New drugs for familiar therapeutic targets: thrombopoietin receptor agonists and immune thrombocytopenic purpura

Translating mechanisms into therapeutic strategies for immune thrombocytopenia (ITP): Lessons from clinical trials

Immune thrombocytopenia (ITP) is an autoimmune disorder that causes a significant reduction in peripheral blood platelet count. Fortunately, due to an increased understanding of ITP, there have been significant improvements in the diagnosis and treatment of these patients. Over the past decade, there have been a variety of proven therapeutic options available for ITP patients, including intravenous immunoglobulins (IVIG), Rituximab, corticosteroids, and thrombopoietin receptor agonists (TPO-RAs). Although the effectiveness of current therapies in treating more than two-thirds of patients, still some patients do not respond well to conventional therapies or fail to achieve long-term remission. Recently, a significant advancement has been made in identifying various mechanisms involved in the pathogenesis of ITP, leading to the development of novel treatments targeting these pathways. It seems that new agents that target plasma cells, Bruton tyrosine kinase, FcRn, platelet desialylation, splenic tyrosine kinase, and classical complement pathways are opening new ways to treat ITP. In this study, we reviewed the pathophysiology of ITP and summarized updates in this population's management and treatment options. We also took a closer look at the 315 ongoing trials to investigate their progress status and compare the effectiveness of interventions. May our comprehensive view of ongoing clinical trials serve as a guiding beacon, illuminating the path towards future trials of different drugs in the treatment of ITP patients.

A Review on Romiplostim Mechanism of Action and the Expressive Approach in E. coli

Immune thrombocytopenic purpura (ITP) is an autoimmune disorder determined by immune-mediated platelet demolition and reduction of platelet production. Romiplostim is a new thrombopoiesis motivating peptibody that binds and stimulates the human thrombopoietin receptor the patent of which was registered in 2008. It is used to treat thrombocytopenia in patients with chronic immune thrombocytopenic purpura. Romiplostim is a 60 kDa peptibody designed to inhibit cross-reacting immune responses. It consists of four high-affinity TPO-receptor binding domains for the Mpl receptor and one human IgG1 Fc domain. Escherichia coli is a good host for the fabrication of recombinant proteins such as romiplostim. The expression of a gene intended in E. coli is dependent on many factors such as a protein's inherent ability to fold, mRNA's secondary structure, its solubility, its toxicity preferential codon use, and its need for post-translational modification (PTM). This review focuses on the structure, function, mechanism of action, and expressive approach to romiplostim in E. coli.

Novel Therapies to Address Unmet Needs in ITP

Primary immune thrombocytopenia (ITP) is an autoimmune disorder that causes low platelet counts and subsequent bleeding risk. Although current corticosteroid-based ITP therapies are able to improve platelet counts, up to 70% of subjects with an ITP diagnosis do not achieve a sustained clinical response in the absence of treatment, thus requiring a second-line therapy option as well as additional care to prevent bleeding. Less than 40% of patients treated with thrombopoietin analogs, 60% of those treated with splenectomy, and 20% or fewer of those treated with rituximab or fostamatinib reach sustained remission in the absence of treatment. Therefore, optimizing therapeutic options for ITP management is mandatory. The pathophysiology of ITP is complex and involves several mechanisms that are apparently unrelated. These include the clearance of autoantibody-coated platelets by splenic macrophages or by the complement system, hepatic desialylated platelet destruction, and the inhibition of platelet production from megakaryocytes. The number of pathways involved may challenge treatment, but, at the same time, offer the possibility of unveiling a variety of new targets as the knowledge of the involved mechanisms progresses. The aim of this work, after revising the limitations of the current treatments, is to perform a thorough review of the mechanisms of action, pharmacokinetics/pharmacodynamics, efficacy, safety, and development stage of the novel ITP therapies under investigation. Hopefully, several of the options included herein may allow us to personalize ITP management according to the needs of each patient in the near future.

Recent advances in the mechanisms and treatment of immune thrombocytopenia

Primary immune thrombocytopenia is an autoimmune disease associated with a reduced peripheral blood platelet count. The phenotype is variable with some patients suffering no bleeding whilst others have severe bleeding which may be fatal. Variability in clinical behaviour and treatment responses reflects its complex underlying pathophysiology. Historically the management has relied heavily on immune suppression. Recent studies have shown that the older empirical immune suppressants fail to alter the natural history of the disease and are associated with a poor quality of life for patients. Newer treatments, such as the thrombopoietin receptor agonists, have transformed ITP care. They have high efficacy, are well tolerated and improve patients’ quality of life. A greater understanding of the underlying pathophysiology of this disorder has helped develop a number of new targeted therapies. These include inhibitors of the neonatal Fc receptor inhibitors, Bruton tyrosine kinase and complement pathway. Here we discuss the mechanisms underlying ITP and the new approach to ITP care.

Efficacy of romiplostim in treatment of thrombocytopenia in children with Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a life-threatening primary immunodeficiency associated with bleeding of variable severity due to thrombocytopenia. Correction of the thrombocytopenia is of paramount importance for most WAS patients. We report a retrospective analysis of the safety and efficacy of romiplostim treatment in reducing thrombocytopenia and bleeding tendency in 67 children (median age 1·3 years) with genetically confirmed WAS, followed in eight months (range, 1-12 months). Complete or partial primary responses regarding platelet counts were observed in 22 (33%) and 18 (27%) subjects, respectively. Yet, even in the non-responder group, the risk of haemorrhagic events decreased significantly, to 21%, after the first month of treatment. The responses tended to be durable and stable over time, with no significant fluctuations in platelets counts. The results of this retrospective study of a large cohort of WAS patients demonstrates that romiplostim can be used to increase platelet counts and reduce the risks of life-threatening bleeding in WAS patients awaiting haematopoietic stem cell transplantation or forgoing the procedure for various reasons.

Thrombopoietin receptor agonists for prevention and treatment of chemotherapy-induced thrombocytopenia in patients with solid tumours

BACKGROUND

Chemotherapy-induced thrombocytopenia (CIT) is defined as a peripheral platelet count less than 100×109/L, with or without bleeding in cancer patients receiving myelosuppressive chemotherapy. CIT is a significant medical problem during chemotherapy, and it carries the risk of sub-optimal overall survival and bleeding. Alternative interventions to platelet transfusion are limited. Different stages of preclinical and clinical studies have examined the thrombopoietin receptor agonists (TPO-RAs) for CIT in patients with solid tumours.

OBJECTIVES

To assess the effects of TPO-RAs to prevent and treat CIT in patients with solid tumours:(1) to prevent CIT in patients without thrombocytopenia before chemotherapy, (2) to prevent recurrence of CIT, and (3) to treat CIT in patients with thrombocytopenia during chemotherapy.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL, to 28 September 2017), MEDLINE (from 1950 to 28 September 2017), as well as online registers of ongoing trials (Clinical Trials, Chinese Clinical Trial Register, Australian New Zealand Clinical Trial Registry, WHO ICTRP Search Portal, International Standard Randomised Controlled Trial Number registry, GlaxoSmithKline Clinical Study Register, and Amgen Clinical Trials) and conference proceedings (American Society of Hematology, American Society of Clinical Oncology, European Hematology Association, European Society of Medical Oncology, and Conference Proceedings Citation Index-Science, from 2002 up to September 2017) for studies.

SELECTION CRITERIA

Randomised controlled trials (RCTs) comparing TPO-RAs alone, or in combination with other drugs, to placebo, no treatment, other drugs, or another TPO-RAs for CIT in patients with solid tumours.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened the results of the search strategies, extracted data, assessed risk of bias, and analysed data according to standard methodological methods expected by Cochrane.

MAIN RESULTS

We identified six trials eligible for inclusion, of which two are ongoing, and one awaiting classification study. The three included trials were conducted at many different sites in Europe, America, and Asia. All of the three studies recruited adult and elder participants (no children were included) with solid tumours, and compared TPO-RAs with placebo. No studies compared TPO-RAs alone, or in combination with other drugs, to no treatment, or other drugs, or another TPO-RAs.We judged the overall risk of bias as high as we found a high risk for detection bias. We assessed the risk of bias arising from inadequate blinding of outcome assessors as high for number and severity of bleeding episodes (one of the primary outcomes).To prevent CIT: We included two trials (206 participants) comparing TPO-RAs (eltrombopag, multiple-dose oral administration with chemotherapy) with placebo. The use of TPO-RAs may make little or no difference to the all-cause mortality at 33 weeks of follow-up (RR 1.35, 95% CI 0.53 to 3.45; one trial, 26 participants; low quality of evidence). There is not enough evidence to determine whether TPO-RAs reduce the number of patients with at least one bleeding episode of any severity (RR 0.62, 95% CI 0.22 to 1.78; two trials, 206 participants; very low quality of evidence). There is not enough evidence to determine whether TPO-RAs reduce the number of patients with at least one severe/life-threatening bleeding episode (RR 0.36, 95% CI 0.06 to 2.06; two trials, 206 participants; very low quality of evidence). No studies were found that looked at overall survival (one of the primary outcomes), the number of treatment cycles with at least one bleeding episode, the number of days on which bleeding occurred, the amount of bleeding, or quality of life.To prevent recurrence of CIT: We included one trial (62 participants) comparing TPO-RAs (romiplostim, single-dose subcutaneous administration with chemotherapy) with placebo. There is not enough evidence to determine whether TPO-RAs reduce the number of patients with at least one bleeding episode of any severity (RR 2.80, 95% CI 0.17 to 47.53; one trial, 62 participants; very low quality of evidence). There is not enough evidence to determine whether TPO-RAs reduce the number of patients with at least one severe/life-threatening bleeding episode (no severe/life-threatening bleeding episodes; one trial, 62 participants; very low quality of evidence). No studies were found that looked at overall survival (one of the primary outcomes), the number of treatment cycles with at least one bleeding episode, the number of days on which bleeding occurred, the amount of bleeding, or quality of life. We found one ongoing study (expected recruitment 74 participants), it is planned to give TPO-RAs (romiplostim, subcutaneous administration with chemotherapy) to participants, but to date this trial has not reported any outcomes.To treat CIT: We found one ongoing study (expected recruitment 83 participants), which is planned to give TPO-RAs (eltrombopag, seven days orally) to participants when their platelet counts are less than 75×109/L during chemotherapy. This trial was originally planned to complete in March 2017, however, the completion date has passed and no results are reported.The one awaiting classification study included patients without thrombocytopenia before chemotherapy (to prevent CIT), patients with thrombocytopenia during chemotherapy (to prevent recurrence of CIT), and other patients during chemotherapy (uncertain whether CIT had happened). There was no evidence for a difference in the number of patients with at least one bleeding episode of any severity (RR 0.27, 95% CI 0.07 to 1.02; one trial, 75 participants). There was no evidence for a difference in the number of patients with at least one severe/life-threatening bleeding episode (RR 0.44, 95% CI 0.03 to 6.77; one trial, 75 participants). This study did not address overall survival or quality of life.

AUTHORS' CONCLUSIONS

No certain conclusions can be drawn due to the lack of strong evidence in the review. The available weak evidence did not support the use of TPO-RAs for preventing CIT or preventing recurrence of CIT in patients with solid tumours. There was no evidence to support the use of TPO-RAs for treating CIT in patients with solid tumours.

Comparative analysis of human ex vivo-generated platelets vs megakaryocyte-generated platelets in mice: a cautionary tale

Thrombopoiesis is the process by which megakaryocytes release platelets that circulate as uniform small, disc-shaped anucleate cytoplasmic fragments with critical roles in hemostasis and related biology. The exact mechanism of thrombopoiesis and the maturation pathways of platelets released into the circulation remain incompletely understood. We showed that ex vivo-generated murine megakaryocytes infused into mice release platelets within the pulmonary vasculature. Here we now show that infused human megakaryocytes also release platelets within the lungs of recipient mice. In addition, we observed a population of platelet-like particles (PLPs) in the infusate, which include platelets released during ex vivo growth conditions. By comparing these 2 platelet populations to human donor platelets, we found marked differences: platelets derived from infused megakaryocytes closely resembled infused donor platelets in morphology, size, and function. On the other hand, the PLP was a mixture of nonplatelet cellular fragments and nonuniform-sized, preactivated platelets mostly lacking surface CD42b that were rapidly cleared by macrophages. These data raise a cautionary note for the clinical use of human platelets released under standard ex vivo conditions. In contrast, human platelets released by intrapulmonary-entrapped megakaryocytes appear more physiologic in nature and nearly comparable to donor platelets for clinical application.

Use of eltrombopag after romiplostim in primary immune thrombocytopenia

The thrombopoietin receptor agonists (THPO-RAs), romiplostim and eltrombopag, are effective and safe in immune thrombocytopenia (ITP). However, the value of their sequential use when no response is achieved or when adverse events occur with one THPO-RA has not been clearly established. Here we retrospectively evaluated 51 primary ITP adult patients treated with romiplostim followed by eltrombopag. The median age of our cohort was 49 (range, 18-83) years. There were 32 women and 19 men. The median duration of romiplostim use before switching to eltrombopag was 12 (interquartile range 5-21) months. The reasons for switching were: lack of efficacy (n = 25), patient preference (n = 16), platelet-count fluctuation (n = 6) and side-effects (n = 4). The response rate to eltrombopag was 80% (41/51), including 67% (n = 35) complete responses. After a median follow-up of 14 months, 31 patients maintained their response. Efficacy was maintained after switching in all patients in the patient preference, platelet-count fluctuation and side-effect groups. 33% of patients experienced one or more adverse events during treatment with eltrombopag. We consider the use of eltrombopag after romiplostim for treating ITP to be effective and safe. Response to eltrombopag was related to the cause of romiplostim discontinuation.

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.

Analysis of the landscape of biologically-derived pharmaceuticals in Europe: dominant production systems, molecule types on the rise and approval trends

A thorough sort of the human drugs approved by the European Medicines Agency (EMA) between its establishment in 1995 until June 2012 is presented herein with a focus on biologically-derived pharmaceuticals. Over 200 (33%) of the 640 approved therapeutic drugs are derived from natural sources, produced via recombinant DNA technology, or generated through virus propagation. A breakdown based on production method, type of molecule and therapeutic category is presented. Current EMA approvals demonstrate that mammalian cells are the only choice for glycoprotein drugs, with Chinese hamster ovary cells being the dominant hosts for their production. On the other hand, bacterial cells and specifically Escherichia coli are the dominant hosts for protein-based drugs, followed by the yeast Saccharomyces cerevisiae. The latter is the dominant host for recombinant vaccine production, although egg-based production is still the main platform of vaccine provision. Our findings suggest that the majority of biologically-derived drugs are prescribed for cancer and related conditions, as well as the treatment of diabetes. The approval rate for biologically-derived drugs shows a strong upward trend for monoclonal antibodies and fusion proteins since 2009, while hormones, antibodies and growth factors remain the most populous categories. Despite a clear pathway for the approval of biosimilars set by the EMA and their potential to drive sales growth, we have only found approved biosimilars for three molecules. In 2012 there appears to be a slow-down in approvals, which coincides with a reported decline in the growth rate of biologics sales.

The role of platelet factor 4 in radiation-induced thrombocytopenia

PURPOSE

Factors affecting the severity of radiation-induced thrombocytopenia (RIT) are not well described. We address whether platelet factor 4 (PF4; a negative paracrine for megakaryopoiesis) affects platelet recovery postradiation.

METHODS AND MATERIALS

Using conditioned media from irradiated bone marrow (BM) cells from transgenic mice overexpressing human (h) PF4 (hPF4+), megakaryocyte colony formation was assessed in the presence of this conditioned media and PF4 blocking agents. In a model of radiation-induced thrombocytopenia, irradiated mice with varying PF4 expression levels were treated with anti-hPF4 and/or thrombopoietin (TPO), and platelet count recovery and survival were examined.

RESULTS

Conditioned media from irradiated BM from hPF4+ mice inhibited megakaryocyte colony formation, suggesting that PF4 is a negative paracrine released in RIT. Blocking with an anti-hPF4 antibody restored colony formation of BM grown in the presence of hPF4+ irradiated media, as did antibodies that block the megakaryocyte receptor for PF4, low-density lipoprotein receptor-related protein 1 (LRP1). Irradiated PF4 knockout mice had higher nadir platelet counts than irradiated hPF4+/knockout litter mates (651 vs. 328 × 106/mcL, p = 0.02) and recovered earlier (15 days vs. 22 days, respectively, p <0.02). When irradiated hPF4+ mice were treated with anti-hPF4 antibody and/or TPO, they showed less severe thrombocytopenia than untreated mice, with improved survival and time to platelet recovery, but no additive effect was seen.

CONCLUSIONS

Our studies show that in RIT, damaged megakaryocytes release PF4 locally, inhibiting platelet recovery. Blocking PF4 enhances recovery while released PF4 from megakaryocytes limits TPO efficacy, potentially because of increased release of PF4 stimulated by TPO. The clinical value of blocking this negative paracrine pathway post-RIT remains to be determined.

TPO receptor agonist for chronic idiopathic thrombocytopenic purpura

BACKGROUND

Chronic idiopathic thrombocytopenic purpura (ITP) is an acquired autoimmune disorder that is characterized predominantly by a low platelet count. Thrombopoietin (TPO) receptor agonists increase production of platelets by stimulating the TPO receptor in people with chronic ITP.

OBJECTIVES

To determine the efficacy and safety of TPO receptor agonists in chronic ITP patients.

SEARCH STRATEGY

We searched MEDLINE (from 1950 to March 2011), EMBASE (from 1974 to March 2011), and the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2011, Issue 3) to identify all randomized trials in chronic ITP. We also contacted authors of included studies and TPO receptor agonists manufacturers.

SELECTION CRITERIA

Randomized controlled trials (RCTs) comparing TPO receptor agonists alone, or in combination with other drugs, to placebo, no treatment, other drugs, splenectomy or another TPO receptor agonist in patients with chronic ITP.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened papers, extracted data and assessed the risk of bias in the included studies.

MAIN RESULTS

Six trials with 808 patients were included. Five studies compared TPO receptor agonists with placebo (romiplostim: 100, eltrombopag: 299, placebo: 175); one study compared TPO receptor agonists with standard of care (SOC) (romiplostim: 157; SOC: 77). SOC included a variety of therapies, such as glucocorticoid, anti-D immune globulin, intravenous immune globulin, rituximab, azathioprine, and so on. Overall survival, one of our primary outcomes, was not studied by these RCTs and we could not estimate number needed to treat (NNT). Another primary outcome, improving significant bleeding events, did not reveal any significant differences between the TPO receptor agonists group and the control group (placebo or SOC) (versus placebo risk ratio (RR) 0.48, 95% confidence interval (CI) 0.20 to 1.15; versus SOC RR 0.49, 95% CI 0.15 to 1.63).For secondary outcomes, TPO receptor agonists statistically significantly improved overall platelet response (versus placebo RR 4.06, 95% CI 2.93 to 5.63; versus SOC RR 1.81, 95% CI 1.37 to 2.37), complete response (versus placebo RR 9.29, 95% CI 2.32 to 37.15) and durable response (versus placebo RR 14.16, 95% CI 2.91 to 69.01). There was a significant reduction in overall bleeding events (WHO grades 1 to 4) when compared to placebo (RR 0.78, 95% CI 0.68 to 0.89), but not when compared to SOC(RR 0.97, 95% CI 0.75 to 1.26).Total adverse events (Grades 1 to 5) were not statistically significantly different between the treatment and control groups(both placebo and SOC) (versus placebo RR 1.04, 95% CI 0.95 to 1.15; versus SOC RR 0.97, 95% CI 0.75 to 1.26). Total serious adverse events (Grade 3 and higher adverse events) were increased when patients receiving treatment with SOC (RR 0.61, 95% CI 0.40 to 0.92), but not receiving treatment with placebo (RR 0.92, 95% CI 0.61 to 1.38).There are selective and performance biases because of open-label and inadequate allocation.

AUTHORS' CONCLUSIONS

There was currently no evidence to support that TPO receptor agonists are effective in chronic ITP. Compared to placebo or SOC, despite significantly increased platelet response, there was no evidence to demonstrate that TPO receptor agonists did improve significant bleeding events in chronic ITP. The effect on overall survival awaits further analysis. Although long-term studies are lacking, current data demonstrated adverse effects of TPO receptor agonists were similar to that of placebo and SOC.

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.

Use of thrombopoietic agents for the thrombocytopenia of liver disease

Thrombocytopenia can be a sign of advanced liver disease. This complicates the management of these patients, as patients with advanced liver disease frequently need therapeutic interventions, which are associated with an increased risk of bleeding. Thrombocytopenia may even be more pronounced in patients with hepatitis C virus (HCV) infection. This can limit current and future treatments for HCV, which remains based on interferon therapy, and this therapy can worsen thrombocytopenia. Thus, the ability to increase platelets to higher and safer levels would be desirable, but no treatment has yet been proven effective and safe in this setting. The small molecule thrombopoietin mimetic, eltrombopag, has demonstrated a dose-response increase in platelet counts in a phase II study in HCV patients without significant side effects, allowing initiation and completion of a 12-week course of pegylated interferon plus ribavirin in 36%, 53%, and 65% of patients receiving 30 mg, 50 mg, or 75 mg eltrombopag daily, respectively, compared to only 6% in the placebo arm. A current phase III study is currently evaluating whether this treatment allows successful outcomes and sustained viral eradication.

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.

Synthesis and biological evaluation of oxoindolin-3-ylidene ethyl benzothiohydrazides as non-peptide TPO mimics

A novel series of oxoindolin-3-ylidene ethyl benzohydrazides were designed, synthesized, and identified as small molecule agonists of thrombopoietin (TPO) receptor c-mpl. Sulfur-oxygen exchange in oxoindolin-3-ylidene ethyl benzohydrazides was found to improve their agonistic activities. Several oxoindolin-3-ylidene ethyl benzothiohydrazides have been identified as full agonists of c-mpl.

Thrombopoietin and platelet production in chronic immune thrombocytopenia

Since 1968, a greater understanding of platelet biology and its regulation by thrombopoietin (TPO) has emerged. It is now recognized that immune thrombocytopenic purpura (ITP) is a disorder of reduced platelet production as well as increased platelet destruction. New therapies for ITP have emerged that have exploited this new pathophysiologic understanding. This article reviews the biology of TPO, the regulation of its circulating level in ITP, the platelet kinetic data supporting inappropriate platelet production in ITP, and the TPO mimetic agents available to treat ITP.

New thrombopoietic growth factors

Thrombopoietin (TPO) is the physiologic regulator of platelet production and works by binding to its receptor on megakaryocyte precursor cells, thereby activating a large number of antiapoptotic and cell maturation pathways. "First-generation" recombinant forms of TPO were developed over a decade ago and were found to increase the platelet count in patients undergoing nonmyeloablative chemotherapy, in patients with immune thrombocytopenic purpura (ITP) and myelodysplasia, as well as in platelet apheresis donors. Thrombopoietin did not improve platelet counts in patients undergoing stem cell transplantation or acute leukemia induction. Further development ended when antibodies formed against one of the recombinant proteins. Subsequently, 2 "second-generation" TPO mimetics have been developed and are entering clinical practice: romiplostim and eltrombopag. Romiplostim is an injectable peptide TPO mimetic that activates the TPO receptor just like native TPO. Eltrombopag is an oral nonpeptide TPO mimetic that activates the TPO receptor by binding to a different region of the TPO receptor that does not compete with TPO binding. Both increased the platelet counts in healthy subjects and in over two thirds of patients with ITP both before and after splenectomy; responses were maintained for at least 1 year. Romiplostim and eltrombopag are now US Food and Drug Administration approved for the second-line treatment of patients with ITP. Adverse events have been few, but long-term assessment for reticulin formation, increased bone marrow blasts, and thromboembolism is ongoing. Studies are under way to assess the efficacy of these drugs in the treatment of other thrombocytopenic disorders associated with chemotherapy, myelodysplasia, and chronic hepatitis.

A prototype nonpeptidyl, hydrazone class, thrombopoietin receptor agonist, SB-559457, is toxic to primary human myeloid leukemia cells

Biologic characterization of SB-559457 (SB), a nonpeptidyl hydrazone class of thrombopoietin receptor (Mpl) agonist, revealed toxicity toward human leukemia cells. Antiproliferative effects followed by significant, nonapoptotic, cell death within 72 hours occurred in 24 of 26 acute myeloid leukemia, 0 of 6 acute lymphoblastic leukemia, and 3 of 6 chronic myeloid leukemia patient samples exposed to SB, but not recombinant human thrombopoietin (rhTpo), in liquid suspension culture. Further investigation revealed increased phosphorylation of p70S6/S6 kinases in SB-, but not in rhTpo-, treated cells. Expression profiling of cells exposed to SB versus rhTpo revealed statistically significant, more than 2-fold changes in GAPDH and REDD1 gene expression, confirmed by quantitative reverse-transcribed polymerase chain reaction. These genes, induced in energy or hypoxia stressed cells, have been implicated in cell death pathways, and may provide important clues to the mechanism of SB-induced, leukemic cell death. These results suggest that nonpeptidyl, hydrazone class Mpl agonists may be clinically useful antileukemic agents by virtue of their combined thrombopoietic and antileukemic effects.

Thrombopoietin agonists for the treatment of thrombocytopenia in liver disease and hepatitis C

Thrombocytopenia is a condition of unusually low level of platelets in blood, resulting from an imbalance between the production and destruction of platelets, and is associated with aplastic anemia, myelodysplasia, and idiopathic thrombocytopenic purpura (ITP). Thrombocytopenia can also be associated with severe chronic liver disease as a result of several factors that may act in concert, including reduced production of the endogenous thrombopoietic growth factor, thrombopoietin (TPO). This article examines the nature of thrombocytopenia, ITP, and TPO.

Platelet factor 4 regulates megakaryopoiesis through low-density lipoprotein receptor-related protein 1 (LRP1) on megakaryocytes

Platelet factor 4 (PF4) is a negative regulator of megakaryopoiesis, but its mechanism of action had not been addressed. Low-density lipoprotein (LDL) receptor-related protein-1 (LRP1) has been shown to mediate endothelial cell responses to PF4 and so we tested this receptor's importance in PF4's role in megakaryopoiesis. We found that LRP1 is absent from megakaryocyte-erythrocyte progenitor cells, is maximally present on large, polyploidy megakaryocytes, and near absent on platelets. Blocking LRP1 with either receptor-associated protein (RAP), an antagonist of LDL family member receptors, or specific anti-LRP1 antibodies reversed the inhibition of megakaryocyte colony growth by PF4. In addition, using shRNA to reduce LRP1 expression was able to restore megakaryocyte colony formation in bone marrow isolated from human PF4-overexpressing mice (hPF4(High)). Further, shRNA knockdown of LRP1 expression was able to limit the effects of PF4 on megakaryopoiesis. Finally, infusion of RAP into hPF4(High) mice was able to increase baseline platelet counts without affecting other lineages, suggesting that this mechanism is important in vivo. These studies extend our understanding of PF4's negative paracrine effect in megakaryopoiesis and its potential clinical implications as well as provide insights into the biology of LRP1, which is transiently expressed during megakaryopoiesis.

Role of growth factors and thrombopoietic agents in the treatment of chronic hepatitis C

Advanced liver disease and interferon-based treatment are both associated with varying degrees of cytopenia in patients with chronic hepatitis C. Growth factors to increase hemoglobin and neutrophils are commonly used in clinical practice, despite the absence of data to indicate benefits in terms of sustained viral response. Thrombocytopenia is observed frequently, is multi-factorial in etiology, and may result in significant limitations on interventional and therapeutic options. A small-molecule thrombopoietin mimetic, eltrombo-pag, has demonstrated a dose-response associated increase in platelet count in a phase 2 study, allowing initiation and completion of a 12-week course of peginterferon plus ribavirin in 36%, 53%, and 65% of patients receiving 30 mg, 50 mg, or 75 mg of eltrombopag daily, respectively, compared with 6% in the placebo arm. Phase 3 studies are currently evaluating whether initiating and maintaining antiviral therapy with eltrombopag will lead to an increase in sustained virologic response in chronic hepatitis C infection.

Hematopoietic growth factor mimetics: from concept to clinic

Hematopoietic growth factor (HGF) mimetics offer a number of attractive advantages as therapeutic agents. Small chemical compounds, in particular, provide reduced cost and oral availability. As many of these mimetics are unrelated in structure to the normal cytokine the immunogenic response is not a significant issue. Isolation of small peptide agonists for erythropoietin (EPO) and thrombopoietin (TPO) receptors has been associated with significant translational challenges and here we summarize approaches used to achieve the potency and stability required for clinical utility. We also compare and contrast the initial screening approaches, and the translational and clinical issues associated with two recently approved TPO mimetics, romiplostim and the orally available eltrombopag. Finally we summarize the development and clinical findings for the EPO mimetic, Hematide, consider alternative approaches, and discuss the future potential for isolation of growth factor (GF) mimetics.