Radiosensitization produced by iododeoxyuridine with high linear energy transfer heavy ion beams

Transcriptomic Signatures of Auger Electron Radioimmunotherapy Using Nuclear Targeting (111)In-Trastuzumab for Potential Combination Therapies

(111)In-labeled trastuzumab modified with nuclear localizing signal (NLS) peptides ((111)In-trastuzumab-NLS) efficiently delivers an Auger electron (AE) emitter (111)In into the cell nucleus and is thus a promising radiopharmaceutical in AE radioimmunotherapy (AE-RIT) for targeted killing of HER2-positive cancer. However, further improvement of its therapeutic efficacy is required. In this study, the authors show a transcriptomic approach to identify potential targets for enhancing the cytotoxic effects of (111)In-trastuzumab-NLS. They generated two types of (111)In-trastuzumab-NLS harboring different numbers of NLS peptides, (111)In-trastuzumab-NLS-S and -L. These radioimmunoconjugates (230 and 460 kBq) showed a significant higher cytotoxicity to SKBR3 human breast cancer cells overexpressing HER2 compared to (111)In-trastuzumab. Microarray analysis revealed that NF-kB-related genes (38 genes) were significantly changed in transcription by (111)In trastuzumab-NLS-L (230 kBq) treatment. Quantitative reverse transcription polymerase chain reaction confirmed the microarray data by showing transcriptional alternation of selected NF-κB target genes in cells treated with (111)In-trastuzumab-NLS-L. Interestingly, bortezomib, a drug known as a NF-κB modulator, significantly enhanced the cytotoxicity of (111)In-trastuzumab-NLS-L in SKBR3 cells. Taken together, the transcriptome data suggest the possibility that the modulation of NF-kB signaling activity is a molecular signature of (111)In-trastuzumab-NLS and coadministration of bortezomib may be efficacious in enhancement of AE-RIT with (111)In-trastuzumab-NLS.

Comparison of the bromodeoxyuridine-mediated sensitization effects between low-LET and high-LET ionizing radiation on DNA double-strand breaks

The incorporation of halogenated pyrmidines such as bromo- and iodo-deoxyuridines (BrdU, IdU) into DNA as thymidine analogs enhances cellular radiosensitivity when high-linear energy transfer (LET) radiation is not used. Although it is known that high-LET ionizing radiation confers fewer biological effects resulting from halogenated pyrimidine incorporation, the exact mechanisms of reduced radiosensitivity with high-LET radiation are not clear. We investigated the radiosensitization effects of halogenated pyrimidines with high-LET radiation using accelerated carbon and iron ions. Cells synchronized into the G1 phase after unifilar (1 cell cycle) and bifilar (2 cell cycles) substitution with 10 µM BrdU were exposed to various degrees of LET with heavy ions and X-rays. We then carried out a colony formation assay to measure cell survival. The γ-H2AX focus formation assay provided a measure of DNA double-strand break (DSB) formation and repair kinetics. Chromosomal aberration formations for the first post-irradiation metaphase were also scored. For both low-LET X-rays and carbon ions (13 keV/µm), BrdU incorporation led to impaired DNA repair kinetics, a larger initial number of DNA DSBs more frequent chromosomal aberrations at the first post-irradiated metaphase, and increased radiosensitivity for cell lethality. The enhancement ratio was higher after bifilar substitution. In contrast, no such synergistic enhancements were observed after high-LET irradiation with carbon and iron ions (70 and 200 keV/µm, respectively), even after bifilar substitution. Our results suggest that BrdU substitution did not modify the number and quality of DNA DSBs produced by high-LET radiation. The incorporation of halogenated pyrimidines may produce more complex/clustered DNA damage along with radicals formed by low-LET ionizing radiation. In contrast, the severity of damage produced by high-LET radiation may undermine the effects of BrdU and account for the observed minimal radiosensitization effects.

Recent advances in the biology of heavy-ion cancer therapy

Superb biological effectiveness and dose conformity represent a rationale for heavy-ion therapy, which has thus far achieved good cancer controllability while sparing critical normal organs. Immediately after irradiation, heavy ions produce dense ionization along their trajectories, cause irreparable clustered DNA damage, and alter cellular ultrastructure. These ions, as a consequence, inactivate cells more effectively with less cell-cycle and oxygen dependence than conventional photons. The modes of heavy ion-induced cell death/inactivation include apoptosis, necrosis, autophagy, premature senescence, accelerated differentiation, delayed reproductive death of progeny cells, and bystander cell death. This paper briefly reviews the current knowledge of the biological aspects of heavy-ion therapy, with emphasis on the authors' recent findings. The topics include (i) repair mechanisms of heavy ion-induced DNA damage, (ii) superior effects of heavy ions on radioresistant tumor cells (intratumor quiescent cell population, TP53-mutated and BCL2-overexpressing tumors), (iii) novel capacity of heavy ions in suppressing cancer metastasis and neoangiogenesis, and (iv) potential of heavy ions to induce secondary (especially breast) cancer.

Recent insights into the biological action of heavy-ion radiation

Biological effectiveness varies with the linear energy transfer (LET) of ionizing radiation. During cancer therapy or long-term interplanetary manned explorations, humans are exposed to high-LET energetic heavy ions that inactivate cells more effectively than low-LET photons like X-rays and gamma-rays. Recent biological studies have illustrated that heavy ions overcome tumor radioresistance caused by Bcl-2 overexpression, p53 mutations and intratumor hypoxia, and possess antiangiogenic and antimetastatic potential. Compared with heavy ions alone, the combination with chemical agents (a Bcl-2 inhibitor HA14-1, an anticancer drug docetaxel, and a halogenated pyrimidine analogue 5-iodo-2'-deoxyuridine) or hyperthermia further enhances tumor cell killing. Beer, its certain constituents, or melatonin ameliorate heavy ion-induced damage to normal cells. In addition to effects in cells directly targeted with heavy ions, there is mounting evidence for nontargeted biological effects in cells that have not themselves been directly irradiated. The bystander effect of heavy ions manifests itself as the loss of clonogenic potential, a transient apoptotic response, delayed p53 phosphorylation, alterations in gene expression profiles, and the elevated frequency of gene mutations, micronuclei and chromosome aberrations, which arise in nonirradiated cells having received signals from irradiated cells. Proposed mediating mechanisms involve gap junctional intercellular communication, reactive oxygen species and nitric oxide. This paper reviews briefly the current knowledge of the biological effects of heavy-ion irradiation with a focus on recent findings regarding its potential benefits for therapeutic use as well as on the bystander effect.

Differential activation of mitogen-activated protein kinases following high and low LET radiation in murine macrophage cell line

Mitogen-activated protein kinases have been shown to respond to various stimuli including cytokines, mitogens and gamma irradiation, leading to cell proliferation, differentiation, or death. The duration of their activation determines the specificity of response to each stimulus in various cells. In this study, the crucial intracellular kinases, ERK, JNK, and p38 kinase involved in cell survival, death, or damage and repair were examined for their activity in RAW 264.7 cells at various time points after irradiation with 2 Gy doses of proton ions or X-rays. This is the first report that shows that the MAPK signaling induced after heavy ion or X-ray exposure is not the same. Unlike gamma irradiation, there was prolonged but marginal activation of prosurvival ERK pathway and significant activation of proapoptotic p38 pathway in response to high LET radiation.

Energetic heavy ions overcome tumor radioresistance caused by overexpression of Bcl-2

BACKGROUND AND PURPOSE

Overexpression of Bcl-2 is frequent in human cancers and has been associated with radioresistance. Here we investigated the potential impact of heavy ions on Bcl-2 overexpressing tumors.

MATERIALS AND METHODS

Bcl-2 cells (Bcl-2 overexpressing HeLa cells) and Neo cells (neomycin resistant gene-expressing HeLa cells) exposed to gamma-rays or heavy ions were assessed for the clonogenic survival, apoptosis and cell cycle distribution.

RESULTS

Whereas Bcl-2 cells were more resistant to gamma-rays (0.2keV/microm) and helium ions (16.2keV/microm) than Neo cells, heavy ions (76.3-1610keV/microm) yielded similar survival regardless of Bcl-2 overexpression. Carbon ions (108keV/microm) decreased the difference in the apoptotic incidence between Bcl-2 and Neo cells, and prolonged G(2)/M arrest that occurred more extensively in Bcl-2 cells than in Neo cells.

CONCLUSIONS

High-LET heavy ions overcome tumor radioresistance caused by Bcl-2 overexpression, which may be explained at least in part by the enhanced apoptotic response and prolonged G(2)/M arrest. Thus, heavy-ion therapy may be a promising modality for Bcl-2 overexpressing radioresistant tumors.

Hyperthermia and incorporation of halogenated pyrimidines: radiosensitization in cultured rodent and human tumor cells

PURPOSE

To investigate the possible benefit of hyperthermia (HT) in combination with radiosensitization by halogenated pyrimidines (HPs) in rodent as well as in human tumor cells.

METHODS AND MATERIALS

Exponentially growing rodent cells, radiosensitive R-1 and MOS cells and radioresistant RUC-II and V79 cells, and human SW1573 cells, were exposed to 0, 1, 2, and 4 microM of chloro- (CldUrd), bromo- (BrdUrd), or iodo-deoxyuridine (IdUrd) in the culture medium. Survival after irradiation with gamma-rays from a 137Cs source and/or hyperthermic treatment (HT, 60 min at 42 degrees C) was determined by clonogenic assay. Linear-quadratic analyses of the radiation survival curves were performed to assess sensitization in the dose range 1 to 3 Gy relevant to radiotherapy.

RESULTS

The incorporation of HPs sensitized all cell lines to HT and resulted in radiosensitization dependent on the percentage of thymidine replacement. At equal levels of thymidine replacement, IdUrd was the most potent radiosensitizer. HT further increased radiation-induced lethality of cells that had incorporated HPs. Linear-quadratic analyses showed that HT further increased the linear parameter of the LQ formula while the quadratic parameter was not significantly changed.

CONCLUSION

The combination of HT and HPs act additively in increasing the radiosensitivity of rodent tumor cell lines with varying radiosensitivities as well as of a human tumor cell line. In particular, the ratio of the linear parameter to the quadratic parameter, relevant for fractionation effects in radiotherapy, was increased.

Irregular variations in radiation sensitivity when the linear energy transfer is increased

Seven cell lines were analyzed for clonogenic survival after irradiation with photons (60Co) or accelerated helium or nitrogen ions. The cell lines showed different sensitivity to photon radiation and most of the differences decreased after irradiation with helium ions with a linear energy transfer (LET) of about 40 keV/microns. However, all cell types had individual LET sensitization patterns and the mean relative biological effectiveness (RBE) at 10% survival ranged from 1.46 +/- 0.12 to 2.41 +/- 0.26 for the helium ions. This difference was significant and the differences increased further when higher survival levels were considered. There was only a weak tendency towards a relation between photon and helium ion sensitivity when the linear component of the survival curves, the alpha-values, were compared, and no relation at all for other parameters. It was not possible to predict the response to an increased LET from the photon responses obtained. Three of the cell lines were also irradiated with nitrogen ions with an LET of 125 keV/microns. These cells were, as expected, sensitized further and the average RBE at 10% survival was 3.67 +/- 0.67. However, one cell line was more resistant than the others in this case. Furthermore, the quadratic component of the survival curves, the beta values, were higher after irradiation with nitrogen than with helium ions. Thus, several irregular and unexpected results were seen when the LET was increased.

Iododeoxyuridine incorporation and radiosensitization in three human tumor cell lines

Iododeoxyuridine is a halogenated pyrimidine and non-hypoxic cell radiosensitizer currently being used in clinical trials. The amount of radiosensitization by IdUrd is related to the amount of incorporation of the drug into a cell's DNA. These experiments were carried out in three human tumor cell lines (lung, glioma, and melanoma) in monolayer culture exposed to concentrations of IdUrd from 0.1-10 microM for one and three cell cycles before irradiation to determine incorporation and sensitization as a function of drug exposure. Except for the lung cell line, which required greater than 1 microM IdUrd, these cells demonstrate radiosensitization when exposed to 0.1 microM or greater of IdUrd. Maximum sensitization occurred at 10 microM IdUrd for all the cell lines at three cell cycles. The percent thymidine replacement by IdUrd increased with increasing concentrations, but was cell line dependent. Maximum percent replacement occurred at 10 microM at three cell cycles for all the cell lines: lung = 22.4%, glioma = 32.0%, and melanoma = 39.1%. The relationships between percent thymidine replacement and sensitization are not identical across these human tumor cell lines. If IdUrd is going to be a successful radiosensitizer in clinical trials, sustained plasma levels of 10 microM or greater for at least three cell cycles should be achieved during irradiation. This may be best accomplished with repeated short exposures to IdUrd (three cell cycles or approximately 4 days in these cell lines) every 1-2 weeks during radiation. Measurements of thymidine replacement in a tumor biopsy should be attempted prior to radiation to develop a predictive assay for radiosensitization.

Radiobiology and clinical application of halogenated pyrimidine radiosensitizers

Halogenated pyrimidines (HP) represent a unique class of non-hypoxic cell radiosensitizers currently under clinical re-investigation. In order for halogenated pyrimidines to sensitize cells to radiation, they must be incorporated into cellular DNA. In the case of human tumors, which have in general rather long cell cycle times, this may require many days of continuous drug infusion to achieve adequate replacement of the DNA base thymidine with HP. In vitro studies support the relationship between the extent of radiosensitization and the percentage of thymidine replacement. Recent clinical studies evaluating the role of iododeoxyuridine (IdUrd) as a radiation sensitizer in large unresectable sarcomas have been extremely encouraging. To support and expand upon these positive clinical findings more information and research is needed regarding: (1) the mechanism of HP-induced radiosensitization; (2) the percentage of HP thymidine replacement in human tumors achievable and how it relates to treatment outcome; (3) the means of increasing HP incorporation in tumor and minimizing incorporation in normal tissues; (4) a better understanding of optimal timing between HP administration and radiation treatment; and (5) methods to evaluate which tumors are appropriate candidates for HP therapy. While presently limited to use in conventional high dose-rate X-ray therapy, laboratory studies suggest that HP might also be effective in low dose-rate brachytherapy and for selected high LET clinical beams. HPs probably will not be 'general' non-hypoxic cell radiosensitizers for all tumor types, but with appropriate tumor-type/anatomical site selection and refinement in their administration, HPs may prove beneficial in cancer treatment.

Correlation of exposure time, concentration and incorporation of IdUrd in V-79 cells with radiation response

These experiments were designed to find the minimum concentration at which incorporation of and sensitization by IdUrd (Iododeoxyuridine) would occur and the effect of concentrations from .1 to 100 microM for exposures of 8 to 96 hr in cultured V-79 cells exposed to 137Cs gamma rays at 2 Gy per minute. At 0.1 microM thymidine replacement averaged 1% and the SER ranged from 1.1 to 1.28, significant at the 95% level. The maximum thymidine replacement was 49% after 48 hr exposure to 30 microM yielding an SER of 2.7. SER generally peaked after 72 hr of exposure. This cell line has an 8 hr cycle time in our hands and thus optimal sensitization would occur after 9 cell cycles. These ranges need testing in human cells in culture and in Phase I clinical trials.