Abstract 1130: PET imaging of tumor-immune status for response prediction and treatment monitoring in syngeneic mouse models
Authors: Kristensen Lotte K. , Christensen Camilla , Nielsen Carsten H. , Kjaer Andreas 
Published: August 2019

Introduction: Cancer immuno-therapy has been established as a powerful way of treating several types of cancer and the degree of immune cell infiltration in tumors seems to reflect the clinical outcome. Accordingly, non-invasive PET imaging of key immune cell subsets could become a valuable tool for tailoring cancer immuno-therapy. Therefore, we developed antibody-based PET tracers specific for CD8a+ and PD-L1 enabling visualization of tumor infiltrating T-cells and tumor PD-L1 status in syngeneic mouse models. Furthermore, we evaluated the ability of theses tracers to predict and monitor combinations of immune-stimulating therapies.

Experimental procedures: Radiotracers were generated from intact antibodies (anti-PD-L1, clone 6E11, humanized cross-reactive to mouse) or antibody fragments (R-anti-M-CD8a+) and were radiolabeled with 89Zr or 64Cu. Tracer specificity was evaluated by immuno-reactive fraction assays and blocking studies and compared to radiolabeled isotype controls. Syngeneic mouse models were selected based on their respective tumor microenvironment (TME) as analyzed by flow cytometry and immunohistochemistry (IHC). Treatment efficacy of external radiation therapy (XRT) and anti-CTLA-4 was evaluated by baseline and post-therapy scans with 64Cu-NOTA-CD8a-F(ab)’2 in CT26 tumor-bearing mice. Prediction of response to therapy with XRT and anti-PD-L1 was evaluated with 89Zr-DFO-6E11 in various syngeneic mouse models. In vivo imaging data was compared with IHC and flow cytometry.

Results: Tracers were highly specific for their targets with an immuno-reactivity &gt;85%. Blocking of endogenous CD8a+ and PD-L1 was successfully reflected by PET imaging with 89Zr-DFO-CD8a-F(ab)’2 and 89Zr-DFO-6E11, respectively. Fractionated XRT induced infiltration of CD8a+ cells in spleen and CT26 tumors which was detected by in vivo 89Zr-DFO-CD8a-F(ab)’2 PET imaging and ex vivo by IHC analysis. Studies with 89Zr-DFO-CD8a-F(ab)’2 PET imaging of combination therapies with fractionated XRT and immune checkpoint inhibitors are ongoing. 89Zr-DFO-6E11 successfully detected various degrees of PD-L1 expression in xenograft mouse models as well as the upregulated PD-L1 expression in syngeneic mouse models following fractionated XRT. Studies with 89Zr-DFO-6E11 PET imaging of combination therapies with fractionated XRT and immune checkpoint inhibitors are ongoing.

Conclusions: The developed radiotracers enable whole-body visualization of CD8a+ T cells and PD-L1 expression in various syngeneic mouse models. These radiotracers can be used as tools to predict efficacy of immuno-therapy in preclinical drug development. In addition, they may be valuable in vivo biomarkers of response to immune-therapy in patients.

Citation Format: Lotte K. Kristensen, Camilla Christensen, Carsten H. Nielsen, Andreas Kjaer. PET imaging of tumor-immune status for response prediction and treatment monitoring in syngeneic mouse models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1130.