Study to Assess Adverse Events, Change in Disease Activity and How Oral ABBV-552 Capsules Moves Through the Body of Participants Aged 50 to 90 Years With Mild Alzheimer's Disease

Alzheimer's disease (AD) is a progressive, irreversible neurological disorder and is the most common cause of dementia in the elderly population. Clinical symptoms of the disease may begin with occasional forgetfulness such as misplacement of items, forgetting important dates or events, and may progress to noticeable memory loss, increased confusion and agitation, and eventually, loss of independence and non-responsiveness. This study will assess how safe and effective ABBV-552 is in treating symptoms of early AD. Adverse events, change in disease activity, how ABBV-552 moves through body of participants and the body response to ABBV-552 will be assessed.

ABBV-552 is an investigational drug being developed for the treatment of Alzheimer's disease (AD). Study doctors put the participants in 1 of 4 groups (3 active dose groups and a placebo group), called treatment arms. Each group receives a different treatment. There is a 1 in 4 chance that participants will be assigned to placebo. Approximately 240 participants aged 50-90 years with mild AD will be enrolled in approximately 60 sites across the world.

Participants will receive oral ABBV-552 or placebo capsules once daily for 12 weeks and followed for 30 days after the last dose of study drug.

There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.