Genotoxicity assessment of an energetic propellant compound, 3-nitro-1,2,4-triazol-5-one (NTO)

Photolysis of 3-Nitro-1,2,4-triazol-5-one: Mechanisms and Products

Insensitive munitions formulations that include 3-nitro-1,2,4-triazol-5-one (NTO) are replacing traditional explosive compounds. While these new formulations have superior safety characteristics, the compounds have greater environmental mobility, raising concern over potential contamination and cleanup of training and manufacturing facilities. Here, we examine the mechanisms and products of NTO photolysis in simulated sunlight to further inform NTO degradation in sunlit surface waters. We demonstrate that NTO produces singlet oxygen and that dissolved oxygen increases the NTO photolysis rate in deionized water. The rate of NTO photolysis is independent of concentration and decreases slightly in the presence of Suwannee River Natural Organic Matter. The apparent quantum yield of NTO generally decreases as pH increases, ranging from 2.0 × 10-5 at pH 12 to 1.3 × 10-3 at pH 2. Bimolecular reaction rate constants for NTO with singlet oxygen and hydroxyl radical were measured to be (1.95 ± 0.15) × 106 and (3.28 ± 0.23) × 1010 M-1 s-1, respectively. Major photolysis reaction products were ammonium, nitrite, and nitrate, with nitrite produced in nearly stoichiometric yield upon the reaction of NTO with singlet oxygen. Environmental half-lives are predicted to span from 1.1 to 5.7 days. Taken together, these data enhance our understanding of NTO photolysis under environmentally relevant conditions.

Outdoor dissolution and photodegradation of insensitive munitions formulations IMX-101 and IMX-104: Photolytic transformation pathway and mechanism study

New munition compounds have been developed to replace traditional explosives to prevent unintended detonations. However, insensitive munitions (IM) can leave large proportion of unexploded charge in the field, where it is subjected to photodegradation and dissolution in precipitation. The photolytic reactions occurring on the surfaces of IMX-101 and IMX-104 formulations and the subsequent fate of photolytic products in the environment were thoroughly investigated. The constituents of IMX-101 and IMX-104 formulations dissolve sequentially under rainfall in the order of aqueous solubility: 3-nitro-1,2,4-triazol-5-one (NTO) > nitroguanidine (NQ) > 2,4-dinitroanisole (DNAN) > 1,3,5-hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). A linear relationship between DNAN dissolution and rainwater volume was observed (r²: 0.86-0.99). It was estimated that it would take 16-228 years to completely dissolve these formulation particles under natural environmental conditions in Oracle, AZ. We used LC/MS/MS and GC/MS to examine the dissolution samples from IMX-101 and 104 particles exposed to rainfall and sunlight and found six DNAN photo-transformation products including 2-methoxy-5-nitrophenol, 4-methoxy-3-nitrophenol, 4-methoxy-3-nitroaniline, 2-methoxy-5-nitroaniline, 2,4-dinitrophenol, and methoxy-dinitrophenol, which are in good agreement with computational modeling results of bond strengths. The main DNAN photodegradation pathways are therefore proposed. Predicted eco-toxicity values suggested that the parent compound DNAN, methoxy-nitrophenols, methoxy-nitroanilines and the other two products (2,4-dinitrophenol and methoxy-dinitrophenol) would be harmful to fish and daphnid. Our study provides improved insight about the rain dissolution and photochemical behavior of IM formulations under natural conditions, which helps to form target-oriented strategies to mitigate explosive contamination in military training sites.

Computational Investigation on Interactions between Some Munition Compounds and Humic Substances

Humic acid substances (HAs) in natural soil and sediment environments affect the retention and degradation of insensitive munition compounds and legacy high explosives (MCs): 2,4-dinitroanisole (DNAN), DNi^-NH₄⁺, N-methyl-p-nitroaniline (nMNA), 1-nitroguanidine (NQ), 3-nitro-1,2,4-triazol-5-one (NTO; neutral and anionic forms), 2,4,6-trinitrotoluene (TNT), and 1,3,5-trinitro-1,3,5-triazinane (RDX). A humic acid model compound has been considered using molecular dynamics, thermodynamic integration, and density functional theory to characterize the munition binding ability, ionization potential, and electron affinity compared to that in the water solution. Humic acids bind most compounds and act as both a sink and source for electrons. Ionization potentials suggest that HAs are more susceptible to oxidation than the MCs studied. The electron affinity of HAs is very conformation-dependent and spans the same range as the munition compounds. When HAs and MCs are complexed, the HAs tend to radicalize first, thus buffering MCs against reductive as well as oxidative attacks.

Flexible porous aerogels decorated with Ag nanoparticles as an effective SERS substrate for label-free trace explosives detection

Sensitive and reliable methods for explosives detection are of significance in homeland security due to the serious threats of explosives in terrorist attack events. However, such suitable sensors are still rare. Herein, porous silica aerogels decorated with silver nanoparticles (SiO2-Ag hybrids) were prepared and applied as a flexible SERS substrate for ultrasensitive explosives detection. It is worth noting that the silica aerogel we prepared had good flexibility compared with traditional silica aerogels, which effectively avoided structural damage during sample collection. Also, because of excellent adsorption performance provided by the silica aerogel, trace explosive 3-nitro-1,2,4-triazol-5-one (NTO) could be enriched and realized by label-free detection. Combined with the plasma enhancement provided by Ag NPs decorated around these porous aerogels, the limit of detection for explosive NTO was as low as 7.94 × 10-10 M. As far as we know, this SiO2-Ag hybrid SERS substrate was firstly used for the detection of explosives. It presented good sensitivity and reproducibility for analyte sensing. Most importantly, this is a label-free method for trace explosives detection and has a good application prospect in homeland security.

Predicting the Impact of Aqueous Ions on Fate and Transport of Munition Compounds in the Environment

A model framework for natural water has been developed using computational chemistry techniques to elucidate the interactions between solvated munition compounds and eight common ions in naturally occurring water sources. The interaction energies, residence times, coordination statistics, and surface preferences of nine munition-related compounds with each ion were evaluated. The propensity of these interactions to increase degradation of the munition compound was predicted using accelerated replica QM/MM simulations. The degradation prediction data qualitatively align with previous quantum mechanical studies. The results suggest that primary ions of interest for fate and transport modeling of munition compounds in natural waters may follow the relative importance of SO₄^2-, Cl^- ≫ HCO₃^-, Na⁺, Mg²⁺ > Ca²⁺, K⁺, and NH₄⁺.

Comparative toxicogenomics of three insensitive munitions constituents 2,4-dinitroanisole, nitroguanidine and nitrotriazolone in the soil nematode Caenorhabditis elegans

BACKGROUND

Ecotoxicological studies on the insensitive munitions formulation IMX-101 and its components 2,4-dinitroanisole (DNAN), nitroguanidine (NQ) and nitrotriazolone (NTO) in various organisms showed that DNAN was the main contributor to the overall toxicity of IMX-101 and suggested that the three compounds acted independently. These results motivated this toxicogenomics study to discern toxicological mechanisms for these compounds at the molecular level.

METHODS

Here we used the soil nematode Caenorhabditis elegans, a well-characterized genomics model, as the test organism and a species-specific, transcriptome-wide 44 K-oligo probe microarray for gene expression analysis. In addition to the control treatment, C. elegans were exposed for 24 h to 6 concentrations of DNAN (1.95-62.5 ppm) or NQ (83-2667 ppm) or 5 concentrations of NTO (187-3000 ppm) with ten replicates per treatment. The nematodes were transferred to a clean environment after exposure. Reproduction endpoints (egg and larvae counts) were measured at three time points (i.e., 24-, 48- and 72-h). Gene expression profiling was performed immediately after 24-h exposure to each chemical at the lowest, medium and highest concentrations plus the control with four replicates per treatment.

RESULTS

Statistical analyses indicated that chemical treatment did not significantly affect nematode reproduction but did induce 2175, 378, and 118 differentially expressed genes (DEGs) in NQ-, DNAN-, and NTO-treated nematodes, respectively. Bioinformatic analysis indicated that the three compounds shared both DEGs and DEG-mapped Reactome pathways. Gene set enrichment analysis further demonstrated that DNAN and NTO significantly altered 12 and 6 KEGG pathways, separately, with three pathways in common. NTO mainly affected carbohydrate, amino acid and xenobiotics metabolism while DNAN disrupted protein processing, ABC transporters and several signal transduction pathways. NQ-induced DEGs were mapped to a wide variety of metabolism, cell cycle, immune system and extracellular matrix organization pathways.

CONCLUSION

Despite the absence of significant effects on apical reproduction endpoints, DNAN, NTO and NQ caused significant alterations in gene expression and pathways at 1.95 ppm, 187 ppm and 83 ppm, respectively. This study provided supporting evidence that the three chemicals may exert independent toxicity by acting on distinct molecular targets and pathways.

Adsorption and oxidation of 3-nitro-1,2,4-triazole-5-one (NTO) and its transformation product (3-amino-1,2,4-triazole-5-one, ATO) at ferrihydrite and birnessite surfaces

The emerging insensitive munitions compound (IMC) 3-nitro-1,2,4-triazole-5-one (NTO) is currently being used to replace conventional explosives such as 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), but the environmental fate of this increasingly widespread IMC remains poorly understood. Upon release from unexploded solid phase ordinances, NTO exhibits high aqueous solubility and, hence, potential mobilization to groundwater. Adsorption and abiotic transformation at metal oxide surfaces are possible mechanisms for natural attenuation. Here, the reactions at ferrihydrite and birnessite surfaces of NTO and its biotransformation product, 3-amino-1, 2, 4-triazol-5-one (ATO), were studied in stirred batch reactor systems at controlled pH (7.0). The study was carried out at metal oxide solid to solution ratios (SSR) of 0.15, 1.5 and 15 g kg^-1. The samples were collected at various time intervals up to 3 h after reaction initiation, and analyzed using HPLC with photodiode array and mass spectrometric detection. We found no detectable adsorption or transformation of NTO upon reaction with birnessite, whereas ATO was highly susceptible to oxidation by the same mineral, showing nearly complete transformation within 5 min at 15 g kg^-1 SSR to urea, CO_2(g) and N_2(g). The mean surface-area-normalized pseudo-first order rate constant (k) for ATO oxidation by birnessite across all SSRs was 0.05 ± 0.022 h^-1 m^-2, and oxidation kinetics were independent of dissolved O₂ concentration. Both NTO and ATO were resistant to oxidation by ferrihydrite. However, NTO showed partial removal from solution upon reaction with ferrihydrite at 0.15 and 1.5 g kg^-1 SSR and complete loss at 15 g kg^-1 SSR due to strong adsorption. Conversely, ATO adsorption to ferrihydrite was much weaker than that measured for NTO.

Characterization of the Testicular Toxicity of 3-Nitro-1,2,4-Triazol-5-One and 2,4-Dinitroanisole in Rats (Rattus norvegicus)

Nitrotriazolone (3-nitro-1,2,4-triazol-5-one; NTO) and dinitroanisole (2,4-dinitroanisole; DNAN), insensitive energetic materials used in explosive formulations, have induced testicular toxicity and oligospermia in repeated-dose oral toxicity tests. To identify the target site of testicular toxicity of NTO and DNAN, Sprague Dawley rats were orally dosed with NTO (500 mg/kg/d) or DNAN (50 or 100 mg/kg/d) in corn oil for 1, 3, 7, or 14 days. Degeneration of germinal epithelium occurred in multiple tubule stages on days 7 and 14 in treated rats. Degeneration increased in severity with time and was characterized by degeneration/apoptosis of pachytene spermatocytes and round and elongating spermatids, depletion of step 19 spermatids, luminal spermatogenic cell sloughing, multinucleate cells, and pronounced Sertoli cell vacuolation. Serum luteinizing hormone and follicle-stimulating hormone did not differ between NTO- and DNAN-treated and control rats on any sampling day. Serum testosterone levels reduced only in rats given 50 mg/kg/d DNAN for 7 days. These results suggest that the initial site of testicular injury for both NTO and DNAN is the Sertoli cell.

Ecotoxicity of the insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) and its reduced metabolite 3-amino-1,2,4-triazol-5-one (ATO)

The insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) was recently approved by the U.S. Army to replace cyclotrimethylene trinitramine (RDX) in conventional explosives. As its use becomes widespread, concern about the potential toxicity of NTO increases. NTO can undergo microbial reduction to 3-amino-1,2,4-triazol-5-one (ATO), which is recalcitrant in waterlogged soils. In this study, the acute toxicity of NTO and ATO towards various organisms, including microorganisms (i.e., methanogenic archaea, aerobic heterotrophs, and Aliivibrio fischeri (Microtox assay)), the microcrustacean Daphnia magna (ATO only), and zebrafish embryos (Danio rerio), was assessed. NTO was notably more inhibitory to methanogens than ATO (IC₅₀=1.2mM,>62.8mM, respectively). NTO and ATO did not cause noteworthy inhibition on aerobic heterotrophs even at the highest concentrations tested (32.0mM). High concentrations of both NTO and ATO were required to inhibit A. fischeri (IC₂₀=19.2, 22.4mM, respectively). D. magna was sensitive to ATO (LC₅₀=0.27mM). Exposure of zebrafish embryos to NTO or ATO (750μM) did not cause lethal or developmental effects (22 endpoints tested). However, both compounds led to swimming behavior abnormalities at low concentrations (7.5μM). The results indicate that the reductive biotransformation of NTO could enhance or lower its toxicity according to the target organism.

Effects of 3-Nitro-1,2,4-triazol-5-one on Survival, Growth and Metamorphosis in the Northern Leopard Frog, Lithobates pipiens

New explosive formulations are being developed to be less sensitive to impact and inadvertent explosion, increasing safety for the warfighter. Since testing and training make environmental releases imminent, the toxicity of 3-nitro-1,2,4-triazol-5-one (NTO), a component of Insensitive Munitions eXplosive (IMX) formulations, was assessed in a one-generation study to the northern leopard frog (Lithobates ( = Rana) pipiens). Because NTO in water creates acidic conditions, acute studies were conducted with non-pH-adjusted NTO, while a long-term (70-d) study was conducted with neutralized NTO. In the acute study, 48-h and 7-d LC₅₀s were ~250 mg NTO/L. In the long-term study, tadpoles were dead by day 2 in 11,350 mg/L NTO, and by day 63 in 8382 mg/L. The 70-d LC₅₀ was 3670 mg (neutralized) NTO/L. The number of organisms reaching complete metamorphosis was reduced by NTO; the lowest IC₂₅ was 1999 mg NTO/L for the Number Completing Metamorphosis. The NOECs for Time to Front Limb Eruption or Time to Metamorphosis were the same at 1346 mg/L. Histopathology did not significantly distinguish between NTO-exposed and unexposed animals, although possible effects on the density of spermatogonia in NTO-exposed males was suggested. The test data indicate that acute toxicity to ambient NTO can be attributed primarily to its acidic nature; relatively low chronic toxicity of neutralized NTO is due to delays in metamorphosis. The consequence from this latter observation may be ecologically significant as delays of even a few days could increase mortality through predation and/or loss of the aquatic medium in temporary water bodies.

Characteristics and products of the reductive degradation of 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN) in a Fe-Cu bimetal system

It has been shown previously that, under acidic conditions, 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN) degrade in the presence of iron/copper bimetal particles; the reactions can be modeled by pseudo-first-order kinetics. This study investigates the reaction mechanisms of the degradation processes under different conditions. Batch studies were conducted using laboratory-prepared solutions and an industrial insensitive munition-laden (IMX) wastewater. The influence of parameters such as initial pH of the solution, copper/iron (Fe-Cu) contact, and solid/liquid ratio were systematically investigated to assess their impact on the reaction kinetics. These parameters were subsequently incorporated into pseudo-first-order decomposition models for NTO and DNAN. The activation energies for the degradation reactions were 27.40 and 30.57 kJ mol^-1, respectively. Degradation intermediates and products were identified. A nitro-to-amino pathway, which ultimately may lead to partial mineralization, is postulated. The amino intermediate, aminonitroanisole, was detected during DNAN degradation, but for NTO, aminotiazolone is suggested. Additionally, urea was identified as a degradation product of NTO.

Toxicokinetic Model Development for the Insensitive Munitions Component 3-Nitro-1,2,4-Triazol-5-One

3-Nitro-1,2,4-triazol-5-one (NTO) is a component of insensitive munitions that are potential replacements for conventional explosives. Toxicokinetic data can aid in the interpretation of toxicity studies and interspecies extrapolation, but only limited data on the toxicokinetics and metabolism of NTO are available. To supplement these limited data, further in vivo studies of NTO in rats were conducted and blood concentrations were measured, tissue distribution of NTO was estimated using an in silico method, and physiologically based pharmacokinetic models of the disposition of NTO in rats and macaques were developed and extrapolated to humans. The model predictions can be used to extrapolate from designated points of departure identified from rat toxicology studies to provide a scientific basis for estimates of acceptable human exposure levels for NTO.

Biotransformation and Degradation of the Insensitive Munitions Compound, 3-Nitro-1,2,4-triazol-5-one, by Soil Bacterial Communities

Insensitive munitions (IM) are a new class of explosives that are increasingly being adopted by the military. The ability of soil microbial communities to degrade IMs is relatively unknown. In this study, microbial communities from a wide range of soils were tested in microcosms for their ability to degrade the IM, 3-nitro-1,2,4-triazol-5-one (NTO). All seven soil inocula tested were able to readily reduce NTO to 3-amino-1,2,4-triazol-5-one (ATO) via 3-hydroxyamino-1,2,4-triazol-5-one (HTO), under anaerobic conditions with H2 as an electron donor. Numerous other electron donors were shown to be suitable for NTO-reducing bacteria. The addition of a small amount of yeast extract (10 mg/L) was critical to diminish lag times and increased the biotransformation rate of NTO in nearly all cases indicating yeast extract provided important nutrients for NTO-reducing bacteria. The main biotransformation product, ATO, was degradable only in aerobic conditions, as evidenced by a rise in the inorganic nitrogen species nitrite and nitrate, indicative of nitrogen-mineralization. NTO was nonbiodegradable in aerobic microcosms with all soil inocula.

Toxicity of the conventional energetics TNT and RDX relative to new insensitive munitions constituents DNAN and NTO in Rana pipiens tadpoles

An initiative within the US military is targeting the replacement of traditional munitions constituents with insensitive munitions to reduce risk of accidental detonation. The purpose of the present study was to comparatively assess toxicity of the traditional munitions constituents 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) with the new insensitive munitions constituents 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO). The following exposure durations were performed with Rana pipiens (leopard frog) tadpoles: TNT and DNAN, 96 h and 28 d; RDX, 10 d and 28 d; NTO, 28 d. The 96-h 50% lethal concentration (LC50) values and 95% confidence intervals for TNT and DNAN were 4.4 mg/L (4.2 mg/L, 4. 7 mg/L) and 24.3 mg/L (21.3 mg/L, 27.6 mg/L), respectively. No significant impacts on survival were observed in the 10-d exposure to RDX up to 25.3 mg/L. Effects on tadpole swimming distance were observed with a lowest-observed-effect concentration (LOEC) of 5.9 mg/L RDX. In the 28-d exposures, the LOECs for survival for TNT, DNAN, and NTO were 0.003 mg/L, 2.4 mg/L, and 5.0 mg/L, respectively. No significant mortality was observed in the RDX chronic 28-d exposure up to the highest treatment level tested of 28.0 mg/L. Neither tadpole developmental stage nor growth was significantly affected in any of the 28-d exposures. Rana pipiens were very sensitive to chronic TNT exposure, with an LOEC 3 orders of magnitude lower than those for insensitive munitions constituents DNAN and NTO.

Oral Toxicity of 3-Nitro-1,2,4-triazol-5-one in Rats

3-Nitro-1,2,4-triazol-5-one (NTO), an insensitive explosive, was evaluated to assess potential environmental and human health effects. A 14-day oral toxicity study in Sprague-Dawley rats was conducted with NTO in polyethylene glycol -200 by gavage at doses of 0, 250, 500, 1000, 1500, or 2000 mg/kg-d. Body mass and food consumption decreased in males (2000 mg/kg-d), and testes mass was reduced at doses of 500 mg/kg-d and greater. Based on the findings in the 14-day study, a 90-day study was conducted at doses of 0, 30, 100, 315, or 1000 mg/kg-d NTO. There was no effect on food consumption, body mass, or neurobehavioral parameters. Males in the 315 and 1000 mg/kg-d groups had reduced testes mass with associated tubular degeneration and atrophy. The testicular effects were the most sensitive adverse effect and were used to derive a benchmark dose (BMD) of 70 mg/kg-d with a 10% effect level (BMDL10) of 40 mg/kg-d.

Biomarkers of oral exposure to 3-nitro-1,2,4-triazol-5-one (NTO) and 2,4-dinitroanisole (DNAN) in blood and urine of rhesus macaques (Macaca mulatta)

The U.S. Department of Defense is using the chemicals 2,4-dinitroanisole (DNAN) and 3-nitro-1, 2,4-triazol-5-one (NTO) in new munitions development. In a screen for biomarkers of exposure, these compounds were measured in urine and blood of male rhesus monkeys after oral doses. NTO peaked at 4 h, with urinary concentrations at least 100-fold higher than that of blood or serum while 4-dinitrophenol (DNP), a metabolite of DNAN, appeared in blood at concentrations 10- to 20-fold higher than the parent compound. For human exposure monitoring, urine is optimal for NTO while the metabolite DNP in blood is best for DNAN.

Genotoxicity assessment of ethylenediamine dinitrate (EDDN) and diethylenetriamine trinitrate (DETN)

Ethylenediamine dinitrate (EDDN) and diethylenetriamine trinitrate (DETN) are relatively insensitive explosive compounds that are being explored as safe alternatives to other more sensitive compounds. When used in combination with other high explosives they are an improvement and may provide additional safety during storage and use. The genetic toxicity of these compounds was evaluated to predict the potential adverse human health effects from exposure by using a standard genetic toxicity test battery which included: a gene mutation test in bacteria (Ames), an in vitro Chinese Hamster Ovary (CHO) cell chromosome aberration test and an in vivo mouse micronucleus test. The results of the Ames test showed that EDDN increased the mean number of revertants per plate with strain TA100, without activation, at 5000μg/plate compared to the solvent control, which indicated a positive result. No positive results were observed with the other tester strains with or without activation in Salmonella typhimurium strains TA98, TA1535, TA1537, and Escherichia coli strain WP2 uvrA. DETN was negative for all Salmonella tester strains and E. coli up to 5000μg/plate both with and without metabolic activation. The CHO cell chromosome aberration assay was performed using EDDN and DETN at concentrations up to 5000μg/mL. The results indicate that these compounds did not induce structural chromosomal aberrations at all tested concentrations in CHO cells, with or without metabolic activation. EDDN and DETN, when tested in vivo in the CD-1 mouse at doses up to 2000mg/kg, did not induce any significant increase in the number of micronuclei in bone marrow erythrocytes. These studies demonstrate that EDDN is mutagenic in one strain of Salmonella (TA100) but was negative in other strains, for in vitro induction of chromosomal aberrations in CHO cells, and for micronuclei in the in vivo mouse micronucleus assay. DETN was not genotoxic in all in vitro and in vivo tests. These results show the in vitro and in vivo genotoxicity potential of these chemicals.