Senile plaques and neurofibrillary tangles (NFTs) are characteristic hallmarks of the neuropathology of Alzheimer's disease. Numerous drug discovery efforts aimed at reducing the production or enhancing the clearance of amyloid-beta peptides, the principal component of senile plaques, have so far provided mixed results in clinical trials. In contrast, drug discovery programs aimed at modulating NFT pathology have been relative sparse although they present a substantial opportunity. NFTs are composed of aggregates of the microtubule associated protein, tau. Tau undergoes a variety of posttranslational modifications that can regulate its aggregation state. One such modification is the addition of O-linked N-acetylglucosamine (O-GlcNAc) moieties to serine and threonine residues by the glycosyltransferase, O-linked N-acetylglucosaminyltransferase. O-GlcNAcylation of tau is a highly dynamic process and the competing removal of O-GlcNAc moieties is mediated the glycoside hydrolase, O linked-N-acetylglucosaminidase (O-GlcNAcase). Recently, it was shown that increasing O-GlcNAcylation of tau in the brain using the O-GlcNAcase inhibitor Thiamet G could decrease tau pathology in transgenic mice and stabilise against aggregation. Although Thiamet G provided the initial conceptual data this inhibitor is a carbohydrate substrate mimetic and as such has poor CNS drug properties. Given these limitations we performed a high throughput screen to identify structurally distinct O-GlcNAcase inhibitor scaffolds with CNS drug properties. A subsequent hit optimization effort enabled us to identify novel, selective and highly brain penetrant O-GlcNAcase inhibitors. These compounds have a unique non-carbohydrate backbone and show a robust pharmacodynamic response in preclinical animal models. The objectives of this grant proposal are now to optimize our lead series to deliver an exploratory development candidate molecule and to perform a chronic proof-of-concept study with this molecule chemically distinct from Thiamet G in a tau transgenic mouse model. A positive outcome will enable the progression of the molecule into IND-enabling toxicity studies and ultimately into man.