PROJECT SUMMARY/ABSTRACT We previously reported (2) on the soluble amyloid precursor protein alpha (sAPPα)-enhancing effects of tropisetron (F03). Highly brain-permeable F03 is approved in 49 countries for the treatment of post-operative nausea and vomiting (PONV) and is a multifunctional ligand: it is a potent 5-HT3 serotonin receptor (5-HT3R) antagonist (Ki ~ 3 nM), a partial α7 nicotinic acetylcholine receptor (α7nAChR) agonist (Ki ~ 450 nM), and binds to the extracellular domain of amyloid precursor protein (eAPP). In our studies, we found F03 increases sAPPα and also significantly decreases the phospho-tau (p-tau)/total tau (t-tau) ratio in two Alzheimer's disease (AD) mouse models. We show that F03 can also reduce corticotropin-releasing factor (CRF) induced p-tau/tau increase in vitro. Hyperphosphorylation of tau leads to toxic tau oligomers and ultimately formation of neurofibrillary tangles (NFTs) in AD brain and is closely correlated with cognitive decline (34); thus, decreasing tau phosphorylation is a critical target for new therapeutic approaches for AD and tauopathies. Based on reports that sAPPα reduces the p-tau/t-tau ratio through suppression of activity of the kinase GSK3β (9), we plan to assess the ability of our sAPPα enhancers to reduce the p-tau/t-tau ratio in vitro and in vivo. A key goal of the project is to identify optimized small molecule sAPPα-enhancing, p-tau/t-tau lowering compounds that improve cognition in murine AD models, for further development as a novel therapy for AD. An additional goal would be to elucidate the underlying mechanism of action (MOA) of sAPPα enhancement and related reduction in the p- tau/t-tau ratio. In Aim 1, we would test F03 and analogs we have in-hand as well as new analogs from Aim 2 in SH-SY5Y cells and 3xTg-AD primary neurons to establish EC50s for sAPPα enhancement and p-tau/t-tau decreases. Prioritized compounds would be evaluated for receptor binding and APP binding. Tertiary testing will be in induced pluripotent stem cell (iPSC)-derived neurons from AD subjects and includes synaptic spine density quantification. In Aim 2, medicinal chemistry would be used to design new chemical entity (NCE) analogs in a iterative fashion with receptor and APP binding along with enhanced drug-like properties and oral brain permeability. In Aim 3, ADME studies on analogs include solubility, microsomal stability, protein binding, parallel artificial membrane permeability assay (PAMPA) analysis, and in vivo pharmacokinetics (PK). Optimal candidates would undergo safety (including hERG) and off-target panel profiling along with analyses of effect on other kinases and of tau phosphorylation sites. In Aim 4 in vivo testing, including acute studies on select candidates to evaluate effect on intracerebroventricular (ICV) delivered CRF induced p-tau/t-tau increases in brain in 3xTg-AD mice. Active candidates would be tested in chronic 4-week efficacy studies in 3xTg-AD and ApoE4-5XFAD AD model mice. Readout...