Quantifying the Abscopal Effect in Breast Cancer Using Granzyme B PET Imaging

Abstract

<h3>Purpose/Objective(s)</h3> Radiation can alter the tumor microenvironment and produce a corresponding immune response in tumors that have not been irradiated by generating a local inflammatory response and enhancing recognition of distant tumor cells, which is termed the "abscopal effect". We previously designed a positron emission tomography (PET) tracer, [⁶⁸Ga]-NOTA-GZP, which non-invasively detects granzyme B, a cytotoxic protease released by cytotoxic CD8⁺ lymphocytes and NK cells that initiates apoptosis of target tumor cells. [⁶⁸Ga]-NOTA-GZP PET imaging has been previously demonstrated to be predictive of response to checkpoint blockade in murine models, and is currently in clinical trials. We hypothesized that the abscopal effect could be induced by a single fraction of 12Gy irradiation and quantified by [⁶⁸Ga]-NOTA-GZP PET imaging. <h3>Materials/Methods</h3> To test this hypothesis, 4T1 breast cancer cells were implanted bilaterally in the mammary fat pad and one of the tumors received a single fraction of 12Gy irradiation followed by combination of PD-1 and CTLA-4 blockade concurrently. Tumor growth of the irradiated and contralateral tumors was measured and compared to irradiation only, immunotherapy only and control groups. Changes in granzyme B were assessed with [⁶⁸Ga]-NOTA-GZP PET imaging every three days until day 9. <h3>Results</h3> From days 0 to 9, there was no obvious change of PET uptake in the control and immunotherapy only groups. However, for the irradiation group and concurrent combination treatment group, PET uptake of the irradiated side increased slightly but not significantly. A significant elevation in the non-irradiated tumor of concurrent combination treatment group (p<0.05) was observed on day 9. Accordingly, a single fraction of 12Gy irradiation caused growth delay of the primary tumors (57.94±20.42 mm³) but had no effect on the contralateral tumors outside the radiation field (782.61±16.37 mm³) by day 30. Additionally, no tumor control was observed in the immunotherapy only group (p=0.7641). For the concurrent combination treatment group, both the irradiated tumor (17.02±5.71 mm³, p <0.0001) and the non-irradiated tumor volume decreased significantly (23.059±1.70 mm³, p <0.0001), consistent with the abscopal effect. The mean number of lung metastasis was lower in concurrent combination treatment group than other groups (p=0.037). <h3>Conclusion</h3> Concurrent combination treatment of a single fraction of 12Gy irradiation and PD-1 and CTLA-4 blockade activated the abscopal effect on non-irradiated tumors. [⁶⁸Ga]-NOTA-GZP PET imaging was able to non-invasively quantify the abscopal effect prior to subsequent tumor volume changes. This clinically relevant non-invasive approach is a promising potential tool to guiding combination radiation and immunotherapy to better understand and activate the abscopal effect in patients.