Thrombopoietins and Thrombopoiesis: a Clinical Perspective

Since the discovery of thrombopoietin four years ago there has been much interest in the clinical use of this growth factor and its impact on platelet transfusions. Two recombinant thrombopoietin molecules are currently under intense clinical investigation. One is a full-length, glycosylated thrombopoietin (rHuTPO) and the other is a non-glycosylated, truncated thrombopoietin coupled to polyethylene glycol (PEG-rHuMGDF). Both bind to the thrombopoietin receptor, c-mpl, and stimulate megakaryocyte growth and platelet production in vitro and in vivo. In early clinical studies these "Mpl ligands" have been effective in reducing thrombocytopenia after non-myeloablative but not after myeloablative chemotherapy. In transfusion medicine, they may serve to increase the yield of stem cell harvests, expand progenitor cells ex vivo and stimulate platelet apheresis donors. Their impact on platelet usage is still unclear but may be less than initially estimated.