Marios
Sotiropoulos
*a,
Nicholas T.
Henthorn
a,
John W.
Warmenhoven
a,
Ranald I.
Mackay
b,
Karen J.
Kirkby
ac and
Michael J.
Merchant
ac
aDivision of Cancer Sciences, School of Medical Sciences, Faculty of Biology, Medicine and Health, The University of Manchester, Manchester, UK. E-mail: marios.sotiropoulos@manchester.ac.uk
bChristie Medical Physics and Engineering, The Christie NHS Foundation Trust, Manchester, UK
cThe Christie NHS Foundation Trust, Manchester, UK
First published on 17th November 2017
Gold nanoparticles have been proven as potential radiosensitizer when combined with protons. Initially the radiosensitization effect was attributed to the physical interactions of radiation with the gold and the production of secondary electrons that induce DNA damage. However, emerging data challenge this hypothesis, supporting the existence of alternative or supplementary radiosensitization mechanisms. In this work we incorporate a realistic cell model with detailed DNA geometry and a realistic gold nanoparticle biodistribution based on experimental data. The DNA single and double strand breaks, and damage complexity are counted under various scenarios of different gold nanoparticle size, biodistribution and concentration, and proton energy. The locality of the effect, i.e. the existence of higher damage at a location close to the gold distribution, is also addressed by investigating the DNA damage at a chromosomal territory. In all the cases we do not observe any significant increase in the single/double strand break yield or damage complexity in the presence of gold nanoparticles under proton irradiation; nor there is a locality to the effect. Our results show for the first time that the physical interactions of protons with the gold nanoparticles should not be considered directly responsible for the observed radiosensitization effect. The model used only accounts for DNA damage from direct interactions, whilst considering the indirect effect, and it is possible the radiosensitization effect to be due to other physical effects, although we consider that possibility unlikely. Our conclusion suggests that other mechanisms might have greater contribution to the radiosensitization effect and further investigation should be conducted.
High atomic number materials have been proven to be able to enhance the local dose of radiation, providing radiosensitization capabilities. However, only recently have advances in the manufacturing of nanoparticles lead to the consideration of metallic nanoparticles as a radiosensitizing agent in radiation therapy. In particular, gold nanoparticles (GNP) constitute a very promising radiosensitization agent as they provide high atomic number (Z = 79), one of the highest biocompatibility and low toxicity, ease of fabrication into a range of sizes and shapes, and high surface area. GNPs have demonstrated in vivo and in vitro radiosensitization potential both for photon and ion beams.1–5
The cell nucleus, and more specifically the nuclear DNA, has been established as the main target for radiation damage. Therefore, to understand the GNP radiosensitization the cell survival has been investigated in relation to the dose deposited to the nucleus and the subsequent DNA damage. Dosimetric studies around a GNP revealed highly inhomogeneous dose distributions,6–8 rendering the macroscopic dose an unsuitable predictor of the cell survival. Instead, the local effect model (LEM) was incorporated to account for the high dose gradient.9–11
In the case of photon irradiation, McMahon9 demonstrated a good agreement between the LEM and the data from Jain et al.12 for the MDA-MB-231 cell line when exposed to 160 kVp X-rays and 1.9 nm GNPs. Lechtman et al.10 also found a good agreement between cell survival and their LEM modification for PC-3 human prostate cancer cells treated with 300 kVp X-rays and 30 nm GNPs. Although the LEM linked the dose distributions produced by the GNPs to the cell survival, it renders difficulties in understanding the underlying radiosensitization mechanisms. To further elucidate the physical mechanisms and study the molecular radiosensitization effect in the nucleus, Xie et al.13 implemented the biophysical Monte Carlo code PARTRACK. They showed that for X-rays ranging from 60 to 200 kVp a significant DNA double strand break enhancement is produced only when the GNPs are located on the nucleus surface.
Chithrani et al.2 reported an increase in the γ-H2AX and 53BP1 foci number of 1.84 (1.45), at 4 h, and 1.28 (1.34) at 24 h post-irradiation, when HeLa cells were treated with 220 kVp X-rays and 50 nm GNP. At the same conditions the enhancement ratio (ER), which is the ratio of the survival fraction with and without GNPs at a specific dose and can be calculated from the cell survival curves, was 2.24 at 4 Gy. For A375 melanoma cells treated with 6 MV X-rays and gold nanorods (∼15 nm × 44 nm), a γ-H2AX foci number increase of 2.10 was observed at 2 Gy, while the ER can be calculated from the data reported by Xu et al.14 to be 1.14. In contrast, Jain et al.12 found no significant 53BP1 foci number increase either 1 h or 24 h post irradiation for MDA-MB-231 breast cancer cells exposed to 160 kVp X-rays and 1.9 nm GNPs. For the same conditions the ER was calculated to be 1.22 at 1.5 Gy. In an attempt to understand the variations between theoretical calculations and experimental measurements, McQuaid et al.15 implemented two separate models. The first model accurately predicted the short term DNA damage, while the second model was in good agreement with the long term cell survival. The need for two models suggests that different mechanisms may operate at different timescale. The fact that the DNA damage was not sufficient to account for the cell survival allowed the speculation of other mechanisms.
Polf et al.4 first demonstrated experimentally the radiosensitization effect under proton irradiation. Specifically they showed an ER of 1.12 at 2 Gy when DU145 prostate cancer cells loaded with 44 nm GNPs were exposed in a 160 MeV proton spread out Bragg peak. Liu et al.3 found an ER of 1.06 at 2 Gy for EMT-6 murine breast cancer cells treated with 3 MeV protons and 6.1 nm GNPs. Li et al.16 studied the effect of 1.3 and 4 MeV protons with 5 or 10 nm GNPs to A431 epidermoid carcinoma cells. They found an ER of 1.22–1.43 at 2 Gy for the 1.3 MeV beam depending on the GNP size, but no significant enhancement for the 4 MeV beam. Similarly, Jeynes et al.17 did not see any radiosensitization effect when RT-112 bladder cancer cells were irradiated with 3 MeV protons, although their result may be an outcome of the relatively low GNP concentration used. In spite of the experimental validation of GNP enhanced proton therapy, computational models have suggested a much lower radiosensitization effect. For instance, Lin et al.11 predicted a significant enhancement in the case of proton irradiations only when the GNPs were inserted in the cell nucleus.
The DNA damage induction in the case of GNP enhanced proton therapy has not been systematically investigated, creating a gap between the observed effect and the related mechanisms. In this work we apply Monte Carlo simulations in order to calculate for the first time direct single and double strand break (SSB and DSB) induction and investigate the effect on the damage complexity in the presence of GNPs. A cell model complete with chromosomal territories and detailed nuclear structure are adopted. A realistic GNP distribution is used, arranging the GNP in clusters of vesicles that contain the GNP. The results provide further evidence that the radiosensitization effect in gold nanoparticle enhanced proton therapy should not be attributed, at least exclusively, to the physical interaction mechanisms but alternative or supplementary mechanisms should be considered.
A second geometry consisting of a chromosomal territory of the first geometry was also used to study the effects on a localized area. The chromosomal territory is a cube with an edge of 3.114 μm. In front of the chromosomal territory a box with dimensions 3.114 μm × 3.114 μm and variable length and filled with GNPs (length: 1 μm) or solid gold (length: 0.5 μm) is placed. The chromosomal territory was also filled with GNPs (while the box remained empty) in order to study the effect of the nuclear internalization of the nanoparticles.
A combination of two physics lists was used as follows: for the gold material the “Livermore” models and for the water material constituting the cell and DNA material the Geant4-DNA physics models were implemented. No nuclear interactions were included in the simulations. The “Livermore” physics list was considered more suitable since we expect the GNPs to be situated close enough to the DNA structures.21
In this study the free radical production was not taken into account, scoring DNA damage only from the direct effect. This decision was based on the premise that the free radicals generated are proportional to the secondary electrons produced. Indeed, in the study by Xie et al.13 the inclusion of the free radical production did not change the trends of radiation enhancement but affected only the absolute numbers of single and double strand breaks. We are unable to accurately model the indirect effect at this time, and acknowledge that this limitation prevents prediction of absolute values of radiation enhancement using this simulation.
The majority of the formulations available do not allow accumulation of the GNPs into the nucleus. Therefore, we assumed that the gold nanoparticles did not enter the nucleus,15,17 and all the nanoparticles were located in the cytoplasm. The concentration of 0.7% wt/wt gold was used as reference, similar to the concentration achieved in a tumour in a mouse model;1 the effect of higher GNP number was also studied (5×, 10×, 20×, and 100× the reference GNPs at the 0.7% gold wt/wt reference concentration, i.e. about 3.4%, 6.6%, 12%, and 42% gold wt/wt) since large variations in the number of GNP uptake has been observed.26 The GNP sizes of 6, 15, 30 nm were studied, with the number of GNPs used for each size given in Table 1. As some nuclear accumulation has been demonstrated,27,28 the chromosomal territory was filled with 1×, 10×, and 20× the GNP number at the reference concentration in an attempt to address the effect of the nuclear accumulation.
Radius (nm) | Number of GNPs in cytoplasm |
---|---|
6 | 587![]() |
15 | 38![]() |
30 | 4700 |
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Fig. 3 Percentage of the contribution of the simple (DSBs) and complex (DSB+) double strand breaks for the irradiation conditions described in Fig. 2. |
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Fig. 7 Percentage of the contribution of the simple (DSBs) and complex (DSB+) for the reference concentration (0.7% wt/wt gold) of 15 nm GNP, when irradiated with 2 Gy of 1, 10, and 50 MeV protons. |
Fig. 10 presents the results for 15 nm GNP at the reference, 10×, and 20× times the GNPs at the reference concentration (0.7% wt/wt), under 10 and 50 MeV proton irradiation. In addition, for the same cases the DNA damage complexity is shown in Fig. 11A. The results show that, under proton irradiation, even when the GNPs are inside the nucleus they do not increase the DNA damage. Although we believe that our model underestimates the strand break yields in the case photons, it is interesting to include a photon irradiation test case. Fig. 10 presents the DSB yield by mono-energetic photon beam of 25 keV, where about a 2, and 25 fold increase in the DSB damage is observed for the 1× and 20× times the reference GNPs, in line with previous findings.11 Notably, for the photon test case, as shown in Fig. 11B higher concentration increases also the damage complexity.
According to our results, the radiosensitization effect under proton irradiation does not depend on the size or distribution of the GNPs. McQuaid et al.,15 for photon irradiation, found a better estimation of the dose enhancement with the local effect model (LEM) when larger GNP were used in the model instead of the nominal value. They attributed this discrepancy to “the aggregation of the gold particles in to clusters and the formation of biological vesicles”. In this study we implemented a realistic distribution allowing GNP to form vesicles (“Peckys” distribution) and we did not find any difference in the DSB induction between the uniform distribution and the formation into vesicles, independently of the GNP size or proton energy. The introduction of a higher amount of gold, even at unrealistic concentrations, did not increase the DNA damage. Interestingly, we found no dependence on the DNA damage enhancement to the proton energy or LET values studied. We suspect that the number and range of the electrons produced are not enough to create a measurable effect.
Since we expect the number and range of the produced secondary electrons to be limited, we hypothesize that the effect may be localized in regions close to the GNP distribution. Therefore, the effect may average over the whole nucleus making the identification of the DNA damage enhancement more difficult. To study the locality of the effect the damage to a chromosomal territory by a GNP or solid gold filled box was investigated. In the case of the GNP filled box impinging the chromosomal territory, no substantial increase in SSB or DSB was observed. We attribute the effect mainly to the low geometrical interaction probability. It is worth noting that although for the high concentration of 100 times the reference GNPs we have 42% gold wt/wt, only 3.7% of the cytoplasm volume is actually gold. When the box is filled with gold, we observe an increase in the SSB for both irradiation energies, but the DSB yield does not increase. This indicates that not only the increase of ionizations is important but the spatial distribution as well. Indeed, the SSBs are directly linked to the ionization number but the DSBs are related to the spatial distribution of the ionizations through the clustering of the SSBs.
Relative to the previous claim is the suitability of dose enhancement as an index of the damage enhancement. In previous works, the macroscopic quantity of dose deposition has been utilized to compare the damage enhancement at the micro-scale, in the presence of the GNPs. We suggest that dose enhancement might have limited applicability when we focus our interest to cell fate; similar to the need for accounting for the relative biological effectiveness in ion therapy.
Unlike photon irradiations, in a proton irradiation a small number of protons are needed to deliver a clinically relevant dose. As a result, the geometrical interaction probability of a proton with a GNP might be extremely low. In addition, not every particle that crosses the GNP will interact with it. This makes the production of secondary electrons a very rare process. To further support our findings we calculated (see section 1 at the ESI†) the (i) geometrical, and (ii) physical interaction probability in the case of the GNP filled box impinging the chromosomal territory. We estimate that 0.5 and 1.7 protons will cross a GNP on average, while 9 and 1.4 more electrons with range higher than 100 nm will be generated for the 10 and 50 MeV proton irradiation respectively, at the dose of 2 Gy. As a result we believe that in the case of proton irradiation the physical contribution to the DNA damage is expected to be very low. Indeed, Martínez-Rovira and Prezado32 with Monte Carlo simulations, and Cho et al.33 with experimental and Monte Carlo calculations studied the physical dose distribution produced by GNPs irradiated with protons and showed that there is no, or very little, physical contribution to the radiosensitization effect, a conclusion that we also see when considering the strand break yields. The absence of DNA damage from the physical interactions of the protons with the GNPs points out to the presence of chemical or biological factors.
In our simulations we took into account only damage resulting from direct interactions of radiation with DNA only, i.e. the free radical production was not simulated. We expect our observations to be similar when the free radical production is taken into account. It has been demonstrated34 under ion irradiation that the inclusion of free radicals mainly affects the absolute numbers of single and double SBs, with minimal effect in the damage complexity. Similar observation can be made on Xie et al.,13 where the inclusion of free radicals resulted to similar trends in the observed single and double SBs. Regardless, since in the case of proton irradiation the probability of an electron generated by the physical interactions with the gold to reach the nucleus is very low, much lower than the case of photon irradiation, we believe that the inclusion of the free radicals might not have a strong effect. Free radicals, on the other hand, might play an important role in the radiosensitization properties of the GNPs which do not relate with DNA damage. Nonetheless, the inclusion of the indirect damage could shed some additional light into the chemical pathway of the radiosensitization, especially when combined with a model of free radical–GNP coating reaction. Sicard-Roselli et al.35 showed that the free radicals react with the GNP coating and produce even more free radicals, with the possibility of contributing to the oxidative stress.
As has been mentioned in the scoring process section and discussed in more detail in section 2 of the ESI† (dependence of the clustering algorithm to the energy threshold values), there is a high sensitivity of the predicted SSB and DSB to the threshold values selected, the conversion scheme, and in particular DNA geometry. This dependence on the parameters and assumptions made in each model has resulted in different threshold values being used between different models. The parameters are optimized in a way that consistency is achieved when compared with well-established experimental values. Our model was mainly developed for proton irradiation, therefore was optimized to achieve consistent yields under protons. Under photon irradiation of the GNP filled chromosomal territory a substantial underestimation of the SSB and DSB yields is observed, implying a track structure dependence of the model. While under photon irradiation our model requires further optimization to ensure agreement with experimental yields, those results provide an indirect validation that our model is sensitive enough to account for the increased DNA damage observed in the presence of GNPs. We expect that a conversion scheme based on the accumulation of energy deposition, rather than the conversion of the energy deposition events at the sugar-phosphate group will produce a more consistent model at a wider radiation quality range.
Since the GNP radiosensitization effect under proton irradiation has been experimentally established in vivo and in vitro,4,5,16 it is interesting to speculate possible mechanisms that might be responsible for the effect. Our work and previous studies32,33 make clear that the radiosensitization under proton irradiations, unlike photon irradiations where physical interactions can also contribute to the effect, is highly unlikely to originate from direct or indirect damage to the DNA. As a result, the alternative or supplementary mechanisms that have been proposed for photon irradiations, such as increased oxidative stress, mitochondria function disruption,36,37 and lysosomal rupture,38 might be more important in the case of proton irradiations, and ultimately the key to understanding radiosensitization with heavy metal nanoparticles bombarded with ions.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr07310k |
This journal is © The Royal Society of Chemistry 2017 |