A Study to Evaluate the Efficacy, Safety, Tolerability and Pharmacokinetics of PTT-621 in Patients With Moderately to Severely Active Ulcerative Colitis

Inclusion Criteria: * 1\. Male or female participants aged 18-70 years inclusive at screening. 2. Diagnosed with ulcerative colitis (UC) at least 60 days prior to the first dose of IP. 3\. Participants have active UC at screening, defined as a modified Mayo score of 5-9. 4\. Endoscopic evidence of UC extending ≥15 cm from the anal verge during the screening period. 5\. Participants have had inadequate/loss of response or intolerance to at least one of the following: 5-aminosalicylic acid, glucocorticoids, immunosuppressants, biologics, and/or small-molecule targeted therapies. 6\. Participants currently receiving the following UC therapies are eligible if they meet stability requirements: Oral glucocorticoids: with stable dosing for ≥2 weeks prior to screening endoscopy；Oral 5-ASA: Stable dose for at least 2 weeks prior to the endoscopic assessment during the screening period. 7\. Female participants of childbearing potential must have a negative serum/urine pregnancy test at screening. 8\. Willing to use at least one highly effective contraceptive method during sexual intercourse throughout the study and until 1 month after the last dose of IP. 9\. Willing to refrain from sperm/ova donation throughout the study and until 1 month after the last dose of IP. 10\. Able and willing to provide written informed consent and comply with all protocol-specified procedures and visit schedules. Exclusion Criteria: * 1\. A current diagnosis of Crohn's disease (CD), indeterminate colitis, infectious colitis, or any other colitis/enteritis that could interfere with efficacy evaluation； 2. Has UC complications (e.g., fulminant colitis, toxic megacolon, prior total or partial colectomy, or other conditions requiring surgery)； 3. Prior or current gastrointestinal dysplasia； 4. History of gastrointestinal malignancy, or any current evidence of gastrointestinal cancer.