A Randomized, Controlled, Open-Label, Rater-Blinded, Phase 3b Study of the Efficacy, Safety, and Tolerability of 6-Week Extended Interval Dosing (EID) of Natalizumab (BG00002) in Subjects With Relapsing-Remitting Multiple Sclerosis witching From Treatment With 4-Week Natalizumab Standard Interval Dosing (SID) in Relation to Continued SID Treatment - Followed by an Open-Label Crossover Extension Study Comprising Subcutaneous and Intravenous Natalizumab Administration

For Part 1: • Ability of the participant to understand the purpose and risks of
the study and provide signed and dated informed consent and authorization to
use confidential health information in accordance with national and local
participant privacy regulations. • Aged 18 to 60 years old, inclusive, at the
time of informed consent • Diagnosis of relapsing remitting multiple sclerosis
(RRMS) according to the McDonald criteria. • Treatment with natalizumab as
disease-modifying monotherapy for RRMS that is consistent with the approved
dosing for a minimum of 12 months prior to randomization. The participant must
have received at least 11 doses of natalizumab in the 12 months prior to
randomization with no missed doses in the 3 months prior to randomization. •
Expanded Disability Status Scale (EDSS) <=5.5 at screening. • No relapses in
the last 12 months prior to randomization, as determined by the enrolling
Investigator For Part 2: • Ability of the participants to understand the
purpose and risks of the study and provide signed and dated informed consent
for Part 2 and authorization to use confidential health information in
accordance with national and local participant privacy regulations. • Completed
Part 1 Week 72 visit while remaining on their randomized treatment assignment
of SID or EID. • The inclusion and exclusion criteria for new participants who
did not participate in Part 1 of the study are the same as those for
participants who did participate in Part 1.

For Part 1:
• Primary and secondary progressive multiple sclerosis (MS).
• MRI positive for Gd-enhancing lesions at screening.
• Participants for whom MRI is contraindicated (e.g., have a contraindicated
pacemaker or other contraindicated implanted metal device, have suffered, or
are at risk for, side effects from Gd, or have claustrophobia that cannot be
medically managed).
• History of any clinically significant (as determined by the Investigator)
cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic
(including diabetes), urologic, pulmonary, neurologic (except for RRMS),
dermatologic, psychiatric, renal, or other major disease that would preclude
participation in a clinical study, in the opinion of the Investigator.
• Presence of anti-natalizumab antibodies at screening.

For Part 2:
• Participants treated with natalizumab EID was reverted to natalizumab SID by
choice or as rescue treatment in Part 1.
• Participant received treatment with any MS disease-modifying therapy other
than natalizumab in Part 1 or in the period between Part 1 and Part 2.
• History of human immunodeficiency virus or history of other immunodeficient
conditions.
• Current enrollment or a plan to enroll in any interventional clinical study
in which an investigational treatment or approved therapy for investigational
use is administered within 30 days (or 5 half-lives of the agent, whichever is
longer) prior to the Baseline Visit or at any time during this study.
• Inability to comply with study requirements.
• Other unspecified reasons that, in the opinion of the Investigator or Biogen,
make the participant unsuitable for enrollment.

The inclusion and exclusion criteria for new participants who did not
participate in Part 1 of the study are the same as those for participants who
did participate in Part 1.