A Novel Translational PET Imaging Approach to Quantifying Distal Tumor Immune Activation After Targeted Radiation Therapy and Checkpoint Blockade

Abstract

ObjectivesThis study aimed to provide a novel non-invasive method to quantify abscopal immune activation and predict combinational treatment response using [68Ga]-NOTA-GZP PET imaging. Materials/Methods4T1 breast cancer cells were implanted bilaterally in the mammary fat pad of Balb/c mice and Lewis's lung cancer cells (LLC) were implanted bilaterally on the shoulders of C57/Bl6 mice. One of the tumors received a single fraction of 12Gy irradiation followed by combination of concurrent PD-1 and CTLA-4 inhibitors or controls. Tumor growth of the irradiated and non-irradiated tumors was measured and compared to 12Gy irradiation only, checkpoint inhibitor only, and no treatment control group. Changes in granzyme B activity were assessed with [68Ga]-NOTA-GZP PET imaging from baseline and every three days until day 9. ResultsIn the 4T1 model, concurrent treatment with dual checkpoint inhibitors (CPI) and radiation resulted in reduction of the irradiated tumor volume at day 30. At this same time point, the non-irradiated tumor volume for combination treatment decreased significantly, consistent with abscopal immune activation. Similarly, in the LLC model, concurrent treatment inhibited tumor growth on the non-irradiated tumor at day 15. Importantly, on day 9, granzyme B PET signal in both 4T1 and LLC models was significantly higher in the non-irradiated tumors that responded to concurrent treatment compared to subsequent non-responding tumors. A similar lack of granzyme B signal was observed in the non-irradiated tumors from mice that received radiation or CPI only and control tumors. ROC analysis identified a PET threshold of 1.505 and 1.233 on day 9 that predicted treatment response in 4T1 and LLC models, respectively. Conclusion[68Ga]-NOTA-GZP PET imaging was able to non-invasively predict abscopal immune activation prior to subsequent tumor volume changes after combination treatment. It provides a potential translational paradigm for investigating distal immune activation post-radiation in a clinical setting.