Study to Evaluate the Clinical Activity and Safety of Oral NX-13 in Moderate to Severe Ulcerative Colitis

* Adult subjects aged 18 to 75 years (inclusive) * Diagnosis of UC ≥ 90 days before screening confirmed by histologic evidence * Active UC defined as a total Mayo Score (MMS) of ≥ 5 (inclusive) at baseline * ES ≥ 2 within 14 days prior to randomization * RBS ≥ 1.

* Severe extensive colitis as evidenced by physician judgment that the participant is likely to require hospitalization for medical care or surgical intervention of any kind for UC (e.g., colectomy) within the 12 weeks after randomization; * Current evidence of fulminant colitis, toxic megacolon or recent history (within 6 months prior to screening) of toxic megacolon, or bowel perforation * Diagnosis of Crohn's disease (CD) or indeterminate colitis, or the presence or history of a fistula consistent with CD * Diagnosis of microscopic colitis, ischemic colitis, or radiation colitis * Bacterial or parasitic pathogenic enteric infection;