Promegapoietin-1a, an engineered chimeric IL-3 and Mpl-L receptor agonist, stimulates hematopoietic recovery in conventional and abbreviated schedules following radiation-induced myelosuppression in nonhuman primates

Radiation countermeasures for hematopoietic acute radiation syndrome: growth factors, cytokines and beyond

PURPOSE

The intent of this article is to report the status of some of the pharmaceuticals currently in late stage development for possible use for individuals unwantedly and acutely injured as a result of radiological/nuclear exposures. The two major questions we attempt to address here are: (a) What medicinals are currently deemed by regulatory authorities (US FDA) to be safe and effective and are being stockpiled? (b) What additional agents might be needed to make the federal/state/local medicinal repositories more robust and useful in effectively managing contingencies involving radiation overexposures?

CONCLUSIONS

A limited number (precisely four) of medicinals have been deemed safe and effective, and are approved by the US FDA for the 'hematopoietic acute radiation syndrome (H-ARS).' These agents are largely recombinant growth factors (e.g. rhuG-CSF/filgrastim, rhuGM-CSF/sargramostim) that target and stimulate myeloid progenitors within bone marrow. Romiplostim, a small molecular agonist that enhances platelet production via stimulation of bone marrow megakaryocytes, has been recently approved and indicated for H-ARS. It is critical that additional agents for other major sub-syndromes of ARS (gastrointestinal-ARS) be approved. Future success in developing such medicinals will undoubtedly entail some form of a polypharmaceutical strategy, or perhaps novel, bioengineered chimeric agents with multiple, radioprotective/radiomitigative functionalities.

Nonhuman primates as models for the discovery and development of radiation countermeasures

INTRODUCTION

Despite significant scientific advances over the past six decades toward the development of safe and effective radiation countermeasures for humans using animal models, only two pharmaceutical agents have been approved by United States Food and Drug Administration (US FDA) for hematopoietic acute radiation syndrome (H-ARS). Additional research efforts are needed to further develop large animal models for improving the prediction of clinical safety and effectiveness of radiation countermeasures for ARS and delayed effects of acute radiation exposure (DEARE) in humans. Area covered: The authors review the suitability of animal models for the development of radiation countermeasures for ARS following the FDA Animal Rule with a special focus on nonhuman primate (NHP) models of ARS. There are seven centers in the United States currently conducting studies with irradiated NHPs, with the majority of studies being conducted with rhesus monkeys. Expert opinion: The NHP model is considered the gold standard animal model for drug development and approval by the FDA. The lack of suitable substitutes for NHP models for predicting response in humans serves as a bottleneck for the development of radiation countermeasures. Additional large animal models need to be characterized to support the development and FDA-approval of new radiation countermeasures.

Danggui sini decoction ameliorates myelosuppression in animal model by upregulating Thrombopoietin expression

Danggui Sini decoction (DSD), a famous Chinese medicine, has been used therapeutically in various diseases. In this study, we tried to investigate whether and how DSD could ameliorate myelosuppression in an animal model, in which myelosuppression is induced by cyclophosphamide treatment. The myelosuppression model was established by intraperitoneal injection of 100 mg/kg cyclophosphamide in mice. Flow cytometry was used to assess cell numbers and evaluate the bone marrow cell cycle distribution. Spleen samples were collected, and the mRNA expression levels of thrombopoietin (TPO) and c-Mpl were analyzed by RT-PCR. Our results demonstrated that DSD could significantly elevate the level of bone marrow hematopoietic stem progenitor cells in myelosuppression mice model. DSD also accelerated cell proliferation by switching cell cycles from G0/G1 phase to S and G2/M phase. Moreover, DSD significantly elevated the mRNA expression level of TPO, but not c-Mpl in spleen. Overall, the present results indicated that DSD is a promising Chinese medicine that is highly potent to ameliorate myelosuppression induced by chemotherapy by upregulating TPO expression.

Early-acting hematopoietic growth factors: biology and clinical experience

Secreted protein growth factors that stimulate the self-renewal, proliferation, and differentiation of the most primitive stem cells are among the most biologically interesting molecules and at least theoretically have diverse applications in the evolving field of regenerative medicine. Among this class of regulators, the early-acting hematopoietic growth factors and their cellular targets are perhaps the best characterized and serve as a paradigm for manipulating other stem cell based tissues. This chapter reviews the preclinical knowledge accumulated over ~40 years, since the discovery of the first such growth factor, and the clinical applications of those that, upon testing in humans, ultimately gained regulatory approval for the treatment of various hematological diseases.

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

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

A strategic crossflow filtration methodology for the initial purification of promegapoietin from inclusion bodies

A novel crossflow filtration methodology is demonstrated for the initial purification of the therapeutic protein, promegapoietin-1a (PMP), produced as inclusion bodies (IBs) in a recombinant Escherichia coli bioprocess. Two strategic separation steps were performed by utilizing a filtration unit with a 1000 kDa polyethersulphone membrane. The first step, aiming for separation of soluble contaminants, resulted in a 50% reduction of the host cell proteins, quantified by total amino acid analysis and a 70% reduction of all DNA, quantified by fluorometry, when washing the particulate material with a 10mM EDTA in 50mM phosphate buffer, pH 8. The second step, aiming for separation of particulate contaminants from solubilized IBs, resulted in a 97-99.5% reduction of endotoxin, used as a marker for cell debris, and was quantified by the kinetic turbidimetric LAL endotoxin assay. The overall PMP yield was 58% and 33% respectively for the two solubilizations investigated, guanidine hydrochloride and arginine, as measured by RP-HPLC. The scope was also to investigate the physical characteristics of the intermediate product/s with regard to the choice of IB solvent. Preliminary results from circular dichroism spectroscopy measurements indicate that the protein secondary structure was restored when arginine was used in the second step.

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.

Cytokine-based treatment of accidentally irradiated victims and new approaches

A major goal of medical management of acute radiation syndrome following accidental exposures to ionizing radiation (IR) is to mitigate the risks of infection and hemorrhage related to the period of bone marrow aplasia. This can be achieved by stimulating the proliferation and differentiation of residual hematopoietic stem and progenitor cells (HSPC) related to either their intrinsic radioresistance or the heterogeneity of dose distribution. This is the rationale for treatment with hematopoietic growth factors. In fact, apoptosis has recently been shown to play a major role in the death of the continuum of more or less radiosensitive HSPC, soon after irradiation. Therefore, administration of antiapoptotic cytokine combinations such as stem cell factor, Flt-3 ligand, thrombopoietin, and interleukin-3 (4F), may be important for multilineage recovery, particularly when these factors are administered early. Moreover, acute exposure to high doses of IR induces sequential, deleterious effects responsible for a delayed multiple organ dysfunction syndrome. These considerations strongly suggest that therapeutics could include tissue-specific cytokines, such as keratinocyte growth factor, and pleiotropic agents, such as erythropoietin, in addition to hematopoietic growth factors to ensure tissue damage repair and mitigate the inflammatory processes. Noncytokine drugs have also been proposed as an alternative to treat hematopoietic or nonhematopoietic radiation effects. To develop more effective treatments for radiation injuries, basic research is required, particularly to improve understanding of stem cell needs within their environment. In the context of radiological terrorism and radiation accidents, new growth promoting molecules need to be approved and available cytokines stockpiled.

Radiation protectants: current status and future prospects

In today's heightened nuclear/biological/chemical threat environment, there is an increased need to have safe and effective means to protect not only special high-risk service groups, but also the general population at large, from the health hazards of unintended ionizing radiation exposures. An unfulfilled dream has been to have a globally effective pharmacologic that could be easily taken orally without any undue side effects prior to a suspected or impending nuclear/radiological event; such an ideal radioprotective agent has yet to be identified, let alone fully developed and approved for human use. No one would argue against the fact that this is problematic and needs to be corrected, but where might the ultimate solution to this difficult problem be found? Without question, representative species of the aminothiol family [e.g., Amifostine (MedImmune, Gaithersburg, Maryland)] have proven to be potent cytoprotectants for normal tissues subjected to irradiation or to radiomimetic chemicals. Although Amifostine is currently used clinically, drug toxicity, limited times of protection, and unfavorable routes of administration, all serve to limit the drug's utility in nonclinical settings. A full range of research and development strategies is being employed currently in the hunt for new safe and effective radioprotectants. These include: (1) large scale screening of new chemical classes or natural products; (2) restructuring/reformulating older protectants with proven efficacies but unwanted toxicities; (3) using nutraceuticals that are only moderately protective but are essentially nontoxic; (4) using low dose combinations of potentially toxic but efficacious agents that protect through different routes to foster radioprotective synergy; and (5) accepting a lower level of drug efficacy in lieu of reduced toxicity, banking on the premise that the protection afforded can be leveraged by post-exposure therapies. Although it is difficult to predict which of these strategies will ultimately prove to be successful, it is certain that the probability of a useful protectant being fielded is increased significantly. This is due to the resurgence of interest in radiation protection, increased resources being expended by federal agencies, and by the Food and Drug Administration's willingness to innovate relative to new approval guidance.

Colony-stimulating factors in the management of neutropenia and its complications

Granulocyte colony-stimulating factor (CSF) and granulocyte-macrophage CSF are potent drugs used to increase neutrophil counts after myelosuppressive chemotherapy. However, in various indications, the use of CSFs has no clinical benefit with regard to morbidity or mortality from infectious complications, frequency of antibiotic use, or rate of hospitalization. Thus, the application of CSFs should be limited to indications with proven clinical benefits or evidence of cost-effectiveness. This review will provide an overview of the state-of-the-art use of CSFs in chemotherapy-associated neutropenia, transplantation, and bone marrow failure syndromes. In addition, recently developed drugs for accelerated hematopoietic recovery will be presented.

Thrombopoietic Factors in Chronic Bone Marrow Failure States: The Platelet Problem Revisited: Table 1

Thrombocytopenia is a serious clinical problem in several different clinical settings. In chronic bone marrow failure states, which include aplastic anemia, myelodysplastic syndrome, and graft failure, the prolonged nature of thrombocytopenia often leads to alloimunization after repeated platelet transfusions, the consequence of which is a platelet-refractory state and enhanced risk of bleeding. Despite the introduction of several thrombopoietic factors into clinical trials, an effective way to alleviate thrombocytopenia has been elusive, and the problem in chronic bone marrow failure states has remained poorly addressed by clinical investigations. Even so, several studies by our group and others suggest that a subset of patients suffering from chronic bone marrow failure can respond to appropriate growth factor therapy. The temporal pace of response appears, however, to be much slower than that observed after administering growth factors which act on neutrophils. On the other hand, durable responses can be secured in some patients given thrombopoietic factors for long periods of time. Herein, we provide an overview of the clinical research investigations of thrombopoietic factors in chronic bone marrow failure, and the emerging insights these studies provide for understanding the process of thrombopoiesis and its therapy in this setting.

Role of cytokines and growth factors in radioprotection

Cytokines and growth factors are growing groups of proteins that are responsible for the communication between cells of the immune system, hematopoietic cells, and other cell types. The cloning and large-scale production in a recombinant form of these agents in pharmacological quantities permitted investigations aimed at assessing the benefit they may provide in preserving and restoring functions of tissues compromised by irradiation. We have extensively examined past investigations which suggest that some cytokines and growth factors protect animals from radiation lethality when given prior to or after irradiation, and even in untreated animals, these cytokines serve in innate defenses against external stimuli. In contrast, some cytokines given before irradiation sensitize the animals to radiation lethality. Unfortunately, due to their adverse side effects, these cytokines were not found suitable as radioprotectors. Recent studies suggest that new approaches may bring cytokines and growth factors in clinic for radiation injury. The information and insight gained about therapeutic potential of cytokine manipulation will allow for more rational design of treatment protocols.

Combinations of Cytokines Promote Survival of Mice and Limit Acute Radiation Damage in Concert with Amelioration of Vascular Damage

Recovery from hematopoietic aplasia is a predominant factor in the survival of total-body-irradiated mice within 30 days after exposure. However, other radiation-induced pathophysiological events have been shown to play a role, among which an inflammatory reaction must be considered. In the present study, we evaluated the therapeutic potential of a hematopoietic growth factor (thrombopoietin, Tpo) and pleiotropic cytokines (Il4 or Il11), used alone or in combination, on the survival of mice, hematopoietic reconstitution, inflammatory reaction and vascular changes. All treatments including Tpo induced a higher level of survival than did treatment with a placebo, with combinations being the most efficient. The increased survival could not be explained solely by an improved hematopoietic recovery. Treatments with Tpo also reduced the level of the chemokine KC in plasma and the level of expression of mRNA for inflammatory and coagulation proteins in the lungs of irradiated mice. In addition, radiation- induced vascular hyperpermeability was reduced with the use of Tpo. In summary, our results show that Tpo may improve survival by limiting vascular leakage, which in turn could limit inflammatory reactions and the ensuing tissue damage.

Single administration of stem cell factor, FLT-3 ligand, megakaryocyte growth and development factor, and interleukin-3 in combination soon after irradiation prevents nonhuman primates from myelosuppression: long-term follow-up of hematopoiesis

Preservation of hematopoietic stem and progenitor cell survival is required for recovery from radiation-induced myelosuppression. We recently showed that short-term injection of antiapoptotic cytokine combinations into mice soon after lethal gamma irradiation promoted survival. The present study investigated the hematopoietic response of cynomolgus monkeys to a single dose of stem cell factor, FLT-3 ligand, megakaryocyte growth and development factor, and interleukin-3 in combination (4F, each factor given intravenously at 50 microg/kg) administered 2 hours after 5-Gy gamma irradiation. Treated monkeys (n = 4) experienced no thrombocytopenia. Only 1 in 4 displayed a transient period of neutropenia (neutrophil [ANC] count < 0.5 x 10(9)/L), whereas all irradiated controls (n = 4) experienced neutropenia (5-12 days) and thrombocytopenia (platelet [PLT] count < 20 x 10(9)/L, 5-31 days). Treated animals exhibited an impressive 2-wave PLT response that peaked at days 8 and 22 after total body irradiation (TBI). Areas under the curve (AUC) of PLTs, ANCs, white blood cells (WBCs), and red blood cells (RBCs) between days 0 and 90 were significantly higher in treated animals than in controls. Humeral bone marrow-derived clonogenic activity was significantly spared at 24 hours and 4 days after TBI in treated monkeys. No apparent impairment of the hematopoietic status and stem cell pool, in terms of long-term culture-initiating cells (LTC-ICs) and side population (SP) cells, was observed after 15 months. These results strongly suggest that the 4F cytokine combination, as a single dose regimen, could act as an emergency treatment for nuclear accident or terrorism victims.

Hematopoietic recovery following autologous bone marrow transplantation in a nonhuman primate: effect of variation in treatment schedule with PEG-rHuMGDF

Mathematical modeling of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) pharmacokinetics (PK) and pharmacodynamics (PD) suggest that variations in the PEG-rHuMGDF treatment schedule could reduce the severity and duration of thrombocytopenia following myeloablation and bone marrow transplant (BMT). We tested this hypothesis in a rhesus monkey model of autologous (Au) bone marrow-derived mononuclear cell (BM-MNC) transplantation following lethal myeloablation. On day 0, animals were myeloablated by total body exposure to 920 cGy, 250 kVp x-irradiation (TBI). Four cohorts of animals were infused with 1 x 10(8) AuBM-MNC/kg body weight within 2 hours of TBI. The AuBMT-alone cohort received no cytokine, the daily dosage cohort received PEG-rHuMGDF (2.5 micro g/kg/day, s.c.) post TBI and AuBMT, and the pre/post-transplant cohort received PEG-rHuMGDF (2.5 micro g/kg/day, s.c.) pre (day -9 to day -5) and post TBI and AuBMT. The post-transplant PEG-rHuMGDF administration in the above cohorts was begun on day 1 post TBI and continued until platelet counts reached 200,000 micro l (range, 15-31 days). Another group received PEG-rHuMGDF (300 micro g/kg/day, s.c.) on days 1 and 3 only following TBI and AuBMT. The TBI controls received neither AuBMT nor cytokine therapy. In this model of AuBMT, with regard to the PEG-rHuMGDF administration schedule, the daily dosage of the post-transplant cohort did not significantly improve platelet recovery; the pre/post-transplant schedule and an abbreviated high-dosage, post-transplant schedule (days 1 and 3) significantly improved the duration and nadir of thrombocytopenia and platelet recovery. These data confirm predictions from PK/PD modeling of PEG-rHuMGDF that thrombocytopenia is preventable following AuBMT.

Short-term injection of antiapoptotic cytokine combinations soon after lethal gamma -irradiation promotes survival

Recovery from radiation-induced (RI) myelosuppression depends on hematopoietic stem and progenitor cell survival and the active proliferation/differentiation process, which requires early cytokine support. Single cytokine or late-acting growth factor therapy has proved to be inefficient in ensuring reconstitution after severe RI damage. This work was aimed at evaluating the in vivo survival effect of combinations of early-acting cytokines whose antiapoptotic activity has been demonstrated in vitro: stem cell factor (SCF [S]), FMS-like tyrosine kinase 3 ligand (FLT-3 ligand [F]), thrombopoietin (TPO [T]), interleukin-3 (IL-3 [3]), and stromal derived factor-1 (SDF-1). B6D2F1 mice underwent total body irradiation at 8 Gy cesium Cs 137 gamma radiation (ie, lethal dose 90% at 30 days) and were treated soon after irradiation, at 2 hours and at 24 hours, with recombinant murine cytokines, each given intraperitoneally at 50 microg/kg per injection. All treatments induced 30-day survival rates significantly higher than control (survival rate, 8.3%). 4F (SFT3) and 5F (4F + SDF-1) were the most efficient combinations (81.2% and 87.5%, respectively), which was better than 3F (SFT, 50%), TPO alone (58.3%), and SDF-1 alone (29.2%) and also better than 4F given at 10 microg/kg per injection (4F10, 45.8%) or as a 50 microg/kg single injection at 2 hours (4Fs, 62.5%). Despite delayed death occurring mainly from day 150 on and possible long-term hematopoiesis impairment, half the 30-day protective effects of 4F and 5F were preserved at 300 days. Our results show that short- and long-term survival after irradiation depends on appropriate multiple cytokine combinations and at optimal concentrations. The proposal is made that an emergency cytokine regimen could be applied to nuclear accident victims as part of longer cytokine treatment, cell therapy, or both.

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.

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.