ATR inhibition with radiation creates an inflammatory tumour microenvironment

Background: ATR inhibitors (ATRi) are in early phase clinical trials and have been shown to sensitise to chemotherapy and radiotherapy (RT) in preclinical data. No data have been published about the effect of these drugs on the tumour microenvironment. We demonstrate significant modulation of the immune microenvironment by ATRi + RT. Methods: We used an immunocompetent mouse model of HPV-driven malignancies (TC1) to investigate the effect of the ATRi, AZD6738, in combination with RT 8 Gy in 4 fractions. Gene expression analysis, flow cytometry and cytokine quantification were used. Results: There was sensitisation to RT by ATRi in this model. ATRi + RT caused a marked increase in a number of adaptive immune cells infiltrating the tumour at 5 days after treatment. There were significantly more dendritic cells, macrophages, and myeloid-derived suppressor cells after ATRi + RT. We also identified increased numbers of CD3+ and NK cells after the combination treatment. Transcriptional analysis found evidence of increased T-cell activation with the combination therapy. We found evidence of an interferon (IFN) response, with transcriptional upregulation of IFN-stimulated genes including those playing a role in nucleic acid sensing: Ddx58/RIG-I, and Ifih1/MDA5, Zbp1/DAI and Ddx60. We identified significant modulation of cytokine gene expression (CCL2, CCL5, CXCL10), and found that cultured tumour cells secreted CCL3, CCL5 and CXCL10 after ATRi + RT. We hypothesise that DNA damage and micronucleus formation caused by ATR + RT (as previously reported by us, Dillon et al. MCT 2016;16:25-34) leads to an IFN response through activation of nucleic acid sensing pathways, leading to influx of innate immune cells. Conclusions: To our knowledge this is the first comprehensive analysis of the immune tumour microenvironment after radiation with a radiosensitising targeted drug. Further understanding of the effect of this combination on immune response may allow modulation to maximise tumour control through activation of anti-tumour immunity. MTD is a CRUK clinical research fellow. AZD6738 provided by AstraZeneca. MM, KJH joint senior authors. Legal entity responsible for the study: The Institute of Cancer Research. Funding: Cancer Research UK. Disclosure: M.T. Dillon: Co-investigator on clinical study of AZD6738 which is partially funded by AstraZeneca. A. Melcher: Compensated consultant/advisor and received honoraria: Amgen, BMS, Merck Serono; Research funding: Oncolytics Biotech Inc. K.J. Harrington: Payment for consulting fees: Amgen, AstraZeneca/Medimmune, BMS, Lytix Biopharma, MSD, Merck, Oncos Therapeutics, Pfizer, Viralytics Inc.; Research grant funding: MSD, Oncolytics Biotech., Viralytics Inc. All other authors have declared no conflicts of interest.