This study will randomize around 200 subjects from approximately 50 sites located world-wide, who have experienced at least one relapse while taking IFN-beta therapy within 48 weeks prior to Screening, irrespective of disability progression. Secondary progressive multiple sclerosis (SPMS) subjects, who are still experiencing relapses, and subjects who have received disease modifying drugs (DMDs), other than IFN-beta therapy, during their MS treatment history, but are currently on IFN-beta therapy and have experienced active MS symptoms (at least 1 relapse) during the 48 weeks prior to Screening, may also be enrolled.