A Randomized, Double-Blind, Placebo-Controlled Study With Pegylated Recombinant Human Megakaryocyte Growth and Development Factor (PEG-rHuMGDF) as an Adjunct to Chemotherapy for Adults With De Novo Acute Myeloid Leukemia

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.

Dual Growth Factor (rhTPO + G-CSF) and Chemotherapy Combination Regimen for Elderly Patients with Acute Myeloid Leukemia: A Phase II Single-Arm Multicenter Study

Acute myeloid leukemia (AML) is a disease affecting older adults, although optimal strategies for treating such patients remain unclear. This prospective phase II, open-label, multicenter study was designed to assess the efficacy and safety of two hematologic growth factors, recombinant human thrombopoietin (rhTPO) and granulocyte colony-stimulating factor (G-CSF), in combination with decitabine, cytarabine, and aclarubicin (D-CTAG regimen) to treat older adults with newly diagnosed AML (Identifier: NCT04168138). The above agents were administered as follows: decitabine (15 mg/m² daily, days 1–5); low-dose cytarabine (10 mg/m² q12 h, days 3–9); rhTPO (15,000 U daily, days 2, 4, 6, 8, 10–24 or until >50×10⁹/L platelets); aclarubicin (14 mg/m² daily, days 3–6); and G-CSF (300 μg daily, days 2–9). We concurrently monitored historic controls treated with decitabine followed by cytarabine, aclarubicin, and G-CSF (D-CAG) only. After the first D-CTAG cycle, the overall response rate (ORR) was 84.2% (16/19), including 13 (73.7%) complete remissions (CRs) and three (15.8%) partial remissions. This CR rate surpassed that of the D-CAG treatment (p < 0.05). Median overall survival (OS) time in the D-CTAG group was 20.2 months (range, 4–31 months), compared with 14 months in the D-CAG group, and 1-year OS was 78%. The proportion of those experiencing grade III–IV thrombocytopenia was significantly lower for D-CTAG (57.9%) than for D-CAG (88.4%; p < 0.05). Ultimately, the curative effect of adding rhTPO was not inferior to that of D-CAG, and D-CTAG proved safer for elderly patients, especially in terms of hematologic toxicity. A prospective phase III randomized study is warranted to confirm these observations.

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.

Mechanisms and therapeutic prospects of thrombopoietin receptor agonists

The second-generation thrombopoietin (TPO) receptor agonists eltrombopag and romiplostim are potent activators of megakaryopoiesis and represent a growing treatment option for patients with thrombocytopenic hematological disorders. Both TPO receptor agonists have been approved worldwide for the treatment of children and adults with chronic immune thrombocytopenia. In the EU and USA, eltrombopag is approved for the treatment of patients with severe aplastic anemia who have had an insufficient response to immunosuppressive therapy and in the USA for the first-line treatment of severe aplastic anemia in combination with immunosuppressive therapy. Eltrombopag has also shown efficacy in several other disease settings, for example, chemotherapy-induced thrombocytopenia, selected inherited thrombocytopenias, and myelodysplastic syndromes. While both TPO receptor agonists stimulate TPO receptor signaling and enhance megakaryopoiesis, their vastly different biochemical structures bestow upon them markedly different molecular and functional properties. Here, we review and discuss results from preclinical and clinical studies on the functional and molecular mechanisms of action of this new class of drug.

Eltrombopag for use in children with immune thrombocytopenia

Eltrombopag is currently the only US Food and Drug Administration-approved thrombopoietin receptor agonist for the treatment of chronic immune thrombocytopenia (ITP) in children. This oral, once-per-day therapy has shown favorable efficacy and adverse effect profiles in children. Two multicenter, double-blind, placebo controlled clinical trials (PETIT [Efficacy and Safety Study of Eltrombopag in Pediatric Patients With Thrombocytopenia From Chronic Idiopathic Thrombocytopenic Purpura (ITP)] and PETIT2 [Study of a New Medication for Childhood Chronic Immune Thrombocytopenia (ITP), a Blood Disorder of Low Platelet Counts That Can Lead to Bruising Easily, Bleeding Gums, and/or Bleeding Inside the Body]) demonstrated efficacy in raising platelet counts, reducing bleeding, and reducing the need for concomitant ITP therapies with relatively few adverse effects. The most commonly reported drug-related adverse effects include headache, nausea, and hepatobiliary laboratory abnormalities. Long-term safety data in children are limited, and studies in adults have not revealed a clinically significant increased incidence of thrombosis, marrow fibrosis, or cataract formation. Eltrombopag has also been approved for treating refractory severe aplastic anemia (AA) and has potential for expanded use in ITP and severe AA as well as in other conditions associated with thrombocytopenia.

Tyrosyl-tRNA synthetase stimulates thrombopoietin-independent hematopoiesis accelerating recovery from thrombocytopenia

Aminoacyl-tRNA synthetases (aaRSs) catalyze aminoacylation of tRNAs in the first step of protein synthesis in the cytoplasm. However, in higher eukaryotes, they acquired additional functions beyond translation. In the present study, we show that an activated form of tyrosyl-tRNA synthetase (YRS^ACT) functions to enhance megakaryopoiesis and platelet production in vitro and in vivo. These findings were confirmed with human megakaryocytes differentiated from peripheral blood CD34⁺ hematopoietic stem cells and with human induced pluripotent stem (iPS) cells. The activity of YRS^ACT is independent of thrombopoietin (TPO), as evidenced by expansion of the megakaryocytes from iPS cell-derived hematopoietic stem cells from a patient deficient in TPO signaling. These findings demonstrate a previously unrecognized function of an aaRS which may have implications for therapeutic interventions.

New mechanisms behind blood cell formation continue to be uncovered, with therapeutic approaches for hematological diseases being of great interest. Here we report an enzyme in protein synthesis, known for cell-based activities beyond translation, is a factor inducing megakaryocyte-biased hematopoiesis, most likely under stress conditions. We show an activated form of tyrosyl-tRNA synthetase (YRS^ACT), prepared either by rationally designed mutagenesis or alternative splicing, induces expansion of a previously unrecognized high-ploidy Sca-1⁺ megakaryocyte population capable of accelerating platelet replenishment after depletion. Moreover, YRS^ACT targets monocytic cells to induce secretion of transacting cytokines that enhance megakaryocyte expansion stimulating the Toll-like receptor/MyD88 pathway. Platelet replenishment by YRS^ACT is independent of thrombopoietin (TPO), as evidenced by expansion of the megakaryocytes from induced pluripotent stem cell-derived hematopoietic stem cells from a patient deficient in TPO signaling. We suggest megakaryocyte-biased hematopoiesis induced by YRS^ACT offers new approaches for treating thrombocytopenia, boosting yields from cell-culture production of platelet concentrates for transfusion, and bridging therapy for hematopoietic stem cell transplantation.

Cytokine control of megakaryopoiesis

Platelets are anuclear blood cells required for haemostasis and are implicated in other processes including inflammation and metastasis. Platelets are produced by megakaryocytes, specialized cells that are themselves generated by a process of controlled differentiation and maturation of bone-marrow stem and progenitor cells. This process of megakaryopoiesis involves the coordinated interplay of transcription factor-controlled cellular programming with extra-cellular cues produced locally in supporting niches or as circulating factors. This review focuses on these external cues, the cytokines and chemokines, that drive production of megakaryocytes and support the terminal process of platelet release. Emphasis is given to thrombopoietin (Tpo), the major cytokine regulator of steady-state megakaryopoiesis, and its specific cell surface receptor, the Mpl protein, including normal and pathological roles as well as clinical application. The potential for alternative or supplementary regulatory mechanisms for platelet production, particularly in times of acute need, or in states of infection or inflammation are also discussed.

Intensive consolidation with G-CSF support: Tolerability, safety, reduced hospitalization, and efficacy in acute myeloid leukemia patients ≥60 years

The aim of this study was to evaluate the efficacy and feasibility of intensified consolidation therapy employing fludarabine and ARA-C in cycle 1 and intermediate-dose ARA-C (IDAC) in cycles 2 through 4, in elderly acute myeloid leukemia (AML) patients and to analyze the effects of pegfilgrastim on the duration of neutropenia, overall toxicity, and hospitalization-time during consolidation in these patients. Thirty nine elderly patients with de novo AML (median age 69.9 years) who achieved complete remission (CR) after induction-chemotherapy were analyzed. To examine the effect of pegfilgrastim on neutropenia and hospitalization, we compared cycles 2 and 4 where pegfilgrastim was given routinely from day 6 (IDAC-P) with cycle 3 where pegfilgrastim was only administered in case of severe infections and/or prolonged neutropenia. All four planned cycles were administered in 23/39 patients (59.0%); 5/39 patients (12.8%) received 3 cycles, 3/39 (7.7%) 2 cycles, and 8/39 (20.5%) one consolidation-cycle. The median duration of severe neutropenia was 7 days in cycle 2 (IDAC-P), 11.5 days in cycle 3 (IDAC), and 7.5 days in cycle 4 (IDAC-P) (P < .05). Median overall survival was 1.1 years and differed significantly between patients aged <75 and ≥75 years (P < .05). The probability to be alive after 5 years was 32%. Together, intensified consolidation can be administered in AML patients ≥60, and those who are <75 may benefit from this therapy. Routine administration of pegfilgrastim during consolidation shortens the time of neutropenia and hospitalization in these patients.

Condensed versus standard schedule of high-dose cytarabine consolidation therapy with pegfilgrastim growth factor support in acute myeloid leukemia

The aim of this cohort study was to compare a condensed schedule of consolidation therapy with high-dose cytarabine on days 1, 2 and 3 (HDAC-123) with the HDAC schedule given on days 1, 3 and 5 (HDAC-135) as well as to evaluate the prophylactic use of pegfilgrastim after chemotherapy in younger patients with acute myeloid leukemia in first complete remission. One hundred and seventy-six patients were treated with HDAC-135 and 392 patients with HDAC-123 with prophylactic pegfilgrastim at days 10 and 8, respectively, in the AMLSG 07-04 and the German AML Intergroup protocol. Time from start to chemotherapy until hematologic recovery with white blood cells >1.0 G/l and neutrophils >0.5 G/l was in median 4 days shorter in patients receiving HDAC-123 compared with HDAC-135 (P<0.0001, each), and further reduced by 2 days (P<0.0001) by pegfilgrastim. Rates of infections were reduced by HDAC-123 (P<0.0001) and pegfilgrastim (P=0.002). Days in hospital and platelet transfusions were significantly reduced by HDAC-123 compared with HDAC-135. Survival was neither affected by HDAC-123 versus HDAC-135 nor by pegfilgrastim. In conclusion, consolidation therapy with HDAC-123 leads to faster hematologic recovery and less infections, platelet transfusions as well as days in hospital without affecting survival.

Alternative agents to prophylactic platelet transfusion for preventing bleeding in people with thrombocytopenia due to chronic bone marrow failure: a meta-analysis and systematic review

BACKGROUND

People with thrombocytopenia due to bone marrow failure are vulnerable to bleeding. Platelet transfusions have limited efficacy in this setting and alternative agents that could replace, or reduce platelet transfusion, and are effective at reducing bleeding are needed.

OBJECTIVES

To compare the relative efficacy of different interventions for patients with thrombocytopenia due to chronic bone marrow failure and to derive a hierarchy of potential alternative treatments to platelet transfusions.

SEARCH METHODS

We searched for randomised controlled trials (RCTs) in the Cochrane Central Register of Controlled Trials (the Cochrane Library 2016, Issue 3), MEDLINE (from 1946), Embase (from 1974), CINAHL (from 1937), the Transfusion Evidence Library (from 1980) and ongoing trial databases to 27 April 2016.

SELECTION CRITERIA

We included randomised controlled trials in people with thrombocytopenia due to chronic bone marrow failure who were allocated to either an alternative to platelet transfusion (artificial platelet substitutes, platelet-poor plasma, fibrinogen concentrate, recombinant activated factor VII (rFVIIa), desmopressin (DDAVP), recombinant factor XIII (rFXIII), recombinant interleukin (rIL)6 or rIL11, or thrombopoietin (TPO) mimetics) or a comparator (placebo, standard of care or platelet transfusion). We excluded people undergoing intensive chemotherapy or stem cell transfusion.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened search results, extracted data and assessed trial quality. We estimated summary risk ratios (RR) for dichotomous outcomes. We planned to use summary mean differences (MD) for continuous outcomes. All summary measures are presented with 95% confidence intervals (CI).We could not perform a network meta-analysis because the included studies had important differences in the baseline severity of disease for the participants and in the number of participants undergoing chemotherapy. This raised important concerns about the plausibility of the transitivity assumption in the final dataset and we could not evaluate transitivity statistically because of the small number of trials per comparison. Therefore, we could only perform direct pairwise meta-analyses of included interventions.We employed a random-effects model for all analyses. We assessed statistical heterogeneity using the I2 statistic and its 95% CI. The risk of bias of each study included was assessed using the Cochrane 'Risk of bias' tool. The quality of the evidence was assessed using GRADE methods.

MAIN RESULTS

We identified seven completed trials (472 participants), and four ongoing trials (recruiting 837 participants) which are due to be completed by December 2020. Of the seven completed trials, five trials (456 participants) compared a TPO mimetic versus placebo (four romiplostim trials, and one eltrombopag trial), one trial (eight participants) compared DDAVP with placebo and one trial (eight participants) compared tranexamic acid with placebo. In the DDAVP trial, the only outcome reported was the bleeding time. In the tranexamic acid trial there were methodological flaws and bleeding definitions were subject to significant bias. Consequently, these trials could not be incorporated into the quantitative synthesis. No randomised trial of artificial platelet substitutes, platelet-poor plasma, fibrinogen concentrate, rFVIIa, rFXIII, rIL6 or rIL11 was identified.We assessed all five trials of TPO mimetics included in this review to be at high risk of bias because the trials were funded by the manufacturers of the TPO mimetics and the authors had financial stakes in the sponsoring companies.The GRADE quality of the evidence was very low to moderate across the different outcomes.There was insufficient evidence to detect a difference in the number of participants with at least one bleeding episode between TPO mimetics and placebo (RR 0.86, 95% CI 0.56 to 1.31, four trials, 206 participants, low-quality evidence).There was insufficient evidence to detect a difference in the risk of a life-threatening bleed between those treated with a TPO mimetic and placebo (RR 0.31, 95% CI 0.04 to 2.26, one trial, 39 participants, low-quality evidence).There was insufficient evidence to detect a difference in the risk of all-cause mortality between those treated with a TPO mimetic and placebo (RR 0.74, 95%CI 0.52 to 1.05, five trials, 456 participants, very low-quality evidence).There was a significant reduction in the number of participants receiving any platelet transfusion between those treated with TPO mimetics and placebo (RR 0.76, 95% CI 0.61 to 0.95, four trials, 206 participants, moderate-quality evidence).There was no evidence for a difference in the incidence of transfusion reactions between those treated with TPO mimetics and placebo (pOR 0.06, 95% CI 0.00 to 3.44, one trial, 98 participants, very low-quality evidence).There was no evidence for a difference in thromboembolic events between TPO mimetics and placebo (RR 1.41, 95%CI 0.39 to 5.01, five trials, 456 participants, very-low quality evidence).There was no evidence for a difference in drug reactions between TPO mimetics and placebo (RR 1.12, 95% CI 0.83 to 1.51, five trials, 455 participants, low-quality evidence).No trial reported the number of days of bleeding per participant, platelet transfusion episodes, mean red cell transfusions per participant, red cell transfusion episodes, transfusion-transmitted infections, formation of antiplatelet antibodies or platelet refractoriness.In order to demonstrate a reduction in bleeding events from 26 in 100 to 16 in 100 participants, a study would need to recruit 514 participants (80% power, 5% significance).

AUTHORS' CONCLUSIONS

There is insufficient evidence at present for thrombopoietin (TPO) mimetics for the prevention of bleeding for people with thrombocytopenia due to chronic bone marrow failure. There is no randomised controlled trial evidence for artificial platelet substitutes, platelet-poor plasma, fibrinogen concentrate, rFVIIa, rFXIII or rIL6 or rIL11, antifibrinolytics or DDAVP in this setting.

What is the role of novel thrombopoietic agents in the management of acute leukemia?

The role of novel thrombopoietic agents in the management of acute leukemia is a tale of two molecules, romiplostim and eltrombopag. Both are thrombopoietin (TPO) receptor agonists with somewhat different mechanisms of action. Romiplostim is a peptide TPO receptor agonist that activates the TPO receptor by binding to it just like TPO. Eltrombopag is a nonpeptide TPO receptor agonist that activates the TPO receptor by binding to the transmembrane domain. Both TPO receptor agonists increase platelet counts in healthy humans and in those with immune thrombocytopenia. This review focuses on the potential these agents may have in supportive care of patients with acute leukemia.

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.

Is ITP a thrombophilic disorder?

Immune thrombocytopenia (ITP) represents the epitome of acquired bleeding diseases for the hematologist. Stemming from the interest for the safety of thrombopoietin-receptor agonists (TPO-ra) romiplostim and eltrombopag, recent data have investigated if thrombotic risk is also increased in this disorder. In patients not treated with TPO-ra, a slightly higher risk of venous thrombosis (VTE) is consistently found in ITP, but not to a rate demanding special attention in the generality of cases. No significant increase of arterial thrombosis (AT) is apparent. However, age, splenectomy, and personal risk factors may put some ITP patient to a particularly higher risk of venous and arterial thrombosis (three to four times higher than the average subject). Patients exposed to TPO-ra present indirect evidence of a much higher risk of both AT and VTE. Unfortunately, no matched control population is available and the prospective and registrative nature of these studies may have emphasized the incidence of thrombosis, which was recorded as adverse event. The clinician should be able to individualize the best treatment for the patient, taking also into account the thrombotic risk, limiting active treatment of ITP to those patients really at risk of bleeding.

Growth factor use in medication-induced hematologic toxicity

Myelosuppression is a dose-limiting adverse effect with antineoplastic therapy and nonchemotherapy medications. Clinicians have data and guidelines to provide direction for the management of neutropenia and thrombocytopenia in patients with malignancies. Clinical situations outside oncology extrapolate these data along with limited data sets for those patients who demonstrate myelosuppressive effects from medications that are not traditionally considered cytotoxic. Pharmacological treatments can be used to help ameliorate the myelosuppressive toxicities. Recombinant technology has provided growth factors to counteract or lessen the degree of toxicity from myelosuppressive medications including chemotherapy. Clinical strategies and future trends on how to mitigate medication-related myelosuppression are discussed.

Thrombocytopenia in cancer patients

Thrombocytopenia is a frequent complication of cancer and its treatment. The causes of thrombocytopenia in cancer patients can be diverse and multifactorial. Systemic chemotherapy is the most frequent cause of thrombocytopenia. The degree and duration thrombocytopenia depends upon whether the chemotherapeutic treatment is myeloablative, as used in stem cell transplants, or non-myeloablative, as typically used in solid non-hematologic malignancies. Additional causes of significant thrombocytopenia include tumor involvement of bone marrow and spleen; microangiopathic disorders such as disseminated intravascular coagulation, thrombotic thrombocytopenic purpura or hemolytic uremia syndrome. Lymphoproliferative malignancies can also be associated with secondary immune thrombocytopenia. Due to the broad differential diagnosis associated with cancer related thrombocytopenia, a careful diagnostic evaluation is indicated. The goal of treatment should be to maintain a safe platelet count to allow effective treatment of the underlying malignancy, prevent bleeding complications and to minimize the use of platelet product transfusion.

The new thrombopoietic agenda: impact on leukemias and MDS

The two generations of thrombopoietin (TPO) receptor (R) agonists have had utility in a number of hematologic conditions. However their use has often been surprisingly complex and drawbacks have been revealed in certain conditions more than in others. The first-generation megakaryocyte growth and development factor (MGDF) was discontinued due to the production of antibodies against it that cross-reacted with native TPO. Nonetheless it was tested in a wide variety of thrombocytopenic conditions and showed unequivocal efficacy in increasing the number of platelets in certain ones. As a result of lessons learned with MGDF, second-generation TPO-R agonists romiplostim and eltrombopag were initially tested and have been approved for the treatment of chronic immune thrombocytopenia (ITP), thrombocytopenia in hepatitis C, and recently aplastic anemia. These agents have had more mixed outcomes in diseases such as acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS). Results of several studies will be discussed.

Recombinant human thrombopoietin alleviates infection-associated thrombocytopenia: a retrospective study in senile patients

OBJECTIVE

To examine the efficacy and adverse events of recombinant human thrombopoietin (rhTPO) in the treatment of infection-associated thrombocytopenia in senile patients.

METHODS

The current study is a retrospective analysis of the patients receiving rhTPO for infection-associated thrombocytopenia in our hospital.

RESULTS

Forty-nine cases were included in the analysis as rhTPO group. The absolute platelet count after treatment, increase in platelet count, and the overall response rate were considerably higher in the rhTPO group than that in the control group. Improvement in bleeding score was higher in the rhTPO treatment group than that in the control group (2.1 ± 5.4 vs 0.4 ± 1.7). Bleeding event was stopped in 68.2% of the patients after rhTPO treatment and in 35% of the patients in the control group (P = .032). A stratified analysis indicated that the therapeutic efficacy is much better in patients without organ failure.

CONCLUSION

Recombinant human TPO is effective in alleviating infection-associated thrombocytopenia and hemorrhage in senile patients, particularly if given prior to the emergence of organ failure.

Unique in vitro and in vivo thrombopoietic activities of ingenol 3,20 dibenzoate, a Ca(++)-independent protein kinase C isoform agonist

Thrombopoiesis following severe bone marrow injury frequently is delayed, thereby resulting in life-threatening thrombocytopenia for which there are limited treatment options. The reasons for these delays in recovery are not well understood. Protein kinase C (PKC) agonists promote megakaryocyte differentiation in leukemia cell lines and primary cells. However, little is known about the megakaryopoietic effects of PKC agonists on primary CD34+ cells grown in culture or in vivo. Here we present evidence that the novel PKC isoform-selective agonist 3,20 ingenol dibenzoate (IDB) potently stimulates early megakaryopoiesis of human CD34+ cells. In contrast, broad spectrum PKC agonists failed to do so. In vivo, a single intraperitoneal injection of IDB selectively increased platelets in mice without affecting hemoglobin or white counts. Finally, IDB strongly mitigated radiation-induced thrombocytopenia, even when administered 24 hours after irradiation. Our data demonstrate that novel PKC isoform agonists such as IDB may represent a unique therapeutic strategy for accelerating the recovery of platelet counts following severe marrow injury.

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..

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.

Thrombocytopenia and platelet transfusions in patients with cancer

Platelet transfusions are a critical component of the supportive care for patients receiving intensive therapy for hematologic malignancies. The platelet count "triggering" prophylactic transfusion has decreased over the years, and studies comparing a prophylactic versus a therapeutic transfusion approach are in progress. The evidence supporting the need for platelet transfusions prior to different invasive procedures is reviewed. Lastly, studies evaluating the use of thrombopoietic stimulating agents to reduce hemorrhage and decrease the need for platelet transfusions are discussed. To date, there is no evidence that this approach is of clinical utility.

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.

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.

Endogenous thrombopoietin levels during the clinical management of acute myeloid leukaemia

Thrombocytopenia represents a major problem in the management of acute myeloid leukaemia (AML). The data regarding the alterations of endogenous thrombopoietin (TPO) regulation during the clinical course of AML are limited. The aim of this study was to investigate endogenous TPO dynamics in association with platelets during the clinical course of AML. We serially measured both TPO and platelets concurrently over the entire treatment period of newly diagnosed patients receiving both remission induction and consolidation chemotherapies. The median concentration of TPO in AML patients at the initial diagnosis was 469.71 pg/ml and increased significantly during the aplastic period due to remission induction chemotherapy (median: 1085.33 pg/ml) but then decreased to a level (median: 45.26 pg/ml) encountered in the healthy control subjects (median: 56.90 pg/ml). In the cytopenic period due to consolidation treatment, TPO level again increased significantly to a high level (median: 891.38 pg/ml) during the platelet nadir, but decreased toward normal (median: 100.75 pg/ml) after the thrombocytopenic period had elapsed. In conclusion, endogenous TPO levels exhibit an inverse fluctuation in relation to platelet counts during the clinical course of AML. Pharmacological stimulation of thrombopoiesis in AML with novel molecules, including the recombinant thrombopoietins and the small peptide agonists, should be based on a critical administration strategy that must consider the endogenous levels of TPO. TPO levels in distinct AML disease states may explain the unsuccessful recombinant TPO trials and could help to design better strategies for 'pharmacological stimulation of thrombopoiesis' in AML.

Historical review: megakaryopoiesis and thrombopoiesis

The study of thrombopoiesis has evolved greatly since an era when platelets were termed "the dust of the blood," only about 100 years ago. During this time megakaryocytes were identified as the origin of blood platelets; marrow-derived megakaryocytic progenitor cells were functionally defined and then purified; and the primary regulator of the process, thrombopoietin, was cloned and characterized and therapeutic thrombopoietic agents developed. During this journey we continue to learn that the physiologic mechanisms that drive proplatelet formation can be recapitulated in cell-free systems and their biochemistry evaluated; the molecular underpinnings of endomitosis are being increasingly understood; the intracellular signals sent by engagement of a large number of megakaryocyte surface receptors have been defined; and many of the transcription factors that drive megakaryocytic fate determination have been identified and experimentally manipulated. While some of these biologic processes mimic those seen in other cell types, megakaryocytes and platelets possess enough unique developmental features that we are virtually assured that continued study of thrombopoiesis will yield innumerable clinical and scientific insights for many decades to come.

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.

Current status of growth factors in the treatment of acute myeloid and lymphoblastic leukemia

The safety of granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) in patients with acute leukemia has been well established in numerous clinical trials. The primary aim of these studies was to determine whether CSFs, when used as adjuncts to intensive chemotherapy, reduced the duration of neutropenia, prevented febrile neutropenia, infections, and hospitalization rates, and improved response and overall outcome in patients with acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL). Despite considerable efforts in divers clinical settings, the potential advantages of hematopoietic growth factors in the management of these leukemias remain inconclusive. In general, individual published trials have shown declines in the incidence and/or duration of neutropenia but have not consistently demonstrated a reduction in the overall frequency of infectious complications or the duration of hospitalization. Most protocols also have failed to show a benefit in terms of disease-free or overall survival. Nevertheless, improvements in "soft" clinical end points, such as incidence of severe infections, may be clinically important and contribute, even if only incrementally, to the patient's quality of life. Selection of those patients likely to benefit from growth factors in a specific clinical setting is a worthwhile endeavour.

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.

Hematopoietic Growth Factors Including Keratinocyte Growth Factor in Allogeneic and Autologous Stem Cell Transplantation

The aim of hematopoietic stem cell transplantation (HSCT) is to cure patients of malignancies, autoimmune diseases, and immunodeficiency disorders by redirecting the immune system: the often described graft-versus-leukemia (GVL) or graft-versus-tumor (GVT) effects. Unfortunately, fulfillment of this goal is often hampered by relapse of the underlying disease, graft-versus-host disease (GVHD), or severe opportunistic infections, which account for the majority of post-transplantation deaths. Moreover, studies of long-term survivors of transplantation indicate an accelerated immune aging due to the transplantation procedure itself, preceding chemo- or radiotherapy, and acute and chronic GVHD. Significant advances have been made towards overcoming these obstacles by enhancing immune reconstitution with hematopoietic growth factors (HGFs) such as granulocyte colony-stimulating factor (G-CSF) or erythropoietin (EPO) or through the application of cytokines. In addition, there are approaches to promote the thymic-dependent development of naive T cells, which are prepared for the interaction with a multitude of pathogens. Examples are the application of keratinocyte growth factor (KGF), neuroendocrine hormones such as growth hormone or prolactin, sex hormone ablation, or the invention of a three-dimensional artificial thymus based on a cytomatrix. Might these measures result in a higher rate of healthy and fully recovered patients? Here we review progress in each of these areas.

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.

Phase II trial of subcutaneous recombinant human interleukin 11 with subcutaneous recombinant human granulocyte-macrophage colony stimulating factor in patients with acute myeloid leukemia (AML) receiving high-dose cytarabine during induction: ECOG 3997

Randomized trials of substituting high-dose cytarabine (HiDAC) for standard dose cytarabine (SDAC) during induction therapy for newly diagnosed AML have not demonstrated an improvement in the complete remission (CR) rate. Phase II trials of the scheduled administration of HiDAC after SDAC suggest an improved outcome. The hematological complications of intensification are considerable. GM-CSF after chemotherapy improved the survival of older patients in a randomized trial. Recombinant human interleukin 11, a thrombopoietic cytokine, reduced the incidence of chemotherapy-induced thrombocytopenia in patients with solid tumors. Therefore, 34 patients were treated, with newly diagnosed AML less than 56 years of age, with daunorubicin 45 mg/m2 on days 1-3, cytarabine 100mg/m2 days 1-7 and cytarabine 2g/m2 for 12 h on days 8-10 (7+3+3). rhIL-11 (50 microg/kg/day,) and GM-CSF (250 microg/kg/day) were administered subcutaneously from day 11 until recovery. The complete remission rate was 59% (90% C.I. 43-73%). The median time to recovery of neutrophils to >500 and platelets to > or =20,000 microl(-1) was 27 days (95% C.I. 27-30 days) and 25 days (95% C.I. 24-29 days), respectively. The trial does not confirm the high CR rate observed in phase II trials, despite optimal supportive care.

Role of cytokines in the treatment of acute leukemias: a review

Myeloid growth factors, such as granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor, have been used to decrease the duration of chemotherapy-induced neutropenia and thereby reduce the incidence and severity of infections in various regimens used to treat acute myeloid leukemia and acute lymphoblastic leukemia. These growth factors have also been used to recruit dormant myeloid leukemia cells into the S phase of cell cycle in order to increase their susceptibility to the antileukemic effects of agents such as cytarabine. Multiple prospective randomized trials have examined the benefit and safety of the addition of growth factors before, during, and after chemotherapy. A reduction in the duration of neutropenia has been the most consistent finding; this has not been associated with stimulation of leukemia cells, the main concern of using this strategy. Unfortunately, few studies have reported a benefit in prolonging the duration of disease-free survival or overall survival. Other cytokines, including interleukins and thrombopoietin, have also been evaluated for their theoretical ability to recruit immune mechanisms to eradicate residual leukemia burden after chemotherapy, and to stimulate platelet production. In this review, we summarize the clinical experience with these growth factors in treating acute leukemias.