Pancreatic ductal adenocarcinoma (PDAC) is an unmet clinical need and innovative strategies are urgently needed to improve clinical outcome. In pre-clinical models, radiotherapy (RT) is associated with a cascade of immune-modulatory changes within the tumour microenvironment (TME), and combining radiotherapy with immune checkpoint blockade can enhance T cell infiltration and T cell mediated tumour cell death. However, the potential for RT to modulate the tumour immune micro-environment in PDAC, using clinically relevant RT doses is unknown. The aim of this project was to investigate the potential of stereotactic ablative body radiotherapy (SABR) to modulate the pancreatic cancer TME through pre-clinical studies and a window of opportunity clinical study with a view to developing combinations of SABR and immunotherapy for clinical translation.

A pre-clinical radiotherapy platform was developed which allowed the precise and accurate delivery of a clinically relevant dose of ablative radiotherapy (35Gy delivered in 5 daily fractions) to mice bearing subcutaneous tumour allografts. In preclinical modes of pancreatic cancer, which varied in the degree and composition of tumour infiltrating immune cells, I demonstrated using flow cytometry and RNA sequencing that the TME is significantly modulated by this dose and schedule of radiotherapy. While radiotherapy appeared to activate an immune response resulting in increased proliferation of cytotoxic CD8⁺ T cells, this was insufficient to increase intra-tumoural CD8⁺ T cell infiltration. Instead, immunosuppressive responses dominated in the TME following radiation delivery with an increase in immunosuppressive Tregs, m-MDSCs and ‘M2’ macrophages seen in tumours following ablative radiotherapy. On myeloid cells PD-L1 expression increased following radiotherapy and the expression of CD39 and CD73, key ectoenzymes involved in the conversion of ATP to immunosuppressive adenosine, was increased on Tregs and myeloid cells respectively. However, combining ablative radiotherapy with dual PD-L1 and CTLA-4 blockade or CD73 inhibition with or without PD-L1 inhibition did not improve the anti-tumour effect of radiotherapy. As a prelude to future interventional studies, the PORTICOtrans clinical study in patients with operable PDAC demonstrated the feasibility and safety of taking intra-operative biopsy samples using a trans-duodenal approach prior to complete devascularisation of the tumour and post-devascularisation samples immediately after tumour resection in the majority of patients. There was no systematic difference in immune infiltrate of tumour samples collected at different time points using multiplex immunofluorescence panels. Gene expression analysis using TempO-Seq, a ligation based assay on FFPE tissue, revealed only 0.2% or fewer genes were differentially expressed (p<0.05) in tumour samples when the different sampling time points were compared. The PORTICO-SABR study was designed to evaluate the safety of pre-operative ablative radiotherapy (35Gy in 5 daily fraction) delivered immediately prior to surgery in operable pancreatic cancer and will characterise immunomodulatory changes in the PDAC TME following radiotherapy. This study will provide unique insights into whether the immunomodulatory changes demonstrated in murine models of PDAC are seen in a clinical setting.