Whatever happened to thrombopoietin? (2002)

The long-awaited discovery of thrombopoietin (TPO) in 1994 has led to great advances in understanding the biology and physiology of megakaryocyte and platelet production.1, 2 TPO has been shown to be the key regulator of platelet production and, in its absence, platelet counts drop to 10 to 15 percent of baseline.3, 4 TPO is produced primarily in the liver and appears to be produced at a constant rate, irrespective of platelet count. This has led to the widely accepted view that the regulation of platelet production by TPO differs from the regulation RBCs by EPO. Rather than having a "sensor" of platelet mass that controls TPO production, the circulating level of TPO is determined by the rate of clearance of TPO by avid TPO receptors (c-Mpl receptors) on platelets and probably also on megakaryocytes.5, 6 In this edition of TRANSFUSION, Scheding et al. analyze how the platelet TPO receptor regulates TPO levels in thrombocytopenic chemotherapy patients. They hypothesized that platelets lacking TPO receptor function, in contrast to normal platelets, would not bind and clear TPO from the circulation after transfusion. After confirming previous findings that TPO levels are 10- to 20-fold above normal when the platelet count drops below 20 × 109 per L, they measured the decrease in the plasma TPO level after transfusion of sufficient platelets to increase the platelet count 2- to 3-fold. The TPO level dropped 20 to 30 percent after transfusion of unmanipulated platelets but was unchanged after the transfusion of platelets that had their TPO receptors bound (and presumably internalized7) by prior incubation with TPO ("TPO receptor-saturated" platelets). This elegant but simple experiment in humans confirms the concept that TPO levels are determined primarily by platelet (and possibly megakaryocyte)-mediated clearance of TPO.5-12 In addition, it puts to rest any remaining concern that there is a platelet "sensor" that regulates the production of TPO. This last statement requires some elaboration. Although hepatic TPO gene transcription does not change during thrombocytopenia, several controversial studies have suggested that the amount of marrow TPO mRNA increases in response to thrombocytopenia and may thereby alter TPO levels.13, 14 The study by Schelding et al. suggests that this mechanism, if present, fails to produce a physiologically significant amount of TPO. If the marrow was producing TPO, then the transfusion of TPO-saturated platelets and the rise in the platelet count would have been expected to decrease the marrow TPO production and cause a drop in the TPO level; this was not seen in these experiments. These data join those of Peck-Radosavljevic et al.,15 who found that thrombocytopenic cirrhotic patients had extremely low TPO levels and that both the platelet count and TPO levels returned to normal after liver transplantation. If a marrow "sensor" existed to increase TPO production by marrow stromal cells, TPO levels should have been higher prior to transplantation. Despite providing enormous insights into the underlying biology of platelets and megakaryocytes, the clinical promise of TPO has not yet come to fruition. At a similar time after the genes encoding G–CSF and EPO were cloned, both factors were in widespread clinical use. Currently, no recombinant TPO is clinically available. Two recombinant forms of TPO have been prepared and extensively studied.16, 17 One is a glycosylated molecule (rHuTPO) with a primary structure identical to endogenous TPO, and the other is a nonglycosylated, pegylated molecule (pegylated rHu megakaryocyte growth and development factor [PEG-rHuMGDF]) composed of the first 161 amino acids of endogenous TPO. Both forms have terminal half-lives of 30 to 40 hours. Both recombinant TPOs markedly increase the production of platelets in a dose-dependent fashion in individuals with normal marrow.18-20 The platelet count starts to rise 5 days after a single injection and peaks after 10 to 12 days. However, in patients undergoing high-dose chemotherapy for acute myeloid leukemia (AML) or progenitor cell transplantation, various TPO administration schedules have failed to improve the time to recovery of platelets to 20 × 109 per L or the need for platelet transfusions, but there is a marked increase in the rebound thrombocytosis following platelet recovery.21-24 In patients undergoing nonmyeloablative chemotherapy for cancer, both recombinant TPOs improve the nadir platelet count, reduce the duration of thrombocytopenia, and decrease the need for platelet transfusions by approximately 50 percent.25-28 The rather unimpressive results in the BMT/AML/chemotherapy trials led to the investigation of TPO in nonchemotherapy settings, in which marrow reserves were less impaired. Six patients with HIV-related thrombocytopenia had their platelet count normalize after eight injections of PEG-rHuMGDF, and three of four immune thrombocytopenic purpura (ITP) patients responded to one to seven daily intravenous doses of PEG-rHuMGDF.29, 30 In a series of 21 patients with myelodysplastic syndrome, the platelet count doubled after treatment with PEG-rHuMGDF, and some patients had multilineage responses.31 The administration of rHuTPO during progenitor cell mobilization with chemotherapy and/or G–CSF markedly increased the yield of CD34+ cells and reduced the number of aphereses needed to attain a target graft dose, but after transplantation, this product showed a significant but minor reduction in platelet transfusions and time to platelet engraftment.32 Finally, in a large series of platelet donors, a single administration of PEG-rHuMGDF increased apheresis yields by 3-fold, and the product was clinically effective.18, 33 These less-than-anticipated findings have led to a loss of enthusiasm for further clinical development of recombinant TPO for two major reasons. The first is the assessment that the current potential market for TPO in oncology is rather small. With no apparent utility in marrow transplantation or AML chemotherapy, a large potential market is gone. Because significant thrombocytopenia is rather uncommon after routine chemotherapy, platelet transfusions are widely available, and transfusion triggers are revised downward,34 the market for recombinant TPO is rather small, despite the proven benefit in reducing the need for platelet transfusions. The second reason for a loss of enthusiasm for recombinant TPO is the realization that the clinical biology of TPO is much more complex than that for either EPO or G–CSF. TPO has a terminal half-life of 30 to 40 hours, and a single dose after routine chemotherapy is as effective as multiple doses. Furthermore, endogenous levels are strikingly elevated in exactly those conditions (progenitor cell transplantation and AML induction chemotherapy) where a platelet growth factor would be of great clinical value but for which recombinant TPO has been shown to be ineffective.35 Conceivably, the elevated endogenous TPO has already maximally stimulated available precursors and/or downregulated the available TPO receptors, rendering pharmacologic doses useless. There are many other peculiar aspects of the clinical biology of TPO. Although it stimulates megakaryocyte-colony forming cells and early megakaryocyte growth, TPO actually inhibits platelet release from mature megakaryocytes.36 In clinical situations such as progenitor cell transplantation, TPO may actually stimulate megakaryocyte growth but then prevent them from releasing platelets. Like EPO or G–CSF, TPO stimulates lineage-specific maturation of late precursors, but unlike EPO and G–CSF, it also affects early precursor cells of all lineages.37-40 How this pluripotential effect of TPO functions in the cytokine chaos of progenitor cell transplantation or AML induction chemotherapy has not been studied. The clinical interactions of TPO with elevations of endogenous EPO, G–CSF, TNF-α, and transforming growth factor-β have been poorly studied; inhibitory effects of elevated levels of transforming growth factor-β on megakaryocyte growth might negate the pharmacologic effects of recombinant TPO.41 Finally, TPO has major antiapoptotic effects, but only when given at critical times. This is best illustrated by the finding that when given within 2 but not 24 hours after semilethal irradiation of mice, multilineage blood cell recovery and survival increase.42-44 Were the apparent complexity of TPO clinical biology not enough, the recent demonstration of the induction of autoantibodies to TPO by PEG-rHuMGDF and the ensuing thrombocytopenia has sorely shaken our confidence in this hematopoietic growth factor.45 Despite millions of administrations in North America, neither recombinant EPO nor G–CSF has induced clinically significant neutralizing antibodies. However, after the injection of two or three subcutaneous doses of PEG-rHuMGDF into normal volunteers, antibodies were produced in up to 9 percent of subjects. The exact mechanism is still under investigation, but preliminary animal data suggest that it is the subcutaneous route and not the molecular form of TPO that mediates this problem. TPO may be a potent mobilizer of dendritic cells and thus may hasten autoantibody formation after subcutaneous injection. What is clear is that the clinical failure of recombinant TPO to affect platelet production significantly in progenitor cell transplantation and AML induction chemotherapy underscores our lack of understanding of the clinical biology of TPO in these situations and has lessened the commercial enthusiasm for development of recombinant TPO. Equally clear, but less well appreciated, is that recombinant TPO does "work" in situations in which the endogenous TPO level is not elevated and adequate marrow reserves are present. The clinical biology in this situation seems rather simple: administer TPO, and the platelet count rises 5 to 10 days later. Chronic ITP, HIV ITP, apheresis donors (directed donors, donors with unusual HLA types, autologous donors with concomitant platelet cryopreservation), and some dose-intense chemotherapy regimens have all shown clinically significant benefits from TPO therapy. Although none is individually a large market, together they comprise a large group of patients with a clinical need for treatment of their thrombocytopenia. In addition, there are the many other thrombocytopenic disorders that would benefit from a recombinant TPO. Many preoperative and postoperative thrombocytopenic surgical patients receive platelet transfusions that might be avoided by the use of TPO. Although liver failure patients have a complex coagulopathy, they are invariably thrombocytopenic and might benefit from TPO administration to reduce bleeding. The preliminary data in myelodysplastic syndrome patients are certainly encouraging and need to be expanded. Gestational thrombocytopenia commonly complicates analgesic efforts at delivery and would certainly respond to TPO. Further studies need to be done in all of these groups but have a high potential to demonstrate clinical benefits. Finally, the issue of safety needs to be addressed. Because thrombosis has not been a problem despite significant thrombocytosis, the major issue is the induction of autoantibody formation by the recombinant TPOs. PEG-rHuMGDF is no longer in clinical development for this reason. Neutralizing antibodies have not been seen with intravenous administration of rHuTPO, and rHuTPO remains in development. Peptide and nonpeptide TPO mimetics, as well as TPO receptor-activating peptides, are just entering the clinical arena.46-48 In preclinical models, these peptides appear to be potent stimulators of platelet production and are not likely to be immunogenic. The first round of the clinical development of TPO is concluded. With time, we will understand the mechanism for the apparent lack of effect of TPO in progenitor cell transplantation and AML induction chemotherapy, and we will possibly devise successful strategies to make TPO work in these settings. While this is being studied, the next round of TPO clinical development should focus on those areas mentioned previously here where marrow reserves are intact, TPO levels are near normal, and the clinical biology is more straightforward. Development of rHuTPO and the TPO mimetics should proceed in the areas of ITP, HIV ITP, certain apheresis donors, some chemotherapy regimens, liver failure, thrombocytopenic surgical patients, and patients with myelodysplastic syndrome. This will meet the clinical needs and provide an adequate market for the pharmaceutical industry.