Study Evaluating Efficacy and Safety of Amiselimod (MT-1303) in Mild to Moderate Ulcerative Colitis

Inclusion Criteria: * Subjects will be eligible if they are male or female aged between 18 to 75 years at time of consent (inclusive) with normal vital signs and a diagnosis of active mild ulcerative colitis (UC) (modified Mayo Score of 3 or 4) or moderate UC (modified Mayo Score of 5 to 8) confirmed at least 12 weeks prior to randomization by clinical and endoscopic evidence and corroborated by a histopathology report. * Subjects must have an endoscopic subscore of ≥2 from and evidence of active UC extending ≥15 cm from the anal verge confirmed by a screening colonoscopy. * If subjects are receiving oral or rectal 5-aminosalicylates (5-ASAs) or oral corticosteroids (≤20 mg prednisolone equivalent) for treatment of their UC, they must be on a stable dose for at least 28 days prior to randomization. * Subjects who complete the Double-Blind Period of the study who, in the opinion of the Investigator, would benefit from continued treatment, may participate in the Open Label Extension (OLE) Period. Exclusion Criteria: * Any of the following: a diagnosis of Crohn's disease, indeterminate colitis, colitis (pseudomembranous, microscopic, or ischemic) or coeliac disease, current or recent (within 12 weeks prior to randomization) evidence of fulminant colitis, proctitis (defined as a rectal inflammation within 15 cm from the anal verge), abdominal abscess, toxic megacolon, bowel obstruction, or bowel perforation; a history or evidence of any colonic resection or subtotal or total c…