Future directions with platelet growth factors

Maillard reaction and immunogenicity of protein therapeutics

The recombinant DNA technology enabled the production of a variety of human therapeutic proteins. Accumulated clinical experience, however, indicates that the formation of antibodies against such proteins is a general phenomenon rather than an exception. The immunogenicity of therapeutic proteins results in inefficient therapy and in the development of undesired, sometimes life-threatening, side reactions. The human proteins, designed for clinical application, usually have the same amino acid sequence as their native prototypes and it is not yet fully clear what the reasons for their immunogenicity are. In previous studies we have demonstrated for the first time that interferon-β (IFN-β) pharmaceuticals, used for treatment of patients with multiple sclerosis, do contain advanced glycation end products (AGEs) that contribute to IFN-β immunogenicity. AGEs are the final products of a chemical reaction known as the Maillard reaction or glycation, which implication in protein drugs’ immunogenicity has been overlooked so far. Therefore, the aim of the present article is to provide a comprehensive overview on the Maillard reaction with emphasis on experimental data and theoretical consideration telling us why the Maillard reaction warrants special attention in the context of the well-documented protein drugs’ immunogenicity.

Current status of thrombopoietic agents

Two different thrombopoietic drugs, romiplostim and eltrombopag, have completed Phase III trials in patients with immune-mediated thrombocytopenia and were observed to successfully improve platelet counts. This review will mostly focus on these two agents. We shall review their preclinical development as well as clinical trial results in immune-mediated thrombocytopenia. Also, we shall discuss their potential clinical use in thrombocytopenias associated with other medical conditions where reduction in platelet production is an integral part of disease manifestations.

Phase II study of low-dose interleukin-11 in patients with myelodysplastic syndrome

Severe thrombocytopenia places patients with myelodysplastic syndrome (MDS) at risk of serious hemorrhage. Currently, therapeutic options are limited to platelet transfusions. The only commercially available growth factor that increases platelet counts is interleukin-11 (IL-11). We report the results of a phase II trial to more accurately assess the clinical response and toxicity data for low-dose IL-11 (10 microg/kg/day) in patients with MDS. In this study, nine of 32 assessable patients (28%) demonstrated increases in their platelet counts after treatment. Of these, five were considered major platelet responses (15%), as defined by World Health Organization criteria. Four patients had minor platelet responses (13%). The median duration of platelet response was 9 months. Low-dose IL-11 was well tolerated, with no observed grade 4 toxicities. Our study provides additional clinical evidence that chronic administration of IL-11, at low doses, can raise platelet counts and reduce platelet transfusion requirements in a subset of patients with MDS.

Effect of IL-10 on the expression of HSC growth factors in hepatic fibrosis rat

AIM: To study the effect of IL-10 on the expression of growth factors - transforming growth factor-β1 (TGF-β1), epidermal growth factor (EGF), hepatocyte growth factor (HGF)and platelet-derived growth factor (PDGF) of hepatic stellate cells (HSCs) of hepatic fibrosis rat and the anti-fibrogenic role of exogenous IL-10.

METHODS: Hepatic fibrosis was induced by CCl4 administration intra-peritoneally. Sixty clean male Sprague-Dawley (SD) rats were randomly divided into three groups: normal control group (GN, 8 rats), hepatic fibrosis model group (GC, 28 rats) and IL-10 treated group (GI, 24 rats). At the beginning of the 7^th and 11^th wk, rats in each group were routinely perfused with pronase E and type IV collag-enase through a portal vein catheter and the suspension obtained from the liver was spun by centrifugation with 11% Nycodenz density gradient to isolate HSCs. Histological examination was used to determine the degree of hepatic fibrosis. RT-PCR was employed to analyze mRNA expression from freshly isolated cells. Immunocytochemistry was performed to detect protein expression in primary cultured HSCs.

RESULTS: Rat hepatic fibrosis was developed with the increase of injection frequency of CCl4, and HSCs were successfully isolated. At the 7^th and 11^th wk, TGF-β1, EGF, and HGF mRNA in GC increased obviously compared with GN (P = 0.001/0.042, 0.001/0.001, 0.001/0.001) and GI (P = 0.001/0.007, 0.002/0.001, 0.001/0.001). For TGF-β1, no difference was observed between GI and GN. For EGF, mRNA level in GI increased compared with GN during the 7^th wk (P = 0.005) and 11^th wk (P = 0.049). For HGF, mRNA level in GI decreased compared with GN at the 7^th wk (P = 0.001) and 11^th wk (P = 0.021). Between these two time points, TGF-β1 expression at the 7^th wk was higher than that of the 11^th wk (P = 0.049), but for EGF, the former was lower than the latter (P = 0.022). As for PDGF mRNA, there was no significant difference between these groups, but difference seemed to exist in protein levels. Results by immunocytochemistry of TGF-β1 and EGF were paralleled with the above findings.

CONCLUSION: The expression of TGF-β1, EGF and HGF increased in HSC of hepatic fibrosis rat and decreased after treatment with IL-10. IL-10 plays an anti-fibrogenic role by suppressing growth factors expression.

Determination of Thrombopoietin-Derived Peptides Recognized by Both Cellular and Humoral Immunities in Healthy Donors and Patients with Thrombocytopenia

Thrombopoietin (TPO) is a cytokine that promotes megakaryocytopoiesis and thrombopoiesis and is considered a drug suitable for patients with thrombocytopenia. However, unexpected severe thrombocytopenia has developed in some healthy individuals participating in phase I clinical trials with a pegylated recombinant human megakaryocyte growth factor (PEG-rHuMGDF) that contained the first 163 amino acids of endogenous TPO, which resulted in hampering the further development of clinical trials. Autoimmune responses to PEG-rHuMGDF, which cross-reacted with endogenous TPO, were suggested to be involved in this rare but severe adverse event, although the immunogenic epitopes have not yet been determined. To better understand the molecular basis of such autoimmune reactions, we investigated the reactivity of 18 TPO-derived peptides with HLA-A2-binding motifs to plasma and T cells, both from patients with thrombocytopenia (n=24) and from healthy donors (HDs) (n=24). Four peptides, including those possessing amino acids in receptor-binding sites, were preferentially reactive to plasma from at least 20% of the patients, whereas one peptide at position 101-109 was equally reactive to those of the patients and the HDs. Each of the five peptides had the ability to induce peptide-specific cytotoxic T lymphocytes (CTLs) in both groups, albeit with less frequency among the patients. More important, each of these five peptides had the ability to induce HLA-A2-restricted and peptide-specific CTL activity reactive to cells that produce TPO. These results may provide new insights to gain a better understanding of autoimmune reactions to TPO.

Opportunities for Trisenox (arsenic trioxide) in the treatment of myelodysplastic syndromes

Arsenic trioxide (ATO) has a long history of efficacy as an antileukemic agent. However, with the advent of modern therapy, it had been relegated to a historical footnote. In the 1990s, investigators in China reported that ATO was safe and had dramatic efficacy in patients with acute promyelocytic leukemia (APL). Preclinical investigations indicate that the biological targets of this novel drug extend to a variety of malignancies other than APL and include induction of apoptosis, nonterminal differentiation, and suppression of proliferation and angiogenesis. The myelodysplastic syndromes (MDSs) present a particular therapeutic challenge. Ineffective hematopoiesis predominates in patients with low-grade prognostic scores. The survival of those patients with high-grade disease is compromised by a high risk of leukemia transformation. Although a number of therapeutic options have been investigated, none has emerged as being broadly efficacious and having an acceptable toxicity profile. No drug has yet received approval by the Food and Drug Administration for this indication. Biologic features of MDS, which include accelerated apoptotic potential, limited maturation capacity, and medullary neovascularity, create a strong scientific rationale for the investigation of ATO in MDS. This report describes the history and scientific basis for ATO treatment of hematologic malignancies, enumerates the potential benefits of ATO in MDS, and discusses the direction of ongoing trials of this novel antineoplastic agent.

Inhibitive effect of cordyceps sinensis on experimental hepatic fibrosis and its possible mechanism

AIM: To investigate the inhibitive effect and its possible mechanism of Cordyceps Sinensis (CS) on CCl₄-plus ethanol-induced hepatic fibrogenesis in experimental rats.

METHODS: Rats were randomly allocated into a normal control group, a model control group and a CS group. The latter two groups were administered with CCl₄ and ethanol solution at the beginning of the experiment to induce hepatic fibrosis. The CS group was also treated with CS 10 days after the beginning of CCl₄ and ethanol administration. All control groups were given corresponding placebo at the same time. At the end of the 9th week, rats in each group were humanely sacrificed. Blood and tissue specimens were taken. Biochemical, radioimmunological, immunohistochemical and molecular biological examinations were used to determine the level change of ALT, AST, HA, LN content in serum and TGFβ₁, PDGF, collagen I and III expression in tissue at either protein or mRNA level or both of them.

RESULTS: As compared with the model control group, serum ALT, AST, HA, and LN content levels were markedly dropped in CS group (86.0 ± 34.4 vs 224.3 ± 178.9, 146.7 ± 60.2 vs 272.6 ± 130.1, 202.0 ± 79.3 vs 316.5 ± 94.1 and 50.4 ± 3.0 vs 59.7 ± 9.8, respectively, P < 0.05). Tissue expression of TGFβ₁ and its mRNA, collagen I mRNA were also markedly decreased (0.2 ± 0.14 vs 1.73 ± 1.40, 1.68 ± 0.47 vs 3.17 ± 1.17, 1.10 ± 0.84 vs 2.64 ± 1.40, respectively, P < 0.05). More dramatical drop could be seen in PDGF expression (0.87 ± 0.43 vs 1.91 ± 0.74, P < 0.01). Although there was no statistical significance, it was still strongly suggested that collagen III mRNA expression was also decreased in CS group as compared with model control group (0.36 ± 0.27 vs 0.95 ± 0.65, P = 0.0615). In this experiment, no significant change could be found in PDGF mRNA expression between two groups (0.35 ± 0.34 vs 0.70 ± 0.46, P > 0.05).

CONCLUSION: Cordyceps sinensis could inhibit hepatic fibrogenesis derived from chronic liver injury, retard the development of cirrhosis, and notably ameliorate the liver function. Its possible mechanism involves inhibiting TGFβ₁ expression, and thereby, down regulating PDGF expression, preventing HSC activation and deposition of procollagen I and III.

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 potential role of thrombopoietin in idiopathic thrombocytopenic purpura

Thrombopoietin (TPO) plays a central role in the pathogenesis of idiopathic thrombocytopenic purpura (ITP), as it does in other immune-mediated thrombocytopenias. Because TPO is bound and internalized by platelets, it is destroyed together with platelets at an accelerated rate in the macrophage system. Because the spleen acts as a TPO sink, compensation of the decreased platelet count by an increased production in the bone marrow is insufficient. This aspect of ITP, as well as the use of TPO in the treatment of ITP, will be the focus of this article.

The end is just the beginning: megakaryocyte apoptosis and platelet release

Under influence of hematopoietic growth factors, particularly thrombopoietin (TPO), hematopoietic stem cells in the bone marrow go through a process of commitment, proliferation, differentiation, and maturation and become mature megakaryocytes. At this critical point, terminally differentiated megakaryocytes face a new fate: ending the old life as mature megakaryocytes by induction of apoptosis and beginning a new life as platelets by fragmentation of the large megakaryocyte cytoplasm. These events are as important as megakaryocyte commitment, proliferation, differentiation, and maturation, but the molecular mechanisms regulating these events are not well established. Although TPO drives megakaryocyte proliferation and differentiation and protects hematopoietic progenitor cells from death, it does not appear to promote platelet release from terminally differentiated megakaryocytes. Although mature megakaryocyte apoptosis is temporally associated with platelet formation, premature megakaryocyte death directly causes thrombocytopenia in cancer therapy and in diseases such as mvelodysplastic syndromes and human immunodeficiency virus infection. Also, genetic studies have shown that accumulation of megakaryocytes in bone marrow is not necessarily sufficient to produce platelets. All of these findings suggest that platelet release from megakaryocytes is an important and regulated aspect of platelet production, in which megakaryocyte apoptosis may also play a role. This review summarizes recent research progress on megakaryocyte apoptosis and platelet release.

Thrombocytopenia caused by the development of antibodies to thrombopoietin

Thrombocytopenia developed in some individuals treated with a recombinant thrombopoietin (TPO), pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF). Three of the subjects who developed severe thrombocytopenia were analyzed in detail to determine the cause of their thrombocytopenia. Except for easy bruising and heavy menses, none of these subjects had major bleeding episodes; none responded to intravenous immunoglobulin or prednisone. Bone marrow examination revealed a marked reduction in megakaryocytes. All 3 thrombocytopenic subjects had antibody to PEG-rHuMGDF that cross-reacted with endogenous TPO and neutralized its biological activity. All anti-TPO antibodies were immunoglobulin G (IgG), with increased amounts of IgG4; no IgM antibodies to TPO were detected at any time. A quantitative assay for IgG antibody to TPO was developed and showed that the antibody concentration varied inversely with the platelet count. Anti-TPO antibody recognized epitopes located in the first 163 amino acids of TPO and prevented TPO from binding to its receptor. In 2 subjects, endogenous TPO levels were elevated, but the TPO circulated as a biologically inactive immune complex with anti-TPO IgG; the endogenous TPO in these complexes had an apparent molecular weight of 95 000, slightly larger than the full-length recombinant TPO. None of the subjects had atypical HLA or platelet antigens, and the TPO cDNA was normal in both that were sequenced. Treatment of one subject with cyclosporine eliminated the antibody and normalized the platelet count. These data demonstrate a new mechanism for thrombocytopenia in which antibody develops to TPO; because endogenous TPO is produced constitutively, thrombocytopenia ensues.