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 toparticipate throughout the duration of study, in the Investigator's judgment, hasfrequent and sufficient contact with the participant, is fluent in the language ofthe tests used at the study site

• Willingness and ability to complete all aspects of the study (including magneticresonance imaging [MRI], lumbar puncture, clinical genotyping, and positron emissiontomography [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 toperform the neuropsychological testing (eye glasses and hearing aids are permitted)

• Probable mild to moderate AD dementia (consistent with National Institute onAging-Alzheimer's Association [NIA-AA] core clinical criteria for probable ADdementia) or prodromal AD (consistent with the NIA-AA diagnostic criteria andguidelines 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 beforebaseline

• Positive amyloid PET scan (cut-off: >50 Centiloid units) within 12 months beforebaseline

• In case of treatment with symptomatic AD medications, dosing regimen must be stablefor at least 8 weeks prior to baseline and until randomization

• Agreement not to donate blood or blood products for transfusion for the duration ofthe study and for 1 year after final dose of study drug

• Agreement not to participate in other research studies for the duration of thisstudy

• Agree to apolipoprotein E (APOE) genotyping

Inclusion criteria for Part 4:

• Completed the treatment period in Part 1, Part 2, or Part 3 of the study

Key exclusion criteria for part 1, 2 and 3:

• Any evidence of other relevant neurological condition, including other (non-AD)neurodegenerative and neuropsychiatric conditions, neurovascular brain disorders,seizure disorders, inflammatory and infectious disorders of the central nervoussystem, trauma and delirium, among several others

• Other relevant medical conditions including significant hematological diseases, anyclinically significant ophthalmologic diseases, decreased visual acuity in eithereye, with a BCVA letter score of less than 20 letters on the Early TreatmentDiabetic Retinopathy Study (ETDRS) chart or the Snellen equivalent of 20/400 if theETDRS chart is not used

• Clinically significant cardiovascular diseases, chronic kidney disease, confirmedand unexplained impaired hepatic function, abnormal thyroid function, among severalothers

• History of hypersensitivity to biologic agents or any of the excipients in theformulation

• Clinically significant abnormalities (as judged by the Investigator) in laboratorytest results (including complete blood count, chemistry panel, routine cerebrospinalfluid [CSF] parameters and urinalysis)

• MRI exclusion criteria: >2 lacunar infarcts, any territorial infarct >1 cm^3, anywhite matter lesion that corresponds to an overall Fazekas score of 3 that requiresat least one confluent hyperintense lesion on the fluid-attenuated inversionrecovery (FLAIR) sequence, which is ≥20 mm in any dimension

• More than 4 microhemorrhages on MRI and/or presence of any focal area ofleptomeningeal hemosiderosis based on the review performed by the central MRI readerprior to randomization

• Presence of any other significant cerebral abnormalities, including amyloid-relatedimaging abnormality-edema/effusion (ARIA-E), as assessed on MRI

• Inability to tolerate MRI procedures or contraindication to MRI

• Inability to undergo ophthalmological assessments

• Contraindication to lumbar puncture

• Contraindication to having a PET scan

Exclusion criteria for Part 4:

• Prematurely discontinued from the treatment period for study (i.e., before the startof the follow-up period of Part 1, Part 2, or Part 3) for any reason or meetingdiscontinuation criteria before the baseline visit of Part 4.

• Received any active investigational treatment other than RO7126209 during or sincecompletion of Part 1, Part 2 or Part 3

• Any passive immunotherapy (immunoglobulin) since completion of Part 1, Part 2, orPart 3 that is meant to prevent or postpone cognitive decline.

• Use of anti-coagulation medications - Evidence of ongoing ARIA-E. In this caseparticipant may enroll into Part 4 once the ARIA-E is resolved - Evidence of ongoinginfusion-related reaction (IRR) or hypersensitivity reaction. In this caseparticipant may enroll into Part 4 once the IRR is resolved.

• MRI evidence of any of the following at OLE baseline: evidence of ongoing ARIA-E,any ARIA-H (leptomeningeal hemosiderosis or microhemorrhages) that would requirepermanent discontinuation of study treatment, > 2 lacunar infarcts, Any territorialinfarct > 1 cm^3, any white matter lesion that corresponds to an overall Fazekasscore of 3 that requires at least one confluent hyperintense lesion on the FLAIRsequence, which is ≥ 20 mm in any dimension

• Any drop in hemoglobin of > 20% compared to predose on Day 1 or hemoglobin valuebelow 10 g/dL