Progress and Challenges in Developing Medical Countermeasures for Chemical, Biological, Radiological, and Nuclear Threat Agents

Abstract

This Commentary delves into the current progress and challenges on ongoing research on medical countermeasures (MCs) for chemical, biologic, radiologic, and nuclear (CBRN) threats. CBRN agents pose a serious risk to human health and safety, with the potential for mass casualties in both military and civilian settings. Chemical threats are toxic compounds that could be used in a terrorist attack, an accidental release, or chemical warfare. They include nerve agents, organophosphates, pulmonary agents, metabolic/cellular agents, vesicants, ocular toxicants, and opioid agents. Developing effective MCs is crucial for mitigating the acute and chronic effects of exposure to CBRN agents. The papers in this special issue of JPET highlights the latest advancements in MC research, showcasing insightful outcomes on experimental models, mechanisms, and translational research on MCs for CBRN threats. They portray several notable contributions, including the development of neurosteroid and combination anticonvulsant therapies for nerve agent poisoning, the exploration of chronic impacts and diagnostic tracers for OP neurotoxicity, the establishment of innovative pediatric OP models, the identification of novel molecules for ocular, pulmonary and vesicant injuries, and the repurposing of existing drugs for the treatment of botulism, cyanide, and OP poisoning. These crucial outcomes underscore the breadth of current research covering a variety of chemical threats. Overall, this collection of articles highlights the importance of ongoing research and development in the field of MCs, emphasizing the potential of these countermeasures to effectively treat and mitigate the effects of toxicant exposures and thereby enhance our preparedness for mass casualty incidents.

SIGNIFICANCE STATEMENT

CBRN agents pose a significant threat to public health. Effective MCs exist for certain chemical threats, but there is a need for new and improved MCs for many others. The research presented in this special issue of JPET highlights the latest advancements in MCs for CBRN threats. This research has the potential to lead to the development of new and repurposed MCs that are more effective, broad-spectrum, and easier to administer to mitigate acute and long-term consequences of chemical exposures.

Introduction

The Chemical Abstracts Service registry, a repository of chemical information, contains 110 million chemicals, with 345,000 categorized as toxic. The U.S. biodefense plan collaboratively addresses threats from chemical, biologic, radiologic, and nuclear (CBRN) agents. Within this plan, the chemical countermeasures research program focuses on medical countermeasures (MCs) and therapeutic interventions for toxic exposures. The U.S. Department of Homeland Security identified “chemicals of concern” that are not only potential agents of terrorism but also may be released from transportation and storage facilities during industrial accidents or natural disasters. Around 200 chemicals of concern, known as threat agents, pose significant health risks and are categorized into toxidrome groups based on their primary toxic effects (see Table 1). Threat agents include chemical warfare agents (CWAs), toxic industrial chemicals, organophosphates (OP), pesticides, and ultra-potent synthetic opioids. Notable threats include nerve agents such as sarin, vesicants such as mustard, and vesicating agents such as sulfur mustard. Defining their toxicity mechanisms and developing efficacious MCs is critical for preventing mass morbidity and mortality.

TABLE 1.

List of common chemical threat agents in medical countermeasure research

Class	Subtype	Chemical threat agent
Nerve agents	G-type	Tabun (GA), sarin (GB), soman (GD), cyclosarin (SF)
	V-type	VX, Russian VX
	Novichok	Organophosphates with the structural moiety of 2-fluoro-1,3,2-dioxophospholane (exact structures of Novichok agents are unclear)
Organophosphates	Pesticides	DFP, parathion, paraoxon, malathion, chlorpyrifos, phorate oxon, aldicarb, monocrotophos, diazinon
Convulsants		Picrotoxin, TETS, strychnine
Vesicants	Mustard	Sulfur mustard, nitrogen mustard
	Others	Lewisite, Acrolein, hydrogen fluoride, phosgene oxime
Pulmonary toxicants	Lower pulmonary	Chlorine, phosgene, phosphine, isocyanate
	Upper pulmonary	Ammonia, sulfur dioxide, hydrogen fluoride
Metabolic and cellular toxicants	Cyanides	Hydrogen cyanide, hydrogen sulfide
	Arsenicals	Arsenic trioxide, thallium sulfate, arsine
Anticoagulants		Brodifacoum, bromadiolone
Pharmaceutical-based agents	Opioids	Fentanyl, diacetyl morphine, carfentanil, acetylfentanyl, sufentanil, remifentanil
Gulf War illness agents	Nerve agents	Sarin
	Other agents	Pyridostigmine bromide, DEET, permethrin

CBRN threats pose a serious risk to public health, whether deployed intentionally or encountered unintentionally. These threats can cause widespread death and destruction, and they are a major concern for many nations and militaries around the world. The impact of chemical attacks is starkly illustrated by the 2013 sarin attacks in Syria, claiming the lives of approximately 1,400 civilians (Dolgin, 2013; Reddy and Colman, 2017). The Tokyo subway sarin incident in 1996 exposed numerous civilians to the deadly gas (Okumura et al., 1996). Recently, a highly dangerous class of chemicals, Novichok, has emerged, surpassing the threats posed by sarin and VX. A Novichok exposure incident in Salisbury, England, garnered media attention for standard detection failures but confirmed toxicity through the patient’s response to specific antidotes (Nepovimova and Kuca, 2018; Steindl et al., 2021). Sulfur mustard was reportedly used in chemical warfare during World Wars or national skirmishes. In addition to the imminent threat from military CWAs, including nerve agents, vesicants, and pulmonary toxicants, thousands of incidents involving OP pesticide poisoning occur annually. These incidents highlight the alarming global trend of chemical exposures. The challenges posed by these threats necessitate ongoing research and therapeutic countermeasure development.

Exposure to chemicals most commonly occurs via inhalation, ingestion, and dermal routes. It can result in injuries mirroring common clinical signs, such as seizures, respiratory depression, neovascularization, fibrosis, acute lung injury, inflammation, and coagulopathy (Jett et al., 2020). Devastating long-term illness develops in most survivors. Due to symptomatic targeting, broad-spectrum MCs can be effective across multiple threat agents and mitigate or reduce the symptoms (Reddy and Reddy, 2015). In addition to the traditional development of targeted novel agents, there is a substantial initiative to repurpose United States Food and Drug Administration (FDA)-approved products for alternative indications as potential medical countermeasures to accelerate the development process (Reddy, 2019; Reddy et al., 2020, 2021). The concept of use, considering timing and administration routes for effective application during a mass casualty emergency, is crucial in determining the suitability of compounds for development for civilian and military populations. While some MCs exist for certain chemical threats, the U.S. government initiated the National Institutes of Health (NIH) Countermeasures Against Chemical Threats (CounterACT) Program to enhance and optimize the nation’s medical response capabilities through dedicated research (Jett, 2016). Many programs within the U.S. Department of Defense support research devoted to the development of MCs for treating CBRN exposures in mass causalities and military settings.

The special issue of JPET (February 2024) focuses on the latest advancements in MCs against CBRN threats. MCs are promising therapeutic agents that can mitigate the acute and/or chronic effects of exposure to various chemical and biologic agents. The issue presents 33 insightful papers on experimental models, toxicity mechanisms, and preclinical research on novel and repurposed MCs for CBRN threats. This commentary highlights the significance of cutting-edge research on diverse chemical agents and translational potential of the latest advancements in MCs for CBRN threats.

Nerve Agents

Developing new MCs is a costly and time-consuming endeavor. Two common approaches to developing innovative MCs involve creating new drugs or repurposing existing drugs already approved for other indications. Counteract programs are prioritizing drug repurposing as a means of rapidly developing treatments for chemical, biologic, radiologic, and nuclear (CBRN) threats. In a succinct and informative article, Quattrochi and colleagues outline the Countering Emerging Threats — Rapid Acquisition and Investigation of Drugs for Repurposing (RAIDR) program, which aims to swiftly address potential military threats by repurposing late-stage or approved drugs (Quattrochi et al., 2024). This approach enhances warfighter resilience by bridging treatment gaps and minimizing development costs through leveraging established safety profiles and existing manufacturing processes. The Rapid Acquisition and Investigation of Drugs for Repurposing program contributes to a cost-effective approach for developing niche MCs in response to the evolving CBRN threat landscape.

In the MC development field, Reddy’s comprehensive evaluation of ganaxolone’s experimental and clinical profile for treating nerve agent seizures and refractory status epilepticus (RSE) is an elegant introduction to breakthrough therapeutics (Reddy, 2024). RSE, a hallmark of nerve agent exposure, carries a high mortality rate and significant neurologic morbidity in survivors. Ganaxolone and related neurosteroids, which enhance synaptic and extrasynaptic GABA-A receptors, have emerged as more effective anticonvulsants than benzodiazepines. This article chronicles the author’s two-decade-long transformative ‘bench-to-clinic’ journey of bringing neurosteroid therapy to patients, with ganaxolone leading the way in Phase 3 trials for RSE and nerve agents. Notably, ganaxolone is the first anticonvulsant to transition from the NIH CounterACT program to the Biomedical Advanced Research and Development Authority) for advanced clinical development as an MC for nerve agents. The product boasts unique features, including broad-spectrum effectiveness, absence of tolerance, rapid onset, and well-established mechanisms and therapeutics. Furthermore, Lein and colleagues delve into the topic of OP neurotoxicity, providing deep insights into the cellular and molecular mechanisms of blood-brain barrier impairment and epilepsy development following acute OP intoxication (Bernardino et al., 2024). They also discuss potential therapeutics for blood-brain barrier stabilization as a neuroprotective strategy for mitigating acute and chronic neurologic dysfunction.

Nerve agents and OP threat compounds, such as diisopropylfluorophosphate (DFP), are potent convulsants that produce neurotoxicity by inhibiting acetylcholinesterase, leading to a cholinergic crisis. Neurologic manifestations following OP exposure include convulsive status epilepticus (SE) and neuronal damage or death. These signs are consistently observed in rodent models of OP intoxication. Sex-dependent differences in neurotoxicity and SE are crucial biologic variables after OP exposure. Reddy and colleagues identified significant sex-dependent differences in electrographic seizure activity and neuronal injury in the DFP model of refractory SE in adult rats (Singh et al., 2024a). Male animals exhibited more severe SE and lower survival rates than females, while females showed greater neuroprotection to midazolam than males. These findings suggest the feasibility of developing sex-specific neuroprotective strategies for OP intoxication and SE. In a comparative study of three OP compounds (paraoxon, DFP, and sarin) in adult rat models, Deshpande and team reported hippocampal pathology and chronic epileptic seizures around 4–6 months after OP exposure (Blair et al., 2024). These outcomes are consistent with hallmark features of chronic epilepsy, such as spontaneous recurrent seizures, hippocampal sclerosis, and mossy fiber sprouting. Thompson and colleagues prepared and demonstrated the brain distribution of paraoxon as a carbon-11 positron emission tomography tracer ([11C] paraoxon), indicating rapid entry into the brain (Hayes et al., 2024). This tracer could be a valuable diagnostic tool for unraveling OP neurotoxicity.

Nerve agent-induced seizures and SE exhibit resistance to delayed treatment with benzodiazepines such as midazolam. Lumley and colleagues demonstrated enhanced efficacy when antiseizure medications lacosamide and rufinamide were administered in combination with ketamine (an N-methyl-D-aspartate receptor antagonist) and midazolam (a benzodiazepine) for controlling soman-induced seizures, epileptogenesis, and brain damage in rats (Lumley et al., 2024). These findings reinforce the emerging concept of GABA-A receptor internalization and N-methyl-D-aspartate receptor upregulation following nerve agent-induced SE. The production of reactive oxygen species is a common biochemical consequence of OP intoxication. Patel and colleagues demonstrated improved efficacy of a reformulated catalytic antioxidant, AEOL10150, in mitigating oxidative stress, neuroinflammation, and neuronal death following DFP exposure in rat models (Liang et al., 2024). This report suggests the drug’s potential to ameliorate both acute and delayed effects of OP exposure. Neuroinflammation and neurodegeneration are prevalent following nerve agent exposure. Cool and team investigated the potential of Q-VD-OPh, a pan-caspase inhibitor, to suppress the inflammatory response in a mouse sarin exposure model (Shah et al., 2024). The compound exhibited protective effects in reducing the sarin-induced increase in pro-inflammatory cytokines.

Current anticonvulsants, such as benzodiazepines, are often ineffective in treating nerve agent-induced seizures, particularly when administered late after exposure. Neurosteroids, a class of naturally occurring steroid molecules, have emerged as promising therapeutic agents for nerve agent intoxication due to their robust ability to modulate neuronal excitability and enhance seizure control. Neurosteroids act as positive allosteric modulators of synaptic and extrasynaptic GABA-A receptors, the primary inhibitory neurotransmitter receptors in the brain, leading to increased inhibitory transmission and reduced neuronal excitability. This strong modulation of GABA-A receptors is thought to underlie the anticonvulsant effects of neurosteroids. This journal issue presents multiple studies demonstrating the efficacy of neurosteroids in protecting against OP and nerve agent neurotoxicity. In a well-established nerve agent model, Lumley and team found that a combination of allopregnanolone, midazolam, and ketamine was effective in reducing seizures and neurodegeneration in a delayed treatment model of soman-induced SE (Nguyen et al., 2024). This regimen was proven partially effective in reducing seizures and neurodegeneration, indicating the need for further studies with better neurosteroid products for maximal efficacy. While allopregnanolone and ganaxolone exhibit remarkable protective effects, they face limitations, such as poor bioavailability, short half-life, and formulation delivery challenges. Researchers have developed novel neurosteroid analogs with enhanced water solubility and improved pharmacokinetic properties to overcome these limitations. Reddy and team have developed novel hydrophilic neurosteroids, such as valaxanolone and lysaxanolone, showing promising results in animal studies. In an antiseizure study, Reddy’s team reported valaxanolone and lysaxanolone effectively protected against DFP-induced epileptic seizures and electrographic biomarkers of ictal activity (Ramakrishnan et al., 2024). In a neuroprotection study, Reddy and the team found that valaxanolone and lysaxanolone significantly improved behavioral and memory deficits, correlated with their ability to reduce neurodegeneration and neuroinflammation (Reddy et al., 2024). These findings provide insights into the potential of hydrophilic neurosteroid analogs to overcome the limitations of current neurosteroid products and provide new options for developing effective treatments for nerve agent-induced seizures and neurodegeneration.

Millions of children encounter OPs through agricultural practices or chemical accidents. However, most studies on the efficacy of MCs against OP poisoning are conducted in adult rodent models. Children are more susceptible to the neurotoxic effects of nerve agents than adults, and there are limited pediatric models to study the toxicokinetic consequences and interventions. Additionally, the long-term neurodevelopmental consequences of OP poisoning in children are not fully understood. This journal issue presents three studies that used pediatric models to test potential MCs and the long-term impact of neonatal exposure to OP agents. Reddy and colleagues established a pediatric rat model of SE induced by the OP compound DFP (Singh et al., 2024b). The observed long-lasting behavioral abnormalities, epileptic seizures, and bilateral brain defects mimic the neurologic sequelae seen in children exposed to OPs. This pediatric model offers a valuable tool for studying pathologic mechanisms and testing potential treatment strategies to alleviate the enduring effects of OP-induced SE in children. Braga and colleagues investigated the efficacy of midazolam and the combination of tezampanel with caramiphen in treating soman-induced SE in infant rat models (Furtado et al., 2024). Despite its indication for nerve agent-induced SE treatment, they found that midazolam does not offer effective neuroprotection. In contrast, blocking α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/N-methyl-D-aspartate receptors with tezampanel and caramiphen provides significant neuroprotection, emphasizing the importance of glutamate receptor antagonists as powerful anticonvulsants for pediatric seizures. Furthermore, Neff and Reddy (2024) demonstrated a pediatric rat model of neonatal DFP exposure that helps screen novel therapies to mitigate the long-term developmental effects of OP exposure in children. They found that neonatal exposure to DFP caused major long-term deficits in behavior and memory in pediatric rats. Neurosteroid treatment with ganaxolone protected against these deficits and correlated well with neuropathological changes. These studies highlight the importance of using pediatric models to study the effects of OP poisoning in children and to develop effective treatment strategies.

Vesicating Agents

Vesicant threat agents, such as sulfur mustard, nitrogen mustard (NM), and phosgene oxime, pose significant health hazards due to their ability to induce blistering and extensive tissue damage upon exposure. Consequently, researchers are exploring various therapeutic approaches to mitigate mustard chemical toxicity, including anti-inflammatory agents, antioxidants, farnesoid receptor activation, immunomodulators, and wound/tissue repair agents. Although mustard agents can cause severe skin and ocular injuries, experimental therapies focus on wound care and ocular protection. Despite the use of mustard as a chemical weapon for over a century, there remains a scarcity of effective treatments suitable for field settings. Nevertheless, numerous studies have investigated medical therapeutics for NM and SM toxicity. This issue presents multiple reports demonstrating the efficacy of new or repurposed drugs in experimental models of vesicant injury.

Ocular injury is the most prevalent form of injury caused by mustard. Focusing on the ocular injury, Fini and colleagues investigated the protective effects of dynasore and dyngo-4a, inhibitors of classic dynamins, against oxidative stress on the ocular surface in an NM-induced human cornea injury model (Pan et al., 2024). While dynasore shows superior cytoprotection, mdivi-1 emerged as a potential countermeasure against NM-induced epitheliopathy, offering insights into distinct pathways of corneal epitheliopathy for novel therapeutic strategies for dry eye and chemical exposure. In another study on mustard’s ocular toxicity, Agarwal and team mitigated injuries induced by NM in an ex vivo human cornea model and evaluated the efficacy of dexamethasone in reversing these injuries (Mishra et al., 2024a). Dexamethasone, administered 2 hours post NM exposure, successfully mitigated NM-induced corneal injuries, suggesting its potential as a therapeutic intervention to counter vesicant-induced ocular toxicities in chemical emergencies. Targeting the skin injury, Huang and team evaluated the potential of the FDA-approved β-blocker carvedilol and its enantiomers to counter NM-induced skin injuries (Shahid et al., 2024). Carvedilol effectively reduced NM-induced cytotoxicity, apoptosis, and inflammation, suggesting its repurposing potential as an MC against mustard-induced skin injuries. These studies highlight the importance of developing effective treatments for mustard-induced ocular and skin injuries.

Focusing on corneal injury, Agarwal and team assessed the efficacy of dexamethasone in countering ocular SM toxicity, evaluating two dosing frequencies (Mishra et al., 2024b). Administering dexamethasone every 8 hours proved more potent than every 12 hours in reversing corneal SM injuries, suggesting a novel and comprehensive treatment regimen for immediate and sustained effects up to 56 days post-exposure. This offers valuable insights for potential countermeasures against sulfur mustard-induced ocular damage. The study by Gorbatyuk and team investigated the mechanism of ocular toxicity induced by NM, a surrogate for sulfur mustard (Zhylkibayev et al., 2024). After topically applying NM to mice, the research reveals a significant decline in retinal function associated with increased cell death in corneal and retinal tissues and differential expression of 215 proteins linked to ferroptosis and necroptosis. Identifying activated unfolded protein response protein kinase RNA-like ER kinase and increased vascular endothelial growth factor secretion in corneal cells and tissues provides valuable insights into the molecular mechanisms underlying NM-induced ocular toxicity. The findings suggest a potential pro-death signaling pathway from the cornea to the retina, highlighting the need to examine NM-induced ocular toxicity further. Shalwitz et al. (2024) investigated INV-102 as a potential topical treatment of ocular injuries caused by sulfur mustard, demonstrating its ability to activate DNA repair mechanisms and improve clinical recovery in a rabbit model, even when treatment begins 24 hours post-exposure. The findings highlight the therapeutic potential of targeting the body’s natural DNA damage response for conditions associated with double-strand breaks, addressing the current lack of therapeutic options for mustard gas-induced corneal injuries. These studies highlight the potential of new therapies that can protect people from the devastating effects of mustard exposure.

Phosgene oxime, a vesicating agent, is considered a potent CWA and produces rapid onset of severe injury as an urticant. This research study by Tewari-Singh and team investigated the skin toxicity model caused by phosgene oxime using SKH-1 and C57BL/6 mice (Goswami et al., 2024). They revealed immediate physiologic changes and skin lesions resembling urticaria, providing valuable insights into phosgene oxime-induced skin injury progression and establishing potential biomarkers for dermal exposure that may help develop targeted interventions. In addition, phenylarsine oxide (PAO) and 2-chloroethyl ethyl sulfide are surrogates of arsenicals and mustards, respectively, which induce similar cutaneous inflammatory responses in skin. Athar and colleagues uncovered a common mechanism of toxic action by vesicants, arsenicals, and mustards using surrogates PAO and 2-chloroethyl ethyl sulfide (Srivastava et al., 2024). Their findings show heat shock protein 90 as a shared molecular signaling factor in the skin damage induced by these two distinct classes of vesicants. Topical treatment with heat shock protein 90 inhibitors significantly attenuates vesicant-induced skin injury, presenting a promising medical countermeasure with broad implications for mitigating cutaneous damage caused by chemical warfare agents. With further research, it may be possible to develop effective therapies that can be used to mitigate the effects of vesicant exposure in field settings.

Pulmonary Agents

Pulmonary threat agents, such as chlorine gas, phosgene, sulfur mustard, ammonia, acrolein, and chloropicrin, pose a significant threat due to their ability to induce severe lung damage, including acute lung injury and acute respiratory distress syndrome, which can lead to respiratory depression days or even weeks later. The development of effective MCs for pulmonary toxic exposures is crucial for protecting individuals from the severe respiratory consequences of exposure to these agents. Therapeutic interventions for pulmonary toxic exposures are often limited or nonexistent, presenting a critical challenge in addressing respiratory injuries caused by pulmonary toxicants. The escalating concern over inhaled chemical threats in civilian and military contexts emphasizes the need to minimize both immediate lung injury and delayed clinical complications resulting from exposures. Current therapeutic approaches often face challenges regarding effectiveness, safety, and suitability for mass casualty scenarios. Thus, there is a great need for improved therapeutic interventions. An excellent overview of this entire area is provided by the review article by Marzec and Nadadur (2024), which describes current developments in understanding the acute toxicity and pathophysiology of key pulmonary threat chemicals. The article explores potential therapeutics in early-stage development and addresses challenges associated with developing countermeasures suitable for mass casualty scenarios. It provides valuable insights into the mechanisms of inhalation irritants, riot-control irritants, and CWAs. The article also highlights issues with therapeutic interventions to mitigate toxicity of pulmonary chemicals of concern. This study by Ahmad and colleagues demonstrated the development of a robust animal model for investigating acute lung injury resulting from cutaneous exposure to the toxic vesicating agent Lewisite, using the less toxic surrogate PAO (Zafar et al., 2024). The findings mimic the effects of cutaneous Lewisite exposure and provide a valuable tool for studying toxicity mechanisms and developing medical countermeasures to mitigate vesicant-induced lung injury.

This issue of the journal presents two reports that have demonstrated the sodium 2-mercaptoethane sulfonate (Mesna) as an antidote for SM exposure and farnesoid receptor activation as a therapeutic approach for NM-induced lung injury. Using an inhalation model of SM-induced pulmonary injury, White and team demonstrated the effectiveness of Mesna, an FDA-approved compound, as a promising antidote for SM exposure (Nick et al., 2024). Mesna demonstrated significant survival benefits, improved cardiopulmonary function, and reduced airway fibrin cast formation in a rat model of SM inhalation, suggesting its potential as an inexpensive and readily available therapeutic option for real-world SM exposure situations. In a rigorous pharmacology study, Laskin and team investigated the impact of farnesoid X receptor (FXR) activation on lung injury, oxidative stress, and fibrosis induced by the cytotoxic vesicant NM (Meshanni et al., 2024). The mustard-induced histopathological changes, fibrosis, aberrations in pulmonary function, and increased oxidative stress and inflammation markers were effectively attenuated by the administration of the FXR synthetic agonist obeticholic acid, indicating the potential therapeutic role of FXR activation in limiting NM-induced lung injury and chronic disease and thereby confirming it as a viable approach for intervention. These studies provide promising evidence for the development of effective therapeutic interventions for SM and NM exposure. Mesna, an already approved drug, could be readily repurposed for the treatment of SM poisoning, while FXR activation offers a novel therapeutic strategy for NM-induced lung injury. Further research is needed to optimize these approaches and address the remaining challenges in treating pulmonary toxicants.

Metabolic and Cellular Agents

Metabolic and cellular agents, such as cyanide and arsenicals, are toxicants that disrupt blood, cellular, and metabolic functions. The development of effective treatments for metabolic and cellular agents is crucial for mitigating the severe health consequences of exposure to these toxicants. While there are available treatments for acute cyanide poisoning, such as hydroxocobalamin, their limitations in mass casualty situations due to slow delivery highlight the critical need for more efficient antidotes. This issue of the journal presents two reports that have demonstrated CoN4[14] as an effective antidote for cyanide and azide poisoning, the limitations of current arsenical antidotes, and metabolic alterations for PAO-induced kidney injury. Peterson and team identified the cobalt (II/III) complex CoN4[14] as a highly effective antidote to both azide and cyanide poisoning in mouse models (Pearce et al., 2024a). Demonstrating superior therapeutic capabilities compared with the FDA-approved hydroxocobalamin, their results highlight a “redox-switching” mechanism in the compound’s interaction with cyanide. This mechanism could be a crucial but overlooked feature in cobalt-based cyanide antidotes, offering valuable insights for developing improved therapies for chemical poisonings. Arsenicals, including the deadly Lewisite used in chemical warfare, induce severe skin burns and blisters. They can also lead to pulmonary damage or hypovolemic shock, with potentially fatal outcomes. While antidotes like British anti-Lewisite exist, their limited application and potential toxic side effects underscore the urgent need for more effective treatments against arsenical exposure. George and colleagues explored the transcriptomic effects of cutaneous exposure to PAO, a common arsenical, on the kidneys of mice (Moore et al., 2024). They found that PAO exposure upregulates metabolic pathways, such as fatty acid oxidation and PPAR-α signaling, and genes in proximal tubule epithelial cells and endothelial cells. This could potentially lead to acute kidney injury. The study suggests that targeting these metabolic alterations could lead to novel therapies for PAO-induced kidney injury. Further research is needed to refine existing antidotes and explore novel therapeutic approaches to protect individuals from the lethal effects of cyanide and arsenical poisons.

Biologic, Radiologic, and Other Agents

In this issue, three reports have demonstrated potential antidotes for the toxicity of radiologic, biologic and other toxicants. CS tear gas (2-chlorobenzalmalononitrile), a common crowd control agent, has been widely used, assuming its effects are transient and minimal. However, a new study by Achanta et al. (2024) reveals that CS exposure can cause acute and persistent skin injuries. Their research identifies transient receptor potential ion channel ankyrin repeat 1 as a key mediator of CS-induced irritation, pain, and neurogenic inflammation. They also found that pharmacologic inhibition of TRPA1 with HC-030031 or A-967079 significantly mitigates pathologic biomarkers in mouse ear skin, suggesting that TRPA1 inhibitors could be effective post-exposure countermeasures. Exposure to ionizing radiation, such as that from nuclear weapons, can have serious and life-threatening consequences. Proliferating bovine pulmonary artery endothelial cells (BPAECs) exhibit transient cell-cycle arrest as their primary response to X-ray irradiation. This arrest is associated with oxidant-linked mitochondrial changes, while apoptosis is negligible. The study by Pearce et al. (2024b) found that culturing cells at 3% oxygen delays X-ray-induced cell-cycle arrest, implicates superoxide as a key cytotoxic species, and demonstrates the BPAEC’s utility for screening potential countermeasures. They identified hydroxyquinoline derivatives as effective antagonists of oxygen/superoxide-mediated radio-sensitizing effects. This research not only sheds light on the radiobiological responses of BPAECs but also suggests a promising direction for developing countermeasures against ionizing radiation effects. These studies highlight the potential long-term health consequences of exposure to CS tear gas and ionizing radiation.

Botulism is a serious and potentially fatal illness caused by toxins produced by the bacterium Clostridium botulinum. It poses a significant public health threat due to the potency of the toxin, which can cause paralysis and respiratory failure. The toxin can be ingested through contaminated food, wound infections, or inhaled in aerosolized form. Currently, there are limited treatment options for botulism, particularly for respiratory paralysis. However, a recent study by McNutt and colleagues suggests that aminopyridines, such as the clinically approved drug 3,4-diaminopyridine, may offer a promising new treatment approach (McClintic et al., 2024). They demonstrate that 3,4-diaminopyridine and other aminopyridines can rapidly reverse respiratory paralysis in botulism-infected mice. This suggests that these drugs could be effective in treating botulism patients with respiratory failure, potentially improving their chances of survival. The results of this study suggest that aminopyridines may offer a valuable new treatment option for botulism, particularly for patients with respiratory paralysis.

Gulf War illness (GWI) is a chronic, complex condition that affects war veterans and is characterized by debilitating pain, fatigue, sleep disturbances, behavioral problems, and cognitive impairments. There is currently no definitive treatment of GWI, making it crucial to develop effective therapeutic interventions for the neuropsychiatric disorders associated with this condition. One suspected cause of GWI is chemical neurotoxicity, which disrupts neuronal function and contributes to the observed symptoms. Epigenetic modifications, such as histone dysregulations, have been found in GWI veterans and may contribute to decreased brain-derived neurotrophic factor (BDNF) expression. BDNF is a key factor in the antidepressant effect of ketamine, a drug recently approved by the FDA for therapy-resistant depression. To investigate whether ketamine’s antidepressant effects in GWI are mediated by epigenetic mechanisms, Deshpande and team conducted a study (Ribeiro-Davis et al., 2024). Their findings show that the antidepressant effects of ketamine in GWI-like depression involve the inhibition of HDAC expression, upregulation of BDNF, and dendritic modifications. These results support the potential use of ketamine for treating depression in GWI patients. Further research is needed to confirm these findings in clinical trials.

Conclusions

Research on MCs plays a pivotal role in the timely and effective development of treatments for toxic exposures. A notable focus is on repurposing FDA-approved drugs as MCs, aiming to expedite the development process. This edition of JPET highlights significant strides in MC development for chemical agents and other CBRN threats, featuring 29 research papers and four review articles authored by reputed experts in the CounterACT research field. These contributions reflect substantial progress in unraveling crucial pathologic insights and identifying potential drug candidates, paving the way for transformative advancements. Some notable highlights from the papers in this issue are outlined in Table 2. As such, the articles in the special issue are very valuable and they reflect persistent enthusiasm for the advancements in MCs for CBRN threats. With the continued CBRN research, therapeutic countermeasures have the potential to save lives and mitigate the long-term health impacts of chemical exposures and thereby enhance our preparedness for chemical incidents.

TABLE 2.

Salient highlights of research papers in the special issue on MCs

Chemical threat	Top highlights
Nerve agents	• Ganaxolone breakthrough for nerve agent seizures and RSE • Sex-dependent differences in OP seizure and neurotoxicity • Chronic effects of OP exposure • Diagnostic tracer for OP neurotoxicity • Combination therapy for soman-induced seizures • Improved antioxidant against DFP exposure • Q-VD-OPh for inhibiting inflammatory response • Novel hydrophilic neurosteroids as promising anticonvulsants • Pediatric models for OP exposure
Vesicating agents	• Dynasore and dyngo-4a against NM-induced ocular injury • Dexamethasone efficacy in nitrogen mustard-induced corneal injuries • Carvedilol for reducing nitrogen mustard-induced skin injuries
Pulmonary agents	• Mesna as a promising antidote for sulfur mustard exposure • FXR activation for limiting nitrogen mustard-induced lung injury
Metabolic and cellular agents	• Cobalt complex CoN4[14] as an effective cyanide antidote • Transcriptomic effects of PAO on kidneys
Biologic, radiologic, and other agents	• TRPA1 inhibitors for the tear gas CS-induced skin injuries • BPAEC utility for screening drugs against ionizing radiation • Aminopyridines as fast-acting treatments for botulism • Ketamine for treating GWI-associated neuropsychiatric disorders

Abbreviations

BDNF

brain derived neurotrophic factor

BPAEC

bovine pulmonary artery endothelial cells

CBRN

chemical, biological, radiological, and nuclear

CounterACT

Countermeasures Against Chemical Threats program

CWA

chemical warfare agents

DFP

diisopropyl-fluorophosphate

FDA

Food and Drug Administration

FXR

farnesoid X receptor

MC

medical countermeasure

Mesna

mercaptoethane sulfonate

NIH

National Institutes of Health

NM

nitrogen mustard

OP

organophosphate

PAO

phenylarsine oxide

RSE

refractory status epilepticus

SE

status epilepticus

Authorship Contributions

Wrote or contributed to the writing of the manuscript: Reddy.