This is a Phase 1, randomized, open label, 4-period, 6-sequence, partial cross-over, single-dose study to evaluate the PK of age-appropriate tofacitinib MR formulations (release rates: MR-Slow, MR-Moderate, and MR-Fast) compared to tofacitinib IR solution under fasting conditions. The effect of food on the PK of MR-Slow and MR-Fast will also be assessed.