A Study to Assess Two Forms of The Study Medicine (Ritlecitinib) in Healthy Adult Participants

The purpose of this study is to compare if two forms of study medicine, Ritlecitinib, get processed differently in healthy adults.

This study is seeking participants who are:

* aged 18 or older;
* male or female who are healthy as determined by medical assessment ;
* Body-mass Index (BMI) of 16 to 32, and a total body weight \> 45kg.

The study will take up to 2.5 months, including the screening period. There will be 5 periods in total for this study. Participants will have to stay at the study clinic for at least 11 days. Participants will take Riltecitinib either as sprinkled in Soft Food or as Intact Blend-In Capsule. On day 1 of each period, participants will take Riltecitinib and have blood samples taken both before and afterwards. Participants will also answer questions for taste assessment purpose. A follow-up phone call will be made at 28 to 35 days after the last study period.

Ritlecitinib is a covalent and irreversible inhibitor of JAK3 with high selectivity over the other JAK isoforms (JAK1, JAK2, and TYK2). Ritlecitinib also inhibits irreversibly the tyrosine kinase expressed in TEC family kinases with selectivity over the broader human kinome. Treatment with ritlecitinib is expected to inhibit the inflammatory pathways mediated by IL 7, IL 15 and IL 21, all implicated in UC, CD, AA, RA, and vitiligo. Moreover, due to lack of activity against the other JAK isoforms, ritlecitinib is expected to spare immunoregulatory cytokines such as IL 10, IL 27 and IL 35, which are critical to the maintenance of immunosuppressive functions and immune homeostasis.

The objective of this study is to estimate the impact of administration methods on the bioavailability of the pediatric ritlecitinib intact BiC formulation. The study will be conducted as a Phase 1, open-label, single dose, randomized, 4-crossover periods and 1-fixed period design in a single cohort of approximately 12 healthy male or female participants at a single center. Participants will be randomized into 1 of 4 sequences of treatment. Blood samples will be collected for PK analysis. A taste assessment will be also conducted.

Participants will participate in the study for up to approximately 2.5 months, with the inclusion of the screening and follow-up period. On Day 1 of each period, participants will receive a single dose of IP. Administration of IP will be via dosing using intact BiCs with water or by emptying the capsule contents on soft food as per dosing instructions.

Participants will be confined in the CRU for a total of at least 11 days and discharged at the discretion of the investigator. A follow-up phone call will be made at least 28 calendar days and up to 35 calendar days after the last administration of the study intervention to capture any potential AE and confirm appropriate contraceptive usage.

Tolerability and safety will be assessed for all treatments by monitoring AEs.

Eligibility Criteria:
Key Inclusion Criteria:

1. Aged 18 or older.
2. Male or female who are healthy as determined by medical assessment.
3. Body-mass Index (BMI) of 16 to 32, and a total body weight \> 45kg.

Key Exclusion Criteria:

1. Any condition possibly affecting drug absorption (eg, gastrectomy, cholecystectomy).
2. Evidence or history of clinically significant hematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, neurological, or allergic disease (including drug allergies, but excluding untreated, asymptomatic, seasonal allergies at the time of dosing).
3. Known immunodeficiency disorder, including positive serology for HIV, or a first degree relative with a hereditary immunodeficiency, or infections (acute or chronic).