⁃ Study Part 1

• Age ≥50 and ≤85 years at screening.

• Diagnosis of prodromal AD: MCI due to AD according to National Institute on Aging Alzheimer's Association (NIA-AA) criteria.

• PET scan at screening consistent with the presence of amyloid pathology.

• Clinical Dementia Rating (CDR)-Global Score of 0.5.

• Subjects either not taking any marketed treatment for AD or receiving a stable dose of an acetylcholinesterase inhibitor (ACHEI) and/or memantine for at least 2 months prior to baseline.

⁃ Study Part 2

• Age ≥35 and ≤50 years at screening (subjects with DS with age ≥35 and ≤39 years may be considered on the condition that there is prior evidence of amyloid results compatible with AD pathology at PET-scan and/or in biofluids).

• Male or female subjects with DS with a cytogenetic diagnosis being either trisomy 21 or complete unbalanced translocation of the chromosome 21.

• PET scan at screening consistent with the presence of amyloid pathology.

• Mild to moderate intellectual disability as per Diagnostic and Statistical Manual of Mental Disorders (DSM-5) classification.

• Subjects must have a study partner who has direct and regular contact, at least 10 hours per week, with the subject and who is able to provide reliable answers to questions related to the subject, according to the study investigator.