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From the Columbia University Medical Center, New York, NY (Dr. Falchi), and the University of Texas MD Anderson Cancer Center, Houston, TX (Dr. Verstovsek).
ABSTRACT
- Objective: To review the clinical aspects and current practices in the management of polycythemia vera (PV) and essential thrombocythemia (ET).
- Methods: Review of the literature.
- Results: PV and ET are rare chronic myeloid disorders. The 2 most important life-limiting complications of PV and ET are thrombohemorrhagic events and myelofibrosis/acute myeloid leukemia (AML) transformation. Vascular events are at least in part preventable with counseling on risk factors, phlebotomy (for patients with PV), antiplatelet therapy, and cytoreduction with hydroxyurea, interferons, or anagrelide (for patients with ET). Ruxolitinib was recently approved for PV after hydroxyurea failure. PV/ET transformation into myelofibrosis or AML is part of the natural history of the disease and no therapy has been shown to prevent it. Treatment of leukemic transformation of myeloproliferative neoplasms (MPN LT) follows recommendations set forth for primary myelofibrosis and AML.
- Conclusion: With appropriate management, patients with PV and ET typically enjoy a long survival and near-normal quality of life. Transformation into myelofibrosis or AML cannot be prevented by current therapies, however. Treatment results with MPN LT are generally poor and novel strategies are needed to improve outcomes.
Key words: myeloproliferative neoplasms; myelofibrosis; leukemic transformation.
Polycythemia vera (PV) and essential thrombocythemia (ET), along with primary myelofibrosis (PMF), belong to the group of Philadelphia-negative myeloproliferative neoplasms (MPN). All these malignancies arise from the clonal proliferation of an aberrant hematopoietic stem cell, but are characterized by distinct clinical phenotypes [1,2]. Although the clinical course of PV and ET is indolent, it can be complicated by thrombohemorrhagic episodes and/or evolution into myelofibrosis and/or acute myeloid leukemia (AML) [3]. Since vascular events are the most frequent life-threatening complications of PV and ET, therapeutic strategies are aimed at reducing this risk. Treatment may also help control other symptoms associated with the disease [4]. No therapy has been shown to prevent evolution of PV or ET into myelofibrosis or AML. The discovery of the Janus kinase 2 (JAK2)/V617F mutation in most patients with PV and over half of those with ET (and PMF) [5,6] has opened new avenues of research and led to the development of targeted therapies, such as the JAK1/2 inhibitor ruxolitinib, for patients with MPN [7,8].
Epidemiology
PV and ET are typically diagnosed in the fifth to seventh decade of life [9]. Although these disorders are generally associated with a long clinical course, survival of patients with PV or ET may be shorter than that of the general population [10–13]. Estimating the incidence and prevalence of MPN is a challenge because most patients remain asymptomatic for long periods of time and do not seek medical attention [13]. The annual incidence rates of PV and ET are estimated at 0.01 to 2.61 and 0.21 to 2.53 per 100,000, respectively. PV occurs slightly more frequently in males, whereas ET has a predilection for females [14]. Given the long course and low mortality associated with these disorders, the prevalence rates of PV and ET are significantly higher than the respective incidence rates: up to 47 and 57 per 100,000, respectively [15–17].
Molecular Pathogenesis
In 2005 researchers discovered a gain-of-function mutation of the JAK2 gene in nearly all patients with PV and more than half of those with ET and PMF [5,6,18,19]. JAK2 is a non-receptor tyrosine kinase that plays a central role in normal hematopoiesis. Substitution of a valine for a phenylalanine at codon 617 (ie, V617F) leads to its constitutive activation and signaling through the JAK-STAT pathway [5,6,18,19]. More rarely (and exclusively in patients with PV), JAK2 mutations involve exon 12 [20–22]. The vast majority of JAK2-negative ET patients harbor mutations in either the myeloproliferative leukemia (MPL) gene, which encodes the thrombopoietin receptor [23–25], or the calreticulin (CALR) gene [26,27], which encodes for a chaperone protein that plays a role in cellular proliferation, differentiation, and apoptosis [28]. Both the MPL and CALR mutations ultimately result in the constitutive activation of the JAK-STAT pathway. Thus, JAK2, MPL, and CALR alterations are collectively referred to as driver mutations. Moreover, because these mutations affect the same oncogenic pathway (ie, JAK-STAT), they are almost always mutually exclusive in a given patient. Patients with ET (or myelofibrosis) who are wild-type for JAK2, MPL, and CALR are referred to as having “triple-negative” disease. Many recurrent non-driver mutations are also found in patients with MPN. These are not exclusive of each other (ie, patients may have many at the same time) and involve for example ten-eleven translocation-2 (TET2), additional sex combs like 1 (ASXL1), enhancer of zeste homolog 2 (EZH2), isocitrate dehydrogenase 1 and isocitrate dehydrogenase 2 (IDH1/2), and DNA methyltransferase 3A (DNMT3A) genes, among others [29]. The biologic and prognostic significance of these non-driver alterations remain to be fully defined in ET and PV.
Diagnostic Criteria
Diagnostic criteria for PV and ET according to the World Health Organization (WHO) classification [30] are summarized in Table 1. Criteria for the diagnosis of prefibrotic myelofibrosis are included as well since this entity was formally recognized as separate from ET and part of the PMF spectrum in the 2016 WHO classification of myeloid tumors [30]. Clinically, both PV and ET generally remain asymptomatic for a long time. PV tends to be more symptomatic than ET and can present with debilitating constitutional symptoms (fatigue, night sweats, and weight loss), microvascular symptoms (headache, lightheadedness, acral paresthesias, erythromelalgia, atypical chest pain, and pruritus) [31], or macrovascular accidents (larger vein thrombosis, stroke, or myocardial ischemia) [32]. ET is often diagnosed incidentally, but patients can suffer from similar general symptoms and vascular complications. Causes of secondary absolute erythrocytosis (altitude, chronic hypoxemia, heavy smoking, cardiomyopathy, use of corticosteroids, erythropoietin, or other anabolic hormones, familial or congenital forms) or thrombocytosis (iron deficiency, acute blood loss, trauma or injury, acute coronary syndrome, systemic autoimmune disorders, chronic kidney failure, other malignancies, splenectomy) should be considered and appropriately excluded. Once the diagnosis is made, symptom assessment tools such as the Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF) [33] or the abbreviated version, the MPN-SAF Total Symptom Score (MPN-SAF TSS) [34], are generally used to assess patients’ symptom burden and response to treatment in everyday practice.
Risk Stratification
Thrombohemorrhagic events, evolution into myelofibrosis, and leukemic transformation (LT) are the most serious complications in the course of PV or ET. Only thrombohemorrhagic events are, at least partially, preventable. Arterial or venous thrombotic complications are observed at rates of 1.8 to 10.9 per 100 patient-years in PV (arterial thrombosis being more common than venous) and 0.74 to 7.7 per 100 patient-years in ET, depending on the risk group [35] and the presence of other factors (see below).
The risk stratification of patients with PV is based on 2 factors: age ≥ 60 years and prior history of thrombosis. If either is present, the patient is assigned to the high-risk category, whereas if none is present the patient is considered at low risk [36]. In addition, high hematocrit [37] and high white blood cell (WBC) count [38], but not thrombocytosis, have been associated with the development of vascular complications. In one study, the risk of new arterial thrombosis was increased by the presence of leukoerythroblastosis, hypertension, and prior arterial thrombosis, while karyotypic abnormalities and prior venous thrombosis were predictors of new venous thrombosis [39]. Another emerging risk factor for thrombosis in patients with PV is high JAK2 allele burden (ie, the normal-to-mutated gene product ratio), although the evidence supporting this conclusion is equivocal [40].
Traditionally, in ET patients, the thrombotic risk was assessed using the same 2 factors (age ≥ 60 years and prior history of thrombosis), separating patients into low- and high-risk groups. However, the prognostication of ET patients has been refined recently with the identification of new relevant factors. In particular, the impact of JAK2 mutations on thrombotic risk has been thoroughly studied. Clinically, the presence of JAK2V617F is associated with older age, higher hemoglobin and hematocrit, lower platelet counts, more frequent need for cytoreductive treatment, and greater tendency to evolve into PV (a rare event) [41,42]. Many [41,43–46], but not all [47–51], studies suggested a correlation between JAK2 mutation and risk of both arterial and venous thrombosis. Although infrequent, a JAK2V617F homozygous state (ie, the mutation is present in both alleles) might confer an even higher thrombotic risk [52]. Moreover, the impact of the JAK2 mutation on vascular events persists over time [53], particularly in patients with high or unstable mutation burden [54]. Based on JAK2V617F’s influence on the thrombotic risk of ET patients, a new prognostic score was proposed, the International Prognostic Score for ET (IPSET)-thrombosis (Table 2). The revised version of this model is currently endorsed by the National Comprehensive Cancer Network and divides patients into 4 risk groups: high, intermediate, low, and very low. Treatment recommendations vary according to the risk group (as described below) [55].
Other thrombotic risk factors have been identified, but deemed not significant enough to be included in the model. Cardiovascular risk factors (hypercholesterolemia, hypertension, smoking, diabetes mellitus) can increase the risk of vascular events [56–59], as can splenomegaly [60] and baseline or persistent leukocytosis [61–63]. Thrombocytosis has been correlated with thrombotic risk in some studies [64–68], whereas others did not support this conclusion and/or suggested a lower rate of thrombosis and, in some cases, increased risk of bleeding in ET patients with platelet counts greater than 1000 × 103/μL (due to acquired von Willebrand syndrome) [51,61,63,68,69].
CALR mutations tend to occur in younger males with lower hemoglobin and WBC count, higher platelet count, and greater marrow megakaryocytic predominance, as compared to JAK2 mutations [26,27,70–72]. The associated incidence of thrombosis was less than 10% at 15 years in patients with CALR mutations, lower than the incidence reported for ET patients with JAK2V617F mutations [73]. The presence of the mutation per se does not appear to affect the thrombotic risk [74–76]. Information on the thrombotic risk associated with MPL mutations or a triple-negative state is scarce. In both instances, however, the risk appears to be lower than with the JAK2 mutation [73,77–79].
Venous thromboembolism (VTE) in patients with PV or ET may occur at unusual sites, such as the splanchnic or cerebral venous systems [80]. Risk factors for unusual VTE include younger age [81], female gender (especially with concomitant use of oral contraceptive pills) [82], and splenomegaly/splenectomy [83]. JAK2 mutation has also been associated with thrombosis at unusual sites. However, the prevalence of MPN or JAK2V617F in patients presenting with splanchnic VTE has varied [80]. In addition, MPN may be occult (ie, no clinical or laboratory abnormalities) in around 15% of patients [84]. Screening for JAK2V617F and underlying MPN is recommended in patients presenting with isolated unexplained splanchnic VTE. Treatment entails long-term anticoagulation therapy. JAK2V617F screening in patients with nonsplanchnic VTE is not recommended, as its prevalence in this group is low (< 3%) [85,86].
Risk-Adapted Therapy
Low-Risk PV
All patients with PV should receive counseling to mitigate cardiovascular risk factors, including smoking cessation, lifestyle modifications, and lipid-lowering therapy, as indicated. Furthermore, all PV patients should receive acetylsalicylic acid (ASA) to decrease their risk for thrombosis and control vasomotor symptoms [55,87]. Aspirin 81 to 100 mg daily is the preferred regimen because it provides adequate antithrombotic effect without the associated bleeding risk of higher-dose aspirin [88]. Low-risk PV patients should also receive periodic phlebotomies to reduce and maintain their hematocrit below 45%. This recommendation is based on the results of the Cytoreductive Therapy in Polycythemia Vera (CYTO PV) randomized controlled trial. In that study, patients receiving more intense therapy to maintain the hematocrit below 45% had a lower incidence of cardiovascular-related deaths or major thrombotic events than those with hematocrit goals of 45% to 50% (2.7% versus 9.8%) [89]. Cytoreduction is an option for low-risk patients who do not tolerate phlebotomy or require frequent phlebotomy, or who have disease-related bleeding, severe symptoms, symptomatic splenomegaly, or progressive leukocytosis [38].
High-Risk PV
Patients older than 60 years and/or with a history of thrombosis should be considered for cytoreductive therapy in addition to the above measures. Frontline cytoreductive therapies include hydroxyurea or interferon (IFN)-alfa [87]. Hydroxyurea is a potent ribonucleotide reductase inhibitor that interferes with DNA repair and is the treatment of choice for most high-risk patients with PV [90]. In a small trial, hydroxyurea reduced the risk of thrombosis compared with historical controls treated with phlebotomy alone [91]. Hydroxyurea is generally well tolerated; common side effects include cytopenias, nail changes, and mucosal and/or skin ulcers. Although never formally proven to be leukemogenic, this agent should be used with caution in younger patients [87]. Indeed, in the original study, the rates of transformation were 5.9% and 1.5% for patients receiving hydroxyurea and phlebotomy alone [92], respectively, although an independent role for hydroxyurea in LT was not supported in the much larger European Collaboration on Low-dose Aspirin in Polycythemia Vera (ECLAP) study [93]. Approximately 70% of patients will have a sustained response to hydroxyurea [94], while the remaining patients become resistant to or intolerant of the drug. Resistant individuals have a higher risk of progression to acute leukemia and death [95].
IFN-alfa is a pleiotropic antitumor agent that has found application in many types of malignancies [96] and is sometimes employed as treatment for patients with newly diagnosed high-risk PV. Early studies showed responses in up to 100% of cases [97,98], albeit at the expense of a high discontinuation rate due to adverse events, such as flu-like symptoms, fatigue, and neuropsychiatric manifestations [99]. A newer formulation of the drug obtained by adding a polyethylene glycol (PEG) moiety to the native IFN-alfa molecule (PEG-IFN alfa) was shown to have a longer half-life, greater stability, less immunogenicity, and, potentially, better tolerability [100]. Pilot phase 2 trials of PEG-IFN-alfa-2a demonstrated its remarkable activity, with symptomatic and hematologic responses seen in most patients (which, in some cases, persisted beyond discontinuation), and reasonable tolerability, with long-term discontinuation rates of 20% to 30% [101–103]. In some patients, JAK2V617F became undetectable over time [104]. Results of 2 ongoing trials, MDP-RC111 (single-arm study, PEG-IFN-alfa-2a in high-risk PV or ET [NCT01259817]) and MPD-RC112 (randomized controlled trial, PEG-IFN-alfa-2a versus hydroxyurea in the same population [NCT01258856]), will shed light on the role of PEG-IFN-alfa in the management of patients with high-risk PV or ET. In two phase 2 studies of PEG-IFN-alfa-2b, complete responses were seen in 70% to 100% of patients and discontinuation occurred in around a third of cases [105,106]. A new, longer-acting formulation of PEG-IFN-alfa-2a (peg-proline INF-alfa-2b, AOP2014) is also undergoing clinical development [107,108].
The approach to treatment of PV based on thrombotic risk level is illustrated in Figure 1.
Very Low- and Low-Risk ET
Individuals with ET should undergo rigorous cardiovascular risk management and generally receive ASA to decrease their thrombotic risk and improve symptom control. Antiplatelet therapy may not be warranted in patients with documented acquired von Willebrand syndrome, with or without extreme thrombocytosis, or in those in the very low-risk category according to the IPSET-thrombosis model [55,87]. The risk/benefit ratio of antiplatelet agents in patients with ET at different thrombotic risk levels was assessed in poor-quality studies and thus remains highly uncertain. Platelet-lowering agents are sometimes recommended in patients with low-risk disease who have platelet counts ≥ 1500 × 103/μL, due to the potential risk of acquired von Willebrand syndrome and a risk of bleeding (this would require stopping ASA) [109]. Cytoreduction may also be used in low-risk patients with progressive symptoms despite ASA, symptomatic or progressive splenomegaly, and progressive leukocytosis.
Intermediate-Risk ET
This category includes patients older than 60 years, but without thrombosis or JAK2 mutations. These individuals would have been considered high risk (and thus candidates for cytoreductive therapy) according to the traditional risk stratification. Guidelines currently recommend ASA as the sole therapy for these patients, while reserving cytoreduction for those who experience thrombosis (ie, become high-risk) or have uncontrolled vasomotor or general symptoms, symptomatic splenomegaly, symptomatic thrombocytosis, or progressive leukocytosis.
High-Risk ET
For patients with ET in need of cytoreductive therapy (ie, those with prior thrombosis or older than 60 years with a JAK2V617F mutation), first-line options include hydroxyurea, IFN, and anagrelide. Hydroxyurea remains the treatment of choice in most patients [110]. In a seminal study, 114 patients with ET were randomly assigned to either observation or hydroxyurea treatment with the goal of maintaining the platelet count below 600 × 103/μL. At a median follow-up of 27 months, patients in the hydroxyurea group had a lower thrombosis rate (3.6% versus 24%, P = 0.003) and longer thrombosis-free survival, regardless of the use of antiplatelet drugs [64].
Anagrelide, a selective inhibitor of megakaryocytic differentiation and proliferation, was compared with hydroxyurea in patients with ET in 2 randomized trials. In the first (n = 809), the group receiving anagrelide had a higher risk of arterial thrombosis, major bleeding, and fibrotic evolution, but lower incidence of venous thrombosis. Hydroxyurea was better tolerated, mainly due to anagrelide-related cardiovascular adverse events [111]. As a result of this study, hydroxyurea is often preferred to anagrelide as frontline therapy for patients with newly diagnosed high-risk ET. In the second, more recent study (n = 259), however, the 2 agents proved equivalent in terms of major or minor arterial or venous thrombosis, as well as discontinuation rate [112]. The discrepancy between the 2 trials may be partly explained by the different ET diagnostic criteria used, with the latter only enrolling patients with WHO-defined true ET and the former utilizing Polycythemia Vera Study Group-ET diagnostic criteria that included patients with increases in other blood counts or varying degrees of marrow fibrosis.
Interferons were studied in ET in parallel with PV. PEG-IFN-alfa-2a proved effective in patients with ET, with responses observed in 80% of patients [103]. PEG-IFN- alfa-2b produced similar results, with responses in 70% to 90% of patients in small studies and discontinuation observed in 20% to 38% of cases [105,106,113]. Because the very long-term leukemogenic potential of hydroxyurea has remained somewhat uncertain, anagrelide or IFN might be preferable choices in younger patients.
The approach to treatment of ET based on thrombotic risk level is illustrated in Figure 2.
Assessing Response to Therapy
For both patients with PV and ET the endpoint of treatment set forth for clinical trials has been the achievement of a clinicohematologic response. However, studies have failed to show a correlation between response and reduction of the thrombohemorrhagic risk [114]. Therefore, proposed clinical trial response criteria were revised to include absence of hemorrhagic or thrombotic events as part of the definition of response (Table 3) [94].
Approach to Patients Refractory to or Intolerant of First-line Therapy
According to the European LeukemiaNet recommendations, an inadequate response to hydroxyurea in patients with PV (or myelofibrosis) is defined as a need for phlebotomy to maintain the hematocrit below < 45%, the platelet count > 400 × 103/μL, and a WBC count > 10,000/μL, or failure to reduce splenomegaly > 10 cm by > 50% at a dose of ≥ 2 g/day or maximum tolerated dose. Historically, treatment options for patients with PV or ET who failed first-line therapy (most commonly hydroxyurea) have included alkylating agents, such as busulfan, chlorambucil, pipobroman, and phosphorus (P)-32. However, the use of these drugs is limited by the associated risk of LT [93,115,116]. IFN (or anagrelide for ET) is often considered in patients previously treated with hydroxyurea, and vice versa.
Ruxolitinib is a JAK1 and JAK2 inhibitor currently approved for the treatment of PV patients refractory to or intolerant of hydroxyurea [7]. Following promising results of a phase 2 trial [117], ruxolitinib 10 mg twice daily was compared with best available therapy in the pivotal RESPONSE trial (n = 222). Ruxolitinib proved superior in achieving hematocrit control, reduction of spleen volume, and improvement of symptoms. Grade 3-4 hematologic toxicity was infrequent and similar in the 2 arms [118]. In addition, longer follow-up of that study suggested a lower rate of thrombotic events in patients receiving ruxolitinib (1.8 versus 8.2 per 100 patient-years) [119]. In a similarly designed randomized phase 3 study in PV patients without splenomegaly (RESPONSE-2), more patients in the ruxolitinib arm had hematocrit reduction without an increase in toxicity. Based on the results of these studies, ruxolitinib can be considered a standard of care for second-line therapy in this post-hydroxyurea patient population [120]. Ruxolitinib is also being tested in patients with high-risk ET who have become resistant to, or were intolerant of hydroxyurea, but currently has no approved indication in this setting [121,122]. Common side effects of ruxolitinib include cytopenias (especially anemia), increased risk of infections, hyperlipidemia, and increased risk of non-melanoma skin cancer.
Novel agents that have been studied in patients with PV and ET are histone deacetylase inhibitors, murine double minute 2 (MDM2, or HDM2 for their human counterpart) inhibitors (which restore the function of p53), Bcl-2 homology domain 3 mimetics such as navitoclax and venetoclax, and, for patients with ET, the telomerase inhibitor imetelstat [123].
Disease Evolution
Post-PV/Post-ET Myelofibrosis
Diagnostic criteria for post-PV and post-ET myelofibrosis are outlined in Table 4. Fibrotic transformation represents a natural evolution of the clinical course of PV or ET. It occurs in up to 15% and 9% of patients with PV and ET, respectively, in western countries [124]. The true percentage of ET patients who develop myelofibrosis is confounded by the inclusion of prefibrotic myelofibrosis cases in earlier series. The survival of patients who develop myelofibrosis is shortened compared to those who do not. In patients with PV, risk factors for myelofibrosis evolution include advanced age, leukocytosis, JAK2V617F homozygosity or higher allele burden, and hydroxyurea therapy. Once post-PV myelofibrosis has occurred, hemoglobin < 10 g/dL, platelet count < 100 × 103/μL, and WBC count > 30,000/μL are associated with worse outcomes [125]. In patients with ET, risk factors for myelofibrosis transformation include age, anemia, bone marrow hypercellularity and increased reticulin, increased lactate dehydrogenase, leukocytosis, and male gender. The management of post-PV/post-ET myelofibrosis recapitulates that of PMF.
Leukemic Transformation
The presence of more than 20% blasts in peripheral blood or bone marrow in a patient with MPN defines LT. This occurs in up to 5% to 10% of patients and may or may not be preceded by a myelofibrosis phase [126]. In cases of extramedullary transformation, a lower percentage of blasts can be seen in the bone marrow compared to the peripheral blood. The pathogenesis of LT has remained elusive, but it is believed to be associated with genetic instability, which facilitates the acquisition of additional mutations, including those of TET2, ASXL1, EZH2 DNMT3, IDH1/2, and TP53 [127].
Clinical risk factors for LT include advanced age, karyotypic abnormalities, prior therapy with alkylating agents or P-32, splenectomy, increased peripheral blood or bone marrow blasts, leukocytosis, anemia, thrombocytopenia, and cytogenetic abnormalities. Hydroxyurea, IFN, and ruxolitinib have not been shown to have leukemogenic potential thus far. Prognosis of LT is uniformly poor and patient survival rarely exceeds 6 months.
There is no standard of care for MPN LT. Treatment options range from low-intensity regimens to more aggressive AML-type induction chemotherapy. No strategy appears clearly superior to others [128]. Hematopoietic stem cell transplantation is the only therapy that provides clinically meaningful benefit to patients [129], but it is applicable only to a minority of patients with chemosensitive disease and good performance status [130]. Notable experimental approaches to MPN LT include hypomethylating agents, such as decitabine [131] or azacytidine [132], with or without ruxolitinib [133–135].
Conclusion
PV and ET are rare, chronic myeloid disorders. Patients typically experience a long clinical course and enjoy near-normal quality of life if properly managed. The 2 most important life-limiting complications of PV and ET are thrombohemorrhagic events and myelofibrosis/AML transformation. Vascular events are at least in part preventable with counseling on risk factors, phlebotomy (for patients with PV), antiplatelet therapy, and cytoreduction with hydroxyurea, IFNs, or anagrelide (for patients with ET). In addition, ruxolitinib was recently approved for PV patients after hydroxyurea failure. PV/ET transformation in myelofibrosis or AML is part of the natural history of the disease and no therapy has been shown to prevent it. Treatment follows recommendations set forth for PMF and AML, but results are generally poorer and novel strategies are needed to improve outcomes.
Corresponding author: Lorenzo Falchi, MD, Columbia University Medical Center, New York, NY.
Financial disclosures: None.
1. Swerdlow SH, Campo E, Harris NL, et al. WHO classification of tumours of haematopoietic and lymphoid tissues. 4th ed. Lyon: IARC; 2008.
2. Alvarez-Larran A, Pereira A, Arellano-Rodrigo E, et al. Cytoreduction plus low-dose aspirin versus cytoreduction alone as primary prophylaxis of thrombosis in patients with high-risk essential thrombocythaemia: an observational study. Br J Haematol 2013;161:865–71.
3. Tefferi A. Polycythemia vera and essential thrombocythemia: 2013 update on diagnosis, risk-stratification, and management. Am J Hematol 2013;88:507–16.
4. Michiels JJ, Berneman Z, Schroyens W, et al. Platelet-mediated erythromelalgic, cerebral, ocular and coronary microvascular ischemic and thrombotic manifestations in patients with essential thrombocythemia and polycythemia vera: a distinct aspirin-responsive and coumadin-resistant arterial thrombophilia. Platelets 2006;17:528–44.
5. James C, Ugo V, Couedic J-PL, et al. A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera. Nature 2005;434:1144–8.
6. Kralovics R, Passamonti F, Buser A, et al. A gain-of-function mutation of JAK2 in myeloproliferative disorders.N Engl J Med 2005;352:1779–90.
7. Verstovsek S, Mesa R, Gotlib J, et al. A double-blind, placebo-controlled trial of ruxolitinib for myelofibrosis. N Engl J Med 2012;366:799–807.
8. Harrison C, Kiladjian J, Al-Ali H, et al. JAK Inhibition with ruxolitinib versus best available therapy for myelofibrosis.N Engl J Med 2012;366:787–98.
9. Berglund S, Zettervall O. Incidence of polycythemia vera in a defined population. Eur J Haematol 1992;48:20–6.
10. Rozman C, Giralt M, Feliu E, et al. Life expectancy of patients with chronic nonleukemic myeloproliferative disorders. Cancer 1991;67:2658–63.
11. Passamonti F, Rumi E, Pungolino E, et al. Life expectancy and prognostic factors for survival in patients with polycythemia vera and essential thrombocythemia. Am J Med 2004;117:755–61.
12. Hultcrantz M, Kristinsson SY, Andersson TM, et al. Patterns of survival among patients with myeloproliferative neoplasms diagnosed in Sweden from 1973 to 2008: a population-based study. J Clin Oncol 2012;30:2995–3001.
13. Wolanskyj AP, Schwager SM, et al. Essential thrombocythemia beyond the first decade: life expectancy, long-term complication rates, and prognostic factors. Mayo Clin Proc 2006;81:159–66.
14. Srour SA, Devesa SS, Morton LM, et al. Incidence and patient survival of myeloproliferative neoplasms and myelodysplastic/myeloproliferative neoplasms in the United States, 2001-12. Br J Haematol 2016;174:382–96.
15. Titmarsh GJ, Duncombe AS, McMullin MF, et al. How common are myeloproliferative neoplasms? A systematic review and meta-analysis. Am J Hematol 2014;89:581–7.
16. Moulard O, Mehta J, Fryzek J, et al. Epidemiology of myelofibrosis, essential thrombocythemia, and polycythemia vera in the European Union. Eur J Haematol 2014;92:289–97.
17. Stein BL, Gotlib J, Arcasoy M, et al. Historical views, conventional approaches, and evolving management strategies for myeloproliferative neoplasms. J Natl Compr Canc Netw 2015;13:424–34.
18. Levine RL, Wadleigh M, Cools J, et al. Activating mutation in the tyrosine kinase JAK2 in polycythemia vera, essential thrombocythemia, and myeloid metaplasia with myelofibrosis. Cancer Cell 2005;7:387–97.
19. Baxter EJ, Scott LM, Campbell PJ, et al. Acquired mutation of the tyrosine kinase JAK2 in human myeloproliferative disorders. Lancet 2005;365:1054–61.
20. Scott LM, Tong W, Levine RL, et al. JAK2 exon 12 mutations in polycythemia vera and idiopathic erythrocytosis. N Engl J Med 2007;356:459–68.
21. Pardanani A, Lasho TL, Finke C, et al. Prevalence and clinicopathologic correlates of JAK2 exon 12 mutations in JAK2V617F-negative polycythemia vera. Leukemia 2007;21:1960–3.
22. Passamonti F, Elena C, Schnittger S, et al. Molecular and clinical features of the myeloproliferative neoplasm associated with JAK2 exon 12 mutations. Blood 2011;117:2813–6.
23. Abe M, Suzuki K, Inagaki O, et al. A novel MPL point mutation resulting in thrombopoietin-independent activation. Leukemia 2002;16:1500–6.
24. Pikman Y, Lee BH, Mercher T, et al. MPLW515L is a novel somatic activating mutation in myelofibrosis with myeloid metaplasia. PLoS Med 2006;3:e270.
25. Pardanani AD, Levine RL, Lasho T, et al. MPL515 mutations in myeloproliferative and other myeloid disorders: a study of 1182 patients. Blood 2006;108:3472–6.
26. Nangalia J, Massie CE, Baxter EJ, et al. Somatic CALR mutations in myeloproliferative neoplasms with nonmutated JAK2. N Engl J Med 2013;369:2391–405.
27. Klampfl T, Gisslinger H, Harutyunyan AS, et al. Somatic mutations of calreticulin in myeloproliferative neoplasms. N Engl J Med 2013;369:2379–90.
28. Wang WA, Groenendyk J, Michalak M. Calreticulin signaling in health and disease. Int J Biochem Cell Biol 2012;44:842–6.
29. Saeidi K. Myeloproliferative neoplasms: Current molecular biology and genetics. Crit Rev Oncol Hematol 2016;98:375–89.
30. Arber DA, Orazi A, Hasserjian R, et al. The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood 2016;127: 2391–405.
31. Mesa RA, Niblack J, Wadleigh M, et al. The burden of fatigue and quality of life in myeloproliferative disorders (MPDs): an international Internet-based survey of 1179 MPD patients. Cancer 2007;109:68–76.
32. Marchioli R, Finazzi G, Landolfi R, et al. Vascular and neoplastic risk in a large cohort of patients with polycythemia vera. J Clin Oncol 2005;23:2224–32.
33. Scherber R, Dueck AC, Johansson P, et al. The Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF): international prospective validation and reliability trial in 402 patients. Blood 2011;118:401–8.
34. Emanuel RM, Dueck AC, Geyer HL, et al. Myeloproliferative neoplasm (MPN) symptom assessment form total symptom score: prospective international assessment of an abbreviated symptom burden scoring system among patients with MPNs. J Clin Oncol 2012;30:4098–103.
35. Casini A, Fontana P, Lecompte TP. Thrombotic complications of myeloproliferative neoplasms: risk assessment and risk-guided management. J Thromb Haemost 2013;11:1215–27.
36. Barbui T, Finazzi G, Falanga A. Myeloproliferative neoplasms and thrombosis. Blood 2013;122:2176-–84.
37. Pearson TC, Wetherley-Mein G. Vascular occlusive episodes and venous haematocrit in primary myeloproliferative polychytemia. Lancet 1978;2:1219–22.
38. Landolfi R, Di Gennaro L, Barbui T, et al. Leukocytosis as a major thrombotic risk factor in patients with polycythemia vera. Blood 2007;109:2446–52.
39. Tefferi A, Rumi E, Finazzi G, et al. Survival and prognosis among 1545 patients with contemporary polycythemia vera: an international study. Leukemia 2013;27:1874–81.
40. Barbui T, Falanga A. Molecular biomarkers of thrombosis in myeloproliferative neoplasms. Thromb Res 2016;140 Suppl 1:S71–75.
41. Campbell PJ, Scott LM, Buck G, et al. Definition of subtypes of essential thrombocythaemia and relation to polycythaemia vera based on JAK2 V617F mutation status: a prospective study. Lancet 2005;366:1945–53.
42. Tefferi A, Guglielmelli P, Larson DR, et al. Long-term survival and blast transformation in molecularly annotated essential thrombocythemia, polycythemia vera, and myelofibrosis. Blood 2014;124:2507–13.
43. Qin Y, Wang X, Zhao C, et al. The impact of JAK2V617F mutation on different types of thrombosis risk in patients with essential thrombocythemia: a meta-analysis. Int J Hematol 2015;102:170–80.
44. Ziakas PD. Effect of JAK2 V617F on thrombotic risk in patients with essential thrombocythemia: measuring the uncertain. Haematologica 2008;93:1412–4.
45. Lussana F, Dentali F, Abbate R, et al. Screening for thrombophilia and antithrombotic prophylaxis in pregnancy: Guidelines of the Italian Society for Haemostasis and Thrombosis (SISET). Thromb Res 2009;124: e19–25.
46. Dahabreh IJ, Zoi K, Giannouli S, et al. Is JAK2 V617F mutation more than a diagnostic index? A meta-analysis of clinical outcomes in essential thrombocythemia. Leuk Res 2009;33:67–73.
47. Cho YU, Chi HS, Lee EH, et al. Comparison of clinicopathologic findings according to JAK2 V617F mutation in patients with essential thrombocythemia. Int J Hematol 2009;89:39–44.
48. Wolanskyj AP, Lasho TL, Schwager SM, et al. JAK2 mutation in essential thrombocythaemia: clinical associations and long-term prognostic relevance. Br J Haematol 2005;131:208–13.
49. Antonioli E, Guglielmelli P, Pancrazzi A, et al. Clinical implications of the JAK2 V617F mutation in essential thrombocythemia. Leukemia 2005;19:1847–9.
50. Palandri F, Catani L, Testoni N, et al. Long-term follow-up of 386 consecutive patients with essential thrombocythemia: safety of cytoreductive therapy. Am J Hematol 2009;84:215–20.
51. Chim CS, Sim JP, Chan CC, et al. Impact of JAK2V617F mutation on thrombosis and myeloid transformation in essential thrombocythemia: a multivariate analysis by Cox regression in 141 patients. Hematology 2010;15: 187–92.
52. Vannucchi AM, Antonioli E, Guglielmelli P, et al. Clinical profile of homozygous JAK2 617V>F mutation in patients with polycythemia vera or essential thrombocythemia. Blood 2007;110:840–6.
53. Carobbio A, Finazzi G, Antonioli E, et al. JAK2V617F allele burden and thrombosis: a direct comparison in essential thrombocythemia and polycythemia vera. Exp Hematol 2009;37:1016–21.
54. Alvarez-Larran A, Bellosillo B, Pereira A, et al. JAK2V617F monitoring in polycythemia vera and essential thrombocythemia: clinical usefulness for predicting myelofibrotic transformation and thrombotic events. Am J Hematol 2014;89:517–23.
55. Barbui T, Vannucchi AM, Buxhofer-Ausch V, et al. Practice-relevant revision of IPSET-thrombosis based on 1019 patients with WHO-defined essential thrombocythemia. Blood Cancer J 2015;5:e369.
56. Carobbio A, Thiele J, Passamonti F, et al. Risk factors for arterial and venous thrombosis in WHO-defined essential thrombocythemia: an international study of 891 patients. Blood 2011;117:5857–9.
57. Alvarez-Larran A, Cervantes F, Bellosillo B, et al. Essential thrombocythemia in young individuals: frequency and risk factors for vascular events and evolution to myelofibrosis in 126 patients. Leukemia 2007;21:1218–23.
58. Jantunen R, Juvonen E, Ikkala E, et al. The predictive value of vascular risk factors and gender for the development of thrombotic complications in essential thrombocythemia. Ann Hematol 2001;80:74–8.
59. Besses C, Cervantes F, Pereira A, et al. Major vascular complications in essential thrombocythemia: a study of the predictive factors in a series of 148 patients. Leukemia 1999;13:150–4.
60. Haider M, Gangat N, Hanson C, Tefferi A. Splenomegaly and thrombosis risk in essential thrombocythemia: the mayo clinic experience. Am J Hematol 2016;91: E296–297.
61. Carobbio A, Finazzi G, Antonioli E, et al. Thrombocytosis and leukocytosis interaction in vascular complications of essential thrombocythemia. Blood 2008;112:3135–7.
62. Palandri F, Polverelli N, Catani L, et al. Impact of leukocytosis on thrombotic risk and survival in 532 patients with essential thrombocythemia: a retrospective study. Ann Hematol 2011;90:933–8.
63. Campbell PJ, MacLean C, Beer PA, et al. Correlation of blood counts with vascular complications in essential thrombocythemia: analysis of the prospective PT1 cohort. Blood 2012;120:1409–11.
64. Cortelazzo S, Finazzi G, Ruggeri M, et al. Hydroxyurea for patients with essential thrombocythemia and a high risk of thrombosis. N Engl J Med 1995;332:1132–6.
65. van Genderen PJ, Mulder PG, Waleboer M, et al. Prevention and treatment of thrombotic complications in essential thrombocythaemia: efficacy and safety of aspirin. Br J Haematol 1997;97:179–84.
66. Storen EC, Tefferi A. Long-term use of anagrelide in young patients with essential thrombocythemia. Blood 2001;97:863–6.
67. De Stefano V, Za T, Rossi E, et al. Recurrent thrombosis in patients with polycythemia vera and essential thrombocythemia: incidence, risk factors, and effect of treatments. Haematologica 2008;93:372–80.
68. Alvarez-Larran A, Cervantes F, Pereira A, et al. Observation versus antiplatelet therapy as primary prophylaxis for thrombosis in low-risk essential thrombocythemia. Blood 2010;116:1205–10.
69. Palandri F, Polverelli N, Catani L, et al. Bleeding in essential thrombocythaemia: a retrospective analysis on 565 patients. Br J Haematol 2012;156:281–4.
70. Rotunno G, Mannarelli C, Guglielmelli P, et al. Impact of calreticulin mutations on clinical and hematological phenotype and outcome in essential thrombocythemia. Blood 2014;123:1552–5.
71. Tefferi A, Wassie EA, Lasho TL, et al. Calreticulin mutations and long-term survival in essential thrombocythemia. Leukemia 2014;28:2300–3.
72. Rumi E, Pietra D, Ferretti V, et al. JAK2 or CALR mutation status defines subtypes of essential thrombocythemia with substantially different clinical course and outcomes. Blood 2014;123:1544–51.
73. Palandri F, Latagliata R, Polverelli N, et al. Mutations and long-term outcome of 217 young patients with essential thrombocythemia or early primary myelofibrosis. Leukemia 2015;29:1344–9.
74. Fu R, Xuan M, Zhou Y, et al. Analysis of calreticulin mutations in Chinese patients with essential thrombocythemia: clinical implications in diagnosis, prognosis and treatment. Leukemia 2014;28:1912–4.
75. Tefferi A, Wassie EA, Guglielmelli P, et al. Type 1 versus Type 2 calreticulin mutations in essential thrombocythemia: a collaborative study of 1027 patients. Am J Hematol 2014;89:E121–4.
76. Pietra D, Rumi E, Ferretti VV, et al. Differential clinical effects of different mutation subtypes in CALR-mutant myeloproliferative neoplasms. Leukemia 2016;30: 431–8.
77. Rumi E, Pietra D, Guglielmelli P, et al. Acquired copy-neutral loss of heterozygosity of chromosome 1p as a molecular event associated with marrow fibrosis in MPL-mutated myeloproliferative neoplasms. Blood 2013;121:4388–95.
78. Beer PA, Campbell PJ, Scott LM, et al. MPL mutations in myeloproliferative disorders: analysis of the PT-1 cohort. Blood 2008;112:141–9.
79. Gangat N, Wassie EA, Lasho TL, et al. Mutations and thrombosis in essential thrombocythemia: prognostic interaction with age and thrombosis history. Eur J Haematol 2015;94:31–6.
80. Sekhar M, McVinnie K, Burroughs AK. Splanchnic vein thrombosis in myeloproliferative neoplasms. Br J Haematol 2013;162:730–47.
81. Stein BL, Saraf S, Sobol U, et al. Age-related differences in disease characteristics and clinical outcomes in polycythemia vera. Leuk Lymph 2013;54:1989–95.
82. Landolfi R, Di Gennaro L, Nicolazzi MA, et al. Polycythemia vera: gender-related phenotypic differences. Intern Emerg Med 2012;7:509–15.
83. Winslow ER, Brunt LM, Drebin JA, et al. Portal vein thrombosis after splenectomy. Am J Surg 2002;184:631–6.
84. Smalberg JH, Arends LR, Valla DC, et al. Myeloproliferative neoplasms in Budd-Chiari syndrome and portal vein thrombosis: a meta-analysis. Blood 2012;120:4921–8.
85. Dentali F, Squizzato A, Brivio L, et al. JAK2V617F mutation for the early diagnosis of Ph- myeloproliferative neoplasms in patients with venous thromboembolism: a meta-analysis. Blood 2009;113:5617–23.
86. Pardanani A, Lasho TL, Hussein K, et al. JAK2V617F mutation screening as part of the hypercoagulable work-up in the absence of splanchnic venous thrombosis or overt myeloproliferative neoplasm: assessment of value in a series of 664 consecutive patients. Mayo Clin Proc 2008;83:457–9.
87. Barbui T, Barosi G, Birgegard G, et al. Philadelphia-negative classical myeloproliferative neoplasms: critical concepts and management recommendations from European LeukemiaNet. J Clin Oncol 2011;29:761–70.
88. Landolfi R, Marchioli R, Kutti J, et al. Efficacy and safety of low-dose aspirin in polycythemia vera. N Engl J Med 2004;350:114–24.
89. Marchioli R, Finazzi G, Specchia G, et al. Cardiovascular events and intensity of treatment in polycythemia vera. N Engl J Med 2013;368:22–33.
90. Kiladjian JJ, Chevret S, Dosquet C, et al. Treatment of polycythemia vera with hydroxyurea and pipobroman: final results of a randomized trial initiated in 1980. J Clin Oncol 2011;29:3907–13.
91. Kaplan ME, Mack K, Goldberg JD, et al. Long-term management of polycythemia vera with hydroxyurea: a progress report. Semin Hematol 1986;23:167–71.
92. Fruchtman SM, Mack K, Kaplan ME, et al. From efficacy to safety: a Polycythemia Vera Study group report on hydroxyurea in patients with polycythemia vera. Semin Hematol 1997;34:17–23.
93. Finazzi G, Caruso V, Marchioli R, et al. Acute leukemia in polycythemia vera: an analysis of 1638 patients enrolled in a prospective observational study. Blood 2005;105: 2664–70.
94. Barosi G, Mesa R, Finazzi G, et al. Revised response criteria for polycythemia vera and essential thrombocythemia: an ELN and IWG-MRT consensus project. Blood 2013;121:4778–81.
95. Alvarez-Larran A, Pereira A, Cervantes F, et al. Assessment and prognostic value of the European LeukemiaNet criteria for clinicohematologic response, resistance, and intolerance to hydroxyurea in polycythemia vera. Blood 2012;119:1363–9.
96. Stein BL, Tiu RV. Biological rationale and clinical use of interferon in the classical BCR-ABL-negative myeloproliferative neoplasms. J Interferon Cytokine Res 2013;33: 145–53.
97. Ludwig H, Cortelezzi A, Van Camp BG, et al. Treatment with recombinant interferon-alpha-2C: multiple myeloma and thrombocythaemia in myeloproliferative diseases. Oncology 1985;42 Suppl 1:19–25.
98. Silver RT. Long-term effects of the treatment of polycythemia vera with recombinant interferon-alpha. Cancer 2006;107:451–8.
99. Kiladjian JJ, Mesa RA, Hoffman R. The renaissance of interferon therapy for the treatment of myeloid malignancies. Blood 2011;117:4706–15.
100. Veronese FM, Mero A. The impact of PEGylation on biological therapies. BioDrugs 2008;22:315–29.
101. Kiladjian JJ, Cassinat B, Chevret S, et al. Pegylated interferon-alfa-2a induces complete hematologic and molecular responses with low toxicity in polycythemia vera. Blood 2008;112:3065–72.
102. Turlure P, Cambier N, Roussel M, et al. Complete hematological, molecular and histological remissions without cytoreductive treatment lasting after pegylated-interferon {alpha}-2a (peg-IFN{alpha}-2a) therapy in polycythemia vera (PV): long term results of a phase 2 trial [abstract]. Blood 2011;118(21). Abstract 280.
103. Quintas-Cardama A, Kantarjian H, Manshouri T, et al. Pegylated interferon alfa-2a yields high rates of hematologic and molecular response in patients with advanced essential thrombocythemia and polycythemia vera. J Clin Oncol 2009;27:5418–24.
104. Quintas-Cardama A, Abdel-Wahab O, Manshouri T, et al. Molecular analysis of patients with polycythemia vera or essential thrombocythemia receiving pegylated interferon a-2a. Blood 2013;122:893–901.
105. Samuelsson J, Hasselbalch H, Bruserud O, et al. A phase II trial of pegylated interferon alpha-2b therapy for polycythemia vera and essential thrombocythemia: feasibility, clinical and biologic effects, and impact on quality of life. Cancer 2006;106:2397–405.
106. Jabbour E, Kantarjian H, Cortes J, et al. PEG-IFN-alpha-2b therapy in BCR-ABL-negative myeloproliferative disorders: final result of a phase 2 study. Cancer 2007; 110:2012–18.
107. Them NC, Bagienski K, Berg T, et al. Molecular responses and chromosomal aberrations in patients with polycythemia vera treated with peg-proline-interferon alpha-2b. Am J Hematol 2015;90:288–94.
108. Gisslinger H, Klade C, Georgiev P, et al. Final results from PROUD-PV a randomized controlled phase 3 trial comparing ropeginterferon alfa-2b to hydroxyurea in polycythemia vera patients [abstract]. Blood 2016;128(suppl 22). Abstract 475.
109. van Genderen PJ, van Vliet HH, Prins FJ, et al. Excessive prolongation of the bleeding time by aspirin in essential thrombocythemia is related to a decrease of large von Willebrand factor multimers in plasma. Ann Hematol 1997;75:215–20.
110. Cortelazzo S, Finazzi G, Ruggeri M, et al. Hydroxyurea for patients with essential thrombocythemia and a high risk of thrombosis. N Engl J Med 1995;332:1132–7.
111. Harrison CN, Campbell PJ, Buck G, et al. Hydroxyurea compared with anagrelide in high-risk essential thrombocythemia. N Engl J Med 2005;353:33–45.
112. Gisslinger H, Gotic M, Holowiecki J, et al. Anagrelide compared with hydroxyurea in WHO-classified essential thrombocythemia: the ANAHYDRET Study, a randomized controlled trial. Blood 2013;121:1720–8.
113. Alvarado Y, Cortes J, Verstovsek S, et al. Pilot study of pegylated interferon-alpha 2b in patients with essential thrombocythemia. Cancer Chemother Pharmacol 2003;51:81–6.
114. Barosi G, Tefferi A, Barbui T, ad hoc committee ‘Definition of clinically relevant outcomes for contemporarily clinical trials in Ph-neg M. Do current response criteria in classical Ph-negative myeloproliferative neoplasms capture benefit for patients? Leukemia 2012;26:1148–9.
115. Bjorkholm M, Derolf AR, Hultcrantz M, et al. Treatment-related risk factors for transformation to acute myeloid leukemia and myelodysplastic syndromes in myeloproliferative neoplasms. J Clin Oncol 2011;29:2410–5.
116. Alvarez-Larran A, Martinez-Aviles L, Hernandez-Boluda JC, et al. Busulfan in patients with polycythemia vera or essential thrombocythemia refractory or intolerant to hydroxyurea. Ann Hematol 2014;93:2037–43.
117. Verstovsek S, Passamonti F, Rambaldi A, et al. A phase 2 study of ruxolitinib, an oral JAK1 and JAK2 Inhibitor, in patients with advanced polycythemia vera who are refractory or intolerant to hydroxyurea. Cancer 2014;120: 513–20.
118. Vannucchi AM, Kiladjian JJ, Griesshammer M, et al. Ruxolitinib in polycythemia vera resistant to or intolerant of hydroxyurea. N Engl J Med 2015; 372:426–35.
119. Verstovsek S, Vannucchi AM, Griesshammer M, et al. Ruxolitinib versus best available therapy in patients with polycythemia vera: 80-week follow-up from the RESPONSE trial. Haematologica 2016;101:821–9.
120. Passamonti F, Griesshammer M, Palandri F, et al. Ruxolitinib for the treatment of inadequately controlled polycythaemia vera without splenomegaly (RESPONSE-2): a randomised, open-label, phase 3b study. Lancet Oncol 2017;18:88–99.
121. Verstovsek S, Passamonti F, Rambaldi A, et al. Long-term results from a phase II open-label study of ruxolitinib in patients with essential thrombocythemia refractory to or intolerant of hydroxyurea [abstract]. Blood 2014;124. Abstract 1847.
122. Harrison CN, Mead AJ, Panchal A, et al. Ruxolitinib versus best available therapy for ET intolerant or resistant to hydroxycarbamide in a randomized trial. Blood 2017 Aug 9. pii: blood-2017-05-785790 .
123. Bose P, Verstovsek S. Drug development pipeline for myeloproliferative neoplasms: potential future impact on guidelines and management. J Natl Compr Canc Netw 2016;14:1613–24.
124. Cerquozzi S, Teffieri A. Blast transformation and fibrotic progression in polycythemia vera and essential thrombocythemia: a literature review of incidence and risk factors. Blood Cancer J 2015;Nov 13;5:e366.
125. Passamonti F, Rumi E, Caramella M, et al. A dynamic prognostic model to predict survival in post-polycythemia vera myelofibrosis. Blood 2008;111:3383–7.
126. Mesa RA, Verstovsek S, Cervantes F, et al. Primary myelofibrosis (PMF), post polycythemia vera myelofibrosis (post- PV MF), post essential thrombocythemia myelofibrosis (post-ET MF), blast phase PMF (PMF-BP): Consensus on terminology by the international working group for myelofibrosis research and treatment (IWG-MRT). Leuk Res 2007;31:737–40.
127. Rampal R, Mascarenhas J. Pathogenesis and management of acute myeloid leukemia that has evolved from a myeloproliferative neoplasm. Curr Opin Hematol 2014;21:65–71.
128. Chihara D, Kantarjian HM, Newberry KJ, et al. Survival outcome of patients with acute myeloid leukemia transformed from myeloproliferative neoplasms [abstract]. Blood 2016;128. Abstract 1940.
129. Tam CS, Nussenzveig RM, Popat U, et al. The natural history and treatment outcome of blast phase BCR-ABL-myeloproliferative neoplasms. Blood 2008;112:1628–37.
130. Kundranda MN, Tibes R, Mesa RA. Transformation of a chronic myeloproliferative neoplasm to acute myelogenous leukemia: does anything work? Curr Hematol Malig Rep 2012;7:78–86.
131. Badar T, Kantarjian HM, Ravandi F, et al. Therapeutic benefit of decitabine, a hypomethylating agent, in patients with high-risk primary myelofibrosis and myeloproliferative neoplasm in accelerated or blastic/acute myeloid leukemia phase. Leuk Res 2015;39:950–6.
132. Thepot S, Itzykson R, Seegers V, et al. Treatment of progression of Philadelphia-negative myeloproliferative neoplasms to myelodysplastic syndrome or acute myeloid leukemia by azacitidine: a report on 54 cases on the behalf of the Groupe Francophone des Myelodysplasies (GFM). Blood 2010;116:3735–42.
133. Pemmaraju N, Kantarjian H, Kadia T, et al. A phase I/II study of the Janus kinase (JAK)1 and 2 inhibitor ruxolitinib in patients with relapsed or refractory acute myeloid leukemia. Clin Lymphoma Myeloma Leuk 2015;15:171–6.
134. Rampal RK, Mascarenhas JO, Kosiorek HE, et al. Safety and efficacy of combined ruxolitinib and decitabine in patients with blast-phase MPN and post-MPN AML: results of a phase I study (Myeloproliferative Disorders Research Consortium 109 trial) [abstract]. Blood 2016;128. Abstract 1124.
135. Bose P, Verstovsek S, Gasior Y, et al. Phase I/II study of ruxolitinib (RUX) with decitabine (DAC) in patients with post-myeloproliferative neoplasm acute myeloid leukemia (post-MPN AML): phase I results [abstract]. Blood 2016;128. Abstract 4262.
From the Columbia University Medical Center, New York, NY (Dr. Falchi), and the University of Texas MD Anderson Cancer Center, Houston, TX (Dr. Verstovsek).
ABSTRACT
- Objective: To review the clinical aspects and current practices in the management of polycythemia vera (PV) and essential thrombocythemia (ET).
- Methods: Review of the literature.
- Results: PV and ET are rare chronic myeloid disorders. The 2 most important life-limiting complications of PV and ET are thrombohemorrhagic events and myelofibrosis/acute myeloid leukemia (AML) transformation. Vascular events are at least in part preventable with counseling on risk factors, phlebotomy (for patients with PV), antiplatelet therapy, and cytoreduction with hydroxyurea, interferons, or anagrelide (for patients with ET). Ruxolitinib was recently approved for PV after hydroxyurea failure. PV/ET transformation into myelofibrosis or AML is part of the natural history of the disease and no therapy has been shown to prevent it. Treatment of leukemic transformation of myeloproliferative neoplasms (MPN LT) follows recommendations set forth for primary myelofibrosis and AML.
- Conclusion: With appropriate management, patients with PV and ET typically enjoy a long survival and near-normal quality of life. Transformation into myelofibrosis or AML cannot be prevented by current therapies, however. Treatment results with MPN LT are generally poor and novel strategies are needed to improve outcomes.
Key words: myeloproliferative neoplasms; myelofibrosis; leukemic transformation.
Polycythemia vera (PV) and essential thrombocythemia (ET), along with primary myelofibrosis (PMF), belong to the group of Philadelphia-negative myeloproliferative neoplasms (MPN). All these malignancies arise from the clonal proliferation of an aberrant hematopoietic stem cell, but are characterized by distinct clinical phenotypes [1,2]. Although the clinical course of PV and ET is indolent, it can be complicated by thrombohemorrhagic episodes and/or evolution into myelofibrosis and/or acute myeloid leukemia (AML) [3]. Since vascular events are the most frequent life-threatening complications of PV and ET, therapeutic strategies are aimed at reducing this risk. Treatment may also help control other symptoms associated with the disease [4]. No therapy has been shown to prevent evolution of PV or ET into myelofibrosis or AML. The discovery of the Janus kinase 2 (JAK2)/V617F mutation in most patients with PV and over half of those with ET (and PMF) [5,6] has opened new avenues of research and led to the development of targeted therapies, such as the JAK1/2 inhibitor ruxolitinib, for patients with MPN [7,8].
Epidemiology
PV and ET are typically diagnosed in the fifth to seventh decade of life [9]. Although these disorders are generally associated with a long clinical course, survival of patients with PV or ET may be shorter than that of the general population [10–13]. Estimating the incidence and prevalence of MPN is a challenge because most patients remain asymptomatic for long periods of time and do not seek medical attention [13]. The annual incidence rates of PV and ET are estimated at 0.01 to 2.61 and 0.21 to 2.53 per 100,000, respectively. PV occurs slightly more frequently in males, whereas ET has a predilection for females [14]. Given the long course and low mortality associated with these disorders, the prevalence rates of PV and ET are significantly higher than the respective incidence rates: up to 47 and 57 per 100,000, respectively [15–17].
Molecular Pathogenesis
In 2005 researchers discovered a gain-of-function mutation of the JAK2 gene in nearly all patients with PV and more than half of those with ET and PMF [5,6,18,19]. JAK2 is a non-receptor tyrosine kinase that plays a central role in normal hematopoiesis. Substitution of a valine for a phenylalanine at codon 617 (ie, V617F) leads to its constitutive activation and signaling through the JAK-STAT pathway [5,6,18,19]. More rarely (and exclusively in patients with PV), JAK2 mutations involve exon 12 [20–22]. The vast majority of JAK2-negative ET patients harbor mutations in either the myeloproliferative leukemia (MPL) gene, which encodes the thrombopoietin receptor [23–25], or the calreticulin (CALR) gene [26,27], which encodes for a chaperone protein that plays a role in cellular proliferation, differentiation, and apoptosis [28]. Both the MPL and CALR mutations ultimately result in the constitutive activation of the JAK-STAT pathway. Thus, JAK2, MPL, and CALR alterations are collectively referred to as driver mutations. Moreover, because these mutations affect the same oncogenic pathway (ie, JAK-STAT), they are almost always mutually exclusive in a given patient. Patients with ET (or myelofibrosis) who are wild-type for JAK2, MPL, and CALR are referred to as having “triple-negative” disease. Many recurrent non-driver mutations are also found in patients with MPN. These are not exclusive of each other (ie, patients may have many at the same time) and involve for example ten-eleven translocation-2 (TET2), additional sex combs like 1 (ASXL1), enhancer of zeste homolog 2 (EZH2), isocitrate dehydrogenase 1 and isocitrate dehydrogenase 2 (IDH1/2), and DNA methyltransferase 3A (DNMT3A) genes, among others [29]. The biologic and prognostic significance of these non-driver alterations remain to be fully defined in ET and PV.
Diagnostic Criteria
Diagnostic criteria for PV and ET according to the World Health Organization (WHO) classification [30] are summarized in Table 1. Criteria for the diagnosis of prefibrotic myelofibrosis are included as well since this entity was formally recognized as separate from ET and part of the PMF spectrum in the 2016 WHO classification of myeloid tumors [30]. Clinically, both PV and ET generally remain asymptomatic for a long time. PV tends to be more symptomatic than ET and can present with debilitating constitutional symptoms (fatigue, night sweats, and weight loss), microvascular symptoms (headache, lightheadedness, acral paresthesias, erythromelalgia, atypical chest pain, and pruritus) [31], or macrovascular accidents (larger vein thrombosis, stroke, or myocardial ischemia) [32]. ET is often diagnosed incidentally, but patients can suffer from similar general symptoms and vascular complications. Causes of secondary absolute erythrocytosis (altitude, chronic hypoxemia, heavy smoking, cardiomyopathy, use of corticosteroids, erythropoietin, or other anabolic hormones, familial or congenital forms) or thrombocytosis (iron deficiency, acute blood loss, trauma or injury, acute coronary syndrome, systemic autoimmune disorders, chronic kidney failure, other malignancies, splenectomy) should be considered and appropriately excluded. Once the diagnosis is made, symptom assessment tools such as the Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF) [33] or the abbreviated version, the MPN-SAF Total Symptom Score (MPN-SAF TSS) [34], are generally used to assess patients’ symptom burden and response to treatment in everyday practice.
Risk Stratification
Thrombohemorrhagic events, evolution into myelofibrosis, and leukemic transformation (LT) are the most serious complications in the course of PV or ET. Only thrombohemorrhagic events are, at least partially, preventable. Arterial or venous thrombotic complications are observed at rates of 1.8 to 10.9 per 100 patient-years in PV (arterial thrombosis being more common than venous) and 0.74 to 7.7 per 100 patient-years in ET, depending on the risk group [35] and the presence of other factors (see below).
The risk stratification of patients with PV is based on 2 factors: age ≥ 60 years and prior history of thrombosis. If either is present, the patient is assigned to the high-risk category, whereas if none is present the patient is considered at low risk [36]. In addition, high hematocrit [37] and high white blood cell (WBC) count [38], but not thrombocytosis, have been associated with the development of vascular complications. In one study, the risk of new arterial thrombosis was increased by the presence of leukoerythroblastosis, hypertension, and prior arterial thrombosis, while karyotypic abnormalities and prior venous thrombosis were predictors of new venous thrombosis [39]. Another emerging risk factor for thrombosis in patients with PV is high JAK2 allele burden (ie, the normal-to-mutated gene product ratio), although the evidence supporting this conclusion is equivocal [40].
Traditionally, in ET patients, the thrombotic risk was assessed using the same 2 factors (age ≥ 60 years and prior history of thrombosis), separating patients into low- and high-risk groups. However, the prognostication of ET patients has been refined recently with the identification of new relevant factors. In particular, the impact of JAK2 mutations on thrombotic risk has been thoroughly studied. Clinically, the presence of JAK2V617F is associated with older age, higher hemoglobin and hematocrit, lower platelet counts, more frequent need for cytoreductive treatment, and greater tendency to evolve into PV (a rare event) [41,42]. Many [41,43–46], but not all [47–51], studies suggested a correlation between JAK2 mutation and risk of both arterial and venous thrombosis. Although infrequent, a JAK2V617F homozygous state (ie, the mutation is present in both alleles) might confer an even higher thrombotic risk [52]. Moreover, the impact of the JAK2 mutation on vascular events persists over time [53], particularly in patients with high or unstable mutation burden [54]. Based on JAK2V617F’s influence on the thrombotic risk of ET patients, a new prognostic score was proposed, the International Prognostic Score for ET (IPSET)-thrombosis (Table 2). The revised version of this model is currently endorsed by the National Comprehensive Cancer Network and divides patients into 4 risk groups: high, intermediate, low, and very low. Treatment recommendations vary according to the risk group (as described below) [55].
Other thrombotic risk factors have been identified, but deemed not significant enough to be included in the model. Cardiovascular risk factors (hypercholesterolemia, hypertension, smoking, diabetes mellitus) can increase the risk of vascular events [56–59], as can splenomegaly [60] and baseline or persistent leukocytosis [61–63]. Thrombocytosis has been correlated with thrombotic risk in some studies [64–68], whereas others did not support this conclusion and/or suggested a lower rate of thrombosis and, in some cases, increased risk of bleeding in ET patients with platelet counts greater than 1000 × 103/μL (due to acquired von Willebrand syndrome) [51,61,63,68,69].
CALR mutations tend to occur in younger males with lower hemoglobin and WBC count, higher platelet count, and greater marrow megakaryocytic predominance, as compared to JAK2 mutations [26,27,70–72]. The associated incidence of thrombosis was less than 10% at 15 years in patients with CALR mutations, lower than the incidence reported for ET patients with JAK2V617F mutations [73]. The presence of the mutation per se does not appear to affect the thrombotic risk [74–76]. Information on the thrombotic risk associated with MPL mutations or a triple-negative state is scarce. In both instances, however, the risk appears to be lower than with the JAK2 mutation [73,77–79].
Venous thromboembolism (VTE) in patients with PV or ET may occur at unusual sites, such as the splanchnic or cerebral venous systems [80]. Risk factors for unusual VTE include younger age [81], female gender (especially with concomitant use of oral contraceptive pills) [82], and splenomegaly/splenectomy [83]. JAK2 mutation has also been associated with thrombosis at unusual sites. However, the prevalence of MPN or JAK2V617F in patients presenting with splanchnic VTE has varied [80]. In addition, MPN may be occult (ie, no clinical or laboratory abnormalities) in around 15% of patients [84]. Screening for JAK2V617F and underlying MPN is recommended in patients presenting with isolated unexplained splanchnic VTE. Treatment entails long-term anticoagulation therapy. JAK2V617F screening in patients with nonsplanchnic VTE is not recommended, as its prevalence in this group is low (< 3%) [85,86].
Risk-Adapted Therapy
Low-Risk PV
All patients with PV should receive counseling to mitigate cardiovascular risk factors, including smoking cessation, lifestyle modifications, and lipid-lowering therapy, as indicated. Furthermore, all PV patients should receive acetylsalicylic acid (ASA) to decrease their risk for thrombosis and control vasomotor symptoms [55,87]. Aspirin 81 to 100 mg daily is the preferred regimen because it provides adequate antithrombotic effect without the associated bleeding risk of higher-dose aspirin [88]. Low-risk PV patients should also receive periodic phlebotomies to reduce and maintain their hematocrit below 45%. This recommendation is based on the results of the Cytoreductive Therapy in Polycythemia Vera (CYTO PV) randomized controlled trial. In that study, patients receiving more intense therapy to maintain the hematocrit below 45% had a lower incidence of cardiovascular-related deaths or major thrombotic events than those with hematocrit goals of 45% to 50% (2.7% versus 9.8%) [89]. Cytoreduction is an option for low-risk patients who do not tolerate phlebotomy or require frequent phlebotomy, or who have disease-related bleeding, severe symptoms, symptomatic splenomegaly, or progressive leukocytosis [38].
High-Risk PV
Patients older than 60 years and/or with a history of thrombosis should be considered for cytoreductive therapy in addition to the above measures. Frontline cytoreductive therapies include hydroxyurea or interferon (IFN)-alfa [87]. Hydroxyurea is a potent ribonucleotide reductase inhibitor that interferes with DNA repair and is the treatment of choice for most high-risk patients with PV [90]. In a small trial, hydroxyurea reduced the risk of thrombosis compared with historical controls treated with phlebotomy alone [91]. Hydroxyurea is generally well tolerated; common side effects include cytopenias, nail changes, and mucosal and/or skin ulcers. Although never formally proven to be leukemogenic, this agent should be used with caution in younger patients [87]. Indeed, in the original study, the rates of transformation were 5.9% and 1.5% for patients receiving hydroxyurea and phlebotomy alone [92], respectively, although an independent role for hydroxyurea in LT was not supported in the much larger European Collaboration on Low-dose Aspirin in Polycythemia Vera (ECLAP) study [93]. Approximately 70% of patients will have a sustained response to hydroxyurea [94], while the remaining patients become resistant to or intolerant of the drug. Resistant individuals have a higher risk of progression to acute leukemia and death [95].
IFN-alfa is a pleiotropic antitumor agent that has found application in many types of malignancies [96] and is sometimes employed as treatment for patients with newly diagnosed high-risk PV. Early studies showed responses in up to 100% of cases [97,98], albeit at the expense of a high discontinuation rate due to adverse events, such as flu-like symptoms, fatigue, and neuropsychiatric manifestations [99]. A newer formulation of the drug obtained by adding a polyethylene glycol (PEG) moiety to the native IFN-alfa molecule (PEG-IFN alfa) was shown to have a longer half-life, greater stability, less immunogenicity, and, potentially, better tolerability [100]. Pilot phase 2 trials of PEG-IFN-alfa-2a demonstrated its remarkable activity, with symptomatic and hematologic responses seen in most patients (which, in some cases, persisted beyond discontinuation), and reasonable tolerability, with long-term discontinuation rates of 20% to 30% [101–103]. In some patients, JAK2V617F became undetectable over time [104]. Results of 2 ongoing trials, MDP-RC111 (single-arm study, PEG-IFN-alfa-2a in high-risk PV or ET [NCT01259817]) and MPD-RC112 (randomized controlled trial, PEG-IFN-alfa-2a versus hydroxyurea in the same population [NCT01258856]), will shed light on the role of PEG-IFN-alfa in the management of patients with high-risk PV or ET. In two phase 2 studies of PEG-IFN-alfa-2b, complete responses were seen in 70% to 100% of patients and discontinuation occurred in around a third of cases [105,106]. A new, longer-acting formulation of PEG-IFN-alfa-2a (peg-proline INF-alfa-2b, AOP2014) is also undergoing clinical development [107,108].
The approach to treatment of PV based on thrombotic risk level is illustrated in Figure 1.
Very Low- and Low-Risk ET
Individuals with ET should undergo rigorous cardiovascular risk management and generally receive ASA to decrease their thrombotic risk and improve symptom control. Antiplatelet therapy may not be warranted in patients with documented acquired von Willebrand syndrome, with or without extreme thrombocytosis, or in those in the very low-risk category according to the IPSET-thrombosis model [55,87]. The risk/benefit ratio of antiplatelet agents in patients with ET at different thrombotic risk levels was assessed in poor-quality studies and thus remains highly uncertain. Platelet-lowering agents are sometimes recommended in patients with low-risk disease who have platelet counts ≥ 1500 × 103/μL, due to the potential risk of acquired von Willebrand syndrome and a risk of bleeding (this would require stopping ASA) [109]. Cytoreduction may also be used in low-risk patients with progressive symptoms despite ASA, symptomatic or progressive splenomegaly, and progressive leukocytosis.
Intermediate-Risk ET
This category includes patients older than 60 years, but without thrombosis or JAK2 mutations. These individuals would have been considered high risk (and thus candidates for cytoreductive therapy) according to the traditional risk stratification. Guidelines currently recommend ASA as the sole therapy for these patients, while reserving cytoreduction for those who experience thrombosis (ie, become high-risk) or have uncontrolled vasomotor or general symptoms, symptomatic splenomegaly, symptomatic thrombocytosis, or progressive leukocytosis.
High-Risk ET
For patients with ET in need of cytoreductive therapy (ie, those with prior thrombosis or older than 60 years with a JAK2V617F mutation), first-line options include hydroxyurea, IFN, and anagrelide. Hydroxyurea remains the treatment of choice in most patients [110]. In a seminal study, 114 patients with ET were randomly assigned to either observation or hydroxyurea treatment with the goal of maintaining the platelet count below 600 × 103/μL. At a median follow-up of 27 months, patients in the hydroxyurea group had a lower thrombosis rate (3.6% versus 24%, P = 0.003) and longer thrombosis-free survival, regardless of the use of antiplatelet drugs [64].
Anagrelide, a selective inhibitor of megakaryocytic differentiation and proliferation, was compared with hydroxyurea in patients with ET in 2 randomized trials. In the first (n = 809), the group receiving anagrelide had a higher risk of arterial thrombosis, major bleeding, and fibrotic evolution, but lower incidence of venous thrombosis. Hydroxyurea was better tolerated, mainly due to anagrelide-related cardiovascular adverse events [111]. As a result of this study, hydroxyurea is often preferred to anagrelide as frontline therapy for patients with newly diagnosed high-risk ET. In the second, more recent study (n = 259), however, the 2 agents proved equivalent in terms of major or minor arterial or venous thrombosis, as well as discontinuation rate [112]. The discrepancy between the 2 trials may be partly explained by the different ET diagnostic criteria used, with the latter only enrolling patients with WHO-defined true ET and the former utilizing Polycythemia Vera Study Group-ET diagnostic criteria that included patients with increases in other blood counts or varying degrees of marrow fibrosis.
Interferons were studied in ET in parallel with PV. PEG-IFN-alfa-2a proved effective in patients with ET, with responses observed in 80% of patients [103]. PEG-IFN- alfa-2b produced similar results, with responses in 70% to 90% of patients in small studies and discontinuation observed in 20% to 38% of cases [105,106,113]. Because the very long-term leukemogenic potential of hydroxyurea has remained somewhat uncertain, anagrelide or IFN might be preferable choices in younger patients.
The approach to treatment of ET based on thrombotic risk level is illustrated in Figure 2.
Assessing Response to Therapy
For both patients with PV and ET the endpoint of treatment set forth for clinical trials has been the achievement of a clinicohematologic response. However, studies have failed to show a correlation between response and reduction of the thrombohemorrhagic risk [114]. Therefore, proposed clinical trial response criteria were revised to include absence of hemorrhagic or thrombotic events as part of the definition of response (Table 3) [94].
Approach to Patients Refractory to or Intolerant of First-line Therapy
According to the European LeukemiaNet recommendations, an inadequate response to hydroxyurea in patients with PV (or myelofibrosis) is defined as a need for phlebotomy to maintain the hematocrit below < 45%, the platelet count > 400 × 103/μL, and a WBC count > 10,000/μL, or failure to reduce splenomegaly > 10 cm by > 50% at a dose of ≥ 2 g/day or maximum tolerated dose. Historically, treatment options for patients with PV or ET who failed first-line therapy (most commonly hydroxyurea) have included alkylating agents, such as busulfan, chlorambucil, pipobroman, and phosphorus (P)-32. However, the use of these drugs is limited by the associated risk of LT [93,115,116]. IFN (or anagrelide for ET) is often considered in patients previously treated with hydroxyurea, and vice versa.
Ruxolitinib is a JAK1 and JAK2 inhibitor currently approved for the treatment of PV patients refractory to or intolerant of hydroxyurea [7]. Following promising results of a phase 2 trial [117], ruxolitinib 10 mg twice daily was compared with best available therapy in the pivotal RESPONSE trial (n = 222). Ruxolitinib proved superior in achieving hematocrit control, reduction of spleen volume, and improvement of symptoms. Grade 3-4 hematologic toxicity was infrequent and similar in the 2 arms [118]. In addition, longer follow-up of that study suggested a lower rate of thrombotic events in patients receiving ruxolitinib (1.8 versus 8.2 per 100 patient-years) [119]. In a similarly designed randomized phase 3 study in PV patients without splenomegaly (RESPONSE-2), more patients in the ruxolitinib arm had hematocrit reduction without an increase in toxicity. Based on the results of these studies, ruxolitinib can be considered a standard of care for second-line therapy in this post-hydroxyurea patient population [120]. Ruxolitinib is also being tested in patients with high-risk ET who have become resistant to, or were intolerant of hydroxyurea, but currently has no approved indication in this setting [121,122]. Common side effects of ruxolitinib include cytopenias (especially anemia), increased risk of infections, hyperlipidemia, and increased risk of non-melanoma skin cancer.
Novel agents that have been studied in patients with PV and ET are histone deacetylase inhibitors, murine double minute 2 (MDM2, or HDM2 for their human counterpart) inhibitors (which restore the function of p53), Bcl-2 homology domain 3 mimetics such as navitoclax and venetoclax, and, for patients with ET, the telomerase inhibitor imetelstat [123].
Disease Evolution
Post-PV/Post-ET Myelofibrosis
Diagnostic criteria for post-PV and post-ET myelofibrosis are outlined in Table 4. Fibrotic transformation represents a natural evolution of the clinical course of PV or ET. It occurs in up to 15% and 9% of patients with PV and ET, respectively, in western countries [124]. The true percentage of ET patients who develop myelofibrosis is confounded by the inclusion of prefibrotic myelofibrosis cases in earlier series. The survival of patients who develop myelofibrosis is shortened compared to those who do not. In patients with PV, risk factors for myelofibrosis evolution include advanced age, leukocytosis, JAK2V617F homozygosity or higher allele burden, and hydroxyurea therapy. Once post-PV myelofibrosis has occurred, hemoglobin < 10 g/dL, platelet count < 100 × 103/μL, and WBC count > 30,000/μL are associated with worse outcomes [125]. In patients with ET, risk factors for myelofibrosis transformation include age, anemia, bone marrow hypercellularity and increased reticulin, increased lactate dehydrogenase, leukocytosis, and male gender. The management of post-PV/post-ET myelofibrosis recapitulates that of PMF.
Leukemic Transformation
The presence of more than 20% blasts in peripheral blood or bone marrow in a patient with MPN defines LT. This occurs in up to 5% to 10% of patients and may or may not be preceded by a myelofibrosis phase [126]. In cases of extramedullary transformation, a lower percentage of blasts can be seen in the bone marrow compared to the peripheral blood. The pathogenesis of LT has remained elusive, but it is believed to be associated with genetic instability, which facilitates the acquisition of additional mutations, including those of TET2, ASXL1, EZH2 DNMT3, IDH1/2, and TP53 [127].
Clinical risk factors for LT include advanced age, karyotypic abnormalities, prior therapy with alkylating agents or P-32, splenectomy, increased peripheral blood or bone marrow blasts, leukocytosis, anemia, thrombocytopenia, and cytogenetic abnormalities. Hydroxyurea, IFN, and ruxolitinib have not been shown to have leukemogenic potential thus far. Prognosis of LT is uniformly poor and patient survival rarely exceeds 6 months.
There is no standard of care for MPN LT. Treatment options range from low-intensity regimens to more aggressive AML-type induction chemotherapy. No strategy appears clearly superior to others [128]. Hematopoietic stem cell transplantation is the only therapy that provides clinically meaningful benefit to patients [129], but it is applicable only to a minority of patients with chemosensitive disease and good performance status [130]. Notable experimental approaches to MPN LT include hypomethylating agents, such as decitabine [131] or azacytidine [132], with or without ruxolitinib [133–135].
Conclusion
PV and ET are rare, chronic myeloid disorders. Patients typically experience a long clinical course and enjoy near-normal quality of life if properly managed. The 2 most important life-limiting complications of PV and ET are thrombohemorrhagic events and myelofibrosis/AML transformation. Vascular events are at least in part preventable with counseling on risk factors, phlebotomy (for patients with PV), antiplatelet therapy, and cytoreduction with hydroxyurea, IFNs, or anagrelide (for patients with ET). In addition, ruxolitinib was recently approved for PV patients after hydroxyurea failure. PV/ET transformation in myelofibrosis or AML is part of the natural history of the disease and no therapy has been shown to prevent it. Treatment follows recommendations set forth for PMF and AML, but results are generally poorer and novel strategies are needed to improve outcomes.
Corresponding author: Lorenzo Falchi, MD, Columbia University Medical Center, New York, NY.
Financial disclosures: None.
From the Columbia University Medical Center, New York, NY (Dr. Falchi), and the University of Texas MD Anderson Cancer Center, Houston, TX (Dr. Verstovsek).
ABSTRACT
- Objective: To review the clinical aspects and current practices in the management of polycythemia vera (PV) and essential thrombocythemia (ET).
- Methods: Review of the literature.
- Results: PV and ET are rare chronic myeloid disorders. The 2 most important life-limiting complications of PV and ET are thrombohemorrhagic events and myelofibrosis/acute myeloid leukemia (AML) transformation. Vascular events are at least in part preventable with counseling on risk factors, phlebotomy (for patients with PV), antiplatelet therapy, and cytoreduction with hydroxyurea, interferons, or anagrelide (for patients with ET). Ruxolitinib was recently approved for PV after hydroxyurea failure. PV/ET transformation into myelofibrosis or AML is part of the natural history of the disease and no therapy has been shown to prevent it. Treatment of leukemic transformation of myeloproliferative neoplasms (MPN LT) follows recommendations set forth for primary myelofibrosis and AML.
- Conclusion: With appropriate management, patients with PV and ET typically enjoy a long survival and near-normal quality of life. Transformation into myelofibrosis or AML cannot be prevented by current therapies, however. Treatment results with MPN LT are generally poor and novel strategies are needed to improve outcomes.
Key words: myeloproliferative neoplasms; myelofibrosis; leukemic transformation.
Polycythemia vera (PV) and essential thrombocythemia (ET), along with primary myelofibrosis (PMF), belong to the group of Philadelphia-negative myeloproliferative neoplasms (MPN). All these malignancies arise from the clonal proliferation of an aberrant hematopoietic stem cell, but are characterized by distinct clinical phenotypes [1,2]. Although the clinical course of PV and ET is indolent, it can be complicated by thrombohemorrhagic episodes and/or evolution into myelofibrosis and/or acute myeloid leukemia (AML) [3]. Since vascular events are the most frequent life-threatening complications of PV and ET, therapeutic strategies are aimed at reducing this risk. Treatment may also help control other symptoms associated with the disease [4]. No therapy has been shown to prevent evolution of PV or ET into myelofibrosis or AML. The discovery of the Janus kinase 2 (JAK2)/V617F mutation in most patients with PV and over half of those with ET (and PMF) [5,6] has opened new avenues of research and led to the development of targeted therapies, such as the JAK1/2 inhibitor ruxolitinib, for patients with MPN [7,8].
Epidemiology
PV and ET are typically diagnosed in the fifth to seventh decade of life [9]. Although these disorders are generally associated with a long clinical course, survival of patients with PV or ET may be shorter than that of the general population [10–13]. Estimating the incidence and prevalence of MPN is a challenge because most patients remain asymptomatic for long periods of time and do not seek medical attention [13]. The annual incidence rates of PV and ET are estimated at 0.01 to 2.61 and 0.21 to 2.53 per 100,000, respectively. PV occurs slightly more frequently in males, whereas ET has a predilection for females [14]. Given the long course and low mortality associated with these disorders, the prevalence rates of PV and ET are significantly higher than the respective incidence rates: up to 47 and 57 per 100,000, respectively [15–17].
Molecular Pathogenesis
In 2005 researchers discovered a gain-of-function mutation of the JAK2 gene in nearly all patients with PV and more than half of those with ET and PMF [5,6,18,19]. JAK2 is a non-receptor tyrosine kinase that plays a central role in normal hematopoiesis. Substitution of a valine for a phenylalanine at codon 617 (ie, V617F) leads to its constitutive activation and signaling through the JAK-STAT pathway [5,6,18,19]. More rarely (and exclusively in patients with PV), JAK2 mutations involve exon 12 [20–22]. The vast majority of JAK2-negative ET patients harbor mutations in either the myeloproliferative leukemia (MPL) gene, which encodes the thrombopoietin receptor [23–25], or the calreticulin (CALR) gene [26,27], which encodes for a chaperone protein that plays a role in cellular proliferation, differentiation, and apoptosis [28]. Both the MPL and CALR mutations ultimately result in the constitutive activation of the JAK-STAT pathway. Thus, JAK2, MPL, and CALR alterations are collectively referred to as driver mutations. Moreover, because these mutations affect the same oncogenic pathway (ie, JAK-STAT), they are almost always mutually exclusive in a given patient. Patients with ET (or myelofibrosis) who are wild-type for JAK2, MPL, and CALR are referred to as having “triple-negative” disease. Many recurrent non-driver mutations are also found in patients with MPN. These are not exclusive of each other (ie, patients may have many at the same time) and involve for example ten-eleven translocation-2 (TET2), additional sex combs like 1 (ASXL1), enhancer of zeste homolog 2 (EZH2), isocitrate dehydrogenase 1 and isocitrate dehydrogenase 2 (IDH1/2), and DNA methyltransferase 3A (DNMT3A) genes, among others [29]. The biologic and prognostic significance of these non-driver alterations remain to be fully defined in ET and PV.
Diagnostic Criteria
Diagnostic criteria for PV and ET according to the World Health Organization (WHO) classification [30] are summarized in Table 1. Criteria for the diagnosis of prefibrotic myelofibrosis are included as well since this entity was formally recognized as separate from ET and part of the PMF spectrum in the 2016 WHO classification of myeloid tumors [30]. Clinically, both PV and ET generally remain asymptomatic for a long time. PV tends to be more symptomatic than ET and can present with debilitating constitutional symptoms (fatigue, night sweats, and weight loss), microvascular symptoms (headache, lightheadedness, acral paresthesias, erythromelalgia, atypical chest pain, and pruritus) [31], or macrovascular accidents (larger vein thrombosis, stroke, or myocardial ischemia) [32]. ET is often diagnosed incidentally, but patients can suffer from similar general symptoms and vascular complications. Causes of secondary absolute erythrocytosis (altitude, chronic hypoxemia, heavy smoking, cardiomyopathy, use of corticosteroids, erythropoietin, or other anabolic hormones, familial or congenital forms) or thrombocytosis (iron deficiency, acute blood loss, trauma or injury, acute coronary syndrome, systemic autoimmune disorders, chronic kidney failure, other malignancies, splenectomy) should be considered and appropriately excluded. Once the diagnosis is made, symptom assessment tools such as the Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF) [33] or the abbreviated version, the MPN-SAF Total Symptom Score (MPN-SAF TSS) [34], are generally used to assess patients’ symptom burden and response to treatment in everyday practice.
Risk Stratification
Thrombohemorrhagic events, evolution into myelofibrosis, and leukemic transformation (LT) are the most serious complications in the course of PV or ET. Only thrombohemorrhagic events are, at least partially, preventable. Arterial or venous thrombotic complications are observed at rates of 1.8 to 10.9 per 100 patient-years in PV (arterial thrombosis being more common than venous) and 0.74 to 7.7 per 100 patient-years in ET, depending on the risk group [35] and the presence of other factors (see below).
The risk stratification of patients with PV is based on 2 factors: age ≥ 60 years and prior history of thrombosis. If either is present, the patient is assigned to the high-risk category, whereas if none is present the patient is considered at low risk [36]. In addition, high hematocrit [37] and high white blood cell (WBC) count [38], but not thrombocytosis, have been associated with the development of vascular complications. In one study, the risk of new arterial thrombosis was increased by the presence of leukoerythroblastosis, hypertension, and prior arterial thrombosis, while karyotypic abnormalities and prior venous thrombosis were predictors of new venous thrombosis [39]. Another emerging risk factor for thrombosis in patients with PV is high JAK2 allele burden (ie, the normal-to-mutated gene product ratio), although the evidence supporting this conclusion is equivocal [40].
Traditionally, in ET patients, the thrombotic risk was assessed using the same 2 factors (age ≥ 60 years and prior history of thrombosis), separating patients into low- and high-risk groups. However, the prognostication of ET patients has been refined recently with the identification of new relevant factors. In particular, the impact of JAK2 mutations on thrombotic risk has been thoroughly studied. Clinically, the presence of JAK2V617F is associated with older age, higher hemoglobin and hematocrit, lower platelet counts, more frequent need for cytoreductive treatment, and greater tendency to evolve into PV (a rare event) [41,42]. Many [41,43–46], but not all [47–51], studies suggested a correlation between JAK2 mutation and risk of both arterial and venous thrombosis. Although infrequent, a JAK2V617F homozygous state (ie, the mutation is present in both alleles) might confer an even higher thrombotic risk [52]. Moreover, the impact of the JAK2 mutation on vascular events persists over time [53], particularly in patients with high or unstable mutation burden [54]. Based on JAK2V617F’s influence on the thrombotic risk of ET patients, a new prognostic score was proposed, the International Prognostic Score for ET (IPSET)-thrombosis (Table 2). The revised version of this model is currently endorsed by the National Comprehensive Cancer Network and divides patients into 4 risk groups: high, intermediate, low, and very low. Treatment recommendations vary according to the risk group (as described below) [55].
Other thrombotic risk factors have been identified, but deemed not significant enough to be included in the model. Cardiovascular risk factors (hypercholesterolemia, hypertension, smoking, diabetes mellitus) can increase the risk of vascular events [56–59], as can splenomegaly [60] and baseline or persistent leukocytosis [61–63]. Thrombocytosis has been correlated with thrombotic risk in some studies [64–68], whereas others did not support this conclusion and/or suggested a lower rate of thrombosis and, in some cases, increased risk of bleeding in ET patients with platelet counts greater than 1000 × 103/μL (due to acquired von Willebrand syndrome) [51,61,63,68,69].
CALR mutations tend to occur in younger males with lower hemoglobin and WBC count, higher platelet count, and greater marrow megakaryocytic predominance, as compared to JAK2 mutations [26,27,70–72]. The associated incidence of thrombosis was less than 10% at 15 years in patients with CALR mutations, lower than the incidence reported for ET patients with JAK2V617F mutations [73]. The presence of the mutation per se does not appear to affect the thrombotic risk [74–76]. Information on the thrombotic risk associated with MPL mutations or a triple-negative state is scarce. In both instances, however, the risk appears to be lower than with the JAK2 mutation [73,77–79].
Venous thromboembolism (VTE) in patients with PV or ET may occur at unusual sites, such as the splanchnic or cerebral venous systems [80]. Risk factors for unusual VTE include younger age [81], female gender (especially with concomitant use of oral contraceptive pills) [82], and splenomegaly/splenectomy [83]. JAK2 mutation has also been associated with thrombosis at unusual sites. However, the prevalence of MPN or JAK2V617F in patients presenting with splanchnic VTE has varied [80]. In addition, MPN may be occult (ie, no clinical or laboratory abnormalities) in around 15% of patients [84]. Screening for JAK2V617F and underlying MPN is recommended in patients presenting with isolated unexplained splanchnic VTE. Treatment entails long-term anticoagulation therapy. JAK2V617F screening in patients with nonsplanchnic VTE is not recommended, as its prevalence in this group is low (< 3%) [85,86].
Risk-Adapted Therapy
Low-Risk PV
All patients with PV should receive counseling to mitigate cardiovascular risk factors, including smoking cessation, lifestyle modifications, and lipid-lowering therapy, as indicated. Furthermore, all PV patients should receive acetylsalicylic acid (ASA) to decrease their risk for thrombosis and control vasomotor symptoms [55,87]. Aspirin 81 to 100 mg daily is the preferred regimen because it provides adequate antithrombotic effect without the associated bleeding risk of higher-dose aspirin [88]. Low-risk PV patients should also receive periodic phlebotomies to reduce and maintain their hematocrit below 45%. This recommendation is based on the results of the Cytoreductive Therapy in Polycythemia Vera (CYTO PV) randomized controlled trial. In that study, patients receiving more intense therapy to maintain the hematocrit below 45% had a lower incidence of cardiovascular-related deaths or major thrombotic events than those with hematocrit goals of 45% to 50% (2.7% versus 9.8%) [89]. Cytoreduction is an option for low-risk patients who do not tolerate phlebotomy or require frequent phlebotomy, or who have disease-related bleeding, severe symptoms, symptomatic splenomegaly, or progressive leukocytosis [38].
High-Risk PV
Patients older than 60 years and/or with a history of thrombosis should be considered for cytoreductive therapy in addition to the above measures. Frontline cytoreductive therapies include hydroxyurea or interferon (IFN)-alfa [87]. Hydroxyurea is a potent ribonucleotide reductase inhibitor that interferes with DNA repair and is the treatment of choice for most high-risk patients with PV [90]. In a small trial, hydroxyurea reduced the risk of thrombosis compared with historical controls treated with phlebotomy alone [91]. Hydroxyurea is generally well tolerated; common side effects include cytopenias, nail changes, and mucosal and/or skin ulcers. Although never formally proven to be leukemogenic, this agent should be used with caution in younger patients [87]. Indeed, in the original study, the rates of transformation were 5.9% and 1.5% for patients receiving hydroxyurea and phlebotomy alone [92], respectively, although an independent role for hydroxyurea in LT was not supported in the much larger European Collaboration on Low-dose Aspirin in Polycythemia Vera (ECLAP) study [93]. Approximately 70% of patients will have a sustained response to hydroxyurea [94], while the remaining patients become resistant to or intolerant of the drug. Resistant individuals have a higher risk of progression to acute leukemia and death [95].
IFN-alfa is a pleiotropic antitumor agent that has found application in many types of malignancies [96] and is sometimes employed as treatment for patients with newly diagnosed high-risk PV. Early studies showed responses in up to 100% of cases [97,98], albeit at the expense of a high discontinuation rate due to adverse events, such as flu-like symptoms, fatigue, and neuropsychiatric manifestations [99]. A newer formulation of the drug obtained by adding a polyethylene glycol (PEG) moiety to the native IFN-alfa molecule (PEG-IFN alfa) was shown to have a longer half-life, greater stability, less immunogenicity, and, potentially, better tolerability [100]. Pilot phase 2 trials of PEG-IFN-alfa-2a demonstrated its remarkable activity, with symptomatic and hematologic responses seen in most patients (which, in some cases, persisted beyond discontinuation), and reasonable tolerability, with long-term discontinuation rates of 20% to 30% [101–103]. In some patients, JAK2V617F became undetectable over time [104]. Results of 2 ongoing trials, MDP-RC111 (single-arm study, PEG-IFN-alfa-2a in high-risk PV or ET [NCT01259817]) and MPD-RC112 (randomized controlled trial, PEG-IFN-alfa-2a versus hydroxyurea in the same population [NCT01258856]), will shed light on the role of PEG-IFN-alfa in the management of patients with high-risk PV or ET. In two phase 2 studies of PEG-IFN-alfa-2b, complete responses were seen in 70% to 100% of patients and discontinuation occurred in around a third of cases [105,106]. A new, longer-acting formulation of PEG-IFN-alfa-2a (peg-proline INF-alfa-2b, AOP2014) is also undergoing clinical development [107,108].
The approach to treatment of PV based on thrombotic risk level is illustrated in Figure 1.
Very Low- and Low-Risk ET
Individuals with ET should undergo rigorous cardiovascular risk management and generally receive ASA to decrease their thrombotic risk and improve symptom control. Antiplatelet therapy may not be warranted in patients with documented acquired von Willebrand syndrome, with or without extreme thrombocytosis, or in those in the very low-risk category according to the IPSET-thrombosis model [55,87]. The risk/benefit ratio of antiplatelet agents in patients with ET at different thrombotic risk levels was assessed in poor-quality studies and thus remains highly uncertain. Platelet-lowering agents are sometimes recommended in patients with low-risk disease who have platelet counts ≥ 1500 × 103/μL, due to the potential risk of acquired von Willebrand syndrome and a risk of bleeding (this would require stopping ASA) [109]. Cytoreduction may also be used in low-risk patients with progressive symptoms despite ASA, symptomatic or progressive splenomegaly, and progressive leukocytosis.
Intermediate-Risk ET
This category includes patients older than 60 years, but without thrombosis or JAK2 mutations. These individuals would have been considered high risk (and thus candidates for cytoreductive therapy) according to the traditional risk stratification. Guidelines currently recommend ASA as the sole therapy for these patients, while reserving cytoreduction for those who experience thrombosis (ie, become high-risk) or have uncontrolled vasomotor or general symptoms, symptomatic splenomegaly, symptomatic thrombocytosis, or progressive leukocytosis.
High-Risk ET
For patients with ET in need of cytoreductive therapy (ie, those with prior thrombosis or older than 60 years with a JAK2V617F mutation), first-line options include hydroxyurea, IFN, and anagrelide. Hydroxyurea remains the treatment of choice in most patients [110]. In a seminal study, 114 patients with ET were randomly assigned to either observation or hydroxyurea treatment with the goal of maintaining the platelet count below 600 × 103/μL. At a median follow-up of 27 months, patients in the hydroxyurea group had a lower thrombosis rate (3.6% versus 24%, P = 0.003) and longer thrombosis-free survival, regardless of the use of antiplatelet drugs [64].
Anagrelide, a selective inhibitor of megakaryocytic differentiation and proliferation, was compared with hydroxyurea in patients with ET in 2 randomized trials. In the first (n = 809), the group receiving anagrelide had a higher risk of arterial thrombosis, major bleeding, and fibrotic evolution, but lower incidence of venous thrombosis. Hydroxyurea was better tolerated, mainly due to anagrelide-related cardiovascular adverse events [111]. As a result of this study, hydroxyurea is often preferred to anagrelide as frontline therapy for patients with newly diagnosed high-risk ET. In the second, more recent study (n = 259), however, the 2 agents proved equivalent in terms of major or minor arterial or venous thrombosis, as well as discontinuation rate [112]. The discrepancy between the 2 trials may be partly explained by the different ET diagnostic criteria used, with the latter only enrolling patients with WHO-defined true ET and the former utilizing Polycythemia Vera Study Group-ET diagnostic criteria that included patients with increases in other blood counts or varying degrees of marrow fibrosis.
Interferons were studied in ET in parallel with PV. PEG-IFN-alfa-2a proved effective in patients with ET, with responses observed in 80% of patients [103]. PEG-IFN- alfa-2b produced similar results, with responses in 70% to 90% of patients in small studies and discontinuation observed in 20% to 38% of cases [105,106,113]. Because the very long-term leukemogenic potential of hydroxyurea has remained somewhat uncertain, anagrelide or IFN might be preferable choices in younger patients.
The approach to treatment of ET based on thrombotic risk level is illustrated in Figure 2.
Assessing Response to Therapy
For both patients with PV and ET the endpoint of treatment set forth for clinical trials has been the achievement of a clinicohematologic response. However, studies have failed to show a correlation between response and reduction of the thrombohemorrhagic risk [114]. Therefore, proposed clinical trial response criteria were revised to include absence of hemorrhagic or thrombotic events as part of the definition of response (Table 3) [94].
Approach to Patients Refractory to or Intolerant of First-line Therapy
According to the European LeukemiaNet recommendations, an inadequate response to hydroxyurea in patients with PV (or myelofibrosis) is defined as a need for phlebotomy to maintain the hematocrit below < 45%, the platelet count > 400 × 103/μL, and a WBC count > 10,000/μL, or failure to reduce splenomegaly > 10 cm by > 50% at a dose of ≥ 2 g/day or maximum tolerated dose. Historically, treatment options for patients with PV or ET who failed first-line therapy (most commonly hydroxyurea) have included alkylating agents, such as busulfan, chlorambucil, pipobroman, and phosphorus (P)-32. However, the use of these drugs is limited by the associated risk of LT [93,115,116]. IFN (or anagrelide for ET) is often considered in patients previously treated with hydroxyurea, and vice versa.
Ruxolitinib is a JAK1 and JAK2 inhibitor currently approved for the treatment of PV patients refractory to or intolerant of hydroxyurea [7]. Following promising results of a phase 2 trial [117], ruxolitinib 10 mg twice daily was compared with best available therapy in the pivotal RESPONSE trial (n = 222). Ruxolitinib proved superior in achieving hematocrit control, reduction of spleen volume, and improvement of symptoms. Grade 3-4 hematologic toxicity was infrequent and similar in the 2 arms [118]. In addition, longer follow-up of that study suggested a lower rate of thrombotic events in patients receiving ruxolitinib (1.8 versus 8.2 per 100 patient-years) [119]. In a similarly designed randomized phase 3 study in PV patients without splenomegaly (RESPONSE-2), more patients in the ruxolitinib arm had hematocrit reduction without an increase in toxicity. Based on the results of these studies, ruxolitinib can be considered a standard of care for second-line therapy in this post-hydroxyurea patient population [120]. Ruxolitinib is also being tested in patients with high-risk ET who have become resistant to, or were intolerant of hydroxyurea, but currently has no approved indication in this setting [121,122]. Common side effects of ruxolitinib include cytopenias (especially anemia), increased risk of infections, hyperlipidemia, and increased risk of non-melanoma skin cancer.
Novel agents that have been studied in patients with PV and ET are histone deacetylase inhibitors, murine double minute 2 (MDM2, or HDM2 for their human counterpart) inhibitors (which restore the function of p53), Bcl-2 homology domain 3 mimetics such as navitoclax and venetoclax, and, for patients with ET, the telomerase inhibitor imetelstat [123].
Disease Evolution
Post-PV/Post-ET Myelofibrosis
Diagnostic criteria for post-PV and post-ET myelofibrosis are outlined in Table 4. Fibrotic transformation represents a natural evolution of the clinical course of PV or ET. It occurs in up to 15% and 9% of patients with PV and ET, respectively, in western countries [124]. The true percentage of ET patients who develop myelofibrosis is confounded by the inclusion of prefibrotic myelofibrosis cases in earlier series. The survival of patients who develop myelofibrosis is shortened compared to those who do not. In patients with PV, risk factors for myelofibrosis evolution include advanced age, leukocytosis, JAK2V617F homozygosity or higher allele burden, and hydroxyurea therapy. Once post-PV myelofibrosis has occurred, hemoglobin < 10 g/dL, platelet count < 100 × 103/μL, and WBC count > 30,000/μL are associated with worse outcomes [125]. In patients with ET, risk factors for myelofibrosis transformation include age, anemia, bone marrow hypercellularity and increased reticulin, increased lactate dehydrogenase, leukocytosis, and male gender. The management of post-PV/post-ET myelofibrosis recapitulates that of PMF.
Leukemic Transformation
The presence of more than 20% blasts in peripheral blood or bone marrow in a patient with MPN defines LT. This occurs in up to 5% to 10% of patients and may or may not be preceded by a myelofibrosis phase [126]. In cases of extramedullary transformation, a lower percentage of blasts can be seen in the bone marrow compared to the peripheral blood. The pathogenesis of LT has remained elusive, but it is believed to be associated with genetic instability, which facilitates the acquisition of additional mutations, including those of TET2, ASXL1, EZH2 DNMT3, IDH1/2, and TP53 [127].
Clinical risk factors for LT include advanced age, karyotypic abnormalities, prior therapy with alkylating agents or P-32, splenectomy, increased peripheral blood or bone marrow blasts, leukocytosis, anemia, thrombocytopenia, and cytogenetic abnormalities. Hydroxyurea, IFN, and ruxolitinib have not been shown to have leukemogenic potential thus far. Prognosis of LT is uniformly poor and patient survival rarely exceeds 6 months.
There is no standard of care for MPN LT. Treatment options range from low-intensity regimens to more aggressive AML-type induction chemotherapy. No strategy appears clearly superior to others [128]. Hematopoietic stem cell transplantation is the only therapy that provides clinically meaningful benefit to patients [129], but it is applicable only to a minority of patients with chemosensitive disease and good performance status [130]. Notable experimental approaches to MPN LT include hypomethylating agents, such as decitabine [131] or azacytidine [132], with or without ruxolitinib [133–135].
Conclusion
PV and ET are rare, chronic myeloid disorders. Patients typically experience a long clinical course and enjoy near-normal quality of life if properly managed. The 2 most important life-limiting complications of PV and ET are thrombohemorrhagic events and myelofibrosis/AML transformation. Vascular events are at least in part preventable with counseling on risk factors, phlebotomy (for patients with PV), antiplatelet therapy, and cytoreduction with hydroxyurea, IFNs, or anagrelide (for patients with ET). In addition, ruxolitinib was recently approved for PV patients after hydroxyurea failure. PV/ET transformation in myelofibrosis or AML is part of the natural history of the disease and no therapy has been shown to prevent it. Treatment follows recommendations set forth for PMF and AML, but results are generally poorer and novel strategies are needed to improve outcomes.
Corresponding author: Lorenzo Falchi, MD, Columbia University Medical Center, New York, NY.
Financial disclosures: None.
1. Swerdlow SH, Campo E, Harris NL, et al. WHO classification of tumours of haematopoietic and lymphoid tissues. 4th ed. Lyon: IARC; 2008.
2. Alvarez-Larran A, Pereira A, Arellano-Rodrigo E, et al. Cytoreduction plus low-dose aspirin versus cytoreduction alone as primary prophylaxis of thrombosis in patients with high-risk essential thrombocythaemia: an observational study. Br J Haematol 2013;161:865–71.
3. Tefferi A. Polycythemia vera and essential thrombocythemia: 2013 update on diagnosis, risk-stratification, and management. Am J Hematol 2013;88:507–16.
4. Michiels JJ, Berneman Z, Schroyens W, et al. Platelet-mediated erythromelalgic, cerebral, ocular and coronary microvascular ischemic and thrombotic manifestations in patients with essential thrombocythemia and polycythemia vera: a distinct aspirin-responsive and coumadin-resistant arterial thrombophilia. Platelets 2006;17:528–44.
5. James C, Ugo V, Couedic J-PL, et al. A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera. Nature 2005;434:1144–8.
6. Kralovics R, Passamonti F, Buser A, et al. A gain-of-function mutation of JAK2 in myeloproliferative disorders.N Engl J Med 2005;352:1779–90.
7. Verstovsek S, Mesa R, Gotlib J, et al. A double-blind, placebo-controlled trial of ruxolitinib for myelofibrosis. N Engl J Med 2012;366:799–807.
8. Harrison C, Kiladjian J, Al-Ali H, et al. JAK Inhibition with ruxolitinib versus best available therapy for myelofibrosis.N Engl J Med 2012;366:787–98.
9. Berglund S, Zettervall O. Incidence of polycythemia vera in a defined population. Eur J Haematol 1992;48:20–6.
10. Rozman C, Giralt M, Feliu E, et al. Life expectancy of patients with chronic nonleukemic myeloproliferative disorders. Cancer 1991;67:2658–63.
11. Passamonti F, Rumi E, Pungolino E, et al. Life expectancy and prognostic factors for survival in patients with polycythemia vera and essential thrombocythemia. Am J Med 2004;117:755–61.
12. Hultcrantz M, Kristinsson SY, Andersson TM, et al. Patterns of survival among patients with myeloproliferative neoplasms diagnosed in Sweden from 1973 to 2008: a population-based study. J Clin Oncol 2012;30:2995–3001.
13. Wolanskyj AP, Schwager SM, et al. Essential thrombocythemia beyond the first decade: life expectancy, long-term complication rates, and prognostic factors. Mayo Clin Proc 2006;81:159–66.
14. Srour SA, Devesa SS, Morton LM, et al. Incidence and patient survival of myeloproliferative neoplasms and myelodysplastic/myeloproliferative neoplasms in the United States, 2001-12. Br J Haematol 2016;174:382–96.
15. Titmarsh GJ, Duncombe AS, McMullin MF, et al. How common are myeloproliferative neoplasms? A systematic review and meta-analysis. Am J Hematol 2014;89:581–7.
16. Moulard O, Mehta J, Fryzek J, et al. Epidemiology of myelofibrosis, essential thrombocythemia, and polycythemia vera in the European Union. Eur J Haematol 2014;92:289–97.
17. Stein BL, Gotlib J, Arcasoy M, et al. Historical views, conventional approaches, and evolving management strategies for myeloproliferative neoplasms. J Natl Compr Canc Netw 2015;13:424–34.
18. Levine RL, Wadleigh M, Cools J, et al. Activating mutation in the tyrosine kinase JAK2 in polycythemia vera, essential thrombocythemia, and myeloid metaplasia with myelofibrosis. Cancer Cell 2005;7:387–97.
19. Baxter EJ, Scott LM, Campbell PJ, et al. Acquired mutation of the tyrosine kinase JAK2 in human myeloproliferative disorders. Lancet 2005;365:1054–61.
20. Scott LM, Tong W, Levine RL, et al. JAK2 exon 12 mutations in polycythemia vera and idiopathic erythrocytosis. N Engl J Med 2007;356:459–68.
21. Pardanani A, Lasho TL, Finke C, et al. Prevalence and clinicopathologic correlates of JAK2 exon 12 mutations in JAK2V617F-negative polycythemia vera. Leukemia 2007;21:1960–3.
22. Passamonti F, Elena C, Schnittger S, et al. Molecular and clinical features of the myeloproliferative neoplasm associated with JAK2 exon 12 mutations. Blood 2011;117:2813–6.
23. Abe M, Suzuki K, Inagaki O, et al. A novel MPL point mutation resulting in thrombopoietin-independent activation. Leukemia 2002;16:1500–6.
24. Pikman Y, Lee BH, Mercher T, et al. MPLW515L is a novel somatic activating mutation in myelofibrosis with myeloid metaplasia. PLoS Med 2006;3:e270.
25. Pardanani AD, Levine RL, Lasho T, et al. MPL515 mutations in myeloproliferative and other myeloid disorders: a study of 1182 patients. Blood 2006;108:3472–6.
26. Nangalia J, Massie CE, Baxter EJ, et al. Somatic CALR mutations in myeloproliferative neoplasms with nonmutated JAK2. N Engl J Med 2013;369:2391–405.
27. Klampfl T, Gisslinger H, Harutyunyan AS, et al. Somatic mutations of calreticulin in myeloproliferative neoplasms. N Engl J Med 2013;369:2379–90.
28. Wang WA, Groenendyk J, Michalak M. Calreticulin signaling in health and disease. Int J Biochem Cell Biol 2012;44:842–6.
29. Saeidi K. Myeloproliferative neoplasms: Current molecular biology and genetics. Crit Rev Oncol Hematol 2016;98:375–89.
30. Arber DA, Orazi A, Hasserjian R, et al. The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood 2016;127: 2391–405.
31. Mesa RA, Niblack J, Wadleigh M, et al. The burden of fatigue and quality of life in myeloproliferative disorders (MPDs): an international Internet-based survey of 1179 MPD patients. Cancer 2007;109:68–76.
32. Marchioli R, Finazzi G, Landolfi R, et al. Vascular and neoplastic risk in a large cohort of patients with polycythemia vera. J Clin Oncol 2005;23:2224–32.
33. Scherber R, Dueck AC, Johansson P, et al. The Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF): international prospective validation and reliability trial in 402 patients. Blood 2011;118:401–8.
34. Emanuel RM, Dueck AC, Geyer HL, et al. Myeloproliferative neoplasm (MPN) symptom assessment form total symptom score: prospective international assessment of an abbreviated symptom burden scoring system among patients with MPNs. J Clin Oncol 2012;30:4098–103.
35. Casini A, Fontana P, Lecompte TP. Thrombotic complications of myeloproliferative neoplasms: risk assessment and risk-guided management. J Thromb Haemost 2013;11:1215–27.
36. Barbui T, Finazzi G, Falanga A. Myeloproliferative neoplasms and thrombosis. Blood 2013;122:2176-–84.
37. Pearson TC, Wetherley-Mein G. Vascular occlusive episodes and venous haematocrit in primary myeloproliferative polychytemia. Lancet 1978;2:1219–22.
38. Landolfi R, Di Gennaro L, Barbui T, et al. Leukocytosis as a major thrombotic risk factor in patients with polycythemia vera. Blood 2007;109:2446–52.
39. Tefferi A, Rumi E, Finazzi G, et al. Survival and prognosis among 1545 patients with contemporary polycythemia vera: an international study. Leukemia 2013;27:1874–81.
40. Barbui T, Falanga A. Molecular biomarkers of thrombosis in myeloproliferative neoplasms. Thromb Res 2016;140 Suppl 1:S71–75.
41. Campbell PJ, Scott LM, Buck G, et al. Definition of subtypes of essential thrombocythaemia and relation to polycythaemia vera based on JAK2 V617F mutation status: a prospective study. Lancet 2005;366:1945–53.
42. Tefferi A, Guglielmelli P, Larson DR, et al. Long-term survival and blast transformation in molecularly annotated essential thrombocythemia, polycythemia vera, and myelofibrosis. Blood 2014;124:2507–13.
43. Qin Y, Wang X, Zhao C, et al. The impact of JAK2V617F mutation on different types of thrombosis risk in patients with essential thrombocythemia: a meta-analysis. Int J Hematol 2015;102:170–80.
44. Ziakas PD. Effect of JAK2 V617F on thrombotic risk in patients with essential thrombocythemia: measuring the uncertain. Haematologica 2008;93:1412–4.
45. Lussana F, Dentali F, Abbate R, et al. Screening for thrombophilia and antithrombotic prophylaxis in pregnancy: Guidelines of the Italian Society for Haemostasis and Thrombosis (SISET). Thromb Res 2009;124: e19–25.
46. Dahabreh IJ, Zoi K, Giannouli S, et al. Is JAK2 V617F mutation more than a diagnostic index? A meta-analysis of clinical outcomes in essential thrombocythemia. Leuk Res 2009;33:67–73.
47. Cho YU, Chi HS, Lee EH, et al. Comparison of clinicopathologic findings according to JAK2 V617F mutation in patients with essential thrombocythemia. Int J Hematol 2009;89:39–44.
48. Wolanskyj AP, Lasho TL, Schwager SM, et al. JAK2 mutation in essential thrombocythaemia: clinical associations and long-term prognostic relevance. Br J Haematol 2005;131:208–13.
49. Antonioli E, Guglielmelli P, Pancrazzi A, et al. Clinical implications of the JAK2 V617F mutation in essential thrombocythemia. Leukemia 2005;19:1847–9.
50. Palandri F, Catani L, Testoni N, et al. Long-term follow-up of 386 consecutive patients with essential thrombocythemia: safety of cytoreductive therapy. Am J Hematol 2009;84:215–20.
51. Chim CS, Sim JP, Chan CC, et al. Impact of JAK2V617F mutation on thrombosis and myeloid transformation in essential thrombocythemia: a multivariate analysis by Cox regression in 141 patients. Hematology 2010;15: 187–92.
52. Vannucchi AM, Antonioli E, Guglielmelli P, et al. Clinical profile of homozygous JAK2 617V>F mutation in patients with polycythemia vera or essential thrombocythemia. Blood 2007;110:840–6.
53. Carobbio A, Finazzi G, Antonioli E, et al. JAK2V617F allele burden and thrombosis: a direct comparison in essential thrombocythemia and polycythemia vera. Exp Hematol 2009;37:1016–21.
54. Alvarez-Larran A, Bellosillo B, Pereira A, et al. JAK2V617F monitoring in polycythemia vera and essential thrombocythemia: clinical usefulness for predicting myelofibrotic transformation and thrombotic events. Am J Hematol 2014;89:517–23.
55. Barbui T, Vannucchi AM, Buxhofer-Ausch V, et al. Practice-relevant revision of IPSET-thrombosis based on 1019 patients with WHO-defined essential thrombocythemia. Blood Cancer J 2015;5:e369.
56. Carobbio A, Thiele J, Passamonti F, et al. Risk factors for arterial and venous thrombosis in WHO-defined essential thrombocythemia: an international study of 891 patients. Blood 2011;117:5857–9.
57. Alvarez-Larran A, Cervantes F, Bellosillo B, et al. Essential thrombocythemia in young individuals: frequency and risk factors for vascular events and evolution to myelofibrosis in 126 patients. Leukemia 2007;21:1218–23.
58. Jantunen R, Juvonen E, Ikkala E, et al. The predictive value of vascular risk factors and gender for the development of thrombotic complications in essential thrombocythemia. Ann Hematol 2001;80:74–8.
59. Besses C, Cervantes F, Pereira A, et al. Major vascular complications in essential thrombocythemia: a study of the predictive factors in a series of 148 patients. Leukemia 1999;13:150–4.
60. Haider M, Gangat N, Hanson C, Tefferi A. Splenomegaly and thrombosis risk in essential thrombocythemia: the mayo clinic experience. Am J Hematol 2016;91: E296–297.
61. Carobbio A, Finazzi G, Antonioli E, et al. Thrombocytosis and leukocytosis interaction in vascular complications of essential thrombocythemia. Blood 2008;112:3135–7.
62. Palandri F, Polverelli N, Catani L, et al. Impact of leukocytosis on thrombotic risk and survival in 532 patients with essential thrombocythemia: a retrospective study. Ann Hematol 2011;90:933–8.
63. Campbell PJ, MacLean C, Beer PA, et al. Correlation of blood counts with vascular complications in essential thrombocythemia: analysis of the prospective PT1 cohort. Blood 2012;120:1409–11.
64. Cortelazzo S, Finazzi G, Ruggeri M, et al. Hydroxyurea for patients with essential thrombocythemia and a high risk of thrombosis. N Engl J Med 1995;332:1132–6.
65. van Genderen PJ, Mulder PG, Waleboer M, et al. Prevention and treatment of thrombotic complications in essential thrombocythaemia: efficacy and safety of aspirin. Br J Haematol 1997;97:179–84.
66. Storen EC, Tefferi A. Long-term use of anagrelide in young patients with essential thrombocythemia. Blood 2001;97:863–6.
67. De Stefano V, Za T, Rossi E, et al. Recurrent thrombosis in patients with polycythemia vera and essential thrombocythemia: incidence, risk factors, and effect of treatments. Haematologica 2008;93:372–80.
68. Alvarez-Larran A, Cervantes F, Pereira A, et al. Observation versus antiplatelet therapy as primary prophylaxis for thrombosis in low-risk essential thrombocythemia. Blood 2010;116:1205–10.
69. Palandri F, Polverelli N, Catani L, et al. Bleeding in essential thrombocythaemia: a retrospective analysis on 565 patients. Br J Haematol 2012;156:281–4.
70. Rotunno G, Mannarelli C, Guglielmelli P, et al. Impact of calreticulin mutations on clinical and hematological phenotype and outcome in essential thrombocythemia. Blood 2014;123:1552–5.
71. Tefferi A, Wassie EA, Lasho TL, et al. Calreticulin mutations and long-term survival in essential thrombocythemia. Leukemia 2014;28:2300–3.
72. Rumi E, Pietra D, Ferretti V, et al. JAK2 or CALR mutation status defines subtypes of essential thrombocythemia with substantially different clinical course and outcomes. Blood 2014;123:1544–51.
73. Palandri F, Latagliata R, Polverelli N, et al. Mutations and long-term outcome of 217 young patients with essential thrombocythemia or early primary myelofibrosis. Leukemia 2015;29:1344–9.
74. Fu R, Xuan M, Zhou Y, et al. Analysis of calreticulin mutations in Chinese patients with essential thrombocythemia: clinical implications in diagnosis, prognosis and treatment. Leukemia 2014;28:1912–4.
75. Tefferi A, Wassie EA, Guglielmelli P, et al. Type 1 versus Type 2 calreticulin mutations in essential thrombocythemia: a collaborative study of 1027 patients. Am J Hematol 2014;89:E121–4.
76. Pietra D, Rumi E, Ferretti VV, et al. Differential clinical effects of different mutation subtypes in CALR-mutant myeloproliferative neoplasms. Leukemia 2016;30: 431–8.
77. Rumi E, Pietra D, Guglielmelli P, et al. Acquired copy-neutral loss of heterozygosity of chromosome 1p as a molecular event associated with marrow fibrosis in MPL-mutated myeloproliferative neoplasms. Blood 2013;121:4388–95.
78. Beer PA, Campbell PJ, Scott LM, et al. MPL mutations in myeloproliferative disorders: analysis of the PT-1 cohort. Blood 2008;112:141–9.
79. Gangat N, Wassie EA, Lasho TL, et al. Mutations and thrombosis in essential thrombocythemia: prognostic interaction with age and thrombosis history. Eur J Haematol 2015;94:31–6.
80. Sekhar M, McVinnie K, Burroughs AK. Splanchnic vein thrombosis in myeloproliferative neoplasms. Br J Haematol 2013;162:730–47.
81. Stein BL, Saraf S, Sobol U, et al. Age-related differences in disease characteristics and clinical outcomes in polycythemia vera. Leuk Lymph 2013;54:1989–95.
82. Landolfi R, Di Gennaro L, Nicolazzi MA, et al. Polycythemia vera: gender-related phenotypic differences. Intern Emerg Med 2012;7:509–15.
83. Winslow ER, Brunt LM, Drebin JA, et al. Portal vein thrombosis after splenectomy. Am J Surg 2002;184:631–6.
84. Smalberg JH, Arends LR, Valla DC, et al. Myeloproliferative neoplasms in Budd-Chiari syndrome and portal vein thrombosis: a meta-analysis. Blood 2012;120:4921–8.
85. Dentali F, Squizzato A, Brivio L, et al. JAK2V617F mutation for the early diagnosis of Ph- myeloproliferative neoplasms in patients with venous thromboembolism: a meta-analysis. Blood 2009;113:5617–23.
86. Pardanani A, Lasho TL, Hussein K, et al. JAK2V617F mutation screening as part of the hypercoagulable work-up in the absence of splanchnic venous thrombosis or overt myeloproliferative neoplasm: assessment of value in a series of 664 consecutive patients. Mayo Clin Proc 2008;83:457–9.
87. Barbui T, Barosi G, Birgegard G, et al. Philadelphia-negative classical myeloproliferative neoplasms: critical concepts and management recommendations from European LeukemiaNet. J Clin Oncol 2011;29:761–70.
88. Landolfi R, Marchioli R, Kutti J, et al. Efficacy and safety of low-dose aspirin in polycythemia vera. N Engl J Med 2004;350:114–24.
89. Marchioli R, Finazzi G, Specchia G, et al. Cardiovascular events and intensity of treatment in polycythemia vera. N Engl J Med 2013;368:22–33.
90. Kiladjian JJ, Chevret S, Dosquet C, et al. Treatment of polycythemia vera with hydroxyurea and pipobroman: final results of a randomized trial initiated in 1980. J Clin Oncol 2011;29:3907–13.
91. Kaplan ME, Mack K, Goldberg JD, et al. Long-term management of polycythemia vera with hydroxyurea: a progress report. Semin Hematol 1986;23:167–71.
92. Fruchtman SM, Mack K, Kaplan ME, et al. From efficacy to safety: a Polycythemia Vera Study group report on hydroxyurea in patients with polycythemia vera. Semin Hematol 1997;34:17–23.
93. Finazzi G, Caruso V, Marchioli R, et al. Acute leukemia in polycythemia vera: an analysis of 1638 patients enrolled in a prospective observational study. Blood 2005;105: 2664–70.
94. Barosi G, Mesa R, Finazzi G, et al. Revised response criteria for polycythemia vera and essential thrombocythemia: an ELN and IWG-MRT consensus project. Blood 2013;121:4778–81.
95. Alvarez-Larran A, Pereira A, Cervantes F, et al. Assessment and prognostic value of the European LeukemiaNet criteria for clinicohematologic response, resistance, and intolerance to hydroxyurea in polycythemia vera. Blood 2012;119:1363–9.
96. Stein BL, Tiu RV. Biological rationale and clinical use of interferon in the classical BCR-ABL-negative myeloproliferative neoplasms. J Interferon Cytokine Res 2013;33: 145–53.
97. Ludwig H, Cortelezzi A, Van Camp BG, et al. Treatment with recombinant interferon-alpha-2C: multiple myeloma and thrombocythaemia in myeloproliferative diseases. Oncology 1985;42 Suppl 1:19–25.
98. Silver RT. Long-term effects of the treatment of polycythemia vera with recombinant interferon-alpha. Cancer 2006;107:451–8.
99. Kiladjian JJ, Mesa RA, Hoffman R. The renaissance of interferon therapy for the treatment of myeloid malignancies. Blood 2011;117:4706–15.
100. Veronese FM, Mero A. The impact of PEGylation on biological therapies. BioDrugs 2008;22:315–29.
101. Kiladjian JJ, Cassinat B, Chevret S, et al. Pegylated interferon-alfa-2a induces complete hematologic and molecular responses with low toxicity in polycythemia vera. Blood 2008;112:3065–72.
102. Turlure P, Cambier N, Roussel M, et al. Complete hematological, molecular and histological remissions without cytoreductive treatment lasting after pegylated-interferon {alpha}-2a (peg-IFN{alpha}-2a) therapy in polycythemia vera (PV): long term results of a phase 2 trial [abstract]. Blood 2011;118(21). Abstract 280.
103. Quintas-Cardama A, Kantarjian H, Manshouri T, et al. Pegylated interferon alfa-2a yields high rates of hematologic and molecular response in patients with advanced essential thrombocythemia and polycythemia vera. J Clin Oncol 2009;27:5418–24.
104. Quintas-Cardama A, Abdel-Wahab O, Manshouri T, et al. Molecular analysis of patients with polycythemia vera or essential thrombocythemia receiving pegylated interferon a-2a. Blood 2013;122:893–901.
105. Samuelsson J, Hasselbalch H, Bruserud O, et al. A phase II trial of pegylated interferon alpha-2b therapy for polycythemia vera and essential thrombocythemia: feasibility, clinical and biologic effects, and impact on quality of life. Cancer 2006;106:2397–405.
106. Jabbour E, Kantarjian H, Cortes J, et al. PEG-IFN-alpha-2b therapy in BCR-ABL-negative myeloproliferative disorders: final result of a phase 2 study. Cancer 2007; 110:2012–18.
107. Them NC, Bagienski K, Berg T, et al. Molecular responses and chromosomal aberrations in patients with polycythemia vera treated with peg-proline-interferon alpha-2b. Am J Hematol 2015;90:288–94.
108. Gisslinger H, Klade C, Georgiev P, et al. Final results from PROUD-PV a randomized controlled phase 3 trial comparing ropeginterferon alfa-2b to hydroxyurea in polycythemia vera patients [abstract]. Blood 2016;128(suppl 22). Abstract 475.
109. van Genderen PJ, van Vliet HH, Prins FJ, et al. Excessive prolongation of the bleeding time by aspirin in essential thrombocythemia is related to a decrease of large von Willebrand factor multimers in plasma. Ann Hematol 1997;75:215–20.
110. Cortelazzo S, Finazzi G, Ruggeri M, et al. Hydroxyurea for patients with essential thrombocythemia and a high risk of thrombosis. N Engl J Med 1995;332:1132–7.
111. Harrison CN, Campbell PJ, Buck G, et al. Hydroxyurea compared with anagrelide in high-risk essential thrombocythemia. N Engl J Med 2005;353:33–45.
112. Gisslinger H, Gotic M, Holowiecki J, et al. Anagrelide compared with hydroxyurea in WHO-classified essential thrombocythemia: the ANAHYDRET Study, a randomized controlled trial. Blood 2013;121:1720–8.
113. Alvarado Y, Cortes J, Verstovsek S, et al. Pilot study of pegylated interferon-alpha 2b in patients with essential thrombocythemia. Cancer Chemother Pharmacol 2003;51:81–6.
114. Barosi G, Tefferi A, Barbui T, ad hoc committee ‘Definition of clinically relevant outcomes for contemporarily clinical trials in Ph-neg M. Do current response criteria in classical Ph-negative myeloproliferative neoplasms capture benefit for patients? Leukemia 2012;26:1148–9.
115. Bjorkholm M, Derolf AR, Hultcrantz M, et al. Treatment-related risk factors for transformation to acute myeloid leukemia and myelodysplastic syndromes in myeloproliferative neoplasms. J Clin Oncol 2011;29:2410–5.
116. Alvarez-Larran A, Martinez-Aviles L, Hernandez-Boluda JC, et al. Busulfan in patients with polycythemia vera or essential thrombocythemia refractory or intolerant to hydroxyurea. Ann Hematol 2014;93:2037–43.
117. Verstovsek S, Passamonti F, Rambaldi A, et al. A phase 2 study of ruxolitinib, an oral JAK1 and JAK2 Inhibitor, in patients with advanced polycythemia vera who are refractory or intolerant to hydroxyurea. Cancer 2014;120: 513–20.
118. Vannucchi AM, Kiladjian JJ, Griesshammer M, et al. Ruxolitinib in polycythemia vera resistant to or intolerant of hydroxyurea. N Engl J Med 2015; 372:426–35.
119. Verstovsek S, Vannucchi AM, Griesshammer M, et al. Ruxolitinib versus best available therapy in patients with polycythemia vera: 80-week follow-up from the RESPONSE trial. Haematologica 2016;101:821–9.
120. Passamonti F, Griesshammer M, Palandri F, et al. Ruxolitinib for the treatment of inadequately controlled polycythaemia vera without splenomegaly (RESPONSE-2): a randomised, open-label, phase 3b study. Lancet Oncol 2017;18:88–99.
121. Verstovsek S, Passamonti F, Rambaldi A, et al. Long-term results from a phase II open-label study of ruxolitinib in patients with essential thrombocythemia refractory to or intolerant of hydroxyurea [abstract]. Blood 2014;124. Abstract 1847.
122. Harrison CN, Mead AJ, Panchal A, et al. Ruxolitinib versus best available therapy for ET intolerant or resistant to hydroxycarbamide in a randomized trial. Blood 2017 Aug 9. pii: blood-2017-05-785790 .
123. Bose P, Verstovsek S. Drug development pipeline for myeloproliferative neoplasms: potential future impact on guidelines and management. J Natl Compr Canc Netw 2016;14:1613–24.
124. Cerquozzi S, Teffieri A. Blast transformation and fibrotic progression in polycythemia vera and essential thrombocythemia: a literature review of incidence and risk factors. Blood Cancer J 2015;Nov 13;5:e366.
125. Passamonti F, Rumi E, Caramella M, et al. A dynamic prognostic model to predict survival in post-polycythemia vera myelofibrosis. Blood 2008;111:3383–7.
126. Mesa RA, Verstovsek S, Cervantes F, et al. Primary myelofibrosis (PMF), post polycythemia vera myelofibrosis (post- PV MF), post essential thrombocythemia myelofibrosis (post-ET MF), blast phase PMF (PMF-BP): Consensus on terminology by the international working group for myelofibrosis research and treatment (IWG-MRT). Leuk Res 2007;31:737–40.
127. Rampal R, Mascarenhas J. Pathogenesis and management of acute myeloid leukemia that has evolved from a myeloproliferative neoplasm. Curr Opin Hematol 2014;21:65–71.
128. Chihara D, Kantarjian HM, Newberry KJ, et al. Survival outcome of patients with acute myeloid leukemia transformed from myeloproliferative neoplasms [abstract]. Blood 2016;128. Abstract 1940.
129. Tam CS, Nussenzveig RM, Popat U, et al. The natural history and treatment outcome of blast phase BCR-ABL-myeloproliferative neoplasms. Blood 2008;112:1628–37.
130. Kundranda MN, Tibes R, Mesa RA. Transformation of a chronic myeloproliferative neoplasm to acute myelogenous leukemia: does anything work? Curr Hematol Malig Rep 2012;7:78–86.
131. Badar T, Kantarjian HM, Ravandi F, et al. Therapeutic benefit of decitabine, a hypomethylating agent, in patients with high-risk primary myelofibrosis and myeloproliferative neoplasm in accelerated or blastic/acute myeloid leukemia phase. Leuk Res 2015;39:950–6.
132. Thepot S, Itzykson R, Seegers V, et al. Treatment of progression of Philadelphia-negative myeloproliferative neoplasms to myelodysplastic syndrome or acute myeloid leukemia by azacitidine: a report on 54 cases on the behalf of the Groupe Francophone des Myelodysplasies (GFM). Blood 2010;116:3735–42.
133. Pemmaraju N, Kantarjian H, Kadia T, et al. A phase I/II study of the Janus kinase (JAK)1 and 2 inhibitor ruxolitinib in patients with relapsed or refractory acute myeloid leukemia. Clin Lymphoma Myeloma Leuk 2015;15:171–6.
134. Rampal RK, Mascarenhas JO, Kosiorek HE, et al. Safety and efficacy of combined ruxolitinib and decitabine in patients with blast-phase MPN and post-MPN AML: results of a phase I study (Myeloproliferative Disorders Research Consortium 109 trial) [abstract]. Blood 2016;128. Abstract 1124.
135. Bose P, Verstovsek S, Gasior Y, et al. Phase I/II study of ruxolitinib (RUX) with decitabine (DAC) in patients with post-myeloproliferative neoplasm acute myeloid leukemia (post-MPN AML): phase I results [abstract]. Blood 2016;128. Abstract 4262.
1. Swerdlow SH, Campo E, Harris NL, et al. WHO classification of tumours of haematopoietic and lymphoid tissues. 4th ed. Lyon: IARC; 2008.
2. Alvarez-Larran A, Pereira A, Arellano-Rodrigo E, et al. Cytoreduction plus low-dose aspirin versus cytoreduction alone as primary prophylaxis of thrombosis in patients with high-risk essential thrombocythaemia: an observational study. Br J Haematol 2013;161:865–71.
3. Tefferi A. Polycythemia vera and essential thrombocythemia: 2013 update on diagnosis, risk-stratification, and management. Am J Hematol 2013;88:507–16.
4. Michiels JJ, Berneman Z, Schroyens W, et al. Platelet-mediated erythromelalgic, cerebral, ocular and coronary microvascular ischemic and thrombotic manifestations in patients with essential thrombocythemia and polycythemia vera: a distinct aspirin-responsive and coumadin-resistant arterial thrombophilia. Platelets 2006;17:528–44.
5. James C, Ugo V, Couedic J-PL, et al. A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera. Nature 2005;434:1144–8.
6. Kralovics R, Passamonti F, Buser A, et al. A gain-of-function mutation of JAK2 in myeloproliferative disorders.N Engl J Med 2005;352:1779–90.
7. Verstovsek S, Mesa R, Gotlib J, et al. A double-blind, placebo-controlled trial of ruxolitinib for myelofibrosis. N Engl J Med 2012;366:799–807.
8. Harrison C, Kiladjian J, Al-Ali H, et al. JAK Inhibition with ruxolitinib versus best available therapy for myelofibrosis.N Engl J Med 2012;366:787–98.
9. Berglund S, Zettervall O. Incidence of polycythemia vera in a defined population. Eur J Haematol 1992;48:20–6.
10. Rozman C, Giralt M, Feliu E, et al. Life expectancy of patients with chronic nonleukemic myeloproliferative disorders. Cancer 1991;67:2658–63.
11. Passamonti F, Rumi E, Pungolino E, et al. Life expectancy and prognostic factors for survival in patients with polycythemia vera and essential thrombocythemia. Am J Med 2004;117:755–61.
12. Hultcrantz M, Kristinsson SY, Andersson TM, et al. Patterns of survival among patients with myeloproliferative neoplasms diagnosed in Sweden from 1973 to 2008: a population-based study. J Clin Oncol 2012;30:2995–3001.
13. Wolanskyj AP, Schwager SM, et al. Essential thrombocythemia beyond the first decade: life expectancy, long-term complication rates, and prognostic factors. Mayo Clin Proc 2006;81:159–66.
14. Srour SA, Devesa SS, Morton LM, et al. Incidence and patient survival of myeloproliferative neoplasms and myelodysplastic/myeloproliferative neoplasms in the United States, 2001-12. Br J Haematol 2016;174:382–96.
15. Titmarsh GJ, Duncombe AS, McMullin MF, et al. How common are myeloproliferative neoplasms? A systematic review and meta-analysis. Am J Hematol 2014;89:581–7.
16. Moulard O, Mehta J, Fryzek J, et al. Epidemiology of myelofibrosis, essential thrombocythemia, and polycythemia vera in the European Union. Eur J Haematol 2014;92:289–97.
17. Stein BL, Gotlib J, Arcasoy M, et al. Historical views, conventional approaches, and evolving management strategies for myeloproliferative neoplasms. J Natl Compr Canc Netw 2015;13:424–34.
18. Levine RL, Wadleigh M, Cools J, et al. Activating mutation in the tyrosine kinase JAK2 in polycythemia vera, essential thrombocythemia, and myeloid metaplasia with myelofibrosis. Cancer Cell 2005;7:387–97.
19. Baxter EJ, Scott LM, Campbell PJ, et al. Acquired mutation of the tyrosine kinase JAK2 in human myeloproliferative disorders. Lancet 2005;365:1054–61.
20. Scott LM, Tong W, Levine RL, et al. JAK2 exon 12 mutations in polycythemia vera and idiopathic erythrocytosis. N Engl J Med 2007;356:459–68.
21. Pardanani A, Lasho TL, Finke C, et al. Prevalence and clinicopathologic correlates of JAK2 exon 12 mutations in JAK2V617F-negative polycythemia vera. Leukemia 2007;21:1960–3.
22. Passamonti F, Elena C, Schnittger S, et al. Molecular and clinical features of the myeloproliferative neoplasm associated with JAK2 exon 12 mutations. Blood 2011;117:2813–6.
23. Abe M, Suzuki K, Inagaki O, et al. A novel MPL point mutation resulting in thrombopoietin-independent activation. Leukemia 2002;16:1500–6.
24. Pikman Y, Lee BH, Mercher T, et al. MPLW515L is a novel somatic activating mutation in myelofibrosis with myeloid metaplasia. PLoS Med 2006;3:e270.
25. Pardanani AD, Levine RL, Lasho T, et al. MPL515 mutations in myeloproliferative and other myeloid disorders: a study of 1182 patients. Blood 2006;108:3472–6.
26. Nangalia J, Massie CE, Baxter EJ, et al. Somatic CALR mutations in myeloproliferative neoplasms with nonmutated JAK2. N Engl J Med 2013;369:2391–405.
27. Klampfl T, Gisslinger H, Harutyunyan AS, et al. Somatic mutations of calreticulin in myeloproliferative neoplasms. N Engl J Med 2013;369:2379–90.
28. Wang WA, Groenendyk J, Michalak M. Calreticulin signaling in health and disease. Int J Biochem Cell Biol 2012;44:842–6.
29. Saeidi K. Myeloproliferative neoplasms: Current molecular biology and genetics. Crit Rev Oncol Hematol 2016;98:375–89.
30. Arber DA, Orazi A, Hasserjian R, et al. The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood 2016;127: 2391–405.
31. Mesa RA, Niblack J, Wadleigh M, et al. The burden of fatigue and quality of life in myeloproliferative disorders (MPDs): an international Internet-based survey of 1179 MPD patients. Cancer 2007;109:68–76.
32. Marchioli R, Finazzi G, Landolfi R, et al. Vascular and neoplastic risk in a large cohort of patients with polycythemia vera. J Clin Oncol 2005;23:2224–32.
33. Scherber R, Dueck AC, Johansson P, et al. The Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF): international prospective validation and reliability trial in 402 patients. Blood 2011;118:401–8.
34. Emanuel RM, Dueck AC, Geyer HL, et al. Myeloproliferative neoplasm (MPN) symptom assessment form total symptom score: prospective international assessment of an abbreviated symptom burden scoring system among patients with MPNs. J Clin Oncol 2012;30:4098–103.
35. Casini A, Fontana P, Lecompte TP. Thrombotic complications of myeloproliferative neoplasms: risk assessment and risk-guided management. J Thromb Haemost 2013;11:1215–27.
36. Barbui T, Finazzi G, Falanga A. Myeloproliferative neoplasms and thrombosis. Blood 2013;122:2176-–84.
37. Pearson TC, Wetherley-Mein G. Vascular occlusive episodes and venous haematocrit in primary myeloproliferative polychytemia. Lancet 1978;2:1219–22.
38. Landolfi R, Di Gennaro L, Barbui T, et al. Leukocytosis as a major thrombotic risk factor in patients with polycythemia vera. Blood 2007;109:2446–52.
39. Tefferi A, Rumi E, Finazzi G, et al. Survival and prognosis among 1545 patients with contemporary polycythemia vera: an international study. Leukemia 2013;27:1874–81.
40. Barbui T, Falanga A. Molecular biomarkers of thrombosis in myeloproliferative neoplasms. Thromb Res 2016;140 Suppl 1:S71–75.
41. Campbell PJ, Scott LM, Buck G, et al. Definition of subtypes of essential thrombocythaemia and relation to polycythaemia vera based on JAK2 V617F mutation status: a prospective study. Lancet 2005;366:1945–53.
42. Tefferi A, Guglielmelli P, Larson DR, et al. Long-term survival and blast transformation in molecularly annotated essential thrombocythemia, polycythemia vera, and myelofibrosis. Blood 2014;124:2507–13.
43. Qin Y, Wang X, Zhao C, et al. The impact of JAK2V617F mutation on different types of thrombosis risk in patients with essential thrombocythemia: a meta-analysis. Int J Hematol 2015;102:170–80.
44. Ziakas PD. Effect of JAK2 V617F on thrombotic risk in patients with essential thrombocythemia: measuring the uncertain. Haematologica 2008;93:1412–4.
45. Lussana F, Dentali F, Abbate R, et al. Screening for thrombophilia and antithrombotic prophylaxis in pregnancy: Guidelines of the Italian Society for Haemostasis and Thrombosis (SISET). Thromb Res 2009;124: e19–25.
46. Dahabreh IJ, Zoi K, Giannouli S, et al. Is JAK2 V617F mutation more than a diagnostic index? A meta-analysis of clinical outcomes in essential thrombocythemia. Leuk Res 2009;33:67–73.
47. Cho YU, Chi HS, Lee EH, et al. Comparison of clinicopathologic findings according to JAK2 V617F mutation in patients with essential thrombocythemia. Int J Hematol 2009;89:39–44.
48. Wolanskyj AP, Lasho TL, Schwager SM, et al. JAK2 mutation in essential thrombocythaemia: clinical associations and long-term prognostic relevance. Br J Haematol 2005;131:208–13.
49. Antonioli E, Guglielmelli P, Pancrazzi A, et al. Clinical implications of the JAK2 V617F mutation in essential thrombocythemia. Leukemia 2005;19:1847–9.
50. Palandri F, Catani L, Testoni N, et al. Long-term follow-up of 386 consecutive patients with essential thrombocythemia: safety of cytoreductive therapy. Am J Hematol 2009;84:215–20.
51. Chim CS, Sim JP, Chan CC, et al. Impact of JAK2V617F mutation on thrombosis and myeloid transformation in essential thrombocythemia: a multivariate analysis by Cox regression in 141 patients. Hematology 2010;15: 187–92.
52. Vannucchi AM, Antonioli E, Guglielmelli P, et al. Clinical profile of homozygous JAK2 617V>F mutation in patients with polycythemia vera or essential thrombocythemia. Blood 2007;110:840–6.
53. Carobbio A, Finazzi G, Antonioli E, et al. JAK2V617F allele burden and thrombosis: a direct comparison in essential thrombocythemia and polycythemia vera. Exp Hematol 2009;37:1016–21.
54. Alvarez-Larran A, Bellosillo B, Pereira A, et al. JAK2V617F monitoring in polycythemia vera and essential thrombocythemia: clinical usefulness for predicting myelofibrotic transformation and thrombotic events. Am J Hematol 2014;89:517–23.
55. Barbui T, Vannucchi AM, Buxhofer-Ausch V, et al. Practice-relevant revision of IPSET-thrombosis based on 1019 patients with WHO-defined essential thrombocythemia. Blood Cancer J 2015;5:e369.
56. Carobbio A, Thiele J, Passamonti F, et al. Risk factors for arterial and venous thrombosis in WHO-defined essential thrombocythemia: an international study of 891 patients. Blood 2011;117:5857–9.
57. Alvarez-Larran A, Cervantes F, Bellosillo B, et al. Essential thrombocythemia in young individuals: frequency and risk factors for vascular events and evolution to myelofibrosis in 126 patients. Leukemia 2007;21:1218–23.
58. Jantunen R, Juvonen E, Ikkala E, et al. The predictive value of vascular risk factors and gender for the development of thrombotic complications in essential thrombocythemia. Ann Hematol 2001;80:74–8.
59. Besses C, Cervantes F, Pereira A, et al. Major vascular complications in essential thrombocythemia: a study of the predictive factors in a series of 148 patients. Leukemia 1999;13:150–4.
60. Haider M, Gangat N, Hanson C, Tefferi A. Splenomegaly and thrombosis risk in essential thrombocythemia: the mayo clinic experience. Am J Hematol 2016;91: E296–297.
61. Carobbio A, Finazzi G, Antonioli E, et al. Thrombocytosis and leukocytosis interaction in vascular complications of essential thrombocythemia. Blood 2008;112:3135–7.
62. Palandri F, Polverelli N, Catani L, et al. Impact of leukocytosis on thrombotic risk and survival in 532 patients with essential thrombocythemia: a retrospective study. Ann Hematol 2011;90:933–8.
63. Campbell PJ, MacLean C, Beer PA, et al. Correlation of blood counts with vascular complications in essential thrombocythemia: analysis of the prospective PT1 cohort. Blood 2012;120:1409–11.
64. Cortelazzo S, Finazzi G, Ruggeri M, et al. Hydroxyurea for patients with essential thrombocythemia and a high risk of thrombosis. N Engl J Med 1995;332:1132–6.
65. van Genderen PJ, Mulder PG, Waleboer M, et al. Prevention and treatment of thrombotic complications in essential thrombocythaemia: efficacy and safety of aspirin. Br J Haematol 1997;97:179–84.
66. Storen EC, Tefferi A. Long-term use of anagrelide in young patients with essential thrombocythemia. Blood 2001;97:863–6.
67. De Stefano V, Za T, Rossi E, et al. Recurrent thrombosis in patients with polycythemia vera and essential thrombocythemia: incidence, risk factors, and effect of treatments. Haematologica 2008;93:372–80.
68. Alvarez-Larran A, Cervantes F, Pereira A, et al. Observation versus antiplatelet therapy as primary prophylaxis for thrombosis in low-risk essential thrombocythemia. Blood 2010;116:1205–10.
69. Palandri F, Polverelli N, Catani L, et al. Bleeding in essential thrombocythaemia: a retrospective analysis on 565 patients. Br J Haematol 2012;156:281–4.
70. Rotunno G, Mannarelli C, Guglielmelli P, et al. Impact of calreticulin mutations on clinical and hematological phenotype and outcome in essential thrombocythemia. Blood 2014;123:1552–5.
71. Tefferi A, Wassie EA, Lasho TL, et al. Calreticulin mutations and long-term survival in essential thrombocythemia. Leukemia 2014;28:2300–3.
72. Rumi E, Pietra D, Ferretti V, et al. JAK2 or CALR mutation status defines subtypes of essential thrombocythemia with substantially different clinical course and outcomes. Blood 2014;123:1544–51.
73. Palandri F, Latagliata R, Polverelli N, et al. Mutations and long-term outcome of 217 young patients with essential thrombocythemia or early primary myelofibrosis. Leukemia 2015;29:1344–9.
74. Fu R, Xuan M, Zhou Y, et al. Analysis of calreticulin mutations in Chinese patients with essential thrombocythemia: clinical implications in diagnosis, prognosis and treatment. Leukemia 2014;28:1912–4.
75. Tefferi A, Wassie EA, Guglielmelli P, et al. Type 1 versus Type 2 calreticulin mutations in essential thrombocythemia: a collaborative study of 1027 patients. Am J Hematol 2014;89:E121–4.
76. Pietra D, Rumi E, Ferretti VV, et al. Differential clinical effects of different mutation subtypes in CALR-mutant myeloproliferative neoplasms. Leukemia 2016;30: 431–8.
77. Rumi E, Pietra D, Guglielmelli P, et al. Acquired copy-neutral loss of heterozygosity of chromosome 1p as a molecular event associated with marrow fibrosis in MPL-mutated myeloproliferative neoplasms. Blood 2013;121:4388–95.
78. Beer PA, Campbell PJ, Scott LM, et al. MPL mutations in myeloproliferative disorders: analysis of the PT-1 cohort. Blood 2008;112:141–9.
79. Gangat N, Wassie EA, Lasho TL, et al. Mutations and thrombosis in essential thrombocythemia: prognostic interaction with age and thrombosis history. Eur J Haematol 2015;94:31–6.
80. Sekhar M, McVinnie K, Burroughs AK. Splanchnic vein thrombosis in myeloproliferative neoplasms. Br J Haematol 2013;162:730–47.
81. Stein BL, Saraf S, Sobol U, et al. Age-related differences in disease characteristics and clinical outcomes in polycythemia vera. Leuk Lymph 2013;54:1989–95.
82. Landolfi R, Di Gennaro L, Nicolazzi MA, et al. Polycythemia vera: gender-related phenotypic differences. Intern Emerg Med 2012;7:509–15.
83. Winslow ER, Brunt LM, Drebin JA, et al. Portal vein thrombosis after splenectomy. Am J Surg 2002;184:631–6.
84. Smalberg JH, Arends LR, Valla DC, et al. Myeloproliferative neoplasms in Budd-Chiari syndrome and portal vein thrombosis: a meta-analysis. Blood 2012;120:4921–8.
85. Dentali F, Squizzato A, Brivio L, et al. JAK2V617F mutation for the early diagnosis of Ph- myeloproliferative neoplasms in patients with venous thromboembolism: a meta-analysis. Blood 2009;113:5617–23.
86. Pardanani A, Lasho TL, Hussein K, et al. JAK2V617F mutation screening as part of the hypercoagulable work-up in the absence of splanchnic venous thrombosis or overt myeloproliferative neoplasm: assessment of value in a series of 664 consecutive patients. Mayo Clin Proc 2008;83:457–9.
87. Barbui T, Barosi G, Birgegard G, et al. Philadelphia-negative classical myeloproliferative neoplasms: critical concepts and management recommendations from European LeukemiaNet. J Clin Oncol 2011;29:761–70.
88. Landolfi R, Marchioli R, Kutti J, et al. Efficacy and safety of low-dose aspirin in polycythemia vera. N Engl J Med 2004;350:114–24.
89. Marchioli R, Finazzi G, Specchia G, et al. Cardiovascular events and intensity of treatment in polycythemia vera. N Engl J Med 2013;368:22–33.
90. Kiladjian JJ, Chevret S, Dosquet C, et al. Treatment of polycythemia vera with hydroxyurea and pipobroman: final results of a randomized trial initiated in 1980. J Clin Oncol 2011;29:3907–13.
91. Kaplan ME, Mack K, Goldberg JD, et al. Long-term management of polycythemia vera with hydroxyurea: a progress report. Semin Hematol 1986;23:167–71.
92. Fruchtman SM, Mack K, Kaplan ME, et al. From efficacy to safety: a Polycythemia Vera Study group report on hydroxyurea in patients with polycythemia vera. Semin Hematol 1997;34:17–23.
93. Finazzi G, Caruso V, Marchioli R, et al. Acute leukemia in polycythemia vera: an analysis of 1638 patients enrolled in a prospective observational study. Blood 2005;105: 2664–70.
94. Barosi G, Mesa R, Finazzi G, et al. Revised response criteria for polycythemia vera and essential thrombocythemia: an ELN and IWG-MRT consensus project. Blood 2013;121:4778–81.
95. Alvarez-Larran A, Pereira A, Cervantes F, et al. Assessment and prognostic value of the European LeukemiaNet criteria for clinicohematologic response, resistance, and intolerance to hydroxyurea in polycythemia vera. Blood 2012;119:1363–9.
96. Stein BL, Tiu RV. Biological rationale and clinical use of interferon in the classical BCR-ABL-negative myeloproliferative neoplasms. J Interferon Cytokine Res 2013;33: 145–53.
97. Ludwig H, Cortelezzi A, Van Camp BG, et al. Treatment with recombinant interferon-alpha-2C: multiple myeloma and thrombocythaemia in myeloproliferative diseases. Oncology 1985;42 Suppl 1:19–25.
98. Silver RT. Long-term effects of the treatment of polycythemia vera with recombinant interferon-alpha. Cancer 2006;107:451–8.
99. Kiladjian JJ, Mesa RA, Hoffman R. The renaissance of interferon therapy for the treatment of myeloid malignancies. Blood 2011;117:4706–15.
100. Veronese FM, Mero A. The impact of PEGylation on biological therapies. BioDrugs 2008;22:315–29.
101. Kiladjian JJ, Cassinat B, Chevret S, et al. Pegylated interferon-alfa-2a induces complete hematologic and molecular responses with low toxicity in polycythemia vera. Blood 2008;112:3065–72.
102. Turlure P, Cambier N, Roussel M, et al. Complete hematological, molecular and histological remissions without cytoreductive treatment lasting after pegylated-interferon {alpha}-2a (peg-IFN{alpha}-2a) therapy in polycythemia vera (PV): long term results of a phase 2 trial [abstract]. Blood 2011;118(21). Abstract 280.
103. Quintas-Cardama A, Kantarjian H, Manshouri T, et al. Pegylated interferon alfa-2a yields high rates of hematologic and molecular response in patients with advanced essential thrombocythemia and polycythemia vera. J Clin Oncol 2009;27:5418–24.
104. Quintas-Cardama A, Abdel-Wahab O, Manshouri T, et al. Molecular analysis of patients with polycythemia vera or essential thrombocythemia receiving pegylated interferon a-2a. Blood 2013;122:893–901.
105. Samuelsson J, Hasselbalch H, Bruserud O, et al. A phase II trial of pegylated interferon alpha-2b therapy for polycythemia vera and essential thrombocythemia: feasibility, clinical and biologic effects, and impact on quality of life. Cancer 2006;106:2397–405.
106. Jabbour E, Kantarjian H, Cortes J, et al. PEG-IFN-alpha-2b therapy in BCR-ABL-negative myeloproliferative disorders: final result of a phase 2 study. Cancer 2007; 110:2012–18.
107. Them NC, Bagienski K, Berg T, et al. Molecular responses and chromosomal aberrations in patients with polycythemia vera treated with peg-proline-interferon alpha-2b. Am J Hematol 2015;90:288–94.
108. Gisslinger H, Klade C, Georgiev P, et al. Final results from PROUD-PV a randomized controlled phase 3 trial comparing ropeginterferon alfa-2b to hydroxyurea in polycythemia vera patients [abstract]. Blood 2016;128(suppl 22). Abstract 475.
109. van Genderen PJ, van Vliet HH, Prins FJ, et al. Excessive prolongation of the bleeding time by aspirin in essential thrombocythemia is related to a decrease of large von Willebrand factor multimers in plasma. Ann Hematol 1997;75:215–20.
110. Cortelazzo S, Finazzi G, Ruggeri M, et al. Hydroxyurea for patients with essential thrombocythemia and a high risk of thrombosis. N Engl J Med 1995;332:1132–7.
111. Harrison CN, Campbell PJ, Buck G, et al. Hydroxyurea compared with anagrelide in high-risk essential thrombocythemia. N Engl J Med 2005;353:33–45.
112. Gisslinger H, Gotic M, Holowiecki J, et al. Anagrelide compared with hydroxyurea in WHO-classified essential thrombocythemia: the ANAHYDRET Study, a randomized controlled trial. Blood 2013;121:1720–8.
113. Alvarado Y, Cortes J, Verstovsek S, et al. Pilot study of pegylated interferon-alpha 2b in patients with essential thrombocythemia. Cancer Chemother Pharmacol 2003;51:81–6.
114. Barosi G, Tefferi A, Barbui T, ad hoc committee ‘Definition of clinically relevant outcomes for contemporarily clinical trials in Ph-neg M. Do current response criteria in classical Ph-negative myeloproliferative neoplasms capture benefit for patients? Leukemia 2012;26:1148–9.
115. Bjorkholm M, Derolf AR, Hultcrantz M, et al. Treatment-related risk factors for transformation to acute myeloid leukemia and myelodysplastic syndromes in myeloproliferative neoplasms. J Clin Oncol 2011;29:2410–5.
116. Alvarez-Larran A, Martinez-Aviles L, Hernandez-Boluda JC, et al. Busulfan in patients with polycythemia vera or essential thrombocythemia refractory or intolerant to hydroxyurea. Ann Hematol 2014;93:2037–43.
117. Verstovsek S, Passamonti F, Rambaldi A, et al. A phase 2 study of ruxolitinib, an oral JAK1 and JAK2 Inhibitor, in patients with advanced polycythemia vera who are refractory or intolerant to hydroxyurea. Cancer 2014;120: 513–20.
118. Vannucchi AM, Kiladjian JJ, Griesshammer M, et al. Ruxolitinib in polycythemia vera resistant to or intolerant of hydroxyurea. N Engl J Med 2015; 372:426–35.
119. Verstovsek S, Vannucchi AM, Griesshammer M, et al. Ruxolitinib versus best available therapy in patients with polycythemia vera: 80-week follow-up from the RESPONSE trial. Haematologica 2016;101:821–9.
120. Passamonti F, Griesshammer M, Palandri F, et al. Ruxolitinib for the treatment of inadequately controlled polycythaemia vera without splenomegaly (RESPONSE-2): a randomised, open-label, phase 3b study. Lancet Oncol 2017;18:88–99.
121. Verstovsek S, Passamonti F, Rambaldi A, et al. Long-term results from a phase II open-label study of ruxolitinib in patients with essential thrombocythemia refractory to or intolerant of hydroxyurea [abstract]. Blood 2014;124. Abstract 1847.
122. Harrison CN, Mead AJ, Panchal A, et al. Ruxolitinib versus best available therapy for ET intolerant or resistant to hydroxycarbamide in a randomized trial. Blood 2017 Aug 9. pii: blood-2017-05-785790 .
123. Bose P, Verstovsek S. Drug development pipeline for myeloproliferative neoplasms: potential future impact on guidelines and management. J Natl Compr Canc Netw 2016;14:1613–24.
124. Cerquozzi S, Teffieri A. Blast transformation and fibrotic progression in polycythemia vera and essential thrombocythemia: a literature review of incidence and risk factors. Blood Cancer J 2015;Nov 13;5:e366.
125. Passamonti F, Rumi E, Caramella M, et al. A dynamic prognostic model to predict survival in post-polycythemia vera myelofibrosis. Blood 2008;111:3383–7.
126. Mesa RA, Verstovsek S, Cervantes F, et al. Primary myelofibrosis (PMF), post polycythemia vera myelofibrosis (post- PV MF), post essential thrombocythemia myelofibrosis (post-ET MF), blast phase PMF (PMF-BP): Consensus on terminology by the international working group for myelofibrosis research and treatment (IWG-MRT). Leuk Res 2007;31:737–40.
127. Rampal R, Mascarenhas J. Pathogenesis and management of acute myeloid leukemia that has evolved from a myeloproliferative neoplasm. Curr Opin Hematol 2014;21:65–71.
128. Chihara D, Kantarjian HM, Newberry KJ, et al. Survival outcome of patients with acute myeloid leukemia transformed from myeloproliferative neoplasms [abstract]. Blood 2016;128. Abstract 1940.
129. Tam CS, Nussenzveig RM, Popat U, et al. The natural history and treatment outcome of blast phase BCR-ABL-myeloproliferative neoplasms. Blood 2008;112:1628–37.
130. Kundranda MN, Tibes R, Mesa RA. Transformation of a chronic myeloproliferative neoplasm to acute myelogenous leukemia: does anything work? Curr Hematol Malig Rep 2012;7:78–86.
131. Badar T, Kantarjian HM, Ravandi F, et al. Therapeutic benefit of decitabine, a hypomethylating agent, in patients with high-risk primary myelofibrosis and myeloproliferative neoplasm in accelerated or blastic/acute myeloid leukemia phase. Leuk Res 2015;39:950–6.
132. Thepot S, Itzykson R, Seegers V, et al. Treatment of progression of Philadelphia-negative myeloproliferative neoplasms to myelodysplastic syndrome or acute myeloid leukemia by azacitidine: a report on 54 cases on the behalf of the Groupe Francophone des Myelodysplasies (GFM). Blood 2010;116:3735–42.
133. Pemmaraju N, Kantarjian H, Kadia T, et al. A phase I/II study of the Janus kinase (JAK)1 and 2 inhibitor ruxolitinib in patients with relapsed or refractory acute myeloid leukemia. Clin Lymphoma Myeloma Leuk 2015;15:171–6.
134. Rampal RK, Mascarenhas JO, Kosiorek HE, et al. Safety and efficacy of combined ruxolitinib and decitabine in patients with blast-phase MPN and post-MPN AML: results of a phase I study (Myeloproliferative Disorders Research Consortium 109 trial) [abstract]. Blood 2016;128. Abstract 1124.
135. Bose P, Verstovsek S, Gasior Y, et al. Phase I/II study of ruxolitinib (RUX) with decitabine (DAC) in patients with post-myeloproliferative neoplasm acute myeloid leukemia (post-MPN AML): phase I results [abstract]. Blood 2016;128. Abstract 4262.