and selective (>10-fold GSK-3β/GSK-3α) GSK-3β
inhibitor known to date. Inhibition of CRMP2T514 and tau
phosphorylation, as well as favorable therapeutic window against WNT/β-catenin
signaling activation, was observed in cells.