Purpose/Objective(s): Nanoparticles, particularly those made of high Z materials like gold or hafnium have shown some promise in radiosensitization approaches in murine models and clinical trials. The results, however, are largely phenomenological and lack mechanistic detail. There is general evidence that a close association between particles and the cell increases radiosensitization. To better understand the true mechanism and potency, gold nanocages were coated on glass coverslips to control the number of nanoparticles contacting each cell membrane but not interacting with the nucleus, enabling precise determination of a dose enhancement factor (DEF).

Materials/Methods: Gold nanocages of ~50 nm edge length were synthesized via galvanic replacement of a sacrificial silver nanocube template. These nanocages were then attached to glass coverslips that had been treated with (3-mercaptopropyl)trimethoxysilane. Following deposition the surfaces were sterilized, and 4T1 murine breast tumor cells were seeded on the surface and allowed to adhere. This model system was then treated with 4 Gy of radiation (150 keV source), and colony formation was assessed after 8 days. Cell size was measured using optical microscopy. Electron trajectories were simulated in water (CASINO software) with an electron energy max of 50 keV.

Results: Surfaces were coated at densities of 0, 17, 34, or 68 AuNC/μm². The cell contact area averaged 308 +/- 15 μm², indicating an average of 0, 0.52, 1.04, or 2.08 x 10⁴ AuNC contacting each cell +/- 5%. Increasing concentration of AuNC was found to significantly enhance radiosensitization with a maximal dose enhancement factor of 2.25 occurring at the 68 AuNC/μm² density. Interestingly, we observed a linear relationship between dose enhancement and number of AuNCs, with ~15,000 AuNCs/cell giving a DEF of 2. Electrons generated from the particle upon interaction with the irradiation photons were predicted travel a max of ~60 µm with a 50 keV initial energy, indicating the electrons can pass completely through 4T1 cells with an average diameter of 10-20 µm while still maintaining enough energy to create biologically relevant collisions.

Conclusion: Gold nanocages were found to provide significant radiosensitization in an in vitro model. Initial results indicate that a particle density and sensitization are linearly related, and the electrons from nanocages on the surface of the cell affect membrane, cytosol and nuclear compartments. We are working to compare dose enhancement caused by distinct gold spheres, rods, or pyramids, and materials such as carbon, iron oxide, hafnium oxide, and silica. The distance between the nucleus and surface will be increased with a polymer layer to quantitatively assess the relative contribution of membrane vs. DNA damage in cell killing, which will also be measured through manipulation of the cell cycle.