A Review of Romiplostim Mechanism of Action and Clinical Applicability.

Thrombocytopenia results from a variety of conditions, including radiation, chemotherapy, autoimmune disease, bone marrow disorders, pathologic conditions associated with surgical procedures, hematopoietic stem cell transplant (HSCT), and hematologic disorders associated with severe aplastic anemia. Immune thrombocytopenia (ITP) is caused by immune reactions that accelerate destruction and reduce production of platelets. Thrombopoietin (TPO) is a critical component of platelet production pathways, and TPO receptor agonists (TPO-RAs) are important for the management of ITP by increasing platelet production and reducing the need for other treatments. Romiplostim is a TPO-RA approved for use in patients with ITP in the United States, European Union, Australia, and several countries in Africa and Asia, as well as for use in patients with refractory aplastic anemia in Japan and Korea. Romiplostim binds to and activates the TPO receptor on megakaryocyte precursors, thus promoting cell proliferation and viability, resulting in increased platelet production. Through this mechanism, romiplostim reduces the need for other treatments and decreases bleeding events in patients with thrombocytopenia. In addition to its efficacy in ITP, studies have shown that romiplostim is effective in improving platelet counts in various settings, thereby highlighting the versatility of romiplostim. The efficacy of romiplostim in such disorders is currently under investigation. Here, we review the structure, mechanism, pharmacokinetics, and pharmacodynamics of romiplostim. We also summarize the clinical evidence supporting its use in ITP and other disorders that involve thrombocytopenia, including chemotherapy-induced thrombocytopenia, aplastic anemia, acute radiation syndrome, perisurgical thrombocytopenia, post-HSCT thrombocytopenia, and liver disease.

Clinical overview and practical considerations for optimizing romiplostim therapy in patients with immune thrombocytopenia.

The fundamental treatment goal for patients with immune thrombocytopenia (ITP) is reduced or ameliorated bleeding. Although various treatment options exist for the management of ITP, recent advances have led to the approval of three thrombopoietin receptor agonists (TPO-RAs; romiplostim, eltrombopag, and avatrombopag) in the United States and European Union. Current treatment guidelines for ITP indicate that medical therapy is preferred over surgical therapy and support the use of TPO-RAs as early as 3 months after disease onset. More recent data are available on the use of romiplostim in patients who have had ITP for <1 year, and romiplostim is now indicated for the treatment of adults who have not responded adequately to initial treatment, as well as children aged ≥1 year who have had ITP for ≥6 months. Here we review the role of romiplostim in the management of ITP, with a focus on efficacy and safety data, emerging data on early use (beginning within 3 months of disease onset) and treatment-free remission, and practical considerations for optimal management of ITP.

Thrombopoietin receptor agonists: ten years later.

The two thrombopoietin receptor agonists (TPO-RA), eltrombopag and romiplostim, were licensed in the US for treatment of immune thrombocytopenia (ITP) in 2008 and, since then, their use has progressively increased around the world; they are currently used in more than 100 countries. The six largest randomized controlled trials conducted in ITP have used one of these two agents. All studies have demonstrated a platelet response rate between 50-90%, depending on the criteria used, with good safety and tolerability. TPO-RA were shown to be effective in reducing bleeding and the need for concomitant or rescue medication. Many other investigations of their mechanism of effect, prospective and retrospective trials, and studies focusing on toxicity have been performed widening our knowledge of these two agents. Initial concerns on issues such as myelofibrosis have not been confirmed. Only a small number of patients develop moderate-severe reticulin fibrosis and/or collagen fibrosis; however, these are usually reversed after discontinuation of TPO-RA. Studies indicate, however, that TPO-RA may increase the risk of venous thromboembolism. Both TPO-RA are currently approved in patients with chronic ITP aged >1-year who are refractory to at least one other treatment. Eltrombopag has acquired two additional indications: severe aplastic anemia refractory to first-line treatment and hepatitis C patients undergoing treatment with interferon-ribavirin. Despite these wide-ranging studies, important questions still need to be answered. This summary review on TPO-RA will summarize what is known regarding efficacy in ITP, evaluate safety concerns in more depth, and focus on the questions that remain.

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.

Computational Design of Antiangiogenic Peptibody by Fusing Human IgG1 Fc Fragment and HRH Peptide: Structural Modeling, Energetic Analysis, and Dynamics Simulation of Its Binding Potency to VEGF Receptor.

Peptibodies represent a new class of biological therapeutics with combination of peptide activity and antibody-like properties. Previously, we discovered a novel peptide HRH that exhibited a dose-dependent angiogenesis-suppressing effect by targeting vascular endothelial growth factor receptors (VEGFRs). Here, we computationally designed an antiangiogenic peptibody, termed as PbHRH, by fusing the HRH peptide to human IgG1 Fc fragment using the first approved peptibody drug Romiplostim as template. The biologically active peptide of Romiplostim is similar with HRH peptide; both of them have close sequence lengths and can fold into a α-helical conformation in free state. Molecular dynamics simulations revealed that the HRH functional domain is highly flexible, which is functionally independent of Fc fragment in the designed PbHRH peptibody. Subsequently, the intermolecular interactions between VEGFR-1 domain 2 (D2) and PbHRH were predicted, clustered and refined into three representatives. Conformational analysis and energetic evaluation unraveled that the PbHRH can adopt multiple binding modes to block the native VEGF-A binding site of VEGFR-1 D2 with its HRH functional domain, although the binding effectiveness of HRH segments in peptibody context seems to be moderately decreased relative to that of free HRH peptide. Overall, it is suggested that integrating HRH peptide into PbHRH peptibody does not promote the direct intermolecular interaction between VEGFR-1 D2 and HRH. Instead, the peptibody may indirectly help to improve the pharmacokinetic profile and bioavailability of HRH.

Spotlight on romiplostim in the treatment of children with chronic immune thrombocytopenia: design, development, and potential place in therapy.

Primary immune thrombocytopenia (ITP) is an autoimmune disorder characterized by isolated thrombocytopenia. In approximately one-third of cases, the duration of thrombocytopenia will extend beyond 12 months consistent with a diagnosis of chronic ITP. Minor bleeding manifestations are common in chronic ITP while severe or life-threatening bleeding complications are uncommon. Moreover, spontaneous resolution occurs in the majority of children with chronic ITP necessitating treatment in only those children with ongoing bleeding manifestations or impairment in health-related quality of life (HRQOL). The characterization of thrombopoietin (TPO) and remarkable advancements in our understanding of the pathophysiology of ITP has led to the development of a new class of agents, the TPO-receptor agonists that have documented efficacy in the amelioration of thrombocytopenia and bleeding manifestations in chronic ITP. Romiplostim is a second-generation TPO-receptor agonist that has undergone limited evaluation in the treatment of chronic ITP in children. Evolving data suggest that romiplostim may be a safe and effective agent in the treatment of chronic ITP in children. Additional data are needed to confirm its ability to increase platelet counts, decrease bleeding manifestation, and improve the HRQOL of children and caregivers impacted by chronic ITP.

An insight into the thermodynamic characteristics of human thrombopoietin complexation with TN1 antibody.

Human thrombopoietin (hTPO) primarily stimulates megakaryocytopoiesis and platelet production and is neutralized by the mouse TN1 antibody. The thermodynamic characteristics of TN1 antibody-hTPO complexation were analyzed by isothermal titration calorimetry (ITC) using an antigen-binding fragment (Fab) derived from the TN1 antibody (TN1-Fab). To clarify the mechanism by which hTPO is recognized by TN1-Fab the conformation of free TN1-Fab was determined to a resolution of 2.0 Å using X-ray crystallography and compared with the hTPO-bound form of TN1-Fab determined by a previous study. This structural comparison revealed that the conformation of TN1-Fab does not substantially change after hTPO binding and a set of 15 water molecules is released from the antigen-binding site (paratope) of TN1-Fab upon hTPO complexation. Interestingly, the heat capacity change (ΔCp) measured by ITC (-1.52 ± 0.05 kJ mol(-1)  K(-1) ) differed significantly from calculations based upon the X-ray structure data of the hTPO-bound and unbound forms of TN1-Fab (-1.02 ∼ 0.25 kJ mol(-1)  K(-1) ) suggesting that hTPO undergoes an induced-fit conformational change combined with significant desolvation upon TN1-Fab binding. The results shed light on the structural biology associated with neutralizing antibody recognition.

The formin DIAPH1 (mDia1) regulates megakaryocyte proplatelet formation by remodeling the actin and microtubule cytoskeletons.

Megakaryocytes are highly specialized precursor cells that produce platelets via cytoplasmic extensions called proplatelets. Proplatelet formation (PPF) requires profound changes in microtubule and actin organization. In this work, we demonstrated that DIAPH1 (mDia1), a mammalian homolog of Drosophila diaphanous that works as an effector of the small GTPase Rho, negatively regulates PPF by controlling the dynamics of the actin and microtubule cytoskeletons. Moreover, we showed that inhibition of both DIAPH1 and the Rho-associated protein kinase (Rock)/myosin pathway increased PPF via coordination of both cytoskeletons. We provide evidence that 2 major effectors of the Rho GTPase pathway (DIAPH1 and Rock/myosin II) are involved not only in Rho-mediated stress fibers assembly, but also in the regulation of microtubule stability and dynamics during PPF.

The pharmacology and clinical application of thrombopoietin receptor agonists.

The discovery and application of thrombopoietin (TPO) and thrombopoietin receptor (TPOR) agonists have changed the clinical treatment of thrombocytopenia. These compounds exert favorable clinical effects without the adverse events caused by traditional treatments (e.g., corticosteroids, immunoglobulins, monoclonal antibodies and splenectomy). This review provides a synopsis of new agents that boost platelet production, especially the TPOR agonists, and highlights their pharmacological characteristics and clinical applications.

Evans syndrome after unrelated bone marrow transplantation for refractory cytopenia of childhood.

Post-transplant ES, which is often resistant to therapies, has seldom been described. This report describes a case of ES after UBMT for RCC. A five-yr-old boy developed RCC with no evidence of monosomy 7. Because no matching family donors were available for SCT and immunosuppressive therapy was ineffective, UBMT was performed when he was six yr old. The conditioning regimen included TAI (3 Gy) and administration of FLU, CY, and rabbit antithymocyte globulin. The recovery of blood cells was good. He displayed grade II acute GVHD involving only the skin. ES developed on day 66, with positive results for Epstein-Barr virus DNA and HHV 6. Cytopenia was resolved with treatment with RTX, GCV, an escalated dose of steroids, high-dose gammaglobulin, and romiplostim. No relapse has occurred since discontinuing steroids on day 177 and romiplostim on day 268. Post-SCT ES after UBMT is rare, and the risk factors and therapies are unclear. Prospective analysis and collection of cases from multiple centers are required for clarification.

Thrombopoietin from beginning to end.

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

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

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

Romiplostim for the treatment of primary immune thrombocytopenia.

Romiplostim is a peptibody designed to evade the immune response to recombinant thrombopoietin and to stimulate the production of new platelets. The pathophysiology underlying primary immune thrombocytopenia (ITP) has shown that not only accelerated peripheral platelet destruction but also suppression of the production of new platelets can be responsible for the persistence of thrombocytopenia. Several prospective and well-designed trials have shown that weekly subcutaneous administration of romiplostim induces more durable platelet responses and less treatment failure compared with placebo or standard of care, not only in adult but also pediatric chronic ITP patients regardless of splenectomy status. Treatment with romiplostim also resulted in less use of rescue medication and permitted the majority of patients receiving concurrent ITP drugs to reduce or discontinue these therapies, avoiding further immunosuppression. Romiplostim has been shown to have overall a very favorable safety profile and to be well tolerated relative to other ITP treatments.

Polyunsaturated fatty acids confer cryoresistance on megakaryocytes generated from cord blood and also enhance megakaryocyte production from cryopreserved cord blood cells.

BACKGROUND AIMS: Previous data have shown that the addition of docosahexanoic acid (DHA)/arachidonic acid (AA) has a beneficial effect on cytokine-mediated in vitro generation of megakaryocytes (MK) from umbilical cord blood (UCB).Cryopreservation forms an inherent part of UCB banking and MK progenitors are known to be very sensitive to the stresses of freezing. It is therefore imperative to generate functional cells from cryopreserved cells, and the generated cells need to be cryopreserved until used. In the present study, cryopreservation of ex vivo-expanded MK as well as MK generation from cryopreserved UCB samples was investigated. METHODS: MK generated with or without DHA/AA were cryopreserved in freezing medium containing 10% dimethyl sulfoxide (DMSO). Freezing efficacy was tested by quantitating MK after revival. Cryopreserved CD34(+) cells were cultured with stem cell factor (SCF) and thrombopoietin (TPO), in the presence and absence of DHA/AA for 10 days, and then quantitated for MK. Results. We observed a 1.5-3-fold increase in MK numbers, their progenitor content and their expression of phenotypic markers and MK-related transcription factors. DHA/AA sets showed a 2-5-fold improved engraftment in NOD/SCID mice. These data showed that the beneficial effect of DHA/AA obtained during MK expansion was not altered after freezing stress. The enhancement in MK generation obtained from fresh cord blood (CB) cells was reproduced with comparable efficiency when we used cryopreserved CB samples. CONCLUSIONS: Taken together, our data suggest that in vitro-generated DHA/AA MK survive cryoinjuries in a functionally better state. DHA/AA support a more efficient generation of MK from cryopreserved UCB.

Identification of the residues in the extracellular domain of thrombopoietin receptor involved in the binding of thrombopoietin and a nuclear distribution protein (human NUDC).

Thrombopoietin (TPO) and its receptor (Mpl) have long been associated with megakaryocyte proliferation, differentiation, and platelet formation. However, studies have also shown that the extracellular domain of Mpl (Mpl-EC) interacts with human (h) NUDC, a protein previously characterized as a human homolog of a fungal nuclear migration protein. This study was undertaken to further delineate the putative binding domain on the Mpl receptor. Using the yeast two-hybrid system assay and co-immunoprecipitation, we identified that within the Mpl-EC domain 1 (Mpl-EC-D1), amino acids 102-251 were strongly involved in ligand binding. We subsequently expressed five subdomains within this region with T7 phage display. Enzyme-linked immunosorbent binding assays identified a short stretch of peptide located between residues 206 and 251 as the minimum binding domain for both TPO and hNUDC. A series of sequential Ala replacement mutations in the region were subsequently used to identify the specific residues most involved in ligand binding. Our results point to two hydrophobic residues, Leu(228) and Leu(230), as having substantial effects on hNUDC binding. For TPO binding, mutations in residues Asp(235) and Leu(239) had the largest effect on binding efficacy. In addition, deletion of the conservative motif WGSWS reduced binding capacity for hNUDC but not for TPO. These separate binding sites on the Mpl receptor for TPO and hNUDC raise interesting implications for the cytokine-receptor interactions.

Zinc is not required for activity of TPO agonists acting at the c-Mpl receptor transmembrane domain.

Molecules that mimic the cytokine thrombopoietin that act by an atypical mechanism of binding to a receptor transmembrane (TM) domain are widely understood to require zinc for their biological activity. We investigated potent thrombopoietin mimetics from three chemical classes including the recently registered drug Eltrombopag, which operate via this novel mechanism, to determine whether zinc is essential for inducing cell proliferation. Using addition of zinc and a potent metal chelator, we show that the existing paradigm is incorrect and the compounds exhibit excellent thrombopoietin-mimetic activity even in the presence of high concentrations of EDTA. The implications of these findings for the mechanism of action are discussed.

Moving towards a new era in the management of chronic immune thrombocytopenia.

Immune thrombocytopenia (ITP) is an organ-specific autoimmune disease in which a low concentration of plasma thrombopoietin (TPO) contributes to the thrombocytopenia. Functional thrombopoietin deficiency in response to thrombocytopenia is central to the pathophysiology of chronic ITP. Decreased platelet production in ITP patients has been described only in recent years, however. Following the development of TPO-mimetics, it has become clear that the augmentation of thrombopoiesis is a key therapeutic target. TPO mimetics are novel effective treatments providing durable platelet responses in ITP. Two agents have reached clinical practice, the 'peptibody' romiplostim (Nplate(R)) approved for treatment of thrombocytopenia in patients with chronic ITP in Europe, Canada, Australia and the USA and the non-peptide TPO mimetic, eltrombopag (Promacta(R)), approved in the USA. This review summarises the background to the development of these agents and presents an update on data from randomised phase III trials and open-label studies. These novel drugs provide a noteworthy treatment option for patients with chronic ITP, in whom thrombocytopenia and bleeding risk have not been controlled by standard treatments. The first candidates for treatment in clinical practice are undoubtedly refractory patients with lack of response to other therapies or at continued risk for bleeding despite treatment. Appropriate inclusion of TPO mimetics into the treatment paradigm will most likely have a positive impact on the long-term outcome of ITP and allow carefully monitored patients to remain well controlled, with good tolerability for prolonged periods.

Review article: second-generation thrombopoietin agents for treatment of chronic idiopathic thrombocytopenic purpura in adults.

OBJECTIVE: To review the pharmacology, pharmacokinetics, efficacy, and safety of two new thrombopoietic (TPO) receptor agonists, romiplostim and eltrombopag, in the treatment of chronic idiopathic thrombocytopenic purpura (ITP) in adults. DATA SOURCES: A MEDLINE search was conducted (1966 to March 2009) using the search terms romiplostim, AMG 531, eltrombopag, SB-497115, idiopathic thrombocytopenic purpura. Articles on phases 1-3 clinical trials in patients with ITP were identified and reviewed. References from manufacturer information, and abstracts from recent hematology meetings, were also evaluated. STUDY SELECTION AND DATA EXTRACTION: Controlled clinical trials evaluating romiplostim and eltrombopag for treatment of chronic ITP in adults were selected from the data sources. All published relevant abstracts were also included. DATA SYNTHESIS: Limited randomized controlled trials and open-label ongoing long-term extension studies for romiplostim and eltrombopag, have shown that both TPO agonists are effective in improving the platelet count and reducing the bleeding episodes in adult patients with ITP unresponsive to at least one standard treatment. The most common adverse events associated with the drugs are mild to moderate headaches. The use of these agents has also been associated with rare but serious side-effects including bone marrow reticulin fibrosis, thrombotic events, and myeloid malignancies. CONCLUSIONS: Until more long-term follow-up data regarding the safety, as well as comparative studies that further define the role of TPO agonists versus other agents in the treatment of chronic ITP are available, these agents should be reserved for patients with ITP refractory or intolerant to standard therapy.

Modelling atypical small-molecule mimics of an important stem cell cytokine, thrombopoietin.

We report the first comprehensive 3D QSAR study of a large, structurally diverse set of compounds that act as atypical thrombopoietin (TPO) mimics by interacting with the transmembrane domain of the TPO receptor, c-MPL. These agonists of c-MPL were superimposed according to a pharmacophore hypothesis, resulting in 3D QSAR models of high statistical significance. The pharmacophore-based superimposition and comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) were used to derive the QSAR models relating structure to the published in vitro bioactivities of the TPO mimics. The CoMFA and CoMSIA models gave high correlation coefficients of the bioactivities with the derived fields, resulting in robust prediction of agonist activity of the superimposed compounds. The models have been interpreted in terms of the requirements for binding to the transmembrane domain of the TPO receptor.