The primary objectives for the initial treatment period of this study are to further evaluate the safety of natalizumab monotherapy by evaluating the risk of hypersensitivity reactions and immunogenicity following re-exposure to natalizumab and confirming the safety of switching from interferon (IFN), glatiramer acetate, or other multiple sclerosis (MS) therapies to natalizumab. The primary objective for the long-term treatment period of this study is to evaluate the long-term impact of natalizumab monotherapy on the progression of disability measured by Expanded Disability Status Scale (EDSS) changes over time.

Study 101-MS-322 (NCT00306592) was conducted to evaluate the safety of natalizumab monotherapy following re-exposure to natalizumab in former clinical trial participants in Studies C-1801 (NCT00027300), C-1802 (NCT00030966), and C-1803 (NCT00097760) and included subjects in North America. In parallel with the conduct of that study, this study (101-MS-321 [NCT00297232]) was initiated for participants in Europe and the rest of the world. In addition, after 48 weeks, participants from 101-MS-322 (NCT00306592) could enter study 101-MS-321 (NCT 00297232), which was considered the Long-Term Treatment Period of 101-MS-322 (NCT00306592).

The primary purpose and primary outcome for both studies are identical; therefore, the combined long-term data from both studies are presented. (Combined Week 48 data from both studies are presented in the 101-MS-322 [NCT00306592] record.)

Key Inclusion Criteria

• MS subjects who completed Study C-1801 (NCT00027300), C-1802 (NCT00030966), or C-1803 (NCT00097760) and a Dosing Suspension Safety Evaluation (neurological examination or a magnetic resonance imaging scan) or participated in the IMA 04001 (STARS) Study
• Subjects who are considered by the Investigator to be free of signs and symptoms suggestive of progressive multifocal leukoencephalopathy and willing to discontinue and remain free from concomitant immunosuppressive or immunomodulatory treatment (including IFN-beta and glatiramer acetate) while being treated with natalizumab during the study.
• In addition, subjects who completed 48 weeks of treatment in Study 101-MS-322 (NCT00306592) in Canada will be allowed to enter this study at the start of the long-term treatment period (Week 52 - 480).

Key Exclusion Criteria

• Considered by the Investigator to be immunocompromised
• History of persistent anti-natalizumab antibodies, based upon testing from prior natalizumab studies
• History of any major disease or malignancy
• Discontinued natalizumab in a previous study due to allergic reaction

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

Our agreement is subject to confidentiality but generally the PI can publish, for noncommercial purposes only, results and methods of the trial, but no other Sponsor Confidential Information. PI must give Sponsor no less than 60 days to review any manuscript for a proposed publication and must delay publication for up to an additional 90 days thereafter if Sponsor needs to file any patent application to protect any of Sponsor's intellectual property contained in the proposed publication.