Identification of MRI Biomarkers and Histopathological Alterations in Response to Combination Therapy with Antiangiogenic Agents and Radiation in a Murine Model of Glioma Tumor

Introduction: Combinatorial therapy using radiation therapy (RT) and antiangiogenic agents (AA) holds great promise in treatment of gliomas; however, the timing, sequence, and duration of the combination therapy has yet to be established. As a result, there is a growing need to develop noninvasive, reproducible, and quantitative biomarkers of response to therapy. This study aims to identify multi-parametric MRI biomarkers that can effectively determine tumor vascular changes in response to AA and RT.

Methods: We compared results between two AAs (Sunitinib and VEGF-Trap) in combination with RT in preclinical glioma models. Intracranial murine glioma models were generated by injection of human glioma cell line into NOD/SCID mice. Mice were treated with AA alone, RT alone, or AA+RT, and some received no treatment for control. Serial multi-parametric MRI analysis included (1) T2-weighted RARE, (2) diffusion-weighted imaging (DWI), (3) DCE-MR, (4) T1 quantification using a saturation recovery RARE (SR-RARE), and (5) contrast-enhanced T1-weighted RARE anatomical imaging at baseline and treatment day 3, 7, 10, and 14. Data analysis included the measurement of initial area under the signal intensity curve at 60 seconds (iAUC60); Ktrans, Kep, and Ve for dynamic contrast-enhanced (DCE) MRI; and apparent diffusion coefficient (ADC) maps for diffusion-weighted imaging. Correlative immunohistochemical analysis was performed using proliferation, hypoxia, and endothelial markers.

Results: We demonstrated a significant reduction in Ktrans (an index of vascular permeability) in the RT and RT+AA groups. This finding is corroborated by a reduction in tumor vascular density and vascular diameter. We found that ADC (reflective of tumor cellularity and extracellular water content) is significantly increased in RT versus non-RT groups and paralleled by diminished tumor cell proliferation and increased intercellular space. VEGF-Trap treatment had a greater effect on tumor growth, tumor vessel density, and tumor vessel permeability compared with Sunitinib. Combination treatment showed a prolonged effect on reducing tumor growth and vascular density compared with RT or AA alone.

Conclusion: The results of this study identify novel biomarkers of response to combination therapy that can be used efficiently to schedule the sequence and extent of AA and RT. Future work will focus on validating these biomarkers in clinical trials for patients with gliomas.