Classical myeloproliferative neoplasms (MPNs), also called Philadelphia-negative MPNs, are clonal hematopoietic disorders characterized by the excessive production of terminally differentiated blood cells . Classical MPNs include three main diseases: PV, ET, and MF, which have frequent disease-related complications such as venous and arterial thrombosis, hemorrhages, and transformation to acute myeloid leukemia . The identification of driver mutations in
Treatment decisions for MPN patients are influenced by the MPN subtype, symptom burden, and risk classification. Even though the main objective of ET and PV therapy is to prevent vascular events, there is a need for disease-modifying drugs that can eradicate clonal hematopoiesis and prevent progression to more aggressive myeloid neoplasms [15-17]. JAK inhibitors are a valuable therapeutic strategy for patients with MF who have splenomegaly and/or disease-related symptoms [4, 5, 18]. However, allogeneic stem cell transplantation remains the only curative treatment for MF, but it is typically limited by age-related comorbidities and high treatment-related mortality [19, 20]. Therefore, a better understanding of the molecular pathogenesis and potential new therapies to modify the natural course of the disease is important.
Here, the present paper reviews the current understanding of the molecular basis of MPNs and potential disease-modifying therapies.
Constitutive activation of the JAK-STAT pathway is a hallmark of classical MPNs [2, 3]. Documented drivers in classical MPNs include
MPL is a cell surface receptor for thrombopoietin (TPO) .
CALR is a protein that resides in the lumen of the endoplasmic reticulum (ER), where it functions as a molecular chaperone for many glycoproteins, assisting in their folding and contributing to calcium homeostasis. The two most frequent mutations are type 1 and type 2 mutations [32, 33]. The type 1 mutation is a 52 bp deletion (c.1092_1143del, p.L367fs*46), whereas the type 2 mutation is a 5 bp insertion (c.1154_ 1155insTTGTC, p.K385fs*47). The distribution of these
The recent development of next-generation sequencing has allowed the identification of several mutations in patients with a variety of myeloid neoplasms, including MPNs [2, 35-45]. The most commonly affected genes are those important in epigenetic regulation, messenger RNA splicing, transcriptional mechanisms, and signal transduction. Although reported somatic mutations lack specificity because they can be found in a broad range of myeloid neoplasms, there is evidence to suggest that the identification of certain non-driver mutations in MPN patients is associated with a greater risk of disease progression or shortened survival [19, 46-48].
Mutations in epigenetic regulators: Mutations targeting DNA methylation regulators (
Mutations in splicing machinery: Mutations in components of the RNA spliceosome machinery, including
Mutations in transcription factors and signal transduction genes: Sequencing a number of genes implicated in myeloid malignancies in samples from patients with post-MPN AML versus chronic MPN phase revealed somatic mutations targeting
Signal transduction gene mutations, such as
Although the efficacy of interferon-α (IFN-α) has shown efficacy in the treatment of ET and PV for more than 20 years and has provided an alternative option to chemotherapeutic agents, toxicity and the need for frequent parenteral application of conventional formulations led to a high proportion of patients discontinuing treatment [63, 64]. Because pegylated (peg) forms have been shown to have increased tolerance and efficacy in IFNα-treated hepatitis patients, there is renewed interest in using pegylated IFN, and clinical trials have been performed to elucidate its role in the upfront and salvage treatment of patients with ET and PV. The objectives included not only high rates of hematologic responses, but also molecular responses in
Unfortunately, since ruxolitinib is insufficient to eliminate the underlying myeloid progenitor clone, ‘add-on’ approaches to ruxolitinib are being developed for increased efficacy and potential disease-modifying effects (Table 1).
Targeting BET protein: BET proteins regulate key oncogenic pathways, including NF-κB and TGF-β signaling, which are important drivers of pro-inflammatory cytokine expression and bone marrow fibrosis, respectively, and are implicated in MF pathogenesis. Preclinical studies suggest that a combination of CPI-0610, a selective and potent small molecule BET inhibitor, together with a JAK inhibitor, can result in the reduction of serum levels of inflammatory cytokines, bone marrow fibrosis (BMF), and mutant cell burden . A phase 2 study of CPI-0610 alone or as an “add-on” to ruxolitinib (CPI-0610+ruxolitinib) (NCT02158858) provided clinical benefits in MF patients with inadequate responses or those refractory to ruxolitinib. Through improvement in BMF and anemia responses, its potential for disease modification has been suggested .
Targeting anti-apoptotic protein Bcl-xL: The anti-apoptotic protein Bcl-xL is regulated by JAKs and the combined targeting of JAK2. Furthermore, it has been demonstrated that Bcl2/-xL is synergistic in preclinical
Targeting phosphatidylinositol-3-kinase delta: The phosphatidylinositol-3-kinase/Akt/mammalian target of the rapamycin (PI3K/Akt/mTOR) cascade integrates cellular growth and proliferation signals downstream of JAK-STAT, and constitutive activation of this pathway is central to MPN pathogenesis . Preclinical studies have shown that inhibitors of this pathway, both alone and synergistically in combination with ruxolitinib or fedratinib, reduce proliferation and induce apoptosis of
Updated results from a trial in which umbralisib, a selective inhibitor of the delta isoform of PI3K with a superior tolerability profile, was “added on” to ruxolitinib (stable dose for ≥8 wk) in patients with an insufficient response to the latter, were recently presented (NCT02493530) . Two out of 23 patients achieved complete response (CR). An additional 11 patients showed clinical improvement based on anemia, spleen, and/or symptom responses. To note, determination of sub-optimal response to ruxolitinib for patient eligibility for this trial was left up to the physician’s discretion. Parsaclisib is another PI3K delta isoform-specific inhibitor that has been studied in combination with ruxolitinib in an ongoing “add on” trial (NCT02718300); however, a sub-optimal response to ruxolitinib is clearly defined in this trial (palpable spleen >10 cm or 5–10 cm, with active symptoms of myelofibrosis after at least 6 months of ruxolitinib and with a stable dose over the preceding 8 weeks or longer) . Parsaclisib exhibited a good tolerability profile in this trial, but a switch from daily to weekly dosing after 8 weeks of combination therapy (to mitigate toxicities) appeared to correlate with some loss of response. A similar phase 1 trial of idelalisib added to ruxolitinib (stable dose for ≥4 wk) has been completed (NCT02436135).
Several investigational agents are being studied as monotherapies in ruxolitinib-resistant or ineligible patients.
Telomerase inhibitor: The telomerase inhibitor imetelstat generated considerable excitement when seven durable complete and partial responses from 33 patients with MF, with reversal of BMF in all four patients who had a complete response, were reported in a pilot study . The results from the phase 2 imetelstat trial in patients with DIPSS intermediate-2/high-risk myelofibrosis who had failed therapy with a JAK inhibitor (NCT02426086) were presented at the 2018 American Society of Hematology (ASH) annual meeting . In this study, two doses of imetelstat (9.4 mg/kg or 4.7 mg/kg IV, every 3 wk) were administered to 107 patients with intermediate-2 or high-risk MF that was relapsed/refractory to prior JAK inhibitor therapy (i.e., either no reduction in splenomegaly after 12 weeks or worsening splenomegaly at any time after the start of the JAK inhibitor therapy). The lower dose (4.7 mg/kg) arm (N=48) was closed to new patient entry due to insufficient activity after an interim analysis, and the patients were allowed a dose escalation. At the time of the clinical cutoff (ASH Annual Meeting 2018), 9.4 mg/kg administered every 3 weeks resulted in a 10.2% spleen response and 32% symptom response. Importantly, after a median follow-up of 22.6 months, the median survival in the 9.4 mg/kg arm was not reached, while the median OS was 19.9 months in the 4.7 mg/kg arm . Although no formal study has reported survival for patients who are truly relapsed/refractory to JAK inhibitors, observed OS after imetelstat therapy was in marked contrast to the 13-14 months reported by several groups studying patients who discontinued ruxolitinib [80-82].
Murine double minute 2 (MDM2) inhibitor: Preclinical studies have shown that JAK2V617F leads to overexpression of murine double minute 2 (MDM2) in MPN , and upregulation of MDM2 protects the clonal HSCs driving the disease from apoptosis. An open-label phase 2 trial with the MDM2 inhibitor KRT-232 is currently enrolling patients who failed JAK inhibitor therapy (NCT03662126).
Megakaryocytes in PMF exhibit impaired maturation associated with downregulation of the transcription factor GATA1 . These atypical megakaryocytes may contribute to bone marrow fibrosis by releasing cytokines such as transforming growth factor (TGF)-β. The aurora kinase A (AURKA) inhibitor alisertib has been shown to promote the differentiation of PMF megakaryocytes and ameliorate bone marrow fibrosis
Fibrosis-driving cells in PMF bone marrow are reported to be clonal, neoplastic, and derived from monocytes . The anti-fibrotic agent, PRM-151, is intravenously administered (every 4 wk) and recombinant pentraxin-2 molecule, also known as serum amyloid protein. The results from 18 patients, 9 of whom received PRM-151 alone and 9 that received a combination with ruxolitinib in an open-label extension study, are presented. The median time of study was 30.9 months, and the drug was well tolerated. The mean best percent change (by palpation) in spleen size from baseline was -37%, with a median percent reduction of -26.1%. The mean best percent improvement in the MPN-SAF TSS was -54%, with a median percent reduction of TSS of -64%. Interestingly, even the patients on PRM-151 monotherapy showed similar benefits in terms of spleen size and symptom burden as those receiving PRM-151+ruxolitinib. In addition, an overall improvement in the BM reticulin and collagen fibrosis grade was observed .
There are some potential strategies aimed at reversing bone marrow fibrosis. Galunisertib, a small-molecule inhibitor of the TGF-β receptor 1 kinase ALK5, blocks excessive collagen production in
The effect of recombinant interferon to prevent the development of marked splenomegaly, anemia, and florid myelofibrosis in early myelofibrosis was tested . Early data on the combination of ruxolitinib and pegylated IFN-α from the ongoing RUXOPEG study showed that this combination was generally well tolerated, and the preliminary efficacy results were encouraging .
Mutant calreticulin binds to the thrombopoietin receptor, MPL (requirement for the lectin-dependent function of mutant calreticulin to bind to the extracellular domain of MPL) to serve as a potential tumor antigen in MPN . This led to the development of novel, vaccine-based approaches to target this immunogenic mutant protein [95, 96], but these have not yet entered clinical practice.
Although the identification of driver mutations in
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