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Auger emitters exhibit interesting features due to their emission of a cascade of short-range Auger electrons. Maximum DNA breakage efficacy is achieved when decays occur near DNA. Studies of double-strand breaks (DSBs) yields in plasmids revealed cutoff distances from DNA axis of 10.5 Å–12 Å, beyond which the mechanism of DSBs moves from direct to indirect effects, and the yield decreases rapidly. Some authors suggested that the average energy deposited in a DNA cylinder could explain such cutoffs. We aimed to study this hypothesis in further detail. Using the Monte Carlo code CELLDOSE, we investigated the influence of the 125I atom position on energy deposits and absorbed doses per decay not only in a DNA cylinder, but also in individual strands, each modeled as 10 spheres encompassing the fragility sites for phosphodiester bond cleavage. The dose per decay decreased much more rapidly for a sphere in the proximal strand than for the DNA cylinder. For example, when moving the 125I source from 10.5 Å to 11.5 Å, the average dose to the sphere dropped by 43%, compared to only 13% in the case of the cylinder. Explaining variations in DSBs yields with 125I position should consider the probability of inducing damage in the proximal strand (nearest to the 125I atom). The energy received by fragility sites in this strand is highly influenced by the isotropic (4π) emission of 125I low-energy Auger electrons. The positioning of Auger emitters for targeted radionuclide therapy can be envisioned accordingly. [ABSTRACT FROM AUTHOR]

Ionizing radiation causes various types of DNA damage e.g. single strand breaks (SSB) and double strand breaks (DSB), whereby the SSB/DSB ratio is shifted toward the DSB with increasing LET. For the DNA-incorporated Auger electron emitter Iodine-125 a SSB/DSB ratio of 5.4:1 is calculated based on computer simulations. In the presented work the SSB/DSB ratio of DNA-incorporated Iodine-125 was experimentally determined and compared to external homogenous γ-irradiation. Iodine-125-iododeoxyuridine (I-125-UdR) was incorporated into the DNA of SCL-II cells and cells were subsequently frozen for decay accumulation. Accordingly, external γ-irradiation (Cs-137) experiments were performed in frozen cells. After exposure the neutral or alkaline Comet Assay was performed to quantify DSB or DSB and SSB, respectively. Automated quantification of the comets was performed using the Olive Tail Moment (Metafer CometScan; MetaSystems). Calculation of absorbed dose for Auger electrons on cellular level is extremely biased due to the exclusive DNA localization of I-125-UdR. To avoid dose calculation the γ-H2AX assay was used in order to allow the comparison of the Comet Assay data between both investigated radiation qualities. For low-LET γ-radiation, a SSB/DSB ratio of 10:1 was determined. In contrast, a lower SSB/DSB ratio of 6:1 was induced by DNA-incorporated Iodine-125 which compares very well to the calculated values of Pomplun and co-authors DNA-incorporated Iodine-125 induces a high-LET type DNA damage pattern in respect to SSB/DSB ratio. [ABSTRACT FROM AUTHOR]

Plaque brachytherapy has become the most common eye and vision sparing treatment for uveal melanoma. This review explores methods to maximize vision retention, local tumor destruction and metastasis-free survival. Revealed are how medical physics and radiation therapy techniques have been used to improve outcomes and minimize risk. Attention was given to plaque selection based on intraocular radiation distribution, tumor size and intraocular location. Pre-treatment dosimetry-based plaque selection (e.g. palladium-103, ruthenium-106, iodine-125, strontium-90) offers an effective way to maximize vision and improve local control. Historical comparisons show how plaque size and isotope selection affect intraocular radiation distribution. Contrasts are made with respect to radiation plaque-source selection. Ocular anatomy-driven management strategies are reviewed for iris, ciliary body, choroid and peripapillary uveal tumors. Methods are described to optimize surgical plaque implantation. Finger's slotted plaque design is shown to overcome epibulbar obstructions; thereby normalizing episcleral plaque position. Expert opinions include how knowledge of normal tissue tolerances, customized plaque construction and standardized outcome measures can be used to improve local control and vision outcomes. SORRT, an ongoing multicenter, international radiation side-effects registry will be mined to compare and contrast radiation sources, improve our knowledge about plaque selection and patient outcomes. [ABSTRACT FROM AUTHOR]

Purpose: The efficacy of DNA-targeting radionuclide therapies might be strongly enhanced by employing short range particle-emitters. However, the gain of effectiveness is not yet well substantiated. We compared the Auger electron emitter I-125 to the ß−-emitter P-32 in terms of biological effectiveness per decay and radiation dose when located in the close proximity to DNA using DNA Triplex-forming oligonucleotides (TFO). The clonogenicity and the induction of DNA double-strand breaks (DSB) were investigated in SCL-II cells after exposure to P-32- or I-125-labeled TFO targeting the glyceraldehyde 3-phosphate dehydrogenase (GAPDH) gene and after external homogeneous exposure to gamma-rays as reference radiation. Materials and methods: TFO were labeled with P-32 or I-125 using the primer extension method. Cell survival was analyzed by colony-forming assay and DNA damage was assessed by microscopic quantification of protein 53 binding protein 1 (53BP1) foci in SCL-II cells. Results: I-125-TFO induced a pronounced decrease of cell survival (D37 at ∼360 accumulated decays per cell, equivalent to 1.22 Gy cell nucleus dose) and a significant increase of 53BP1 foci with increasing decays. The P-32-labeled TFO induced neither a strong decrease of cell survival nor an increase of 53BP1 foci up to ∼4000 accumulated decays per cell, equivalent to ∼1 Gy cell nucleus dose. The RBE for I-125-TFO was in the range of 3–4 for both biological endpoints. Conclusions: I-125-TFO proved to be much more radiotoxic than P-32-TFO per decay and per unit dose although targeting the same sequence in the GAPDH gene. This might be well explained by the high number of low energy Auger electrons emitted by I-125 per decay, leading to a high ionization density in the immediate vicinity of the decay site, probably producing highly complex DNA lesions overcharging DNA repair mechanisms. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the ability of human lymphocytes labeled with DNA-incorporated 125I to exert an inhibitory (antiproliferative) bystander effect on co-cultured human colon adenocarcinoma LS174T cells in vitro. Materials and methods: Human peripheral blood lymphocytes were stimulated to synthesize DNA in the presence of phytohemagglutinin (PHA) and labeled with 5-[125I]iodo-2′-deoxyuridine. Human colon adenocarcinoma LS174T cells were co-cultured with the 125I-labeled lymphocytes in various ratios for 5 days and the proliferation of the LS174T cells was assessed. Further, the supernatant media from these co-cultures were: (i) Transferred to LS174T cells and their proliferation measured after 5 days, (ii) used to assess the clonogenic survival of LS174T cells, and (iii) screened for factors that suppress growth. Results: A significant reduction in the proliferation of LS174T cells was observed when co-cultured either with 125I-labeled lymphocytes (56 ± 3.5%) or the supernatant media (52.5 ± 1.3%) obtained from these co-cultures. Clonogenic survival of LS174T cells grown in the supernatant media corroborated the decrease in tumor cell growth. Conclusion: The observed reduction in the proliferation of LS174T cells in presence of 125I-labeled lymphocytes or media obtained from such co-cultures can be attributed to an inhibitory (antiproliferative) bystander effect, probably mediated by factor(s) released from the dying 125I-labeled lymphocytes. [ABSTRACT FROM AUTHOR]

Purpose: The frequency of DNA strand breaks produced by the decay of Auger electron-emitting radionuclides is inversely proportional to the distance of DNA nucleotides from the decay site; and thus is very sensitive to changes in the local conformation of the DNA. Analysis of the frequency of DNA breaks, or radioprobing, gives valuable information about the local DNA structure. More than 10 years ago, we demonstrated the feasibility of radioprobing using a DNA-repressor complex with a known structure. Herein, we used radioprobing to study the conformation of DNA in complex with the tumor suppressor protein 53 (p53). Several structures of p53-DNA complexes have been solved by X-ray crystallography. These structures, obtained with the p53 DNA binding domain, a truncated form, laid the groundwork for understanding p53-DNA interactions and their relation to p53 functions. However, whether all observed stereochemical details are relevant to the native p53-DNA complex remains unclear. A common theme of the crystallographic structures is the lack of significant bending in the central part of the DNA response element. In contrast, gel electrophoresis and electron microscopy data showed strong DNA bending and overtwisting upon binding to the native p53 tetramer. Methods: To analyze DNA in complex with p53, we incorporated 125I-dCTP in two different positions of synthetic duplexes containing the consensus p53-binding site. Results: The most significant changes in the break frequency distributions were detected close to the center of the binding site, which is consistent with an increase in DNA twisting in this region and local DNA bending and sliding. Conclusions: Our data confirm the main results of the studies made in solution and lay a foundation for systematic examination of interactions between DNA and native p53 using 125I radioprobing. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Purpose: To put radioprobing into context as a relatively new method of determining structural detail in deoxyribonucleic acid (DNA), and to review its use since first proposed in 1997. The key feature of the method is that, by experiment or simulation, a radionuclide such as iodine-125 (125I) is placed near the DNA at a known point relative to the DNA base sequence, and the number of resulting strand breaks in each nucleotide is determined. As the intensity of damage declines consistently with distance from the radionuclide, relative distances between the emitter and the nucleotides can be deduced, and hence potentially the topology or structural detail of the DNA. For simulation, appropriate software includes a Molecular Dynamics package, analysis and visualization tools, and a Monte Carlo track structure program. Conclusions: A review of published work and our own recent unpublished studies have shown that radioprobing is sufficiently sensitive and consistent to determine structural detail such as internal folding topology and flexing behavior, and can be applied to DNA or a DNA-protein complex in an approximation to its normal biological environment. [ABSTRACT FROM AUTHOR]

Purpose This study investigated the ultrastructural changes in extraocular muscles under which radioactive plaques had been placed for the treatment of uveal melanoma. Methods At the time of plaque removal, biopsies were taken from four horizontal recti that had been left in situ over plaques and from one lateral rectus muscle that had been disinserted before brachytherapy. Normal lateral recti from enucleated eyes were used as controls. Iodine-125 seeds were used with a mean total activity of 54.04 mCi, remaining for an average of 149.62 hours over the sclera. Results Muscles that had been left in situ over the radioactive plaques demonstrated a focal decrease in muscular tissue and increased fibroblasts and collagen. Electron microscopy showed increased collagen, loss of sarcoplasmic reticulum and swollen mitochondria. The disinserted muscle in the plaque group appeared unaffected. Conclusions Despite the theoretical shielding properties of plaques, leaving an extraocular muscle over the plaque may lead to several non-specific ultrastructural changes. [ABSTRACT FROM AUTHOR]

Purpose: One important obstacle in modeling the biological damage of 125 I lies in the collective effects of the emitted electrons. The huge number of possible different ways of atomic rearrangement causes a strong variability in the number and energies of the emitted electrons. The aim of this work is to tabulate, separately for each atomic rearrangement pathway, the absorbed doses and the single-strand breaks (SSB) probabilities for different size nanospheres of liquid water. Methods: A simple deterministic (non-stochastic) atomic rearrangement model of only 265 pathways is appropriate for modeling the electron-capture and internal conversion processes in 125 I. The feasibility of the atomic rearrangement model is verified by comparing the SSB yield for ultrasoft X-ray sources with Lobachevsky's experimental data. Results: A linear equation is sufficient for describing the SSB probability as a function of the dose absorbed in a small volume surrounding the labelled nucleotide 125 I-dC. Conclusions: The application of a deterministic atomic rearrangement model to Auger electron emission spectra gives a new perspective to the study of low linear energy transfer (LET) radiation effects on DNA. The damages induced by point electron-capture (EC) sources and by macroscopic X-ray sources are compared. [ABSTRACT FROM AUTHOR]