(662d) The Quaternary State of Polymerized Human Hemoglobin Regulates Oxygenation of Breast Cancer Solid Tumors: A Theoretical and Experimental Study

Purpose: A major constraint in chemo-and radio-therapeutic cancer treatment is inadequate oxygenation of solid tumors. Hypoxic conditions in the tumor micro-environment induce quiescence in cancer cells, which reduces the effectiveness of cancer therapies. Consequently, alleviating hypoxia in solid tumors is considered a promising target for improving the efficacy of anti-cancer therapeutics such as chemotherapy. Polymerized human hemoglobin (PolyhHb) can be transfused to increase solid tumor oxygenation and improve the efficacy of anti-cancer therapeutics. In this study, we analyzed the biophysical properties of synthesized PolyhHbs with low and high oxygen (O₂) affinity. Transfusion of PolyhHbs may alter micro-vascular hemodynamics, which could improve O₂ transport into the tumor. We hypothesize that computational models may be used to guide the dosage and type of PolyhHb as a function of tumor O₂ consumption and O₂ tension.

Methods: We used PolyhHb synthesized with low (T-state) and high (R-state) oxygen affinities. Here, blood fluid flow was modeled with the non-Newtonian constitutive law. In the simulation, we modeled PolyhHb delivery as a top-load transfusion. The inlet partial pressure of dissolved O₂ (pO_2,in) was varied from normal conditions (90 mm Hg) to hypoxic conditions (1 mm Hg). In addition, diameter of the arteriole, maximum rate of O₂ consumption, and thickness of the tissue space were each varied. The fluid velocity profiles, apparent viscosity, wall shear stress, O₂ distributions, O₂ transfer rate, O₂ consumption rate, and model sensitivity were each analyzed. To compliment this study, an animal study was performed by periodically delivering PolyhHb in the T- and R-state to mice bearing 1 mm³ MDA-MB-231 human breast cancer tumors. Hypoxic mediated gene expression, tumor size, vascular density, and tissue iron histopathology were each analyzed.

Results: We found that increasing the dosage of PolyhHb may decrease the apparent viscosity of blood in arterioles. In addition, we found that PolyhHb transfusion decreased the wall shear stress at large diameters (> 20 μm) but increases wall shear stress for small diameters (< 10 μm). Both T- and R-state PolyhHb exchange transfusion may lead to elevated O₂ delivery at low pO_2,in. In addition, R-state PolyhHb exchange transfusion may be more effective than T-state PolyhHb at similar exchange volumes. The pO₂ pressure drop per unit length signifies that while the O₂ flux across the vessel wall is similar at high pO_2,in, there is significantly more O₂ lost at high pO₂s but more O₂ retained at low pO₂s. At low pO_2,in the radius of the arteriole had the greatest effect on O₂ delivery. At high pO_2,in the maximum rate of oxygen consumption had the greatest effect on O₂ delivery. Interestingly, R-state PolyhHb is much less sensitive to arteriole radius than T-state PolyhHb under hypoxic conditions ( < 10 mm Hg). In the experimental MDA-MB-231 mouse model, HBOC transfusion led to decreased angiogenesis, tumor growth, and hypoxic gene expression. Tissue iron histopathology revealed that the primary route of PolyhHb clearance occurred in the liver and spleen indicating a minimal risk for renal damage.

Conclusions: Decreases in the apparent viscosity resulting from PolyhHb exchange transfusion may result in significant changes in flow distributions throughout the tumor micro-circulatory network. The difference in wall shear stress implies that PolyhHb may have a more significant effect on capillary beds. The increased O₂ flux and decreased pO₂ drop per unit length indicates that both PolyhHbs are suited to deliver O₂ under hypoxic conditions.