Abstract 2816: Immuno-PET monitoring of CD8+ T cell infiltration post anti-ICOS agonist antibody treatment alone and in combination with PD-1 blocking antibody using a 89Zr anti-CD8+ mouse minibody in EMT 6 syngeneic tumor mouse
Authors: Alsaid Hasan , Cheng Shih-Hsun , Bi Meixia , Rambo Mary V. , Skedzielewski Tinamarie , Hoang Bao , Mohanan Sunish , Gehman Andrew , Hsu Chih-Yang , Doan Minh , Xie Fang , Groseclose M. Reid , Hopson Christopher , Brett Sara , Wilson Ian A. , Nicholls Andrew , Ballas Marc , Waight Jeremy D. , Jucker Beat M. , Hoos Axel 
Published: July 2021

Introduction Inducible T cell co-stimulator (ICOS) is a co-stimulatory receptor that is important for promoting immune activation and function. Despite reported clinical activity and a wide range of non-clinical studies supporting a role for ICOS in lymphocyte activation, proliferation and pro-inflammatory cytokine secretion, little is known regarding the potential of monoclonal agonist antibody-mediated ICOS signaling to drive cytotoxic T cell infiltration into tumors. Feladilimab (GSK3359609) is a non-depleting IgG4 ICOS agonist antibody currently being evaluated in pivotal clinical trials. Here, we used PET/CT imaging to evaluate CD8+ T cell infiltration following treatment with a rodent surrogate of feladilimab (7E.17G9 mouse [m] IgG1) alone or in combination with anti-PD-1 mAb (RMP-14 rat IgG2a) in a syngeneic model of breast cancer (EMT6).

Method Female BALB/c mice with established tumors (~150 mm3) received 10µg, IP of either IgG control mAbs, ICOS mAb, or ICOS + PD-1 mAbs on day 0, 3, 5, 7, 9, 10, or 14. Imaging was performed at 24 & 48 hrs post IV dose of 89Zr labeled CD8 minibody (IAB42M1-14, ImaginAb, CA) on day 0, 3, 5, 9, or 14. In addition to the CD8 minibody uptake in tumor & tumor-draining lymph node (TDLN), 3D radiomic features were extracted from PET/CT images. Top ranked features were used for hierarchical clustering to identify treatment effects.

Results Tumor size regressed in all treated groups relative to IgG control, with a number of mice clearing tumors (ICOS: 4 mice, ICOS + PD-1: 9 mice). The in vivo uptake of CD8 minibody in TDLN was significantly higher in the ICOS + PD-1 group on day 4, 6, & 7 relative to IgG control P&lt;0.05. The CD8 minibody uptake in tumor was significantly higher in the ICOS group on day 6, 11, & 16, and in the ICOS + PD-1 group on day 11 compared to IgG control P&lt;0.05. Top ranked CT radiomic features were predictive for treatment effects at earlier days (day 3 - 5), while PET features were predictive at later days (6 - 10). Texture features in TDLN were consistently selected at earlier days and shape features in tumor were consistently selected in later days.

Conclusions Herein, we demonstrated for the first time that treatment of tumor-bearing mice with ICOS agonist mAb alone or in combination with PD-1 blockade can increase CD8+ T cell infiltration into tumors & TDLN, and is correlated with reduced tumor burden. Notably, radiomics features predicted an effect of treatment on CD8+ T cell infiltration earlier than the detection of absolute changes in the CD8 minibody uptake in tumor & TDLN. Overall, these data support the ongoing pivotal investigation of feladilimab. Moreover, this translational imaging method may be a useful tool to non-invasively monitor CD8+ T cell in response to immunotherapies and understand the temporal relationship between CD8+ T cell flux in tumor and in TDLN.

Citation Format: Hasan Alsaid, Shih-Hsun Cheng, Meixia Bi, Mary V. Rambo, Tinamarie Skedzielewski, Bao Hoang, Sunish Mohanan, Andrew Gehman, Chih-Yang Hsu, Minh Doan, Fang Xie, M. Reid Groseclose, Christopher Hopson, Sara Brett, Ian A. Wilson, Andrew Nicholls, Marc Ballas, Jeremy D. Waight, Beat M. Jucker, Axel Hoos. Immuno-PET monitoring of CD8+ T cell infiltration post anti-ICOS agonist antibody treatment alone and in combination with PD-1 blocking antibody using a 89Zr anti-CD8+ mouse minibody in EMT 6 syngeneic tumor mouse [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2816.