The main aim of the study is to assess if TAK-101 can reduce gluten related symptoms and immune activation in adult participants with celiac disease (CeD) on a gluten-free diet (GFD).

Participants will receive TAK-101 and/or placebo through the vein on Day 1 and Day 8. All participants will receive active treatment at Week 24.

The drug being tested in this study is called TAK-101. TAK-101 is being tested to treat people who have celiac disease. The study has 2 cohorts planned. Cohort 1 has 1 dose level and the Cohort 2 may include 1 or 2 additional dose levels, depending on safety, tolerability, and activity observed in Cohort 1. Dosing in the Cohort 2 will be based on data from Cohort 1.

The study will enroll approximately 90 patients. In Cohort 1, approximately 45 participants will be randomly assigned in 1:2:2 ratio in one of the three arm groups which will remain undisclosed to the participant and study doctor during the study (unless there is an urgent medical need). Eligible participants in Cohort 1 will receive:

• Group A: 2 infusion doses of placebo, 1 infusion dose on Day 1 and 1 infusion dose on Day 8, followed by 1 infusion dose of 25 micrograms per kilogram (µg/kg) Gluten Epitopes (GE) TAK-101 at Week 24.
• Group B: 1 infusion dose of 25 µg/kg GE TAK-101 on Day 1 followed by 1 infusion dose of placebo on Day 8, followed by 1 infusion dose of 25 µg/kg GE TAK-101 at Week 24.
• Group C: 2 infusion doses of 25 µg/kg GE TAK-101, 1 infusion dose on Day 1 and 1 infusion dose on Day 8, followed by 1 infusion dose of 25 µg/kg GE TAK-101 at Week 24.

If it is deemed appropriate to enroll both the TAK-101 50 µg/kg GE and the 12.5 µg/kg GE dose levels in Cohort 2, approximately 45 participants may be randomly assigned in 1:2:2 ratio in Cohort 2 to receive:

• Group A: Two infusion doses of placebo, 1 infusion dose on Day 1 and 1 infusion dose on Day 8, followed by 1 infusion dose of 25 µg/kg GE TAK-101 at Week 24.
• Group D: Two infusion doses of 50 µg/kg GE TAK-101, 1 infusion dose on Day 1 and 1 infusion dose on Day 8, followed by 1 infusion dose of 50 µg/kg GE TAK-101 at Week 24.
• Group F: Two infusion doses of 12.5 µg/kg GE TAK-101, 1 infusion dose on Day 1 and 1 infusion dose on Day 8, followed by 1 infusion dose of 12.5 µg/kg GE TAK-101 at Week 24.

If it is decided not to open the second cohort at the 50 µg/kg GE dose level and if 1 2.5 µg/kg GE dose is recommended to be tested by the independent data monitoring committee (IDMC), approximately 27 participants will be randomly assigned in 1:2 ratio to receive:

• Group E: Two infusion doses of placebo, 1 infusion dose on Day 1 and 1 infusion dose on Day 8, followed by 1 infusion dose of 12.5 µg/kg GE TAK-101 at Week 24.
• Group F: Two infusion doses of 12.5 µg/kg GE TAK-101, 1 infusion dose on Day 1 and 1 infusion dose on Day 8, followed by 1 infusion dose of 12.5 µg/kg TAK-101 at Week 24.

This trial will be conducted globally. The overall time to participate in this study is approximately 34 weeks. Participants will make multiple visits to the clinic, and will be contacted by telephone OR plus a final visit after receiving their last dose of study drug for a follow-up assessment.

Inclusion Criteria:

1. Biopsy-confirmed CeD that is well-controlled, defined as mild or with no ongoing signs or symptoms felt to be related to active CeD and with immunoglobulin A (IgA) tissue transglutaminase (tTG) <2 × upper limit of normal (ULN) and IgG deamidated gliadin peptide (DGP) <3 × ULN.

   Note: Participants may be retested for IgA tTG and IgG DGP to meet eligibility criteria at the discretion of the investigator. Intermittent symptoms would not exclude participants from participation as long as symptoms are generally well controlled in the opinion of the investigator, and as long as symptoms are back to baseline for 2 weeks before the run-in gluten challenge.

2. Must be able to maintain a gluten-free diet (GFD) for ≥6 months.

3. Must be HLA-DQ2.5 and/or HLA-DQ8 positive during screening laboratory testing.

Exclusion Criteria:

1. Has received any investigational compound within 12 weeks (84 days) or 5 half-lives, whichever is longer, before signing of the informed consent form (ICF).

2. Has received TAK-101 in a previous clinical study.

3. Has presence of other inflammatory gastrointestinal (GI) disorders or systemic autoimmune diseases, that either have the potential to cause persistent GI symptoms similar to CeD or are not well controlled without the use of excluded medications.

   • Examples of conditions that are exclusionary include: inflammatory bowel disease, eosinophilic gastroenteritis or colitis, and microscopic colitis requiring treatment in the 6 months before screening.
   • Examples of conditions that may be permissible after discussion with the medical monitor/or sponsor include: systemic autoimmune disease such as scleroderma, psoriatic or rheumatoid arthritis, lupus that is stable and without GI involvement, well controlled autoimmune thyroid disease, well controlled type 1 diabetes or proton pump inhibitor -responsive eosinophilic esophagitis in symptomatic and histologically confirmed remission.

4. Has known or suspected refractory CeD or ulcerative jejunitis.

5. Has known or suspected allergy to wheat, such as hypersensitivity and/or anaphylaxis including wheat-dependent-exercise induced anaphylaxis (WDEIA). If there is a possible history of urticaria, angioedema, or anaphylaxis to wheat, investigators should perform testing for wheat anti-Immunoglobulin (anti-IgE) antibodies or refer to an allergist for evaluation prior to enrollment to rule out any of these allergies.

6. Ongoing systemic immunosuppressant, systemic (oral or IV) corticosteroid treatment, or has received treatment with systemic immunosuppressants or corticosteroids in the 12 weeks before run-in gluten challenge.

7. Has known or suspected clinically significant liver disease or positive test result for hepatitis B or C.

8. Has intolerable symptoms after the run-in gluten challenge and is unwilling to undergo subsequent post treatment gluten challenges.

9. Has known allergy to or intolerance of TAK-101 or any of its ingredients or excipients. Also, any subject with a symptomatic allergic reaction that is confirmed by laboratory serology such as elevated tryptase levels following the administration of TAK-101 will be excluded from future dosing.

10. Has a current diagnosis of active malignancy or malignancy diagnosed in the 5 years prior to screening or is receiving ongoing treatment for malignancy.

Takeda provides access to the de-identified individual participant data (IPD) for eligible studies to aid qualified researchers in addressing legitimate scientific objectives (Takeda's data sharing commitment is available on https://clinicaltrials.takeda.com/takedas-commitment?commitment=5). These IPDs will be provided in a secure research environment following approval of a data sharing request, and under the terms of a data sharing agreement.

IPD from eligible studies will be shared with qualified researchers according to the criteria and process described on https://vivli.org/ourmember/takeda/. For approved requests, the researchers will be provided access to anonymized data (to respect patient privacy in line with applicable laws and regulations) and with information necessary to address the research objectives under the terms of a data sharing agreement.