XL-019 is a potent inhibitor of JAK2 (IC50 = 2 nM), and is selective for JAK2 versus the other members of the JAK kinase family (JAK1 IC50 = 130 nM, JAK3 IC50 = 250 nM, TYK2 IC50 = 340 nM). It is active against both wild type and mutationally activated forms of JAK2, and showed good oral bioavailability and pharmacodynamic properties in preclinical studies. Activating mutations in JAK2 are frequently observed in patients with myeloproliferative disorders such as myelofibrosis, polycythemia vera and essential thrombocythemia. JAK2 activity is also upregulated via multiple mechanisms in many lymphomas and solid tumors. XL-019 was being developed by Exelixis for the treatment of myelofibrosis and polycythaemia vera.