A Study to Learn About the Safety of Diranersen (BIIB080) Injections and Whether They Can Improve Symptoms of Participants With Mild Cognitive Impairment Due to Alzheimer's Disease (AD) or Mild AD Dementia Between 50 to 80 Years of Age

Dose response in Change From Baseline to Week 76 on the CDR-SB

Timeframe: Baseline to Week 76

The Clinical Dementia Rating (CDR) scale is a clinician-rated dementia staging system that tracks the progression of cognitive impairment in 6 categories (memory, orientation, judgment and problem solving, community affairs, home and hobbies, and personal care). Each category is scored on a 5-point scale in which None=0, Questionable=0.5, Mild=1, Moderate=2, and Severe=3. The global CDR score is established by clinical scoring rules and has values of 0 (no dementia), 0.5, (questionable dementia), 1 (mild dementia), 2 (moderate dementia), and 3 (severe dementia). The CDR-SB is obtained by adding the ratings in each of the 6 categories and ranges from 0 to 18 with higher scores indicative of greater impairment.

Change From Baseline to Week 76 on the CDR-SB

Timeframe: Baseline to Week 76

The CDR scale is a clinician-rated dementia staging system that tracks the progression of cognitive impairment in 6 categories (memory, orientation, judgment and problem solving, community affairs, home and hobbies, and personal care). Each category is scored on a 5-point scale in which None=0, Questionable=0.5, Mild=1, Moderate=2, and Severe=3. The global CDR score is established by clinical scoring rules and has values of 0 (no dementia), 0.5, (questionable dementia), 1 (mild dementia), 2 (moderate dementia), and 3 (severe dementia). The CDR-SB is obtained by adding the ratings in each of the 6 categories and ranges from 0 to 18 with higher scores indicative of greater impairment. Positive change from baseline indicates clinical decline.

Change From Baseline to Week 76 on the Alzheimer's Disease Cooperative Study Activities of Daily Living for Mild Cognitive Impairment (ADCS-ADL-MCI)

Timeframe: Baseline to Week 76

The ADCS-ADL-MCI consists of 17 instrumental items (e.g., shopping, preparing meals, using household appliances) and 1 basic item (getting dressed). Ratings reflect caregiver observations about the participant's actual functioning and provide an assessment of change in the functional state of the participant over time. The total score ranges from 0 to 53, with lower values over time reflecting functional deterioration. Positive change from baseline indicates clinical improvement.

Change From Baseline to Week 76 on the Alzheimer's Disease Assessment Scale Cognitive Subscale (ADAS-Cog 13)

Timeframe: Baseline to Week 76

ADAS-Cog13 comprises both cognitive tasks and clinical ratings of cognitive performance. The scale items capture word recall, ability to follow commands, the ability to correctly copy or draw an image, naming, the ability to interact with everyday objects, orientation, word recognition, memory, comprehension of spoken language, word-finding, and language ability, with a measure for delayed word recall and concentration/distractibility. The total score ranges from 0 to 85. An increase in score over time indicates increasing cognitive impairment. Positive change from baseline indicates clinical decline.

Change From Baseline to Week 76 on the Mini Mental State Examination (MMSE)

Timeframe: Baseline to Week 76

The MMSE is a widely used performance-based test of global cognitive status. It consists of 11 tasks that assess orientation, word recall, attention and calculation, language abilities, and visuospatial functions. The scores from the 11 tests are combined to obtain the total score, which ranges from 0 to 30, with lower scores over time indicating increasing cognitive impairment. Positive change from baseline indicates clinical improvement.

Change From Baseline to Week 76 on the Modified Integrated Alzheimer's Disease Rating Scale (iADRS)

Timeframe: Baseline to Week 76

iADRS is a composite and is calculated as a linear combination of total scores of ADAS-Cog13 and Alzheimer's Disease Cooperative Study Instrumental Activities of Daily Living Inventory (ADCS-iADL) that measures cognition and daily function. ADCS-iADL is calculated from a subset of questions from ADCS-ADL. Range for ADCS-iADL is 0-59 and higher scores reflect better performance. ADAS-Cog13 comprises cognitive tasks and clinical ratings of cognitive performance. Scale items capture word recall, ability to follow commands, ability to correctly copy/draw, naming, ability to interact with everyday objects, orientation, word recognition, memory, spoken language comprehension, word-finding, and language ability, with a measure for delayed word recall and concentration/distractibility. Score ranges from 0 to 85 with higher scores reflecting cognitive impairment. iADRS score range is 0-144 and higher scores indicate greater impairment. Positive change from baseline indicates clinical decline.

Change From Baseline to Week 76 on the Alzheimer's Disease Composite Score (ADCOMS)

Timeframe: Baseline to Week 76

ADCOMS is a composite score comprised of ADAS-cog (4 items), MMSE (2 items) and CDR-SB (6 items). The total scores on the scale range from 0 to 1.97 with higher scores indicating greater impairment. Positive change from baseline indicates clinical decline.

Number of Participants With Treatment-Emergent Adverse Events (TEAEs) and Serious Adverse Events (SAEs)

Timeframe: From first dose of study drug up to end of study of placebo-controlled period (up to Week 96)

An AE is any untoward medical occurrence in a patient or clinical investigation participant administered a pharmaceutical product and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including abnormal laboratory finding), symptom, or disease temporally associated with use of a medicinal (investigational) product, whether or not related to medicinal (investigational) product. TEAE is any AE that started or worsened on or after the administration of the first dose of study drug through the end of follow-up period. SAE is any untoward medical occurrence that at any dose results in death, in the view of investigator, places the participant at immediate risk of death (life-threatening event), requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, results in congenital anomaly/birth defect or is medically important event.