PROJECT SUMMARY After decades of intensive research and drug development efforts, there are still no cures or substantially effective treatments for Alzheimer’s disease (AD). Currently available medications, including cholinesterase inhibitors, NMDA receptor antagonists, and amyloid beta (Aβ) monoclonal antibodies (mAbs), can only reduce certain symptoms or slightly slow down disease progression in mild to moderate AD patients; and their side effects, particularly those of Aβ mAbs, further limit their benefit. The most recently reported efficacy and safety profile of another Aβ mAb donanemab does not add much enthusiasm to the field. Studies of AD have been mainly focused on Aβ plaques and tau protein-formed neurofibrillary tangles in the brain. However, in recent years, it is increasingly clear that the development of AD is the collective consequence of the toxicities induced by Aβ plaques, tau protein-formed neurofibrillary tangles, and malfunctions of microglia. Nacetylcysteine (NAC) is a derivative of amino acid cysteine; it has been studied for its potential effects on AD, largely due to its antioxidant and anti-inflammatory activities, particularly its beneficial effects on microglia. However, NAC cannot penetrate the BBB and reach the brain sufficiently. To realize NAC’s potential as an AD treatment, we developed a brain targeted NAC delivery system (TN-NAC). Our preliminary data demonstrate that TN-NAC can effectively cross the BBB, quench the elevated ROS, attenuate the activation of microglia, reduce Aβ burden, and eliminate senescent cells in the brain in a 5xFAD mouse model, and improve the performance of the animals in behavior tests. Based on these exciting data, in this Phase I STTR study, we will test the feasibility to develop TN-NAC as a therapy for AD through two specific aims. In SA1 we will evaluate the toxicity and pharmacokinetic properties of TN-NAC in mice. The maximum tolerated dose (MTD) of TN-NAC will be first determined in mice; and then the PK properties of TN-NAC at various doses will be examined. In SA2 we will test the therapeutic efficacy of TN-NAC in a 3xTg-AD mouse model. We will assess the effects of TNNAC on biochemical, pathological, and behavioral alterations in the AD mice. We will also further evaluate the systemic toxicity of TN-NAC in AD mice. The successful completion of the proposed studies will serve as a milestone for the further development effort. A Go/No Go decision will be made based on 1) if TN-NAC can achieve sufficient blood and brain NAC concentrations at a certain dose without causing evident toxicity in mice; and 2) if TN-NAC at that dose can significantly attenuate AD pathology and behavioral impairment in the AD mice. A Go decision will lead us in a STTR Phase 2 study to: 1) using large animal models to find a dose range that can be extrapolated to humans and test the safety and efficacy of TN-NAC for the treatment of AD in a GLP setting using large animal models, and 2) carrying out cGM...