Radiation therapy is delivered to approximately 50% of all cancer patients. Although the effects of radiation on tumour volume are well described with DNA damage leading to cell death and senescence, the effects of radiation on immune responses are variable in preclinical models, and clinical data is limited. Lately, there has been renewed interest in the potential use of irradiation in colorectal and other cancers to stimulate immunogenic modulation. Furthermore, immunotherapy that targets the immunosuppressive interaction between programmed cell death 1 (PD-1) and its ligand PD-L1 has recently been approved for malignancies including non-small cell lung cancer, melanoma and head and neck squamous cell carcinoma.
PD-1 is a cell surface molecule expressed on activated CD4+ and CD8+ T cells, B cells, monocytes, natural killer cells (NK cells) and some dendritic cells. Elevated expression of PD-1 is associated with T cell exhaustion. Its ligand (PD-L1) is a transmembrane protein that is undetectable in most normal tissues, but is induced by inflammatory cytokines, especially type I and type II IFNs. Accordingly, PD-L1 expression is increased after ionizing radiation (IR).
ATR is an essential DNA damage–signalling kinase activated at damaged replication forks and resected DNA double-strand breaks. Research has proven that ATR kinase inhibitors can sensitise cancer cells to cisplatin and IR in tissue culture. Furthermore, ATR kinase activity is also increased by hypoxia, and ATR kinase inhibitors can sensitise radiation-resistant hypoxic cells to radiation.
A recent study by Vendetti et al used the syngeneic CT26 colon cancer animal model in BALB/c mice to examine the role of ATR kinase inhibition on tumour growth and immune modulation in combination with radiotherapy. Mice were subcutaneously engrafted with 0.5x106 CT26 tumour cells and treated with the ATR kinase inhibitor AZD6738 (75mg/kg) on days 1 - 3, 2Gy of conformal radiation (tumour) on days 1 and 2 or the combination of the two treatments. Tumour growth over time was monitored and tumours were harvested at different time points to characterise the tumour infiltrating cell populations using flow cytometry. In addition, experiments using CD8+ depleted tumour bearing mice, and mice that had completely responded to AZD6738 plus radiation treatment, were performed to investigate immune system mediated treatment effects.
Vendetti et al found that:
Treatment with the vehicle or AZD6738 alone had no impact on tumour growth. However, irradiation alone resulted in a 47.6% reduction in tumour growth, while AZD6738 plus radiation resulted in a 78.0% tumour growth inhibition relative to the vehicle control on day 15.
All complete responder mice rejected CT26 tumours upon rechallenge, whereas tumour-naïve BALB/c mice exhibited normal tumour growth, which implied that the improved efficacy of AZD6738 plus radiation was mediated by the immune system.
Depletion of CD8+ cells using an anti-CD8 antibody resulted in significantly accelerated CT26 tumour growth suggesting that AZD6738 plus radiation generates a CD8+ T cell-dependent response. However, CD8+ T cell-independent mechanisms are also likely to contribute to the overall anti-tumour response as the combination treatment with AZD6738 and radiation in these mice still resulted in inhibited tumour growth.
AZD6738 reduced the radiation-induced PD-L1 expression in CT26 tumours. Furthermore, AZD6738 plus radiation reduced the percentage of tumour infiltrating CD8+ T cells that produced INF-γ or IL-2 following stimulation compared to radiation alone.
AZD6738 plus radiation led to an increase in proliferating tumour-infiltrating CD8+ T cells and supressed the radiation-induced coexpression of T cell exhaustion markers, suggesting that the combination treatment promoted an increase in functional tumour-infiltrating CD8+ T cells, especially at the later time points.
Both the total and the proliferating regulatory T cell (Treg) numbers in the CT26 tumours were reduced after treatment with AZD6738 plus radiation and the CD8+/Treg ratios were elevated compared to vehicle control at each time point.
In this study, the authors show that the ATR kinase inhibitor combined with local tumour radiation generated a durable CD8+ T cell-dependent anti-tumour response in the syngeneic CT26 mouse model, associated with attenuation of radiation-induced CD8+ T cell exhaustion and the potentiation of CD8+ T cell activity in the tumour microenvironment. Furthermore, AZD6738 treatment in combination with radiation generated an immunologic memory in complete responder mice. There are several ongoing clinical trials with AZD6738 in combination with chemotherapy or irradiation, and the findings in this paper suggest that effects on immune modulation should be taken into consideration when interrogating the results from these trials.
Vendetti et al, ATR kinase inhibitor AZD6738 potentiates CD8+ T cell–dependent antitumor activity following radiation. J Clin Invest, 2018; 128(9):3926-3940