This is a Phase Ⅱb, multicenter, randomized, parallel-group, double-blind, placebo-controlled, dose-finding study of AC-101 tablets in patients with moderately to severely active ulcerative colitis (UC). The study will evaluate the efficacy, safety, and pharmacokinetics of AC-101 compared with placebo in patients who have had an inadequate response, loss of response, or intolerance to prior advanced therapies or to prior conventional therapies. The study consists of a screening period (up to 4 weeks), a 12-week induction period, a 40-week extension period, and a 4-week safety follow-up period.