Natalizumab as an Efficacy Switch in Participants With Relapsing Multiple Sclerosis After Failure on Other Therapies

Key Inclusion Criteria: * Subjects of childbearing potential must practice effective contraception from Day -1 and be willing and able to continue contraception for duration of the study. * Must have documented diagnosis of relapsing MS (McDonald 2010 Criteria \[Polman 2011\]) at Screening. * Must have been treated with Gilenya or Betaseron, Rebif, Avonex, Copaxone, Extavia, Tecfidera (BRACET) for at least the 12 months prior to Screening with no interruption of treatment greater than 1 month. Prior treatment with natalizumab is allowed; however, there must be a minimum 1 year since last natalizumab infusion and the Screening visit of this study, and if discontinuation of natalizumab in the past was not due to intolerance, anti-natalizumab antibodies, or efficacy loss. * Must have had disease activity in the 6 months prior to Screening while on Gilenya or BRACET (as defined by at least 1 gadolinium enhancing lesion OR at least 2 new T2 lesions compared with magnetic resonance imaging done within 12 months of screening OR clinical relapse, or Expanded Disability Status Scale \[EDSS\] progression of 1 point) * Must have an EDSS score from 0 to 5.5 inclusive at Screening. * Must have lymphocyte count that is documented as at or above the lower limit of normal (LLN) by the day before the first Tysabri infusion. If lymphocytes have not returned to LLN or above the day before the first Tysabri infusion (day 0), the subject has screen failed. The subject who screen fails is eligibl…