A non-peptide compound which can mimic the effect of thrombopoietin via c-Mpl

The structure, function, and clinical use of the thrombopoietin receptor agonist avatrombopag

Thrombopoietin regulates platelet production through activation of the thrombopoietin receptor (TPO-R). TPO-R agonists (TPO-RAs) are available to treat thrombocytopenia in chronic immune thrombocytopenia (ITP), chronic liver disease (CLD) patients who are undergoing a procedure, severe aplastic anemia (SAA), and hepatitis C virus (HCV) infection. There are four TPO-RAs approved in the US and Europe: romiplostim (ITP), eltrombopag (ITP, SAA, HCV), avatrombopag (ITP, CLD), and lusutrombopag (CLD). It is important to understand pharmacological characteristics of these agents when evaluating treatment options. Avatrombopag interacts with the transmembrane domain of the TPO-RA and does not compete with endogenous thrombopoietin for TPO-R binding. Structural differences between avatrombopag and other TPO-RAs may impart differential downstream effects on cell signaling pathways, potentially resulting in clinically relevant differences in outcome. Avatrombopag has a favorable pharmacological profile with similar exposure in Japanese, Chinese, or Caucasian patients and no drug-drug interactions, food interactions, or potential for chelation.

Understanding cytokine and growth factor receptor activation mechanisms

Our understanding of the detailed mechanism of action of cytokine and growth factor receptors - and particularly our quantitative understanding of the link between structure, mechanism and function - lags significantly behind our knowledge of comparable functional protein classes such as enzymes, G protein-coupled receptors, and ion channels. In particular, it remains controversial whether such receptors are activated by a mechanism of ligand-induced oligomerization, versus a mechanism in which the ligand binds to a pre-associated receptor dimer or oligomer that becomes activated through subsequent conformational rearrangement. A major limitation to progress has been the relative paucity of methods for performing quantitative mechanistic experiments on unmodified receptors expressed at endogenous levels on live cells. In this article, we review the current state of knowledge on the activation mechanisms of cytokine and growth factor receptors, critically evaluate the evidence for and against the different proposed mechanisms, and highlight other key questions that remain unanswered. New approaches and techniques have led to rapid recent progress in this area, and the field is poised for major advances in the coming years which promise to revolutionize our understanding of this large and biologically and medically important class of receptors.

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.

Peptide immobilized on gold particles enhances cell growth

A multivalent ligand of thrombopoietin (TPO) was prepared by immobilization of mimetic peptides on gold particles. An effective peptide ligand containing cysteine was designed to enhance the growth of TPO-sensitive cells. The peptide was then immobilized on gold particles by self assembly. The multivalent ligand enhanced the growth of TPO-dependent cells and its activity was more than that of the monovalent ligand.

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.

AKR-501 (YM477) in combination with thrombopoietin enhances human megakaryocytopoiesis

OBJECTIVE

AKR-501 (YM477) is an orally active thrombopoietin (TPO) receptor agonist that mimics the biological effect of TPO in vitro and in vivo. Here, we report that AKR-501 in combination with TPO has additive effect on megakaryocytopoiesis.

MATERIALS AND METHODS

Granulocyte colony-stimulating factor-mobilized human peripheral blood CD34+ cells were cultured with AKR-501, TPO, or a combination of the two in serum-free liquid culture system. The numbers of hematopoietic progenitor cells, megakaryocytic progenitor cells, and megakaryocytes were measured using flow cytometry. Further, the effect of AKR-501 on TPO binding to TPO receptor was examined.

RESULTS

Both AKR-501 and TPO alone increased the number of megakaryocytes, and the maximum activities of AKR-501 and TPO were similar. Interestingly, in the presence of TPO concentrations producing maximal stimulation, the addition of AKR-501 increased the number of megakaryocytes to about 200% of that generated with TPO only. In the time course experiment, the combination of AKR-501 and TPO augmented the numbers of hematopoietic progenitor cells and colony-forming unit in culture in the early stages. Thus, the combination of AKR-501 and TPO enhanced not only the differentiation into megakaryocytes, but also the expansion of human hematopoietic progenitor cells. Further, AKR-501 did not inhibit TPO binding to the TPO receptor. This result indicated the possibility that AKR-501 and TPO may act simultaneously on the TPO receptor, and this could be responsible for their additive effect of on megakaryocytopoiesis.

CONCLUSIONS

This study suggests that AKR-501 would be useful for the treatment of thrombocytopenia even at high plasma levels of endogenous TPO following chemotherapy.

Characterization of novel non-peptide thrombopoietin mimetics, their species specificity and the activation mechanism of the thrombopoietin receptor

A series of non-peptide small compounds discovered to be thrombopoietin receptor agonists showed species specificity to humans. Compound I could induce megakaryocyte lineage from human bone marrow cells, but not from mouse, guinea pig or cynomolgus monkey bone marrow cells. To elucidate the mechanism, we identified the pivotal amino acid residue for the receptor activation by compound I by taking advantage of its species specificity. The response of compound I to three human/mouse chimeric receptors indicated the importance of the transmembrane domain. Comparison of amino acid sequences of the transmembrane domain of the thrombopoietin receptor between human and three animal species led us to hypothesize that histidine 499 is necessary for the reactivity to the thrombopoietin mimetics. We verified the hypothesis using two mutant receptors: the human thrombopoietin receptor mutant His499Leu and the mouse thrombopoietin receptor mutant Leu490His. These results should be helpful for structure-activity relationship studies and conducting in vivo studies of thrombopoietin mimetics.

Structure-activity relationships of xanthocillin derivatives as thrombopoietin receptor agonist

Xanthocillin derivatives, which show thrombopoietin receptor agonist activity, were synthesized through our developed method. Bioassay data suggest the importance of alkene geometry, the presence of substituents at the benzene ring that support hydrophobic character, and the moderate size of the molecule. One of the two isonitrile group of the natural product appears to be dispensable.

Small, nonpeptide p75NTR ligands induce survival signaling and inhibit proNGF-induced death

Studies showing that neurotrophin binding to p75NTR can promote cell survival in the absence of Trk (tropomyosin-related kinase) receptors, together with recent structural data indicating that NGF may bind to p75NTR in a monovalent manner, raise the possibility that small molecule p75NTR ligands that positively regulate survival might be found. A pharmacophore designed to capture selected structural and physical chemical features of a neurotrophin domain known to interact with p75NTR was applied to in silico screening of small molecule libraries. Small, nonpeptide, monomeric compounds were identified that interact with p75NTR. In cells showing trophic responses to neurotrophins, the compounds promoted survival signaling through p75NTR-dependent mechanisms. In cells susceptible to proneurotrophin-induced death, compounds did not induce apoptosis but inhibited proneurotrophin-mediated death. These studies identify a unique range of p75NTR behaviors that can result from isolated receptor liganding and establish several novel therapeutic leads.

[Effects of liquid crystal-related compounds on human megakaryocytopoiesis and thrombopoiesis]

In the present study, the effects of liquid crystal-related compounds on the megakaryocytopoiesis and thrombopoiesis were evaluated in vitro using CD34+ cells prepared from human placental and umbilical cord blood (CB). About 20 kinds of compounds were tested for their effects on the clonal growth of CB CD34+ megakaryocytic progenitor cells (CFU-Meg) in plasma clot culture. The compounds, dissolved in DMSO, were added to the cultures within a concentration range of 10-100 nM. When used alone, none of the compounds supported the clonal growth of CFU-Meg. However, when thrombopoietin (TPO) was used as a growth factor, three compounds increased CFU-Meg clonal growth significantly, producing approximately 1.3-1.4 fold increases in the total number of megakaryocyte colonies in comparison with the control. These compounds promoted mainly mature CFU-Meg-derived small colonies, suggesting that their target is relatively mature CFU-Meg. These effective compounds were examined in liquid culture supplemented with TPO alone for 14 days. Although there was no evident promotion of the total number of cells harvested from the culture, two compounds suppressed cell growth significantly. Only one compound enhanced the generation of CFU-Meg in the harvested cells. Although these results do not indicate a strong correlation between the chemical structure of each compound and biological effectiveness, the incorporation of phenylpyridine and phenylpyrimidine and binding of a hydroxyl residue into the structure may play an important role in the activity. Thus, liquid crystal-related compounds whose biological action was previously unknown have been shown to act as regulators of hematopoiesis.

A novel nonpeptidyl human c-Mpl activator stimulates human megakaryopoiesis and thrombopoiesis

NIP-004 is a novel synthetic compound developed to display human thrombopoietin (TPO) receptor (c-Mpl) agonist activity. NIP-004 displays species specificity, stimulating proliferation or differentiation of human c-Mpl–expressing cells such as UT-7/TPO and human CD34+ cells but not murine c-Mpl–expressing cells or cynomolgus monkey cells. To test the mechanism of its action, we constructed mutant forms of c-Mpl; murine c-MplL490H dis-played a response to NIP-004, whereas human c-MplH499L lost this response, indicating that histidine in the transmembrane domain of c-Mpl is essential for its activity. Because histidine is not present in the c-Mpl transmembrane domain of rats, hamsters, rhesus macaques, and cynomolgus monkeys, we examined the in vivo efficacy of NIP-004 using mice that received xenotransplants. In immunodeficient nonobese diabetic (NOD)/Shi-scid, IL-2Rγnull (NOG) mice receiving transplants of umbilical cord blood–derived CD34+ cells, NIP-004 increased human megakaryoblasts, mature megakaryocytes, and circulating human platelets 6-fold, the latter being morphologically and functionally indistinguishable from normal human platelets. These observations indicate that NIP-004 is a novel human c-Mpl activator and induces human thrombopoiesis.

New benzimidazoles as thrombopoietin receptor agonists

A novel benzimidazole series of small-molecule thrombopoietin receptor agonists has been discovered. Herein, we discuss the preliminary exploration of structure-activity relationships within this chemotype.

Xanthocillins as thrombopoietin mimetic small molecules

Four xanthocillins (1-4), including a new compound 4, were isolated from cultured marine fungus Basipetospora sp. as thrombopoietin (TPO) mimics. Compounds 1-4 promoted the proliferation of a TPO-sensitive human leukemia cell line, UT-7/TPO, and UT-7/EPO-mpl, genetically engineered to express c-Mpl, a receptor for TPO in dose-dependent manners. However, the proliferation of UT-7/EPO, a parental cell line of UT-7/EPO-mpl that was devoid of TPO receptor, was not affected by them. Thrombopoietic action of compound 1 was nearly as potent as that of TPO, inducing cell proliferation at a concentration ranging from 1 to 100nM. Compound 1 also induced the phosphorylation of several proteins, including Janus kinase 2 (Jak2), signal transducers, and activators of transcription-3 (STAT3) and STAT5 in the UT-7/EPO-mpl cell line, but not in the UT-7/EPO cell line. These data indicated that xanthocillins are putative agonists for c-Mpl, as their cellular actions were analogous to those of TPO.

Primary and secondary thrombocytosis in childhood

This review summarizes current data on the pathomechanisms and clinical aspects of primary and secondary thrombocytosis in childhood. Primary thrombocytosis is extremely rare in childhood, mostly diagnosed at the beginning of the second decade of life. As in adults, the criteria of the Polycythemia Vera Group are appropriate to diagnose primary thrombocytosis. The pathomechansims of non-familial forms are complex and include spontaneous formation of megakaryopoietic progenitors and increased sensitivity to thrombopoietin (Tpo). Familial forms can be caused by mutations in Tpo or Tpo receptor (c-mpl) genes. These mutations result in overexpression of Tpo, sustained intracellular signalling or disturbed regulation of circulating Tpo. Treatment of primary thrombocytosis is not recommended if platelet counts are <1500/nl and bleeding or thrombosis did not occur in patient's history. In severe cases, decision on treatment should weigh potential risks of treatment options (hydroxyurea, anagrelide) against expected benefits for preventing thrombosis or haemorrhage. Secondary thrombocytosis is frequent in children, in particular in the first decade of life. Hepatic Tpo production is stimulated in acute response reaction to a variety of disorders. Thrombosis prophylaxis is not required, even at platelet counts >1000/nl, except for cases with additional prothrombotic risk factors.

Reversal of antigen-dependent signaling by two mutations in antibody/receptor chimera: implication of inverse agonism in cytokine receptor superfamily

Understanding the receptor activation mechanism is essential for the rational design of pharmacologically active ligand molecules. However, the activation mechanism of most cytokine receptors remains still unclear, and while agonism and antagonism have been described for ligand-mimetic peptides, there has been no report of inverse agonism that has been characterized for G protein-coupled receptors (GPCRs). To explore the activation mechanism of cytokine receptors, here we tried to investigate how agonism and antagonism could be altered by randomizing antibody variable region of an antibody/cytokine receptor chimera recognizing hen egg lysozyme (HEL) as an agonist. Based on our previous finding that the co-expression of V(H)-gp130 and V(L)-erythropoietin receptor (EpoR) chimeras transduced strict and efficient HEL-dependent cell growth signal, a V(H)-gp130 library encoding four randomized CDR2 residues was retrovirally infected to IL-3-dependent Ba/F3 cells already transfected with V(L)-EpoR. The selection without IL-3 resulted in a clonal expansion of the transduced cells, and interestingly some of which showed HEL dose-dependent growth suppression. Our results clearly indicate that agonism and antagonism of the antibody/cytokine receptor chimera can be readily switched by a subtle modification of the ligand binding domain as well as that of GPCRs, also implying the existence of inverse agonism in cytokine receptor superfamily.

Induction of megakaryocytopoiesis and thrombocytopoiesis by JTZ-132, a novel small molecule with thrombopoietin mimetic activities

We report in this paper that a novel small molecule, JTZ-132, induced growth and differentiation of megakaryocytic progenitor cells and improved thrombocytopenia in myelosuppressed mice. JTZ-132 stimulated proliferation of UT-7/TPO cells, a cell line highly sensitive to thrombopoietin (TPO), and exhibited full efficacy comparable to TPO with an approximate EC50 (median effective concentration) value of 0.43 μM, whereas little proliferation was observed in a TPO-insensitive cell line, UT-7/EPO, and human carcinoma cell line, HCT116. Signal transduction studies revealed that JTZ-132 induced tyrosine phosphorylation of c-Mpl, Janus kinase-2 (JAK2), and signal transducers and activators of transcription 5 (STAT5) in UT-7/TPO cells as well as TPO. JTZ-132 increased the number of megakaryocyte-specific marker, CD61+ and CD41+, cells in cultures of mouse and human bone marrow cells, respectively, and the colonyforming unit megakaryocytes in mouse bone marrow cells. In vivo experiments in x-ray irradiation– or busulfan injection–induced myelosuppressed mice demonstrated that subcutaneously injected JTZ-132 at 30 mg/kg showed significantly higher platelet number at nadir and accelerated platelet recovery without affecting white blood cell number. These data suggest that JTZ-132 is a novel stimulator of megakaryocytopoiesis and thrombocytopoiesis in vitro and in vivo with TPO mimetic activities and that it is useful for the treatment of thrombocytopenia.

A potential therapeutic role for small nonpeptidyl compounds that mimic human granulocyte colony-stimulating factor

Granulocyte colony-stimulating factor (G-CSF) stimulates the proliferation of bone marrow granulocytic progenitor cells and promotes their differentiation into granulocytes. G-CSF is therefore an important component of immune defense against pathogenic microorganisms: recombinant human G-CSF (rhG-CSF) is used to treat patients with a variety of neutropenias. In the present study, we screened approximately 10 000 small nonpeptidyl compounds and found 3 small compounds that mimic G-CSF in several in vitro and in vivo assays. These compounds induced G-CSF-dependent proliferation, but had no effect on interleukin-3-dependent, interleukin-2-dependent, interleukin-10-dependent, thrombopoietin (TPO)-dependent, or erythropoietin (EPO)-dependent proliferation. Each compound induced the phosphorylation of signal transducers and activators of transcription-3 (STAT3) and mitogen-activated protein kinase (MAPK) in a G-CSF-dependent cell line and in human neutrophils. In addition, these compounds induced hematopoietic colony formation from primary rat bone marrow cells in vitro. When subcutaneously injected into normal rats, they caused an increase in peripheral blood neutrophil counts. Furthermore, when they were administered to cyclophosphamide-induced neutropenic rats, blood neutrophil levels increased and remained elevated up to day 8. We therefore suggest that these small nonpeptidyl compounds mimic the activity of G-CSF and may be useful in the treatment of neutropenic patients.

A peptide mimetic of human interferon (IFN)-beta

Type I interferons (IFNs) are cytokines that are used clinically as antiviral and antitumour agents. The interaction of IFNs with their heterodimeric type I IFN receptor comprised of IFNAR1 and IFNAR2 is a first step to inducing biological actions. Here, we describe the successful mimicry of IFN-beta by a peptide isolated by phage-display screening using a neutralizing anti-IFN-beta monoclonal antibody. The 15-mer peptide, designated SYR6, was shown to compete with IFN-beta for binding to type I IFN receptor in a concentration-dependent manner, and was shown to elicit antiviral activity on cultured cells. This antiviral activity was not eliminated in the presence of neutralizing monoclonal antibodies to IFN-alpha, -beta and -gamma, and a low concentration of soluble type I IFN receptor, suggesting that it was not due to IFN contamination or the induction of endogenous IFNs by SYR6. This peptide might be a potent agonist to provide a mechanism of activating heterodimeric cytokine receptors.

Thrombopoietin as a drug: biologic expectations, clinical realities, and future directions

After the cloning of thrombopoietin (c-mpl ligand, Tpo) in 1994, 2 recombinant thrombopoietic growth factors, full-length glycosylated recombinant human Tpo (reHuTPO) and polyethylene glycol conjugated megakaryocyte growth and development factor (PEG-reHuMGDF), have been studied in humans in a variety of clinical settings. Both thrombopoietins are generally well tolerated if administered intravenously (IV). The c-mpl ligands produce a dose-related enhancement of platelet levels, reduce nonmyeloablative chemotherapy-induced mild thrombocytopenia, and mobilize hematopoietic progenitors. On September 11, 1998, the development of PEG-reHuMGDF was suspended in the U.S., due to formation of the neutralizing anti-Tpo antibody. Those neutralizing antibodies lead to thrombocytopenia and pancytopenia in some patients receiving subcutaneous (SC) PEG-reHuMGDF. Japanese investigators indicate that the probability of antibody formation against PEG-reHuMGDF is low when the drug is administered IV instead of SC. reHuTPO has a more favorable safety profile from the point of antibody production. The c-mpl ligands can improve apheresis yields when administered to normal platelet donors. Preliminary data about the use of PEG-reHuMGDF in myelodysplasia, aplastic anemia, and immune thrombocytopenic purpura are promising. Tpo is usually not effective in myeloablative thrombocytopenia when bone marrow hematopoietic progenitors are not present. The major obstacle for the thrombopoietins is their delayed action for managing clinical thrombocytopenia. This review will focus on the biologic basis, current clinical experience, and future directions for the use of thrombopoietic molecules as drugs. The identification of a safe, effective, and potent pharmacologic platelet growth factor could significantly improve the management of thrombocytopenia-induced bleeding.

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

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

Cytokines as therapeutic drugs

Cytokines are a growing group of proteins that are responsible for the communication of cells of the immune system, hematopoietic cells, and other cell types. They play a dominant role in various diseases, particularly in promoting and perpetuating inflammation. Cytokine production is a reaction of the body to a pathologic state to restore homeostasis. In such cases, the therapeutic intervention should support the reaction of the body by giving the cytokine itself (agonistic therapeutics). In other cases, manifestation of a disease results from an overproduction of cytokines, making cytokine antagonists desirable therapeutic drugs. Furthermore, cytokines may be good candidates as cancer therapeutics, especially to support the restoration of blood cell populations after chemotherapy or radiation.

Erythropoietin mimetics derived from solution phase combinatorial libraries

The erythropoietin receptor (EPOr) is activated by ligand-induced homodimerization, which leads to the proliferation and differentiation of erythroid progenitors. Through the screening of combinatorial libraries of dimeric iminodiacetic acid diamides, novel small molecule binders of EPOr were identified in a protein binding assay. Evaluation of a series of analogues led to optimization of binding subunits, and these were utilized in the synthesis of higher order dimer, trimer, and tetramer libraries. Several of the most active EPOr binders were found to be partial agonists and induced concentration-dependent proliferation of an EPO-dependent cell line (UT-7/EPO) while having no effect on a cell line lacking the EPOr (FDC-P1). An additional compound library, based on a symmetrical isoindoline-5,6-dicarboxylic acid template and including the optimized binding subunits, was synthesized and screened leading to the identification of additional EPO mimetics.

Properties of small molecules affecting insulin receptor function

Small molecules with insulin mimetic effects and oral availability are of interest for potential substitution of insulin injections in the treatment of diabetes. We have searched databases for compounds capable of mimicking one epitope of the insulin molecule known to be involved in binding to the insulin receptor (IR). This approach identifies thymolphthalein, which is an apparent weak agonist that displaces insulin from its receptor, stimulates auto- and substrate phosphorylation of IR, and potentiates lipogenesis in adipocytes in the presence of submaximal concentrations of insulin. The various effects are observed in the 10(-5)-10(-3) M range of ligand concentration and result in partial insulin activity. Furthermore, analogues of the related phenol red and fluorescein molecules fully displace insulin from the IR ectodomain, however, without insulin agonistic effects. The interactions are further characterized by NMR, UV-vis, and fluorescence spectroscopies. It is shown that both fluorescence and UV-vis changes in the ligand spectra induced by IR fragments occur with Kd values similar to those obtained in the displacement assay. Nevertheless, insulin itself cannot completely abolish binding of the small molecules. Determination of the binding stoichiometry reveals multiple binding sites for ligands of which one overlaps with the insulin binding site on the receptor.

Recent progress in the pathogenesis and management of essential thrombocythemia

In the last decade, the diagnosis of essential thrombocythemia (ET) has been refined by appreciation of the occurrence of karyotypically occult but molecularly evident chronic myelogenous leukemia and morphologically subtle myelodysplastic syndrome (MDS) and cellular-phase agnogenic myeloid metaplasia (AMM). Although ET continues to be defined by the presence of nonreactive thrombocythemia that is not accounted for by another chronic myeloid disorder, recent studies of clonality and other laboratory parameters have suggested clinically relevant biologic heterogeneity among affected patients. Furthermore, randomized, prospective, and controlled retrospective data have provided additional clinical information that has resulted in the development of risk categories and risk-adjusted treatment recommendations.

Cytokine receptor dimerization and activation: prospects for small molecule agonists

Ligand-induced dimerization of cell surface receptors has emerged as a general mechanism for the initiation of signal transduction. A number of therapeutically important receptor families are believed to be activated by this process. Recently available structural information, particularly for the erythropoietin receptor, has provided insight into the mechanism of receptor activation. These findings have also revealed important constraints on the nature of receptor-agonist complexes. The prospects of discovering small-molecule mimetics of such receptor agonists are discussed, including strategies which have led to the identification of a small number of peptide and non-peptide cytokine mimetics.

Discovery of cytokine mimics using cell-based systems

The successful cloning and subsequent clinical application of recombinant cytokines and/or growth factors has generated a number of important therapeutics. In contrast to the G-protein-coupled receptors, identification of small-molecule agonists of the cytokine and/or growth factor receptor family has proved difficult. The first small peptides and non-peptidic small-molecule agonists for several receptors have recently been reported. The initial identification and/or crucial characterization of these molecules as true mimics was dependent on the use of cell-based functional assays. This article will review recent cell-based assay technologies that are suitable for HTS and that are being applied to the discovery of novel cytokine and growth factor mimics.

The structure, organization, activation and plasticity of the erythropoietin receptor

Dimerization of the erythropoietin receptor has long been accepted as the singular step in its mechanism of activation. Recent studies have revealed a regulator process for activation that is dependent on the actual configuration of the receptor-ligand dimer assembly. This aspect of the receptor subunit assembly appears to extend to the unliganded receptor, which can dimerize on the cell surface and diminish any spontaneous background signaling in the absence of ligand. This self-recognition, as well as the multiple ligand binding capabilities of the receptor binding site, is consistent with an emerging theme of plasticity in protein-protein and ligand-receptor interactions.

Small molecule cytokine mimetics

A number of reports describe small peptides, and even bona fide small organic molecules, that activate homodimeric cytokine receptors and show cytokine-like activity in vitro and in vivo. These cases can be examined in light of the mechanistic and thermodynamic principles that govern cytokine-receptor activation.