In vitro activity: PF-06651600, as a potent JAK3-selective inhibitor, is highly efficacious at inhibiting γc cytokine signaling, which is dependent on both JAK1 and JAK3. In vitro, 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.

Kinase Assay: PF-06651600 is a new and potent JAK3-specific inhibitor which can inhibit the JAK3 kinase activity with IC50 value of 33.1 nM but does not act (IC50>10 000 nM) against JAK1, JAK2, and TYK2. 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. PF-06651600 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 PF-06651600 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. PF-06651600 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.

Cell Assay: To study the effect of PF-06651600 on Th17 cells post differentiation, skewed Th17 cells are washed, rested with medium for overnight and resuspended in medium containing the same concentrations of cytokines as during skewing but without anti-CD3 or anti-CD28 antibodies, in the presence of PF-06651600 at 10 different concentrations for 2 additional days. On Day 9, supernatant is harvested from each well and IL-17A is determined.