Efficacy of the JAK2 inhibitor INCB16562 in a murine model of MPLW515L-induced thrombocytosis and myelofibrosis

Abstract

IntroductionPolycythemia vera (PV), essential thrombocytosis (ET), and primary myelofibrosis (PMF) are classified as BCR-ABL Ϫ myeloproliferative neoplasms (MPNs), typified by clonal proliferation of 1 or more myeloid lineages. 1,2 There are approximately 130 000 to 150 000 patients with MPN in the United States, which makes these disorders among the most common hematopoietic malignancies. 3 Patients with MPN are at high risk for several disease-related complications, including bleeding, thrombosis, splenomegaly, progressive bone marrow failure, and transformation to acute myeloid leukemia (AML). Current therapies for PV and ET are largely empiric, and include antiplatelet therapy, phlebotomy, hydroxyurea, anagrelide, and IFN-␥. 4 For patients with PMF or with post-PV/ET myelofibrosis, treatment options are limited, with the notable exception of allogeneic stem cell transplantation for the subset of patients in which age and/or comorbidities do not exclude transplantation as a therapeutic option. 5,6 There is therefore a need for novel therapies for patients with these disorders.Although previous studies had demonstrated the clonal stem cell origin of these disorders, 7,8 the genetic basis of these disorders was not known until several groups reported the identification of a recurrent somatic mutation in JAK2 (JAK2V617F) in approximately 90% to 95% of patients with PV and in approximately 50% to 60% of patients with Expression of JAK2V617F in vitro transforms hematopoietic cells to cytokine-independent growth and leads to constitutive activation of downstream signaling pathways. 9,15 In addition, expression of JAK2V617F in vivo using the murine bone marrow transplantation (BMT) assay results in a short latency, fully penetrant MPN notable for marked polycythemia, hepatosplenomegaly, and variable progression to myelofibrosis. [16][17][18][19] These data demonstrate the importance of JAK2V617F to the pathogenesis of JAK2V617F-positive MPN.Although the discovery of JAK2V617F mutations in almost all patients with PV and approximately half of those with ET and PMF provided important insight into the molecular basis of these MPNs, the etiology of JAK2V617F Ϫ MPN remained unknown. Investigators subsequently identified somatic activating mutations in exon 12 of JAK2 in patients with JAK2V617F Ϫ PV; 20 however, alternate JAK2 mutations were not identified in JAK2V617F Ϫ ET and PMF. Based on the observation that the JAK2V617F kinase requires expression of a type I homodimeric cytokine receptor (EPOR, MPL, GCSFR) to efficiently transform hematopoietic cells, 15 investigators sequenced these cytokine receptors in patients with MPN and identified somatic mutations at codon 515 of the thrombopoietin receptor (MPLW515L) in ET and PMF. 21 Subsequent to the initial identification of the MPLW515L allele, additional somatic mutations at codon 515 (MPLW515K, MPLW515A) 22,23 and at codon 505 (MPLS505N) 24 21 More importantly, overexpression of MPLW515L in the murine BMT assay results in development of an acute myeloproliferative ne...

HSP90 is a therapeutic target in JAK2-dependent myeloproliferative neoplasms in mice and humans

“…We also performed Western blot analysis of JAK2 protein levels in normal and MPLW515L splenocytes after a single dose of PU-H71. Consistent with the pharmacokinetic data, we observed potent degradation of JAK2 but does not result in reduction in the size of the malignant clone (55). We therefore wished to determine whether HSP90 inhibition with PU-H71 was able to reduce mutant allele burden in this model.…”

Genetic studies reveal an unexpected negative regulatory role for Jak2 in thrombopoiesis

“…27,28 We hypothesized that Jak2 loss in PLTs and MKs would compromise TPO internalization and maintain high steady state TPO despite thrombocytosis. We assessed TPO levels in Pf4-Cre-positive Jak2 f/f mice, controls, and mice expressing MPLW515L, 29 which develop a similar extent of thrombocytosis and MK expansion as Pf4-Cre-positive Jak2 f/f mice. Serum TPO was markedly reduced in MPLW515L mice (TPO 230 1/2 37 pg/mL in MPLW515L vs 2787 1/2 204 pg/mL in controls, P , .0001), but not in Pf4-Cre-positive Jak2 f/f mice (TPO 2524 1/2 230 pg/mL, P , .0001) (n 5 8 to 13 mice/group; Figure 2I).…”

EXEL-8232, a small-molecule JAK2 inhibitor, effectively treats thrombocytosis and extramedullary hematopoiesis in a murine model of myeloproliferative neoplasm induced by MPLW515L

“…In vitro data of both studies show sufficient sensitivity and specificity of two different smallmolecule JAK2 inhibitors, pathway specificity in vitro and in vivo, as well as response to treatment, such as improved platelet counts, WBC, body weights, decreased splenic weights; however both studies were not able to show elimination of the malignant clone. 11 A significant proportion (B50%) of ET and MF patients lack the JAK2V617F mutation. 1,2,[12][13][14] The MPLW515L and JAK2 Exon 12 alleles account for an additional B10% and B2%, respectively, of the JAK2V617F-negative patients 4,14 and less commonly TET2, ASXL1, CBL, IDH and IKZF1.…”

“…5,9 In contrast, robust thrombocytosis is induced in the MPLW515L murine BMT model. 4,11 We therefore used this system to evaluate platelet response to the JAK2-specific smallmolecule inhibitor EXEL-8232. We designed these studies by treating animals for 28 days with EXEL-8232 from day 10 post BMT and demonstrated dose-dependent responses to therapy as well as insights into potential side effects.…”