Romiplostim: A second-generation thrombopoietin agonist

Thrombopoietin (TPO) is the major regulator of both megakaryopoiesis and platelet production. TPO is a glycoprotein primarily produced in the liver. TPO, when binding to its receptor (c-Mpl), triggers a signaling cascade that leads to the differentiation and proliferation of megakaryocytes, with a concomitant increase in platelets. The cloning and characterization of TPO in 1994 led to the production of a full length, glycosylated recombinant human TPO (rhTPO) and a pegylated, truncated protein (PEGrHuMGDF). These first-generation TPO drugs stimulated megakaryocyte production and increased platelet counts in healthy volunteers. Successful clinical trials followed in cancer patients, patients with immune thrombocytopenic purpura (ITP), and cancer patients receiving non-myeloablative chemotherapy. Neither rhTPO nor PEG-rHuMGDF raised platelet counts in myeloablated chemotherapy patients, probably due to a lack of megakaryocyte progenitor cells in their bone marrow. Unfortunately, neutralizing antibodies developed against TPO in 13 subjects who had received multiple injections of PEG-rHuMGDF. The resulting thrombocytopenia in these individuals ended all clinical trials with both drugs. A second generation of TPO growth factors have been developed and are in clinical trials. Researchers screened peptide libraries to find random, unrelated peptides that could stimulate TPO-dependent cell lines without also causing neutralizing antibody production. These peptides were then conjugated to various carrier molecules to increase their half-lives. This strategy led to the synthesis of romiplostim (AMG-531), with 2 sets of identical peptides linked to the Fc moiety of an Immunoglobulin G (IgG) antibody. This TPO peptide mimetic has shown success in clinical trials with healthy volunteers and individuals with ITP. No neutralizing antibodies have developed against AMG-531, however some thromboembolic events have occurred in high risk patients, and potentially reversible increases in bone marrow reticulin have been reported. Other TPO nonpeptide mimetics have been created by using a similar strategy with libraries of nonpeptide molecules that can stimulate TPO-dependent cell lines. Eltrombopag and AKR-501 are two drugs of this type that have shown positive results in clinical trials. In addition, antibodies that can stimulate the c-Mpl receptor are being engineered to act as potent TPO agonists. These and other drugs in preclinical development represent a new line of therapy for thrombocytopenic patients.