Phase II trial of panobinostat, an oral pan-deacetylase inhibitor in patients with primary myelofibrosis, post-essential thrombocythaemia, and post-polycythaemia vera myelofibrosis

Biological and structural investigation of tetrahydro-β-carboline-based selective HDAC6 inhibitors with improved stability

Our previously reported HDAC6 inhibitor (HDAC6i) Marbostat-100 (4) has provided many arguments for further clinical evaluation. By the substitution of the acidic hydrogen of 4 for different carbon residues, we were able to generate an all-carbon stereocenter, which significantly improves the hydrolytic stability of the inhibitor. Further asymmetric synthesis has shown that the (S)-configured inhibitors preferentially bind to HDAC6. This led to the highly selective and potent methyl-substituted derivative S-29b, which elicited a long-lasting tubulin hyperacetylation in MV4-11 cells. Finally, a crystal structure of the HDAC6/S-29b complex provided mechanistic explanation for the high potency and stereoselectivity of synthesized compound series.

Design, synthesis and mechanistic study of new dual targeting HDAC/tubulin inhibitors

Aim: The purpose of this work is to create and synthesize a new class of chemicals: 3-cyano-2-substituted pyridine compounds with expected multitarget inhibition of histone deacetylase (HDAC) and tubulin. Materials & methods: The target compounds (3a-c, 4a-c and 5a-c) were synthesized utilizing 6-(4-methoxyphenyl)-2-oxo-4-(3,4,5-trimethoxyphenyl)-3-cyanopyridine, with various linkers and zinc-binding groups (ZBGs). Results: Most of the tested compounds showed promising growth inhibition, and hydroxamic acid-containing hybrids possessed higher HDAC inhibition than other ZBGs. Compound 4b possessed the highest potency; however, it showed the most tubulin polymerization inhibition. Docking studies displayed good binding into HDAC1 and six pockets and tubulin polymerization protein. Conclusion: Compound 4b could be considered a good antitumor candidate to go further into in vivo and clinical studies.

Integrated molecular modeling and dynamics approaches revealed the mechanism of selective inhibition of HDAC6/8

The high structural homology of histone deacetylases 6 and 8 (HDAC6/8) poses a challenge in achieving isoform selectivity and has resulted in adverse side effects due to pan-inhibition in clinical applications. Additionally, the rational design of dual-target inhibitors, centered on HDAC6/8, demands a profound understanding of their selectivity mechanisms. Addressing the urgent need for enhanced specificity in the development of inhibitors targeting specific isoforms, we elucidate the mechanism underpinning the selective inhibition of HDAC6/8 inhibitors through in-silico strategies. The hydrogen bonding interaction with Asp101 and Tyr306 is a key factor that enables compound 12b to selectively inhibit HDAC8. Its favorable spatial orientation places the Cap group of 12b between Tyr306 and Tyr100, resulting in an overall L-shaped conformation. These two factors significantly contribute to the selective inhibitory activity of 12b against HDAC8. The zinc binding group (ZBG) of compound NN-390 forms a hydrogen bond with His610, a key residue of HDAC6, facilitating stable chelation with zinc ions. In addition, the Cap group of NN-390 interacts with Phe620 and Phe680 via van der Waals forces, leading to an overall Y-shaped conformation. The aforementioned factors are the main reasons for the selective inhibition of HDAC6 by NN-390. Furthermore, whether the Cap group is in the para or meta-position will influence the selective inhibition of either HDAC6 or HDAC8. We believe these clues can offer valuable insights for the rational design of selective inhibitors targeting HDAC6/8 and pave the way for rational design of dual-target HDAC6/8-based inhibitors.Communicated by Ramaswamy H. Sarma.

Pharmacological Modulation of the Crosstalk between Aberrant Janus Kinase Signaling and Epigenetic Modifiers of the Histone Deacetylase Family to Treat Cancer

Hyperactivated Janus kinase (JAK) signaling is an appreciated drug target in human cancers. Numerous mutant JAK molecules as well as inherent and acquired drug resistance mechanisms limit the efficacy of JAK inhibitors (JAKi). There is accumulating evidence that epigenetic mechanisms control JAK-dependent signaling cascades. Like JAKs, epigenetic modifiers of the histone deacetylase (HDAC) family regulate the growth and development of cells and are often dysregulated in cancer cells. The notion that inhibitors of histone deacetylases (HDACi) abrogate oncogenic JAK-dependent signaling cascades illustrates an intricate crosstalk between JAKs and HDACs. Here, we summarize how structurally divergent, broad-acting as well as isoenzyme-specific HDACi, hybrid fusion pharmacophores containing JAKi and HDACi, and proteolysis targeting chimeras for JAKs inactivate the four JAK proteins JAK1, JAK2, JAK3, and tyrosine kinase-2. These agents suppress aberrant JAK activity through specific transcription-dependent processes and mechanisms that alter the phosphorylation and stability of JAKs. Pharmacological inhibition of HDACs abrogates allosteric activation of JAKs, overcomes limitations of ATP-competitive type 1 and type 2 JAKi, and interacts favorably with JAKi. Since such findings were collected in cultured cells, experimental animals, and cancer patients, we condense preclinical and translational relevance. We also discuss how future research on acetylation-dependent mechanisms that regulate JAKs might allow the rational design of improved treatments for cancer patients. SIGNIFICANCE STATEMENT: Reversible lysine-ɛ-N acetylation and deacetylation cycles control phosphorylation-dependent Janus kinase-signal transducer and activator of transcription signaling. The intricate crosstalk between these fundamental molecular mechanisms provides opportunities for pharmacological intervention strategies with modern small molecule inhibitors. This could help patients suffering from cancer.

Janus Kinases in Leukemia

Janus kinases (JAKs) transduce signals from dozens of extracellular cytokines and function as critical regulators of cell growth, differentiation, gene expression, and immune responses. Deregulation of JAK/STAT signaling is a central component in several human diseases including various types of leukemia and other malignancies and autoimmune diseases. Different types of leukemia harbor genomic aberrations in all four JAKs (JAK1, JAK2, JAK3, and TYK2), most of which are activating somatic mutations and less frequently translocations resulting in constitutively active JAK fusion proteins. JAKs have become important therapeutic targets and currently, six JAK inhibitors have been approved by the FDA for the treatment of both autoimmune diseases and hematological malignancies. However, the efficacy of the current drugs is not optimal and the full potential of JAK modulators in leukemia is yet to be harnessed. This review discusses the deregulation of JAK-STAT signaling that underlie the pathogenesis of leukemia, i.e., mutations and other mechanisms causing hyperactive cytokine signaling, as well as JAK inhibitors used in clinic and under clinical development.

A Review of Progress in Histone Deacetylase 6 Inhibitors Research: Structural Specificity and Functional Diversity

Histone deacetylases (HDACs) are essential for maintaining homeostasis by catalyzing histone deacetylation. Aberrant expression of HDACs is associated with various human diseases. Although HDAC inhibitors are used as effective chemotherapeutic agents in clinical practice, their applications remain limited due to associated side effects induced by weak isoform selectivity. HDAC6 displays unique structure and cellular localization as well as diverse substrates and exhibits a wider range of biological functions than other isoforms. HDAC6 inhibitors have been effectively used to treat cancers, neurodegenerative diseases, and autoimmune disorders without exerting significant toxic effects. Progress has been made in defining the crystal structures of HDAC6 catalytic domains which has influenced the structure-based drug design of HDAC6 inhibitors. This review summarizes recent literature on HDAC6 inhibitors with particular reference to structural specificity and functional diversity. It may provide up-to-date guidance for the development of HDAC6 inhibitors and perspectives for optimization of therapeutic applications.

Targeting Abnormal Hematopoietic Stem Cells in Chronic Myeloid Leukemia and Philadelphia Chromosome-Negative Classical Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are unique hematopoietic stem cell disorders sharing mutations that constitutively activate the signal-transduction pathways involved in haematopoiesis. They are characterized by stem cell-derived clonal myeloproliferation. The key MPNs comprise chronic myeloid leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). CML is defined by the presence of the Philadelphia (Ph) chromosome and BCR-ABL1 fusion gene. Despite effective cytoreductive agents and targeted therapy, complete CML/MPN stem cell eradication is rarely achieved. In this review article, we discuss the novel agents and combination therapy that can potentially abnormal hematopoietic stem cells in CML and MPNs and the CML/MPN stem cell-sustaining bone marrow microenvironment.

Metabolic Vulnerabilities and Epigenetic Dysregulation in Myeloproliferative Neoplasms

The Janus kinase 2 (JAK2)-driven myeloproliferative neoplasms (MPNs) are associated with clonal myelopoiesis, elevated risk of death due to thrombotic complications, and transformation to acute myeloid leukemia (AML). JAK2 inhibitors improve the quality of life for MPN patients, but these approved therapeutics do not readily reduce the natural course of disease or antagonize the neoplastic clone. An understanding of the molecular and cellular changes requisite for MPN development and progression are needed to develop improved therapies. Recently, murine MPN models were demonstrated to exhibit metabolic vulnerabilities due to a high dependence on glucose. Neoplastic hematopoietic progenitor cells in these mice express elevated levels of glycolytic enzymes and exhibit enhanced levels of glycolysis and oxidative phosphorylation, and the disease phenotype of these MPN model mice is antagonized by glycolytic inhibition. While all MPN-driving mutations lead to aberrant JAK2 activation, these mutations often co-exist with mutations in genes that encode epigenetic regulators, including loss of function mutations known to enhance MPN progression. In this perspective we discuss how altered activity of epigenetic regulators (e.g., methylation and acetylation) in MPN-driving stem and progenitor cells may alter cellular metabolism and contribute to the MPN phenotype and progression of disease. Specific metabolic changes associated with epigenetic deregulation may identify patient populations that exhibit specific metabolic vulnerabilities that are absent in normal hematopoietic cells, and thus provide a potential basis for the development of more effective personalized therapeutic approaches.

Bifunctional HDAC Therapeutics: One Drug to Rule Them All?

Histone deacetylase (HDAC) enzymes play crucial roles in epigenetic gene expression and are an attractive therapeutic target. Five HDAC inhibitors have been approved for cancer treatment to date, however, clinical applications have been limited due to poor single-agent drug efficacy and side effects associated with a lack of HDAC isoform or complex selectivity. An emerging strategy aiming to address these limitations is the development of bifunctional HDAC therapeutics-single molecules comprising a HDAC inhibitor conjugated to another specificity targeting moiety. This review summarises the recent advancements in novel types of dual-targeting HDAC modulators, including proteolysis-targeting chimeras (PROTACs), with a focus on HDAC isoform and complex selectivity, and the future potential of such bifunctional molecules in achieving enhanced drug efficacy and therapeutic benefits in treating disease.

HDAC11 deficiency disrupts oncogene-induced hematopoiesis in myeloproliferative neoplasms

Protein acetylation is an important contributor to cancer initiation. Histone deacetylase 6 (HDAC6) controls JAK2 translation and protein stability and has been implicated in JAK2-driven diseases best exemplified by myeloproliferative neoplasms (MPNs). By using novel classes of highly selective HDAC inhibitors and genetically deficient mouse models, we discovered that HDAC11 rather than HDAC6 is necessary for the proliferation and survival of oncogenic JAK2-driven MPN cells and patient samples. Notably, HDAC11 is variably expressed in primitive stem cells and is expressed largely upon lineage commitment. Although Hdac11is dispensable for normal homeostatic hematopoietic stem and progenitor cell differentiation based on chimeric bone marrow reconstitution, Hdac11 deficiency significantly reduced the abnormal megakaryocyte population, improved splenic architecture, reduced fibrosis, and increased survival in the MPLW515L-MPN mouse model during primary and secondary transplantation. Therefore, inhibitors of HDAC11 are an attractive therapy for treating patients with MPN. Although JAK2 inhibitor therapy provides substantial clinical benefit in MPN patients, the identification of alternative therapeutic targets is needed to reverse MPN pathogenesis and control malignant hematopoiesis. This study establishes HDAC11 as a unique type of target molecule that has therapeutic potential in MPN.

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

Ruxolitinib, a selective JAK1/JAK2 inhibitor, is the current first line therapy for myelofibrosis (MF), which reduces symptomatology and splenomegaly, but does not clearly modify disease course. Panobinostat, a histone deacetylase inhibitor, was shown to be safe and tolerable in phase I and II trials and demonstrated clinical activity in approximately a third of treated patients. Combination therapy of ruxolitinib and panobinostat showed synergistic activity in a preclinical MF model, which prompted clinical evaluation of this combination in both ruxolitinib naïve and treated MF patients. Herein, we report the results of an investigator-initiated, dose escalation, phase I trial of ruxolitinib and panobinostat in 15 patients with primary MF and post-polycythemia vera/essential thrombocythemia MF. This combination treatment proved to be safe and tolerable without dose limiting thrombocytopenia and a maximum tolerated dose of both agents in combination was not determined. The majority of patients maintained stable disease with this combination treatment and 40 % attained a clinical improvement (spleen n = 5, anemia n = 1) by modified IWG-MRT at the end of 6 cycles. This is one of the first attempts of rationally designed, JAK inhibitor-based, combination therapy studies and exemplifies the feasibility of such an approach in patients with advanced MF.

A phase 2 study of pracinostat combined with ruxolitinib in patients with myelofibrosis

Although ruxolitinib improves symptoms and splenomegaly in patients with advanced myelofibrosis, whether this agent is truly disease-modifying remains unclear. Histone deacetylase inhibitors (HDACi) downregulate JAK2 via interference with chaperone function. Pracinostat, a pan-HDACi, has modest single-agent activity in myelofibrosis. We conducted a single-institution, phase 2, investigator-initiated trial of ruxolitinib plus pracinostat (begun after 12 weeks of ruxolitinib) in 25 patients with myelofibrosis, of whom 20 received both agents. Sixteen (80%) patients had objective responses (all 'clinical improvement'). The rate of spleen response (by palpation) was 74%, and that of symptom response 80%. Most responses occurred prior to pracinostat initiation. Three patients experienced improvement in bone marrow fibrosis, and one a near-complete molecular response after two years on study treatment. All patients discontinued pracinostat and are currently off-study. Pracinostat interruptions and dose reductions were frequent, often due to worsening anemia. These findings do not support continued development of pracinostat in myelofibrosis.

SOHO State of the Art Updates and Next Questions: Myelofibrosis

The discovery of a mutation in the Janus Kinase 2 gene in 2005 spurred significant progress in the field of myeloproliferative neoplasms. A comprehensive description of genomic factors at play in the malignant clone in myeloproliferative neoplasms, particularly myelofibrosis (MF), have recently led to more precise, personalized prognostic tools. Despite this, understanding of the disease pathogenesis remains relatively limited. We continue to lack a detailed description of the interaction between the hematopoietic stem cell clone, abnormal bone marrow niche cells, and circulating signaling molecules and an understanding of how they cooperate to promote cell proliferation, fibrogenesis, and extramedullary hematopoiesis. Despite our knowledge gaps, recent research in MF has led to promising clinical translation. In this article, we summarize recent insights into MF pathophysiology, progress in the development of novel therapeutics, and opportunities for further advancement of the field.

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.

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.

Developmental Therapeutics in Myeloproliferative Neoplasms

The unprecedented success of the Janus kinase (JAK) 1/2 inhibitor ruxolitinib in myelofibrosis (MF) provided much-needed impetus for clinical drug development for the Philadelphia chromosome-negative myeloproliferative neoplasms. The survival benefit conferred by this agent, along with its marked efficacy with regard to spleen volume and symptom reduction, have made ruxolitinib the cornerstone of drug therapy in MF. However, there remain significant unmet needs in the treatment of patients with MF, and many novel classes of agents continue to be investigated in efforts to build on the progress made with ruxolitinib. These include inhibitors of histone deacetylases (HDACs) and DNA methyltransferases, phosphatidylinositol-3-kinase isoforms, heat shock protein 90, cyclin-dependent kinases 4/6, and Hedgehog signaling, among others. In parallel, other JAK inhibitors with potential for less myelosuppression or even improvement of anemia, greater selectivity for JAK1 or JAK2, and the ability to overcome JAK inhibitor persistence are in various stages of development. First-in-class agents such as the activin receptor IIA ligand trap sotatercept (for anemia of MF), the telomerase inhibitor imetelstat, and the antifibrotic agent PRM-151 (recombinant human pentraxin-2) are also in clinical trials. In polycythemia vera, a novel interferon administered every 2 weeks is being developed for front-line therapy in high-risk individuals, and inhibitors of human double minute 2 (HDM2) have shown promise in preclinical studies, as have HDAC inhibitors such as givinostat (both in the laboratory and in the clinic). Ruxolitinib is approved for second-line therapy of polycythemia vera and is being developed for essential thrombocythemia.

Management of myelofibrosis: JAK inhibition and beyond

INTRODUCTION

Myelofibrosis (MF) is characterized by bone marrow fibrosis with subsequent extramedullary hematopoiesis and abnormal cytokine expression leading to splenomegaly, constitutional symptoms and cytopenias. The discovery of the JAK2 V617F mutation in the majority of MF patients has been followed by significant progress in drug development for MF. Areas covered: In this article, we review advances in the understanding of the underlying disease biology, prognostic assessment and therapeutic modalities for MF. We provide clinical trial evidence behind using the JAK2 inhibitor ruxolitinib, erythropoiesis stimulating agents, androgens, immunomodulatory drugs, interferon, cytoreductive drugs and hypomethylating agents in MF. Finally, we review novel therapeutic options for MF including the new JAK1/2 inhibitors, ruxolitinib based combination approaches as well as novel therapeutic agents. Expert commentary: Despite significant reduction of splenomegaly and improvement of symptom burden and a signal for survival improvement, ruxolitinib does not lead to major reductions in JAK2 V617F allele burden and bone marrow fibrosis. No ruxolitinib-based combination approach has so far demonstrated superiority over ruxolitinib monotherapy. The novel JAK2 inhibitors pacritinib and momelotinib, other JAK inhibitors, telomerase inhibitors, anti-fibrosis agents and hsp90 inhibitors are in different stages of development.

Myelofibrosis: an update on drug therapy in 2016

INTRODUCTION

Primary myelofibrosis (PMF) is the least common but the most aggressive of the classic Philadelphia chromosome-negative myeloproliferative neoplasms. Survival is much shorter in PMF than in polycythemia vera (PV) or essential thrombocythemia (ET). Post-PV/ET myelofibrosis (MF) is clinically indistinguishable from PMF and approached similarly. Areas covered: Current pharmacologic therapy of MF revolves around the Janus kinase 1/2 (JAK1/2) inhibitor ruxolitinib, which dramatically improves constitutional symptoms and splenomegaly in the majority of patients, and improves overall survival (OS). However, allogeneic stem cell transplantation remains the only potential cure. Other JAK inhibitors continue to be developed for MF, and momelotinib and pacritinib are in phase III clinical trials. Anemia is common in MF, and initially worsened by ruxolitinib. Momelotinib and pacritinib may prove advantageous in this regard. Current strategies for managing anemia of MF include danazol, immunomodulatory drugs and erythroid stimulating agents, either alone or in combination with ruxolitinib. Expert opinion: A number of other agents, representing diverse drug classes, are in various stages of development for MF. These include newer JAK inhibitors, other signaling inhibitors, epigenetic modifiers, anti-fibrotic agents, telomerase inhibitors, and activin receptor ligand traps (for anemia). Hopefully, these novel therapies will further extend the clinical benefits of ruxolitinib.

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.

Treatment of Myelofibrosis: A Moving Target

Myelofibrosis (MF) is a myeloproliferative neoplasm that presents either as a primary disease or evolves secondarily from polycythemia vera or essential thrombocythemia to post-polycythemia vera MF or post-essential thrombocythemia MF, respectively. Myelofibrosis is characterized by stem cell-derived clonal myeloproliferation, abnormal cytokine expression, bone marrow fibrosis, anemia, splenomegaly, extramedullary hematopoiesis, constitutional symptoms, cachexia, leukemic progression, and shortened survival. Therapeutic options for patients with MF have been limited to the use of cytoreductive agents, predominantly hydroxyurea; splenectomy and splenic irradiation for treatment of splenomegaly; and management of anemia with transfusions, erythropoiesis-stimulating agents, androgens, and immunomodulatory agents along with steroids. The only curative option is allogeneic stem cell transplantation (ASCT), which is associated with high morbidity and mortality risks. Recently, JAK (Janus kinase) inhibitor therapies have become available and proven to be palliative in primary MF patients with hydroxyurea-refractory splenomegaly and severe constitutional symptoms. The purpose of this article is to review the clinical features of MF; discuss different treatment strategies, including ASCT; and discuss the potential danger and benefit of using JAK inhibitors prior to ASCT.

Investigational histone deacetylase inhibitors (HDACi) in myeloproliferative neoplasms

INTRODUCTION

The Philadelphia chromosome negative myeloproliferative neoplasms (MPN) mainly comprise polycythemia vera (PV), essential thrombocythemia (ET) and myelofibrosis (MF, primary or post-PV/ET). Therapy in PV and ET focuses on minimizing thrombosis and bleeding risk, while in MF, prolongation of survival is an important goal. Different cytoreductive agents are employed in high risk PV and ET, while the JAK inhibtior ruxolitinib is the cornerstone of therapy in MF. Histone deacetylase inhibitors (HDACi) are pleiotropic agents with diverse epigenetic and non-epigenetic actions, selectively in transformed cells. A number of HDACi have been or are being investigated in MPN. Areas covered: The mechanisms of action of HDACI in neoplastic cells are summarized, and the preclinical rationale and data supporting their development in MPN specifically examined, particularly their synergism with JAK inhibitors. Major findings of clinical trials of HDACi, both alone and in combination with ruxolitinib, in MPN are then discussed, with particular attention to their toxicities and disease-modifying effects. Expert opinion: HDACi are clearly active in MPN, and there is good preclinical rationale for this. Their combination with ruxolitinib in MF is promising, but the long-term tolerability of these agents is an important concern. Further development in PV or ET appears unlikely.

Myeloproliferative Neoplasms: Molecular Drivers and Therapeutics

Activating mutations in genes that drive neoplastic cell growth are numerous and widespread in cancer, and specific genetic alterations are associated with certain types of cancer. For example, classic myeloproliferative neoplasms (MPNs) are hematopoietic stem cell disorders that affect cells of the myeloid lineage, including erythrocytes, platelets, and granulocytes. An activating mutation in the JAK2 tyrosine kinase is prevalent in these diseases. In MPN patients that lack such a mutation, other genetic changes that lead to activation of the JAK2 signaling pathway are present, indicating deregulation of JAK2 signaling plays an etiological driving role in MPNs, a concept supported by significant evidence from in vivo experimental MPN systems. Thus, small molecules that inhibit JAK2 activity are ideal drugs to impede the progression of disease in MPN patients. However, even though JAK inhibitors provide significant symptomatic relief, they have failed as a remission-inducing therapy. Nonetheless, the progress made understanding the molecular etiology of MPNs since 2005 is significant and has provided insight for the development and testing of novel molecular targeted therapeutic approaches. The current understanding of driver mutations in MPNs and an overview of current and potential therapeutic strategies for MPN patients will be discussed.

Role of tyrosine-kinase inhibitors in myeloproliferative neoplasms: comparative lessons learned

An important pathogenetic distinction in the classification of myeloproliferative neoplasms (MPNs) is the presence or absence of the BCR–ABL fusion gene, which encodes a unique oncogenic tyrosine kinase. The BCR–ABL fusion, caused by the formation of the Philadelphia chromosome (Ph) through translocation, constitutes the disease-initiating event in chronic myeloid leukemia. The development of successive BCR–ABL-targeted tyrosine-kinase inhibitors has led to greatly improved outcomes in patients with chronic myeloid leukemia, including high rates of complete hematologic, cytogenetic, and molecular responses. Such levels of treatment success have long been elusive for patients with Ph-negative MPNs, because of the difficulties in identifying specific driver proteins suitable as drug targets. However, in recent years an improved understanding of the complex pathobiology of classic Ph-negative MPNs, characterized by variable, overlapping multimutation profiles, has prompted the development of better and more broadly targeted (to pathway rather than protein) treatment options, particularly JAK inhibitors. In classic Ph-negative MPNs, overactivation of JAK-dependent signaling pathways is a central pathogenic mechanism, and mutually exclusive mutations in JAK2, MPL, and CALR linked to aberrant JAK activation are now recognized as key drivers of disease progression in myelofibrosis (MF). In clinical trials, the JAK1/JAK2 inhibitor ruxolitinib – the first therapy approved for MF worldwide – improved disease-related splenomegaly and symptoms independent of JAK2^V617F mutational status, and prolonged survival compared with placebo or standard therapy in patients with advanced MF. In separate trials, ruxolitinib also provided comprehensive hematologic control in patients with another Ph-negative MPN – polycythemia vera. However, complete cytogenetic or molecular responses with JAK inhibitors alone are normally not observed, underscoring the need for novel combination therapies of JAK inhibitors and complementary agents that better address the complexity of the pathobiology of classic Ph-negative MPNs. Here, we discuss the role of tyrosine-kinase inhibitors in the current MPN-treatment landscape.

A Phase II Trial of a Histone Deacetylase Inhibitor Panobinostat in Patients With Low-Grade Neuroendocrine Tumors

Lessons Learned

Background.

The most common subtypes of neuroendocrine tumors (NETs) are pancreatic islet cell tumors and carcinoids, which represent only 2% of all gastrointestinal malignancies. Histone deacetylase (HDAC) inhibitors have already been shown to suppress tumor growth and induce apoptosis in various malignancies. In NET cells, HDAC inhibitors have resulted in increased Notch1 expression and subsequent inhibition of growth. We present here a phase II study of the novel HDAC inhibitor panobinostat in patients with low-grade NET.

Methods.

Adult patients with histologically confirmed, metastatic, low-grade NETs and an Eastern Cooperative Oncology Group (ECOG) performance status of ≤2 were treated with oral panobinostat 20 mg once daily three times per week. Treatment was continued until patients experienced unacceptable toxicities or disease progression. The study was stopped at planned interim analysis based on a Simon two-stage design.

Results.

Fifteen patients were accrued, and 13 were evaluable for response. No responses were seen, but the stable disease rate was 100%. The median progression-free survival (PFS) was 9.9 months, and the median overall survival was 47.3 months. Fatigue (27%), thrombocytopenia (20%), diarrhea (13%), and nausea (13%) were the most common related grade 3 toxicities. There was one grade 4 thrombocytopenia (7%). These results did not meet the prespecified criteria to open the study to full accrual.

Conclusion.

The HDAC inhibitor panobinostat has a high stable disease rate and reasonable PFS in low-grade NET, but has a low response rate.

Therapeutic approaches in myelofibrosis and myelodysplastic/myeloproliferative overlap syndromes

The discovery of JAK2 (V617F) a decade ago led to optimism for a rapidly developing treatment revolution in Ph(-) myeloproliferative neoplasms. Unlike BCR-ABL, however, JAK2 was found to have a more heterogeneous role in carcinogenesis. Therefore, for years, development of new therapies was slow, despite standard treatment options that did not address the overwhelming symptom burden in patients with primary myelofibrosis (MF), post-essential thrombocythemia MF, post-polycythemia vera MF, and myelodysplastic syndrome (MDS)/myeloproliferative neoplasm (MPN) syndromes. JAK-STAT inhibitors have changed this, drastically ameliorating symptoms and ultimately beginning to show evidence of impact on survival. Now, the genetic foundations of myelofibrosis and MDS/MPN are rapidly being elucidated and contributing to targeted therapy development. This has been empowered through updated response criteria for MDS/MPN and refined prognostic scoring systems in these diseases. The aim of this article is to summarize concisely the current and rationally designed investigational therapeutics directed at JAK-STAT, hedgehog, PI3K-Akt, bone marrow fibrosis, telomerase, and rogue epigenetic signaling. The revolution in immunotherapy and novel treatments aimed at previously untargeted signaling pathways provides hope for considerable advancement in therapy options for those with chronic myeloid disease.

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.

The Bone Marrow-Mediated Protection of Myeloproliferative Neoplastic Cells to Vorinostat and Ruxolitinib Relies on the Activation of JNK and PI3K Signalling Pathways

The classical BCR-ABL-negative Myeloproliferative Neoplasms (MPN) are a group of heterogeneous haematological diseases characterized by constitutive JAK-STAT pathway activation. Targeted therapy with Ruxolitinib, a JAK1/2-specific inhibitor, achieves symptomatic improvement but does not eliminate the neoplastic clone. Similar effects are seen with histone deacetylase inhibitors (HDACi), albeit with poorer tolerance. Here, we show that bone marrow (BM) stromal cells (HS-5) protected MPN-derived cell lines (SET-2; HEL and UKE-1) and MPN patient-derived BM cells from the cytotoxic effects of Ruxolitinib and the HDACi Vorinostat. This protective effect was mediated, at least in part, by the secretion of soluble factors from the BM stroma. In addition, it correlated with the activation of signalling pathways important for cellular homeostasis, such as JAK-STAT, PI3K, JNK, MEK-ERK and NF-κB. Importantly, the pharmacological inhibition of JNK and PI3K pathways completely abrogated the BM protective effect on MPN cell lines and MPN patient samples. Our findings shed light on mechanisms of tumour survival and may indicate novel therapeutic approaches for the treatment of MPN.

Proinflammatory Cytokine IL-6 and JAK-STAT Signaling Pathway in Myeloproliferative Neoplasms

The recent JAK1/2 inhibitor trial in myeloproliferative neoplasms (MPNs) showed that reducing inflammation can be more beneficial than targeting gene mutants. We evaluated the proinflammatory IL-6 cytokine and JAK-STAT signaling pathway related genes in circulating CD34⁺ cells of MPNs. Regarding laboratory data, leukocytosis has been observed in polycythemia vera (PV) and JAK2V617F mutation positive versus negative primary myelofibrosis (PMF) patients. Moreover, thrombocytosis was reduced by JAK2V617F allele burden in essential thrombocythemia (ET) and PMF. 261 significantly changed genes have been detected in PV, 82 in ET, and 94 genes in PMF. The following JAK-STAT signaling pathway related genes had augmented expression in CD34⁺ cells of MPNs: CCND3 and IL23A regardless of JAK2V617F allele burden; CSF3R, IL6ST, and STAT1/2 in ET and PV with JAK2V617F mutation; and AKT2, IFNGR2, PIM1, PTPN11, and STAT3 only in PV. STAT5A gene expression was generally reduced in MPNs. IL-6 cytokine levels were increased in plasma, as well as IL-6 protein levels in bone marrow stroma of MPNs, dependent on JAK2V617F mutation presence in ET and PMF patients. Therefore, the JAK2V617F mutant allele burden participated in inflammation biomarkers induction and related signaling pathways activation in MPNs.

Clinical potential of pacritinib in the treatment of myelofibrosis

Myelofibrosis (MF) is a myeloid disorder caused by a clonal hematopoietic stem-cell proliferation associated with activation of the Janus kinase (JAK) signal transducer and activator of transcription (STAT) signaling pathways. Patients with MF often develop severe splenomegaly, marked symptom burden and significant cytopenias, with a consequent marked negative impact on quality of life and survival. The management of MF patients has dramatically improved with the development of a group of drugs that inhibit JAK signaling. The first of these agents to be approved was ruxolitinib, a JAK1/JAK2 inhibitor, which has been shown to improve both spleen size and symptoms in patients with MF. However, myelotoxicity, particularly of the platelet lineage, significantly limits the patient population who can benefit from this agent. Thus, there is an unmet need for novel agents with limited myelotoxicity to treat MF. Pacritinib, a JAK2 and FMS-like tyrosine kinase 3 (FLT3) inhibitor, has shown promising results in early phase trials with limited myelotoxicity and clinical responses that are comparable with those seen with ruxolitinib, even in patients with severe thrombocytopenia. Currently there are two large phase III clinical trials of pacritinib in MF, including patients with thrombocytopenia, and those previously treated with ruxolitinib. If the encouraging results observed in early phase clinical trials are confirmed, pacritinib will represent a new and exciting treatment option for patients with MF and particularly patients with significant cytopenias.

Setting Appropriate Goals for the Next Generation of Clinical Trials in Myelofibrosis

New targeted therapies administered in phase II and phase III studies have produced substantial improvements in outcomes of myelofibrosis (MF). However, strong documentation that the new agents modify the natural history of the disease is lacking, and a number of therapeutic indications of new drugs remain unaddressed. Overall survival (OS) improvement is the major goal of next-generation clinical trials in MF. This may be attained if an adequate population of patients and an unambiguous design of the trial will be selected. Another goal is preventing disease progression in early MF: this requires a controlled clinical trial with an accessible endpoint and a clinically relevant definition of disease progression. Improvement in the documentation of responsiveness of patient-reported outcomes (PROs) will allow to use them as a critical endpoint of new trials. Finally, exploiting the clinical utility of biomarkers should become a major goal of future clinical experimentation in MF.

Emerging drugs for the treatment of myelofibrosis

INTRODUCTION

Myelofibrosis (MF) is a myeloproliferative neoplasm associated with significant disease burden composed of splenomegaly, constitutional symptoms and a reduced life expectancy. The advent of targeted treatments has provided new means by which to improve MF associated splenomegaly, symptoms, health-related quality of life and even mortality.

AREAS COVERED

We discuss the spectrum of targeted treatments currently under investigation for MF. We furthermore compare their effects on improving anemia, reducing fibrosis and splenomegaly and enhancing symptom control.

EXPERT OPINION

MF is a complex disorder, partly attributable to its heterogeneity. Although the severity of patient symptoms correlates with risk category, high symptom burden may also be observed in low-risk patients. Serial use of PRO tools allows clinicians to objectively evaluate the MF symptom burden, compare efficacy of therapies and adjust medications to improve symptom control. Novel targeted agents have proven superior to historic treatment regimens for symptom management. Promising treatment categories include JAK2 inhibitors, histone deacetylase inhibitors, hypomethylating agents, heat shock protein-90 inhibitors, hedgehog inhibitors, PI3-AKT-mTOR inhibitors, antifibrosing agents and telomerase inhibitors. The majority of therapies remain under investigation, either alone or in combination with other treatments. It is anticipated that these agents will be increasingly integrated into standard treatment algorithms for MF symptom management.

Novel therapies for myelofibrosis

Myelofibrosis (MF), including primary, post-essential thrombocythemia and post-polycythemia vera MF, associates with a reduced quality of life and shortened life expectancy. Dysregulation of the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway is prominent, even in the absence of the JAK2(V617F) mutation. Therefore, all symptomatic MF patients may potentially derive benefit from JAK inhibitors. Despite the efficacy of JAK inhibitors in controlling signs and symptoms of MF, they do not eradicate the disease. Therefore, JAK inhibitors are currently being tested in combination with other novel therapies, a strategy which may be more effective in reducing disease burden, either by overcoming JAK inhibitor resistance or targeting additional mechanisms of pathogenesis. Additional targets include modulators of epigenetic regulation, pathways that work downstream from JAK/STAT (i.e. mammalian target of rapamycin/AKT/phosphoinositide 3-kinase) heat shock protein 90, hedgehog signaling, pro-fibrotic factors, abnormal megakaryocytes and telomerase. In this review, we discuss novel MF therapeutic strategies.

Looking forward: novel therapeutic approaches in chronic and advanced phases of myelofibrosis

Myelofibrosis (MF) is complex at the pathobiologic level and heterogeneous at the clinical level. The advances in molecular characterization of MF provide important insight into the mechanisms driving this chronic myeloid malignancy, refine risk stratification, offer novel therapeutic targets, and serve to measure therapeutic response. Although JAK2 inhibition has been the focus of laboratory and clinical efforts over the last decade, current experimental therapeutic approaches have broadened to include inhibitors of key alternative signaling pathways, epigenetic modulators, anti-fibrotics, and immunotherapies. Based on compelling preclinical rationale, a number of JAK2 inhibitor based combination therapies are now actively being evaluated in the clinic with the goal of disease course modification. The role and timing of hematopoietic stem cell transplant (HSCT) for MF has been challenged with the availability of commercial ruxolitinib and the plethora of experimental treatment options that exist. Integration of preconditioning JAK2 inhibition, reduced intensity conditioning regimens, and alternative donor sources are all being explored in an attempt to optimize this potentially curative modality. This review will summarize modern MF risk stratification, current clinical research approaches to chronic and advance phase MF focusing on novel agents alone and in combination, and update the reader on new directions in HSCT.

Dysregulation of the MIRLET7/HMGA2 axis with methylation of the CDKN2A promoter in myeloproliferative neoplasms

Overexpression of high mobility group AT-hook 2 (Hmga2), which is negatively regulated by MIRLET7 micro RNAs through 3'-untranslated region (3'UTR), causes proliferative haematopoiesis mimicking myeloproliferative neoplasms (MPNs) and contributes to progression of myelofibrosis in mice. Thus, we investigated HMGA2 mRNA expression in 66 patients with MPNs including 23 polycythaemia vera (PV), 33 essential thrombocythaemia (ET) and 10 primary myelofibrosis (PMF). HMGA2 mRNA expression, especially variant 1 with 3'UTR that contains MIRLET7-specific sites, rather than variant 2 lacking 3'UTR, is frequently deregulated due to decreased MIRLET7 expression in granulocytes from over 20% of PV and ET, and in either granulocytes or CD34(+) cells from 100% of PMF. Patients with deregulated HMGA2 mRNA expression were significantly more likely to show splenomegaly, high serum lactate dehydrogenase values, and methylation of the CDKN2A promoter compared with other patients without deregulation of HMGA2. A histone deacetylase inhibitor, panobinostat, significantly increased MIRLET7 expression and reduced variant 1 of HMGA2 mRNA expression, but not variant 2, in both U937 cells and PMF-derived CD34(+) cells. Moreover, both panobinostat and small interfering RNA of HMGA2 demethylated the CDKN2A promoter in U937 cells. In conclusion, the frequently dysregulated MIRLET7/HMGA2 axis could be a therapeutic target in MPNs.

Epigenetic diversity in hematopoietic neoplasms

Tumor cell populations display a remarkable extent of variability in non-genetic characteristics such as DNA methylation, histone modification patterns, and differentiation levels of individual cells. It remains to be elucidated whether non-genetic heterogeneity is simply a byproduct of tumor evolution or instead a manifestation of a higher-order tissue organization that is maintained within the neoplasm to establish a differentiation hierarchy, a favorable microenvironment, or a buffer against changing selection pressures during tumorigenesis. Here, we review recent findings on epigenetic diversity, particularly heterogeneity in DNA methylation patterns in hematologic malignancies. We also address the implications of epigenetic heterogeneity for the clonal evolution of tumors and discuss its effects on gene expression and other genome functions in cancer.

Rationale for combination therapy in myelofibrosis

Agents targeting the JAK-STAT pathway have dominated the investigational therapeutic portfolio over the last five years resulting in the first and only approved agent for the treatment of patients with myelofibrosis (MF). However, chromatin modifying agents, anti-fibrosing agents, and other signaling pathway inhibitors have also demonstrated activity and offer the potential to improve upon the clinical success of JAK2 inhibition. Due to the complex pathobiological mechanisms underlying MF, it is likely that a combination of biologically active therapies will be required to target the MF hematopoietic stem cell in order to achieve significant disease course modification. Ruxolitinib in partnership with panobinostat, decitabine, and LDE225 are being evaluated in current combination therapy trials based on pre-clinical studies that provide strong scientific rationale. The rationale of combination of danazol or lenalidomide with ruxolitinib is mainly based on mitigation of anti-JAK2-mediated myelosuppression. Combination trials of ruxolitinib and novel anti-fibrosing agents such as PRM-151 represent an attempt to address therapeutic limitations of JAK2 inhibitors such as reversal of bone marrow fibrosis. Ruxolitinib is also being incorporated in novel treatment strategies in the setting of hematopoietic stem cell transplantation for MF. As the pathogenetic mechanisms are better understood, potential drug combinations in MF will increase dramatically and demonstration of biologic activity in effective preclinical models will be required to efficiently evaluate the most active combinations with least toxicity in future trials. This manuscript will address the proposed goals of combination therapy approach and review the state of the art in combination experimental therapy for MF.

Novel myelofibrosis treatment strategies: potential partners for combination therapies

Of the myeloproliferative neoplasms (MPNs), myelofibrosis (MF) is associated with the greatest symptom burden and poorest prognosis and is characterized by constitutional symptoms, cytopenias, splenomegaly and bone marrow fibrosis. A hallmark of MF is dysregulation of the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway that has led to the development of JAK inhibitors targeting this pathway. Calreticulin gene mutations have recently been identified in JAK2 mutation-negative patients with MF. Identification of JAK inhibitor resistance and broad contributions to MF disease pathogenesis from epigenetic deregulators, pathways that work in concert with JAK/STAT (that is, mammalian target of rapamycin/AKT/phosphoinositide 3-kinase, RAS/RAF/MEK, PIM kinase), fibrosis-promoting factors and the MF megakaryocyte, suggest that numerous options may be partnered with a JAK inhibitor. Therefore, we will discuss logical and potential partners for combination therapies for the treatment of patients with MF.

Efficacy of ruxolitinib for myelofibrosis

INTRODUCTION

The discovery of the activating JAK2 V617F mutation in patients with myelofibrosis (MF) led to the development of JAK2 inhibitors. The first such inhibitor to enter clinical trials was ruxolitinib . This review summarizes preclinical and clinical data of ruxolitinib in MF.

AREAS COVERED

A literature search through Medline employing the terms 'ruxolitinib,' 'INCB018424' and 'myelofibrosis' was undertaken. The results from Phase I/II studies in patients with MF showed that ruxolitinib led to durable improvements in splenomegaly, and symptoms associated with MF. Two Phase III trials have compared ruxolitinib against placebo and best available therapy, and in both studies ruxolitinib demonstrated superior rates of spleen control and symptom improvement, and additional analysis demonstrated a survival benefit with ruxolitinib treatment. The main toxicities seen with ruxolitinib are cytopenias, which are managed with dose adjustments. Recent reports documented sporadic cases of immunosuppression-related infections. Ruxolitinib is the first drug ever approved for the therapy of patients with MF.

EXPERT OPINION

Understanding the factors that predict the rate and duration of response to ruxolitinib would improve our ability to manage patients treated with this medication. Clinical trials combining ruxolitinib with novel compounds that are also active in MF will further improve therapy for this disease.