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Essential thrombocythemia (ET) is characterized by abnormal megakaryopoiesis and enhanced thrombotic risk. Once-daily low-dose aspirin is the recommended antithrombotic regimen, but accelerated platelet generation may reduce the duration of platelet cyclooxygenase-1 (COX-1) inhibition. We performed a multicenter double-blind trial to investigate the efficacy of 3 aspirin regimens in optimizing platelet COX-1 inhibition while preserving COX-2–dependent vascular thromboresistance. Patients on chronic once-daily low-dose aspirin (n = 245) were randomized (1:1:1) to receive 100 mg of aspirin 1, 2, or 3 times daily for 2 weeks. Serum thromboxane B2 (sTXB2), a validated biomarker of platelet COX-1 activity, and urinary prostacyclin metabolite (PGIM) excretion were measured at randomization and after 2 weeks, as primary surrogate end points of efficacy and safety, respectively. Urinary TX metabolite (TXM) excretion, gastrointestinal tolerance, and ET-related symptoms were also investigated. Evaluable patients assigned to the twice-daily and thrice-daily regimens showed substantially reduced interindividual variability and lower median (interquartile range) values for sTXB2 (ng/mL) compared with the once-daily arm: 4 (2.1-6.7; n = 79), 2.5 (1.4-5.65, n = 79), and 19.3 (9.7-40; n = 85), respectively. Urinary PGIM was comparable in the 3 arms. Urinary TXM was reduced by 35% in both experimental arms. Patients in the thrice-daily arm reported a higher abdominal discomfort score. In conclusion, the currently recommended aspirin regimen of 75 to 100 once daily for cardiovascular prophylaxis appears to be largely inadequate in reducing platelet activation in the vast majority of patients with ET. The antiplatelet response to low-dose aspirin can be markedly improved by shortening the dosing interval to 12 hours, with no improvement with further reductions (EudraCT 2016-002885-30).

Essential thrombocythemia (ET) is characterized by abnormal megakaryopoiesis and enhanced thrombotic risk. Once-daily low-dose aspirin is the recommended antithrombotic regimen, but accelerated platelet generation may reduce the duration of platelet cyclooxygenase-1 (COX-1) inhibition. We performed a multicenter double-blind trial to investigate the efficacy of 3 aspirin regimens in optimizing platelet COX-1 inhibition while preserving COX-2–dependent vascular thromboresistance. Patients on chronic once-daily low-dose aspirin (n = 245) were randomized (1:1:1) to receive 100 mg of aspirin 1, 2, or 3 times daily for 2 weeks. Serum thromboxane B2(sTXB2), a validated biomarker of platelet COX-1 activity, and urinary prostacyclin metabolite (PGIM) excretion were measured at randomization and after 2 weeks, as primary surrogate end points of efficacy and safety, respectively. Urinary TX metabolite (TXM) excretion, gastrointestinal tolerance, and ET-related symptoms were also investigated. Evaluable patients assigned to the twice-daily and thrice-daily regimens showed substantially reduced interindividual variability and lower median (interquartile range) values for sTXB2(ng/mL) compared with the once-daily arm: 4 (2.1-6.7; n = 79), 2.5 (1.4-5.65, n = 79), and 19.3 (9.7-40; n = 85), respectively. Urinary PGIM was comparable in the 3 arms. Urinary TXM was reduced by 35% in both experimental arms. Patients in the thrice-daily arm reported a higher abdominal discomfort score. In conclusion, the currently recommended aspirin regimen of 75 to 100 once daily for cardiovascular prophylaxis appears to be largely inadequate in reducing platelet activation in the vast majority of patients with ET. The antiplatelet response to low-dose aspirin can be markedly improved by shortening the dosing interval to 12 hours, with no improvement with further reductions (EudraCT 2016-002885-30).

•HD-DXM allows a better initial response than PDN, but less long-lasting over time, useful when a rapid platelet increase is required.•PDN seems to increase long-lasting responses but exhibits a less safe profile than HD-DXM.

Motta: Sanofi Genzyme:Honoraria.Barcellini:Agios:Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding;Novartis:Honoraria, Other: invited speaker , Research Funding;Bioverativ:Membership on an entity's Board of Directors or advisory committees;Incyte:Membership on an entity's Board of Directors or advisory committees;Alexion:Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: invited speaker , Research Funding.Cappellini:BMS:Honoraria;Genzyme/Sanofi:Honoraria, Membership on an entity's Board of Directors or advisory committees;CRISPR Therapeutics, Novartis, Vifor Pharma:Membership on an entity's Board of Directors or advisory committees.

Introduction. Polycythemia Vera (PV) is a myeloproliferative neoplasm (MPN) whose major features are peripheral erythrocytosis and increased thrombotic risk, affecting morbidity and mortality. Essential Thrombocythemia (ET), a MPN characterized by high platelet generation rate, has been shown to be associated with a reduction in von Willebrand factor (vWf) large multimers and activity which depends on platelet turnover (Lancellotti et al, JTH 2015;13:1226-37). The acquired vWf type 2A defect in ET is thought to influence bleeding rate. Data on vWf and on a possible acquired von Willebrand deficiency in PV are less clear. Previous studies included PV patients with marked thrombocytosis, with bleeding history or included treated and untreated patients, with heterogeneous results. Therefore, the phenotypic features of vWf in PV patients with well-controlled haematocrit, routinely accessing the outpatient clinics remain largely undefined. In addition, whether the acquired vWf defect is similar between PV and ET remains poorly defined.

Gugliotta: SHIRE Co.: Honoraria. Gugliotta:Bristol Myers Squibb: Consultancy, Honoraria; Novartis: Consultancy, Honoraria.

Background: the Registro Italiano Trombocitemie (RIT) was activated mainly to evaluate the diagnosis and therapy appropriateness in the thrombocythemic patients with Ph-negative myeloproliferative neoplasm (MPN) observed in the adhering centers.