A Randomized, Double-blind, Placebo-controlled Study to Assess Safety, Tolerability, Pharmacokinetics, Immunogenicity, and Pharmacodynamic Response of Repeated Intravenous Infusions of BAN2401 in Subjects With Mild Cognitive Impairment Due to Alzheimer's Disease and Mild Alzheimer's Disease

Inclusion Criteria MCI due to AD 1. Subjects who have clinical and cognitive symptoms consistent with the National Institute on Aging-Alzheimer's Association (NIA-AA) core criteria for MCI 2. Subjects who have a Clinical Dementia Rating (CDR) of 0.5 and a memory box score of 0.5 or greater at Screening 3. Subjects who report a history of subjective memory decline with slow progression at least 1 year before Screening, or subjects whose information provider or attending physician reports a history of memory decline with slow progression at least 1 year before Screening 4. Subjects with objective impairment in episodic memory as indicated by 1-1.5 standard deviations below age-adjusted mean in the Wechsler Memory Scale-Revised (WMS-R) logical memory II (delayed recall) at Screening: * less than or equal to 15 for age 50 to 64 years * less than or equal to 12 for age 65 to 69 years * less than or equal to 11 for age 70 to 74 years * less than or equal to 9 for age 75 to 79 years * less than or equal to 7 for age 80 to 90 years Mild AD 5. Subjects who meet the NIA-AA core clinical criteria for probable AD 6. Subjects who have a CDR of 0.5 or 1.0 and a memory box score of 0.5 or greater at Screening All subjects 7. Male or female subjects aged between 50 and 90 years, inclusive, at obtaining informed consent 8. Subjects who have an Mini Mental State Examination (MMSE) score greater than or equal to 22 and less than or equal to 30 at Screening 9. Body Mass Index (BMI) less than 35 kg/m2 at Screening 10. Females must not be pregnant or lactating, and specified contraceptive precautions must be followed 11. Subjects must have identified caregivers/informants 12. Must have an informant or a caregiver who will provide written informed consent voluntarily and is able to spend 3 days a week with the subject (4 hours per day), and is able to support the subject during the study period by providing necessary patient information, assisting treatment compliance, and accompanying the subject to all scheduled visits (if needed) throughout the study. 13. Provide voluntary written informed consent (obtaining as much as possible from subjects, but mandatory from their legal guardians). 14. Willing and able to comply with all aspects of the protocol. Exclusion Criteria 1. Any neurological condition that may affect cognitive impairment 2. History of transient ischemic attacks (TIA), stroke, or seizures within 12 months of Screening 3. Any psychiatric diagnosis or symptoms (e.g., hallucinations, major depression, or delusions) that could interfere with study procedures in the subject 4. Any medical devices contraindicated for MRI scanning (e.g., cardiac pacemaker/defibrillator, ferromagnetic metal implants, any devices other than those approved as safe for use in MRI scanners) 5. Evidence of infection, tumor, stroke or other clinically significant lesions that could indicate a dementia diagnosis other than AD on brain MRI at Screening 6. Evidence of other clinically significant lesions that could indicate a dementia diagnosis other than AD on brain MRI at Screening, or other significant pathological findings on brain MRI at Screening 7. A prolonged QT interval (QTcF greater than or equal to 450 ms) as demonstrated by a repeated ECG at Screening 8. Any other clinically significant conditions (e.g., cardiac, respiratory, gastrointestinal, renal disease) that in the opinion of the investigator(s) could affect the subject's safety or interfere with the study assessments 9. Severe visual or hearing impairment that would prevent the subject from performing psychometric tests accurately.