Residual Gammah2ax Foci in Head and Neck Squamous Cell Carcinomas As Predictors for Tumour Radiosensitivity: Evaluation in Pre-Clinical Xenograft Models and Clinical Specimens.

Background and purpose: Predictive biomarkers can be instrumental to treatment individualisation of cancer patients and improve therapy outcome. Residual gamma H2AX foci represent a promising biomarker to predict tumour radiosensitivity. In this pre-clinical study, the slope of the dose-response curve was evaluated for its predictive relevance in head and neck squamous cell carcinoma xenografts (HNSCC). Additionally, the feasibility of the translated assay was tested in a clinical setting in patient derived HNSCC samples, and associations between residual gamma H2AX foci and clinical parameters were analysed. Materials and methods: Seven HNSCC xenografts models (FaDu, SAS, SKX, UT-SCC-5, UT-SCC-14, UT-SCC-45, XF354) were used. Tumour bearing NMRI nude mice were randomly distributed to five treatment arms (0-8 Gy). Residual gamma H2AX foci (24 h post irradiation) were counted by visual scoring in a micromilieu dependent manner (assessed with BrdU and pimonidazole). The local tumour control values measured as TCD50 (tumour control dose 50%) have previously been published. Patient derived HNSCC biopsies were cultivated ex vivo for 24 h including 4 h of pimonidazole and BrdU treatment, subsequently irradiated with 0-8 Gy and fixed after 24 h. Results: In the pre-clinical study, the dose-response curve slopes negatively correlated with the tumour control dose after fractionated irradiation (TCD50, fx, R-2 = 0.63, p = 0.032) and after single dose irradiation under homogeneous hypoxia (TCD50, SD, clamp, R-2 = 0.66, p = 0.027). The cH2AX assay in clinical HNSCC samples showed a dose-response relationship, with the values of the slopes ranging from 0.099 Gy(-1) to 0.920 Gy(-1) (coefficient of variation = 52.8%). Slopes derived from patients were in the same ranges as the sensitive, moderate and resistant models of the pre-clinical study. Statistical analysis revealed a significant negative correlation between the slope and the patients' age (R-2 = 0.65, p = 0.001). Conclusion: These results further support the promise of the slope of the residual gamma H2AX foci doseresponse as a biomarker for radiosensitivity. In the clinical samples, the variation in the slopes reveals patients' specific repair capacities, which could hold potential value for treatment individualisation. (C) 2019 Elsevier B.V. All rights reserved.