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Guest Editorial

Artificial Oxygen Carriers—A Clinical Point of View

Artificial oxygen carriers (AOCs) as blood substi-tutes have long been studied as a projected shortageof blood or blood products appears imminent (1).Although it is desirable that AOCs will resolvecurrent problems and assuage future concerns, thereplacement of red blood cells (RBCs) represents amajor hurdle (2), perhaps comparable with the com-plexities of the development of artificial heart orblood vessels; none of them have yet successfullyreplaced native tissues/organs. The situation is evenless favorable for AOCs as transfusion medicine hasbeen well established and is much more successfulthan any other organ/tissue transplantation.Although recent reports suggesting deterioration ofdonated blood after 2 weeks of storage (3) maypropel quests for AOCs, the synthesis of humanhemoglobin (Hb) remains far behind the efficiency ofblood donation. Thus, human RBCs remain thesource of human Hb as in the liposome-encapsulatedHb (LEH), that is, Hb-vesicle (4) or liposomal-Hb (5)unless xenogeneic Hb is used to manufacture cell-free Hb-based oxygen carriers (HBOCs) (6–8). Asother alternatives, modified myoglobin (9) or totallysynthetic molecules (10) await development. Thisalso means that AOCs will require additional benefitsover RBCs in contrast to the current understandingof AOCs being inferior to RBCs (2). While the struc-ture of artificial organs does not necessarily resemblethe natural material (Hb) or native tissue (RBCs),physical characteristics disparate from Hb or RBCmay work better in some way that is not expectedfrom native tissue. In this respect, there are promisingapplications reported in the present issue not only asRBC substitutes but also as new therapeutics asexamined experimentally in treating diseases orcounteracting pathologic pathways.

SAFETY ISSUES OF TRANSFUSIONSUBSTITUTE

One of the common characteristics of AOCs is thelack of surface antigens, which may allow transfusion

for patients with rare blood types or emergencytransfusion in cases with hemorrhagic shock (11).Although a purely theoretical analysis (12)may indicate that AOCs with lower O2 affinity(P50O2 = 40 mm Hg) are more efficient in oxygen(O2) transfer than AOCs with higher O2 affinity,uncontrolled hemorrhage resulting in progressiveanemia naturally renders tissue perfusion insufficientand energy metabolism inefficient, and therefore, theefficacy of AOCs is no longer the same as is theoreti-cally expected in normoxic conditions; Ikegawa et al.(13) reported that LEH with low O2 affinity iscapable of maintaining an O2 consumption and O2

delivery relationship comparable with RBCs byreplacing 86% of circulatory blood with LEHs,whereas systemic acidosis and tissue hypoxia devel-oped probably due to premature O2 unloading,causing precapillary arteriolar spasm and distal mal-perfusion (13). In any event, there are few clinicalsituations that would use 90% or more of circulatoryblood with AOC alone when AOCs are consideredinferior to RBCs (2) or even potentially hazardous(4) by overloading the reticuloendothelial system(RES). Nonetheless, we believe that AOCs may beadvantageous in acting as RBC substitutes only untilidentical RBCs become available for transfusion(11). For such a “bridging” use, the average amount oftype O Rh-positive washed/packed RBCs for hemor-rhagic shock patients was 814 mL or approximately15 mL/kg. Moreover, patients receiving such emer-gency transfusion within 60 min of hemorrhage had asignificantly better survival rate than those receivingit later (over 60 min). This emphasized the impor-tance of early transfusion to shorten the time ofsevere ischemia/anemia at the cost of possible minormismatch transfusion, which turned out to be notdetrimental (11). In order to minimize such a meta-bolic load, Sakai et al. (14) developed a system toremove Hb-vesicles from the circulation. Suppressedphagocytosis (4) or a possible competitive inhibitionof the antigen-presenting ability of the RES mayraise other concerns as emergency transfusion isapplied to patients with severe hemorrhage duemainly to accidents when contamination with patho-genic organisms is generally unavoidable. In thisdoi:10.1111/j.1525-1594.2011.01357.x

Artificial Organs36(2):127–129, Wiley Periodicals, Inc.© 2012, Copyright the AuthorArtificial Organs © 2012, International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc.

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regard, transgenic mice were tested and found to becapable of antigen presentation against third-partyantigens in the presence of 20 mL/kg of LEH (15),suggesting the persistence of antigen-presentingcapability even after a large amount of LEHadministration.Although new cell-free HBOCs (6–8)have been proposed not for transfusion but forvarious other indications including the reduction ofoxidative stress, their xenogeneic Hb remained to betested for the absence of immunogenicity.

PARTICLE SIZE

The nanometer size of AOCs is considered to beadvantageous for maintaining microcirculation infocal ischemia and reperfusion. In neural ischemia,LEH has been reported to be protective in reducingbrain edema (16), attenuating the extent of infarction(17,18) or lessening hearing loss due to functionaldamage to the cochlea (19). Such a phenomenon wasreported in skeletal muscle undergoing ischemia andreperfusion using 31P nuclear magnetic resonanceimaging imaging in a rodent model (20), where LEHadministration maintained significantly better intrac-ellular pH during ischemia and better energy statusas expressed by PCr/Pi after reperfusion than inanimals receiving RBCs, saline, or no treatment. Thiswas the first study to clearly demonstrate improve-ments immediately after LEH administration as allother previous studies detected effects of HBOCs asintegrated benefits later for morphological protec-tion (16–18) or functional recovery (19). HBOCshave been reported to be more protective than RBCtransfusion or saline in cerebral ischemia (16–18),skeletal muscle (20), skin ulcer (21) as well as gastricwound healing (manuscript in preparation), suggest-ing that AOCs are advantageous over RBCs in termsof a particle size allowing them to function as aplasma oxygenizer.

OXYGEN AFFINITY

Contrary to mathematical assumption (12), high O2

affinity is considered to be superior in the treatment ofischemic and/or hypoxic tissues (17). There was asignificant difference in protective effects amongAOCs with different O2 affinities; the higher the affin-ity, the more efficacious in treating cochlear ischemia(19) or the smaller amount required for the samebenefits in cerebral ischemia (17). By the same ratio-nale, AOCs have been tried in the oxygenization oftumors to enhance radiotherapy or chemotherapy.Because tumor tissue PO2 has been reported to be lessthan 10 mm Hg, most AOCs tested for thisutility had high O2 affinity (low P50 value around

5–12 mm Hg) as reported by Murayama et al. (22),who used LEH with high O2 affinity (P50 = 12 mm Hg)and found a significant synergistic effect with radio-therapy in suppressing tumor growth, suppressingexpression of hypoxia-inducible factor 1-alpha up to 3days after LEH treatment. Of interest would bewhether LEH with low O2 affinity might work asefficiently as LEH with high O2 affinity (22).

DOSAGE

Because a surplus of O2 in ischemic tissue wouldproduce reactive oxygen species, reperfusion injurycould be aggravated (23). It might be speculated thatAOCs with high O2 affinity (low P50 value) mayremove extra O2 from tissue by binding with O2, or bynot releasing O2, and thus attenuate oxidative stressat reperfusion. Although this theory may seem rea-sonable, it might not account for the benefits of AOCswith low O2 affinity without reperfusion (16) or theabsence of aggravation when a larger amount ofAOCs is used as compared with RBCs or saline atreperfusion (17,18). Furthermore, the amount of O2

delivered by RBCs (hematocrit > 40%) at reperfu-sion apparently far exceeds the amount of O2 thatcould be removed by AOCs with high O2 affinity(volume < 3%). In other experiments using positronemission tomography to titrate O2 metabolism incerebral ischemia and reperfusion (18), there were nodifferences in the changes of cerebral blood flow, O2

extract fraction, or cerebral metabolic rate of O2 until3 h after reperfusion regardless of the absence orpresence of various amounts of AOCs with low O2

affinity (0.4 to 10 mL/kg). In the study, 2 mL/kg wasfound to be more protective than 10 mL/kg, support-ing the concept that an appropriate amount of O2 isimportant. In tumor radiotherapy (22), 10 mL/kg ofLEH with high O2 affinity works better than 20 mL/kg. Finally, the amount of AOCs necessary to benefitcerebral ischemia/reperfusion was less for AOCs withhigh O2 affinity (17). Thus, at present, it seems appro-priate to assume that it is important to deliver anappropriate amount of O2 that could be metabolizedby ischemic tissue with defective or inefficient O2

metabolism.

REPERFUSION INJURY

There is an impression that pathologic conditionsassociated with reperfusion receive greater benefit bytreatment with AOCs. The effects of AOCs onderanged circulation, not permanent ischemia, suchas the operative wound in the stomach, esophagus,bronchus, and skin (21), have been studied in ourlaboratory and showed ameliorated and accelerated

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wound healing. It is reasonable to hypothesize thatthe amount of tissues protected by AOCs duringischemia would become conspicuous as severelyischemic tissues become even more damaged at rep-erfusion; ischemic tissues may be edematous fromtheir occlusive vasculature, where nanoparticles mayperfuse similar to plasma, better than RBCs, to sup-press reperfusion injury. This may account at least inpart for the nonlinear (23) or reversed U-shapeddose-response relationship (17,18) discussed earlier.Therefore, antioxidant effects (6–8,24) of AOCs maycontribute to their benefits in pathological conditionsinvolving tissue ischemia and reperfusion (22).

CONCLUSION

After data demonstrating the inferiority of AOCsas compared with RBCs (2) or even with asanguine-ous solutions as RBC substitutes, it is encouragingthat these emerging and new applications are beingexamined and reported as promising in respect tocellular Hb such as LEH (4,5,13–22) or new genera-tion cell-free Hb (6–8,24), where RBCs are not effec-tive or beneficial. Because interest in AOCs haschanged from that of RBC substitute for transfusionto that in new therapeutics with multiple potentialclinical applications, the development and testing ofAOCs must be followed closely.

Akira T. Kawaguchi, MD, PhDCell Transplantation and Regenerative Medicine

Tokai University School of MedicineShimokasuya 143

IseharaKanagawa 259-1193, Japan

E-mail: akira@is.icc.u-tokai.ac.jp

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