Data Package for study.

The primary objective of Part A is to determine the safety and tolerability of natalizumab administered over 24 weeks in Japanese participants with relapsing-remitting multiple sclerosis (MS). The endpoints for this will include assessment of adverse evetns (AEs), changes in laboratory evaluations, vital signs, Expanded Disability Status Scale (EDSS) scores, and changes in physical and neurological examination findings. The secondary objectives of Part A are to characterize the pharmacokinetics (PK) profile and pharmacodynamics (PD) of natalizumab. The primary objective of Part B is to determine if natalizumab, when compared to placebo, is effective in treating Japanese participants with relapsing-remitting MS, as measured by new active lesions on cranial magnetic resonance imaging (MRI) scans over 24 weeks. New active lesions are the sum of the gadolinium-enhancing (Gd+) lesions and any new or newly-enlarging T2-hyperintense lesions that do not enhance. The primary endpoint is the rate of development of new active lesions over 24 weeks. Secondary objectives of Part B are to determine over 24 weeks whether natalizumab, when compared to placebo, is effective in reducing the frequency of clinical exacerbations, reducing the number of Gd+ lesions, reducing the number of new or newly-enlarging T2-hyperintense lesions on brain MRI scans, increasing the proportion of relapse-free participants, and improving outcomes on visual analog scale (VAS) assessing the participant's global impression of his/her well-being. Additional objectives are to assess the safety and tolerability, the incidence of serum antibodies to natalizumab and the PK profile of natalizumab.