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 (<10 mm in diameter) or superficial siderosis.
• The participant carries two copies of the apolipoprotein-E epsilon 4 (APOE4) allele (APOE4/4).
• The participant is currently receiving or has previously received any anti-amyloid immunotherapy (including, but not limited to, aducanumab, lecanemab, or donanemab) or triggering receptor expressed on myeloid cells 2 (TREM-2) targeting therapy.
• The participant is currently receiving anticoagulant therapies.
• The participant has had malignancy in the 3 years prior to Screening, excluding basal cell carcinoma, squamous cell carcinoma of the skin, melanoma in situ, Stage 1 or 2 prostate cancer, fully resected renal cell cancers, or cervical carcinoma in situ that has been successfully treated.

The above information is not intended to contain all considerations relevant to a participant's potential participation in a clinical trial.

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org

Part A and optional dose 2 cohort is a double-blind study in which participants, care providers, Treating Investigator, independent rater, clinical site staff, and Sponsor's clinical trial team members are blinded to study intervention. Part B is an open-label extension.