A Study to Evaluate Safety and Efficacy of BIIB091 in Participants With Relapsing Forms of Multiple Sclerosis

The primary objectives are to investigate the safety and tolerability of BIIB091 monotherapy in participants with relapsing multiple sclerosis (RMS) (Part 1), and to evaluate the effects of BIIB091 combination therapy with Diroximel Fumarate (DRF) compared with the DRF monotherapy arm, on the key Magnetic Resonance Imaging (MRI) measure of active Central Nervous System (CNS) inflammation (Part 2). The secondary objectives are to evaluate the effects of BIIB091 monotherapy on the MRI measures of active CNS inflammation, to evaluate the effects of BIIB091 combination therapy with DRF compared with the DRF monotherapy arm on additional MRI measures of active CNS inflammation, to investigate the safety and tolerability of BIIB091 combination therapy with DRF in participants with RMS.