Use of thrombopoietic growth factors in acute leukemia.

Several hematopoietic growth factors have been shown to affect megakaryocyte development, and two, interleukin (IL)-11 and thrombopoietin (TPO) are presently being evaluated for use in patients with thrombocytopenia. In two studies patients who required one or more platelet transfusions during their first course of chemotherapy were found to require fewer platelet transfusions if their second cycle was augmented with IL-11. The drug was generally safe, with cardiovascular compromise the only significant complication occurring in a minority of patients. Although these reports included patients with various malignancies, studies of IL-11 in patients with myeloproliferative disorders have not been presented. In several clinical trials in cancer patients treatment with TPO was safe, and when administered early following a moderately aggressive cytotoxic insult was effective in accelerating platelet recovery. In addition, in both pre-clinical and clinical trials, TPO given to stem cell donors during mobilization lead to accelerated hematopoietic recovery. Finally, TPO appears safe when administered to patients with acute myelogenous leukemia (AML), both with respect to acute toxicity and long-term outcome of the leukemia. However, when used following a 7-day course of standard chemotherapy, the agent does not appear to accelerate platelet recovery. As such, additional clinical trials to test different growth factor regimens are ongoing. A number of studies have suggested that megakaryocytic growth factors may play a role in the biology of myeloproliferative disorders. Given the potential for adversely affecting patients with these disorders, the affects of IL-11 or TPO in patients with AML must continue to be carefully studied.

Thrombopoietin: the primary regulator of megakaryocyte and platelet production.

The development of blood cells requires the interplay of hematopoietic stem and progenitor cells, marrow stroma and polypeptide growth factors. Although many proteins support the expansion of megakaryocytic precursor cells, identification of the late acting, lineage specific growth factor for platelet production, termed thrombopoietin, has remained elusive. Recently, characterization of the proto-oncogene c-mpl revealed structural homology with the hematopoietic cytokine receptor family. Based on the cell of origin of its cDNA, we hypothesized that the ligand for c-Mpl might be identical with Tpo, and together with scientists at ZymoGenetics, Inc. recently cloned its cDNA. Using recombinant protein we have shown that the mpl-ligand displays all of the expected biological properties of the major regulator of megakaryocyte development, and proposed that it be termed thrombopoietin.

Physiology and preclinical studies of thrombopoietin.

Until recently, the molecular basis for the control of platelet production was largely unknown. In the past year, several groups have obtained complementary DNA for thrombopoietin, the substance first theorized nearly 40 years ago to regulate this process. Cellular and molecular studies have confirmed many of the properties previously attributed to this molecule, and have revealed some surprises. It is now clear that thrombopoietin is the critical regulator of platelet production. Detailed study of the molecule will likely yield important physiologic insights into megakaryocyte biology, and its application to states of iatrogenic and natural marrow failure will almost certainly provide therapeutic advances.

Detection of thrombopoietic activity in plasma by stimulation of suppressed thrombopoiesis.

Rabbits in which thrombocytosis had been produced by five daily transfusions of platelet concentrates had suppressed endogenous thrombopoiesis, as reflected by decreased incorporation of selenomethionine-(75)Se ((75)SeM) into the circulating platelet mass. Rabbits in which endogenous thrombopoiesis had been suppressed by transfusion-induced thrombocytosis were used to detect thrombopoietic activity in rabbit plasma. Thrombopoietic activity was demonstrated in the plasma of both normal and thrombocytopenic donor rabbits. A dose response relationship was observed between the incorporation of (75)SeM into platelets and the dose of plasma administered. Infusion of 20-150 ml of plasma from thrombocytopenic donors increased the incorporation of (75)SeM into platelets from 52 to 107% above control values. A dose response effect also was seen after infusion of normal plasma, but normal plasma produced less effect than comparable doses of plasma from thrombocytopenic donors. Rabbits with transfusion-induced thrombocytosis appear to be more sensitive assay animals for the detection of thrombopoietic activity than animals with normal platelet counts. Changes in the rate of appearance and levels of (75)SeM may primarily indicate changes in platelet protein or platelet size and are apparently more sensitive indicators of the state of thrombopoiesis than are alterations in the numbers of circulating platelets. The results strongly support the concept of a humoral agent, i.e. thrombopoietin, that acts on megakaryocytes to regulate platelet production.