PF-06651600 is a newly discovered irreversible covalent JAK3-selective inhibitor. A high level of selectivity towards JAK3 is achieved by the covalent interaction of PF-06651600 with a unique cysteine residue (Cys-909) in the catalytic domain of JAK3, which is replaced by a serine residue in the other JAK isoforms. PF-06651600 allowed the comparison of JAK3-selective inhibition to pan-JAK or JAK1-selective inhibition, in relevant immune cells to a level that could not be achieved previously without such potency and selectivity. In vitro, PF-06651600 inhibits Th1 and Th17 cell differentiation and function, and in vivo it reduces disease pathology in rat adjuvant-induced arthritis as well as in mouse experimental autoimmune encephalomyelitis models. Preclinical data demonstrates that inhibition of the cytolytic function of CD8+ T cells and NK cells by PF-06651600 is driven by the inhibition of TEC kinases. Based on the underlying pathophysiology of inflammatory diseases such as rheumatoid arthritis, inflammatory bowel disease, alopecia areata and vitiligo, the dual activity of PF06651600 towards JAK3 and the TEC kinase family may provide a beneficial inhibitory profile for therapeutic intervention. PF-06651600 is in phase III clinical trial for the treatment of alopecia areata and in phase II clinical trial for the treatment of Crohn's disease, Rheumatoid arthritis, Ulcerative colitis and Vitiligo.