Megakaryocyte growth and development factor (MGDF) moderately enhances in-vitro platelet aggregation

Thrombopoietin as biomarker and mediator of cardiovascular damage in critical diseases

Thrombopoietin (TPO) is a humoral growth factor originally identified for its ability to stimulate the proliferation and differentiation of megakaryocytes. In addition to its actions on thrombopoiesis, TPO directly modulates the homeostatic potential of mature platelets by influencing their response to several stimuli. In particular, TPO does not induce platelet aggregation per se but is able to enhance platelet aggregation in response to different agonists (“priming effect”). Our research group was actively involved, in the last years, in characterizing the effects of TPO in several human critical diseases. In particular, we found that TPO enhances platelet activation and monocyte-platelet interaction in patients with unstable angina, chronic cigarette smokers, and patients with burn injury and burn injury complicated with sepsis. Moreover, we showed that TPO negatively modulates myocardial contractility by stimulating its receptor c-Mpl on cardiomyocytes and the subsequent production of NO, and it mediates the cardiodepressant activity exerted in vitro by serum of septic shock patients by cooperating with TNF-α and IL-1β. This paper will summarize the most recent results obtained by our research group on the pathogenic role of elevated TPO levels in these diseases and discuss them together with other recently published important studies on this topic.

Application of reticulated platelets to transfusion management during autologous stem cell transplantation

BACKGROUND

The immature (or reticulated) platelet fraction (IPF) is rich in nucleic acids, especially RNA, and can be used as a predictive factor for platelet recovery in platelet immunomediated consumption or in postchemotherapy myelosuppression. Our aim was to determine if transfusions with IPF-rich solutions, during autologous peripheral blood stem cell transplantation, reduce the occurrence of bleeding and hemorrhagic complications.

PATIENTS AND METHODS

Transfusions were administered to 40 children, affected with hematological pathologies, who underwent autologous peripheral hematopoietic progenitor cell transplantation. There were two groups of 20 patients, one group treated with IPF-poor and the other with IPF-rich solutions. In the two groups, the conditioning regimen was the same for the same pathology (hematological pathologies: 14 acute lymphoblastic leukemia; twelve acute myelocytic leukemia; four non-Hodgkin's lymphoma; two Hodgkin's lymphoma; eight solid tumors). A new automated analyzer was used to quantify the IPF: the XE2100 (Sysmex, Kobe, Japan) blood cell counter with upgraded software.

RESULTS

The 20 patients who received solutions with a high percentage of IPF (3%-9% of total number of infused platelets) required fewer transfusions than the 20 patients who received transfusions with a low percentage of IPF (0%-1% of total number of infused platelets): 83 versus 129 (mean of number of transfusions 4.15 versus 6.45) and a significant difference was found between the two groups by using the Mann-Whitney test (P < 0.001). The prophylactic transfusions decreased from three to two per week. There was only one case of massive hemorrhage.

CONCLUSION

The use of IPF solutions reduces the number of transfusions and bleedings after peripheral blood stem cell transplantation in pediatric patients.

The effect of MGDF on platelet function and thrombosis in animal models

Recombinant human megakaryocyte growth and development factor (rHuMGDF) is a recombinant form of the endogenous c-mpl ligand, thrombopoietin (TPO). rHuMGDF (and c-mpl ligands in general) can produce a measurable sensitization of platelets to known platelet agonists. Our laboratory has observed this sensitization in vitro in both platelet-rich plasma and whole blood and ex vivo in platelets from animals receiving rHuMGDF. Concurrently, clear increases in the tyrosine phosphorylation of Jak2 and c-mpl receptor can be observed both in vitro and ex vivo. To assess the in vivo prothrombotic potential of rHuMGDF, a rabbit carotid artery model of cyclic flow reduction (CFR) was used. Intravenous administration of platelet-sensitizing dosages of rHuMGDF had no effect on the CFR pattern, whereas control experiments demonstrated that the CFR pattern can be modulated by both platelet sensitizing (epinephrine) and antithrombotic (aspirin and ketanserin) agents. We conclude that thrombopoiesis can be observed at dosages that do not sensitize platelets, and further, that platelet-sensitizing dosages of rHuMGDF do not necessarily enhance platelet-dependent thrombosis in this model.

Effects of Mpl ligands on platelet production and function in nonhuman primates

Endogenous thrombopoietin (TPO) stimulates platelet production in nonhuman primates dose-dependentbyinducing megakaryocyte development from early marrow hematopoietic progenitors and subsequent proliferation and endoreduplication. Recombinant human TPO, nonpegylated or pegylated recombinant human megakaryocyte growth and development factor produce log-linear responses in peak peripheral platelet counts (or peripheral platelet mass turnover), platelet TPO receptor density, and marrow megakaryocyte volume, ploidy, number and mass. Mpl ligand therapy sustains normal peripheral platelet concentrations following myelosuppressive chemotherapy in baboons and corrects peripheral platelet counts in HIV-infected chimpanzees with severe thrombocytopenia. Whereas Mpl ligands do not directly induce platelet aggregation in vitro, they enhance aggregatory responsiveness of platelets to physiologic agonists both in vitro and transiently ex vivo following treatment with Mpl ligands. However, platelet recruitment into forming thrombus is not augmented by these agents when evaluated in quantitative rabbit or baboon models of platelet-dependent thrombus formation, except for the direct effect of platelet concentration per se. These findings indicate that appropriate dosing of these agents prevents thrombocytopenia without increasing the risk of platelet-dependent thrombo-occlusive complications.

Defective c-Mpl signaling in the syndrome of thrombocytopenia with absent radii

Thrombocytopenia with absent radii (TAR) syndrome is a rare congenital defect with severe hypomegakaryocytic thrombocytopenia and bilateral radial aplasia. To elucidate a possible relationship between thrombocytopenia in TAR and defects in the thrombopoietin (TPO)/c-Mpl system, we examined TPO activity in sera from six patients and in vitro reactivity of the patients' platelets to recombinant human TPO. We found elevated TPO serum levels in all patients, excluding a TPO production defect as a pathomechanism for the thrombocytopenia. In contrast to healthy controls, however, platelets of TAR patients failed to respond to recombinant TPO as measured by testing TPO synergism to suboptimal concentration of platelet activators. Most interestingly, TPO-induced tyrosine phosphorylation of platelet proteins was completely absent (four out of five) or markedly decreased (one out of five). More detailed investigations of the signal cascades of c-Mpl demonstrated the absence of Jak2 phosphorylation after TPO stimulation in a TAR patient's platelets. A defect in the early events of c-Mpl signal transduction might be the reason for impaired megakaryocytopoiesis in TAR syndrome.

Myelofibrosis: experimental models and human studies

Thrombopoietin (TPO) is the central regulator of megakaryocytopoiesis and thrombocytopoiesis. Preclinical data and human studies have so far shown that the recombinant molecule is safe to administer and associated with very little toxicity. Nevertheless, different experimental animal models have revealed that a chronic exposure to very high doses of TPO could result in myeloproliferative syndromes with a spectrum of pathological features in common with human idiopathic myelofibrosis (PMF). A number of investigators have researched whether TPO or its receptor Mpl were involved in the pathogenesis of human myeloproliferative syndromes which are also characterized by a predominant megakaryocytic involvement, in PMF and primitive essential thrombocythemia. In both diseases, megakaryocyte (MK) progenitors develop autonomously in serum-deprived cultures. This spontaneous MK development is also observed at limiting dilution demonstrating that MK escape the normal regulatory controls. Furthermore, this abnormal MK proliferation and maturation is neither due to an autocrine stimulation by TPO nor by point mutation or deletion in the coding region of the c-mpl gene. This paper will review the data that have been reported to date on the effects of an overexpression of Mpl ligand and related molecules on the induction of experimental myelofibrosis and highlight recent insights into the pathogenesis of PMF.

Single administration of thrombopoietin to lethally irradiated mice prevents infectious and thrombotic events leading to mortality

A sufficiently high dose of thrombopoietin to overcome initial c-mpl-mediated clearance stimulates hematopoietic reconstitution following myelosuppressive treatment. We studied the efficacy of thrombopoietin on survival after supralethal total body irradiation (9 Gy) of C57BL6/J mice and the occurrence of infectious and thrombotic complications in comparison with a bone marrow graft or prophylactic antibiotic treatment. Administration of 0.3 microg thrombopoietin, 2 hours after irradiation, protected 62% of the mice as opposed to no survival in placebo controls. A graft with a supraoptimal number of syngeneic bone marrow cells (10(6) cells) fully prevented mortality, whereas antibiotic treatment was ineffective. Blood cell recovery was observed in the thrombopoietin-treated mice but not in the placebo or antibiotic-treated group. Bone marrow and spleen cellularity as well as colony-forming unit granulocyte-macrophage and burst-forming unit erythroid were considerably increased in thrombopoietin-treated mice relative to controls. Histologic examination at day 11 revealed numerous petechiae and vascular obstructions within the brain microvasculature of placebo-treated mice, which was correlated with hypercoagulation and hypofibrinolysis. Thrombopoietin treatment prevented coagulation/fibrinolysis disorder and vascular thrombosis. High fibrinogen levels were related to bacterial infections in 67% of placebo-treated mice and predicted mortality, whereas the majority of the thrombopoietin-treated mice did not show high fibrinogen levels and endotoxin was not detectable in plasma. We conclude that thrombopoietin administration prevents mortality in mice subjected to 9-Gy total body irradiation both by interfering in the cascade leading to thrombotic complications and by amelioration of neutrophil and platelet recovery and thus protects against infections and hemorrhages.

Effects of Recombinant Human Megakaryocyte Growth and Development Factor (rHuMGDF) on Platelet Production, Platelet Aggregation, and Thrombosis

Recombinant human megakaryocyte growth and development factor (rHuMGDF) is a c-mpl ligand that promotes the differentiation of CD34+ precursor cells into megakaryocyte, and then platelets. In experimental animals, injection of this and other c-mpl ligands leads to profound increases in the circulating platelet count in a matter of days. However, c-mpl ligands have also been shown to sensitize platelets to aggregating agents in vitro, raising the possibility that c-mpI ligands may have prothrombotic effects in vivo. Therefore, characterizing rHuMGDF in an in vivo model of thrombosis is a necessary and critical step in defining the in vivo pharmacology of this novel and important hernatopoietic factor, a pegylated form of which is currently in clinical trials. To determine the biologically effective doses in the rabbit, daily subcutaneous injections of rHuMGDF at 0.1, 1.0, or 10 µg/kg were administered ever 7 days. Daily injection of 10 µ/kg produced an approximate fourfold increase in platelet count and 1.0 µ/kg doubled platelet count over the injection period, both of which were statistically significant. The serum concentrations of rHuMGDF were determined 10 minutes following a single intravenous injection with 0.1, 1.0, and 10 µ/kg, and were 0.05 +/- 0.02, 0.98 +/- 0.07, and 21.32 +/- 21.35 ng/ml. To determine whether rHuMGDF can sensitize platelets in vivo, platelet aggregometry was performed on platelets isolated from animals immediately before and 10 minutes after they had been injected intravenously with rHuMGDF (0.1, 1.0, and 10 µ/kg). Intravenous injection of 10 µ/kg produced measurable changes in platelet aggregometry ex vivo, as evidenced by an increased sensitivity of platelets to adenosine diphosphate (ADP). To assess. the in vivo prothrombotic potential of rHuMGDF, a rabbit carotid artery model of cyclic flow reduction (CFR) was used to measure the effect of intravenous rHuMGDF administration on the rate of thrombus formation as assessed by CFR slope and frequency. Intravenous administration of rHuMGDF had no effect on CFR slope or frequency when administered in doses ranging from 0.1 to 10 µ/kg. Control experiments demonstrated that CFR slope and frequency can be enhanced by intravenous infusion of epinephrine and can be abolished by the combined administration of aspirin and ketanserin, indicating that potentially prothrombotic and antithrombotic agents can be identified in this model. We conclude that biologically active doses of rHuMGDF used in this study (1.0 and 10 µ/kg) produce measurable serum levels, induce a thrombopoietic effect, and sensitize platelets in vivo, as determined by ex vivo aggregometry, at 10 µ/kg. Despite the sensitization of platelets to aggregation induced by ADP, it is clear that rHuMGDF does not alter the pattern of CFRs observed in the rabbit carotid artery, whereas agents known to sensitize platelets (epinepherine) and to inhibit platelets (aspirin and ketanserin) readily affected the CFR pattern. These findings indicate that intravenous rHuMIGDF administration, while capable of sensitizing pIatelets, does not enhance platelet-dependent thrombosis in vivo.

A Randomized, Double-Blind, Placebo-Controlled Study With Pegylated Recombinant Human Megakaryocyte Growth and Development Factor (PEG-rHuMGDF) as an Adjunct to Chemotherapy for Adults With De Novo Acute Myeloid Leukemia

To determine the safety, biologic, and clinical benefits of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF; Amgen, Thousand Oaks, CA) after myelosuppressive chemotherapy in acute myeloid leukemia (AML), 108 adult patients with de novo AML were randomized to receive either PEG-rHuMGDF (2.5 μg/kg/d or 5 μg/kg/d) for up to 21 doses (group A), a single dose of 2.5 μg/kg PEG-rHuMGDF, 7 daily doses of 2.5 μg/kg PEG-rHuMGDF (group B), or placebo. The greatest biologic activity was seen in group A with a median peak platelet count of 1,084 × 109/L, occurring at a median 9 days after the last dose of study drug, compared with 517 × 109/L and 390 × 109/L in group B and placebo group, respectively. Thrombocytosis (platelets &gt;1,000 × 109/L) was seen at rates of 52%, 8%, and 9% in groups A, B, and placebo, respectively, but were not associated with any adverse event. There was no effect on median time to transfusion independent platelet recovery (≥20 × 109/L). The median time to neutrophil recovery (≥500/μL) and red blood cell transfusion requirements were similar in all groups, and there was no apparent stimulation of leukemia. PEG-rHuMGDF was biologically active and well tolerated. Further investigation of dose and scheduling is required, specifically earlier dosing before and during chemotherapy.

A Randomized, Double-Blind, Placebo-Controlled Study With Pegylated Recombinant Human Megakaryocyte Growth and Development Factor (PEG-rHuMGDF) as an Adjunct to Chemotherapy for Adults With De Novo Acute Myeloid Leukemia

To determine the safety, biologic, and clinical benefits of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF; Amgen, Thousand Oaks, CA) after myelosuppressive chemotherapy in acute myeloid leukemia (AML), 108 adult patients with de novo AML were randomized to receive either PEG-rHuMGDF (2.5 μg/kg/d or 5 μg/kg/d) for up to 21 doses (group A), a single dose of 2.5 μg/kg PEG-rHuMGDF, 7 daily doses of 2.5 μg/kg PEG-rHuMGDF (group B), or placebo. The greatest biologic activity was seen in group A with a median peak platelet count of 1,084 × 109/L, occurring at a median 9 days after the last dose of study drug, compared with 517 × 109/L and 390 × 109/L in group B and placebo group, respectively. Thrombocytosis (platelets >1,000 × 109/L) was seen at rates of 52%, 8%, and 9% in groups A, B, and placebo, respectively, but were not associated with any adverse event. There was no effect on median time to transfusion independent platelet recovery (≥20 × 109/L). The median time to neutrophil recovery (≥500/μL) and red blood cell transfusion requirements were similar in all groups, and there was no apparent stimulation of leukemia. PEG-rHuMGDF was biologically active and well tolerated. Further investigation of dose and scheduling is required, specifically earlier dosing before and during chemotherapy.

Thrombopoietin potentiates agonist-stimulated activation of p38 mitogen-activated protein kinase in human platelets

Thrombopoietin (TPO) plays a crucial role in megakaryocyte differentiation and platelet production. c-Mpl, a receptor for TPO, is also expressed in terminally differentiated platelets. We investigated the effects of TPO on activation of p38 mitogen-activated protein kinase in human platelets. Thrombin, a thrombin receptor agonist peptide, a thromboxane A(2) analogue, collagen, crosslinking the glycoprotein VI, ADP, and epinephrine, but not phorbol 12, 13-dibutyrate activated p38. TPO did not activate p38 by itself, whereas TPO pretreatment potentiated the agonist-induced activation of p38. TPO did not promote phosphorylation of Hsp27 and cytosolic phospholipase A(2) by itself, but enhanced thrombin-induced phosphorylation of them. The specific p38 inhibitor SB203580 strongly inhibited such phosphorylation. Thus, TPO possesses the priming effect on p38 activation in human platelets and could affect platelet functions through the p38 pathway.

Effect of recombinant human megakaryocyte growth and development factor coupled with polyethylene glycol on the platelet storage lesion

BACKGROUND

Platelet production is regulated by a thrombopoietic growth factor (Mpl ligand). The receptor for this platelet growth factor (Mpl) is expressed on the platelet surface membrane. A recombinant thrombopoietic cytokine, recombinant human megakaryocyte growth and development factor coupled with polyethylene glycol (PEG-rHuMGDF), was added to apheresis platelets in vitro to determine whether Mpl ligand-receptor binding produced any beneficial or adverse effect on the development of the platelet storage lesion during 5 days of storage.

STUDY DESIGN AND METHODS

This study was designed as a dose-response protocol to determine the effects of adding increasing concentrations of PEG-rHuMGDF (0.0 [control], 2.5, 25, and 250 ng/mL) to apheresis platelets stored in two types of plastic storage containers. The increasing concentrations of PEG-rHuMGDF used simulated the theoretical peak plasma level attained in vivo, with an intravenous dose of 0, 0.1, 1.0 and 10 microg per kg of PEG-rHuMGDF. The platelets were stored with agitation at 20 to 24 degrees C for 5 days. A battery of in vitro assays was performed on storage Days 1 and 5, including pH, blood gases, platelet count, lactate dehydrogenase, mean platelet volume, glucose, lactate, osmotic recovery, morphology score, CD62P, and one-dimensional polyacrylamide gel electrophoresis analyses.

RESULTS

Analysis of results on both Day 1 and Day 5 showed no significant differences among any of the three PEG-rHuMGDF doses and the control group, for any in vitro assay. One-dimensional polyacrylamide gel electrophoresis showed no changes among the platelet protein patterns for the three PEG-rHuMGDF doses studied or the control. Storage-induced changes, however, did occur equally in all four groups of platelets over the 5 days of storage.

CONCLUSION

The addition to stored apheresis platelets of up to 10 microg per kg of PEG-rHuMGDF (250 ng/mL), followed by 5 days of storage at standard conditions, does not appear to promote or retard development of the platelet storage lesion.

Thrombopoietin

Thrombopoietin (TPO) was first described as an activity in thrombocytopenic serum that could stimulate platelet production upon transfer into other animals. The molecular cloning of TPO has allowed a detailed characterisation of its precise biological activities, both in vitro and in vivo. TPO binds its specific receptor, the c-Mpl protein, which is expressed on the surface of target cells, and induces receptor dimerization and activation of intracellular signalling pathways including the Janus kinase (JAK)-signal transducers and activators of transcription (STAT) and Ras cascades. TPO is a lineage-dominant cytokine, stimulating megakaryocyte production and maturation in vitro and inducing the greatest elevation in platelet numbers of any known cytokine upon administration to humans or laboratory animals. Indeed, gene targeting studies have established that the TPO/c-Mpl signalling system is the major physiological regulator of steady-state megakaryocyte and platelet production. These studies have also revealed a largely unanticipated role for TPO in the regulation of hemopoietic stem cells. Preclinical studies in animal models have indicated the capacity of c-Mpl ligands to overcome the thrombocytopenia associated with chemo/radiotherapy and clinical trials have already demonstrated the safety and efficacy of Mpl-ligands in elevating platelet counts in humans. It is likely that these molecules will lead to new therapeutics that will find utility in a range of clinical contexts.

The prolonged hematologic effects of a single injection of PEG-rHuMGDF in normal and thrombocytopenic mice

A single injection of > or =10 microg/kg PEG-rHuMGDF in mice causes a dose-dependent increase in circulating platelets beginning on day 3 and peaking on days 5-6. The mean platelet volume and platelet distribution width at doses > or =100 microg/kg initially increase in a dose-dependent fashion and later decrease. However, the mean platelet volume does not change when platelets are incubated with PEG-rHuMGDF in vitro. The number of marrow megakaryocytes increases in a dose-dependent fashion as early as day 1 and peaks on day 3. Marrow megakaryocyte colony-forming units (CFU-Meg) do not increase on days 1-3 at a dose of 100 microg/kg (a dose that increases platelet numbers two- to threefold and may be clinically relevant), but the relative frequency of high ploidy megakaryocytes and the proportion of large marrow megakaryocytes (29-50 microm in diameter) increases. After a dose of 1,000 microg/kg the percentage of megakaryocytes in mitosis peaks at 24-48 hours and the percentage of megakaryocytes incorporating BrdU is maximal at 48 hours, the relatively delayed peak of BrdU incorporation most likely representing endomitosis. The relative frequency of type II and III megakaryocytes peaks on days 3 and 4, respectively. Pharmacokinetic analysis of PEG-rHuMGDF shows peak serum concentrations at 2-4 hours and a terminal half-life of 11.4+/-2.5 hours. A single injection of PEG-rHuMGDF ameliorates carboplatin-induced megakaryocytopenia and thrombocytopenia in a dose-response dependent fashion. In conclusion, a single injection of PEG-rHuMGDF increases megakaryocyte and platelet production in normal and myelo-suppressed mice.

PEGylation prevents the N-terminal degradation of megakaryocyte growth and development factor

PURPOSE

Determine the effect of PEGylation on in-vitro degradation for recombinant human Megakaryocyte Growth and Development Factor (rHuMGDF) in the neutral pH range.

METHODS

Degradation products were characterized by cation-exchange HPLC, N-terminal sequencing and mass spectrometry.

RESULTS

The main route of degradation was through non-enzymatic cyclization of the first two amino acids and subsequent cleavage to form a diketopiperazine and des(Ser, Pro)rHuMGDE This reaction was prevented by alkylation of the N-terminus by polyethylene glycol (PEG).

CONCLUSIONS

PEGylation of proteins is commonly performed to achieve increased in-vivo circulation half-lives. For rHuMGDF, an additional advantage of PEGylation was enhanced in-vitro shelf-life stability.

Recombinant human megakaryocyte growth and development factor attenuates postbypass thrombocytopenia

BACKGROUND

Cardiopulmonary bypass contributes to platelet loss and dysfunction by exposure to shear stresses, foreign surfaces, and hypothermia. This study tested the hypothesis that pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) accelerates recovery of the platelet population after hypothermic extracorporeal circulation (HEC).

METHODS

In a blinded study, subcutaneous injections of drug or placebo were given to dogs daily for 3 days preoperatively (day 0, 1, and 2) with no drug on day 3. On day 4, the animal was prepared for arteriovenous HEC. After heparinization, HEC was initiated at 30 to 40 mL x kg(-1) x min(-1). Hypothermic extracorporeal circulation (25 degrees C) was continued for 90 minutes.

RESULTS

Preoperative platelet count (x10(3) platelets/microL) did not differ from predrug count in placebo (256+/-27 versus 255+/-20) or PEG-rHuMGDF (271+/-30 versus 291+/-38). During 60 minutes of HEC, the platelet count decreased to approximately 10% of baseline in placebo (29+/-5) and PEG-rHuMGDF (46+/-8), and recovered to approximately 70% of baseline after rewarming (90 minutes of HEC: placebo, 185+/-17, versus PEG-rHuMGDF, 169+/-22). After HEC, platelet count was greater in PEG-rHuMGDF-treated animals (p < 0.05) without altering function (aggregation responses). Within the first 6 hours after HEC, platelet count in PEG-rHuMGDF-treated animals was rising and increased to 260+/-29 (p < 0.01), but was unchanged in placebo animals (186+/-21). Thereafter, platelet count in placebo animals declined to a nadir of 124+/-15 (72 hours after HEC), whereas platelet count in PEG-rHuMGDF animals approximated the preoperative value (>200) at all times.

CONCLUSIONS

Appropriately timed presurgical administration of PEG-rHuMGDF counteracts post-HEC relative thrombocytopenia without increasing platelet population and enhancing aggregation preoperatively or during extracorporeal circulation.

Therapeutic cytokine stimulation of thrombocytopoiesis

Endogenous thrombopoietin (TPO) stimulates platelet production by inducing dose-dependent megakaryocyte development from early marrow hematopoietic progenitors and subsequent proliferation and endoreduplication. Recombinant human (rHu) TPO or pegylated recombinant human megakaryocyte growth and development factor (PEG-rHu MGDF) produce log-linear responses in experimental animals and humans when administered over the dose range of 0.05-25 micrograms/kg/day with respect to peak peripheral platelet counts (or peripheral platelet mass turnover) and marrow megakaryocyte volume, ploidy, number and mass. Other cytokines stimulating megakaryocytopoiesis in various species using different dosing regimens include Interleukin 1 (IL-1), Interleukin 3 (IL-3), Interleukin 6 (IL-6), Interleukin 11 (IL-11), stem cell factor (SCF), granulocytemacrophage colony stimulating factor, (GM-CSF), fusion proteins, such as promegapoietin, and a peptide agonist of TPO receptor. Whereas neither rHuTPO nor PEG-rHuMGDF induce platelet aggregation in vitro, they transiently enhance aggregatory responsiveness of platelets to physiologic agonists from several different experimental animals both in vitro and ex vivo. However, platelet recruitment into forming thrombus is not augmented by these agents when evaluated in quantitative rabbit or baboon models of platelet-dependent thrombus formation, except for the effect of platelet concentration per se. These findings indicate that appropriate dosing of these agents following marrow suppression prevents thrombocytopenia without increasing the risk of platelet-dependent thrombo-occlusive complications.

Mpl ligand or thrombopoietin: biological activities

Thrombopoietin (TPO) or Mpl ligand is the primary physiological regulator of platelet production. This cytokine is the most potent stimulator of the proliferation and differentiation of MK progenitor and precursor cells in vitro. It also acts additively or synergistically with several cytokines on progenitor cells from various hematopoietic lineages, including the primitive stem cells. The factor is an extremely potent thrombocytopoietic agent when administrated to normal animals, and it accelerates platelet and erythropoietic recovery in several models of myelosuppression. Phase I/II clinical trials are ongoing with no detectable adverse effects. Mpl ligand does not induce platelet aggregation, but it lowers the platelet sensitivity to physiological dose of agonists. In experimental mouse models, high and chronic dose of Mpl ligand results in myelofibrosis. TPO is constantly produced by the liver and the kidney; its plasmatic clearance occurs by binding to its receptor expressed on megakaryocytes and platelets. However, the full spectrum of the biological effects of this new cytokine is not fully understood, in particular its the role in the terminal stage of platelet production. In the near future, it is likely that new insights will be obtained in the physiopathological mechanisms underlying abnormal platelet production in human.

Removal of the fluorescent 4-(aminosulfonyl)-2,1,3-benzoxadiazole label from cysteine-containing peptides

The complete removal of the fluorescent cysteine derivative 4-(aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (ABD-F) from an intact protein has not been demonstrated even after extended treatment with a reducing agent. It has been suggested that this may be due to incomplete denaturation under the conditions employed. We were interested in investigating this phenomenon utilizing small peptides containing individual ABD-labeled cysteine residues. After incubating the fluorescent peptides in the presence of a reductant, it was shown that the ABD label could be completely removed from all of the cysteine-containing peptides investigated. Therefore, delabeling irreversibility is due to residual structure in proteins. Electrospray ionization mass spectrometry (ESI-MS) was used to determine the molecular mass of each peptide after removal of the ABD lavel. The ESI-MS data were consistent with the generation of a free sulfhydryl. The generation of the free sulfhydryl was further substantiated when a delabeled peptide was completely relabeled with ABD-F in the absence of reductant.

Transgenic Mice Overexpressing Human c-mpl Ligand Exhibit Chronic Thrombocytosis and Display Enhanced Recovery From 5-Fluorouracil or Antiplatelet Serum Treatment

The consequences of long-term in vivo expression of human c-mpl ligand in a mouse model were examined. Transgenic mice expressing the human full-length cDNA in the liver exhibited a fourfold increase in circulating platelet count that persisted stably over the life of the animals. Transgenic animals thrived and appeared healthy for at least 500 days. Transgenic platelets appeared normal with respect to surface antigens and response to platelet aggregation agonists. The highest-expressing transgenic line maintained human c-mpl ligand serum levels of 3 ng/mL. Megakaryocyte numbers in bone marrow and spleen were elevated, as were bone marrow and spleen megakaryocyte colony-forming cells (MEG-CFC). Megakaryocytes were observed in the bone marrow, spleen, liver, and lung, but in no other sites. Circulating myeloid and lymphoid cell populations were increased twofold. Additionally, the animals had a slight but significant anemia despite an increase in marrow colony-forming units-erythroid (CFU-E). No evidence of myelofibrosis was observed in the bone marrow. The platelet nadir in response to administration of either antiplatelet serum (APS) or 5-fluorouracil (5FU) was significantly reduced relative to the control level. Furthermore, the red blood cell (RBC) nadir was reduced relative to control levels in both models, suggesting that c-mpl ligand can directly or indirectly support the maintenance of erythrocyte levels following thrombopoietic insult.

Transgenic Mice Overexpressing Human c-mpl Ligand Exhibit Chronic Thrombocytosis and Display Enhanced Recovery From 5-Fluorouracil or Antiplatelet Serum Treatment

The consequences of long-term in vivo expression of human c-mpl ligand in a mouse model were examined. Transgenic mice expressing the human full-length cDNA in the liver exhibited a fourfold increase in circulating platelet count that persisted stably over the life of the animals. Transgenic animals thrived and appeared healthy for at least 500 days. Transgenic platelets appeared normal with respect to surface antigens and response to platelet aggregation agonists. The highest-expressing transgenic line maintained human c-mpl ligand serum levels of 3 ng/mL. Megakaryocyte numbers in bone marrow and spleen were elevated, as were bone marrow and spleen megakaryocyte colony-forming cells (MEG-CFC). Megakaryocytes were observed in the bone marrow, spleen, liver, and lung, but in no other sites. Circulating myeloid and lymphoid cell populations were increased twofold. Additionally, the animals had a slight but significant anemia despite an increase in marrow colony-forming units-erythroid (CFU-E). No evidence of myelofibrosis was observed in the bone marrow. The platelet nadir in response to administration of either antiplatelet serum (APS) or 5-fluorouracil (5FU) was significantly reduced relative to the control level. Furthermore, the red blood cell (RBC) nadir was reduced relative to control levels in both models, suggesting that c-mpl ligand can directly or indirectly support the maintenance of erythrocyte levels following thrombopoietic insult.

Effects of polyethylene glycol-conjugated recombinant human megakaryocyte growth and development factor on platelet counts after chemotherapy for lung cancer

BACKGROUND

Polyethylene glycol (PEG)-conjugated recombinant human megakaryocyte growth and development factor (MGDF, also known as PEG-rHuMGDF), a recombinant molecule related to thrombopoietin, specifically stimulates megakaryopoiesis and platelet production and reduces the severity of thrombocytopenia in animals receiving myelosuppressive chemotherapy.

METHODS

We conducted a randomized, double-blind, placebo-controlled dose-escalation study of MGDF in 53 patients with lung cancer who were treated with carboplatin and paclitaxel. The patients were randomly assigned in blocks of 4 in a 1:3 ratio to receive either placebo or MGDF (0.03, 0.1, 0.3, 1.0, 3.0, or 5.0 microg per kilogram of body weight per day), injected subcutaneously. No other marrow-active cytokines were given.

RESULTS

In the 38 patients who received MGDF after chemotherapy, the median nadir platelet count was 188,000 per cubic millimeter (range, 68,000 to 373,000), as compared with 111,000 per cubic millimeter (range, 21,000 to 307,000) in 12 patients receiving placebo (P = 0.013). The platelet count recovered to base-line levels in 14 days in the treated patients as compared with more than 21 days in those receiving placebo (P<0.001). Among all 40 patients treated with MGDF, 1 had deep venous thrombosis and pulmonary embolism, and another had superficial thrombophlebitis.

CONCLUSIONS

MGDF has potent stimulatory effects on platelet production in patients with chemotherapy-induced thrombocytopenia.

Mpl ligand (MGDF) alone and in combination with stem cell factor (SCF) promotes proliferation and survival of human megakaryocyte, erythroid and granulocyte/macrophage progenitors

We examined cytokine-stimulated proliferation and survival of human megakaryocyte progenitor cells. We used a reliable, immunoenzymatic method of labeling CD41a-, CD42b-stained megakaryocytes in intact agar cultures to specifically identify and enumerate all megakaryocyte-containing colonies. We examined a previously defined population of cells enriched for megakaryocyte progenitors that coexpress CD34 and the megakaryocyte/platelet marker CD61. These CD34+61+ cells displayed clonogenicity of approximately 30% and contained myeloid, erythroid and megakaryocyte progenitor cells. With single CD34+61+ cells, megakaryocyte growth and development factor ([MGDF], also known as mpl-ligand or thrombopoietin) stimulated 9% of cells to complete their first cell division by day 2 (versus 21% with stem cell factor [SCF], or 13% with interleukin 3 alone). MGDF showed an additive effect with SCF and interleukin 3 to increase this number at least twofold. In purified CD34+61+ cells, MGDF stimulated the survival of megakaryocyte-colony forming cells (CFC) when addition of other proliferative factors was delayed for 5, 10 and 15 days (all p < 0.0001 versus saline control). MGDF also promoted survival of BFU-E and granulocyte-macrophage-CFC for at least 10 days (p < or = 0.0013 and p < or = 0.0362, respectively). SCF alone prolonged survival of CD34+61+ progenitor cells, however, MGDF + SCF was significantly more active. Whereas the action of MGDF on megakaryocyte-CFC was evident both in stimulating proliferation and survival, its ability to promote survival was between two- and fivefold greater than its action to stimulate proliferation. Thus MGDF alone, and in combination with SCF, was active in promoting the survival and proliferation of human progenitor cells of multiple hemopoietic lineages.

The physiologic role and therapeutic potential of the Mpl-ligand in thrombopoiesis

The constant and appropriate production of megakaryocytes, and subsequently platelets, is critical for maintenance of hemostasis. Inadequate megakaryopoiesis and/or thrombopoiesis can lead to serious bleeding disorders. The humoral factors regulating these processes have been the subject of study for several decades. Although many cytokines have been shown to influence megakaryocyte development and platelet production, none appeared to do so in a lineage-dominant fashion analogous to the situation with erythrocyte and neutrophil production. More recently, a ligand for the hematopoietic cytokine receptor encoded by the c-mpl gene (Mpl ligand) has been shown to have profound effects on megakaryocyte growth and development. These effects appear to include the expansion of megakaryocyte progenitors (i.e. megakaryocyte-colony stimulating activity), and induction of megakaryocyte maturation to the point of platelet production (i.e. thrombopoietin). Administration of recombinant Mpl-ligand to rodents or primates treated with myelosuppressive agents abrogates or alleviates the severity and the duration of the resultant thrombocytopenias. The in vitro and in vivo data to date indicate that this new cytokine holds tremendous promise as a therapeutic agent for the treatment of thrombocytopenia associated with cancer therapies.