A Selective and Potent Oral Inhibitor of the JAK2 Tyrosine Kinase Reverses Polycythemia and Leukocytosis Induced by JAK2 V617F in a Mouse Model.

Abstract

A somatic mutation (V617F) in the pseudokinase domain of the non-receptor tyrosine kinase JAK2 is found in virtually all patients with the myeloproliferative disease (MPD) polycythemia vera (PV), and about half of those with essential thrombocythemia and chronic idiopathic myelofibrosis. When expressed in mice by retroviral bone marrow transduction and transplantation, JAK2 V617F but not JAK2 wild-type induces leukocytosis with neutrophilia and recapitulates the entire erythroid phenotype of PV, with polycythemia, reticulocytosis, low plasma Epo, and endogenous erythroid colonies (Zaleskas et al., PLoS ONE 2006; 1:e18; Wernig et al., Blood 2006; 107:4274; Lacout et al., Blood 2006; 108:1652). By analogy to BCR-ABL in chronic myeloid leukemia, JAK2 V617F is therefore a rational target for therapy in PV and other JAK2 V617F+ MPDs. Previous studies with a nonselective and parenteral tyrphostin JAK2 inhibitor suggested that short-term inhibition of JAK2 could decrease reticulocytosis in recipients of JAK2 V617F-transduced marrow (Zaleskas et al., op cit.). Here, we studied one of a novel series of inhibitors of JAK2, AZ-01, which potently inhibited JAK2 kinase activity (IC50 < 1nM) and suppressed proliferation of cell lines carrying the JAK2 V617F mutation. We treated cohorts of Balb/c mice with JAK2 V617F-induced polycythemia and leukocytosis with AZ-01 at 10 mg/kg by oral gavage twice daily, or with vehicle alone. After 28 days of treatment, AZ-01-treated mice had significant reductions in hematocrit, hemoglobin, reticulocytes, and splenomegaly, as well as normalization of peripheral blood leukocyte counts (Figure 1). These results demonstrate that oral administration of a potent and selective JAK2 inhibitor can reverse the hematopoietic abnormalities of JAK2 V617F-induced MPD in mice, and validate JAK2 as a target for therapy of human JAK2 V617F+ MPD. [Display omitted] An orally available JAK2 inhibitor decreases the leukocyte count, hematocrit, and reticulocyte count in mice with JAK2 V617F-induced MPD. Leukocyte count (left), hematocrit (center) and reticulocyte counts (right) of cohorts of mice with JAK2 V617F-induced MPD treated with vehicle or drug at 10 mg/kg BID. Blood counts were assessed at 15 and 28 days of treatment. Decreases in d28 hematocrit and retic count in the drug-treated cohorts were significant (p=0.008 and p<0.0001, respectively, t-tests).