Study With Lu AF87908 in Healthy Participants and Participants With Alzheimer's Disease

The study will be divided into three parts: Part A, Part B and Part C.

Part A consists of up to 6 sequential cohorts (Cohorts A1 to A6):

-Cohorts A1 to A6: 8 healthy participants per cohort (aiming for an equal number of men and women): 6 randomized to Lu AF87908 and 2 randomized to placebo.

Part B consists of up to 3 sequential cohorts (Cohorts B1 to B3):

-Cohorts B1 to B3: 4 Japanese and 4 Chinese healthy participants per cohort: 6 participants randomized to Lu AF87908 and 2 participants randomized to placebo.

Part C consists of 2 sequential cohorts (Cohorts C1 and C4):

-Cohort C1 will consist of 6 participants with Alzheimer's disease with 3:1 randomization scheme for active: placebo. Cohort C4 will consist of up to 8 participants. Cohort C4 participants will be randomized based on the randomization scheme from Cohort C1.

Inclusion criteria

Healthy Participants:

-Men and women ≥18 and ≤65 years of age with a body mass index (BMI) ≥18 and ≤32 kilograms (kg)/square meter (m^2) and a minimum weight of 50 kg.

Participants with Alzheimer's disease:

• Men and women with a clinical diagnosis of Alzheimer's disease, Stages 3-4 according to the FDA guidelines.
• Mini-Mental State Examination (MMSE) of 15-30.
• Clinical Dementia Scale (CDR) global score up to and including 2.0.
• Confirmed or determined (via amyloid positron emission tomography [PET] scan) to be amyloid positive.
• If on FDA approved Alzheimer's disease medication, the treatment has been stable for 4 months prior to Day 1.

-≥50 years of age.

• BMI ≥18 and ≤40 kg/m^2 and a minimum weight of 50 kg.

Exclusion criteria

• Clinically relevant structural brain abnormality as assessed using magnetic resonance imaging (MRI).
• Any past or current treatment with an anti-Abeta or anti-tau active vaccine.
• Any past or current treatment with a monoclonal anti-tau antibody or a tau anti-sense oligomer within the last 6 months.
• Treatment with covid-19, influenza or pneumonia vaccine within the last 30 days prior to dosing of investigational medicinal product (IMP).

Other eligibility criteria may apply.