This is a Phase 1, single-center study conducted in 2 parts:

Part 1a, single ascending dose (SAD-TDU16519): Double-blind, randomized, placebo-controlled sequential ascending single oral doses including up to 6 cohorts. Each cohort will include 8 participants (6 receiving SAR443820 and 2 placebo).

Part 1b (TDU16519): - Open label, single SAR443820 dose in one or two separated cohort(s) for SAR443820 measurements in CSF and in plasma.

Part 2, multiple ascending dose (MAD -TDR16520): Double-blind, randomized, placebo-controlled, sequential ascending repeated oral doses for 14 days, including up to 4 cohorts. Each cohort will include 10 participants (8 receiving SAR443820 and 2 placebo).

The duration of the study for a participant will include:

Screening Period: up to 28 days

Part 1a:

Treatment in fasted condition: 1 day (Day 1). Study observation Period from Day -2/Day -1 to Day 3. Follow-up with the end of study: from Day 5 to Day 7. Total duration from screening per participant: up to 5 weeks.

Part 1b:

Treatment in fed condition: 1 day (Day 1). Study observation Period from Day -1/Day1 to Day 2. Follow-up with the end of study: from Day 5 to Day 7. Total duration from screening per participant: up to 5 weeks.

Part 2:

Treatment: 14 days (Day 1 to Day 14). Study observation Period from Day -2/Day -1 to Day 17. Follow-up with the end of study: from Day 19 to Day 21. Total duration from screening per participant: up to 7 weeks.

Inclusion Criteria:

Male and/or female participant, between 18 and 55 years of age, inclusive. Body weight between 50.0 and 100.0 kg, inclusive, if male, and between 40.0 and 90.0 kg, inclusive, if female, body mass index between 18.0 and 30.0 kg/m2, inclusive.

Certified as healthy by a comprehensive clinical assessment (detailed medical history and complete physical examination).

Having given written informed consent prior to undertaking any study-related procedure.

Not under any administrative or legal supervision or under institutionalization due to regulatory or juridical order.

Exclusion Criteria:

Any history or presence of clinically relevant cardiovascular, pulmonary, gastrointestinal, hepatic, renal, metabolic, hematological, neurological, osteo-muscular, articular, psychiatric, systemic, ocular, gynecologic (if female), or infectious disease, or signs of acute illness.

Personal medical history of seizure.

Frequent headaches and/or migraine, recurrent nausea and/or vomiting (for vomiting only: more than twice a month).

Any medication (including St John's Wort) within 14 days before inclusion or within 5 times the elimination half-life or pharmacodynamic half-life of the medication, with the exception of hormonal contraception or menopausal hormone replacement therapy; any vaccination within the last 28 days and any biologics (antibody or its derivatives) given within 4 months before inclusion.

Positive result for hepatitis B, C or HIV

Positive result on urine drug screen

Positive alcohol test.

Any consumption of citrus fruits or their juices within 5 days before inclusion.

Current psychiatric disorder, suicidal ideation in the previous 6 months (as assessed by the C-SSRS), or a lifetime suicide attempt.

Additional exclusion criteria applied, and specially for Part 1b, criteria related to the study procedure of lumbar puncture.

The above information is not intended to contain all considerations relevant to a patient'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