Ratio of complex double strand break damage induced by 125IUdR and 123IUdR correlates with experimental in vitro cell killing effectiveness

Induction of Chromosomal Aberrations after Exposure to the Auger Electron Emitter Iodine-125, the β--emitter Tritium and Cesium-137 γ rays

High-LET-type cell survival curves have been observed in cells that were allowed to incorporate 125I-UdR into their DNA. Incorporation of tritiated thymidine into the DNA of cells has also been shown to result in an increase in relative biological effectiveness in cell survival experiments, but the increase is smaller than observed after incorporation of 125I-UdR. These findings are explained in the literature by the overall complexity of the induced DNA damage resulting from energies of the ejected electron(s) during the decay of 3H and 125I. Chromosomal aberrations (CA) are defined as morphological or structural changes of one or more chromosomes, and can be induced by ionizing radiation. Whether the number of CA is associated with the linear energy transfer (LET) of the radiation and/or the actual complexity of the induced DNA double-strand breaks (DSB) remains elusive. In this study, we investigated whether DNA lesions induced at different cell cycle stages and by different radiation types [Auger-electrons (125I), β- particles (3H), or γ radiation (137Cs)] have an impact on the number of CA induced after induction of the same number of DSB as determined by the γ-H2AX foci assay. Cells were synchronized and pulse-labeled in S phase with low activities of 125I-UdR or tritiated thymidine. For decay accumulation, cells were cryopreserved either after pulse-labeling in S phase or after progression to G2/M or G1 phase. Experiments with γ irradiation (137Cs) were performed with synchronized and cryopreserved cells in S, G2/M or G1 phase. After thawing, a CA assay was performed. All experiments were performed after a similar number of DSB were induced. CA induction after 125I-UdR was incorporated was 2.9-fold and 1.7-fold greater compared to exposure to γ radiation and radiation from incorporated tritiated thymidine, respectively, when measured in G2/M cells. In addition, measurement of CA in G2/M cells after incorporation of 125I-UdR was 2.5-fold greater when compared to cells in G1 phase. In contrast, no differences were observed between the three radiation qualities with respect to exposure after cryopreservation in S or G1 phase. The data indicate that the 3D organization of replicated DNA in G2/M cells seems to be more sensitive to induction of more complex DNA lesions compared to the DNA architecture in S or G1 cells. Whether this is due to the DNA organization itself or differences in DNA repair capability remains unclear.

Comet Assay analysis of DNA strand breaks after exposure to the DNA-incorporated Auger Electron Emitter Iodine-125

PURPOSE

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.

MATERIALS AND METHODS

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.

RESULTS

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.

CONCLUSION

DNA-incorporated Iodine-125 induces a high-LET type DNA damage pattern in respect to SSB/DSB ratio.

Norepinephrine-Transporter-Targeted and DNA-Co-Targeted Theranostic Guanidines

High risk neuroblastoma often recurs, even with aggressive treatments. Clinical evidence suggests that proliferative activities are predictive of poor outcomes. This report describes syntheses, characterization, and biological properties of theranostic guanidines that target norepinephrine transporter and undergo intracellular processing, and subsequently their catabolites are efficiently incorporated into DNA of proliferating neuroblastoma cells. Radioactive guanidines are synthesized from 5-radioiodo-2'-deoxyuridine, a molecular radiotherapy platform with clinically proven minimal toxicities and DNA-targeting properties. The transport of radioactive guanidines into neuroblastoma cells is active as indicated by the competitive suppression of cellular uptake by meta-iodobenzylguanidine. The rate of intracellular processing and DNA uptake is influenced by the agent's catabolic stability and cell population doubling times. The radiotoxicity is directly proportional to DNA uptake and duration of exposure. Biodistribution of 5-[¹²⁵I]iodo-3'-O-(ε-guanidinohexanoyl)-2'-deoxyuridine in a mouse neuroblastoma model shows significant tumor retention of radioactivity. Neuroblastoma xenografts regress in response to the clinically achievable doses of this agent.

Comparative gene expression analysis after exposure to 123I-iododeoxyuridine, γ- and α-radiation-potential biomarkers for the discrimination of radiation qualities

Gene expression analysis was carried out in Jurkat cells in order to identify candidate genes showing significant gene expression alterations allowing robust discrimination of the Auger emitter 123I, incorporated into the DNA as 123I-iododeoxyuridine (123IUdR), from α- and γ-radiation. The γ-H2AX foci assay was used to determine equi-effect doses or activity, and gene expression analysis was carried out at similar levels of foci induction. Comparative gene expression analysis was performed employing whole human genome DNA microarrays. Candidate genes had to show significant expression changes and no altered gene regulation or opposite regulation after exposure to the radiation quality to be compared. The gene expression of all candidate genes was validated by quantitative real-time PCR. The functional categorization of significantly deregulated genes revealed that chromatin organization and apoptosis were generally affected. After exposure to 123IUdR, α-particles and γ-rays, at equi-effect doses/activity, 155, 316 and 982 genes were exclusively regulated, respectively. Applying the stringent requirements for candidate genes, four (PPP1R14C, TNFAIP8L1, DNAJC1 and PRTFDC1), one (KLF10) and one (TNFAIP8L1) gene(s) were identified, respectively allowing reliable discrimination between γ- and 123IUdR exposure, γ- and α-radiation, and α- and 123IUdR exposure, respectively. The Auger emitter 123I induced specific gene expression patterns in Jurkat cells when compared with γ- and α-irradiation, suggesting a unique cellular response after 123IUdR exposure. Gene expression analysis might be an effective tool for identifying biomarkers for discriminating different radiation qualities and, furthermore, might help to explain the varying biological effectiveness at the mechanistic level.

99mTc-labeled HYNIC-DAPI causes plasmid DNA damage with high efficiency

^99mTc is the standard radionuclide used for nuclear medicine imaging. In addition to gamma irradiation, ^99mTc emits low-energy Auger and conversion electrons that deposit their energy within nanometers of the decay site. To study the potential for DNA damage, direct DNA binding is required. Plasmid DNA enables the investigation of the unprotected interactions between molecules and DNA that result in single-strand breaks (SSBs) or double-strand breaks (DSBs); the resulting DNA fragments can be separated by gel electrophoresis and quantified by fluorescent staining. This study aimed to compare the plasmid DNA damage potential of a ^99mTc-labeled HYNIC-DAPI compound with that of ^99mTc pertechnetate (^99mTcO₄⁻). pUC19 plasmid DNA was irradiated for 2 or 24 hours. Direct and radical-induced DNA damage were evaluated in the presence or absence of the radical scavenger DMSO. For both compounds, an increase in applied activity enhanced plasmid DNA damage, which was evidenced by an increase in the open circular and linear DNA fractions and a reduction in the supercoiled DNA fraction. The number of SSBs elicited by ^99mTc-HYNIC-DAPI (1.03) was twice that caused by ^99mTcO₄⁻ (0.51), and the number of DSBs increased fivefold in the ^99mTc-HYNIC-DAPI-treated sample compared with the ^99mTcO₄⁻ treated sample (0.02 to 0.10). In the presence of DMSO, the numbers of SSBs and DSBs decreased to 0.03 and 0.00, respectively, in the ^99mTcO₄^– treated samples, whereas the numbers of SSBs and DSBs were slightly reduced to 0.95 and 0.06, respectively, in the ^99mTc-HYNIC-DAPI-treated samples. These results indicated that ^99mTc-HYNIC-DAPI induced SSBs and DSBs via a direct interaction of the ^99mTc-labeled compound with DNA. In contrast to these results, ^99mTcO₄⁻ induced SSBs via radical formation, and DSBs were formed by two nearby SSBs. The biological effectiveness of ^99mTc-HYNIC-DAPI increased by approximately 4-fold in terms of inducing SSBs and by approximately 10-fold in terms of inducing DSBs.

Doses and risks from uranium are not increased significantly by interactions with natural background photon radiation

The impact of depleted uranium (DU) on human health has been the subject of much conjecture. Both the chemical and radiological aspects of its behaviour in the human body have previously been investigated in detail, with the radiological impact being assumed to be linked to the alpha decay of uranium. More recently, it has been proposed that the accumulation in tissue of high-Z materials, such as DU, may give rise to enhanced local energy deposition in the presence of natural background photon radiation due to the high photoelectric interaction cross sections of high-Z atoms. It is speculated that, in addition to producing short-range photoelectrons, these events will be followed by intense Auger and Coster-Kronig electron emission, thereby causing levels of cell damage that are unaccounted for in conventional models of radiological risk. In this study, the physical and biological bases of these claims are investigated. The potential magnitudes of any effect are evaluated and discussed, and compared with the risks from other radiological or chemical hazards. Monte Carlo calculations are performed to estimate likely energy depositions due to the presence of uranium in human tissues in photon fields: whole body doses, organ doses in anthropomorphic phantoms and nano-/micro-dosimetric scenarios are each considered. The proposal is shown generally to be based on sound physics, but overall the impact on human health is expected to be negligible.

Radiolabeled cyclosaligenyl monophosphates of 5-iodo-2'-deoxyuridine, 5-iodo-3'-fluoro-2',3'-dideoxyuridine, and 3'-fluorothymidine for molecular radiotherapy of cancer: synthesis and biological evaluation

Targeted molecular radiotherapy opens unprecedented opportunities to eradicate cancer cells with minimal irradiation of normal tissues. Described in this study are radioactive cyclosaligenyl monophosphates designed to deliver lethal doses of radiation to cancer cells. These compounds can be radiolabeled with SPECT- and PET-compatible radionuclides as well as radionuclides suitable for Auger electron therapies. This characteristic provides an avenue for the personalized and comprehensive treatment strategy that comprises diagnostic imaging to identify sites of disease, followed by the targeted molecular radiotherapy based on the imaging results. The developed radiosynthetic methods produce no-carrier-added products with high radiochemical yield and purity. The interaction of these compounds with their target, butyrylcholinesterase, depends on the stereochemistry around the P atom. IC(50) values are in the nanomolar range. In vitro studies indicate that radiation doses delivered to the cell nucleus are sufficient to kill cells of several difficult to treat malignancies including glioblastoma and ovarian and colorectal cancers.

The large-scale digital cell analysis system: an open system for nonperturbing live cell imaging

The Large-Scale Digital Cell Analysis System (LSDCAS) was designed to provide a highly extensible open source live cell imaging system. Analysis of cell growth data has demonstrated a lack of perturbation in cells imaged using LSDCAS, through reference to cell growth data from cells growing in CO(2) incubators. LSDCAS consists of data acquisition, data management and data analysis software, and is currently a Core research facility at the Holden Comprehensive Cancer Center at the University of Iowa. Using LSDCAS analysis software, this report and others show that although phase-contrast imaging has no apparent effect on cell growth kinetics and viability, fluorescent image acquisition in the cell lines tested caused a measurable level of growth perturbation using LSDCAS. This report describes the current design of the system, reasons for the implemented design, and details its basic functionality. The LSDCAS software runs on the GNU/Linux operating system, and provides easy to use, graphical programs for data acquisition and quantitative analysis of cells imaged with phase-contrast or fluorescence microscopy (alone or in combination), and complete source code is freely available under the terms of the GNU Public Software License at the project website (http://lsdcas.engineering.uiowa.edu).

Analysis and simulation of the relative lethality of Auger-electron-emitting radionuclides with a liquid-scintillation counter

PURPOSE

The efficiency of strand-break induction and the counting efficiency of a liquid-scintillation counter can both be described similarly in terms of Poisson statistics. The aim of this work is to relate these two concepts, developing a simple method to simulate with a liquid-scintillation counter the relative biological effects between two different electron-emitting radionuclides.

METHODS

A gel scintillator can be used to confine the decaying nuclei into nanoscale structures of liquid water (micelles). Because the fluorescing agents of the gel lay outside the micelle structure, the low-energy electrons emitted by the decaying nucleus lose part of their energy within the micelle structure before being detected, resulting in a negative increment of the counting efficiency. The difference in the counting efficiency between two gels with micelles of different characteristic sizes is applied to simulate the relative lethality of the radionuclides.

RESULTS

The results are only qualitatively successful. A better accuracy cannot be achieved for commercial liquid-scintillation spectrometers, which have two photomultiplier tubes of identical gain. Also the comparison cannot be extended to low-Z Auger-electron-emitting radionuclides such as (55)Fe, since the micelle size effect is significantly increased by the interference of the L-Auger electrons.

CONCLUSIONS

A liquid-scintillation counter with a gain decreased by a factor of 2.5 in one of the two photomultiplier tubes would be necessary to improve the simulation of the damaging efficiency.

DNA Breakage by Decay of Auger Electron Emitters: Experiments with123I-iodoHoechst 33258 and Plasmid DNA

The Auger electron-emitting isotope 123I is of interest in the context of potential exploitation of Auger electron emitters in radioimmunotherapy. The efficiency of induction of cytotoxic lesions by decay of DNA-associated 125I, the prototype Auger electron emitter, is well established, but its long half-life (60 days) is a limitation. However, the advantage of the much shorter half-life of 123I (13.2 h) might be outweighed by its "weaker" Auger electron cascade with an average of 8-11 Auger electrons, compared to about 15-21 electrons for 125I. Accordingly, the efficiency of DNA breakage for DNA-associated 123I was investigated by incubation of 123I-iodoHoechst 33258 with plasmid DNA. The efficiency of double-strand break induction by decay of 123I was 0.62 compared to 0.82 per decay of 125I in the same experimental system. In the presence of dimethylsulfoxide, the values were 0.54 and 0.65 for decay of 123I and 125I, respectively. The results also showed that at a very low ligand/plasmid molar ratio (<1), the majority of cleavage seemed to occur at a particular site on the plasmid molecule, indicating preferential binding of the 123I-ligand to a unique site or a cluster of neighboring sites.