(22c) Evaluating Chemo-Sensitizing Potential of Oxygen Delivery Facilitated By Transfused Polymerized Hemoglobins on Vascularized Solid Tumors

Background: In the United States, over 60% of patients diagnosed with stage III and IV solid tumors will undergo chemo- and/or radio-therapy during their treatment. However, inadequate oxygenation of the solid tumor mass can result in chemotherapeutic resistance within the tumor mass. Thus, increasing O₂ delivery to tumor tissue is a promising strategy to decrease tumor hypoxia and increase the effectiveness of chemotherapeutics. One strategy proposed to facilitate increases in tumor O₂ delivery is transfusion of hemoglobin based O₂ carriers (HBOC). Unfortunately, the efficacy of HBOC co-administration is sensitive to the heterogeneous fluid flow and mass transport within the aberrant tumor vascular architecture and tissue structure. These topological variations within the tumor microenvironment plague HBOC treatment modalities which has resulted in varying therapeutic efficacy over the past 30 years. Despite this, no research has quantified how micro-scale O₂ transport translates to macro-scale variations in tumor hypoxia during treatment with O₂ carriers. This is because evaluating micro-scale transport phenomena in vivo is hindered by the spatial resolution of modern non-invasive imaging and measurement strategies. Thus, it is difficult to accurately quantity the fluid and mass transport associated with the aberrant microvascular architecture, heterogeneous blood flow, and rapid tumor expansion. To better examine these variations in transport phenomena, we implemented three-dimensional computational simulations of tumor growth and treatment with synthesized HBOCs. These simulations were validated against data from intravital microscopy measurements in animal models transfused with the HBOCs. Using these multiscale models of transport phenomena throughout vascular networks, we then translate the micro-scale performance to clinically relevant, macro-scale, diagnostic parameters.

Methodology: For this study we synthesized polymerized hemoglobin (PolyHb), an HBOC currently undergoing development as a red blood cell substitute and chemosensitizer. During the synthesis of PolyHb, we can control the O₂ concentration within the reactor to effectively lock the PolyHb either in the high affinity relaxed quaternary state or the low affinity tense quaternary state. For this study, a library of PolyHbs was synthesized with varying sizes and O₂ affinities for use in animal studies. The measured biophysical properties including O₂ equilibrium, deoxygenation kinetics, rheology, and hydrodynamic radius of the resulting PolyHbs were used as parameters for computational simulations of O₂ delivery throughout the tumor vasculature. For the simulations, three-dimensional vascularized tumor constructs were stochastically generated within simulated 6 mm synthetic arterio-venous networks of vasculature within the host tissue. Each of these networks include vessels from the arterial (100 μm ID) to the capillary (2.5 μm ID) scale. We then used these systems to incorporate the effects of the initial tumor position relative to the arteries and veins within the microenvironments of the host tissue. Each tumor construct was generated in a multi-scale model of multiphasic tumor expansion coupled with vessel growth, adaptation, co-option, and regression. Following the simulated growth, we computationally evaluated the enhanced O₂ transport and delivery throughout the tumor vascular networks facilitated by systemic treatment with either the high or low affinity PolyHbs. The computational model developed here was validated with intravital microscopy within a mouse chamber window model. Mice were transfused with either the low or high affinity PolyHbs. The changes in blood flow, vascular morphology, and the O₂ concentration distributions within the vasculature and tissue spaces were each measured and compared with the simulated tumor systems to validate accurate performance of the simulated tumor systems. Once validated, the data from the simulation was used to compute averaged macro-scale tumor properties such as regional blood flow, tissue hemoglobin concentration, vascular oxygen saturation, and hypoxic volume. We were then able to examine how coadministration of low and high affinity PolyHb would modulate tumor growth and chemotherapeutic effectiveness in animal tumor models.

Results and Conclusions: Despite variations in the O₂ equilibrium between the low and high affinity PolyHbs, both facilitate exponential increases in O₂ delivery under hypoxic conditions (pO₂ < 10 mm Hg). With these increases in O₂ delivery, both species are expected to modulate O₂ delivery to hypoxic tumors. When these species were analyzed in the simulated system, treatment of hypoxic tumors resulted in decreased hypoxia. This was confirmed with intravital microscopy measurements and a decrease in the tumor growth rate. In contrast, we found that transfusion of PolyHb to normoxic tumors resulted in increased hypoxic volume. Animal models of PolyHb treatment of normoxic tumors resulted in increased tumor expansion. This indicates that the simulated model can be used to predict target tissue types for PolyHb treatment. Additionally, we found that increases in the tissue hemoglobin concentration and regional blood flow would lead to improved oxygenation. Taken together, the resulting multiscale fluid and mass transport simulation of the tumor vascular architecture is a powerful tool to relate clinically measurable tumor parameters as predictors for therapeutic effectiveness when designing HBOC co-administered chemo-therapy regimens.