The reason for this study is to collect safety and efficacy information regarding the study drug remternetug in participants with early symptomatic Alzheimer's disease (AD).

TRAILRUNNER-ALZ 1 is a Phase 3, randomized, double-blind, placebo-controlled study to evaluate the safety and efficacy of remternetug in participants with early symptomatic AD. Initially, 600 participants will enroll in the double-blind treatment period. Participants in the double-blind treatment period will receive remternetug or placebo administered via subcutaneous injection or intravenous infusion.

Following the 52-week main study period, participants will continue participation for up to an additional 76 weeks in an extension period. Participants who previously received remternetug will receive placebo and participants who previously received placebo will receive remternetug. Thus, all participants will receive remternetug if they complete the study.

An additional 974 participants with early Alzheimer's disease will be enrolled to an addendum safety cohort. The participants will be administered open label remternetug via subcutaneous injection or intravenous infusion. Participants enrolled into the addendum safety cohort are not eligible for the extension period.

Inclusion Criteria:

• Gradual and progressive change in cognitive function ≥6 months prior to screening.
• A Mini-Mental (MMSE) score of 20 to 30 (inclusive) at screening.
• Has an amyloid PET scan result consistent with the eligibility criteria.
• Have a reliable study partner who will provide written informed consent to participate and is in frequent contact with the participant.
• Have adequate literacy, vision, and hearing for the neuropsychological testing in the opinion of the investigator at the time of screening.
• Are reliable and willing to make themselves available for the duration of the study and are willing to follow study procedures.
• Males and females will be eligible for this study.
• Women not of childbearing potential (WNOCBP) may participate in this trial.

Exclusion Criteria:

• Current serious or unstable illnesses that, in the investigator's opinion, could interfere with the analyses in this study; or has a life expectancy of <24 months.
• History of cancer with high risk of recurrence and preventing completion of the trial.
• Participants with any current primary psychiatric diagnosis other than AD that in the investigator's opinion, is likely to confound interpretation of the drug effect, affect cognitive assessment, or affect the participant's ability to complete the study.
• History of clinically significant multiple or severe drug allergies.
• Have any clinically important abnormality at screening, as determined by investigator that could be detrimental to the participant, could compromise the study, or show evidence of other etiologies for dementia.
• Screening magnetic resonance imaging (MRI) which shows evidence of significant abnormality that would suggest another potential etiology for progressive dementia or a clinically significant finding that may impact the participant's ability to safely participate in the study.
• Have any contraindications for MRI or positron emission tomography (PET).
• Have had prior treatment with a passive anti-amyloid immunotherapy.
• Have received active immunization against Aβ in any other study.
• Have known allergies to remternetug related compounds, or any components of the formulation.

Anonymized individual patient level data will be provided in a secure access environment upon approval of a research proposal and a signed data sharing agreement.

Data are available 6 months after the primary publication and approval of the indication studied in the US and EU, whichever is later. Data will be indefinitely available for requesting

A research proposal must be approved by an independent review panel and researchers must sign a data sharing agreement.

The main treatment period is double-blinded and the addendum is open-label.