 This is a randomized study drug assigned by chance, double-blind neither the researchers nor the participants know what treatment the participant is receiving, multi-center more than one hospital or medical school team work on a medical research study, placebo-controlled, parallel-group study in participants who are asymptomatic and at risk for developing Alzheimers dementia. The study will consist of a Screening Phase approximately 90 days, treatment Phase 54 months and follow-up Phase 7 to 28 days. In treatment Phase eligible Participants will be randomized to receive study drug or placebo once daily for up to 4.5 years. The maximum study duration for a participant will be 58 months. Participants safety will be monitored throughout the study. 