A Study to Investigate the Safety and Effectiveness of SAR448851 in Participants With Early Alzheimer's Disease

This is a randomized, placebo-controlled Phase 2 study to evaluate the efficacy and safety of SAR448851 in early Alzheimer's disease (AD) participants. The purpose of this study is to measure efficacy and safety with once daily oral SAR448851 compared to placebo in participants with mild cognitive impairment due to AD or mild AD dementia and with evidence of cerebral amyloid pathology. This Phase 2 study has 2 parts: Part A is a randomized, double-blind, parallel-group, placebo-controlled study with SAR448851 oral once daily. Part B is an open-label extension. All participants who complete Part A may continue to Part B. An optional dose 2 cohort will be considered to evaluate the efficacy and safety of SAR448851 dose 2 oral once daily. The study duration will be up to 111 weeks for Part A and B, and up to 63 weeks for the dose 2 cohort. The treatment duration will be up to 96 weeks for Part A and B, and up to 48 weeks for the dose 2 cohort. Up to 160 participants will be included in this study.

Inclusion Criteria: * Participant must be 55 to 85 years (inclusive) of age, at the time of signing the informed consent. * Diagnosis of mild cognitive impairment (MCI) due to Alzheimer's disease (AD) (National Institute on Aging-Alzheimer's Association \[NIA-AA\] Stage 3) or mild AD dementia (NIA-AA Stage 4). * Have a global Clinical Dementia Rating (CDR) score 0.5 to 1.0 and a CDR-Memory Box score ≥ 0.5 at screening. * Have a study partner who must provide separate written informed consent at screening. Study partner should be \>18 years of age, have known participant for at least 1 year, and have a minimum of 8 hours per week contact with participant. * The participant has evidence of cerebral amyloid pathology confirmed by positive visual read on amyloid positron emission tomography (PET) at screening. Exclusion Criteria: * The participant has any history of significant neurological disease including but not limited to, frontotemporal dementia, Lewy body dementia, Huntington's disease, serious infection of the brain, Parkinson's disease, multiple concussions, multiple sclerosis, or epilepsy or recurrent seizures (except febrile childhood seizures). * The participant has evidence of more than 4 microhemorrhages (\