Development of a new minipig model to study radiation-induced gastrointestinal syndrome and its application in clinical research

An Overview of Radiation Countermeasure Development in Radiation Research from 1954 to 2024

Preparation for medical responses to major radiation accidents, further driven by increases in the threat of nuclear warfare, has led to a pressing need to understand the underlying mechanisms of radiation injury (RI) alone or in combination with other trauma (combined injury, CI). The identification of these mechanisms suggests molecules and signaling pathways that can be targeted to develop radiation medical countermeasures. Thus far, the United States Food and Drug Administration (U.S. FDA) has approved seven countermeasures to mitigate hematopoietic acute radiation syndrome (H-ARS), but no drugs are available for prophylaxis and no agents have been approved to combat the other sub-syndromes of ARS, let alone delayed effects of acute radiation exposure or the effects of combined injury. From its inception, Radiation Research has significantly contributed to the understanding of the underlying mechanisms of radiation injury and combined injury, and to the development of radiation medical countermeasures for these indications through the publication of peer-reviewed research and review articles.

Severity scoring systems for radiation-induced GI injury - Prioritization for use of GI-ARS medical countermeasures

PURPOSE

Severity scoring systems for ionizing radiation-induced gastrointestinal injury have been used in animal radiation models, human studies involving the use of radiation therapy, and radiation accidents. Various radiation exposure scenarios (i.e., total body irradiation, total abdominal irradiation, etc.) have been used to investigate ionizing radiation-induced gastrointestinal injury. These radiation-induced GI severity scoring systems are based on clinical signs and symptoms and gastrointestinal-specific biomarkers (i.e., citrulline, etc.). In addition, the time course for radiation-induced changes in blood citrulline levels were compared across various animal (i.e., mice, minipigs, Rhesus Macaque, etc.) and human model systems.

CONCLUSIONS

A worksheet tool was developed to prioritize individuals with severe life-threatening gastrointestinal acute radiation syndrome, based on the design of the Exposure and Symptom Tool addressing hematopoietic acute radiation syndrome, to rescue individuals from potential gastrointestinal acute radiation syndrome injury. This tool provides a triage diagnostic approach to assist first-responders to assess individuals suspected of showing gastrointestinal acute radiation syndrome severity to guide medical management, hence enhancing medical readiness for managing radiological casualties.

Combined Administration of Pravastatin and Metformin Attenuates Acute Radiation-Induced Intestinal Injury in Mouse and Minipig Models

Radiation-induced gastrointestinal (GI) damage is one of the critical factors that serve as basis for the lethality of nuclear accidents or terrorism. Further, there are no Food and Drug Administration-approved agents available to mitigate radiation-induced intestinal injury. Although pravastatin (PS) has been shown to exhibit anti-inflammatory and epithelial reconstructive effects following radiation exposure using mouse and minipig models, the treatment failed to improve the survival rate of high-dose irradiated intestinal injury. Moreover, we previously found that metformin (MF), a common drug used for treating type 2 diabetes mellitus, has a mitigating effect on radiation-induced enteropathy by promoting stem cell properties. In this study, we investigated whether the combined administration of PS and MF could achieve therapeutic effects on acute radiation-induced intestinal injury in mouse and minipig models. We found that the combined treatment markedly increased the survival rate and attenuated histological damage in a radiation-induced intestinal injury mouse model, in addition to epithelial barrier recovery, anti-inflammatory effects, and improved epithelial proliferation with stem cell properties. Furthermore, in minipig models, combined treatment with PS and MF ameliorates gross pathological damage in abdominal organs and attenuated radiation-induced intestinal histological damage. Therefore, the combination of PS and MF effectively alleviated radiation-induced intestinal injury in the mouse and minipig models. We believe that the combined use of PS and MF is a promising therapeutic approach for treating radiation-induced intestinal injury.

Metallothionein 2 activation by pravastatin reinforces epithelial integrity and ameliorates radiation-induced enteropathy

Background

Radiotherapy or accidental exposure to ionizing radiation causes severe damage of healthy intestinal tissues. Intestinal barrier function is highly sensitive to ionizing radiation, and loss of epithelial integrity results in mucosal inflammation, bacterial translocation, and endotoxemia. Few studies have of epithelial integrity as a therapeutic target to treat radiation toxicity. Here, we examined the effects of pravastatin (PS) and the molecular mechanisms underlying epithelial integrity on radiation-induced enteropathy.

Methods

The radio-mitigative effects of PS were evaluated in a minipig model by quantifying clinical symptoms, and performing histological and serological analyses and mRNA sequencing in intestinal tissues. To evaluate the role of intercellular junctions on radiation damage, we used tight junction regulator and metallothionein 2 (MT2) as treatments in a mouse model of radiation-induced enteropathy. Caco-2 monolayers were used to examine functional epithelial integrityand intercellular junction expression.

Finding

Using a minipig model of pharmaceutical oral bioavailability, we found that PS mitigated acute radiation-induced enteropathy. PS-treated irradiated minipigs had mild clinical symptoms, lower intestinal inflammation and endotoxin levels, and improved gastrointestinal integrity, compared with control group animals. The results of mRNA sequencing analysis indicated that PS treatment markedly influenced intercellular junctions by inhibiting p38 MAPK signaling in the irradiated intestinal epithelium. The PS-regulated gene MT2 improved the epithelial barrier via enhancement of intercellular junctions in radiation-induced enteropathy.

Interpretation

PS regulated epithelial integrity by modulating MT2 in radiation-damaged epithelial cells. These findings suggested that maintenance of epithelial integrity is a novel therapeutic target for treatment of radiation-induced gastrointestinal damage.

Funding

As stated in the Acknowledgments

L-citrulline: A preclinical safety biomarker for the small intestine in rats and dogs in repeat dose toxicity studies

INTRODUCTION

Gastrointestinal (GI) toxicity is still an issue within drug development, especially for novel oncology drugs. The identification of GI mucosal damage at an early stage with high sensitivity and specificity across preclinical species and humans remains difficult. To date, in preclinical studies, no qualified mechanistic, diagnostic or prognostic biomarkers exist for GI mucosal toxicity. L-citrulline is one of the most promising biomarker candidates used in clinical settings to quantify enterocyte integrity in various small intestinal diseases. L-citrulline is an intermediate metabolic amino acid produced mainly by functional enterocytes of the small intestine, whereby enterocyte loss will cause a drop in circulating L-citrulline.

METHODS

In several repeat-dose toxicity studies, plasma L-citrulline has been evaluated as a potential safety biomarker for intestinal toxicity in beagle dogs and Wistar (Han) rats treated with different oncological drug candidates in drug development. Clinical observations and body weight determinations were performed during the pretreatment, treatment and treatment-free recovery period as well as toxicokinetic, gross and histopathology examinations. The quantitative determination of plasma L-citrulline levels during the pretreatment (only dogs), treatment and treatment-free recovery period were performed using an HPLC MS/MS assay. In cynomolgus monkeys, the first investigations on baseline L-citrulline levels were performed.

RESULTS

In dogs, a dose- and exposure-dependent decrease of up to 50% in plasma L-citrulline was seen without histopathological alterations. However, a decrease of more than 50% in comparison to the individual animal pretreatment value of L-citrulline correlated very well with histopathological findings (intestinal crypt necrosis, villus atrophy, enterocyte loss) and clinical signs (bloody faeces and diarrhoea). During a treatment-free recovery period, a trend of increasing levels was observed in dogs. In rats, absolute L-citrulline plasma levels of treated animals decreased compared to the values of the concurrent control group. This decrease also correlated with the histopathological findings in the small intestine (single cell necrosis and mucosa atrophy). Because of a large physiological variation in L-citrulline plasma levels in dogs and rats, a clear cut-off value for absolute L-citrulline levels predictive of intestinal mucosal toxicity was difficult to establish. However, a > 50% decrease in L-citrulline plasma levels during the treatment period strongly correlated with histopathological findings.

DISCUSSION

Based on the performed analysis, a longitudinal investigation of L-citrulline plasma levels for individual animals in the control and treatment groups is essential and pretreatment values of L-citrulline levels in rodents would be highly informative. Overall, further cross-species comparison (Cynomolgus monkey, mouse) and implementation in clinical trials as exploratory biomarker is essential to foster the hypothesis and to understand completely the clinical relevance of L-citrulline as a small intestine biomarker.

Whole blood gene expression within days after total-body irradiation predicts long term survival in Gottingen minipigs

Gottingen minipigs mirror the physiological radiation response observed in humans and hence make an ideal candidate model for studying radiation biodosimetry for both limited-sized and mass casualty incidents. We examined the whole blood gene expression profiles starting one day after total-body irradiation with increasing doses of gamma-rays. The minipigs were monitored for up to 45 days or time to euthanasia necessitated by radiation effects. We successfully identified dose- and time-agnostic (over a 1-7 day period after radiation), survival-predictive gene expression signatures derived using machine-learning algorithms with high sensitivity and specificity. These survival-predictive signatures fare better than an optimally performing dose-differentiating signature or blood cellular profiles. These findings suggest that prediction of survival is a much more useful parameter for making triage, resource-utilization and treatment decisions in a resource-constrained environment compared to predictions of total dose received. It should hopefully be possible to build such classifiers for humans in the future.

l-citrulline: A preclinical safety biomarker for the small intestine in rats and dogs in repeat dose toxicity studies

INTRODUCTION

Gastrointestinal (GI) toxicity is still an issue within drug development, especially for novel oncology drugs. The identification of GI mucosal damage at an early stage with high sensitivity and specificity across preclinical species and humans remains difficult. To date, in preclinical studies, no qualified mechanistic, diagnostic or prognostic biomarkers exist for GI mucosal toxicity. l-citrulline is one of the most promising biomarker candidates used in clinical settings to quantify enterocyte integrity in various small intestinal diseases. l-citrulline is an intermediate metabolic amino acid produced mainly by functional enterocytes of the small intestine, whereby enterocyte loss will cause a drop in circulating l-citrulline.

METHODS

In several repeat-dose toxicity studies, plasma l-citrulline has been evaluated as a potential safety biomarker for intestinal toxicity in beagle dogs and Wistar (Han) rats treated with different oncological drug candidates in drug development. Clinical observations and body weight determinations were performed during the pretreatment, treatment and treatment-free recovery period as well as toxicokinetic, gross and histopathology examinations. The quantitative determination of plasma l-citrulline levels during the pretreatment (only dogs), treatment and treatment-free recovery period were performed using an HPLC MS/MS assay. In cynomolgus monkeys, the first investigations on baseline l-citrulline levels were performed.

RESULTS

In dogs, a dose- and exposure-dependent decrease of up to 50% in plasma l-citrulline was seen without histopathological alterations. However, a decrease of more than 50% in comparison to the individual animal pretreatment value of l-citrulline correlated very well with histopathological findings (intestinal crypt necrosis, villus atrophy, enterocyte loss) and clinical signs (bloody faeces and diarrhoea). During a treatment-free recovery period, a trend of increasing levels was observed in dogs. In rats, absolute l-citrulline plasma levels of treated animals decreased compared to the values of the concurrent control group. This decrease also correlated with the histopathological findings in the small intestine (single cell necrosis and mucosa atrophy). Because of a large physiological variation in l-citrulline plasma levels in dogs and rats, a clear cut-off value for absolute l-citrulline levels predictive of intestinal mucosal toxicity was difficult to establish. However, a > 50% decrease in l-citrulline plasma levels during the treatment period strongly correlated with histopathological findings.

DISCUSSION

Based on the performed analysis, a longitudinal investigation of l-citrulline plasma levels for individual animals in the control and treatment groups is essential and pretreatment values of l-citrulline levels in rodents would be highly informative. Overall, further cross-species comparison (Cynomolgus monkey, mouse) and implementation in clinical trials as exploratory biomarker is essential to foster the hypothesis and to understand completely the clinical relevance of l-citrulline as a small intestine biomarker.

Evaluation of the radiation response and regenerative effects of mesenchymal stem cell-conditioned medium in an intestinal organoid system

Intestinal organoids have recently emerged as an in vitro model relevant to the gut system owing to their recapitulation of the native intestinal epithelium with crypt-villus architecture. However, it is unclear whether intestinal organoids reflect the physiology of the in vivo stress response. Here, we systemically investigated the radiation response in organoids and animal models using mesenchymal stem cell-conditioned medium (MSC-CM), which contains secreted paracrine factors. Irradiated organoids exhibited sequential induction of viability loss and regrowth after irradiation (within 12 days), similar to the response of the native intestinal epithelium. Notably, treatment with MSC-CM facilitated the reproliferation of intestinal stem cells (ISCs) and restoration of damaged crypt-villus structures in both models. Furthermore, Wnt/Notch signaling pathways were commonly upregulated by MSC-CM, but not radiation, and pharmacologically selective inhibition of Wnt or Notch signaling attenuated the enhanced recovery of irradiated organoids, with increases in ISCs, following MSC-CM treatment. Interestingly, the expression of Wnt4, Wnt7a, and active β-catenin was increased, but not notch family members, in MSC-CM-treated organoid after irradiation. Treatment of recombinant mouse Wnt4 and Wnt7a after irradiation improved to some extent intestinal epithelial regeneration both in vitro and in vivo. Overall, these results suggested that intestinal organoids recapitulated the physiological stress response of the intestinal epithelium in vivo. Thus, our findings provided important insights into the physiology of intestinal organoids and may contribute to the development of strategies to enhance the functional maturation of engineered organoids.

A Systematic Review of Metabolomic and Lipidomic Candidates for Biomarkers in Radiation Injury

A large-scale nuclear event has the ability to inflict mass casualties requiring point-of-care and laboratory-based diagnostic and prognostic biomarkers to inform victim triage and appropriate medical intervention. Extensive progress has been made to develop post-exposure point-of-care biodosimetry assays and to identify biomarkers that may be used in early phase testing to predict the course of the disease. Screening for biomarkers has recently extended to identify specific metabolomic and lipidomic responses to radiation using animal models. The objective of this review was to determine which metabolites or lipids most frequently experienced perturbations post-ionizing irradiation (IR) in preclinical studies using animal models of acute radiation sickness (ARS) and delayed effects of acute radiation exposure (DEARE). Upon review of approximately 65 manuscripts published in the peer-reviewed literature, the most frequently referenced metabolites showing clear changes in IR induced injury were found to be citrulline, citric acid, creatine, taurine, carnitine, xanthine, creatinine, hypoxanthine, uric acid, and threonine. Each metabolite was evaluated by specific study parameters to determine whether trends were in agreement across several studies. A select few show agreement across variable animal models, IR doses and timepoints, indicating that they may be ubiquitous and appropriate for use in diagnostic or prognostic biomarker panels.

The hunt for radiation biomarkers: current situation

Purpose: The possibility of a large-scale acute radiation exposure necessitates the development of new methods that could provide a rapid assessment of the doses received by individuals using high-throughput technologies. There is also a great interest in developing new biomarkers of dose exposure, which could be used in large molecular epidemiological studies in order to correlate estimated doses received and health effects. The goal of this review was to summarize current literature focused on biological dosimetry, namely radiation-responsive biomarkers.Methods: The studies involved in this review were thoroughly selected according to the determined criteria and PRISMA guidelines.Results: We described briefly recent advances in radiation genomics and metabolomics, giving particular emphasis to proteomic analysis. The majority of studies were performed on animal models (rats, mice, and non-human primates). They have provided much beneficial information, but the most relevant tests have been done on human (oncological) patients. By inspecting the radiaiton biodosimetry literate of the last 10 years, we identified a panel of candidate markers for each -omic approach involved.Conslusions: We reviewed different methodological approaches and various biological materials, which can be exploited for dose-effect prediction. The protein biomarkers from human plasma are ideal for this specific purpose. From a plethora of candidate markers, FDXR is a very promising transcriptomic candidate, and importantly this biomarker was also confirmed by some studies at protein level in humans.

Morphological and functional impairment in the gut in a partial body irradiation minipig model of GI-ARS

Purpose: Göttingen minipig (G-MP) displays classic gastrointestinal acute radiation syndrome (GI-ARS) following total body irradiation (TBI) at GI doses which are lethal by 10-14 days. In collaboration with BARDA, we are developing a hemi-body/partial body irradiation (PBI) model by exposing only the abdomen and lower extremities to study GI structure/function impairment, natural history of injury and recovery, as well as correlative biomarkers out to 30 days.Materials and methods: Twenty-four G-MP were exposed to either 12 or 16 Gy (LINAC Elekta); head, forelimbs, and thorax were outside the irradiation field, sparing ∼50% of the bone marrow. Animals were followed for 30 days with euthanasia scheduled at pre-set intervals to study the time course of GI injury and recovery. Hematological profiles, clinical symptoms, gross- and histo-pathology including markers of proliferation and apoptosis in the small intestines, gut function parameters (food tolerance, digestion, absorption, citrulline production), and levels of two biomarkers, CRP and IGF-1, were evaluated.Results: PBI at 16 Gy yielded higher lethality than 12 Gy. Unlike TBI, PBI did not cause severe pancytopenia or external hemorrhage, as expected, and allowed to focus the injury on GI organs while sparing the radiation sensitive heart and lung. Compromised animals showed inactivity, anorexia, vomiting, diarrhea, and weight loss. Histology revealed that in 12 Gy irradiated animals, lesions recovered overtime. In 16 Gy irradiated animals, lesions were more pronounced and persistent. BrdU and Ki67 labelling demonstrated dose-dependent loss of crypts and subsequent mucosal ulceration which recovered over time. Minimal apoptosis was observed at both doses. Reductions in food tolerance, digestion, absorption, and citrulline production were time and dose-dependent. Loss of citrulline reached a nadir between 6-12 days and then recovered partially. CRP and IGF-1 were upregulated following PBI at GI doses.Conclusions: This lower hemi-body irradiation model allowed for extended survival at GI-specific ARS doses and development of a well-controlled GI syndrome with minimal hematopoietic injury or confounding mortality from cardiopulmonary damage. A dose-dependent impairment in the intestinal structure resulted in overall decreased gut functionality followed by a partial recovery. However, while the structure appeared to be recovered, not all functionality was attained. PBI induced systemic inflammation and altered the IGF-1 hormone indicating that these can be used as biomarkers in the minipig even under partial body conditions. This PBI model aligns with other minipig models under BARDA's large animal consortium to test medical countermeasure efficacy against a less complex GI-specific ARS injury.

Pilot Study of Radiation-induced Gastrointestinal Injury in a Hemi-body Shielded Göttingen Minipig Model

Development of medical countermeasures (MCMs) for gastrointestinal (GI) injury following acute radiation exposure requires well-characterized models that can assess not only survival but also secondary endpoints, including structural and functional characteristics of GI damage and recovery that ultimately contribute to long-term survival. The authors conducted a pilot study in a hemi-body shielded Göttingen minipig model of radiation-induced GI injury that enables radiation damage to the GI tract to be evaluated and reduces the potential for hemorrhage and/or damage in other more sensitive organ systems. With shielding of the head, chest, and front legs, radiation dose levels of 14 Gy were required to see significant GI-related morbidity, while dose levels of 16 Gy resulted in significant mortality by day 45 post-irradiation. Periodic scheduled necropsies showed significant reduction in and slow recovery of intestinal crypt count at 14 and 16 Gy. Intestinal proliferative activity was initially increased and then gradually decreased over the course of the study. Histological evidence of marked inflammatory infiltrates was noted in the GI tract at day 5, while collagen deposition, indicative of fibrosis, was observed as early as day 15, peaking at day 30. The radiation dose-responsive indicators of GI damage identified in this model (i.e., intestinal crypt count and proliferative activity) may serve as useful endpoints for evaluation of the efficacy of potential MCMs.

Expanded Characterization of a Hemi-Body Shielded Göttingen Minipig Model of Radiation-induced Gastrointestinal Injury Incorporating Oral Dosing Procedures

In collaboration with the Biomedical Advanced Research and Development Authority (BARDA), the authors recently conducted a pilot study in a hemi-body shielded model of radiation-induced gastrointestinal (GI) injury in Göttingen minipigs following exposure to radiation dose levels between 8-16 Gy. Herein, the impact of oral dosing procedures is assessed, as well as the specific causes of death in animals exposed to radiation doses of 14 and 16 Gy (n = 64; 32 male, 32 female, between 6 and 8 mo of age). Oral dosing using a 2-tablet placebo system comprised of both immediate release and enteric-coated tablets starting 24 h post-irradiation resulted in inhibited gastric emptying of the enteric-coated tablets, which were found to be retained in the stomach and/or regurgitated. This finding appears to be species-specific, as similar findings have not been reported for other large animal species (e.g., non-human primates). Mortality was primarily dictated by decreased activity, body weight loss (>35%), and/or respiratory distress, despite shielding of the lung. The cause of respiratory distress in animals that were pre-terminally euthanized varied according to the timing of death, with interstitial inflammation and extensive fibrosis observed >20 days post-irradiation. Kidney damage was also identified in most animals after day 10. Changes in the GI tract were consistent with previous studies and included collagen deposition/fibrosis. Observations of inflammatory infiltrates and interstitial inflammation/fibrosis in both shielded and unshielded organs support a strong secondary inflammatory syndrome post-irradiation.

Development of a minipig physical phantom from CT data

Quantification of pathological progression of radiation-induced injury is essential in development of treatment methods, and a proper animal model is necessary for relevant radiological and medical studies. A minipig is a current animal model selected because of its similarities to humans in anatomy and pathology. In the present study, a minipig physical phantom was developed using computed tomography (CT) data. For dosimetry purposes, the minipig physical phantom was constructed on a slice-by-slice basis, with an array of holes to accommodate dosimeters. The phantom is constituted of three major organs, i.e. bone, lung, and remaining soft tissue, and the organs are clearly distinguishable on each 20-mm-thick axial slice. The quality of the tissue-equivalent (TE) substitutes was analyzed in terms of the atomic compositions and Hounsfield units (HUs). The density (in g/cm3) and effective atomic number of TE substitutes for the bone, lung, and soft tissue are 1.4 and 7.9, 0.5 and 10.0, and 1.0 and 5.9, respectively. Although the TE substitutes have slightly different physical properties, we think the phantom is acceptable because the HU values of the TE substitutes lie in the HU range of real tissues.

A review of radiation countermeasures focusing on injury-specific medicinals and regulatory approval status: part I. Radiation sub-syndromes, animal models and FDA-approved countermeasures

PURPOSE

The increasing global risk of nuclear and radiological accidents or attacks has driven renewed research interest in developing medical countermeasures to potentially injurious exposures to acute irradiation. Clinical symptoms and signs of a developing acute radiation injury, i.e. the acute radiation syndrome, are grouped into three sub-syndromes named after the dominant organ system affected, namely the hematopoietic, gastrointestinal, and neurovascular systems. The availability of safe and effective countermeasures against the above threats currently represents a significant unmet medical need. This is the first article within a three-part series covering the nature of the radiation sub-syndromes, various animal models for radiation countermeasure development, and the agents currently approved by the United States Food and Drug Administration for countering the medical consequences of several of these prominent radiation exposure-associated syndromes.

CONCLUSIONS

From the U.S. and global perspectives, biomedical research concerning medical countermeasure development is quite robust, largely due to increased government funding following the 9/11 incidence and subsequent rise of terrorist-associated threats. A wide spectrum of radiation countermeasures for specific types of radiation injuries is currently under investigation. However, only a few radiation countermeasures have been fully approved by regulatory agencies for human use during radiological/nuclear contingencies. Additional research effort, with additional funding, clearly will be needed in order to fill this significant, unmet medical health problem.

Long-term culture-induced phenotypic difference and efficient cryopreservation of small intestinal organoids by treatment timing of Rho kinase inhibitor

AIM

To investigate a suitable long-term culture system and optimal cryopreservation of intestinal organoid to improve organoid-based therapy by acquiring large numbers of cells.

METHODS

Crypts were isolated from jejunum of C57BL/6 mouse. Two hundred crypts were cultured in organoid medium with either epidermal growth factor/Noggin/R-spondin1 (ENR) or ENR/CHIR99021/VPA (ENR-CV). For subculture, organoids cultured on day 7 were passaged using enzyme-free cell dissociation buffer (STEMCELL Technologies). The passage was performed once per week until indicated passage. For cryopreservation, undissociated and dissociated organoids were resuspended in freezing medium with or without Rho kinase inhibitor subjected to different treatment times. The characteristics of intestinal organoids upon extended passage and freeze-thaw were analyzed using EdU staining, methyl thiazolyl tetrazolium assay, qPCR and time-lapse live cell imaging.

RESULTS

We established a three-dimensional culture system for murine small intestinal organoids using ENR and ENR-CV media. Both conditions yielded organoids with a crypt-villus architecture exhibiting Lgr5⁺ cells and differentiated intestinal epithelial cells as shown by morphological and biochemical analysis. However, during extended passage (more than 3 mo), a comparative analysis revealed that continuous passaging under ENR-CV conditions, but not ENR conditions induced phenotypic changes as observed by morphological transition, reduced numbers of Lgr5⁺ cells and inconsistent expression of markers for differentiated intestinal epithelial cell types. We also found that recovery of long-term cryopreserved organoids was significantly affected by the organoid state, i.e., whether dissociation was applied, and the timing of treatment with the Rho-kinase inhibitor Y-27632. Furthermore, the retention of typical morphological characteristics of intestinal organoids such as the crypt-villus structure from freeze-thawed cells was observed by live cell imaging.

CONCLUSION

The maintenance of the characteristics of intestinal organoids upon extended passage is mediated by ENR condition, but not ENR-CV condition. Identified long-term cryopreservation may contribute to the establishment of standardized cryopreservation protocols for intestinal organoids for use in clinical applications.

Metabolomic applications in radiation biodosimetry: exploring radiation effects through small molecules

PURPOSE

Exposure of the general population to ionizing radiation has increased in the past decades, primarily due to long distance travel and medical procedures. On the other hand, accidental exposures, nuclear accidents, and elevated threats of terrorism with the potential detonation of a radiological dispersal device or improvised nuclear device in a major city, all have led to increased needs for rapid biodosimetry and assessment of exposure to different radiation qualities and scenarios. Metabolomics, the qualitative and quantitative assessment of small molecules in a given biological specimen, has emerged as a promising technology to allow for rapid determination of an individual's exposure level and metabolic phenotype. Advancements in mass spectrometry techniques have led to untargeted (discovery phase, global assessment) and targeted (quantitative phase) methods not only to identify biomarkers of radiation exposure, but also to assess general perturbations of metabolism with potential long-term consequences, such as cancer, cardiovascular, and pulmonary disease.

CONCLUSIONS

Metabolomics of radiation exposure has provided a highly informative snapshot of metabolic dysregulation. Biomarkers in easily accessible biofluids and biospecimens (urine, blood, saliva, sebum, fecal material) from mouse, rat, and minipig models, to non-human primates and humans have provided the basis for determination of a radiation signature to assess the need for medical intervention. Here we provide a comprehensive description of the current status of radiation metabolomic studies for the purpose of rapid high-throughput radiation biodosimetry in easily accessible biofluids and discuss future directions of radiation metabolomics research.

A Model for Precise and Uniform Pelvic- and Limb-Sparing Abdominal Irradiation to Study the Radiation-Induced Gastrointestinal Syndrome in Mice Using Small Animal Irradiation Systems

Background and Purpose:

Currently, no readily available mitigators exist for acute abdominal radiation injury. Here, we present an animal model for precise and homogenous limb-sparing abdominal irradiation (LSAIR) to study the radiation-induced gastrointestinal syndrome (RIGS).

Materials and Methods:

The LSAIR technique was developed using the small animal radiation research platform (SARRP) with image guidance capabilities. We delivered LSAIR at doses between 14 and 18 Gy on 8- to 10-week-old male C57BL/6 mice. Histological analysis was performed to confirm that the observed mortality was due to acute abdominal radiation injury.

Results:

A steep dose–response relationship was found for survival, with no deaths seen at doses below 16 Gy and 100% mortality at above 17 Gy. All deaths occurred between 6 and 10 days after irradiation, consistent with the onset of RIGS. This was further confirmed by histological analysis showing clear differences in the number of regenerative intestinal crypts between animals receiving sublethal (14 Gy) and 100% lethal (18 Gy) radiation.

Conclusion:

The developed LSAIR technique provides uniform dose delivery with a clear dose response, consistent with acute abdominal radiation injury on histological examination. This model can provide a useful tool for researchers investigating the development of mitigators for accidental or clinical high-dose abdominal irradiation.

Citrulline as a Biomarker for Gastrointestinal-Acute Radiation Syndrome: Species Differences and Experimental Condition Effects

Animal models of hematopoietic and gastrointestinal acute radiation syndromes (ARS) have been characterized to develop medical countermeasures. Acute radiation-induced decrease of intestinal absorptive function has been correlated to a decrease in the number of intestinal crypt cells resulting from apoptosis and enterocyte mass reduction. Citrulline, a noncoded amino acid, is produced almost exclusively by the enterocytes of the small intestine. Citrullinemia has been identified as a simple, sensitive and suitable biomarker for radiation-induced injury associated with gastrointestinal ARS (GI-ARS). Here we discuss the effect of radiation on plasma citrulline levels in three different species, C57BL/6 mice, Göttingen minipigs and rhesus nonhuman primates (NHPs), measured by liquid chromatography tandem mass spectrometry (LC-MS/MS). The effects of experimental study conditions such as feeding and anesthesia were also examined on plasma citrulline levels in the NHPs. Both the mice and Göttingen minipigs were partial-body irradiated (PBI) with doses from 13-17 Gy and 8-16 Gy, respectively, whereas NHPs were total-body irradiated (TBI) with doses from 6.72-13 Gy. Blood samples were taken at different time points and plasma citrulline levels were measured in the three species at baseline and after irradiation. Basal plasma citrulline concentrations (mean ± SEM) in mice and minipigs were 57.8 ± 2.8 μM and 63.1 ± 2.1 μM, respectively. NHPs showed a basal plasma citrulline concentration of 32.6 ± 0.7 μM, very similar to that of humans (∼40 μM). Plasma citrulline progressively decreased after irradiation, reaching nadir values between day 3.5 and 7. The onset of citrulline recovery was observed earlier at lower radiation doses, while only partial citrulline recovery was noted at higher radiation doses in minipigs and NHPs, complete recovery was noted in mice at all doses. Plasma citrulline levels in NHPs anesthetized with ketamine and acepromazine significantly decreased by 35.5% (P = 0.0017), compared to unanesthetized NHPs. In the postprandial state, citrulline concentrations in NHPs were slightly but significantly decreased by 12.2% (P = 0.0287). These results suggest that plasma citrulline is affected by experimental conditions such as anesthesia and feeding.

Establishment of a mouse model of 70% lethal dose by total-body irradiation

Whereas increasing concerns about radiation exposure to nuclear disasters or side effects of anticancer radiotherapy, relatively little research for radiation damages or remedy has been done. The purpose of this study was to establish level of LD70/30 (a lethal dose for 70% of mice within 30 days) by total-body γ irradiation (TBI) in a mouse model. For this purpose, at first, 8-week-old male ICR and C57BL/6N mice from A and B companies were received high dose (10, 11, 12 Gy) TBI. After irradiation, the body weight and survival rate were monitored for 30 days consecutively. In next experiment, 5-week-old male ICR and C57BL/6N mice from B company were received same dose irradiation. Results showed that survival rate and body weight change rate in inbred C57BL/6N mice were similar between A and B company. In ICR mice, however, survival rate and body weight change rate were completely different among the companies. Significant difference of survival rate both ICR and C57BL6N mice was not observed in between 5-week-old and 8-week-old groups receiving 10 or 12 Gy TBI. Our results indicate that the strain and age of mice, and even purchasing company (especially outbred), should be matched over experimental groups in TBI experiment. Based on our results, 8-week-old male ICR mice from B company subjected to 12 Gy of TBI showed LD70/30 and suitable as a mouse model for further development of new drug using the ideal total-body irradiation model.

Development of a minipig model for lung injury induced by a single high-dose radiation exposure and evaluation with thoracic computed tomography

Radiation-induced lung injury (RILI) due to nuclear or radiological exposure remains difficult to treat because of insufficient clinical data. The goal of this study was to establish an appropriate and efficient minipig model and introduce a thoracic computed tomography (CT)-based method to measure the progression of RILI. Göttingen minipigs were allocated to control and irradiation groups. The most obvious changes in the CT images after irradiation were peribronchial opacification, interlobular septal thickening, and lung volume loss. Hounsfield units (HU) in the irradiation group reached a maximum level at 6 weeks and decreased thereafter, but remained higher than those of the control group. Both lung area and cardiac right lateral shift showed significant changes at 22 weeks post irradiation. The white blood cell (WBC) count, a marker of pneumonitis, increased and reached a maximum at 6 weeks in both peripheral blood and bronchial alveolar lavage fluid. Microscopic findings at 22 weeks post irradiation were characterized by widening of the interlobular septum, with dense fibrosis and an increase in the radiation dose-dependent fibrotic score. Our results also showed that WBC counts and microscopic findings were positively correlated with the three CT parameters. In conclusion, the minipig model can provide useful clinical data regarding RILI caused by the adverse effects of high-dose radiotherapy. Peribronchial opacification, interlobular septal thickening, and lung volume loss are three quantifiable CT parameters that can be used as a simple method for monitoring the progression of RILI.

Use of biomarkers for assessing radiation injury and efficacy of countermeasures

Several candidate drugs for acute radiation syndrome (ARS) have been identified that have low toxicity and significant radioprotective and radiomitigative efficacy. Inasmuch as exposing healthy human volunteers to injurious levels of radiation is unethical, development and approval of new radiation countermeasures for ARS are therefore presently based on animal studies and Phase I safety study in healthy volunteers. The Animal Efficacy Rule, which underlies the Food and Drug Administration approval pathway, requires a sound understanding of the mechanisms of injury, drug efficacy, and efficacy biomarkers. In this context, it is important to identify biomarkers for radiation injury and drug efficacy that can extrapolate animal efficacy results, and can be used to convert drug doses deduced from animal studies to those that can be efficacious when used in humans. Here, we summarize the progress of studies to identify candidate biomarkers for the extent of radiation injury and for evaluation of countermeasure efficacy.

Citrulline as a Biomarker in the Murine Total-Body Irradiation Model: Correlation of Circulating and Tissue Citrulline to Small Intestine Epithelial Histopathology

The use of plasma citrulline as a biomarker for gastrointestinal acute radiation syndrome via exposure to total-body irradiation in a murine model was investigated. The radiation exposure covered lethal, mid-lethal, and sub-lethal gastrointestinal acute radiation syndrome. Plasma citrulline profiles were generated over the first 6 d following total-body irradiation exposure of 6-15 Gy. In addition, plasma citrulline was comprehensively evaluated in the context of matching small intestine citrulline and histopathology. Higher plasma citrulline was significantly associated with lower irradiation doses over the first 6 d following the irradiation insult. Furthermore, higher plasma citrulline was significantly associated with higher crypt survival. The correlation of the plasma citrulline to crypt survival was more robust for higher irradiation doses and for later time points. The data suggested plasma citrulline was most informative for reflecting gastrointestinal injury resulting from exposure to 9-15 Gy total-body irradiation covering time-points 2-5 d post the irradiation insult.

Radiation countermeasure agents: an update (2011-2014)

INTRODUCTION

Despite significant scientific advances over the past 60 years towards the development of a safe, nontoxic and effective radiation countermeasure for the acute radiation syndrome (ARS), no drug has been approved by the US FDA. A radiation countermeasure to protect the population at large from the effects of lethal radiation exposure remains a significant unmet medical need of the US citizenry and, thus, has been recognized as a high priority area by the government.

AREA COVERED

This article reviews relevant publications and patents for recent developments and progress for potential ARS treatments in the area of radiation countermeasures. Emphasis is placed on the advanced development of existing agents since 2011 and new agents identified as radiation countermeasure for ARS during this period.

EXPERT OPINION

A number of promising radiation countermeasures are currently under development, seven of which have received US FDA investigational new drug status for clinical investigation. Four of these agents, CBLB502, Ex-RAD, HemaMax and OrbeShield, are progressing with large animal studies and clinical trials. G-CSF has high potential and well-documented therapeutic effects in countering myelosuppression and may receive full licensing approval by the US FDA in the future.