Janus-associated kinases (JAKs) are cytoplasmic tyrosine kinases that are required for activating the signaling of certain cytokines and growth factor receptors. JAK1 mostly activates IL-6, whereas JAK1 and JAK3 trigger IL-2 and IL-4 and JAK1 and JAK2 stimulates IFN-γ. JAK Inhibitor I is a pyridine-containing tetracycle that interferes with JAK kinase activity by interacting within the ATP-binding cleft. It inhibits JAK1, 2, and 3 with IC50 values of 15, 1, and 5 nM, respectively, while displaying signi?cantly weaker affinities (IC50s = 130 nM - > 10 μM) for other protein tyrosine kinases. It was shown to block IL-2 and IL-4-dependent proliferation of mouse T-cell lymphoma cells with IC50 values of 50-100 nM. In a mouse model of atopic dermatitis, JAK Inhibitor I, supplied at 2 mg encapsulated in a PLGA nanoparticle, was shown to suppress IFN-γ/STAT1, IL-2/STAT5, and IL-4/STAT6 signaling pathways.
