A Study of Gantenerumab in Participants With Mild Alzheimer Disease

• Clinical diagnosis of probable mild Alzheimer disease (AD) based on National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS/ADRDA) criteria or major NCD based whether or not receiving AD approved medication
• Cerebral spinal fluid (CSF) result consistent with the presence of amyloid pathology
• Availability of a person ('caregiver') who in the investigator's judgment has frequent and sufficient contact with the participant, and is able to provide accurate information regarding the participant's cognitive and functional abilities
• Fluency in the language of the tests used at the study site
• Willingness and ability to complete all aspects of the study
• Adequate visual and auditory acuity, in the investigator's judgment, sufficient to perform the neuropsychological testing (eye glasses and hearing aids are permitted)
• If currently receiving approved medications for AD, the dosing regimen must have been stable for 3 months prior to screening
• Agreement not to participate in other research studies for the duration of this trial and its associated substudies

PART 2 - All participants who have been randomized and are actively participating in the study are eligible for Part 2

• Dementia or neurocognitive disorder (NCD) due to a condition other than AD, including, but not limited to, frontotemporal dementia, Parkinson disease, dementia with Lewy bodies, Huntington disease, or vascular dementia
• History or presence of clinically evident vascular disease potentially affecting the brain that in the opinion of the investigator has the potential to affect cognitive function
• History or presence of stroke within the past 2 years or documented history of transient ischemic attack within the last 12 months
• History or presence of systemic autoimmune disorders potentially causing progressive neurologic disease with associated cognitive deficits
• History of schizophrenia, schizoaffective disorder, or bipolar disorder
• Alcohol and/or substance use disorder (according to the DSM-5) within the past 2 years (nicotine use is allowed)
• History or presence of atrial fibrillation
• Within the last 2 years, unstable or clinically significant cardiovascular disease (e.g., myocardial infarction, angina pectoris, cardiac failure New York Heart Association Class II or higher)
• Uncontrolled hypertension
• Chronic kidney disease
• Impaired hepatic function

PET imaging substudy, in addition to above:

• Prior participation in other research study or clinical care within the last year such that the total radiation exposure would exceed the local or national annual limits

Part 2 Participants who have been discontinued from the study