Future directions with platelet growth factors

Since the purification of thrombopoietin 6 years ago, c-Mpl ligands such as recombinant human thrombopoietin (rhTPO) and pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) have undergone extensive clinical investigation. Both recombinant forms have been shown to reduce the thrombocytopenia associated with nonmyeloablative chemotherapy. Several areas of research have been identified for further clinical development of c-Mpl ligands. One future direction is to continue to explore the intravenous route of administration of rhTPO and PEG-rHuMGDF, as well as fusion proteins of interleukin-3-thrombopoietin and thrombopoietin peptide mimetics, which may be as potent as thrombopoietin, but may lack antigenicity. Another focus would be on the use of these molecules in treating non-chemotherapy-induced thrombocytopenia associated with myelodysplastic syndrome (MDS), idiopathic thrombocytopenic purpura (ITP), human immunodeficiency virus (HIV)-related ITP, and liver disease. Additionally, c-Mpl ligands may have a role in improving apheresis yields when administered to normal platelet donors. Considerable data demonstrate the effectiveness of PEG-rHuMGDF in raising the platelet yields in apheresis donors. In the past few years, investigation into the use of thrombopoietin for ex vivo expansion of pluripotent stem cells has been extensive. Last, thrombopoietin may serve as a radioprotectant by preventing radiation-induced apoptosis of pluripotent stem cells. In the coming years, the clinical role of rhTPO, PEG-rHuMGDF, and related molecules such as the thrombopoietin peptide mimetics will probably be established for both chemotherapeutic and nonchemotherapeutic indications.