Effects of Antiplatelet Treatment with Aspirin (ASA) on Platelet Reactivity in Patients with Essential Thrombocythemia

INTRODUCTION: Essential Thrombocythemia (ET) is a myeloproliferative neoplasm characterized by an increased rate of thrombotic complications. Antithrombotic prophylaxis with aspirin (ASA), alone or in combination with cytoreduction with hydroxyurea (HU), is widely utilized in ET patients. However, thrombosis occurrence/recurrence in spite of antithrombotic prophylaxis remains a relevant issue. Growing data support the possible contribution to this failure of the inter-individual variability of pharmacological ASA response.

AIM: Aim of this study was to characterize, in a group of ET patients receiving 100 mg/d ASA, the platelet reactivity in terms of platelet aggregation and activation properties.

MATERIALS AND METHODS: Venous blood samples were obtained from 77 ET patients (26M/51F), and two control groups, i.e., one including 72 non-ET patients receiving chronic ASA prophylaxis, and the other including 111 healthy control subjects (57M/54F). The mutational status of ET was: 35 patients were JAK2V617F⁺, 22 CALR⁺, 3 MPL⁺, and 17 triple negative. Thirty-three ET patients were on ASA+HU, 23 on ASA alone, 5 on HU alone, and 16 were not receiving any of these drugs. Platelet aggregation was assessed in whole blood by the Multiplate® analyzer (Roche). The platelet response to the thrombin receptor activating peptide (TRAP) trigger was the measure of the overall platelet aggregation capacity, while the response to the arachidonic acid (AA) trigger was the measure of ASA effect on platelet aggregation. A normalized AA-induced aggregation (r-AA-agg), defined as AA/TRAP ratio, was calculated for each sample to reflect the individual variation of platelet inhibition by ASA. The platelet activation status was evaluated before and after aggregation by measuring the surface expression of CD62P (P-selectin) by flow cytometry (Accuri™ C6, BD Bioscience).

RESULTS: The analysis of subgroups according to treatments shows that AA-induced platelet aggregation in ASA- and ASA+HU-treated ET patients was significantly lower compared to non-ASA ET subjects (p<0.001), and was significantly greater compared to ASA-treated non-ET patients (145±85 AU; p<0.001). The same results were observed with TRAP-induced platelet aggregation. Accordingly, the r-AA-agg. was greater in ET subjects on ASA (=53%) or ASA+HU (=50%) as compared to non-ET ASA-treated individuals (=19%). Furthermore, among ET patients on ASA±HU, those with platelet >450x10⁹/L showed AA-induced aggregation significantly greater than subjects with platelet <450x10⁹/L. The increment of platelet surface CD62P expression after AA stimulation (as a marker of platelet activation) was not influenced by anti-platelet therapy, but was significantly associated with JAK2V617F mutation.

CONCLUSIONS: Our data show that in more than 70% of ET patients, in spite of ASA intake, the platelet reactivity remains higher than in non-ET patients receiving the same drug regimen. This phenomenon, together with the so-called "turnover" resistance, i.e. increased platelet turnover associated to short aspirin half-life, may contribute to aspirin failure in ET. Studies are necessary to evaluate the efficacy and safety of a different dose or timing of ASA administration in these patients.

Project funded by "AIRC-IG2013" grant Nr. 14505 from the "Italian Association for Cancer Research" (A.I.R.C.).