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

This is a Phase 2B randomized, double-blind, placebo-controlled, multiple dose, multicenter
study designed to evaluate the efficacy, safety and microbiome alterations associated with
two dose levels of SER-287, after pre-treatment with vancomycin, in adult subjects, age
18-80, with active mild-to-moderate ulcerative colitis (UC).

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