Thrombopoietin receptor agonists in refractory immune thrombocytopenia: differential responses to eltrombopag and romiplostim: a case report and possible explanations.

Objective: Although new thrombopoietin (TPO) receptor agonist drugs, such as romiplostim and eltrombopag, are highly effective and well tolerated for patients with immune thrombocytopenia (ITP) refractory to first-line treatments such as prednisolone, the cross-resistance of these two TPO receptor agonists is still unknown.

Methods: An 84-year-old Japanese female patient with steroid-refractory ITP received eltrombopag with a gradually increasing dose schedule from 12.5 to 25 mg/day, 37.5 mg/day and finally 50 mg/day. As no increase in platelet count was observed even at the maximum dose of 50 mg/day, and eltrombopag-related grade 3 elevation of aspartate aminotransferase was observed, another TPO receptor agonist, romiplostim, was administered at 1 μg/kg/week subcutaneously. A rapid increase in platelet count was observed 1 week after the first injection. The dose of romiplostim was escalated to 4 μg/kg according to the platelet count and a complete response was achieved 7 weeks after the first injection without any adverse events.

Conclusions: The successful treatment of ITP refractory to eltrombopag with romiplostim strongly suggests that the absence of cross-resistance between these two approved TPO receptor agonists and possible differences in mechanism of action. Further study of the mechanisms of action of TPO receptor agonists is called for along with further exploration of the potential of romiplostim in refractory ITP.