Acute, whole-body exposure to ionizing radiation has the potential to induce widespread injury, compromising multiple organ systems and predisposing the individual to a variety of pathologies. For medical countermeasures to advance towards regulatory approval, the United States Food and Drug Administration Animal Rule requires the use of validated large animal models of acute radiation syndrome (ARS). The rhesus macaque (Macaca mulatta) serves as a primary model for therapeutic development due to its close genetic, anatomic, and physiological homology to humans. Characterizing the nature of ARS is critical prior to drug development as it provides detailed insights into the system-specific responses to radiation exposure. In this study, we assessed the effects of 5.8 and 6.5 Gy total-body irradiation (TBI) on multiple organ systems in male and female rhesus macaques through histological evaluations of various tissues, clinical parameters, complete blood counts, and serum biochemistry. A total of 31 nonhuman primates were used to investigate the injury cascade of ARS. Our results demonstrate that TBI induces consistent lung injury along with gastrointestinal and hematopoietic alterations. Of note, severe splenic depletion in non-surviving animals, alongside unexpected tissue-specific responses such as reversed trends in sternum cellularity, were observed. Hematopoietic suppression was dose-dependent, with profound leukopenia, neutropenia, and thrombocytopenia, while red blood cell parameters declined more gradually. Serum biochemistry further revealed dynamic changes in various renal, hepatic, and pancreatic markers. Comparisons between 5.8 and 6.5 Gy exposures, as well as between males and females, revealed differences in injury severity and recovery, underscoring the need to account for both dose and sex in ARS model characterization.

Despite significant radiobiological advancements following World War II, only a limited number of medical countermeasures (MCMs) have been approved by the United States Food and Drug Administration (US FDA) for acute radiation exposure related illnesses. Accordingly, well-characterized and validated animal models, both large and small, are still very much needed to develop safe and effective countermeasures. Animal models that are used for such purposes need to reflect not only the clinical and pathogenic features of those seen in radiation exposed humans, but also comparable radiation dose- and time-dependent relationships. The objective of the present study therefore was to further characterize the response patterns of rhesus nonhuman primates exposed to total-body, potentially lethal, radiation doses using the Armed Forces Radiobiology Research Institute high level cobalt-60 gamma-radiation source. Response patterns of male and female rhesus macaques were assessed following acute, total-body exposures to potentially lethal, gamma rays (5.8, 6.5, and 7.2 Gy). Groups of 15, 16, and 8 animals were exposed to the three radiation doses, respectively. All animals were provided a minimum, subject-based supportive care, that excluded the use of blood products. Blood products were excluded in order to replicate a large scale radiological/nuclear scenario treatment option in which access to blood products may be limited or unavailable. This is also relevant for military scenarios, in which medical facilities may not have the appropriate capabilities for blood transfusions. All animals were clinically monitored for 60 days post-irradiation. Survival was the primary endpoint of this study, while secondary endpoints included recovery of various hematopoietic elements. The mortality rates of the rhesus macaques were 33%, 37.5% and 50%, respectively, for the three radiation doses (i.e., 5.8, 6.5 and 7.2 Gy). Within the surviving animals, hematological blood values had returned largely to pre-exposure levels by the end of the study period. The results of this study provides foundational data on the use of the rhesus macaque model for subsequent development and testing of new radiation MCMs, as per required by the US FDA Animal Rule.

PLX-R18 is a novel cell-based product of ex vivo expanded adherent human-placenta-derived stromal cells. After intramuscular administration, these living cells are capable of secreting various cytokines that produce a therapeutic benefit. The endogenously secreted cytokines facilitate the recovery of hematopoietic progenitor cells and regenerate multiple blood lineage cells. Preclinical studies have demonstrated that PLX-R18 cells can prevent and also mitigate hematopoietic acute radiation syndrome in experimental animal models. This agent has an open US Food and Drug Administration investigational drug status for hematopoietic system-associated ARS (H-ARS). A phase I study using patients with bone marrow failure demonstrated the safety of the agent while promoting hematopoietic regeneration in humans.

Well-characterized animal models of acute radiation syndrome are needed for the development of radiation medical countermeasures to mitigate injury due to acute exposure to high doses of total- or partial-body radiation. Such animal models must reveal a radiation dose- and time-dependent relationship, pathogenesis of injury, and clinical presentation similar to humans. The focus of this study was to investigate clinical responses, principally lethality patterns, of cynomolgus macaques acutely exposed to relatively high doses of total-body radiation. Such investigations are currently relevant due to the limited availability of rhesus macaques, the dominant and preferred macaque subspecies, due to limited supply and their use in other high-priority areas. In this study employing cynomolgus macaques, a preliminary dose-response relationship was determined using three different radiation doses (4.7, 5.8 and 6.5 Gy, n = 24, n = 8/radiation dose) at a dose rate of 0.6 Gy/min. Animals were provided subject-based supportive care excluding blood products and were monitored for 60 days postirradiation for survival, which was the primary endpoint and the secondary endpoint was hematopoietic recovery. The lethality curve suggested LD30/60, LD50/60, and LD70/60 values as 4.8, 5.3, and 5.8 Gy, respectively. The initial results of this study are deemed critical for future efficacy assessments of newly developed medical countermeasures for acute radiation injuries by making use of an alternative subspecies of macaques, namely cynomolgus macaques (Macaca fascicularis).

INTRODUCTION:The availability of well-characterized small and large animal models is critical for the discovery and development of new drugs that counter the negative health effects of unwanted, acute ionizing radiation exposures. AREA COVERED:This article discusses the opportunities and challenges of small and large animal models for the development and regulatory approval of novel drugs for acute radiation syndrome (ARS). Various animal models of ARS have been analyzed for both strengths and weaknesses relative to the development of drugs for ARS following the Food and Drug Administration (FDA) Animal Rule. This article is based on a search of literature utilizing PubMed, covering the period up to March 2025. EXPERT OPINION:Relative to large animal models, the rhesus macaque model is currently the most used and best characterized for translational relevance. Other large animal models (e.g. minipig) are currently used as well to evaluate other specific types of acute injury, such as cutaneous injuries. Due to the limited supply of rhesus macaques for studying radiation injury and countermeasure development, it is of some urgency to further characterize and consider the use of alternative models, especially large animal models, for advanced research and subsequent regulatory approval of ARS countering drugs.

Radiation medical countermeasure development under the United States Food and Drug Administration Animal Rule needs validated large animal models of acute radiation syndrome. Such a well-established large animal model is the rhesus nonhuman primate. The potential use of the rhesus for other high priority areas and limited supply of such animals emphasizes the need to validate other large animal models, in particular other macaque models, in order to compensate for the lack of rhesus macaques for radiation countermeasure development. Based on existing data, cynomolgus macaques, Macaca fascicularis, are a viable alternative, but need further characterization. Reliance on such animal models requires that the models are well validated. Data gathered from rhesus and cynomolgus macaques under the same experimental conditions are not available; therefore, the authors compared and contrasted here the radiosensitivity of both macaques receiving same levels of clinical support and exposed to same doses of total-body gamma-radiation, and at same dose rates using same radiation source. Under matched experimental conditions, significant differences between acutely irradiated rhesus and cynomolgus macaques relative to the rates of survival and blood cell changes were observed. The presented data demonstrate that the cynomolgus macaque is more sensitive to ionizing radiation exposure. Overall, data supports the concept that the cynomolgus macaque is a viable, potentially useful alternative large animal model for the evaluation of radiation medical countermeasures. For comparative purposes however, additional studies with both macaques under identical experimental conditions, such as levels of clinical support and different radiation qualities with males and females ran concurrently in future will be critically important.

The rhesus macaque (Macaca mulatta) is the primary nonhuman primate (NHP) model used for the development of radiation medical countermeasures (MCMs), but due to the limited supply of rhesus macaques that has resulted from their need in other high priority medical research areas, alternative animal models for MCM development have been sought. The cynomolgus macaque (Macaca fascicularis) is less well characterized and less commonly used, but represents another quite viable, large animal NHP model for investigating MCMs. We have investigated the nature of injuries within selected organ systems induced by two potentially lethal doses (5.8 and 6.5 Gy) of ionizing radiation delivered as a total-body exposure to both rhesus and cynomolgus NHPs. Results suggest that the injuries within organs with strong self-renewing capacities (gastrointestinal and lymphohematopoietic systems) were comparable between the two NHP species, although the severity of the injuries differed. By contrast, the nature and seriousness of noted tissue pathologies were more comparable for other tissues with more limited self-renewal. In aggregate, however, the observed radiation-associated pathologies in various organs appeared to be more prominent within cynomolgus NHPs and hence, were somewhat more sensitive to the radiation exposures compared to rhesus NHPs.

INTRODUCTION:Radiation-exposed victims are often subjected to additional traumas such as wounds, burns, hemorrhages, or infections, commonly referred to as radiation combined injury (RCI). Though significant advances have been made over the last three decades toward the development of effective drugs for RCI, no specific agent for the syndrome has yet been approved by the Food and Drug Administration (FDA). AREAS COVERED:This article covers the development of drugs to treat RCI with critical evaluation of both small (mouse, rats) and large (canines and swine) animal models. These models of RCI have been analyzed for strengths and weaknesses relative to drug development. The major categories of medicinals of interest include new classes of a) anti-radiation agents (prophylactics/mitigators/therapeutics), b) tissue-reparative recombinants (growth factors/cytokines), c) blood products (artificial blood cells, stem cells), and d) new generation(s) of broad-spectrum antibiotics (ciprofloxacin). This review is based on the PubMed search of literature covering the period up to October 2025. EXPERT OPINION:Several animal models are currently being developed to study RCI and drugs for its treatment. These animal models are important for regulatory approval of RCI drugs designed to enhance survival outcomes.

Exposure to high, marginally lethal doses or higher of ionizing radiation, either intentional or accidental, results in injury to various organs. Currently, there is only a limited number of safe and effective radiation countermeasures approved by US Food and Drug Administration for such injuries. These approved agents are effective for only the hematopoietic component of the acute radiation syndrome and must be administered only after the exposure event: currently, there is no FDA-approved agent that can be used prophylactically. The nutraceutical, gamma-tocotrienol (GT3) has been found to be a promising radioprotector of such exposure-related injuries, especially those of a hematopoietic nature, when tested in either rodents or nonhuman primates. We investigated the nature of injuries and the possible protective effects of GT3 within select organ systems/tissues caused by both non-lethal level (4.0 Gy), as well as potentially lethal level (5.8 Gy) of ionizing radiation, delivered as total-body or partial-body exposure. Results indicated that the most severe, dose-dependent injuries occurred within those organ systems with strong self-renewing capacities (e.g., the lymphohematopoietic and gastrointestinal systems), while in other tissues (e.g., liver, kidney, lung) endowed with less self-renewal, the pathologies noted tended to be less pronounced and less dependent on the level of exposure dose or on the applied exposure regimen. The prophylactic use of the test nutraceutical, GT3, appeared to limit the extent of irradiation-associated pathology within blood forming tissues and, to some extent, within the small intestine of the gastrointestinal tract. No distinct, global pattern of bodily protection was noted with the agent’s use, although a hint of a possible radioprotective benefit was suggested not only by a lessening of apparent injury within select organ systems, but also by way of noting the lack of early onset of moribundity within select GT3-treated animals.

Despite remarkable scientific progress over the past six decades within the medical arts and in radiobiology in general, limited radiation medical countermeasures (MCMs) have been approved by the United States Food and Drug Administration for the acute radiation syndrome (ARS). Additional effort is needed to develop large animal models for improving the prediction of clinical safety and effectiveness of MCMs for acute and delayed effects of radiation in humans. Nonhuman primates (NHPs) are considered the animal models that reproduce the most appropriate representation of human disease and are considered the gold standard for drug development and regulatory approval. The clinical and histopathological effects of supralethal, total- or partial-body irradiations (12 Gy) of NHPs were assessed, along with possible protective actions of a promising radiation MCM, gamma-tocotrienol (GT3). Results show that these supralethal radiation exposures induce severe injuries that manifest both clinically as well as pathologically, as evidenced by the noted functionally crippling lesions within various major organ systems of experimental NHPs. The MCM, GT3, has limited radioprotective efficacy against such supralethal radiation doses.

Threats of radiological or nuclear disasters are of serious concern and a top priority for government agencies involved in domestic security and public health preparedness. There is a need for sensitive bioassays for biodosimetric assessments of radiation exposures originating from unanticipated nuclear/radiological events. The Food and Drug Administration Animal Rule approval pathway requires an in-depth understanding of the mechanisms of radiation injury, drug efficacy and biomarkers for radiation medical countermeasure approval. Biomarkers can be helpful for extrapolating the efficacious countermeasure dose in animals to humans. We summarised here our studies to identify candidate biomarkers for the acute radiation injury using various omic platforms (metabolomics/lipidomics, proteomics, microbiome and transcriptomics/microRNA) using murine and non-human primate models conducted in our laboratory. Multi-omic platforms appear to be highly useful in assessing radiation exposure levels and for identifying biomarkers of radiation injury and countermeasure efficacy, which can expedite the regulatory approval of countermeasures.

BACKGROUND:Animal models are vital for the development of radiation medical countermeasures for the prophylaxis or treatment of acute radiation syndrome and for the delayed effects of acute radiation exposure. Nonhuman primates (NHPs) play an important role in the regulatory approval of such agents by the United States Food and Drug Administration following the Animal Rule. Reliance on such animal models requires that such models are well characterized. METHODS:Data gathered from both male and female animals under the same conditions and gathered concurrently are limited; therefore, the authors compared and contrasted here the radiosensitivity of both male and female NHPs provided different levels of clinical support over a range of acute, total-body gamma irradiation, as well as the influence of age and body weight. RESULTS:Under matched experimental conditions, the authors observed only marginal, but clearly evident differences between acutely irradiated male and female NHPs relative to the measured response endpoints (rates of survival, blood cell changes, and cytokine fluctuations). These differences appeared to be accentuated by the level of exposure as well as by the nature of clinical support. CONCLUSION:Additional studies with both sexes under various experimental conditions and different radiation qualities run concurrently are needed.

Keywords: Acute radiation syndromegamma-radiationinterleukin 11medical countermeasurespolypharmacy approachradiation injury

IntroductionRadiological/nuclear accidents, hostile military activity, or terrorist strikes have the potential to expose a large number of civilians and military personnel to high doses of radiation resulting in the development of acute radiation syndrome and delayed effects of exposure. Thus, there is an urgent need for sensitive and specific assays to assess the levels of radiation exposure to individuals. Such radiation exposures are expected to alter primary cellular proteomic processes, resulting in multifaceted biological responses.Areas coveredThis article covers the application of proteomics, a promising and fast developing technology based on quantitative and qualitative measurements of protein molecules for possible rapid measurement of radiation exposure levels. Recent advancements in high-resolution chromatography, mass spectrometry, high-throughput, and bioinformatics have resulted in comprehensive (relative quantitation) and precise (absolute quantitation) approaches for the discovery and accuracy of key protein biomarkers of radiation exposure. Such proteome biomarkers might prove useful for assessing radiation exposure levels as well as for extrapolating the pharmaceutical dose of countermeasures for humans based on efficacy data generated using animal models.Expert opinionThe field of proteomics promises to be a valuable asset in evaluating levels of radiation exposure and characterizing radiation injury biomarkers.

Introduction The possibility of exposure to high doses of total- or partial-body ionizing radiation at a high dose rate due to radiological/nuclear accidents or terrorist attacks is increasing. Despite research and development during the last six decades, there is a shortage of nontoxic, safe, and effective radiation medical countermeasures (MCMs) for radiological and nuclear emergencies. To date, the US Food and Drug Administration (US FDA) has approved only four agents for the mitigation of hematopoietic acute radiation syndrome (H-ARS).Area covered We present the current status of a promising radiation countermeasure, gamma-tocotrienol (GT3; a component of vitamin E) as a radiation MCM that has been investigated in murine and nonhuman primate models of H-ARS. There is significant work with this agent using various omic platforms during the last few years to identify its efficacy biomarkers.Expert opinion GT3 is a newer type of radioprotector having significant injury-countering potential and is currently under advanced development for H-ARS. As a pre-exposure drug, it requires only single doses, lacks significant toxicity, and has minimal, ambient temperature storage requirements; thus, GT3 appears to be an ideal MCM for military and first responders as well as for storage in the Strategic National Stockpile.

Moderate-to-high doses of ionizing irradiation can lead to potentially life-threatening morbidities and increase mortality risk. In preclinical testing, 5-androstenediol has been shown to be effective in protecting against hematopoietic acute radiation syndrome. This agent is important for innate immunity, serves to modulate cell cycle progression, reduces radiation-induced apoptosis, and regulates DNA repair. The drug has been evaluated clinically for its pharmacokinetics and safety. The United States Food and Drug Administration granted investigational new drug status to its injectable depot formulation (NEUMUNE). Its safety and efficacy profiles make it an attractive candidate for further development as a radiation countermeasure.

There is a need to develop and deploy medical countermeasures (MCMs) in order to support astronauts during space missions against excessive exposures to ionizing radiation exposure. The radiation environment of extraterrestrial space is complex and is characterized by nearly constant fluences of elemental atomic particles (protons being a dominant particle type) with widely different energies and ionization potentials. Chronic exposure to such ionizing radiation carries both near- and long-term health risks, which are generally related to the relative intensity and duration of exposure. These radiation-associated health risks can be managed only to a limited extent by physical means, but perhaps they might be more effectively managed biomedically. The Armed Forces Radiobiology Research Institute/Uniformed Services University of the Health Sciences has a long history of researching and developing MCMs specifically designed to support terrestrial-based military missions involving a radiation-threat component. The development of MCMs for both low and high doses of radiation are major aims of current research, and as such can provide lessons learned for the development of countermeasures applicable to future space missions and its extraterrestrial radiation environment.

Detonation of an improvised nuclear weapon, or a radiological dispersal device by terrorists, or an unintended radiological/nuclear accident in populated areas would result in a mass casualty scenario with radiation exposures of different severities. Such incidences are perceived as national security threats of major consequences. Acute radiation syndrome (ARS) is triggered by an exposure to a high dose of penetrating ionizing radiation during a short time window. In humans, moderate exposure to 2 to 4 Gy of ionizing radiation results in clinically manageable hematopoietic ARS (H-ARS), characterized by severe depletion of vital blood cells and bone marrow progenitors. Since 2015, the United States Food and Drug Administration (U.S. FDA) has approved four radiation medical countermeasures for H-ARS following the Animal Rule; namely, Neupogen, Neulasta, Leukine and Nplate (romiplostim). Here, we briefly present the treatment modalities for H-ARS. We have discussed the latest FDA-approved agent, romiplostim, as a treatment modality for H-ARS. The nature of this agent and the preclinical and clinical work that preceded its FDA approval as a radiation medical countermeasure are discussed, as are the development and use of related thrombopoietic agents for the treatment of radiation-exposed victims.

Introduction The high attrition rate during drug development remains a challenge that costs a significant amount of time and money. Improving the probabilities of success during the early stages of radiation medical countermeasure (MCM) development for approval by the United States Food and Drug Administration (US FDA) following the Animal Rule will reduce this burden.Area covered This article focuses on new technologies involving various organ-on-chip platforms. Of late, there has been rapid development of these technologies, especially in terms of mimicking both normal and abnormal physiological conditions. Here, we suggest possible applications of these novel systems for the discovery and development of radiation MCMs for the acute radiation syndrome (ARS).Expert opinion Each organ-on-a-chip system has its own strengths and shortcomings. As such, the system selected for MCM discovery, development, and regulatory approval should be carefully considered and optimized to the fullest extent in order to augment successful drug testing and the minimization of attrition rates of candidate agents. The recent encouraging progress with organ-on-a-chip technology will likely lead to additional radiation MCMs for ARS. The acceptance of organ-on-a-chip technology may be a promising step toward improving the success rate of pharmaceuticals in MCM development.