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Preclinical data have shown modulation of the tumour immune microenvironment by ATRi and radiation, through enhanced DNA damage leading to an interferon response triggered by cytoplasmic nucleic acid.  We analysed clinical trial samples to determine whether there is evidence for this in humans.

Patients were treated within the PATRIOT study, a non-randomised phase 1 study of ATRi with ceralasertib and palliative radiotherapy, or ceralasertib alone.  Radiation dose was 30 Gy in 15 fractions. In combination with radiotherapy, patients were treated with 80 mg BD ceralasertib (the RP2D in combination), or 160 mg BD as monotherapy (the RP2D monotherapy dose).  Peripheral blood was taken at various time points and circulating immune cells analysed by flow cytometry.  Plasma cytokines were analysed using multiplex bead-based flow.  Two patients had tumour biopsies before treatment and prior to the second fraction of radiation, differential gene expression between these samples was analysed by RNAseq.

There was a significant increase in the proportion of activated CD8 T-cells (CD69+. CD95+), NK (CD95+, TIM-3+), and unconventional T-cells (CD69+, TIM-3+) in peripheral blood with combination treatment.  With ATRi alone, a trend towards increased NK activation was seen.  Both monotherapy and combination therapy resulted in modulation of plasma cytokine levels, with an increase in CCL2 and a decrease in CCL5 noted in monotherapy, and an increase in CXCL10 with combination therapy.  Paired tumour biopsy analysis found significant differential gene expression in multiple immune-related pathways in both innate and adaptive immunity after combination treatment and cell-type deconvolution inferred an expansion of natural killer cells and macrophages after combination treatment.  The number of differentially expressed genes was higher in patients receiving combination therapy than in those receiving ceralasertib alone.

We have found evidence that there is marked modulation of the tumour immune microenvironment by the combination of ATRi and radiotherapy.  This is reflected in circulating immune cells.  The changes that we observe are similar to those previously noted in preclinical models and support further investigation of combinations of DNA damage response inhibitors, radiation and immune-targeted therapies.