Inclusion Criteria:

• Diagnosis of UC established ≥3 months before baseline by clinical and endoscopic evidence and corroborated by a histopathology report
• Moderate to severe active UC as defined by a total Mayo score of 6 to 12 with a centrally read rectosigmoidoscopy score ≥2 prior to baseline
• Demonstrated an inadequate response to, loss of response to, or intolerance to at least one of the following treatments:
  • Immunomodulators
  • Anti-TNF agents
  • Corticosteroids (non-US sites only)
• Neurological exam free of clinically significant, unexplained signs or symptoms during screening and no clinically significant change prior to randomization
• No known history of active tuberculosis and negative test for tuberculosis during screening
• Other inclusion criteria as per protocol

Exclusion Criteria:

• Disease limited to the rectum (ie, within 10 cm of the anal verge)
• Toxic megacolon
• Crohn's Disease
• History of subtotal colectomy with ileorectostomy or colectomy with ileoanal pouch, Koch pouch, or ileostomy for UC
• Planned bowel surgery within 24 weeks from baseline
• Stool positive for C. Difficile toxin at screening
• History of gastrointestinal surgery within 8 weeks of baseline
• Primary Sclerosing Cholangitis
• Any uncontrolled or clinically significant systemic disease
• Condition or disease that, in the opinion of the investigator would pose a risk to subject safety or interfere with study evaluation, procedures or completion
• Known to have tested positive for hepatitis B virus surface antigen, hepatitis C virus antibody or HIV
• Underlying condition that predisposes subject to infections (eg, uncontrolled diabetes; history of splenectomy)
• Known history of drug or alcohol abuse within 1 year of screening
• Malignancy (other than resected cutaneous basal or cutaneous squamous cell carcinoma, or treated in situ cervical cancer considered cured) within 5 years of screening visit (if a malignancy occurred > 5 years ago, subject is eligible with documentation of disease free state since treatment)
• Immunosuppressive therapy with either cyclosporine A, tacrolimus, or mycophenolate mofetil, within 1 month prior to baseline
• Prior exposure to anti TNF agents, within 2 months, or 5 times the respective elimination half life (whichever is longer) prior to baseline
• Any prior exposure to vedolizumab, rituximab, efalizumab, natalizumab
• Use of topical (rectal) aminosalicylic acid (eg, mesalamine) or topical (rectal) steroids within 2 weeks prior to baseline
• Use of intravenous or intramuscular corticosteroids within 2 weeks prior to screening and during screening
• Previously treated with AMG 181
• Received any type of live attenuated vaccine < 1 month prior to baseline or is planning to receive any such live attenuated vaccine over the course of the study
• Treatment of infection with intravenous (within 30 days of baseline) or oral (within 14 days prior to baseline) antibiotics, antivirals, or antifungals
• Abnormal laboratory results at screening
• Any other laboratory abnormality, which, in the opinion of the investigator, will prevent the subject from completing the study or will interfere with the interpretation of the study results
• Currently enrolled in another investigational device or drug study, or less than 30 days since ending another investigational device or drug study(s), or receiving other investigational agent(s).
• Other investigational procedures are excluded
• Pregnant or breast feeding
• Other exclusion criteria as per protocol