Abstract 1492: Trastuzumab-induced normalization in a HER2+ murine model of breast cancer
Authors: Sorace Anna G. , Whisenant Jennifer G. , McIntyre J. Oliver , Yankeelov Thomas E. 
Published: August 2015

Abstract
Introduction: The primary goal of this study is to quantitatively map the vascular and cellular response to trastuzumab treatment in a murine model of HER2+ breast cancer. We report longitudinal dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) and immunohistochemical studies hypothesized to identify timing windows of trastuzumab-induced vascular normalization.
Experimental Design: Mice were subcutaneously implanted with BT474 breast cancer cells and randomly assigned to treated (10 mg/kg trastuzumab) and control (saline) groups. After tumors reached ∼225 mm3, animals (n = 20) were imaged (7.0 T MRI) before treatment (day 0), and 24 hours after each treatment (day 1 and day 4). Subgroups were sacrificed for histology between days 0 through 10 (n = 45). DCE-MRI analysis yielded parametric maps of blood vessel perfusion and permeability (Ktrans) and the extravascular extracellular volume fraction (ve). Tumor sections were paraffin-embedded and stained with CD31, α-SMA, Ki67 and H&E. Slides were scanned in high resolution (20×) and quantitatively analyzed with Leica SCN400 software.
Results: Compared to controls, treated tumors exhibited a significant increase in Ktrans (p = 0.03) on day 4, indicative of enhanced vessel perfusion and/or permeability. Additionally at day 4, treated tumors exhibited a significant increase in extravascular extracellular volume fraction measured via ve (p = 0.01), indicating increased cell death. These significant changes occurred prior to a significant change in tumor size at day 4 (p = 0.11). Decreased Ki67 proliferation staining (p = 0.02) at day 3 and a significant decrease in tumor volume at day 7 (p = 0.04) in the treated group confirmed tumor response to trastuzumab. The increase in Ktrans suggests properties of vessel normalization with potential to improve delivery of systemic therapies. Immunohistochemical analyses showed treated tumors have a significant decrease in microvessel density (CD31 staining, p &lt; 0.01), with a simultaneous significant increase in α-SMA staining (pericyte coverage, p = 0.05) on day 4; thus, there was an overall increase in the “vessel maturation index” (ratio of α-SMA to CD31 staining) compared to controls (p = 0.01).
Conclusion: These concordant (though preliminary) imaging and histological data suggest that trastuzumab treatment promotes vascular normalization, providing motivation for future studies designed to improve the efficacy of combination therapies in HER2+ breast cancer. For example, DCE-MRI has the potential to identify windows of vascular normalization thereby indicating optimal timing for treatment dosing with a cytotoxic therapy. As trastuzumab is currently given clinically in combination with various chemotherapies for HER2+ cancer, trastuzumab-induced vascular normalization could be utilized to improve drug delivery efficiency and enhance overall patient response without increasing dose or systemic toxicity.
Citation Format: Anna G. Sorace, Jennifer G. Whisenant, J. Oliver McIntyre, Thomas E. Yankeelov. Trastuzumab-induced normalization in a HER2+ murine model of breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1492. doi:10.1158/1538-7445.AM2015-1492