Confirmatory Clinical Study in Active Ulcerative Colitis

Inclusion Criteria: * Documented diagnosis (histologic diagnosis and either endoscopic or radiographic diagnosis) of ulcerative colitis (UC) at least 3 months prior to Screening. * Diagnosis of active mild-to-moderate UC at Screening as defined by an mMS of 4 to 7, including a MES ≥2 (confirmed by central reading), a Mayo Rectal Bleeding score of 1 or 2, and a Mayo Stool Frequency score ≥1. * UC lesions extending ≥10 cm from the anal verge. * Participant must either receive a stable dose of orally administered 5-aminosalicylic acid (5-ASA); have failed, due to insufficient efficacy, oral 5-ASA; have a documented intolerance or poor tolerance to an aminosalicylic acid treatment, including 5-ASA, or be contra-indicated to receive 5-ASA treatment per local labeling. * Participant must provide written informed consent or assent (parent or legal guardian must provide consent for a participant \<18 years of age who has assented to participate in the study, or as required per local regulations). * In countries not allowing females of childbearing potential (FOCBP) to participate without an acceptable contraceptive method, the FOCBP must agree to abide to local requirements and eg, use at least an acceptable method of contraception until the end of treatment. Exclusion Criteria: * Diagnosis of Crohn's disease, undetermined colitis, ischemic colitis, fulminant colitis, or toxic megacolon. * Evidence of a clinically significant, active infection of the gastrointestinal tract. * Severe UC (mMS\>7), meeting modified Truelove Witts' criteria and/or RB score of 3, participant with ulcerative proctitis only, or participant in whom colitis is most severe in the transverse colon or ascending colon, or if any hospitalization is planned at the time of Screening. * Total colectomy, stoma, or ileo-anal pouch, or history of extensive colonic resection leaving less than 30 cm of colon. * Presence of intra-abdominal fistula, abscesses, diverticulitis, or gastrointestinal bleeding unrelated to UC. * History of colon carcinoma or high-grade dysplasia. * Previous use of any advanced UC treatment, including any anti-TNF (eg, infliximab), antiintegrin (eg, vedolizumab) or anti-IL-12/23 (eg, ustekinumab) agent, anti-IL23 (eg, risankizumab), Janus kinase inhibitors (eg, tofacitinib), and sphingosine-1-phosphate receptor modulators (eg, etrasimod). * Use of sulfasalazine ≤4 weeks prior to randomization. * Use of corticosteroids or any disease-modifying antirheumatic drugs (DMARD), including thiopurines, ≤6 weeks prior to randomization into the study, except for a stable, low dose of oral corticosteroids (≤10 mg prednisolone/day) for at least 2 weeks prior to Screening colonoscopy and up to at least the Week 12 visit. * Use of antibiotics (except for local use), prebiotics, or probiotics ≤4 weeks prior to randomization or anticipated during study participation, or concomitant, chronic use of an antidiarrheal drug, or concomitant use of any rectal treatment. * Use of fecal microbiota transplantation (FMT) ≤52 weeks prior to randomization. * Treatment with another investigational drug or intervention within 30 days prior to Screening, or within 5 times the elimination half-life of the investigational drug (whichever is longest). * Any immunocompromised state, including conditions linked to severe immunosuppression (eg, active human immunodeficiency virus, malignancies, liver cirrhosis, systemic chemotherapy). * Leukopenia (total white blood cell count \<3000/μL) and/or neutropenia (absolute neutrophil count \<1000/μL), anemia (hemoglobin \<10.0 g/dL), thrombocytopenia (peripheral blood platelet count \<100 × 10\^9/L), and/or any coagulation disorder with significantly increased risk of bleeding. * Ongoing or recent (\<3 months) renal disease or insufficiency as manifested, eg, by medical history and/or clinical examination and/or (calculated or measured) glomerular filtration rate ≤60 mL/min. * Ongoing or recent (\<3 months) advanced hepatic dysfunction defined as a Child Pugh score ≥10 (Class C), or increase ≥2 times the upper limit of normal in aspartate aminotransferase (AST), alanine aminotransferase (ALT), total bilirubin (TB), prothrombin time (PT) or international normalised ratio (INR). * Clinically significant bone marrow disease if progressive or not controlled, or any history of solid organ or bone marrow transplantation. * Active intravenous drug abuse or alcohol abuse disorder as assessed by the Investigator. * Pregnancy or lactation at study entry. Note: Participants who become pregnant or start to breastfeed during the study may continue the study per the Investigator's discretion. * Participants who are inappropriate for the study per the Investigator's discretion. * An employee (or a relative of) of the Investigator, study center, contract research organization, or Sponsor.