A novel therapeutic approach for thrombocytopenia by minibody agonist of the thrombopoietin receptor

cMPL-Based Purification and Depletion of Human Hematopoietic Stem Cells: Implications for Pre-Transplant Conditioning

The transplantation of gene-modified autologous hematopoietic stem and progenitor cells (HSPCs) offers a promising therapeutic approach for hematological and immunological disorders. However, this strategy is often limited by the toxicities associated with traditional conditioning regimens. Antibody-based conditioning strategies targeting cKIT and CD45 antigens have shown potential in mitigating these toxicities, but their long-term safety and efficacy in clinical settings require further validation. In this study, we investigate the thrombopoietin (TPO) receptor, cMPL, as a novel target for conditioning protocols. We demonstrate that high surface expression of cMPL is a hallmark feature of long-term repopulating hematopoietic stem cells (LT-HSCs) within the adult human CD34+ HSPC subset. Targeting the cMPL receptor facilitates the separation of human LT-HSCs from mature progenitors, a delineation not achievable with cKIT. Leveraging this finding, we developed a cMPL-targeting immunotoxin, demonstrating its ability to selectively deplete host cMPLhigh LT-HSCs with a favorable safety profile and rapid clearance within 24 hours post-infusion in rhesus macaques. These findings present significant potential to advance our understanding of human hematopoiesis and enhance the therapeutic outcomes of ex vivo autologous HSPC gene therapies.

Monoclonal antibodies used for the management of haematological disorders

INTRODUCTION

Monoclonal antibodies Ab (MoAb) are increasingly becoming part of therapeutic armamentarium for haematologists and haemato-oncologists. This review brings together commonly used antibodies in one place for brevity and novel understanding.

AREAS COVERED

Pubmed and Scopus databases were explored focusing on MoAb in clinical haematological practice. Emphasis was given to current review articles. The data base was searched from 1997 till present. 24 different antibodies, most of which are in use were discussed. Antibodies are used for diverse conditions i.e. malignant and benign haematological conditions, treatment at various phases of stem cell transplantation. These antibodies were used both alone or in combination with various chemotherapy, targeted small molecules or as immunoconjugates. Some of the side effect profiles of these antibodies were common and some were unique. Unusual infections or organ dysfunctions were noted. Improved function of antibodies by protein engineering is also advancing rapidly. Dosage, frequency and route of administration depended on the convenience and condition for which the antibody is used.

EXPERT OPINION

: MoAbs are increasingly used in haematology practice either alone or in combination with other types of therapy for improved out come in various haematological conditions.

Selection of a Full Agonist Combinatorial Antibody that Rescues Leptin Deficiency In Vivo

Growth factor deficiency in adulthood constitutes a distinct clinical syndrome with significant morbidities including abnormal body composition, reduced energy, affective disturbances, dyslipidemia, and increased cardiovascular risk. Protein replacement therapies using recombinant proteins or enzymes represent the only approved treatment. Combinatorial antibodies have shown great promise as a new class of therapeutic molecules because they act as "mechanism-based antibodies" with both agonist and antagonist activities. Using leptin, a key hormone in energy metabolism, as an example, a function-guided approach is developed to select combinatorial antibodies with high potency and full agonist activity that substitute natural growth factors in vivo. The identified antibody shows identical biochemical properties and cellular profiles as leptin, and rescues leptin-deficiency in ob/ob mice. Remarkably, the antibody activates leptin receptors that are otherwise nonfunctional because of mutations (L372A and A409E). Combinatorial antibodies have significant advantages over recombinant proteins for chronical usage in terms of immunological tolerance and biological stability.

Anti-GPRC5D/CD3 Bispecific T-Cell-Redirecting Antibody for the Treatment of Multiple Myeloma

Although treatment advances over recent decades have significantly improved survival of patients with multiple myeloma, there is still an unmet medical need for more effective treatments. In this study, we identified G-protein-coupled receptor family C group 5 member D (GPRC5D) expression on the surface of malignant cells involved in multiple myeloma, but except for plasma cells and B cells, not at appreciable levels on normal hematopoietic cells and bone marrow progenitors, including hematopoietic stem cells. In addition, we constructed IgG-based anti-GPRC5D/CD3 bispecific T-cell-redirecting antibodies (GPRC5D TRAB), which suppressed the tumor growth of GPRC5D-positive myeloma cells through the activation of T cells in vitro and in vivo in xenograft models. Collectively, these findings suggest that GPRC5D is an antigen specific to multiple myeloma and a potential target of TRAB therapy.

Thrombocytopenia in Patients with Chronic Hepatitis C Virus Infection

Thrombocytopenia in patients with chronic hepatitis C virus (HCV) infection is a major problem. The pathophysiology is multifactorial, with auto-immunogenicity, direct bone marrow suppression, hypersplenism, decreased production of thrombopoietin and therapeutic adverse effect all contributing to thrombocytopenia in different measures. The greatest challenge in the care of chronic HCV patients with thrombocytopenia is the difficulty in initiating or maintaining IFN containing anti-viral therapy. Although at present, it is possible to avoid this challenge with the use of the sole Direct Antiviral Agents (DAAs) as the primary treatment modality, thrombocytopenia remains of particular interest, especially in cases of advanced liver disease. The increased risk of bleeding with thrombocytopenia may also impede the initiation and maintenance of different invasive diagnostic and therapeutic procedures. While eradication of HCV infection itself is the most practical strategy for the remission of thrombocytopenia, various pharmacological and non-pharmacological therapeutic options, which vary in their effectiveness and adverse effect profiles, are available. Sustained increase in platelet count is seen with splenectomy and splenic artery embolization, in contrast to only transient rise with platelet transfusion. However, their routine use is limited by complications. Different thrombopoietin analogues have been tried. The use of synthetic thrombopoietins, such as recombinant human TPO and pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMDGF), has been hampered by the development of neutralizing antibodies. Thrombopoietin-mimetic agents, in particular, eltrombopag and romiplostim, have been shown to be safe and effective for HCV-related thrombocytopenia in various studies, and they increase platelet count without eliciting any immunogenicity Other treatment modalities including newer TPO analogues-AMG-51, PEG-TPOmp and AKR-501, recombinant human IL-11 (rhIL-11, Oprelvekin), recombinant human erythropoietin (rhEPO), danazol and L-carnitine have shown promising early result with improving thrombocytopenia. Thrombocytopenia in chronic HCV infection remain a major problem, however the recent change in DAAs without IFN, as the frontline therapy for HCV, permit to avoid the dilemmas associated with initiating or maintaining IFN based anti-viral therapy.

The Thrombopoietin Receptor: Structural Basis of Traffic and Activation by Ligand, Mutations, Agonists, and Mutated Calreticulin

A well-functioning hematopoietic system requires a certain robustness and flexibility to maintain appropriate quantities of functional mature blood cells, such as red blood cells and platelets. This review focuses on the cytokine receptor that plays a significant role in thrombopoiesis: the receptor for thrombopoietin (TPO-R; also known as MPL). Here, we survey the work to date to understand how this receptor functions at a molecular level throughout its lifecycle, from traffic to the cell surface, dimerization and binding cognate cytokine via its extracellular domain, through to its subsequent activation of associated Janus kinases and initiation of downstream signaling pathways, as well as the regulation of these processes. Atomic level resolution structures of TPO-R have remained elusive. The identification of disease-causing mutations in the receptor has, however, offered some insight into structure and function relationships, as has artificial means of receptor activation, through TPO mimetics, transmembrane-targeting receptor agonists, and engineering in dimerization domains. More recently, a novel activation mechanism was identified whereby mutated forms of calreticulin form complexes with TPO-R via its extracellular N-glycosylated domain. Such complexes traffic pathologically in the cell and persistently activate JAK2, downstream signal transducers and activators of transcription (STATs), and other pathways. This pathologic TPO-R activation is associated with a large fraction of human myeloproliferative neoplasms.

A Novel Peptide Thrombopoietin Mimetic Designing and Optimization Using Computational Approach

Thrombopoietin receptor (TPOR) is a cytokine receptor protein present on the cell surface. The activation of TPOR by thrombopoietin (TPO) (a glycoprotein hormone) triggers an intracellular cascade of megakaryocytopoiesis for the formation of platelets. Recent studies on ex vivo megakaryocytopoiesis have evolved the possibilities of therapeutics uses. These findings have paved the way for the development of various TPO alternatives (recombinant TPO, peptide, and non-peptide TPO mimetics), which are useful in regenerative medicine. However, these alternatives possess various limitations such as induction of autoimmune effects, high production cost, low specificity, and hence activity. In the present study, a novel peptidic TPO mimetic was designed through computational studies by studying the binding sites of TPO and TMP to TPOR and analogs of known mimetics. Screening of combinatorial library was done through molecular docking using ClusPro. These studies indicated mimetic-9 as a significant molecule since it was found to have better binding score of −938.8 kcal/mol with seven hydrogen bonds and a high number of hydrophobic interactions, than known mimetic TMP with docking score of −798.4 kcal/mol and TMP dimer with docking score of −811.9 kcal/mol for TPOR. Mimetic9-TPOR complex was further assessed by the molecular dynamics simulation, and their complex was found to be stable with an RMSD value of 0.091 Å. While studying the parameters, mimetic-9 was found to have overall good physiochemical properties with positive grand average hydropathy (GRAVY) score and high instability index score and was found to be localized in the extracellular region. The designed mimetic-9 might prove to be a useful lead molecule for mimicking the role of TPO for in vitro platelet production with higher efficiency.

Anti-EGFR scFv tetramer (tetrabody) with a stable monodisperse structure, strong anticancer effect, and a long in vivo half-life

The development of single-chain variable fragments (scFvs) as therapeutic agents has the potential to reduce the high cost of antibody production, but the development process often impairs scFv functions such as binding affinity and pharmacokinetics. Multimerization is one strategy for recovering or enhancing these lost functions. Previously, we constructed several antiepidermal growth factor receptor (EGFR) scFv multimers by modifying linker length and domain order. Antitumor effects comparable with those of the currently approved anti-EGFR therapeutic antibodies were observed for scFv trimers. In the present study, we fractionated an anti-EGFR scFv tetramer from the intracellular soluble fraction of an Escherichia coli transformant. Compared with the trimer, the tetramer showed higher affinity, greater cancer cell growth inhibition, and prolonged blood retention time. Furthermore, the tetramer did not dissociate into the trimer or other smaller species during long-term storage (up to 33 weeks). Thus, our developed scFv tetramer is an attractive candidate next-generation anti-EGFR therapeutic antibody that can be produced via a low-cost bacterial expression system.

Calcium-dependent antigen binding as a novel modality for antibody recycling by endosomal antigen dissociation

The pH-dependent antigen binding antibody, termed a recycling antibody, has recently been reported as an attractive type of second-generation engineered therapeutic antibody. A recycling antibody can dissociate antigen in the acidic endosome, and thus bind to its antigen multiple times. As a consequence, a recycling antibody can neutralize large amounts of antigen in plasma. Because this approach relies on histidine residues to achieve pH-dependent antigen binding, which could limit the epitopes that can be targeted and affect the rate of antigen dissociation in the endosome, we explored an alternative approach for generating recycling antibodies. Since calcium ion concentration is known to be lower in endosome than in plasma, we hypothesized that an antibody with antigen-binding properties that are calcium-dependent could be used as recycling antibody. Here, we report a novel anti-interleukin-6 receptor (IL-6R) antibody, identified from a phage library that binds to IL-6R only in the presence of a calcium ion. Thermal dynamics and a crystal structure study revealed that the calcium ion binds to the heavy chain CDR3 region (HCDR3), which changes and possibly stabilizes the structure of HCDR3 to make it bind to antigen calcium dependently (PDB 5AZE). In vitro and in vivo studies confirmed that this calcium-dependent antigen-binding antibody can dissociate its antigen in the endosome and accelerate antigen clearance from plasma, making it a novel approach for generating recycling antibody.

The thrombopoietin receptor P106L mutation functionally separates receptor signaling activity from thrombopoietin homeostasis

The interaction between thrombopoietin (THPO) and its receptor c-Mpl regulates downstream cytokine signaling and platelet homeostasis. Hereditary mutations of c-Mpl can either result in loss-of-function and thrombocytopenia or in gain-of-function and thrombocythemia (HT), and are important models to analyze the mechanism of c-Mpl activity. We have analyzed the effect of the c-Mpl P106L gain-of-function and the nearby loss-of-function R102P and F104S mutations, which cause HT or thrombocytopenia, respectively, on posttranslational processing, intracellular trafficking, cell surface expression, and cell proliferation. In contrast to R102P and F104S, the P106L mutant confers cytokine-independent growth and stimulates downstream signaling after THPO treatment in Ba/F3 cells. Despite their opposite function, R102P and P106L, both lead to abnormal subcellular receptor distribution, lack of membrane localization, impaired glycosylation, and elevated THPO serum levels in effected patients. These findings indicate that the activation of downstream signaling by c-Mpl P106L does not require correct processing, trafficking, and cell surface expression of c-Mpl, whereas the negative feedback loop controlling THPO serum levels requires cell surface expression of the receptor. Thus, we propose that the P106L mutation functionally separates the activity of c-Mpl in downstream signaling from that in maintaining platelet homeostasis.

hGH promotes megakaryocyte differentiation and exerts a complementary effect with c-Mpl ligands on thrombopoiesis

hGH has a distinct capacity to promote the differentiation, especially the terminal differentiation of human primary megakaryocytes. hGH exerts a complementary and synergistic effect with c-Mpl ligands on thrombopoiesis.

Humanized VB22B minibody for human Mpl stimulates human megakaryopoiesis but does not enhance platelet aggregation

OBJECTIVE

Thrombopoietin stimulates megakaryopoiesis and platelet production by binding to its receptor, Mpl, on hematopoietic progenitor cells. Previously, a murine VB22B minibody for Mpl was shown to stimulate megakaryocyte colony formation in vitro and increase the platelet count in cynomolgus monkeys. In this study, we directly compared the effects of a humanized VB22B minibody (huVB22B) with those of thrombopoietin and eltrombopag under the hypothesis that Mpl agonists might have different biological effects on megakaryopoiesis, platelet production, intracellular signal transduction, and platelet function.

MATERIALS AND METHODS

Human bone marrow-derived CD34(+) cells were used for colony formation assays and proplatelet formation assays in vitro. The DNA ploidy in megakaryocytes was analyzed by flow cytometry. Phosphorylation of signal transducers and activators of transcription and mitogen-activated protein kinase was detected by Western blotting using specific antibodies. The effects of the Mpl agonists on platelet aggregation were analyzed by aggregometry using human platelets.

RESULTS

HuVB22B was as potent as thrombopoietin and eltrombopag in its ability to form mature megakaryocytes using human CD34(+) cells in vitro. It did not affect granulocyte-macrophage or erythroid colony formation. HuVB22B increased the number of proplatelet-forming megakaryocytes more efficiently than thrombopoietin or eltrombopag. Despite stronger phosphorylation of signal transducers and activators of transcription and mitogen-activated protein kinase compared with thrombopoietin in human platelets, huVB22B did not enhance adenosine diphosphate- or collagen-induced platelet aggregation. Eltrombopag did not enhance agonist-induced platelet aggregation.

CONCLUSIONS

We found that huVB22B, eltrombopag, and thrombopoietin have different effects on megakaryopoiesis, platelet function, and intracellular signaling. The precise mechanisms for these different biological effects regarding stimulation through the same receptor, Mpl, remain to be elucidated.

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.

Epithelial membrane protein-2 is a novel therapeutic target in ovarian cancer

PURPOSE

The tetraspan protein epithelial membrane protein-2 (EMP2) has been shown to regulate the surface display and signaling from select integrin pairs, and it was recently identified as a prognostic biomarker in human endometrial cancer. In this study, we assessed the role of EMP2 in human ovarian cancer.

EXPERIMENTAL DESIGN

We examined the expression of EMP2 within a population of women with ovarian cancer using tissue microarray assay technology. We evaluated the efficacy of EMP2-directed antibody therapy using a fully human recombinant bivalent antibody fragment (diabody) in vitro and ovarian cancer xenograft models in vivo.

RESULTS

EMP2 was found to be highly expressed in >70% of serous and endometrioid ovarian tumors compared with nonmalignant ovarian epithelium using a human ovarian cancer tissue microarray. Using anti-EMP2 diabody, we evaluated the in vitro response of nine human ovarian cancer cell lines with detectable EMP2 expression. Treatment of human ovarian cancer cell lines with anti-EMP2 diabodies induced cell death and retarded cell growth, and these response rates correlated with cellular EMP2 expression. We next assessed the effects of anti-EMP2 diabodies in mice bearing xenografts from the ovarian endometrioid carcinoma cell line OVCAR5. Anti-EMP2 diabodies significantly suppressed tumor growth and induced cell death in OVCAR5 xenografts.

CONCLUSIONS

These findings indicate that EMP2 is expressed in the majority of ovarian tumors and may be a feasible target in vivo.

New thrombopoietic growth factors

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

Development of thrombopoietin receptor agonists for clinical use

Thrombopoietin (TPO) is an essential hematopoietic cytokine for megakaryopoiesis. In 2002, we demonstrated that pegylated-recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) increased platelet counts in patients with chronic immune thrombocytopenic purpura (ITP) in a Phase I/II clinical trial. After the cessation of clinical trials of PEG-rHuMGDF because of severe thrombocytopenia or pancytopenia due to the development of the neutralizing antibody cross-reacting with endogenous TPO, second generation non-immunogenic TPO receptor agonists have been developed. A small molecule eltrombopag and Romiplostim were approved for clinical use by FDA in 2008 to treat patients with chronic ITP who are refractory to the prior therapy. Although the efficacy of both TPO receptor agonists is convincing for the refractory ITP, further investigation is necessary to assess the potential long-term side effects and clinical applications of these therapies for other thrombocytopenic conditions.

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

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

New-generation drugs that stimulate platelet production in chronic immune thrombocytopenic purpura

Idiopathic thrombocytopenic purpura is an acquired disease characterised by a low platelet count. Development of autoantibodies is a main cause of the disease. Although many patients have few symptoms, life-threatening bleeding can arise and hence, when platelet counts fall to unacceptable values treatment should be initiated. However, conventional immunosuppressive approaches can fail, perhaps because of the heterogeneous nature of the disease. Newly developed agents that increase platelet production by stimulating megakaryocytes-such as drugs that bind to the thrombopoietin receptor c-MPL-offer an alternative treatment strategy. Although initial thrombopoietin analogues caused adverse immune reactions, second-generation thrombopoietin-receptor agonists that are in late-stage clinical development seem promising. In particular, eltrombopag and romiplostim safely increase and maintain platelet production in patients with refractory disease. However, long-term side-effects are being assessed and the exact role of these agents in the overall treatment strategy of chronic idiopathic thrombocytopenic purpura remains to be established.

Advances in the understanding of congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (MIM #604498) is an extremely rare inherited bone marrow failure syndrome, usually presenting as a severe thrombocytopenia at birth due to ineffective megakaryocytopoiesis and no characteristic physical anomalies. Usually the isolated thrombocytopenia progresses to pancytopenia during the first years of life. The only curative therapy to date is haematopoietic stem cell transplantation. Most of the cases of congenital amegakaryocytic thrombocytopenia are caused by defective expression or function of the thrombopoietin receptor due to homozygous or compound heterozygous mutations in the gene MPL. The essential roles of thrombopoietin as a lineage specific regulator of platelet production and as a regulator of haematopoietic stem cell function are reflected in the haematological defects seen in affected individuals.

Hematopoietic growth factor mimetics: from concept to clinic

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

Discovery of novel non-peptide thrombopoietin mimetic compounds that induce megakaryocytopoiesis

We have identified a series of novel non-peptide compounds that activate the thrombopoietin-dependent cell line Ba/F3-huMPL. The compounds stimulated proliferation of Ba/F3-huMPL in the absence of other growth factors, but did not promote proliferation of the thrombopoietin-independent parent cell line Ba/F3. The thrombopoietin-mimetic compounds elicited signal-transduction responses comparable with recombinant human thrombopoietin, such as tyrosine phosphorylation of the thrombopoietin receptor, JAK (Janus kinase) 2, Tyk2 (tyrosine kinase 2), STAT (signal transducer and activator of transcription) 3, STAT5, MAPKs (mitogen-activated protein kinases), PLCγ (phospholipase Cγ), Grb2 (growth-factor-receptor-bound protein 2), Shc (Src homology and collagen homology), Vav, Cbl and SHP-2 (Src homology 2 domain-containing protein tyrosine phosphatase 2) and increased the number of CD41+ cells (megakaryocyte lineage) in cultures of human CD34+ bone-marrow cells (haematopoietic stem cells). These findings suggest that this series of compounds are novel agonists of the human thrombopoietin receptor and are possible lead compounds for the generation of anti-thrombocytopaenia drugs.

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.

The Pathophysiology of ITP Revisited: Ineffective Thrombopoiesis and the Emerging Role of Thrombopoietin Receptor Agonists in the Management of Chronic Immune Thrombocytopenic Purpura

Autoimmune thrombocytopenia (ITP) is characterized by autoantibody-mediated platelet destruction that can be demonstrated by shortened radiolabeled platelet survival. An additional role of ineffective thrombopoiesis was suggested by autologous platelet kinetic studies performed in the 1980s. Sera of patients with ITP have been demonstrated to inhibit megakaryocyte growth in culture supporting the concept of suboptimal platelet production as a contributing factor to the thrombocytopenia. The relatively modest rise in thrombopoietin (TPO) levels in thrombocytopenic patients with ITP has helped to identify the TPO receptor as a potential target for the treatment of ITP. Initial studies with recombinant TPO in patients with ITP were encouraging, and novel compounds designed to stimulate the TPO receptor and resultant pathways have been shown in randomized trials to be effective in raising the platelet count and sustaining it at safe levels. Adverse effects of these agents have been relatively mild, although rare serious events including increased bone marrow reticulin deposition, increased numbers of circulating blasts and thrombosis have occurred, and theoretic risks of stimulation of megakaryocytopoiesis and platelet activation remain a concern. As these agents become available it will be important to identify those patients who will most benefit from their use. The place of these drugs in the current management algorithms of ITP will evolve over time as results of clinical trials with these agents and experience with their use in the clinic clarify short-term and long-term efficacy and potential toxicities.

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

BACKGROUND

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

AIM

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

RESULTS

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

CONCLUSION

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

Cytokines for the Treatment of Thrombocytopenia

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

New thrombopoietic growth factors

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

Grafting of thrombopoietin-mimetic peptides into cystine knot miniproteins yields high-affinity thrombopoietin antagonists and agonists

Thrombopoietin is the primary regulator of platelet production. We exploited two naturally occurring miniproteins of the inhibitor cystine knot family as stable and rigid scaffolds for the incorporation of peptide sequences that have been shown to act as high-affinity thrombopoietin antagonists. Several miniproteins that antagonistically block thrombopoietin-mediated receptor activation were identified using a microscale reporter assay. Covalent miniprotein dimerization yielded potent bivalent c-Mpl receptor agonists with EC(50) values in the low nanomolar or picomolar range. One selected miniprotein-derived thrombopoietin agonist was almost as active as natural thrombopoietin with regard to stimulation of megakaryocyte colony formation from human bone marrow mononuclear cells, and elicited doubling of platelet counts in mice. Our data suggest that dimeric cystine knot miniproteins have considerable potential for the future development of small and stable receptor agonists. This approach may provide a promising strategy for pharmaceutical interference with other receptors activated by ligand-induced dimerization.

Megakaryocyte development and platelet production

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, and the proliferation, maturation and terminal differentiation of the megakaryocytic progenitors. Circulating levels of thrombopoietin (TPO), the primary growth-factor for the megakaryocyte (MK) lineage, induce concentration-dependent proliferation and maturation of MK progenitors by binding to the c-Mpl receptor and signalling induction. Decreased platelet turnover rates results in increased concentration of free TPO, enabling the compensatory response of marrow MKs to increased platelet production. C-Mpl activity is orchestrated by a complex cascade of signalling molecules that induces the action of specific transcription factors to drive MK proliferation and maturation. Mature MKs form proplatelet projections that are fragmented into circulating particles. Newly developed thrombopoietic agents operating via c-Mpl receptor may prove useful in supporting platelet production in thrombocytopenic state. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease state, and the new approaches to thrombopoietic therapy.

A rationally designed agonist antibody fragment that functionally mimics thrombopoietin

By using rational design, antibody fragments (Fabs) that mimic thrombopoietin (TPO) were created. A peptide with cMpl receptor-binding capability was grafted into different complementarity-determining regions of a fully human Fab scaffold. Functional presentation of the peptide was optimized by using phage display and cell-based panning. Select antibodies and fragments containing two grafted peptides were assayed for their ability to stimulate the cMpl receptor in vitro. Several candidates demonstrated agonist activity in an in vitro cMpl receptor signaling reporter assay, including Fab59, which was estimated to be equipotent to TPO. Fab59 additionally was able to effectively stimulate platelet production in normal mice. These rationally designed mimetic Fabs may provide a therapeutic intervention for thrombocytopenia while avoiding the potential generation of neutralizing antibodies to endogenous TPO. Furthermore, this study demonstrates a method by which short-lived linear peptides with binding activity may be converted to more stable and potent agonists capable of activating cell surface receptors.