A Study to Evaluate the Safety and Biomarker Effects of RO7269162 in Participants at Risk for or at the Prodromal Stage of Alzheimer's Disease (AD)

This clinical trial is recruiting people who either are at risk of AD - have build-up of beta-amyloid, but have no clinical symptoms, or with a diagnosis of mild cognitive impairment. People can take part if they have a certain level of plaques (beta-amyloid) in the brain, shown by a positron emission tomography (PET) scan, a medical imaging technique in which tracers are injected to visualize specific pathological processes in the brain. People who take part in this clinical trial (participants) will be given RO7269162 OR placebo for up to about 1 and a half years. The clinical trial team will see them every 3 weeks in the first 3 months and then every 6 weeks until the end of the trial. These hospital visits will include checks to see how the participant responds to the treatment and any side effects they may have. The total time of participation in the clinical trial will be 90 weeks.

Inclusion Criteria: * Body Mass Index (BMI) between 18 to 35 kg/m\^2 inclusive at screening * Participants must be either cognitively unimpaired or with a diagnosis of MCI due to AD, according to the National Institute on Aging - Alzheimer's Association (NIA - AA) research framework * Clinical Dementia Rating-Global Score (CDR-GS) of 0 or 0.5 * Positive amyloid PET scan based on a cut-off of ≥24 CL units * Availability of a person (referred as a "study partner" throughout the protocol) who: (a) has frequent and sufficient contact (e.g., minimum twice a week in-person, via telephone, video calls, by e-mail or other electronic means) with the participant, and is willing and able to provide accurate information regarding the participant's cognitive and functional abilities, signs the necessary ICF(s), and has sufficient cognitive capacity to accurately report on the participant's cognitive and functional abilities; (b) is in sufficient good general health to have a high likelihood of maintaining the same level of interaction with the participant and participation in study procedures throughout the duration of the study; and (c) is fluent in the language of the tests used at the study site. Please note that the study partner does not need to be a family member. Every effort should be made to keep the same study partner throughout the study * In case of treatment with symptomatic AD medications, dosing regimen must be stable for at least eight weeks prior to baseline Exclusion Criteria: * Any medical history or evidence of a condition other than AD that may affect cognition * History or presence of significant cardiovascular conditions and/or significant hematological disease * History or presence of chronic kidney disease and/or impaired hepatic function * Uncontrolled/poorly controlled diabetes * History of or active inflammatory bowel disease * Have received any passive or active immunotherapy (immunoglobulin) or other long-acting biologic agent that is under evaluation or approved to prevent or postpone cognitive decline administered within 1 year prior to baseline, and/or any other investigational treatment within five half-lives or 16 weeks prior to screening, whichever is longer