A Study to Learn About the Safety of BIIB091 and Its Effect on Brain Inflammation When Taken Alone or With Diroximel Fumarate (DRF) in Adults With Relapsing Forms of Multiple Sclerosis (MS)

The primary objectives of this study are to investigate the safety and tolerability of BIIB091 monotherapy in participants with relapsing multiple sclerosis (RMS) in Part 1 and to evaluate the effects of BIIB091 combination therapy with DRF compared with the DRF monotherapy arm on the key MRI measure of active central nervous system (CNS) inflammation in Part 2. The secondary objectives of Part 1 of the study are to evaluate the effects of BIIB091 monotherapy on the MRI measures of active CNS inflammation and to assess the effect of BIIB091 monotherapy on QTc and other ECG parameters. The secondary objectives of Part 2 are to evaluate the effects of BIIB091 combination therapy with DRF compared with the DRF monotherapy arm on additional MRI measures of active CNS inflammation, the safety and tolerability of BIIB091 combination therapy with DRF in participants with RMS, and the effect of BIIB091 combination therapy with DRF on QTc and other ECG parameters. Part 1 of the study was concluded normally. Sponsor decision was to not conduct part 2.