A Study to Assess Efficacy and Safety of SER-287 in Adults With Active Mild-to-Moderate Ulcerative Colitis

Inclusion Criteria: * Documented diagnosis of UC at least three months prior to screening, and with a minimum disease extent of 15 cm from the anal verge * Active mild-to-moderate UC * Inadequate response to, loss of response to, or intolerance of, at least one of the following conventional therapies: 5-ASA compounds, corticosteroids, 6-mercaptopurine (6-MP) or azathioprine (AZA), anti-TNFα, anti-integrin or tofacitinib Exclusion Criteria: * Known history of Crohn's disease * No previous history of treatment for UC (treatment-naïve) * Subjects on steroid medication who are unable to have steroids tapered and be completely off steroids at least two weeks prior to screening * Unable to stop steroid enemas or suppositories, or 5-ASA enemas or suppositories, at least two weeks prior to screening * Subjects who have received any investigational or approved biologic therapy within eight weeks or five half-lives prior to screening (whichever is longer) * Subjects who have received any investigational or approved non-biologic therapy, except for those specifically listed in the Permitted Concomitant Medications, for the treatment of underlying disease, within 30 days or five half-lives prior to screening (whichever is longer) * Major gastrointestinal surgery (not including appendectomy or cholecystectomy) within two months before screening, or any history of total colectomy