Gold Nanoparticles as Potential Radiosensitisers in Head and Neck Squamous Cell Carcinomas

Head and neck cancer refers to cancers that develop in the squamous cells that line mucosal surfaces in the head and neck, and as such they are collectively known as head and neck squamous cell carcinoma (HNSCC). HNSCC are the 8th most common cancer in the UK and are more likely to affect populations from poor socioeconomic backgrounds; the number of cases is predicted to increase in the future by an estimated 42 %. Radiotherapy a primary treatment method for HNSCC, however, due to exposure and damage to nearby healthy tissues by X-ray ionising radiation (IR) during treatment side effects that impact the patient’s quality of life such as difficulty swallowing and speech impairment are likely. Therefore, research to improve treatment and reduce the negative side effects has led to the investigation of potential radiosensitisers, one example of which is gold nanoparticles (AuNPs). In this work, the use of AuNPs as radiosensitisers for HNSCC was examined and the potential mechanisms of this radiosensitisation in HNSCC cell lines began to be determined. AuNPs of 7, 17 and 35 nm in diameter and with either citrate, poly(ethylene glycol) monomethyl ether thiol Mn ~800 or ~6000 g mol-1 (PEG18-SH or PEG136-SH) surface stabilisers were synthesised and characterised to provide material that could be tested in clonogenic assays to determine their viability as radiosensitisers. This required the development of optimal experimental conditions for HNSCC cell lines - FaDu (hypopharyngeal cancer), UMSCC6 (oropharyngeal cancer) and UMSCC12 (laryngeal cancer); due to the variability of conditions used across previous studies making direct comparisons difficult. Once these were established, the synthesised AuNPs were tested to identify the most promising radiosensitisers and this began to show the important characteristics required for a AuNP radiosensitiser. After treatment with 4 Gy X-ray IR, 35 nm AuNPs led to the greatest radiosensitisation, with increases between 6-12 % in the reduction of surviving fractions of HNSCC cells compared to untreated cells. Mechanistic studies highlighted differences between methods of radiosensitisation across cell lines and showed that AuNPs can inhibit repair of deoxyribonucleic acid (DNA) damage following treatment with X-ray IR, with other potential mechanisms of radiosensitisation, such as increased DNA damage and increased production of reactive oxygen species (ROS), ruled out. Finally, AuNPs were exposed to other cells that they would encounter in the body such as THP-1 monocytes, a type of white blood cell. The cytotoxicity of these AuNPs on monocytes was examined, and showed the size and surface stabiliser of the AuNP could lead to increased amounts of lactate dehydrogenase (LDH) being released by THP-1 cells when incubated with different concentrations of AuNPs. The inflammatory response of THP-1 cells in response to AuNPs showed that some AuNPs led to a higher levels of inflammatory markers caspase-1 and interleukin-1β (IL-1β). It was also shown that AuNPs were able to either increase or decrease production of IL-1β in THP-1 cells that had already had an inflammatory response activated by an inflammatory agonist.