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.

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.

Key Inclusion Criteria:

1. Diagnosis of RMS [relapsing-remitting multiple sclerosis (RRMS) or active secondary progressive multiple sclerosis (SPMS)] in accordance with the 2017 Revised McDonald criteria.

2. Time since MS symptom onset is <20 years.

3. Must have expanded disability status scale (EDSS) score of 0 through 5.0 at screening and baseline.

4. 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.

Key Exclusion Criteria:

1. Diagnosis of primary progressive multiple sclerosis (PPMS) in accordance with the 2017 Revised McDonald criteria.

2. An MS relapse that has occurred within 30 days prior to Baseline (Day 1) or the participant has not stabilized from a previous relapse at the time of screening.

3. History of severe allergic, anaphylactic reactions or hypersensitivity reaction to BIIB091 or DRF, the excipients contained in the formulation, and if appropriate, any diagnostic agents to be administered during the study, including the following:

   • Known hypersensitivity to any components of the study treatment
   • Known hypersensitivity to previous fumarate or bruton's tyrosine kinase (BTK) inhibitor treatments
   • History of hypersensitivity to parenteral administration of Gd-based contrast agents

4. Evidence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection within the past 4 weeks prior to Baseline.

5. History of human immunodeficiency virus (HIV) infection or a positive or indeterminate test result at screening for HIV.

6. Current or history of hepatitis C infection regardless of viral load.

7. Current or history of hepatitis B infection.

8. Current enrollment or plan to enroll in any other drug, biological, device, clinical study, or treatment with an investigational drug or approved therapy for investigational use within 90 days prior to randomization or 5 half-lives of the drug or therapy, whichever is longer.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.

In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on https://www.biogentrialtransparency.com/