PF-06651600

PF-06651600 is a novel potent, selective JAK3 inhibitor with IC50 of 0.346 nM, displays >1,000-fold selectivity over JAK1 and JAK2; inhibits Th1 and Th17 cell differentiation and function, reduces disease pathology in rat adjuvant-induced arthritis as well as in mouse experimental autoimmune encephalomyelitis models; selectively targets γc cytokine pathways while preserving JAK1-dependent anti-inflammatory signaling such as the IL-10 suppressive functions following LPS treatment in macrophages and the suppression of TNFα and IL-1β production in IL-27-primed macrophages.Rheumatoid Arthritis Phase 2 Clinical (In Vitro) :Ritlecitinib is a potent JAK3-selective inhibitor which can inhibit the JAK3 kinase activity with an IC50 of 33.1 nM but without activity (IC50>10 000 nM) against JAK1, JAK2, and TYK2. Ritlecitinib 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. Ritlecitinib also inhibits the phosphorylation of STAT3 elicited by IL-21 with an IC50 of 355 nM. Functional assessment in T-cell differentiation assays demonstrate that Ritlecitinib suppresses Th1 and Th17 differentiation as measured by IFNγ, after 5 days under Th1 conditions, and IL-17 production, after 6 days under Th17 conditions, with IC50 values of 30 nM and 167 nM, respectively. Ritlecitinib also suppresses Th1 and Th17 function as measured by the inhibition of IFNγ production (IC50=48 nM) and IL-17 production (IC50=269 nM) in cells that have been previously differentiated and rested before being treated with PF-06651600. (In Vivo) :In the rat adjuvant-induced arthritis (AIA) model, Ritlecitinib reduces paw swelling with an unbound EC50 of 169 nM. Similarly, Ritlecitinib significantly reduces disease severity in the experimental autoimmune encephalomyelitis (EAE) mouse model when dosed either therapeutically at 30 or 100 mg/kg or prophylactically at 20 and 60 mg/kg. The efficacy of Ritlecitinib in these two rodent models of inflammatory and autoimmune diseases illustrates that JAK3-selective inhibition can be sufficient to have disease modifying effects in human diseases.