Transferrin soluble receptor: a possible probe for detection of erythropoietin abuse by athletes

Current implementation and future of the Athlete Biological Passport

During the last four decades, the main instrument at the disposal of anti-doping authorities has been the detection of prohibited substances in biological samples collected from athletes. However, the availability of substances identical to those produced by the human body, such as EPO, testosterone and GH, necessitated a new drug-testing paradigm. From the early 2000's, the Athlete Biological Passport (ABP) was proposed as an alternative means to drug testing. Doping leaves a characteristic fingerprint on the biology of the athlete and the ABP is used to prove the act of doping from the detection of that fingerprint. Once a biomarker of doping is implemented in the ABP, it will continue to remain valid and should be able to detect the physiological changes brought on by performance-enhancing drugs that have not yet been invented. However, the sensitivity of the ABP to detect doping is limited if the physiological result of a low level of doping remains within the individual's own reference range. Recent advances in proteomics and metabolomics show the huge potential of the ABP.

Erythropoietin regulations in humans under different environmental and experimental conditions

In the adult human, the kidney is the main organ for the production and release of erythropoietin (EPO). EPO is stimulating erythropoiesis by increasing the proliferation, differentiation and maturation of the erythroid precursors. In the last decades, enormous efforts were made in the purification, molecular encoding and description of the EPO gene. This led to an incredible increase in the understanding of the EPO-feedback-regulation loop at a molecular level, especially the oxygen-dependent EPO gene expression, a key function in the regulation loop. However, studies in humans at a systemic level are still very scanty. Therefore, it is the purpose of the present review to report on the main recent investigations on EPO production and release in humans under different environmental and experimental conditions, including: (i) studies on EPO circadian, monthly and even annual variations, (ii) studies in connection with short-, medium- and long-term exercise at sea-level which will be followed (iii) by studies performed at moderate and high altitude.

Blood doping: present procedures and detection techniques

Blood doping represents a serious risk in endurance athletes. Blood transfusion practices (either autologous or homologous) have been used since 1960 and, despite the significant improvement in the laboratory methods, only homologous blood transfusion can be detected currently, while for autologous blood transfusion, no validated methods exist. In the last 15 years, a number of drugs have been developed to treat anemic patients. From recombinant erythropoietin to synthetic hemoglobin, all the developed tools are potentially useful to increase the oxygen transport to peripheral tissues in endurance athletes. Thus, the availability of doping-detection methods can only be sustained by the knowledge of any novel therapeutic approach in this field. The identification of the doping molecule is the gold standard of any antidoping campaign; despite this, indirect methods based on the detection of the effects induced by the doping procedure will be a very powerful tool in the near future. Nevertheless, while direct methods are only affected by the sensitivity and the specificity of the method itself (deterministic methods), indirect approaches are affected by the statistic weight of the results (probabilistic methods). Thus, blood doping will be better controlled by the combination of the two approaches.

Pharmacokinetic-pharmacodynamic modelling of recombinant human erythropoietin in athletes : a population approach

OBJECTIVE

To develop a pharmacokinetic model able to take into account the negative feedback loop of endogenous erythropoietin production observed after repeated administration of recombinant human erythropoietin (rHuEPO), and to propose a pharmacokinetic-pharmacodynamic model capable of assessing and quantifying the relationship between changes in: (i) serum soluble transferrin receptor (sTfR) levels, (ii) reticulocyte haematocrit (RetHct), and (iii) percentage macrocytes (%Macro) secondary to repeated administration of rHuEPO.

SUBJECTS AND METHODS

Eighteen trained athletes (three females and 15 males) participated in this study. They received subcutaneous injections of rHuEPO-α 50 U/kg bodyweight for 26 days (days 1, 3, 5, 9, 10, 12, 15, 17, 19, 22, 24 and 26) with iron supplementation. Venous blood samples were collected before, during and after rHuEPO treatment for determination of serum erythropoietin concentrations, haematological parameters (RetHct, %Macro) and sTfR levels. Population pharmacokinetic-pharmacodynamic calculations were performed using NONMEM® software.

RESULTS

The serum erythropoietin concentration-time profile was compatible with a one-compartment open model and first-order input rate. The mean half-lives calculated from the first and the terminal log-linear parts of the curves were 5.2 and 35.8 hours, respectively. After subcutaneous administration of rHuEPO, the terminal part of the curve should correspond to the absorption rather than the elimination phase ('flip-flop' phenomenon). The total clearance divided by bio-availability was 4.33 L/h. The pharmacodynamic relationship based on a sigmoid E(max) model can be reasonably used to relate changes observed in haematological and biochemical markers after rHuEPO administration to changes in serum erythropoietin concentrations. rHuEPO induces a delayed increase in sTfR levels, RetHct and %Macro. The half-life (t1/2) k(0) (equilibration delay) values were 10.2 days for sTfR, 2 days for RetHct and 10.2 days for %Macro. The pharmaco-kinetic-pharmacodynamic approach developed in this study allowed below-base-line decreases in RetHct levels (i.e. from days 10-26 after the end of rHuEPO treatment) to be taken into account. A negative-feedback loop of red blood cell production further to high haemoglobin and haematocrit values could explain this decrease.

CONCLUSIONS

The approach described here may provide an additional tool in the war against drug abuse by athletes; indeed, the model could be useful for simulating pharmacokinetic-pharmacodynamic relationships according to different rHuEPO dosage schedules.

Strategies for rhEPO detection in sport

This article examines available strategies for the detection of recombinant erythropoietin (rhEPO) abuse in sport. RhEPO was quickly recognized as an effective but hazardous performance-enhancing agent. In the absence of a valid procedure to detect rhEPO doping, at-competition health checks were introduced, which excluded athletes from competition when their hemoglobin or hematocrit values exceeded an arbitrary limit. This limited the danger to athletes, but did nothing to eliminate the use of rhEPO. Through the last decade, both direct and indirect methods for detecting rhEPO were investigated. No single indirect marker was found that satisfactorily demonstrated rhEPO use. A combination of blood and urine tests together formed the procedure and strategy approved by the International Olympic Committee (IOC) for detecting rhEPO use at the Sydney Olympics. However strategies for testing for EPO are as important as the developed laboratory analytical procedures. The use of extensive out-of-competition testing and analysis within the IOC accredited laboratory system is critical to any testing program. At-competition blood tests have merit as true health checks and will also be needed to detect acutely useful agents such as hemoglobin-based oxygen carriers. However the persistence of the "health check" rationale for on-site at-competition rhEPO testing has led to much wasted testing effort, as rhEPO use by athletes will rarely occur near to or at the time of the competition for fear of detection. Thus, direct testing methods (such as the rhEPO urine test) especially will fail due to the completed metabolism and elimination of administered rhEPO before the test, unless the international sporting federations use the information gathered to assist in targeted out-of-competition testing. This article discusses the limitations of testing at competition and proposed strategies for dealing with various phases of EPO doping in detail, concluding that no one single currently used strategy will detect all users of rhEPO. The development of strategies to diagnose rhEPO abuse may serve as a model to detect other biological agents.

Detection of DNA-recombinant human epoetin-alfa as a pharmacological ergogenic aid

The use of DNA-recombinant human epoetin-alfa (rhEPO) as a pharmacological ergogenic aid for the enhancement of aerobic performance is estimated to be practised by at least 3 to 7% of elite endurance sport athletes. rhEPO is synthesised from Chinese hamster ovary cells, and is nearly identical biochemically and immunologically to endogenous epoetin-alfa (EPO). In a clinical setting, rhEPO is used to stimulate erythrocyte production in patients with end-stage renal disease and anaemia. A limited number of human studies have suggested that rhEPO provides a significant erythropoietic and ergogenic benefit in trained individuals as evidenced by increments in haemoglobin, haematocrit, maximal oxygen uptake (VO2max) and exercise endurance time. The purpose of this review is to summarise the various technologies and methodologies currently available for the detection of illicit use of rhEPO in athletes. The International Olympic Committee (IOC) banned the use of rhEPO as an ergogenic aid in 1990. Since then a number of methods have been proposed as potential techniques for detecting the illegal use of rhEPO. Most of these techniques use indirect markers to detect rhEPO in blood samples. These indirect markers include macrocytic hypochromatic erythrocytes and serum soluble transferrin receptor (sTfr) concentration. Another indirect technique uses a combination of 5 markers of enhanced erythropoiesis (haematocrit, reticulocyte haematocrit, percentage of macrocytic red blood cells, serum EPO, sTfr) to detect rhEPO. The electrophoretic mobility technique provides a direct measurement of urine and serum levels of rhEPO, and is based on the principle that the rhEPO molecule is less negatively charged versus the endogenous EPO molecule. Isoelectric patterning/focusing has emerged recently as a potential method for the direct analysis of rhEPO in urine. Among these various methodologies, the indirect technique that utilises multiple markers of enhanced erythropoiesis appears to be the most valid, reliable and feasible protocol currently available for the detection of rhEPO in athletes. In August 2000, the IOC Medical Commission approved this protocol known as the 'ON model', and it was subsequently used in combination with a second, confirmatory test (isoelectric patterning) to detect rhEPO abusers competing in the 2000 Sydney Summer Olympics. This combined blood and urine test was approved with modifications by the IOC in November 2001 for use in the 2002 Salt Lake City Winter Olympics.

Soluble transferrin receptor (sTfR): biological variations and reference limits

The aim of this study was to establish soluble serum transferrin receptor (sTfR) reference limits. sTfR was measured in 885 healthy subjects from 3 to 91 years old (433 men, 409 women), without hematological abnormalities, using an immunonephelometric assay. The sTfR median concentrations in our population decreased gradually from the group aged 3-10 years to the group aged 21-40 years, then there were no changes in the older groups except for the females >60 years of age. The interindividual variability ranged from 12.6% to 30.3% among different age groups, and the analytical variability was 5%. Biological factors and other factors associated with sTfR concentration variation were examined and accounted for 35% of the sTfR variability in men aged 20 years or less, and 18% in those older than 20 years. Also, they accounted for 45% of the variability in women aged 20 years or less and 14% in those older than 20 years. The main factors statistically associated with sTfR concentration in males were ferritin, orosomucoid, hemoglobin, and tobacco in all age groups and only mean corpuscular volume (MCV) in males less than 20 years old. In the females the main factors were age, orosomucoid, and hemoglobin in all age groups, MCV and tobacco in females less than 20 years old, and ferritin and physical activity in females more than 20 years old. These factors were used to define the exclusion and partition criteria for obtaining the reference samples. Medians for reference values were: 1.60 mg/l in the 3-10-year old group (males and females); 1.42 mg/l in males between 11 and 20 years of age, and 1.33 mg/l in females of the same age. In the other age groups, the median of the reference values was 1.16 mg/l, except in females over 60 years old, for whom it was 1.26 mg/l.

Laboratory screening for erythropoietin abuse in sport: an emerging challenge

The growing diffusion of banned practice to improve the athletic performances is forcing clinical laboratories to identify and standardize reliable assays to detect potential unfairness. Among the doping practices, the use of recombinant human erythropoietin is becoming fairly popular, due to simplicity and safeties of administration and troublesome detection. The heterogeneous response rate, the presence of a little but significant amount of naturally occurring hormone, the short half-life exhibited by recombinant human erythropoietin and the lack of standardization of commercial assays appear the main problems to overcome. Aim of the present article is to provide a critical review of some of the more widespread laboratory techniques currently available for the screening for erythropoietin abuse in sport.

The measurement of serum transferrin receptor

The concentration of the soluble fragment of transferrin receptor in serum is an important new hematological parameter. Clinical and laboratory studies have shown that this serum form of the receptor reflects the total body mass of cellular transferrin receptor, 80% of which is contained in the erythroid marrow. The two disorders that result in an elevation in the serum transferrin receptor are anemias associated with enhanced erythropoiesis and tissue iron deficiency. The concentration of soluble transferrin receptor provides a useful quantitative measure of the erythroid marrow mass and thereby assists clinically in categorizing the type of anemia. The most important clinical use of the serum transferrin receptor is in determining the cause of iron deficient erythropoiesis (that is, identifying iron deficiency anemia whether it occurs alone or in the presence of the anemia of chronic disease). Present evidence supports the routine use of the serum transferrin receptor in the clinical evaluation of anemic patients.

Other ergogenic agents

Amid the surrounding chaos of the supplement blitz, athlete, coach, and physician alike must step back and place the issue of supplements into perspective. There is no legal supplement that can substantially alter performance to date in the same way as illegal drugs. Effectiveness, safety, legality, and purity of compounds are all issues that should be addressed when approaching the use of any supplement. Education about the validity of the claims of supplements is important. Research is lending useful and helpful information despite the many new products continually appearing on the market. Because there is no mechanism for investigations to adequately research every supplement, many of the supplements should be approached with caution and skepticism. In addition, supplements in and of themselves should not be viewed as the sole answer to performance improvement. There is some promise to an extremely small number of supplements that appear to enhance performance, yet they do so in the realm of complete athletic training, including hard work, sports-specific training and strength training, psychological preparedness, and good nutritional intake.