In Vivo Effects of Eltrombopag on Human Platelet Function.

Eltrombopag, an orally-administered small-molecule agonist of the thrombopoietin receptor (c-Mpl), is under investigation as a treatment for immune thrombocytopenic purpura (ITP). Studies have indicated that eltrombopag does not ‘prime’ platelets for activation in vitro, and eltrombopag administration to healthy volunteers does not increase platelet surface P-selectin or activated integrin αIIbβ3 (Jenkins J. Blood 2007). However, the effects of eltrombopag on platelet function in thrombocytopenic patients in vivo, either by direct binding to c-Mpl receptors on platelets or indirectly by altering the dynamics of platelet production and causing an influx of young, large platelets, is unknown. Whole blood flow cytometry, unlike other assays of platelet function, enables measurement of platelet function in the setting of marked thrombocytopenia (Michelson. Platelets, Elsevier, 2007). As a substudy of larger treatment studies, 17 adult patients with chronic ITP received eltrombopag at a starting dose of 50 mg daily, with the possibility of an increase to 75 mg daily after 3 weeks. Blood samples were drawn pre-treatment, and after 7 and 28 days of therapy. Platelet count, mean platelet volume (MPV), and the immature platelet fraction (IPF, or reticulated platelet count) were measured using a Sysmex XE-2100. Platelet surface P-selectin and activated integrin αIIbβ3 (reported by monoclonal antibody PAC1) were measured by whole blood flow cytometry in the presence and absence of 0.5 μM ADP, 20 μM ADP, 1.5 μM thrombin receptor activating peptide (TRAP), or 20 μM TRAP. Bleeding was quantified by a comprehensive scale that allocates grades of 0 (no), 1 (minor) or 2 (marked) bleeding at 10 anatomical sites according to physical examination and/or history (Page, L.K. Br J Haematol 2007). Eleven of the 17 patients responded to eltrombopag with a rise in platelet count of >30 x 10⁹/L. The IPF increased in responders but not non-responders (table 1). Response to eltrombopag was not predicted by pretreatment MPV or IPF. The ITP bleeding score decreased in responders over the study period in parallel with the increases in platelet count (table 1). As determined by platelet surface P-selectin and activated integrin αIIbβ3, eltrombopag did not result in platelet activation or augment ADP- or TRAP-induced platelet activation (table 2). In summary, eltrombopag increases the platelet count and reduces bleeding in responding adult patients with chronic ITP through the release of new platelets into the circulation. While bleeding is reduced in responders, eltrombopag does not result in platelet activation or augmentation of platelet activation by ADP or TRAP. This suggests that the newer platelets released by eltrombopag stimulation are not hyper-functional (or are only transiently so prior to day 7).