PROJECT 3 ABSTRACT Medications are an important exposure in the life course epidemiology framework that ties the Adult Changes in Thought (ACT) U19 Program together. Medications for chronic conditions are taken over several years, often beginning in mid-life, and chronic regimens achieve consistent blood levels, so toxic or protective brain effects of medication exposures are plausible. The Adult Changes in Thought (ACT) study has served as an incredible resource for cutting-edge dementia pharmacoepidemiology research for over 20 years. We take a multifaceted approach to examine links between common drugs and brain health by using an array of brain-related measures in the same cohort to deepen our understanding about these relationships and mechanisms. A considerable methodologic issue inherent in pharmocoepidemiology research is confounding by indication, where the condition for which a drug is prescribed is associated with dementia rather than the drug itself. In this Project, we take an innovative and translational approach to complement our existing pharmacoepidemology methods by deploying molecular assays that will directly address confounding by indication bias with a cell-based model using human induced pluripotent stem cell (hiPSC)-derived neurons (hiPSC-Ns) from ACT participants. The following Aims will be accomplished by working directly with all Project Cores. Aim 1: Deploy a human stem cell- based molecular assay to directly test mechanisms of neurotoxicity from anticholinergics (AChs) and address confounding by indication. We will deploy assays that measure four cellular outcomes: AD pathological molecules, Ab and pTau; neurotoxicity; and neuronal function. Aim 2: To determine comparative associations of antihypertensives (AHTs) with dementia and AD (2A), neuropathology (2B), and neuroimaging outcomes (2C), and test mechanisms of neuroprotection (2D). We will test the hypotheses that after controlling for their effects on blood pressure, exposure to Ang-II↑ drugs compared with Ang-II↓ drugs is associated with lower risk of dementia and AD, neuropathology and neuroimaging outcomes. We will test the hypothesis that Ang-II↑ drugs will have positive effects on cellular outcomes (i.e: Aβ and pTau, neurotoxicity and neuronal function) compared with an Ang-II↓ drug using the group of hiPSC-derived neurons developed in Aim 1. Our Project has important