Isolation and characterization of a new hemoglobin derivative cross-linked between the alpha chains (lysine 99 alpha 1----lysine 99 alpha 2)

Recombinant hemoglobin betaG83C-F41Y. An octameric protein

We have engineered a stable octameric hemoglobin (Hb) of molecular mass 129 kDa, a dimer of recombinant hemoglobin (rHb betaG83C-F41Y) tetramers joined by disulfide bonds at the beta83 position. One of the major problems with oxygen carriers based on acellular hemoglobin solutions is vasoactivity, a limitation which may be overcome by increasing the molecular size of the carrier. The oxygen equilibrium curves showed that the octameric rHb betaG83C-F41Y exhibited an increased oxygen affinity and a decreased cooperativity. The CO rebinding kinetics, auto-oxidation kinetics, and size exclusion chromatography did not show the usual dependence on protein concentration, indicating that this octamer was stable and did not dissociate easily into tetramers or dimers at low concentration. These results were corroborated by the experiments with haptoglobin showing no interaction between octameric rHb betaG83C-F41Y and haptoglobin, a plasma glycoprotein that binds the Hb dimers and permits their elimination from blood circulation. The lack of dimers could be explained if there are two disulfide bridges per octamer, which would be in agreement with the lack of reactivity of the additional cysteine residues. The kinetics of reduction of the disulfide bridge by reduced glutathione showed a rate of 1000 M(-1) x h(-1) (observed time coefficient of 1 h at 1 mM glutathione) at 25 degrees C. Under air, the cysteines are oxidized and the disulfide bridge forms spontaneously; the kinetics of the tetramer to octamer reaction displayed a bimolecular reaction of time coefficient of 2 h at 11 microM Hb and 25 degrees C. In addition, the octameric rHb betaG83C-F41Y was resistant to potential reducing agents present in fresh plasma.

What happened to blood substitutes?

Concerns about the safety and adequacy of the blood supply have fostered twenty years of research into the so-called "blood substitutes" among them the oxygen carriers based on modified hemoglobin. Although none of these materials has yet been licensed for use in North America or Europe, the results of research and clinical trials have increased our understanding of oxygen delivery and its regulation. In particular, the examination of the basis for the vasoactivity observed with some of the hemoglobin based oxygen carriers has led to the insight that several colligative properties of hemoglobin solutions, such as their diffusion coefficient for oxygen, viscosity and colloid oncotic pressure, are important determinants of efficacy.

Heterogeneous nucleation in sickle hemoglobin: experimental validation of a structural mechanism

Sickle hemoglobin polymerizes by two types of nucleation: homogeneous nucleation of aggregates in solution, and heterogeneous nucleation on preexisting polymers. It has been proposed that the same contact that is made in the interior of the polymer between the mutant site beta6 and its receptor pocket on an adjacent molecule is the primary contact site for the heterogeneous nucleus. We have constructed cross-linked hybrid molecules in which one beta-subunit is from HbA with Glu at beta6, and the other is from HbS with a Val at beta6. We measured solubility (using sedimentation) and polymerization kinetics (using laser photolysis) on cross-linked hybrids, and cross-linked HbS as controls. We find approximately 4000 times less heterogeneous nucleation in the cross-linked AS molecules than in cross-linked HbS, in strong confirmation of the proposal. In addition, changes in stability of the nucleus support a further proposal that more than one beta6 contact is involved in the homogeneous nucleus.

Current Status of Artificial o2 Carriers

This article describes currently evaluated artificial O2 carriers, summarizes their efficacy, and discusses their side effects, based on and restricted to published data. For compounds in phase III testing, approximately 500 to 1000 patients have been dosed, and similar numbers of control patients have been investigated. For compounds in phase I or II testing, the number of patients dosed is significantly less. Unfortunately, there is a significant amount of nonpublished data, which renders the overall assessment difficult, and the direct comparison among different types of artificial O2 carriers is significantly limited by the virtual nonexistence of studies that directly compare different products.

Recent advances in the development of haemoglobin-based blood substitutes

The term 'blood substitute' is commonly used to describe products which can carry and deliver oxygen. These products are also referred to as 'oxygen carriers' or 'oxygen therapeutics'. Blood substitutes are a new generation of oxygen therapeutics and their introduction will redefine treatment approaches in a wide range of medical and surgical practices. There are two major classes of this new generation of oxygen therapeutics (1) modified haemoglobin solutions, referred to as haemoglobin-based blood substitutes (HBBS) and (2) perfluorocarbon emulsions. Tremendous progress has been made in the past four years with the development of HBBS. In comparison, not much progress has been made in the development of perfluorocarbons as oxygen carriers. In the present review we have limited our discussion to the development of HBBS. Several types of HBBS have been developed and are in different phases of clinical trials. Free haemoglobin has been crosslinked, conjugated, polymerised or encapsulated to prevent its dissociation into dimers. The stability and purity of HBBS are extremely important in overcoming most of the significant toxicities of these products. Commercial manufacturers have utilised better proprietary formulations and purification technologies, and HBBS developed by these organisations have demonstrated safety in both preclinical and clinical studies. Recent research activities suggest a broad range of therapeutic applications for these new generation of oxygen therapeutics, 'blood substitutes'. The introduction of HBBS in critical care medicine will introduce a new approach of not only improving perfusion, but delivering oxygen to tissues.

Hemoglobin substitutes

Orthopaedic patients frequently require blood transfusions to treat peri-operative anemia. Research in the area of hemoglobin substitutes has been of great interest since it holds the promise of reducing the reliance on allogeneic blood transfusions. The three categories of hemoglobin substitutes are (1) cell-free, extracellular hemoglobin preparations made from human or bovine hemoglobin (hemoglobin-based oxygen carriers or HBOCs); (2) fluorine-substituted linear or cyclic carbon chains with a high oxygen-carrying capacity (perfluorocarbons); and (3) liposome-encapsulated hemoglobin. Of the three, HBOCs have been the most extensively studied and tested in preclinical and clinical trials that have shown success in diminishing the number of blood transfusions as well as an overall favorable side-effect profile. This has been demonstrated in vascular, cardiothoracic, and orthopaedic patients. HBOC-201, which is a preparation of cell-free bovine hemoglobin, has been approved for clinical use in South Africa. These products may well become an important tool for physicians treating peri-operative anemia in orthopaedic patients.

Pharmacokinetics of PEG-hemoglobin SB1, a hemoglobin-based oxygen carrier, after its intravenous administration in beagle dogs

The purpose of the present study was to investigate the pharmacokinetics of PEG-hemoglobin SB1, a modified bovine hemoglobin with polyethylene glycol, after its single and multiple administration in beagle dogs. For this purpose, the analytical method of free hemoglobin in the plasma was developed and validated. Excellent linearity (r2=0.999) was observed in the calibration curve data, with the limit of quantification of 0.005 g/dL. The precision and the deviation of the theoretical values for accuracy were always within +/-15% in both the between- and the within-day results. The method was tested by measuring the plasma concentrations following intravenous administration to beagle dogs and was shown to be suitable for pharmacokinetic studies. In a single dose study, the plasma half-life (t1/2) increased and the total body clearance (CLt) decreased with the dose (i.e., 0.017 to 0.75 gHb/kg as PEG-hemoglobin SB1) in both sexes. The volume of distribution at steady-state (Vd(ss)) showed no difference with the dose. In contrast, the values of t1/2, CLt and the area under the plasma concentration-time curve (AUC) after the multiple dose were significantly different from those of the single dose administration. The values of t1/2 in the multiple administration were about two times higher than that of the single dose. As a result, t1/2 of hemoglobin after the administration of PEG-hemoglobin SB1 was about 15-30 h, indicating the PEG modification of the hemoglobin lead to a prolongation of plasma concentration of the protein. Therefore, these observations suggested that the PEG modification of hemoglobin is potentially applicable in the hemoglobin-based therapeutics.

Oxygen therapeutics: can we tame haemoglobin?

Chemically modified or genetically engineered haemoglobins (Hbs) developed as oxygen therapeutics (often termed 'blood substitutes') are designed to correct oxygen deficit due to ischaemia in a variety of clinical settings. These modifications are intended to stabilize Hb outside its natural environment--red blood cells--in a functional tetrameric and/or polymeric form. Uncontrolled haem-mediated oxidative reactions of cell-free Hb and its reactions with various oxidant/antioxidant and cell signalling systems have emerged as an important pathway of toxicity. Current protective strategies designed to produce safe Hb-based products are focused on controlling or suppressing the 'radical' nature of Hb while retaining its oxygen-carrying function.

Hemoglobin solutions

OBJECTIVE

To review current knowledge about cell-free hemoglobin solutions.

DATA SOURCE

A computerized MEDLINE search was used to retrieve all studies concerning cell-free hemoglobin solutions from 1990 to 2003. The reference lists of all available review articles and primary studies were also reviewed to identify references not identified in the computerized search.

STUDY SELECTION

All clinical and experimental studies involving cell-free hemoglobin solutions were included.

DATA EXTRACTION

From the selected studies, information was obtained regarding the experimental model or the study population in which cell-free hemoglobin solutions were investigated, the type of cell-free hemoglobin solution used, their deleterious or beneficial effects, and their possible indications.

DATA SYNTHESIS

In many studies, hemoglobin solutions were considered as efficient resuscitative agents and good alternatives to red blood cell transfusion, owing to their marked vasopressor effect, coupled with their capacity to improve the microcirculation and rapidly restore metabolic parameters. The main problems identified include excessive systemic vasoconstriction and oxidative damage. Initial enthusiasm in the development of hemoglobin solutions has been tempered recently by the negative results of a U.S. multicenter trial studying the early infusion of diaspirin cross-linked hemoglobin in trauma patients. Nevertheless, the properties of diaspirin cross-linked hemoglobin (and particularly the strong vasopressor effects) cannot be attributed to all hemoglobin solutions, and results of new clinical studies are eagerly awaited to evaluate the potential benefit of such solutions in the management of trauma patients.

CONCLUSIONS

Today, we are aware of the effects of the first generation of blood substitutes. Further research is ongoing into newer solutions. One area of interest is the development of new molecular structures to decrease nitric oxide binding, thus minimizing any adverse events and maximizing potential benefits. Nevertheless, possible adverse effects need to be carefully evaluated before these agents can be widely administered.

Direct observation of photolysis-induced tertiary structural changes in hemoglobin

Human Hb, an alpha2beta2 tetrameric oxygen transport protein that switches from a T (tense) to an R (relaxed) quaternary structure during oxygenation, has long served as a model for studying protein allostery in general. Time-resolved spectroscopic measurements after photodissociation of CO-liganded Hb have played a central role in exploring both protein dynamical responses and molecular cooperativity, but the direct visualization and the structural consequences of photodeligation have not yet been reported. Here we present an x-ray study of structural changes induced by photodissociation of half-liganded T-state and fully liganded R-state human Hb at cryogenic temperatures (25-35 K). On photodissociation of CO, structural changes involving the heme and the F-helix are more marked in the alpha subunit than in the beta subunit, and more subtle in the R state than in the T state. Photodeligation causes a significant sliding motion of the T-state beta heme. Our results establish that the structural basis of the low affinity of the T state is radically different between the subunits, because of differences in the packing and chemical tension at the hemes.

Hemoglobin-based oxygen carriers

Three types of materials have been studied as candidate blood substitutes: the perfluorocarbons, modified hemoglobins, and liposome-encapsulated hemoglobin. Progress has been greatest with the hemoglobin-based oxygen carriers. Hemoglobin is a highly active molecule; hence, modification has been required to avoid potential deleterious effects. Although there has been considerable progress toward bringing such a product to the clinic, its development has challenged understanding of oxygen delivery and use. The study of these molecules has provided new insights into basic physiologic processes.

Vascular response to infusions of a nonextravasating hemoglobin polymer

The clinical utility of cross-linked tetrameric hemoglobin solutions is limited by peripheral vasoconstriction thought to be due to scavenging of nitric oxide. In addition, transfusion of crude preparations of hemoglobin polymers can cause arterial hypertension. We tested the hypothesis that eliminating low-molecular-weight components from the polymer solution would prevent extravasation and its associated pressor response. A zero-link polymer of bovine hemoglobin was developed without chemical linkers left between the tetramers. Transfusion of unprocessed preparations of these polymers in rats resulted in appearance of the polymer in the renal hilar lymph. However, eliminating the low-molecular-weight components with a 300-kDa diafiltration resulted in an average hydrodynamic radius of 250 A and in undetectable levels of polymer in hilar lymph. Exchange transfusion in anesthetized rats and cats and in awake cats produced no increase in arterial pressure. In anesthetized cats, exchange transfusion with an albumin solution reduced hematocrit from 30 to 18%, increased cerebral blood flow, and dilated pial arterioles. In contrast, reducing hematocrit by transfusing the diafiltered polymer did not increase cerebral blood flow as pial arterioles constricted. These results are consistent with the hypothesis that the increase in arterial pressure associated with cell-free hemoglobin transfusion depends on hemoglobin extravasation. Constriction observed in the cerebrovascular bed with a nonextravasating hemoglobin polymer at low hematocrit is presumably a regulatory response to prevent overoxygenation at low blood viscosity.

The oxidative effect of bacterial lipopolysaccharide on native and cross-linked human hemoglobin as a function of the structure of the lipopolysaccharide

The binding of lipopolysaccharide (LPS, also known as bacterial endotoxin) to human hemoglobin is known to result in oxidation of hemoglobin to methemoglobin and hemichrome. We have investigated the effects of the LPSs from smooth and rough Escherichia coli and Salmonella minnesota on the rate of oxidation of native oxyhemoglobin A0 and hemoglobin cross-linked between the alpha-99 lysines. For cross-linked hemoglobin, both smooth LPSs produced a rate of oxidation faster than the corresponding rough LPSs, indicating the importance of the binding of LPS to the hemoglobin. The effect of the LPS appeared to be largely on the initial fast phase of the oxidation reaction, suggesting modification of the heme pocket of the alpha chains. For hemoglobin A0, the rates of oxidation produced by rough and smooth LPSs were very similar, suggesting the possibility that the effect of the LPSs was to cause dissociation of hemoglobin into dimers. The participation of cupric ion in the oxidation process was demonstrated in most cases. In contrast, the rate of oxidation of cross-linked hemoglobin by the LPSs of both the rough and smooth E. coli was not affected by the presence of chelators, suggesting that cupric ion had previously bound to these LPSs. Overall, these data suggest that the physiological effectiveness of hemoglobin solutions now being developed for clinical use may be decreased by the presence of lipopolysaccharide in the circulation of recipients.

Effects of hemoglobin-based blood substitutes on vasoactivity of rat aortic rings

Our objective is to characterize the vasoactive properties of a 10% alphaalpha diaspirin cross-linked human hemoglobin (alphaalphaHb) and to test the hypothesis that sodium nitroprusside (SNP)-induced relaxation is inhibited in the presence of alphaalphaHb. Experiments were performed on aortic rings from 18 Wistar rats; the rings were suspended in aerated Krebs solution. Changes in isometric tension were measured to increasing concentrations of alphaalphaHb (1.8 x 10(-9) to 10(-4) M) on phenylephrine (PE)-induced contraction (3 x 10(-7) M), on acetylcholine (ACh)-induced relaxation (10(-8) to 10(-6) M), on SNP-induced relaxation (10(-9) and 10(-8) M), and on PE-induced contraction with an endothelin-1 (ET1) receptor antagonist, BQ123 (10(-5) M). Control rings received no alphaalphaHb. A concentration-dependent increase of the PE-precontraction (1.3%, 6.8%, 17.4%, and 34%, respectively) as well as the inhibition and reversal of ACh-induced relaxation was observed after alphaalphaHb. The presence of alphaalphaHb decreased the SNP-induced relaxation in the presence or absence of endothelium. The relaxation induced by SNP was reduced with time in the presence, but not in the absence, of alphaalphaHb. In conclusion, although pharmacological modulation of the vasoconstriction is possible with nitric oxide donors, our findings suggest that in the clinical setting, large sustained donor doses may be required.

Molecular engineering of a polymer of tetrameric hemoglobins

We have engineered a recombinant mutant human hemoglobin, Hb Prisca beta(S9C+C93A+C112G), which assembles in a polymeric form. The polymerization is obtained through the formation of intermolecular S-S bonds between cysteine residues introduced at position beta9, on the model of Hb Porto Alegre (beta9Ser --> Cys) (Bonaventura and Riggs, Science 1967;155:800-802). Cbeta93 and Cbeta112 were replaced in order to prevent formation of spurious S&bond;S bonds during the expression, assembly, and polymerization events. Dynamic light scattering measurements indicate that the final polymerization product is mainly formed by 6 to 8 tetrameric hemoglobin molecules. The sample polydispersity Q = 0.07 +/- 0.02, is similar to that of purified human hemoglobin (Q = 0.02 +/- 0.02), consistent with a good degree of homogeneity. In the presence of strong reducing agents, the polymer reverts to its tetrameric form. During the depolymerization process, a direct correlation is observed between the hydrodynamic radius and the light scattering of the system, which, in turn, is proportional to the mass of the protein. We interpret this to indicate that the hemoglobin molecules are tightly packed in the polymer with no empty spaces. The tight packing of the hemoglobin molecules suggests that the polymer has a globular shape and, thus, allows estimation of its radius. An illustration of an arrangement of a finite number of tetrameric hemoglobin molecules is presented. The conformational and functional characteristics of this polymer, such as heme pocket conformation, stability to denaturation, autoxidation rate, oxygen affinity, and cooperativity, remain similar to those of tetrameric human hemoglobin.

Carbon monoxide from heme catabolism protects against hepatobiliary dysfunction in endotoxin-treated rat liver

BACKGROUND AND AIMS

Liver is a major organ for heme detoxification under disease conditions, but its self-protective mechanisms against the toxicity are unknown. This study aimed to examine roles of carbon monoxide (CO), the gaseous product of heme oxygenase (HO), in ameliorating hepatobiliary dysfunction during catabolism of heme molecules in endotoxemic livers.

METHODS

Vascular resistance and biliary flux of bilirubin-IXalpha, an index of HO-derived CO generation, were monitored in perfused livers of endotoxemic rats. Livers were perfused with HbO(2), which captures nitric oxide (NO) and CO, or metHb, a reagent trapping NO but not CO.

RESULTS

In endotoxin-pretreated livers where inducible NO synthase and HO-1 overproduced NO and CO, HbO(2) caused marked vasoconstriction and cholestasis. These changes were not reproduced by the NO synthase inhibitor aminoguanidine alone, but by coadministration of zinc protoporphyrin-IX, an HO inhibitor. CO supplementation attenuated the events caused by aminoguanidine plus zinc protoporphyrin-IX, suggesting that simultaneous elimination of these vasorelaxing gases accounts for a mechanism for HbO(2)-induced changes. This concept was supported by observation that metHb did not cause any cholestasis; the reagent captures NO but triggers CO overproduction through rapid degradation of the heme by HO-1.

CONCLUSIONS

These results suggest protective roles of CO against hepatobiliary dysfunction caused by heme overloading under stress conditions.

Effect of the blood substitute diaspirin crosslinked hemoglobin in rat mesenteric and human radial collateral arteries

The actions of the blood substitute diaspirin crosslinked hemoglobin (DCLHb) were investigated in rat (small mesenteric artery) and human (radial collateral artery) resistance vessels mounted in a wire myograph for isometric tension recording. DCLHb did not contract resting vessels from rats, but vasoconstrictor responses were observed in isolated arteries and perfused mesenteric beds prestimulated with threshold concentrations of methoxamine. The DCLHb contractile responses were greatly attenuated by N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) or endothelial removal, whereas BQ-123 (endothelin A receptor antagonist), prazosin (alpha1-adrenoceptor antagonist), or indomethacin (cyclooxygenase inhibitor) had no effect. Endothelium-dependent relaxations to carbachol in both rat mesenteric and human radial collateral artery were inhibited by DCLHb. Relaxations to carbachol were studied in the presence of L-NAME or 25 mM KCl to investigate the effect of DCLHb on endothelium-derived hyperpolarizing factor (EDHF) and nitric oxide, respectively. In both rat and human vessels, EDHF-mediated relaxations were not affected by DCLHb preincubation, whereas the nitric oxide component of carbachol-induced relaxations was practically abolished. In conclusion, inhibition of the effects of basal nitric oxide release underpins the vasoconstrictor effects of DCLHb. DCLHb effectively abolishes the nitric oxide component of carbachol-induced relaxation, with no effect on the EDHF-mediated component in both isolated rat mesenteric and human radial collateral arteries.

Pharmacokinetics of diaspirin cross-linked haemoglobin in a rat model of hepatic cirrhosis

The aim of the study was to evaluate the effect of cirrhosis on the disposition of the haemoglobin-based oxygen carrier, diaspirin cross-linked haemoglobin (DCLHb). Cirrhosis was induced in male Sprague-Dawley rats (200-250 g) by inhalational exposure to carbon tetrachloride (CCl4), over a period of 6 weeks. Pharmacokinetic evaluation was performed after a single intravenous bolus administration of DCLHb (400 mg kg(-1)). Serum biochemistry, including aspartate transaminase, alkaline phosphatase, bile acids, serum albumin, and serum creatinine, were measured in CCl4-treated (n = 6) and age-matched control (n = 6) rats. After 6 weeks, the jugular vein and carotid artery were cannulated for bolus DCLHb administration (400 mg kg(-1)) and blood sampling, respectively, in both groups of rats. Cirrhosis produced significant (P < 0.05) elevations in alkaline phosphatase (497.4 +/- 84.8 U L(-1) vs 241.2 +/- 5.1 U L(-1)), aspartate transaminase (920.5 +/- 190.9 U L(-1) vs 238.2 +/- 118.1 U L(-1)) and bile acids (333.8 +/- 77.3 mg dL(-1) vs 43.8 +/- 4.2 mg dL(-1)) compared with the control group. No significant renal dysfunction was observed as a result of CCl4 exposure. Plasma DCLHb concentrations declined approximately log-linearly. Systemic clearance of DCLHb was estimated to be 2.2 +/- 0.7 mL h(-1) in the treatment group and was slightly, but not significantly, less in the control group (3.6 +/- 1.7 mL h(-1)). There was also a trend toward a longer elimination half-life in the treatment group (4.7 +/- 2.2 h) compared with the control group (3.8 +/- 0.8 h), although this difference was not statistically significant. Cirrhosis does not significantly alter the disposition of DCLHb perhaps due to increased extra-hepatic metabolism by the reticulo-endothelial system.

Diaspirin cross-linked hemoglobin improves oxygen extraction capabilities in endotoxic shock

We studied the effects of diaspirin cross-linked hemoglobin (DCLHb), a cell-free hemoglobin derived from human erythrocytes, on blood flow distribution and tissue oxygen extraction capabilities in endotoxic shock. Eighteen pentobarbital sodium-anesthetized, mechanically ventilated dogs received 2 mg/kg of E. coli endotoxin, followed by saline resuscitation to restore cardiac filling pressures to baseline levels. The animals were randomly divided into three groups: six served as control, six received DCLHb at a dose of 500 mg/kg (group 1) and six DCLHb at a dose of 1,000 mg/kg (group 2). Cardiac tamponade was then induced by saline injection in the pericardial sac to progressively reduce cardiac index and thereby allow study of tissue oxygen extraction capabilities. DCLHb had a dose-dependent vasopressor effect but did not significantly alter cardiac index or regional blood flow. During cardiac tamponade, critical oxygen delivery was 12.8 +/- 0.7 ml. kg(-1). min(-1) in the control group, but 8.6 +/- 0.9 and 8.2 +/- 0.7 ml. kg(-1). min(-1) in groups 1 and 2, respectively (both P < 0.05 vs. control group). The critical oxygen extraction ratio was 39.1 +/- 3.1% in the control group but 58.7 +/- 12.8% and 60.2 +/- 9.0% in groups 1 and 2, respectively. We conclude that DCLHb can improve whole body oxygen extraction capabilities during endotoxic shock in dogs.

Diaspirin cross-linked Hb and norepinephrine prevent the sepsis-induced increase in critical O(2) delivery

We hypothesized that support of arterial perfusion pressure with diaspirin cross-linked Hb (DCLHb) would prevent the sepsis-induced attenuation in the systemic O(2) delivery-O(2) uptake relationship. Awake septic rats were treated with a chronic infusion of DCLHb or a reference treatment [norepinephrine (NE)] to increase mean arterial pressure by 10-20% over 18 h. Septic and sham control groups received normal saline. Isovolemic hemodilution to create anemic hypoxia was then performed in a metabolic box during continuous measurement of systemic O(2) uptake. O(2) delivery was calculated from hemodynamic variables, and the critical point of O(2) delivery (DO(2 crit)) was determined using piecewise regression analysis of the O(2) delivery-O(2) uptake relationship. Sepsis increased DO(2 crit) from 4.99 +/- 0.17 to 6.69 +/- 0.42 ml x min(-1) x 100 g(-1) (P < 0.01), while O(2) extraction capacity was decreased (P < 0.05). DCLHb and NE infusion prevented the sepsis-induced increase in DO(2 crit) [4.56 +/- 0.42 ml x min(-1) x 100 g(-1) (P < 0.01) and 5.04 +/- 0.56 ml x min(-1) x 100 g(-1) (P < 0.05), respectively]. This was explained by a 59% increase in O(2) extraction capacity in the DCLHb group compared with septic controls (P < 0.05), whereas NE treatment decreased systemic O(2) uptake in anemic hypoxia (1.51 +/- 0.08 vs. 1.87 +/- 0.1 ml x min(-1) x 100 g(-1) in septic controls, P < 0.05). We conclude that DCLHb ameliorated O(2) extraction capacity in the septic microcirculation, whereas NE decreased the metabolic demands of the tissues.

Acellular hemoglobin solution enters compressed lung capillaries more readily than red blood cells

High lung inflation pressures compress alveolar septal capillaries, impede red cell transit, and interfere with oxygenation. However, recently introduced acellular hemoglobin solutions may enter compressed lung capillaries more easily than red blood cells. To test this hypothesis, we perfused isolated rat lungs with fluorescently labeled diaspirin cross-linked hemoglobin (DCLHb; 10%) and/ or autologous red cells (hematocrit, 20). Septal capillaries were compressed by setting lung inflation pressure above vascular pressures (zone 1). Examination by confocal microscopy showed that DCLHb was distributed throughout alveolar septa. Furthermore, this distribution was not affected by adding red blood cells to the perfusate. We estimated the maximum acellular hemoglobin mass within septa to be equivalent to that of 15 red blood cells. By comparison, we found an average of 2.7 +/- 4.6 red cells per septum in zone 1. These values increased to 30.4 +/- 25.8 and 50.4 +/- 22.1 cells per septum in zones 2 and 3, respectively. We conclude that perfusion in zone 1 with a 10% acellular hemoglobin solution may increase the hemoglobin concentration per septum up to fivefold compared with red cell perfusion.

Effect of alpha-alpha diaspirin crosslinked hemoglobin (DCLHb) on the potency of sodium nitroprusside and nitroglycerine to decrease blood pressure in rats: a dose-response study

The nitrovasodilators, sodium nitroprusside and nitroglycerine, effect a dose-dependent decrease in mean arterial blood pressure (MABP) by liberating nitric oxide. Alpha-alpha diaspirin crosslinked hemoglobin (DCLHb) is known to bind nitric oxide. We studied the effect of DCLHb on the potency of sodium nitroprusside (n=36) and nitroglycerine (n=36) to decrease MABP in rats which received 1, 10, 100, 1,000, or 10,000 mg/kg of the DCLHb, or normal saline as the Control. Six doses of sodium nitroprusside or nitroglycerine were given to each rat in a systematically varied sequence. For both drugs, in rats given 1, 10, or 100 mg/kg of DCLHb there were no between groups differences in the change in MABP compared to the Control group. For rats that received 1,000 or 10,000 mg/kg of DCLHb, the potency of nitroglycerine and sodium nitroprusside to decrease MABP was less (p<0.05) than the other groups. These data support the hypothesis that small doses of DCLHb effect a minimal change in the potency of nitrovasodilators to reduce blood pressure. However, they suggest that clinically relevant doses of DCLHb attenuate the vasodilatory ability of sodium nitroprusside and nitroglycerine.

Haemoglobin-based oxygen carriers

Haemoglobin-based oxygen carriers are being developed for use in blood replacement therapies, either for perioperative haemodilution or for resuscitation from haemorrhagic blood loss. There is a high demand for these products because of risks associated with blood transfusions and pending worldwide blood shortages. Development of these products has required new technologies in protein engineering; since the haemoglobin is cell-free in solution, the molecule must be modified to be retained within blood circulation. Three classes of haemoglobin are under development: intramolecular cross-linked, intermolecular polymerised and surface conjugated with polyethylene glycol. Two products based on cross-linking chemistry have been discontinued because of serious adverse events and/or increased mortality rate in Phase III clinical trials. Three products based on polymerisation chemistry are in ongoing Phase III clinical trials. A new product based on surface conjugation is in preclinical evaluation. Although cross-linked and polymerised products have shown to be safe in preclinical and early Phase I/II clinical trials, they have had difficulty in proving efficacy. The primary adverse effect for the majority of cross-linked or polymerised products is a haemodynamic response, leading to increased vascular resistance to blood flow. The physiological mechanisms are still incompletely understood, so that safety and efficacy cannot be completely dissociated. New understandings on the mode of action of these products will help to define their utility and application. New products are under development, designed specifically to maximise blood flow and tissue perfusion and therefore, oxygenation.

Molecular dimensions of Hb-based O(2) carriers determine constriction of resistance arteries and hypertension

The effect of molecular dimension of hemoglobin (Hb)-based O(2) carriers on the diameter of resistance arteries (A(0), 158 +/- 21 microm) and arterial blood pressure were studied in the conscious hamster dorsal skinfold model. Cross-linked Hb (XLHb), polyethylene glycol (PEG)-conjugated Hb, hydroxyethylstarch-conjugated XLHb, polymerized XLHb, and PEG-modified Hb vesicles (PEG-HbV) were synthesized. Their molecular diameters were 7, 22, 47, 68, and 224 nm, respectively. The bolus infusion of 7 ml/kg of XLHb (5 g/dl) caused an immediate hypertension (+34 +/- 13 mmHg at 3 h) with a simultaneous decrease in A(0) diameter (79 +/- 8% of basal value) and a blood flow decrease throughout the microvascular network. The diameter of smaller arterioles did not change significantly. Infusion of larger O(2) carriers resulted in lesser vasoconstriction and hypertension, with PEG-HbV showing the smallest changes. Constriction of resistance arteries was found to be correlated with the level of hypertension, and the responses were proportional to the molecular dimensions of the O(2) carriers. The underlying mechanism is not evident from these experiments; however, it is likely that the effects are related to the diffusion properties of the different Hb molecules.

Hemoglobin solutions--not just red blood cell substitutes

OBJECTIVE

To review current knowledge about cell-free hemoglobin solutions.

DATA SOURCES

All studies involving cell-free hemoglobin were retrieved from a computerized MEDLINE search from 1980 to 1998. We also reviewed the reference lists of all available review articles and primary studies to identify references not found in the computerized search.

STUDY SELECTION

Clinical and experimental studies in which cell-free hemoglobin solutions were studied.

DATA EXTRACTION

From the selected studies, information was obtained regarding the experimental model or the study population in which cell-free hemoglobin solutions were investigated, the type of cell-free hemoglobin solution, their deleterious or beneficial effects, and their possible indications.

DATA SYNTHESIS

In many studies, hemoglobin solutions were considered as efficient resuscitative agents and good alternatives to red blood cell transfusion, because of their marked vasopressor effect coupled with their capacity to improve the microcirculation and quickly restore metabolic parameters. Nevertheless, potential problems include an increased susceptibility to infection, immunosuppression, oxidative damage, excessive pulmonary and systemic vasoconstriction, and platelet activation.

CONCLUSIONS

Hemoglobin solutions are more than mere blood substitutes. Promising effects on oxygen transport and the microcirculation need to be confirmed, and the results of continuing research are eagerly awaited.

Nonideality and the nucleation of sickle hemoglobin

The homogeneous and heterogeneous nucleation kinetics of sickle hemoglobin (HbS) have been studied for various degrees of solution crowding by substitution of cross-linked hemoglobin A, amounting to 50% of the total hemoglobin. By cross-linking hemoglobin A, hybrid formation between hemoglobin A and hemoglobin S was prevented, thus simplifying the analysis of the results. Polymerization was induced by laser photolysis, and homogeneous nucleation kinetics were determined by observation of the stochastic behavior of the onset of light scattering. Heterogeneous nucleation was determined by observing the exponential growth of the progress curves, monitored by light scattering. At concentrations between 4 and 5 mM tetramer (i.e., approximately 30 g/dl), the substitution of 50% HbA for HbS slows the reaction by a factor of 10(3) to 10(4). Using scaled particle theory to account for the crowding of HbA, the observed decrease in the homogeneous nucleation rate was accurately predicted, with no variation of parameters required. Heterogeneous nucleation, on the other hand, is not well described in the present formulation, and the theory for this process appears to require modification of the way in which nonideality is introduced. Nonetheless, the accuracy of the homogeneous nucleation description suggests that such an approach may be useful for other assembly processes that occur in a crowded intracellular milieu.

Evaluation and validation of virus removal by ultrafiltration during the production of diaspirin crosslinked haemoglobin (DCLHb)

Virus retention during ultrafiltration through A/G Technology filter cartridges was investigated to characterize the removal process and validate the degree of virus titre reduction during the filtration of red blood cell haemolysates performed as part of the production of diaspirin crosslinked haemoglobin (DCLHb). When viruses were suspended in phosphate buffered saline solution, retention was greater with larger sized viruses and smaller filter pore size. Virus titre was maintained at starting levels in the filter retentate circuit during the course of filtration, suggesting that the virus removal mechanism is predominantly size exclusion. Evaluation of specific processing variables indicated that the retention of phiX174 virus was increased in the presence of red blood cell haemolysate or at high membrane crossflow rates and transmembrane pressures, while the retention of EMC virus was less sensitive to variations in these parameters. Using these results to design a validation protocol, log reduction values of >7.9 were demonstrated for the retention of human immunodeficiency virus, pseudorabies virus and bovine viral diarrhoea viruses, 7.6 for hepatitis A virus, and 4.2 for porcine parvovirus. It was also shown that the retention of viruses was maintained during repetitive use of the same filter cartridge.

Pyridoxalated hemoglobin polyoxyethylene: a nitric oxide scavenger with antioxidant activity for the treatment of nitric oxide-induced shock

Hemoglobins modified for therapeutic use as either hemoglobin-based oxygen carriers or scavengers of nitric oxide are currently being evaluated in clinical trials. One such product, pyridoxalated hemoglobin polyoxyethylene conjugate (PHP), is a human-derived and chemically modified hemoglobin that has yielded promising results in Phase II clinical trials, and is entering a pivotal Phase III clinical trial for the treatment of shock associated with systemic inflammatory response syndrome (SIRS). Shock associated with SIRS is a NO-induced shock. PHP, a new mechanism-based therapy, has been demonstrated in clinical trials to have the expected hemodynamic activity of raising blood pressure and reducing catecholamine use, consistent with its mechanism of action as a NO scavenger. PHP is conjugated with polyoxyethylene, which results in a surface-decorated molecule with enhanced circulation time and stability as well as in attachment of soluble red blood cell enzymes, including catalase and superoxide dismutase. PHP thus contains an antioxidant profile similar to the intact red blood cell and is therefore resistant to both initial oxidative modification by oxidants such as hydrogen peroxide and subsequent ferrylhemoglobin formation. These studies suggest both that the redox activity of modified hemoglobins can be attenuated and that modified hemoglobins containing endogenous antioxidants, such as PHP, may have reduced pro-oxidant potential. These antioxidant properties, in addition to the NO-scavenging properties, may allow the use of PHP in other indications in which excess NO, superoxide, or hydrogen peroxide is involved, including ischemia-reperfusion injury and hemorrhagic shock.

Diaspirin cross-linked hemoglobin improves mucosal perfusion in the ileum of septic rats

OBJECTIVE

To determine the effect of a bolus infusion of diaspirin cross-linked hemoglobin (DCLHb or hemoglobin crosfumaril) on the ileal mucosal microcirculation in septic rats.

DESIGN

Prospective, randomized, single-blinded study.

SETTING

University-affiliated animal research laboratory.

SUBJECTS

Twenty-four male Sprague-Dawley rats, weighing 320-380 g.

INTERVENTIONS

Under inhalational anesthesia, arterial and venous catheters were inserted and sepsis was created by cecal ligation and perforation (CLP). Twenty-four hours later, animals were reanesthetized and ventilated. Via midline abdominal incision, the ileum was mobilized and prepared for intravital microscopy. Post-CLP hemodynamic values were obtained, and videomicroscopy was performed on four to ten villi. Animals were then randomized to receive 2 mL of DCLHb solution (100 mg/mL; n = 12) or pentastarch (n = 12) intravenously, and measurements were repeated after 20 mins. Rats treated with DCLHb then received nitroprusside to restore mean arterial pressure to post-CLP levels, and final measurements were obtained 15 mins later.

MEASUREMENTS AND MAIN RESULTS

Cardiac index increased with both treatments (p < .001), whereas systemic vascular resistance index and mean arterial blood pressure were augmented only with DCLHb (p < .0001 compared with pentastarch). Intercapillary areas (ICA; inversely related to capillary density) were determined using computerized image analysis. ICA size decreased after treatment, from 974 +/- 79 to 791 +/- 106 microm2 with DCLHb and from 1044 +/- 90 to 840 +/- 82 microm2 with pentastarch (both p < .05). Red blood cell velocity in terminal arterioles, as assessed by velocimetry from the recorded images, increased by 15% with both treatments (p < .05). Restoration of mean arterial pressure to post-CLP levels in DCLHb animals by nitroprusside infusion abolished the effects of the hemoglobin solution on ICA size and red blood cell velocity.

CONCLUSION

Both DCLHb and pentastarch infusion improved microcirculatory perfusion in the ileum of septic rats. In addition, DCLHb also exhibited vasopressor properties, which in combination with improved perfusion may be particularly useful in the treatment of sepsis.

Hemoglobin stimulates the release of proinflammatory cytokines from leukocytes in whole blood

Isolated mononuclear leukocytes, when incubated with purified hemoglobin Ao (HbAo), release the proinflammatory cytokines interleukin-8 (IL-8) and tumor necrosis factor-alpha (TNF-alpha). In this study we examined whether leukocytes in whole blood, when incubated with HbAo, release IL-8, TNF-alpha, and IL-6. Leukocytes in whole blood incubated with HbAo for 4 hours at 37 degrees C, 5% CO2, and 95% humidity released 187, 1313, and 50 pg/mL of IL-6, IL-8, and TNF-alpha, respectively, as compared with 6, 192, and 2 pg/mL released by leukocytes in blood incubated with human serum albumin (HSA). Furthermore, plasma from blood incubated with HbAo exhibited chemotactic activity and stimulated human umbilical vein endothelial cells to become adherent to neutrophils. These activities were 3.3 and 2.6 times those measured in plasma from blood incubated with HSA. Hydrocortisone (0.05 micromol/L to 50 micromol/L) inhibited cytokine release in a dose-dependent manner with ED50 values of 0.23 micromol/L, 0.19 micromol/L, and 0.10 micromol/L for IL-6, IL-8, and TNF-alpha, respectively. The release of proinflammatory cytokines in whole blood after exposure to hemoglobin solutions is consistent with the possibility that an inflammatory reaction could develop on infusion of hemoglobin, whereas inhibition of cytokine release by hydrocortisone suggests that the inclusion of anti-inflammatory compounds in hemoglobin solutions may prevent undesirable effects caused by inflammation after infusion.

Purification and chemical modifications of hemoglobin in developing hemoglobin based oxygen carriers

The efficacy of blood substitutes, as a whole, has been readily demonstrated, in animals as well as clinical studies. It is well known that stroma free hemoglobin (SF-Hb) is very toxic, due to effects on renal and coagulation functions and vascular tone. Several modifications have been made to the hemoglobin tetramer in an attempt to eliminate its toxicity. Conjugation, cross-linking, polymerization, and recombinant technology have all been used to reduce toxicity, while aiming to optimize the therapeutic value of hemoglobin based blood substitutes. The remaining issue seems to be the hypertensive response seen in many hemoglobin solutions. The cause of the hypertensive response, and hence what chemical modifications are suitable to alleviate it are still under debate.

Site-specific modifications and toxicity of blood substitutes. The case of diaspirin cross-linked hemoglobin

Safe and effective hemoglobin-based blood substitutes may be advantageous over conventional therapies for certain clinical settings requiring short term blood replacement such as emergency resuscitation and hemodilution in surgery. Many advances have been made in developing these oxygen therapeutics, however safety concerns continue to slow their clinical progress. An important and often overlooked consideration in evaluating the safety of modified hemoglobins is the impact of chemical and/or genetic modifications on the redox chemistry of these proteins. Diaspirin cross-linked hemoglobin (DBBF-Hb) has been extensively evaluated in vitro and in animal models, and thus represents a useful model to explore possible correlations between structural-functional alterations and toxicity of hemoglobin-based blood substitutes.

Blood substitutes

Red cell substitutes are a group of oxygen carriers designed to temporarily replace transfused blood. Each product is unique in its limitations and advantages. Research and development has been slow because of the far-reaching consequences of replacing an oxygen carrier outside of the red cell. Nevertheless, a number of products are in advanced clinical trials and nearing the market. When they are available for use it is likely that development will accelerate and even better products will substantially alleviate the world-wide shortage of blood for transfusion and enable the delivery of medical care to underserved populations. An important consequence of the development of these products has been a better understanding of how oxygen is delivered to tissues.

Current aspects in pharmacology of modified hemoglobins

Blood substitutes are products that are designed to replace whole blood (or) red blood cells in the field of transfusion medicine. There are two major classes that belong to this new therapeutics: (i) modified hemoglobins and (ii) perfluorocarbons. Modified hemoglobins have made tremendous progress in the past decade and are being considered for a wide variety of conditions like trauma, elective surgery, oxygenation of tumors to make them more sensitive to radiation therapy, stroke etc. Although, these agents are primarily used for oxygen delivery, their pharmacological actions have been significantly important. Several mechanisms are being explored to explain these pharmacological effects. Modified hemoglobins suffer several drawbacks including hypertension, renal toxicity, and pulmonary hypertension that restrict their development. This review deals with the clinical status and pharmacological actions of modified hemoglobins presently in advanced stages of development and some of the newer generation hemoglobin based therapeutics are also discussed.

Cys-93-betabeta-succinimidophenyl polyethylene glycol 2000 hemoglobin A. Intramolecular cross-bridging of hemoglobin outside the central cavity

Bis(maleidophenyl)-PEG2000 (Bis-Mal-PEG2000), a new bifunctional protein cross-linker targeted to sulfhydryl groups, introduces intra-tetrameric cross-links into oxy-HbA in nearly quantitative yields. Structural as well as crystallographic analyses of the cross-linked species, Bis-Mal-PEG2000 HbA, identified Cys-93(beta) as the site of intramolecular cross-linking. The cross-bridging had only a limited influence on the O(2) affinity and cooperativity of HbA in 50 mM BisTris acetate, pH 7.4. However, the Bohr effect was reduced by approximately 60%. Bis-Mal-PEG2000 HbA retained sensitivity to the presence of allosteric effectors 2, 3-diphosphoglycerate, IHP, and chloride, albeit to a lesser degree compared with HbA. Crystallographic analysis revealed the overall structure of deoxy-Bis-Mal-PEG2000 HbA to be similar to deoxy-HbA but for the loss of the salt bridge between Asp-94(beta) and His-146(beta). The large influence of the cross-bridging on the alkaline Bohr effect of HbA is consistent with the loss of this salt bridge. Unlike the "central cavity cross-bridges" described previously, the cross-link introduced by Bis-Mal-PEG2000 into HbA is an "outside the central cavity cross-bridge." In view of its oxy-conformational specificity and limited influence on O(2) affinity, this new cross-linking strategy holds promise for the stabilization of new designer low O(2) affinity Hbs generated by recombinant DNA technology for applications as Hb based therapeutics.

Chemical characterization of pyridoxalated hemoglobin polyoxyethylene conjugate

Pyridoxalated hemoglobin polyoxyethylene conjugate (PHP) was developed in the 1980s as an oxygen carrier and is now under development for treatment of nitric oxide-dependent, volume refractory shock. PHP is made by derivatizing human stroma-free hemoglobin with pyridoxal-5-phosphate and polyoxyethylene (POE). A unique aspect of using POE for modification is that unlike its mono-methoxy polyethylene glycol (PEG) relatives, POE is bifunctional. The result of derivatization of stroma-free hemoglobin is a complex mixture of modified hemoglobin and other red cell proteins. The molecular weight profile, based on size exclusion chromatography, is bimodal and has a number average molecular weight of approximately 105¿ omitted¿000 and a weight average molecular weight of approximately 187¿ omitted¿000. The mixture of hemoglobin molecules has on average 3.3 pyridoxal and 5.0 polyoxyethylene units per tetramer. A portion of the tetramers are linked by POE crosslinks. The hemoglobin tetramers retain their ability to dissociate into dimer pairs and only a small percentage of the dimer pairs are not modified with POE. The SDS-PAGE profile exhibits the ladder-like appearance commonly associated with polyethylene glycol-modified proteins. The isoelectric focusing profile is broad, demonstrating a pI range of 5.0-6.5. The hydrodynamic size of PHP was determined to be approximately 7.2 nm by dynamic light scattering. Soluble red blood cell proteins, such as catalase, superoxide dismutase, and carbonic anhydrase, are present in PHP and are also modified by POE.

Analysis of cross-linked human hemoglobin by conventional isoelectric focusing, immobilized pH gradients, capillary electrophoresis, and mass spectrometry

Diaspirin cross-linked hemoglobin (DCLHb), a hemoglobin-based oxygen carrier exhibiting near physiological oxygen binding capability and devoid of nephrotoxic side effects, was previously found, by gel permeation, reversed-phase high performance liquid chromatography (RP-HPLC) and mass spectrometry, to consist of ca. 94% cross-linked product (reacted on the Lys 99 of two alpha-chains), accompanied by ca. 6% cross-linked Hb, which also reacted on the Lys 132 and/or Lys-144 of the beta-chains and a small amount of intermolecularly cross-linked dimers. However, conventional isoelectric focusing in carrier ampholyte buffers (CA-IEF) gave an unexpected spectrum of four major, almost equally represented, pI species in the pH range of 6.82-7.01, a band of mid-intensity with a pI of 7.11, and two minor components with pls of 6.73 and 6.77. This extraordinary polydispersity was reevaluated by other surface charge probes, such as immobilized pH gradients (IPG) and capillary zone electrophoresis (CZE) of native and denatured globin chains. IPGs of DCLHb gave the expected spectrum of bands, consisting of a main component (92%) with pl 7.337 and three additional minor bands, with lower pIs, representing ca. 8% of the total. These data were in agreement with CZE profiles of native DCLHb, which resolved, in addition to the main DCLHb peak, 3-4 minor components representing ca. 10% of the total. Also, CZE of denatured, heme-free globin chains gave the expected pattern with only traces of minor, extrareacted species. The latter technique, in addition to resolving alpha- and beta-globin chains in a 1:1 ratio in control Hb, resolved a free beta- and the alpha-alpha-dimer in DCLHb. In a 1:1 mixture of control and DCLHb, three peaks were observed, eluting in the order alpha-, alpha-alpha- and beta-globin chains. The identity of the major DCLHb and of the minor species was ascertained by mass spectrometry.