Erythropoietin(EPO), a member of the type I cytokine superfamily, was first identified as the hormone that stimulates erythroid progenitors within the bone marrow to mature into erythrocytes. The main site of production of EPO is from the kidney and to a much lesser extent from the liver. In the kidney, certain interstitial fibroblasts appear to be a major source of EPO; however, other studies suggest an important role of proximal tubular cells as well.
EPO acts synergistically with other cytokines to promote the proliferation, differentiation, and survival of progenitor cells in the erythroid lineage and boosts the production of erythrocytes. It does not influence the fate of the pluripotent stem cell, but acts on the colony forming unit-erythroid (CFU-E) cells to prevent their apoptosis and induce expression of erythroid specific proteins.
The EPO binding to its receptor results in homodimerization of the receptor, followed by activation of several signal transduction pathways: JAK2/STAT5 system, G-protein (RAS), calcium channel, and kinases [Figure ). A gain of function mutation in JAK2 has been reported in patients with polycythemia vera and other myeloproliferative diseases. EPO also acts on angiogenesis, vasculogenesis, regulation of vascular resistance, and neuroprotection.
Pharmacology of the Current Preparations of Recombinant Human Erythropoietin Available for use
- Erythropoietin alpha: Epoetin alpha is an isoform of recombinant DNA-derived erythropoietin (rEPO), synthesized in Chinese hamster ovary (CHO) cells. It differs from the beta isoform in its migration on isoelectric focusing (IEF) and in a range of lectin-binding assays.
- Erythropoietin beta: Epoetin beta is also synthesized by CHO cell lines and differs from epoetin alpha in containing:
- a greater proportion of more basic isoforms,
- a greater proportion of EPO binding to Erythrina cristagalli agglutinin, and
- isoforms with higher in vivo: In vitro bioactivity.
- Darbepoetin alpha (a hyperglycosylated rhEPO): It has five N-glycosylation sites as compared to three in the rhEPO. This is created by a process called “site mutagenesis” and confers higher negative charge and threefold longer half-life. This is based on the principle that increase in number of glycosylation sites may enhance its activity. This helps in giving once a week dosing strategy.
- Continuous erythropoietin receptor activator (CERA): Methoxy polyethylene glycol-epoetin beta is a third-generation molecule, incorporating a large polymer chain and has an elimination half-life in humans that is up to 6 times longer than darbepoetin alpha and up to 20 times longer than epoetin, making it possible for once in 2 weeks to once a month dosing strategy. The successful conversion of patients on epoetin or darbepoetin to CERA has been successfully demonstrated.
Clinical Applications of Recombinant Human Erythropoietin
· Anemia associated with chronic kidney disease on dialysis
· Anemia of chronic disease
· Anemia in HIV-infected patients
· Patients on hepatitis C treatment
· Cancer/chemotherapy related anemia
· Use of erythropoietin in myelodysplastic syndrome
Use of erythropoietin in research settings
Heart failure: Recombinant EPO therapy has been found to be useful in patients with heart failure, especially with the cardio-renal anemia syndrome. Some recent studies show reduction in cardiac remodeling, Brain Natriuretic eptide levels, and hospitalization rate, resulting in improvement in left and right ventricular systolic function.
Stroke: There is a lot of interest in the role of EPO as a neuroprotective agent in ischemic stroke based on preclinical studies and one pilot study; however, a recent study failed to show any benefit and raised some doubts regarding the safety of EPO in such patients.
Acute kidney injury: The role of EPO in acute kidney injury (AKI) is undergoing active research and animal studies have revealed a physiological basis for the use of erythropoietin in AKI; however, a recent study failed to show any benefit.
Adverse effects of recombinant human erythropoietin
- Flu-like symptoms: Commonest side effect which subsides within 24 hours
- Allergic and anaphylactic reactions
- Seizures and hyperkalemia: Rare
- Thrombosis: A meta-analysis involving nearly 10,000 cancer patients indicates that treatment with rhEPO increases the risk of thrombosis
- Possibility of cancer progression: There is somewhat less convincing evidence that rhEPO enhances tumor progression
- Pure red cell aplasia (mainly reported in patients with CKD): Autoantibodies in the serum can neutralize both rhEPO and endogenous EPO. This was mainly observed in CKD patients, especially after SC injection. Its incidence after 2000 has reduced, especially with the IV formulations
Recent Areas of Research in Erythropoietin
With the discovery of EPO-R in non-erythroid tissue, pleiotropic effects of EPO were understood. Some areas of research with EPO as a novel therapeutic agent are mentioned below:
Spinal cord injury (SCI): Recently, research has focused on rhEPO and its effects on SCI treatment as well as the mechanisms such as anti-apoptotic, anti-inflammatory, and edema reduction, leading to neuronal and oligodendrocytes’ survival and restoration of vascular integrity.
EPO in depression: A current study is underway to evaluate the potential for EPO to alleviate depression and neurocognitive deficits in affective disorders among treatment-resistant cases.
EPO in diabetes: EPO has been found to affect all phases of wound healing and shows encouraging results for chronic wound healing in experimental animal and human studies, especially in the management of patients with chronic diabetic wounds.
EPO as an immunomodulating agent: A recent article shows that macrophages act as direct targets of EPO which enhances the pro-inflammatory activity and function of these cells.
Abuse of Erythropoietin in Competitive Sports
To improve physical fitness and endurance exercise in sports: Administration of rhEPO increases the body’s maximum oxygen consumption capacity, thus increasing endurance and physical fitness. This has led to the misuse of rhEPO in sports. In 1990, the International Olympic Committee (IOC) prohibited the use of EPO in sports. With different types of EPO available in the market, it is even more challenging to detect them from the law enforcement point of view.
The most important recombinant EPOs and analogues misused in sports are:
- Darbepoetin alpha