In vivo effects of eltrombopag on platelet function in immune thrombocytopenia: no evidence of platelet activation

Abstract

The effects of eltrombopag, a thrombopoietin-receptor agonist, on platelet function in immune thrombocytopenia (ITP) are not fully characterized. This study used whole blood flow cytometry to examine platelet function in 20 patients receiving eltrombopag treatment at days 0, 7, and 28. Platelet surface expression of activated GPIIb/IIIa, P-selectin, and GPIb was measured with and without low and high adenosine diphosphate (ADP) and thrombin receptor activating peptide (TRAP) concentrations. Before eltrombopag treatment with no ex vivo agonist, platelet activation was higher in ITP patients than controls. Platelet GPIb and activated GPIIb/IIIa expression without added agonist was unchanged following eltrombopag treatment, whereas a slight increase in P-selectin was observed. Expression of P-selectin and activated GPIIb/IIIa in response to high-dose ADP was lower during eltrombopag treatment than at baseline. Eltrombopag led to a slight increase in platelet reactivity to TRAP only in responders to eltrombopag but not to levels above those in controls; whole blood experiments demonstrated that this increase was probably because of higher platelet counts rather than higher platelet reactivity. In conclusion, although thrombocytopenic ITP patients have higher baseline platelet activation than controls, eltrombopag did not cause platelet activation or hyper-reactivity, irrespective of whether the platelet count increased.

Figures

Figure 1

Platelet parameters in healthy controls and in patients with ITP at 0, 7, and 28 days of eltrombopag treatment. (A) Platelet count, (B) mean platelet volume (MPV), (C) immature platelet fraction (IPF) %, and (D) absolute immature platelet fraction (AIPF). Data are mean ± SEM (*P < .05; **P < .02; ***P < .001).

Figure 2

Platelet parameters in responders to eltrombopag treatment. (A) Platelet count, (B) mean platelet volume (MPV), (C) immature platelet fraction (IPF) %, and (D) absolute immature platelet fraction (AIPF). Responders (black columns) and nonresponders (gray columns). Data are mean ± SEM (*P < .05; **P < .02; ***P < .001).

Figure 3

Platelet surface expression of activation markers in controls and ITP patients at days 0, 7, and 28 of eltrombopag treatment with no added agonist and in response to low and high dose ADP and TRAP stimulation. (A) Activated GPIIb/IIIa (PAC-1), (B) P-selectin, and (C) GPIb. C indicates healthy controls; D0, ITP patients pretreatment with eltrombopag; and D7 and D28, 7 and 28 days after initiation of eltrombopag treatment in ITP patients. Black-capped bars indicate unpaired (ITP at baseline vs controls) and gray bars indicate pairwise comparisons (ITP patients at different study days). Data are mean ± SEM (*P < .05; **P < .02; ***P < .001).

Figure 4

Platelet surface markers of activation in responders and nonresponders to eltrombopag treatment with no added agonist and in response to low and high dose ADP and TRAP stimulation. (A-C) Responders and (D-F) nonresponders. (A,D) Activated GPIIb/IIIa (PAC-1), (B,E) P-selectin, and (C,F) GPIb. C indicates healthy controls; D0, ITP patients pretreatment with eltrombopag; D7 and D28, 7 and 28 days after initiation of eltrombopag treatment in ITP patients. Data are mean ± SEM (*P < .05; **P < .02; ***P < .001).

Figure 5

Platelet surface markers of activation in controls following in vitro manipulation of platelet counts. (A) Activated GPIIb/IIIa, (B) P-selectin, and (C) GPIb. Data are mean ± SEM, n = 5 (*P < .05, **P < .02) for agonist-induced platelet integrin expression in thrombocytopenic samples compared with that at “normal” platelet counts (ie, 175 × 109/L).