Placebo matching IV afimkibart. Placebo matching SC afimkibart.

Percentage of participants achieving endoscopic subscore of 0 or 1 (normal appearance of mucosa or mild disease) at Week 12.

Percentage of participants achieving endoscopic subscore of 0 (normal appearance of mucosa) at Week 12.

Percentage of participants achieving a decrease in mMS of at least 2 points and 30% from baseline and either a decrease in RBS >= 1 or RBS = 0 or 1 (no blood seen or stool with streaks of blood) at Week 12. mMS is a composite score of ulcerative colitis disease activity, given by the sum of three subscores: SFS, RBS and ES. SFS is measured on a scale from 0 (normal number of stools) to 3 (5 or more stools than normal). RBS is measured on a scale from 0 (no blood seen) to 3 (blood alone passed). ES is measured on a scale from 0 (normal appearance of mucosa) to 3 (severe disease).

Percentage of participants achieving a histologic improvement, defined as Geboes <=3.1 at Week 12. Geboes is a grading system for histologic ulcerative colitis disease activity with scores ranging from 0 (no activity) to 5.4 (ulcer or granulation tissue).

Percentage of participants achieving a histologic remission, defined as Geboes <2B at Week 12. Geboes is a grading system for histologic ulcerative colitis disease activity with scores ranging from 0 (no activity) to 5.4 (ulcer or granulation tissue).

Percentage of participants achieving Geboes <= 3.1 and ES = 0 or 1 (normal appearance of mucosa or mild disease) at Week 12. Geboes is a grading system for histologic ulcerative colitis disease activity with scores ranging from 0 (no activity) to 5.4 (ulcer or granulation tissue).

Percentage of participants achieving Geboes < 2 and ES = 0 or 1 (normal appearance of mucosa or mild disease) at Week 12. Geboes is a grading system for histologic ulcerative colitis disease activity with scores ranging from 0 (no activity) to 5.4 (ulcer or granulation tissue).

Percentage of participants achieving mMS <= 2 with SFS = 0 or 1 (up to 1-2 stools more than normal), RBS = 0 (no blood seen) and ES = 0 or 1 (normal appearance of mucosa or mild disease) at Week 12 in biomarker-defined subgroups. mMS is a composite score of ulcerative colitis disease activity, given by the sum of three subscores: SFS, RBS and ES. Each subscore is measured on a scale from 0 to 3, with higher values associated with greater severity.

Percentage of participants achieving endoscopic subscore of 0 or 1 (normal appearance of mucosa or mild disease) at Week 12 in biomarker-defined subgroups.

Change in bowel urgency from baseline through Week 12. Bowel urgency is measured on a scale from 0 (None) to 4 (Severe).

Change in abdominal pain from baseline through Week 12. Abdominal pain is measured on a scale from 0 (None) to 4 (Severe).

Change in fatigue as measured by the Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-Fatigue) from baseline to Week 12. FACIT-Fatigue is a 13-item self-reported assessment of the level and impact of fatigue. The overall FACIT-Fatigue score ranges between 0 and 52, with higher scores associated with better quality of life concerns related to fatigue.

Change in Inflammatory Bowel Disease Questionnaire (IBDQ) score from baseline to Week 12. IBDQ is a 32-item self-reported assessment of health-related quality of life in participants with inflammatory bowel disease. The overall IBDQ score ranges from 32 to 224, with higher scores associated with better health-related quality of life.

Patient Global Impression of Change (PGIC) from baseline to Weeks 2 and 12. PGIC measures overall change in ulcerative colitis symptoms from "Much better" to"Much worse".

Patient Global Impression of Severity (PGIS) from baseline to Weeks 2 and 12. PGIS measures severity of ulcerative colitis symptoms from "None" to "Very severe".

Incidence and severity of AEs, including serious AEs, AEs leading to treatment discontinuation and AEs of special interest.