Milestones in understanding platelet production: a historical overview.

The discovery of thrombopoietin (TPO, also termed THPO) in 1994 was a major achievement in understanding the regulation of platelet production. In prior decades, physiological studies had demonstrated that platelets were produced from bone marrow megakaryocytes and that the megakaryocytes responded to thrombocytopenia by increasing their number, size and DNA ploidy. In 1958, it was proposed that a 'thrombopoietin' must exist that regulated this interaction between the circulating platelet mass and the bone marrow megakaryocytes. After over three decades of effort, TPO was finally purified by five independent laboratories. TPO stimulated megakaryocyte colony-forming cell growth and increased the number, size and ploidy of megakaryocytes. When the genes for TPO or TPO receptor were eliminated in mice, megakaryocytes grew and platelets were made, but at 15% of their normal number. A first generation of recombinant human (rh) TPO molecules [rhTPO and pegylated recombinant human megakaryocyte growth and development factor (PEG-rhMGDF)] rapidly entered clinical trials in 1995 and increased platelet counts in humans undergoing non-myeloablative chemotherapy but not in those undergoing stem cell transplantation. Antibodies developed against PEG-rhMGDF and development of these recombinant thrombopoietins ended. A second generation of TPO receptor agonists (romiplostim and eltrombopag) was then developed. Neither of these TPO receptor agonists demonstrated any significant untoward effects and both are now licensed in many countries for the treatment of immune thrombocytopenia. This review describes the significant experiments that have surrounded the discovery of TPO and its clinical development.

Development of romiplostim for the treatment of patients with chronic immune thrombocytopenia: from bench to bedside.

Immune thrombocytopenia (ITP) is an autoimmune disorder characterised by abnormally low platelet counts (<100 x 10(9)/l), purpura, and bleeding episodes, and can be categorised in three phases: newly-diagnosed, persistent, and chronic. As many patients become refractory to standard treatments (corticosteroids, danazol, azathioprine, splenectomy), there is an urgent need for alternative treatments. The successful isolation and cloning of thrombopoietin (TPO) in the mid-1990s and identification of its key role in platelet production was a major breakthrough, rapidly followed by the development of the recombinant thrombopoietins, recombinant human TPO and a pegylated truncated product, PEG-rHuMGDF. Both agents increased platelet counts but development was halted because of the development of antibodies that cross-reacted with native TPO, resulting in prolonged treatment-refractory thrombocytopenia. Experimentation with novel platforms for extending the circulating half-life of therapeutic peptides by combining them with antibody fragment crystallisable (Fc) constructs led to the development of a new family of molecules termed 'peptibodies'. The 60Da recombinant peptibody romiplostim was finally produced by linking several copies of an active TPO-binding peptide sequence to a carrier Fc fragment. In clinical trials, romiplostim was effective in ameliorating thrombocytopenia in patients with chronic ITP, was well tolerated and did not elicit cross-reacting antibodies. Romiplostim has recently been approved for the treatment of adults with chronic ITP.

An overview of thrombopoietin: with a historical perspective.

A physiologically relevant thrombopoietin (TPO) must be a humoral regulator with lineage specificity for megakaryocytes and their precursors. It should be capable of stimulating platelet production in normal animals, and elevated levels of TPO should be detectable in the plasma following acute, severe thrombocytopenia. Acute thrombocytopenia provides a model system that is likely to predict the effects of TPO, since many of the effects on megakaryocytes and platelets observed after induction of acute thrombocytopenia would be mediated by TPO. Important questions remain to be answered. Do the currently available data for the c-Mpl ligand explain previously published data that describe elevated levels of Meg-CSF in the circulation following production of bone marrow aplasia? Does the c-Mpl ligand account for all of the megakaryocyte stimulatory factors that have been described? Is there another factor that accounts for at least some of the acute alterations in megakaryocytopoiesis that occur immediately following a decrease in platelet levels?