Sequential treatment of CD34+ cells from patients with primary myelofibrosis with chromatin-modifying agents eliminate JAK2V617F-positive NOD/SCID marrow repopulating cells

Oral Decitabine/Cedazuridine Is an Effective Ambulatory Therapy for Patients With Myelofibrosis Refractory to JAK2 Inhibitor Therapy

BACKGROUND

Outcomes are dismal for patients with myelofibrosis (MF) who are no longer responsive to JAK2 inhibitors (JAKi) and/or have increasing blast cell numbers. Although prior reports have suggested the benefits of intravenous decitabine (DAC) combined with ruxolitinib for patients with Myeloproliferative Neoplasm (MPN) accelerated/blast phase (AP/BP), decitabine-cedazuridine (DEC-C), an oral fixed-dose combination providing equivalent pharmacokinetic exposure, has not been evaluated in MF.

METHODS

We conducted a retrospective analysis of 14 patients with high-risk MF refractory to ruxolitinib or MPN-AP (10-19% blasts) treated with DEC-C +/- JAKi at Mount Sinai Hospital from 2021 to 2024.

RESULTS

The cohort was elderly (median age,76 years) and almost uniformly possessed high risk mutations with 13 of the 14 patients progressing on JAKi therapy. With a median follow-up of 9.4 months, the median overall survival (OS) was 29 months for the entire cohort. Median OS was 10.8 months for MPN-AP and was not reached for ruxolitinib refractory MF patients. All patients (n = 9) receiving > 4 cycles of DEC-C had clinical benefit exemplified by a reduction in blast cell numbers, spleen size, and lack of progression to MPN-BP (78%). Furthermore, 3/14 patients proceeded to allogeneic stem cell transplant. Myelosuppression was a common adverse event which was managed by reducing the number of days of administration of DEC-C from 5 to 3 per cycle.

CONCLUSIONS

This report demonstrates the feasibility, tolerability, and clinical benefit of an exclusively ambulatory regimen for high-risk, elderly patients with advanced MF which warrants further evaluation in a prospective clinical trial.

IGFBP-6/sonic hedgehog/TLR4 signalling axis drives bone marrow fibrotic transformation in primary myelofibrosis

Primary myelofibrosis is a Ph-negative chronic myeloproliferative neoplasm characterized by bone marrow fibrosis and associated with the involvement of several pathways, in addition to bone marrow microenvironment alterations, mostly driven by the activation of the cytokine receptor/JAK2 pathway. Identification of driver mutations has led to the development of targeted therapy for myelofibrosis, contributing to reducing inflammation, although this currently does not translate into bone marrow fibrosis remission. Therefore, understanding the clear molecular cut underlying this pathology is now necessary to improve the clinical outcome of patients. The present study aims to investigate the involvement of IGFBP-6/sonic hedgehog /Toll-like receptor 4 axis in the microenvironment alterations of primary myelofibrosis. We observed a significant increase in IGFBP-6 expression levels in primary myelofibrosis patients, coupled with a reduction to near-normal levels in primary myelofibrosis patients with JAK2V617F mutation. We also found that both IGFBP-6 and purmorphamine, a SHH activator, were able to induce mesenchymal stromal cells differentiation with an up-regulation of cancer-associated fibroblasts markers. Furthermore, TLR4 signaling was also activated after IGFBP-6 and purmorphamine exposure and reverted by cyclopamine exposure, an inhibitor of the SHH pathway, confirming that SHH is involved in TLR4 activation and microenvironment alterations. In conclusion, our results suggest that the IGFBP-6/SHH/TLR4 axis is implicated in alterations of the primary myelofibrosis microenvironment and that IGFBP-6 may play a central role in activating SHH pathway during the fibrotic process.

Imetelstat, a telomerase inhibitor, is capable of depleting myelofibrosis stem and progenitor cells

Clinical trials of imetelstat therapy have indicated that this telomerase inhibitor might have disease-modifying effects in a subset of patients with myelofibrosis (MF). The mechanism by which imetelstat induces such clinical responses has not been clearly elucidated. Using in vitro hematopoietic progenitor cell (HPC) assays and in vivo hematopoietic stem cell (HSC) assays, we examined the effects of imetelstat on primary normal and MF HSCs/HPCs. Treatment of CD34⁺ cells with imetelstat reduced the numbers of MF but not cord blood HPCs (colony-forming unit-granulocyte/macrophage, burst-forming unit-erythroid, and colony-forming unit-granulocyte/erythroid/macrophage/megakaryocyte) as well as MF but not normal CD34⁺ALDH⁺ cells irrespective of the patient's mutational status. Moreover, imetelstat treatment resulted in depletion of mutated HPCs from JAK2V617F⁺ MF patients. Furthermore, treatment of immunodeficient mice that had been previously transplanted with MF splenic CD34⁺ cells with imetelstat at a dose of 15 mg/kg, 3 times per week for 4 weeks had a limited effect on the degree of chimerism achieved by normal severe combined immunodeficiency repopulating cells but resulted in a significant reduction in the degree of human MF cell chimerism as well as the proportion of mutated donor cells. These effects were sustained for at least 3 months after drug treatment was discontinued. These actions of imetelstat on MF HSCs/HPCs were associated with inhibition of telomerase activity and the induction of apoptosis. Our findings indicate that the effects of imetelstat therapy observed in MF patients are likely attributable to the greater sensitivity of imetelstat against MF as compared with normal HSCs/HPCs as well as the intensity of the imetelstat dose schedule.

Effects of trichostatin A on pig SCNT blastocyst formation rate and cell number: A meta-analysis

Although somatic cell nuclear transfer (SCNT) can be used to create transgenic pigs for human xenotransplantation, low efficiency limits its use. Trichostatin A (TSA) promotes SCNT embryo development, but whether TSA modifies SCNT blastocyst numbers is unclear. Thus, there is an urgent need to understand whether TSA modifies the rate and number of embryos that grow from oocytes to blastocysts in culture and what types of cell signaling pathways may be involved. Thus, we identified 63 reports, of which 13 are included in this meta-analysis. Data show that TSA significantly increased the SCNT blastocyst formation rate, but did not change blastocyst cell number. Due to study heterogeneity (I²>50%), we hypothesized that donor cells were of different backgrounds so we analyzed two donor cell subgroups: fetal and adult fibroblasts. Analysis of the fetal fibroblast subgroups showed no heterogeneity, but the adult fibroblast subgroups were heterogeneous, suggesting epigenetic reprogramming of fetal fibroblasts by TSA. Adult fibroblast heterogeneity may be complex and reprogramming by TSA is more difficult. Thus, TSA fibroblasts reprogramming is the source of heterogeneity in this meta-analysis. More work is needed to better understand how TSA influences SCNT pig embryonic development, and histone deacetylase inhibitors can be assessed with respect to SCNT pig embryos. Finally, efforts in epigenetic research may improve SCNT pig embryo outcomes.

A humanized bone marrow ossicle xenotransplantation model enables improved engraftment of healthy and leukemic human hematopoietic cells

Xenotransplantation models represent powerful tools for the investigation of healthy and malignant human hematopoiesis. However, current models do not fully mimic the components of the human bone marrow (BM) microenvironment, and they enable only limited engraftment of samples from some human malignancies. Here we show that a xenotransplantation model bearing subcutaneous humanized ossicles with an accessible BM microenvironment, formed by in situ differentiation of human BM-derived mesenchymal stromal cells, enables the robust engraftment of healthy human hematopoietic stem and progenitor cells, as well as primary acute myeloid leukemia (AML) samples, at levels much greater than those in unmanipulated mice. Direct intraossicle transplantation accelerated engraftment and resulted in the detection of substantially higher leukemia-initiating cell (LIC) frequencies. We also observed robust engraftment of acute promyelocytic leukemia (APL) and myelofibrosis (MF) samples, and identified LICs in these malignancies. This humanized ossicle xenotransplantation approach provides a system for modeling a wide variety of human hematological diseases.

A thrombopoietin receptor antagonist is capable of depleting myelofibrosis hematopoietic stem and progenitor cells

Recently, interactions between thrombopoietin (TPO) and its receptor, the myeloproliferative leukemia (MPL) virus oncogene, have been shown to play a role in the development and progression of myeloproliferative neoplasms including myelofibrosis (MF). These observations have led to the development of strategies to disrupt the association of TPO with its receptor as a means of targeting MF hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs). In this report, we show that although both splenic and peripheral blood MF CD34(+) cells expressed lower levels of MPL than normal CD34(+) cells, TPO promoted the proliferation of MF CD34(+) cells and HPCs in a dose-dependent fashion. Furthermore, the treatment of MF but not normal CD34(+) cells with a synthesized MPL antagonist, LCP4, decreased the number of CD34(+)Lin(-) cells and all classes of assayable HPCs (colony-forming unit-megakaryocyte [CFU-MK], CFU-granulocyte/macrophage, burst-forming unit-erythroid/CFU-erythroid, and CFU-granulocyte/erythroid/macrophage/MK) irrespective of their mutational status. In addition, LCP4 treatment resulted in the depletion of the number of MF HPCs that were JAK2V617F(+) Moreover, the degree of human cell chimerism and the proportion of malignant donor cells were significantly reduced in immunodeficient mice transplanted with MF CD34(+) cell grafts treated with LCP4. These effects of LCP4 on MF HSCs/HPCs were associated with inhibition of JAK-STAT activity, leading to the induction of apoptosis. These findings demonstrate that such specific anti-cytokine receptor antagonists represent a new class of drugs that are capable of targeting MF HSCs.

Evolving Therapeutic Strategies for the Classic Philadelphia-Negative Myeloproliferative Neoplasms

Despite the emergence of JAK inhibitors, there is a need for disease-modifying treatments for Philadelphia-negative myeloproliferative neoplasms (MPNs). JAK inhibitors ameliorate symptoms and address splenomegaly, but because of the heterogeneous contributors to the disease process, JAK inhibitor monotherapy incompletely addresses the burden of disease. The ever-growing understanding of MPN pathogenesis has provided the rationale for testing novel and targeted therapeutic agents, as monotherapies or in combination, in preclinical and clinical settings. A number of intriguing options have emerged, and it is hoped that further progress will lead to significant changes in the natural history of MPNs.

Allo-SCT for myelofibrosis: reversing the chronic phase in the JAK inhibitor era?

At present, allo-SCT is the only curative treatment for patients with myelofibrosis (MF). Unfortunately, a significant proportion of candidate patients are considered transplant ineligible due to their poor general condition and advanced age at the time of diagnosis. The approval of the first JAK inhibitor, ruxolitinib, for patients with advanced MF in 2011 has had a qualified impact on the treatment algorithm. The drug affords substantial improvement in MF-associated symptoms and splenomegaly but no major effect on the natural history. There has, therefore, been considerable support for assessing the drug's candidacy in the peritransplant period. The drug's precise impact on clinical outcome following allo-SCT is currently not known; nor are the drug's long-term efficacy and safety known. Considering the rarity of MF and the small proportion of patients who undergo allo-SCT, well designed collaborative efforts are required. In order to address some of the principal challenges, an expert panel of laboratory and clinical experts in this field was established, and an independent workshop held during the 54th American Society of Hematology Annual Meeting in New Orleans, USA on 6 December 2013, and the European Hematology Association's Annual Meeting in Milan, Italy on 13 June 2014. This document summarizes the results of these efforts.

C-X-C motif chemokine 12 influences the development of extramedullary hematopoiesis in the spleens of myelofibrosis patients

Myelofibrosis (MF) is characterized by the constitutive mobilization of hematopoietic stem cells (HSC) and hematopoietic progenitor cells (HPC) and the establishment of extramedullary hematopoiesis. The mechanisms underlying this abnormal HSC/HPC trafficking pattern remain poorly understood. We demonstrated that both splenic and peripheral blood (PB) MF CD34(+) cells equally share a defective ability to home to the marrow, but not to the spleens, of NOD/LtSz-Prkdc(scid) mice. This trafficking pattern could not be attributed to discordant expression of integrins or chemokine receptors other than the downregulation of C-X-C chemokine receptor type 4 by both PB and splenic MF CD34(+) cells. The number of both splenic MF CD34(+) cells and HPCs that migrated toward splenic MF plasma was, however, significantly greater than the number that migrated toward PB MF plasma. The concentration of the intact HSC/HPC chemoattractant C-X-C motif chemokine 12 (CXCL12) was greater in splenic MF plasma than PB MF plasma, as quantified using mass spectrometry. Functionally inactive truncated products of CXCL12, which are the product of proteolytic degradation by serine proteases, were detected at similar levels in both splenic and PB MF plasma. Treatment with an anti-CXCL12 neutralizing antibody resulted in a reduction in the degree of migration of splenic MF CD34(+) cells toward both PB and splenic MF plasma, validating the role of CXCL12 as a functional chemoattractant. Our data indicate that the MF splenic microenvironment is characterized by increased levels of intact, functional CXCL12, which contributes to the localization of MF CD34(+) cells to the spleen and the establishment of extramedullary hematopoiesis.

JAK2 inhibitors do not affect stem cells present in the spleens of patients with myelofibrosis

Dysregulation of Janus kinase (JAK)-signal transducer and activator of transcription signaling is central to the pathogenesis of myelofibrosis (MF). JAK2 inhibitor therapy in MF patients results in a rapid reduction of the degree of splenomegaly, yet the mechanism underlying this effect remains unknown. The in vitro treatment of splenic and peripheral blood MF CD34(+) cells with the JAK1/2/3 inhibitor, AZD1480, reduced the absolute number of CD34(+), CD34(+)CD90(+), and CD34(+)CXCR4(+) cells as well as assayable hematopoietic progenitor cells (HPCs) irrespective of the JAK2 and calreticulin mutational status. Furthermore, AZD1480 treatment resulted in only a modest reduction in the proportion of HPCs that were JAK2V617F(+) or had a chromosomal abnormality. To study the effect of the drug on MF stem cells (MF-SCs), splenic CD34(+) cells were treated with AZD1480 and transplanted into immunodeficient mice. JAK2 inhibitor therapy did not affect the degree of human cell chimerism or the proportion of malignant donor cells. These data indicate that JAK2 inhibitor treatment affects a subpopulation of MF-HPCs, while sparing another HPC subpopulation as well as MF-SCs. This pattern of activity might account for the reduction in spleen size observed with JAK2 inhibitor therapy as well as the rapid increase in spleen size observed frequently with its discontinuation.

Differential effects of epigenetic modifiers on the expansion and maintenance of human cord blood stem/progenitor cells

Epigenetic therapies, including DNA methyltransferase and histone deacetylase (HDAC) inhibitors, are increasingly being considered to treat hematological malignancies, but their effects on normal hematopoietic stem cells (HSCs) remain largely unexplored. We compared the effects of several HDAC inhibitors, including valproic acid (VPA) and trichostatin A (TSA), alone or in combination with 5-aza-2'-deoxycytidine (5azaD) on the expansion of HSCs. VPA induced the highest expansion of CD34+CD90+ cells and progenitor cells compared with other HDAC inhibitors or the sequential addition of 5azaD/TSA in culture. Xenotransplantation studies demonstrated that VPA prevents HSC loss, whereas 5azaD/TSA treatment leads to a net expansion of HSCs that retain serial transplantation ability. 5azaD/TSA-mediated HSC expansion was associated with increased histone acetylation and transient DNA demethylation, which corresponded with higher gene transcript levels. However, some genes with increased transcript levels lacked changes in methylation. Importantly, a global microarray analysis revealed a set of differentially expressed genes in 5azaD/TSA- and VPA-expanded CD34+ cells that might be involved in the expansion and maintenance of transplantable HSCs, respectively. In summary, our data indicate that treatment of HSCs with different chromatin-modifying agents results in either the expansion or maintenance of HSCs, an observation of potential therapeutic importance.

A phase II study of Givinostat in combination with hydroxycarbamide in patients with polycythaemia vera unresponsive to hydroxycarbamide monotherapy

Givinostat, a histone-deacetylase inhibitor (HDACi), inhibits proliferation of cells bearing the JAK2 V617F mutation and has shown significant activity with good tolerability in patients with chronic myeloproliferative neoplasms (MPN). In this multicentre, open-label, phase II study, 44 patients with polycythaemia vera (PV), unresponsive to the maximum tolerated doses (MTD) of hydroxycarbamide (HC), were treated with Givinostat (50 or 100 mg/d) in combination with MTD of HC. The European LeukaemiaNet response criteria were used to assess the primary endpoint after 12 weeks of treatment. Complete or partial response was observed in 55% and 50% of patients receiving 50 or 100 mg of Givinostat, respectively. Control of pruritus was observed in 64% and 67% of patients in the 50 and 100 mg groups, respectively. The combination of Givinostat and HC was well tolerated: eight patients (18%) discontinued, four in each treatment arm; grade 3 adverse events were reported in one patient (4·5%) in each treatment arm. The combined use of Givinostat and HC was safe and clinically effective in HC-unresponsive PV patients.

Spleens of myelofibrosis patients contain malignant hematopoietic stem cells

Cancer stem cell behavior is thought to be largely determined by intrinsic properties and by regulatory signals provided by the microenvironment. Myelofibrosis (MF) is characterized by hematopoiesis occurring not only in the marrow but also in extramedullary sites such as the spleen. In order to study the effects of these different microenvironments on primitive malignant hematopoietic cells, we phenotypically and functionally characterized splenic and peripheral blood (PB) MF CD34+ cells from patients with MF. MF spleens contained greater numbers of malignant primitive HPCs than PB. Transplantation of PB MF CD34+ cells into immunodeficient (NOD/SCID/IL2Rγ(null)) mice resulted in a limited degree of donor cell chimerism and a differentiation program skewed toward myeloid lineages. By contrast, transplanted splenic MF CD34+ cells achieved a higher level of chimerism and generated both myeloid and lymphoid cells that contained molecular or cytogenetic abnormalities indicating their malignant nature. Only splenic MF CD34+ cells were able to sustain hematopoiesis for prolonged periods (9 months) and were able to engraft secondary recipients. These data document the existence of MF stem cells (MF-SCs) that reside in the spleens of MF patients and demonstrate that these MF-SCs retain a differentiation program identical to that of normal hematopoietic stem cells.

Role of additional novel therapies in myeloproliferative neoplasms

The recent approval of ruxolitinib (INCB018424) for myelofibrosis and the preclinical/clinical development of several additional janus kinase (JAK)-targeted agents have ushered in an era of novel therapies for advanced myeloproliferative neoplasms (MPN), which are associated with constitutive activation of the JAK-signal transducer and activation of transcription (STAT) signaling pathway. Collectively, these novel therapeutic approaches could rapidly broaden the spectrum of available therapies, with potential for improved clinical outcome for patients with advanced MPN. This review covers the recent developments in the testing of novel therapeutic agents other than JAK inhibitors that target signaling pathways in addition to JAK/STAT, or target the deregulated epigenetic mechanisms in MPN.

Efficacy of vorinostat in a murine model of polycythemia vera

The discovery of the JAK2V617F mutation in most patients with Ph-negative myeloproliferative neoplasms has led to the development of JAK2 kinase inhibitors. However, JAK2 inhibitor therapy has shown limited efficacy and dose-limiting hematopoietic toxicities in clinical trials. In the present study, we describe the effects of vorinostat, a small-molecule inhibitor of histone deacetylase, against cells expressing JAK2V617F and in an animal model of polycythemia vera (PV). We found that vorinostat markedly inhibited proliferation and induced apoptosis in cells expressing JAK2V617F. In addition, vorinostat significantly inhibited JAK2V617F-expressing mouse and human PV hematopoietic progenitors. Biochemical analyses revealed significant inhibition of phosphorylation of JAK2, Stat5, Stat3, Akt, and Erk1/2 in vorinostat-treated, JAK2V617F-expressing human erythroleukemia (HEL) cells. Expression of JAK2V617F and several other genes, including GATA1, KLF1, FOG1, SCL, C/EPBα, PU.1, and NF-E2, was significantly down-regulated, whereas the expression of SOCS1 and SOCS3 was up-regulated by vorinostat treatment. More importantly, we observed that vorinostat treatment normalized the peripheral blood counts and markedly reduced splenomegaly in Jak2V617F knock-in mice compared with placebo treatment. Vorinostat treatment also decreased the mutant allele burden in mice. Our results suggest that vorinostat may have therapeutic potential for the treatment of PV and other JAK2V617F-associated myeloproliferative neoplasms.

Targeting JAK2 in the therapy of myeloproliferative neoplasms

INTRODUCTION

Myeloproliferative neoplasms (MPNs) are a group of stem cell diseases, including polycythemia vera, essential thrombocythemia and primary myelofibrosis. Currently, there is no curative therapy for these diseases other than bone marrow transplant; therefore there is an apparent need for palliative treatment. MPNs are frequently associated with activating mutations in JAK2; small-molecule drugs targeting this molecule have entered clinical trials.

AREAS COVERED

In this review novel JAK2 inhibitors are discussed and alternative approaches to inhibiting their transforming potential are highlighted. Current clinical approaches do not only aim at blocking JAK2 activity, but also at reducing its stability and expression are highlighted, including inhibition of heat shock protein 90 (HSP90) and deacetylases (DAC) have the potential to significantly enhance the efficacy of JAK2 inhibitors.

EXPERT OPINION

Preliminary results from clinical trials indicate the feasibility and efficacy of JAK2-targeted approaches. However, JAK2 inhibitor treatment is limited by dose-dependent toxicity and combination treatment might be required. The discovery of JAK2 mutations that cause secondary resistance in vitro would further highlight the need for the development of next-generation JAK2 inhibitors and novel synergistic approaches.

A novel murine model of myeloproliferative disorders generated by overexpression of the transcription factor NF-E2

Mice expressing a transgene encoding the transcription factor NF-E2 in hematopoietic cells exhibit features of myeloproliferative neoplasms, including thrombocytosis, Epo-independent colony formation, stem and progenitor cell overabundance, leukocytosis, and progression to acute myeloid leukemia.

The molecular pathophysiology of myeloproliferative neoplasms (MPNs) remains poorly understood. Based on the observation that the transcription factor NF-E2 is often overexpressed in MPN patients, independent of the presence of other molecular aberrations, we generated mice expressing an NF-E2 transgene in hematopoietic cells. These mice exhibit many features of MPNs, including thrombocytosis, leukocytosis, Epo-independent colony formation, characteristic bone marrow histology, expansion of stem and progenitor compartments, and spontaneous transformation to acute myeloid leukemia. The MPN phenotype is transplantable to secondary recipient mice. NF-E2 can alter histone modifications, and NF-E2 transgenic mice show hypoacetylation of histone H3. Treatment of mice with the histone deacetylase inhibitor (HDAC-I) vorinostat restored physiological levels of histone H3 acetylation, decreased NF-E2 expression, and normalized platelet numbers. Similarly, MPN patients treated with an HDAC-I exhibited a decrease in NF-E2 expression. These data establish a role for NF-E2 in the pathophysiology of MPNs and provide a molecular rationale for investigating epigenetic alterations as novel targets for rationally designed MPN therapies.

Epigenomics in hematopoietic transplantation: novel treatment strategies

Allogeneic hematopoietic stem cell transplantation is a high risk but curative treatment option for leukemia, myelodysplasia and other hematological malignancies. After high dose radio- or chemo-therapy, recipient’s hematopoiesis is replaced by a new immunosystem and residual malignant cells are eliminated by the graft-versus-leukemia reaction. The benefit of this immunological effect is limited by the most frequent complication of hematopoietic stem cell transplantation: graft-versus-host disease. In addition to their well-known anti-tumor activity, epigenetic drugs mediate immunotolerance without reducing alloreactivity or even enhance graft-versus-leukemia effect without inducing graft-versus-host disease by regulating cytokine release, increasing the circulating number of regulatory T cells and interacting with natural killer cells. We focus on the use of epigenetic drugs in the allogeneic transplantation setting in relation to their anti-tumor and immunomodulatory potential.

Emerging drug discovery approaches for selective targeting of "precursor" metastatic breast cancer cells: highlights and perspectives

Breast cancer is a prevalent disease and a major cause of morbidity and cancer-related deaths among women worldwide. A significant number of patients at the time of primary diagnosis present metastatic disease, at least to locoregional lymph nodes, which results in somewhat unpredictable prognosis that often prompts adjuvant systemic therapies of various kinds. The time course of distant recurrence is also unpredictable with some patients sustaining a recurrence within months after diagnosis, even during adjuvant treatments, while others can experience recurrence years or decades after initial diagnosis. To date, clinically approved therapeutics yielded marginal benefits for patients with systemic metastatic breast disease, since despite high clinical responses to various therapies, the patients virtually always become resistant and tumor relapses. Molecular profiling studies established that breast cancer is highly heterogeneous and encompasses diverse histological and molecular subtypes with distinct biological and clinical implications in particular in relation to the incidence of progression to metastasis. The latter has been recognized to result from late genetic events during the multistep progression proposed by the dominant theory of carcinogenesis. However, there is evidence that the dissemination of primary cancer can also be initiated at a very early stage of cancer development, originating from rare cell variants, possibly cancer stem-like cells (CSC), with invasive potential. These precursor metastatic cancer cells with stem-like properties are defined by their ability to self-renew and to regenerate cell variants, which have high plasticity and intrinsic invasive properties required for dissemination and tropism toward specific organs. Equally relevant to the CSC hypothesis for metastasis formation is the epithelial-mesenchymal transition (EMT) process, which is critical for the acquisition of cancer cell invasive behavior and for selection/gain of CSC properties. These exciting concepts have led to the formulation of various approaches for targeting precursor metastatic cells, and these have taken on greater priority in therapeutic drug discovery research by both academia and pharmaceuticals. In this review, we focus on current efforts in medicinal chemistry to develop small molecules able to target precursor metastatic cells via interference with the CSC/EMT differentiation program, self-renewal, and survival. It is not meant to be comprehensive and the reader is referred to selected reviews that provide coverage of related basic aspects. Rather, emphasis is given to promising molecules with CSC/EMT signaling at the preclinical stage and in clinical trials that are paving the way to new generations of anti-metastasis drugs.

New mutations and pathogenesis of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are clonal disorders characterized by excessive production of mature blood cells. In the majority of classic MPN—polycythemia vera, essential thrombocythemia, and primitive myelofibrosis—driver oncogenic mutations affecting Janus kinase 2 (JAK2) or MPL lead to constitutive activation of cytokine-regulated intracellular signaling pathways. LNK, c-CBL, or SOCSs (all negative regulators of signaling pathways), although infrequently targeted, may either drive the disease or synergize with JAK2 and MPL mutations. IZF1 deletions or TP53 mutations are mainly found at transformation phases and are present at greater frequency than in de novo acute myeloid leukemias. Loss-of-function mutations in 3 genes involved in epigenetic regulation, TET2, ASXL1, and EZH2, may be early events preceding JAK2V617F but may also occur late during disease progression. They are more frequently observed in PMF than PV and ET and are also present in other types of malignant myeloid diseases. A likely hypothesis is that they facilitate clonal selection, allowing the dominance of the JAK2V617F subclone during the chronic phase and, together with cooperating mutations, promote blast crisis. Their precise roles in hematopoiesis and in the pathogenesis of MPN, as well as their prognostic impact and potential as a therapeutic target, are currently under investigation.

Epigenetic abnormalities in myeloproliferative neoplasms: a target for novel therapeutic strategies

The myeloproliferative neoplasms (MPNs) are a group of clonal hematological malignancies characterized by a hypercellular bone marrow and a tendency to develop thrombotic complications and to evolve to myelofibrosis and acute leukemia. Unlike chronic myelogenous leukemia, where a single disease-initiating genetic event has been identified, a more complicated series of genetic mutations appear to be responsible for the BCR-ABL1-negative MPNs which include polycythemia vera, essential thrombocythemia, and primary myelofibrosis. Recent studies have revealed a number of epigenetic alterations that also likely contribute to disease pathogenesis and determine clinical outcome. Increasing evidence indicates that alterations in DNA methylation, histone modification, and microRNA expression patterns can collectively influence gene expression and potentially contribute to MPN pathogenesis. Examples include mutations in genes encoding proteins that modify chromatin structure (EZH2, ASXL1, IDH1/2, JAK2V617F, and IKZF1) as well as epigenetic modification of genes critical for cell proliferation and survival (suppressors of cytokine signaling, polycythemia rubra vera-1, CXC chemokine receptor 4, and histone deacetylase (HDAC)). These epigenetic lesions serve as novel targets for experimental therapeutic interventions. Clinical trials are currently underway evaluating HDAC inhibitors and DNA methyltransferase inhibitors for the treatment of patients with MPNs.

Recurring mutations in myeloproliferative neoplasms alter epigenetic regulation of gene expression

The prevalence of activating JAK2 mutations in myeloproliferative neoplasms (MPNs) suggests that aberrant gene expression due to deregulated signaling of the JAK2/STAT pathway plays an important role in the etiology of these diseases. While likely true, recent work has uncovered some fascinating new insights into both the function of mutationally-activated JAK2 as well as other mutated gene products in MPNs, and how these mutations may affect gene expression. In addition to being a cytoplasmic tyrosine kinase that relays signals from cytokine receptors, activated JAK2 can also function in the nucleus where it phosphorylates histones and deregulates binding of the transcriptional repressor HP1α. In addition, MPN-associated JAK2 mutants phosphorylate PRMT5 and inhibit its histone methyltransferase activity. Thus, in addition to the classical JAK/STAT pathway, JAK2 activating mutations in MPNs may deregulate gene expression by altering epigenetic mechanisms. Studies aimed at identifying the biochemical ramifications of other recurring MPN mutations also suggest deregulated epigenetic modifications may be important in MPN formation. Mutant TET2, as well as IDH1/2, impairs the hydroxylation of methylcytosine, thus affecting DNA methylation. Likewise, mutations in EZH2, a histone methyl transferase, ASXL1, which functions in chromatin modifier complexes, and the DNA methyltransferase DNMT3A, appear to inactivate the functions of these proteins toward regulating the epigenetic state of genes. Thus, it is likely that the control of gene expression by epigenetic mechanisms plays an important role in MPNs, since multiple recurring mutations in MPNs alter normal epigenetic regulatory mechanisms.

New ways to use DNA methyltransferase inhibitors for the treatment of myelodysplastic syndrome

Ongoing analysis of the seminal AZA-001 study has taught many important lessons in the use of DNA methyltransferase (DNMT) inhibitors. The data emphasize the importance of patience in the use of these drugs, with several cycles required for the manifestations of hematologic responses. Improved survival in patients with high-risk myelodysplastic syndrome (MDS) treated with azacitidine extends to patients with any International Working Group-defined hematologic response; however, the benefit to patients with stable disease is less clear. A great deal remains to be learned about the optimal dosing and scheduling of the DNMT inhibitors, alone and in combination. New information on the impact of DNMT inhibitors on the immune system and on stem cells will likely lead to novel uses of these drugs in MDS and other hematologic and nonhematologic malignancies.

Ruxolitinib, a selective JAK1 and JAK2 inhibitor for the treatment of myeloproliferative neoplasms and psoriasis

Ruxolitinib (INCB-018424) is a potent, orally available, selective inhibitor of both JAK1 and JAK2 of the JAK-STAT signaling pathway, being developed by Incyte Corp and Novartis AG. Ruxolitinib was initially developed to target the constitutive activation of the JAK-STAT pathway in patients with myeloproliferative neoplasms (MPNs). Meaningful reductions in spleen size and constitutional symptoms have been noted in patients with myelofibrosis (both primary and post-essential thrombocythemia/polycythemia vera). Data from a phase I/II clinical trial led to ongoing registration trials in the US and Europe. Toxicity (primarily decreased erythropoiesis and thrombocytopoiesis) has been managed by close control of dosing. The inhibition of inflammatory cytokine signaling through JAK1 inhibition has led to intriguing results in patients with rheumatoid arthritis and psoriasis (using a topical cream formulation). Ruxolitinib is a well tolerated, first-in-class JAK2 inhibitor with various potential clinical indications.