A Prospective, Open-label, Non-randomized, Clinical Trial to Determine if Natalizumab (Tysabri®) Improves Ambulatory Measures in Relapsing-remitting Multiple Sclerosis (RRMS) Patients

Key Inclusion Criteria: * Must give written informed consent and provide all authorizations required by local law (for example, Protected Health Information \[PHI\]) * Men or women between 18 and 60 years of age, inclusive * Must have Expanded Disability Status Scale (EDSS) less than or equal to 5.5 at baseline * Must be able to walk at least 100 m without assistive devices * Must be natalizumab-naïve * Must have a documented diagnosis of a relapsing remitting form of multiple sclerosis (MS0 as defined by the revised McDonald Committee criteria (Polman et al., 2005) * Must have had a recent (within 3 months from baseline) magnetic resonance imaging (MRI) * Must have had at least 1 relapse in the previous year and must satisfy the locally approved therapeutic indications for Tysabri. If Tysabri is not yet approved in a specific country, patients must fulfill the following criteria: * Patients with high disease activity despite treatment with a beta-interferon defined as patients who have failed to respond to a full and adequate course of a beta-interferon * Patients must have had at least 1 relapse in the previous year while on therapy, and have at least 9 T2 hyperintense lesions in cranial MRI or at least 1 gadolinium (Gd)-enhancing lesion * Patients with rapidly evolving severe relapsing remitting multiple sclerosis defined as patients who have had 2 or more disabling relapses in one year and 1 or more Gd-enhancing lesions on brain MRI or significant increase in T2 lesions as compared to a previous MRI * Must be stable in disability for at least 30 days prior to enrollment to the study * Must be stable in symptomatic management of the disease, specifically spasticity, depression and fatigue for at least 30 days prior to enrollment to the study * Must be considered by the Investigator to be free of signs and symptoms suggestive of progressive multifocal leukoencephalopathy (PML) based on medical history, physical examination, or laboratory testing * Must be willing to discontinue and remain free from concomitant immunosuppressive or immunomodulatory treatment (including interferon \[IFN\] and glatiramer acetate \[GA\]) while being treated with natalizumab during the study. Key Exclusion Criteria: Unless otherwise specified, candidates will be excluded from study entry if any of the following exclusion criteria exist at the time of the Screening Visit: * Onset of a relapse within 50 days prior to first infusion * Considered by the Investigator to be immunocompromised, based on medical history, physical examination, or laboratory testing or due to prior immunosuppressive treatment * History of, or available abnormal laboratory results indicative of, any significant cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, gastrointestinal, dermatologic, psychiatric (including major depression), renal, and/or other major disease that would preclude the administration of a recombinant humanized antibody immunomodulating agent. The Investigator must re-review the subject's medical fitness for participation and consider any diseases that would preclude treatment * History of malignancy (subjects with basal cell carcinoma that has been completely excised prior to study entry remain eligible) * Known history of human immunodeficiency virus infection or hematological malignancy * History of organ transplantation (including anti-rejection therapy) * A clinically significant infectious illness (cellulitis, abscess, pneumonia, septicemia) within 30 days prior to the Screening Visit * Treatment with immunosuppressant medications (mitoxantrone, cyclophosphamide, cyclosporine, azathioprine, methotrexate) within 6 months prior to Screening * Female subjects who are not postmenopausal for at least 1 year, surgically sterile (does not include tubal ligation), or willing to practice effective contraception (as defined by the Investigator) during the study * Women who are breastfeeding, pregnant, or planning to become pregnant while on study * Current enrollment in any other study treatment or disease study * Unwillingness or inability to comply with the requirements of this protocol, including the presence of any condition (physical, mental, or social) that is likely to affect the subject's ability to comply with the study protocol * Subjects with walking impairment due to causes other than MS * Other unspecified reasons that, in the opinion of the Investigator and/or Biogen Idec, make the subject unsuitable for enrollment into this study NOTE: Other eligibility criteria may apply.