Blood substitutes

Antihypoxic oxygenation agents with respiratory hyperoxia to improve cancer immunotherapy

Hypoxia/HIF-1α- and extracellular adenosine/A2 adenosine receptor-mediated immunosuppression protects tissues from collateral damage by antipathogen immune cells. However, this mechanism also protects cancerous tissues by inhibiting antitumor immune cells in hypoxic and extracellular adenosine-rich tumors that are the most resistant to current therapies. Here, we explain a potentially novel, antiimmunosuppressive reasoning to justify strategies using respiratory hyperoxia and oxygenation agents in cancer treatment. Earlier attempts to use oxygenation of tumors as a monotherapy or to improve radiotherapy have failed because oxygenation protocols were not combined with immunotherapies of cancer. In contrast, the proposal for therapeutic use of antihypoxic oxygenation described here was motivated by the need to prevent the hypoxia/HIF-1α-driven accumulation of extracellular adenosine to (a) unleash antitumor immune cells from inhibition by intracellular cAMP and (b) prevent immunosuppressive transcription of cAMP response element- and hypoxia response element-containing immunosuppressive gene products (e.g., TGF-β). Use of oxygenation agents together with inhibitors of the A2A adenosine receptor may be required to enable the most effective cancer immunotherapy. The emerging outcomes of clinical trials of cancer patients refractory to all other treatments provide support for the molecular and immunological mechanism-based approach to cancer immunotherapy described here.

Oxygenation and A2AR blockade to eliminate hypoxia/HIF-1α-adenosinergic immunosuppressive axis and improve cancer immunotherapy

The promising results of the first in-human clinical study using A2AR antagonists for treatment of renal cell carcinoma highlight two decades of research into the hypoxia-A2-adenosinergic pathway. Importantly, clinical responses have been observed in patients who previously progressed on anti-PD-1/PDL-1 therapy, emphasizing the clinical importance of targeting A2AR signaling in cancer immunotherapies. Recently, it has been shown that systemic oxygenation weakens all known stages of the hypoxia-A2-adenosinergic axis. Therefore, we advocate the clinical use of systemic oxygenation and oxygenation agents in combination with A2AR blockade to further improve cancer immunotherapies. This approach is expected to completely eliminate the upstream (hypoxia-HIF-1α) and downstream (adenosine-A2AR) stages of the immunosuppressive hypoxia-adenosinergic signaling axis. This might be a necessary strategy to maximize the therapeutic benefits of A2AR antagonists and increase susceptibility of tumors to cancer treatments.

Low oxygen-affinity hemoglobin solution increases oxygenation of partially ischemic tissue during acute anemia

BACKGROUND

Maintenance of postsurgical tissue oxygenation depends on the ability of the specific tissue to recruit perfusion and oxygen (O(2)) supply. When native O(2)-carrying capacity is lacking, fluids to improve O(2)-carrying capacity based in hemoglobin (Hb) could prevent partially ischemic tissue hypoxia by increasing O(2) release from the remaining red blood cells.

STUDY DESIGN

Responses to facilitated O(2) transport after exchange transfusion with polymerized bovine Hb (PBH) were studied in a chronic partially ischemic tissue model, induced by large feeding arteriole ligation during hamster window chamber model implantation. PBH effects in microvascular perfusion and tissue oxygenation were studied after exchange transfusion of 40% of animal's blood volume. Experimental groups were defined by the concentration of PBH used, ie, PBH at 13 g/dL (PBH13); PBH at 4 g/dL in albumin solution to matching colloidal osmotic pressure (COP) (PBH4); and no PBH, only albumin solution at matching COP (PBH0).

RESULTS

Restitution of O(2)-carrying capacity with PBH13 increased blood pressure and produced vasoconstriction compared with PBH4 and PBH0. On the other hand, PBH4 maintained blood pressure without substantial vasoconstriction, increased tissue partial pressure of O(2), arteriolar O(2) supply, and extraction to the partially ischemic tissue compared with PBH0 and PBH13.

CONCLUSIONS

Results suggest the existence of an optimal concentration of low O(2)-affinity acellular Hb to increase oxygenation of partially ischemic tissue during anemic conditions.

Balance between vasoconstriction and enhanced oxygen delivery

BACKGROUND

Hemoglobin (Hb) solutions are potential alternatives to blood transfusion when native oxygen (O(2))-carrying capacity is lacking. Polymerized bovine Hb (PBH) solutions are characterized by its vasoactivity, low O(2) affinity, oncotic effect, prolonged shelf life, and stability. Responses to facilitated O(2) transport, after exchange transfusion with PBH, were studied in the hamster window chamber model during acute extreme anemia to determine how PBH affects microvascular perfusion and tissue oxygenation.

STUDY DESIGN AND METHODS

An anemic state was induced by hemodilution with a plasma expander (70-kDa dextran). After hemodilution, animals were randomly assigned to exchange transfusion groups based on the concentration of PBH used, namely, PBH at 13 g(Hb) per dL (PBH13), PBH diluted to 8 or 4 g(Hb) per dL in albumin solution at matching colloidal osmotic pressure (COP; PBH8 and PBH4), and no PBH only albumin solution at matching COP (PBH0). Measurement of systemic variables, microvascular hemodynamics, capillary perfusion, and intravascular and tissue O(2) levels was performed at 11 percent Hct.

RESULTS

Restitution of O(2)-carrying capacity with PBH13 restored higher arterial pressure and triggered vasoconstriction, low perfusion, and higher peripheral resistance. Groups PBH4 and PBH0 had lower arterial pressures than Group PBH13. Groups PBH4 and PBH8 maintained higher perfusion and functional capillary density than Groups PBH13 and PBH0. In all groups, blood gas variables and acid-base balance were recovered proportional to microvascular perfusion. Arterial O(2) tensions were improved for Groups PBH4 and PBH8 by preventing O(2) precapillary release and increasing O(2) reserve.

CONCLUSION

Further studies to establish acellular Hb optimal dosage, efficacy, safety, and effects on outcome are indicated before these solutions are implemented in routine practice.

Multicenter comparative study of conventional mechanical gas ventilation to tidal liquid ventilation in oleic acid injured sheep

We performed a multicenter study to test the hypothesis that tidal liquid ventilation (TLV) would improve cardiopulmonary, lung histomorphological, and inflammatory profiles compared with conventional mechanical gas ventilation (CMV). Sheep were studied using the same volume-controlled, pressure-limited ventilator systems, protocols, and treatment strategies in three independent laboratories. Following baseline measurements, oleic acid lung injury was induced and animals were randomized to 4 hours of CMV or TLV targeted to "best PaO2" and PaCO2 35 to 60 mm Hg. The following were significantly higher (p < 0.01) during TLV than CMV: PaO2, venous oxygen saturation, respiratory compliance, cardiac output, stroke volume, oxygen delivery, ventilatory efficiency index; alveolar area, lung % gas exchange space, and expansion index. The following were lower (p < 0.01) during TLV compared with CMV: inspiratory and expiratory pause pressures, mean airway pressure, minute ventilation, physiologic shunt, plasma lactate, lung interleukin-6, interleukin-8, myeloperoxidase, and composite total injury score. No significant laboratories by treatment group interactions were found. In summary, TLV resulted in improved cardiopulmonary physiology at lower ventilatory requirements with more favorable histological and inflammatory profiles than CMV. As such, TLV offers a feasible ventilatory alternative as a lung protective strategy in this model of acute lung injury.

Fluids as oxygen carriers and the potential role in trauma resuscitation

Patients with major trauma present a challenge, often using large quantities of banked blood both at the time of injury and during their hospital stay. Blood transfusion is not without risk and is associated with high costs; it is immunosuppressive, rendering patients more susceptible to infection. In the western world, banked blood is fully screened and relatively safe; the same is not true in parts of the developing world, where high rates of HIV carriage make blood transfusion a risky undertaking. Additionally, blood transfusion as a vector for transmission of illnesses such as prion disease is a distinct possibility, for both the developed and developing world alike. The introduction of artificial blood substitutes would ameliorate some risk and also remove the cost of extensive blood testing. For trauma outside hospital, blood substitutes could compete directly with fluid resuscitation as donated blood is not usually available. Patients with prolonged transport times would appear to be the most obvious beneficiaries and volume expansion, along with improvement in oxygen-carrying capacity would be the ultimate goal. All clinicians confronted with the need for transfusion of homologous blood would welcome the development of a safe and reliable alternative to red blood cells in order to ensure oxygen transport to the tissues. However, even though research on red cell substitutes started more than 100 years ago, even now none of the heavily investigated compounds based on haemoglobin or perfluorocarbons has been released in Europe or the USA for routine clinical use.

A computational model of oxygen delivery by hemoglobin-based oxygen carriers in three-dimensional microvascular networks

A detailed computational model is developed to simulate oxygen transport from a three-dimensional (3D) microvascular network to the surrounding tissue in the presence of hemoglobin-based oxygen carriers. The model accounts for nonlinear O(2) consumption, myoglobin-facilitated diffusion and nonlinear oxyhemoglobin dissociation in the RBCs and plasma. It also includes a detailed description of intravascular resistance to O(2) transport and is capable of incorporating realistic 3D microvascular network geometries. Simulations in this study were performed using a computer-generated microvascular architecture that mimics morphometric parameters for the hamster cheek pouch retractor muscle. Theoretical results are presented next to corresponding experimental data. Phosphorescence quenching microscopy provided PO(2) measurements at the arteriolar and venular ends of capillaries in the hamster retractor muscle before and after isovolemic hemodilution with three different hemodilutents: a non-oxygen-carrying plasma expander and two hemoglobin solutions with different oxygen affinities. Sample results in a microvascular network show an enhancement of diffusive shunting between arterioles, venules and capillaries and a decrease in hemoglobin's effectiveness for tissue oxygenation when its affinity for O(2) is decreased. Model simulations suggest that microvascular network anatomy can affect the optimal hemoglobin affinity for reducing tissue hypoxia. O(2) transport simulations in realistic representations of microvascular networks should provide a theoretical framework for choosing optimal parameter values in the development of hemoglobin-based blood substitutes.

MICROVASCULAR PERSPECTIVE OF OXYGEN-CARRYING AND -NONCARRYING BLOOD SUBSTITUTES

The development of an alternative to natural blood has evolved from the initial goal of replicating blood properties to the current objective of formulating a fluid that can be used to replace blood while preserving microvascular function and delivering oxygen. The properties of this fluid are counterintuitive and different from blood because it has high viscosity, oxygen affinity, and a low oxygen carrier concentration when compared with blood. The optimal oxygen carrier devised presently is poly-ethylene-conjugated human hemoglobin, a material demonstrated to be vasoinactive and void of the toxicities present in previous hemoglobin formulations. A feature of this material is that it is effective in small quantities, and therefore amplifies the equivalent supply of blood derived from blood donations.

Perioperative blood transfusion and outcome

PURPOSE OF REVIEW

As a result of advances in pathogen testing and transfusion standards over the last decade, the risk of disease transmission through allogeneic blood transfusions has decreased markedly. The effects of allogeneic blood transfusions on the immune system, however, have received more attention, as they appear to influence outcome. The following review summarizes the effects of allogeneic blood transfusions on selected outcome parameters and the influence of white blood cell reduction on these parameters.

RECENT FINDINGS

Adverse effects of allogeneic blood transfusions on outcome variables such as postoperative infection, cancer recurrence, pulmonary function, length of stay, and mortality have been shown in multiple trials, but most were not randomized or blinded. One proposed approach to reduce unwanted side-effects is to reduce the donor's white blood cell count before transfusion. This can be done either by individual bedside filtration or by pre-storage (or post-storage) universal white blood cell reduction. Studies investigating this approach have yielded conflicting results.

SUMMARY

Although the results of a number of studies suggest a negative impact of allogeneic blood transfusions on immune function and consequently outcome parameters, this has not been proven in rigorously controlled randomized trial, or in meta-analyses. Reduction of white blood cells might be beneficial in selected patient populations, but at this time does not appear warranted in the general surgical population. As universal white blood cell reduction is a very costly process, it probably should not be implemented until such a benefit is proven.

Rationale for blood conservation

BACKGROUND

Exposure of patients to allogeneic blood transfusion can be minimized or avoided by the systematic use of multiple blood conservation techniques. Current use of these technologies is variable.

METHODS

Review of pertinent English language literature.

RESULTS

Enthusiasm for preoperative autologous blood donation (PAD) has declined considerably, perhaps due to increased cost and inconvenience to patients. Acute normovolemic hemodilution (ANH) has several practical advantages over PAD, but has not become generally accepted as a blood conservation strategy. Erythropoietin, iron, and artificial oxygen carriers are pharmacologic alternatives.

CONCLUSIONS

Pharmacologic stimulation of erythropoiesis offers substantial potential to progress toward a goal of bloodless medicine. The potential of artificial blood substitutes is still being defined.

Oxygen-carrying blood substitutes: a microvascular perspective

Development of a viable blood substitute began by focusing on recreating the oxygen-carrying capacity of blood, leading to the recognition that haemoglobin (Hb) is presently unequalled for this function. However, as human Hb is the only realistic source of this protein, the production of a blood substitute that solves transfusional blood availability problems and shortages must introduce a multiplying factor between supply of natural blood and blood substitute, while maintaining equivalency of function/efficacy. In other words, a unit of blood should produce several units of equivalent blood substitute. This expansion is now possible because of new understanding of how blood delivers oxygen in the microcirculation and the consequences of reducing oxygen-carrying capacity in haemorrhage. This information is used to provide improved resuscitation capacity and maintenance of tissue metabolism by tailoring the properties of a blood substitute to the task of maintaining microvascular function, rather than oxygen delivery capacity. Resuscitation in an organism that is haemorrhaging requires maintenance perfusion, a process directly linked to the maintenance of adequate levels of shear stress on the endothelium, induced by either increased blood/plasma viscosity or increased blood flow velocity in the microcirculation. This process must also be intimately coupled with the requirement that no portion of the tissue is anoxic. This disparate set of requirements can be satisfied with high viscosity Hb solutions that have high affinity for oxygen, a combination of properties that causes the microcirculation to remain functional, and a requirement that supersedes restoration of oxygen-carrying capacity in the treatment of haemorrhage.

Reduced phagocytosis of colloidal carriers using soluble CD47

PURPOSE

This study was designed to illustrate the feasibility of using soluble CD47 protein to antagonize phagocytosis of colloidal drug carriers by macrophages.

METHODS

Expression of CD47-streptavidin (CD47-SA) fusion protein was achieved in B21CodonPlus host cells following IPTG induction. Murine macrophage cell line J774A.1, expressing high levels of SIRPalpha, was selected as the biologic model system for phagocytosis. FITC-labeled perfluorocarbon (PFC) emulsions were used as the colloidal carriers to trigger phagocytosis. Microscopy (inverted light and UV-fluorescence) and flow cytometry were used to qualitatively and quantitatively determine the degree of phagocytosis, respectively.

RESULTS

The bacterially expressed, purified CD47-SA had neither cytotoxic nor cytostatic effects when incubated with J774A.1 cells up to a concentration of 400 nM for 24 h. Phagocytosis of FITC-labeled PFC emulsions was significantly diminished when macrophages were pretreated with 100 nM CD47-SA for 1 h.

CONCLUSIONS

We demonstrated that soluble CD47-SA antagonized phagocytosis of colloidal carriers to a significant degree by interaction with macrophage SIRPalpha.

Hemodilution With Stoma-Free Hemoglobin at Physiologically Maintained Viscosity Delays the Onset of Vasoconstriction

Solutions of modified cell-free hemoglobin, prepared from outdated red blood cells, have been developed during the past decade to circumvent the increasing need for allogeneic blood. Despite improvements in the safety and efficacy of these solutions, undesirable effects such as an increase in vascular tone leading to hypertension have not been fully resolved, which might hinder their clinical usefulness. To discriminate between the pharmacological and rheological effects of cell-free hemoglobin, we compared the effects of blood/cell-free hemoglobin mixtures of high versus low viscosity on hemodynamics and vascular hindrance, an index of vascular tone, which was normalized for blood viscosity. Anesthetized rats were subjected to 50% exchange transfusion with (1) high-viscosity solutions: whole blood (n=5) or red blood cells mixed with cell-free hemoglobin (Hb-Hv group, n=5); (2) low-viscosity solutions: cell-free hemoglobin (Hb-Lv group, n=5) or human albumin (n=5). Two hours after hemodilution, vascular hindrance remained unchanged in animals transfused with whole blood and albumin. Hb-Lv induced an immediate and sustained increase in vascular hindrance (208%). Conversely, in Hb-Hv animals, the vascular hindrance increase was delayed and smaller (27% to 147%), whereas peripheral resistance increased gradually (94% after 2 hours). Our results demonstrate the beneficial effects of cell-free hemoglobin in the presence of the animals' own red blood cells in maintaining physiological viscosity and limiting vasoconstriction because of the pharmacological properties of cell-free hemoglobin.

No scavenging and the hypertensive effect of hemoglobin-based blood substitutes

The major pathway for nitric oxide scavenging in red cells involves the direct reaction of the gas with HbO2 to form nitrate and the ferric form of the protein, metHb. Because both atoms of O2 are incorporated into nitrate, this process is called NO dioxygenation (NOD). The NOD reaction involves an initial, very rapid bimolecular addition of NO to bound O2 to form a transient Fe(III)-peroxynitrite complex, which can be observed spectrally at alkaline pH. This intermediate rapidly isomerizes at pH 7 (t1/2 <== 1 ms) to metHb and NO3-, which is nontoxic and readily transported out of red cells and excreted. The rate of NO consumption by intracellular HbO2 during normal blood flow is limited by diffusion up to and into the red cells and is too slow to interfere significantly with vasoregulation. In contrast, extracellular HbO2 is highly vasoconstrictive, and the resultant hypertension is a significant side effect of most hemoglobin-based blood substitutes. The major cause of this blood pressure effect seems to be the high rate of NO dioxygenation by cell-free HbO2, which can extravasate into the vessel walls and interfere directly with NO signaling between endothelial and smooth muscle cells. This interpretation is supported by a strong linear correlation between the magnitude of the blood pressure effect caused by infusion of cross-linked recombinant hemoglobin tetramers in vivo and the rate of NO dioxygenation by these proteins measured in vitro.

Drugs for increasing oxygen and their potential use in doping: a review

Blood oxygenation is a fundamental factor in optimising muscular activity. Enhancement of oxygen delivery to tissues is associated with a substantial improvement in athletic performance, particularly in endurance sports. Progress in medical research has led to the identification of new chemicals for the treatment of severe anaemia. Effective and promising molecules have been created and sometimes used for doping purposes. The aim of this review is to present methods, and drugs, known to be (or that might be) used by athletes to increase oxygen transport in an attempt to improve endurance capacity. These methods and drugs include: (i) blood transfusion; (ii) endogenous stimulation of red blood cell production at altitude, or using hypoxic rooms, erythropoietins (EPOs), EPO gene therapy or EPO mimetics; (iii) allosteric effectors of haemoglobin; and (iv) blood substitutes such as modified haemoglobin solutions and perfluorochemicals. Often, new chemicals are used before safety tests have been completed and athletes are taking great health risks. Such new chemicals have also created the need for new instrumental strategies in doping control laboratories, but not all of these chemicals are detectable. Further progress in analytical research is necessary.

Benefit and risk perceptions in transfusion medicine: blood and blood substitutes

Blood transfusion is a remarkably safe, routine procedure in clinical medicine. However, little attention has focused on the perceptions of risk associated with the receipt of blood, blood products or 'blood substitutes'. It is pertinent to ask (i) what key stakeholder groups know about transfusion, (ii) how safe they perceive blood/blood products to be, (iii) how the latter information might influence their own and others' perceptions of risk linked to transfusion, and (iv) the extent to which approved blood substitutes might be preferred over autologous or donor blood. An appreciation of what stakeholders perceive to be the benefits and risks of the receipt of blood and blood substitutes will inform future transfusion strategies. To obtain such information, a programme of research has been initiated at Nottingham. Surveys have targeted key stakeholder groups, namely, UK adult blood donors and nondonors, anaesthetists, general practitioners and health care journalists. Experimental studies examining message framing and cueing have also been conducted with undergraduate students. Such research will improve misunderstandings about current issues associated with blood donation and transfusion against the backdrop of changing public trust of health care professionals and attitudes and expectations on blood safety and benefits of blood substitutes.

Transfusion medicine: looking to the future

The evolution of transfusion medicine into a clinically oriented discipline emphasising patient care has been accompanied by challenges that need to be faced as specialists look to the future. Emerging issues that affect blood safety and blood supply, such as pathogen inactivation and more stringent donor screening questions, bring new pressures on the availability of an affordable blood supply. Imminent alternatives for management of anaemia, such as oxygen carriers, hold great promise but, if available, will require close oversight. With current estimates of HIV or hepatitis C viral (HCV) transmission approaching one in 2000000 units transfused, keeping to a minimum bacterial contamination of platelet products (one in 2000) and errors in transfusion, with its estimated one in 800000 mortality rate, assume great urgency. Finally, serious difficulties in blood safety and availability for poor, developing countries require innovative strategies and commitment of resources.

Theoretical analysis of effects of blood substitute affinity and cooperativity on organ oxygen transport

Hemoglobin-based O(2) carriers (HBOCs), which are developed as an alternative to blood transfusion, provide O(2) delivery. At present, there is no model to predict the O(2) transport for a red blood cell-HBOC mixture on a whole organ basis. On the basis of the first principles of mass balance, a model of O(2) transport for an organ was derived to calculate venous Po(2) (Pv(O(2))) for a given inlet arterial Po(2) (Pa(O(2))), blood flow, and oxygen consumption. The model was validated by using several in vivo animal studies on HBOC administration for a wide range of HBOC oxygen-binding parameters and predicted Pv(O(2)) for various Pa(O(2)) in the same species. The model was also used to predict the effect of HBOC affinity and cooperativity on Pv(O(2)) for humans. The results indicate that Pv(O(2)) can be increased at a constant blood flow-to-oxygen consumption ratio by reducing the affinity of HBOC for normoxia and mild hypoxia; however, a high-affinity HBOC would be more efficient in maintaining higher Pv(O(2)) for severe hypoxia (Pa(O(2)) < 40 Torr).

RSR13 e modificação alostérica da afinidade hemoglobina-oxigênio: abuso entre atletas

O ácido metilpropiônico (RSR13) é um modificador alostérico da hemoglobina, com a qual se liga de forma não-covalente, diminuindo sua afinidade pelo oxigênio de modo dose-dependente e, conseqüentemente, aumentando a oxigenação periférica. O objetivo deste artigo é apresentar brevemente as evidências científicas acerca das características farmacológicas e funcionais, indicações médicas e efeitos adversos do uso do RSR13 por atletas, a mais recente alternativa de aumento artificial do desempenho. Estudos experimentais preliminares verificaram algum efeito positivo do RSR13 sobre a recuperação do miocárdio isquêmico e sobre a extensão da isquemia cerebral, mas as principais indicações estudadas atualmente são a cirurgia com hipotermia e cardioplegia durante circulação extracorpórea e o uso como agente coadjuvante potenciador da radioterapia para certos tumores sólidos. Somente um estudo em modelo canino mostrou aumento do consumo máximo de oxigênio em músculo isolado, não existindo evidências de que o RSR13 possa efetivamente melhorar o desempenho em humanos. Em realidade, já foram descritos efeitos adversos, como diminuição da perfusão sanguínea, elevação da pressão arterial e diminuição da função renal. Antecipando o potencial aumento da utilização do RSR13 por atletas, métodos já foram desenvolvidos para sua detecção em amostras de urina humana.