Thrombopoietin and thrombopoietin mimetics in the treatment of thrombocytopenia.

Although the thrombopoietin receptor was discovered in 1991 and thrombopoietin (TPO) was purified in 1994, the development of a clinically useful TPO was hampered by the appearance of neutralizing antibodies to some forms of recombinant TPO. However, in 2008 two new drugs that mimic the effect of TPO became available to treat thrombocytopenia. Romiplostim is a TPO peptide mimetic given by subcutaneous injection that activates the TPO receptor by binding to the distal hematopoietic receptor domain just like TPO. Eltrombopag is a TPO nonpeptide mimetic administered orally that activates the TPO receptor by binding to the transmembrane domain. Both increase the platelet count in healthy humans as well as in >80% of patients with immune thrombocytopenic purpura (ITP). Although initially restricted to the second-line treatment of ITP, both agents could help treat many thrombocytopenic disorders. Both agents are well tolerated, with mild headache being the most common complaint. Potential long-term complications include thrombosis, increased bone marrow reticulin, rebound worsening of thrombocytopenia upon discontinuation, and increased blast formation. Ongoing studies should establish the incidence of these complications and determine the efficacy of these new agents in a variety of other thrombocytopenic conditions.