Anti-transforming growth factor-β therapy in patients with myelofibrosis

A Phase Ib Trial of AVID200, a TGFβ 1/3 Trap, in Patients with Myelofibrosis

PURPOSE

Myelofibrosis (MF) is a clonal myeloproliferative neoplasm characterized by systemic symptoms, cytopenias, organomegaly, and bone marrow fibrosis. JAK2 inhibitors afford symptom and spleen burden reduction but do not alter the disease course and frequently lead to thrombocytopenia. TGFβ, a pleiotropic cytokine elaborated by the MF clone, negatively regulates normal hematopoiesis, downregulates antitumor immunity, and promotes bone marrow fibrosis. Our group previously showed that AVID200, a potent and selective TGFβ 1/3 trap, reduced TGFβ1-induced proliferation of human mesenchymal stromal cells, phosphorylation of SMAD2, and collagen expression. Moreover, treatment of MF mononuclear cells with AVID200 led to increased numbers of progenitor cells (PC) with wild-type JAK2 rather than JAK2V617F.

PATIENTS AND METHODS

We conducted an investigator-initiated, multicenter, phase Ib trial of AVID200 monotherapy in 21 patients with advanced MF.

RESULTS

No dose-limiting toxicity was identified at the three dose levels tested, and grade 3/4 anemia and thrombocytopenia occurred in 28.6% and 19.0% of treated patients, respectively. After six cycles of therapy, two patients attained a clinical benefit by IWG-MRT criteria. Spleen and symptom benefits were observed across treatment cycles. Unlike other MF-directed therapies, increases in platelet counts were noted in 81% of treated patients with three patients achieving normalization. Treatment with AVID200 resulted in potent suppression of plasma TGFβ1 levels and pSMAD2 in MF cells.

CONCLUSIONS

AVID200 is a well-tolerated, rational, therapeutic agent for the treatment of patients with MF and should be evaluated further in patients with thrombocytopenic MF in combination with agents that target aberrant MF intracellular signaling pathways.

Therapeutic activity of GARP:TGF-β1 blockade in murine primary myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm characterized by the clonal expansion of myeloid cells, notably megakaryocytes (MKs), and an aberrant cytokine production leading to bone marrow (BM) fibrosis and insufficiency. Current treatment options are limited. TGF-β1, a profibrotic and immunosuppressive cytokine, is involved in PMF pathogenesis. While all cell types secrete inactive, latent TGF-β1, only a few activate the cytokine via cell type-specific mechanisms. The cellular source of the active TGF-β1 implicated in PMF is not known. Transmembrane protein GARP binds and activates latent TGF-β1 on the surface of regulatory T lymphocytes (Tregs) and MKs or platelets. Here, we found an increased expression of GARP in the BM and spleen of mice with PMF and tested the therapeutic potential of a monoclonal antibody (mAb) that blocks TGF-β1 activation by GARP-expressing cells. GARP:TGF-β1 blockade reduced not only fibrosis but also the clonal expansion of transformed cells. Using mice carrying a genetic deletion of Garp in either Tregs or MKs, we found that the therapeutic effects of GARP:TGF-β1 blockade in PMF imply targeting GARP on Tregs. These therapeutic effects, accompanied by increased IFN-γ signals in the spleen, were lost upon CD8 T-cell depletion. Our results suggest that the selective blockade of TGF-β1 activation by GARP-expressing Tregs increases a CD8 T-cell-mediated immune reaction that limits transformed cell expansion, providing a novel approach that could be tested to treat patients with myeloproliferative neoplasms.

Extracellular Vesicles in Myeloid Neoplasms

Myeloid neoplasms arise from malignant primitive cells, which exhibit growth advantage within the bone marrow microenvironment (BMM). The interaction between these malignant cells and BMM cells is critical for the progression of these diseases. Extracellular vesicles (EVs) are lipid bound vesicles secreted into the extracellular space and involved in intercellular communication. Recent studies have described RNA and protein alterations in EVs isolated from myeloid neoplasm patients compared to healthy controls. The altered expression of various micro-RNAs is the best-described feature of EVs of these patients. Some of these micro-RNAs induce growth-related pathways such as AKT/mTOR and promote the acquisition of stem cell-like features by malignant cells. Another well-described characteristic of EVs in myeloid neoplasms is their ability to suppress healthy hematopoiesis either via direct effect on healthy CD34+ cells or via alteration of the differentiation of BMM cells. These results support a role of EVs in the pathogenesis of myeloid neoplasms. mainly through mediating the interaction between malignant and BMM cells, and warrant further study to better understand their biology. In this review, we describe the reported alterations of EV composition in myeloid neoplasms and the recent discoveries supporting their involvement in the development and progression of these diseases.

SOHO State of the Art Updates and Next Questions: Novel Therapies in Development for Myelofibrosis

Myeloproliferative neoplasms research has entered a dynamic and exciting era as we witness exponential growth of novel agents in advanced/early phase clinical trials for myelofibrosis (MF). Building on the success and pivotal role of ruxolitinib, many novel agents, spanning a wide range of mechanisms/targets (epigenetic regulation, apoptotic/intracellular signaling pathways, telomerase, bone marrow fibrosis) are in clinical development; several are studied in registrational trials and hold great potential to expand the therapeutic arsenal/shift the treatment paradigm if regulatory approval is granted. Insight into MF pathogenesis and its molecular underpinnings, preclinical studies demonstrating synergism of ruxolitinib with investigational agents, urgent unmet clinical needs (cytopenias, loss of response to JAK inhibitors); and progressive disease fueled the rapid rise of innovative therapeutics. New strategies include pairing ruxolitinib with erythroid maturation agents to manage anemia (luspatercept), designing rational combinations with ruxolitinib to boost responses in both the frontline and suboptimal response settings (pelabresib, navitoclax, parsaclisib), treatment with non-JAK inhibitor monotherapy in the second-line setting (navtemadlin, imetelstat), novel JAK inhibitors tailored to subgroups with challenging unmet needs (momelotinib and pacritinib for anemia and thrombocytopenia, respectively); and agents potentially enhancing longevity (imetelstat). Beyond typical endpoints evaluated in MF clinical trials (spleen volume reduction ≥ 35%, total symptom score reduction ≥ 50%) thus far, emerging endpoints include overall survival, progression-free survival, transfusion independence, anemia benefits, bone marrow fibrosis and driver mutation allele burden reduction. Novel biomarkers and additional clinical features are being sought to assess new agents and tailor emerging therapies to appropriate patients. New strategies are needed to optimize the design of clinical trials comparing novel combinations to standard agent monotherapy.

Inhibition of proinflammatory signaling impairs fibrosis of bone marrow mesenchymal stromal cells in myeloproliferative neoplasms

Although bone marrow-derived mesenchymal stromal cells (BM-MSCs) have been identified as a major cellular source of fibrosis, the exact molecular mechanism and signaling pathways involved have not been identified thus far. Here, we show that BM-MSCs contribute to fibrosis in myeloproliferative neoplasms (MPNs) by differentiating into αSMA-positive myofibroblasts. These cells display a dysregulated extracellular matrix with increased FN1 production and secretion of profibrotic MMP9 compared to healthy donor cells. Fibrogenic TGFβ and inflammatory JAK2/STAT3 and NFκB signaling pathway activity is increased in BM-MSCs of MPN patients. Moreover, coculture with mononuclear cells from MPN patients was sufficient to induce fibrosis in healthy BM-MSCs. Inhibition of JAK1/2, SMAD3 or NFκB significantly reduced the fibrotic phenotype of MPN BM-MSCs and was able to prevent the development of fibrosis induced by coculture of healthy BM-MSCs and MPN mononuclear cells with overly active JAK/STAT signaling, underlining their involvement in fibrosis. Combined treatment with JAK1/2 and SMAD3 inhibitors showed synergistic and the most favorable effects on αSMA and FN1 expression in BM-MSCs. These results support the combined inhibition of TGFβ and inflammatory signaling to extenuate fibrosis in MPN.

The treatment of fibrosis in patients with rare bone marrow disorders could be improved with a combined therapy that targets inflammatory pathways. Myeloproliferative neoplasms (MPN) are a group of bone marrow disorders characterized by the over-production of blood cells, which can lead to fibrosis in the bone marrow. Vladan Čokić at the University of Belgrade, Serbia, and co-workers examined how stem cells known as mesenchymal stromal cells from the bone marrow contribute to MPN fibrosis. They found an increase in three pro-inflammatory signaling pathways in MPN patients, resulting in the stromal cells differentiating into cells with dysregulated extracellular matrices. The differentiated cells did not behave correctly nor degrade properly, triggering fibrosis. The team combined two drugs that target the inflammatory signaling pathways, and successfully inhibited the development of fibrosis in MPN cell cultures.

SOHO State of the Art Updates and Next Questions: Identifying and Treating "Progression" in Myelofibrosis

Over the last decade, the Janus kinase (JAK) 1/2 inhibitor ruxolitinib has become widely established as the cornerstone of pharmacologic therapy for most patients with myelofibrosis (MF), providing dramatic and durable benefits in terms of splenomegaly and symptoms, and prolonging survival. Ruxolitinib does not address all aspects of the disease, however; notably cytopenias, and its ability to modify the underlying biology of the disease remains in question. Furthermore, patients eventually lose response to ruxolitinib. Multiple groups have reported the median overall survival of MF patients after ruxolitinib discontinuation to be 13 to 14 months. While consensus criteria only recognize splenic and blast progression as "progressive disease" in patients with MF, disease progression can occur in a variety of ways. Besides increasing splenomegaly and progression to accelerated phase/leukemic transformation, patients may develop worsening disease-related symptoms, cytopenias, progressive leukocytosis, extramedullary hematopoiesis, etc. As in the frontline setting, treatment needs to be tailored to the clinical needs of the patient. Current treatment options for patients with MF who fail ruxolitinib remain unsatisfactory, and this continues to represent an area of major unmet medical need. The regulatory approval of fedratinib has introduced an important option in the postruxolitinib setting. Fortunately, a plethora of novel agents, both new JAK inhibitors and drugs from other classes, eg, bromodomain and extraterminal (BET), murine double minute 2 (MDM2) and telomerase inhibitors, activin receptor ligand traps, BH3-mimetics and more, are poised to greatly expand the therapeutic armamentarium for patients with MF if successful in pivotal trials.

TGFβ1 protein trap AVID200 beneficially affects hematopoiesis and bone marrow fibrosis in myelofibrosis

Myelofibrosis (MF) is a progressive chronic myeloproliferative neoplasm characterized by hyperactivation of JAK/STAT signaling and dysregulation of the transcription factor GATA1 in megakaryocytes (MKs). TGF-β plays a pivotal role in the pathobiology of MF by promoting BM fibrosis and collagen deposition and by enhancing the dormancy of normal hematopoietic stem cells (HSCs). In this study, we show that MF-MKs elaborated significantly greater levels of TGF-β1 than TGF-β2 and TGF-β3 to a varying degree, and we evaluated the ability of AVID200, a potent TGF-β1/TGF-β3 protein trap, to block the excessive TGF-β signaling. Treatment of human mesenchymal stromal cells with AVID200 significantly reduced their proliferation, decreased phosphorylation of SMAD2, and interfered with the ability of TGF-β1 to induce collagen expression. Moreover, treatment of MF mononuclear cells with AVID200 led to increased numbers of progenitor cells (PCs) with WT JAK2 rather than mutated JAK2V617F. This effect of AVID200 on MF PCs was attributed to its ability to block TGF-β1–induced p57^Kip2 expression and SMAD2 activation, thereby allowing normal rather than MF PCs to preferentially proliferate and form hematopoietic colonies. To assess the in vivo effects of AVID200, Gata1^lo mice, a murine model of MF, were treated with AVID200, resulting in the reduction in BM fibrosis and an increase in BM cellularity. AVID200 treatment also increased the frequency and numbers of murine progenitor cells as well as short-term and long-term HSCs. Collectively, these data provide the rationale for TGF-β1 blockade, with AVID200 as a therapeutic strategy for patients with MF.

Myelofibrosis: challenges for preclinical models and emerging therapeutic targets

Introduction: Myelofibrosis (MF) is characterized by anemia, splenomegaly, constitutional symptoms and bone marrow fibrosis. MF has no curative treatment to date, except for a small subset of patients that are eligible for allogeneic hematopoietic stem cell transplant. The discovery in recent years of the MF mutational landscape and the role of bone marrow microenvironment in disease pathogenesis has led to further insights into disease biology and consequentially rationally derived therapies.Areas covered: We searched PubMed/Medline/American Society of Hematology (ASH) abstracts until November 2020 using the following terms: myelofibrosis, mouse models, pre-clinical studies and clinical trials. The development of targeted therapies is aimed to modify the history of the disease. Although JAK inhibitors showed encouraging results in terms of spleen and symptoms response, long term remissions and disease modifying ability is lacking. Beyond JAK inhibitors, a range of agents targeting proliferative, metabolic, apoptotic pathways, the microenvironment, epigenetic modification and immunomodulation are in various stages of investigations. We review pre-clinical data, preliminary clinical results of these agents, and finally offer insights on the management of MF patients.Expert opinion: MF patients refractory or with suboptimal response to JAK inhibitors, may be managed by addition of agents with differing mechanisms, such as bromodomain (BET), lysine demethylase 1 (LSD1), MDM2, or Bcl-Xl inhibitors which could prevent emergence of resistance. Immunotherapies as long-acting interferons, and calreticulin directed antibodies or peptide vaccination are eagerly awaited. Historically, therapeutic challenges in MF have arisen due to the fact that rationally derived therapies that are based on murine models have limited impact on fibrosis and underlying disease biology in human studies, the latter illustrates the complex multi-faceted disease pathogenesis of MF. Together, we not only suggest individualized therapy in MF that is guided by genomic signature but also its early implementation potentially in prefibrotic MF.

Lessons from mouse models of MPN

Over the past decades, a variety of MPN mouse models have been developed to express in HSC the main mutations identified in patients: JAK2^V617F, CALRdel52 or ins5 and MPL^W515L. These models mimic quite faithfully human PV or ET with their natural evolutions into MF and their hemostasis complications, demonstrating the driver function of these mutations in MPN. Here, we review these models and show how they have improved our general understanding of MPN regarding (1) the mechanisms of fibrosis, thrombosis/hemorrhages and disease initiation, (2) the roles of additional mutations and signaling pathways in disease progression and (3) the preclinical development of novel therapies. We also address controversial results between these models and remind how these models may differ from human MPN onset and also how basically mice are not humans, encouraging caution when one draw lessons from mice to humans. Furthermore, the contribution of germline genetic predisposition, HSC and niche aging, metabolic, oxidative, replicative or genotoxic stress, inflammation, immune escape and additional mutations need to be considered in further investigations to encompass the full complexity of human MPN in mice.

Inflammatory Microenvironment and Specific T Cells in Myeloproliferative Neoplasms: Immunopathogenesis and Novel Immunotherapies

The Philadelphia-negative myeloproliferative neoplasms (MPNs) are malignancies of the hematopoietic stem cell (HSC) arising as a consequence of clonal proliferation driven by somatically acquired driver mutations in discrete genes (JAK2, CALR, MPL). In recent years, along with the advances in molecular characterization, the role of immune dysregulation has been achieving increasing relevance in the pathogenesis and evolution of MPNs. In particular, a growing number of studies have shown that MPNs are often associated with detrimental cytokine milieu, expansion of the monocyte/macrophage compartment and myeloid-derived suppressor cells, as well as altered functions of T cells, dendritic cells and NK cells. Moreover, akin to solid tumors and other hematological malignancies, MPNs are able to evade T cell immune surveillance by engaging the PD-1/PD-L1 axis, whose pharmacological blockade with checkpoint inhibitors can successfully restore effective antitumor responses. A further interesting cue is provided by the recent discovery of the high immunogenic potential of JAK2V617F and CALR exon 9 mutations, that could be harnessed as intriguing targets for innovative adoptive immunotherapies. This review focuses on the recent insights in the immunological dysfunctions contributing to the pathogenesis of MPNs and outlines the potential impact of related immunotherapeutic approaches.

Inflammatory Microenvironment and Specific T Cells in Myeloproliferative Neoplasms: Immunopathogenesis and Novel Immunotherapies

The Philadelphia-negative myeloproliferative neoplasms (MPNs) are malignancies of the hematopoietic stem cell (HSC) arising as a consequence of clonal proliferation driven by somatically acquired driver mutations in discrete genes (JAK2, CALR, MPL). In recent years, along with the advances in molecular characterization, the role of immune dysregulation has been achieving increasing relevance in the pathogenesis and evolution of MPNs. In particular, a growing number of studies have shown that MPNs are often associated with detrimental cytokine milieu, expansion of the monocyte/macrophage compartment and myeloid-derived suppressor cells, as well as altered functions of T cells, dendritic cells and NK cells. Moreover, akin to solid tumors and other hematological malignancies, MPNs are able to evade T cell immune surveillance by engaging the PD-1/PD-L1 axis, whose pharmacological blockade with checkpoint inhibitors can successfully restore effective antitumor responses. A further interesting cue is provided by the recent discovery of the high immunogenic potential of JAK2V617F and CALR exon 9 mutations, that could be harnessed as intriguing targets for innovative adoptive immunotherapies. This review focuses on the recent insights in the immunological dysfunctions contributing to the pathogenesis of MPNs and outlines the potential impact of related immunotherapeutic approaches.

Cytotoxic T cells isolated from healthy donors and cancer patients kill TGFβ-expressing cancer cells in a TGFβ-dependent manner

Transforming growth factor-beta (TGFβ) is a highly potent immunosuppressive cytokine. Although TGFβ is a tumor suppressor in early/premalignant cancer lesions, the cytokine has several tumor-promoting effects in advanced cancer; abrogation of the antitumor immune response is one of the most important tumor-promoting effects. As several immunoregulatory mechanisms have recently been shown to be targets of specific T cells, we hypothesized that TGFβ is targeted by naturally occurring specific T cells and thus could be a potential target for immunomodulatory cancer vaccination. Hence, we tested healthy donor and cancer patient T cells for spontaneous T-cell responses specifically targeting 38 20-mer epitopes derived from TGFβ1. We identified numerous CD4+ and CD8+ T-cell responses against several epitopes in TGFβ. Additionally, several ex vivo responses were identified. By enriching specific T cells from different donors, we produced highly specific cultures specific to several TGFβ-derived epitopes. Cytotoxic CD8+ T-cell clones specific for both a 20-mer epitope and a 9-mer HLA-A2 restricted killed epitope peptide were pulsed in HLA-A2+ target cells and killed the HLA-A2+ cancer cell lines THP-1 and UKE-1. Additionally, stimulation of THP-1 cancer cells with cytokines that increased TGFβ expression increased the fraction of killed cells. In conclusion, we have shown that healthy donors and cancer patients harbor CD4+ and CD8+ T cells specific for TGFβ-derived epitopes and that cytotoxic T cells with specificity toward TGFβ-derived epitopes are able to recognize and kill cancer cell lines in a TGFβ-dependent manner.

Bone marrow niche dysregulation in myeloproliferative neoplasms

The bone marrow niche is a complex and dynamic structure composed of a multitude of cell types which functionally create an interactive network facilitating hematopoietic stem cell development and maintenance. Its specific role in the pathogenesis, response to therapy, and transformation of myeloproliferative neoplasms has only recently been explored. Niche functionality is likely affected not only by the genomic background of the myeloproliferative neoplasm-associated mutated hematopoietic stem cells, but also by disease-associated 'chronic inflammation', and subsequent adaptive and innate immune responses. 'Cross-talk' between mutated hematopoietic stem cells and multiple niche components may contribute to propagating disease progression and mediating drug resistance. In this timely article, we will review current knowledge surrounding the deregulated bone marrow niche in myeloproliferative neoplasms and suggest how this may be targeted, either directly or indirectly, potentially influencing therapeutic choices both now and in the future.

Forging ahead or moving back: dilemmas and disappointments of novel agents for myeloproliferative neoplasms

The common 'Philadelphia chromosome'-negative myeloproliferative neoplasms (MPN) comprise essential thrombocythaemia, polycythaemia vera and myelofibrosis. These are clinically diverse disorders and present many challenges during their course, ranging from the management of very indolent, chronic-phase disease through to very aggressive stages frequently associated with poor quality of life, heavy symptom burdens and potentially life expectancies of <18 months. Their management also requires expertise in thrombosis and haemostasis in addition to marrow failure, debilitating symptom control and balancing the 'pros and cons' of intensive therapy such as allogeneic stem cell transplant versus novel and established therapies. In the past 15 years this field has seen rapid advances following an understanding of the pivotal importance of constitutive Janus kinase/signal transducers and activators of transcription (JAK/STAT) signalling, the interplay of the wider genomic landscape and the development of updated diagnostic criteria, prognostic scores and targeted therapies. In this article, we review the successes and failures of novel agents and approaches to MPN management.

Early post-transplantation factors predict survival outcomes in patients undergoing allogeneic hematopoietic cell transplantation for myelofibrosis

Factors predicting allogeneic hematopoietic cell transplantation (HCT) outcomes in myelofibrosis in the early post-HCT period have not been defined thus far. We attempt to study such factors that can help identify patients at a higher risk of relapse or death. This retrospective study included 79 patients who underwent first HCT for myelofibrosis at three centers between 2005 and 2016. Univariate analysis showed that red blood cell (RBC) transfusion dependence (HR 9.02, 95% CI 4.0–20.35), platelet transfusion dependence (HR 8.17, 95%CI 3.83–17.37), 100% donor chimerism in CD33 + cells (HR 0.21, 95%CI 0.07–0.62), unfavorable molecular status (HR 4.41, 95%CI 1.87–10.39), normal spleen size (HR 0.42, 95%CI 0.19–0.94), grade ≥ 2 bone marrow fibrosis (vs. grade ≤ 1; HR 2.7, 95%CI 1.1–6.93) and poor graft function (HR 2.6, 95%CI 1.22–5.53) at day +100 were statistically significantly associated with relapse-free survival (RFS). RBC transfusion dependence and unfavorable molecular status were also statistically significant in the multivariate analysis. Patients in whom both of these factors were present had a significantly worse RFS when compared to those with one or none. While limited by a small sample size, we demonstrate the significance of transfusion dependence and molecular status at day +100 in predicting outcomes.

Standard care and investigational drugs in the treatment of myelofibrosis

Myelofibrosis (MF) is a heterogeneous disorder characterized by splenomegaly, constitutional symptoms, ineffective hematopoiesis, and an increased risk of leukemic transformation. The ongoing research in understanding the pathophysiology of the disease has allowed for the development of targeted drugs optimizing patient management. Furthermore, disease prognostication has significantly improved. Current therapeutic interventions are only partially effective with only allogeneic stem cell transplant potentially curative. Ruxolitinib is the only approved therapy for MF by the US Food and Drug Administration. However, despite efficacy in reducing splenomegaly and controlling symptomatology, it is not associated with consistent molecular or pathologic responses. Drug discontinuation is associated with a dismal outcome. The therapeutic landscape in MF has significantly improved, and emerging drugs with different target pathways, alone or in combination with ruxolitinib, seem promising.

Advances in potential treatment options for myeloproliferative neoplasm associated myelofibrosis

INTRODUCTION

The Janus kinase (JAK)1/2 inhibitor ruxolitinib provides rapid, sustained and often dramatic benefits to patients with myelofibrosis, inducing spleen shrinkage and ameliorating symptoms, and improves survival. However, the drug has little effect on the underlying bone marrow fibrosis or on mutant allele burden, and clinical resistance eventually develops. Furthermore, ruxolitinib-induced cytopenias can be challenging in everyday practice.

AREAS COVERED

The developmental therapeutics landscape in MF is discussed. This includes potential partners for ruxolitinib being developed with an aim to improve cytopenias, or to enhance its disease-modifying effects. The development of other JAK inhibitors with efficacy post-ruxolitinib or other unique attributes is being pursued in earnest. Agents with novel mechanisms of action are being studied in patients whose disease responds sub-optimally to, is refractory to or progresses after ruxolitinib.

EXPERT OPINION

The JAK inhibitors fedratinib, pacritinib and momelotinib are clearly active, and it is expected that one or more of these will become licensed in the future. The activin receptor ligand traps are promising as treatments for anemia. Imetelstat has shown interesting activity post-ruxolitinib, and azactidine may be a useful partner for ruxolitinib in some patients. Appropriately, multiple pre-clinical and clinical leads are being pursued in this difficult therapeutic area.

Novel strategies for the treatment of myelofibrosis driven by recent advances in understanding the role of the microenvironment in its etiology

Myelofibrosis is the advanced stage of the Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), characterized by systemic inflammation, hematopoietic failure in the bone marrow, and development of extramedullary hematopoiesis, mainly in the spleen. The only potentially curative therapy for this disease is hematopoietic stem cell transplantation, an option that may be offered only to those patients with a compatible donor and with an age and functional status that may face its toxicity. By contrast, with the Philadelphia-positive MPNs that can be dramatically modified by inhibitors of the novel BCR-ABL fusion-protein generated by its genetic lesion, the identification of the molecular lesions that lead to the development of myelofibrosis has not yet translated into a treatment that can modify the natural history of the disease. Therefore, the cure of myelofibrosis remains an unmet clinical need. However, the excitement raised by the discovery of the genetic lesions has inspired additional studies aimed at elucidating the mechanisms driving these neoplasms towards their final stage. These studies have generated the feeling that the cure of myelofibrosis will require targeting both the malignant stem cell clone and its supportive microenvironment. We will summarize here some of the biochemical alterations recently identified in MPNs and the novel therapeutic approaches currently under investigation inspired by these discoveries.

Increased SLAMF7high monocytes in myelofibrosis patients harboring JAK2V617F provide a therapeutic target of elotuzumab

Monocyte-derived fibrocytes recently garnered attention because the novel pathogenesis of myelofibrosis (MF), and suppression of fibrocyte differentiation by serum amyloid P remarkably improved MF. We previously revealed that human fibrocytes highly expressed signaling lymphocytic activation molecule F7 (SLAMF7) compared with macrophages and that SLAMF7^high monocytes in the peripheral blood (PB) of MF patients were significantly elevated relative to those in healthy controls (HCs). In this study, we evaluated SLAMF7^high monocyte percentage in the PB of HCs, myeloproliferative neoplasm (MPN) patients with MF, and MPN patients without MF by using a cross-sectional approach. We found that MPN patients with MF who harbored JAK2V617F had a significantly elevated SLAMF7^high monocyte percentage, which correlated positively with the JAK2V617F allele burden. In addition, the serum concentration of interleukin-1ra (IL-1ra) was significantly correlated with the SLAMF7^high monocyte percentage and JAK2V617F allele burden. These findings suggest that both SLAMF7^high monocytes and IL-1ra could be useful noninvasive markers of MF onset. Furthermore, the JAK2V617F allele burden of SLAMF7^high monocytes was significantly higher than that of SLAMF7^low monocytes and could be a potential target of elotuzumab (Elo), an anti-SLAMF7 antibody used for treating multiple myeloma. Elo independently inhibited differentiation of fibrocytes derived not only from HCs but also from MF patients in vitro. Elo also ameliorated MF and splenomegaly induced by romiplostim administration in humanized NOG mice. In conclusion, an increase of SLAMF7^high monocytes with higher JAK2V617F allele burden was associated with the onset of MF in MPN patients harboring JAK2V617F, and Elo could be a therapeutic agent for MPN patients with MF who harbor JAK2V617F.

Pharmacotherapy of Myelofibrosis

Myelofibrosis (MF) is a myeloproliferative neoplasm that is pathologically characterized by bone marrow myeloproliferation, reticulin and collagen fibrosis, and extramedullary hematopoiesis. Constitutive activation of the Janus associated kinase (JAK)-signal transducers and activators of transcription signaling pathway with resultant elevation in pro-inflammatory cytokine levels is the pathogenic hallmark of MF. JAK inhibitors, namely ruxolitinib, have been successful in alleviating symptoms and reducing splenomegaly, but therapy-related myelosuppression has led to the further development of highly selective JAK2 inhibitors. Additionally, ruxolitinib does not appear to affect the malignant hematopoietic clone substantially, evidenced by lack of molecular remissions, bone marrow histopathologic responses, and a proportion of treated patients developing progressive disease and leukemic transformation while receiving therapy. A number of other pharmacotherapeutic strategies are currently being explored in the clinic. Non-JAK inhibitor strategies being evaluated in MF include non-JAK signaling pathway inhibitors, epigenetic-directed therapies, immune-modulating agents, anti-fibrotic agents, and telomerase inhibitors. This review highlights the current landscape of MF pharmacotherapy and explores therapeutic advances underway.

Combined Inhibition of DYRK1A, SMAD, and Trithorax Pathways Synergizes to Induce Robust Replication in Adult Human Beta Cells

Small-molecule inhibitors of dual-specificity tyrosine-regulated kinase 1A (DYRK1A) induce human beta cells to proliferate, generating a labeling index of 1.5%-3%. Here, we demonstrate that combined pharmacologic inhibition of DYRK1A and transforming growth factor beta superfamily (TGFβSF)/SMAD signaling generates remarkable further synergistic increases in human beta cell proliferation (average labeling index, 5%-8%, and as high as 15%-18%), and increases in both mouse and human beta cell numbers. This synergy reflects activation of cyclins and cdks by DYRK1A inhibition, accompanied by simultaneous reductions in key cell-cycle inhibitors (CDKN1C and CDKN1A). The latter results from interference with the basal Trithorax- and SMAD-mediated transactivation of CDKN1C and CDKN1A. Notably, combined DYRK1A and TGFβ inhibition allows preservation of beta cell differentiated function. These beneficial effects extend from normal human beta cells and stem cell-derived human beta cells to those from people with type 2 diabetes, and occur both in vitro and in vivo.

The Molecular Mechanism of Transforming Growth Factor-β Signaling for Intestinal Fibrosis: A Mini-Review

Inflammatory bowel disease is known as the most chronic inflammatory disorder in colon, which subsequently progresses to intestinal obstruction and fistula formation. Many studies to date for the treatment of IBD have been focused on inflammation. However, most of the anti-inflammatory agents do not have anti-fibrotic effects and could not relieve intestinal stricture in IBD patients. Because preventing or reversing intestinal fibrosis in IBD is a major therapeutic target, we analyzed the papers focusing on TGF-β signaling in intestinal fibrosis. TGF-β is a good candidate to treat the intestinal fibrosis in IBD which involves TGF-β signaling pathway, EMT, EndMT, ECM, and other regulators. Understanding the mechanism involved in TGF-β signaling will contribute to the treatment and diagnosis of intestinal fibrosis occurring in IBD as well as the understanding of the molecular mechanisms underlying the pathogenesis.

New Concepts of Treatment for Patients with Myelofibrosis

OPINION STATEMENT

Seven years after the approval of the Janus kinase 1/2 (JAK1/2) inhibitor ruxolitinib, it remains the only drug licensed for the treatment of myelofibrosis. Patients who discontinue ruxolitinib have a dismal outcome, and this is, therefore, an area of significant unmet need. Given the central role that JAK-signal transducer and activator of transcription (STAT) activation plays in disease pathogenesis, there have been many other JAK inhibitors tested, but most have been abandoned, for a variety of reasons. The JAK2-selective inhibitor fedratinib has recently been resurrected, and there has been a resurgence of interest in the failed JAK1/2 inhibitor momelotinib, which possibly improves anemia. Pacritinib, a non-myelosuppressive JAK2-selective inhibitor, is currently in a dose-ranging study mandated by regulatory authorities. A plethora of other targeted agents, most backed by preclinical data, are in various stages of investigation. These include epigenetic and immune therapies, agents targeting cellular survival, metabolic and apoptotic pathways, the cell cycle, DNA repair, and protein folding and degradation, among others. However, at this time, none of these is close to registration or even in a pivotal trial, illustrating the difficulties in recapitulating the clinical disease in preclinical models. Most current clinical trials are testing the addition of a novel agent to ruxolitinib, either in the frontline setting or in the context of an insufficient response to ruxolitinib, or attempting to study new drugs in the second-line, "ruxolitinib failure" setting. Emerging data supports the addition of azacitidine to ruxolitinib in some patients. Other strategies have focused on improving cytopenias, through amelioration of bone marrow fibrosis or other mechanisms. This is important, because cytopenias are the commonest reason for ruxolitinib interruption and/or dose reduction, and dose optimization of ruxolitinib is tied to its survival benefit. The activin receptor ligand trap, sotatercept, and the anti-fibrotic agent, PRM-151, have shown promise in this regard.

Management of Myelofibrosis-Related Cytopenias

PURPOSE OF REVIEW

Cytopenias, particularly anemia, are frequently encountered in patients with myelofibrosis. Management of cytopenias in myelofibrosis can be very challenging because current therapeutic interventions are only of modest efficacy and ruxolitinib, the only approved drug for myelofibrosis, is myelosuppressive. Yet, dose optimization of ruxolitinib is important for its survival benefit in patients with advanced disease. We sought to summarize the data on treatments for cytopenias available at present and review promising agents in development and emerging strategies.

RECENT FINDINGS

The activin receptor ligand traps hold considerable promise for the treatment of anemia and could represent an attractive combination strategy with ruxolitinib. Low-dose thalidomide, which could offset both anemia and thrombocytopenia caused by ruxolitinib, represents another potential partner for ruxolitinib. The anti-fibrotic agent PRM-151 produced sustained improvements in cytopenias in some patients, and further data on this drug are eagerly awaited. Finally, several preclinical leads with translational potential are worthy of clinical investigation as strategies to halt/reverse bone marrow fibrosis and thereby improve cytopenias. Cytopenias remain a significant hurdle in myelofibrosis management, but several novel investigational agents hold considerable promise for the future.

GATA1 insufficiencies in primary myelofibrosis and other hematopoietic disorders: consequences for therapy

INTRODUCTION

GATA1, the founding member of a family of transcription factors, plays important roles in the development of hematopoietic cells of several lineages. Although loss of GATA1 has been known to impair hematopoiesis in animal models for nearly 25 years, the link between GATA1 defects and human blood diseases has only recently been realized. Areas covered: Here the current understanding of the functions of GATA1 in normal hematopoiesis and how it is altered in disease is reviewed. GATA1 is indispensable mainly for erythroid and megakaryocyte differentiation. In erythroid cells, GATA1 regulates early stages of differentiation, and its deficiency results in apoptosis. In megakaryocytes, GATA1 controls terminal maturation and its deficiency induces proliferation. GATA1 alterations are often found in diseases involving these two lineages, such as congenital erythroid and/or megakaryocyte deficiencies, including Diamond Blackfan Anemia (DBA), and acquired neoplasms, such as acute megakaryocytic leukemia (AMKL) and the myeloproliferative neoplasms (MPNs). Expert commentary: Since the first discovery of GATA1 mutations in AMKL, the number of diseases that are associated with impaired GATA1 function has increased to include DBA and MPNs. With respect to the latter, we are only just now appreciating the link between enhanced JAK/STAT signaling, GATA1 deficiency and disease pathogenesis.

Myelofibrosis-Related Anemia: Current and Emerging Therapeutic Strategies

Myelofibrosis (MF) is a clonal hematopoietic stem cell disorder characterized by pathological myeloproliferation and aberrant cytokine production resulting in progressive fibrosis, inflammation, and functional compromise of the bone marrow niche. Patients with MF develop splenomegaly (due to extramedullary hematopoiesis), hypercatabolic symptoms (due to overexpression of inflammatory cytokines), and anemia (due to bone marrow failure and splenic sequestration). MF remains curable only with allogeneic hematopoietic stem cell transplantation (ASCT), a therapy that few MF patients are deemed fit to undergo. The goals of treatment are thus often palliative. The approval of the JAK inhibitor ruxolitinib has done much to address the burden of splenomegaly and constitutional symptoms of patients with MF; however, therapy-related anemia is often an anticipated downside. Anemia thus remains a challenge in the management of MF and represents a major unmet need. Intractable anemia depresses quality of life, portends poor outcomes, and can act to restrict access to palliative JAK inhibition in some patients. While therapies for MF-related anemia do exist, they are limited in their efficacy, durability, and tolerability. Therapies currently in development promise improved anemia-specific outcomes; however, are still early in the pathway to regulatory approval and regular clinical use. In this review, we will discuss established and emerging treatments for MF-related anemia. We will give particular attention to developmental therapies which herald significant progress in the understanding and management of MF-related anemia.

Current treatment algorithm for the management of patients with myelofibrosis, JAK inhibitors, and beyond

Myelofibrosis (MF) is a heterogeneous disorder characterized by splenomegaly, constitutional symptoms, ineffective hematopoiesis, and an inherent risk of leukemic transformation. The past decade has seen a massive shift in available therapeutic options for our patients and we are learning how and when to use novel agents, either alone or in combination, during the disease course. This has translated into improved management of splenomegaly, significant amelioration in disease-related symptom burden for many, and may lead to improved survival. But limitations in the therapeutic options remain. Moreover, enhanced delineation of the mutational landscape of MF has offered both increasingly complex prognostic algorithms and yielded further potential therapeutic avenues. In this review, we will focus on stratifying both standard and experimental therapeutic management in 2017 and 2018 and postulate on the emerging treatments that will likely become part of our increasingly complex treatment algorithm.

Novel Therapies for Myelofibrosis

PURPOSE OF REVIEW

The purpose of the review was to provide a contemporary update of novel agents and targets under investigation in myelofibrosis in the Janus kinase (JAK) inhibitor era.

RECENT FINDINGS

Myelofibrosis (MF) is a clonal stem cell disease characterized by marrow fibrosis and a heterogeneous disease phenotype with a variable degree of splenomegaly, cytopenias, and constitutional symptoms that significantly impact quality of life and survival. Overactive JAK/STAT signaling is a hallmark of MF. The only approved therapy for MF, JAK1/2 inhibitor ruxolitinib, can ameliorate splenomegaly, improve symptoms, and prolong survival in some patients. Therapeutic challenges remain, however. Myelosuppression limits the use of ruxolitinib in some patients, eventual drug resistance is common, and the underlying malignant clone persists despite therapy. A deeper understanding of the pathogenesis of MF has informed the development of additional agents. Promising targets under investigation include JAK1 and JAK2 and downstream intermediates in related signaling pathways, epigenetic modifiers, pro-inflammatory cytokines, and immune regulators.

Efficacy of ALK5 inhibition in myelofibrosis

Myelofibrosis (MF) is a bone marrow disorder characterized by clonal myeloproliferation, aberrant cytokine production, extramedullary hematopoiesis, and bone marrow fibrosis. Although somatic mutations in JAK2, MPL, and CALR have been identified in the pathogenesis of these diseases, inhibitors of the Jak2 pathway have not demonstrated efficacy in ameliorating MF in patients. TGF-β family members are profibrotic cytokines and we observed significant TGF-β1 isoform overexpression in a large cohort of primary MF patient samples. Significant overexpression of TGF-β1 was also observed in murine clonal MPL^W515L megakaryocytic cells. TGF-β1 stimulated the deposition of excessive collagen by mesenchymal stromal cells (MSCs) by activating the TGF-β receptor I kinase (ALK5)/Smad3 pathway. MSCs derived from MPL^W515L mice demonstrated sustained overproduction of both collagen I and collagen III, effects that were abrogated by ALK5 inhibition in vitro and in vivo. Importantly, use of galunisertib, a clinically active ALK5 inhibitor, significantly improved MF in both MPL^W515L and JAK2^V617F mouse models. These data demonstrate the role of malignant hematopoietic stem cell (HSC)/TGF-β/MSC axis in the pathogenesis of MF, and provide a preclinical rationale for ALK5 blockade as a therapeutic strategy in MF.

Myeloproliferative leukemia protein activation directly induces fibrocyte differentiation to cause myelofibrosis

Myelofibrosis (MF) may be caused by various pathogenic mechanisms such as elevation in circulating cytokine levels, cellular interactions and genetic mutations. However, the underlying mechanism of MF still remains unknown. Recent studies have revealed that fibrocytes, the spindle-shaped fibroblast-like hematopoietic cells, and the thrombopoietin (TPO)/myeloproliferative leukemia protein (MPL; TPO receptor) signaling pathway play a certain role in the development of MF. In the present study, we aimed to investigate the relationship between fibrocytes and MPL activation. We showed that TPO or a TPO receptor agonist directly induces fibrocyte differentiation using murine fibrocyte cell lines and a murine MF model. Conversely, elimination of macrophages expressing MPL by clodronate liposomes reversed the MF phenotype of the murine model, suggesting that fibrocyte differentiation induced by MPL activation contributes to the progression of MF. Furthermore, we revealed that SLAMF7^high MPL^high monocytes in human peripheral blood mononuclear cells were possible fibrocyte precursors and that these cells increased in number in MF patients not treated with ruxolitinib. Our findings confirmed a link between fibrocytes and the TPO/MPL signaling pathway, which could result in a greater understanding of the pathogenesis of MF and lead to the development of novel therapeutic interventions.

Emerging treatments for classical myeloproliferative neoplasms

There has been a major revolution in the management of patients with myeloproliferative neoplasms (MPN), and in particular those with myelofibrosis and extensive splenomegaly and symptomatic burden, after the introduction of the JAK1 and JAK2 inhibitor ruxolitinib. The drug also has been approved as second-line therapy for polycythemia vera (PV). However, the therapeutic armamentarium for MPN is still largely inadequate for coping with patients' major unmet needs, which include normalization of life span (myelofibrosis and some patients with PV), reduction of cardiovascular complications (mainly PV and essential thrombocythemia), prevention of hematological progression, and improved quality of life (all MPN). In fact, none of the available drugs has shown clear evidence of disease-modifying activity, even if some patients treated with interferon and ruxolitinib showed reduction of mutated allele burden, and ruxolitinib might extend survival of patients with higher-risk myelofibrosis. Raised awareness of the molecular abnormalities and cellular pathways involved in the pathogenesis of MPN is facilitating the development of clinical trials with novel target drugs, either alone or in combination with ruxolitinib. Although for most of these molecules a convincing preclinical rationale was provided, the results of early phase 1 and 2 clinical trials have been quite disappointing to date, and toxicities sometimes have been limiting. In this review, we critically illustrate the current landscape of novel therapies that are under evaluation for patients with MPN on the basis of current guidelines, patient risk stratification criteria, and previous experience, looking ahead to the chance of a cure for these disorders.

Preclinical rationale for TGF-β inhibition as a therapeutic target for the treatment of myelofibrosis

To assess the role of abnormal transforming growth factor-beta (TGF-β) signaling in the pathogenesis of primary myelofibrosis (PMF), the effects of the TGF-β receptor-1 kinase inhibitor SB431542 on ex vivo expansion of hematopoietic cells in cultures from patients with JAK2V617+-polycythemia vera (PV) or PMF (JAK2V617F+, CALRpQ365f+, or unknown) and from normal sources (adult blood, AB, or cord blood, CB) were compared. In cultures of normal sources, SB431542 significantly increased by 2.5-fold the number of progenitor cells generated by days 1-2 (CD34+) and 6 (colony-forming cells) (CB) and that of precursor cells, mostly immature erythroblasts, by days 14-17 (AB and CB). In cultures of JAK2V617F+-PV, SB431542 increased by twofold the numbers of progenitor cells by day 10 and had no effect on that of precursors cells by days 12-17 (∼fourfold increase in all cases). In contrast, SB431542 had no effect on the number of either progenitor or precursor cells in cultures of JAK2V617F+ and CALR pQ365fs+ PMF. These ontogenetic- and disease-specific effects were associated with variegation in the ability of SB431542 to induce CD34+ cells from AB (increased), CB (decreased), or PV and PMF (unaffected) into cycle and erythroblasts in proliferation (increased for AB and PV and unaffected for CB and PMF). Differences in expansion of erythroblasts from AB, CB, and PV were associated with differences in activation of TGF-β signaling (SHCY317, SMAD2S245/250/255, and SMAD1S/S/SMAD5S/S/SMAD8S/S) detectable in these cells by phosphoproteomic profiling. In conclusion, treatment with TGF-β receptor-1 kinase inhibitors may reactivate normal hematopoiesis in PMF patients, providing a proliferative advantage over the unresponsive malignant clone.

A phase II study of panobinostat in patients with primary myelofibrosis (PMF) and post-polycythemia vera/essential thrombocythemia myelofibrosis (post-PV/ET MF)

Myelofibrosis is a chronic and progressive myeloproliferative neoplasm characterized by anemia, splenomegaly, debilitating symptoms and leukemic transformation. Ruxolitinib, an oral JAK1/2 inhibitor, is highly effective in ameliorating systemic symptoms and reducing splenomegaly. Current clinical research is focused on the evaluation of agents based on pre-clinical rationale that can result in disease course modification. Panobinostat is a pan-histone deacetylase inhibitor that has demonstrated clinical activity as a single agent in early phase trials of myelofibrosis. We previously conducted a phase I trial of panobinostat monotherapy in patients with myelofibrosis and determined 25mg thrice weekly as the recommended phase II dose. We then completed an investigator initiated, Simon 2-stage, phase II trial of 22 myelofibrosis patients at our single institution. After 6 cycles of therapy, the overall response rate by IWG-MRT criteria was 36% (8/22; 95% CI: 16-56%). The median percent reduction in spleen volume was 34% (range, 1.6%-73%) in eight evaluable patients. The average reduction in JAK2V617F allele burden was 6.8% (Range; -4.0% to 20.2%) and one patient obtained a complete molecular response. Six patients remained on therapy in the extension phase for a median of 18 months (range, 7-44). Treatment discontinuation was frequent due to patient/physician perception of therapy ineffectiveness. The optimal dosing of panobinostat for the treatment of MF remains somewhat ill-defined but appears to be most effective and better tolerated when administered at lower doses over a prolonged duration of therapy.

Bone marrow fibrosis in myelofibrosis: pathogenesis, prognosis and targeted strategies

Bone marrow fibrosis is a central pathological feature and World Health Organization major diagnostic criterion of myelofibrosis. Although bone marrow fibrosis is seen in a variety of malignant and non-malignant disease states, the deposition of reticulin and collagen fibrosis in the bone marrow of patients with myelofibrosis is believed to be mediated by the myelofibrosis hematopoietic stem/progenitor cell, contributing to an impaired microenvironment favoring malignant over normal hematopoiesis. Increased expression of inflammatory cytokines, lysyl oxidase, transforming growth factor-β, impaired megakaryocyte function, and aberrant JAK-STAT signaling have all been implicated in the pathogenesis of bone marrow fibrosis. A number of studies indicate that bone marrow fibrosis is an adverse prognostic variable in myeloproliferative neoplasms. However, modern myelofibrosis prognostication systems utilized in risk-adapted treatment approaches do not include bone marrow fibrosis as a prognostic variable. The specific effect on bone marrow fibrosis of JAK2 inhibition, and other rationally based therapies currently being evaluated in myelofibrosis, has yet to be fully elucidated. Hematopoietic stem cell transplantation remains the only curative therapeutic approach that reliably results in resolution of bone marrow fibrosis in patients with myelofibrosis. Here we review the pathogenesis, biological consequences, and prognostic impact of bone marrow fibrosis. We discuss the rationale of various anti-fibrogenic treatment strategies targeting the clonal hematopoietic stem/progenitor cell, aberrant signaling pathways, fibrogenic cytokines, and the tumor microenvironment.

New Strategies in Myeloproliferative Neoplasms: The Evolving Genetic and Therapeutic Landscape

The classical BCR-ABL1-negative myeloproliferative neoplasms (MPN) include essential thrombocythemia (ET), polycythemia vera (PV), and myelofibrosis (MF). Although these clonal disorders share certain clinical and genetic features, MF in particular is distinct for its complex mutational landscape, severe disease phenotype, and poor prognosis. The genetic complexity inherent to MF has made this disease extremely challenging to treat. Pharmacologic JAK inhibition has proven to be a transformative therapy in MPNs, alleviating symptom burden and improving survival, but has been hampered by off-target toxicities and, as monotherapy, has shown limited effects on mutant allele burden. In this review, we discuss the genetic heterogeneity contributing to the pathogenesis of MPNs, focusing on novel driver and epigenetic mutations and how they relate to combination therapeutic strategies. We discuss results from ongoing studies of new JAK inhibitors and report on new drugs and drug combinations that have demonstrated success in early preclinical and clinical trials, including type II JAK inhibitors, antifibrotic agents, and telomerase inhibitors.

Bone marrow fibrosis in primary myelofibrosis: pathogenic mechanisms and the role of TGF-β

Primary myelofibrosis (PMF) is a Philadelphia chromosome negative myeloproliferative neoplasm (MPN) with adverse prognosis and is associated with bone marrow fibrosis and extramedullary hematopoiesis. Even though the discovery of the Janus kinase 2 (JAK2), thrombopoietin receptor (MPL) and calreticulin (CALR) mutations have brought new insights into the complex pathogenesis of MPNs, the etiology of fibrosis is not well understood. Furthermore, since JAK2 inhibitors do not lead to reversal of fibrosis further understanding of the biology of fibrotic process is needed for future therapeutic discovery. Transforming growth factor beta (TGF-β) is implicated as an important cytokine in pathogenesis of bone marrow fibrosis. Various mouse models have been developed and have established the role of TGF-β in the pathogenesis of fibrosis. Understanding the molecular alterations that lead to TGF-β mediated effects on bone marrow microenvironment can uncover newer therapeutic targets against myelofibrosis. Inhibition of the TGF-β pathway in conjunction with other therapies might prove useful in the reversal of bone marrow fibrosis in PMF.

P-Selectin Sustains Extramedullary Hematopoiesis in the Gata1 low Model of Myelofibrosis

Splenomegaly is a major manifestation of primary myelofibrosis (PMF) contributing to clinical symptoms and hematologic abnormalities. The spleen from PMF patients contains increased numbers of hematopoietic stem cells (HSC) and megakaryocytes (MK). These MK express high levels of P-selectin (P-sel) that, by triggering neutrophil emperipolesis, may cause TGF-β release and disease progression. This hypothesis was tested by deleting the P-sel gene in the myelofibrosis mouse model carrying the hypomorphic Gata1(low) mutation that induces megakaryocyte abnormalities that recapitulate those observed in PMF. P-sel(null) Gata1(low) mice survived splenectomy and lived 3 months longer than P-sel(WT) Gata1(low) littermates and expressed limited fibrosis and osteosclerosis in the marrow or splenomegaly. Furthermore, deletion of P-sel disrupted megakaryocyte/neutrophil interactions in spleen, reduced TGF-β content, and corrected the HSC distribution that in Gata1(low) mice, as in PMF patients, is abnormally expanded in spleen. Conversely, pharmacological inhibition of TGF-β reduced P-sel expression in MK and corrected HSC distribution. Spleens, but not marrow, of Gata1(low) mice contained numerous cKIT(pos) activated fibrocytes, probably of dendritic cell origin, whose membrane protrusions interacted with MK establishing niches hosting immature cKIT(pos) hematopoietic cells. These activated fibrocytes were not detected in spleens from P-sel(null) Gata1(low) or TGF-β-inhibited Gata1(low) littermates and were observed in spleen, but not in marrow, from PMF patients. Therefore, in Gata1(low) mice, and possibly in PMF, abnormal P-sel expression in MK may mediate the pathological cell interactions that increase TGF-β content in MK and favor establishment of a microenvironment that supports myelofibrosis-related HSC in spleen.

Janus Kinase Inhibitors and Stem Cell Transplantation in Myelofibrosis

Myelofibrosis (MF) is characterized by splenomegaly, blood count abnormalities, particularly cytopenias, and a propensity for transformation to acute leukemia. The current treatment approach is to ameliorate symptoms due to these abnormalities. Treatment with Janus kinase 2 inhibitors reduces spleen size and improves symptoms in patients with MF, but most of the patients eventually have disease progression and stop responding. Allogeneic stem cell transplantation remains the only curative option. However, its efficacy must be balanced against the risk of treatment-related death and long-term sequelae of transplant like chronic graft versus host disease. The challenge is to integrate treatment with Janus kinase inhibitors with allogeneic stem cell transplantation.

Activation of non-canonical TGF-β1 signaling indicates an autoimmune mechanism for bone marrow fibrosis in primary myelofibrosis

Primary myelofibrosis (PMF) is characterized by megakaryocyte hyperplasia, dysplasia and death with progressive reticulin/collagen fibrosis in marrow and hematopoiesis in extramedullary sites. The mechanism of fibrosis was investigated by comparing TGF-β1 signaling of marrow and spleen of patients with PMF and of non-diseased individuals. Expression of 39 (23 up-regulated and 16 down-regulated) and 38 (8 up-regulated and 30 down-regulated) TGF-β1 signaling genes was altered in the marrow and spleen of PMF patients, respectively. Abnormalities included genes of TGF-β1 signaling, cell cycling and abnormal in chronic myeloid leukemia (EVI1 and p21(CIP)) (both marrow and spleen) and Hedgehog (marrow only) and p53 (spleen only) signaling. Pathway analyses of these alterations predict an increased osteoblast differentiation, ineffective hematopoiesis and fibrosis driven by non-canonical TGF-β1 signaling in marrow and increased proliferation and defective DNA repair in spleen. Since activation of non-canonical TGF-β1 signaling is associated with fibrosis in autoimmune diseases, the hypothesis that fibrosis in PMF results from an autoimmune process triggered by dead megakaryocytes was tested by determining that PMF patients expressed plasma levels of mitochondrial DNA and anti-mitochondrial antibodies greater than normal controls. These data identify autoimmunity as a possible cause of marrow fibrosis in PMF.

How many JAK inhibitors in myelofibrosis?

The discovery of the activating mutation JAK2 V617F ushered a new era in MPN which included new diagnostic and prognostic criteria as well as a potential therapeutic target. JAK2 inhibition became a reality with first patients receiving drugs that targeted JAK2 in 2007 and was marked by the first approval in 2011 of Ruxolitinib a JAK 1 and 2-inhibitor to treat myelofibrosis (MF). In this article entitled "How many JAK inhibitors for myelofibrosis" we discuss JAK2 as a target, review briefly the benefits to patients with MF of JAK inhibition and highlight some of the differences between the number of JAK inhibitors currently being evaluated. Reflecting upon what we have learnt from the chronic myeloid leukaemia field and for MF regarding disease complexity as well as individual patient factors including resistance we discuss why it is likely we will need several different agents with JAK inhibitory activity. The next chapter discusses combination therapies for myelofibrosis which is a logical step in both trying to cure this disease and improve patient outcome and toxicities with JAK inhibitors.