Dose-finding Study of MT-1303

Inclusion Criteria: * RRMS as defined by the revised McDonald criteria * Evidence of recent MS activity defined as either: * at least one documented relapse in the previous 12 months, OR * a positive gadolinium (Gd)-enhanced MRI scan within 3 months prior to screening, OR * at least two documented relapses in the previous 24 months with a positive Gd-enhanced MRI scan within the previous 12 months * Expanded Disability Status Score (EDSS) score ≥0.0 and ≤5.5 points. Exclusion Criteria: * Primary progressive, secondary progressive or progressive relapsing MS at screening * Disease duration \>15 years combined with an EDSS score ≤2.0 * Relapse of MS during the Screening Period * History or known presence of other neurological disorders likely to render the subject unsuitable for the study * History of any of a list of pre-defined cardiovascular diseases * History or known presence of any significant central nervous system, infectious, metabolic, oncological, ophthalmological or respiratory system disease or illness likely to render the subject unsuitable for the study * Previous exposure to any sphingosine 1-phosphate receptor modulator * Receipt of a live vaccine or systemic corticosteroid use within 28 days prior to randomisation * Previous treatment with beta-interferons or glatiramer acetate within 14 days prior to randomisation * Previous treatment with intravenous immunoglobulin, plasmapheresis, certain immunosuppressants, lymphocyte-depleting therapy, total body irradiation or bone marrow transplantation * Need, or likely need for, treatment with Class I or III anti-arrhythmic drugs or with heart-rate-lowering beta-blockers or calcium-channel blockers, or with any other drugs which can reduce the heart rate * Evidence of significant anaemia, thrombocytopenia, leucopoenia or lymphocytopenia, renal or hepatic impairment * Clinically significant electrocardiogram (ECG) findings.