This is a randomised, double-blind, placebo-controlled, proof of clinical principle study to explore the efficacy and safety of orally administered BBT-401-1S in subjects with ulcerative colitis.

Inclusion Criteria:

• Male or female, of any race, ≥18 and ≤60 years of age.
• Have been diagnosed with active UC for ≥3 months prior to Day 1, as determined by clinical and endoscopic evidence and documented in a histopathology evaluation.
• Have a total Mayo score ≥6, an endoscopic subscore ≥2, rectal bleeding subscore ≥1, and a stool frequency subscore ≥1, regardless of standard of care history.
• Able to comprehend and willing to voluntarily sign an ICF and to abide by the study restrictions.

Exclusion Criteria:

• Have received:

  1. intravenous corticosteroids, rectally administered corticosteroids, or rectally administered 5-aminosalicylic acid within 3 weeks, or
  2. Janus kinase (JAK) inhibitors within 2 weeks, or
  3. cyclosporine, mycophenolate, tacrolimus, or methotrexate within 5 weeks, or
  4. anti-TNF-α biologics within 9 weeks, or
  5. any other biologics (including ustekinumab and vedolizumab) for the treatment of UC within 12 weeks.

• Have received orally administered azathioprine or 6-mercaptopurine that has been stable for <8 weeks.

• Have received orally administered 5-aminosalicylic acid, sulphasalazine, or low-dose corticosteroids (prednisolone ≤20 mg/day or equivalent) that have been stable for <5 weeks.

• Have received any other concomitant medications for UC that have been stable (ie, have not started dosing with a new drug or had a change to their dosing regimen) for <7 days or 5 half-lives, whichever is longer.

• Have Crohn's disease, indeterminate colitis, ischaemic colitis, fulminant colitis, toxic megacolon, chronic (as determined by the investigator) pancolitis, confined proctitis (distal, ≤15 cm), or symptomatic intestinal stenosis.

• Have a history of extensive colonic resection (subtotal or total colectomy) or are anticipated to require surgical intervention for UC.

• Have an ileostomy, colostomy, or known fixed symptomatic stenosis of the intestine.

• Have a positive test for Clostridium difficile, or have evidence of treatment for Clostridium difficile infection or other pathogenic bowel infection within 60 days or for another intestinal pathogen within 30 days prior to Day 1.