Evaluation of aspirin responsiveness in healthy subjects and essential thrombocythemia patients

Introduction. Essential thrombocythemia (ET) is a myeloproliferative neoplasm characterized by an increased number of platelets in the circulating blood, which is associated with an increased risk for both bleeding and thrombotic complications (1). Low-dose acetylsalicylic acid (ASA, 75-100 mg/d), which inhibits the platelet thromboxane A2 (TxA2) biosynthesis, is used for preventing thrombotic complications in ET patients at risk. The pharmacodynamics (PD) effects of ASA are blunted in some ET patients (ET-Non responders, ET-NR) (2). Aim of this study was to identify the causes of poor responsiveness to Aspirin in ET-NR. Methods. ET responders (ET-R, n=10), ET-NR (n=9) and healthy controls (n=11) were enrolled in the study. Using ID-LC-MS-MS for determination of ASA and salicylic acid (SA) we evaluated: 1) the in vitro activity of plasma and blood esterases, that hydrolyze ASA to SA; 2) the in vivo kinetics of ASA and SA at different times after the administration of 100 mg ASA (enteric-coated formulation). Serum TxB2 (stable metabolite of TxA2) was measured by ELISA at different time points after drug intake. In vitro effects of ASA on platelet aggregation (Multiplate) and TxB2 production (ELISA) induced by collagen in whole blood were measured in ET-NR and healthy controls. Results. Esterase activity was similar in the three study groups. Pharmacokinetics of ASA was very variable in all subjects: ASA maximum plasma concentration ranged between 500 and 1400 ng/mL 2-6 hours after intake. SA had similar trend. Half of ET-NR did not show ASA absorption within the 8 hours observation period. In these patients serum TxB2 was persistently high. However, in vitro addition of ASA (100 µM) inhibited TxB2 production at the same rate as in controls, excluding an impaired pharmacodynamics effect (3). Conclusions. Excluding enzymatic or pharmacodynamics effects, causes of inadequate response to ASA in ET patients need some more investigations on gastro-intestinal availability. References 1) Patrono C., Rocca B. and De Stefano V. Platelet activation and inhibition in polycythemia vera and essential thrombocythemia. Blood. 2013;121 (10):1701-1711 2) Cattaneo M. Aspirin and Clopidogrel. Efficacy, Safety, and the Issue of Drug Resistance. Arterioscler Thromb Vasc Biol. 2004; 24:1980-1987. 3) Frelinger AL, Furman MI, Linden MD, Li Y, Fox ML, Barnard MR, Michelson AD. Residual arachidonic acid-induced platelet activation via an adenosine diphosphate-dependent but cyclooxygenase-1- and cyclooxygenase-2-independent pathway: a 700-patient study of aspirin resistance. Circulation. 2006 Jun 27;113(25):2888-96.