Erythropoietin

Erythropoietin (Epo) is a hormone produced by the kidneys in response to hypoxia, and is also a prescription drug used for treating anemia. It is essential for normal development and maturation of red blood cells (RBC), and abnormally high levels of either the endogenous or drug form can lead to dangerously high hematocrit values.

History

Carnot and DeFlandre made initial observations in rabbits that suggested the existence of a factor in peripheral blood that could stimulate production of reticulocytes. Their experiment involved bleeding a rabbit to induce accelerated RBC production, they then transferred some of the plasma to a recipient animal. The key observation of increased reticulocytes in the recipient animal prompted the search for a substance, which they named hemopoietin, that regulated the rate of RBC production.

A major breakthrough came in 1977, when small amounts of erythropoietin were purified from the urine of patients with aplastic anemia. Amino acid sequence data from this protein were used in subsequent efforts to clone the gene for erythropoietin in 1983. The gene was then inserted into a suitable mammalian cell line, Chinese hamster ovary cells, allowing large-scale manufactureof the protein as a commercial product. It was approved for use in 1991. About $10B was spent worldwide in 2006 for treatment of patients with rHuEpo, with about $2B for the cost of treating Medicare patients on dialysis.

Nomenclature

Erythropoietin exists in several forms and goes by several names. The endogenous form is also referred to as 'epoetin alfa' and sometimes spelled as 'erythropoetin'; it can be abbreviated to EPO, Epo, or EP. Various synthetic forms of recombinant (r) human (h) Epo are available, collectively referred to as rHuEpo or rhEpo. These include:

• Procrit, the trade name of epoetin alfa marketed in the US by Ortho Biotech Products, L.P., a member of the Johnson & Johnson Family of Companies. It is approved for treatment of chemotherapy-related anemia in patients with most types of cancer; for the treatment of anemia in chronic kidney disease patients who are not on dialysis; for treatment of anemia related to zidovudine treatment in HIV patients; and for reducing the need for transfusions in patients undergoing some types surgery who are anemic or at significant risk for blood loss.
• Eprex, the trade name of epoetin alfa marketed outside the US by Ortho Biotech Products, L.P., a member of the Johnson & Johnson Family of Companies.
• Epogen, the trade name of epoetin alfa made and marketed by Amgen in the US for treatment of anemia in patients with chronic renal failure on dialysis.
• NeoRecormon, the trade name of epoetin beta marketed by Roche in Europe for treatment of anemia in patients with chronic renal failure and for treatment of anemia in people with solid tumours who are receiving platinum-based chemotherapy. Even though epoetin alfa and epoetin beta are both synthesized in Chinese hamster ovary cells, they differ in their erythropoietin isoform compositions and biological properties.
• Aranesp, the trade name of darbepoetin alfa, a hyperglycosylated mutant form of Epo produced and marketed by Amgen for treating anemia associated with chronic renal failure (CRF), including patients on dialysis and patients not on dialysis, and for treating anemia in patients with nonmyeloid malignancies where anemia is due to the effect of concomitantly administered chemotherapy.

Structure

Epo is a glycoprotein with a molecular mass of 30.4 kD. Its structure includes a 165-amino acid backbone with three N-linked carbohydrates attached to asparagines at amino acid positions 24, 38, and 83 and one O-linked carbohydrate attached to Ser126 . The carbohydrate residues allow for many possible isoforms and contribute to the stability of the hormone in vivo. Darbepoetin (see above) was created through site-directed mutation of two amino acid residues, allowing for two additional N-linked carbohydrate chains.

Production

Epo is produced by peritubular cells in the adult kidney, and in hepatocytes in the fetus. In adults, a small amount is also produced by the liver. The rate of Epo synthesis and secretion depends on local oxygen concentrations; hypoxia is the main stimulus for Epo production. The serum concentration of Epo in adults is normally 4-27 mU/mL. In adults with non-renal anemias, the serum concentration tends to increase with the severity of the anemia.

Actions

Epo's activities depend on successful interaction with its receptor, which is prominent on the surface of developing RBC in the bone marrow. Epo signaling acts to prevent or retard apoptosis, i.e., it acts as a survival factor for developing cells. The increase in RBC mass brought about by Epo stimulation of the bone marrow completes a self-regulating feedback loop, since (other things being equal), the increased RBC mass would lessen the hypoxia experienced by the kidney and thus, lessen Epo production.

Pharmaceutical Epo

EPO as a blood doping agent

Pharmaceutical Epo is sometimes used by nonanemic athletes to increase their body's oxygen-carrying capacity and thus gain an unfair advantage in competition. Besides the risk of disqualification for cheating, athletes who participate in this illicit use of Epo risk the complications of abnormally high RBC concentrations, which include abnormal blood clotting. Detection of illicit Epo use is challenging because endogenous and exogenous (pharmaceutical) Epo are almost identical. Several tests rely on altered patterns of glycosylation of Epo molecules shed in the urine. Other detection methods rely on altered parameters of RBC production such as hematocrit, reticulocyte hematocrit, the proportion of abnormally large RBC, the serum Epo level, and the soluble transferrin receptor concentration.

Potential future applications

Epo's activity in the bone marrow to increase RBC production hinges on its ability to inhibit apoptosis. Experimental treatment of diseases in which apoptosis is prominent have yielded promising initial results. For example, Epo has been proposed as being both safe and beneficial in acute stroke.