The study (NCT03352557) was terminated based on lack of efficacy following the placebo-controlled period readout.

The primary objective of the placebo-controlled period is to evaluate the safety and tolerability of BIIB092 in participants with mild cognitive impairment (MCI) due to Alzheimer's disease (AD) or with mild AD. The secondary objectives of the placebo-controlled period are to evaluate the efficacy of multiple doses of BIIB092 in slowing cognitive and functional impairment in participants with MCI due to AD or with mild AD, and to evaluate the immunogenicity of BIIB092 after multiple doses in participants with MCI due to AD or with mild AD.

The primary objective of the long-term extension period is to evaluate the long-term safety and tolerability of BIIB092 in participants with MCI due to AD or with mild AD.

Key Inclusion Criteria:

• Must have a gradual and progressive change in memory function over more than 6 months.
• Must meet all of the clinical criteria for mild cognitive impairment (MCI) due to Alzheimer's disease (AD) or mild AD and must have
• Objective evidence of cognitive impairment at Screening
• Clinical Dementia Rating Scale (CDR) global score of 0.5 for MCI due to AD or 0.5 or 1 for mild AD
• Mini-Mental State Examination (MMSE) score of 22 to 30 (inclusive)
• CDR Memory Box score of ≥0.5
• Must consent to apolipoprotein E (ApoE) genotyping
• Must have 1 informant/study partner
• Must have amyloid beta positivity confirmed at Screening

Key Exclusion Criteria:

• Any medical or neurological/neurodegenerative condition (other than AD) that, in the opinion of the Investigator, might be a contributing cause to the participant's cognitive impairment or could lead to discontinuation, lack of compliance, interference with study assessments, or safety concerns
• Clinically significant, unstable psychiatric illness
• Have had a stroke or Transient Ischemic Attack (TIA) or unexplained loss of consciousness in the past 1 year
• Relevant brain hemorrhage, bleeding disorder and cerebrovascular abnormalities
• History of unstable angina, myocardial infarction, chronic heart failure or clinically significant conduction abnormalities within 1 year prior to Screening Visit 1
• Indication of impaired renal or liver function
• Alcohol or substance abuse in past 1 year
• Clinically significant systemic illness or serious infection within 30 days prior to or during the screening period
• Use of allowed medications for chronic conditions at doses that have not been stable for at least 4 weeks prior to Screening Visit 1 and during the screening period up to Study Day 1, or use of AD medications at doses that have not been stable for at least 8 weeks prior to Screening Visit 1 and during the screening period up to Study Day 1.
• Use of any medications that, in the opinion of the Investigator, may contribute to cognitive impairment, put the participants at higher risk for adverse events (AEs), or impair the participant's ability to perform cognitive testing or complete study procedures.
• Contraindications to study procedures

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply

In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on http://clinicalresearch.biogen.com/