Inclusion Criteria:

• Able and willing to provide written informed consent.
• 18 - 80 years of age at time of consent.
• For women who are not postmenopausal (at least 12 months of non-therapy-induced amenorrhea) or surgically sterile (e.g., absence of ovaries and/or uterus): agreement to remain abstinent or use a highly effective method of contraception (e.g., combined oral contraceptive pill or transdermal patch, spermicide and barrier [condoms], intrauterine device, implants for contraception, injections for contraception [with prolonged release], hormonal vaginal device, sterilization, or surgical tubal ligation for the duration of the study [i.e., during the treatment period and for at least 24 weeks after the last dose of study drug]. 
  • Abstinence is acceptable only if it is in line with the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.
• For men: agreement to remain abstinent or use a condom, as well as not donate sperm during the treatment period and for at least 24 weeks after the last dose of study drug.
  • Abstinence is acceptable only if it is in line with the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods for the partner) and withdrawal are not acceptable methods of contraception.
• Diagnosis of CD based on clinical, histopathological, and endoscopic evidence established ≥ 3 months prior to screening visit.
  • The Medical Monitor should be consulted in cases where CD was established at least 6 months prior to screening and a histopathology report is not available.  The eligibility of the patient will be considered based on the weight of evidence supporting diagnosis and excluding other potential diagnoses.
• Moderately-to-severely active disease defined in the Screening Phase by:
  • Clinical signs and symptoms resulting in a CDAI score of ≥ 220 to ≤ 480 calculated on the day of randomization, requiring a minimum of 4 days of e-diary PRO data from the 7 days prior to randomization; and
  • A mean daily SF score ≥ 6; or
  • A mean daily SF > 3 and a mean daily AP score > 1 calculated on the day of randomization, requiring a minimum of 4 days of e-diary PRO data from the 7 days prior to randomization (applicable to Cohort 3, the pivotal cohort, only); and
  • The presence of active inflammation on screening ileocolonoscopy defined as a SES-CD of ≥ 7 or ≤ 4 in cases of isolated ileitis or post-ileocecal resection as determined by the central read model.
• Involvement of ileum and/or colon with at least four colonic segments traversable by a pediatric endoscope or three segments (colon and/or ileum) for patients who have undergone a bowel resection for CD
• Meets the following surveillance colonoscopy requirements:
  • Surveillance was undertaken at screening or ≤ 12 months prior in patients with colonic disease for > 10 years (regardless of any risk factors for bowel cancer).
  • Surveillance was undertaken at screening or ≤ 5 years prior in patients with colonic disease for ≤ 10 years who have risk factors for bowel cancer.
  • Note:  local colonic surveillance guidelines can be followed if patients have no risk factors and colonic disease for ≤ 10 years.
• Have experienced intolerance, refractory disease, or no response (as defined below) to at least one of the following therapies within 5 years from screening:
• CS Therapy
  • Refractory:
    • Has signs/symptoms of persistently active disease despite a history of at least one 4-week induction regimen including a dose equivalent to ≥ 30 mg/day prednisone for 2 weeks if oral or 1 week if IV or ≥ 9 mg/day oral budesonide.
  • Intolerance to CS therapy:
    • History including, but not limited to, Cushing’s syndrome, osteopenia/osteoporosis, hyperglycemia, insomnia, and infection.
• IS Therapy
  • Refractory:
    • Has signs/symptoms of persistently active disease despite a history of at least one 12-week regimen of oral AZA (or equivalent) (≥ 1.5 mg/kg) or 6-MP (or equivalent) (≥ 0.75 mg/kg) or MTX (≥ 15 mg/week).
  • Intolerance to 6-MP (or equivalent), AZA (or equivalent), or MTX:
    • History of intolerance to AZA (or equivalent), 6-MP (or equivalent), and/or MTX (including, but not limited to, infection, nausea/vomiting, abdominal pain, pancreatitis, liver function test abnormalities, lymphopenia, and thiopurine methyltransferase genetic polymorphism).
• Anti-TNF Therapy
  • Inadequate primary non-response:
    • Did not respond (as evidenced by persistent signs/symptoms related to CD after receiving ≥ 2 induction doses of either infliximab [≥ 5 mg/kg] or adalimumab [160 mg/80 mg or 80 mg/40 mg] or certolizumab pegol [≥ 400 mg]).
  • Inadequate secondary non-response:
    • Initially responded to induction therapy with infliximab (≥ 5 mg/kg) or adalimumab (≥ 40 mg) or certolizumab pegol (≥ 400 mg), but experienced signs/symptoms related to recurrence of CD during maintenance.
  • Intolerance:
    • Experienced a significant injection-site reaction, congestive heart failure, infection, or other condition that precluded continuing use of anti-TNF therapy at any time
• Patients who have not previously demonstrated inadequate response or intolerance to one or more anti-TNF therapies are eligible to participate in the study provided they are intolerant or refractory to corticosteroid or immunosuppressant therapy.

Exclusion Criteria:

• Gastrointestinal Health
  • Has undergone subtotal colectomy with ileorectal anastomosis or has undergone total colectomy.
  • Short-bowel syndrome.
  • Has an ileostomy or colostomy.
  • Has evidence of fixed stenosis or small-bowel stenosis with prestenotic dilation that precludes adequate endoscopic assessment of the bowel.
  • Diagnosis of UC or indeterminate colitis.
  • Suspicion of ischaemic colitis, radiation colitis, or microscopic colitis.
  • Evidence of abdominal or perianal abscess.
  • Sinus tract with evidence for infection (e.g., purulent discharge) in the clinical judgment of the investigator. Fistulas related to Crohn’s disease are not exclusionary.
  • Expected to require surgery to manage CD-related complications during the study.
  • A history or evidence of adenomatous colonic polyps that have not been removed.
  • Past or present disease-related colonic mucosal dysplasia.
• Prior or Concomitant Therapy
  • Any of the following treatments for CD within 8 weeks prior to randomization:
    • Adalimumab;
    • Certolizumab pegol;
    • Infliximab.
  • Any prior treatment with ustekinumab within 14 weeks prior to randomization.
  • Any prior treatment with etrolizumab or other anti-integrin agents (including vedolizumab, natalizumab, and efalizumab).
  • Any prior treatment with anti-adhesion molecules (e.g., anti-MAdCAM-1).
  • Prior treatment with T cell- or B cell-depleting agents (e.g., rituximab, alemtuzumab, or visilizumab) within ≤ 12 months prior to randomization, with the exception of AZA and 6-MP (or equivalent).
  • Any investigational treatment that included investigational vaccines within 12 weeks prior to randomization in the study or five half-lives of the investigational product, whichever is greater.
  • History of moderate or severe allergic or anaphylactic/anaphylactoid reactions to chimeric, human, or humanized antibodies, fusion proteins, or murine proteins or hypersensitivity to etrolizumab (active drug substance)  or any of the excipients (L-histidine, L-arginine, succinic acid, polysorbate 20).
  • Treatment with corticosteroid enemas/suppositories and/or topical (rectal) 5-aminiosalicylate (5-ASA) preparations ≤ 2 weeks prior to randomization.
  • Continued tube feeding, defined formula diets, and/or parenteral alimentation/nutrition as treatment for CD ≥ 3 weeks prior to randomization.
  • Expectation of tube feeding, defined formula diets, and/or parenteral alimentation/nutrition as treatment for CD during the study.
  • Any live or attenuated vaccines  4 weeks prior to randomization.
  • Use of IV steroids during screening, with the exception of a single IV steroid dose administered in the Emergency Department.
  • Use of cyclosporine, tacrolimus, sirolimus, or mycophenolate mofetil ≤ 4 weeks prior to randomization.
  • Chronic use of nonsteroidal anti-inflammatory drugs (NSAIDs). Prophylactic aspirin use up to 325 mg/day is permitted, as is occasional use of NSAIDs for conditions such as headache, arthritis, myalgia, and menstrual cramps.
  • If receiving oral CSs, patients will be excluded unless the dose is stable at ≤ 20mg/day prednisone or equivalent for ≥ 2 weeks immediately prior to randomization.
  • If receiving ongoing treatment with oral 5-ASA, patients will be excluded if the dose is not stable for ≥ 2 weeks immediately prior to randomization.
  • If receiving ongoing treatment with probiotics (e.g., Culturelle, Saccharomyces boulardii) or over-the-counter supplements (e.g., N-acetyl glucosamine, curcumin), patients will be excluded if the dose is not stable for ≥ 2 weeks immediately prior to randomization.
  • If receiving ongoing treatment with ISs (e.g., 6-MP, AZA, or MTX), patients will be excluded if the dose is not stable for ≥ 8 weeks immediately prior to randomization.
  • If receiving ongoing treatment with antibiotics for the treatment of CD, patients will be excluded if the dose is not stable for ≥ 2 weeks immediately prior to randomization.
  • Patients may continue to receive ongoing treatment with anti-diarrheals (e.g., loperamide or diphenoxylate with atropine), preferably achieving a stable dose for ≥ 2 weeks prior to randomization. However, if the patient and/or the treating physician decides to change the dose or course of anti-diarrheals at any time during screening, these patients will be allowed to participate in study.
• Infection Risk
  • Congenital or acquired immune deficiency.
  • Patients must undergo screening for HIV and test negative for preliminary and confirmatory tests.
  • Positive hepatitis C virus (HCV) antibody test result, unless the patient:
    • has undetectable HCV RNA levels for > 6 months after completing a successful course of HCV antiviral treatment and an undetectable HCV. RNA at screening; or
    • has a known history of HCV antibody positivity with history of undetectable HCV RNA and undetectable HCV RNA at screening in the absence of history of HCV anti-viral treatment.
  • In the screening hepatitis B assessment (which consists of testing for hepatitis B surface antigen [HBsAg], hepatitis B core anti-body [HBcAb], and if required, hepatitis B virus [HBV] DNA), patients who test positive for HBsAg are excluded from the study. Patients who test positive for HBcAb but negative for HBsAg must have a confirmed negative HBV DNA test result to be eligible for the study and will be required to undergo periodic monitoring for HBV DNA during the study.
  • Positive stool test result for ova or parasites or positive stool culture for pathogens at time of screening.
  • Evidence of infection with and/or treatment for Clostridium difficile or other intestinal pathogen treatment within 8 weeks prior to randomization.
  • A history of active or latent tuberculosis (TB) confirmed by one of the following screening tests:
    • Positive tuberculin (purified protein derivative-PPD) skin test; or
    • Positive QuantiFERON® TB Gold test.
  • Patients with a documented history of BCG vaccination must have a negative QuantiFERON test result and negative chest radiograph (see below) to be eligible.
  • Suspicion of active TB on chest radiograph (X-ray, posteroanterior and lateral) taken within 3 months of randomization.
  • History of recurrent opportunistic infections and/or history of severe or disseminated viral infections.
  • Any serious opportunistic infections that occurred ≤ 6 months prior to screening.
  • Any current or recent signs or symptoms (≤ 8 weeks before screening) of infection, except for the following:
    • Minor infections (e.g., common cold) that have, in the investigator’s judgment, completely resolved prior to randomization;
    • Fungal infections of the nail beds;
    • Oral or vaginal candidiasis that has resolved with or without treatment prior to randomization;
    • Any major episode of infection requiring treatment with IV antibiotics ≤ 8 weeks prior to screening or oral antibiotics ≤ 4 weeks prior to screening. Treatment with antibiotics as adjunctive therapy for CD in the absence of documented infection is not exclusionary.
• General Safety
  • Pregnancy or lactation.
  • Lack of peripheral venous access.
  • Hospitalization (other than for elective reasons) within 4 weeks prior to randomization.
  • Inability to comply with study protocol, in the opinion of the investigator.
  • Significant uncontrolled comorbidity such as neurological, cardiac (e.g., moderate to severe heart failure New York Heart Association Class III/IV), pulmonary, renal, hepatic, endocrine, or GI disorders (other than CD).
  • Neurological conditions or diseases that may interfere with monitoring for PML.
  • Clinically significant abnormalities on screening neurologic examination.
  • History of demyelinating disease.
  • History of major neurological disorders, including stroke, MS, brain tumor, neurodegenerative disease, or poorly controlled epilepsy.
  • History of alcohol, drug, or chemical abuse ≤ 6 months prior to screening.
  • Conditions other than CD that could require treatment with > 20 mg/day of prednisone (or equivalent) during the course of the study.
  • History of cancer, including hematologic malignancy, solid tumors, and carcinoma in situ within 5 years before screening.
  • Non-serious basal cell or squamous cell carcinoma of the skin that has been excised and is considered cured is not exclusionary.
  • A history of chronic myelogenous leukemia, hairy cell leukemia, melanoma, renal cell carcinoma, or Kaposi sarcoma is exclusionary irrespective of the duration of time before screening.
  • History of cervical smear result at any time that indicated the presence of adenocarcinoma in situ (AIS), high-grade squamous intraepithelial lesions (HSIL), or cervical intraepithelial neoplasia (CIN) of Grade > 1.
  • History of organ transplant or cell transplantation.
  • Presence of metal in the body that could a pose hazard during any potential scanning in patients for whom a magnetic resonance imaging (MRI) scan is considered unsafe.
• Laboratory Values (at Screening)
  • Serum creatinine > 2 times the upper limit of normal (ULN).
  • Impaired hepatic function defined by one of the following:
    • Serum transaminases > 3 x ULN;
    • Alkaline phosphatase > 3 x ULN;
    • Total bilirubin > 2.5 x ULN (excluding inherited deficiencies such as Gilbert’s disease);
    • Platelet count < 100,000/µL;
    • Hemoglobin < 8 g/dL;
    • Absolute neutrophil count < 1500/µL;
    • Absolute lymphocyte count < 500/µL.