The current generation of blood substitutes employed in clinical trials are fundamentally red blood cell (RBC) substitutes; i.e., they are designed primarily to transport oxygen. The products that are now being tested in advanced phase clinical trials are derived from hemoglobin and often referred to as hemoglobin based oxygen carriers (HBOCs). The potential clinical benefits of HBOCs are well known. The objectives of this brief overview are to outline potential clinical applications in trauma care and review the results of HBOCs in recent patient trials.   

Potential Role of Hemoglobin-Based Oxygen Carriers in Trauma Care

FDA approval of a new product proceeds through phase I, II, and III studies designed to establish safety and efficacy. FDA regulation defines efficacy as… a reasonable expectation that. . .pharmacologic or other effects of the biologic product. . .will serve a chemically significant function in the diagnosis, cure, mitigation, treatment or prevention of disease in man.” The Center for Biologics Evaluation and Research (CBER) is the review body for the FDA in the arena of biologies and has published a comprehensive listing of “points to consider in the safety and efficacy assessment of HBOCs.” Specific recommendations for clinical studies were in three areas: perioperative applications, acute hemorrhagic shock, and regional perfusion. Field trials for severe trauma, where RBCs are not available, were warned as difficult because of safety and ethical issues. Decreased perioperative allogeneic RBC transfusion was regarded as a clinical benefit, but the potential risks of HBOCs would have to be defined and evaluated as well.

Clinical Evaluation of Modified Tetrameric Hemoglobin in Trauma Care

Of the modified Hb tetrameric solutions that looked promising in the late 1980s, only one formulation was authorized by the FDA for a phase III study in trauma. HemAssist (Baxter Healthcare, Boulder, CO) consisted of Hb tetramers crossed linked between alpha subunits with bis 3,5 diabromosalicyl fumarate to prevent dissociation into dimers and reduce oxygen affinity. Unfortunately this product failed. Regarded by some as a major setback for the clinical implementation of HBOCs, it is important to emphasize that this US multicenter trial of diaspirin cross-linked Hb (DCLHb) for the treatment of severe traumatic hemorrhagic shock was based on the explicit proposal that “DCLHb was tested not as a substitute for blood but rather as an adjunct to the currently used therapies for enhancing oxygen delivery: fluids, blood, and operative intervention.” Although an unexpected outcome raises the issue of comparable study groups, the difference in the primary study endpoint was concerning: the 28-day mortality for the DCLHb group was 46% (24 of 52), compared with 17% for the control (normal saline) group (8 of 46).

Clinical Safety of Polymerized Hemoglobin in Trauma Care 

At this moment, the most promising HBOCs clinically are polymerized Hb solutions. Perhaps a coincidence but polymerization addresses several of the problems inherent in tetrameric Hb; i.e., enhanced intravascular retention and reduced colloid osmotic activity. Polymerization also appears to attenuate vasoconstriction associated with the infusion of Hb solutions. A proposed explanation is that tetrameric Hb (65 KDa) extravasates through the endothelium to bind abluminal NO, leading to unopposed vasoconstriction; but polymerized Hb (>130 KDa) remains in the vasculature to bind only luminal NO. Polymerized HBOCs have undergone extensive preclinical and clinical testing for safety.

Hemopure (Biopure Corp, Cambridge, MA), a polymer of bovine Hb, has been used successfully to reduce allogeneic RBC transfusion in elective cardiac, aortic, and hepatic surgery. Animal studies designed to replicate prehospital hypotensive resuscitation for hemorrhagic shock have also been encouraging although the issue of compromised tissue perfusion because of vasoconstriction has surfaced. Hemopure has been approved for replacement of acute blood loss in South Africa, but there are no published results to date.  PolyHeme (Northfield lab, Evanston, IL) has been the only product to be evaluated in trauma patients to date. Under FDA guidance, we initiated clinical trials in trauma to confirm safety with escalating doses of PolyHeme. Cognizant of the vasoconstriction associated with the DCLHb clinical trial, we designed a study to specifically evaluate the pulmonary vascular response to PolyHeme infusion in acutely injured patients.  

Clinical Efficacy of Polymerized Hemoglobin in Trauma Care

Perioperative Applications: Reduce Allogeneic RBC Transfusions in Trauma Care 

Prompted by FDA guidelines to demonstrate efficacy, all HBOC companies have pursued what appears to be the simplest clinically; i.e., to reduce the need for allogeneic RBC transfusions. In collaboration with David B Hoyt, MD, we conducted a randomized trial in patients requiring urgent transfusion. The total number of allogeneic RBC transfusions for the control versus PolyHeme was 10.4 + 0.9 units versus 6.8 + 0.9 units (p < 0.05), respectively, through day 1, and 11.3 + 0.9 units versus  7.8 + 0.9 units (p = 0.06), respectively, through day 3.  With our long-term interest in the pathogenesis of postinjury MOF, we then pursued the hypothesis that PolyHeme, in lieu of stored RBCs during initial resuscitation, would attenuate the adverse immunoinflammatory effects of allogeneic RBC transfusion. In preparation for these clinical trials, we conducted in vitro studies to test our hypothesis. The plasma fraction from three or more units of stored RBCs primed the human PMNs for enhanced superoxide production and elastase release but this was not seen with equivalent volumes of PolyHeme. In our subsequent clinical trial, circulating PMNs from patients resuscitated with stored RBCs manifested evidence of priming through increased CD11b/CD18 expression and enhanced superoxide production. Furthermore, patients resuscitated with stored RBCs had higher levels of the proinflammatory cytokines IL₆ and IL₈, and higher levels of the counterregulatory cytokine IL_10. 

Acute Hemorrhagic Shock: When Stored RBCs are Unavailable in Trauma Care 

The most compelling indication for an HBOC is the scenario in which stored RBCs are unavailable. This potential benefit for military use has largely driven the development of HBOCs, but there are also a number of key applications in civilian trauma care. Despite the appeal, the scientific design and ethical conduct of clinical trials to establish efficacy of HBOCs when RBCs are unavailable remain a challenge. To best approximate this scenario, we compared the 30-day mortality in 171 trauma patients given up to 20 units (1,000 g) of PolyHeme, compared with a historic control of 300 surgical patients who refused stored RBCs on religious grounds. The 30-day mortality was 25.0% (10 of 40 patients), compared with 64.5% (20 of 31 patients) in the control patients. 

With this background, a large multicenter prehospital trial was initiated in the U. S.  in January 2003. Severely injured patients, blunt or penetrating, with a SBP < 90 mm Hg  due to acute blood loss are randomized at the scene to receive either the standard crystalloid resuscitation or PolyHeme. The study is conducted, by necessity, with exception to informed consent.  In the hospital, the control group receives stored RBCs as needed while the study group is administered PolyHeme up to six units and then stored RBCs as needed. The primary study endpoint is 30 day mortality; the secondary endpoints include incidence of ARDS and MOF as well as amount of stored RBC transfusion. At this moment, 25 trauma centers in the US are participating in this trial projected for 720 patients. The final interim analysis of 500 patients enrolled to date is in progress today.