Particle therapy, ATM and PD-1 inhibition for pMMR rectal cancer

Background and Aims: Colorectal cancer is a leading cause of cancer mortality and most rectal cancers are mismatch repair proficient (pMMR), making them largely resistant to PD-1 inhibitors (PD1i). While PD1i achieves excellent complete response rates in MMR-deficient tumors, these represent <5% of rectal cancers. For the remaining 95% of patients, effective immunotherapy strategies are urgently needed. We evaluated two high-LET radiation approaches to prime pMMR rectal cancers to PD1i and enhance local control: 1) Diffusing alpha emitter Radiation Therapy (DaRT) implants, and 2) LET-optimized (Bragg Peak) protons + ATM inhibitor. An ATM inhibitor was evaluated with protons to evaluate whether it could produce a more alpha/carbon-like effect given the lower comparative LETd achievable in clinical treatment plans. Methods: In vitro proton irradiation of HCT116, HT29, SW480, and CT26 was performed as previously with dose averaged LET (LETd) values of 2.2 keV/um for ENT and 7.0 keV/um for BP protons. Female Balb/C mice were subcutaneously inoculated with 1×10^6 CT26 cells on one flank and 1×10^5 cells on the contralateral flank to establish primary and secondary (abscopal) tumors, respectively. PD1i monoclonal antibody (RMP1-14 Bio X Cell) was administered i.p. 100ug every 72 hours for 4 total doses. The ATM inhibitor AZD1390 was delivered via oral gavage at 20 mg/kg, PO 1 hour prior to radiation. For in vivo studies, radiation was delivered using a PXI SmART small animal irradiator with opposing lateral beams (225 kVp, 20 mA, 0.3mm Cu filter). Proton irradiation plans were produced as previously described. We used mass cytometry by time of flight (CyTOF) using the TME CD45 lymphocyte subpopulations based on multiple cell markers. Result: In our preliminary data we demonstrate that both DaRT and BP protons+ATMi synergize with PD1i to dramatically increase complete response rates at both primary and abscopal tumors by increasing DNA damage, activating innate immune pathways, and reducing immunosuppression within the tumor microenvironment. Critically, this effect was observed in a pMMR model which did not respond to PD1i or conventional X-rays+PD1i, highlighting the innovation and potential impact of this work. Conclusion: DaRT and BP protons ATMi prime pMMR colorectal cancer to PD1i, and all elements are clinically translatable.