A Study to Evaluate the Safety, Pharmacokinetics, and Pharmacodynamics of DNL921 in Healthy Participants and Participants With Alzheimer's Disease

Key Inclusion Criteria (Healthy Participants): * Are male or female of non-childbearing potential and are aged 18 through 60 years at the time of screening * Have a BMI of 18 through 32 kg/m2 and a body weight of at least 50 kg * For female participants: Are of non-childbearing potential * For male participants: Are surgically sterile by bilateral orchidectomy, or are abstinent, or agree to use two acceptable forms of contraception Key Exclusion Criteria (Healthy Participants): * Have any history of clinically significant neurological, psychiatric, endocrine, pulmonary, cardiovascular, gastrointestinal, hepatic, pancreatic, renal, metabolic, hematological, immunological, or allergic disease, or other major disorders * Have a positive serum pregnancy test or are currently lactating or breastfeeding * Have been hospitalized during the 4 weeks prior to screening Key Inclusion Criteria (Participants with AD): * Are male or female of non-childbearing potential and aged 50 to 75 years at the time of screening * Have a BMI between 18 and 32 kg/m2 and a body weight of at least 45 kg * Have a diagnosis of probable mild to moderate AD dementia based on NIA AA 2011 criteria, including amnestic or nonamnestic presentation at screening or prodromal AD (consistent with the NIA-AA diagnostic criteria and guidelines for mild cognitive impairment due to AD) * Have supportive evidence of AD pathology by the following: * Plasma ptau217/Abeta42 positive * Positive amyloid PET scan * Fo…