"As the first hematopoietic growth factor approved for human therapy, recombinant human EPO (rHuEPO) has been used for the treatment of anemia resulting from chronic renal failure, cancers (primarily chemotherapy-associated anemia), autoimmune diseases, AIDS, surgery, bone marrow transplantation and myelodysplastic syndromes, etc. Interestingly, recent studies have also observed that rHuEPO has non-blood system functions and shows the potential of being used as a neuroprotective drug for cerebral ischemia, brain trauma, inflammatory disease and neural degenerative disorders .
"Currently, three kinds of rHuEPO or rHuEPO analogs are commercially available, namely rHuEPO alpha, rHuEPO beta, and darbepoetin alfa . These three recombinant proteins bind to the same erythropoietin receptor, but differ in structure, degree of glycosylation, receptor-binding affinity and in vivo metabolism. Since the initial introduction of rHuEPO-alpha in the 1980s, clinicians quickly recognized the frequent dose/injection requirement of the drug as a significant shortcoming. The mean in vivo half-lives of rHuEPO alpha and rHuEPO beta administered intravenously or subcutaneously are only 8.5 and 17 hours respectively [9, 10]. Patients therefore need an injection schedule of daily, twice weekly or three times per week which imposes a burden on both patients and health care providers. Thus, there has been a longstanding need to develop recombinant EPO analogs having a longer in vivo half-life and/or enhanced erythropoietic activity.
"Attempts have been made in the prior art to genetically change or chemically modify the structure of the native EPO protein to either slow down its in vivo metabolism or improve its therapeutic properties. For example, there appears to be a direct correlation between the amounts of sialic acid-containing carbohydrates on the EPO molecule and its in vivo metabolism and functional activity. Increasing the carbohydrate content of the EPO molecule thus results in a longer half-life and enhanced activities in vivo [11, 12].
"Other attempts to extend the half-life of EPO have focused on increasing the molecular weight of the EPO protein through chemical conjugation with polyethylene glycol (PEGylation) and the like. PEGylated-EPO has a much larger molecular weight and is protected from being cleared from circulation and therefore has a longer plasma half-life . However, PEGylation may alter the protein structure resulting in unanticipated changes of function and specificity of the EPO moiety. There are also reports of increasing the molecular weight of EPO by other methods, such as to link the EPO molecule to a carrier protein (human albumin), or to form the homodimerization of two complete EPO molecules by using linking peptides (3- to 17-amino acids) or by chemical cross-linking reagents [17, 18, 19, 20]. While all these methods have achieved some success in extending the half-life and enhancing the activities of EPO, combining the EPO molecule with the Fc fragment of human immunoglobulin (IgG) in a fusion protein as described in the present application achieves unique advantages.
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