Dimethyl fumarate (DMF) demonstrated favorable benefit-risk in relapsing-remitting multiple sclerosis (RRMS) patients in phase-III DEFINE and CONFIRM trials, and ENDORSE extension.

The main aim of this study is assessing DMF safety/efficacy up to 13 years in ENDORSE.

Randomized patients received DMF 240 mg twice daily or placebo (PBO; Years 0-2), then DMF (Years 3-10; continuous DMF/DMF or PBO/DMF); maximum follow-up (combined studies), 13 years.

By January 2020, 1736 patients enrolled/dosed in ENDORSE (median follow-up 8.76 years (ENDORSE range: 0.04-10.98) in DEFINE/CONFIRM and ENDORSE); 52% treated in ENDORSE for ⩾6 years. Overall, 551 (32%) patients experienced serious adverse events (mostly multiple sclerosis (MS) relapse or fall; one progressive multifocal leukoencephalopathy); 243 (14%) discontinued treatment due to adverse events (4% gastrointestinal (GI) disorders). Rare opportunistic infections, malignancies, and serious herpes zoster occurred, irrespective of lymphocyte count. For DMF/DMF ( = 501), overall annualized relapse rate (ARR) remained low (0.143 (95% confidence interval (CI), 0.120-0.169)), while for PBO/DMF ( = 249), ARR decreased after initiating DMF and remained low throughout (ARR 0-2 years, 0.330 (95% CI, 0.266-0.408); overall ARR (ENDORSE, 0.151 (95% CI, 0.118-0.194)). Over 10 years, 72% DMF/DMF and 73% PBO/DMF had no 24-week confirmed disability worsening.

Sustained DMF safety/efficacy was observed in patients followed up to 13 years, supporting DMF's positive benefit/risk profile for long-term RRMS treatment.