Erythropoietin’s Erythropoietic Analogues
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Erythropoietin’s erythropoietic effect is desirable for several medical conditions as well as for the performance of athletes. Erythropoiesis directly increases oxygen capacity and Vo2max, thereby increasing maximal cycling power, though it has been shown not to improve submaximal exercise performance. It is tempting to speculate that it may also improve recovery for athletes, though this has not been studied.
The human EPO gene was first isolated and cloned in 1985. Glycosylation and sulfation of the resulting protein depend on the enzymatic structure of the cells that produced it, thus, commercial EPO is followed by a Greek letter to indicate its glycosylation pattern.
In 1989, epoetin alpha, produced by CHO cells, was the first recombinant erythropoietin (rEPO) to be commercially available and was later FDA approved for the treatment of chemotherapy-induced anemia, for antiviral treatment of HIV patients, and for the preparation of blood donation prior to surgery. In 1990, epoetin beta, also produced by CHO cells, though with a different glycosylation pattern, became commercially available. Subsequent products included the third epoetin, omega, produced by BHK cells, and delta, produced by HT-1080 cells. Epoetin alpha, beta, deta, and omega had a half-life of about 8 and 24 hours when administered intravenously (IV) or subcutaneously (SC), respectively, necessitating thrice-weekly injections for patients requiring erythropoietic effects.
As the pharmaceutical industry required a product with a longer half-life, the novel erythropoiesis-stimulating protein (Anaresp or darbepoetin alfa) was developed by mutating five amino acids in the EPO protein. Darbepoetin alfa had less affinity at the EPOR but a much longer half-life of 21 and 49 hours when administered IV and SC, respectively. As IV administration is preferred, darbepoetin alfa is 3.6x more potent than previous rEPOs when administered thrice weekly and up to 14x more potent when administered once weekly; consequently, it could be dosed less frequently.
In 2007, continuous erythropoietin receptor activator (CERA), composed of CHO cell-derived epoetin beta attached to a methoxy polyethylene glycol (PEG) was launched commercially in Europe by Roche. The linkage to PEG lowered affinity for the EPOR but also reduced glomerular filtration by the kidneys, extending its half-life to 135 and 139 hours when delivered IV and SC, respectively.
USE OF rEPO IN ENDURANCE SPORTS
rEPO became a commercial success between 1987 and 1989 and was already banned by the International Olympic Committee in 1990, although there had yet to be a method to test for it. It was only in 2000 that a test was developed to distinguish between natural and rEPO and in the same year, a method was established to test for rEPO via serum markers of erythropoiesis. With the establishment of the Athlete Biological Passport (ABP) in 2008, athletes grew to microdose EPO to limit fluctuations in their blood markers on the ABP.
ALTERNATIVE MEANS OF IMPROVING ERYTHROPOETIC ACTIVITY
In addition to epoetin, darbepoetin alfa, and CERA, there are four other ways to increase erythropoiesis systemically. First, erythropoietin-mimetic peptides (EMPs), most notable of which is peginesatide (brand name Hematide or Takeda), are biomimetic peptides that differ in structure from EPO but activate the same pathways to induce erythropoiesis. Second, sotatrecept (ACE-011) includes a portion of the transforming growth factor beta activin tied to a segment of human immunoglobulin G1 that interferes with the SMAD pathway to trap activin. Originally developed to enhance bone mineral density, it was serendipitously found to increase hematocrit and red blood cell levels and exhibits a long half-life.
Recall that hypoxia-inducible factors (HIFs) coordinate the body’s response to hypoxia, inducing the expression of genes that lead to erythropoiesis. The third method of increasing erythropoiesis are HIF stabilizers, competitors to 2-oxoglutarate that block HIF prolyl hydroxylases and stabilize HIF a. HIF stabilizers are consumed orally and raise erythropoietin to physiologic levels, as opposed to the super-physiologic levels that can be reached with rEPO. Though still in development by pharmaceutical companies (e.g. FG-4592 and FG-2216 by FibroGen, AKB-6548 by Akebia, GSK 1278863 by GlaxoSmithKline, Bay 85-3934 by Bayer), HIF stabilizers are already available on the black market and were banned by WADA in 2011.
The final method to increase systemic EPO is gene doping. Humans with chronic kidney disease have been infected with a cytomegalovirus promoter that was designed to express EPO through an adenovector, successfully increasing EPO levels for two weeks. The technique is so promising that WADA had already banned gene therapy in 2006. Finally, it is interesting to note that testosterone administration increases hematopoiesis in manners mediated and not mediated by EPO production.
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