Enhancment of the immune response induced by Alpha Radiation- Based Brachytherapy can Inhibit Triple Negative Breast Cancer tumor development and Cure Colon Tumors in Mice

Introduction

Alpha radiation efficiently kills tumor cells by creating bulky double strand breaks, thereby releasing tumor antigens and DAMP signals. Diffusing Alpha emitters Radiation Therapy (DaRT), employs Radium-224 loaded sources (DaRT seeds), which are inserted into the tumor and release, by recoil, short-lived alpha-particle emitting atoms, 2-3 mm from the source. It was previously shown that in situ ablation by DaRT induces a systemic antitumor immune response, as a monotherapy. Here strategies to boost this response by TLR agonists and inhibition of immune suppressor cells were investigated. In addition, the specificity of the response was demonstrated.

Materials and methods

4T1 and CT26 bearing mice were treated with DaRT in combination with TLR3/9 agonists. In the CT26 model DaRT and TLR3 agonist were combined with Tregs/MDSC inhibition achieved by low dose cyclophosphamide and Sildenafil, respectively. The specificity of the antitumor immune response was investigated by challenge and Winn assays.

Results and discussion

It was observed that in a non-immunogenic triple negative breast cancer tumor model, 4T1, DaRT combined with intratumoral administration of the TLR3 agonist polyIC significantly retarded tumor growth compared to DaRT alone. In colon cancer mouse model, CT26, DaRT in combination with TLR3,9 agonists retarded tumor progression and prolonged overall survival. Adding Tregs/MDSC inhibition to DaRT and the TLR9 agonist, CpG, resulted in the cure of 51% of the mice, compared to only 6% cure of mice treated with non-radioactive seeds and the immune enhancers. This effect was mediated by a specific immune memory against tumor antigens, demonstrated by 100% resistance only to CT26 tumor challenge and high percentage of protection by splenocytes from cured mice specifically against CT26 inoculation in naïve mice.

Conclusion

The current results indicate that ablation by DaRT is required to enhance a specific antitumor immune response, which vaccinate the host against tumor antigens. The concurrent use of DaRT, Tregs/MDSCs inhibitors and immunoadjuvants, resulting in the cure of tumor bearing mice, synergize probably throw activation of antigen presenting cells and creation of immune memory against specific tumor antigens.