Curative treatment of advanced, poorly immunogenic metastatic murine cancer models using an in situ vaccine, low-dose radiopharmaceutical, and immune checkpoint inhibition

We hypothesized that combining ISV to prime and low-dose TRT to propagate anti-tumor immunity could promote response to dual immune checkpoint blockade (DCP; anti-PD-L1 and anti-CTLA-4) in murine models of poorly immunogenic metastatic disease.

Methods

C57BL/6 mice were engrafted with a primary tumor (right flank), a secondary tumor (left flank), and intravenously injected tumor cells to model advanced metastatic disease using the syngeneic MOC2 head and neck squamous cell carcinoma model or the B78 melanoma model. Mice were randomized to receive monotherapy or double or triple combinations of low-dose TRT, ISV targeting the primary tumor, and DCP. Tumor growth and survival were measured and mice rendered tumor-free mice were re-challenged with tumor engraftment to assess immune memory.

Immune response was assessed by immunohistochemistry, flow cytometry, and quantitative PCR. Results: In two poorly immunogenic, syngeneic murine tumor models of advanced metastatic cancer, when compared to dual combinations or monotherapies, a triple combination of TRT+DCP+ISV enabled eradication of bulky well-established tumors, prevented development of lung metastases from circulating tumor cells (4 of 7 mice with circulating B16 melanoma and 14 of 15 mice with circulating MOC2 HNSCC developed no lung metastases), and extended survival (p < 0.01).

This combination conferred T cell-dependent, tumor-specific immune memory, enabling uniform rejection of tumor re-engraftment (0% vs 100% for both B78 melanoma and MOC2 re-engraftment in disease-free triple-therapy treated mice vs age matched control mice). TRT+DCP+ISV activated both innate and adaptive immune response pathways in the primary and secondary tumors. Conclusions: Combining ISV and low-dose TRT is a promising approach to prime and propagate anti-tumor immunity and to promote response to checkpoint inhibition in settings of poorly immunogenic metastatic cancers.