Low-dose radiopharmaceutical therapy enhances the efficacy of B7-H3 CAR T cells in murine metastatic neuroblastoma

Background: Chimeric antigen receptor (CAR) T cell therapy has had clinical success in hematologic malignancies, but limited efficacy is seen in solid tumors. In this study, we investigated whether systemic CAR T cell therapy could be enhanced in metastatic models of neuroblastoma when combined with radiopharmaceutical therapy (RPT). Methods: Non-irradiated or irradiated tumor cells were co-cultured with CAR T cells (1:1) in vitro and supernatant media was subsequently collected for cytokines analyses. CAR T cell phenotypes were characterized by flow cytometry including checkpoint marker expression. Xenograft models of metastatic neuroblastoma were generated in NOD-Rag1nullIL2rgnull (NRG) mice. Tumor-bearing mice received 1.8 Gy of radiation delivered by 177Lu-NM600 RPT five days after tumor implantation. Nine days after RPT, CAR T cells were administered intravenously. To evaluate tumor burden, mice were imaged weekly for 4 weeks. Results: In models of metastatic neuroblastoma, 177Lu-NM600 RPT significantly increased overall survival when combined with CAR T cell therapy in vivo. Pre-treatment of tumor cells with 177Lu also significantly increased CAR T cell cytotoxicity while decreasing production of IL-4 and IL-10 in vitro. Co-culture of CAR T cells with irradiated tumors led to increases in PD-1+TIM3+LAG3+ T cells, suggesting that further combination with immune checkpoint inhibitors may enhance clinical efficacy. Conclusions: Our findings demonstrate that low-dose RPT can potentiate the anti-tumor efficacy of CAR T cells in metastatic neuroblastoma. To our knowledge, this is the first report of dosimetry-based RPT being combined with CAR T cells in a metastatic solid tumor setting. These findings underscore the potential of combining RPT and CAR T cells to overcome the unique challenges of solid tumors, particularly when treating metastatic disease.