(191bz) Computational Analysis of Solid Tumor Oxygenation Facilitated By Polymerized Human Hemoglobins

Authors: 
Belcher, D., The Ohio State University
Palmer, A., The Ohio State University
Banerjee, U., The Ohio State University
Baehr, C., University of California Davis
Purpose: In the U.S., over 60% of patients diagnosed with stage III and IV solid tumors will undergo chemo- and/or radio-therapy during the course of their treatment. A major constraint in chemo-and radio-therapeutic cancer treatment is inadequate oxygenation of solid tumors. Hypoxic conditions in the tumor microenvironment 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. 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. By locking PolyhHb in the relaxed (R)-state, O2 offloading at low O2 tensions (<20 mmHg) may be increased, while O2 offloading at high O2 tensions (>20 mm Hg) is facilitated with tense (T)-state PolyhHb. Therefore, R-state PolyhHb may deliver significantly more O2 to hypoxic tissues. Furthermore, transfusion of PolyhHbs may alter microvascular hemodynamics, which could improve O2 transport into the tumor. We hypothesize that in silico models of the tumor microenvironment may be used to guide the dosage and type of PolyhHb as a function of tumor O2 consumption and O2 tension.

Methods: To yield low and high affinity PolyhHbs respectively, we first polymerized tense (T) and relaxed (R) state PolyhHb via glutaraldehyde. Clarification, purification, and concentration were each performed with tangential flow filtration. The diameter, cooperativity, O2 tension at 50% saturation (p50), and rapid offloading kinetics of the synthesized PolyhHb were each analyzed. The resulting biophysical parameters were used to populate an O2 diffusion model into the tumor tissue. Here, blood fluid flow was modeled with the Quemeda constitutive law and the radius of the red blood cell rich core was approximated from experimental data. Starling flow was modeled with Brinkman’s equation for flow through the blood vessel wall and tissue space. The O2 equilibrium for all hemoglobin species was handled with the Hill Equation. In the simulation, we modeled PolyhHb delivery at various transfusion volumes. The inlet partial pressure of dissolved O2 (pO2,in) was varied from normal conditions (90 mm Hg) to extremely hypoxic conditions (1 mm Hg). The fluid velocity profiles, apparent viscosity through the lumen, wall shear stress, O2 distributions in the tissue space, O2 flux through the blood vessel, hypoxic volume were each analyzed. Sensitivity to environmental conditions were examined by varying the diameter of the arteriole, maximum rate of O2 consumption, Michaelis-Menten Constant, and thickness of the tissue space.

Results: In general, we found that increasing the volume of PolyhHb transfusion decreased the apparent viscosity of blood in the arteriole. In addition, we found that PolyhHb transfusion decreased the wall shear stress at large arteriole diameters (> 20 μm) but increased wall shear stress for small arteriole diameters (< 10 μm). Both T- and R-state PolyhHb transfusions may lead to elevated O2 delivery at low pO2,in. In addition, R-state PolyhHb transfusions may be more effective than T-state PolyhHb for O2 delivery at similar transfusion volumes. R-state PolyhHb was less effective at maintaining O2 delivery under normoxic conditions. The pO2 pressure drop per unit length signifies that while the O2 flux across the vessel wall is similar at high pO2,in, there is significantly more O2 lost at high pO2s but more O2 retained at low pO2s. At low pO2,in the radius of the arteriole had the greatest effect on O2 delivery. At high pO2,in the maximum rate of O2 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).

Conclusions: Decreases in the apparent viscosity resulting from PolyhHb transfusion may result in significant changes in flow distributions throughout the tumor microcirculatory network. The difference in wall shear stress implies that PolyhHb may have a more significant effect on capillary beds through mechano-transduction. The increased O2 flux and decreased pO2 drop per unit length indicates that both PolyhHbs are suited to deliver O2 under hypoxic conditions. However, the hypoxic volumes estimated here are inconsistent with the literature. This indicates that the assumptions in the Krogh tissue cylinder model may not adequately describe the tumor microenvironment. The system was more sensitive to changes in the tumor microenvironment than to the PolyhHb which indicates that a more descriptive model should be developed to match results from previous studies. Furthermore, the limited oxygenation results of this study may indicate that increases in oxygenation alone may not resolve tumor hypoxia. Therefore, future work should further evaluate the hemodynamic effects of PolyhHb in tumor capillary networks.