Mitigating the risk of radiation-induced cancers: limitations and paradigms in drug development

Cosmic Ionizing Radiation: A DNA Damaging Agent That May Underly Excess Cancer in Flight Crews

In the United States, the Federal Aviation Administration has officially classified flight crews (FC) consisting of commercial pilots, cabin crew, or flight attendants as "radiation workers" since 1994 due to the potential for cosmic ionizing radiation (CIR) exposure at cruising altitudes originating from solar activity and galactic sources. Several epidemiological studies have documented elevated incidence and mortality for several cancers in FC, but it has not yet been possible to establish whether this is attributable to CIR. CIR and its constituents are known to cause a myriad of DNA lesions, which can lead to carcinogenesis unless DNA repair mechanisms remove them. But critical knowledge gaps exist with regard to the dosimetry of CIR, the role of other genotoxic exposures among FC, and whether possible biological mechanisms underlying higher cancer rates observed in FC exist. This review summarizes our understanding of the role of DNA damage and repair responses relevant to exposure to CIR in FC. We aimed to stimulate new research directions and provide information that will be useful for guiding regulatory, public health, and medical decision-making to protect and mitigate the risks for those who travel by air.

Temporary Knockdown of p53 During Focal Limb Irradiation Increases the Development of Sarcomas

Approximately half of patients with cancer receive radiotherapy and, as cancer survivorship increases, the low rate of radiation-associated sarcomas is rising. Pharmacologic inhibition of p53 has been proposed as an approach to ameliorate acute injury of normal tissues from genotoxic therapies, but how this might impact the risk of therapy-induced cancer and normal tissue injuries remains unclear. We utilized mice that express a doxycycline (dox)-inducible p53 short hairpin RNA to reduce Trp53 expression temporarily during irradiation. Mice were placed on a dox diet 10 days prior to receiving 30 or 40 Gy hind limb irradiation in a single fraction and then returned to normal chow. Mice were examined weekly for sarcoma development and scored for radiation-induced normal tissue injuries. Radiation-induced sarcomas were subjected to RNA sequencing. Following single high-dose irradiation, 21% of animals with temporary p53 knockdown during irradiation developed a sarcoma in the radiation field compared with 2% of control animals. Following high-dose irradiation, p53 knockdown preserves muscle stem cells, and increases sarcoma development. Mice with severe acute radiation-induced injuries exhibit an increased risk of developing late persistent wounds, which were associated with sarcomagenesis. RNA sequencing revealed radiation-induced sarcomas upregulate genes related to translation, epithelial-mesenchymal transition (EMT), inflammation, and the cell cycle. Comparison of the transcriptomes of human and mouse sarcomas that arose in irradiated tissues revealed regulation of common gene programs, including elevated EMT pathway gene expression. These results suggest that blocking p53 during radiotherapy could minimize acute toxicity while exacerbating late effects including second cancers.

SIGNIFICANCE

Strategies to prevent or mitigate acute radiation toxicities include pharmacologic inhibition of p53 and other cell death pathways. Our data show that temporarily reducing p53 during irradiation increases late effects including sarcomagenesis.

Inter-agency perspective: Translating advances in biomarker discovery and medical countermeasures development between terrestrial and space radiation environments

Over the past 20+ years, the U.S. Government has made significant strides in establishing research funding and initiating a portfolio consisting of subject matter experts on radiation-induced biological effects in normal tissues. Research supported by the National Cancer Institute (NCI) provided much of the early findings on identifying cellular pathways involved in radiation injuries, due to the need to push the boundaries to kill tumor cells while minimizing damage to intervening normal tissues. By protecting normal tissue surrounding the tumors, physicians can deliver a higher radiation dose to tumors and reduce adverse effects related to the treatment. Initially relying on this critical NCI research, the National Institute of Allergy and Infectious Diseases (NIAID), first tasked with developing radiation medical countermeasures in 2004, has provided bridge funding to move basic research toward advanced development and translation. The goal of the NIAID program is to fund approaches that can one day be employed to protect civilian populations during a radiological or nuclear incident. In addition, with the reality of long-term space flights and the possibility of radiation exposures to both acute, high-intensity, and chronic lower-dose levels, the National Aeronautics and Space Administration (NASA) has identified requirements to discover and develop radioprotectors and mitigators to protect their astronauts during space missions. In sustained partnership with sister agencies, these three organizations must continue to leverage funding and findings in their overlapping research areas to accelerate biomarker identification and product development to help safeguard these different and yet undeniably similar human populations - cancer patients, public citizens, and astronauts.

KMRC011, an agonist of toll-like receptor 5, mitigates irradiation-induced tissue damage and mortality in cynomolgus monkeys

In the study here, the potential applicability of KMRC011 - an agonist of toll-like receptor-5 - as a countermeasure for radiation toxicities was evaluated. Following a single 5.5 Gy total body irradiation (TBI, surface absorbed dose = 7 Gy) of Co⁶⁰ γ-rays, mortality rates and degrees of pathological lesions that developed over 80 days were compared in monkeys that received TBI only and a group that was injected once with KMRC011 (10 μg/kg) after TBI. Compared to the TBI-only hosts (80%), the death rate was significantly improved by the use of KMRC011 (40%), all deaths in both groups occurred in the period from Days 19-24 post-TBI. Further analysis of monkeys that survived until the end of the experiment showed that AST and ALT levels were elevated only in the TBI group, and that radiation-induced tissue damage was alleviated by the KMRC011 injection. Additionally, expression of cell death-related proteins was lower in tissues from the KMRC011-treated hosts than in those in the TBI-only group. Other measured parameters, including body weight, food uptake, and hematological values did not significantly differ between the two groups over the entire period. The results of this study, thus demonstrate that KMRC011 could potentially be used as a medical countermeasure for the treatment of acute radiation exposure.

Medical countermeasures for radiation induced health effects: report of an Interagency Panel Session held at the NASA Human Research Program Investigator's Workshop, 26 January 2017

An Interagency Panel Session organized by the NASA Human Research Program (HRP) Space Radiation Program Element (SRPE) was held during the NASA HRP Investigator’s Workshop (IWS) in Galveston, Texas on 26 January 2017 to identify complementary research areas that will advance the testing and development of medical countermeasures (MCMs) in support of radioprotection and radiation mitigation on the ground and in space. There were several areas of common interest identified among the various participating agencies. This report provides a summary of the topics discussed by each agency along with potential areas of intersection for mutual collaboration opportunities. Common goals induded repurposing of pharmaceuticals, nutraceuticals for use as radioprotectors and/or mitigators, low-dose/chronic exposure paradigms, late effects post-radiation exposure, mixed-field exposures of gamma-neutron, performance decrements, and methods to determine individual exposure levels.

Predicting the Public Health Consequences of a Nuclear Terrorism Attack: Drawing on The Experiences of Hiroshima and Fukushima

The increasing threat of nuclear terrorism warrants consideration of the health consequences of a terrorist incident should preventive measures fail. Although there has not yet been a nuclear terrorist attack of any kind, experiences with the aftermath of the bombing of Hiroshima and the core meltdowns at Fukushima can provide useful insight and allow some inferences to be made regarding the types of casualties that might be sustained and the rescue efforts that might be required. There are many parallels between the events at Hiroshima and what might be expected from an improvised nuclear device, and there are parallels between the radioactivity released to the environment at Fukushima and the aftermath of a radiological dispersal device attack. Nevertheless, there are some unique aspects to a ground-detonated improvised nuclear device that pose health threats beyond those seen at Hiroshima (i.e., fallout). And psychological health may be impacted more than physical health in the case of a radiological dispersal device. Preparedness requires consideration of all of these various health hazards in order to determine how best to mitigate the consequences of a nuclear terrorism attack.

Ionizing radiation exposure: hazards, prevention, and biomarker screening

Radiation is a form of energy derived from a source that is propagated through material in space. It consists of ionizing radiation or nonionizing radiation. Ionizing radiation is a feature of the environment and an important tool in medical treatment, but it can cause serious damage to organisms. A number of protective measures and standards of protection have been proposed to protect against radiation. There is also a need for biomarkers to rapidly assess individual doses of radiation, which can not only estimate the dose of radiation but also determine its effects on health. Proteomics, genomics, metabolomics, and lipidomics have been widely used in the search for such biomarkers. These topics are discussed in depth in this review.

Defined Sensing Mechanisms and Signaling Pathways Contribute to the Global Inflammatory Gene Expression Output Elicited by Ionizing Radiation

Environmental insults are often detected by multiple sensors that activate diverse signaling pathways and transcriptional regulators, leading to a tailored transcriptional output. To understand how a tailored response is coordinated, we examined the inflammatory response elicited in mouse macrophages by ionizing radiation (IR). RNA-sequencing studies revealed that most radiation-induced genes were strongly dependent on only one of a small number of sensors and signaling pathways, notably the DNA damage-induced kinase ATM, which regulated many IR-response genes, including interferon response genes, via an atypical IRF1-dependent, STING-independent mechanism. Moreover, small, defined sets of genes activated by p53 and NRF2 accounted for the selective response to radiation in comparison to a microbial inducer of inflammation. Our findings reveal that genes comprising an environmental response are activated by defined sensing mechanisms with a high degree of selectivity, and they identify distinct components of the radiation response that might be susceptible to therapeutic perturbation.

The Radiation Stress Response: Of the People, By the People and For the People

The radiation stress response can have broad impact. In this Failla Award presentation it is discussed in three components using terms relevant to the current political season as to how the radiation stress response can be applied to the benefit for cancer care and as service to society. Of the people refers to the impact of radiation on cells, tissues and patients. The paradigm our laboratory uses is radiation as a drug, called "focused biology", and physics as "nano-IMRT" because at the nanometer level physics and biology merge. By the people refers to how the general population often reacts to the word "radiation" and how the Radiation Research Society can better enable society to deal with the current realities of radiation in our lives. For the people refers to the potential for radiation oncology and radiation sciences to improve the lives of millions of people globally who are now beyond benefits of cancer treatment and research.

Biomarkers of Ionizing Radiation Exposure: A Multiparametric Approach

Humans are exposed to ionizing radiation not only through background radiation but also through the ubiquitous presence of devices and sources that generate radiation. With the expanded use of radiation in day-to-day life, the chances of accidents or misuse only increase. Therefore, a thorough understanding of the dynamic effects of radiation exposure on biological entities is necessary. The biological effects of radiation exposure on human cells depend on much variability such as level of exposure, dose rate, and the physiological state of the cells. During potential scenarios of a large-scale radiological event which results in mass casualties, dose estimates are essential to assign medical attention according to individual needs. Many attempts have been made to identify biomarkers which can be used for high throughput biodosimetry screening. In this study, we compare the results of different biodosimetry methods on the same irradiated cells to assess the suitability of current biomarkers and push forward the idea of employing a multiparametric approach to achieve an accurate dose and risk estimation.

Does Imaging Technology Cause Cancer? Debunking the Linear No-Threshold Model of Radiation Carcinogenesis

In the past several years, there has been a great deal of attention from the popular media focusing on the alleged carcinogenicity of low-dose radiation exposures received by patients undergoing medical imaging studies such as X-rays, computed tomography scans, and nuclear medicine scintigraphy. The media has based its reporting on the plethora of articles published in the scientific literature that claim that there is "no safe dose" of ionizing radiation, while essentially ignoring all the literature demonstrating the opposite point of view. But this reported "scientific" literature in turn bases its estimates of cancer induction on the linear no-threshold hypothesis of radiation carcinogenesis. The use of the linear no-threshold model has yielded hundreds of articles, all of which predict a definite carcinogenic effect of any dose of radiation, regardless of how small. Therefore, hospitals and professional societies have begun campaigns and policies aiming to reduce the use of certain medical imaging studies based on perceived risk:benefit ratio assumptions. However, as they are essentially all based on the linear no-threshold model of radiation carcinogenesis, the risk:benefit ratio models used to calculate the hazards of radiological imaging studies may be grossly inaccurate if the linear no-threshold hypothesis is wrong. Here, we review the myriad inadequacies of the linear no-threshold model and cast doubt on the various studies based on this overly simplistic model.