Improved oxygenation in ischemic hamster flap tissue is correlated with increasing hemodilution with Hb vesicles and their O2 affinity

Synthetic blood and blood products for combat casualty care and beyond

ABSTRACT

Synthetic biology adopts an engineering design approach to create innovative treatments that are reliable, scalable, and customizable to individual patients. Interest in substitutes for allogenic blood components, primarily red blood cells and platelets, increased in the 1980s because of concerns over infectious disease transmission. However, only now, with emerging synthetic approaches, are such substitutes showing genuine promise. Affordable alternatives to donated blood would be of enormous benefit worldwide. Several approaches to replacing the oxygen-carrying function of red cells are under advanced investigation. Hemoglobin-based oxygen carriers incorporate modifications to reduce the renal toxicity and nitric oxide scavenging of free hemoglobin. While use of earlier-generation hemoglobin-based oxygen carriers may be limited to circumstances in which blood transfusion is not an option, recent advances in chemical modification of hemoglobin may eventually overcome such problems. Another approach encases hemoglobin molecules in biocompatible synthetic nanoparticles. An alternative is the ex vivo production of red cells in bioreactors, with or without genetic manipulation, that offers the potential of a universal donor product. Various strategies to manufacture synthetic platelets are also underway, ranging from simple phospholipid liposomes encapsulating adenosine diphosphate and decorated with fibrinogen fragments, to more complex capsules with multiple receptor peptide sequences. Ex vivo production of platelets in bioreactors is also possible including, for example, platelets derived from induced pluripotent stem cells that are differentiated into a megakaryocytic lineage. Prior to clinical use, trials assessing synthetic blood components must evaluate meaningful safety and effectiveness outcomes in relatively large numbers of critically ill patients. Overcoming these challenges may be as much a hurdle as product design. This article reviews the state of the science of the synthetic biology approach to developing blood component substitutes.

Erythropoietin enhances oxygenation in critically perfused tissue through modulation of nitric oxide synthase

The aim of this study was to investigate the effect of human recombinant erythropoietin (EPO) on the microcirculation and oxygenation of critically ischemic tissue and to elucidate the role of endothelial NO synthase in EPO-mediated tissue protection. Island flaps were dissected from the back skin of anesthetized male Syrian golden hamsters including a critically ischemic, hypoxic area that was perfused via a collateralized vasculature. Before ischemia, animals received an injection of epoetin beta at a dose of 5,000 U/kg body weight with (n = 7) or without (n = 7) blocking NO synthase by 30 mg/kg body weight L-NAME (Nomega-nitro-L-arginine methyl ester hydrochloride). Saline-treated animals served as control (n = 7). Ischemic tissue damage was characterized by severe hypoperfusion and inflammation, hypoxia, and accumulation of apoptotic cell nuclei after 5 h of collateralization. Erythropoietin pretreatment increased arteriolar and venular blood flow by 33% and 37%, respectively (P < 0.05), and attenuated leukocytic inflammation by approximately 75% (P < 0.05). Furthermore, partial tissue oxygen tension in the ischemic tissue increased from 8.2 to 15.8 mmHg (P < 0.05), which was paralleled by a 21% increased density of patent capillaries (P < 0.05) and a 50% reduced apoptotic cell count (P < 0.05). The improved microcirculation and oxygenation were associated with a 2.2-fold (P < 0.05) increase of endothelial NO synthase protein expression. Of interest, L-NAME completely abolished all the beneficial effects of EPO pretreatment. Our study demonstrates that, in critically ischemic and hypoxic collateralized tissue, EPO pretreatment improves tissue perfusion and oxygenation in vivo. This effect may be attributed to NO-dependent vasodilative effects and anti-inflammatory actions on the altered vascular endothelium.

Hemoglobin vesicles improve wound healing and tissue survival in critically ischemic skin in mice

Local hypoxia, as due to trauma, surgery, or arterial occlusive disease, may severely jeopardize the survival of the affected tissue and its wound-healing capacity. Initially developed to replace blood transfusions, artificial oxygen carriers have emerged as oxygen therapeutics in such conditions. The aim of this study was to target primary wound healing and survival in critically ischemic skin by the systemic application of left-shifted liposomal hemoglobin vesicles (HbVs). This was tested in bilateral, cranially based dorsal skin flaps in mice treated with a HbV solution with an oxygen affinity that was increased to a P(50) (partial oxygen tension at which the hemoglobin becomes 50% saturated with oxygen) of 9 mmHg. Twenty percent of the total blood volume of the HbV solution was injected immediately and 24 h after surgery. On the first postoperative day, oxygen saturation in the critically ischemic middle flap portions was increased from 23% (untreated control) to 39% in the HbV-treated animals (P < 0.05). Six days postoperatively, flap tissue survival was increased from 33% (control) to 57% (P < 0.01) and primary healing of the ischemic wound margins from 6.6 to 12.7 mm (P < 0.05) after HbV injection. In addition, higher capillary counts and endothelial nitric oxide synthase expression (both P < 0.01) were found in the immunostained flap tissue. We conclude that left-shifted HbVs may ameliorate the survival and primary wound healing in critically ischemic skin, possibly mediated by endothelial nitric oxide synthase-induced neovascularization.

Haemoglobin-vesicles as artificial oxygen carriers: present situation and future visions

During the long history of development of haemoglobin (Hb)-based O2 carriers (HBOCs), many side effects of Hb molecules have become apparent. They imply the physiological importance of the cellular structure of red blood cells. Hb-vesicles (HbV) are artificial O2 carriers that encapsulate concentrated Hb solution with a thin lipid membrane. We have overcome the intrinsic issues of the suspension of HbV as a molecular assembly, such as stability for storage and in blood circulation, blood compatibility and prompt degradation in the reticuloendothelial system. Animal tests clarified the efficacy of HbV as a transfusion alternative and the possibility for other clinical applications. The results of ongoing HbV research make us confident in advancing further development of HbV, with the expectation of its eventual realization.

Rheological properties of hemoglobin vesicles (artificial oxygen carriers) suspended in a series of plasma-substitute solutions

Hemoglobin vesicles (HbV) or liposome-encapsulated Hbs are artificial oxygen carriers that have been developed for use as transfusion alternatives. The extremely high concentration of the HbV suspension (solutes, ca. 16 g/dL; volume fraction, ca. 40 vol %) gives it an oxygen-carrying capacity that is comparable to that of blood. The HbV suspension does not possess a colloid osmotic pressure. Therefore, HbV must be suspended in or co-injected with an aqueous solution of a plasma substitute (water-soluble polymer), which might interact with HbV. This article describes our study of the rheological properties of HbV suspended in a series of plasma substitute solutions of various molecular weights: recombinant human serum albumin (rHSA), dextran (DEX), modified fluid gelatin (MFG), and hydroxylethyl starch (HES). The HbV suspended in rHSA was nearly Newtonian. Other polymers-HES, DEX, and MFG-induced HbV flocculation, possibly by depletion interaction, and rendered the suspensions as non-Newtonian with a shear-thinning profile (10(-4)-10(3) s(-1)). These HbV suspensions showed a high storage modulus (G') because of the presence of flocculated HbV. However, HbV suspended in rHSA exhibited a very low G'. The viscosities of HbV suspended in DEX, MFG, and high-molecular-weight HES solutions responded quickly to rapid step changes in shear rates of 0.1-100 s(-1) and a return to 0.1 s(-1), indicating that flocculation is both rapid and reversible. Microscopically, the flow pattern of the flocculated HbV that perfused through microchannels (4.5 microm deep, 7 microm wide, 20 cmH2O applied pressure) showed no plugging. Furthermore, the time required for passage was simply proportional to the viscosity. Collectively, the HbV suspension viscosity was influenced by the presence of plasma substitutes. The HbV suspension provides a unique opportunity to manipulate rheological properties for various clinical applications in addition to its use as a transfusion alternative.

Is hemoglobin in hemoglobin vesicles infused for isovolemic hemodilution necessary to improve oxygenation in critically ischemic hamster skin?

The aim of this study was to test the influence of hemoglobin, encapsulated in phospholipid vesicles as an oxygen carrier, given in the course of isovolemic hemodilution to improve oxygenation in critically ischemic hamster flap tissue. Capillary hemodynamics and macromolecular leakage were investigated with intravital microscopy and analyzed off-line with the CapImage software. Partial tissue oxygen tension was measured with fluorescence quenching electrodes. The occurrence of apoptosis was assessed with the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay. Vesicles with (HbV) or without (V) encapsulated Hb were suspended in 6% hydroxyethyl starch (HES) used for the 33% blood exchange. In the ischemic tissue, hemodilution led to an increase in functional capillary density by 31% for HES (P < 0.01 vs. other groups), 66% for V-HES, and 62% for HbV-HES (all P < 0.01 vs. control). Capillary diameters behaved inversely proportional to capillary microhemodynamics. The 20% increase in macromolecular leakage found over time in control animals was completely abolished in the vesicles groups (P < 0.01) but not with HES. Oxygen tension was improved from 10.7 to 16.0 mmHg after HbV-HES (P < 0.01 vs. baseline and other groups). Compared with the other groups, apoptosis was significantly reduced after HbV-HES (P < 0.01). We conclude that the encapsulation of Hb was essential to attenuate hypoxia and subsequent cell death in the critically ischemic tissue. However, the effect was partly attributed to the rheological changes exerted by the vesicles.

Cardioprotection before revascularization in ischemic myocardial injury and the potential role of hemoglobin-based oxygen carriers

Despite the availability of interventional catheterization for patients with acute coronary syndromes, there is an unavoidable delay until the occluded coronary artery(s) can be revascularized, during which time persistent ischemia may lead to irreversible myocardial damage despite subsequently high patency rates. Accordingly, there has been an intense effort to develop early interventions that will preserve the viability of ischemic myocardium before revascularization. A number of novel strategies have been studied, including hemoglobin-based oxygen carriers. These compounds transport oxygen in the plasma to help maintain more normal oxygen delivery to the myocardium supplied by a thrombosed vessel, and they also release oxygen to tissue more efficiently than intraerythrocytic hemoglobin.

Oxygen release from low and normal P50 Hb vesicles in transiently occluded arterioles of the hamster window model

A phospholipid vesicle encapsulating Hb [Hb vesicle (HbV)] has been developed as a transfusion alternative. One characteristic of HbV is that the O(2) affinity [Po(2) at which Hb is 50% saturated (P(50))] of Hb can be easily regulated by the amount of the coencapsulated allosteric effector pyridoxal 5'-phosphate. In this study, we prepared two HbVs with different P(50)s (8 and 29 mmHg, termed HbV(8) and HbV(29), respectively) and observed their O(2)-releasing behavior from an occluded arteriole in a hamster skinfold window model. Conscious hamsters received HbV(8) or HbV(29) at a dose rate of 7 ml/kg. In the microscopic view, an arteriole (diameter: 53.0 +/- 6.6 mum) was occluded transcutaneously by a glass pipette on a manipulator, and the reduction of the intra-arteriolar Po(2) 100 mum down from the occlusion was measured by the phosphorescence quenching of preinfused Pd-porphyrin. The baseline arteriolar Po(2) (50-52 mmHg) decreased to about 5 mmHg for all the groups. Occlusion after HbV(8) infusion showed a slightly slower rate of Po(2) reduction compared with that after HbV(29) infusion. The arteriolar O(2) content was calculated at each reducing Po(2) in combination with the O(2) equilibrium curves of HbVs, and it was clarified that HbV(8) showed a significantly slower rate of O(2) release compared with HbV(29) and was a primary source of O(2) (maximum fraction, 0.55) overwhelming red blood cells when the Po(2) was reduced (e.g., <10 mmHg) despite a small dosage of HbV. This result supports the possible utilization of Hb-based O(2) carriers with lower P(50) for oxygenation of ischemic tissues.

RESUSCITATION FROM HEMORRHAGIC SHOCK WITH MalPEG-ALBUMIN: COMPARISON WITH MalPEG-HEMOGLOBIN

Our aim was to determine the efficacy of polyethylene glycol-conjugated human albumin (MalPEG-Alb) in restoring circulatory volume after 1 h of hemorrhagic shock. Experiments were performed in the awake condition in the hamster skin fold preparation. Microhemodynamic parameters and tissue Po2 were assessed with intravital microscopy and the use of the phosphorescence quenching technique. One hour after shock induction by withdrawal of 50% of the blood volume, animals were resuscitated with MalPEG-Alb (n = 6). Systemic and microhemodynamic parameters following resuscitation were identical to those obtained with the same protocol using MalPEG-Hb (1). However, parameters related to microvascular oxygen distribution were significantly lower in the MalPEG-Alb group compared with the previous data from the MalPEG-Hb group in that tissue oxygen partial pressure was 5 +/- 2 mmHg (vs. 8 +/- 3 mmHg, P < 0.05), oxygen delivery was reduced to 60 +/- 27% (P < 0.05), and oxygen consumption was reduced to 69 +/- 28% (P < 0.05). Both molecules were matched in composition (4.2 g/dL) and surface chemistry. MalPEG-Alb colloid osmotic pressure was 37 mmHg (vs. 49 mmHg for MalPEG-Hb), and viscosity was 2.7 cP (vs. 2.5 cP for MalPEG-Hb). The present results show that both solutions are efficacious plasma expanders and that the hemoglobin-based solution provides improved oxygen distribution and tissue Po2 in the hamster chamber model.

Improving Microcirculation is More Effective Than Substitution of Red Blood Cells to Correct Metabolic Disorder in Experimental Hemorrhagic Shock

Microcirculatory perfusion deficits and impaired tissue oxygenation in nonvital organs frequently occur after hemorrhage and they contribute to potentially lethal complications. The aim of this study was to test the influence of colloid osmotic pressure, viscosity, and red blood cell (RBC) content of the resuscitative fluid on metabolic disorder, perfusion, and oxygenation in peripheral tissues. Awake hamsters were subjected to hemorrhage of 50% and were resuscitated with 25% of blood volume with solutions containing 6% pegylated bovine albumin only (PEG-BSA 0) and 6% PEG-BSA mixed with autologous RBCs to reach 4 g/dL (PEG-BSA 4) and 8 g/dL (PEG-BSA 8) of hemoglobin. PEG-BSA had a viscosity of 4.2 cP and a COP of 116 mmHg. Microhemodynamics and tissue pO2 were assessed in the hamster chamber window preparation with intravital microscopy. Arterial base excess tended to be lower than baseline for PEG-BSA 0 and PEG-BSA 4 (ns), whereas base deficit remained significantly decreased for PEG-BSA 8 (P<0.05 vs. baseline). Oxygen extraction was 91% +/- 2% of the oxygen delivery for PEG-BSA 0 compared with 85% +/- 2% for PEG-BSA 8 (P<0.05). Functional capillary density was 61%, 47%, and 45% for PEG-BSA 0 (P<0.05 vs. other groups), PEG-BSA 4 and PEG-BSA 8, respectively. We conclude that arterial base excess and oxygen extraction ratio in the tissue was better restored if a higher fraction of PEG-BSA and less RBCs were infused. This was attributed to a more homogeneous distribution of oxygen, as reflected by functional capillary density. Our results suggest that the transfusion trigger in hemorrhagic shock may be shifted toward lower hemoglobin concentrations if highly viscous and oncotic solutions are used.

O2 release from Hb vesicles evaluated using an artificial, narrow O2-permeable tube: comparison with RBCs and acellular Hbs

A phospholipid vesicle that encapsulates a concentrated hemoglobin (Hb) solution and pyridoxal 5'-phosphate as an allosteric effector [Hb vesicle (HbV) diameter, 250 nm] has been developed to provide an O2 carrying ability to plasma expanders. The O2 release from flowing HbVs was examined using an O2-permeable, fluorinated ethylenepropylene copolymer tube (inner diameter, 28 microm) exposed to a deoxygenated environment. Measurement of O2 release was performed using an apparatus that consisted of an inverted microscope and a scanning-grating spectrophotometer with a photon-count detector, and the rate of O2 release was determined based on the visible absorption spectrum in the Q band of Hb. HbVs and fresh human red blood cells (RBCs) were mixed in various volume ratios at a Hb concentration of 10 g/dl in isotonic saline that contained 5 g/dl albumin, and the suspension was perfused at the centerline flow velocity of 1 mm/s through the narrow tube. The mixtures of acellular Hb solution and RBCs were also tested. Because HbVs were homogeneously dispersed in the albumin solution, increasing the volume of the HbV suspension resulted in a thicker marginal RBC-free layer. Irrespective of the mixing ratio, the rate of O2 release from the HbV/RBC mixtures was similar to that of RBCs alone. On the other hand, the addition of 50 vol% of acellular Hb solution to RBCs significantly enhanced the rate of deoxygenation. This outstanding difference in the rate of O2 release between the HbV suspension and the acellular Hb solution should mainly be due to the difference in the particle size (250 vs. 7 nm) that affects their diffusion for the facilitated O2 transport.