Artificial blood: current status of hemoglobin solutions.

Attempts to develop a hemoglobin-based red cell substitute have spanned many decades, but no clinically useful product has been produced to date. The issues preventing clinical application primarily are ones of safety--not efficacy. Numerous animal studies have documented the efficacy of SFH. Although effective, the solution has limitations that have caused concern. Oncotic considerations limit the concentration of the infusate SFH to 6 to 8 g/dL, or half-normal. Owing to the loss of organic phosphate modulators of P50, such as 2,3-DPG, the P50 of SFH is typically between 12 and 14 mm Hg, which is also half the normal value. And finally, the intravascular half-life of SFH is too short, ranging only from 2 to 6 hr. Polymerization provides a means of correcting these limitations. The high oxygen affinity can be greatly diminished by covalent binding of pyridoxal-5'-phosphate to the N-terminal of the chains. Colloid osmotic pressure exerted by a protein solution is proportional to the number of discrete colloid particles. Through polymerization, the number of colloid particles is reduced, leading to a decrease in COP. Data show that this can be achieved in a reproducible fashion. The rate at which COP diminishes determines the yield of polymeric species, as well as their molecular weight distribution. Polymerization can be controlled to result in a yield of 75% to 85% polymers with a molecular weight distribution of 128 to 400 kd. The number average and the weight average molecular weights indicate that the large proportion of polymers represent the cross linking of two tetramers. The data that reflect the interaction of oxygen with poly-SFH-P indicate that the oxygen carrying function of hemoglobin has not been significantly altered by the chemical modifications. The binding coefficient of oxygen is unchanged. As anticipated, there is a loss of cooperativity (diminished Hill coefficient) between the hemoglobin chains, suggesting structural restrictions in the polymeric species because of cross linking. A reduced alkaline Bohr effect is the expected result, and data confirm this. Finally, some increase in oxygen affinity is to be expected with polymerization. This is indeed the case, although the P50 of poly-SFH-P is comparable to banked blood (18 to 22 mm Hg). To be clinically useful, a modified hemoglobin solution requires a reasonable shelf-life.(ABSTRACT TRUNCATED AT 400 WORDS)