SPECIAL EDITORIAL
Blood Substitutes: The Need of Hour Shrikant Balasaheb Mali, MDS
I
n critical care, blood transfusions are needed the most, yet blood is in a constant short supply. Most cases of this are due to a unit’s short shelf life of 42 days and group/Rh incompatibilities. The risks of allogeneic transfusion extend beyond microbial transmission to include allergy, alloimmunization, bacterial sepsis, graft-versus-host disease, transfusionrelated acute lung injury, renal failure, volume overload, and immunosuppression. Reactions commonly associated with proinflammatory responses to transfusion are attributable to donor leukocytes that can also release inflammatory mediators, contributing to adverse outcomes. A continuing concern with the blood supply relates to emerging blood-borne pathogens such as West Nile virus, variant Creutzfeldt-Jakob disease, and prion diseases. Since the 1980 s, as the danger of human immunodeficiency virus transmission by blood transfusion has come to light, an increased interest is being given to blood substitutes. Unlike natural blood, the only function provided by synthetic blood substitutes is gas transportation, that is, providing oxygen to cells and carrying out exchange of carbon dioxide. This becomes the 1 critical thing to be dealt with when a large amount of blood is lost because of massive trauma. Blood substitutes differ from conventional crystalloid or colloid solutions in that they must (by definition) transport oxygen in excess of what can be dissolved into balanced salt buffer. Many blood substitutes use natural hemoglobin (Hb) chemistry to achieve oxygen transport and are termed hemoglobin-based oxygen carriers (HBOCs). Although HBOCs may decrease or eliminate the need for allogeneic blood transfusions in patients under relevant clinical conditions, liberation of the Hb molecule from its usual position inside a red cell membrane may be associated with adverse effects. An ideal blood substitute should have the oxygen-carrying capacity similar to natural Hb in terms of oxygen and carbon dioxide transport; be less antigenic; require no compatibility testing; have a long shelf life, preferably at room temperature; have a long intravascular half-life; and be free of toxicity, side effects, and pathogens. It should have acceptable erythrocyte sedimentation rate and excretion rate values and should not cause
What Is This Box? A QR Code is a matrix barcode readable by QR scanners, mobile phones with cameras, and smartphones. The QR Code links to the online version of the article.
From the Oral and Maxillofacial Surgery, MGV KBH Dental College, Nashik, India. Received October 1, 2014. Accepted for publication January 21, 2015. Address correspondence and reprint requests to Shrikant Balasaheb Mali, MDS, Flat No. 2, Jyoti Savitri Apartment, Above Bank of Maharashtra, Ashoka Marg, Nashik, India; E-mail: [email protected] The author reports no conflicts of interest. Copyright # 2015 by Mutaz B. Habal, MD ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000001611
1006
increases in arterial or pulmonary blood pressure and reticuloendothelial system overload. It should not form methemoglobin, activate complement, increase white blood cell count, and react with plasma substitutes or platelets. Substances tried as blood substitutes include milk, salt or saline solutions, Hb and animal plasma, and Ringer’s solution. Modified Hb from humans and animals as well as recombinant origin and fluorocarbon emulsions have been the 2 main approaches used to exploit artificial blood substitutes. The Hb approach is the most appealing because it most directly mimics the mechanism of oxygen in natural blood. Genetic engineering, chemical crosslinking, and attaching synthetic protective coating are used in artificial blood substitutes to mimic the function of red blood cells (RBCs) and reduce the potential toxicity of Hb as contained in cells. Perfluorocarbons (PFCs) show their potential to work by getting back circulated from the reticuloendothelial system, with the help of lipoproteins and chylomicrons, to the lungs, from which these are excreted. The small size of microemulsions (0.1 mm) allows them to pass through the small capillaries, and therefore, microemulsions can exist as more stable singlephase preparations because they do not cause crenation or hemolysis of RBCs. The emulsions used also scavenge particulate and gaseous microemboli, benefits that may be important during routine use and during cardiopulmonary bypass or in the treatment of the bends in deep sea diving. Another potential advantage relates to cancer therapy, in which PFCs could increase the oxygenation of tumors to augment radiation and/ or chemotherapy effects. The adverse effects of PFCs include limited shelf life, flulike symptoms, as well as complement and phagocytic activation (not believed to occur with second-generation emulsions). Instability, environmental degradation, toxicity of phospholipids, and rapid degradation during storage are certain obstacles that prevent use of PFCs. Limitations associated with toxicity induced by the PFCs are the driving force for finding better blood substitutes that overcome these problems. Size of particle plays an important role in toxicity and efficacy of fluorocarbon emulsions because, as the particle size is increased beyond 0.03 mm, toxicity increases and retention time of the particle in the bloodstream decreases. Apart from particle size, PFCs having 9 to 11 carbon atoms should be used as a material for artificial blood substitutes to be completely eliminated from the body after parenteral administration. Hemoglobin-based oxygen carriers are solutions that contain Hb from purified human, animal, or recombinant sources. These contain the real Hb molecule at their heart. Initial use of purified Hb in the late 1940 s led to renal failure and hypertension. Therefore, newer forms were tried by polymerizing Hb or attaching it to various compounds. Five different types of HBOCs have been evaluated in clinical studies. Hemolink, Hemopure, and PolyHeme are human purified cross-linked Hb polymers. Others are Hemospan and Diaspirin cross-linked Hb. It is only inside RBCs that a heme group binds 1 oxygen molecule, causing conformational changes that further increase the affinity of Hb for added oxygen. Hemoglobin can be crosslinked and polymerized with glutaraldehyde and O-raffinose to
The Journal of Craniofacial Surgery
Volume 26, Number 4, June 2015
Copyright © 2015 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
The Journal of Craniofacial Surgery
Volume 26, Number 4, June 2015
increase its ability to deliver oxygen and increase its duration of action in circulation. The limited duration of efficacy is due to increased clearance of free Hb in the circulation and to autooxidation to methemoglobin. Nitric oxide (NO) is ordinarily abundant at the vascular endothelium, where it is synthesized and insulated from contact with Hb by virtue of the latter’s containment within RBCs. Nitric oxide acts as a potent vasodilator and also as an inhibitor of platelet activation. Dissolved HBOCs readily reach the vascular endothelium and scavenge NO, thereby increasing systemic and pulmonary artery pressures. Hemoglobin solutions have many adverse effects such as oxidation on storage; interference with macrophage function; gastrointestinal distress; and neurotoxicity, which may lead to activation of complement, kinin, and coagulation; concerns exist regarding hemochromatosis and iron overload as well as production of free radicals. Nephrotoxicity is possible: stroma-free Hb resulted in oliguria secondary to acute tubular necrosis. The polymerized or cross-linked HBOCs in current phase III trials do not seem to have this toxicity. First-generation RBC substitutes are just oxygen carriers, based on the use of modified Hb without the presence of RBC enzymes or RBC membrane. Second-generation RBC substitutes are more than oxygen carriers; these use modified Hb containing RBC antioxidant enzymes. Third-generation RBC blood substitutes are closer to RBCs because these contain nanotechnology-based polyhemoglobin (PolyHb) and conjugated Hb and continue to show promise in clinical trials. In the first- and second-generation RBC substitutes, Hb is in direct contact with blood and therefore needs to be highly purified to remove trace contaminants. Furthermore, Hb outside the RBC is more reactive and has a short circulation time; therefore, first- and second-generation RBC substitutes are useful only for short-term applications. Attempts have been made to study third-generation RBC substitutes with all the contents of RBC but replacing the RBC membrane with synthetic membrane. Encapsulation of Hb in polymeric shells, which are a suspension of fine particles of artificial blood substitutes, avoids drawbacks such as nephrotoxicity and activation of complement C3a associated with liposomes as carriers for delivery of artificial blood substitutes. In addition to its ability to carry poorly soluble agents, Hb encapsulated in polymeric shells has high binding capacity for oxygen. Polymeric shells are completely degraded by proteolytic enzymes because the polymer present is protein, which results in minimal side effects. Coating of polymeric shells with poly(ethylene glycol) results in increased circulation time to maintain high levels of artificial blood substitutes. Circulation time of polymerized Hb can be increased by encapsulating it within a micellar system, forming an amphiphilic block copolymer ranging between 30 and 100 nm in diameter. Vesicular approaches seem to be promising for the design of a universal oxygen carrier because it has no blood group antigens on its surface, is nonimmunogenic, has increased circulation time, and could be stored for long periods. One solution to the problems associated with hemoglobin-based blood substitutes regardless of the source is the encapsulation of hemoglobin within a liposome, PEGylated transfersomes, dendrimers, or polymerosomes. These have the potential to reduce the toxicity of hemoglobin whether in the tetrameric, cross-linked, polymerized, or recombinant forms as these systems have been proven to reduce the toxicity of a number of bioactive agents. The use of nanobiotechnology-based PolyHb with modified Hb of less than 1% molecular dimension did not show vasopressor effects even when large volumes of 10 L were infused. Liposome-encapsulated Hb is a novel artificial oxygen carrier that has been developed for clinical applications to enable long-term storage and subsequent transfusion. #
2015 Mutaz B. Habal, MD
Special Editorial
Dendrimers are definite globular-sized nanostructures that are chemically stable, flexible, have low cytotoxicity, and have a hydrophilic exterior to mimic blood plasma. These varied characteristics make them a good choice in the field of blood substitutes, for example, polyamidoamino dendrimers. Polymersomes are synthetic polymer vesicles that are formed in nanometric dimensions that can efficiently encapsulate oxygencarrying proteins such as Hb. Because Hb solutions have the capability to transport oxygen and because of their oncotic activity, they are therefore tested to be used as artificial blood substitutes and plasma expanders, respectively. The reversible oxygenation property of these solutions makes them a choice for rapid initial treatment of hypovolemia and tissue hypoxia. Stroma-free Hb solutions have been found to overcome the problems of toxicity, shelf life, and stability by many folds. Aqueous solutions have additional benefits such as presence of a polysaccharide oncotic agent, which makes them ideal to be used as synthetic plasma expanders. Recombinant human gelatinlike protein having an isoelectric point near to 8 is an ideal method to control the clearance of artificial blood substitutes. Slow gelling due to hydroxypropline content results in the composition, which allows the use of high– molecular-weight protein, which helps to maintain suitable colloidal osmotic pressure. This can also prevent risk for anaphylactic shock, which exists with most commercially available formulations. The first nanobiotechnologic approach reported in the literature is the cross-linking of Hb into ultrathin PolyHb membrane for artificial RBC membrane. New generations of this approach include the nanobiotechnologic assembly of Hb, catalase, and superoxide dismutase into a soluble nanodimension complex. This acts as both oxygen carrier and antioxidant for those conditions with potentials for ischemia-reperfusion injuries. Another recent novel approach is the assembling of Hb and fibrinogen into a soluble nanodimension PolyHb-fibrinogen complex that acts as an oxygen carrier with plateletlike activity. This is potentially useful in cases of extensive blood loss requiring massive replacement using blood substitutes resulting in the need for platelet and clotting factors replacement. Nanodimension artificial cells can also be formed in the form of nanodimension biodegradable polymeric membrane artificial cells containing Hb and RBC enzymes. Hemerythrin-based blood substitutes use, as an active ingredient, a protein responsible for oxygen transport in marine invertebrates that uses a non–heme diiron active site. Hemerythrin was shown to avoid reactivity toward hydrogen peroxide, NO, and nitrite. Another advantage is the higher molecular weight than that of Hb, which should lead to lower levels of extravasation or elimination through the kidney. Until now, glutaraldehyde-polymerized hemerythrin and polyethylene glycol–derivatized hemerythrin were successfully obtained. The effects of these chemical modifications on molecular weight, auto-oxidation rate, and oxygen affinity seem to be favorable for blood substitute application. The advantages of blood substitutes are as follows. They are synthetic and can be produced in large quantities. They do not have transmissible diseases such as human immunodeficiency virus, hepatitis B virus, hepatitis C virus, syphilis, human T lymphocyte virus, or West Nile virus. They can be sterilized before use. Group and rhesus (Rh) problems are not present here because both classes are group neutral. There is a longer storage capability, especially for HBOCs. Some of them can be kept at room temperature for 3 years. A normal blood unit can be kept for only 42 days in the refrigerator. They have immediate reoxygenating capacity compared with naturally transfused blood, which can take up to 24 hours to reach its maximum. This is mainly due to the loss of 2, 3, diphosphoglycerate in natural blood over time. Smaller molecules of substitutes can perfuse tissues better than RBCs
1007
Copyright © 2015 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
Special Editorial
The Journal of Craniofacial Surgery
can, such as in the case of stroke patients. They can better perfuse obstructed microcirculation such as in stroke, myocardial infarction, ischemic limbs, sickle cell anemia, and other conditions. They can also better perfuse disturbed microcirculations such as in tumors, hemorrhagic shock, and other conditions. They can be enhanced with higher enzyme levels than that of RBC to be more effective than RBC against severe ischemia-reperfusion injury, fatal elevation of tissue PCO2, and other conditions. They can be freeze dried into powder form. In this form, they are light and compact, with ease of transport especially in case of emergency, major disaster, or war. Nanobiotechnology can combine PolyHb with other enzymes and other bioreactants for specially designed oxygen therapeutics. Polyhemoglobin, being in solution, can more readily go through the obstruction to supply the needed oxygen. However, this has to be done early because, if there is much delay, PolyHb alone might result in the production of oxygen radicals that can cause tissue injury (ischemia-reperfusion injuries). Even antioxidant enzymes normally present in RBCs are not enough to prevent this problem in severe sustained hemorrhagic shock. The disadvantages are as follows. These are costlier than conventional blood. They can only carry oxygen, whereas blood
1008
Volume 26, Number 4, June 2015
can perform other functions as well (volume expansion, plasma proteins, etc). These will also decrease the available amounts of fresh frozen plasma and cryoprecipitate in blood banks. Although no HBOC is anticipated to replace allogeneic blood, a safe HBOC would facilitate hemodynamic stabilization until blood is available and do so without concern for infectious agent transmission or transfusion reaction. Hemoglobin-based oxygen carriers also have long shelf lives, a benefit when blood is in short supply or unavailable. Apart from the use of blood substitutes to reduce allogeneic blood transfusions in surgery, there are numerous other potential future indications based on their potential to augment tissue oxygenation. Such future indications will probably include treatment and prevention of cerebral ischemia, stroke, cardiopulmonary bypass–related cerebral adverse events, spinal cord ischemia, myocardial ischemia due to acute infarction, percutaneous coronary angioplasty, acute limb ischemia, emergency surgery and trauma as long as no allogeneic blood is available, and decompression sickness. These render tumors more sensitive to radiation and chemotherapy, prevent or treat sequelae of air embolism, and, finally, improve organ preservation for subsequent organ transplantation.
#
2015 Mutaz B. Habal, MD
Copyright © 2015 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
Blood Substitutes: The Need of Hour Shrikant Balasaheb Mali, MDS
I
n critical care, blood transfusions are needed the most, yet blood is in a constant short supply. Most cases of this are due to a unit’s short shelf life of 42 days and group/Rh incompatibilities. The risks of allogeneic transfusion extend beyond microbial transmission to include allergy, alloimmunization, bacterial sepsis, graft-versus-host disease, transfusionrelated acute lung injury, renal failure, volume overload, and immunosuppression. Reactions commonly associated with proinflammatory responses to transfusion are attributable to donor leukocytes that can also release inflammatory mediators, contributing to adverse outcomes. A continuing concern with the blood supply relates to emerging blood-borne pathogens such as West Nile virus, variant Creutzfeldt-Jakob disease, and prion diseases. Since the 1980 s, as the danger of human immunodeficiency virus transmission by blood transfusion has come to light, an increased interest is being given to blood substitutes. Unlike natural blood, the only function provided by synthetic blood substitutes is gas transportation, that is, providing oxygen to cells and carrying out exchange of carbon dioxide. This becomes the 1 critical thing to be dealt with when a large amount of blood is lost because of massive trauma. Blood substitutes differ from conventional crystalloid or colloid solutions in that they must (by definition) transport oxygen in excess of what can be dissolved into balanced salt buffer. Many blood substitutes use natural hemoglobin (Hb) chemistry to achieve oxygen transport and are termed hemoglobin-based oxygen carriers (HBOCs). Although HBOCs may decrease or eliminate the need for allogeneic blood transfusions in patients under relevant clinical conditions, liberation of the Hb molecule from its usual position inside a red cell membrane may be associated with adverse effects. An ideal blood substitute should have the oxygen-carrying capacity similar to natural Hb in terms of oxygen and carbon dioxide transport; be less antigenic; require no compatibility testing; have a long shelf life, preferably at room temperature; have a long intravascular half-life; and be free of toxicity, side effects, and pathogens. It should have acceptable erythrocyte sedimentation rate and excretion rate values and should not cause
What Is This Box? A QR Code is a matrix barcode readable by QR scanners, mobile phones with cameras, and smartphones. The QR Code links to the online version of the article.
From the Oral and Maxillofacial Surgery, MGV KBH Dental College, Nashik, India. Received October 1, 2014. Accepted for publication January 21, 2015. Address correspondence and reprint requests to Shrikant Balasaheb Mali, MDS, Flat No. 2, Jyoti Savitri Apartment, Above Bank of Maharashtra, Ashoka Marg, Nashik, India; E-mail: [email protected] The author reports no conflicts of interest. Copyright # 2015 by Mutaz B. Habal, MD ISSN: 1049-2275 DOI: 10.1097/SCS.0000000000001611
1006
increases in arterial or pulmonary blood pressure and reticuloendothelial system overload. It should not form methemoglobin, activate complement, increase white blood cell count, and react with plasma substitutes or platelets. Substances tried as blood substitutes include milk, salt or saline solutions, Hb and animal plasma, and Ringer’s solution. Modified Hb from humans and animals as well as recombinant origin and fluorocarbon emulsions have been the 2 main approaches used to exploit artificial blood substitutes. The Hb approach is the most appealing because it most directly mimics the mechanism of oxygen in natural blood. Genetic engineering, chemical crosslinking, and attaching synthetic protective coating are used in artificial blood substitutes to mimic the function of red blood cells (RBCs) and reduce the potential toxicity of Hb as contained in cells. Perfluorocarbons (PFCs) show their potential to work by getting back circulated from the reticuloendothelial system, with the help of lipoproteins and chylomicrons, to the lungs, from which these are excreted. The small size of microemulsions (0.1 mm) allows them to pass through the small capillaries, and therefore, microemulsions can exist as more stable singlephase preparations because they do not cause crenation or hemolysis of RBCs. The emulsions used also scavenge particulate and gaseous microemboli, benefits that may be important during routine use and during cardiopulmonary bypass or in the treatment of the bends in deep sea diving. Another potential advantage relates to cancer therapy, in which PFCs could increase the oxygenation of tumors to augment radiation and/ or chemotherapy effects. The adverse effects of PFCs include limited shelf life, flulike symptoms, as well as complement and phagocytic activation (not believed to occur with second-generation emulsions). Instability, environmental degradation, toxicity of phospholipids, and rapid degradation during storage are certain obstacles that prevent use of PFCs. Limitations associated with toxicity induced by the PFCs are the driving force for finding better blood substitutes that overcome these problems. Size of particle plays an important role in toxicity and efficacy of fluorocarbon emulsions because, as the particle size is increased beyond 0.03 mm, toxicity increases and retention time of the particle in the bloodstream decreases. Apart from particle size, PFCs having 9 to 11 carbon atoms should be used as a material for artificial blood substitutes to be completely eliminated from the body after parenteral administration. Hemoglobin-based oxygen carriers are solutions that contain Hb from purified human, animal, or recombinant sources. These contain the real Hb molecule at their heart. Initial use of purified Hb in the late 1940 s led to renal failure and hypertension. Therefore, newer forms were tried by polymerizing Hb or attaching it to various compounds. Five different types of HBOCs have been evaluated in clinical studies. Hemolink, Hemopure, and PolyHeme are human purified cross-linked Hb polymers. Others are Hemospan and Diaspirin cross-linked Hb. It is only inside RBCs that a heme group binds 1 oxygen molecule, causing conformational changes that further increase the affinity of Hb for added oxygen. Hemoglobin can be crosslinked and polymerized with glutaraldehyde and O-raffinose to
The Journal of Craniofacial Surgery
Volume 26, Number 4, June 2015
Copyright © 2015 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
The Journal of Craniofacial Surgery
Volume 26, Number 4, June 2015
increase its ability to deliver oxygen and increase its duration of action in circulation. The limited duration of efficacy is due to increased clearance of free Hb in the circulation and to autooxidation to methemoglobin. Nitric oxide (NO) is ordinarily abundant at the vascular endothelium, where it is synthesized and insulated from contact with Hb by virtue of the latter’s containment within RBCs. Nitric oxide acts as a potent vasodilator and also as an inhibitor of platelet activation. Dissolved HBOCs readily reach the vascular endothelium and scavenge NO, thereby increasing systemic and pulmonary artery pressures. Hemoglobin solutions have many adverse effects such as oxidation on storage; interference with macrophage function; gastrointestinal distress; and neurotoxicity, which may lead to activation of complement, kinin, and coagulation; concerns exist regarding hemochromatosis and iron overload as well as production of free radicals. Nephrotoxicity is possible: stroma-free Hb resulted in oliguria secondary to acute tubular necrosis. The polymerized or cross-linked HBOCs in current phase III trials do not seem to have this toxicity. First-generation RBC substitutes are just oxygen carriers, based on the use of modified Hb without the presence of RBC enzymes or RBC membrane. Second-generation RBC substitutes are more than oxygen carriers; these use modified Hb containing RBC antioxidant enzymes. Third-generation RBC blood substitutes are closer to RBCs because these contain nanotechnology-based polyhemoglobin (PolyHb) and conjugated Hb and continue to show promise in clinical trials. In the first- and second-generation RBC substitutes, Hb is in direct contact with blood and therefore needs to be highly purified to remove trace contaminants. Furthermore, Hb outside the RBC is more reactive and has a short circulation time; therefore, first- and second-generation RBC substitutes are useful only for short-term applications. Attempts have been made to study third-generation RBC substitutes with all the contents of RBC but replacing the RBC membrane with synthetic membrane. Encapsulation of Hb in polymeric shells, which are a suspension of fine particles of artificial blood substitutes, avoids drawbacks such as nephrotoxicity and activation of complement C3a associated with liposomes as carriers for delivery of artificial blood substitutes. In addition to its ability to carry poorly soluble agents, Hb encapsulated in polymeric shells has high binding capacity for oxygen. Polymeric shells are completely degraded by proteolytic enzymes because the polymer present is protein, which results in minimal side effects. Coating of polymeric shells with poly(ethylene glycol) results in increased circulation time to maintain high levels of artificial blood substitutes. Circulation time of polymerized Hb can be increased by encapsulating it within a micellar system, forming an amphiphilic block copolymer ranging between 30 and 100 nm in diameter. Vesicular approaches seem to be promising for the design of a universal oxygen carrier because it has no blood group antigens on its surface, is nonimmunogenic, has increased circulation time, and could be stored for long periods. One solution to the problems associated with hemoglobin-based blood substitutes regardless of the source is the encapsulation of hemoglobin within a liposome, PEGylated transfersomes, dendrimers, or polymerosomes. These have the potential to reduce the toxicity of hemoglobin whether in the tetrameric, cross-linked, polymerized, or recombinant forms as these systems have been proven to reduce the toxicity of a number of bioactive agents. The use of nanobiotechnology-based PolyHb with modified Hb of less than 1% molecular dimension did not show vasopressor effects even when large volumes of 10 L were infused. Liposome-encapsulated Hb is a novel artificial oxygen carrier that has been developed for clinical applications to enable long-term storage and subsequent transfusion. #
2015 Mutaz B. Habal, MD
Special Editorial
Dendrimers are definite globular-sized nanostructures that are chemically stable, flexible, have low cytotoxicity, and have a hydrophilic exterior to mimic blood plasma. These varied characteristics make them a good choice in the field of blood substitutes, for example, polyamidoamino dendrimers. Polymersomes are synthetic polymer vesicles that are formed in nanometric dimensions that can efficiently encapsulate oxygencarrying proteins such as Hb. Because Hb solutions have the capability to transport oxygen and because of their oncotic activity, they are therefore tested to be used as artificial blood substitutes and plasma expanders, respectively. The reversible oxygenation property of these solutions makes them a choice for rapid initial treatment of hypovolemia and tissue hypoxia. Stroma-free Hb solutions have been found to overcome the problems of toxicity, shelf life, and stability by many folds. Aqueous solutions have additional benefits such as presence of a polysaccharide oncotic agent, which makes them ideal to be used as synthetic plasma expanders. Recombinant human gelatinlike protein having an isoelectric point near to 8 is an ideal method to control the clearance of artificial blood substitutes. Slow gelling due to hydroxypropline content results in the composition, which allows the use of high– molecular-weight protein, which helps to maintain suitable colloidal osmotic pressure. This can also prevent risk for anaphylactic shock, which exists with most commercially available formulations. The first nanobiotechnologic approach reported in the literature is the cross-linking of Hb into ultrathin PolyHb membrane for artificial RBC membrane. New generations of this approach include the nanobiotechnologic assembly of Hb, catalase, and superoxide dismutase into a soluble nanodimension complex. This acts as both oxygen carrier and antioxidant for those conditions with potentials for ischemia-reperfusion injuries. Another recent novel approach is the assembling of Hb and fibrinogen into a soluble nanodimension PolyHb-fibrinogen complex that acts as an oxygen carrier with plateletlike activity. This is potentially useful in cases of extensive blood loss requiring massive replacement using blood substitutes resulting in the need for platelet and clotting factors replacement. Nanodimension artificial cells can also be formed in the form of nanodimension biodegradable polymeric membrane artificial cells containing Hb and RBC enzymes. Hemerythrin-based blood substitutes use, as an active ingredient, a protein responsible for oxygen transport in marine invertebrates that uses a non–heme diiron active site. Hemerythrin was shown to avoid reactivity toward hydrogen peroxide, NO, and nitrite. Another advantage is the higher molecular weight than that of Hb, which should lead to lower levels of extravasation or elimination through the kidney. Until now, glutaraldehyde-polymerized hemerythrin and polyethylene glycol–derivatized hemerythrin were successfully obtained. The effects of these chemical modifications on molecular weight, auto-oxidation rate, and oxygen affinity seem to be favorable for blood substitute application. The advantages of blood substitutes are as follows. They are synthetic and can be produced in large quantities. They do not have transmissible diseases such as human immunodeficiency virus, hepatitis B virus, hepatitis C virus, syphilis, human T lymphocyte virus, or West Nile virus. They can be sterilized before use. Group and rhesus (Rh) problems are not present here because both classes are group neutral. There is a longer storage capability, especially for HBOCs. Some of them can be kept at room temperature for 3 years. A normal blood unit can be kept for only 42 days in the refrigerator. They have immediate reoxygenating capacity compared with naturally transfused blood, which can take up to 24 hours to reach its maximum. This is mainly due to the loss of 2, 3, diphosphoglycerate in natural blood over time. Smaller molecules of substitutes can perfuse tissues better than RBCs
1007
Copyright © 2015 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
Special Editorial
The Journal of Craniofacial Surgery
can, such as in the case of stroke patients. They can better perfuse obstructed microcirculation such as in stroke, myocardial infarction, ischemic limbs, sickle cell anemia, and other conditions. They can also better perfuse disturbed microcirculations such as in tumors, hemorrhagic shock, and other conditions. They can be enhanced with higher enzyme levels than that of RBC to be more effective than RBC against severe ischemia-reperfusion injury, fatal elevation of tissue PCO2, and other conditions. They can be freeze dried into powder form. In this form, they are light and compact, with ease of transport especially in case of emergency, major disaster, or war. Nanobiotechnology can combine PolyHb with other enzymes and other bioreactants for specially designed oxygen therapeutics. Polyhemoglobin, being in solution, can more readily go through the obstruction to supply the needed oxygen. However, this has to be done early because, if there is much delay, PolyHb alone might result in the production of oxygen radicals that can cause tissue injury (ischemia-reperfusion injuries). Even antioxidant enzymes normally present in RBCs are not enough to prevent this problem in severe sustained hemorrhagic shock. The disadvantages are as follows. These are costlier than conventional blood. They can only carry oxygen, whereas blood
1008
Volume 26, Number 4, June 2015
can perform other functions as well (volume expansion, plasma proteins, etc). These will also decrease the available amounts of fresh frozen plasma and cryoprecipitate in blood banks. Although no HBOC is anticipated to replace allogeneic blood, a safe HBOC would facilitate hemodynamic stabilization until blood is available and do so without concern for infectious agent transmission or transfusion reaction. Hemoglobin-based oxygen carriers also have long shelf lives, a benefit when blood is in short supply or unavailable. Apart from the use of blood substitutes to reduce allogeneic blood transfusions in surgery, there are numerous other potential future indications based on their potential to augment tissue oxygenation. Such future indications will probably include treatment and prevention of cerebral ischemia, stroke, cardiopulmonary bypass–related cerebral adverse events, spinal cord ischemia, myocardial ischemia due to acute infarction, percutaneous coronary angioplasty, acute limb ischemia, emergency surgery and trauma as long as no allogeneic blood is available, and decompression sickness. These render tumors more sensitive to radiation and chemotherapy, prevent or treat sequelae of air embolism, and, finally, improve organ preservation for subsequent organ transplantation.
#
2015 Mutaz B. Habal, MD
Copyright © 2015 Mutaz B. Habal, MD. Unauthorized reproduction of this article is prohibited.
