A Study to Evaluate the Safety, Tolerability, PK and Preliminary Efficacy of HX15001 Combined With Etrasimod in Moderate-severe Ulcerative Colitis

The purpose of this study is to evaluate the safety, tolerability, pharmacokinetics, and preliminary efficacy of HX15001 injection combined with Etrasimod tablets in participants with moderately to severely active ulcerative colitis (UC).

Inclusion Criteria: * Patient able to read and understand, and willing to sign the informed consent form; * Weight≥40kg; * Clinical diagnosis of Ulcerative colitis (UC) that has been present for at least 3 years before the screening visit; * Mayo score of 5-9 at the baseline visits, including Mayo Endoscopic Score ≥2 and Rectal Bleeding Subscore ≥1, and disease extent from the anal verge ≥ 15 cm at screening. * Participants with a history of extensive colitis of ≥8 years' duration, or left-sided colitis of ≥10 years' duration, must have had a complete coloscopy within the past 1 year to exclude the presence fo dysplastic lesions. * Participants who have had an inadequate response to prior treatment with biologic agents or JAK inhibitors. * Willing and able to comply with clinic visits and study-related procedures. * Willing to use adequate birth control, if of reproductive potential and sexually active Exclusion Criteria: * Evidence or history (within 6 months) of fulminant colitis, toxic megacolon, or intestinal perforation. * Prior colectomy (partial or total), or anticipated need for surgical intervention for UC during the study. * Prior or current diagnosis of Crohn's disease, fistulas/abscesses, indeterminate colitis, unclassified IBD, microscopic colitis, ischemic colitis, infectious colitis, radiation colitis, active diverticulitis, or other colitis/enteritis that may confound efficacy assessment. * Current unresolved colonic dysplasia, adenoma, adenomatous polyps, or neoplastic lesions. Patients with history of adenomatous polyps are eligible if polyps were completely removed (documented) and screening colonoscopy/histology shows no residual polyps or dysplasia. * Concomitant primary sclerosing cholangitis or autoimmune hepatitis. * Malignancy within 5 years prior to screening, except fully resected non-metastatic basal cell carcinoma, squamous cell carcinoma, cervical carcinoma in situ, breast ductal carcinoma in situ, or papillary thyroid carcinoma with no recurrence. * Major surgery within 6 weeks prior to screening, or planned surgery during the study, unless judged not to increase patient risk or affect study compliance. * Any condition precluding endoscopic evaluation. * Unstable or poorly controlled cardiovascular (including atrial fibrillation requiring Class IA/III antiarrhythmics or QT-prolonging agents; unstable ischemic heart disease; Class I/II heart failure; cardiac arrest; cerebrovascular disease; uncontrolled hypertension; symptomatic bradycardia; recurrent cardiogenic syncope; untreated severe sleep apnea; history of second-degree AV block without a functional pacemaker), respiratory, gastrointestinal (excluding UC), hepatic, renal, endocrine, hematologic, neurologic, or psychiatric disease that may compromise patient safety or confound efficacy assessment. - Patients requiring systemic corticosteroids for non-UC conditions (e.g., asthma, adrenal insufficiency, post-transplant) or with peripheral venous access issues are excluded. * Diabetes mellitus, uveitis, history of ocular surgery/laser, history of maculopathy, or active/uncontrolled macular edema at screening. * Clinically significant 12-lead ECG abnormalities that may affect safety or interpretation, including QTcF \>450 msec (males) or \>470 msec (females), QRS \>120 msec, or resting heart rate \