Molecular volume and HBOC-induced vasoconstriction

Comment on Tsai et al, page 3603

HBOC molecular volume plays a critical role in limiting vasoconstriction.

Tsai and colleagues have evaluated for the first time the vasoconstrictive and hypertensive effects of 3 acellular hemoglobin-based oxygen carriers (HBOCs) with approximately the same NO dioxygenation rate constant (23-31 μM^–1s^–1) but varying molecular volume (127-3368 nm³) in a hamster window chamber model of the microcirculation. Their results demonstrate that HBOC molecular volume plays a critical role in mitigating the vasoconstrictive effect previously observed with first-generation HBOCs.¹  This is significant, since the root cause of vasoconstriction and hypertension observed in phase 3 clinical trials of acellular HBOCs continues to be a hotly debated topic and the results of this work could lead to a better understanding of these phenomena.¹  On one hand, the vasoconstrictive effect is hypothesized to stem from NO scavenging.²  On the other hand, it may stem from overoxygenating tissues surrounding blood vessels and depend on the molecular volume of the HBOC.³ 

In support of the NO-scavenging hypothesis, Olson et al²  have shown in a series of very elegant experiments that it is possible to control the hypertensive effect elicited by the hemoglobin (Hb) tetramer by reducing the NO dioxygenation rate constant via protein engineering. In fact, their work refutes the overoxygenation origin of the hypertensive effect by evaluating the hypertensive effect of Hb mutants with variable oxygen affinity and NO dioxygenation rate constants. Their results clearly show that Hb mutants with low P₅₀s (high oxygen affinity) but high NO dioxygenation rate constants elicit the hypertensive effect, whereas Hb mutants with variable oxygen affinity but with low NO dioxygenation rate constants do not.

As the current article clearly points out, another way to limit the vasoconstrictive effect would be to engineer the size of the HBOC. It is widely known that tetrameric Hb can extravasate through the vasculature and scavenge NO from the surrounding smooth muscle cells, thereby disrupting the NO signaling cascade between endothelial cells and smooth muscle cells.³  In order to limit Hb extravasation, the molecular diameter of HBOCs should be engineered to be greater than the capillary pore diameter (∼ 7 nm). It should come as no surprise that NO scavenging was found to be similar for all HBOCs examined, since the NO dioxygenation rate constants for all HBOCs were similar in magnitude. However, the results of this work show that HBOC molecular volume plays a critical role in HBOC extravasation and that facilitated diffusion-related mechanisms are important factors that also determine the extent of the vasoconstrictive effect. In this article, Hb modification via polymerization and conjugation with polyethylene glycol were used to increase the molecular volume of HBOCs. However, encapsulation of Hb within natural or synthetic amphiphiles is another approach that could have been used to increase the molecular volume of HBOCs without modifying the chemical structure of the hemoglobin tetramer.^4,5 

It is my opinion that both hypotheses for the origin of the hypertensive effect are valid and that either or a combination of both approaches can be used to reduce the hypertensive effect, which is the major impediment facing Food and Drug Administration (FDA) approval of HBOCs.▪