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.
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).