Auger electron-induced double-strand breaks depend on DNA topology

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.

Auger electrons and DNA double-strand breaks studied by using iodine-containing chemicals

Irradiation of high Z elements such as iodine, gold, gadolinium with monochromatic X-rays causes photoelectric effects that include the release of Auger electrons. Decay of radioactive iodine such as I-123 and I-125 also results in multiple events and some involve the generation of Auger electrons. These electrons have low energy and travel only a short distance but have a strong effect on DNA damage including the generation of double-strand breaks. In this chapter, we focus on iodine and discuss various studies that used iodine-containing chemicals to generate Auger electrons and cause DNA double-strand breaks. First, DNA synthesis precursors containing iodine were used to place iodine on DNA. DNA binding dyes such as iodine Hoechst were investigated for Auger electron generation and DNA breaks. More recently, iodine containing nanoparticles were developed. We describe our study using tumor spheroids loaded with iodine nanoparticles and synchrotron-generated monochromatic X-rays. This study led to the demonstration that an optimum effect on DNA double-strand break formation is observed with a 33.2keV X-ray which is just above the K-edge energy of iodine.

Cisplatin-Resistant CD44+ Lung Cancer Cells Are Sensitive to Auger Electrons

Cancer stem cells (CSCs) are resistant to conventional therapy and present a major clinical challenge since they are responsible for the relapse of many cancers, including non-small cell lung cancer (NSCLC). Hence, future successful therapy should also eradicate CSCs. Auger electrons have demonstrated promising therapeutic potential and can induce DNA damage while sparing surrounding cells. Here, we sort primary patient-derived NSCLC cells based on their expression of the CSC-marker CD44 and investigate the effects of cisplatin and a thymidine analog (deoxyuridine) labeled with an Auger electron emitter (125I). We show that the CD44+ populations are more resistant to cisplatin than the CD44− populations. Interestingly, incubation with the thymidine analog 5-[125I]iodo-2′-deoxyuridine ([125I]I-UdR) induces equal DNA damage, G2/M cell cycle arrest, and apoptosis in the CD44− and CD44+ populations. Our results suggest that Auger electron emitters can also eradicate resistant lung cancer CD44+ populations.

Modular Nanotransporters for Nuclear-Targeted Delivery of Auger Electron Emitters

This review describes artificial modular nanotransporters (MNTs) delivering their cargos into target cells and then into the nuclei – the most vulnerable cell compartment for most anticancer agents and especially for radionuclides emitting short-range particles. The MNT strategy uses natural subcellular transport processes inherent in practically all cells including cancer cells. The MNTs use these processes just as a passenger who purchased tickets for a multiple-transfer trip making use of different kinds of public transport to reach the desired destination. The MNTs are fusion polypeptides consisting of several parts, replaceable modules, accomplishing binding to a specific receptor on the cell and subsequent internalization, endosomal escape and transport into the cell nucleus. Radionuclides emitting short-range particles, like Auger electron emitters, acquire cell specificity and significantly higher cytotoxicity both in vitro and in vivo when delivered by the MNTs into the nuclei of cancer cells. MNT modules are interchangeable, allowing replacement of receptor recognition modules, which permits their use for different types of cancer cells and, as a cocktail of several MNTs, for targeting several tumor-specific molecules for personalized medicine.

Sodium-iodide symporter positive cells after intracellular uptake of 99mTc versus α-emitter 211At

Purpose: We evaluated the DNA damaging potential of Auger electrons emitted in the decay of 99mTc compared to α-particles of 211At. Material and methods: The impact of 99mTc and 211At was monitored in a NIS-expressing rat thyroid cell model PC Cl3 with varying, yet defined intra- and extracellular radionuclide distribution (using ± perchlorate). The radiotoxicity of 99mTc and 211At was studied by the comet assay under neutral and alkaline conditions and colony formation. Results: In the presence of perchlorate, the radioactivity yielding 37 % cellular survival, A37, was estimated to be (0.27 ± 0.02) MBq/ml and (450 ± 30) MBq/ml for 211At and 99mTc, respectively. In absence of perchlorate, cellular radiotracer uptake was similar for both radionuclides (2.2 %, 2.7 %), yet the A37 was reduced by 82% for the α-emitter and by 95 % for 99mTc. Cellular dose increased by a factor of 5 (211At) and 38 (99mTc). Comet assays revealed an increased DNA damage after intracellular uptake of both radiotracers. Conclusions: The data indicate damage to the cell to occur from absorbed dose without recognizable contribution from intracellular heterogeneity of radionuclide distribution. Comet assay under alkaline and neutral conditions did not reveal any shift to more complex DNA damage after radionuclide uptake. Cellular uptake of 99mTc and 211At increased cellular dose and reduced clonogenic survival.

Gold nanoparticles as a potent radiosensitizer in neutron therapy

The purpose of this study was to investigate the potential of gold nanoparticles as radiosensitizer for use in neutron therapy against hepatocellular carcinoma.

The hepatocellular carcinoma cells lines Huh7 and HepG2 were irradiated with γ and neutron radiation in the presence or absence of gold nanoparticles. Effects were evaluated by transmission electron microscopy, cell survival, cell cycle, DNA damage, migration, and invasiveness.

Gold nanoparticles significantly enhanced the radiosensitivity of Huh7 and HepG2 cells to γ-rays by 1.41- and 1.16-fold, respectively, and by 1.80- and 1.35-fold to neutron radiation, which has high linear energy transfer. Accordingly, exposure to neutron radiation in the presence of gold nanoparticles induced cell cycle arrest, DNA damage, and cell death to a significantly higher extent, and suppressed cell migration and invasiveness more robustly. These effects are presumably due to the ability of gold nanoparticles to amplify the effective dose from neutron radiation more efficiently.

The data suggest that gold nanoparticles may be clinically useful in combination therapy against hepatocellular carcinoma by enhancing the toxicity of radiation with high linear energy transfer.

A Multifunctional Radiotheranostic Agent for Dual Targeting of Breast Cancer Cells

A straightforward synthetic route for a new multifunctional 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) derivative is described. To demonstrate the versatility of this pro-chelator for the preparation of radiolabeled hybrid compounds containing two different biological targeting moieties, an antitumor agent (e.g., a DNA-intercalating agent) and an estrogen receptor (ER) ligand (e.g., LXXLL-based peptide) were regiospecifically conjugated to the DOTA derivative. The bifunctional probe was radiolabeled with the auger electron emitter indium-111, and the resulting radioconjugate was demonstrated to induce DNA damage in vitro, which, along with the nuclear internalization exhibited in breast cancer cells, might enhance its therapeutic activity. This favorable in vitro performance suggests that these hybrid compounds could be attractive probes for theranostic applications.

Re-assessing gallium-67 as a therapeutic radionuclide

INTRODUCTION

Despite its desirable half-life and low energy Auger electrons that travel further than for other radionuclides, 67Ga has been neglected as a therapeutic radionuclide. Here, 67Ga is compared with Auger electron emitter 111In as a potential therapeutic radionuclide.

METHODS

Plasmid pBR322 studies allowed direct comparison between 67Ga and 111In (1MBq) in causing DNA damage, including the effect of chelators (EDTA and DTPA) and the effects of a free radical scavenger (DMSO). The cytotoxicity of internalized (by means of delivery in the form of oxine complexes) and non-internalized 67Ga and 111In was measured in DU145 prostate cancer cells after a one-hour incubation using cell viability (trypan blue) and clonogenic studies. MDA-MB-231 and HCC1954 cells were also used.

RESULTS

Plasmid DNA damage was caused by 67Ga and was comparable to that caused by 111In; it was reduced in the presence of EDTA, DTPA and DMSO. The A50 values (internalized activity of oxine complexes per cell required to kill 50% of cells) as determined by trypan blue staining was 1.0Bq/cell for both 67Ga and 111In; the A50 values determined by clonogenic assay were 0.7Bq/cell and 0.3Bq/cell for 111In and 67Ga respectively. At the concentrations required to achieve these uptake levels, non-internalized 67Ga and 111In caused no cellular toxicity. Qualitatively similar results were found for MDA-MB-231 and HCC1954 cells.

CONCLUSION

67Ga causes as much damage as 111In to plasmid DNA in solution and shows similar toxicity as 111In at equivalent internalized activity per cell. 67Ga therefore deserves further evaluation for radionuclide therapy.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The data presented here is at the basic level of science. If future in vivo and clinical studies are successful, 67Ga could become a useful radionuclide with little healthy tissue toxicity in the arsenal of weapons for treating cancer.

Neutron activation of In(iii) complexes with thiosemicarbazones leads to the production of potential radiopharmaceuticals for the treatment of breast cancer

^114mIn(iii) complexes with 2-acetylpyridine-derived thiosemicarbazones show potent cytotoxic activity.

Direct and Auger Electron-Induced, Single- and Double-Strand Breaks on Plasmid DNA Caused by 99mTc-Labeled Pyrene Derivatives and the Effect of Bonding Distance

It is evident that ^99mTc causes radical-mediated DNA damage due to Auger electrons, which were emitted simultaneously with the known γ-emission of ^99mTc. We have synthesized a series of new ^99mTc-labeled pyrene derivatives with varied distances between the pyrene moiety and the radionuclide. The pyrene motif is a common DNA intercalator and allowed us to test the influence of the radionuclide distance on damages of the DNA helix. In general, pUC 19 plasmid DNA enables the investigation of the unprotected interactions between the radiotracers and DNA that results in single-strand breaks (SSB) or double-strand breaks (DSB). The resulting DNA fragments were separated by gel electrophoresis and quantified by fluorescent staining. Direct DNA damage and radical-induced indirect DNA damage by radiolysis products of water were evaluated in the presence or absence of the radical scavenger DMSO. We demonstrated that Auger electrons directly induced both SSB and DSB in high efficiency when ^99mTc was tightly bound to the plasmid DNA and this damage could not be completely prevented by DMSO, a free radical scavenger. For the first time, we were able to minimize this effect by increasing the carbon chain lengths between the pyrene moiety and the ^99mTc nuclide. However, a critical distance between the ^99mTc atom and the DNA helix could not be determined due to the significantly lowered DSB generation resulting from the interaction which is dependent on the type of the ^99mTc binding motif. The effect of variable DNA damage caused by the different chain length between the pyrene residue and the Tc-core as well as the possible conformations of the applied Tc-complexes was supplemented with molecular dynamics (MD) calculations. The effectiveness of the DNA-binding ^99mTc-labeled pyrene derivatives was demonstrated by comparison to non-DNA-binding ^99mTcO₄^–, since nearly all DNA damage caused by ^99mTcO₄^– was prevented by incubating with DMSO.

⁹⁹mTc pyrene derivative complex causes double-strand breaks in dsDNA mainly through cluster-mediated indirect effect in aqueous solution

Radiation therapy for cancer patients works by ionizing damage to nuclear DNA, primarily by creating double-strand breaks (DSB). A major shortcoming of traditional radiation therapy is the set of side effect associated with its long-range interaction with nearby tissues. Low-energy Auger electrons have the advantage of an extremely short effective range, minimizing damage to healthy tissue. Consequently, the isotope ^99mTc, an Auger electron source, is currently being studied for its beneficial potential in cancer treatment. We examined the dose effect of a pyrene derivative ^99mTc complex on plasmid DNA by using gel electrophoresis in both aqueous and methanol solutions. In aqueous solutions, the average yield per decay for double-strand breaks is 0.011±0.005 at low dose range, decreasing to 0.0005±0.0003 in the presence of 1 M dimethyl sulfoxide (DMSO). The apparent yield per decay for single-strand breaks (SSB) is 0.04±0.02, decreasing to approximately a fifth with 1 M DMSO. In methanol, the average yield per decay of DSB is 0.54±0.06 and drops to undetectable levels in 2 M DMSO. The SSB yield per decay is 7.2±0.2, changing to 0.4±0.2 in the presence of 2 M DMSO. The 95% decrease in the yield of DSB in DMSO indicates that the main mechanism for DSB formation is through indirect effect, possibly by cooperative binding or clustering of intercalators. In the presence of non-radioactive ligands at a near saturation concentration, where radioactive Tc compounds do not form large clusters, the yield of SSB stays the same while the yield of DSB decreases to the value in DMSO. DSBs generated by ^99mTc conjugated to intercalators are primarily caused by indirect effects through clustering.

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.

Effect of distance between decaying (125)I and DNA on Auger-electron induced double-strand break yield

PURPOSE

To determine the possible effects of (125)I-to-DNA distance on the magnitude and mechanism of Auger-electron induced-double-strand break (DSB) production.

MATERIALS AND METHODS

We have synthesized a series of (125)I-labeled Hoechst (H) derivatives ((125)IE-H, (125)IB-H, (125)I-C(8)-H and (125)I-C(12)-H). While all four molecules share a common DNA minor groove binding bis-benzimidazole motif, they are designed to position (125)I at varying distances from the DNA helix. Each Hoechst derivative was incubated at 4°C in phosphate buffered saline (PBS) together with supercoiled (SC) (3)H-pUC19 plasmid DNA (ratio 3:1) ± the •OH scavenger dimethyl sulfoxide (DMSO) (0.2 M). Aliquots were analyzed on agarose gels over time and DSB yields per decay of (125)I atom were determined. Docking of the iodinated compounds on a DNA molecule was carried out to determine the distance between the iodine atom and the central axis of DNA.

RESULTS

In the absence of DMSO, the results show that the DSB yields decrease monotonically as the (125)I atom is distanced - by 10.5 Å to 13.9 Å - from the DNA helix ((125)IEH: 0.52 ± 0.01; (125)IB-H: 0.24 ± 0.03; (125)I-C(8)-H: 0.18 ± 0.02; (125)I-C(12)-H: 0.10 ± 0.00). In the presence of DMSO, DSB yields for (125)IEH (0.49 ± 0.02) and (125)IB-H (0.26 ± 0.04) remain largely unchanged indicating that DSB are entirely produced by direct effects. Strikingly, (125)I-C(8)-H or (125)I-C(12)-H, did not produce detectable DSB in the presence of DMSO under similar conditions suggesting when (125)I atom is positioned > 12 Å from the DNA, DSB are entirely produced by indirect effects.

CONCLUSION

These results suggest that at a critical distance between the (125)I atom and the DNA helix, DSB production switches from an 'all' direct to an 'all' indirect mechanism, the latter situation being comparable to the decay of (125)I free in solution. These experimental findings were correlated with theoretical expectations based on microdosimetry.

Molecular and cellular radiobiological effects of Auger emitting radionuclides

Although the general radiobiologic principles underlying external beam therapy and radionuclide therapy are similar, significant differences in the biophysical and radiobiologic effects from the two types of radiation continue to accumulate. Here, I will address the unique features that distinguish the molecular and cellular radiobiological effects of Auger electron-emitting radionuclides consequent to (1) the physical characteristics of the decaying atom and its subcellular localisation, (2) DNA topology and (3) the bystander effect. Based on these experimental findings, I postulate that the ability of track structural simulations as primary tools in modelling DNA damage and cellular survival at the molecular level would be greatly enhanced when these contributions are factored in.

Analysis of the contribution of charge transport in iodine-125-induced DNA damage

Auger electron emitters like (125)I are the radionuclides of choice for gene-targeted radiotherapy. The highly localized damage they induce in DNA is produced by three mechanisms: direct damage by the emitted Auger electrons, indirect damage by diffusible free radicals produced by Auger electrons traveling in water, and charge neutralization of the residual, highly positively charged tellurium daughter atom by stripping electrons from covalent bonds of neighboring residues. The purpose of our work was to determine whether these mechanisms proceed through an intermediate energy transfer step along DNA. It was proposed that this intermediate step proceeds through the charge transport mechanism in DNA. Conventional charge transport has been described as either a hopping mechanism initiated by charge injection into DNA and propagated by charge migration along the DNA or a tunneling mechanism in which charge moves directly from a donor to an acceptor within DNA. Well-known barriers for the hopping mechanism were used to probe the role of charge transport in (125)I-induced DNA damage. We studied their effect on the distribution of DNA breaks produced by the decay of (125)I in samples frozen at -80 degrees C. We found that these barriers had no measurable effect on the distribution of (125)I-induced breaks.

DNA double-strand breaks induced by decay of (123)I-labeled Hoechst 33342: role of DNA topology

PURPOSE

To determine double-strand-break (DSB) yields produced by decay of minor-groove-bound (123)I-labeled Hoechst 33342 ((123)IEH) in supercoiled (SC) and linear (L) forms of pUC19 DNA, to compare strand-break efficiency of (123)IEH with that of (125)IEH, and to examine the role of DNA topology in DSB induction by these Auger electron emitters.

MATERIALS AND METHODS

Tritium-labeled SC and L pUC19 DNA were incubated with (123)IEH (0-10.9 MBq) at 4 degrees C. After (123)I had completely decayed (10 days), samples were analyzed on agarose gel, and single-strand-break (SSB) and DSB yields were measured.

RESULTS

Each (123)I decay in SC DNA produces a DSB yield of 0.18 +/- 0.01. On the basis of DSB yields for (125)IEH (0.52 +/- 0.02 for SC and 1.62 +/- 0.07 for L, reported previously) and dosimetric expectations, a DSB yield of approximately 0.5 (3 x 0.18) per (123)I decay is expected for L DNA. However, no DSB are observed for the L form, even after approximately 2 x 10(11) decays of (123)I per microg DNA, whereas a similar number of (125)I decays produces DSB in approximately 40% of L DNA.

CONCLUSION

(123)IEH-induced DSB yield for SC but not L DNA is consistent with the dosimetric expectations for Auger electron emitters. These studies highlight the role of DNA topology in DSB production by Auger emitters and underscore the failure of current theoretical dosimetric methods per se to predict the magnitude of DSB.

Method to eliminate linear DNA from mixture containing nicked circular, supercoiled, and linear plasmid DNA

Preparations of circular plasmid DNA in either supercoiled or nicked circular form often are contaminated with undesired linear DNA fragments arising from shearing/degradation of chromosomal DNA or linearization of plasmid DNA itself. We report a simple enzymatic method, using a combination of lambda exonuclease and RecJ(f), for the selective removal of linear DNA from such mixtures. lambda exonuclease digests one strand of linear duplex DNA in the 5' to 3' direction, whereas RecJ(f), a single-strand-specific exonuclease, digests the remaining complementary single strand into mononucleotides. This combination of exonucleases can remove linear DNA from a mixture of linear and supercoiled DNA, leaving the supercoiled form intact. Furthermore, the inability of lambda exonuclease to initiate digestion at nicks or gaps enables the removal of undesired linear DNA when nicked circular DNA has been enzymatically prepared from supercoiled DNA. This method can be useful in the preparation of homogeneous circular plasmid DNA required for therapeutic applications and biophysical studies.