Study of GS-1427 in Participants With Moderately to Severely Active Ulcerative Colitis

Key Inclusion Criteria:

• Individuals have Ulcerative Colitis (UC) with symptoms of at least 90 days durationbefore randomization, with the diagnosis confirmed by endoscopy and histology at anytime prior to randomization. Documentation of endoscopy and histology consistentwith the diagnosis of UC must be available in the source documents.

• Individuals have UC with minimum disease extent of 15 cm from the anal verge.

• Individuals have moderately to severely active UC as determined by endoscopyoccurring during screening with a total modified Mayo Clinic Score (mMCS) of 5 to 9points, including a centrally read endoscopic subscore of at least 2.

• Individuals have an inadequate response or loss of response or are intolerant to atleast 1 of the following UC treatments: corticosteroids, immunomodulators, oradvanced therapy.

• Individuals have an inadequate response or loss of response or are intolerant to < 3AT mechanisms of action for UC (use of 2 or more AT with the same mechanism ofaction, eg, 2 TNF-α inhibitors, counts as 1 mechanism of action)

Key Exclusion Criteria:

• Have a current diagnosis of Crohn's Disease (CD) or clinical findings suggestive ofCD, diagnosis of indeterminate colitis due to etiologies such as an entericpathogen, or lymphocytic or collagenous colitis.

• Have a current diagnosis of toxic megacolon, symptomatic colonic stricture, acutesevere colitis, fulminant colitis, or abdominal abscess at screening orrandomization.

• Have any history of exposure to vedolizumab or other integrin antagonists

• Requirement for ongoing therapy with or use of any prohibited medication asspecified in the protocol

Note: Other protocol defined Inclusion/Exclusion criteria may apply