A 2-Part, Multicenter, Randomized, Blinded, Active-Controlled Phase 2 Study to Sequentially Evaluate the Safety and Efficacy of BIIB091 Monotherapy and BIIB091 Combination Therapy With Diroximel Fumarate in Participants With Relapsing Forms of Multiple Sclerosis
In this study, researchers will learn more about a study drug called BIIB091 in participants with MS who may be experiencing relapses. It is a 2-part study. In Part 1, one set of participants will take either BIIB091 or diroximel fumarate (DRF). In Part 2, a different set of participants will take either a combination of BIIB091 and DRF or DRF alone. The goal of the study is to learn more about the safety of BIIB091 and to compare the effects of the study drug when taken alone or together with DRF. The main question researchers are trying to answer are: * How many participants have new or worsening medical problems (adverse events) after taking BIIB091 or DRF? * How many new areas of inflammation occur in the brain after treatment with BIIB091 and DRF? Researchers will use magnetic resonance imaging (MRI) scans to compare images of the brain before and after treatment. They will also explore the effect of BIIB091 and DRF on the heart using electrocardiograms (ECGs). The study will be done as follows: * After screening, participants who joined Part 1 will be randomly assigned to receive either a high or low dose of BIIB091, or the standard dose of DRF. * The results of Part 1 will be used to choose the best dose of BIIB091 to use in Part 2. * Participants who join Part 2 will be randomly assigned to receive either a standard dose of DRF, a combo of BIIB091 and the standard dose of DRF, or a combo of BIIB91 with a low dose of DRF. * Neither the researchers nor the participants will know which drug or dose the participants will receive in either part of the study. * The treatment period will last 48 weeks in each part of the study. Participants will take the drugs by mouth 2 times a day. * Each part will also have a follow-up safety period that lasts up to 2 weeks. * In total, participants in each part will have 20 study visits, or more if they have a relapse. The total study duration for participants will be up to 54 weeks.
• Diagnosis of RMS \[relapsing-remitting multiple sclerosis (RRMS) or active secondary progressive multiple sclerosis (SPMS)\] in accordance with the 2017 Revised McDonald criteria.
• Time since MS symptom onset is \<20 years.
• Must have expanded disability status scale (EDSS) score of 0 through 5.0 at screening and baseline.
• Must have at least 1 of the following occurring prior to Baseline (Day 1):
‣ ≥2 clinical relapses in the last 24 months (but not within 30 days prior to Baseline \[Day 1\]) with at least 1 relapse during the last 12 months prior to randomization.
⁃ ≥1 clinical relapse within the past 24 months (but not within 30 days prior to Baseline \[Day 1\]) and ≥1 new brain MRI lesion (Gd-positive and/or new or enlarging T2 hyperintense lesion) within the past 12 months prior to randomization. The screening MRI could be used to satisfy this criterion (if needed for inclusion, local reading is required). For new or enlarging T2 hyperintense lesions, the reference scan cannot be \>12 months prior to randomization.
⁃ ≥1 GdE lesion on brain MRI within 6 months prior to randomization.