Red Blood Cell-Mimicking Synthetic Biomaterial Particles: The New Frontier of Blood Doping?

Blood doping has become an integral part of endurance sports over the past decades. It has been and it is still administered with the aim to boost the red blood cell (RBC) mass, thereby increasing the maximal aerobic power, ameliorating oxygen transport and delivery to the muscles, and ultimately improving the aerobic performance [6]. This unfair practice includes a variety of methods or substances, such as blood transfusions, administration of recombinant human erythropoietin and novel erythropoiesis-stimulating substances such as the continuous erythropoiesis receptor activator (CERA), artificial oxygen carriers, allosteric hemoglobin modulators, as well as various altitude training simulator systems [4] [5] [8]. Since most of these techniques produce substantial variations of the hematological profile, and can be now be reliably detected by direct [2] [3] and indirect techniques (e. g., the hematological passport) [7], plasma volume expanders [11] as well as other hemodiluting agents (e. g., desmopressin) [10] can also be occasionally administered to attenuate the artificial increase in hematological values induced by the different blood doping practices.

The use of blood transfusion, an extremely straightforward, practical and effective mean for increasing the red blood-cell supply, achieved broad popularity in the 1970s, but its use suddenly declined in the early 1980s, being replaced by more sophisticated strategies, such as the administration of recombinant human erythropoietin and other erythropoiesis-stimulating substances. Following implementation of reliable tests to screen for these drugs, blood transfusions have undergone unexpected resurgence in the mid 2000s, forcing the scientific community to develop new diagnostic techniques to detect this form of cheating. As such, the antidoping methods currently available allow to clearly identify mixed red blood cell populations in homologous blood transfusion samples containing 0.3–2.0% of donor blood [9] [12], so that the unfair athlete might be in search of alternative options to cheat.

There is a long history of science seeking to develop artificial substitutes for damaged body parts. While some body parts such as teeth and limbs may be commonly replaced by „imitations” without major loss of functionality, the development of a suitable RBC substitute has been proven elusive for decades. Nevertheless, the advent of new technologies has allowed production of biomaterials that provide the ideal technological platform for drug delivery, medical imaging and regenerative medicine. A variety of biomaterials including polymeric nanoparticles and liposomes have been developed, some of which are already available in the market. Inspired by an actual clinical need, that is infection-free emergency transfusions, Doshi et al. recently adopted a biomimetic strategy to efficiently synthesize particles that mimic the key structural and functional features of healthy RBCs [1]. Similar to their natural counterparts, RBC-mimicking particles were further fortified with additional, uncrosslinked hemoglobin to enhance oxygen carrying capacity. Even more interestingly, these particles retain the ability to flow through capillaries smaller than their resting diameter, stretch in response to flow and possess the ability to carry a large amount of oxygen to the peripheral tissue [1]. It is hence easily understandable that these RBC-mimicking synthetic biomaterials might represent an ideal and novel form of blood doping, since they reproduce all the main structural and biological features of natural RBC (i. e., size, shape, elastic modulus, ability to deform under flow, oxygen-carrying capacity), but are as yet virtually undetectable by the current antidoping techniques.

Although there are no studies demonstrating the influence of these new products on animal and human biology so far, given the elite athletes’ innate nature to experiment with novel ergogenic strategies at any cost, further research should be timely planned to be proactive against this new sinister perspective of blood doping. Translational research (i. e., experimental studies on the animal model) is urgently needed to identify the metabolism of artificial blood, the beneficial effects on oxygen delivery in vivo, their potential adverse effects as well as their influence on the parameters currently included in antidoping protocols ([Table 1] ). Little is known so far on the behaviour of RBC-mimicking particles after administration in vivo, so that experimental studies should be planned to assess dosage, half-life, catabolic pathways and potential interactions with other blood components. As with similar „blood substitutes” previously developed (i. e., polymerized hemoglobin solutions, perfluorocarbon emulsions), there is a chance that RBC-mimicking particles might in fact possess unpredictable biological properties that can be deleterious to the human health, such as a short half-life and a rapid clearance by the reticulo-endothelial system where these artificial particles might trigger a severe inflammatory or autoimmune response, the activation of primary hemostasis (i. e., platelet hyperaggregability), the induction of an incontrollable vasoconstrictor response due to endogenous nitric oxide scavenging (i. e., ischemia and infarction), and a large accumulation in the kidneys where they might produce renal toxicity (5).

Table 1 Aims of future translational research on Red Blood Cells-mimicking particles. 1. Define biological properties in vivo (i. e., dose, half-life, catabolism, interaction with other blood cells). 2. Characterize oxygen delivery features in vivo. 3. Recognize potential side effects (i. e., renal toxicity, autoimmune and inflammatory reactions, platelet hyperaggregability, vasoconstrictor response). 4. Identify the influence on parameters currently included in antidoping protocols

At this stage of development, the question of whether some athletes will be able to benefit from these synthetic RBCs seems to overwhelm the point if laboratory testing would currently detect their usage. Nevertheless, the scientific community must be proactive, to avoid repetition of negative experience such as that with recombinant erythropoietin, which became commonplace in sports before an antidoping test had been developed. It is plausible, however, that detection of antigenically distinct donor and recipient RBCs by flow cytometry would lead to wasted efforts in this specific circumstance. No study has been carried out thus far to verify how the hematological parameters, especially those included in the biological passport of the athletes, might vary in response to the administration of these synthetic agents. It is also unclear whether the different hematological instruments can detect or distinguish RBC-mimicking particles from their natural counterparts. As such, experimental studies in animals should also be encouraged to address these crucial analytical issues.

Staying ahead in the fight against doping requires to staying abreast in translational research. There is overwhelming evidence of unsuspectable and nearly undetectable agents being used, and the potential list now includes a new issue, that is RBC-mimicking synthetic biomaterials. Awareness of this tangible risk demands a strict cooperation between basic scientists, sports physicians, laboratory professionals and sport federations to put forward an effective strategy combining prevention and repression, so that education on the risks and reliable antidoping methods can be developed much before this new threat appears in sports.

References

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