Crohn's disease (CD) is a serious, long-term condition that can cause swelling (inflammation) and painful ulcers anywhere in the gut. This study will investigate whether zasocitinib can improve CD biology at the cellular and molecular level in adults. The main aims of this study are to better understand zasocitinib's mechanism of action in CD and evaluate how safe this treatment is.

The participants will be treated with zasocitinib for 3 months (12 weeks) and will need to undergo a test where a doctor uses a thin, flexible tube with a tiny camera to look inside the large bowel (colon) and the end of the small bowel (ileum); this test is called ileocolonoscopy.

During the study, participants will visit the study clinic several times.

Zasocitinib will be tested in this mechanistic study in adult participants with active Crohn's disease (CD). Peripheral blood and affected intestinal biopsy samples will be evaluated at the cellular and molecular levels, before and during zasocitinib treatment. The study will assess therapeutic biology, biomarkers, and the safety of zasocitinib.

The study will enroll approximately 20 participants. This is a single center study. The overall duration is up to approximately 5 months (20 weeks) including a 4-week safety follow-up period.

Inclusion Criteria:

1. Men and women aged 18 to 75 years can participate in the study.
2. Must have been diagnosed with Crohn's disease (CD) at least 1 month (30 days) before study start.
3. Must have received other treatments for CD earlier but no longer tolerate these treatments or the treatment no longer helps.

Exclusion Criteria

1. Cannot have another condition that causes swelling (inflammation) in the bowel.
2. Cannot have complications of CD which may require surgery.
3. Cannot have current abscesses or abscesses that have been treated within 6 weeks before study start.
4. Cannot have had a removal of parts of the bowel within half a year (6 months) before study start.
5. Cannot have had any surgery inside the belly (intraabdominal) within 3 months before study start.
6. Cannot have or had a J-pouch.
7. Cannot have had infections up to 2 months (8 weeks) before study start.
8. Cannot have given blood or plasma within 1 month (30 days) before study start.
9. Cannot tolerate having blood taken with a needle from a vein (venipuncture).

Additional entry criteria will be discussed with the study doctor.

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.