An Efficacy and Safety Study of PENTASA in Chinese Patients With Left-sided Active Ulcerative Colitis Followed by a 24-Week Open-Label Extension Phase

Inclusion Criteria: * Newly diagnosed or recurrent mild to moderate left-sided active ulcerative colitis * Mayo score of at least 4 but not greater than 10 points and a score of ≥ 2 for colonoscopy * Extent of colonic involvement and endoscopy subscore of Mayo score as confirmed by colonoscopy * The patient is compliant with Patient Daily Diary by having adequately responded to the questions on ≥5 of the most recent full 7 days before the Randomization Visit * Negative stool test at screening to rule out parasites and bacterial pathogens Exclusion Criteria: * Patients receiving \> 2g/day of oral 5-aminosalicylic acid (5-ASA) products, or receiving topical rectal 5-ASA \> 3g/week, within 7 days prior to screening * Severe/fulminant ulcerative colitis or toxic dilatation of the colon * Prior bowel resection surgery * Patients allergic to 5-ASA and derivative, any of the excipients, aspirin, or salicylates * Patients with one or more of the diseases: bacillary dysentery, amebic dysentery, chronic schistosomiasis, intestinal tuberculosis and Crohn's disease * Patients who are unable to fill in the Patient Daily Diary or follow data-capturing procedures