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Overall, therefore, rhEPO has proved both effective and safe in the treatment of a variety of clinical conditions and its range of therapeutic applications is likely to increase over the coming years.
Thrombopoietin (TPO) is the haemopoietic growth factor now shown to be the primary physiological regulator of platelet production. Although its existence had been inferred for several decades, its purification from blood proved an almost impossible task, due to its low production levels and the availability of only an extremely cumbersome TPO bioassay. Its existence was finally proved in the mid-1990s when thrombopoietin cDNA was cloned. This molecule is likely to represent an important future therapeutic agent in combating depressed plasma platelet levels, although this remains to be proved by clinical trials.
Platelets (thrombocytes) carry out several functions in the body, all of which relate to the arrest of bleeding. They are disc-shaped structures 1-2 mm in diameter, and are present in the blood of healthy individuals at levels of approximately 250 x 109/l. They are formed by a lineage-specific stem cell differentiation process, as depicted in Figure 6.8. The terminal stages of this process entails the maturation of large progenitor cells termed ‘megakaryocytes’. Platelets represent small vesicles which bud off from the megakaryocyte cell surface and enter the circulation.
A number of disorders have been identified that are primarily caused by the presence of abnormal platelet levels in the blood. Thrombocythaemia is a disease characterized by abnormal megakaryocyte proliferation, leading to elevated blood platelet levels. In many instances, this results in an elevated risk of spontaneous clot formation within blood vessels. In other instances, the platelets produced are defective, which can increase the risk of spontaneous or prolonged bleeding events.
Thrombocytopenia, on the other hand, is a condition characterized by reduced blood platelet levels. Spontaneous bruising, bleeding into the skin (purpura) and prolonged bleeding after injury represent typical symptoms. Thrombocytopenia is induced by a number of clinical conditions, including:
Figure 6.8. Simplified representation of the production of platelets from stem cells. CFU-megakaryocytes and in particular, mature megakaryocytes, are most sensitive to the stimulatory actions of TPO. These two cell types also display a limited response to IL-6, IL-11 and LIF
• bone marrow failure;
• chemotherapy (or radiotherapy);
• various viral infections.
TPO should alleviate thrombocytopenia in most instances by encouraging platelet production. Currently, the standard therapy for the condition entails administration of 5 units of platelets to the sufferer (1 unit equals the quantity of platelets derived in one sitting from a single blood donor). TPO therapy is a particularly attractive potential alternative because:
• it eliminates the possibility of accidental transmission of disease via transfusions;
• platelets harvested from blood donations have a short shelf-life (5 days), and must be stored during that time at 22°C on mechanical shakers;
• platelets exhibit surface antigens, and can thus promote antibody production. Repeat administrations may thus be less effective, due to the potential presence of neutralizing antibodies.
The most likely initial TPO therapeutic target is thrombocytopenia induced by cancer chemo- or radiotherapy. This indication generally accounts for up to 80% of all platelet transfusions undertaken. In the USA alone, close to 2 million people receive platelet transfusions annually. Human TPO is a 332 amino acid, 60 kDa glycoprotein, containing six potential N-linked glycosylation sites. These are all localized towards the C-terminus of the molecule. The N-terminal half exhibits a high degree of amino acid homology with EPO and represents the biologically active domain of the molecule.
Sources of TPO include kidney and skeletal muscle cells but it is primarily produced by the liver, from where it is excreted constantly into the blood. This regulatory factor supports the proliferation, differentiation and maturation of megakaryocytes and their progenitors and
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promotes the production of platelets from megakaryocytes (Figure 6.8). It also appears to modulate the growth/differentiation of progenitor cells, which eventually give rise to both erythrocytes and macrophages.
TPO induces its characteristic effects by binding to a specific TPO receptor present on the surface of sensitive cells. The receptor, also known as c-mpl, is a single chain, 610 amino acid transmembrane glycoprotein. The mechanism of signal transduction triggered upon TPO-binding remains to be elucidated.
Currently, at least two recombinant TPO-based products are being assessed in clinical trials. One product is a recombinant glycosylated form produced in a mammalian cell line, the other is a non-glycosylated variant, expressed in E. coli. The latter molecule, also known as megakaryocyte growth and development factor (MGDF), is PEGylated subsequent to its purification. PEGylation, as already described in the context of several other protein-based therapeutics, extends the molecule’s plasma half-life.