ABSTRACT: After an almost 40-year search for a primary regulatory of platelet production, thrombopoietin has recently been purified and cloned. Thrombopoietin regulates all stages in the production of platelets by promoting both the proliferation of megakaryocyte progenitors and their maturation into platelet-producing megakaryocytes. In preclinical studies in normal mice and nonhuman primates, administration of thrombopoietin resulted in a rapid rise in platelet counts to levels previously unattainable with other thrombopoietic cytokines. In myelosuppressed animal models, use of thrombopoietin following chemotherapy, radiation, or stem-cell transplantation accelerated megakaryocyte and platelet recovery. Thrombopoietin has rapidly moved from the laboratory to the clinic in the last 3 years. Preliminary results of clinical trials using truncated or full-length forms of the molecule indicate that thrombopoietin is a powerful stimulus to the production of megakaryocytes and normal platelets in humans and enhances platelet recovery following chemotherapy. Although the peripheral effect is selective on platelet lineage, thrombopoietin mediates stimulatory effects on progenitors of multiple cell lineages at the bone marrow level and mobilizes progenitor cells into the peripheral blood. These biological effects suggest that thrombopoietin holds promise as a useful agent for the prevention and treatment of thrombocytopenia in cancer patients and for other disorders of thrombocytopenia. [ONCOLOGY 12(11):1597-1608, 1998]
Thrombocytopenia is a well-recognized problem that occurs in patients with malignancies. Although multiple etiologic factors of primary (eg, intrinsic bone marrow disease) or secondary (eg, tumor-induced coagulopathy, immune-mediated thrombocytopenia, hypersplenism-related platelet sequestration) origin can contribute to thrombocytopenia in cancer patients, the most common etiology of clinically significant thrombocytopenia is the increasing use of myelosuppressive chemotherapy.
Administration of antibiotics, transfusion of blood products, and modifications in chemotherapy doses have been the major means of combating chemotherapy-induced hematologic toxicity. Over the past decade, the clinical availability of hematopoietic growth factors, such as granulocyte colony-stimulating factor (G-CSF, filgrastim [Neupogen]), granulocyte-macrophage colony-stimulating factor (GM-CSF, sargramostim [Leukine]), and erythropoietin (Epogen, Procrit) have helped reduce clinical complications associated with granulocyte and erythroid cell lineages. Thrombocytopenia has been managed primarily by platelet transfusions and chemotherapy dose reductions.
Severe thrombocytopenia is a frequent clinical problem in the management of leukemia and pediatric malignancies, as well as in the setting of bone marrow or peripheral blood progenitor-cell transplantation. Although clinically significant thrombocytopenia does not commonly occur with most standard chemotherapeutic regimens used in the treatment of solid tumors, cumulative thrombocytopenia can be problematic with multiple cycles of certain chemotherapeutic regimens used in the treatment of lymphoma, sarcoma, breast cancer, ovarian cancer, and germ cell tumors (Table 1).[1-11] Furthermore, the concept that the dose intensity of chemotherapy may be important in improving overall outcome has led to the use of more aggressive, potentially curative regimens in patients with chemosensitive malignancies.
The use of peripheral blood progenitor cells has significantly reduced the duration of thrombocytopenia in patients receiving myeloablative or highly myelosuppressive therapy. However, 80% of patients undergoing autologous or allogeneic marrow transplantation continue to require transfusions 3 weeks after transplantation, as compared with 20% of patients receiving autologous peripheral blood progenitor cell transplants.
The use of platelet transfusions to manage severe thrombocytopenia has increased approproximately twofold in the United States from 1982 to 1992.[13,14] Advances in organ transplantation and cardiac surgery and the use of aggressive chemotherapy in patients with treatment- responsive malignancies have all contributed to this marked increase in the need for platelets and to rising health-care costs.
The total national direct medical cost of treating chemotherapy-induced thrombocytopenia was estimated to range from $146 to $233 million in 1993. In a recently conducted, observational study of platelet utilization in patients undergoing myeloablative treatment and stem-cell transplantation at 18 transplant centers in the United States and Canada, which used conservative cost figures for platelet transfusions, the average 60-day platelet cost per patient was estimated at $3,500 for autologous peripheral blood progenitor-cell support and $9,200 for allogeneic bone marrow transplantation (BMT).
Although platelet transfusions may decrease the risk of fatal bleeding complications, repeated use of platelets increases the risks of transmission of infectious diseases, transfusion reactions, alloimmunization, and graft-vs-host disease. All of these factors have motivated the search for an agent that could stimulate platelet production and prevent or ameliorate thrombocytopenia.
Platelets are small, anucleated cells that play an essential role in maintaining normal hemostasis. The normal circulating platelet count (range, 150 to 450 × 103/L) varies greatly between individuals but remains fairly constant in any one individual; there is minimum day-to-day variation in platelet count throughout life unless altered by a disease process or a change in physiologic condition.
The body maintains a constant circulating mass of platelets through the process of megakaryocytopoiesis. Peripheral blood platelets are derived from megakaryocytes, which are located primarily in the bone marrow. However, their large size and ability to attain a multilobular nucleus with a polyploid complement of DNA and abundant cytoplasm allow megakaryocytes to produce several thousand platelets per cell.
During megakaryocytopoiesis, pluripotent hematopoietic cells undergo commitment to the megakaryocyte lineage. The burst-forming unit-megakaryocyte (BFU-MK) is the most primitive megakaryocyte progenitor cell. The BFU-MKs take 21 days to develop in culture, and their immunophenotypic cell surface markers are CD34+, c-kit+ and HLA-DR-. The BFU-MKs differentiate and produce colony-forming unit-megakaryocytes (CFU-MKs), which take 12 days to appear in culture and carry cell surface markers CD34+, c-kit+, and HLA-DR+.
Colony-forming unit-megakaryocytes give rise to megakaryocytes (MKs), which lose their capacity to undergo mitosis but retain their capacity for endoreduplication. Megakaryocytes become large polyploid cells (up to 128N) with maturation of cytoplasm and acquisition of biochemical properties characteristic of functional platelets. Platelets are produced by transendothelial cytoplasmic fragmentation and have a life span of approximately 10 days.
The concept of a lineage-specific regulator involved in platelet development originated nearly 40 years ago. The term "thrombopoietin" was first used in 1958 to describe a humoral regulator present in thrombocytopenic plasma that restored platelet counts to a normal level after the onset of the thrombocytopenic state. Since then, many groups have attempted to isolate and purify thrombopoietin from thrombocytopenic plasma, serum, and other physiologic sources.
A major step in the search for thrombopoietin came in 1992 with the cloning of the cellular homolog of the viral oncogene, c-Mpl. This line of research began in 1986 with the identification of the murine retrovirus myeloproliferative leukemia virus (MPLV). The responsible viral oncogene, v-Mpl, was cloned in 1990 and the corresponding cellular proto-oncogene, c-Mpl, was identified in 1992. Sequence analysis of c-Mpl suggested homology to the genes encoding a family of hematopoietic growth factor receptors.
Subsequent studies established the link between c-Mpl and megakaryocytopoiesis. The expression of c-Mpl appeared to be restricted to CD34+ hematopoietic stem cells, megakaryocytes, and platelets. Furthermore, c-Mpl antisense oligonucleotides selectively inhibited megakaryocytic colony form- ation in vitro without affecting the growth of erythroid or granulocyte-macrophage colonies, indicating that a putative ligand for c-Mpl may be a lineage-specific regulator of megakaryocytopoiesis.
From that point on, the search for the c-Mpl ligand was aggressively pursued. In 1994, five different groups [24-28] using three distinct strategies reported on the identification, purification, and cloning of cDNA for the Mpl ligand. The observation that all of the megakaryocyte colony- and platelet-stimulating activity from thrombocytopenic plasma can be removed by recombinant c-Mpl further confirmed that this ligand is, indeed, thrombopoietin; the ligand was also named megakaryocyte growth and development factor (MGDF) and megapoietin by different investigators.
The human thrombopoietin gene is located on chromosome 3q27 and consists of seven exons joined to six introns.[28,30] The precursor protein product is made of 353 amino acids, and cleavage of a signal peptide at the amino terminus produces a mature peptide of 332 amino acids with two domains (Figure 1).
The amino terminal domain consists of 153 amino acids, including 4 cysteine residues, and has 23% homology with erythropoietin. The similarity increases to 50% when conservative amino acids are taken into account.
The carboxy terminal domain consists of 179 amino acids and has several N-linked glycosylation sites.[17,31] Whereas the amino terminal domain is sufficient for the thrombopoietic effects of the molecule, the carboxy terminal domain is likely important in maintaining its circulating half-life.
The major site of thrombopoietin production is the liver (fetal and adult).[25,27,30] Other sites expressing thrombopoietin mRNA include bone marrow, kidney, brain, testes, and spleen.[17,25,27,30]
The c-Mpl receptor, the receptor for thrombopoietin, is a transmembrane protein with an N-terminal extracellular domain and a C-terminal intracellular domain. The receptor expression is largely restricted to the tissues that support hematopoiesis, ie, bone marrow cells, spleen, and fetal liver, and is mostly expressed on CD34+ cells, cells of megakaryocyte lineage, and platelets.[20,23,32] The gene for the c-Mpl receptor is located on mouse chromosome 4 and human chromosome 1p34.
The c-Mpl receptor binds to the ligand (thrombopoietin), which results in proliferation and differentiation of megakaryocyte progenitors. Activation of the c-Mpl receptor results in the activation of the JAK/STAT pathway and possibly the Ras signal transduction pathway. With receptor activation, there is an increase in megakaryocyte number, increased nuclear mass, increased ploidy, and cytoplasmic maturation and release of platelets.
In Vitro Properties
In vitro studies have demonstrated that thrombopoietin stimulates all stages of megakaryocytopoiesis.[26,31,35,36] It acts as a megakaryocyte colony-stimulating factor in the early stages of megakaryocytopoiesis to stimulate the proliferation of megakaryocytic progenitors and in later stages to promote maturation of megakaryocytes into large polyploid cells capable of producing platelets (Figure 2). In suspension cultures, thrombopoietin increases the size and ploidy of megakaryocytes and the expression of Ib and IIb/IIIa glycoproteins on the cell surface.[18,37,38] Thrombopoietin-induced megakaryocytes express increased formation of demarcation membranes, platelet-specific granules, and platelet territories.[39,40]
Studies have demonstrated that thrombopoietin is essential for full megakaryocyte maturation in cell cultures. In these studies, the soluble form of the Mpl receptor was used to neutralize all of thrombopoietin’s biological activity. The addition of this soluble receptor terminated megakaryocyte formation despite the presence of C-kit ligand (stem cell factor), interleukin-6 (IL-6), or inter-leukin-11 (IL-11). However, in the presence of interleukin-3 (IL-3), production of megakaryocytes decreased but did not cease entirely; in addition, megakaryocytes lacked demarcation membranes and platelet-specific granules and had low ploidy (4N) levels.
The effects of thrombopoietin are not limited to megakaryocytopoiesis. In vitro studies demonstrate that, in the presence of erythropoietin, thrombopoietin enhances the growth of erythroid progenitor cells.[41,42] In addition, thrombopoietin directly stimulates primitive hematopoietic stem cells in the presence of early-acting cytokines (IL-3 or C-kit ligand) and exhibits both proliferative and differentiative effects on these progenitor cells.[42-44]
Physiologic Role and Regulation
The role of thrombopoietin as a physiologic regulator of platelet production has been supported by the inverse relationship between the platelet count and serum thrombopoietin level in animal models of thrombocytopenia induced by antiplatelet-antiserum or by chemotherapy or radiation treatment, as well as in cancer patients following myeloablative chemotherapy and bone-marrow transplantation.[45-49]
The importance of the c-Mpl ligand and receptor system in physiologic regulation of platelet production in vivo is further demonstrated by the generation of c-Mpl and thrombopoietin knock-out mice. Mice genetically altered to be defective in c-Mpl or the thrombopoietin gene exhibit an 85% reduction in peripheral platelet count, as well as decreased bone marrow and splenic megakaryocytes.[50,51]
In addition, the lack of thrombopoietin or c-Mpl results in significant reductions in the levels of erythroid, myeloid, and multipotential progenitor cells. This indicates that thrombopoietin acts not only on cells committed to megakaryocyte lineage but also on primitive hematopoietic progenitor cells.[52,53]
As in other thrombocytopenic animal models, circulating thrombopoietin levels are elevated in c-mpl-/- mice.[50,54] However, there is no detectable difference between the c-mpl-/-and c-mpl+/+ mice with respect to thrombopoietin mRNA levels in any tissue. This suggests that transcriptional regulation of the thrombopoietin gene is not involved in the increased level of thrombopoietin observed during thrombocytopenia.
An alternative theory of thrombopoietin regulation states that circulating thrombopoietin levels are regulated by platelet mass. Recent studies support this hypothesis.[16,17,55] Injection of normal platelets into c-mpl-/- mice resulted in a decrease in plasma thrombopoietin level and normalization of platelet levels.[54,55] These findings suggest that thrombopoietin is constitutively synthesized and released into the circulation by the liver and/or kidney. Under normal circumstances, platelets clear most of the thrombopoietin from the circulation via binding to the c-Mpl receptors.[51,54,55]
In disorders of platelet production, such as aplastic anemia, or in patients undergoing chemotherapy or radiotherapy, plasma thrombopoietin levels are elevated. This is due most likely to a decrease in the clearance mechanism related to the low platelet mass. However, in conditions in which low platelet numbers are accompanied by normal or increased numbers of megakaryocytes (such as idiopathic thrombocytopenic purpura), circulating thrombopoietin levels remain normal since thrombopoietin binds to megakaryocytes via c-Mpl receptors.
In normal mice, administration of thrombopoietin causes a rapid, four- to sixfold rise in circulating platelet count, along with increases in number, size, and ploidy of bone marrow and splenic megakaryocytes.[27,35,36] The magnitude of platelet response to thrombopoietin is higher than has previously been achieved with other thrombopoietic cytokines. In addition, thrombopoietin also increases the number of burst-forming unit-erythroid (BFU-E) in the bone marrow and spleen.
Although, in normal animals, thrombopoietin exerts its effects exclusively on platelets, in the myelosuppressed murine model, administration of recombinant murine thrombopoietin accelerated not only platelet recovery but also red blood cell recovery.[57,59,60] Thrombopoietin-treated animals showed an increase in megakaryocytic as well as myeloid and erythroid progenitors in the bone marrow and spleen. Thus, the effect of thrombopoietin on multiple cell lineages at the progenitor-cell level, along with increased levels of other endogenous cytokines in a severely myelosuppressed model, may enhance recovery of other cell lineages.
In the myelosuppressed nonhuman primate model, thrombopoietin also ameliorated platelet nadirs following whole-body irradiation. These preclinical trials have shown major activity of this cytokine without significant toxicity.
1. Elias A, Ryan L, Aisner J, et al: Mesna, Doxorubicin, ifosfamide,
dacarbazine (MAID) regimen for adults with advanced sarcoma. Semin
Oncol 17(2 suppl 4):41-49, 1990.
2. Schutte J, Mouridsen H, Stewart W, et al: Ifosfamide plus
doxorubicin in previously untreated patients with advanced soft
tissue sarcoma. Cancer Chemother Pharmacol 31(suppl 2):204-209, 1993.
3. Hill M, Macfarlane V, Moore J, et al:
Taxane/platinum/anthracycline combination therapy in advanced
epithelial ovarian cancer. Semin Oncol 24(1; suppl 2):34-37, 1997.
4. Veldhuis G, Williams P, Beijnew J, et al: Paclitaxel, ifosfamide
and cisplatin with granulocyte colony-stimulating factor or
recombinant human interleukin 3 and granulocyte colony stimulating
factor in ovarian cancer: A feasibility study. Br J Cancer
5. Williams S, Birch R, Einhorn L, et al: Treatment of disseminated
germ-cell tumor with cisplatin, bleomycin, and either vinblastine or
etoposide. N Engl J Med 316:1435-1440, 1987.
6. Loehrer P, Lauer R, Roth B, et al: Salvage therapy in recurrent
germ cell cancer: Ifosfamide and cisplatin plus either vinblastine or
etoposide. Ann Intern Med 109:540-546, 1988.
7. Krigel R, Palackdharry C, Padavic K, et al: Ifosfamide,
carboplatin and etoposide plus granulocyte colony-stimulating factor:
A phase I study with apparent activity in non-small-cell lung cancer.
J Clin Oncol 12:1251-1258, 1994.
8. Bonadonna G, Valagussar P, Santos A: Alternating non-cross
resistant combination chemotherapy or MOPP in stage IV Hodgkin’s
disease. Ann Intern Med 104:739-746, 1986.
9. Goss P, Shepherd F, Scott G, et al: Dexamethasone/ ifosfamide/
cisplatin/etoposide (DICE) as therapy for patients with advanced
refractory non-Hodgkin’s lymphoma: Preliminary report of a phase
II study. Ann Oncol 2(suppl 1):43-46, 1991.
10. Dana B, Dahlberg S, Miller T, et al: M-BACOD treatment for
intermediate and high-grade malignant lymphoma: A Southwest Oncology
Group phase II trial. J Clin Oncol 8:1155-1162, 1990.
11. Pruesser P, Wilke H, Achterrath W, et al: Phase II study with the
combination etoposide, doxorubicin, and cisplatin in advanced
measurable gastric cancer. J Clin Oncol 7(9):1310-1317, 1989.
12. Bernstein SH, Nademanee A, Vose J, et al: A multicenter study of
platelet recovery and utilization in patients after myeloablative
therapy and hematopoietic stem-cell transplantation: Cytokine growth
factors in hematology and oncology. Blood 91(9):3509-3517, 1998.
13. Wallace EL, Churchill WH, Suregenor DM, et al: Collection and
transfusion of blood and blood components in the United States, 1992.
Transfusion 35:802-812, 1995.
14. Heyman MR, Schiffer CA: Platelet transfusion therapy for the
cancer patient. Semin Oncol 17:198-209, 1990.
15. Malone D, Sullivan S, Black D, et al: The cost of treating
chemotherapy-induced thrombocytopenia (abstract). Proc Am Soc Clin
Oncol 14:305, 1995.
16. Kuter DJ: The physiology of platelet production. Stem Cells
14(suppl 1):88-101, 1996.
17. Gewirtz A: Megakaryocytopoiesis: The state of the art. Thromb
Haemost 74(1):204-209, 1995.
18. Hoffman R, Murray L, Young J, et al: Hierarchical structure of
human megakaryocyte progenitor cells. Stem Cells 14(suppl l):75-81, 1996.
19. Kelemen E, Cserhati I, Tanos B: Demonstration and some properties
of human thrombopoietin in thrombocythaemic sera. Acta Haematol
20. Vignon I, Mornon J, Cocault L, et al: Molecular cloning and
characterizations of Mpl, the human homolog of the v-mpl oncogene:
Identification of a member of the hematopoietic growth factor
superfamily. Proc Natl Acad Sci USA 89:5640, 1992.
21. Wendling F, Varlet P, Charon M, et al: A retrovirus complex
inducing an acute myeloproliferative leukemia disorder in mice.
Virology 149:242, 1986.
22. Souyri M, Vigon I, Penciolelli JF, et al: A putative truncated
cytokine receptor gene transduced by the myeloproliferative leukemia
virus immortalizes hematopoietic progenitors. Cell 63:1137-1147, 1990.
23. Methia N, Louache F, Vainchenker W, et al: Oligodeoxynucleotides
antisense to the proto-oncogene c-Mpl specifically inhibits in vitro
megakaryocytopoiesis. Blood 82:1395-1401, 1993.
24. Bartley TD, Bogenberg J, Hunt P, et al: Identification and
cloning of a megakaryocyte growth and development factor that is a
ligand for the cytokine receptor Mpl. Cell 77:1117-1124, 1994.
25. de Savage FJ, Hass PB, Spencer SD, et al: Stimulation of
megakaryocytopoiesis and thrombopoiesis by the c-Mpl ligand. Nature
26. Kuter DJ, Beeler DL, Rosenberg RD: The purification of
megapoietin: A physiological regulator of megakaryocyte growth and
platelet production. Proc Natl Acad Sci USA 91:11104-11108, 1994.
27. Lok S, Kaushansky K, Holly RD, et al: Cloning and expression of
murine thrombopoietin CDNA and stimulation of platelet production in
vivo. Nature 369:565-568, 1994.
28. Sohma Y, Akahori H, Seki N, et al: Molecular cloning and
chromosomal localization of the human thrombopoietin gene. Federation
of European Biochemical Societies Lett 353:57-61, 1994.
29. Wendling F, Maraskovsky E, Debili N, et al: c-Mpl ligand as a
humoral regulator of megakaryopoiesis. Nature 369:571-574, 1994.
30. Chang M, McNinch J, Basu R, et al: Cloning and characterization
of the human megakaryocyte growth and development factor (MGDF) gene.
J Biol Chem 270(2):511-514, 1995.
31. Foster D, Lok S: Biological roles for the second domain of
thrombopoietin. Stem Cells 14(suppl 1):102-107, 1996.
32. Debili N, Wendling F, Cosman, et al: The Mpl receptor is
expressed in the megakaryocyte lineage from later progenitors to
platelets. Blood 85:391, 1995.
33. Le Conniat M, Souyri M, Vigon I, et al: The human homolog of the
myeloproliferative leukemia virus maps to chromosome band 1p34. Hum
Genet 83:194-196, 1989.
34. Gurney A, de Sauvage F: Dissection of c-Mpl and thrombopoietin
function: Studies of knockout mice and receptor signal transduction.
Stem Cells 14(suppl 1):116-123, 1996.
35. Kaushansky K, Lok S, Holly RD, et al: Promotion of megakaryocyte
progenitor expansion and differentiation by the c-Mpl ligand
thrombopoietin. Nature 369:568-571, 1994.
36. Broudy VC, Lin NL, Kaushansky K: Thrombopoietin (c-Mpl ligand)
acts synergistically with erythropoietin, stem cell factor, and
interleukin-11 to enhance murine megakaryocyte colony growth and
increases megakaryocyte ploidy in vitro. Blood 85:1719, 1995.
37. Kaushansky K, Broudy VC, Lin N, et al: Thrombopoietin, the
Mpl-ligand, is essential for full megakaryocyte development. Proc
Natl Acad Sci USA 92:3234-3238, 1995.
38. Ziegler FC, de Sauvage F, Widmer HR, et al: In vitro
megakaryocytopoietic and thrombopoietic activity of c-Mpl ligand
(TPO) on purified murine hematopoietic stem cells. Blood 84:4045, 1994.
39. Zucker-Franklin D: Megakaryocyte and platelet structure in
thrombocytopoiesis: The effect of cytokines. Stem Cells 14(suppl
40. Kaushansky K: Thrombopoietin: The primary regulator of platelet
production. Blood 86(2):419-431, 1995.
41. Kobayashi M, Layer HJ, Kato T, et al: Recombinant human
thrombopoietin (Mpl ligand) enhances proliferation of erythroid
progenitors. Blood 86:2494-2499, 1995.
42. Sitnicka B, Lin N, Priestley GV, et al: The effect of
thrombopoietin on the proliferation and differentiation of murine
hematopoietic stem cells. Blood 87:87:4998-5005, 1996.
43. Young JC, Bruno B, Luens KM, et al: Thrombopoietin stimulates
megakaryocytopoiesis, myelopoiesis and expansion of CD34+Thy-l+Lin
primitive progenitor cells. Blood 88:1619-1631, 1996.
44. Ramsfjell V, Borge OJ, Vieby OP, et al: Thrombopoietin, but not
erythropoietin, directly stimulates multi-lineage growth of primitive
murine bone marrow progenitor cells in synergy with early acting
cytokines: Distant interactions with the ligands for c-kit and FLT3.
Blood 88(12):4481- 4492, 1996.
45. Kuter DJ, Rosenberg RD: The appearance of a megakaryocyte growth
promoting activity, megapoietin, during acute thrombocytopenia in the
rabbit. Blood 84:1464-1472, 1995.
46. Cohen-Solal K, Villeval J-L, Titeux M: Constitutive expression of
Mpl ligand transcripts during thrombocytopenia or thrombocytosis.
Blood 88:2578-2584, 1996.
47. Nichol JL, Hokom MM, Homkohl A, et al: Megakaryocyte growth and
development factor: Analyses of in vitro effects on human
megakaryopoiesis and endogenous serum levels during
chemotherapy-induced thrombocytopenia. J Clin Invest 95:2973-2979, 1995.
48. Meng YG, Martin TG, Peterson ML: Circulating thrombopoietin
concentrations in thrombocytopenic patients, including cancer
patients following chemotherapy, with or without peripheral blood
progenitor cell transplantation. Br J Haematol 95(3):535-544, 1996.
49. Emmons R, Reid DM, Cohen RL: Human thrombopoietin levels are high
when thrombocytopenia is due to megakaryocyte deficiency and low when
due to increased platelet destruction. Blood 87:4068-4071, 1996.
50. Gurney AL, Caver-Moore K, de Sauvage FJ, et al: Thrombocytopenia
in c-Mpl-deficient mice. Science 265:1445-1447, 1994.
51. de Sauvage F: Physiologic regulation of early and late stages of
megakaryocytosis by thrombopoietin. J Exp Med 183:651-656, 1996.
52. Alexander WS, Roberts AW, Nicola NA: Deficiencies in progenitor
cells of multiple hematopoietic lineages and defective
megakaryocytopoiesis in mice lacking the thrombopoietin receptor
c-Mpl. Blood 87:2162-2170, 1996.
53. Carver-Moore K, Broxmeyer HE, Luoh SM: Low levels of erythroid
and myeloid progenitors in thrombopoietin and c-Mpl-deficient mice.
Blood 88: 803-808, 1996.
54. Gurney A, deSauvage F: Dissection of c-Mpl and thrombopoietin
function: Studies of knockout mice and receptor signal transduction.
Stem Cells 14(suppl 1):116-123, 1996.
55. Fielder P, Gurney A, Stefanich E, et al: Regulation of
thrombopoietin levels by c-Mpl-mediated binding to platelets. Blood
56. Tahara T, Usuki K, Sato H, et al: A sensitive sandwich ELISA for
measuring thrombopoietin in human serum: Serum thrombopoietin levels
in healthy volunteers and in patients with haematopoietic disorders.
Br J Haematol 93:783-788, 1996.
57. Kaushansky K, Lin N, Grossman A, et al: Thrombopoietin expands
erythroid, granulocyte-macrophage and megakaryocytic progenitor cells
in normal myelosuppressed mice. Exp Hematol 24(2):265-269, 1996.
58. Ulich TR, del Castillo J, Yin S, et al: Megakaryocyte growth and
development factor ameliorates carboplatin-induced thrombocytopenia
in mice. Blood 86:971-976, 1995.
59. Hokom MM, Lacey D, Kinstler O, et al: Pegylated megakaryocyte
growth and development factor abrogates the lethal thrombocytopenia
associated with carboplatin and irradiation in mice. Blood
60. Thomas GR, Thibodeaux H, Errett CJ, et al: In vivo biological
effects of various forms of thrombopoietin in a murine model of
transient pancytopenia. Stem Cells 1:245-266, 1996.
61. Farase A, Hunt P, Boone T, et al: Recombinant human megakaryocyte
growth and development factor stimulates thrombocytopoiesis in normal
nonhuman primates. Blood 86:54-59, 1995.
62. Nichol J: Preclinical biology of megakaryocyte growth and
development factor: A summary. Stem Cells 14(suppl 1):48-52, 1996.
63. Basser R, Rasko J, Clarke K, et al: Thrombopoietic effects of
pegylated recombinant human megakaryocyte growth and development
factor (PEG-HuMGDF) in patients with advanced cancer. Lancet
64. Basser R, Rasko J, Clarke K, et al: Randomized, blinded,
placebo-controlled phase I trial of pegylated recombinant human
megakaryocyte growth and development factor with filgrastim after
dose-intensive chemotherapy in patients with advanced cancer. Blood
65. Fanucchi M, Glaspy J, Crawford J, et al: Effects of polyethylene
glycol-conjugated recombinant human megakaryocyte growth and
development factor on platelet counts after chemotherapy for lung
cancer. N Engl J Med 336:404-409, 1997.
66. Crawford J, Glaspy J, Belani C, et al: A randomized,
placebo-controlled, blinded, dose scheduling trial of pegylated
recombinant human megakaryocyte growth and development factor
(PEG-rHuMGDF) with filgrastim support in non-small-cell lung cancer
(NSCLC) patients treated with paclitaxel and carboplatin during
multiple cycles of chemotherapy (abstract). Proc Am Soc Clin Oncol
67. Vadhan-Raj S, Patel S, Broxmeyer HE, et al: Phase I-II
investigation of recombinant human thrombopoietin (rhTPO) in patients
with sarcoma receiving high-dose chemotherapy with Adriamycin and
ifosfamide. Blood 88:448a, 1996.
68. Vadhan-Raj S, Murray L, Bueso-Rumos C, et al: Stimulation of
megakaryocyte and platelet production by a single dose of recombinant
human thrombopoietin in patients with cancer. Ann Intern Med
69. Vadhan-Raj S, Verschraegen C, McGarry L, et al: Recombinant human
thrombopoitin (rhTPO) attenuates high-dose carboplatin (C)-induced
thrombocytopenia in patients with gynecologic malignancy (abstract).
Blood 90:580a, 1997.
70. Connors J, Curie LM, Allan H, et al: Recovery of in vitro
functional activity of platelet concentrates stored at 4°C and
treated with second-messenger effectors. Transfusion 36:691-698, 1996.
71. Currie LM, Vadhan-Raj S, and Connor J: Cryopreservation of
platelets from recombinant human thrombopoietin (rhTPO)-treated
donors using thrombosol and 2% DMSO. J Am Soc Hematol 88(suppl
72. Kuter D: Thrombopoietin: Biology, clinical applications, role in
the donor setting. J Clin Apheresis 11(3):149-159, 1996.
73. Siemensma NP, Bathal PS, Penington DG: The effect of massive
liver resection on platelet kinetics in the rat. J Lab Clin Med
74. Ezumi Y, Takayama H, Okuma M: Thrombopoietin, c-Mpl ligand,
induces tyrosine phosphorylation of Tyk2, JAK2 and STAT3, and
enhances agonist-induced aggregation in platelets in vitro.
Federation of European Biochemical Societies Lett 3 74:48-52, 1995.
75. Toombs CF, Young CH, Glaspy JA: Megakaryocyte growth and
development factor (MGDF) moderately enhances in vitro platelet
aggregation. Thromb Res 80:23-33, 1995.
76. Yan X-Q, Lacey D, Fletcher, et al: Chronic exposure to retroviral
vector encoded MGDF (mpl-Ligand) induces lineage-specific growth and
differentiation of megakarocytes in mice. Blood 88:4025-4033, 1995.
77. Columbyova L, Loda M, Scadden DT: Thrombopoietin receptor
expression in human cancer cell lines and primary tissues. Cancer Res
78. Vigon I, Dreyfus F, Melle J, et al: Expression of the c-Mpl
protooncogene in human hematologic malignancies. Blood 82:877-883, 1993.