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In this study, researchers will learn more about a study drug called BIIB080. The study will focus on participants with mild cognitive impairment or mild dementia due to AD.

The main question researchers are trying to answer is if BIIB080 can slow the worsening of AD more than placebo. It will focus on what dose of BIIB080 slows worsening of AD the most. To help answer this question, researchers will use the Clinical Dementia Rating-Sum of Boxes, also known as the CDR-SB. - Clinicians use the CDR-SB to measure several categories of dementia symptoms. - The results for each category are added together for a total score. Lower scores are better. Researchers will also learn more about the safety of BIIB080. The study will be split into 2 parts. The 1st part is the Placebo-Controlled Period. The 2nd part is the Long-Term Extension Period. The 2nd part of the study will help researchers learn about the long-term safety of BIIB080, and how it affects the participant's daily life, thinking, and memory abilities in the longer term. A description of how the study will be done is given below. - After screening, participants will first receive either a low dose or high dose of BIIB080, or a placebo, as an injection into the fluid around the spinal cord (cerebrospinal fluid). A placebo looks like the study drug but contains no real medicine. - Participants will receive BIIB080 or placebo once every 12 weeks or 24 weeks. - After 76 weeks of treatment in the Placebo-Controlled Period, eligible participants will move onto the Extension Treatment period, which will last 96 weeks. - In the extension period, participants who received placebo will be switched to high dose BIIB080 every 12 or 24 weeks. - Participants may be in the study for up to 201 weeks, or about 4 years. This includes the screening and follow-up periods. - Participants can continue to take certain medications for AD. Participants must be on the same dose of medication for at least 8 weeks before the screening period. - After the screening period, most participants will visit the clinic every 6 weeks.