ABX464 in Subjects With Moderate to Severe Active Ulcerative Colitis

Inclusion Criteria: * Diagnosis of moderate to severe active UC confirmed by endoscopy and histology at least 12 weeks prior to screening visit. Moderate to severe active UC defined by Mayo Clinic Score (MCS) of 6 to 12 inclusive (on a scale of 0-12). Moderate to severe active UC should be confirmed at screening visit with a centrally read MCS endoscopy score of at least 2 (on a scale of 0-3); * Subjects receiving oral corticosteroids must have been on a stable dose of prednisone or prednisone equivalent ≤20 mg/day) or on beclomethasone diproprionate (≤5mg/day) or on budesonide MMX (≤9mg/day), for ≥2 weeks before first dosing (i.e. baseline); * Topical corticosteroids and topical 5-aminosalicylic acid preparations must have been withdrawn ≥2 weeks before first dosing (i.e. baseline); * Subjects who are on oral 5-aminosalicylic acid must have been on a stable dose ≥4 weeks before first dosing (i.e. baseline); * Subjects who are receiving immunosuppressants in the form of azathioprine, 6-mercaptopurine, or methotrexate needed to be on a stable dose for 4 weeks before first dosing (i.e. baseline). Subjects taking methotrexate also are advised to take folic acid 1 mg/day (or equivalent) supplementation if there is no contraindication; * Subjects on probiotics (e.g., Culturelle® \[Lactobacillus GG, i-Health, Inc.\], Saccharomyces boulardii) must be on stable doses for 2 weeks before first dosing (i.e. baseline); * Subjects on antidiarrheals (e.g., loperamide, diphenoxylate with atropine) must be on stable doses for 2 weeks before first dosing (i.e. baseline); * Subjects who have previously received anti-tumor necrosis factor (TNF) therapy or vedolizumab must have discontinued therapy ≥8 weeks before first dosing (i.e. baseline); * Subjects previously treated with cyclosporine or tacrolimus must have discontinued therapy ≥4 weeks before first dosing (i.e. baseline); * Subjects previously treated with tube feeding, defined formula diets, or parenteral alimentation/nutrition must have discontinued treatment 3 weeks before first dosing (i.e. baseline). Exclusion Criteria: * Subject with Crohn's Disease (CD), indeterminate colitis (IC) or presence or history of fistula with CD; * History of toxic megacolon, abdominal abscess, symptomatic colonic stricture or stoma; history or is at imminent risk of colectomy; * History or current evidence of colonic dysplasia or adenomatous colonic polyps. Subject with severe gastrointestinal complications; e.g., short bowel syndromes, obstructing strictures, recent or planned bowel surgery, Ileostomy and/or colostomy, recent bowel perforation; * Subject with significant and known active infections at screening such as Infected abscess, positive for Clostridium difficile (stool antigen and toxin), CMV, TB and recent infectious hospitalization;