This will be a Phase 1, 2 period, fixed sequence, multiple-dose, open-label study of the effect of ritlecitinib on caffeine PK in healthy participants. Approximately 12 healthy male and/or female participants will be enrolled in the study.

Inclusion Criteria:

-Male and female participants must be 18 to 55 years of age, inclusive, at the time of signing the ICD.

-BMI of 17.5 to 30.5 kg/m2; and a total body weight >50 kg (110 lb). -

• Male and female participants who are healthy as determined by medical evaluation including medical history, physical examination, laboratory tests, and 12-lead ECG.
• Participants who are willing and able to comply with all scheduled visits, treatment plan, laboratory tests, lifestyle considerations, and other study procedures.

Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply:

• Evidence or history of clinically significant hematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, neurological, dermatological, or allergic disease (including drug allergies, but excluding untreated, asymptomatic, seasonal allergies at the time of dosing).
• History of febrile illness within 5 days prior to the first dose of investigational product.
• History of any lymphoproliferative disorder such as EBV related lymphoproliferative disorder, history of lymphoma, history of leukemia, or signs or symptoms suggestive of current lymphatic or lymphoid disease.
• Known presence or a history of malignancy other than a successfully treated or excised non metastatic basal cell or squamous cell cancer of the skin or cervical carcinoma in situ.
• Other medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study.
• Participants who, according to the information provided on Drugs.com, would be at increased safety risk if dosed with caffeine.
• History of hypersensitivity to caffeine.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.