The primary objectives of the study are:

• To evaluate the effects of three oral doses of MT-1303 compared to placebo given for a period of 24 weeks in subjects with relapsing-remitting multiple sclerosis (RRMS) on MRI parameters
• To evaluate the safety and tolerability of three oral doses of MT-1303 compared to placebo given for a period of 24 weeks in subjects with RRMS.

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.