Abstract 2781: Diffusion weighted MRI evaluation of response to immunotherapy and radiotherapy in CT26 and 4T1 syngeneic mouse models of cancer
Authors: Lipowska-Bhalla Grazyna , McHugh Damien J. , Babur Muhammad , Martin Isabel Peset , Berks Michael , Little Ross A. , Cheung Susan , Watson Yvonne , Alferez Denis G. , Williams Kaye J. , Honeychurch Jamie , O'Connor James P. 
Published: September 2020

Abstract
Immunotherapy has potential to improve outcome for cancer patients. The MRI biomarker apparent diffusion co-efficient (ADC) can map response to radiotherapy (RT) through its sensitivity to changes in tumor fluid and cellularity. The role of ADC in evaluating immunotherapy agents is unknown, despite investigators using ADC change as an exploratory endpoint in early phase clinical trials.
Our study sought to evaluate and validate ADC changes induced by both RT and immunotherapy agents in syngeneic mouse tumor models. Three experiments were performed in BALB/c mice bearing CT26 colorectal cancers: (1) single 10Gy fraction RT versus control (sham); (2) TLR 7/8 agonist R848 versus control (saline); (3) anti-PD-L1 antibody versus control (saline). We acquired MRI data at 7T Bruker system at days 0, 3, 7, +/- 10 after therapy start, with tumors measuring between 250-300 mm3 at day 0. Median ADC and the inter-quartile range (IQR; a measure of tumor heterogeneity) were derived. Three further equivalent experiments were performed in BALB/c mice bearing triple negative 4T1 breast tumors. All CT26 and 4T1 tumors were bisected at cull for immunohistochemistry and FACS analysis.
For CT26 model, RT-induced tumor growth inhibition (p&lt;0.001) and increased median ADC and IQR at days 7-10 (p&lt;0.05) were accompanied by increased numbers of CD8 cells at days 7-10 on both immunohistochemistry and FACS (p&lt;0.01) and increased necrosis (P&lt;0.05), relative to control. Neither R848 nor anti-PD-L1 modified tumor growth, CD8 cells or macrophages on FACS. R848 induced marked increase in median ADC and IQR at day 3 (p&lt;0.01) accompanied by decrease in CD4 cells at day 3. In distinction, the increased ADC in 4T1 tumors treated with RT (p&lt;0.05 at d7 and d10) and R848 (p&lt;0.05 at d3) were not associated with any change of immune cell populations in tumors as determined by FACS. Anti-PD-L1 therapy did not alter median ADC in either CT26 or 4T1 models, although diffusion heterogeneity, measured by the IQR of ADC was increased markedly in 3/7 CT26 tumors.
In conclusion, our data are the first to evaluate ADC changes induced by RT and immunotherapy agents in syngeneic mouse models of cancer. RT, R848 and anti-PD-L1 induced different ADC responses, each with varied relationships to immune cells. This highlights the need for extensive validation before diffusion weighted MRI biomarkers can be used in clinical trials to monitor response to immunotherapies alone or in combination with RT.
Citation Format: Grazyna Lipowska-Bhalla, Damien J. McHugh, Muhammad Babur, Isabel Peset Martin, Michael Berks, Ross A. Little, Susan Cheung, Yvonne Watson, Denis G. Alferez, Kaye J. Williams, Jamie Honeychurch, James P. O'Connor. Diffusion weighted MRI evaluation of response to immunotherapy and radiotherapy in CT26 and 4T1 syngeneic mouse models of cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2781.