Alberto Álvarez‐Larrán

BACKGROUND AND OBJECTIVES: The mechanisms accounting for the increased risk of thrombosis in patients with essential thrombocythemia (ET) are not well known. The aim of the present study was to ascertain the role of platelet and leukocyte activation in the thrombosis of ET. DESIGN AND METHODS: The activation status of platelets and leukocytes was assessed by flow cytometry studies in 49 patients with ET (22 with previous thrombosis and 27 without a history of thrombosis) and in a group of age- and sex-matched healthy individuals. The assessment included platelet P-selectin expression (measured both at baseline and after stimulation with ADP, thrombin, arachidonic acid (AA), and collagen), platelet-neutrophil and platelet-monocyte complexes, determination of CD11b in the neutrophils and monocytes, and expression of tissue factor in the monocytes (mTF). The JAK2 V617F mutation was studied and correlated with platelet and leukocyte activation. RESULTS: As compared with controls, ET patients had significantly higher values of baseline P-selectin and thrombin- and AA-induced platelet P-selectin expression, as well as higher platelet-neutrophil and platelet-monocyte complexes, neutrophil CD11b expression and baseline mTF expression. Platelet P-selectin, monocyte CD11b, and lipopolysaccharide-induced mTF expression was significantly higher in ET patients with a history of thrombosis than in patients without thrombosis. Patients with the JAK2 V617F mutation or thrombosis showed higher baseline and AA-induced platelet P-selectin expression than did those without thrombosis. INTERPRETATION AND CONCLUSIONS: These results would support a role for platelet and monocyte activation in the thrombosis of ET. In these patients, the presence of the JAK2 V617F mutation is associated with higher platelet activation.

We assessed frequency and predictive factors for major cardiovascular (CV) events in 707 patients with primary myelofibrosis (PMF) followed in 4 European institutions. A total of 236 deaths (33%) were recorded for an overall mortality of 7.7% patient-years (pt-yr). Fatal and nonfatal thromboses were registered in 51 (7.2%) patients, with a rate of 1.75% pt-yr. If deaths from non-CV causes were considered as competing events, we estimated that the adjusted rate of major thrombotic events would have been 2.2% pt-yr. In a multivariable model, age older than 60 years (hazard ratio [HR], 2.34; 95% confidence interval [CI], 1.24-4.39, P = .01) and JAK2 mutational status (HR, 1.92; 95% CI, 1.10-3.34; P = .02) were significantly associated with thrombosis, whereas the strength of the association between leukocyte count higher than 15 x 10(9)/L and CV events was of borderline significance (HR, 1.72; 95% CI, 0.97-2.72; P = .06). The highest incidence of fatal and nonfatal thrombosis was observed when the mutation was present along with leukocytosis (3.9% pt-yr; HR, 3.13; 95% CI, 1.26-7.81). This study is the largest hitherto carried out in this setting and shows that the rate of major CV events in PMF is comparable with that reported in essential thrombocythemia, and it is increased in aged patients and those with JAK2 V617F mutation and leukocytosis.

The effectiveness of antiplatelet therapy as primary prophylaxis for thrombosis in low-risk essential thrombocythemia (ET) is not proven. In this study, the incidence rates of arterial and venous thrombosis were retrospectively analyzed in 300 low-risk patients with ET treated with antiplatelet drugs as monotherapy (n = 198) or followed with careful observation (n = 102). Follow-up was 802 and 848 person-years for antiplatelet therapy and observation, respectively. Rates of thrombotic events were 21.2 and 17.7 per 1000 person-years for antiplatelet therapy and observation, respectively (P = .6). JAK2 V617F-positive patients not receiving antiplatelet medication showed an increased risk of venous thrombosis (incidence rate ratio [IRR]: 4.0; 95% CI: 1.2-12.9; P = .02). Patients with cardiovascular risk factors had increased rates of arterial thrombosis while on observation (IRR: 2.5; 95% CI: 1.02-6.1; P = .047). An increased risk of major bleeding was observed in patients with platelet count greater than 1000 x 10(9)/L under antiplatelet therapy (IRR: 5.4; 95% CI: 1.7-17.2; P = .004). In conclusion, antiplatelet therapy reduces the incidence of venous thrombosis in patients with JAK2-positive ET and the rate of arterial thrombosis in patients with associated cardiovascular risk factors. In the remaining low-risk patients, this therapy is not effective as primary prophylaxis of thrombosis, and observation may be an adequate option.

Criteria of response and definition of resistance and intolerance to hydroxyurea (HU) in polycythemia vera (PV) were proposed by the European LeukemiaNet (ELN). Such criteria were evaluated in 261 PV patients (median follow-up, 7.2 years) treated with HU for a median of 4.4 years. Complete response, partial response, and no response were observed in 24%, 66%, and 10% of patients, respectively. Achieving ELN response (complete or partial) or hematocrit response did not result in better survival or less thrombosis and bleeding. On the contrary, having no response in leukocyte count was associated with higher risk of death (HR, 2.7; 95% confidence interval [CI], 1.3%-5.4%; P = .007), whereas lack of response in platelet count involved a higher risk of thrombosis and bleeding. Resistance and intolerance to HU was registered in 11% and 13% of patients, respectively. Resistance to HU was associated with higher risk of death (HR, 5.6; 95% CI, 2.7%-11.9%; P < .001) and transformation (HR, 6.8; 95% CI, 3.0%-15.4%; P < .001). In summary, fulfilling the ELN definition for response to HU was not associated with a benefit in the clinical outcome in PV, whereas response in platelet and white blood cell counts were predictive of less thrombohemorrhagic complications and better prognosis, respectively. Resistance to HU was an adverse prognostic factor.

Myelofibrotic transformation is a known complication of essential thrombocythaemia (ET), but information on its incidence, presenting features and evolution is scarce. In a series of 195 patients with ET followed for a median of 7.2 years (range: 1.9-24), evolution into myelofibrosis with myeloid metaplasia (MMM) occurred in 13 cases, a median of 8 years (range: 3.6-20.2) from diagnosis. The actuarial probability of this complication was 2.7% (95% CI: 2.4-2.9) at 5 years, 8.3% (95% CI: 7.8-8.9) at 10 years, and 15.3% (95% CI: 6.1-24.5) at 15 years. Four patients had not been treated before developing MMM. The main features indicating this condition were the appearance of immature myeloid precursors in the peripheral blood, a decrease in the Hb value not related to treatment and increased serum lactate dehydrogenase levels, followed by a progressive decrease in the platelet count, increasing leucocytosis and progressive splenomegaly. No patient had constitutional symptoms, and none of five evaluable cases showed chromosome abnormalities in bone marrow or unstimulated blood. After a median the myelofibrotic transformation, three patients have died and four have not required treatment for MMM as yet.