Cotreatment with panobinostat and JAK2 inhibitor TG101209 attenuates JAK2V617F levels and signaling and exerts synergistic cytotoxic effects against human myeloproliferative neoplastic cells

Abstract

IntroductionPhiladelphia chromosome-negative myeloproliferative neoplasms (MPNs) are a group of clonal hematopoietic disorders that includes polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). 1,2 Recent studies have confirmed the pathogenetic involvement of an acquired, somatic, gain-offunction, activating, point mutation JAK2V617F in MPNs. [3][4][5][6] This represents a guanine to thymidine mutation in exon 14 resulting in a valine to phenylalanine substitution at codon 617 in the JH2 or pseudokinase domain of the JAK2 gene (a member of the Janus kinase [JAK] family of nonreceptor tyrosine kinases, JAK1, JAK2, JAK3, and TYK2). 2,6 Highly sensitive assays for JAK2 have determined that the JAK2V617F mutation is present in 90% of patients with PV and approximately 50% to 60% of patients with ET or PMF. 7 In addition, a subset of patients, most commonly with PV, are homozygous for the JAK2V617F allele, the result of copy-neutral loss of heterozygosity at the JAK2 locus, especially in patients with PV. 2,7,8 Mutations in exon 12 of JAK2 are present in almost all patients with PV who are JAK2V617F negative. 9,10 The JAK proteins function in the cytoplasm to relay signals initiated by membrane-bound cytokine receptors. Engagement of the receptor results in the phosphorylation of the receptor and JAK2, which recruits its substrate proteins such as signal transducers and activators of transcription (STATs). 11,12 STATs, especially STAT3 and STAT5, translocate to the nucleus and transactivate many genes involved in cell proliferation and survival (eg, Bcl-xL, cyclin D1, and PIM1). 8,11,12 The V617F mutation in JAK2 also activates the downstream signaling pathways through the phosphatidylinositol 3-kinase (PI3K) and extracellular signal-regulated kinase (ERK). This contributes to diminished apoptosis of the hematopoietic progenitor cells (HPCs). 2,8 Overexpression of JAK2V617F in murine Ba/F3 cells with coexpression of the erythropoietin receptor (EpoR) confers in vitro cytokine-independent growth. 3,13 Recently, it was shown that enforced expression of JAK2V617F in human hematopoietic stem cells and myeloid progenitors directed differentiation toward the erythroid lineage, along with increased expression and phosphorylation of GATA-1 and decreased expression of PU.1. 14-16 JAK2V617F expression in retroviral models and in transgenic mice is sufficient to cause myeloproliferative disorders in the mice that recapitulate many clinicopathologic features observed in human PV, ET, and PMF. [17][18][19][20][21] 22,23 In vivo studies in mouse models have also shown that mutant JAK2V617F represents a novel target for therapeutic intervention with JAK2-selective tyrosine kinase inhibitors in MPNs. 21,24 For example, TG101348 inhibits myeloproliferation and myelofibrosis in a murine model of JAK2V617F-induced polycythemia. 21,22 Early clinical trials of several JAK2-selective kinase inhibitors (eg, XL019, TG101348, and INCB18424) are under way in JAK2-driven MPNs with poor prognosis (eg, PMF). ...

Emerging treatments for classical myeloproliferative neoplasms

“…Panobinostat has been shown to promote JAK2V617F degradation synergistically with TG101209. 91 A phase 1 study of the drug used alone in MF demonstrated clinical effects at low doses given during a long period of time with minimal adverse effects. 92 There are currently 2 phase 1/2 trials assessing the safety and tolerability of the combination of panobinostat and ruxolitinib (Table 2).…”

JAK inhibitors for myeloproliferative neoplasms: clarifying facts from myths

“…29 In other words, in human MPN, especially in MF, targeting a signaling network or multiple oncogenic pathways, rather than an isolated JAK-STAT pathway, might be necessary to secure histopathologic remissions and eradication of the ancestral clone. [65][66][67] The absence of drug-induced selective clonal suppression is not a trivial shortcoming because of the implications regarding the prospect of JAK inhibitor therapy to modify disease rather than simply palliate symptoms. Current information from ruxolitinib clinical trials suggests a mechanism of action that includes a nonspecific myelosuppressive effect (possibly responsible for its salutary effect on leukocytosis, thrombocytosis, and splenomegaly and its side effects of anemia and thrombocytopenia) and an anti-JAK-STAT-mediated down-regulation of inflammatory cytokine activity (possibly responsible for its palliative effect on constitutional symptoms and cachexia; Figure 1).…”

Efficacy of vorinostat in a murine model of polycythemia vera

“…32 Other HDAC inhibitors, including ITF2357 (givinostat) and panobinostat, also showed potent antiproliferative and proapoptotic activity against murine and human cells expressing JAK2V617F. 24,33 Therefore, inhibition of HDAC could be useful in treating MPNs. In the present study, we tested the efficacy of vorinostat in an animal model of Jak2V617F ϩ MPN.…”