Efficient mutation induction by 125I and 131I decays in DNA of human cells

To examine the role of radiation energy deposition in DNA on cellular effects, we investigated the ability of 125IdUrd and 131IdUrd to kill cells and induce mutations at the hprt locus. We employed human lymphoblastoid cells proficient (TK6) or deficient (SE30) in the ability to incorporate a thymidine analog into DNA by way of the thymidine kinase (TK) scavenger pathway. Iodine-125 releases a shower of low-energy Auger electrons upon decay which deposit most of their energy within 20 nm of the decay site, whereas 131I is a high-energy beta/gamma emitter that is generally considered to emit sparsely ionizing radiation. Although 125IdUrd incorporated into cellular DNA was very effective at producing toxic and mutagenic effects in TK6 cells, virtually no effect was seen in TK-deficient cells incubated with similar levels of 125IdUrd in the extracellular medium. In response to 131IdUrd treatment, 0.45 X 10(-6) mutants were induced per centigray dose deposited within the nucleus in TK-proficient cells, whereas few mutations were induced in TK-deficient cells at doses up to 38 cGy from 131I decays occurring in the medium. The differences in biological response between TK6 and SE30 cells cannot be explained by differential radiosensitivity or IdUrd sensitization of the cell lines involved. We conclude that both 125I and 131I decays occurring while incorporated into DNA are more effective at inducing cell killing and mutations in human cells than either nonincorporated decays or low-LET radiations. These results suggest that localized energy deposition is an important factor in producing biologically important damage by both of these isotopes, and that residual lesions following the decay of DNA-incorporated radioisotopes may contribute to the toxic and mutagenic effects observed in TK-proficient cells. Furthermore, they emphasize that certain beta/gamma-emitting isotopes such as 131I may be particularly hazardous when incorporated into DNA.     

Mechanisms underlying production of double-strand breaks in plasmid DNA after decay of 125I-Hoechst

Previously, the kinetics of strand break production by (125)I-labeled m-iodo-p-ethoxyHoechst 33342 ((125)IEH) in supercoiled (SC) plasmid DNA had demonstrated that approximately 1 DSB is produced per (125)I decay both in the presence and absence of the hydroxyl radical scavenger DMSO. In these experiments, an (125)IEH:DNA molar ratio of 42:1 was used. We now hypothesize that this DSB yield (but not the SSB yield) may be an overestimate due to subsequent decays occurring in any of the 41 (125)IEH molecules still bound to nicked (N) DNA. To test our hypothesis, (125)IEH was incubated with SC pUC19 plasmids ((125)IEH:DNA ratio of approximately 3:1) and the SSB and DSB yields were quantified after the decay of (125)I. As predicted, the number of DSBs produced per (125)I decay is one-half that reported previously ( approximately 0.5 compared to approximately 1, +/- DMSO) whereas the number of SSBs ( approximately 3/(125)I decay) is similar to that obtained previously ( approximately 90% are generated by OH radicals). Direct visualization by atomic force microscopy confirms formation of L and N DNA after (125)IEH decays in SC DNA and supports the strand break yields reported. These findings indicate that although SSB production is independent of the number of (125)IEH bound to DNA, the DSB yield can be augmented erroneously by (125)I decays occurring in N DNA. Further analysis indicates that 17% of SSBs and 100% of DSBs take place within the plasmid molecule in which an (125)IEH molecule decays, whereas 83% of SSBs are formed in neighboring plasmid DNA molecules.     

A method of calculating initial DNA strand breakage following the decay of incorporated 125I

Two sources of individual Auger electron spectra and an electron track code were used with a simple model of the DNA to successfully simulate the single-strand DNA breakage measured by Martin and Haseltine (1981). The conditions of the calculation were then extended to examine patterns of single-strand breaks in both strands of the DNA duplex to score double-strand breaks. The occurrences of five types of break were scored. The total number of double-strand breaks (dsb) per decay at the site of the decay was 0.90 and 0.65 for the different Auger electron spectra. It was shown that for mammalian cells an additional source of double-strand breaks from low LET radiation added approximately 0.17 dsb/decay to each, giving a final total of 1.07 and 0.85 dsb/decay for mammalian cells depending on the electron spectrum. Further is is shown that the energy deposition in the DNA from the iodine decay is very complex, with a broad range of energy depositions and products. Even for a particular energy deposited in the DNA different types of strand break are produced. These are identified and their probabilities calculated.     

Radioprobing of DNA: distribution of DNA breaks produced by decay of 125I incorporated into a triplex-forming oligonucleotide correlates with geometry of the triplex.

The distribution of breaks produced in both strands of a DNA duplex by the decay of 125I carried by a triplex-forming DNA oligonucleotide was studied at single nucleotide resolution. The 125I atom was located in the C5 position of a single cytosine residue of an oligonucleotide designed to form a triple helix with the target sequence duplex. The majority of the breaks (90%) are located within 10 bp around the decay site. The addition of the free radical scavenger DMSO produces an insignificant effect on the yield and distribution of the breaks. These results suggest that the majority of these breaks are produced by the direct action of radiation and are not mediated by diffusible free radicals. The frequency of breaks in the purine strand was two times higher that in the pyrimidine strand. This asymmetry in the yield of breaks correlates with the geometry of this type of triplex; the C5 of the cytosine in the third strand is closer to the sugar-phosphate backbone of the purine strand. Moreover, study of molecular models shows that the yield of breaks at individual bases correlates with distance from the 125I decay site. We suggest the possible use of 125I decay as a probe for the structure of nucleic acids and nucleoprotein complexes.     

Radiotoxicity of 5-[123I]iodo-2'-deoxyuridine in V79 cells: a comparison with 5-[125I]iodo-2'-deoxyuridine

The toxic effects of the short-lived (T 1/2 = 13.2 h) Auger-electron-emitting isotope 123I, incorporated in the form of 123IUdR into the DNA of V79 cells in vitro, have been investigated and compared to those of 125IUdR. For the concentrations tested, the rate of incorporation of 123IUdR at any time is proportional to the concentration of extracellular radioactivity. The curve for survival of clonogenic cells decreases exponentially and exhibits no shoulder at low doses. The mean lethal dose (D37) to the nucleus is 79 +/- 9 cGy and is about the same as that obtained previously with 125IUdR. However, the total number of decays needed to produce this D37 with 123IUdR is about twice that required with 125IUdR, approximately equal to the ratio of the energy deposited in microscopic volumes by 125I and 123I, respectively. This correlation suggests that nuclear recoil, electronic excitation, and chemical transmutation are probably of minor importance to the observed biological toxicity with either isotope. The results also indicate that there are no saturation effects in the decay of 125IUdR in the DNA of V79 cells (i.e., all of the emitted energy is biologically effective) and that each of the two steps involved in the 125I decay is equally effective in causing biological damage.     

Site-specific strand breaks in RNA produced by (125)I radiodecay

Decay of ¹²⁵I produces a shower of low energy electrons (Auger electrons) that cause strand breaks in DNA in a distance-dependent manner with 90% of the breaks located within 10 bp from the decay site. We studied strand breaks in RNA molecules produced by decay of ¹²⁵I incorporated into complementary DNA oligonucleotides forming RNA/DNA duplexes with the target RNA. The frequencies and distribution of the breaks were unaffected by the presence of the free radical scavenger dimethyl sulfoxide (DMSO) or by freezing of the samples. Therefore, as was the case with DNA, most of the breaks in RNA were direct rather than caused by diffusible free radicals produced in water. The distribution of break frequencies at individual bases in RNA molecules is narrower, with a maximum shifted to the 3′-end with respect to the distribution of breaks in DNA molecules of the same sequence. This correlates with the distances from the radioiodine to the sugars of the corresponding bases in A-form (RNA/DNA duplex) and B-form (DNA/DNA duplex) DNA. Interestingly, when ¹²⁵I was located close to the end of the antisense DNA oligonucleotide, we observed breaks in RNA beyond the RNA/DNA duplex region. This was not the case for a control DNA/DNA hybrid of the same sequence. We assume that for the RNA there is an interaction between the RNA/DNA duplex region and the single-stranded RNA tail, and we propose a model for such an interaction. This report demonstrates that ¹²⁵I radioprobing of RNA could be a powerful method to study both local conformation and global folding of RNA molecules.     

Strand breaks in plasmid DNA after positional changes of Auger electron-emitting iodine-125: direct compared to indirect effects.

To elucidate the nature and kinetics of DNA strand breaks caused by low-energy Auger electron emitters, we compared the yields of DNA breaks in supercoiled pUC19 DNA in the presence of the (.)OH scavenger dimethyl sulfoxide (DMSO) after the decay of (125)I (1) in proximity to DNA after minor-groove binding ((125)I-iodoHoechst 33342, (125)IH) and (2) at a distance from DNA ((125)I-iodoantipyrine, (125)IAP). DMSO is efficient at protecting supercoiled plasmid DNA from the decay of (125)I free in solution (dose modification factor, DMF = 59 +/- 4) and less effective when the (125)I decays occur close to DNA (DMF = 3.8 +/- 0.3). This difference is due mainly to the inability of DMSO to protect DNA from the double-strand breaks produced by groove-bound (125)I (DMF = 1.0 +/- 0.2). Additionally, the fragmentation of plasmid DNA beyond the production of single-strand and double-strand breaks that is seen after the decay of (125)IH and not (125)IAP (Kassis et al., Radiat. Res. 151, 167-176, 1999) cannot be modified by DMSO. These results demonstrate that the mechanisms underlying double-strand breaks caused by the decay of (125)IH differ in nature from those caused by the decay of (125)IAP.     

Measurement of double-strand breaks in Chinese hamster cell DNA by neutral filter elution: calibration by 125I decay

We labeled the DNA of Chinese hamster lung V79 cells with 125I in the form of iododeoxyuridine and subsequently measured the elution of the DNA through polycarbonate filters at pH 9.6 and pH 7.2. Since decay of incorporated 125I produces predominantly double-strand breaks (DSB) in DNA at a rate close to one DSB per 125I decay, this measurement provides an absolute calibration for the assay of DSBs by neutral filter elution. Neutral elution profiles are not first order with respect to elution time; thus we have examined the relationships between accumulated 125I decays and several functions of retention of DNA on the filter at various times during the elution process. At both pH 9.6 and pH 7.2 there were linear relationships between accumulated decays and certain retention functions. The retention function most closely correlated to 125I decays for both pH values was the logarithm of the ratio of the retention of control DNA to that of 125I-labeled DNA, both evaluated at the 9th fraction (13.5 h of elution). The linear relationship between this ratio and 125I decays allows DSB induction to be determined directly from retention values. The calibration was used to measure DSBs induced by X rays.     

Iodine-125 decay in a synthetic oligodeoxynucleotide. I. Fragment size distribution and evaluation of breakage probability

Lobachevsky, P. N. and Martin, R. F. Iodine-125 Decay in a Synthetic Oligodeoxynucleotide. I. Fragment Size Distribution and Evaluation of Breakage Probability. Incorporation of (125)I-dC into a defined location of a double-stranded oligodeoxynucleotide was used to investigate DNA breaks arising from decay of the Auger electron-emitting isotope. Samples of the oligodeoxynucleotide were also labeled with (32)P at either the 5' or 3' end of either the (125)I-dC-containing (so-called top) or opposite (bottom) strand and incubated in 20 mM phosphate buffer or the same buffer plus 2 M dimethylsulfoxide at 4 degrees C during 18-20 days. The (32)P-end-labeled fragments produced by (125)I decays were separated on denaturing polyacrylamide gels, and the (32)P activity in each fragment was determined by scintillation counting after elution of fragments from the gel. The relative fragment size distributions were then normalized on a per decay basis and converted to a distribution of single-strand break probabilities as a function of distance from the (125)I-dC. The results of three to five experiments for each of eight possible combinations of labels and incubation conditions are presented as a table showing the relative numbers of (32)P counts in different fragments as well as graphs of normalized fragment size distributions and probabilities of breakage. The average numbers of single-strand breaks per (125)I decay are 3. 3 and 3.7 in the top strand and 1.3 and 1.5 in the bottom strand with and without dimethylsulfoxide, respectively. Every (125)I decay event produces a break in the top strand, and breakage of the bottom strand occurs in 75-80% of the events. Thus a double-strand break is produced by (125)I decay with a probability of approximately 0.8.     

DNA-incorporated 125I induces more than one double-strand break per decay in mammalian cells

The Auger-electron emitter 125I releases cascades of 20 electrons per decay that deposit a great amount of local energy, and for DNA-incorporated 125I, approximately one DNA double-strand break (DSB) is produced close to the decay site. To investigate the potential of 125I to induce additional DSBs within adjacent chromatin structures in mammalian cells, we applied DNA fragment-size analysis based on pulsed-field gel electrophoresis (PFGE) of hamster V79-379A cells exposed to DNA-incorporated 125IdU. After accumulation of decays at -70 degrees C in the presence of 10% DMSO, there was a non-random distribution of DNA fragments with an excess of fragments <0.5 Mbp and the measured yield was 1.6 DSBs/decay. However, since these experiments were performed under high scavenging conditions (DMSO) that reduce indirect effects, the yield in cells exposed to 125IdU under physiological conditions would most likely be even higher. In contrast, using a conventional low-resolution assay without measurement of smaller DNA fragments, the yield was close to one DSB/decay. We conclude that a large fraction of the DSBs induced by DNA-incorporated 125I are nonrandomly distributed and that significantly more than one DSB/decay is induced in an intact cell. Thus, in addition to DSBs produced close to the decay site, DSBs may also be induced within neighboring chromatin fibers, releasing smaller DNA fragments that are not detected by conventional DSB assays.     

125I-induced DNA double strand breaks: use in calibration of the neutral filter elution technique and comparison with X-ray induced breaks

The neutral filter elution assay, for measurement of DNA double strand breakage, has been calibrated using mouse L cells and Chinese hamster V79 cells labelled with [125I]dUrd and then held at liquid nitrogen temperature to accumulate decays. The basis of the calibration is the observation that each 125I decay, occurring in DNA, produces a DNA double strand break. Linear relationships between 125I decays per cell and lethal lesions per cell (minus natural logarithm survival) and the level of elution, were found. Using the calibration data, it was calculated that the yield of DNA double strand breaks after X-irradiation of both cell types was from 6 to 9 X 10(-12) DNA double strand breaks per Gy per dalton of DNA, for doses greater than 6 Gy. Neutral filter elution and survival data for X-irradiated and 125I-labelled cells suggested that the relationships between lethal lesions and DNA double strand breakage were significantly different for both cell types. An attempt was made to study the repair kinetics for 125I-induced DNA double strand breaks, but was frustrated by the rapid DNA degradation which occurs in cells that have been killed by the freezing-thawing process.     

Paradoxical effects of iodine-125 decays in parent and daughter DNA: a new target model for radiation damage

Chinese hamster ovary cells were synchronized at the G(1)/S-phase boundary of the cell cycle and were pulse-labeled with (125)I-iododeoxyuridine 30 min after they entered the S phase. Cell samples were harvested and frozen for accumulation of (125)I decays during the first and second G(2) phase after labeling. Cell aliquots that had accumulated the desired number of decays were thawed and plated for evaluation micronucleus formation and cell death. Cells subjected to (125)I decays during the first G(2) phase after labeling exhibited single-hit kinetics of cell killing (n = 1, D(0) 41 decays/cell). In contrast, decays accumulated during the second G(2) phase killed cells with dual-hit kinetics (n = 1.9, D(0) 81 decays/cell). A similar divergence in the action of (125)I was noted for micronucleus formation. These findings indicate that the effects of (125)I varied depending on whether the decays occurred in daughter DNA (first G(2) phase) or parent DNA (second G(2) phase). Control studies with external X rays showed no such divergence of the action of radiation. To account for this paradox, a model is proposed that invokes higher-order chromatin structures as radiation targets. This model implies differential spatial arrangements for parent and daughter DNA in the genome, with DNA strands organized such that a single (125)I decay originating in daughter DNA damages two targets during the first G(2) phase, but identical decays occurring during the second G(2) phase damage only one of the targets.     

Role of mitochondrial DNA in cell death induced by 125I decay

The role of mitochondrial DNA in radiation-induced cell death was determined by selective [125I]iododeoxyuridine (125IUdR) incorporation into exclusively nuclear sites compared to labelling in both nuclear and mitochondrial DNA of Chinese hamster cells. Such selectivity was achieved by using berenil (25 micrograms/ml for 24 h), a drug which inhibits mitochondrial DNA synthesis without affecting incorporation of 125IUdR into nuclear DNA but does not result in reduced clonogenicity or cell cycle perturbations or alteration in the X-ray response of cells. There was no difference in cell killing between cells with nuclear labelling alone compared with nuclear plus mitochondrial labelling. The absence of decays in mitochondrial DNA does not affect the ability of 125I to induce lethal cell damage. The two treatment groups have superimposable curves with a D0 of 96 decays/cell. These findings indicate that mitochondrial DNA is not the most sensitive target for radiation-induced cell death from 125I decay.     

Cell lethality after selective irradiation of the DNA replication fork

Summary It has been suggested that nascent DNA located at the DNA replication fork may exhibit enhanced sensitivity to radiation damage. To evaluate this hypothesis, Chinese hamster ovary cells (CHO) were labeled with¹²⁵I-iododeoxyuridine (¹²⁵IUdR) either in the presence or absence of aphidicolin. Aphidicolin (5 µg/ml) reduced cellular¹²⁵IUdR incorporation to 3–5% of the control value. The residual¹²⁵I incorporation appeared to be restricted to low molecular weight (sub-replicon sized) fragments of DNA which were more sensitive to micrococcal nuclease attack and less sensitive to high salt DNase I digestion than randomly labeled DNA. These findings suggest that DNA replicated in the presence of aphidicolin remains localized at the replication fork adjacent to the nuclear matrix.Based on these observations an attempt was made to compare the lethal consequences of¹²⁵I decays at the replication fork to that of¹²⁵I decays randomly distributed over the entire genome. Regardless of the distribution of decay events, all treatment groups exhibited identical dose-response curves (D₀: 101¹²⁵I decays/cell). Since differential irradiation of the replication complex did not result in enhanced cell lethality, it can be concluded that neither the nascent DNA nor the protein components (replicative enzymes, nuclear protein matrix) associated with the DNA replication site constitute key radiosensitive targets within the cellular genome.     

Iodine-125 decay in a synthetic oligodeoxynucleotide. II. The role of auger electron irradiation compared to charge neutralization in DNA breakage

Lobachevsky, P. N. and Martin, R. F. Iodine-125 Decay in a Synthetic Oligodeoxynucleotide. II. The Role of Auger Electron Irradiation Compared to Charge Neutralization in DNA Breakage. The dramatic chemical and biological effects of the decay of DNA-incorporated (125)I stem from two consequences of the Auger electron cascades associated with the decay of the isotope: high local deposition of radiation energy from short-range Auger electrons, and neutralization of the multiply charged tellurium atom. We have analyzed the extensive data reported in the companion paper (Radiat. Res. 153, 000-000, 2000), in which DNA breakage was measured after (125)I decay in a 41-bp oligoDNA. The experimental data collected under scavenging conditions (2 M dimethylsulfoxide) were deconvoluted into two components denoted as radiation and nonradiation, the former being attributed to energy deposition by Auger electrons. The contribution of the components was estimated by adopting various assumptions, the principal one being that DNA breakage due to the radiation mechanism is dependent on the distance between the decaying (125)I atom and the cleaved deoxyribosyl unit, while the nonradiation mechanism, associated with neutralization of the multiply charged tellurium atom, contributes equally at corresponding nucleotides starting from the (125)I-incorporating nucleotide. Comparison of the experimental data sets collected under scavenging and nonscavenging (without dimethylsulfoxide) conditions was used to estimate the radiation-scavengeable component. Our analysis showed that the nonradiation component plays the major role in causing breakage within 4-5 nucleotides from the site of (125)I incorporation and produces about 50% of all single-stranded breaks. This overall result is consistent with the relative amounts of energy associated with Auger electrons and the charged tellurium atom. However, the nonradiation component accounts for almost four times more breaks in the top strand, to which the (125)I is bound covalently, than in the bottom strand, thus suggesting an important role of covalent bonds in the energy transfer from the charged tellurium atom. The radiation component dominates at the distances beyond 8-9 nucleotides, and 36% of the radiation-induced breaks are scavengeable.     

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

From a structural perspective, the factors controlling and the mechanisms underlying the toxic effects of ionizing radiation remain elusive. We have studied the consequences of superhelical/torsional stress on the magnitude and mechanism of DSBs induced by low-energy, short-range, high-LET Auger electrons emitted by (125)I, targeted to plasmid DNA by m-[(125)I]iodo-p-ethoxyHoechst 33342 ((125)IEH). DSB yields per (125)I decay for torsionally relaxed nicked (relaxed circular) and linear DNA (1.74+/-0.11 and 1.62+/-0.07, respectively) are approximately threefold higher than that for torsionally strained supercoiled DNA (0.52+/-0.02), despite the same affinity of all forms for (125)IEH. In the presence of DMSO, the DSB yield for the supercoiled form remains unchanged, whereas that for nicked and linear forms decreases to 1.05+/-0.07 and 0.76+/-0.03 per (125)I decay, respectively. DSBs in supercoiled DNA therefore result exclusively from direct mechanisms, and those in nicked and linear DNA, additionally, from hydroxyl radical-mediated indirect effects. Iodine-125 decays produce hydroxyl radicals along the tracks of Auger electrons in small isolated pockets around the decay site. We propose that relaxation of superhelical stress after radical attack could move a single-strand break lesion away from these pockets, thereby preventing further breaks in the complementary strand that could lead to DSBs.     

Inhomogeneity of the nucleus to 125IUdR cytotoxicity

Synchronized suspension cultures of Chinese hamster ovary (CHO) cells were used to determine the lethal effects produced by the decay of 125I incorporated into different subfractions of the nuclear genome. Such a shift in nuclear incorporation pattern was achieved by using the drug aphidicolin, which inhibits 95% of all nuclear DNA synthesis, is nontoxic to cells in a colony-forming assay, and does not modify the radiation response of CHO cells to X irradiation. In addition to shifting incorporation of 125I to only 5% of the nuclear genome, both nuclease digestions to characterize the molecular location of 125I and electron microscope autoradiography show an inhomogeneous distribution of sites of 125I incorporation in the presence of 5 micrograms/ml aphidicolin. These data in combination with survival curves of CHO cells labeled with 125I-iododeoxyuridine (125IUdR) either with or without aphidicolin showed a dramatic change in the survival response (DO: 30 decays/cell and 96 decays/cell, respectively). It is concluded, therefore, that the nucleus is not a homogeneous target for radiation-induced cell death because when subfractions of the nuclear genome are labeled, radically different levels in cell survival are obtained.     

Using Hoechst 33342 to target radioactivity to the cell nucleus

We have explored the use of Hoechst 33342 (H33342) to carry radioactivity to the cell nucleus. H33342 enters cells and targets DNA at adenine-thymine-rich regions of the minor groove. Considerable membrane blebbing and ruffling occur in CHO cells within minutes after its addition to the culture medium in micromolar quantities. Blue vesicles are apparent in the cell cytoplasm, and by 30 min the nuclei are stained dark blue. Upon its binding to DNA, a visible emission shift of the dye can be observed with fluorescence microscopy. We have radioiodinated (125I) H33342 and specifically irradiated nuclear DNA by incubating CHO cells with 125I-H33342 at 37 degrees C and accumulating 125I decays at -90 degrees C. At various times, the cells are thawed and assayed for survival (clonogenicity) and DSB (gamma-H2AX) formation. 125I-H33342 decay leads to a monoexponential decrease in cell survival with a D0 of 122 125I decays per cell and a linear increase in DNA DSB induction (equivalent to 15 gamma-H2AX foci/cell). Cell death is not modified by the radioprotective effects of H33342 because we use considerably lower concentrations than those that provide a slight protection against gamma radiation. We conclude that cell killing by 125I-H33342 and the induction of gamma-H2AX foci are highly correlated.     

Track structures and dose distributions from decays of (131)I and (125)I in and around water spheres simulating micrometastases of differentiated thyroid cancer

The disintegration of the radionuclides (131)I and (125)I and the subsequent charged-particle tracks left behind in water (as a model substance for a biological cell) are simulated by the Monte Carlo track structure simulation code PARTRAC, using new inelastic electron scattering cross sections for condensed water. Every photon and electron emitted was followed in detail, event by event, down to 10 eV. From the spatial information on the track structures, absorbed dose distributions per (131)I and (125)I decay were calculated in and around water spheres simulating micrometastases as well as in the tissue surrounding such metastases. These radionuclides were assumed to be distributed uniformly inside spheres of different diameters (0.01, 0.03, 0.1, 0.3, 1.0 and 3.0 mm). The respective electron degradation spectra, the nearest-neighbor distance distributions between inelastic events, and the distance distributions for all activations for both iodine radionuclides were calculated. The absorbed fractions of the initial electron energies, absorbed doses and energy depositions, and single-event distributions, F(1)(epsilon), inside the six water spheres described above and in the surrounding tissue were also calculated. The absorbed doses per decay inside the six water spheres, i.e., the calculated S values (listed from 0.01 to 3.0 mm), were 6.8 x 10(-4), 7.2 x 10(-5), 5.5 x 10(-6), 4.9 x 10(-7), 3.1 x 10(-8) and 1.8 x 10(-9) Gy Bq(-1) s(-1) for (131)I, and 3.4 x 10(-3), 1.7 x 10(-4), 5.1 x 10(-6), 2.0 x 10(-7), 5.6 x 10(-9) and 2.2 x 10(-10) Gy Bq(-1) s(-1) for (125)I. It is concluded that, in the treatment of thyroid cancer, the geometrical track structure properties of (125)I might be superior to those of (131)I in micrometastases with diameters less than 0.1 mm; however, in this medical context, many other factors also have to be considered.     

Relative biological effectiveness of accumulated 125IdU and 125I-estrogen decays in estrogen receptor-expressing MCF-7 human breast cancer cells

The therapeutic potential for delivering a cytotoxic dose of radiation (using the decay of Auger-electron emitters) to the cell nucleus of cancer cells that express estrogen receptors (ERs) by radiolabeled estrogen was investigated in the ER-expressing human breast cancer cell line, MCF-7. The radiolabeled estrogen/ER complex irradiates the cell nucleus by binding specific DNA sequences called estrogen response elements (EREs). Cell clonogenicity and induction of DNA double-strand breaks (DSBs) by gamma radiation or accumulation of (125)I-iododeoxyuridine ((125)IdU) or E-17alpha[(125)I]iodovinyl-11betamethoxyestradiol ((125)IVME2) decays were determined. MCF-7 cells were efficiently killed by accumulation of (125)IdU (D(0) = 30 decays per cell) and (125)IVME2 decays (D(0) = 28 decays per cell). DNA DSBs were induced by the accumulation of (125)IdU (approximately 3750 decays per cell required to reduce the mean value of the elution profile to 50%) or (125)IVME2 decays (approximately 465 decays per cell required to reduce the mean value to 50%). For survival of MCF-7 cells after gamma irradiation, the D(0) was 1 Gy, and approximately 65 Gy was required to reduce the mean value to 50% for induction of DSBs. The RBE values for cell killing and induction of DSBs by (125)IVME2 relative to gamma radiation were 4.8 and 18.8, respectively. The RBE values for cell killing and induction of DSBs by (125)IdU relative to gamma radiation were 4.5 and 2.3, respectively. Cell killing in a manner similar to that induced by high-LET radiation and the high RBE for induction of DSBs by (125)IVME2 in the ER-expressing MCF-7 cells provide a biological rationale for the use of Auger electron-emitting radionuclides covalently bound to estrogen to deliver a cytotoxic dose of radiation to ER-positive cancers.