Information Ritlecitinib (PF-06651600) is a potent and irreversibleJAK3-selectiveinhibitor with an IC50 of 33.1 nM but without activity (IC50 > 10 000 nM) against JAK1, JAK2, and TYK2. Targets JAK3 (Cell-free assay) 33.1 nM In vitro PF-06651600, a newly discovered potent JAK3-selective inhibitor, is highly efficacious at inhibiting γc cytokine signaling, which is dependent on both JAK1 and JAK3. it inhibits Th1 and Th17 cell differentiation and function. In total lymphocytes in human whole blood, PF-06651600 inhibits the phosphorylation of STAT5 elicited by IL-2, IL-4, IL-7, and IL-15 with IC50 values of 244, 340, 407, and 266 nM, respectively, and it inhibits the phosphorylation of STAT3 elicited by IL-21 with an IC50 of 355 nM. In vivo PF-06651600 reduces disease pathology in rat adjuvant-induced arthritis as well as in mouse experimental autoimmune encephalomyelitis models. PF-06651600 has suitable pharmacokinetics and pharmacodynamics properties for preclinical and clinical evaluations. Human pharmacokinetics of PF-06651600 are predicted to show approximate values of blood clearance of 5.6 mL/min/kg, a steady state volume of distribution of 1.3 L/kg, oral bioavailability of 90%, and a systemic half-life of 2 h.