Abstract

Artificial blood is herein defined as consisting of red cell substitutes. Red cell substitutes are solutions intended for use in patients whose red cells are either not available or their use is to be avoided for other reasons. More than 100 years of research have been invested in the quest to produce a solution that can be used safely in humans.

Hemoglobin can exist in either of two structural conformations, corresponding to the oxy (R relaxed) or deoxy (T tense) states. The key differences between these two structures are that the constrained T state has a much lower oxygen affinity than the R state and the T state has a reduced tendency to dissociate into subunits that can be filtered in the kidneys. Traditional efforts to modify hemoglobin to serve as a cell-free O₂ carrier have been directed at stabilization of the T conformation, which would be expected to lower O₂ affinity of native hemoglobin and to reduce its renal excretion. Currently, the optimal O₂ affinity of a cell-free hemoglobin has recently been called into question, but there is no doubt that subunit dissociation needs to be controlled or eliminated.

Differences in the reactions considered to be useful in the production of hemoglobin-based blood substitutes are determined by the dimensions and reactivity of the cross-linking reagents. Even small differences among structures of the reagents can yield products having very different properties. In addition, the conditions of the reaction are very important, not only in regard to the state of ligation, ie, oxygen saturation, but also in regard to the presence of agents or molecules that block or compete for certain reactive sites.

Hemoglobin exists in red blood cells in a highly purified form. However, red cell membranes and plasma contain components that could potentially cause toxicity in recipients of large volumes of hemoglobin preparations. Rabiner's method for the filtration purification of hemoglobin was a significant advance over older centrifugation methods, but some hemoglobins prepared in this way have still caused unwanted reactions.

In addition to human blood, bovine and recombinant hemoglobins have also been used as starting materials for products. These are not without their own problems, such as the fear of bovine spongiform encephalitis (BSE) virus in bovine blood and the extensive purification needed to isolate hemoglobin from bacterial or yeast cultures. As of this writing, one product is before the Federal Department of Agriculture (FDA) for final marketing approval, one is in prolonged clinical trials, two have failed dramatically, and at least two new and novel products are in preclinical or early clinical stages.