imately 100-fold the IC 50 of JAK1 and JAK2. INCB018424 inhibited the proliferation of BaF/3 cells (IC 50 = 126 nM) and HEL cells (IC 50 = 186 nM) with JAK2 mutation, but not TF-1 cells transformed with BCR-ABL, or cell lines expressing activating mutations in c-KIT at concentrations up to 8 mM. INCB018424 inhibited hematopoietic progenitor cell colony formation from CD34 + cells isolated from PV patients and did so more potently than with cells from normal donors, particularly when studied in the absence of saturating levels of hematopoietic growth factors. 25 In a murine model of JAK2V617F-driven sumatriptan malignancy, INCB  8424 treatment resulted in significant attenuation of spleen growth and significantly increased mice survival compared with mice treated with vehicle alone.

25 This was accompanied by a dramatic decrease in circulating levels of pro-inflamma
293, E965 69 27. Kusminski, C. M., McTernan, P. G., Schraw, T., Kos, K., Ore, J. P., Ahima, R., Kumar, S., and Scherer, P. E. (2007) Diabetologia 50, 634 642 28. Kos, K., Harte, A. L., da Silva, N. F., Tonchev, A., Chaldakov, G., James, S., Snead, D. R., Hoggart, B., Ore, J. P., McTernan, P. G., and Kumar, S. (2007) J. Clin. Endocrinol. Metab. 92, 1129 tovok 136 29. Ebinuma, H., Miida, T., Yamauchi, T., Hada, Y., Hara, K., Kubota, N., and Kadowaki, T. (2007) Clin. Chem. 53, 1541544 30. Pan, W., Tu, H., and Kastin, A. J. (2006) Peptides 27, 91116 DECEMBER 30, 2011 VOLUME 286 NUMBER 52 JOURNAL OF BIOLOGICAL CHEMISTRY 44919 Downloaded from jbc at NYU School of Medicine Library, on March 6, 2012 Page 7 T-cell precursor acute lymphoblastic leukemia (ALL). 9 ALL patients with JAK1 mutation had significantly reduced disease-free survival and overall survival, as compared with patients without the mutation.

Discovery of an activating tyrosine kinase mutation known as JAK2V617F in myeloproliferative neoplasms (MPNs)polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF)as generated a great deal of interest in targeting JAK2 as a potential approach to treat MPNs. 10 The mutation occurs in the pseudokinase domain of an enzyme JAK2 and results in the impaired ability of the mutated pseudokinase domain to negatively regulate the kinase domain (an active part of an enzyme) of JAK2. 11 Unchecked JAK2 activation, a key signaling enzyme for many growth factors and purchase AZD2171 cytokines, is believed to play a major role in the pathophysiology of MPNs. Nearly all patients with PV and more than half of the patients with ET and PMF have the JAK2V617F mutation, albeit at different levels of allele burden (the ratio between mutated JAK2V617F DNA and total JAK2 DNA). 12 Al- though JAK2V617F appears to be the most common mutation associated with MPNs, other mutations that also abnormally activate JAK2 have been identified (such as MPLW515L/Kin the MPL receptor, 13 and additional JAK2 mutations residing in exon 12) 14 in the small percentage of patients with MPN who did not have JAK2V617F mutation.

All these mutations confer hypersensitivity to, or indepen- dence from, hematopoietic cytokines, resulting in abnormal proliferation and survival of affected stem cells. Enforced expression of these mutations in mice, either by transgenic expression or by retroviral order AZD2171 transfer to the bone marrow stem cells, results in PV, ET and post-PV/ET myelofibrosis (MF) phenotypes, suggesting a potentially causal role for these mutations in MPNs. Recently three independent groups published studies identifying a genetic haplotype that predisposes to the subsequent development of JAK2V617F mutation and MPN, thereby providing evidence for an inheritable predisposition to developing Pharmacology JAK2V617F mutation through somatic mutation. 15-17 These reports further contributed to the growing evidence that JAK2V617F mutation is not an abnormality causing MPN, but rather a contributing factor for disease existence. 18,19 Identification of the abnormalities that lead to the existence of MPN