Brainshuttle AD: A Multiple Ascending Dose Study to Investigate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of RO7126209 Following Intravenous Infusion in Participants With Prodromal or Mild to Moderate Alzheimer's Disease

Key inclusion criteria for part 1, 2 and 3: * Ability to provide written consent signed by the participant * Availability of a person (referred to as the "study partner") who: consents to participate throughout the duration of study, in the Investigator's judgment, has frequent and sufficient contact with the participant, is fluent in the language of the tests used at the study site * Willingness and ability to complete all aspects of the study (including magnetic resonance imaging \[MRI\], lumbar puncture, clinical genotyping, and positron emission tomography \[PET\] imaging) * Capable of completing assessments either alone or with the help of the study partner * Adequate visual and auditory acuity, in the Investigator's judgment, sufficient to perform the neuropsychological testing (eye glasses and hearing aids are permitted) * Probable mild to moderate AD dementia (consistent with National Institute on Aging-Alzheimer's Association \[NIA-AA\] core clinical criteria for probable AD dementia) or prodromal AD (consistent with the NIA-AA diagnostic criteria and guidelines for mild cognitive impairment due to AD) * Screening Mini-Mental State Examination (MMSE) score of 18 to 28 points, inclusive, within 84 days before baseline * Clinical Dementia Rating-Global Score (CDR-GS) of 0.5, 1, or 2 within 84 days before baseline * Positive amyloid PET scan (cut-off: \>50 Centiloid units) within 12 months before baseline * In case of treatment with symptomatic AD medications, dosing r…