A Study of AC-101 Tablets in Participants With Moderate to Severe Active Ulcerative Colitis

Inclusion Criteria: * Diagnosis of Ulcerative Colitis (UC) or suspected UC symptom for at least 3 months prior to enrolment and confirmed diagnosis of UC by endoscopic and histopathological examinations during screening. * Evidence of active UC defined by modified Mayo score of 5 to 9 points (inclusive) with 3 subscores meeting the followings: 1. Stool frequency (SF) subscore of .≥ 2 points, and 2. Rectal bleeding (RB) subscore of .≥ 1 points, and 3. Endoscopic (ES) subscore of ≥ 2 points (excluding friability), confirmed by screening endoscopy * Disease extension of ≥ 15cm from anal verge, confirmed by screening endoscopy * Currently using concomitant 5-salicylates (5-ASA) or oral corticosteroid (≤ 20 mg prednisone or equivalent, ≤ 9 mg budesonide MMX or equivalent) should keep stable doses from 2 weeks prior to enrolment till study completion. * Biologic-naïve or previous biological treatment for more than 5 half-lives. Exclusion Criteria: * Previous/current documented diagnosis of Crohn's Disease (CD), indeterminate colitis, severe UC required hospitalization or corticosteroid pulse therapy, ulcerative proctitis, fulminant colitis, ischemic colitis and other intestinal diseases. * Previous received 2 or more types of advanced treatment including biologics (e.g., TNF-α antibodies, IL-12/23 antibodies, integrin-α4β7 antibodies) and small molecule (e.g., JAK inhibitors, S1P receptor modulator), and all deemed by investigator as treatment failure. * Intravenous/rectal administration of steroids or topical administration of 5-ASA within 2 weeks prior to enrolment; systemic administration of small molecule (e.g., Tofacitinib, Upadacitinib, Ozanimod) within 4 weeks prior to enrolment. * Clostridium difficile infection or other enteric pathogen infection within 30 days prior to endoscopy, or positive for Clostridium difficile or other enteric pathogens prior to enrolment.