Experimental therapeutics for patients with myeloproliferative neoplasias

Authors


Abstract

Philadelphia chromosome (Ph)-negative myeloproliferative neoplasms (MPNs) are characterized by stem cell-derived, unrestrained clonal myeloproliferation. The World Health Organization classification system, proposed in 2008, identifies 7 distinct categories of Ph-negative MPNs including essential thrombocythemia (ET); polycythemia vera (PV); primary myelofibrosis (PMF); mastocytosis; chronic eosinophilic leukemia; chronic neutrophilic leukemia; and MPN, unclassifiable. For many years, the treatment of ET, PV, and PMF, the most frequently diagnosed Ph-negative MPNs, has been largely supportive. In recent years, that paradigm has been challenged because of the discovery of a recurrent point mutation in the Janus kinase 2 (JAK2) gene (JAK2V617F). This mutation can be detected in the vast majority of patients with PV and approximately half of patients with ET or PMF and serves as both a diagnostic marker as well as representing a putative molecular target for drug development. Several putative targeted agents with significant in vitro JAK2 inhibitory activity and various degrees of JAK2 specificity are currently undergoing clinical evaluation. Furthermore, other investigational non-tyrosine kinase inhibitor approaches such as immunomodulatory agents and pegylated interferon-α have also shown promising results in MPNs. Cancer 2011. © 2010 American Cancer Society.

Philadelphia chromosome (Ph)-negative myeloproliferative neoplasms (MPNs) are clonal proliferative disorders arising from hematopoietic stem or progenitor cells and characterized by an increased number of terminally differentiated myeloid elements.1 Several disorders fall in this category, including polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF), chronic eosinophilic leukemia, chronic myelomonocytic leukemia, and systemic mastocytosis.2 Taken together, PV, ET, and PMF, the 3 most common types of Ph-negative MPNs, are more prevalent than Ph-positive chronic myeloid leukemia (CML), afflicting >100,000 people in the United States alone. As posited by Dameshek in 1951, there are remarkable similarities between all of the MPNs.3 Although patients with PV can have an increased red cell mass, low erythropoietin (Epo) levels, and endogenous erythroid colony (EEC) formation in vitro; and those with ET present with high platelet counts and increased bone marrow megakaryocytic mass; and patients with PMF present with anemia, splenomegaly, leukoerythroblastosis, constitutional symptoms, and marked collagen and reticulin bone marrow fibrosis, it is known that all of these MPNs share clinical and laboratory features, making them overlap phenotypically. As examples, isolated thrombocytosis can be the presenting manifestation of PV, ET, and PMF4, 5; myelofibrosis (MF)6, 7 is a well-known complication of PV; PMF can evolve into PV8; and it has been shown that there is no clear separation by bone marrow morphology, EEC, or serum Epo levels.9, 10 Indeed, all 3 categories of MPNs have the potential to evolve into acute myeloid leukemia and exhibit a propensity to develop thrombotic complications. The inter-relatedness of these disorders has been substantiated in recent years by the discovery of a common pathogenetic underpinning consisting of a recurrent somatic point mutation in the pseudokinase domain of the Januskinase2(JAK2) gene.11-15

Indeed, prospective and retrospective studies indicate that JAK2V617F-positive ET can resemble PV and that patients with ET with this mutation have a higher hemoglobin (Hb) and white blood cell count, an increased bone marrow cellularity, an increased thrombotic risk, a higher rate of transformation to PV, and a greater sensitivity to hydroxyurea (HU).16 However, these findings are brought into question because it has been shown that when “PV-like” ET patients had red blood cell mass studies performed, these patients usually were found to have PV.17 Another study also demonstrated that patients with ET who were positive for JAK2V617F had a phenotype that was more like PV than their JAK2V617F-negative counterparts.16

Patients with JAK2V617F-negative ET may resemble patients with PMF and frequently exhibit isolated thrombocytosis and may present with splenomegaly; cytogenetic abnormalities; megakaryocytic dysplasia; and an increased susceptibility to leukemic transformation, MF), and clonal hematopoiesis.18 However, these patients with JAK2V617F-negative ET may not actually present with features of PMF, but may actually have PMF.19

The finding that mutant JAK2 is endowed with constitutively active kinase activity has spurred great interest in the development of therapeutic agents aimed at inhibiting this putative pathogenic event. It is interesting to note that a mutation in JAK2 can lead to 3 different phenotypes (PV, ET, and PMF). Although the reasons for this are not entirely clear, it is believed that the JAK2V617F allele burden in combination with yet unidentified host gene modifiers can lead to a different clinical phenotype. In ET, the JAK2V617F allelic burden is usually low and heterozygosity for the mutation is the rule.20 The malignant clonal stem cells are not shown to be increased in number compared with the normal stem cells in the bone marrow.21 Alternatively, in PV there is a high JAK2 allelic burden. Although most PV patients are homozygous for the mutation, not all are because not all have uniparental disomy.22 In PV, the malignant clone predominates in the bone marrow.21 In PMF, homozygosity for the JAK2V617F mutation is predominant.23 Taken together, homozygosity of the JAK2 mutation is perhaps not a distinguishing feature for the phenotype of these disorders. It is has been shown that some patients with JAK2V617F–positive erythrocytosis never develop the full clinical features of PV. Gender differences may also play a role because ET and PV are more frequent among women, whereas PMF predominates among men.16 These findings suggest that it is the individual host's characteristics that may play the definitive role in determining the ultimate clinical phenotype.

Importance of the JAK2V617F Mutation in PV, ET, and PMF

The discovery of the JAK2V617F mutation in patients with PV, ET, or PMF has had a significant impact on the taxonomy of MPNs. The JAK family of proteins encompasses 4 members (JAK1, JAK2, JAK3, and [tyrosine kinase] TYK2), which are differentially activated in response to various cytokines. JAK2 has been shown to be predominantly activated in response to an array of cytokines, including Epo, interleukin (IL)-3, granulocyte-macrophage–colony-stimulating factor (GM-CSF), IL-5, and thrombopoietin.24 JAK kinases contain 7 homologous domains (JH1-JH7), including both a catalytic kinase domain (JH1) and a catalytically inactive pseudokinase domain (JH2). In 2005, a G-to-T substitution at the 1849 position, resulting in the substitution of valine for phenylalanine at codon 617 of JAK2 (JAK2V617F), was shown to cause constitutive kinase activity.11-15 This somatic mutation that is present in hematopoietic cells, and occasionally in different hematopoietic compartments including B and T lymphoid cells, occurs with a frequency of >99% in patients with PV and in approximately 50% of patients with ET or PMF. Although most do, not all PV patients express JAK2V617F, and in familial PV, this is particularly the case; it has been shown that multiple genetic defects are implicated in the early pathogenesis of PV.25 Approximately 15% of patients with familial PV are negative for JAK2V617F and this holds true for patients with ET who convert to PV.26, 27JAK2V617F has also been detected in other conditions, including refractory anemia with ringed sideroblasts, acute lymphoblastic leukemia (ALL), and acute myeloid leukemia (AML).28JAK2V617F constitutive kinase activity confers cytokine hypersensitivity and cytokine-independent growth of hematopoietic cells. This is a characteristic feature of hematopoietic colonies obtained from patients with PV and is most marked in hematopoietic cells that co-express Epo-R, myeloproliferative leukemia virus oncogene homolog (MPL), or granulocyte–colony-stimulating factor (G-CSF).29 Importantly, mice carrying the JAK2V617F mutation recapitulate multiple features of human PV, including marked erythrocytosis and a proclivity to develop marked MF over time, thus supporting an important pathogenetic role of this mutation in human MPNs.30 In vitro studies demonstrate that ligand binding to JAK2 results in kinase phosphorylation with subsequent activation of downstream signaling proteins. JAK2 leads to activation of signal transducer and activator of transcription (STAT), mitogen-activated protein kinase (MAPK), and phosphatidylinositol 3-kinase (PI3K) signaling pathways, which could lead to transformation of hematopoietic progenitors.31 However, JAK2 can be absent in the blast cells at transformation, indicating that JAK2V617F–negative leukemia can arise in patients with JAK2V617F-positive MPNs.32 Importantly, the expression of constitutively active STAT5 or the antiapoptotic gene BCLXL results in Epo-independent colony formation.33 Blood samples taken from patients with PV have demonstrated STAT3 activation and BCLXL overexpression.

JAK2V617F-negative MPNs

In light of the finding that rare patients with PV and approximately half of those with ET or PMF are negative for JAK2V617F, systematic screenings have been conducted in patients with JAK2V617F–negative MPNs in search of somatic mutations in the JAK-STAT pathway.34 Four novel somatic mutations in exon 12 of JAK2 were identified. Three of these were small deletions or insertions involving codons 538 to 543 and the fourth was a point mutation resulting in the substitution of lysine for leucine at codon 539. The most frequent of all exon 12 mutations is N542-E543del, which was found in 17 of 52 studied cases.35 JAK2 exon 12 mutant alleles can induce cytokine-independent proliferation in Epo receptor-expressing cell lines and lead to the constitutive activation of JAK-STAT signaling. Patients with PV who carry JAK2 exon 12 mutations often present with isolated erythrocytosis and a subnormal serum Epo level, and harbor EECs. Of diagnostic and pathogenetic importance is that JAK2 exon 12 mutations are mutually exclusive from JAK2V617F. Approximately 10% of patients with PMF who are JAK2 negative display an activating mutation in the MPL gene encoding for the thrombopoietin receptor.36 MPL belongs to the hematopoietic receptor superfamily and on interaction with its ligand, thrombopoietin, triggers megakaryocyte growth and differentiation. A recently identified allele carries a point mutation at codon 515 of the transmembrane-juxtamembrane junction of MPL (MPLW515L), a mutation leading to the substitution of either leucine or lysine for tryptophan.37 Another identified mutation, MPLW515K, involves the same codon.36 The prevalence of MPL mutations is approximately 5% in PMF and 1% in ET. MPL515 mutations are early, stem cell-derived events involving both myeloid and lymphoid progenitors.38 In vitro studies demonstrate that expression of MPLW515L constitutively activates JAK2 and downstream signaling pathways in a similar fashion to JAK2V617F. In vivo studies have shown that expression of this allele results in a distinct phenotype marked by thrombocytosis and MF.37

Current Treatment of MPN

Standard therapy for patients with MPNs has not been firmly defined and is mostly supportive. For patients with low-risk PV, phlebotomy is customarily used with the goal of maintaining a hematocrit <45% in men and 42% in women, and is often used as a first-line therapy for these patients. The European Collaboration on Low-Dose Aspirin in Polycythemia Vera (ECLAP) study was a large, prospective multicenter project of 1638 PV patients that evaluated treatment strategies to achieve a hematocrit <45% and a platelet count <400 ×109/L. Reanalysis of this trial via multivariable analysis indicated that a hematocrit between 40% and 55% was not associated with the occurrence of thrombotic events or mortality.39 This trial suggested a lack of prognostic significance for the hematocrit value and the platelet count in patients with PV and challenged the need for aggressive control of these parameters in patients for preventing thrombotic and hemorrhagic complications. However, there are several concerns with regard to this trial. First, no clear diagnostic criteria were used in this study. The follow-up of patients in the study was too short (mean duration of follow-up, 2.7 years) to truly establish a correlation between the hematocrit values and thrombosis. Lastly, it is known that there is no correlation between hematocrit and the red cell mass. Retrospective analyses among patients with Chuvash polycythemia have not demonstrated a benefit for therapy with phlebotomy.40 However, this cannot be used as proof that phlebotomy is not effective in PV because Chuvash polycythemia is a congenital disorder, the pathophysiology of which is demonstrably different from that of PV. In addition, it is unclear whether phlebotomy therapy was conducted properly in this population.

The results of a controlled, prospective clinical trial demonstrated that HU did not prolong survival or prevent complications of PV, including thrombosis and MF.41 HU does appear to be effective in preventing transient ischemic attack (TIA) in patients unresponsive to aspirin, and this constitutes the only selective indication for this drug.42 Although aspirin can be effective in relieving microvascular complications of PV, including ocular migraine and erythromelalgia, a recent review suggests that the use of aspirin in patients with PV leads to a nonsignificant reduction in the risk of fatal thrombotic events, without an increased risk of major bleeding, compared with patients with PV who are receiving no treatment.43 Patients with low-risk ET normally do not require therapy. The PT-1 study demonstrated that for patients with high-risk ET, HU was no more effective than anagrelide in preventing thrombotic events, with the exception of TIA.44 A reason for this observation is that anagrelide is similar to the platelet inhibitor dipyridamole, and thus it stands to reason that anagrelide would have effectiveness against venous thrombosis. To the best of our knowledge, there is no established target platelet count and there is no correlation between the platelet count and thrombosis. As such, the targeted count should be one in which a patient has no microvascular complications or bleeding.45 Aspirin is recommended for patients considered to have intermediate or high-risk disease.46 Multiple supportive strategies have been shown to be of limited activity in patients with PMF, including HU, steroids, and androgen preparations. Recent publications have demonstrated the efficacy of allogeneic transplantation in patients with PMF.47, 48 One prospective, phase 2 study of 103 patients with PMF or post-ET/PV MF indicated that by using reduced intensity conditioning (33 related donors or 70 unrelated donors),49 the estimated 5-year event-free survival and overall survival rates were 51% and 67%, respectively. As seen earlier, although allogeneic transplantation has been shown to prolong survival, donor availability and treatment-related mortality limit this treatment modality to a small number of patients.50 It is therefore clear that novel strategies aimed at eliminating the malignant clones are needed for the management of patients with MPNs. A series of agents with activity against the mutant JAK2V617F kinase are currently undergoing clinical testing (Table 1).

Table 1. JAK2 Inhibitors Currently in Clinical Trials
JAK2 InhibitorManufacturerTargetClinical ActivityIC50, nMCurrent Stage of Clinical Development
  1. JAK indicates Janus kinase; IC50, concentration that inhibits 50%; HEL, human erythroleukemia; TYK, tyrosine kinase; MPL, myeloproliferative leukemia virus oncogene homolog; FLT, fms-like tyrosine kinase receptor; STAT5, signal transducer and activator of transcription-5.

INCB01842455-60IncyteJAK1, JAK2, JAK3, TYK2Decreased spleen size, improved quality of life, decreased inflammatory cytokine levels No significant effect on JAK2V617F allele burdenJAK1: 2.7 JAK2: 4.5 JAK3: 322Phase 3
TG10134861, 62TargeGenJAK1, JAK2, JAK3Dose-dependent reduction in spleen size and leukocytosisJAK1: 105 JAK2: 3 JAK3: 996Phase 1/2
XL01963, 64ExelixisJAK1, JAK2, JAK3, TYK2Decreased spleen size only in patients with JAK2V617F or MPL mutations, decreased pruritus, decrease in circulating blasts in peripheral bloodJAK1: 132 JAK2: 2 JAK3: 250Development halted
CEP-701 (Lestaurtinib)66-70CephalonFLT3, JAK2Decreased spleen sizeJAK2: 1 JAK3: 3Phase 2
SB151871, 72S*BioJAK1, JAK2, JAK2Reduction of leukocytosis, hepatosplenomegaly, and phospho-STAT5hJAK1: 1276 JAK2: 22 JAK3: 1392Phase 1

Targeted JAK2 Inhibitors

The JAK family (JAK1, JAK2, JAK3, and TYK2) are cytosolic kinases that relay signals from surface receptors to the cell nucleus.51 Figure 1 Top illustrates the JAK family and the receptors they engage.52 JAK kinases are comprised of 7 domains (Jhk1-7).53 IL, interferon (IFN), and G-CSF are JAK1-dependent for signal transduction. Erythropoietin, thrombopoietin, and GM-CSF are dependent on JAK2 for signal transduction. JAK3 is essential for the signal transduction of many ILs and is involved in immune control.51 At the time of ligand binding to its receptor, the latter is activated, leading to activation of the JAK2 tyrosine kinase, which in turn leads to phosphorylation of the receptor and recruitment of STAT proteins. JAK then phosphorylates STAT proteins, leading to STAT dimerization and migration to the nucleus, which results in gene transcription. Patients with MPNs with JAK2V617F have mutations in the Jhk2 domain, which has inhibitory JAK2 properties.54 Figure 1 Bottom shows the position of mutations. This mutation leads to constitutive JAK2 activation, which results in increased downstream signaling and proliferation. JAK2 inhibitors work by interfering with JAK2. In doing so, they abrogate JAK2 activation of downstream signals, including the STAT proteins. This disenables the increased transcription of cellular growth signals and cytokines (Fig. 2).

Figure 1.

Relation of Janus kinase (JAK) and cytokine receptors is shown. (Top) Each cytokine receptor has a predilection for a particular JAK. (Bottom) Diagram of structure of a JAK2 kinase along with the position of JAK2 point mutations found in myeloproliferative neoplasms is shown. TYK indicates tyrosine kinase.

Figure 2.

Mechanism of action of a Janus kinase2 (JAK2) inhibitor is shown. (A) Activation of the JAK-signal transducer and activator of transcription (STAT) signaling pathway is shown. At the time of cytokine binding, JAK2 molecules engage with cytokine receptors and are phosphorylated, which causes transphosphorylation of STAT molecules. In turn, phosphorylated STAT translocates to the nucleus, where it binds cognate DNA sequences, thus resulting in the expression of genes that regulate cell proliferation and survival. (B) Putative mechanism of action of a JAK2 inhibitor is shown. JAK2 inhibitors inhibit the kinase activity of both native and mutant JAK2, which abrogates STAT activation, resulting in the inhibition of cell proliferation. P indicates phosphorylated.

INCB018424

The JAK2 inhibitor furthest along in clinical development is INCB018424, which is a potent and selective JAK1 (concentration that inhibits 50% [IC50] = 2.7 nM), JAK2 (IC50 = 4.5 nM), and JAK3 (IC50 = 322). It has been found to inhibit colony formation from healthy donor cells at a concentration of 400 nM.55 In a phase 2 study, INCB018424 was administered to 155 patients with PMF, post-PV, or post-ET MF.56 Seventy-six patients received treatment for >12 months. Optimization of the dosing regimen using the platelet count helped determine the starting dose (either 10 or 15 mg twice daily) and allowed for dose increases after 1 or 2 months, with most patients subsequently receiving doses of 15 or 20 mg twice daily. The majority of patients remained on treatment at the time of last follow-up (115 of 155 patients; 74%). With the optimized dosing regimen, a reduction in spleen volume occurred as early as 1 month and was durable over 6 months of therapy. Eleven of 23 patients with MF (48%) had a ≥35% reduction in spleen size. INCB018424 treatment resulted in an increased exercise capacity along with marked improvement in constitutional symptoms including fatigue, abdominal pain, and pruritus in >50% of patients.

Of note is that INCB018424 is not specific for JAK2V617F. Given that the JAK2-STAT pathway is highly active in patients with MPNs, even without JAK2 mutation, the drug may be equally effective regardless of JAK2 mutational status through inhibition of the JAK2-STAT axis. Indeed, a subset analysis of 53 patients with MF demonstrated that the improvement in clinical symptoms derived by INCB018424 treatment coincided with a sustained and remarkable reduction of fibrogenic, proinflammatory, and angiogenic growth factors,57 including IL-6 and tumor necrosis factor-α which have both been linked to the pathogenesis of PMF. A prospective, uncontrolled phase 2 trial demonstrated that of 86 evaluable patients with PMF enrolled on study, INCB018424 therapy resulted in a significant and rapid reduction of PMF-related symptoms, with 53%, 66%, 46%, 50%, and 85% of patients experiencing improvements in fatigue, abdominal pain, bone pain, quality of life, and pruritus, respectively.58 Another study evaluated the JAK2V617F allele burden in the blood and bone marrow at baseline and after ≥3 months of therapy with INCB018424 in 19 patients with PMF (n = 9) and post-PV/ET MF (n = 10). Despite significant clinical improvement (CI) in these patients (reduction in splenomegaly, improved constitutional symptoms), there was only a modest decrease noted in the JAK2V617F allele burden.59

On the basis of the activity observed in patients with PMF, a phase 2 trial evaluated the efficacy of INCB018424 in patients with advanced PV (n = 34) or ET (n = 39) who were refractory to or intolerant of HU.60 The starting doses were 10 mg twice daily in patients with PV and 25 mg twice daily in patients with ET. At the time of last follow-up, all 34 (100%) patients with PV had completed 3 months of treatment, with 20 (59%) receiving the drug for >6 months. Thirty-two patients (94%) had attained a partial response (PR) or a complete response (CR). All 24 patients who were phlebotomy-dependent at 6 months before therapy became phlebotomy-independent within 2 weeks of the initiation of INCB018424. Of 21 patients with splenomegaly at baseline, 60% had a ≥50% reduction in spleen size within the first month. All 26 patients with pruritus had symptom resolution along with improvements in bone pain and fever. All 39 (100%) patients with ET had completed at least 3 months of treatment and 17 (44%) had been treated for more than 6 months. Twenty-four patients (61%) had attained a PR or CR. All 4 patients who entered the study with splenomegaly experienced >50% reduction in spleen size. Overall, INCB018424 was found to be well tolerated both by patients with PV or ET. Given the promising phase 2 findings delineated above, a phase 3, randomized, placebo-controlled trial using this JAK2 inhibitor is currently underway.

TG101348

TG101348 is an oral JAK2 inhibitor that is both a potent and selective JAK2 inhibitor (IC50 = 3 nM), but also has JAK1 (IC50 = 105) and JAK3 (IC50 = 996) inhibitory activity.61 It inhibits the growth of hematopoietic colonies and induces apoptosis in human erythroleukemia (HEL) and Ba/F3 cells expressing JAK2V617F, MPLW515K, or JAK2 exon 12 mutations. In a murine model of PV, TG101348 decreased both hematocrit and spleen size and prolonged survival.61 These preclinical studies led to a phase 1 multicenter dose escalation study,62 in which the maximum tolerated dose was found to be 680 mg daily. A total of 59 patients (44 with PMF, 12 with post-PV MF, and 3 with post-ET MF, 86% of whom were JAK2V617F positive) were enrolled (28 in the dose escalation phase and 31 in the dose confirmation phase). Forty patients (68%) initiated treatment at a dose of ≥680 mg daily. After a median follow-up of 12 weeks, 18 patients (31%) had discontinued treatment (due to toxicity, comorbidities, withdrawal of consent, and noncompliance). The 41 patients remaining on study were receiving doses ranging from 240 mg to 680 mg and 33 patients who started at a dose of ≥680 mg daily had completed at least 3 cycles of therapy at the time of last follow-up. At 3 months, 22 patients (67%) had at least a 50% reduction in spleen size. All 21 patients with pretreatment leukocytosis had a reduction in their white blood cell count. Of 51 patients harboring the JAK2V617F mutation, 48 had completed at least 1 cycle of treatment and were evaluable for a molecular response at the time of last follow-up; the median decrease in the mutant allele burden was 48%. Twenty-one patients (44%) had a ≥50% reduction in spleen size and there was an improvement noted in symptoms including fatigue, early satiety, and pruritus. Of the 40 patients (68%) who started at doses of 680 mg, grade 3/4 neutropenia was observed in 15%/0% and grade 3/4 thrombocytopenia was noted in 20%/10% patients. Twenty-four (60%) patients did not require red cell transfusions at baseline; of these patients, grade 3/4 anemia occurred in 42% and 8% of cases, respectively. Important nonhematologic toxicity included 1 patient each with grade 3 nausea and vomiting, and 3 patients with grade 3 diarrhea. Although this drug caused significant myelosuppression, the drug overall was found to be well tolerated with manageable side effects.

XL019

XL019 is a potent and highly selective JAK2 inhibitor. It inhibits JAK2 kinase at low concentrations (IC50 <2 nM) and other JAK kinases at higher concentrations (JAK1: IC50 = 132 nM and JAK3: IC50 = 250 nM).63 One phase 1/2 study evaluated XL019 in 17 (57) patients with PMF and 13 (43) with post-PV/ET MF. Approximately 80% of patients harbored JAK2V617F or the MPLW515 mutation and 37% were transfusion-dependent.64, 65 Initial phase 1 dose escalation began with a starting dose of 100 mg daily for 21 days of a 28-day cycle with dose escalation up to 300 mg. Neurotoxicity was observed in all patients given a dose >100 mg; as such, the doses in subsequent patients were between 25 and 50 mg daily, or 25 mg given 3 times per week. Of the 30 patients who were enrolled, 21 were given a dose ≤50 mg. With the dose schedule of 25 to 50 mg daily, all 12 patients carrying a mutation in JAK2 or MPL experienced a reduction in spleen size. Conversely, no patients with wild-type JAK2 experienced a response. Three of 4 patients who had 10% to 19% blasts in the peripheral blood had a reduction in their blast count after therapy. Twenty-one of 30 (70%) patients had discontinued the drug at the time of last follow-up, including several instances of nerve conduction abnormalities and altered mentation. Because of the high rates of neurologic toxicity, the development of XL019 has been discontinued.

Lestaurtinib

Lestaurtinib (CEP-701) is an orally available compound that inhibits the growth of cell lines carrying both the wild-type and mutated JAK2 tyrosine kinase, and inhibits downstream signaling proteins such as BclXL and cyclin D1/D2.66 In in vitro and in xenograft murine models, lestaurtinib (IC50 = 30-100 nM) inhibits the growth of HEL cells, which are dependent on mutant JAK2 activity for growth.67 In 15 of 18 patients, erythroid cells expanded from CD34-positive cells from patients with MPNs were inhibited by lestaurtinib at a concentration of 100 nM. Conversely, the erythroid cells cultured from 3 healthy patients were not found to be significantly inhibited. At clinically achievable concentrations, lestaurtinib inhibits the proliferation and JAK2/STAT5 signaling in cells obtained from patients with MPNs.67

A multicenter study evaluated the safety and efficacy of lestaurtinib in patients with ET or PV who carried the JAK2V617F mutation. To date, 20 patients (11 with PV and 9 with ET) have been enrolled. Lestaurtinib was administered in escalating doses of 80 to 120 mg twice daily. Five of 8 patients with splenomegaly experienced a reduction in spleen size within 6 weeks of the initiation of therapy. It was well tolerated, with the main adverse effects being gastrointestinal related. At the time of last follow-up, 5 patients had discontinued study participation (1 because of disease progression and 4 because of toxicity). Seven patients had completed 18 weeks of therapy and 6 of these continued to receive lestaurtinib on the extension phase of the trial.68 Another phase 2 study evaluated this drug given at a dose of 80 mg twice daily in patients with PMF and post-PV//ET MF, all of whom carried the JAK2V617F mutation. Of the 22 patients treated, 6 (27%) had a CI that lasted for a median of 14 months. No improvement was seen with regard to bone marrow fibrosis or JAK2V617F allele burden. Grade 3/4 anemia and thrombocytopenia occurred in 14% and 23% of patients, respectively.69 The Myeloproliferative Disorders Research Consortium is currently conducting a multicenter, open-label phase 1 study of lestaurtinib in patients with MF (PMF or post-ET/PV) who carry the JAK2V617F mutation.70 The drug is being administered twice daily at doses ranging from 80 mg to 160 mg. The minimum treatment period is 28 days, but patients may continue for up to 6 months if they experience clinical benefit. Thirteen of 19 (68%) patients had PMF and 7 of these (37%) continued to receive the study drug (range, 1 to ≥54 weeks). The median reduction of spleen size was 6.4 cm. The mutant JAK2 allele burden in peripheral blood decreased slightly from a pretreatment median percentage of 66.4% to 49.6% at the time of last follow-up. These data suggest that lestaurtinib is active in patients with MPNs, particularly with regard to reducing spleen size.

SB1518

SB1518 is a potent and selective orally active adenosine-5′-triphosphate (ATP)-competitive inhibitor of both JAK2 kinase (IC50 = 22 nM) and the mutant JAK2V617F (IC50 = 19 nM).71 It selectively inhibits the proliferation of cell lines driven by JAK2 and its mutants with an IC50 of 81 nM for the murine Ba/F3 cell line transfected to express Epo receptor and JAK2V617F. SB1518 was evaluated in a murine model of MPN that was established via intravenous injection of Ba/F3-JAK2V617F cells. Significant therapeutic effects were observed, including normalization of leukocytosis, reduction of green fluorescent protein (GFP)-labeled Ba/F3 cells in the peripheral blood, resolution of hepatosplenomegaly, reduction of phospho-STAT5, alleviation of anemia and thrombocytopenia, and prolonged survival.71 A phase 1 dose escalation study of SB1518 with 6 dose levels ranging from 100 mg to 600 mg daily was given to 31 patients with MF. Twenty-one patients with MF were evaluable for a response. Seven of 17 (41%) patients with splenomegaly had a decrease in spleen size by ≥35%. Overall, SB1518 was well tolerated at doses up to 500 mg daily in patients with advanced MF and was found to lead to a reduction in spleen size.72

JAK2 Inhibitors in Preclinical Development

Several preclinical studies are currently underway examining compounds with JAK2 inhibitory activity (Table 2).73-83 Some of them, such as AZ-01,73 EXEL-8232,76 TG101209,78 CYT387,80 and R72381, 82 have demonstrated important activity in murine models of MPNs, whereas others have only been tested in cell-based systems. Although JAK inhibitors hold promise, the long-term safety of these agents needs to be determined, particularly those with JAK3 inhibitory activity because JAK3 has been shown to play a crucial role in T-cell development and the homeostasis of the immune system through its association with the common γ chain of cytokine receptors. The long-term side effects on the immune system are unknown and warrant monitoring. In addition, at the time of withdrawal of a JAK2 inhibitor, there is potential for a cytokine rebound that could lead to a hyperinflammatory state.

Table 2. JAK2 Inhibitors Currently in Preclinical Development{TC}
Study DrugFindings
  1. JAK indicates Janus kinase; IC50, concentration that inhibits 50%; HEL, human erythroleukemia; ET, essential thrombocythemia; MF, myelofibrosis; STAT5, signal transducer and activator of transcription-5; ATP, adenosine-5'-triphosphate.

AG49073Induction of apoptosis at 50 μM in peripheral blood of patients homozygous for JAKV617F
AZ-0174Inhibited JAK2 at IC50 <1 nM in cell lines carrying JAK2V617F
Decreased spleen size and hemoglobin level at 10 mg/kg twice daily in a murine model carrying JAK2V617F
Z375Inhibited JAK2V617F (IC50 = 2 μM) and JAK2 autophosphorylation
Inhibited proliferation of HEL cells
EXEL-823276Structurally similar to XL019
Potent and selective against JAK2 (IC50=2 nm)
Effective in a murine model of ET and MF
AZ6077Decreased proliferation of JAK2V617F–positive cells (IC50 = 25 nM)
Decreased phosphor-STAT5 levels
TG10120978Inhibits JAK2 (IC50 = 6 nM) and JAK3 (IC50 = 169 nM)
Preferentially suppressed growth of cells carrying JAK2V617F and decreased STAT5 phosphorylation in a murine model
AT928379Potent inhibitor (IC50 < 5 nM) of JAK2, JAK3, T315IAbl, and Aurora kinases A and B
Inhibition of phosphorylation of JAK2 and STAT5 in JAK2V617F-positive HEL cell line
CYT38780Pyrimidine derivative
Inhibits JAK1, JAK2, and JAK3 (IC50 = 11 nM, 18 nM, and 162 nM, respectively)
Inhibits the growth of hematopoietic cell lines expressing JAK2V617F
R72381, 82Potent inhibition of proliferation (IC50 = 130-200 nM) of murine (BaF3) and human (UKE1 and SET2) JAKV617F–positive cell lines
Inhibits JAK1, JAK2, and JAK3 (IC50 = 740 nM, 2 nM, and 26 nM, respectively)Selective inhibition of constitutive STAT5 phosphorylation in SET2 and Ba/F3
Nonspecific antiproliferative activity against JAK2-independent cell lines.
LS10483Non-ATP competitive JAK2V617F kinase inhibitor (IC50 = 2 μM)
Inhibits JAK2 kinase activity and downstream targets
Selectively induces apoptosis in Ba/F3-EpoR JAK2V617Fcells

Non-JAK2 Targeted Agents

Although JAK2 inhibitors hold promise as therapy for patients with MPNs harboring a JAK2 mutation, not all patients with MPNs carry the mutation, thus suggesting the presence of other oncogenic events. Evidence now exists for additional and/or antecedent mutations in JAK2V617F mutant MPNs such as mutations involving the ten-eleven tranlocation-2 (TET2) gene.84 It is perhaps through the inhibition of alternative signaling pathways important for the maintenance of the malignant clone that some non-JAK2 inhibitor agents have proven efficacious in patients with MPNs. Alternatively, these agents might impact the malignant clone by depriving the latter from critical signals emanating from the bone marrow niche. Non-JAK2 inhibitor agents offer an alternative treatment option for patients either with or without JAK2 mutations (Table 3).

Table 3. Non-JAK2 Inhibitor Investigational Agents Currently in Clinical Trials for MPN
DrugMain Clinical FindingsStage of Development
  • JAK indicates Janus kinase; MPN, myeloproliferative neoplasms; PV, polycythemia vera; ET, essential thrombocythemia; PMF, primary myelofibrosis; CR, complete response; PR, partial response; NR, no response; MF, myelofibrosis; IFN, interferon; HR, hematologic response; CHR, complete hematologic response; AML, acute myeloid leukemia.

  • a

    Pretreatment hemoglobin <10 g/dL or transfusion dependent.

  • b

    Pomalidomide was given up to 12 cycles (1 cycle=28 days) and prednisone at a dose of 30 mg daily with taper during the first 3 cycles.

ITF235786, 8726 patients with JAK2V617F-positive MPN: PV (n=12), ET (n=1), PMF (n=4), and post-PV/ET MF (n=9)Phase 2
All completed at least 1 mo of therapy (13 completed 12 wk)
PV and ET: 3 CR, 8 PR, and 3 NR; 6 of 8 (75%) reduced spleen size
10 of 11 patients had improvement in pruritus, 7 of 8 with a 12% median reduction in mutant JAK2V617F at 24 wk
MF: 5 responses (3 major, 1 moderate, and 1 minor)
Pegylated IFN-α-2a8979 total patients treated (40 with PV and 39 with ET)Phase 3
Median follow-up, 21 mo (range, 2-45 mo)
Overall HR: 80% in PV, 81% in ET
D to CHR: 47 (range, 3-350 d)
Evaluable JAK2V617F-positive patients: 51 (16 with ET and 35 with PV)
Overall molecular response: ET (6 of 16; 38%)
PV (19 of 35; 54%)
Complete molecular response: ET (1 of 16; 6%)
PV (5 of 35; 14%)
Lenalidomide + prednisone9540 patients with PMFPhase 2
Median follow-up, 22 mo (range, 6-27 mo)
Overall response: 30% (12 of 40 patients)
Anemiaa: 30% (7 of 23 patients)
Splenomegaly: 42% (10 of 24 patients)
Improvement in bone marrow fibrosis
Median time to response, 12 wk (range, 2-32 wk)
No deaths or transformations to AML
Pomalidomide ± prednisone9884 patients with MF and anemia: (60 with PMF, 14 with post-ET MF, and 10 with post-PV MF)Phase 2
Four treatment armsb:
1) Pomalidomide, 2 mg daily, and placebo (n=22)
2) Pomalidomide, 2 mg daily, plus prednisone (n=19)
3) Pomalidomide, 0.5 mg/daily, plus prednisone (n=22)
4) Prednisone plus placebo (n=21)
20 patients (24%) had anemia response, 15 (18%) with transfusion independence
Response rates per treatment arm/response in patients treated with ≥3 cycles (n=62):
Arm 1: 23%/38%
Arm 2: 16%/23%
Arm 3: 36%/40%
Arm 4: 19%/25%

Histone deacetylase inhibitors

It has been recently reported that both JAK2 and the mutant JAK2V617F kinases can be found in both the cytoplasm and the nucleus of various human leukemic cell lines and primary CD34-positive hematopoietic progenitors.85 In the nucleus, JAK2 phosphorylates histone H3 at tyrosine 41 (H3Y41), which reduces the affinity of H3 to the transcriptional repressor HP1α.85 Thus, the JAK2-H3Y41-HP1α pathway links JAK2 kinase activity to histone phosphorylation, aberrant gene expression, and genome instability, and provides the rationale for the use of histone deacetylase inhibitors (HDACi) for the treatment of JAK2-driven malignancies. ITF2357 is a synthetic class I and class II HDACi that inhibits the autonomous proliferation of hematopoietic cells from patients with PV and ET via downmodulation of JAK2V617F.86 A phase 2 trial evaluated ITF2357 in patients with PV, ET, MF, and post-PV/ET MF.87 A total of 26 patients (12 males and 14 females) with JAK2V617F positivity in PV (12 patients), ET (1 patient), and PMF (4 patients) and post-PV/ET MF (9 patients) were enrolled. At the time of analysis, all patients had completed at least 1 month of therapy and 13 patients had completed 12 weeks of treatment. In patients with PV and ET, there were 3 CRs and 8 PRs reported (3 patients demonstrated no response). A significant reduction in spleen size was reported in 6 of 8 patients with splenomegaly, whereas 10 of 11 patients had an improvement in pruritus. An 8% median reduction in the mutant JAK2V617F allele burden in 10 of 17 patients at 12 weeks and a 12% reduction in allele burden in 7 of 8 patients at 24 weeks was reported. Among the 13 patients with MF, 5 responses were reported (3 major, 1 moderate, and 1 minor response). The main adverse effects reported were mainly gastrointestinal, but 1 patient developed grade 3 neutropenia.

Pegylated IFN-α

Despite its activity in MPNs, the use of IFN-α has been severely hampered because of its unfavorable toxicity profile and inconvenient dosing schedule. Pegylated formulations of IFN-α (PEG-IFN-α), endowed with longer half-lives, have allowed for a weekly administration with a more acceptable toxicity profile compared with standard IFN-α. Studies with PEG-IFN–2a in patients with MPNs have shown it to have remarkable clinical activity. A phase 2 multicenter study evaluated the efficacy of PEG-IFN–2a in 40 patients with PV.88 Patients were followed for a median of 31 months. At 12 months, all 37 evaluable patients had achieved a hematologic response (95% with CR) and only 3 patients (8%) had discontinued treatment. Twenty-six (90%) of 29 evaluable patients had a decrease in the JAK2V617F allele burden. Before treatment, the median JAK2V617F allele burden was 45%, which decreased after 1 year and 3 years to 22.5% and 3%, respectively. A molecular CR (undetectable JAK2V617F) was achieved in 7 patients and persisted after PEG-IFN–2a was discontinued in 5 patients. These results demonstrated the efficacy of PEG-IFN–2a not only in achieving hematologic responses, but also in inducing deep levels of molecular response, and in some cases the complete elimination of the JAK2V617F-expressing clone. Simultaneously, a phase 2 study studied the activity of PEG-IFN–2a in patients with either PV or ET.89 Seventy-nine patients (39 with ET and 40 with PV) were treated. The median time from diagnosis to treatment was 54 months in patients with PV and 33 months in patients with ET; approximately 81% of patients had prior MPN-directed treatment. The JAK2V617F mutation was detected in 18 patients with ET and 38 patients with PV. The median allele burden was 64% (range, 18.5%-94.6%) and 23% (range, 2.9% -55.5%), respectively, for patients with PV and ET. The initial PEG-IFN–2a dose was 450 μg weekly, but because of poor tolerance, the dose was decreased gradually to a starting dose of 90 μg weekly. Seventy-seven patients were evaluable after a median of 21 months. The hematologic response rate was 80% in patients with PV and 81% in patients with ET (CR in 70% and 76%, respectively), with the majority of responses being achieved within 3 months of the initiation of therapy. Fifty-one of these patients were considered evaluable for a molecular response (16 with ET and 35 with PV). The molecular response rate was 38% (6% complete molecular response rate) in ET patients and 54% (14% complete molecular response) in PV patients. The JAK2V617F mutant allele burden continued to decrease with no evidence of plateau, suggesting at last follow-up that there was selective targeting of the JAK2V617F malignant clone. Phase 3 studies are currently ongoing to further define the role of PEG-IFN–2a for the treatment of patients with PV and ET.

Thalidomide

Thalidomide is a drug with known antiangiogenic and cytokine inhibitor properties that has shown efficacy in the treatment of MF. Results of 2 sequential, phase 2 studies analyzed the outcomes of 36 patients with symptomatic MF (28 with PMF, 3 with post-PV MF, and 5 with post-ET MF) who received either thalidomide at a dose of 200 mg daily (n = 15) or low-dose thalidomide (50 mg daily) with prednisone (0.5 mg/kg for the first month, 0.25 mg/kg for the second month, and 0.125 mg/kg for the third month) (n = 21).90 Of the 36 patients, 20 (56%) had a response with regard to anemia, thrombocytopenia, and/or splenomegaly. After a median follow-up of 25 months, 10 of 36 (28%) patients had an ongoing response. Twenty-two patients (61%) were still alive at the time of last follow-up, with 14 having died of disease-related complications. In comparing the 2 arms, the thalidomide plus prednisone arm was found to be more effective and better tolerated. The toxicity dropout rate was 8 of 15 (53%) patients in the thalidomide alone arm compared with none of the 21 patients in the thalidomide and prednisone arm. Although the results of this study demonstrated some patients achieving a durable remission, the toxicity of the treatment arm using thalidomide at a dose of 200 mg was significant.

A phase 2 dose escalation trial was undertaken assessing the efficacy of low-dose thalidomide (50 mg daily with dose escalation up to 400 mg) in 63 patients with MF.91 An improvement in anemia was observed in 22% of patients, and 39% of transfusion-dependent patients became transfusion-independent. For those patients with a platelet count <100 ×109/L, the platelet level increased by ≥50 ×109/L in 22%. The spleen size also decreased ≥50% in 19% of patients. However, there was significant toxicity noted with this regimen and the dropout rate at 6 months was 51%.

Another prospective phase 2, randomized, double-blind multicenter trial compared thalidomide at a dose of 400 mg daily (n = 26 patients) with placebo (n = 26 patients) administered for 180 days in 52 patients with MF who had anemia (Hb ≤9 g/dL) or who were transfusion-dependent). No difference was noted between the thalidomide versus placebo group with regard to either anemia (1 in each group) or red cell transfusions (3 patients vs 5 patients). The spleen size measured via ultrasound decreased in the thalidomide arm (P < .05). However, there was significantly more somnolence and edema reported with thalidomide and >50% of patients discontinued thalidomide after 4 months of treatment because of adverse events. Only 10 patients were able to complete the 6 months of therapy. This trial demonstrated the efficacy of thalidomide in decreasing spleen size, but underscored the substantial toxicity profile of this drug in patients with PMF.92 Lastly, a phase 2 trial evaluated thalidomide at a dose of 200 mg daily (with dose escalation) in 41 patients with MF (31 with PMF).93 A CR was seen in 4 patients (10%); PR in 4 (10%) patients; and an improvement in anemia, thrombocytopenia, and/or splenomegaly in 9 patients (22%). A reduction in spleen size was found in 9 of 29 (31%) evaluable patients, with a CR reported in 5 patients. Toxicity was notable, with19 patients developing grade 3/4 pneumonia, 9 developing peripheral edema, and 5 developing pulmonary embolism. Although thalidomide has demonstrated activity in patients with PMF, the side effect profile is substantial, which spurred the investigation of better tolerated immunomodulatory agents for this indication.

Lenalidomide

Lenalidomide is an immunomodulatory agent. Two jointly reported phase 2 trials evaluated the efficacy of single-agent lenalidomide in 68 patients with MF.94 The initial lenalidomide dose was 10 mg daily (5 mg daily if the platelet count was <100 ×109/L) for 3 months to 4 months (with planned continued treatment for additional 3 or 24 months in the case of response). The response rate was 22% for anemia, 33% for splenomegaly, and 50% for thrombocytopenia. Eight patients with anemia demonstrated a marked response, with improvement of transfusion dependency or a Hb >10 g/dL. However, the splenomegaly response was only 2%. Other findings of note included resolution of leukoerythroblastosis in 4 patients, a decrease in medullary fibrosis in 2 patients, and cytogenetic remission of del(5)(q13q33) accompanied by a reduction in the JAK2V617F mutation burden in 1 patient. However, most responses were lost, most likely because of the administration of lenalidomide for a limited period of time (<6 months). Grade 3 and 4 neutropenia and thrombocytopenia occurred in 31% and 19% of patients, respectively. On the basis of these results, a subsequent phase 2 study evaluated the efficacy of the combination of lenalidomide and prednisone in 40 patients with MF, but in this case, lenalidomide was administered for a minimum of 6 months and indefinitely in patients demonstrating clinical benefit.95 The initial dose schedule of lenalidomide was 10 mg orally daily on a 28-day cycle (21 days on/7 days off). Oral prednisone was given at a dose of 30 mg daily during Cycle 1, 15 mg daily during Cycle 2, and 15 mg every other day during Cycle 3. The median time from diagnosis of MF to the initiation of the combination therapy was 10 months (>3 years in 11 patients); 75% of patients had received some form of therapy, and 75% entered the study with splenomegaly. The JAK2V617F mutation was found in 18 of 36 (50%) patients tested and 20 of 40 (50%) patients were found to have abnormal cytogenetics. With a median follow-up of 22 months (range, 6 months-27 months), 12 (30%) patients responded according to the stringent International Working Group (IWG) criteria,96 including 8 who carried the JAK2V617F mutation, 3 who were treatment-naive, and 2 who failed prior therapy with thalidomide. The median time to response was 12 weeks. Three patients (7.5%) achieved a PR and 9 (22.5%) demonstrated CI for a median of 18 months. These endpoints are clinically significant because the reduction in disease burden can lead to amelioration of symptoms and an improved quality of life. Anemia improved in 7 of 23 patients (30%) (3 with PRs and 4 with CI) with a pretreatment Hb <10 g/dL or who were transfusion-dependent. Splenomegaly significantly decreased in 10 of 24 patients (42%) (2 with PRs and 8 with CI). Responses were ongoing in 10 of 12 responders who were still receiving lenalidomide at the time of last follow-up. It is interesting to note that 12 months of therapy induced a significant decrease in the JAK2V617F allele burden among the 8 JAK2V617F-positive responders (P = .03). Four of 8 JAK2V617F-positive responders experienced a >50% reduction in the mutant allele burden, which became undetectable in 1 patient. No transformation to acute leukemia was observed with lenalidomide. Ten of 11 assessable responders who had grade 4 reticulin fibrosis before the initiation of therapy had reductions to a score of ≤2. The most frequent grade 3/4 toxicities were hematologic: neutropenia (58%), anemia (42%), and thrombocytopenia (13%). Thirty (75%) patients discontinued therapy (15 because of lack of response and 9 because of grade 3/4 toxicity). Although lenalidomide is associated with significant hematologic toxicity, it should be noted that the number of patients discontinuing therapy because of side effects (9 of 40; 22.5%) was less than that found in the trials with thalidomide, in which the toxicity dropout rate was as high as 53% (thalidomide alone arm),90 mostly because of nonhematologic toxicity. In the absence of randomized trials, it is difficult to determine which immunomodulatory agent, thalidomide or lenalidomide, is superior in patients with PMF. However, it must be emphasized that responses in the lenalidomide trials have been reported according to a more stringent response criteria set (IWG) than those reported in the thalidomide trials.

Pomalidomide

Pomalidomide is a new immunomodulatory agent that has been shown to be well tolerated in several phase 1/2 studies.97 A phase 2 randomized, double-blind study evaluated the efficacy of pomalidomide in reversing anemia in 84 patients with MF (60 with PMF, 14 with post-ET MF, and 10 with post-PV MF). Four treatment arms were tested: 1) pomalidomide at a dose of 2 mg daily and placebo (n = 22); 2) pomalidomide at a dose of 2 mg daily plus prednisone (n = 19); 3) pomalidomide at a dose of 0.5 mg daily plus prednisone (n = 22); and 4) prednisone plus placebo (n = 21). Pomalidomide was given up to 12 cycles (1 cycle was 28 days) and prednisone at a dose of 30 mg daily was given during the first 3 cycles.98 After a median treatment period of 4.6 months for all patients and 10 months for patients still on study, 20 patients (24%) experienced an improvement in their anemia, including 15 (18%) who attained transfusion independence. It is interesting to note that none of these patients experienced a reduction in spleen size. For the 62 patients receiving ≥3 cycles of therapy, the response rates were 38%, 23%, 40%, and 25%, respectively. The median response duration was 6.5 months (range, 2.3 months-16.9 months), whereas the median response duration for the 16 patients who responded to pomalidomide with or without prednisone was 7.8 months (range, 3.2 months to ≥16.9 months). Univariate analysis demonstrated that of the 63 patients receiving pomalidomide with or without prednisone, there was no correlation noted between response and MF subtype (PMF vs post -PV/ET MF; P = .50), platelet count (P = .64), red cell transfusion dependence at baseline (P = .58), or the presence or absence of the JAK2V617F mutation (31% vs 19%; P = .28). Only an elevated white blood cell count remained significant on multivariate analysis. No significant changes were observed regarding changes in bone marrow fibrosis or JAK2V617F allele burden. Although therapy was overall well tolerated, there were several ≥ grade 3 events, including fatigue (12%), neutropenia (8%), thrombocytopenia (11%), anemia (10%), pneumonia/sepsis (11%), and venous thrombosis (4%). Unlike thalidomide and lenalidomide, pomalidomide appeared to have little effect on splenomegaly, which suggests that the clinical use of this immunomodulatory agent may be restricted to the treatment of patients with PMF in whom the primary clinical manifestation is anemia. Indeed, the pomalidomide (at a dose of 0.5 mg daily) plus prednisone arm led to a 36% anemia response rate (8 of 22 patients), which appears to compare favorably with both single-agent thalidomide (20% anemia response rate)90 and single-agent lenalidomide (22% anemia response rate).94

Conclusions

The discovery of the JAK2V617F mutation in patients with MPNs has provided insight into the pathogenesis of these diseases. The high incidence of this mutation in MPNs has spurred interest in developing agents containing JAK2 tyrosine kinase inhibitors, of which several are currently in clinical trials whereas others have reached advanced stages of preclinical development. Although several JAK2 inhibitors have demonstrated significant activity in patients with MPNs, it is important to emphasize several points. First, none of the currently available JAK2 inhibitors is a specific inhibitor of JAK2V617F kinase and all inhibit native JAK2 kinase and, to different extents, other members of the JAK family of kinases. Not surprisingly, these agents have proven active both in patients with and without the JAK2V617F mutation. Therefore, the mechanism of action of these agents remains largely unknown and may be related to their ability to inhibit JAK-STAT–mediated cytokine activities. Second, the activity of these agents has been limited to the improvement of constitutional symptoms and reductions in spleen size, with nearly negligible activity in other clinical manifestations of MPNs such as cytopenia, JAK2V617F allele burden, or bone marrow fibrosis. Finally, although active, these agents have yet to prove that they can change the natural history of MPNs, particularly of those such as PV or ET, in which the median survival for certain subsets of patients approaches that of matched aged controls. These questions notwithstanding, the clinical activity demonstrated by JAK2 inhibitors in MPNs holds great promise and brings great excitement to the field of MPN research, which for decades has been in desperate need of novel active therapies.

CONFLICT OF INTEREST DISCLOSURES

The authors made no disclosures.

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