(P-CB-4) Abstract Title: Re-defining blood substitute quality metrics - the concept of oxygen delivery potency Session Title: Cell Biology, Immunology and Biochemistry (Basic and Preclinical Research)

The principle therapeutic effect for which blood substitutes are prescribed is to enhance oxygen (O₂) transport from the lungs and delivery to tissue. Astonishingly, the current FDA metric used to assess blood product quality does not even consider therapeutic effect (i.e., O₂ transport from the lung and delivery to tissue), simply assessing remaining stored RBCs still circulating at 72 hours. We propose that the concept of potency be applied to blood products. All therapeutics have a potency, i.e., their effectiveness at performing a task as a function of blood level. Likewise, blood products should be assessed the same way – effectiveness at transporting O₂ from the lung and delivering it to the tissue as a function of blood level.


Study

Design/Methods: We developed an in vitro O₂ delivery potency metric from oxygen dissociation/association curve measurements (HEMOX Analyzer, TCS Scientific Corp, PA USA), performed at 2 fixed pH values (lung ~7.6 and tissue ~6.8). We converted ODC HbO₂ affinity data into blood O₂ content (ml O₂ per gram Hb or ml O₂ per L blood) using previously well accepted calculations. Using the ml O₂ per L blood data we determined the potential O₂ delivery delta from the blood O₂ content at pH 7.6 and 100 mmHg pO₂ (representing physiologic conditions in the lung) and subtracting each value from the curve at pH 6.8 between 99 to 1 mmHg pO₂ – plotting this against the corresponding pO₂ delta. These data, which integrate O₂-carrying capacity and HbO₂ affinity, quantify total potential O₂ delivery (i.e., potency) across a physiologic arterio-venous pO₂ gradient, which we term lung to tissue O₂ flux. To test this metric, we developed a simple bench top assay simulating massive “transfusion” by mixing fresh whole blood with stored type O whole blood (day 25) or HBOC, allowing us to compare performance using the same metric.

Results/Findings:

Compared to fresh healthy control blood, storage resulted in significantly reduced (1) p50 (i.e., reduced HbO₂ affinity), (2) delta p50 (between ODCs at different pH), (3) lung to tissue O₂ flux per gram Hb (i.e., loss of 2,3 DPG), and (4) lung to tissue O₂ flux per liter blood. In terms of potency, stored blood (day 25) reduced O₂ delivery potency ~38% compared to fresh whole blood (ANOVA; p < 0.0001). HBOC performance was like fresh blood.


Conclusions:

We demonstrate use of a novel metric to quantify blood O₂ delivery potency, which clearly differentiates the O₂delivery potency of healthy fresh and stored whole blood. Furthermore, it can also be used to benchmark HBOC O₂delivery potency. We propose that this should become the new metric to define blood substitute quality.