Abstract 3938: Acoustic immune priming of radiation therapy eradicates primary tumor and reduces distant metastases in triple-negative murine breast cancer

Introduction: Recently, we have shown that low energy focused ultrasound (FUS) treatment reversed tumor-induced T cell tolerance in the B16F10 melanoma model. FUS also induced protein unfolding and translocation of cellular chaperones on the cell surface, thereby providing danger signals for immune activation. To further study immune priming using FUS, we designed a novel immune priming ablation therapy, whereby non-ablative low energy FUS was delivered before ablative radiation therapy in murine triple-negative breast cancer for generating an in situ vaccine. Methods: Orthotopic, palpable (>5mm) 4T1 tumors, a poorly immunogenic, aggressive, triple-negative murine metastatic mammary carcinoma in Balb/c mice, were treated with FUS (15W (100 W/cm2) and 75W (500W/cm2); 100% duty cycle for 3 seconds using custom transducer in multi-frequency mode with or without RT (20Gy) for three consecutive days. Tumor growth, survival, and metastases were monitored. The tumor immune-infiltrate population was analyzed at day 7 post-treatment using flow cytometry. 18F-FDG PET was used to image distant metastases. For the abscopal study, mice were implanted with a primary (treated) and secondary (non-treated) contralateral tumor and monitored for tumor growth on both sites. Results: FUS treatment (75W) in combination with RT eradicated 86% (6/7) of tumors. Tumor growth was also considerably reduced in RT alone and 15W+RT groups compared to non-treated, with a relapse of 43% (3 of 7) in both the groups. There was significant reduction in regional metastatic spread in draining lymph nodes (DLN) (RT, 50% versus FUS (75W)+RT, 22%) and pulmonary metastases (RT, 60% versus FUS (75W)+RT, 22%). Tumors relapsed in DLN and lungs after FUS+RT treatment in only 2 out of 9 mice. At 7 weeks, FDG-PET scans showed no uptake in FUS+RT treated groups, compared to metastatic uptakes in lungs, lymph nodes, and liver in 3 out of 7 mice, treated with RT alone. There was a significant increase in tumor-infiltrating CD3+ lymphocytes, IFNg+CD8 T cells, Granzyme B+ CD8+ T cells and NKT cells, upon treatment with FUS+RT compared to the RT alone group. In contrast to RT alone, FUS+RT promoted suppression of untreated abscopal tumors in contralateral flank. Summary: In this study, we found that local treatment with FUS+RT resulted in primary tumor control, a significant reduction in distant metastasis with improved long-term survival and suppression of unirradiated, abscopal tumors, indicating that focal