SUMMARY: Polycythemia Vera (P. Vera) is a clonal myeloproliferative neoplasm characterized by isolated erythrocytosis in a majority of the patients, with the remaining demonstrating leukocytosis and/or thrombocytosis along with erythrocytosis. Patients usually present with this disorder in their sixth decade and are often asymptomatic, with the diagnosis made incidentally on routine laboratory evaluation. About 30% of the patients however, may initially present with a thrombotic episode, whereas a small percentage of patients may present with disease related symptoms such as pruritus and fatigue. The conventional risk factors for thrombotic events in MyeloProliferative Neoplasms (MPN) are age more than 60 years and prior thrombosis, and the presence of both these risk factors is associated with a 7-fold increased risk of thrombosis.
Overactivation of the JAK-STAT signal transduction pathway caused by V617F mutation has been implicated in majority of the patients with P. Vera. This pathway normally is responsible for passing information from outside the cell through the cell membrane to the DNA in the nucleus for gene transcription. Janus Kinase (JAK) family of tyrosine kinases are cytoplasmic proteins and include JAK1, JAK2, JAK3 and TYK2. JAK1 helps propagate the signaling of inflammatory cytokines whereas JAK2 is essential for growth and differentiation of hematopoietic stem cells. These tyrosine kinases mediate cell signaling by recruiting STATs (Signal Transducer and Activator of Transcription), with resulting modulation of gene expression. In patients with P. Vera, the aberrant myeloproliferation is the result of dysregulated JAK2-STAT signaling as well as excess production of inflammatory cytokines, associated with this abnormal signaling. JAK2 mutations such as JAK2 V617F are seen in approximately 95% of patients with P. Vera.
Studies have shown that JAK2 mutations that result in the overproduction of erythrocytes, leukocytes, and platelets in P. Vera also promote direct activation of leukocytes and platelets. Activated platelets and leukocytes bind to each other and activate endothelial cells, which may in turn contribute to the prothrombotic state. The prospective CYTO-PV trial published in 2011, established that maintaining hematocrit less than 45% through phlebotomies and/or cytoreductive drugs significantly decreased the risk of thrombotic events in P. Vera patients. Even though several retrospective analyses strongly suggest an association between leukocytosis and thrombosis and leukocytosis particularly at the time of the thrombotic event in P. Vera patients, no prospective trial has been conducted to assess the impact of WBC counts on thrombotic risk in P. Vera.
The REVEAL study is a large, real-world, multicenter, prospective, noninterventional, observational study, in which patients with P. Vera from US community practice and academic centers were enrolled, to evaluate the association between elevated blood counts and occurrence of thrombotic events in patients with P. Vera, using data from the REVEAL study.
This study analyzed the data of 2271 eligible patients for this analysis (78% high risk and 22% low risk). The median patient age was 66 years and 54% were male. The median disease duration was 4.1 years, 20% had a history of thrombotic events and majority of patients (53%) were receiving Hydroxyurea. Patient data was collected at diagnosis, at a 6-month period, and during follow up, 3 years from last patient enrollment, between July 2014 and August 2019 and the researchers analyzed the association between blood counts and thrombotic events. Out of 106 patients who had thrombotic events, 30 had arterial thrombotic events, most commonly, Transient Ischemic Attack and 76 had venous thrombotic events, most commonly, Deep Vein Thrombosis.
It was noted that hematocrit greater than 45% versus 45% or less (P=0.0028), WBC more than 11×109/L versus 11×109/L or less (P<0.0001), and Platelet counts more than 400×109/L versus 400×109/L or less (P=0.0170) were each associated with increased risk of thrombotic events. A WBC count of 11×109/L or more was associated with the highest thrombotic event risk compared with WBC count less than 7×109/L (P<0.0001). When the HCT was controlled at 45% or less, an elevated WBC count (more than 12 × 109/L) was significantly associated with increased risk of thromboembolism (HR=1.95; P=0.03). In all models analyzed, advanced age and history of thrombotic events, were associated with increased thrombotic event risk.
The authors concluded that in this analysis of the largest real-world cohort of P. Vera patients to date, hematocrit more than 45%, as well as WBC more than 11×109/L and Platelet counts more than 400×109/L, were each associated with increased risk of thrombotic events. WBC more than 12×109/L was significantly associated with increased thrombotic risk, even when the HCT was controlled, suggesting that thrombotic risk may be reduced by controlling both the WBC count and HCT level. The authors added that these data support the incorporation of blood count values into risk stratification and treatment strategies for patients with P. Vera in clinical practice, and to move beyond the conventional risk model.
A Real-World Evaluation of the Association between Elevated Blood Counts and Thrombotic Events in Polycythemia Vera: An Analysis of Data from the REVEAL Study). Gerds AT, Mesa RA, Burke JM, et al. Presented at: 2022 European Hematology Association Congress; June 10, 2022; Vienna, AT. Poster # P1062.
Overactivation of the JAK-STAT signal transduction pathway caused by V617F mutation, has been implicated in majority of the patients with P. Vera. This pathway normally is responsible for passing information from outside the cell through the cell membrane to the DNA in the nucleus, for gene transcription. Janus Kinase (JAK) family of tyrosine kinases are cytoplasmic proteins and include JAK1, JAK2, JAK3 and TYK2. JAK1 helps propagate the signaling of inflammatory cytokines whereas JAK2 is essential for growth and differentiation of hematopoietic stem cells. These tyrosine kinases mediate cell signaling by recruiting STAT’s (Signal Transducer and Activator of Transcription), with resulting modulation of gene expression. In patients with P. Vera, the aberrant myeloproliferation is the result of dysregulated JAK2-STAT signaling as well as excess production of inflammatory cytokines, associated with this abnormal signaling. JAK2 mutations such as JAK2 V617F are seen in approximately 95% of patients with P. Vera. The goals of therapy in P. Vera are to maintain the hematocrit at less than 45% and decrease the risk of thrombosis and bleeding. P. Vera is presently managed with periodic phlebotomies, cytoreductive therapy with oral antimetabolite, Hydroxyurea and antiplatelet agents such as low dose Aspirin. However, a significant number of patients on these therapies become intolerant or resistant to these treatments, leading to an increased risk of progression.
JAKAFI® is a potent JAK1 and JAK2 inhibitor and exerts its mechanism of action by targeting and inhibiting the dysregulated JAK2-STAT signaling pathway. The RESPONSE trial is a phase III prospective randomized study in which patients with P. Vera, who were refractory or intolerant of Hydroxyurea were randomized to receive JAKAFI® 10 mg PO, BID (N=110) or Best Available Therapy (BAT), which consisted of investigator choice of monotherapy or observation only (N=112). Eligible patients were phlebotomy dependent and had splenomegaly (> 450 cubic cm). Patients receiving BAT were allowed to cross over to JAKAFI® group from week 32 onwards. The primary endpoint of this study (composite primary endpoint) was the proportion of patients whose hematocrit was controlled without phlebotomy and whose spleen volume was reduced by 35% or more from baseline, as assessed by MRI imaging at 32 weeks. Secondary endpoints included durable response, Complete Hematological Remission and safety. The primary analysis was conducted when all patients reached week 48 or discontinued therapy. The proportion of patients in the JAKAFI® group who achieved the composite primary endpoint was 21% compared to 1% in the BAT group (P < 0.0001) and 91% in the JAKAFI® group maintained their response at week 48. Seventy seven percent (77%) of the patients in the JAKAFI® group achieved at least one of the two major components of the composite primary endpoint. Put another way, 60% of the patients in the JAKAFI® arm were able to achieve the target hematocrit level in the absence of phlebotomy, compared to 20% in the BAT group. Reduction in the spleen volume by 35% or more was noted in 38% of the patients in the JAKAFI® group compared to 1% in the BAT group. Complete Hematological Remission defined as continuous hematocrit below 45%, as well as normal white blood cells and platelets counts, was achieved in 24% and 9% of patients in JAKAFI® and BAT group respectively (P=0.003). More patients assigned to JAKAFI® group also demonstrated 50% or more improvement in the Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF) 14-item total symptom score, at week 32 compared to BAT (49% vs 5%). Thromboembolic events occurred in one patient assigned to the JAKAFI® group as compared to six patients in the BAT group. The authors concluded that JAKAFI® may represent a new option for treating high risk patients with Polycythemia Vera, who are refractory or intolerant of Hydroxyurea. JAKAFI® is superior to Best Available Therapy (BAT) in controlling hematocrit without phlebotomies as well as Splenic Volume. Further, JAKAFI® is also effective in improving P. Vera associated symptoms. Verstovsek S, Kiladjian J, Griesshammer M, et al. J Clin Oncol 32:5s, 2014 (suppl; abstr 7026)
Over activation of the JAK-STAT signal transduction pathway caused by V617F mutation, has been implicated in majority of the patients with P. Vera. This pathway normally is responsible for passing information from outside the cell through the cell membrane to the DNA in the nucleus for gene transcription. Janus Kinase (JAK) family of tyrosine kinases are cytoplasmic proteins and include JAK1, JAK2, JAK3 and TYK2. JAK1 helps propagate the signaling of inflammatory cytokines whereas JAK2 is essential for growth and differentiation of hematopoietic stem cells. These tyrosine kinases mediate cell signaling by recruiting STAT’s (Signal Transducer and Activator of Transcription), with resulting modulation of gene expression. In patients with P. Vera, the aberrant myeloproliferation is the result of dysregulated JAK2-STAT signaling as well as excess production of inflammatory cytokines associated with this abnormal signaling. JAK2 mutations such as JAK2 V617F are seen in approximately 95% of patients with P. Vera. The goals of therapy in P. Vera are to maintain the hematocrit at less than 45% and decrease the risk of thrombosis and bleeding. P. Vera is presently managed with periodic phlebotomies, cytoreductive therapy with oral antimetabolite, Hydroxyurea and antiplatelet agents such as low dose aspirin. However, a significant number of patients on these therapies become intolerant or resistant to these treatments, leading to an increased risk of progression. JAKAFI® is a potent JAK1 and JAK2 inhibitor and exerts its mechanism of action by targeting and inhibiting the dysregulated JAK2-STAT signaling pathway. 
The RESPONSE trial is a phase III prospective randomized study in which patients with P. Vera, who were refractory or intolerant of Hydroxyurea were randomized to receive JAKAFI® 10 mg PO, bid (N=110) or Best Available Therapy (BAT), which consisted of investigator choice of monotherapy or observation only (N=112). Eligible patients were phlebotomy dependent patients with splenomegaly (> 450 cubic cm). Patients receiving BAT were allowed to cross over to JAKAFI® group from week 32 onwards. The primary endpoint of this study (composite primary endpoint) was the proportion of patients whose hematocrit was controlled without phlebotomy and whose spleen volume was reduced by 35% or more from baseline, as assessed by MRI imaging at 32 weeks. Secondary endpoints included durable response, Complete Hematological Remission and safety. The primary analysis was conducted when all patients reached week 48 or discontinued therapy. The proportion of patients in the JAKAFI® group who achieved the composite primary endpoint was 21% compared to 1% in the BAT group (P < 0.0001). Seventy seven percent (77%) of the patients in the JAKAFI® group achieved at least one of the two major components of the composite primary endpoint. Put another way, 60% of the patients in the JAKAFI® arm were able to achieve the target hematocrit level in the absence of phlebotomy, compared to 20% in the BAT group. Reduction in the spleen volume by 35% or more was noted in 38% of the patients in the JAKAFI® group compared to 1% in the BAT group. Complete Hematological Remission defined as continuous hematocrit below 45%, as well as normal white blood cells and platelets counts, was achieved in 24% and 9% of patients in JAKAFI® and BAT group respectively (P=0.003). More patients assigned to JAKAFI® group also demonstrated 50% or more improvement in the Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF) 14-item total symptom score, at week 32 compared to BAT (49% vs 5%). Thromboembolic events occurred in one patient assigned to the JAKAFI® group as compared to six patients in the BAT group. The authors concluded that JAKAFI® may represent a new option for treating high risk patients with Polycythemia Vera, who are refractory or intolerant of Hydroxyurea. Jakafi® is superior to Best Available Therapy (BAT) in controlling hematocrit without phlebotomies as well as Splenic Volume. Further, JAKAFI® is also effective in improving P. Vera associated symptoms. Verstovsek S, Kiladjian J, Griesshammer M, et al. J Clin Oncol 32:5s, 2014 (suppl; abstr 7026)