Platelet autoantibody specificity and response to rhTPO treatment in patients with primary immune thrombocytopenia.

This retrospective study aimed to evaluate the relationship between plasma autoantibody species and rhTPO response in adult ITP patients who failed the first-line treatments. Plasma anti-glycoprotein (GP) IIb/IIIa and anti-GPIb/IX autoantibodies were detected in 47·2% and 40·6% of the 123 patients, respectively. Overall response rate to rhTPO treatment in patients without anti-GPIb/IX autoantibodies was significantly higher than patients with anti-GPIb/IX autoantibodies (82·2% vs. 60·0%, P = 0·006). By contrast, no statistical difference in response rate was observed between patients with or without anti-GPIIb/IIIa autoantibodies (74·1% vs. 72·3%, P = 0·819). Therefore, the presence of anti-GPIb/IX autoantibodies might serve as a predictive factor for poor response to rhTPO treatment in ITP.

Romiplostim in adults with newly diagnosed or persistent immune thrombocytopenia.

Introduction: Three distinct phases are recognized in immune thrombocytopenia (ITP): newly diagnosed (≤3 months after diagnosis), persistent (>3-12 months after diagnosis), and chronic (>12 months). Several international guidelines/expert recommendations have been released in the past 2 years regarding the treatment of newly diagnosed/persistent ITP. Areas covered: Across the guidelines/expert recommendations, thrombopoietin receptor agonists (TPO-RAs), including romiplostim (the focus of this review), are recommended in newly diagnosed or persistent ITP for patients who fail to respond to corticosteroids or intravenous immunoglobulin (or where these are contraindicated). To identify data relating to romiplostim in adults with newly diagnosed or persistent ITP, we conducted a search of PubMed (with no time limit applied) and abstracts from 2019 EHA/ASH meetings using the term 'romiplostim.' Expert opinion: The findings from nine clinical trials, six real-world studies and ten case reports provide insight into the early use of romiplostim, which could help to reduce exposure to the adverse effects associated with prolonged corticosteroid use, as well as reduce the risk of severe bleeding. Additionally, given the durable responses observed in patients with newly diagnosed/persistent ITP, as well as the potential for treatment-free responses following discontinuation, romiplostim might help to avoid the need for subsequent treatment.

Assessment of romiplostim immunogenicity in adult patients in clinical trials and in a global postmarketing registry.

Antibodies to first-generation recombinant thrombopoietin (TPO) neutralized endogenous TPO and caused thrombocytopenia in some healthy subjects and chemotherapy patients. The second-generation TPO receptor agonist romiplostim, having no sequence homology to TPO, was developed to avoid immunogenicity. This analysis examined development of binding and neutralising antibodies to romiplostim or TPO among adults with immune thrombocytopenia (ITP) in 13 clinical trials and a global postmarketing registry. 60/961 (6·2%) patients from clinical trials developed anti-romiplostim-binding antibodies post-baseline. The first positive binding antibody was detected 14 weeks (median) after starting romiplostim, at median romiplostim dose of 2 µg/kg and median platelet count of 29.5 × 109 /l; most subjects had ≥98·5% of platelet assessments showing response. Neutralising antibodies to romiplostim developed in 0·4% of patients, but were unrelated to romiplostim dose and did not affect platelet count. Thirty-three patients (3·4%) developed anti-TPO-binding antibodies; none developed anti-TPO-neutralising antibodies. In the global postmarketing registry, 9/184 (4·9%) patients with spontaneously submitted samples had binding antibodies. One patient with loss of response had anti-romiplostim-neutralising antibodies (negative at follow-up). Collectively, anti-romiplostim-binding antibodies developed infrequently. In the few patients who developed neutralising antibodies to romiplostim, there was no cross-reactivity with TPO and no associated loss of platelet response.

A Pilot Study on the Effect of Anti-Thrombopoietin Antibody on Platelet Count in Patients with Type 2 Diabetes.

Thrombopoietin (THPO) is a circulatory cytokine that plays an important role in platelet production. The presence of anti-THPO antibody relates to thrombocytopenia and is rarely seen in hematopoietic and autoimmune diseases. To date, there had been no reports that focused on the anti-THPO antibody in patients with type 2 diabetes mellitus (T2DM). To evaluate prevalence of the anti-THPO antibody in patients with T2DM and the relationship between anti-THPO antibody and platelet count, a cross-sectional study was performed on 82 patients with T2DM. The anti-THPO antibody was measured by ELISA using preserved sera and detected in 13 patients. The average platelet count was significantly lower in patients with the anti-THPO antibody than in those without the anti-THPO antibody. Multivariate linear regression analyses showed a significant relationship between the anti-THPO antibody and platelet count, after adjusting for other variables. To our best knowledge, this was the first report on the effect of the anti-THPO antibody on platelet count in patients with T2DM. Further investigation is needed to validate the prevalence and pathological significance of the anti-THPO antibody in patients with T2DM.

The non-glycosylated N-terminal domain of human thrombopoietin is a molten globule under native conditions.

Human thrombopoietin (hTPO) is a primary hematopoietic growth factor that regulates megakaryocytopoiesis and platelet production. The non-glycosylated form of 1-163 residues of hTPO (hTPO163 ) including the N-terminal active site domain (1-153 residues) is a candidate for treating thrombocytopenia. However, the autoantigenicity level of hTPO163 is higher than that of the full-length glycosylated hTPO (ghTPO332 ). In order to clarify the structural and physicochemical properties of hTPO163 , circular dichroism (CD) and differential scanning calorimetry (DSC) analyses were performed. CD analysis indicated that hTPO163 undergoes an induced-fit conformational change (+19.0% for helix and -16.7% for ß-strand) upon binding to the neutralizing antibody TN1 in a manner similar to the coupled folding and binding mechanism. Moreover, DSC analysis showed that the thermal transition process of hTPO163 is a multistate transition; hTPO163 is thermally stabilized upon receptor (c-Mpl) binding, as indicated with raising the midpoint (Tm ) temperature of the transition by at least +9.5 K. The conformational variability and stability of hTPO163 indicate that hTPO163 exists as a molten globule under native conditions, which may enable the induced-fit conformational change according to the type of ligands (antibodies and receptor). Additionally, CD and computational analyses indicated that the C-terminal domain (154-332 residues) and glycosylation assists the folding of the N-terminal domain. These observations suggest that the antibody affinity and autoantigenicity of hTPO163 might be reduced, if the conformational variability of hTPO163 is restricted by mutation and/or by the addition of C-terminal domain with glycosylation to keep its conformation suitable for the c-Mpl recognition.

Pathophysiology of immune thrombocytopenia.

PURPOSE OF REVIEW: Immune thrombocytopenia (ITP) is a common autoimmune bleeding disorder with as of yet, no established clinical prognostic or diagnostic biomarkers. Patients frequently experience a markedly decreased quality of life and may be at risk for severe/fatal haemorrhage. Here, we address discoveries in the pathogenesis of ITP, and novel therapeutic strategies in mouse models and human patients. Consolidation of these findings should be important in providing insight to establish future prognostic protocols as well as cutting-edge therapeutics to target refractory ITP. RECENT FINDINGS: It is unknown why a significant portion of ITP patients are refractory to standard treatments. Recent findings suggest distinct heterogeneity in ITP including antibody-mediated platelet activation, Fc-independent desialylated platelet clearance, attenuation of platelet-mediated hepatic thrombopoietin generation, and decreased CD8 T-suppressor generation. These mechanisms may partially explain clinical observations of increased refractoriness to standard therapies targeting classical Fc-dependent pathways. Moreover, these have initiated investigations into platelet desialylation as a diagnostic/prognostic marker and therapeutic target. SUMMARY: Recent evidence of distinct ITP pathophysiology has opened new exploratory avenues for disease management. We will discuss the utility of investigations into these mechanisms of ITP and its potential impact in our understanding of pathogenesis and future treatment strategies.

HSP27 is a partner of JAK2-STAT5 and a potential therapeutic target in myelofibrosis.

Heat shock protein 27 (HSP27/HSPB1) is a stress-inducible chaperone that facilitates cancer development by its proliferative and anti-apoptotic functions. The OGX-427 antisense oligonucleotide against HSP27 has been reported to be beneficial against idiopathic pulmonary fibrosis. Here we show that OGX-427 is effective in two murine models of thrombopoietin- and JAKV617F-induced myelofibrosis. OGX-427 limits disease progression and is associated with a reduction in spleen weight, in megakaryocyte expansion and, for the JAKV617F model, in fibrosis. HSP27 regulates the proliferation of JAK2V617F-positive cells and interacts directly with JAK2/STAT5. We also show that its expression is increased in both CD34+ circulating progenitors and in the serum of patients with JAK2-dependent myeloproliferative neoplasms with fibrosis. Our data suggest that HSP27 plays a key role in the pathophysiology of myelofibrosis and represents a new potential therapeutic target for patients with myeloproliferative neoplasms.

Top-down motif engineering of a cytokine receptor for directing ex vivo expansion of hematopoietic stem cells.

The technique to expand hematopoietic stem cells (HSCs) ex vivo is eagerly anticipated to secure an enough amount of HSCs for clinical applications. Previously we developed a scFv-thrombopoietin receptor (c-Mpl) chimera, named S-Mpl, which can transduce a proliferation signal in HSCs in response to a cognate antigen. However, a remaining concern of the S-Mpl chimera may be the magnitude of the cellular expansion level driven by this molecule, which was significantly less than that mediated by endogenous wild-type c-Mpl. In this study, we engineered a tyrosine motif located in the intracellular domain of S-Mpl based on a top-down approach in order to change the signaling properties of the chimera. The truncated mutant (trunc.) and an amino-acid substitution mutant (Q to L) of S-Mpl were constructed to investigate the ability of these mutants to expand HSCs. The result showed that the truncated and Q to L mutants gave higher and considerably lower number of the cells than unmodified S-Mpl, respectively. The proliferation level through the truncated mutant was even higher than that of non-transduced HSCs with the stimulation of a native cytokine, thrombopoietin. Moreover, we analyzed the signaling properties of the S-Mpl mutants in detail using a pro-B cell line Ba/F3. The data indicated that the STAT3 and STAT5 activation levels through the truncated mutant increased, whereas activation of the Q to L mutant was inhibited by a negative regulator of intracellular signaling, SHP-1. This is the first demonstration that a non-natural artificial mutant of a cytokine receptor is effective for ex vivo expansion of hematopoietic cells compared with a native cytokine receptor.

Involvement of thrombopoietin in acinar cell necrosis in L-arginine-induced acute pancreatitis in mice.

Thrombopoietin (TPO) plays an important role in injuries of different tissues. However, the role of TPO in acute pancreatitis (AP) is not yet known. The aim of the study was to determine the involvement of TPO in AP. Serum TPO was assayed in necrotizing pancreatitis induced by L-arginine in mice. Recombinant TPO and anti-TPO antibody were given to mice with necrotizing pancreatitis. Amylase, lipase, lactate dehydrogenase, myeloperoxidase activity and pancreatic water content were assayed in serum and tissue samples. Pancreas and lung tissue samples were also collected for histological evaluation. Immunohistochemistry of amylase α and PCNA were applied for the study of acinar regeneration and TUNEL assay for the detection of apoptosis in the pancreas. Increased levels of serum TPO were found in necrotizing pancreatitis. After TPO administration, more severe acinar necrosis was found and blockade of TPO reduced the acinar necrosis in this AP model. Acinar regeneration and apoptosis in the pancreas were affected by TPO and antibody treatment in necrotizing pancreatitis. The severity of pancreatitis-associated lung injury was worsened after TPO treatment, but attenuated after Anti-TPO antibody treatment. In conclusion, serum TPO is up-regulated in the necrotizing pancreatitis induced by L-arginine in mice and may be a risk factor for the pancreatic acinar necrosis in AP. As a pro-necrotic factor, blockade of TPO can attenuate the acinar necrosis in AP and may be a possible therapeutic intervention for AP.

Anti-erythropoietin and anti-thrombopoietin antibodies induced after administration of recombinant human erythropoietin.

Recombinant human erythropoietin (rhEPO) has been successfully used for correcting renal anemia. However, recent studies have raised some concerns about the safety of rhEPO treatment due to its immunogenic side effect - pure red cell aplasia (PRCA). We now report a case of development of anti-EPO neutralizing antibodies (Abs) implicated in thrombocytopenia as well as erythrocytopenia. A 35-year-old man had a history of administering rhEPO (epoetin alfa, epoetin beta and darbepoetin alfa) for 2years to treat renal anemia. The hematological parameters were collected. Anti-EPO, anti-platelet, and anti-thrombopoietin (TPO) Ab assays were performed to test the presence of autoreactive Abs. After performing antibody assays due to severe resistance to rhEPO treatment, a high titer of anti-EPO neutralizing Abs was detected. However, unexpectedly, this patient also showed thrombocytopenia rather than PRCA. We investigated the cause of the marked thrombocytopenia and found anti-TPO Abs in patient serum. To our best knowledge, this is the first report of the development of anti-TPO Abs during rhEPO treatment for anemia.

Optimal conditions for lentiviral transduction of engrafting human CD34+ cells.

Cytokines are required for γ-retroviral transduction of human CD34+ cells. However, cytokines may reduce engraftment of CD34+ cells and may not be necessary for their lentiviral transduction. We sought to optimize transduction and engraftment of human CD34+ cells using lentiviral vectors. Single 24 h transduction of human CD34+ cells with human immunodeficiency virus type 1 (HIV1)-based lentiviral vectors in media containing stem cell factor (SCF), FMS-like tyrosine kinase 3 (FLT3) ligand, thrombopoietin (each 100 ng ml⁻¹) and 10% fetal bovine serum was compared with various cytokine conditions during ex vivo culture and assayed using humanized xenograft mice for 6 months after transplantation. Serum-free media improved transduction efficiency of human CD34+ cells. Interleukin-3 (20 ng ml⁻¹) had little effect on transduction efficiency or engraftment. Threefold higher cytokine mixture (each 300 ng ml⁻¹) reduced engraftment of CD34+ cells. SCF alone (100 ng ml⁻¹) proved insufficient for maintaining engraftment ability and reduced transduction efficiency. Short-term prestimulation had little effect on transduction efficiency or engraftment, yet 24 h prestimulation showed higher transduction efficiency, higher gene expression levels and lower engraftment. In summary, 24 h prestimulation followed by single 24-h lentiviral transduction in serum-free media with SCF, FLT3 ligand and thrombopoietin yields high transduction efficiency to engrafting human CD34+ cells, and is applicable in human clinical gene therapy trials.

G-CSF-mediated thrombopoietin release triggers neutrophil motility and mobilization from bone marrow via induction of Cxcr2 ligands.

Emergency mobilization of neutrophil granulocytes (neutrophils) from the bone marrow (BM) is a key event of early cellular immunity. The hematopoietic cytokine granulocyte-colony stimulating factor (G-CSF) stimulates this process, but it is unknown how individual neutrophils respond in situ. We show by intravital 2-photon microscopy that a systemic dose of human clinical-grade G-CSF rapidly induces the motility and entry of neutrophils into blood vessels within the tibial BM of mice. Simultaneously, the neutrophil-attracting chemokine KC (Cxcl1) spikes in the blood. In mice lacking the KC receptor Cxcr2, G-CSF fails to mobilize neutrophils and antibody blockade of Cxcr2 inhibits the mobilization and induction of neutrophil motility in the BM. KC is expressed by megakaryocytes and endothelial cells in situ and is released in vitro by megakaryocytes isolated directly from BM. This production of KC is strongly increased by thrombopoietin (TPO). Systemic G-CSF rapidly induces the increased production of TPO in BM. Accordingly, a single injection of TPO mobilizes neutrophils with kinetics similar to G-CSF, and mice lacking the TPO receptor show impaired neutrophil mobilization after short-term G-CSF administration. Thus, a network of signaling molecules, chemokines, and cells controls neutrophil release from the BM, and their mobilization involves rapidly induced Cxcr2-mediated motility controlled by TPO as a pacemaker.

[The clinical significance of immune-related marker detection in idiopathic thrombocytopenic purpura].

OBJECTIVE: To assess the clinical significance of detecting the immune markers in idiopathic thrombocytopenic purpura (ITP). METHODS: The frequencies of circulating B cells secreting platelet-specific antibody, platelet-specific antibody, the percentage of T lymphocyte subsets, the percentage of reticulated platelet and the level of thrombopoietin in 64 ITP patients and 31 healthy controls were measured with enzyme-linked immunospot assay (ELISPOT), modified monoclonal antibody immobilization of platelet antigens assay (MAIPA), flow cytometry and sandwich enzyme-linked immunosorbent assay respectively. RESULTS: Compared with the controls [1.3 ± 0.5/10(5) peripheral blood mononuclear cell (PBMC), (0.33 ± 0.06, 0.41 ± 0.03), (22.08 ± 4.54)% and (8.19 ± 2.46)%], the frequencies of circulating B cells secreting platelet-specific antibody (7.6 ± 4.6/10(5) PBMC in acute ITP group, 5.3 ± 3.0/10(5) PBMC in chronic ITP group), platelet-specific antibody (including the anti-GPIIb/IIIa antibody, anti-GPIb/IX antibody) (0.51 ± 0.11, 0.48 ± 0.06 in acute ITP group; 0.49 ± 0.10, 0.46 ± 0.09 in chronic ITP group), the percentage of CD(8)(+) T Lymphocyte (27.09 ± 9.86)%, the percentage of reticulated platelet in ITP patients [the megakaryocyte cytosis group (24.85 ± 19.18)%, the normal megakaryocyte group (23.89 ± 18.90)%] were significantly increased (all P < 0.05). The frequencies of circulating B cells secreting platelet-specific antibody in acute ITP patients were notably increased (P < 0.05) compared to the chronic ITP patients. In T lymphocyte subsets, the percentage of CD(3)(+) T lymphocyte and CD(4)(+) T lymphocyte and the ratio of CD(4)(+)/CD(8)(+) in the patients with ITP [(60.88 ± 14.59)%, (28.41 ± 10.55)%, 1.18 ± 0.59] were notably decreased than those in the healthy controls [(69.89 ± 6.43)%, (35.38 ± 5.05)%, 1.64 ± 0.29, P < 0.05]. There was no apparent difference of the level of thrombopoietin between ITP patients with megakaryocyte cytosis (72.09 ± 41.64) and health controls (75.37 ± 26.32, P > 0.05), however, the level of thrombopoietin of ITP patients with normal megakaryocyte apparently increased (118.60 ± 70.72, P < 0.05). CONCLUSION: Detecting the frequencies of circulating B cells secreting platelet-specific antibody, platelet-specific antibody, the percentage of T lymphocyte subsets, the percentage of reticulated platelet and the level of thrombopoietin in the patients with ITP may improve the diagnosis and guide clinical therapy.

Treatment options for chronic refractory idiopathic thrombocytopenic purpura in adults: focus on romiplostim and eltrombopag.

Idiopathic thrombocytopenic purpura (ITP) is a platelet disorder that affects approximately 1 in 10,000 people. In adults, the rate of spontaneous remission is only 5%, and generally, it is a chronic disease persisting for more than 6 months. Chronic refractory ITP may be defined as the failure of any modality to keep the platelet count above 20 x 10(3)/mm(3) for an appreciable time without unacceptable toxicity. Many pharmacologic treatments have been used to manage chronic refractory ITP by attempting to increase platelet counts by decreasing the rate of destruction of these cells. They include, but are not limited to, azathioprine, danazol, dapsone, combination chemotherapy, cyclosporine, and rituximab. However, these therapies offer modest response rates and can cause adverse events that necessitate drug discontinuation. The recent United States Food and Drug Administration approval of the thrombopoietin mimetics, romiplostim and eltrombopag, has provided clinicians with a novel approach for treating chronic refractory ITP. By stimulating platelet production, these drugs offer patients with this disease an alternative to the other agents. The preapproval phase III study with subcutaneous romiplostim showed significantly higher overall response rates versus placebo in both splenectomized and nonsplenectomized patients (83% for romiplostim vs 7% for placebo, p<0.0001). Twenty-five percent of patients receiving romiplostim achieved a platelet count greater than 50 x 10(3)/mm(3) after 1 week, and 50% achieved this platelet count within 2-3 weeks. The preapproval phase III study with oral eltrombopag demonstrated that 70% of patients receiving 50 mg/day and 81% of patients receiving 75 mg/day achieved a platelet count of at least 50 x 10(3)/mm(3) by day 43 (p<0.001 vs placebo for both 50 and 75 mg). Forty-four percent and 62% of patients achieved a platelet count of at least 50 x 10(3)/mm(3) by day 8 with eltrombopag 50 and 75 mg/day, respectively. When deciding which of these agents to prescribe, considerations include oral versus injectable dosage form, adverse-event profiles, and patient adherence with both taking the drug and keeping clinic appointments for monitoring of platelet counts. Several studies are under way to evaluate these drugs in chronic refractory ITP as well as other disease states. Long-term data will also be needed to assess the safety and efficacy of these agents.

Inflammation stimulates thrombopoietin (Tpo) expression in rat brain-derived microvascular endothelial cells, but suppresses Tpo in astrocytes and microglia.

Thrombopoietin (Tpo) and its receptor (c-Mpl; TpoR), which primary regulate megakaryopoiesis and platelet production, are also expressed in the central nervous system (CNS). Increased Tpo concentrations are present in the cerebrospinal fluid (CSF) of some patients with bacterial or viral meningitis. Since previous data implicated a proapoptotic role of Tpo on newly generated neuronal cells, we herein elucidated the regulation of Tpo in primary rat neurons (e17), astrocytes, and microglia (p0-p3), as well as in brain-derived vascular endothelial cells of 3-week-old rats after exposure to bacterial lipopolysaccharide (LPS). LPS inhibited Tpo gene expression in astrocytes and microglia, but not in neurons, most likely due to absence of Toll-like receptor 4 in neurons. While Tpo mRNA expression recovered in astrocytes after 24 h, it remained suppressed in microglia. Furthermore, we detected Tpo mRNA expression in primary brain-derived vascular endothelial cells, which also express the TpoR. In these cells, LPS significantly up-regulated Tpo mRNA expression. TpoR mRNA and protein expression remained constitutive in all cell types. Thus, our data provide evidence for a cell-type-specific modulation of Tpo mRNA expression by inflammation in brain-derived cells. Transient down-regulation of Tpo expression in astrocytes and microglia may limit Tpo-induced neuronal cell death in inflammatory brain disorders.

Assessment of immunogenicity of romiplostim in clinical studies with ITP subjects.

Romiplostim is an Fc-peptide fusion protein that activates intracellular transcriptional pathways via the thrombopoietin (TPO) receptor leading to increased platelet production. Romiplostim has been engineered to have no amino acid sequence homology to endogenous TPO. Recombinant protein therapeutics can be at a risk of development of an antibody response that can impact efficacy and safety. Hence, a strategy to detect potential antibody formation to the drug and to related endogenous molecules can be useful. The immunogenicity assessment strategy involved both the detection and characterization of binding and neutralizing antibodies. The method for detection was based on a surface plasmon resonance biosensor platform using the Biacore 3000. Samples that tested positive for binding antibodies in the Biacore immunoassay were then tested in a neutralization assay. Serum samples from 225 subjects with immune thrombocytopenic purpura (ITP) dosed with romiplostim and 45 ITP subjects dosed with placebo were tested for romiplostim and TPO antibodies. Prior to romiplostim treatment, 17 subjects (7%) tested romiplostim antibody positive and 12 subjects (5%) tested TPO antibody positive for pre-existing binding antibodies. After romiplostim exposure, 11% of the subjects exhibited binding antibodies against romiplostim and 5% of the subjects with ITP showed binding antibodies against TPO. The antibodies against romiplostim did not cross-react with TPO and vice versa. No cases of anti-TPO neutralizing antibodies were detected in romiplostim-treated subjects. The incidence of anti-romiplostim neutralizing antibodies to romiplostim was 0.4% (one subject); this subject tested negative at the time of follow-up 4 months later. No impact on platelet profiles were apparent in subjects that had antibodies to romiplostim to date. In summary, administration of romiplostim in ITP subjects resulted in the development of a binding antibody response against romiplostim and TPO ligand. One subject developed a neutralizing antibody response to romiplostim that impacted the platelet counts of this subject. No neutralizing antibodies to endogenous TPO were observed.

Novel thrombopoietic agents: a review of their use in idiopathic thrombocytopenic purpura.

The underlying problem in idiopathic thrombocytopenic purpura (ITP) has traditionally been recognized as accelerated platelet destruction. However, recent studies have provided evidence that the pathophysiology of ITP is more complex, and impaired platelet production has emerged as one of the mechanisms contributing to the thrombocytopenia. On these grounds, second-generation thrombopoietic agents have been used in clinical trials to stimulate platelet production in ITP patients who are not responsive to standard treatments. These new molecules bear no structural resemblance to thrombopoietin (TPO) but still bind and activate the TPO receptor. Studies have been completed for two TPO receptor agonists: romiplostim (formerly AMG 531) and eltrombopag (formerly SB497115). Romiplostim is a recombinant protein defined as a peptibody. Results of phase I-II trials published recently demonstrated that romiplostim given as a weekly subcutaneous injection for 1-6 weeks results in doubling of platelet counts and an increase to >50 x 10(9)/L in most treated patients with minimal adverse effects. Eltrombopag is an orally available, small organic compound. In a randomized, double-blind, placebo-controlled phase III trial, ITP patients were given daily oral treatment with placebo or eltrombopag 50 mg. Platelet responses were observed in 59% of eltrombopag-treated patients and in 16% of patients in the placebo arm. No significant adverse events were seen. Other thrombopoietic agents in development, such as AKR-501 (formerly YM 477), appear promising in healthy volunteers. Ongoing phase III clinical trials will reveal the potential of these agents in the management of ITP prior to splenectomy and for long-term maintenance therapy, as well as their relative benefit compared with standard of care treatment.

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

Various agents to treat immune thrombocytopenic purpura (ITP) have been developed on the principle that stimulating the thrombopoietin (TPO) receptor would increase platelet production. First-generation agents--recombinant human thrombopoietin (rHuTPO) and pegylated recombinant human megakaryocyte growth and development factor (PEG rHuMGDF)--showed promise, but observations of antibody formation to PEG rHuMGDF led to the discontinuation of development of both agents. Second-generation agents--the TPO peptide mimetics, TPO non-peptide mimetics, and TPO agonist antibodies--have been developed to reduce or eliminate the problem of antigenicity. Clinical studies for some of these agents, such as AMG 531 (romiplosim, Nplate) and eltrombopag (Promacta), are demonstrating their relative safety and efficacy in increasing platelet counts in patients with ITP; AMG 531 and eltrombopag are in late-stage clinical development and are able to stimulate platelet production in patients with ITP. Some differences in safety profiles have been described and are undergoing further study. There are currently seven second-generation TPO receptor agonists that have been reported in the literature, representing the potential advantages--and continuing challenges--with this novel class of platelet-stimulating therapies for ITP and possibly thrombocytopenia in other disease states as well.