Background: Treatment options for recurrent head and neck tumours in the previously irradiated area are limited, including re-irradiation due to radioresistance of the recurrent tumour and previous dose received by surrounding normal tissues. As an in vitro model to study radioresistance mechanisms, isogenic cells with different radiosensitivity can be used. However, they are not readily available. Therefore, our objective was to establish and characterize radioresistant isogenic human pharyngeal squamous carcinoma cells and to evaluate early radiation response in isogenic parental, radioresistant and radiosensitive cells. Methods: Radioresistant cells were derived from parental FaDu cells by repeated exposure to ionizing radiation. Radiosensitivity of the established isogenic radioresistant FaDu-RR cells was evaluated by clonogenic assay and compared to isogenic parental FaDu and radiosensitive 2A3 cells. Additional phenotypic characterization of these isogenic cells with different radiosensitivity included evaluation of chemosensitivity, cell proliferation, cell cycle, radiation-induced apoptosis, resolution of DNA double-strand breaks, and DNA damage and repair signalling gene expression before and after irradiation. Results: In the newly established radioresistant cells in response to 5 Gy irradiation, we observed no alteration in cell cycle regulation, but delayed induction and enhanced resolution of DNA double-strand breaks, lower induction of apoptosis, and pronounced over-expression of DNA damage signalling genes in comparison to parental cells. On the other hand, radiosensitive 2A3 cells were arrested in G 2 /M-phase in response to 5 Gy irradiation, had a prominent accumulation of and slower resolution of DNA double-strand breaks, and no change in DNA damage signalling genes expression. Conclusions: We concluded that the emergence of the radioresistance in the established radioresistant isogenic cells can be at least partially attributed to the enhanced DNA double-strand break repair, altered expression of DNA damage signalling and repair genes. On the other hand, in radiosensitive isogenic cells the reduced ability to repair a high number of induced DNA double-strand breaks and no transcriptional response in DNA damage signalling genes indicate on a lack of adaptive response to irradiation. Altogether, our results confirmed that these isogenic cells with different radiosensitivity are an appropriate model to study the mechanisms of radioresistance.

Human papillomavirus (HPV) is an important etiological factor in oropharyngeal squamous cell carcinoma (SCC). Compared to HPV-negative tumors, HPV-positive oropharyngeal SCC has shown a better response to nonsurgical treatments. In this study, we determined the dose-modifying factors for HPV-positive tumors with single-dose irradiation, with or without low radiosensitizing doses of cisplatin. In vitro, we determined an increased radiosensitivity of HPV-positive SCC, which might be a consequence of HPV-induced changes in the cell cycle regulation and DNA damage response, leading to increased cell death. Additionally, compared to HPV-negative tumors, 30% higher radiosensitivity of HPV-positive tumors was determined by tumor growth delay monitoring in immunodeficient mice in vivo. Concurrent cisplatin treatment had an additive effect in both HPV-negative and HPV-positive tumors, resulting in 20% better response in HPV-positive tumors than in HPV-negative tumors.

Pre-clinical and clinical data indicate differences in the responses of melanoma and carcinoma tumours to electrochemotherapy. The purpose of this study was to investigate the origin of this difference, whether it is due to the intrinsic difference in tumour cell susceptibility to the chemotherapeutic, or due to the tumour micro-environment. For this purpose, we performed a pre-clinical study in B16F1 melanoma and TS/A carcinoma tumours in mice, in which the antitumour effectiveness of electrochemotherapy with bleomycin, the intrinsic sensitivity of tumour cells in vitro, the pharmacokinetics of bleomycin in plasma and tumours, and the vascularization of tumours in vivo were evaluated. The results of the treatment show that carcinoma was significantly more responsive to electrochemotherapy than melanoma. This effect cannot be ascribed to the intrinsic sensitivity of these cells, as melanoma cells were more sensitive than carcinoma cells in vitro. The difference in responses could be ascribed to differences in the pharmacokinetics of bleomycin; at the time of electroporation in carcinomas, more bleomycin was accumulated. This effect could be due to differences in tumour vascularization, as carcinoma tumours had numerous well-distributed, small blood vessels, while melanomas were less vascularized, exhibiting predominantly larger vessels. In conclusion, this study provides evidence on the importance of the tumour micro-environment, particularly the tumour vasculature, in the responses of the tumours to bleomycin electrochemotherapy. Vasculature is important for the pharmacokinetics of bleomycin, influencing drug accumulation and drug distribution in tumours, and might be used as a predictive factor for the tumour response to electrochemotherapy.

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