Novel strategies for patients with chronic myeloproliferative disorders

A Journey Through JAK Inhibitors for the Treatment of Myeloproliferative Diseases

PURPOSE OF REVIEW

Chronic myeloproliferative neoplasms (MPN) represent a group of diseases characterised by constitutive activation of the JAK/STAT pathway in a clonal myeloid precursor. The therapeutic approach aims to treat the symptom burden (headache, itching, debilitation), splenomegaly, slow down the fibrotic proliferation in the bone marrow and reduce the risk of thrombosis/bleeding whilst avoiding leukaemic transformation.

RECENT FINDINGS

In recent years, the advent of JAK inhibitors (JAKi) has significantly broadened treatment options for these patients. In myelofibrosis, symptom control and splenomegaly reduction can improve quality of life with improved overall survival, not impacting progression into acute leukaemia. Several JAKi are available and used worldwide, and combination approaches are now being explored. In this chapter, we review the approved JAKi, highlighting its strengths, exploring potential guidelines in choosing which one to use and reasoning towards future perspectives, where the combinations of therapies seem to promise the best results.

Evaluation of plitidepsin in patients with primary myelofibrosis and post polycythemia vera/essential thrombocythemia myelofibrosis: results of preclinical studies and a phase II clinical trial

Previous data established that plitidepsin, a cyclic depsipeptide, exerted activity in a mouse model of myelofibrosis (MF). New preclinical experiments reported herein found that low nanomolar plitidepsin concentrations potently inhibited the proliferation of JAK2V617F-mutated cell lines and reduced colony formation by CD34+ cells of individuals with MF, at least in part through modulation of p27 levels. Cells of MF patients had significantly reduced p27 content, that were modestly increased upon plitidepsin exposure. On these premise, an exploratory phase II trial evaluated plitidepsin 5 mg/m(2) 3-h intravenous infusion administered on days 1 and 15 every 4 weeks (q4wk). Response rate (RR) according to the International Working Group for Myelofibrosis Research and Treatment consensus criteria was 9.1% (95% CI, 0.2-41.3%) in 11 evaluable patients during the first trial stage. The single responder achieved a red cell transfusion independence and stable disease was reported in nine additional patients (81.8%). Eight patients underwent a short-lasting improvement of splenomegaly. In conclusion, plitidepsin 5 mg/m(2) 3-h infusion q4wk was well tolerated but had a modest activity in patients with primary, post-polycythaemia vera or post-essential thrombocythaemia MF. Therefore, this trial was prematurely terminated and we concluded that further clinical trials with plitidepsin as single agent in MF are not warranted.

Apoptosis deregulation in myeloproliferative neoplasms

Philadelphia-chromosome negative chronic myeloproliferative neoplasms are clonal hematologic diseases characterized by hematopoietic progenitor independence from or hypersensitivity to cytokines. The cellular and molecular mechanisms involved in the pathophysiology of myeloproliferative neoplasms have not yet been fully clarified. Pathophysiologic findings relevant for myeloproliferative neoplasms are associated with genetic alterations, such as, somatic mutation in the gene that codifies JAK-2 (JAK V617F). Deregulation of the process of programmed cellular death, called apoptosis, seems to participate in the pathogenesis of these disorders. It is known that expression deregulation of pro- and anti-apoptotic genes promotes cell resistance to apoptosis, culminating with the accumulation of myeloid cells and establishing neoplasms. This review will focus on the alterations in apoptosis regulation in myeloproliferative neoplasms, and the importance of a better understanding of this mechanism for the development of new therapies for these diseases.

FLT3-mediated p38-MAPK activation participates in the control of megakaryopoiesis in primary myelofibrosis

Primary myelofibrosis (PMF) is characterized by increased number of hematopoietic progenitors and a dysmegakaryopoiesis which supports the stromal reaction defining this disease. We showed that increased ligand (FL) levels in plasma, hematopoietic progenitors, and stromal cells from PMF patients were associated with upregulation of the cognate Flt3 receptor on megakaryocytic (MK) cells. This connection prompted us to study a functional role for the FL/Flt3 couple in PMF dysmegakaryopoiesis, as a route to reveal insights into pathobiology and therapy in this disease. Analysis of PMF CD34(+) and MK cell transcriptomes revealed deregulation of the mitogen-activated protein kinase (MAPK) pathway along with Flt3 expression. In PMF patients, a higher proportion of circulating Flt3(+)CD34(+)CD41(+) cells exhibited an increased MAPK effector phosphorylation independently of Jak2(V617F) mutation. Activation of FL/Flt3 axis in PMF MK cell cultures, in response to FL, induced activation of the p38-MAPK cascade, which is known to be involved in inflammation, also increasing expression of its target genes (NFATC4, p53, AP-1, IL-8). Inhibiting Flt3 or MAPK or especially p38 by chemical, antibody, or silencing strategies restored megakaryopoiesis and reduced phosphorylation of Flt3 and p38 pathway effectors, confirming the involvement of Flt3 in PMF dysmegakaryopoiesis via p38 activation. In addition, in contrast to healthy donors, MK cells derived from PMF CD34(+) cells exhibited an FL-induced migration that could be reversed by p38 inhibition. Taken together, our results implicate the FL/Flt3 ligand-receptor complex in PMF dysmegakaryopoiesis through persistent p38-MAPK activation, with implications for therapeutic prospects to correct altered megakaryopoiesis in an inflammatory context.

Polycythemia vera

The diagnostic approach to a patient with polycythemia has been greatly simplified by the introduction of new genetic testing in addition to traditional tests, such as measurement of red cell mass and serum erythropoietin (Epo) level. Clonal erythrocytosis, which is the diagnostic feature of polycythemia vera (PV), is almost always associated with a JAK2 mutation (JAK2V617F or exon 12). Therefore, in a patient with acquired erythrocytosis, it is reasonable to begin the diagnostic work-up with JAK2 mutation analysis to distinguish PV from secondary erythrocytosis. The clinical course of PV is marked by a high incidence of thrombotic complications that represent the main cause of morbidity and mortality in these patients. Blood hyperviscosity as well as platelet and leukocyte quantitative, and qualitative abnormalities play a major role in the pathogenesis of thrombophilia. Prevention of vascular events and minimizing the risk of disease transition into acute leukaemia are the main targets of the whole PV treatment strategy. This can rely on the use of low-dose aspirin in most patients, while the choice of the optimal cytoreductive strategy is based on the individual vascular risk. Phlebotomy is still the preferred treatment in subjects at low risk, while hydroxyurea or pipobroman is usually administered to most elderly subjects or subjects with a previous vascular history. The use of pegylated interferon, imatinib, and JAK2 inhibitors is currently being evaluated.

Recent developments on JAK2 inhibitors: a patent review

IMPORTANCE OF THE FIELD

JAK2 is one of the most promising targets against neoplastic growth. A somatic mutation (V617F) resulting in enhanced JAK2 kinase activity can be frequently found in patients with serious myeloproliferative neoplasms such as polycythemia vera, essential thrombocythemia and primary myelofibrosis. Preclinical results strongly support that JAK2 inhibitors could be effectively used in these indications. Pharmaceutical companies and academic groups have developed a number of potent JAK2 inhibitors during the last decade. Tolerability and effectiveness of the most promising compounds are currently being investigated in clinical trials.

AREAS COVERED IN THIS REVIEW

In this paper, we aim to give a comprehensive review of the currently available patent literature of JAK2 inhibitors.

WHAT THE READER WILL GAIN

We tried to collect the published core structures possessing JAK2 inhibitory potency including compounds developed by academic and industrial research groups. We review the currently available patent literature as well as the key papers containing additional information about the described JAK2 inhibitors. Clinical status data were collected by searching the Prous Integrity and Pharmaprojects databases.

TAKE HOME MESSAGE

The significant number of JAK2 inhibitors published and numerous clinical trials involving these compounds suggest that some of them might be approved in the next few years and can serve as novel drugs for the treatment of JAK2-dependent pathologies.