Molecular and cellular effects of Auger emitters: 2008–2011

Abstract

Purpose: To review recent Auger emitter research with an emphasis on a review of findings on targeting and accumulation of Auger emitters in tumor cells.

Conclusion: Significant progress can be reported for targeting methods and improvements in methods to accumulate Auger emitters in the target cells, as well as advances in dose calculations. These studies further our understanding of how Auger emitters induce cell death at a cellular and molecular level, supporting the targeted radiomedical applications of Auger emitters.     

Necrosis is not induced by gadolinium neutron capture in glioblastoma multiforme cells

Abstract

Purpose: A comparative study of the effects of different radiation modalities on cell death was performed.

Materials and methods: Radiation modalities included γ-rays, fast neutrons, a mixed energy neutron beam called the modified enhanced thermal neutron beam and the mixed beam including Auger electron irradiation by gadolinium neutron capture. U87 (human brain tumor cells) cell survival curve data were modeled to predict how cells died. Transmission electron microscopy (TEM) images were assembled into a morphology of cell death (MCD) database and used to determine the fraction of necrotic or autophagic cells.

Results: Linear energy transfer (LET) differences for the different radiation modalities were revealed by modeling. All radiation modalities induced autophagy but only fast neutrons induced significant levels of necrosis. No necrosis, above control levels, was found in cells irradiated with mixed beam irradiation including Auger electrons. The number of autophagosomes increased with increasing time after exposure to all radiation modalities indicating progression of autophagy but only cells irradiated with the mixed beam plus Auger electrons exhibited extreme autophagy.

Conclusions: Mixed neutron beam irradiation plus Auger electron irradiation from gadolinium neutron capture is a moderately high LET modality that kills U87 cells without the induction of necrosis and with progression of autophagy to an extreme state.     

Charge build-up during decay of DNA-incorporated 123/125I: Consequences for labeled molecular structures

Abstract

Purpose: To further elucidate the mechanisms behind the strong biological effectiveness of DNA-incorporated Auger electron emitters ¹²³I and ¹²⁵I, which are mostly attributed to the shower of low-energy electrons released during the decay. A second, frequently mentioned cause can be seen in the charges accumulated during the Auger cascade on the decaying nuclide and its subsequent intra-molecular redistribution leading to a Coulomb explosion.

Methods: To assess the size of the charge and the dimensions of DNA damage thus determined, the first Auger cascade was simulated by Monte Carlo methods. The consequences of intra-molecular charge transfer in terms of structural molecular alterations were estimated by density functional theory (DFT) calculations and folding with the results of the Monte Carlo studies.

Results: Charge distributions of ¹²³I and ¹²⁵I were found to be very similar with values between +?1 and +?15 and a mean value of +?6.4. The molecules could tolerate charges up to +?5 (base), +?2 (nucleoside) and +?7 (nucleotide) without being destroyed.

Conclusions: The strong molecular DNA damage after ¹²³I and ¹²⁵I decay depends very much on the size of the DNA molecule involved in the calculation. In general, not every decay can be expected to lead to a Coulomb explosion.     

Photon-induced Auger effect in biological systems: a review

Purpose: In this paper, we review interesting findings reported in the studies of the biological effects induced by inner-shell ionization with the aim of interpreting them from a mechanistic viewpoint, in particular, of the Auger effect of atoms on different biological systems.

Materials and methods: More than 70 published papers are cited on the Auger effects ranging from DNA-related elements (carbon, nitrogen, oxygen and phosphorus) to mammalian cells. Externally administrated bromine, iodine, and platinum have also been cited for the present endeavor. Those significant works all needed a highly monochromatized X-rays from brilliant synchrotron light sources.

Results: We have assembled a coherent view on the inner-shell effects of the Auger process that contrasts to the overall effects with the outer-shell ionization processes.

Conclusion: Some of these studies have reported that the Auger effect significantly enhances biological effects as compared with irradiation at below K-ionization energy. The Auger-specific molecular degradation mode of DNA, involving extensive fragmentation of the deoxypentose moiety, has also been revealed. We conclude that the selectively localized effect on the specified atoms through inner-shell ionization followed by the Auger process should have a definite impact on the current radiation effect studies, which are largely based on non-selective outer-shell ionizations.     

Monte Carlo-simulated Auger electron spectra for nuclides of radiobiological and medical interest – a validation with noble gas ionization data

Abstract

Purpose: To further validate Monte Carlo calculation codes simulating cascades of Auger electron transitions in radionuclides that decay by electron capture or internal conversion. In particular, the need for an appropriate kinetic energy determination of the Auger electrons emitted from multiple-ionized atoms as well as the consideration of shake-off electrons would be investigated implicitly.

Methods: Charge distributions of noble gases after photoionization for different photon energies were calculated and compared with experimental data from the literature. In addition, new electron emission spectra were generated for ^99mTc and ¹²³I.

Results: By including strict energy book-keeping and allowing shake-off electrons, the agreement between experimentally detected charge distributions and Monte Carlo simulations was very good. On this basis, the number of emitted electrons per decay was found to be between 1 and 17 with a mean of 4.0 for ^99mTc and between 1 and 26 with a mean of 7.4 for ¹²³I.

Conclusions: Because of the good agreement with the experimental findings, the validation can be considered to be successful.     

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.     

Tumour targeting of Auger emitters using DNA ligands conjugated to octreotate

Abstract

Purpose: The objective of the study was to conjugate the DNA binding ligand para-[¹²⁵I]-iodoHoechst to octreotate, and to explore the tumour targeting potential of this conjugate in the octreotate-somatostatin receptor system.

Methods: We synthesized a Hoechst analogue containing a tri-butylstannyl group in the para position of phenyl ring, conjugated it to the N-terminal amino group of octreotate and prepared ¹²⁵I-labelled conjugate by iododestannylation. We used the somatostatin receptor (SSTR2) over-expressing cell line A427-7 derived from its parent A427 human non-small cell lung carcinoma cell line to investigate SSTR2 affinity and receptor-mediated internalisation of the conjugate, and the mouse A427-7 tumour xenograft model for in vivo biodistribution studies of the radiolabelled conjugate.

Results: A method was developed for convenient preparation of high specific activity radioiodinated conjugate which retains affinity for somatostatin receptors and is internalised into A427-7 SSTR2 over-expressing cells via a receptor-mediated mechanism. The conjugate accumulates in mouse A427-7 tumour xenografts following intravenous administration.

Conclusions: A dual targeting strategy for Auger endoradiotherapy, in which a DNA ligand is used to target the Auger decay to DNA, in conjunction with receptor-mediated targeting to specific receptors, using a labelled DNA ligand/peptide conjugate, has been demonstrated for the octreotate-somatostatin receptor system.     

A compilation of microdosimetry for uniformly distributed Auger emitters used in medicine

Purpose: To provide a compilation of microdosimetric characteristics for 12 Auger emitters commonly used in medicine.

Materials and methods: Monte Carlo electron track structure simulations are performed for 12 Auger emitters. They are ⁵⁵Fe, ⁶⁷Ga, ^99mTc, ¹¹¹In, ^113mIn, ^115mIn, ¹²³I, ¹²⁵I, ^193mPt, ^195mPt, ²⁰¹Tl, and ²⁰³Pb. Proximity functions of 12 Auger emitters are calculated from the simulated track structures and compared with that of gamma rays from ⁶⁰Co.

Results: Some of those Auger emitters are highly radiotoxic compared to hard gamma rays from ⁶⁰Co. The more electrons per decay and the lower electron energies, the more effective an Auger emitter could be.

Conclusions: The high radiotoxicity of Auger emitters is due to correlations of low-energy electrons released from decay processes. If these correlations were disregarded, Auger emitters would not differ significantly from other low linear energy transfer (LET) radiation sources. Even in the case of uniform distribution, some of those Auger emitters are highly radiotoxic compared to hard gamma rays. For Auger emitters to bond to radiosensitive sites in cell nucleus, much higher radiation effectiveness could be expected.     

Production of low kinetic energy electrons and energetic ion pairs by Intermolecular Coulombic Decay

Abstract

Purpose: The paper gives an introduction into Interatomic and Intermolecular Coulombic Decay (ICD). ICD is an autoionization process, which contrary to Auger decay involves neighbouring sites of the initial vacancy as an integral part of the decay transition. As a result of ICD, slow electrons are produced which generally are known to be active in radiation damage. The author summarizes the properties of ICD and reviews a number of important experiments performed in recent years.

Materials and methods: Intermolecular Coulombic Decay can generally take place in weakly bonded aggregates in the presence of ionizing particles or ionizing radiation. Examples collected here mostly use soft X-rays produced by synchrotron radiation to ionize, and use rare-gas clusters, water clusters or solutes in a liquid jet to observe ICD after irradiation.

Results: Intermolecular Coulombic Decay is initiated by single ionization into an excited state. The subsequent relaxation proceeds via an ultra-fast energy transfer to a neighbouring site, where a second ionization occurs. Secondary electrons from ICD have clearly been identified in numerous systems. ICD can take place after primary ionization, as the second step of a decay cascade which also involves Auger decay, or after resonant excitation with an energy which exceeds the ionization potential of the system.

Conclusions: ICD is expected to play a role whenever particles or radiation with photon energies above the ionization energies for inner valence electrons are present in weakly bonded matter, e.g., biological tissue. The process produces at the same time a slow electron and two charged atomic or molecular fragments, which will lead to structural changes around the ionized site.     

The Auger effect in physical and biological research

Purpose: The paper reports on progress in physics of radiationless transitions and new Auger spectra of ¹²⁵I and ¹²⁴I. We report progress in Monte Carlo track structure simulation of low energy electrons comprising majority electrons released in decay most Auger emitters.

Materials and methods: The input data for electron capture (EC) and internal conversion(IC) were obtained from various physics data libraries. Monte Carlo technique was used for the simulation of Auger electron spectra. Similarly, electron tracks were generated using Monte Carlo track structure methods.

Results: Data are presented for the EC, IC and binding energy (BE) of radionuclides ¹²⁴I and ¹²⁵I. For each of the radionuclides ¹²⁵I and ¹²⁴I some examples of electron spectra of individual decays are given. Because most Auger electrons are low energy and short range, data and a short discussion are presented on recent Monte Carlo track structure development in condensed media and their accuracy.

Conclusions: Accuracy of electron spectra calculated in the decay of electron shower by Auger emitting radionuclides depends on availability of accurate physics data. There are many gaps in these libraries and there is a need for detailed comparison between analytical method and Monte Carlo calculations to refine the method of calculations. On simulation of electron tracks, although improved models for sub-keV electron interaction cross sections for liquid water are now available, more experimental data are needed for benchmarking. In addition, it is desirable to make data and programs for calculations of Auger spectra available online for use by students and researchers.     

The effect of 111In radionuclide distance and auger electron energy on direct induction of DNA double-strand breaks: a Monte Carlo study using Geant4 toolkit

Purpose: In this study, the effect of ¹¹¹In position and Auger electron energy on direct induction of DSBs was investigated.

Materials and methods: The Geant4-DNA simulation toolkit was applied using a simple B-DNA form extracted from PDBlib library. First, the simulation was performed for electrons with energies of ¹¹¹In and equal emission probabilities to find the most effective electron energies. Then, ¹¹¹In Auger electrons’ actual spectrum was considered and their contribution in DSB induction analysed.

Results: The results showed that the most effective electron energy is 183?eV, but due to the higher emission probability of 350?eV electrons, most of the DSBs were induced by the latter electrons. Also, it was observed that most of the DSBs are induced by electrons emitted within 4?nm of the central axis of the DNA and were mainly due to breaks with <4 base pairs distance in opposing strands. Whilst, when ¹¹¹In atoms are very close to the DNA, 1.3 DSBs have been obtained per decay of ¹¹¹In atoms.

Conclusions: The results show that the most effective Auger electrons are the 350?eV electrons from ¹¹¹In atoms with <4?nm distance from the central axis of the DNA which induce ～1.3 DSBs per decay when bound to the DNA. This value seems reasonable when compared with the reported experimental data.     

Study of charge transport mechanisms in 125I-induced DNA damage at various temperatures

Abstract

Purpose: Iodine-125 decay induces localized DNA damage by three major mechanisms: (1) Direct damage by the emitted Auger electrons, (2) indirect damage by diffusible free radicals, and (3) charge neutralization of the residual, highly positively charged, tellurium daughter atom by stripping electrons from neighboring residues. The charge neutralization mechanism of ¹²⁵I-induced DNA damage is poorly understood. Charge transport along a DNA molecules can occur by either a hopping mechanism initiated by charge injection into DNA and propagated by charge migration through DNA bases along the DNA length, or by a tunneling mechanism in which charge transfers directly from a donor to an acceptor residue. In the first case additional damage in DNA nucleotides can be inflicted by the traveling charge; therefore, it is important to learn if charge hopping plays a role in ¹²⁵I-decay-induced DNA damage. In our previous work, we determined that at 193K the charge hopping mechanism was not an appreciable component of the mechanism of ¹²⁵I-induced DNA damage. However, the question whether this is also the case at higher temperatures remained open.

Methods: In the current study we used a well-known chemical barrier for charge hopping, 8-oxo-7, 8,-dihydroguanine (8-oxo-G), to assess the role of this mechanism in ¹²⁵I-decay-induced DNA damage at the following temperatures: 198, 253, 277 and 298 K.

Results: We found that varying the temperature had little effect on the distribution of ¹²⁵I-induced DNA breaks, as well as on the breaks found at the 8-oxo-G probe both with and without piperidine treatment.

Conclusions: We thus conclude that charge transport by the hopping mechanism is not a major factor in ¹²⁵I-decay-induced DNA damage at biologically relevant temperatures.     

Simulation of the relative damaging effects of Auger cascades with gel scintillators

Purpose: The aim of this work is to study the relative damaging effects of DNA-incorporated radionuclides by analyzing the behavior of the liquid-scintillation counting efficiency in volumes of nanometer size.

Methods: A liquid scintillation counter can detect changes in the micelle size when different percentages of an aqueous solution containing an Auger-electron-emitting radionuclide are incorporated to a gel scintillator. The counting efficiency can be used as an indicator of the nature of the stochastic processes occurring within the micelle structure.

Results: Because a large variation in the micelle size only perturbates the counting efficiency slightly, the accuracy of the method is poor. The application of tracing methods, which involve the calculation of the Auger cascades and the deposition of energy within nanometric spheres, can improve the accuracy of the results.

Conclusions: Some steps in the complete simulation of the damaging efficiency can be obviated with the use of a tracer.     

Biological basis of radiation protection needs rejuvenation

Abstract

Purpose: Human beings encounter radiation in many different situations – from proximity to radioactive waste sites to participation in medical procedures using X-rays etc. Limits for radiation exposures are legally regulated; however, current radiation protection policy does not explicitly acknowledge that biological, cellular and molecular effects of low doses and low dose rates of radiation differ from effects induced by medium and high dose radiation exposures. Recent technical developments in biology and medicine, from single cell techniques to big data computational research, have enabled new approaches for study of biology of low doses of radiation. Results of the work done so far support the idea that low doses of radiation have effects that differ from those associated with high dose exposures; this work, however, is far from sufficient for the development of a new theoretical framework needed for the understanding of low dose radiation exposures.

Conclusions: Mechanistic understanding of radiation effects at low doses is necessary in order to develop better radiation protection policy.     

Auger processes in the 21st century

Purpose: The extreme radiotoxicity of Auger electrons and their exquisite capacity to irradiate specific molecular sites has prompted scientists to extensively investigate their radiobiological effects. Their efforts have been punctuated by quadrennial international symposia that have focused on biophysical aspects of Auger processes. The latest meeting, the 6th International Symposium on Physical, Molecular, Cellular, and Medical Aspects of Auger Processes, was held 5–6 July 2007 at Harvard Medical School in Boston, Massachusetts, USA. This article provides a review of the research in this field that was published during the years 2004–2007, the period that has elapsed since the previous meeting.

Conclusion: The field has advanced considerably. A glimpse of the potential of this unique form of ionizing radiation to contribute to future progress in a variety of fields of study is proffered.     

Antisense radiotherapy: targeting full‐size mdr1 mRNA with 125I‐labelled oligonucleotides

Purpose: Antisense radiotherapy is an approach based on the targeting of mRNA of specific genes by complementary oligonucleotide probes labelled with an Auger‐electron‐emitting radioisotope. Decay of the Auger emitter should specifically destroy the targeted mRNA while producing minimal damage to the rest of mRNA pool and the nuclear DNA. The feasibility of this approach was investigated by using full‐length human multidrug‐resistance gene (mdr1) mRNA as a target.

Materials and methods: Antisense oligonucleotides were labelled with [¹²⁵I] I‐dCTP by primer extension and annealed to target mRNA. Breaks in the target mRNA were analysed by denaturing polyacrylamide gel electriphoresis.

Results: The efficiency of ¹²⁵I‐labelled antisense oligonucleotides in producing RNA strand breaks was tested on short synthetic RNA and DNA targets. The position and specificity of ¹²⁵I‐induced breaks in the full‐length mRNA were then tested and compared with the cleavage of the target by RNase H. The distribution of the breaks in the longer mRNA is different from that in the short RNA targets, most likely due to a complex folding of RNA strands in the full‐length mRNA.

Conclusions: The authors posit that ¹²⁵I‐labelled antisense probes could be useful not only for targeting mRNA, but also as probes for mRNA folding in vivo.     

Efficiencies of Induction of DNA Double Strand Breaks in Solution by Photoabsorption at Phosphorus and Platinum

Purpose: This paper aims at determining and comparing the cross sections and quantum yields for DNA strand break induction by the Auger effect at the K‐shell of phosphorus and at the L_III‐shell of platinum.

Materials and methods: Using synchrotron radiation, free and Pt‐bound pBR322 plasmid DNA were irradiated in solution with monochromatic X‐rays, the energies of which were 2.153 and 2.147?keV, corresponding to “on” and “below” the phosphorus K‐shell photoabsorption, and 11.566 and 11.542?keV for “above” and “below” the L_III‐shell photoabsorption of platinum, respectively. To suppress indirect effects by hydroxyl radicals, DMSO (1M) was used as a scavenger.

Results: The inner‐shell photoabsorption of phosphorus and of platinum significantly increased the induction of DNA double strand breaks (DSB), whereas it had little effect on single strand break (SSB) induction. The quantum yields for the induction of DSB were calculated to be 0.017 and 1.13, in the case of phosphorus and platinum, respectively.

Conclusions: The value of the quantum yield for the DSB induction of platinum was about 66‐fold larger than that for the phosphorus. These results clearly demonstrate that the quantum yield of DSB depends upon the magnitude of the Auger cascade.     

Effects of Monoenergetic X-rays with Resonance Energy of Bromine K-absorption Edge on Bromouracil-labelled E. Coli Cells

Summary

In order to examine enhanced killing that might be induced by Auger cascades in the incorporated atoms in cells, bromouracil(BrU)-labelled E. coli cells were irradiated with monoenergetic X-rays at 13·49 and 12·40 keV, just above and below the K-absorption edge of bromine. In both cases BrU-labelled cells were more sensitive for killing than were normal cells. However, when the degree of BrU-sensitization was compared between the two energies of X-rays, the enhanced killing at 13·49 keV was only small, 2 ± 8 per cent based on the D₀ value in saline. By the addition of DMSO, which is believed to suppress radical-mediated effects, killing of BrU-labelled cells was enhanced at 13·49 keV by 8 ± 4 per cent as compared with 12·40 keV, based on D₀. These results have been examined in terms of absorbed energy in BrU-labelled cells and in terms of the number of induced Auger events.