A Study to Assess the Safety and Effects of ABBV-1758 Following Subcutaneous or Intravenous Injections in Participants With 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. The purpose of this study is to test how safe ABBV-1758 is, how well it works, how the body processes it and what effects it has on the body. ABBV-1758 is an investigational drug being developed for the treatment of Alzheimer's disease. This study is conducted in 3 stages. Stage A is a multiple ascending dose study with a 1 in 5 chance (4:1 randomization) that participants are assigned to receive placebo. Stage B is a dose expansion phase, also using 4:1 randomization for ABBV-1758 or placebo. Stage C enrolls Japanese and Chinese participants with the same randomization scheme. Approximately 210 participants will be enrolled at about 55 sites in the United States, China, and Japan. Participants will receive intravenous (IV) or subcutaneous (SC) doses of ABBV-1758 or placebo once every 4 weeks (Q4W) for 24 weeks and will be followed for additional 12 weeks in the Follow-up Period. Participants will have the option of participating in a 12-month, blinded Extension Period receiving ABBV-1758 or placebo based on amyloid PET results. There may be higher treatment burden for participants in this trial compared to their standard of care due to study procedures. Participants will attend regular visits during the study at a hospital or clinic. The safety of the treatment will be checked by medical assessments, blood tests, and completing questionnaires.