Imaging and radiation effects of gold nanoparticles in tumour cells.

Gold nanoparticle radiosensitization represents a novel technique in enhancement of ionising radiation dose and its effect on biological systems. Variation between theoretical predictions and experimental measurement is significant enough that the mechanism leading to an increase in cell killing and DNA damage is still not clear. We present the first experimental results that take into account both the measured biodistribution of gold nanoparticles at the cellular level and the range of the product electrons responsible for energy deposition. Combining synchrotron-generated monoenergetic X-rays, intracellular gold particle imaging and DNA damage assays, has enabled a DNA damage model to be generated that includes the production of intermediate electrons. We can therefore show for the first time good agreement between the prediction of biological outcomes from both the Local Effect Model and a DNA damage model with experimentally observed cell killing and DNA damage induction via the combination of X-rays and GNPs. However, the requirement of two distinct models as indicated by this mechanistic study, one for short-term DNA damage and another for cell survival, indicates that, at least for nanoparticle enhancement, it is not safe to equate the lethal lesions invoked in the local effect model with DNA damage events.

Gold nanoparticle cellular uptake, toxicity and radiosensitisation in hypoxic conditions.

BACKGROUND AND PURPOSE: Gold nanoparticles (GNPs) are novel agents that have been shown to cause radiosensitisation in vitro and in vivo. Tumour hypoxia is associated with radiation resistance and reduced survival in cancer patients. The interaction of GNPs with cells in hypoxia is explored. MATERIALS AND METHODS: GNP uptake, localization, toxicity and radiosensitisation were assessed in vitro under oxic and hypoxic conditions. RESULTS: GNP cellular uptake was significantly lower under hypoxic than oxic conditions. A significant reduction in cell proliferation in hypoxic MDA-MB-231 breast cancer cells exposed to GNPs was observed. In these cells significant radiosensitisation occurred in normoxia and moderate hypoxia. However, in near anoxia no significant sensitisation occurred. CONCLUSIONS: GNP uptake occurred in hypoxic conditions, causing radiosensitisation in moderate, but not extreme hypoxia in a breast cancer cell line. These findings may be important for the development of GNPs for cancer therapy.

Cell type-dependent uptake, localization, and cytotoxicity of 1.9 nm gold nanoparticles.

BACKGROUND: This follow-up study aims to determine the physical parameters which govern the differential radiosensitization capacity of two tumor cell lines and one immortalized normal cell line to 1.9 nm gold nanoparticles. In addition to comparing the uptake potential, localization, and cytotoxicity of 1.9 nm gold nanoparticles, the current study also draws on comparisons between nanoparticle size and total nanoparticle uptake based on previously published data. METHODS: We quantified gold nanoparticle uptake using atomic emission spectroscopy and imaged intracellular localization by transmission electron microscopy. Cell growth delay and clonogenic assays were used to determine cytotoxicity and radiosensitization potential, respectively. Mechanistic data were obtained by Western blot, flow cytometry, and assays for reactive oxygen species. RESULTS: Gold nanoparticle uptake was preferentially observed in tumor cells, resulting in an increased expression of cleaved caspase proteins and an accumulation of cells in sub G(1) phase. Despite this, gold nanoparticle cytotoxicity remained low, with immortalized normal cells exhibiting an LD(50) concentration approximately 14 times higher than tumor cells. The surviving fraction for gold nanoparticle-treated cells at 3 Gy compared with that of untreated control cells indicated a strong dependence on cell type in respect to radiosensitization potential. CONCLUSION: Gold nanoparticles were most avidly endocytosed and localized within cytoplasmic vesicles during the first 6 hours of exposure. The lack of significant cytotoxicity in the absence of radiation, and the generation of gold nanoparticle-induced reactive oxygen species provide a potential mechanism for previously reported radiosensitization at megavoltage energies.

Nanodosimetric effects of gold nanoparticles in megavoltage radiation therapy.

BACKGROUND AND PURPOSE: The addition of gold nanoparticles (GNPs) to tumours leads to an increase in dose due to their high density and energy absorption coefficient, making it a potential radiosensitiser. However, experiments have observed radiosensitisations significantly larger than the increase in dose alone, including at megavoltage energies where gold's relative energy absorption is lowest. This work investigates whether GNPs create dose inhomogeneities on a sub-cellular scale which combine with non-linear dose dependence of cell survival to be the source of radiosensitisation at megavoltage energies. MATERIALS AND METHODS: Monte Carlo simulations were carried out to calculate dose in the vicinity of a single GNP on the nanoscale. The effect of this nanoscale dose distribution was then modelled for MDA-MB-231 cells exposed to 2 nm GNPs, and compared to experimental results. RESULTS: Dramatic dose inhomogeneities occur around GNPs exposed to megavoltage radiation. When analysed using the Local Effect Model, these inhomogeneities lead to significant radiosensitisation, in agreement with experimental results. CONCLUSIONS: This work suggests that GNP radiosensitisation is driven by inhomogeneities in dose on the nanoscale, rather than changes in dose over the entire cell, which may contribute to the similar radiosensitisation observed in megavoltage and kilovoltage experiments. The short range of these inhomogeneities and the variation in enhancement in different cells suggests sub-cellular localisation is important in determining GNP radiosensitisation.

Biological consequences of nanoscale energy deposition near irradiated heavy atom nanoparticles.

Gold nanoparticles (GNPs) are being proposed as contrast agents to enhance X-ray imaging and radiotherapy, seeking to take advantage of the increased X-ray absorption of gold compared to soft tissue. However, there is a great discrepancy between physically predicted increases in X-ray energy deposition and experimentally observed increases in cell killing. In this work, we present the first calculations which take into account the structure of energy deposition in the nanoscale vicinity of GNPs and relate this to biological outcomes, and show for the first time good agreement with experimentally observed cell killing by the combination of X-rays and GNPs. These results are not only relevant to radiotherapy, but also have implications for applications of heavy atom nanoparticles in biological settings or where human exposure is possible because the localised energy deposition high-lighted by these results may cause complex DNA damage, leading to mutation and carcinogenesis.

Cell-specific radiosensitization by gold nanoparticles at megavoltage radiation energies.

PURPOSE: Gold nanoparticles (GNPs) have been shown to cause sensitization with kilovoltage (kV) radiation. Differences in the absorption coefficient between gold and soft tissue, as a function of photon energy, predict that maximum enhancement should occur in the kilovoltage (kV) range, with almost no enhancement at megavoltage (MV) energies. Recent studies have shown that GNPs are not biologically inert, causing oxidative stress and even cell death, suggesting a possible biological mechanism for sensitization. The purpose of this study was to assess GNP radiosensitization at clinically relevant MV X-ray energies. METHODS AND MATERIALS: Cellular uptake, intracellular localization, and cytotoxicity of GNPs were assessed in normal L132, prostate cancer DU145, and breast cancer MDA-MB-231 cells. Radiosensitization was measured by clonogenic survival at kV and MV photon energies and MV electron energies. Intracellular DNA double-strand break (DSB) induction and DNA repair were determined and GNP chemosensitization was assessed using the radiomimetic agent bleomycin. RESULTS: GNP uptake occurred in all cell lines and was greatest in MDA-MB-231 cells with nanoparticles accumulating in cytoplasmic lysosomes. In MDA-MB-231 cells, radiation sensitizer enhancement ratios (SERs) of 1.41, 1.29, and 1.16 were achieved using 160 kVp, 6 MV, and 15 MV X-ray energies, respectively. No significant effect was observed in L132 or DU145 cells at kV or MV energies (SER 0.97-1.08). GNP exposure did not increase radiation-induced DSB formation or inhibit DNA repair; however, GNP chemosensitization was observed in MDA-MB-231 cells treated with bleomycin (SER 1.38). CONCLUSIONS: We have demonstrated radiosensitization in MDA-MB-231 cells at MV X-ray energies. The sensitization was cell-specific with comparable effects at kV and MV energies, no increase in DSB formation, and GNP chemopotentiation with bleomycin, suggesting a possible biological mechanism of radiosensitization.