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)

Key Inclusion Criteria:

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

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

3. Must have expanded disability status scale (EDSS) score of 0 through 5.0 atscreening 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 toBaseline [Day 1]) with at least 1 relapse during the last 12 months prior torandomization.

• ≥1 clinical relapse within the past 24 months (but not within 30 days prior toBaseline [Day 1]) and ≥1 new brain MRI lesion (Gd-positive and/or new orenlarging T2 hyperintense lesion) within the past 12 months prior torandomization. The screening MRI could be used to satisfy this criterion (ifneeded for inclusion, local reading is required). For new or enlarging T2hyperintense lesions, the reference scan cannot be >12 months prior torandomization.

• ≥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 theparticipant has not stabilized from a previous relapse at the time of screening.

3. History of severe allergic, anaphylactic reactions or hypersensitivity reaction toBIIB091 or DRF, the excipients contained in the formulation, and if appropriate, anydiagnostic 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 contrastagents

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

5. History of human immunodeficiency virus (HIV) infection or a positive orindeterminate 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, clinicalstudy, or treatment with an investigational drug or approved therapy forinvestigational use within 90 days prior to randomization or 5 half-lives of thedrug or therapy, whichever is longer.

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