Is disseminated intravascular coagulation the major cause of mortality from radiation at relatively low whole body doses?

Investigating the Multi-Faceted Nature of Radiation-Induced Coagulopathies in a Göttingen Minipig Model of Hematopoietic Acute Radiation Syndrome

Coagulopathies are well documented after acute radiation exposure at hematopoietic doses, and radiation-induced bleeding is notably one of the two main causes of mortality in the hematopoietic acute radiation syndrome. Despite this, understanding of the mechanisms by which radiation alters hemostasis and induces bleeding is still lacking. Here, male Göttingen minipigs received hematopoietic doses of 60Co gamma irradiation (total body) and coagulopathies were characterized by assessing bleeding, blood cytopenia, fibrin deposition, changes in hemostatic properties, coagulant/anticoagulant enzyme levels, and markers of inflammation, endothelial dysfunction, and barrier integrity to understand if a relationship exists between bleeding, hemostatic defects, bone marrow aplasia, inflammation, endothelial dysfunction and loss of barrier integrity. Acute radiation exposure induced coagulopathies in the Göttingen minipig model of hematopoietic acute radiation syndrome; instances of bleeding were not dependent upon thrombocytopenia. Neutropenia, alterations in hemostatic parameters and damage to the glycocalyx occurred in all animals irrespective of occurrence of bleeding. Radiation-induced bleeding was concurrent with simultaneous thrombocytopenia, anemia, neutropenia, inflammation, increased heart rate, decreased nitric oxide bioavailability and endothelial dysfunction; bleeding was not observed with the sole occurrence of a single aforementioned parameter in the absence of the others. Alteration of barrier function or clotting proteins was not observed in all cases of bleeding. Additionally, fibrin deposition was observed in the heart and lungs of decedent animals but no evidence of DIC was noted, suggesting a unique pathophysiology of radiation-induced coagulopathies. These findings suggest radiation-induced coagulopathies are the result of simultaneous damage to several key organs and biological functions, including the immune system, the inflammatory response, the bone marrow and the cardiovasculature.

Commonalities Between COVID-19 and Radiation Injury

As the multi-systemic components of COVID-19 emerge, parallel etiologies can be drawn between SARS-CoV-2 infection and radiation injuries. While some SARS-CoV-2-infected individuals present as asymptomatic, others exhibit mild symptoms that may include fever, cough, chills, and unusual symptoms like loss of taste and smell and reddening in the extremities (e.g., "COVID toes," suggestive of microvessel damage). Still others alarm healthcare providers with extreme and rapid onset of high-risk indicators of mortality that include acute respiratory distress syndrome (ARDS), multi-organ hypercoagulation, hypoxia and cardiovascular damage. Researchers are quickly refocusing their science to address this enigmatic virus that seems to unveil itself in new ways without discrimination. As investigators begin to identify early markers of disease, identification of common threads with other pathologies may provide some clues. Interestingly, years of research in the field of radiation biology documents the complex multiorgan nature of another disease state that occurs after exposure to high doses of radiation: the acute radiation syndrome (ARS). Inflammation is a key common player in COVID-19 and ARS, and drives the multi-system damage that dramatically alters biological homeostasis. Both conditions initiate a cytokine storm, with similar pro-inflammatory molecules increased and other anti-inflammatory molecules decreased. These changes manifest in a variety of ways, with a demonstrably higher health impact in patients having underlying medical conditions. The potentially dramatic human impact of ARS has guided the science that has identified many biomarkers of radiation exposure, established medical management strategies for ARS, and led to the development of medical countermeasures for use in the event of a radiation public health emergency. These efforts can now be leveraged to help elucidate mechanisms of action of COVID-19 injuries. Furthermore, this intersection between COVID-19 and ARS may point to approaches that could accelerate the discovery of treatments for both.

Plasma D-Dimer Levels are Elevated in Radiation Oncology Patients

D-dimer plasma levels were evaluated to determine whether they are altered by radiation. D-dimer levels were measured in radiation oncology patients, who were diagnosed with prostate, breast or lung cancer, or leukemia, as well as in healthy subjects serving as controls. Blood samples from radiotherapy patients were taken at three different time points: pre-, on- and post-radiotherapy. For the patients, considered together, differences between the D-dimer levels at these three time points compared to controls were statistically significant. Compared to the pre-radiotherapy measurements, radiation exposure was associated with a significant increase in the D-dimer levels at the on- and post-radiotherapy time points. At the post-radiotherapy time point, D-dimer levels in the patients were not significantly reduced compared to the on-radiotherapy levels, indicating that the risk for developing disseminated intravascular coagulation (DIC) may be increased in some radiation oncology patients. Of particular concern are the post-radiotherapy results observed for the D-dimer levels in the leukemia patients, in which the average fold increase in the D-dimer levels was 5.43 (compared to the pre-radiotherapy levels). These results suggest that leukemia patients might benefit from frequent assessment of their D-dimer levels after their total-body irradiation-conditioning regimen to detect early signs of DIC development. It is hoped that the results described here will lead to heightened awareness in the radiation oncology community that the risk of DIC development is greatly increased in some of these patients.

Pharmacodynamics of romiplostim alone and in combination with pegfilgrastim on acute radiation-induced thrombocytopenia and neutropenia in non-human primates

Purpose: Evaluation of the pharmacodynamics (PD) and pharmacokinetics (PK) of romiplostim alone and in combination with pegfilgrastim in a non-human primate (NHP) model of acute radiation syndrome (ARS).Materials and methods: Male and female rhesus macaques were subjected to Cobalt-60 γ irradiation, at a dose of 550 cGy 24 h prior to subcutaneous administration of either romiplostim alone as a single (2.5 or 5.0 mg/kg on Day 1) or repeat dose (5.0 mg/kg on Days 1 and 8), pegfilgrastim alone as a repeat dose (0.3 µg/kg on Day 1 and 8), or a combination of both agents (romiplostim 5.0 mg/kg on Day 1; pegfilgrastim 0.3 µg/kg on Days 1 and 8). Clinical outcome, hematological parameters and PK were assessed throughout the 45 d study period post-irradiation.Results: Administration of romiplostim, pegfilgrastim or the combination of both resulted in significant improvements in hematological parameters, notably prevention of severe thrombocytopenia, compared with irradiated, vehicle control-treated NHPs. The largest hematologic benefit was observed when romiplostim and pegfilgrastim were administered as a combination therapy with much greater effects on both platelet and neutrophil recovery following irradiation compared to single agents alone.Conclusions: These results indicate that romiplostim alone or in combination with pegfilgrastim is effective at improving hematological parameters in an NHP model of ARS. This study supports further study of romiplostim as a medical countermeasure to improve primary hemostasis and survival in ARS.

Radiation, inflammation and the immune response in cancer

Radiation is an important component of cancer treatment with more than half of all patients receive radiotherapy during their cancer experience. While the impact of radiation on tumour morphology is routinely examined in the pre-clinical and clinical setting, the impact of radiation on the tumour microenvironment and more specifically the inflammatory/immune response is less well characterised. Inflammation is a key contributor to short- and long-term cancer eradication, with significant tumour and normal tissue consequences. Therefore, the role of radiation in modulating the inflammatory response is highly topical given the current wave of targeted and immuno-therapeutic treatments for cancer. This review provides a general overview of how radiation modulates the inflammatory and immune response—(i) how radiation induces the inflammatory/immune system, (ii) the cellular changes that take place, (iii) how radiation dose delivery affects the immune response, and (iv) a discussion on research directions to improve patient survival, reduce side effects, improve quality of life, and reduce financial costs in the immediate future. Harnessing the benefits of radiation on the immune response will enhance its maximal therapeutic benefit and reduce radiation-induced toxicity.

Radiation-Induced Endothelial Vascular Injury: A Review of Possible Mechanisms

In radiation therapy for cancer, the therapeutic ratio represents an optimal balance between tumor control and normal tissue complications. As improvements in the therapeutic arsenal against cancer extend longevity, the importance of late effects of radiation increases, particularly those caused by vascular endothelial injury. Radiation both initiates and accelerates atherosclerosis, leading to vascular events like stroke, coronary artery disease, and peripheral artery disease. Increased levels of proinflammatory cytokines in the blood of long-term survivors of the atomic bomb suggest that radiation evokes a systemic inflammatory state responsible for chronic vascular side effects. In this review, the authors offer an overview of potential mechanisms implicated in radiation-induced vascular injury.

Application of a Novel Murine Ear Vein Model to Evaluate the Effects of a Vascular Radioprotectant on Radiation-Induced Vascular Permeability and Leukocyte Adhesion

Vascular injury after radiation exposure contributes to multiple types of tissue injury through a cascade of events. Some of the earliest consequences of radiation damage include increased vascular permeability and promotion of inflammation, which is partially manifested by increased leukocyte-endothelial (L/E) interactions. We describe herein a novel intravital imaging method to evaluate L/E interactions, as a function of shear stress, and vascular permeability at multiple time points after local irradiation to the ear. This model permitted analysis of quiescent vasculature that was not perturbed by any surgical manipulation prior to imaging. To evaluate the effects of radiation on vascular integrity, fluorescent dextran was injected intravenously and its extravasation in the extravascular space surrounding the ear vasculature was measured at days 3 and 7 after 6 Gy irradiation. The vascular permeability rate increased approximately twofold at both days 3 and 7 postirradiation ( P < 0.05). Leukocyte rolling, which is indicative of L/E interactions, was significantly increased in mice at 24 h postirradiation compared to that of nonirradiated mice. To assess our model, as a means for assessing vascular radioprotectants, we treated additional cohorts of mice with a thrombopoietin mimetic, TPOm (RWJ-800088). In addition to stimulating platelet formation, thrombopoietin can protect vasculature after several forms of injury. Thus, we hypothesized that TPOm would reduce vascular permeability and L/E adhesion after localized irradiation to the ear vasculature of mice. If TPOm reduced these consequences of radiation, it would validate the utility of our intravital imaging method. TPOm reduced radiation-induced vascular leakage to control levels at day 7. Furthermore, L/E cell interactions were also reduced in irradiated mice treated with TPOm, compared with mice receiving irradiation alone, particularly at high shear stress ( P = 0.03, Kruskal-Wallis). We conclude that the ear model is useful for monitoring quiescent normal tissue vascular injury after radiation exposure. Furthermore, the application of TPOm, for preventing early inflammatory response created by damage to vascular endothelium, suggests that this drug may prove useful in reducing toxicities from radiotherapy, which damage microvasculature that critically important to tissue function.

Limitations in predicting the space radiation health risk for exploration astronauts

Despite years of research, understanding of the space radiation environment and the risk it poses to long-duration astronauts remains limited. There is a disparity between research results and observed empirical effects seen in human astronaut crews, likely due to the numerous factors that limit terrestrial simulation of the complex space environment and extrapolation of human clinical consequences from varied animal models. Given the intended future of human spaceflight, with efforts now to rapidly expand capabilities for human missions to the moon and Mars, there is a pressing need to improve upon the understanding of the space radiation risk, predict likely clinical outcomes of interplanetary radiation exposure, and develop appropriate and effective mitigation strategies for future missions. To achieve this goal, the space radiation and aerospace community must recognize the historical limitations of radiation research and how such limitations could be addressed in future research endeavors. We have sought to highlight the numerous factors that limit understanding of the risk of space radiation for human crews and to identify ways in which these limitations could be addressed for improved understanding and appropriate risk posture regarding future human spaceflight.

Hypercoagulation and complement: Connected players in tumor development and metastases

Hypercoagulation is a common feature of several tumors to the extent that individuals with coagulation defects often present with occult visceral cancers. Recent evidence has shown that hypercoagulation is not just a mere secondary effect due to the presence of the tumor, rather it actively contributes to tumor development and dissemination. Among the numerous mechanisms that can contribute to cancer-associated hypercoagulation, the ones involving immune-mediated processes are gaining increasing attention. In particular, complement cascade and hypercoagulation are one inducing the other in a vicious circle that involves neutrophil extracellular traps (NETs) formation. Together, in this feedback loop, they can promote the protumorigenic phenotype of immune cells and the protection of tumor cells from immune attack, ultimately favouring tumor development, progression and metastases formation. In this review, we summarize the role of these processes in cancer development and highlight new possible intervention strategies based on anticoagulants that can arrest this vicious circle.

A Review of Radiation-Induced Coagulopathy and New Findings to Support Potential Prevention Strategies and Treatments

Results from our recent studies have led to the novel hypothesis that radiation-induced coagulopathy (RIC) and associated hemorrhage occurring as part of the acute radiation syndrome (ARS) is a major cause of death resulting from radiation exposure in large mammals, including humans. This article contains information related to RIC, as well as potential strategies for the prevention and treatment of RIC. In addition, new findings are reported here on the occurrence of RIC biomarkers in humans exposed to radiation. To determine whether irradiated humans have RIC biomarkers, blood samples were obtained from radiotherapy patients who received treatment for different types of malignancies. Blood samples from allogeneic hematopoietic cell transplantation (allo-HCT) patients obtained before, during and after irradiation indicated that exposure led to prolonged clot formation times, increased levels of thrombin-antithrombin III (TAT) complex and increased circulating nucleosome/histone (cNH) levels, which suggest potential coagulopathies in the allo-HCT patients. Since these allo-HCT patients received chemotherapy prior to radiotherapy, it is possible that the chemical agents could have influenced the observed results. Frozen plasma samples from radiotherapy patients with prostate, lung and breast cancer were also obtained for analyses of cNH levels. The results indicated that some of these patients had very high cNH blood levels. Analysis of cNH levels in plasma samples from irradiated ferrets also indicated increased cNH levels compared to preirradiation baseline levels. The results from irradiated animals and some radiotherapy patients suggest the possibility that anti-histone antibodies, which block the toxic effects of elevated cNH levels in the blood, might be useful as therapeutic agents for adverse biological radiation-induced effects. The detection of increased levels of cNH in some radiotherapy patient blood samples demonstrates its potential as a biomarker for diagnosing and/or predicting the propensity for developing coagulopathies/hemorrhage, offering possible treatment options with personalized medicine therapies for cancer patients.

Understanding the Pathophysiology and Challenges of Development of Medical Countermeasures for Radiation-Induced Vascular/Endothelial Cell Injuries: Report of a NIAID Workshop, August 20, 2015

After the events of September 11, 2001, a decade of research on the development of medical countermeasures (MCMs) to treat victims of a radiological incident has yielded two FDA-approved agents to mitigate acute radiation syndrome. These licensed agents specifically target the mitigation of radiation-induced neutropenia and infection potential, while the ramifications of the exposure event in a public health emergency incident could include the entire body, causing additional acute and/or delayed organ/tissue injuries. Anecdotal data as well as recent findings from both radiation accident survivors and animal experiments implicate radiation-induced injury or dysfunction of the vascular endothelium leading to tissue and organ injuries. There are significant gaps in our understanding of the disease processes and progression, as well as the optimum approaches to develop medical countermeasures to mitigate radiation vascular injury. To address this issue, the Radiation and Nuclear Countermeasures Program of the National Institute of Allergy and Infectious Diseases (NIAID) organized a one-day workshop to examine the current state of the science in radiation-induced vascular injuries and organ dysfunction, the natural history of the pathophysiology and the product development maturity of potential medical countermeasures to treat these injuries. Meeting presentations were followed by a NIAID-led open discussion among academic investigators, industry researchers and government agency representatives. This article provides a summary of these presentations and subsequent discussion from the workshop.

Subject-Based versus Population-Based Care after Radiation Exposure

In a mass casualty radiation event situation, individualized therapy may overwhelm available resources and feasibility issues suggest a need for the development of population-based strategies. To investigate the efficacy of a population-based strategy, Chinese macaques (n = 46) underwent total-body irradiation and received preemptive antibiotics, IV hydration on predetermined postirradiation days and were then compared to macaques (n = 48) that received subject-based care in which blood transfusions, IV hydration, nutritional supplementation and antibiotic supportive measures were provided. Estimated radiation doses for LD30/60, LD50/60 and LD70/60 of animals with subject-based care: 6.83 Gy (6.21, 7.59), 7.44 Gy (6.99, 7.88) and 8.05 Gy (7.46, 8.64), respectively, and for population-based care: 5.61 Gy (5.28, 6.17), 6.62 Gy (6.13, 7.18) and 7.63 Gy (7.21, 8.20), respectively. Analysis of four time periods, 0-9, 10-15, 16-25 and 26-60 days postirradiation, identified significant mortality differences during the period of 10-15 days. A subset analysis of higher radiation doses (6.75-7.20 Gy, n = 32) indicated hydration, nutrition and septic status were not significantly different between treatments. Whole blood transfusion treatment, administered only in subject-supportive care, was associated with significantly higher platelet and absolute neutrophil counts. Median platelet counts greater than 5,670 cells/μl and absolute neutrophil counts greater than 26 cells/μl during this period correlated with survival. We observed that the population-based treatment increased the LD50/60 compared to nontreatment (6.62 Gy vs. 4.92 Gy) and may be further optimized during days 10-15, where strategic blood transfusions or other strategies to achieve increases in neutrophil and platelet counts may further increase survival rates in subjects exposed to high doses of radiation.

Effects of a granulocyte colony stimulating factor, Neulasta, in mini pigs exposed to total body proton irradiation

Astronauts could be exposed to solar particle event (SPE) radiation, which is comprised mostly of proton radiation. Proton radiation is also a treatment option for certain cancers. Both astronauts and clinical patients exposed to ionizing radiation are at risk for loss of white blood cells (WBCs), which are the body's main defense against infection. In this report, the effect of Neulasta treatment, a granulocyte colony stimulating factor, after proton radiation exposure is discussed. Mini pigs exposed to total body proton irradiation at a dose of 2 Gy received 4 treatments of either Neulasta or saline injections. Peripheral blood cell counts and thromboelastography parameters were recorded up to 30 days post-irradiation. Neulasta significantly improved WBC loss, specifically neutrophils, in irradiated animals by approximately 60% three days after the first injection, compared to the saline treated, irradiated animals. Blood cell counts quickly decreased after the last Neulasta injection, suggesting a transient effect on WBC stimulation. Statistically significant changes in hemostasis parameters were observed after proton radiation exposure in both the saline and Neulasta treated irradiated groups, as well as internal organ complications such as pulmonary changes. In conclusion, Neulasta treatment temporarily alleviates proton radiation-induced WBC loss, but has no effect on altered hemostatic responses.

Evidence of Disseminated Intravascular Coagulation in a Porcine Model Following Radiation Exposure

Recent evidence has suggested that disseminated intravascular coagulation (DIC) plays an integral role in death at the LD₅₀ dose of either gamma or solar particle event (SPE)-like proton radiation in ferrets. In these studies, Yucatan minipigs were evaluated to determine whether they were susceptible to the development of radiation induced DIC. Yucatan minipigs were exposed to a dose of 2.5 Gray (Gy) with x-rays and monitored over the course of 30 days. Evidence of DIC was evaluated by way of thromboelastometry parameters, platelet counts, fibrinogen concentration, and the d-dimer assay. Pigs exposed to x-rays developed signs of DIC within 2 days post-irradiation. The development of DIC was exacerbated over the course of the studies, and one of the pigs died at day 14 and another had to be euthanized on day 16 post-irradiation. For both of these pigs, DIC was evident at the time of death. The following observations were indicated or were suggestive of DIC: whole blood clotting was impaired (as evidenced by thromboelastometry alterations), there were decreased platelet counts, elevated d-dimer concentrations in the blood, and/or hemorrhaging and the presence of fibrin in tissues observed during post-mortem examination. The extrapolation of data from these studies, in combination with other published data, have led to the hypothesis that there could be a correlation between the propensity to develop DIC, as indicated by hemorrhaging at death at relatively low doses of radiation, and the LD₅₀ for a particular species. Our data suggest that the development of DIC may contribute to death at the LD₅₀ dose in large mammals.

Biological Effects of Space Radiation and Development of Effective Countermeasures

As part of a program to assess the adverse biological effects expected from astronaut exposure to space radiation, numerous different biological effects relating to astronaut health have been evaluated. There has been major focus recently on the assessment of risks related to exposure to solar particle event (SPE) radiation. The effects related to various types of space radiation exposure that have been evaluated are: gene expression changes (primarily associated with programmed cell death and extracellular matrix (ECM) remodeling), oxidative stress, gastrointestinal tract bacterial translocation and immune system activation, peripheral hematopoietic cell counts, emesis, blood coagulation, skin, behavior/fatigue (including social exploration, submaximal exercise treadmill and spontaneous locomotor activity), heart functions, alterations in biological endpoints related to astronaut vision problems (lumbar puncture/intracranial pressure, ocular ultrasound and histopathology studies), and survival, as well as long-term effects such as cancer and cataract development. A number of different countermeasures have been identified that can potentially mitigate or prevent the adverse biological effects resulting from exposure to space radiation.