Nanoscale Energy Deposition by X-ray Absorbing Nanostructures

Rapid, one-pot procedure to synthesise103Pd:Pd@Au nanoparticles en route for radiosensitisation and radiotherapeutic applications

Radioactive gold–palladium nanoparticles for radiotherapy.

Hypoxia and cellular localization influence the radiosensitizing effect of gold nanoparticles (AuNPs) in breast cancer cells

Hypoxia exists in all solid tumors and leads to clinical radioresistance and adverse prognosis. We hypothesized that hypoxia and cellular localization of gold nanoparticles (AuNPs) could be modifiers of AuNP-mediated radiosensitization. The possible mechanistic effect of AuNPs on cell cycle distribution and DNA double-strand break (DSB) repair postirradiation were also studied. Clonogenic survival data revealed that internalized and extracellular AuNPs at 0.5 mg/mL resulted in dose enhancement factors of 1.39 ± 0.07 and 1.09 ± 0.01, respectively. Radiosensitization by AuNPs was greatest in cells under oxia, followed by chronic and then acute hypoxia. The presence of AuNPs inhibited postirradiation DNA DSB repair, but did not lead to cell cycle synchronization. The relative radiosensitivity of chronic hypoxic cells is attributed to defective DSB repair (homologous recombination) due to decreased (RAD51)-associated protein expression. Our results support the need for further study of AuNPs for clinical development in cancer therapy since their efficacy is not limited in chronic hypoxic cells.

Gold nanoparticles functionalization notably decreases radiosensitization through hydroxyl radical production under ionizing radiation

The purpose of this work was to study the influence of gold nanoparticles (GNP) coating on hydroxyl radical (HO) production under ionizing radiation. Though radiosensitizing mechanisms are still unknown, radical oxygen species are likely to be involved, especially HO. We synthesized six different types of GNP, choosing relevant ligands such as polyethylene glycol or human serum albumin. A great attention was paid to characterize these GNP in terms of size, charge and number of atoms in the coating. Our results show that functionalization dramatically decreases HO production, which is correlated to reduced plasmidic DNA damages. These findings are of high importance as GNP translation from fundamental research to applied medicine requires their functionalization to increase blood circulation time and specific cancerous cells addressing. We suggest that to keep GNP efficient for radiotherapy, a wispy coating is required.

A new mechanism for hydroxyl radical production in irradiated nanoparticle solutions

The absolute yield of hydroxyl radicals per unit of deposited X-ray energy is determined for the first time for irradiated aqueous solutions containing metal nanoparticles based on a "reference" protocol. Measurements are made as a function of dose rate and nanoparticle concentration. Possible mechanisms for hydroxyl radical production are considered in turn: energy deposition in the nanoparticles followed by its transport into the surrounding environment is unable to account for observed yield whereas energy deposition in the water followed by a catalytic-like reaction at the water-nanoparticle interface can account for the total yield and its dependence on dose rate and nanoparticle concentration. This finding is important because current models used to account for nanoparticle enhancement to radiobiological damage only consider the primary interaction with the nanoparticle, not with the surrounding media. Nothing about the new mechanism appears to be specific to gold, the main requirements being the formation of a structured water layer in the vicinity of the nanoparticle possibly through the interaction of its charge and the water dipoles. The massive hydroxyl radical production is relevant to a number of application fields, particularly nanomedicine since the hydroxyl radical is responsible for the majority of radiation-induced DNA damage.

The potential effectiveness of nanoparticles as radio sensitizers for radiotherapy

INTRODUCTION

Application of nanoparticles as radio sensitizer is a promising field to improve efficiency of radiotherapy.

METHODS

This study was conducted to review nano radio sensitizers. PubMed, Ovid Medline, Science Direct, Scopus, ISI web of knowledge, and Springer databases were searched from 2000 to May 2013 to identify relevant studies. Search was restricted to English language.

RESULTS

We included any study that evaluated nanoparticles, volunteer of radio enhancement at radiotherapy on animals or cell lines. Nanoparticles can increase radio sensitivity of tumor cells. This effect was shown in vivo and in vitro, at kilovltage or megavoltage energies, in 24 reviewed studies. Focus of studies was on gold nanoparticles. Radio sensitizing effects of nanoparticles depend on nanoparticles' size, type, concentration, intracellular localization, used irradiation energy and tested cell line.

CONCLUSION

Literature suggests potency of nanoparticles for increasing cell radio sensitivity. Reviewed results are promising and warrant future clinical trials.

Photoluminescence of cerium fluoride and cerium-doped lanthanum fluoride nanoparticles and investigation of energy transfer to photosensitizer molecules

Ce_xLa_1−xF₃ nanoparticles have been proposed for use in nanoscintillator–photosensitizer systems, aiming to combine the effects of radiotherapy and photodynamic therapy.

Gold nanoparticles: a paradigm shift in biomedical applications

In the medical field, majority of the active ingredients exists in the form of solid particle (90% of all medicines). Nanotechnology had grabbed the attention of many scientists working in different aspects and gave them a vivid imagination in order to utilize the nanotechnology in an innovative way according to their needs. One of the major applications of nanotechnology is drug delivery through nanoparticles which is on boom for the researchers and gives a challenging environment for the researchers. Among them upcoming challenge is the use of inorganic nanoparticles for the drug delivery and related aspects. There is growing interests in usage of inorganic nanoparticles in medicine due to their size, and unique physical properties that make them different from other nanoparticulate systems. This review will lay special emphasis on the uniqueness of inorganic nanoparticles especially gold nanoparticles as a drug delivery vehicle and moreover will present a wide spread scenario of gold nanoparticles that has been used for treatment of life threatening diseases like cancer.

Quantitative analysis of dendron-conjugated cisplatin-complexed gold nanoparticles using scanning particle mobility mass spectrometry

We report a high-resolution and traceable method to quantify the drug loading on nanoparticle-based cancer therapeutics, and demonstrate this method using a model cisplatin functionalized dendron-gold nanoparticle (AuNP) conjugate. Electrospray differential mobility analysis (ES-DMA) provides upstream size classification based on the electrical mobility of AuNP conjugates in aerosol form following electrospray conversion from the aqueous suspension. A condensation particle counter (CPC) and inductively coupled plasma mass spectrometer (ICP-MS) provide the principal downstream quantification. CPC and ICP-MS yield complementary number-based and elemental mass-based particle size distributions, respectively. Conjugation using three different dendron formulations was differentiated based on changes in the mean mobility particle size. The subsequent cisplatin complexation to the dendron conjugates was quantified by coupling ES-DMA with ICP-MS. Discrete AuNP clusters (e.g., dimers, trimers) could be resolved from the relative quantity of atoms (i.e., Au and Pt) per particle after separation by ES-DMA. Surface density of cisplatin on Au was shown to be proportional to the density of carboxylic groups present and was independent of the state of AuNP clustering. Additionally, we found that colloidal stability of the conjugate is inversely proportional to the surface loading of cisplatin. This study demonstrates a prototype methodology to provide traceable quantification and to determine other important formulation factors relevant to therapeutic performance.

Surface-engineered nanomaterials as X-ray absorbing adjuvant agents for Auger-mediated chemo-radiation

We report a prototype approach to formulate gold nanoparticle-based X-ray absorbing agents through surface-engineering of a cisplatin pharmacophore with modified polyacrylate. The resulting agents exhibit both chemo-therapeutic potency to cancer cells and Auger-mediated secondary electron emission, showing great potential to improve the therapeutic efficacy of chemo-radiation.

Radiosensitization by gold nanoparticles

Recent years brought increasing use of gold nano particles (GNP) as a model platform for interaction of irradiation and GNPs aiming radiosensitization. Endocytosis seems to be one of the major pathways for cellular uptake of GNPs. Internalization mechanism of GNPs is likely receptor-mediated endocytosis, influenced by GNP size, shape, its coating and surface charging. Many showed that DNA damage can occur as a consequence of metal-enhanced production of low energy electrons, Auger electrons and alike. Kilovoltage radiotherapy (RT) carries significantly higher dose enhancement factor (DEF) that is observed with megavoltage irradiations, the latter usually been at the order of 1.1-1.2. Higher gold concentrations seem to carry higher risk of toxicity, while with lower concentrations the DEF can be reduced. Adding a chemotherapeutic agent could increase level of enhancement. Clinical trials are eagerly awaited with a promise of gaining more knowledge deemed necessary for more successful transition to widespread clinical practice.

Modern micro and nanoparticle-based imaging techniques

The requirements for early diagnostics as well as effective treatment of insidious diseases such as cancer constantly increase the pressure on development of efficient and reliable methods for targeted drug/gene delivery as well as imaging of the treatment success/failure. One of the most recent approaches covering both the drug delivery as well as the imaging aspects is benefitting from the unique properties of nanomaterials. Therefore a new field called nanomedicine is attracting continuously growing attention. Nanoparticles, including fluorescent semiconductor nanocrystals (quantum dots) and magnetic nanoparticles, have proven their excellent properties for in vivo imaging techniques in a number of modalities such as magnetic resonance and fluorescence imaging, respectively. In this article, we review the main properties and applications of nanoparticles in various in vitro imaging techniques, including microscopy and/or laser breakdown spectroscopy and in vivo methods such as magnetic resonance imaging and/or fluorescence-based imaging. Moreover the advantages of the drug delivery performed by nanocarriers such as iron oxides, gold, biodegradable polymers, dendrimers, lipid based carriers such as liposomes or micelles are also highlighted.

Nanoparticle location and material dependent dose enhancement in X-ray radiation therapy

Nanoparticles of high atomic number (Z) materials can act as radiosensitizers to enhance radiation dose delivered to tumors. An analytical approach is used to calculate dose enhancements to tumor endothelial cells and their nuclei for a series of nanoparticles (bismuth, gold and platinum) located at different locations relative to nuclei by considering contributions from both photoelectrons and Auger electrons. The ratio of the dose delivered to cells with and without the nanoparticles is known as the dose enhancement factor (DEF). DEFs depend on material composition, size and location of nanoparticles with respect to the cell and the nucleus. Energy of irradiating X-ray beam affects X-ray absorption by nanoparticles and plays an important role in dose enhancements. For diagnostic X-ray sources, bismuth nanoparticles provide higher dose enhancements than gold and platinum nanoparticles for a given nanoparticle size, concentration and location. The highest DEFs are achieved for nanoparticles located closest to the nucleus where energy depositions from short range Auger electrons are maximum. With nanoparticles ranging in diameter between 2-400 nm, the dose enhancement increases with decrease in particle size. The results are useful in finding optimized conditions for nanoparticle enhanced X-ray radiation therapy of cancer.

Photon activated therapy (PAT) using monochromatic synchrotron X-rays and iron oxide nanoparticles in a mouse tumor model: feasibility study of PAT for the treatment of superficial malignancy

Background

X-rays are known to interact with metallic nanoparticles, producing photoelectric species as radiosensitizing effects, and have been exploited in vivo mainly with gold nanoparticles. The purpose of this study was to investigate the potential of sensitizing effect of iron oxide nanoparticles for photon activated therapy.

Methods

X-rays photon activated therapy (PAT) was studied by treating CT26 tumor cells and CT26 tumor-bearing mice loaded with 13-nm diameter FeO NP, and irradiating them at 7.1 keV near the Fe K-edge using synchrotron x-rays radiation. Survival of cells was determined by MTT assay, and tumor regression assay was performed for in vivo model experiment. The results of PAT treated groups were compared with x-rays alone control groups.

Results

A more significant reduction in viability and damage was observed in the FeO NP-treated irradiated cells, compared to the radiation alone group (p < 0.04). Injection of FeO NP (100 mg/kg) 30 min prior to irradiation elevated the tumor concentration of magnetite to 40 μg of Fe/g tissue, with a tumor-to-muscle ratio of 17.4. The group receiving FeO NP and radiation of 10 Gy showed 80% complete tumor regression (CTR) after 15–35 days and relapse-free survival for up to 6 months, compared to the control group, which showed growth retardation, resulting in 80% fatality. The group receiving radiation of 40 Gy showed 100% CTR in all cases irrespective of the presence of FeO NP, but CTR was achieved earlier in the PAT-treated group compared with the radiation alone group.

Conclusions

An iron oxide nanoparticle enhanced therapeutic effect with relatively low tissue concentration of iron and 10 Gy of monochromatic X-rays. Since 7.1 keV X-rays is attenuated very sharply in the tissue, FeO NP-PAT may have promise as a potent treatment option for superficial malignancies in the skin, like chest wall recurrence of breast cancer.

Physical basis and biological mechanisms of gold nanoparticle radiosensitization

The unique properties of nanomaterials, in particular gold nanoparticles (GNPs) have applications for a wide range of biomedical applications. GNPs have been proposed as novel radiosensitizing agents due to their strong photoelectric absorption coefficient. Experimental evidence supporting the application of GNPs as radiosensitizing agents has been provided from extensive in vitro investigation and a relatively limited number of in vivo studies. Whilst these studies provide experimental evidence for the use of GNPs in combination with ionising radiation, there is an apparent disparity between the observed experimental findings and the level of radiosensitization predicted by mass energy absorption and GNP concentration. This review summarises experimental findings and attempts to highlight potential underlying biological mechanisms of response in GNP radiosensitization.

Synthesis and characterization of silver/alanine nanocomposites for radiation detection in medical applications: the influence of particle size on the detection properties

Silver/alanine nanocomposites with varying mass percentage of silver have been produced. The size of the silver nanoparticles seems to drive the formation of the nanocomposite, yielding a homogeneous dispersion of the silver nanoparticles in the alanine matrix or flocs of silver nanoparticles segregated from the alanine crystals. The alanine crystalline orientation is modified according to the particle size of the silver nanoparticles. Concerning a mass percentage of silver below 0.1%, the nanocomposites are homogeneous, and there is no particle aggregation. As the mass percentage of silver is increased, the system becomes unstable, and there is particle flocculation with subsequent segregation of the alanine crystals. The nanocomposites have been analyzed by transmission electron microscopy (TEM), UV-Vis absorption spectroscopy, X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy and they have been tested as radiation detectors by means of electron spin resonance (ESR) spectroscopy in order to detect the paramagnetic centers created by the radiation. In fact, the sensitivity of the radiation detectors is optimized in the case of systems containing small particles (30 nm) that are well dispersed in the alanine matrix. As the agglomeration increases, particle growth (up to 1.5 μm) and segregation diminish the sensitivity. In conclusion, nanostructured materials can be used for optimization of alanine sensitivity, by taking into account the influence of the particles size of the silver nanoparticles on the detection properties of the alanine radiation detectors, thus contributing to the construction of small-sized detectors.

Gold nanoparticles as novel agents for cancer therapy

Gold nanoparticles are emerging as promising agents for cancer therapy and are being investigated as drug carriers, photothermal agents, contrast agents and radiosensitisers. This review introduces the field of nanotechnology with a focus on recent gold nanoparticle research which has led to early-phase clinical trials. In particular, the pre-clinical evidence for gold nanoparticles as sensitisers with ionising radiation in vitro and in vivo at kilovoltage and megavoltage energies is discussed.

Radiation-induced melting in coherent X-ray diffractive imaging at the nanoscale

Coherent X-ray diffraction techniques play an increasingly significant role in the imaging of nanoscale structures, ranging from metallic and semiconductor to biological objects. In material science, X-rays are usually considered to be of a low-destructive nature, but under certain conditions they can cause significant radiation damage and heat loading on the samples. The qualitative literature data concerning the tolerance of nanostructured samples to synchrotron radiation in coherent diffraction imaging experiments are scarce. In this work the experimental evidence of a complete destruction of polymer and gold nanosamples by the synchrotron beam is reported in the case of imaging at 1-10 nm spatial resolution. Numerical simulations based on a heat-transfer model demonstrate the high sensitivity of temperature distribution in samples to macroscopic experimental parameters such as the conduction properties of materials, radiation heat transfer and convection. However, for realistic experimental conditions the calculated rates of temperature rise alone cannot explain the melting transitions observed in the nanosamples. Comparison of these results with the literature data allows a specific scenario of the sample destruction in each particular case to be presented, and a strategy for damage reduction to be proposed.

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.

Gold nanostructures as a platform for combinational therapy in future cancer therapeutics

The field of nanotechnology is currently undergoing explosive development on many fronts. The technology is expected to generate innovations and play a critical role in cancer therapeutics. Among other nanoparticle (NP) systems, there has been tremendous progress made in the use of spherical gold NPs (GNPs), gold nanorods (GNRs), gold nanoshells (GNSs) and gold nanocages (GNCs) in cancer therapeutics. In treating cancer, radiation therapy and chemotherapy remain the most widely used treatment options and recent developments in cancer research show that the incorporation of gold nanostructures into these protocols has enhanced tumor cell killing. These nanostructures further provide strategies for better loading, targeting, and controlling the release of drugs to minimize the side effects of highly toxic anticancer drugs used in chemotherapy and photodynamic therapy. In addition, the heat generation capability of gold nanostructures upon exposure to UV or near infrared light is being used to damage tumor cells locally in photothermal therapy. Hence, gold nanostructures provide a versatile platform to integrate many therapeutic options leading to effective combinational therapy in the fight against cancer. In this review article, the recent progress in the development of gold-based NPs towards improved therapeutics will be discussed. A multifunctional platform based on gold nanostructures with targeting ligands, therapeutic molecules, and imaging contrast agents, holds an array of promising directions for cancer research.

Therapeutic application of metallic nanoparticles combined with particle-induced x-ray emission effect

Metallic nanoparticles (MNP) are able to release localized x-rays when activated with a high energy proton beam by the particle-induced x-ray emission (PIXE) effect. The exploitation of this phenomenon in the therapeutic irradiation of tumors has been investigated. PIXE-based x-ray emission directed at CT26 tumor cells in vitro, when administered with either gold (average diameter 2 and 13 nm) or iron (average diameter 14 nm) nanoparticles (GNP or SNP), increased with MNP solution concentration over the range of 0.1-2 mg ml(-1). With irradiation by a 45 MeV proton therapy (PT) beam, higher concentrations had a decreased cell survival fraction. An in vivo study in CT26 mouse tumor models with tumor regression assay demonstrated significant tumor dose enhancement, thought to be a result of the PIXE effect when compared to conventional PT without MNP (radiation-only group) using a 45 MeV proton beam (p < 0.02). Those receiving GNP or SNP injection doses of 300 mg kg(-1) body weight before proton beam therapy demonstrated 90% or 75% tumor volume reduction (TVR) in 20 days post-PT while the radiation-only group showed only 18% TVR and re-growth of tumor volume after 20 days. Higher complete tumor regression (CTR) was observed in 14-24 days after a single treatment of PT with an average rate of 33-65% for those receiving MNP compared with 25% for the radiation-only group. A lower bound of therapeutic effective MNP concentration range, in vivo, was estimated as 30-79 µg g(-1) tissue for both gold and iron nanoparticles. The tumor dose enhancement may compensate for an increase in entrance dose associated with conventional PT when treating large, solid tumors with a spread-out Bragg peak (SOBP) technique. The use of a combined high energy Bragg peak PT with PIXE generated by MNP, or PIXE alone, may result in new treatment options for infiltrative metastatic tumors and other diffuse inflammatory diseases.

Preferential tumour accumulation of gold nanoparticles, visualised by Magnetic Resonance Imaging: radiosensitisation studies in vivo and in vitro

PURPOSE

To investigate the radiosensitisation of gold nanoparticles (GNP) with an average diameter of 5 nm coated with the gadolinium chelating agent dithiolated diethylenetriaminepentaacetic gadolinium (Au@DTDTPA:Gd) in vitro and in mice bearing tumours (MC7-L1).

MATERIALS AND METHODS

In vivo, the gadolinium chelate coating allows one to perform real-time Magnetic Resonance Imaging (MRI) pharmacokinetic analysis during intravenous infusion. Experiments were performed following treatment with 10 Gy of 150 kVp X-rays. In vitro experiments were also performed with clonogenic assays to generate dose response curves for the same cells.

RESULTS

We observed a preferential accumulation of Au@DTDTPA:Gd in tumours; a substantial toxicity for tumour cells in vitro, but no obvious toxicity for mice; and the absence of a synergistic effect with Au@DTDTPA:Gd and radiation in all experiments.

CONCLUSIONS

The additional absorption of radiation and the subsequent increase in secondary electrons, attributable to the presence of gold in Au@DTDTPA:Gd, does not lead to radiosensitisation. However, this chelating agent exhibits a chemotherapeutic action which warrants further investigation. When compared to positive results obtained by others on radiosensitisation by GNP, the present study suggests that the chemotherapeutic and radiosensitising properties of GNP may depend strongly on the nature of the coating.

Fragmentation and plasmid strand breaks in pure and gold-doped DNA irradiated by beams of fast hydrogen atoms

The results of an investigation into the damage caused to dry plasmid DNA after irradiation by fast (keV) hydrogen atoms are presented. Agarose gel electrophoresis was used to assess single and double strand break yields as a function of dose in dry DNA samples deposited on a mica substrate. Damage levels were observed to increase with beam energy. Strand break yields demonstrated a considerable dependence on sample structure and the method of sample preparation. Additionally, the effect of high-Z nanoparticles on damage levels was investigated by irradiating DNA samples containing controlled amounts of gold nanoparticles. In contrast to previous (photonic) studies, no enhancement of strand break yields was observed with the particles showing a slight radioprotective effect. A model of DNA damage as a function of dose has been constructed in terms of the probability for the creation of single and double strand breaks, per unit ion flux. This model provides quantitative conclusions about the effects of both gold nanoparticles and the different buffers used in performing the assays and, in addition, infers the proportion of multiply damaged fragments.

Nanoscintillator conjugates as photodynamic therapy-based radiosensitizers: calculation of required physical parameters

The recent demonstration of nanoscale scintillators has led to interest in the combination of radiation and photodynamic therapy. In this model, scintillating nanoparticles conjugated to photosensitizers and molecular targeting agents would enhance the targeting and improve the efficacy of radiotherapy and extend the application of photodynamic therapy to deeply seated tumors. In this study, we calculated the physical parameters required for these nanoparticle conjugates to deliver cytotoxic levels of singlet oxygen at therapeutic radiation doses, drawing on the published literature from several disparate fields. Although uncertainties remain, it appears that the light yield of the nanoscintillators, the efficiency of energy transfer to the photosensitizers, and the cellular uptake of the nanoparticles all need to be fairly well optimized to observe a cytotoxic effect. Even so, the efficacy of the combination therapy will likely be restricted to X-ray energies below 300 keV, which limits the application to brachytherapy.

Quantum dots and nanoparticles for photodynamic and radiation therapies of cancer

Semiconductor quantum dots and nanoparticles composed of metals, lipids or polymers have emerged with promising applications for early detection and therapy of cancer. Quantum dots with unique optical properties are commonly composed of cadmium contained semiconductors. Cadmium is potentially hazardous, and toxicity of such quantum dots to living cells, and humans, is not yet systematically investigated. Therefore, search for less toxic materials with similar targeting and optical properties is of further interest. Whereas, the investigation of luminescence nanoparticles as light sources for cancer therapy is very interesting. Despite advances in neurosurgery and radiotherapy the prognosis for patients with malignant gliomas has changed little for the last decades. Cancer treatment requires high accuracy in delivering ionizing radiation to reduce toxicity to surrounding tissues. Recently some research has been focused in developing photosensitizing quantum dots for production of radicals upon absorption of visible light. In spite of the fact that visible light is safe, this approach is suitable to treat only superficial tumours. Ionizing radiation (X-rays and gamma rays) penetrate much deeper thus offering a big advantage in treating patients with tumours in internal organs. Such concept of using quantum dots and nanoparticles to yield electrons and radicals in photodynamic and radiation therapies as well their combination is reviewed in this article.

Variation of strand break yield for plasmid DNA irradiated with high-Z metal nanoparticles

Using agarose gel electrophoresis, we measured the effectiveness of high-Z metal particles of different sizes on SSB and DSB yields for plasmid DNA irradiated with 160 kVp X rays. For plasmid samples prepared in Tris-EDTA buffer, gold nanoparticles were shown to increase G'(SSB) typically by a factor of greater than 2 while G'(DSB) increased by a factor of less than 2. Similar dose-modifying effects were also observed using gold microspheres. Addition of 10(-1) M DMSO typically decreased damage yields by a factor of less than 0.5. Plasmid samples prepared in PBS showed significantly different damage yields compared to those prepared in Tris-EDTA (P < 0.001) with G'(SSB) and G'(DSB) increasing by factors of 100 and 48, respectively. Furthermore, addition of gold nanoparticles to samples prepared in PBS decreased G'(SSB) and G'(DSB) by factors of 0.2 and 0.3, respectively. The results show plasmid damage yields to be highly dependent on differences in particle size between the micro- and nanometer scale, atomic number (Z) of the particle, and scavenging capacity of preparation buffers. This study provides further evidence using a plasmid DNA model system for the potential of high-Z metal nanoparticles as local dose-modifying agents.