Analysis of megakaryocyte growth and development factor (thrombopoietin) effects on blast cell and megakaryocyte growth in myelodysplasia

Identification of Plasma Thrombopoietin Level and Its Significance in Patients with Aplastic Anemia and Myelodysplastic Syndrome

Objective  Our objective was to investigate the concentration of plasma thrombopoietin (TPO) in patients with aplastic anemia (AA) and myelodysplastic syndrome (MDS), as well as its relationship with patients' responses to recombined human TPO (rhTPO) therapy. Methods  We detected the concentration of plasma TPO in 31 patients with AA, 27 patients with MDS, and 11 normal controls using enzyme-linked immunosorbent assay. Results  The median concentration of plasma TPO in patients with AA, MDS, and controls was (841.08 ± 768.64), (212.41 ± 338.93), and (35.09 ± 18.21) pg/mL, respectively. The TPO concentration in patients with AA and MDS was significantly higher than that in controls ( p  < 0.05). The median platelet (PLT) counts were (184 ± 34) ×10 9 /L in the control group and (24 ± 19) ×10 9 /L and (80 ± 71) ×10 9 /L in AA and MDS patients, respectively. Negative correlations were found between plasma TPO concentration and PLT counts as well as megakaryocytes in bone marrow ( p  < 0.05). In AA patients treated with rhTPO, a negative correlation was observed between increased PLT counts and pretreatment TPO levels ( p  < 0.05). Conclusion  Plasma TPO concentration in AA and MDS was significantly higher than that in normal controls. Plasma TPO was negatively correlated with peripheral blood PLT counts and bone marrow megakaryocyte counts. The pretreatment TPO level may serve as a prognostic indicator for the therapeutic effect of rhTPO in AA patients.

Clinical findings with the first generation of thrombopoietic agents

Thrombocytopenia is a common problem in hematology/oncology patients. In the past two decades a number of thrombopoietic growth factors and related cytokines have become available for clinical investigations. Unfortunately, most of the pleiotropic cytokines have been limited by their modest activity and toxicity profile. The discovery of thrombopoietin (TPO), a key regulator of platelet production, led to the clinical development of two recombinant versions of the molecule: full-length, recombinant human thrombopoietin (rhTPO), and truncated and pegylated, megakaryocyte growth and development factor (Peg-rHuMGDF). Both agents showed significant biologic activity in various clinical settings, including nonmyeloablative chemotherapy, mobilization of progenitors, platelet apheresis, and treatment of thrombocytopenia related to other conditions. Despite promising thrombopoietic activity, the clinical development of the first generation of recombinant TPOs was discontinued due to the neutralizing antibodies observed with PEG-rHuMGDF. This has led to the development of TPO agonists with no sequence homology to TPO, which can bind to the TPO receptors and activate signaling, leading to an increase in platelet production. The clinical experience with the first generation of thrombopoietic agents has provided insight into the biology and future directions for a second generation of thrombopoietic agents in various disorders of thrombocytopenia.

Safety and efficacy of romiplostim in patients with lower-risk myelodysplastic syndrome and thrombocytopenia

PURPOSE

To assess the safety and efficacy of romiplostim, a peptibody that increases platelet production, for treatment of thrombocytopenic patients with myelodysplastic syndromes (MDS).

PATIENTS AND METHODS

Eligible patients had lower-risk MDS (International Prognostic Scoring System low or intermediate 1), a mean baseline platelet count <or= 50 x 10(9)/L, and were only receiving supportive care. Patients received three injections of 300, 700, 1,000, or 1,500 microg romiplostim at weekly intervals. After evaluation of platelet response at week 4, patients could continue to receive romiplostim in a treatment extension phase for up to 1 year.

RESULTS

All 44 patients who enrolled completed the treatment phase; 41 patients continued into the extension phase. Median platelet counts increased throughout the study, from fewer than 30 x 10(9)/L at baseline to 60, 73, 38, and 58 x 10(9)/L at week 4 for the 300-, 700-, 1,000-, and 1,500 -microg dose cohorts, respectively. A durable platelet response (per International Working Group 2000 criteria for 8 consecutive weeks independent of platelet transfusions) was achieved by 19 patients (46%). The incidence of bleeding events and platelet transfusions was less common among patients who achieved a durable platelet response than those who did not (4.3 v 39.3 per 100 patient-weeks). Forty-three patients (98%) reported one or more adverse events. Treatment-related serious adverse events were reported in five patients (11%), all of whom were in the 1,500-microg dose cohort. Two patients progressed to acute myeloid leukemia during the study. No neutralizing antibodies to either romiplostim or endogenous thrombopoietin were seen.

CONCLUSION

Romiplostim appeared well-tolerated in this study and may be a useful treatment for patients with MDS and thrombocytopenia.

Phase II study of low-dose interleukin-11 in patients with myelodysplastic syndrome

Severe thrombocytopenia places patients with myelodysplastic syndrome (MDS) at risk of serious hemorrhage. Currently, therapeutic options are limited to platelet transfusions. The only commercially available growth factor that increases platelet counts is interleukin-11 (IL-11). We report the results of a phase II trial to more accurately assess the clinical response and toxicity data for low-dose IL-11 (10 microg/kg/day) in patients with MDS. In this study, nine of 32 assessable patients (28%) demonstrated increases in their platelet counts after treatment. Of these, five were considered major platelet responses (15%), as defined by World Health Organization criteria. Four patients had minor platelet responses (13%). The median duration of platelet response was 9 months. Low-dose IL-11 was well tolerated, with no observed grade 4 toxicities. Our study provides additional clinical evidence that chronic administration of IL-11, at low doses, can raise platelet counts and reduce platelet transfusion requirements in a subset of patients with MDS.

Thrombocytopenia in myelodysplastic syndromes and myelofibrosis

Myelodysplastic syndromes (MDS) are a group of hematopoietic stem cell disorders characterized by ineffective hematopoeisis and an increased risk of transforming to acute myelogenous leukemia (AML). Determining the molecular basis of the disease has been hampered by its heterogeneity. Thrombocytopenia is often a manifestation of MDS and needs to be monitored and treated accordingly. Treating the underlying disorder with a variety of differentiation and immunosuppressive agents alleviates the problem in a small percentage of patients but more often complicates the issue. Several treatments used for primary immune thrombocytopenic purpura (ITP) have been tried in MDS patients, though with only modest success rates. Preliminary studies suggest that the use of a thrombopoietic growth factor may afford substantial increases in platelet levels without excessive deleterious side effects. Primary myelofibrosis (MF) is a chronic myeloproliferative disorder associated with hepatosplenomegaly and refractory cytopenias. Immunomodulatory agents have shown promise in treating the anemia associated with this MF. However, there are currently no standard therapies to treat the thrombocytopenia that is often found in patients with this disease.

Recombinant human thrombopoietin: basic biology and evaluation of clinical studies

Thrombocytopenia is a common medical problem for which the main treatment is platelet transfusion. Given the increasing use of platelets and the declining donor population, identification of a safe and effective platelet growth factor could improve the management of thrombocytopenia. Thrombopoietin (TPO), the c-Mpl ligand, is the primary physiologic regulator of megakaryocyte and platelet development. Since the purification of TPO in 1994, 2 recombinant forms of the c-Mpl ligand--recombinant human thrombopoietin (rhTPO) and pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF)--have undergone extensive clinical investigation. Both have been shown to be potent stimulators of megakaryocyte growth and platelet production and are biologically active in reducing the thrombocytopenia of nonmyeloablative chemotherapy. However, neither TPO has demonstrated benefit in stem cell transplantation or leukemia chemotherapy. Other clinical studies have investigated the use of TPO in treating chronic nonchemotherapy-induced thrombocytopenia associated with myelodysplastic syndromes, idiopathic thrombocytopenic purpura, thrombocytopenia due to human immunodeficiency virus, and liver disease. Based solely on animal studies, TPO may be effective in reducing surgical thrombocytopenia and bleeding, ex vivo expansion of pluripotent stem cells, and as a radioprotectant. Ongoing and future studies will help define the clinical role of recombinant TPO and TPO mimetics in the treatment of chemotherapy- and nonchemotherapy-induced thrombocytopenia.

Effect of thrombopoietin on proliferation of blasts from patients with myelodysplastic syndromes

Thrombopoietin (TPO), a major cytokine involved in megakaryocytopoiesis/thrombopoiesis, may be effective for treatment of the thrombocytopenia associated with myelodysplastic syndromes (MDS). However, it has been unclear whether TPO stimulates proliferation of MDS blasts, as observed in de novo acute myeloid leukemia. This study examined this concern. When marrow cells from 37 MDS cases were cultured with or without recombinant human PEGylated TPO, TPO increased the blast number (stimulation index > or =1.5) in 9 of 16 high-risk MDS cases (refractory anemia with excess blasts [RAEB] and RAEB in transformation) and 4 of 10 cases with MDS transformed to acute leukemia (MDS-AL), but none of 11 cases with low-risk MDS (RA and RA with ringed sideroblasts). When the cell cycle of cultured cells was determined by three-color flow cytometry, TPO activated the cell cycle of MDS cells (causing a decrease in G(0)-phase cells) in most of the cases whose blast number increased in response to TPO. Reverse transcriptase-polymerase chain reaction analysis detected TPO receptor messenger RNA in purified blasts from all six cases examined, irrespective of the response of their blasts to TPO in culture. Analysis of the patients' characteristics identified a high-serum lactate dehydrogenase (LDH) value as being associated with blast proliferation in high-risk MDS cases (p = 0.0036). We conclude that TPO stimulates in vitro proliferation of blasts from a fraction of MDS patients. High-risk MDS patients, especially those who have a high-serum LDH value, and MDS-AL patients should be monitored with particular care in clinical trials of TPO for MDS.

Cloning and functional characterization of a novel c-mpl variant expressed in human CD34 cells and platelets

The thrombopoietin receptor, c-mpl, is a crucial element not only in thrombopoietin (TPO)-initiated signaling pathways but also in the regulation of the circulating amount of TPO. We have identified a new c-mpl isoform, called c-mpl-del, that lacks 72 bp (24 amino acids) in the extracellular region of c-mpl and arises as a consequence of alternative RNA splicing between exons 8 and 9. c-mpl-del is expressed along with c-mpl-wt in blood mononuclear cells, CD34(+)cells, megakaryocytes, and platelets prepared from either normal donors or ET patients, although its relative expression appears to increase with megakaryocyte differentiation. The c-mpl-del-transfected cells expressed greater amounts of c-mpl-del RNA and protein than the comparable c-mpl-wt-transfected cells, however flow cytometry analysis could not detect any c-mpl receptor on the surface of the c-mpl-del-transfected cells. Further evidence for the absence of surface c-mpl-del was that in contrast to cells transfected with c-mpl-wt, those transfected with c-mpl-del did not grow in response to TPO, failed to undergo tyrosine phosphorylation of TPO-specific signal molecules, and did not bind(125)I-rHuTPO. Taken together, these results demonstrate that c-mpl-del, a naturally occurring variant of c-mpl, fails to be incorporated into the cell membrane but might serve as a mechanism to decrease the overall expression of functional c-mpl late in megakaryocyte differentiation.

Defective megakaryocytic development in myelodysplastic syndromes

Megakaryocytic proliferation and differentiation is typically abnormal in patients with myelodysplastic syndromes (MDS). The underlying mechanisms for this finding are not known, but may involve defects at the level of the thrombopoietin-receptor (c-mpl) or post-receptor signaling pathways in megakaryocyte progenitor cells. Premature apoptosis of the bone marrow cells and inhibitory effects of cytokines such as tumor necrosis factor alpha have been implicated as contributing to altered megakaryopoiesis in MDS, but their significance remains unclear. The availability of thrombopoietin (TPO) has facilitated more detailed analysis of megakaryocytic biology using several experimental in-vitro systems. However numerous studies have shown that the developmental abnormalities of MDS megakaryocytes could not be corrected by TPO. Increasing investigations are being extended to the evaluation of signal transduction pathways of c-mpl both in cell lines and human hematopoietic cells in order to identify the molecular mechanisms responsible for the defective megakaryocytic development in MDS.

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.

Megakaryopoiesis in vitro in myelodysplastic syndromes and acute myeloid leukaemia: effect of pegylated recombinant human megakaryocyte growth and development factor in combination with other growth factors

Pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) can stimulate megakaryopoiesis in vitro in some myelodysplastic syndrome (MDS) and acute myeloid leukaemia (AML) patients. We assessed PEG-rHuMGDF combined with granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF (GM-CSF), interleukin 3 (IL-3), IL6, stem cell factor (SCF) or erythropoietin in 40 MDS, 33 AML and 16 normal bone marrow samples. CD61-positive cells in suspension cultures increased with PEG-rHuMGDF alone in 20/25 RA + RAS, 11/14 RAEB + RAEBt and 29/33 AML cases. Further increases when IL-3 and/or SCF were added to PEG-rHuMGDF occurred in 14/20 RA + RAS, 8/13 RAEB + RAEBt and 18/26 AML cases. CFU-Mk growth was poor overall, but could be enhanced by PEG-rHuMGDF combinations in some patients. Stimulation of megakaryopoiesis by PEG-rHuMGDF can be augmented by IL-3 and SCF in many MDS and AML patients.