Enhancement of platelet recovery after myelosuppressive chemotherapy by recombinant human megakaryocyte growth and development factor in patients with advanced cancer

Emerging data on thrombopoietin receptor agonists for management of chemotherapy-induced thrombocytopenia

INTRODUCTION

Chemotherapy-induced thrombocytopenia (CIT) is a common complication of cancer treatment, frequently leading to reduced relative dose intensity, and is associated with reduced survival. Given the lack of FDA-approved therapies for CIT, thrombopoietin receptor agonists (TPO-RAs) have received significant attention for treatment and prevention of CIT.

AREAS COVERED

This review will summarize the development of prior agents for treatment of CIT, discuss the existing literature investigating the use of TPO-RAs in CIT primarily in patients with solid tumor malignancies, and offer insights on the future direction of TPO-RAs and other therapeutics for CIT.

EXPERT OPINION

In alignment with NCCN guidelines, we recommend that patients with CIT participate in a clinical trial for consideration of TPO-RA treatment or consider off-label use of romiplostim when participation in clinical trials is not possible. The literature to date supports the use of TPO-RAs for treatment of persistent CIT. Further data is needed to describe the long-term efficacy, safety, and prescribing practices of TPO-RAs in a diverse patient population with a variety of tumor types and chemotherapy regimens in addition to exploring the underlying biology of CIT.

Systematic literature review and meta-analysis on use of Thrombopoietic agents for chemotherapy-induced thrombocytopenia

Background

Currently, there are no approved options to prevent or treat chemotherapy-induced thrombocytopenia (CIT). We performed a systematic literature review and meta-analysis on use of thrombopoietic agents for CIT.

Patients and methods

We searched Cochrane Central Register of Controlled Trials, Cochrane Database of Systematic Reviews, PubMed, EMBASE, ClinicalTrials.gov, and health technology assessments from January 1995 to March 2021 for studies evaluating thrombopoietic agents for CIT, including recombinant human thrombopoietin (rhTPO), megakaryocyte growth and development factor (MGDF), romiplostim, and eltrombopag. Random effects meta-analyses were conducted for efficacy and safety endpoints.

Results

We screened 1503 titles/abstracts, assessed 138 articles, and abstracted data from 39 publications (14 recombinant human thrombopoietin, 7 megakaryocyte growth and development factor, 9 romiplostim, 8 eltrombopag, and 1 romiplostim/eltrombopag). Random effects meta-analyses of data from multiple studies comparing thrombopoietic agents versus control (comparator, placebo, or no treatment) showed that thrombopoietic agents did not significantly improve chemotherapy dose delays and/or reductions (21.1% vs 40.4%, P = 0.364), grade 3/4 thrombocytopenia (39.3% vs 34.8%; P = 0.789), platelet transfusions (16.7% vs 31.7%, P = 0.111), grade ≥ 2 bleeding (6.7% vs 16.5%; P = 0.250), or thrombosis (7.6% vs 12.5%; P = 0.131). However, among individual studies comparing thrombopoietic agents with placebo or no treatment, thrombopoietic agents positively improved outcomes in some studies, including significantly increasing mean peak platelet counts (186 x 10⁹/L with rhTPO vs 122 x 10⁹/L with no treatment; P < 0.05) in one study and significantly increasing platelet count at nadir (56 x 10⁹/L with rhTPO vs 28 x 10⁹/L with not treatment; P < 0.05) in another study. Safety findings included thrombosis (n = 23 studies) and bleeding (n = 11), with no evidence of increased thrombosis risk with thrombopoietic agents.

Conclusion

Our analyses generate the hypothesis that thrombopoietic agents may benefit patients with CIT. Further studies with well-characterized bleeding and platelet thresholds are warranted to explore the possible benefits of thrombopoietic agents for CIT.

Treatment of drug-induced immune thrombocytopenias

Several therapeutic agents can cause thrombocytopenia by either immune-mediated or non-immune-mediated mechanisms. Non-immune-mediated thrombocytopenia is due to direct toxicity of drug molecules to platelets or megakaryocytes. Immune-mediated thrombocytopenia, on the other hand, involves the formation of antibodies that react to platelet-specific glycoprotein complexes, as in classic drug-induced immune thrombocytopenia (DITP), or to platelet factor 4, as in heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombotic thrombocytopenia (VITT). Clinical signs include a rapid drop in platelet count, bleeding or thrombosis. Since the patient's condition can deteriorate rapidly, prompt diagnosis and management are critical. However, the necessary diagnostic tests are only available in specialized laboratories. Therefore, the most demanding step in treatment is to identify the agent responsible for thrombocytopenia, which often proves difficult because many patients are taking multiple medications and have comorbidities that can themselves also cause thrombocytopenia. While DITP is commonly associated with an increased risk of bleeding, HIT and VITT have a high mortality rate due to the high incidence of thromboembolic complications. A structured approach to drug-associated thrombocytopenia/thrombosis can lead to successful treatment and a lower mortality rate. In addition to describing the treatment of DITP, HIT, VITT, and vaccine-associated immune thrombocytopenia, this review also provides the pathophysiological and clinical information necessary for correct patient management.

Treatment of chemotherapy-induced thrombocytopenia in patients with non-hematologic malignancies

Chemotherapy-induced thrombocytopenia (CIT) is a common complication of the treatment of non-hematologic malignancies. Many patient-related variables (e.g., age, tumor type, number of prior chemotherapy cycles, amount of bone marrow tumor involvement) determine the extent of CIT. CIT is related to the type and dose of chemotherapy, with regimens containing gemcitabine, platinum, or temozolomide producing it most commonly. Bleeding and the need for platelet transfusions in CIT are rather uncommon except in patients with platelet counts below 25x10⁹/L in whom bleeding rates increase significantly and platelet transfusions are the only treatment. Nonetheless, platelet counts below 70x10⁹/L present a challenge. In patients with such counts, it is important to exclude other causes of thrombocytopenia (medications, infection, thrombotic microangiopathy, post-transfusion purpura, coagulopathy and immune thrombocytopenia). If these are not present, the common approach is to reduce chemotherapy dose intensity or switch to other agents. Unfortunately decreasing relative dose intensity is associated with reduced tumor response and remission rates. Thrombopoietic growth factors (recombinant human thrombopoietin, pegylated human megakaryocyte growth and development factor, romiplostim, eltrombopag, avatrombopag and hetrombopag) improve pretreatment and nadir platelet counts, reduce the need for platelet transfusions, and enable chemotherapy dose intensity to be maintained. National Comprehensive Cancer Network guidelines permit their use but their widespread adoption awaits adequate phase III randomized, placebo-controlled studies demonstrating maintenance of relative dose intensity, reduction of platelet transfusions and bleeding, and possibly improved survival. Their potential appropriate use also depends on consensus by the oncology community as to what constitutes an appropriate pretreatment platelet count as well as identification of patient-related and treatment variables that might predict bleeding.

Clinical challenges and promising therapies for chemotherapy-induced thrombocytopenia

Introduction: Chemotherapy-induced thrombocytop enia (CIT) is a common complication of cancer treatment causing chemotherapy delays, dose reductions, and treatment discontinuation, negatively impacting treatment outcomes and putting patients at risk for bleeding complications. There is no FDA-approved agent available to manage CIT.Areas covered: This article covers the diagnosis, definitions, and clinical challenges of CIT, and then focuses on the therapeutics developed to manage CIT. The first-generation thrombopoietic agents (oprelvekin and recombinant human thrombopoietins) are reviewed for critical background and context, followed by a detailed discussion of the data for the thrombopoietin receptor agonists (TPO-RAs) to manage CIT. Efficacy of TPO-RAs in treatment and prevention of CIT, as well as safety concerns such as the risk of thromboembolic complications, are reviewed in detail. For this review, a PubMed/MEDLINE literature search was undertaken for relevant articles published from 1995-2021.Expert opinion: After over two decades of drug development for CIT, multiple clinical trials and observational studies have found TPO-RAs, in particular romiplostim, to be safe and effective agents to manage patients with CIT, although no agent is yet FDA-approved for this indication. Active management of CIT with TPO-RAs is likely to improve oncologic outcomes, although additional data are needed. Phase 3 trials are ongoing.

A multicenter study of romiplostim for chemotherapy-induced thrombocytopenia in solid tumors and hematologic malignancies

Chemotherapy-induced thrombocytopenia (CIT) frequently complicates cancer treatment causing chemotherapy delays, dose reductions, and discontinuation. There is no FDA-approved agent available to manage CIT. This study retrospectively evaluated patients with CIT treated on institutional romiplostim treatment pathways at 4 U.S. centers. The primary outcome was achievement of a romiplostim response [median on-romiplostim platelet count (Plt) ≥75x109/L and ≥30x109/L above baseline]. Secondary outcomes included time to Plt≥100x109/L and rates of the following: Plt<100x109/L, Plt<75x109/L, Plt<50x109/L, thrombocytosis, chemotherapy dose reduction/treatment delay, platelet transfusion, bleeding, and thromboembolism. Multivariable regression was used to identify predictors of romiplostim non-response and compare weekly dosing with intracycle/intermittent dosing. 173 patients (153 solid tumor, 20 lymphoma or myeloma) were treated, with 170 (98%) receiving a median of 4 (range, 1-36) additional chemotherapy cycles on romiplostim. Romiplostim was effective in solid tumor patients: 71% of patients achieved a romiplostim response, 79% avoided chemotherapy dose reductions/treatment delays and 89% avoided platelet transfusions. Median per-patient Plt on romiplostim was significantly higher than baseline (116x109/L vs. 60x109/L, P<0.001). Bone marrow tumor invasion, prior pelvic irradiation, and prior temozolomide predicted romiplostim non-response. Bleeding rates were lower than historical CIT cohorts and thrombosis rates were not elevated. Weekly dosing was superior to intracycle dosing with higher response rates and less chemotherapy dose reductions/treatment delays (IRR 3.00, 95% CI 1.30-6.91, P=0.010) or bleeding (IRR 4.84, 95% CI 1.18-19.89, P=0.029). Blunted response (10% response rate) was seen in non-myeloid hematologic malignancy patients with bone marrow involvement. In conclusion, romiplostim was safe and effective for CIT in most solid tumor patients.

Triplet therapies - the new standard of care for multiple myeloma: how to manage common toxicities

Introduction: Multiple three drug combination regimens have been approved for the treatment of multiple myeloma in the last few years. Triplets have become the new standard of care for transplant eligible and ineligible patients with newly diagnosed as well as relapsed multiple myeloma. Novel agents have a unique profile of side effects. The management of toxicities is important to maintain quality of life and maximize treatment duration and benefit. Areas covered: This article reviews efficacy data, incidence of key adverse events and provide recommendations and expert opinion regarding how to manage common toxicities in triplet therapies. Relevant publications and abstracts were searched in PubMed, ASH, ASCO and EHA meetings. Guidelines from IMWG, NCCN, ESMO and ASCO, published trial protocols and prescribing information were used to formulate recommendations for the management of toxicities. Expert commentary: Side effects are a critical factor guiding the selection of optimal chemotherapy regimens for multiple myeloma. The majority of toxicities encountered with triplet therapies are reversible and can be readily managed with supportive care and dose modifications.

Alternatives, and adjuncts, to prophylactic platelet transfusion for people with haematological malignancies undergoing intensive chemotherapy or stem cell transplantation

BACKGROUND

Platelet transfusions are used in modern clinical practice to prevent and treat bleeding in people with thrombocytopenia. Although considerable advances have been made in platelet transfusion therapy since the mid-1970s, some areas continue to provoke debate especially concerning the use of prophylactic platelet transfusions for the prevention of thrombocytopenic bleeding.

OBJECTIVES

To determine whether agents that can be used as alternatives, or adjuncts, to platelet transfusions for people with haematological malignancies undergoing intensive chemotherapy or stem cell transplantation are safe and effective at preventing bleeding.

SEARCH METHODS

We searched 11 bibliographic databases and four ongoing trials databases including the Cochrane Central Register of Controlled Trials (CENTRAL, 2016, Issue 4), MEDLINE (OvidSP, 1946 to 19 May 2016), Embase (OvidSP, 1974 to 19 May 2016), PubMed (e-publications only: searched 19 May 2016), ClinicalTrials.gov, World Health Organization (WHO) ICTRP and the ISRCTN Register (searched 19 May 2016).

SELECTION CRITERIA

We included randomised controlled trials in people with haematological malignancies undergoing intensive chemotherapy or stem cell transplantation who were allocated to either an alternative to platelet transfusion (artificial platelet substitutes, platelet-poor plasma, fibrinogen concentrate, recombinant activated factor VII, desmopressin (DDAVP), or thrombopoietin (TPO) mimetics) or a comparator (placebo, standard care or platelet transfusion). We excluded studies of antifibrinolytic drugs, as they were the focus of another review.

DATA COLLECTION AND ANALYSIS

Two review authors screened all electronically derived citations and abstracts of papers identified by the review search strategy. Two review authors assessed risk of bias in the included studies and extracted data independently.

MAIN RESULTS

We identified 16 eligible trials. Four trials are ongoing and two have been completed but the results have not yet been published (trial completion dates: April 2012 to February 2017). Therefore, the review included 10 trials in eight references with 554 participants. Six trials (336 participants) only included participants with acute myeloid leukaemia undergoing intensive chemotherapy, two trials (38 participants) included participants with lymphoma undergoing intensive chemotherapy and two trials (180 participants) reported participants undergoing allogeneic stem cell transplantation. Men and women were equally well represented in the trials. The age range of participants included in the trials was from 16 years to 81 years. All trials took place in high-income countries. The manufacturers of the agent sponsored eight trials that were under investigation, and two trials did not report their source of funding.No trials assessed artificial platelet substitutes, fibrinogen concentrate, recombinant activated factor VII or desmopressin.Nine trials compared a TPO mimetic to placebo or standard care; seven of these used pegylated recombinant human megakaryocyte growth and differentiation factor (PEG-rHuMGDF) and two used recombinant human thrombopoietin (rhTPO).One trial compared platelet-poor plasma to platelet transfusion.We considered that all the trials included in this review were at high risk of bias and meta-analysis was not possible in seven trials due to problems with the way data were reported.We are very uncertain whether TPO mimetics reduce the number of participants with any bleeding episode (odds ratio (OR) 0.40, 95% confidence interval (CI) 0.10 to 1.62, one trial, 120 participants, very low quality evidence). We are very uncertain whether TPO mimetics reduce the risk of a life-threatening bleed after 30 days (OR 1.46, 95% CI 0.06 to 33.14, three trials, 209 participants, very low quality evidence); or after 90 days (OR 1.00, 95% CI 0.06 to 16.37, one trial, 120 participants, very low quality evidence). We are very uncertain whether TPO mimetics reduce platelet transfusion requirements after 30 days (mean difference -3.00 units, 95% CI -5.39 to -0.61, one trial, 120 participants, very low quality evidence). No deaths occurred in either group after 30 days (one trial, 120 participants, very low quality evidence). We are very uncertain whether TPO mimetics reduce all-cause mortality at 90 days (OR 1.00, 95% CI 0.24 to 4.20, one trial, 120 participants, very low quality evidence). No thromboembolic events occurred for participants treated with TPO mimetics or control at 30 days (two trials, 209 participants, very low quality evidence). We found no trials that looked at: number of days on which bleeding occurred, time from randomisation to first bleed or quality of life.One trial with 18 participants compared platelet-poor plasma transfusion with platelet transfusion. We are very uncertain whether platelet-poor plasma reduces the number of participants with any bleeding episode (OR 16.00, 95% CI 1.32 to 194.62, one trial, 18 participants, very low quality evidence). We are very uncertain whether platelet-poor plasma reduces the number of participants with severe or life-threatening bleeding (OR 4.00, 95% CI 0.56 to 28.40, one trial, 18 participants, very low quality evidence). We found no trials that looked at: number of days on which bleeding occurred, time from randomisation to first bleed, number of platelet transfusions, all-cause mortality, thromboembolic events or quality of life.

AUTHORS' CONCLUSIONS

There is insufficient evidence to determine if platelet-poor plasma or TPO mimetics reduce bleeding for participants with haematological malignancies undergoing intensive chemotherapy or stem cell transplantation. To detect a decrease in the proportion of participants with clinically significant bleeding from 12 in 100 to 6 in 100 would require a trial containing at least 708 participants (80% power, 5% significance). The six ongoing trials will provide additional information about the TPO mimetic comparison (424 participants) but this will still be underpowered to demonstrate this level of reduction in bleeding. None of the included or ongoing trials include children. There are no completed or ongoing trials assessing artificial platelet substitutes, fibrinogen concentrate, recombinant activated factor VII or desmopressin in people undergoing intensive chemotherapy or stem cell transplantation for haematological malignancies.

Deciphering the molecular and biologic processes that mediate histone deacetylase inhibitor–induced thrombocytopenia

Histone deacetylase inhibitor (HDACI)–induced thrombocytopenia (TCP) is a major dose-limiting toxicity of this new class of drugs. Using preclinical models to study the molecular and biologic events that underpin this effect of HDACI, we found that C57BL/6 mice treated with both the HDAC1/2-selective HDACI romidepsin and the pan-HDACI panobinostat developed significant TCP. HDACI-induced TCP was not due to myelosuppression or reduced platelet lifespan, but to decreased platelet release from megakaryocytes. Cultured primary murine megakaryocytes showed reductions in proplatelet extensions after HDACI exposure and a dose-dependent increase in the phosphorylation of myosin light chain 2 (MLC2). Phosphorylation of MLC to phospho-MLC (pMLC) and subsequent proplatelet formation in megakaryocytes is regulated by the Rho-GTPase proteins Rac1, CDC42, and RhoA. Primary mouse megakaryocytes and the human megakaryoblastic cell line Meg-01 showed reductions in Rac1, CDC42, and RhoA protein levels after treatment with HDACIs. We were able to overcome HDACI-induced TCP by administering the mouse-specific thrombopoietin (TPO) mimetic AMP-4, which improved platelet numbers to levels similar to untreated controls. Our report provides the first detailed account of the molecular and biologic processes involved in HDACI-mediated TCP. Moreover, our preclinical studies provide evidence that dose-limiting TCP induced by HDACIs may be circumvented using a TPO mimetic.

Toxicities of the thrombopoietic growth factors

The thrombopoietic growth factors (TGFs) are a novel class of compounds for the treatment of chronic immune thrombocytopenia (ITP). The first of these agents to receive regulatory approval, romiplostim and eltrombopag, have demonstrated impressive efficacy and tolerability in randomized controlled trials and open-label extension studies of several years duration and stand poised to revolutionize the management of ITP. Nonetheless, critical questions regarding the safety of these agents, particularly after long-term administration, remain partially unanswered. The objective of this review is to describe the reported and potential toxicities of the TGFs, including bone marrow fibrosis, thrombosis, rebound thrombocytopenia, hematologic malignancy, neutralizing antibody formation, hepatotoxicity, cataract formation, and common adverse events. The incidence and clinical implications of these toxicities as well as strategies for patient safety monitoring are examined.

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.

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.

Development of romiplostim for the treatment of patients with chronic immune thrombocytopenia: from bench to bedside

Immune thrombocytopenia (ITP) is an autoimmune disorder characterised by abnormally low platelet counts (<100 x 10(9)/l), purpura, and bleeding episodes, and can be categorised in three phases: newly-diagnosed, persistent, and chronic. As many patients become refractory to standard treatments (corticosteroids, danazol, azathioprine, splenectomy), there is an urgent need for alternative treatments. The successful isolation and cloning of thrombopoietin (TPO) in the mid-1990s and identification of its key role in platelet production was a major breakthrough, rapidly followed by the development of the recombinant thrombopoietins, recombinant human TPO and a pegylated truncated product, PEG-rHuMGDF. Both agents increased platelet counts but development was halted because of the development of antibodies that cross-reacted with native TPO, resulting in prolonged treatment-refractory thrombocytopenia. Experimentation with novel platforms for extending the circulating half-life of therapeutic peptides by combining them with antibody fragment crystallisable (Fc) constructs led to the development of a new family of molecules termed 'peptibodies'. The 60Da recombinant peptibody romiplostim was finally produced by linking several copies of an active TPO-binding peptide sequence to a carrier Fc fragment. In clinical trials, romiplostim was effective in ameliorating thrombocytopenia in patients with chronic ITP, was well tolerated and did not elicit cross-reacting antibodies. Romiplostim has recently been approved for the treatment of adults with chronic ITP.

Considerations in the management of hepatitis C virus-related thrombocytopenia with eltrombopag

Thrombocytopenia is a common clinical problem in HCV-infected cases. Multiple studies have consistently shown a rise in platelet count following a successful HCV treatment thus proving a cause-effect relationship between the two. Although, many therapeutic strategies have been tried in the past to treat HCV-related thrombocytopenia (e.g. interferon dose reductions, oral steroids, intravenous immunoglobulins, splenectomy etc), the success rates have been variable and not always reproducible. After the cessation of clinical trials of PEG-rHuMGDF due to immunogenecity issues, the introduction of non-immunogenic second-generation thrombopoietin-mimetics (eltrombopag and Romiplostim) has opened up a novel way to treat HCV-related thrombocytopenia. Although the data is still sparse, eltrombopag therapy has shown to successfully achieve the primary endpoint platelet counts of >/=50,000/muL in phase II& III, randomized, double-blind, placebo-controlled trials. Likewise, though it is premature to claim safety of this drug especially in high-risk patient groups, reported side effects in the published literature were of insufficient severity to require discontinuation of the drug. Based on the current and emerging evidence, a review of the pharmacologic basis, pharmacokinetics, therapeutic efficacy, safety profile and future considerations of eltrombopag in the context of HCV-related thrombocytopenia is given in this article. A MEDLINE search was conducted (1990 to August 2009) using the search terms eltrombopag, HCV, thrombocytopenia.

Thrombopoietin mimetic agents in the management of immune thrombocytopenic purpura

Thrombopoietin (TPO) is a potent endogenous cytokine and the principal regulator of platelet production. Advances in the understanding of the structure of TPO enabled development of the first generation of thrombopoietic growth factors, recombinant human thrombopoietin (rhTPO) and pegylated human recombinant megakaryocyte growth and development factor (PEG-rHuMGDF). Clinical results showed that these agents were effective in promoting increases in platelet counts in a variety of thrombocytopenic disorders. However, clinical development was halted when studies demonstrated risk for autoantibody formation with cross-reactivity to endogenous TPO. A second generation of thrombopoietic growth factors, including TPO peptide and nonpeptide mimetics and TPO agonist antibodies, utilizing different mechanisms from recombinant growth factors to promote platelet production, are currently in development. The TPO peptide mimetic AMG 531 and the nonpeptide mimetic eltrombopag are in advanced clinical trials and have both resulted in dose-dependent increases in platelets in healthy subjects and in significant increases in platelets in patients with chronic immune thrombocytopenic purpura (ITP). Clinical trials are also being conducted to examine the efficacy and safety of eltrombopag to treat thrombocytopenia in hepatitis C virus (HCV)-infected individuals. These agents appear to be well tolerated and the formation of autoantibodies appears to be limited to first-generation growth factors. Increases in marrow reticulin have been demonstrated with some growth factors, but this appears to be a reversible phenomenon and is not associated with formation of collagen fibrosis. There appears to be no increased incidence of thrombotic events in patients who achieve high platelet counts with growth factor treatments, and although occasional thrombotic events have been reported, their association to the treatment is uncertain. While there is evidence that activation of signaling pathways involved in platelet production may result in reduction in the threshold for platelet activation, this appears to have minimal clinical relevance for the use of thrombopoietin growth factors.

Review article: pharmacological approaches for the treatment of thrombocytopenia in patients with chronic liver disease and hepatitis C infection

BACKGROUND

Patients with chronic liver disease and hepatitis C virus (HCV) frequently experience thrombocytopenia that complicates the management of their disease. Traditional therapy for thrombocytopenia consists of platelet transfusion, which can be associated with significant safety and economic issues. Consequently, efforts have been directed toward developing novel approaches for the treatment of thrombocytopenia.

AIM

To summarize the available data on the limitations of traditional therapies and the effects of novel therapies currently in clinical development for the treatment of thrombocytopenia.

RESULTS

Recent research has begun to reveal the complex mechanisms that regulate thrombopoiesis. Cytokines and growth factors, such as interleukin-11 and thrombopoietin (TPO), play a key role in the production of platelets. A number of recent clinical studies have provided evidence that pharmacologic agents that target megakaryocyte precursors and stimulate thrombopoiesis can effectively reverse thrombocytopenia. Here, we review the regulation of thrombopoiesis, the role of TPO, and a number of novel compounds that stimulate platelet production by acting through the TPO receptor. Agents that stimulate TPO include the orally available nonpeptidic agonists eltrombopag and AKR-501, peptidic agonists AMG-531 and Peg-TPOmp, and small engineered antibodies.

CONCLUSION

Results from clinical trials with these agents in healthy subjects confirm that activation of thrombopoiesis via the TPO pathway is an effective method of stimulating platelet production. This approach may provide safer, more effective treatment for thrombocytopenia in patients with chronic liver disease. Several of these agents are currently being tested in large scale trials.

Cytokines for the Treatment of Thrombocytopenia

Multiple cytokines affect the cellular processes that occur during the transition of a hematopoietic stem cell (HSC) to a platelet. Thrombopoietin (TPO) is the physiological regulator of thrombopoiesis. Although a number of cytokines (interleukin [IL]-1, IL-3, and IL-6) were first evaluated for their ability to lessen the degree of thrombocytopenia occurring during a variety of clinical scenarios, their clinical development was abandoned due to their limited effectiveness or excessive toxicity. Clinical results with TPO and a truncated pegylated form of TPO, megakaryocyte growth and development factor (MGDF), were more promising, but the repeated use of MGDF resulted in the development of neutralizing antibodies. This adverse event halted the further clinical development of not only MGDF but also TPO. IL-11 also affects various stages of megakaryocytopoiesis and thrombopoiesis and its use has been shown to shorten the duration of chemotherapy-induced thrombocytopenia, which led to its approval by the US Food and Drug Administration (FDA). A growing number of new non-immunogenic peptides and non-peptide TPO agonists recently have entered clinical trials. These small molecules appear to be effective therapies and have acceptable toxicity, but additional clinical evaluation will be required prior to their approval for clinical use.

New thrombopoietic growth factors

Although development of first-generation thrombopoietic growth factors (recombinant human thrombopoietin [TPO] and pegylated recombinant human megakaryocyte growth and development factor [PEG-rHuMGDF]) was stopped due to development of antibodies to PEG-rHuMGDF, nonimmunogenic second-generation thrombopoietic growth factors with unique pharmacologic properties have been developed. TPO peptide mimetics contain TPO receptor-activating peptides inserted into complementarity-determining regions of Fab (Fab 59), attached to the IgG Fc region (AMG 531), or pegylated (Peg-TPOmp). Orally available, TPO nonpeptide mimetics (eltrombopag, AKR-501) bind and activate the TPO receptor by a mechanism different from TPO and may have an additive effect to TPO. TPO agonist antibodies are monoclonal antibodies activating the TPO receptor but modified in size [TPO minibodies; ie, VB22B sc(Fv)(2)] or immunoglobuln type (domain subclass-converted TPO agonist antibodies; ie, MA01G4G344). All second-generation thrombopoietic growth factors stimulate growth of TPO-dependent cell lines via JAK2/STAT signaling pathways and increase platelet counts in animals. When tested in healthy humans, TPO peptide and nonpeptide mimetics produced a dose-dependent rise in platelet count. AMG 531 and eltrombopag markedly increase platelet counts in patients with immune thrombocytopenic purpura, without significant adverse effects. One or more second-generation thrombopoietic growth factors should soon be clinically available for treating thrombocytopenic disorders.

Novel Thrombopoietic Agents

Thrombocytopenia is a primary manifestation of immune thrombocytopenic purpura (ITP) and may occur as a result of hepatitis C, malignancy, and treatment with chemotherapy. There is a need for additional means to treat thrombocytopenia in these settings. Recombinant thrombopoietin-like agents became available after the cloning of thrombopoietin in 1994. In clinical trials, these agents showed some efficacy in chemotherapy-induced thrombocytopenia, but their use was ultimately discontinued due to the development of neutralizing antibodies that cross-reacted with endogenous thrombopoietin and caused thrombocytopenia in healthy blood donors and other recipients. Subsequently, “second-generation” thrombopoietic agents without homology to thrombopoietin were developed. In the past 5 years, these second-generation thrombopoeitic growth factors have undergone substantial clinical development and have demonstrated safety, tolerability and efficacy in subjects with ITP and hepatitis C–related thrombocytopenia. These completed studies, many of which are available only in abstract form, and other ongoing studies suggest that thrombopoietic agents will enhance the hematologist’s ability to manage these and other causes of thrombocytopenia.

Haematopoietic stem and progenitor cell-targeted therapies for thrombocytopenia

This review discusses the present outlook for new thrombocytopenia therapies that induce haematopoietic stem and progenitor cells to proliferate, differentiate and produce functional platelets. A brief overview of megakaryopoiesis and its regulation by thrombopoietin (TPO) is followed by a discussion of how early experience with recombinant TPO therapies stimulated the search for novel TPO receptor ligands. A summary is then provided of the results of Phase I clinical trials with the new small molecule and peptide TPO mimetics that are in development at present. Finally, recent developments in the ex vivo expansion of primitive haematopoietic cells and the potential enhancement of cell-based therapies by haematopoietic growth factors in vivo are briefly summarised as part of a look towards the future.

Sequential interleukin 3 and granulocyte-macrophage-colony stimulating factor therapy in patients with bone marrow failure with long-term follow-up of responses

BACKGROUND

Interleukin-3 (IL-3) and granulocyte-macrophage-colony stimulating factor (GM-CSF) have synergistic, hematopoietic growth-promoting activity in preclinical studies. Because of the paucity of effective therapies for patients with chronic bone marrow failure states, the authors studied the biologic activity of sequential IL-3/GM-CSF in such patients.

METHODS

IL-3 was given subcutaneously for 5 days (at escalating doses of 0.15 microg/kg, 0.3 microg/kg, 0.6 microg/kg, 1.2 microg/kg, 2.5 microg/kg, 5.0 microg/kg, 10.0 microg/kg, or 15.0 microg/kg per day), and GM-CSF for was given subcutaneously for 9 days (at a dose of 5 microg/kg per day; Phase I 3 + 3 design) followed by 14 days of rest (total, 2 courses), then maintenance therapy.

RESULTS

The majority of 38 evaluable patients had aplastic anemia or myelodysplastic syndrome. Most patients (79%) had neutrophil responses. Ten patients (26%), all of whom were treated with IL-3 doses >/= 1.2 microg/kg per day, had platelet responses, with a median increase of 132 x 10(9)/L (range, 41-180 x 10(9)/L) over baseline in responders. Six patients (16%) had trilineage recovery, which could be durable (the longest ongoing at 6.5 years after therapy completion). The most common toxicities were low-grade fever, headache, and fatigue. The maximum tolerated doses were IL-3 at 10 microg/kg per day and GM-CSF at 5 microg/kg per day.

CONCLUSIONS

Sequential IL-3/GM-CSF effectively raised blood counts in some patients with bone marrow failure at doses that were tolerated well. These results indicate that early-acting growth factors can induce durable, multilineage responses in a subset of individuals with bone marrow failure.

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.

The glycan domain of thrombopoietin (TPO) acts in trans to enhance secretion of the hormone and other cytokines

Thrombopoietin (TPO), the primary regulator of platelet production, is composed of an amino-terminal 152 amino acids, sufficient for activity, and a carboxyl-terminal region rich in carbohydrates (183 residues) that enhances secretion of the molecule. Full-length TPO is secreted at levels 10-20-fold greater than truncated TPO. By introducing into mammalian cells a novel cDNA encoding the TPO secretory leader linked to its carboxyl-terminal domain (TPO glycan domain (TGD)), we tested whether TGD could function in trans to enhance secretion of TPO. The artificial TGD was secreted, inactive in proliferation assays, and did not inhibit TPO activity. However, when co-transfected with a cDNA encoding truncated TPO, TGD enhanced secretion 4-fold, measured by specific bioassay and immunoassay. TGD also enhanced secretion of granulocyte monocyte colony-stimulating factor and stem cell factor but did not affect the production of erythropoietin, interleukin-3, growth hormone, or of full-length TPO. To localize TGD function, we added an endoplasmic reticulum (ER) retention signal to TGD and, separately, deleted the secretory leader. Deletion of the secretory leader attenuated the secretory function of TGD, whereas addition of the ER retention signal did not alter its function. To investigate the physiologic role of TGD in folding and proteasomal protection, we tested full-length and truncated TPO in assays of protein refolding, and we examined protein stability in the presence of proteasome inhibitors. We found that truncated TGD re-folds readily and that proteasome-mediated degradation contributes to the poor secretion of truncated TPO. We conclude that TGD enhances secretion of TPO and can additionally function as an inter-molecular chaperone, in part because of its ability to prevent degradation of the hormone. The cellular location of TGD action is likely to be within the ER or earlier in the secretory pathway.

The molecular and cellular biology of thrombopoietin: the primary regulator of platelet production

The term thrombopoietin (TPO) was first coined in 1958 and used to describe the humoral substance responsible for causing the platelet count to rise in response to thrombocytopenic stimuli. Despite much progress during the 1980s in the purification and characterization of the humoral regulators of lymphocyte, erythrocyte, monocyte and granulocyte production, the successful search to purify and molecularly clone thrombopoietin did not begin until the oncogene v-mpl was discovered in 1990. Since that time the proto-oncogene c-mpl was identified and, based on homology arguments, believed to encode a hematopoietic cytokine receptor, a hypothesis later proven when the cytoplasmic domain was linked to the ligand binding domain of the IL-4 receptor and shown to support the IL-4 induced growth of hematopoietic cells (Skoda et al., 1993). Finally, two different strategies using c-mpl lead to the identification of a novel ligand for the receptor in 1994 (de Sauvage et al., 1994; Lok et al., 1994; Bartley et al., 1994), a protein that displays all the biologic properties of TPO. This review attempts to distill what has been learned of the molecular and cellular biology of TPO and its receptor during the past several years, and links this information to several new insights into human disease and its treatment.

Development of pancytopenia with neutralizing antibodies to thrombopoietin after multicycle chemotherapy supported by megakaryocyte growth and development factor

Clinical trials of thrombopoietin (TPO), the central regulator of megakaryocytopoiesis, have revealed few side effects associated with its use. We here report a case of pancytopenia associated with the development of neutralizing antibodies to TPO that occurred in a patient who had undergone multicycle chemotherapy with multiple cycles of subcutaneous administration of pegylated recombinant human megakaryocyte growth and development factor. Samples of the patient's bone marrow showed trilineage hypoplasia with absence of myeloid, erythroid, and megakaryocyte progenitor cells but with elevated endogenous levels of erythropoietin, granulocyte colony-stimulating factor, and stem-cell factor. To our knowledge, this is the first report of an aplastic anemia-like syndrome associated with neutralizing antibodies to TPO.

Ex vivo expansion of megakaryocyte progenitor cells from normal bone marrow and peripheral blood and from patients with haematological malignancies

A number of haematological and non-haematological malignancies can be successfully treated using high-dose chemotherapy +/- irradiation followed by haematopoietic progenitor cell transplantation. Post transplant, thrombocytopenia and neutropenia always occur and patients require platelet transfusions. It may be possible to reduce the period of thrombocytopenia by re-infusion of ex vivo expanded megakaryocyte progenitors (MP), derived from the progenitor cell graft. We have investigated the expansion of MP from CD34+ enriched cells from normal bone marrow (NBM) and peripheral blood (PB) and remission BM or PB samples from patients with haematological malignancies. CD34+ cells were cultured in serum-free medium supplemented with thrombopoietin (TPO), interleukin 1 (IL-1), IL-6 and stem cell factor (SCF) for 7 d, then cell proliferation was assessed by flow cytometry using lineage-specific markers. It was possible to significantly expand the number of MP cells from all sources. There were no major differences in yields of MP from normal BM or PB, or BM from multiple myeloma and non-Hodgkin's lymphoma patients. However, expansion of MP in acute myeloid leukaemia samples was lower than all other samples and the number of megakaryocyte colony-forming units was reduced. Several cytokine combinations were evaluated to optimize MP expansion from NBM. Equivalent yields of MP were obtained using TPO and one of IL-1, IL-3, granulocyte-macrophage colony-stimulating factor or SCF, suggesting that large cytokine combinations are not necessary for this procedure. It should be possible to scale up the culture conditions described to produce effective MP doses for clinical transplantation.

Cisplatin, fludarabine, and cytarabine: a novel, pharmacologically designed salvage therapy for patients with refractory, histologically aggressive or mantle cell non-Hodgkin's lymphoma

BACKGROUND

Based on in vitro synergism, the combination of cytarabine (ara-C) and cisplatin is the basis of many salvage regimens for patients with aggressive non-Hodgkin lymphoma (NHL). However, patients with previously refractory disease are significantly less likely to respond, stimulating the search for novel salvage regimens. In vitro, fludarabine enhances the cytotoxicity of both ara-C and cisplatin, increasing ara-C incorporation into DNA and inhibiting repair of platinum/DNA adducts, suggesting that the combination of cisplatin, fludarabine, and ara-C (PFA) may have clinical utility.

METHODS

A Phase-II study of a 96 hour continuous infusion of cisplatin with two timed-sequential couplets of fludarabine and ara-C together with granulocyte colony stimulating factor was performed in 45 patients with previously refractory, histologically aggressive or mantle cell NHL.

RESULTS

Patients had predominantly diffuse large cell and/or immunoblastic NHL or its variants (80%), or they had mantle cell lymphoma (18%). Overall, 93% of patients had previously refractory disease, with a median International Prognostic Index score of 3. A median of 2 cycles per patient were delivered (range, 1-4 cycles) with significant myelosuppression; there were medians of 2 days of neutropenia < 0.5 x 10(9)/L (range, 0-12 days) and 3 days of thrombocytopenia < 20 x 10(9)/L (range, 0-24 days). This was more severe in older patients and was cumulative with successive cycles. Thirty-five percent of cycles were complicated by infections, nausea and emesis were prominent, but other nonhematologic toxicity was mild. Peripheral blood progenitor cells were mobilized adequately after the first cycle, but collections were impaired after more prolonged therapy. The overall response rate was 48% (7% of patients had complete responses, and 41% of patients had partial responses), with one toxic death due to tumor-lysis syndrome. Patients with mantle cell lymphoma were more likely to respond than patients with other histologies (88% vs. 39%, respectively; P = 0.019), although three of eight patients had relapsed rather than refractory disease. The median remission duration was 4 months, with 28% of potentially eligible patients able to proceed to subsequent high dose therapy. The actuarial 2 year survival rates were 20% +/- 6% overall and 50 +/- 18% for patients with mantle cell lymphoma.

CONCLUSIONS

Given the adverse outlook for these patients, the results are promising, particularly for patients with mantle cell lymphoma, and suggest that the addition of fludarabine as a potential biochemical modulator may enhance the activity of cisplatin and ara-C. This is associated with significant cumulative (but manageable) myelosuppression. This paradigm, in which a nucleoside analogue is used to inhibit the repair of platinum/DNA adducts, also may be applicable for the treatment of patients with other types of platinum-sensitive tumors.

Pegylated cytokines: potential application in immunotherapy of cancer

Conjugation of the polymer polyethylene glycol (PEG) to proteins can significantly decrease their clearance from plasma, thus increasing their half-lives in vivo. The increased half-life of PEG-proteins is directly proportional to the total molecular weight of the construct. This approach has been used to design cytokine constructs that can be administered once a week, rather than on a daily or alternate-day schedule. Two cytokines for which this approach appears to be successful are PEG-interferon-alpha-2a (PEG-IFNalpha-2a) and PEG-granulocyte colony- stimulating factor (PEG-G-CSF). Both use high molecular weight PEG (20 to 40kD) to give sufficiently long duration in vivo. In the case of PEG-G-CSF conjugates, the in vivo efficacy is directly proportional to molecular weight, whereas the in vitro activity is inversely proportional, suggesting that overall duration of contact is more important than the affinity of the interaction. Conjugates of a number of other cytokines have been prepared, but until recently, few have used the high molecular weight polymers. In the future, as this approach is taken to make new PEG-cytokine constructs, thorough pharmacokinetic studies will be essential for their development and clinical use.

Failing to live up to the fanfare? A personal perspective on obstacles to the clinical development of thrombopoietic agents

A number of hematopoietic growth factors have been identified that are active on megakaryocytes and platelets, but only 2, interleukin-11 (IL-11) and thrombopoietin, are being actively pursued clinically, with IL-11 approved for treatment of thrombocytopenia. The development of these agents in general has been disappointing, and in part this reflects the inherent biology of these factors with a failure to match clinical need with physiological function. The delayed action of these factors is also a consequence of the intrinsic biology of megakaryocytes and platelets, and thus is likely to be limiting regardless of which factor is employed. In addition, the development of these agents has occurred at a time when there is something of a decreasing demand for platelets, at least in the context of chemotherapy-induced thrombocytopenia. This decrease is the result of increased use of blood stem cells to support intensive chemotherapy procedures, reduced thresholds for platelet transfusion, and a decreasing role for intensive chemotherapy. These issues are discussed.