Gadolinium neutron capture in glioblastoma multiforme cells

Purpose: A proof of principle for cell killing by Gadolinium (Gd) neutron capture in Magnevist® preloaded Glioblastoma multiforme (GBM) cells is provided.

Materials and methods: U87cells were pre-loaded with 5 mg/ml Magnevist® (Gd containing compound) and irradiated using an enhanced neutron beam developed at NIU Institute for Neutron Therapy at Fermilab. These experiments were possible because of an enhanced fast neutron therapy assembly designed to use the fast neutron beam at Fermilab to deliver a neutron beam containing a greater fraction of thermal neutrons and because of the development of improved calculations for dose for the enhanced neutron beam. Clonogenic response was determined.

Results: U87 cell survival after γ irradiation, fast neutron irradiation and irradiation with the enhanced neutron beam in the presence or absence of Magnevist® were determined.

Conclusions: U87 cells were the least sensitive to γ radiation, and increasingly sensitive to fast neutron irradiation, irradiation with the enhanced neutron beam and finally a significant enhancement in cell killing was observed for U87 cells preloaded with Magnevist®. The sensitivity of U87 cells pre-loaded with Magnevist® and then irradiated with the enhanced neutron beam can at least in part be attributed to the Auger electrons emitted by the neutron capture event.     

Calculated DNA Damage from Gadolinium Auger Electrons and Relation to Dose Distributions in a Head Phantom

Purpose: To calculate the number of ¹⁵⁷Gadolinium (¹⁵⁷Gd) neutron capture induced DNA double strand breaks (DSB) in tumor cells resulting from epithermal neutron irradiation of a human head when the peak tissue dose is 10?Gy. To assess the lethality of these Gd induced DSB.

Materials and Methods: DNA single and double strand breaks from Auger electrons emitted during ¹⁵⁷Gd(n,gamma) events were calculated using an atomistic model of B‐DNA with higher‐order structure. When combined with gadolinium neutron capture reaction rates and neutron and photon physical dose rates calculated from the radiation transport through a model of the human head with explicit tumors, peak tissue dose can be related to the number of Auger electron induced DSB in tumor cell DNA. The lethality of these DNA DSB were assessed through a comparison with incorporated ¹²⁵I decay cell survival curves and second comparison with the number of DSB resulting from neutron and photon interactions.

Results: These calculations on a molecular scale (microscopic calculations) indicate that for incorporated ¹⁵⁷Gd, each neutron capture reaction results in an average of 1.56±0.16 DNA single strand breaks (SSB) and 0.21±0.04 DBS in the immediate vicinity (～40?nm) of the neutron capture. In an example case of Gd Neutron Capture Therapy (GdNCT), a 1?cm radius midline tumor, peak normal tissue dose of 10?Gy, and a tumor concentration of 1000?ppm Gd, result in a maximum of 140±27 DSBs per tumor cell.

Conclusions: The number of DSB from the background radiation components is one order of magnitude lower than the Gd Auger electron induced DSB. The cell survival of mammalian cell lines with a similar amount of complex DSB induced from incorporated ¹²⁵I decay yield one to two magnitudes of cell killing. These two points indicate that gadolinium auger electrons could significantly contribute to cell killing in GdNCT.     

Dose-rate effect was observed in T98G glioma cells following BNCT

BackgroundIt is generally said that low LET radiation produce high dose-rate effect, on the other hand, no significant dose rate effect is observed in high LET radiation. Although high LET radiations are produced in BNCT, little is known about dose-rate effect of BNCT.Materials and methodsT98G cells, which were tumor cells, were irradiated by neutron mixed beam with BPA. As normal tissue derived cells, Chinese hamster ovary (CHO-K1) cells and DNA double strand breaks (DNA-DSBs) repair deficient cells, xrs5 cells were irradiated by the neutrons (not including BPA). To DNA-DSBs analysis, T98G cells were stained immunochemically with 53BP1 antibody. The number of DNA-DSBs was determined by counting 53BP1 foci.ResultsThere was no dose-rate effect in xrs5 cells. D₀ difference between 4 cGy/min and 20 cGy/min irradiation were 0.5 and 5.9 at the neutron and gamma-ray irradiation for CHO-K1, and 0.3 at the neutron for T98G cells. D₀ difference between 20 cGy/min and 80 cGy/min irradiation for T98G cells were 1.2 and 0.6 at neutron irradiation plus BPA and gamma-ray. The differences between neutron irradiations at the dose rate in T98G cells were supported by not only the cell viability but also 53BP1 foci assay at 24 h following irradiation to monitor DNA-DSBs.ConclusionDose-rate effect of BNCT when T98G cells include 20 ppm BPA was greater than that of gamma-ray irradiation. Moreover, Dose-rate effect of the neutron beam when CHO-K1 cells did not include BPA was less than that of gamma-ray irradiation These present results may suggest the importance of dose-rate effect for more efficient BNCT and the side effect reduction.     

Radiation effect of gadolinium-neutron capture reactions on the survival of Chinese hamster cells

In gadolinium-neutron capture reactions, prompt gamma rays with an energy spectrum of up to 7 MeV, X-rays and electrons are released. We measured the effect of radiation as a result of capture reactions on cultured Chinese hamster cells. Cells in the medium containing 5000 ppm gadolinium were exposed to thermal neutrons from a nuclear reactor. The survival curve for those cells exhibited a shoulder in the low neutron fluence region. The survival curve for cells exposed to thermal neutrons in the absence of gadolinium was a simple exponential function. To obtain 10% survival levels, 5.4 x 10(12) neutrons/cm2 were required for cells irradiated in the absence of gadolinium, and 1.55 x 10(12) neutrons/cm2 for those irradiated in the presence of gadolinium. The therapeutic ratio in gadolinium-neutron capture therapy depends on the difference in 157Gd concentrations between the tumor and normal tissues. Thus, our current effort has been to develop a method of selectively delivering 157Gd to tumors.     

A novel vertebrate system for the examination and direct comparison of the relative biological effectiveness for different radiation qualities and sources

Purpose: The recent rapid increase of hadron therapy applications requires the development of high performance, reliable in vivo models for preclinical research on the biological effects of high linear energy transfer (LET) particle radiation.

Aim: The aim of this paper was to test the relative biological effectiveness (RBE) of the zebrafish embryo system at two neutron facilities.

Material and Methods: Series of viable zebrafish embryos at 24-hour post-fertilization (hpf) were exposed to single fraction, whole-body, photon and neutron (reactor fission neutrons (<En?=?1?MeV>) and (p (18?MeV)+Be, <En>?=?3.5?MeV) fast neutron) irradiation. The survival and morphologic abnormalities of each embryo were assessed at 24-hour intervals from the point of fertilization up to 192 hpf and then compared to conventional 6?MV photon beam irradiation results.

Results: The higher energy of the fast neutron beams represents lower RBE (ref. source LINAC 6?MV photon). The lethality rate in the zebrafish embryo model was 10 times higher for 1?MeV fission neutrons and 2.5 times greater for p (18?MeV)+Be cyclotron generated fast neutron beam when compared to photon irradiation results. Dose-dependent organ perturbations (shortening of the body length, spine curvature, microcephaly, micro-ophthalmia, pericardial edema and inhibition of yolk sac resorption) and microscopic (marked cellular changes in eyes, brain, liver, muscle and the gastrointestinal system) changes scale together with the dose response.

Conclusion: The zebrafish embryo system is a powerful and versatile model for assessing the effect of ionizing radiation with different LET values on viability, organ and tissue development.     

Interaction between the biological effects of high- and low-LET radiation dose components in a mixed field exposure

Abstract

Purpose: The relative biological effectiveness of two epithermal neutron sources, a reactor based source at Studsvik, Sweden, and a proton accelerator-based source in Birmingham, UK, was studied in relation to the proportional absorbed dose distribution as a function of neutron energy. Evidence for any interactions between the effects of biological damage induced by high- and low-linear energy transfer (LET) dose components, in this ‘mixed field’ irradiation, was also examined

Materials and methods: Clonogenic survival in Chinese Hamster-derived V79 cells was used to assess biological effectiveness in this study. Cells were irradiated in suspension at 4°C at depths of 20, 35, 50 and 65 mm in a water phantom. This prevented the repair of sublethal damage, predominantly that produced by both incident and induced γ-rays in the field, over the variable periods of exposure required to irradiate cells with the same total absorbed dose. Cell survival, as a function of the absorbed radiation dose and depth in the phantom, was compared with Monte Carlo N-Particle (MCNP) calculations of the proportional absorbed dose distribution as a function of neutron energy for the two sources.

Results: In terms of the dose-related reduction in clonogenic cell survival, the epithermal neutron source at Studsvik was more biologically effective than the Birmingham source at all depths considered in the phantom. Although the contribution from the high-LET dose component was greater for the Studsvik source at 20 mm depth in the phantom, at greater depths the dose contribution from the high-LET dose component at Studsvik overlap with those for the Birmingham source. However, the most striking difference is in the fast neutron component to the dose of the two sources, neutron energies > 1 MeV were only associated with the Studsvik source. The relative biological effectiveness (RBE) of both sources declined slightly with depth in the phantom, as the total high-LET dose component declined. The maximum source RBE for Studsvik was 2.70 ± 0.50 at 20 mm; reduced to 2.10 ± 0.35 at depths of 50 and 65 mm. The corresponding values for Birmingham were 1.68 ± 0.25 and 1.31 ± 0.19, all values relate only to the surviving fraction of V79 cells at 37%, since RBE values are only applicable to the selected endpoint. Based on a dose reduction factor (DRF) of 1.0 for the total low-LET component to the absorbed dose, the RBE values for the high-LET dose component (fast neutrons and induced protons from the nitrogen capture reaction) was 14.5 and 7.05 for the Studsvik and Birmingham neutron sources, respectively. This is well outside the range of RBE historically reported values for V79 cells for the same level of cell survival for fast neutrons. The calculation of RBE values, based on the proportional absorbed dose distribution as a function of neutron energy, from historical data, and using a RBE of 1.8 for the dose from the nitrogen capture reaction, suggests RBE values for the total high-LET dose component of 3.1–2.8 and 2.5–2.0 for Studsvik and Birmingham, respectively, values again declining with depth in the phantom.

Conclusions: The overall biological effectiveness of the mixed field irradiation from an epithermal neutron sources depends on the composition and quality of the different dose components. The experimentally derived RBE values for the total high-LET dose components in these ‘mixed field’ irradiations are well in excess of historical data for fast neutrons. The difference between the historically expected and the observed RBE values is attributed to the interactions between the damage produced by high- and low-LET radiation.     

Calculated DNA damage from gadolinium Auger electrons and relation to dose distributions in a head phantom

PURPOSE: To calculate the number of 157Gadolinium (157Gd) neutron capture induced DNA double strand breaks (DSB) in tumor cells resulting from epithermal neutron irradiation of a human head when the peak tissue dose is 10 Gy. To assess the lethality of these Gd induced DSB. MATRIALS AND METHODS: DNA single and double strand breaks from Auger electrons emitted during 157Gd(n,gamma) events were calculated using an atomistic model of B-DNA with higher-order structure. When combined with gadolinium neutron capture reaction rates and neutron and photon physical dose rates calculated from the radiation transport through a model of the human head with explicit tumors, peak tissue dose can be related to the number of Auger electron induced DSB in tumor cell DNA. The lethality of these DNA DSB were assessed through a comparison with incorporated 125I decay cell survival curves and second comparison with the number of DSB resulting from neutron and photon interactions. RESULTS: These calculations on a molecular scale (microscopic calculations) indicate that for incorporated 157Gd, each neutron capture reaction results in an average of 1.56 +/- 0.16 DNA single strand breaks (SSB) and 0.21 +/- 0.04 DBS in the immediate vicinity (approximately 40 nm) of the neutron capture. In an example case of Gd Neutron Capture Therapy (GdNCT), a 1 cm radius midline tumor, peak normal tissue dose of 10 Gy, and a tumor concentration of 1000 ppm Gd, result in a maximum of 140 +/- 27 DSBs per tumor cell. CONCLUSIONS: The number of DSB from the background radiation components is one order of magnitude lower than the Gd Auger electron induced DSB. The cell survival of mammalian cell lines with a similar amount of complex DSB induced from incorporated 125I decay yield one to two magnitudes of cell killing. These two points indicate that gadolinium auger electrons could significantly contribute to cell killing in GdNCT.     

Neutrons applications in cancer treatment and in specific diagnostics

The major effect of ionizing radiation in cells is to destroy the ability of cells to divide by damaging their DNA strands. Extensive researches are leading to an understanding that the characteristics of high LET radiations such as fast neutrons and low LET radiations like protons, photons and electrons are different; because of different types of their interactions with tissue. Low LET radiations mostly damage tissue by producing free radicals. Oxygen has an effect of enhancing free radical formation in cells. Indeed hypoxic cells, which exist in malignant tumors, are radio resistant under irradiation with low LET radiations. In contrast, neutron interacts with tissue primarily via nuclear interactions, so its biological effectiveness is not affected on the presence of oxygen. The required dose to kill the same number of cancerous cells by neutrons is about one third in comparison with photons. Clinical reports show that a full course of treatment with neutrons consists of 12 treatment sessions, compared to 30-40 treatments with photons or electrons. In conclusion, in this review we describe which cancers or tumors could be better treated with neutrons. We also refer to whether neutrons could be used for diagnosis.     

The sensitivity of the in vitro cytokinesis-blocked micronucleus assay in lymphocytes for different and combined radiation qualities

Purpose

The dose-response relationship and the relative biological effectiveness (RBE) for the induction of micronuclei in lymphocytes was analyzed after irradiation in vitro with a 6-MeV neutron beam that was followed by 240-kV X-rays. The dose range of the combined exposure comprised 1 to 3 Gy. For reference, the dose-effect relationships found after X-ray (0.5 to 5 Gy)-and neutron (0.5 to 4 Gy) exposure applied separately are presented. The possibility of an interaction between the 2 radiation qualities is investigated by the method of isobole calculation termed “envelope of additivity”.

Methods

Micronuclei were analyzed in PHA-stimulated, cytokinesis-blocked human lymphocytes.

Results

The dose-response relationships for the micronucleus frequencies induced by the neutron irradiation, as well as by the mixed exposure, were linear. A saturation effect was indicated after neutron doses higher than 3 Gy. After low LET exposure the dose-response curves were describable by a linear-quadratic model. For neutron-induced micronucleus frequencies, RBE-values of 2 to 3 and for the combined exposure RBE values of 1.5 to 2 were calculated for a range of effect of 0.5 to 1.5 micronuclei/binucleated lymphocyte. No indication was found for an interaction between the damage induced by X-rays and that produced by neutrons under our experimental conditions.

Conclusions

These studies demonstrate a clear dependence of micronucleus induction on radiation quality and empha-size the usefulness of the micronucleus assay in biological dosimetry, also in cases in which high LET radiation or a mixed beam is involved as the radiation source.     

Neutrons do not produce a bystander effect in zebrafish irradiated in vivo

Abstract

Purpose: Neutron irradiations at the McMaster Tandetron Accelerator were performed to study direct and bystander effects of neutrons in a live organism.

Methods: The neutrons were produced through ⁷Li(p,n)⁷Be reaction. Although the gamma contamination of the neutron beam cannot be completely eliminated, it was designed to be as low as possible and remain below a threshold already established for bystander effects. Microdosimetric methods using a tissue-equivalent proportional counter have been used to measure the neutron and gamma doses for the cell irradiation. Previous data for a cell line exposed in vitro suggested that neutrons did not produce bystander effects at doses below 300 mGy. The current experiments sought to confirm this using a live whole organism (zebrafish) where tissue samples harvested 2 h after exposure were examined for direct evidence of apoptosis and tested for secretion of bystander factors using an established bioassay. Fish were either exposed directly to the beam or were allowed to swim with or in water previously occupied by irradiated fish.

Results: Using the zebrafish model it was found that there was significant direct cell death seen both by apoptosis scores and clonogenic assay when the neutron dose was approximately 100 mGy. An equivalent dose of gamma rays produced a more toxic effect. It was further found that neutrons did not induce a bystander effect in fish receiving signals from irradiated fish.

Conclusion: The results confirm in vitro experiments which suggest neutrons do not induce bystander signaling. In fact they may suppress gamma induced signaling suggesting a possible intriguing new and as yet unclear mechanism.     

Interpretation of the Increase in the Frequency of Neoplastic Transformations Observed for Some Ionising Radiations at Low Dose Rates

Summary

The anomalous increase of transformation frequency with decreasing dose rate observed by Hill et al. (1982, 1984b) for mouse fibroblast cells irradiated with fission neutrons cannot be satisfactorily explained by current models of radiation action. Recently a new model has been proposed which predicts the enhancement of damage with prolongation of irradiation, for equal doses. This is applied to the transformation studies in an attempt to interpret the enhancement observed for some radiations but not for others. Evidence is presented which suggests that repaired double-strand breaks in the DNA of cells which survive are the precursors of transformation. A critical physical factor is the total irradiation time rather than the dose rate. Approximately 1 per cent of repaired surviving cells go on to transform. From the results an explanation emerges of why transformation enhancement at low dose rates is not observed for natural alpha radiation and for photons or electrons, but is observed for fission neutrons and fast iron ions.     

Auger radiation targeted into DNA: a therapy perspective

Background Auger electron emitters that can be targeted into DNA of tumour cells represent an attractive systemic radiation therapy goal. In the situation of DNA-associated decay, the high linear energy transfer (LET) of Auger electrons gives a high relative biological efficacy similar to that of α particles. In contrast to α radiation, however, Auger radiation is of low toxicity when decaying outside the cell nucleus, as in cytoplasm or outside cells during blood transport. The challenge for such therapies is the requirement to target a high percentage of all cancer cells. An overview of Auger radiation therapy approaches of the past decade shows several research directions and various targeting vehicles. The latter include hormones, peptides, halogenated nucleotides, oligonucleotides and internalising antibodies.Discussion Here, we will discuss the basic principles of Auger electron therapy as compared with vector-guided α and β radiation. We also review some radioprotection issues and briefly present the main advantages and disadvantages of the different targeting modalities that are under investigation.     

Treatment results of fast neutron irradiation in soft tissue sarcomas

Purpose

Surgery is the standard treatment of soft-tissue sarcomas. Adjuvant radiotherapy with photons after less radical resection can improve local control. The rate of tumor control achieved in patients with G1 and G2 soft tissue sarcomas incompletely resected and treated postoperatively with neutron irradiation is similar to that seen in patients undergoing complete tumor resection and adjuvant photon irridiation.

Patients and Methods

At the Department of Radiotherapy and Radiation Oncology of the University of Münster, 61 patients with soft tissue sarcomas were irradiated postoperatively with fast neutrons. Mainly tumors of low or intermediate malignancy (R0; 27%; R1, 21%; R2, 52%) were treated. Malignant fibrous histiocytoma, liposarcoma, and neurogenic sarcomas dominated. 46 patients were irradiated with fast neutrons alone, and 15 patients were treated with mixed beam therapy (photons and neutrons).

Results

The median follow-up period was 44 months. Overall five-year survival probability analysed by Kaplan-Meyer method was 42.5%. The local control rate was 57.7%. 15 patients showed complete remission, 18 patients had a partial remission. Only 11% of the patients showed grade III and IV side effects during neutron irradiation.

Conclusion

Neutron irradiation is efficocious in treating highly and intermediately differentiated soft tissue sarcomas. The result of surgical resection seems to be a very important prognostic factor for patients with soft tissue sarcomas.

     

Development of a simulation method for dynamics of electrons ejected from DNA molecules irradiated with X-rays

Abstract

Purpose: To develop a method for simulating the dynamics of the photoelectrons and Auger electrons ejected from DNA molecules irradiated with pulsed monochromatic X-rays.

Materials and methods: A 30-base-pair (bp) DNA molecule was used as the target model, and the X-rays were assumed to have a Gaussian-shaped time distribution. Photoionization and Auger decay were considered as the atomic processes. The atoms from which the photoelectrons or Auger electrons were emitted were specified in the DNA molecule (or DNA ion) using the Monte Carlo method, and the trajectory of each electron in the electric field formed around the positively charged DNA molecule was calculated with a Newtonian equation. The kinetics of the electrons produced by irradiation with X-rays at an intensity ranging from 1 × 10¹² to 1 × 10¹⁶ photons/mm² and energies of 380 eV (below the carbon K-edge), 435 eV (above the nitrogen K-edge), and 560 eV (above the oxygen K-edge) were evaluated.

Results: It was found that at an X-ray intensity of 1 × 10¹⁴ photons/mm² or less, all the produced electrons escaped from the target. However, above an X-ray intensity of 1 × 10¹⁵ photons/mm² and an energy of 560 eV, some photoelectrons that were ejected from the oxygen atoms were trapped near the target DNA.

Conclusions: A simulation method for studying the trajectories of electrons ejected from a 30-bp DNA molecule irradiated with pulsed monochromatic X-rays has been developed. The present results show that electron dynamics are strongly dependent on the charged density induced in DNA by pulsed X-ray irradiation.     

Formation and repair of clustered damaged DNA sites in high LET irradiated cells

Purpose: Radiation with high linear energy transfer (LET) produces clustering of DNA double-strand breaks (DSB) as well as non-DSB lesions. Heat-labile sites (HLS) are non-DSB lesions in irradiated cells that may convert into DSB at elevated temperature during preparation of naked DNA for electrophoretic assays and here we studied the initial formation and repair of these clustered damaged sites after irradiation with high LET ions.

Materials and methods: Induction and repair of DSB were studied in normal human skin fibroblast (GM5758) after irradiation with accelerated carbon and nitrogen ions at an LET of 125 eV/nm. DNA fragmentation was analyzed by pulsed-field gel electrophoresis (PFGE) and by varying the lysis condition we could differentiate between prompt DSB and heat-released DSB.

Results: Before repair (t = 0 h), the 125 eV/nm ions produced a significant fraction of heat-released DSB, which appeared clustered on DNA fragments with sizes of 1 Mbp or less. These heat-released DSB increased the total number of DSB by 30–40%. This increase is similar to what has been found in low-LET irradiated cells, suggesting that the relative biological effectiveness (RBE) for DSB induction will not be largely affected by the lysis temperature. After 1–2 hours repair, a large fraction of DSB was still unrejoined but there was essentially no heat-released DSB present.

Conclusions: These results suggest that high LET radiation, as low LET gamma radiation, induces a significant fraction of heat-labile sites which can be converted into DSB, and these heat-released DSB may affect both induction yields and estimates of repair.     

Cell death pathways in directly irradiated cells and cells exposed to medium from irradiated cells

Abstract

Purpose: The aim of this study was to compare levels of apoptosis, necrosis, mitotic cell death and senescence after treatment with both direct radiation and irradiated cell conditioned medium.

Materials and methods: Human keratinocytes (HaCaT cell line) were irradiated (0.005, 0.05 and 0.5 Gy) using a cobalt 60 teletherapy unit. For bystander experiments, the medium was harvested from donor HaCaT cells 1 hour after irradiation and transferred to recipient HaCaT cells. Clonogenic assay, apoptosis, necrosis, mitotic cell death, senescence and cell cycle analysis were measured in both directly irradiated cells and bystander cells

Results: A reduction in cell survival was observed for both directly irradiated cells and irradiated cell conditioned medium (ICCM)-treated cells. Early apoptosis and necrosis was observed predominantly after direct irradiation. An increase in the number of cells in G2/M phase was observed at 6 and 12 h which led to mitotic cell death after 72 h following direct irradiation and ICCM treatment. No senescence was observed in the HaCaT cell line following either direct irradiation or treatment with ICCM.

Conclusion: This study has shown that directly irradiated cells undergo apoptosis, necrosis and mitotic cell death whereas ICCM-treated cells predominantly undergo mitotic cell death.     

Fast He2+ ion irradiation of DNA loaded with platinum-containing molecules

Purpose:?The association of radiotherapy and chemotherapy is an attractive approach to improve the therapeutic index of the treatment of tumors. A lot of work has been devoted to investigate the effects of X-ray, γ-ray and neutron irradiation of DNA or living cells loaded with different chemical compounds containing heavy atoms like platinum. No such studies exist presently when fast atomic ions are chosen as ionizing particles. In the present work, we investigate quantitatively the increase of DNA breaks in complexes of plasmid-DNA loaded with platinum atoms under irradiation by fast atomic He²⁺ ions.

Materials and methods:?DNA Plasmids (pBR322) are incubated in solutions containing different concentrations of terpyridine platinum (PtTC). In some preparations, dimethyl sulfoxide (DMSO), a free radical scavenger, has been added in order to investigate the role of the free radicals. The complexes of DNA plasmids loaded with high-Z atoms are irradiated under atmospheric conditions by He²⁺ ions at an energy of 143 MeV/amu and a linear energy transfert (LET) of 2.24 keV/μm. Analysis of DNA damage – single and double strand breaks – is made by electrophoresis on agarose gels.

Results:?The results show a significant increase in DNA strand breaks when platinum is present, indicating a radiosensitization by the high Z atoms. The increase in DNA damages is attributed to inner-shell ionization of a platinum atom by secondary electrons emitted along the He²⁺ tracks followed by an Auger deexcitation, leading, thus, to a local amplification of the radiative effects close to the DNA. The contributions of scavengeable – solvant mediated – indirect effects and non-scavengeable effects (direct ionization) are quantitatively evaluated.

Conclusion:?Enhancement of DNA breaks in plasmids loaded with heavy atoms like platinum and irradiated by atomic ions are observed. This finding suggests an enhancement of cell death rate will occur under irradiation by atomic ions when the cells contain high-Z atoms located close to DNA due to the increase of the DNA breaks.     

Liposome-based delivery of a boron-containing cholesteryl ester for high-LET particle-induced damage of prostate cancer cells: A boron neutron capture therapy study

Abstract

Purpose: The efficacy of a boron-containing cholesteryl ester compound (BCH) as a boron neutron capture therapy (BNCT) agent for the targeted irradiation of PC-3 human prostate cancer cells was examined.

Materials and methods: Liposome-based delivery of BCH was quantified with inductively coupled plasma-mass spectrometry (ICP-MS) and high-performance liquid chromatography (HPLC). Cytotoxicity of the BCH-containing liposomes was evaluated with neutral red, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS), and lactate dehydrogenase assays. Colony formation assays were utilized to evaluate the decrease in cell survival due to high-linear energy transfer (LET) particles resulting from ¹⁰B thermal neutron capture.

Results: BCH delivery by means of encapsulation in a lipid bilayer resulted in a boron uptake of 35.2 ± 4.3 μg/10⁹ cells, with minimal cytotoxic effects. PC-3 cells treated with BCH and exposed to a 9.4 × 10¹¹ n/cm² thermal neutron fluence yielded a 20–25% decrease in clonogenic capacity. The decreased survival is attributed to the generation of high-LET α particles and ⁷Li nuclei that deposit energy in densely ionizing radiation tracks.

Conclusion: Liposome-based delivery of BCH is capable of introducing sufficient boron to PC-3 cells for BNCT. High-LET α particles and ⁷Li nuclei generated from ¹⁰B thermal neutron capture significantly decrease colony formation ability in the targeted PC-3 cells.     

EGF-receptor targeted liposomes with boronated acridine: Growth inhibition of cultured glioma cells after neutron irradiation

Purpose: To study survival of cultured U-343MGaCl 2:6 glioma cells after incubation with boron-containing liposomes targeting the epidermal growth factor receptor following neutron irradiation.

Materials and methods: Epidermal growth factor-tagged liposomes were loaded with water-soluble boronated acridine developed for boron neutron capture therapy, (BNCT). Cellular uptake and distribution were studied. Further, cells were placed at 3 cm depth in a phantom and exposed to an epithermal neutron beam to study clonogenic cell survival.

Results: The cellular uptake of boron reached 90 ppm and it was determined by subcellular fractionation that most of the cell-associated boron was located outside of the nucleus. For clonogenic survival, the cells were incubated with epidermal growth factor receptor-targeted liposomes for 4 hours resulting in a cellular concentration of 55 ppm boron (11 ppm ¹⁰B). At a fluence of 3 × 10¹² neutrons/cm² the cell killing effect of the boron-containing epidermal growth factor-liposomes was about ten times higher than for neutrons only. Furthermore, theoretical calculation of the survival by enriched compound (55 ppm ¹⁰B), using the parameters from non-enriched compound (11 ppm ¹⁰B), shows that the killing effect in this case would be approximately five orders of magnitude higher than for neutrons only.

Conclusion: The results in this study show that epidermal growth factor-receptor targeted liposomes are suitable as tumor-cell delivery agents of boron for BNCT and support further studies to demonstrate their effectiveness in vivo.