From hemoglobin allostery to hemoglobin-based oxygen carriers

Hemoglobin (Hb) plays its vital role through structural and functional properties evolutionarily optimized to work within red blood cells, i.e., the tetrameric assembly, well-defined oxygen affinity, positive cooperativity, and heterotropic allosteric regulation by protons, chloride and 2,3-diphosphoglycerate. Outside red blood cells, the Hb tetramer dissociates into dimers, which exhibit high oxygen affinity and neither cooperativity nor allosteric regulation. They are prone to extravasate, thus scavenging endothelial NO and causing hypertension, and cause nephrotoxicity. In addition, they are more prone to autoxidation, generating radicals. The need to overcome the adverse effects associated with cell-free Hb has always been a major hurdle in the development of substitutes of allogeneic blood transfusions for all clinical situations where blood is unavailable or cannot be used due to, for example, religious objections. This class of therapeutics, indicated as hemoglobin-based oxygen carriers (HBOCs), is formed by genetically and/or chemically modified Hbs. Many efforts were devoted to the exploitation of the wealth of biochemical and biophysical information available on Hb structure, function, and dynamics to design safe HBOCs, overcoming the negative effects of free plasma Hb. Unfortunately, so far, no HBOC has been approved by FDA and EMA, except for compassionate use. However, the unmet clinical needs that triggered intensive investigations more than fifty years ago are still awaiting an answer. Recently, HBOCs "repositioning" has led to their successful application in organ perfusion fluids.

Blood transfusion and alternatives in Jehovah's Witness patients

PURPOSE OF REVIEW

More than 8.5 million people in the world observe the Jehovah's Witness faith, and require unique consideration for perioperative blood management as they generally refuse transfusion of blood and blood products. This review addresses a collaborative approach to each patient throughout the perioperative arena. The principles of this approach include optimization of hemoglobin levels preoperatively, attention to blood-salvaging methods intraoperatively, and minimization of blood draws postoperatively. In addition, we review the technologies currently in development as transfusion alternatives, including hemoglobin-based oxygen carriers.

RECENT FINDINGS

Progress has been made recently in the field of synthetic blood alternatives and hemoglobin-based oxygen carriers, which may lead to improved outcomes in this patient population.

SUMMARY

Utilization of multiple prevention and mitigation strategies to optimize oxygen supply and decrease oxygen demand will lead to decreased incidence of critical anemia and subsequent improved mortality in Jehovah's Witness patients.