The purification of megapoietin: a physiological regulator of megakaryocyte growth and platelet production

The Centenary of Immune Thrombocytopenia - Part 1: Revising Nomenclature and Pathogenesis

The natural history of the immune thrombocytopenia (ITP) is interesting and intriguing because it traces different steps underlying autoimmune diseases. The review points out the main steps that have accompanied the stages of its history and the consequential changes related to its terminology. ITP is an autoimmune disease resulting from platelet antibody-mediated destruction and impaired megakaryocyte and platelet production. However, research advances highlight that a complex dysregulation of the immune system is involved in the pathogenesis of this condition. The review examines the role of the multiple immune components involved in the autoimmunity process, focusing on the more recent mechanisms, which could be new promising therapeutic targets for ITP patients.

Thrombopoietin induces production of nucleated thrombocytes from liver cells in Xenopus laevis

The development of mammalian megakaryocytes (MKs) and platelets, which are thought to be absent in non-mammals, is primarily regulated by the thrombopoietin (TPO)/Mpl system. Although non-mammals possess nucleated thrombocytes instead of platelets, the features of nucleated thrombocyte progenitors remain to be clarified. Here, we provide the general features of TPO using Xenopus laevis TPO (xlTPO). Hepatic and splenic cells were cultured in liquid suspension with recombinant xlTPO. These cells differentiated into large, round, polyploid CD41-expressing cells and were classified as X. laevis MKs, comparable to mammalian MKs. The subsequent culture of MKs after removal of xlTPO produced mature, spindle-shaped thrombocytes that were activated by thrombin, thereby altering their morphology. XlTPO induced MKs in cultured hepatic cells for at least three weeks; however, this was not observed in splenic cells; this result demonstrates the origin of early haematopoietic progenitors in the liver rather than the spleen. Additionally, xlTPO enhanced viability of peripheral thrombocytes, indicating the xlTPO-Mpl pathway stimulates anti-apoptotic in peripheral thrombocytes. The development of thrombocytes from MKs via the TPO-Mpl system in X. laevis plays a crucial role in their development from MKs, comparable to mammalian thrombopoiesis. Thus, our results offer insight into the cellular evolution of platelets/MKs in vertebrates. (200/200).

[Impact of recombinant human thrombopoietin (rhTPO) on short-term response of immunosuppressive therapy in patients with newly diagnosed acquired severe aplastic anemia]

OBJECTIVE

To evaluate the impact of recombinant human thrombopoietin (rhTPO) on short-term response of immunosuppressive therapy (IST) in patients with newly diagnosed acquired severe aplastic anemia (SAA).

METHODS

The clinical data of forty adult acquired SAA patients, who treated with IST combined with rhTPO, were retrospective analyzed and the hematologic recovery were compared with patients by the IST alone during the same period. The factors affecting the short-term response were also analyzed.

RESULTS

At 3 months after IST, both the total response rate and CR+GPR rate in rhTPO group were much higher than those in control group (75.0% vs 50.0%, P=0.022; and 17.5% vs 2.5%, P=0.025). At 6 months after IST, there was no difference of total hematologic response rate in rhTPO group and control group (77.5% vs 57.5%, P=0.058), while the CR+GPR rate was still higher in rhTPO group (45.0% vs 22.5%, P=0.033). The median time of platelet transfusion independence was much shorter in rhTPO group [33(0-90) vs 53(0-75) d, P=0.019]. Patients in rhTPO group needed less platelets transfusion support. The median platelet count in rhTPO group was 29(4-95)×10⁹/L at 3 months after IST, which was much higher than that in control group [29(4-95)×10⁹/L, P=0.006]. There was no significant difference regarding overall survival between the two groups (100.0% vs 91.0%, P=0.276).

CONCLUSION

rhTPO is effective in promoting platelet recovery and improving the hematopoietic response for SAA patients with IST.

Romiplostim as a treatment for immune thrombocytopenia: a review

“Immune thrombocytopenia” (ITP) is an autoimmune disorder that leads to peripheral destruction, as well as a decreased production of platelets. ITP most commonly presents as mild mucocutaneous bleeding. Though it is rare, the leading cause of mortality in persons with ITP is intracranial hemorrhage and those that do not respond to therapy are at increased risk. Our understanding of the pathophysiology of ITP has evolved immensely, especially over the last 60 years. The discovery of the platelet-production stimulator, thrombopoietin (TPO), lent clarity to an earlier hypothesis that inhibition of platelet production at the level of the megakaryocyte, at least in part, accounts for thrombocytopenia in adults with ITP. This facilitated the development of TPO-based therapies to treat ITP. Thrombopoietin receptor agonists are one of the most recent treatments to enter the landscape. Original production of a recombinant human TPO was halted after clinical trials revealed the untoward effect of autoantibodies to the recombinant human TPO with cross-reactivity to endogenous TPO. Next-step development focused on stimulation of the TPO receptor with fewer immunogenic agents. Currently, two such thrombopoietin receptor agonists, romiplostim and eltrombopag, are licensed in the USA to treat thrombocytopenia in adults with persistent or chronic ITP. Ongoing research will assess their efficacy in other immune-mediated and nonimmune-mediated primary and secondary thrombocytopenias.

Development of romiplostim: a novel engineered peptibody

Thrombopoietin (TPO) is a growth factor that stimulates megakaryocytes to increases platelet counts. Due to concerns around the development of autoantibodies in clinical studies of TPO, a novel peptide that bound the TPO receptor was used to develop a TPO mimetic. This peptide has no shared homology with TPO, and therefore avoids the risk of development of antibodies that cross-react with endogenous TPO. The ability of the artificial peptide to bind and stimulate the TPO receptor was improved by dimerization of the peptide, and the stability of the peptide was improved by linking the peptides to the Fc portion of an antibody. Romiplostim (Nplate, Amgen, Inc, Thousand Oaks, CA) is the first fully engineered fusion of a novel peptide and antibody or "peptibody" that can stimulate platelet production.

Treatment of pediatric primary immune thrombocytopenia with thrombopoietin receptor agonists

Chronic immune thrombocytopenia (ITP) occurs in approximately one fifth of children with primary ITP and is characterized by a significant lack of clinical data. A minority of these children exhibit bleeding and need treatment. Often standard therapy used for patients with newly diagnosed ITP is administered to stop bleeding and to increase the platelet count. These drugs are associated with adverse effects, which is particularly evident when used during long time. In adult patients with chronic ITP, thrombopoietin receptor agonists (TPO-RAs) demonstrated efficacy in approximately 80% of patients. These drugs have been studied intensely for registration purposes; however, for children and adolescents they are not yet approved and studies are ongoing. First experiences with these drugs show similar effects and safety as in adults, though based on very small numbers of children. These drugs have the potential to be used during long time, in order to increase platelets, to stop or prevent bleeding and to augment quality of life, making long-term safety an important issue.

Severe Romiplostim-Induced Rebound Thrombocytopenia After Splenectomy for Refractory ITP

Objective: To report a case of severe rebound thrombocytopenia after temporary discontinuation of romiplostim during splenectomy in the context of refractory immune (idiopathic) thrombocytopenic purpura (ITP). Case Summary: A 65-year-old man with a history of severe refractory ITP failing multiple treatments was considered for romiplostim therapy. He was initiated on 1 µg/kg and titrated upward to 4 µg/kg to elevate and stabilize his platelet levels prior to splenectomy. On day 74 of his clinical course, his platelets increased to 434 × 109/L, and his scheduled dose of romiplostim was withheld on day 75 for fear of romiplostim-induced postsplenectomy rebound thrombocytosis. On day 78, his platelets dropped precipitously to 9 × 109/L, and he experienced multiple episodes of epistaxis. He was reinitiated at 5 µg/kg and soon recovered. He later missed a scheduled dose of romiplostim, and his platelets fell to 23 × 109/L. After resuming romiplostim at 8 µg/kg, his platelets continued to recover. Discussion: Romiplostim, a thrombopoietin mimetic is directly regulated by megakaryocytes and existing circulating platelets via a negative feedback mechanism. This explains the theoretical risk of rapid clearance of romiplostim caused by an increased platelet pool. Clinically, alternative causes of his severe postoperative thrombocytopenia were considered and deemed unlikely. The rebound effect was observed after romiplostim was withdrawn on 2 occasions, and platelet counts improved after restarting romiplostim. The Naranjo Adverse Drug Reaction Probability Score of 7 suggests a probable adverse drug reaction. Conclusion: Physicians using romiplostim as a bridge to splenectomy should be cautious about withholding a scheduled dose around the time of surgery.

Resuscitating a dying marrow: the role of hematopoietic growth factors

The treatment landscape for myelodysplastic syndromes (MDS) has evolved over the last two decades, with a better understanding of the disease pathophysiology and the use of newer or combination therapies. For lower-risk MDS patients, hematopoietic growth factors have continued to be the mainstay of therapy. However, better patient selection criteria and decision tools to predict responses have made these therapies more beneficial to patients. As the range of newer drugs continues to expand in our treatment armamentarium for lower-risk MDS, questions still remain regarding the safety of these drugs with long-term use. This review will discuss the role of growth factors in MDS, focusing on dosing and combination strategies to improve responses, selecting the appropriate patient population, and recognizing the safety profile based on evidence from published literature.

Milestones in understanding platelet production: a historical overview

The discovery of thrombopoietin (TPO, also termed THPO) in 1994 was a major achievement in understanding the regulation of platelet production. In prior decades, physiological studies had demonstrated that platelets were produced from bone marrow megakaryocytes and that the megakaryocytes responded to thrombocytopenia by increasing their number, size and DNA ploidy. In 1958, it was proposed that a 'thrombopoietin' must exist that regulated this interaction between the circulating platelet mass and the bone marrow megakaryocytes. After over three decades of effort, TPO was finally purified by five independent laboratories. TPO stimulated megakaryocyte colony-forming cell growth and increased the number, size and ploidy of megakaryocytes. When the genes for TPO or TPO receptor were eliminated in mice, megakaryocytes grew and platelets were made, but at 15% of their normal number. A first generation of recombinant human (rh) TPO molecules [rhTPO and pegylated recombinant human megakaryocyte growth and development factor (PEG-rhMGDF)] rapidly entered clinical trials in 1995 and increased platelet counts in humans undergoing non-myeloablative chemotherapy but not in those undergoing stem cell transplantation. Antibodies developed against PEG-rhMGDF and development of these recombinant thrombopoietins ended. A second generation of TPO receptor agonists (romiplostim and eltrombopag) was then developed. Neither of these TPO receptor agonists demonstrated any significant untoward effects and both are now licensed in many countries for the treatment of immune thrombocytopenia. This review describes the significant experiments that have surrounded the discovery of TPO and its clinical development.

Thrombopoietin from beginning to end

In the two decades since its cloning, thrombopoietin (TPO) has emerged not only as a critical haematopoietic cytokine, but also serves as a great example of bench-to-bedside research. Thrombopoietin, produced by the liver, is the primary regulator of megakaryocyte progenitor expansion and differentiation. Additionally, as TPO is vital for the maintenance of haematopoietic stem cells, it can truly be described as a pan-haematopoietic cytokine. Since recombinant TPO became available, the molecular mechanisms of TPO function have been the subject of extensive research. Via its receptor, c-Mpl (also termed MPL), TPO activates a wide array of downstream signalling pathways, promoting cellular survival and proliferation. Due to its central, non-redundant role in haematopoiesis, alterations of both the hormone and its receptor contribute to human disease; congenital and acquired states of thrombocytosis and thrombocytopenia and aplastic anaemia as a result from dysregulated TPO expression or functional alterations of c-Mpl. With TPO mimetics now in clinical use, the story of this haematopoietic cytokine represents a great success for biomedical research.

Interpreting the developmental dance of the megakaryocyte: a review of the cellular and molecular processes mediating platelet formation

Platelets are essential for haemostasis, and thrombocytopenia (platelet counts <150 × 10(9) /l) is a major clinical problem encountered across a number of conditions, including immune thrombocytopenic purpura, myelodysplastic syndromes, chemotherapy, aplastic anaemia, human immunodeficiency virus infection, complications during pregnancy and delivery, and surgery. Circulating blood platelets are specialized cells that function to prevent bleeding and minimize blood vessel injury. Platelets circulate in their quiescent form, and upon stimulation, activate to release their granule contents and spread on the affected tissue to create a physical barrier that prevents blood loss. The current model of platelet formation states that large progenitor cells in the bone marrow, called megakaryocytes, release platelets by extending long, branching processes, designated proplatelets, into sinusoidal blood vessels. This review will focus on different factors that impact megakaryocyte development, proplatelet formation and platelet release. It will highlight recent studies on thrombopoeitin-dependent megakaryocyte maturation, endomitosis and granule formation, cytoskeletal contributions to proplatelet formation, the role of apoptosis, and terminal platelet formation and release.

Clinical indications for thrombopoietin and thrombopoietin-receptor agonists

Thrombocytopenia is a common hematologic disorder. Stimulation of thrombopoiesis may reduce the risk for thrombocytopenia-induced bleeding, prevent severe thrombocytopenia, and reduce the need for platelet transfusion. The key cytokine is thrombopoietin (TPO). It regulates proliferation and maturation of megakaryocytes as well as platelet production. TPO is synthesized in the liver. Development of TPO from the laboratory into a therapeutic tool has turned out to be an unexpected challenge. Clinical trials on first-generation thrombopoietic growth factors were stopped in 2001. At present, second-generation thrombopoiesis-stimulating agents have only been approved as orphan drugs for third-line therapy of patients with chronic immune thrombocytopenia. Larger groups in need are patients with myelodysplastic syndrome, chemotherapy-induced thrombocytopenia, other forms of hereditary and acquired bone marrow failure, hepatitis C infections, or liver cirrhosis.

Exome sequencing reveals a thrombopoietin ligand mutation in a Micronesian family with autosomal recessive aplastic anemia

We recently identified 2 siblings afflicted with idiopathic, autosomal recessive aplastic anemia. Whole-exome sequencing identified a novel homozygous missense mutation in thrombopoietin (THPO, c.112C>T) in both affected siblings. This mutation encodes an arginine to cysteine substitution at residue 38 or residue 17 excluding the 21-amino acid signal peptide of THPO receptor binding domain (RBD). THPO has 4 conserved cysteines in its RBD that form 2 disulfide bonds. Our in silico modeling predicts that introduction of a fifth cysteine may disrupt normal disulfide bonding to cause poor receptor binding. In functional assays, the mutant-THPO-containing media shows two- to threefold reduced ability to sustain UT7-TPO cells, which require THPO for proliferation. Both parents and a sibling with heterozygous R17C change have reduced platelet counts, whereas a sibling with wild-type sequence has normal platelet count. Thus, the R17C partial loss-of-function allele results in aplastic anemia in the homozygous state and mild thrombocytopenia in the heterozygous state in our family. Together with the recent identification of THPO receptor (MPL) mutations and the effects of THPO agonists in aplastic anemia, our results have clinical implications in the diagnosis and treatment of patients with aplastic anemia and highlight a role for the THPO-MPL pathway in hematopoiesis in vivo.

The incredible journey: From megakaryocyte development to platelet formation

Circulating blood platelets are specialized cells that prevent bleeding and minimize blood vessel injury. Large progenitor cells in the bone marrow called megakaryocytes (MKs) are the source of platelets. MKs release platelets through a series of fascinating cell biological events. During maturation, they become polyploid and accumulate massive amounts of protein and membrane. Then, in a cytoskeletal-driven process, they extend long branching processes, designated proplatelets, into sinusoidal blood vessels where they undergo fission to release platelets. Given the need for platelets in many pathological situations, understanding how this process occurs is an active area of research with important clinical applications.

The biology of thrombopoietin and thrombopoietin receptor agonists

Thrombopoietin (TPO) is the major physiological regulator of platelet production. TPO binds the TPO receptor, activates JAK and STAT pathways, thus stimulating megakaryocyte growth and platelet production. There is no "sensor" of the platelet count; rather TPO is produced in the liver at a constant rate and cleared by TPO receptors on platelets. TPO levels are inversely proportional to the rate of platelet production. Early recombinant TPO molecules were potent stimulators of platelet production and increased platelets in patients with immune thrombocytopenia, chemotherapy-induced thrombocytopenia, myelodysplastic syndromes and platelet apheresis donors. Neutralizing antibodies formed against one recombinant protein and ended their development. A second generation of TPO receptor agonists, romiplostim and eltrombopag, has been developed. Romiplostim is an IgG heavy chain into which four TPO agonist peptides have been inserted. Eltrombopag is an oral small molecule. These activate the TPO receptor by different mechanisms to increase megakaryocyte growth and platelet production. After administration of either to healthy volunteers, there is a delay of 5 days before the platelet count rises and subsequently reaches a peak after 12-14 days. Both have been highly effective in treating ITP and hepatitis C thrombocytopenia. Studies in a wide variety of other thrombocytopenic conditions are underway.

The efficacy and safety of romiplostim in adult patients with chronic immune thrombocytopenia

Romiplostim is a thrombopoietin peptide mimetic agent recently approved for the treatment of adults with chronic immune thrombocytopenia (ITP). In randomized controlled studies, this agent has demonstrated an increase in the platelet count in the majority of patients with ITP, both in splenectomized and nonsplenectomized patients. In up to 5 years of follow up, its safety profile has been favorable, suggesting this could be a novel therapeutic agent in patients with ITP. This article summarizes the development of this new agent and explores its safety and efficacy in adults with chronic ITP.

Platelet apoptosis and agonist-mediated activation in myelodysplastic syndromes

Patients with myelodysplastic syndromes (MDS) have a defect in the differentiation of bone marrow multipotent progenitor cells. Thrombocytopenia in MDS patients may be due to premature megakaryocyte death, but platelet apoptotic mechanisms may also occur. This study aimed to study function and apoptotic state of platelets from MDS patients with different platelet count. Reticulated platelets, platelet activation, activated caspases and annexin-V binding were evaluated by flow cytometry. Pro-apoptotic Bax and Bak proteins were determined by western blots and plasma thrombopoietin by ELISA. Microparticle-associated procoagulant activity and thrombin generation capacity of plasma were determined by an activity kit and calibrated automated thrombography, respectively. High plasma thrombopoietin levels and low immature circulating platelet count showed a pattern of hypoplastic thrombocytopenia in MDS patients. Platelets from MDS patients showed reduced activation capacity and more apoptosis signs than controls. Patients with the lowest platelet count showed less platelet activation and the highest extent of platelet apoptosis. On this basis, patients with thrombocytopenia should suffer more haemorrhagic episodes than is actually observed. Consequently, we tested whether there were some compensatory mechanisms to counteract their expected bleeding tendency. Microparticle-associated procoagulant activity was enhanced in MDS patients with thrombocytopenia, whereas their plasma thrombin generation capacity was similar to control group. This research shows a hypoplastic thrombocytopenia that platelets from MDS patients possess an impaired ability to be stimulated and more apoptosis markers than those from healthy controls, indicating that MDS is a stem cell disorder, and then, both number and function of progeny cells, might be affected.

Cytokines, platelet production and hemostasis

A number of hematopoietic growth factors and other cytokines are capable of altering platelet production and function. Enhancement of these processes may be exploited to ameliorate bleeding propensity in thrombocytopenic patients. Under certain circumstances, cytokines may have adverse effects on the hemostatic system, potentially involved in thrombogenesis and atherogenesis. Inhibition of those cytokines may prove to be a useful experimental approach to investigate their potential pathophysiologic role.

Induction of functional platelets from mouse and human fibroblasts by p45NF-E2/Maf

Determinant factors leading from stem cells to megakaryocytes (MKs) and subsequently platelets have yet to be identified. We now report that a combination of nuclear factor erythroid-derived 2 p45 unit (p45NF-E2), Maf G, and Maf K can convert mouse fibroblast 3T3 cells and adult human dermal fibroblasts into MKs. To screen MK-inducing factors, gene expressions were compared between 3T3 cells that do not differentiate into MKs and 3T3-L1 cells known to differentiate into MKs. 3T3 cells transfected with candidate factors were cultured in a defined MK lineage induction medium. Among the tested factors, transfection with p45NF-E2/MafG/MafK lead to the highest frequency of CD41-positive cells. Adult human dermal fibroblasts transfected with these genes were cultured in MK lineage induction medium. Cultured cells had megakaryocytic features, including surface markers, ploidy, and morphology. More than 90% of MK-sized cells expressed CD41, designated induced MK (iMK). Infusion of these iMK cells into immunodeficient mice led to a time-dependent appearance of CD41-positive, platelet-sized particles. Blood samples from iMK-infused into thrombocytopenic immunodeficient mice were perfused on a collagen-coated chip, and human CD41-positive platelets were incorporated into thrombi on the chip, demonstrating their functionality. These findings demonstrate that a combination of p45NF-E2, Maf G, and Maf K is a key determinant of both megakaryopoiesis and thrombopoiesis.

Thrombopoietin as biomarker and mediator of cardiovascular damage in critical diseases

Thrombopoietin (TPO) is a humoral growth factor originally identified for its ability to stimulate the proliferation and differentiation of megakaryocytes. In addition to its actions on thrombopoiesis, TPO directly modulates the homeostatic potential of mature platelets by influencing their response to several stimuli. In particular, TPO does not induce platelet aggregation per se but is able to enhance platelet aggregation in response to different agonists (“priming effect”). Our research group was actively involved, in the last years, in characterizing the effects of TPO in several human critical diseases. In particular, we found that TPO enhances platelet activation and monocyte-platelet interaction in patients with unstable angina, chronic cigarette smokers, and patients with burn injury and burn injury complicated with sepsis. Moreover, we showed that TPO negatively modulates myocardial contractility by stimulating its receptor c-Mpl on cardiomyocytes and the subsequent production of NO, and it mediates the cardiodepressant activity exerted in vitro by serum of septic shock patients by cooperating with TNF-α and IL-1β. This paper will summarize the most recent results obtained by our research group on the pathogenic role of elevated TPO levels in these diseases and discuss them together with other recently published important studies on this topic.

Megakaryopoiesis and thrombopoiesis: an update on cytokines and lineage surface markers

Megakaryopoiesis is the process by which mature megakaryocytes (MKs) develop from hematopoietic stem cells (HSCs). The biological function of MKs is to produce platelets, which play critical roles in hemostasis and contribute to angiogenesis and wound healing. The generation of platelets from MKs is termed thrombopoiesis. The cytokine thrombopoietin (TPO) is the major regulator of megakaryopoiesis and thrombopoiesis. It binds to its surface receptor, c-Mpl, and acts through multiple downstream signaling pathways, including the PI-3 kinase-Akt, MAPK, and ERK1/ERK2 pathways. However, non-TPO pathways, such as the SDF1/CXCR4 axis, Notch signaling, src family kinases, integrin signaling, and Platelet Factor 4/low-density lipoprotein receptor-related protein 1, have more recently been recognized to influence megakaryopoiesis and thrombopoiesis in vitro and in vivo. In this chapter, we review megakaryopoiesis and thrombopoiesis with emphasis on cell surface marker changes during their differentiation from HSCs, and the classical cytokines that affect these developmental stages. We also discuss non-TPO regulators and their effects on in vitro culture systems.

Percutaneous endoscopic gastrostomy complication rates and compliance with the American Society for Gastrointestinal Endoscopy guidelines for the management of antithrombotic therapy

BACKGROUND

The American Society for Gastrointestinal Endoscopy (ASGE) has published recommendations in regards to anticoagulant (AC) and antiplatelet (AP) therapy management during endoscopic procedures. So far, no study has assessed either ASGE recommendation compliance during percutaneous endoscopic gastrostomy (PEG) placement or procedure-associated complication rates as related to the observance of these recommendations. The aims of this study were to compare the incidence and type of complications during PEG placement in patients receiving or not receiving AC and/or AP therapy and to determine the compliance with ASGE's AC and AP management guidelines.

METHODS

Medical files of patients who underwent PEG placement from January 2004 to December 2008 were reviewed. Clinical and procedure-related data were recorded. Patients were separated into 1 of 2 groups: patients under AP and/or AC therapy prior to PEG placement (n = 51) and a control group of patients (n = 40) not receiving any AP and/or AC treatment at least 6 months prior to the procedure.

RESULTS

A total of 91 patients (51 cases) were included. Groups were comparable in demographics and clinical characteristics. No differences in the frequency and type of complications were found between groups. ASGE's recommendations were not followed in any of these patients.

CONCLUSIONS

Overall PEG placement complication rate was 13.7%. AP therapy may be safely discontinued closer to the time of endoscopic procedure than the time currently recommended by the ASGE guidelines.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

Thrombopoietin and hematopoietic stem cells

Thrombopoietin (TPO) is the cytokine that is chiefly responsible for megakaryocyte production but increasingly attention has turned to its role in maintaining hematopoietic stem cells (HSCs). HSCs are required to initiate the production of all mature hematopoietic cells, but this differentiation needs to be balanced against self-renewal and quiescence to maintain the stem cell pool throughout life. TPO has been shown to support HSC quiescence during adult hematopoiesis, with the loss of TPO signaling associated with bone marrow failure and thrombocytopenia. Recent studies have shown that constitutive activation mutations in Mpl contribute to myeloproliferative disease. In this review, we will discuss TPO signaling pathways, regulation of TPO levels and the role of TPO in normal hematopoiesis and during myeloproliferative disease.

Romiplostim

Thrombocytopenia is a common clinical problem associated with a wide range of medical conditions including immune thrombocytopenia (ITP), chemotherapy-induced thrombocytopenia (CIT), hepatitis C-related thrombocytopenia, and myelodysplastic syndromes (MDS). Until recently, the only treatments for thrombocytopenia were to alleviate the underlying cause or to provide platelet transfusions. With the discovery and recent clinical availability of thrombopoietin (TPO) mimetics, a new treatment option has emerged. Two TPO mimetics are currently clinically available for treating ITP: romiplostim (an injectable peptide TPO mimetic) and eltrombopag (a non-peptide, orally available TPO mimetic). This chapter reviews the development, biology, and clinical trials with romiplostim. With few adverse effects, romiplostim is effective in raising the platelet count in over 80% of ITP patients, allowing them to discontinue other therapies, reduce the need for splenectomy, and improve their quality of life. Long-term theoretical side effects of romiplostim treatment include reticulin formation, thromboembolism, and antibody formation to romiplostim. A practical way of using romiplostim is provided: a higher starting dose of 3 mg/kg is recommended along with efforts to avoid withholding the dose. Future studies will assess the utility of romiplostim in CIT, hepatitis-C related thrombocytopenia, and MDS.

Early-acting hematopoietic growth factors: biology and clinical experience

Secreted protein growth factors that stimulate the self-renewal, proliferation, and differentiation of the most primitive stem cells are among the most biologically interesting molecules and at least theoretically have diverse applications in the evolving field of regenerative medicine. Among this class of regulators, the early-acting hematopoietic growth factors and their cellular targets are perhaps the best characterized and serve as a paradigm for manipulating other stem cell based tissues. This chapter reviews the preclinical knowledge accumulated over ~40 years, since the discovery of the first such growth factor, and the clinical applications of those that, upon testing in humans, ultimately gained regulatory approval for the treatment of various hematological diseases.

Thrombopoietin factors

Megakaryopoiesis and thrombopoiesis are the central biological processes of platelet generation. Severe thrombocytopenia is a major morbidity and mortality factor in several diseases and represents a significant unmet medical need. Since the discovery of thrombopoietin (TPO) as the primary physiological regulator of megakaryopoiesis, a number of therapeutics have been developed for thrombocytopenia and been tested in preclinical models and human clinical trials. The TPO mimetics romiplostim (Nplate(®) or AMG531) and eltrombopag (Promacta(®)) have recently been approved for the treatment of adult chronic idiopathic (immune) thrombocytopenic purpura (ITP) and are successful examples of these endeavors. This chapter will review scientific progress over the last 20 years on various thrombopoietic factors with an emphasis on the biology, physiology, and pharmacology of TPO, its cognate receptor, c-Mpl, and various TPO mimetics.

Eltrombopag--a novel approach for the treatment of chronic immune thrombocytopenic purpura: review and safety considerations

Eltrombopag is one of a number of novel agents recently developed for use in the treatment of patients with immune thrombocytopenia (ITP). Rather than preventing destruction of platelets, these agents increase the production of platelets, presumably overwhelming the immune system resulting in normal platelet counts in individuals refractory to or dependent on other therapies. These treatments are well tolerated and in randomized controlled trials show an improvement in platelet counts and a reduction in bleeding in refractory patients. This article summarizes the development of this new class of drug and evaluates the safety and efficacy of eltrombopag in patients with ITP.

Biology and chemistry of thrombopoietic agents

Endogenous thrombopoietin (eTPO) regulates platelet production by increasing the number, ploidy, and maturation rate of bone marrow megakaryocytes. Early attempts to treat thrombocytopenia by the administration of recombinant TPO were successful but were complicated by the development of antibodies to one of the recombinant proteins. Two new TPO mimetics have recently been approved by the US Food and Drug Administration (FDA) for the treatment of immune thrombocytopenia (ITP). Romiplostim is a peptide TPO mimetic composed of an IgG Fc fragment to which are attached four 14-amino acid TPO peptides that activate the TPO receptor by binding to the extracytoplasmic domain just like eTPO. Romiplostim is administered as a weekly subcutaneous injection. Eltrombopag, a nonpeptide TPO mimetic, is a 442-d drug that binds to a transmembrane site on the TPO receptor and thereby activates it. It is administered daily as an oral tablet. Administration of both romiplostim and eltrombopag to healthy volunteers produced a dose-dependent rise in platelet count beginning on day 5 and peaking at days 12 to 15. Both have been highly effective in increasing the platelet count in patients with ITP and are currently being studied in the treatment of other thrombocytopenic conditions (myelodysplastic syndrome, chemotherapy, liver disease).