This study will compare the effect of A4I antagonist and placebo on MRI lesions,on clinical endpoints, and safety in patients with relapsing Multiple Sclerosis (MS). Eligible patients will be randomized to receive placebo or A4I antagonist, 20mg, 80mg or 300mg, po bid. Patients will undergo MRI brain scans and MS clinical evaluations at intervals throughout the study. The anticipated time on study treatment is 3-12 months, and the target sample size is 100-500 individuals.

Inclusion Criteria:

• adult patients, 18-59 years of age;
• relapsing multiple sclerosis patients with types 1-4 (established through McDonald criteria);
• >=1 MS attack or a Gd-enhancing MRI lesion between 1 month and 1 year before enrollment;
• EDSS score of <=6.5;
• inadequate response to approved treatment(Canada only).

Exclusion Criteria:

• MS attack within 1 month before enrollment;
• systemic corticosteroids within 1 month before enrollment;
• MS treatments (non-symptomatic) within specified periods before enrollment;
• an infection requiring systemic anti-infective treatment or vaccination with a live vaccine within 1 month before enrollment.