A Multicenter, Randomized, Double-Blind, Placebo-Controlled Study With Optional Open-Label Extension in Subjects With Relapsing Multiple Sclerosis to Evaluate the Efficacy and Safety of BIIB033 as an Add-On Therapy to Anti-Inflammatory Disease-Modifying Therapies

The primary objective of Part 1 of this study is to evaluate the effects of BIIB033 versus placebo on disability improvement over 72 weeks. The primary objective of Part 2 of this study is to evaluate the long-term safety profile of BIIB033 as an add-on therapy in participants with MS.The secondary objective of Part 1 is to evaluate the effects of BIIB033 versus placebo on additional measures of disability improvement. The secondary objective of Part 2 is to investigate long-term efficacy (disability improvement) and additional safety measures of BIIB033 as an add-on therapy in participants with MS.