This Phase IIa, multicenter, single-arm trial evaluates the efficacy, safety, tolerability, and pharmacokinetics of PTT-621 tablets in adults (18-70 years) with moderate-to-severe active ulcerative colitis (UC) who have inadequate response/intolerance to standard therapies. Participants receive PTT-621 for 12 weeks. The primary endpoint is the proportion of participants achieving endoscopic improvement at Week 12. Secondary endpoints include safety, clinical remission, clinical response, symptom response, pharmacokinetics, and inflammatory biomarkers. The study is sponsored by Pyrotech Therapeutics, with ethical approval from Renji Hospital's IRB.