A Study to Learn About the Study Medicine (Called Ritlecitinib) For the Potential Treatment of Severe Alopecia Areata (AA) In Children 6 To Less Than 12 Years of Age

The purpose of the study is to evaluate the pharmacokinetics (how the medicine is changed and eliminated from your body after you take it) and pharmacodynamics (effects of the medicine in the body) of the study medicine (called Ritlecitinib) in children of 6 to \<12 years of age with Alopecia Areata, a condition of scalp hair loss. 12 children with alopecia areata will be participating in this study. All participants will receive study medicine with a dose of 20 milligram (mg) orally once daily for 7 days. 5 blood samples will be collected on day 7 for pharmacokinetic evaluation and 2 blood samples each at screening and on Day 7 will be collected for pharmacodynamic evaluation. Participants will take part in the study for about 10 weeks.

This is an interventional, Pharmacokinetic (PK), Pharmacodynamic (PD), phase 1, open label study in children 6 to less than 12 years of age with ≥50% scalp hair loss due to severe alopecia areata. The purpose of the study is to collect data to support dose selection for subsequent studies in the same population.

Participants will be screened and, if all eligibility criteria are met, will receive the first dose of Investigational product within 28 days after the screening visit.

Participants will receive 20 mg ritlecitinib in one dose, daily, for 7 consecutive days. Blood samples for pharmacodynamic evaluation will be collected on screening and Day 7. Blood samples for pharmacokinetic evaluation will be collected on Day 7 at: 0 hr (pre-dose), 0.5 hr, 1 hr, 3 hrs, and 8 hrs after dosing.

At least 12 evaluable participants with respect to the primary endpoint will be enrolled in the study.

Participants and their parents/legal guardians will be required to visit the study site 3 times during the study (Screening, Day 1 and Day 7) A safety follow-up visit will be conducted by phone, 28 to 35 days after the last dose of ritlecitinib.

Eligibility Criteria:
Key Inclusion criteria:

1. Participants who are 6 to less than12 years old at the baseline visit.
2. A diagnosis of severe AA, including AT and AU, with ≥50% scalp hair loss due to AA (ie, a SALT score of ≥50) at both the Screening and Baseline visits, without evidence of terminal hair regrowth within the previous 12 months.

Key Exclusion Criteria:

1. A known congenital cause of AA, other systemic diseases that may cause hair loss (eg, lupus erythematosus, thyroiditis, systemic sclerosis, lichen planus, etc) or other etiology of hair loss (eg, telogen effluvium, androgenetic alopecia, etc).
2. Any present malignancies or history of malignancies, history of any lymphoproliferative disorder
3. History (one or more episodes) of CMV, varicella, herpes zoster (shingles) or disseminated herpes simplex.
4. Other medical or psychiatric condition (including recent \[within the past year\] or active suicidal ideation/behavior) that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study.
5. Not up to date with all age appropriate vaccines (including 2-dose vaccination for varicella) or vaccination with attenuated live vaccine within 6 weeks of first dose of study medicine.