Myelofibrosis

Exploring dysfunctional pathways, mechanisms, and biomarkers in myelofibrosis to discover new insights into the progression of the disease.

1.1 : 100,000

Estimated incidence rate of new cases of MF in the US.1

5-6 Years

Median Overall Survival in MF Patients3

Incidence & Mortality

In the most comprehensive epidemiologic study to date, the rate of incidence of myelofibrosis (MF) (including primary MF, Post-PV MF and Post-ET MF) was about 1.1 cases of MF per 100,000 adults per year in the US.1

  • Primary MF is very rare with an incidence of about 0.3 per 100,000 person-years.2

Median survival is about 5-6 years from diagnosis and the disease typically affects the elderly with a median age at diagnosis of 65 years, although MF can occur at any age.3

Males have a higher risk of MF with about 63% of new cases being diagnosed in male patients.3

High-risk characteristics include unfavorable karyotype (15%), thrombocytopenia at diagnosis (28%), and requiring blood transfusions at the time of referral to a treatment center (39%).3

MF also has a 10-year risk of about 20% of conversion to a more aggressive disease such as acute myeloid leukemia (AML).3

Primary myelofibrosis (PMF) is part of a group of diseases called myeloproliferative neoplasms (MPN), and it is classified as BCR-ABL Philadelphia chromosome negative (Ph-) and characterized by abnormal blood cells which usually harbor mutations in the JAK pathway.7,8

  • JAK2-V617F is the best characterized mutation seen in these disorders with prevalence of 40–50% in primary MF.7
  • Philadelphia chromosome-negative (Ph-) MPNs are derived from blood-forming stem cells, resulting from the excessive proliferation of aberrant megakaryocytes.

MPNs are serious and rare bone marrow disorders disrupting the normal production of blood cells.7,8

  • While MF is the least common MPN it is the most aggressive.
  • MF is associated with bone marrow fibrosis, characterized by increased deposition of reticulin fibers and in some cases collagen fibers, caused by megakaryocyte-induced overproduction of cytokines.

In primary myelofibrosis (PMF), megakaryocyte dysplasia/hyperplasia determines the release of inflammatory cytokines that, in turn, stimulate stromal cells and induce bone marrow fibrosis.7,9

  • Aberrant megakaryocytes produce excessive fibrotic growth factors, resulting in bone marrow fibrosis and impaired hematopoiesis.

Pathogenesis of fibrosis: constitutive activation of JAK-STAT pathway in the MPN stem cell cohort leads to cytokine independent growth of megakaryocytes. Emperipolesis of neutrophils occurs that leads to release of TGF-β, PDGF and bFGF from alpha granules present in megakaryocytes. These cytokines cause fibrosis and angiogenesis and also lead to osteosclerosis by causing the stromal cells to release osteoprotegerin.7

MPNs, including MF, frequently have an activating mutation in the gene encoding Janus kinase 2 (JAK2).7,10,11

  • The exact etiology of MF is unknown and there are no known risk factors for the disease, although overactive Janus-associated kinase (JAK) pathway signaling is present in all patients.
  • Somatic mutations PMF are classified into "driver" and "other" mutations; the former include JAK2 (~65%), CALR (~25%), and MPL (~10%) and the latter ASXL1, SRSF2, and U2AF1, among others.11
  • A minority of patients do not have any of the 3 main "driver mutations" and are referred to as Triple Negative disease (10-15%).11
  • The pathogenesis of MPNs is poorly understood and even though mutations in JAK2, MPL, and CALR are seen and can lead to constitutive action of the JAK-STAT pathway, it has not been elucidated how these mutations can cause different clinical phenotypes.7

About 85% of cases of MF are primary MF (PMF), with the remaining 15% developing from another myeloproliferative disease (i.e. secondary MF, post-ET MF, post-PV MF).8,12

  • Patients with polycythemia vera have about a 10-15% risk of progressing to MF, while essential thrombocythemia (ET) patients have a <4% risk.13,14

PMF develops spontaneously and is further sub-classified as "overtly fibrotic" or "prefibrotic" based on the degree or presence of bone marrow fibrosis.8

  • Prefibrotic/early PMF was added in order to distinguish de novo, but not yet fibrotic PMF from essential thrombocythemia, thus allowing for more tailored treatment decisions.8

PMF carries the worst prognosis among the MPNs.7,11,15

  • PMF presents with anemia/cytopenias, (hepato)splenomegaly (enlarged spleen and liver), and constitutional symptoms, composed of common symptoms of inflammation caused by the overproduction of cytokines such as fatigue, fever, bone pain, pruritus, and thrombosis.
  • PMF is associated with a median survival of 6.5 years.
  • Extramedullary hematopoiesis is seen not only in the liver and spleen but also in the lymph nodes, serosal surfaces, urogenital system and epidural and paraspinal spaces.
  • Bone marrow histology shows fibrosis, angiogenesis and osteosclerosis.
  • Advanced reticulin or collagen fibrosis is associated with classic stages of PMF, but a diagnosis of MF can be established without obvious fibrosis (prefibrotic/early PMF).
  • Bone marrow fibrosis is the most important feature causing increased morbidity and mortality in these patients.

Contemporary prognostic modeling in PMF started with the development of the International Prognostic Scoring System (IPSS) in 2009.11

  • The IPSS for PMF was designed for use at time of initial diagnosis and applies five independent predictors of inferior survival: age > 65 years, hemoglobin <10 g/dL, leukocyte count >25 × 109/L, circulating blasts ≥1% and presence of constitutional symptoms.
  • The presence of 0, 1, 2 and ≥3 adverse factors defined low, intermediate-1, intermediate-2 and high risk disease; the corresponding median survivals were reported at 11.3, 7.9, 4 and 2.3 years

The IWG-MRT subsequently developed a dynamic prognostic model (DIPSS) that utilizes the same prognostic variables used in IPSS but can be applied at any time during the disease course.11

  • DIPSS assigned two, instead of one, adverse points for hemoglobin <10 g/dL and risk categorization was accordingly modified: low (0 adverse points), intermediate-1 (1 or 2 points), intermediate-2 (3 or 4 points) and high (5 or 6 points).
  • The corresponding median survivals were not reached, 14.2, 4 and 1.5 years.

Current conventional drug therapy, including JAK2 inhibitors, for MF is mostly palliative in scope and has not been shown to favorably modify disease natural history or prolong survival.11,16

  • The treatment algorithm in myelofibrosis is based on risk stratification according to the mutation- and karyotype-enhanced international prognostic scoring system (MIPSS70+ version 2.0).11,16
  • There is no evidence to support the value of specific therapy in asymptomatic patients with MIPSS70+ version 2.0 "low" or "very low" risk disease.11,16
  • Allogenic Stem Cell Transplant (ASCT) is the preferred treatment of choice for MIPSS70+ version 2.0 "high" or "very high" risk disease; clinical trial participation is the most appropriate alternative in nontransplant candidates.11,16
  • Symptom-directed conventional therapy is reasonable to consider in treatment requiring MIPSS70+ version 2.0 intermediate-risk disease and in higher risk patients that are not eligible for either ASCT or investigational drug therapy.11,16

JAK-STAT signaling drives proliferation and stimulates expression of pro-survival BCL-family proteins (BCL-XL and MCL-1).17

  • Therefore, somatic mutations in JAK/STAT pathways lead to BCL-XL overexpression.
  • BCL-XL elevation has been associated with resistance to JAK2 inhibitors.

Inhibiting the JAK2 signaling network at two nodal points, both the initiating stage (JAK2) and the effector stage (BCL-XL/BCL-2) may reduce tumor burden while minimizing resistance.17-20

  • BCL-XL/BCL-2 inhibition has shown the ability to overcome JAK2i- resistance.18
  • Conversely, aberrant JAK2-STAT signaling has been linked to over-expression of MCL-1, a well-known BCL-XL/BCL-2 inhibitor resistance factor17
  • JAK2 inhibitors have been shown to reduce MCL-1 expression.17

Preclinical hematologic cancer models have shown combined targeting of JAK2 and BCL-2/BCL-XL may circumvent and overcome acquired resistance to single-agent JAK2 inhibitor treatment.17-20

Relevant Biomarker Pathways

  1. Mehta J, et al. Epidemiology of myeloproliferative neoplasms in the United States. Leukemia & Lymphoma. 2014;55(3):595-600.
  2. Shallis RM, et al. Epidemiology of the classical myeloproliferative neoplasms: The four corners of an expansive and complex map. Blood Reviews. 2020 Jul;42:100706. DOI:10.1016/j.blre.2020.100706.
  3. Gangat N, et al. DIPSS Plus: A Refined Dynamic International Prognostic Scoring System for Primary Myelofibrosis That Incorporates Prognostic Information From Karyotype, Platelet Count, and Transfusion Status. Journal of Clinical Oncology. 2011;29(4):392-397.
  4. Howlader N, et al.SEER Cancer Statistics Review1975-2017; https://seer.cancer.gov/csr/1975_2017/browse_csr.php?sectionSEL=30&pageSEL=sect_30_table.01.. Accessed November 2020.
  5. https://seer.cancer.gov/explorer/application.html?site=409&data_type=1&graph_type=2&compareBy=sex&chk_sex_1=1&race=1&age_range=1&hdn_stage=101&hdn_rate_type=1&advopt_precision=1&advopt_display=2
  6. https://www.medpagetoday.com/resource-centers/polycythemia-vera/incidence-myeloproliferative-neoplasms-population-based-us-data/1155
  7. Agarwal A, et al. Bone marrow fibrosis in primary myelofibrosis: pathogenic mechanisms and the role of TGF-β. Stem Cell Investig 2016;3:5. (image)
  8. Barbui T, et al. The 2016 WHO classification and diagnostic criteria for myeloproliferative neoplasms: document summary and in-depth discussion. Blood Cancer J. 2018;8(2):15.
  9. Malara A, et al. Megakaryocyte Contribution to Bone Marrow Fibrosis: many Arrows in the Quiver. Mediterr J Hematol Infect Dis. 2018;10(1):e2018068.
  10. Winter P, et al. RAS signaling promotes resistance to JAK inhibitors by suppressing BAD-mediated apoptosis. Science Signaling 2014. 7(357):ra122.
  11. Tefferi A, et al. Primary myelofibrosis: 2019 update on diagnosis, risk-stratification and management. Am J Hematol. 2018;93:1551–1560.
  12. The Leukemia & Lymphoma Society. Myelofibrosis Facts. https://www.lls.org/sites/default/files/file_assets/FS14_Myelofibrosis_Fact%20Sheet_Final9.12.pdf. Updated April 2012. Accessed November 2020.
  13. Alvarez-Larran A, et al. Postpolycythaemic myelofibrosis: frequency and risk factors for this complication in 116 patients. Br J Haematol 2009;146(5):504-509.
  14. Tefferi A. Essential thrombocythemia, polycythemia vera, and myelofibrosis: current management and the prospect of targeted therapy. Am J Hematol. 2008;83:491-497.
  15. Liu C, Hao S. (2017). ‘Chapter 8 – Primary Myelofibrosis', in Chang C-C, Ohgami RS.(ed.) Precision Molecular Pathology of Myeloid Neoplasms. Springer, pp. 155-179.
  16. Tefferi A, et al. MIPSS70+ Version 2.0: Mutation and Karyotype-Enhanced International Prognostic Scoring System for Primary Myelofibrosis. J Clin Oncol 2018;36(17):1769-1770.
  17. Waibel M, et al. Combined Targeting of JAK2 and Bcl-2/Bcl-xL to Cure Mutant JAK2-Driven Malignancies and Overcome Acquired Resistance to JAK2 Inhibitors Cell Rep. 2013;5(4):1047-1059.
  18. Zhang M, et al. Selective targeting of JAK/STAT signaling is potentiated by Bcl-xL blockade in IL-2–dependent adult T-cell leukemia. Proc Natl Acad Sci U S A. 2015;112(40):12480-12485.
  19. De Freitas RM, et al. Myeloproliferative neoplasms and the JAK/STAT signaling pathway: an overview. Rev Bras Hematol Hemoter. 2015;37(5):348–353.
  20. Zeuner A, et al. Activity of the BH3 mimetic ABT-737 on polycythemia vera erythroid precursor cells.Blood. 2009;113(7):1522-5.

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