Discovery Of A Potent And Selective Jak2 Inhibitor That Suppresses Gm-Csf-Induced Stat5 Phosphorylation And Erythropoietin-Induced Reticulocytosis In Vivo

Essential thrombocythemia (ET), polythemia vera (PV) and myelofibrosis (MF) are myeloproliferative disorders (MPDs) characterized by a chronic excessive production of cells from one or more myeloid lineages and/or bone marrow fibrosis, and have the potential to progress to AML. Recently, gain-of-function mutations in intracellular tyrosine kinase Janus kinase 2 (Jak2) were identified to be associated with MPDs. Jak2 is a member of the Jak family of kinases including Jak1, Jak3, and Tyk2 and is the most proximal signaling component for a number of cytokine receptors. The most common Jak2 mutation identified in MPDs is a substitution of valine to phenylalanine at codon 617 (V617F) located in the pseudokinase domain, which results in the loss of its repressive function, and subsequently leads to the constitutive activation of Jak2 and downstream signaling pathways (STAT, MAP kinase, and PI3 kinase) which affect the survival, differentiation and proliferation of hematopoietic progenitors. Highly selective Jak2 inhibitors may provide a better therapeutic window in chronic dosing settings (ET and PV patients) than non-selective Jak inhibitors. Here we report the discovery of a potent and highly selective Jak2 inhibitor, AMG-Jak2-01. In enzyme assays, AMG-Jak2-01 was potent (3 nM) and selective over other Jak family kinases (> 2000-fold over Jak1, 10-fold over Jak3 and > 400-fold over Tyk2). In isogenic BaF3 cell lines expressing constitutively active Jak kinases (Tel-Jak fusion), AMG-Jak2-01 showed over 40-fold Jak2 selectivity against Jak1 or Tyk2 and 10-fold selectivity against Jak3 based on the inhibition of STAT5 phosphorylation (pSTAT5 – detected with AlphaScreen®). In cytokine-stimulated primary PBMC or whole blood assays, AMG-Jak2-01 demonstrated more than 25-fold Jak2 selectivity over Jak1and Jak3 based on inhibition of pSTAT5 (detected by flow cytometry). In vivo, oral administration of AMG-Jak2-01 at 100 mg/kg for 1 hr inhibited GM-CSF-induced pSTAT5 by approximately 70% (p In summary, we have discovered a potent Jak2 inhibitor, AMG-Jak2-01, which is selective over other Jak family members in both enzyme and cell-based assays and demonstrates inhibitory activity in GM-CSF-induced STAT5 phosphorylation and erythropoietin-induced reticulocytosis in vivo. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1975. doi:10.1158/1538-7445.AM2011-1975