Biology and chemistry of thrombopoietic agents

Endogenous thrombopoietin (eTPO) regulates platelet production by increasing the number, ploidy, and maturation rate of bone marrow megakaryocytes. Early attempts to treat thrombocytopenia by the administration of recombinant TPO were successful but were complicated by the development of antibodies to one of the recombinant proteins. Two new TPO mimetics have recently been approved by the US Food and Drug Administration (FDA) for the treatment of immune thrombocytopenia (ITP). Romiplostim is a peptide TPO mimetic composed of an IgG Fc fragment to which are attached four 14-amino acid TPO peptides that activate the TPO receptor by binding to the extracytoplasmic domain just like eTPO. Romiplostim is administered as a weekly subcutaneous injection. Eltrombopag, a nonpeptide TPO mimetic, is a 442-d drug that binds to a transmembrane site on the TPO receptor and thereby activates it. It is administered daily as an oral tablet. Administration of both romiplostim and eltrombopag to healthy volunteers produced a dose-dependent rise in platelet count beginning on day 5 and peaking at days 12 to 15. Both have been highly effective in increasing the platelet count in patients with ITP and are currently being studied in the treatment of other thrombocytopenic conditions (myelodysplastic syndrome, chemotherapy, liver disease).