The study will assess the efficacy, safety, and tolerability of add-on therapy with ponesimod 20 mg vs placebo in adult participants with active relapsing multiple sclerosis (RMS) who are treated with dimethyl fumarate (DMF). Approximately 600 participants who have been receiving DMF for at least 6 months will be randomized in a 1:1 ratio to ponesimod 20 mg or placebo. The study consists of the following study periods: Pre-randomization period; Treatment period; Post-treatment observation period. The study includes one ponesimod treatment arm at the maintenance dose of 20 mg o.d. corresponding to the optimal dose when used as monotherapy based on the Phase 2 dose-finding trial and its ongoing extension. The study includes a placebo comparator arm, but all patients will remain on DMF background therapy throughout the study. Moreover, participants who experience a confirmed relapse or an event of 24-week confirmed disability accumulation (DMF) while on study drug will have the option to switch to an alternative treatment. The treatment period has a variable duration from a minimum of 60 weeks (for the last subject randomized) to a maximum of 156 weeks for the first subjects randomized in the trial and includes a gradual up-titration of ponesimod from a 2 mg starting dose to a 20 mg maintenance dose over a period of 14 days. The total duration of the study will be approximately up to 167 weeks.
