Relative biological effectiveness (RBE) of 210 Po alpha-particles versus X-rays on lethality in bovine endothelial cells

Noncancer Effects of Ionizing Radiation Exposure on the Eye, the Circulatory System and beyond: Developments made since the 2011 ICRP Statement on Tissue Reactions

For radiation protection purposes, noncancer effects with a threshold-type dose-response relationship have been classified as tissue reactions (formerly called nonstochastic or deterministic effects), and equivalent dose limits aim to prevent occurrence of such tissue reactions. Accumulating evidence demonstrates increased risks for several late occurring noncancer effects at doses and dose rates much lower than previously considered. In 2011, the International Commission on Radiological Protection (ICRP) issued a statement on tissue reactions to recommend a threshold of 0.5 Gy to the lens of the eye for cataracts and to the heart and brain for diseases of the circulatory system (DCS), independent of dose rate. Literature published thereafter continues to provide updated knowledge. Increased risks for cataracts below 0.5 Gy have been reported in several cohorts (e.g., including in those receiving protracted or chronic exposures). A dose threshold for cataracts is less evident with longer follow-up, with limited evidence available for risk of cataract removal surgery. There is emerging evidence for risk of normal-tension glaucoma and diabetic retinopathy, but the long-held tenet that the lens represents among the most radiosensitive tissues in the eye and in the body seems to remain unchanged. For DCS, increased risks have been reported in various cohorts, but the existence or otherwise of a dose threshold is unclear. The level of risk is less uncertain at lower dose and lower dose rate, with the possibility that risk per unit dose is greater at lower doses and dose rates. Target organs and tissues for DCS are also unknown, but may include heart, large blood vessels and kidneys. Identification of potential factors (e.g., sex, age, lifestyle factors, coexposures, comorbidities, genetics and epigenetics) that may modify radiation risk of cataracts and DCS would be important. Other noncancer effects on the radar include neurological effects (e.g., Parkinson's disease, Alzheimer's disease and dementia) of which elevated risk has increasingly been reported. These late occurring noncancer effects tend to deviate from the definition of tissue reactions, necessitating more scientific developments to reconsider the radiation effect classification system and risk management. This paper gives an overview of historical developments made in ICRP prior to the 2011 statement and an update on relevant developments made since the 2011 ICRP statement.

An approach to assessing the contribution of the high LET effect in strategies for Auger endoradiotherapy

Purpose: The interest in exploiting Auger emitters in cancer therapy stems from their high linear energy transfer (LET)-type radiation damage to DNA. However, the design of Auger-emitter labeled vehicles that target the Auger cascade specifically to the DNA of tumour cells is challenging. Here we suggest a possible approach to evaluate tumour-targeting Auger-labeled conjugates by assessing the impact of a radioprotector known to be effective in protecting from low LET radiation, but not high LET radiation. Given some similarity between the energy spectrum of Auger electrons and that of secondary electrons from soft X-rays, we report the results of radioprotection experiments with 25 kVp X-rays. Materials and methods: Clonogenic survival curves for cultured human keratinocytes were established for three different irradiation conditions: ¹³⁷Cs γ-rays, 25 kVp X-rays and 320 kVp X-rays, and the effect of including a new radioprotector, denoted "2PH", was investigated.Results: The extent of radioprotection by 2PH was comparable for all radiation conditions, although RBE was higher (about 1.7) for soft X-rays. Conclusions: Radioprotectors like 2PH will help to evaluate Auger endoradiotherapy strategies, by determining the relative contributions of the high-LET effects (not protected), compared to other components, such as Auger electrons not effectively targeted to DNA.

Transcriptional benchmark dose modeling: Exploring how advances in chemical risk assessment may be applied to the radiation field

Recent advances in "-omics" technologies have simplified capacity to concurrently assess expression profiles of thousands of targets in a cellular system. However, compilation and analysis of "omics" data in support of human health protection remains a challenge. Benchmark dose (BMD) modeling is currently being employed in chemical risk assessment to estimate acceptable levels of exposure. Although typically applied to conventional endpoints, newer software has enabled this application to be extended to transcriptomic datasets. BMD analytical tools now have the capacity to model transcriptional dose-response data to derive meaningful BMD values for genes, pathways and gene ontologies. In this report, radiation data obtained from the Gene Expression Omnibus (GEO) were analyzed to generate BMD values for transcriptional responses. The datasets comprised microarray analyses of human blood gamma-irradiated ex vivo (0-20 Gy) and human-derived cell lines exposed to alpha particle radiation (0.5-1.5 Gy). The distributions of BMDs for statistically significant genes and pathways in response to radiation exposure were examined and compared across studies. BMD modeling could identify pathway/gene sensitivities across wide radiation dose ranges, experimental conditions (time-points, cell types) and radiation qualities. BMD analysis offered a new approach to examine transcriptional data. The results were shown to provide information on transcriptional thresholds of effects to support refined risk assessments for low dose ionizing radiation exposures, derive gene-based values for relative biological effectiveness and identify pathways involved in radiation sensitivities across cell types which may extend to applications a clinical setting. Environ. Mol. Mutagen. 57:589-604, 2016. © 2016 Wiley Periodicals, Inc.

Chronic exposure by ingestion of environmentally relevant doses of (226)Ra leads to transient growth perturbations in fathead minnow (Pimephales promelas, Rafinesque, 1820)

PURPOSE

To assess the impact of environmentally relevant levels of ingested (226)Ra on a common freshwater fish species.

METHODS

Fathead minnow (Pimephales promelas, Rafinesque) were obtained at the first feeding stage and established on a commercial fish food diet containing (226)Ra in the activity range 10 mBq/g(-1), -10,000 mBq/g(-1). They remained on this diet for 24 months and were sampled invasively at 1,6,18 and 24 months to assess growth, biochemical indices and accumulated dose and non-invasively also at 12 and 15 months to assess growth.

RESULTS

Fish fed 10 and 100 mBq/g(-1) diet showed a small transitory deregulation of growth at 6 and 12 months. Fish fed higher activities showed less significant or insignificant effects. There was a trend at 18 months which was stronger at 24 months for the population distribution to change in all of the (226)Ra fed groups so that smaller fish were smaller and bigger fish were bigger than the controls. There were also significant differences in the ratios of protein:DNA at 24 months which were seen as a trend but were not significant at earlier time points.

CONCLUSIONS

Fish fed a radium diet for 2 years show a small and transitory growth dysregulation at 6 and 12 months. The effects predominate at the lower activities suggesting hormetic or homeostatic adjustments. There was no effect on growth of exposure to the high activities (226)Ra. This suggests that radium does not have a serious impact on the ecology of the system and the level of radium that would be transferred to humans is very low. The results may be important in the assessment of long-term environmental impacts of (226)Ra exposure.

Bystander effect between zebrafish embryos in vivo induced by high-dose X-rays

We employed embryos of the zebrafish, Danio rerio, for our studies on the in vivo bystander effect between embryos irradiated with high-dose X-rays and naive unirradiated embryos. The effects on the naive whole embryos were studied through quantification of apoptotic signals at 25 h post fertilization (hpf) through the terminal dUTP transferase-mediated nick end-labeling (TUNEL) assay followed by counting the stained cells under a microscope. We report data showing that embryos at 5 hpf subjected to a 4-Gy X-ray irradiation could release a stress signal into the medium, which could induce a bystander effect in partnered naive embryos sharing the same medium. We further demonstrated that this bystander effect (induced through partnering) could be successfully suppressed through the addition of the nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) into the medium but not through the addition of the CO liberator tricarbonylchloro(glycinato)ruthenium(II) (CORM-3). This shows that NO was involved in the bystander response between zebrafish embryos induced through X-ray irradiation. We also report data showing that the bystander effect could be successfully induced in naive embryos by introducing them into the irradiated embryo conditioned medium (IECM) alone, i.e., without partnering with the irradiated embryos. The IECM was harvested from the medium that had conditioned the zebrafish embryos irradiated at 5 hpf with 4-Gy X-ray until the irradiated embryos developed into 29 hpf. NO released from the irradiated embryos was unlikely to be involved in the bystander effect induced through the IECM because of the short life of NO. We further revealed that this bystander effect (induced through IECM) was rapidly abolished through diluting the IECM by a factor of 2× or greater, which agreed with the proposal that the bystander effect was an on/off response with a threshold.

Differential effects of alpha-particle radiation and x-irradiation on genes associated with apoptosis

This study examined differential effects of alpha-(α-) particle radiation and X-rays on apoptosis and associated changes in gene expression. Human monocytic cells were exposed to α-particle radiation and X-rays from 0 to 1.5 Gy. Four days postexposure, cell death was measured by flow cytometry and 84 genes related to apoptosis were analyzed using real-time PCR. On average, 33% of the cells were apoptotic at 1.5 Gy of α-particle radiation. Transcript profiling showed statistical expression of 15 genes at all three doses tested. Cells exposed to X-rays were <5% apoptotic at ~1.5 Gy and induced less than a 2-fold expression in 6 apoptotic genes at the higher doses of radiation. Among these 6 genes, Fas and TNF-α were common to the α-irradiated cells. This data suggests that α-particle radiation initiates cell death by TNF-α and Fas activation and through intermediate signalling mediators that are distinct from X-irradiated cells.

Ultra-low dose beta-irradiation induces constriction of rabbit carotid arteries via the endothelium

PURPOSE

To investigate the action of ultra-low dose beta-radiation (ULDBR) on isolated segments of blood vessels.

MATERIALS AND METHODS

We used the pharmacological model of isolated rabbit carotid arteries with intact or mechanically removed endothelium. Specific vascular responses to beta-irradiation were registered after addition of (90)Sr in the concentration range between 12 and 96 microCi l(-1) to the organ bath containing physiological salt solution (PSS).

RESULTS

Intact vascular rings, preconstricted with 20 mM K(+)-PSS, developed an additional constriction upon the addition of (90)Sr depending on the absorbed radiation dose (21.5, 42.9, 85.8, and 171.6 microGy) and the dose rate (51.5, 103.0, 206.0 and 412.0 microGy h(-1)). The vasoconstriction due to (90)Sr was absent in the endothelium-denuded vascular segments indicating the endothelium dependent action of ULDBR. Irradiation did not alter the endothelium dependent relaxation of rabbit carotid arteries induced by acetylcholine. The endothelium dependent responses to ULDBR were abolished after increasing the extra-cellular K(+) to 40 mM.

CONCLUSIONS

ULDBR acts on rabbit carotid arteries as a pharmacological signalling agent because ULDBR effects were completely reversible. ULDBR-mediated contractile responses of the vessels are endothelium dependent. The resistance of acetylcholine endothelium-dependent relaxation of rabbit carotid arteries to ULDBR indicates that the polyphosphoinositide-nitric oxide (NO) signalling cascade is not impaired by ULDBR in endothelial cells.

Relative biologic effects of low-dose-rate alpha-emitting 227Th-rituximab and beta-emitting 90Y-tiuexetan-ibritumomab versus external beam X-radiation

PURPOSE

To determine the relative biologic effects (RBE) of alpha-particle radiation from 227Th-rituximab and of beta-radiation from 90Y-tiuexetan-ibritumomab (Zevalin) compared with external beam X-radiation in the Raji lymphoma xenograft model.

METHODS AND MATERIALS

Radioimmunoconjugates were administered intravenously in nude mice with Raji lymphoma xenografts at different levels of activity. Absorbed dose to tumor was estimated by separate biodistribution experiments for 227Th-rituximab and Zevalin. Tumor growth was measured two to three times per week after injection or X-radiation. Treatment-induced increase in growth delay to reach tumor volumes of 500 and 1,000 mm3, respectively, was used as an end point.

RESULTS

The absorbed radiation dose-rate in tumor was slightly more than 0.1 Gy/d for the first week following injection of 227Th-rituximab, and thereafter gradually decreased to 0.03 Gy/d at 21 days after injection. For treatment with Zevalin the maximum dose-rate in tumor was achieved already 6 h after injection (0.2 Gy/d), and thereafter decreased to 0.01 Gy/d after 7 days. The relative biologic effect was between 2.5 and 7.2 for 227Th-rituximab and between 1 and 1.3 for Zevalin.

CONCLUSIONS

Both at low doses and low-dose-rates, the 227Th-rituximab treatment was more effective per absorbed radiation dose unit than the two other treatments. The considerable effect at low doses suggests that the best way to administer low-dose-rates, alpha-emitting radioimmunoconjugates is via multiple injections.

Relative biological effectiveness (RBE) of alpha radiation in cultured porcine aortic endothelial cells

PURPOSE

Northern peoples can receive elevated radiation doses (1- 10 mSv/y) from transfer of polonium-210 (210Po) through the lichen-caribou-human food chain. Ingested 210Po is primarily blood-borne and thus many of its short range alpha particles irradiate the endothelial cells lining the blood vessels. The relative biological effectiveness (RBE) of alpha particles vs. x-rays was examined in porcine aortic endothelial cells as a surrogate for understanding what might happen to human endothelial cells in northern populations consuming traditional foods.

MATERIALS AND METHODS

Cultured porcine aortic endothelial cells were exposed to x-ray and 210Po alpha particle radiation. Alpha irradiation was applied to the cell cultures internally via the culture medium and externally, using thin-bottomed culture dishes. The results given here are based on the external irradiation method, which was found to be more reliable. Dose-response curves were compared for four lethal endpoints (cell viability, live cell fraction, release of lactate dehydrogenase [LDH] and clonogenic survival) to determine the relative biological effectiveness (RBE) of alpha radiation.

RESULTS

The alpha RBE for porcine cells varied from 1.6-21, depending on the endpoint: 21.2+/-4.5 for cell viability, 12.9+/-2.7 for decrease in live cell number, 5.3+/-0.4 for LDH release to the medium but only 1.6 +/-0.1 for clonogenic survival. The low RBE of 1.6 was due to x-ray hypersensitivity of endothelial cells at low doses.

Choosing an alpha radiation weighting factor for doses to non-human biota

The risk to non-human biota from exposure to ionizing radiation is of current international interest. In calculating radiation doses to humans, it is common to multiply the absorbed dose by a factor to account for the relative biological effectiveness (RBE) of the radiation type. However, there is no international consensus on the appropriate value of such a factor for weighting doses to non-human biota. This paper summarizes our review of the literature on experimentally determined RBEs for internally deposited alpha-emitting radionuclides. The relevancy of each experimental result in selecting a radiation weighting factor for doses from alpha particles in biota was judged on the basis of criteria established a priori. We recommend a nominal alpha radiation weighting factor of 5 for population-relevant deterministic and stochastic endpoints, but to reflect the limitations in the experimental data, uncertainty ranges of 1-10 and 1-20 were selected for population-relevant deterministic and stochastic endpoints, respectively.