The name of this trial is MissionAD1. This 24-month treatment, multicenter, double-blind, placebo-controlled, parallel group, Phase 3 study in participants with Early Alzheimer's Disease (EAD) including mild cognitive impairment (MCI) due to AD/Prodromal AD and the early stages of mild AD will be conducted to evaluate the efficacy and safety of elenbecestat (proposed international nonproprietary name [INN]) (E2609).