Major Inclusion Criteria:
- Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information (PHI) in accordance with national and local subject privacy regulations.
- Be between the ages of 18 and 58, inclusive, at the time of informed consent.
- SPMS defined as relapsing-remitting disease followed by progression of disability independent of or not explained by MS relapses for at least 2 years.
- EDSS score of 3.0 to 6.5, inclusive.
- MS Severity Score (MSSS) of 4 or higher.
- Documented confirmed evidence of disease progression independent of clinical relapses over the 1 year prior to enrollment as defined in the Study Reference Guide.
Major Exclusion Criteria:
- RRMS or primary progressive MS as defined by the revised McDonald Committee criteria.
- Clinical relapse (within 3 months) prior to randomization.
- T25FW test of >30 seconds during the screening.
- Any value below the lower limit of normal for blood levels of leukocytes, lymphocytes, or neutrophils.
- Considered by the Investigator to be immunocompromised based on medical history, physical examination, laboratory testing, or any other testing required by local guidelines, or due to prior immunosuppressive or immunomodulating treatment.
- Subjects for whom MRI is contraindicated (i.e., have pacemakers or other contraindicated implanted metal devices, are allergic to gadolinium, or have claustrophobia that cannot be medically managed).
- History of any clinically significant (as determined by the Investigator) cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic (other than MS), dermatologic, psychiatric, and renal, or other major disease that would preclude participation in a clinical study.
- History of malignant disease, including solid tumors and hematologic malignancies (with the exception of basal cell and squamous cell carcinomas of the skin that have been completely excised and are considered cured).
- Known history of or positive test result for Human Immunodeficiency Virus (HIV).
- Positive test result for hepatitis C virus (test for hepatitis C virus antibody [HCV Ab]) or hepatitis B virus (test for hepatitis B surface antigen [HBsAg] and/or hepatitis B core antibody [HBcAb]).
- History of transplantation or any anti-rejection therapy.
- Presence of any infectious disease (e.g., cellulitis, abscess, pneumonia, septicemia) within 30 days prior to screening.
- History of PML or other opportunistic infections.
- Any prior treatment with cell-depleting therapies, including total lymphoid irradiation, cladribine, rituximab, alemtuzumab, or bone marrow ablation.
- Any prior treatment with natalizumab.
- Treatment with mitoxantrone, cyclophosphamide, cyclosporine, azathioprine, methotrexate, mycophenolate mofetil, T cell or T cell receptor vaccination, fingolimod, daclizumab, or cytapheresis within 6 months prior to randomization.
- Treatment with IV or oral corticosteroids, intravenous immunoglobulin (IVIg), or plasmapheresis for treatment of MS within the 3 months prior to randomization.
- Treatment with glatiramer acetate or any interferon beta preparations within 4 weeks prior to randomization.
- Treatment with 4-aminopyridine within 30 days prior to randomization, unless a stable dose has been maintained for at least 30 days prior to randomization and will be continued for the course of this study.
This study is being sponsored by Biogen-Idec