ESTRO 2023 - Abstract Book

S329

Sunday 14 May 2023

ESTRO 2023

To assess the effect of different radiotherapy (RT) strategies on peripheral B, T, and Natural killer lymphocytes at precise longitudinal time points in prostate cancer (PCa). Materials and Methods Blood collection were prospectively collected from 18 PCa patients at baseline, 3 hours after the first dose of RT, midpoint, RT completion, 6 and 12 months after treatment-end. Accrued patients were treated as it follows: stereotactic body radiation therapy (SBRT) (prostate only, 40 Gy/3FRX, N=5), definitive moderate-hypofractionation (prostate + seminal vesicle,62 Gy/20FRX, N=6), post-operative conventional-fractionation RT (prostate bed + pelvic lymph-nodes, 66-69 Gy/30FRX, N=7). A complete blood count and an in-depth immune cells profile were performed. Fresh blood and isolated peripheral blood mononuclear cells were evaluated by multicolor flow cytometry in terms of immune cell composition, differentiation stage, cytokine production and inhibitory receptor (IR) expression. Results The immunomonitoring of patients revealed that RT affects the balance of systemic immune cells, with the main differences observed between SBRT and conventionally fractionated RT. SBRT favorably impacts immune response in term of increased B cells (CD19+), central-memory (CCR7+CD45RA-) and effector-memory (CCR7-CD45RA-) CD8+ T cells, along with decreased Treg cells (CD3+CD4+CD25highCD127-) after treatment. On the contrary, conventional fractionated RT had a long-term negative effect on the systemic immune profile, including a decrease of total lymphocyte counts accompanied by an increase of neutrophils-to-lymphocytes ratio. Total B and T cells decreased and Treg-to-CD8+ ratio increased. Functionality of T lymphocytes were not affected by any of the 3-fractionation schedules. Interestingly, SBRT significantly up-regulates the expression of V-domain immunoglobulin suppressor of T cell activation (VISTA) in CD8+ T cells in the absence of other IRs. Conclusion Our results indicate the relevance of systematic immunomonitoring during RT to identify novel immune-related target to design trials of combined radio-immunotherapy as a promising strategy in the clinical management of PCa. Purpose or Objective Pancreatic ductal adenocarcinoma (PDAC) is radioresistant histology, and inhibition of cyclin-dependent kinase 1 (CDK1) in combination with radiation therapy (RT) is a potentially attractive strategy to overcome radioresistance. Materials and Methods Standard clonogenic assays, γ H2aX foci staining, HR-GFP reporter assay, and western blot analysis of DNA damage response proteins were performed in MIA-PaCa2 (MP2) and PANC-1 cells following drug targeting of CDK1 through the use of a selective CDK1 inhibitor, RO-3306. Knockdown of CDK1 was achieved in PDAC cells using a doxycycline Tet-On 3G promoter of the CDK1 shRNA gene. One million PDAC cells with the inducible sh-CDK1 were implanted orthotopically in the pancreas of athymic nude mice along with 50 uL BioXmark liquid fiducial adjacent to the injection. After one week of tumor formation, tumors with Luciferase expression were treated with imaged-guided radiation therapy (IGRT) on a Xstrahl small animal radiation research platform (SARRP) to a dose of 20 Gy in 5 daily fractions. Tumor growth was measured via bioluminescence on IVIS Lumina every 2 weeks. Results We found CDK1 inhibition therapy combined with RT lead to greater cell death than either treatment alone. Our results show that treatment of PDAC cells in vitro with a selective CDK1 inhibitor, RO-3306, reduced activated CDK1 and results in a 2-fold increase of DNA damage following 4 Gy of radiation via measurement of gamma H2AX nuclear foci. Furthermore, we found that RO-3306 (10 µM) treatment concurrent with radiation resulted in decreased clonogenic survival with increasing radiation dose. DNA repair markers and HR repair efficacy were both reduced following CDK1 inhibition. Mice with orthotopic inducible shCDK1 PDAC tumors treated with doxycycline 48 hours prior to IGRT showed significantly longer tumor growth delay following in vivo IGRT compared to the CDK1 control tumors (2.5-fold increase, p<0.001). After the last dose of RT, expression of the DNA damage marker yH2aX in tumors was significantly higher in the CDK1 knockdown tumors compared to controls indicating reduced DNA repair in tumors deficient in CDK1. Conclusion Our analysis indicates a novel role of CDK1 targeting to overcome PDAC radioresistance. Targeting CDK1 prior to RT is a promising strategy to overcome the radioresistance of PDAC. OC-0428 Dietary fibre both radiosensitises tumours and spares intestines from acute radiation toxicity A. Kiltie 1 , C.K. Then 2 , S. Paillas 3 , D. Sescu 4 , R. Saito 5 , X. Wang 6 , M. Misheva 7 , J. McCullagh 7 , K. Foster 6 , K. Vallis 2 1 University of Aberdeen, Rowett Institute, Aberdeen, United Kingdom; 2 University of Oxford, MRC Oxford Institute for Radiation Oncology, Oxford, United Kingdom; 3 University of Oxford, Oxford Institute for Radiation Oncology, Oxford, United Kingdom; 4 University of Aberdeen, School of Medicine, Medical Sciences and Nutrition, Aberdeen, United Kingdom; 5 University of North Carolina at Chapel Hill, Lineberger Comprehensive Cancer Center, Chapel Hill, USA; 6 University of Oxford, Department of Zoology, Oxford, United Kingdom; 7 University of Oxford, Chemistry Research Laboratory, Oxford, United Kingdom Purpose or Objective Dietary fibre intake and the gut microbiota have been shown to modulate the efficacies of immunotherapy and chemotherapy treatments, but there have been very few studies in radiotherapy. We previously found that high fibre diets sensitised RT112 xenografts to irradiation by modifying the gut microbiota in immunocompromised mice. As prebiotics can enhance anti-cancer immunity, we hypothesised that dietary fibre and its modification of the gut microbiota can radiosensitise tumours via microbiota secretion of metabolites and/or immunomodulation, in immunoproficient mice. OC-0427 Targeting Cyclin-Dependent Kinase 1 to Radiosensitize Pancreatic Cancer A. Wolfe 1 , S. Van Matre 1 , R. Eluvathingal 1 , O. Zuniga 1 , H. Rodrigues 1 , F. Xia 1 1 University of Arkansas for Medical Sciences, Radiation Oncology, Little Rock, USA