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erythropoietin
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| Identifiers | |
| Symbol(s) | EPO |
| Entrez | 2056 |
| OMIM | 133170 |
| RefSeq | NM_000799 |
| UniProt | P01588 |
| Other data | |
| Locus | Chr. 7 q21 |
Erythropoietin (IPA pronunciation: [ɪˌɹɪθ.ɹoˈpo.ɪ.tɪn], alternative pronunciations: [ɪˌrɪθroʊˈpɔɪtn, əˌrɪθroʊ-, əˌriθroʊ-]) or EPO is a glycoprotein hormone that is a cytokine for erythrocyte (red blood cell) precursors in the bone marrow. Also called hematopoietin or hemopoietin, it is produced by the kidney, and is the hormone regulating red blood cell production. Erythropoietin is available as a therapeutic agent produced by recombinant DNA technology in mammalian cell culture. It is used in treating anemia resulting from chronic renal failure or from cancer chemotherapy. Its use is also believed to be common as a blood doping agent in endurance sports such as bicycle racing, triathlons and marathon running.
Contents |
Discovery and biological role
The existence of a humoral factor regulating red blood cell production was first postulated in 1906 based on transfusion experiments in rabbits. In 1950, the still unidentified erythropoietic factor was found to be stimulated in rats breathing a low-oxygen atmosphere, thus establishing the elements of its biological regulation. In the 1960s its source was identified as the kidneys. Human EPO was first purified from human urine by T. Miyake, C. K. Kung and E. Goldwasser at the University of Chicago in 1977. Limited quantitites of the native human protein were used experimentally to treat patients with anemia.
EPO has now been identified as a glycoprotein with a molecular mass of about 30,000 Daltons. It has a 165 amino acid chain with four oligosaccharide side chains and circulates in the blood plasma at a very low concentration (about 5 pmol/L).
In adult humans, EPO is produced primarily by peritubular cells in the kidneys, where its production is stimulated by low oxygen levels in the blood, also known as hypoxia. Some EPO is also produced by the liver, which is the primary source in the fetus.
EPO acts by binding to a specific erythropoietin receptor (EpoR) on the surface of red cell precursors in the bone marrow, stimulating them to transform into mature red blood cells. As a result the oxygen level in blood reaching the kidney rises and the amount of EPO produced decreases.
Because the kidneys are the primary source of erythropoietin, chronic kidney disease often results in a systemic deficiency of EPO and consequent anemia. Anemia can also occur in cancer patients, sometimes as a direct result of the malignancy but usually as an adverse effect of chemotherapy.
Also, in patients who may require a blood transfusion or undergo surgery where blood loss is expected, EPO is given in advance as a precaution. The bone marrow produces more red blood cells, and if blood is lost during the operation, there is still enough to sustain the patient.
EPO as a therapeutic agent
Therapeutic human erythropoietin was initially isolated and purified from urine in 1977. In 1983, the gene coding for erythropoietin was identified by a team headed by Fu-Kuen Lin at U.S. biotechnology company Amgen. Researchers at The Genetics Insitute (now part of Wyeth) independently discovered the gene at approximately the same time. The resulting patent dispute led to Amgen gaining exclusive marketing rights for erythropoietin in the U.S. Recombinant DNA technology was used to express the protein in Chinese hamster ovary cells, which allowed a synthetic form of EPO (rEPO) to be produced in commercial quantities for the first time.
Recombinant EPO was launched as a pharmaceutical product by Amgen for treatment of anemia resulting from chronic renal failure in 1989 under the brand name Epogen. In 1991 it was also approved for treating anemia resulting from cancer chemotherapy. However, recent clinical studies on patients with head and neck squamous cell carcinoma (HNSCC) and breast carcinoma, in addition to relevant basic science research have demonstrated that patients whose tumors are positive for the erythropoietin receptor (EpoR) may be at increased risk of tumor progression (metastatic spread) if treated with Epo. Johnson & Johnson (J&J), an American pharmaceutical company, markets EPO under license from Amgen for cancer chemotherapy under the brand name Procrit. Amgen’s patents have so far prevented other companies from entering the U.S. market. Even though the patents are all based on work done in the early 1980s, the last of them will not expire until 2015, thirty-two years after the date of the original application.
A longer-acting erythropoietin analogue, darbepoetin (dEPO), also known as novel erythropoiesis-stimulating protein (NESP), was launched by Amgen under the brand name Aranesp in 2001. Relative to rEPO, dEPO has a slightly different amino acid sequence and a greater number of oligosaccharide residues. Aranesp comes in 40mcg and 60mcg 1mL vials, and 100mcg and 200mcg prefilled syringes. It's primary advantage is it can be used only once every two weeks in patients without ESRD (end-stage renal disease).
Outside the U.S. Amgen’s patents did not prevail and two other brands of EPO are available: Eprex (J&J) and NeoRecormon. (Roche). Two new long-acting forms may be launched in Europe in 2006: CERA (Roche) and Dynepo (Shire)
EPO is generally injected subcutaneously (under the skin) by the patient, although it may also be given intravenously. Several injections weekly are required for the original forms, but the long-acting forms may require injections only once every two weeks. EPO cannot be used by patients with leukemia and should be used cautiously by patients with uncontrolled high blood pressure. An extremely rare but severe adverse effect of EPO is acquired pure red cell aplasia (PRCA), an auto-immune condition in which the bone marrow loses its ability to produce red blood cells, leaving the patient dependent on blood transfusions. Certain lots of the J&J Eprex product were associated with a higher incidence of this problem. Studies at J&J showed that the PRCA was caused by interaction of EPO protein with a rubber compound in the pre-filled syringe used to distribute the product. Substitution of a teflon-coated syringe plunger eliminated the problem. By far the most common side-effect for any EPO products is fever. Also, use of EPO products can lead to an increased chance of the formation of blood clots (see below). Headache, nausea, vomiting, and delerium are also common side-effects. These generally subside after 2-3 days. Fever lasting longer than 3 days or rising above 103 degrees Fahrenheit (39.5 °C) should be reported to your physician immediately. Concomminant use of aspirin with EPO products is encouraged to reduce the chance of clotting.
All forms of recombinant erythropoeitin are expensive. A dialysis patient, who can expect to require lifelong EPO treatment, will pay up to $10,000 per year for the drug in the U.S. Cancer chemotherapy patients, who require EPO for shorter periods, pay about $1,000 per month in the U.S. Worldwide revenues for sales of EPO were over USD$10 billion in 2004[1].
Erythropoietin as a blood doping agent
Although pharmaceutical EPO has benefited many thousands of anemic patients, it also has a converse side as a blood doping agent sometimes used by healthy athletes to gain a competitive advantage. By increasing the oxygen-carrying capacity of the blood it increases the aerobic respiratory capacity of the muscles, making it appealing to participants in endurance sports such as cycling and long-distance running. It is considered to be especially valuable in multi-stage bicycle races, where it can offset the decrease in red blood cells that occurs over several weeks of racing.
EPO doping probably became common in Grand Tour cycling after pharmaceutical EPO became available in the late 1980s, but there was at the time no way to prove its use in the absence of physical evidence of EPO possession. In retrospect, the deaths of a dozen or more elite cyclists in the early 1990s from heart failure while sleeping, were grim evidence of its overuse.
Excessive use of pharmaceutical EPO can lead to so many red blood cells that the blood becomes thick enough to strain the heart, especially during sleep when the heart rate is low. Beyond a certain point, the increase in haematocrit is actually detrimental to the oxygen carrying capacity due to negative haemodynamic effects, and increases the likelihood of developing symptoms similar to chronic mountain sickness.
The extent of the doping problem became undeniable after the Festina team scandal in the 1998 Tour de France, in which Willy Voet, a soigneur for the team, was apprehended with a huge cache of doping materials, including EPO. He[2] and others[3] later wrote about the widespread use of drugs in cycling.
In the wake of the Festina affair, the Union Cycliste Internationale (UCI) sought to limit EPO doping by testing riders' red blood cell concentrations as measured by hematocrit(HCT), the proportion by volume of blood cells that are red blood cells. A normal HCT in adult men is 41-50%. Under the UCI rule, any rider with a HCT over 50% was temporarily banned from racing. An independent organization, the World Anti-Doping Agency (WADA), was also created.
The response of EPO-doping athletes has been to monitor their HCT using portable blood centrifugation equipment and engineer their use of pharmaceutical EPO to maintain a HCT of just under 50%. When necessary, blood-thinning infusions are used to reduce HCT to a legal level, ideally 49.9%. Team management, doctors and coaches have in some cases been accused of complicity.
In the 2000 Tour de France, a laboratory test to detect residues of pharmaceutical EPO in urine was introduced for the first time as an anti-doping measure. It was developed by the French National Laboratory for Doping Detection (LNDD). The test method relies on laboratory methods for distinguishing pharmaceutical EPO from the endogenous EPO normally present in an athlete’s urine after strenuous exercise. Because existing brands of pharmaceutical EPO are made in cultured animal cells, they have a different pattern of oligosaccharide residues than the native human form.
Although widely applied, the test is controversial. In 2005, Rutger Beke, a Belgian triathlete successfully challenged his conviction for EPO doping by presenting scientific evidence that the test was unreliable in his case. Several other athletes have since made similar defenses. In the highest profile case to date, Spanish rider Roberto Heras was stripped of what would have been his fourth win of the Vuelta a España in 2005 after a positive urine test for pharmaceutical EPO. He has asserted his innocence and vowed to challenge the conviction based on the handling of his sample and alleged weaknesses in the test method.
If Dynepo becomes available as a pharmaceutical product in 2006 as expected, it may be a boon to doping athletes. It is to be manufactured in cultured human cells and should thus have an authentic pattern of human glycosylation, making it undetectable by the current test method.
Erythropoietin and Jehovah's Witnesses
Jehovah's Witnesses believe that they are to "abstain from blood"; this belief results in the refusal to accept transfusions of blood or blood components.
The use of erythropoietin to increase the number of red blood cells is one suggested alternative to a blood transfusion.
However, the presence of human albumin in the erythropoietin mixture may preclude its use in some Jehovah's Witnesses.
References
- ^ See http://www.i-s-b.net/business/rec_sales.htm Info Service Biotechnology; Sales of Recombinant Drugs.
- ^ Willy Voet, Massacre à la chaîne.
- ^ Christophe Bassons, Positif. Paul Kimmage, Rough Ride. Erwann Menthéour, Secret défonce.
- Miyake T, Kung CK, Goldwasser E. Purification of human erythropoietin. J Biol Chem. 1977 Aug 10;252(15):5558-64.
