JAK-IN-1 shows improved selectivity for JAK3 over JAK1. JAK-IN-1 is a JAK1/2/3 inhibitor with IC50s of 0.26, 0.8 and 3.2 nM, respectively.

JAK3:3.2 nM , JAK1:0.26 nM , JAK2:0.8 nM

JAK-IN-1 is active in both mechanistic and functional cell-based assays using T-cells, one of the major cell types in which JAK3 is potentially relevant.JAK-IN-1 inhibits the proliferation of human CD4 and CD8 T cells in a dose-dependent manner upon stimulation by anti-CD3/anti-CD28 antibody-coated beads partially mimicking the activation signals brought to a Tcell by an antigen-presenting cell.

JAK-IN-1 prevents IL-2-driven STAT5 phosphorylation in a dose- and concentration-dependent manner, with approximately 50% inhibition observed at the 10 mg/kg dose (plasma concentration ～480 nM).JAK-IN-1 is JAK3 selective in vivo, as judged by higher potency inhibiting JAK1/JAK3- vs JAK2- or JAK1/JAK2/TYK2-driven signaling in whole blood assays. JAK-IN-1 potently inhibits IL-2 stimulated plasma concentrations of JAK-IN-1 for each dose.