Comparative Toxicological Evaluation of the Insensitive Munition (IM) 1-methyl-3-nitroguanidine versus its UV-Degradation Products in Fathead Minnow

Abstract: The Army is replacing traditional munitions with insensitive munitions (IM) resistant to accidental detonation. Although the parent IM compound nitroguanidine (NQ) is generally not acutely toxic at concentrations >1000 mg/L in aquatic exposures, products formed by intensive UV-degradation resulted in of multiple-order of magnitude increase in toxicity. A methylated congener of NQ, 1-methyl-3-nitroguanidine (MeNQ), is also being assessed for potential use in IM explosive formulations and consequently was here investigated for UV-degradation hazard and bioaccumulation potential. While up to 716 mg/L parent MeNQ caused no significant mortality or effects on growth in in larval P. promelas in 7-d exposures, the same concentration of MeNQ subjected to UV-treatment resulted in 85% mortality. The UV-treatment degraded only 3.3% of the MeNQ (5,800 mg/L stock, UV-treated for 6h at UV 12x > sunlight), indicating that MeNQ degradation products have potentially high potency. The parent MeNQ exposure decreased transcriptional expression of genes within the significantly enriched insulin metabolic pathway suggesting antagonism of bioenergetics pathways which compliments observed, although non-significant, decreases in body weights. Transcriptional expression in the UV-degraded MeNQ exposures resulted in significant enrichment of pathways and functions related to cell cycle, but also erythrocyte function related to O2/CO2 exchange. These functions likely represent the mechanistic source(s) of increased toxicity observed in the UV-degraded MeNQ exposures, which are distinct from previously observed mechanisms underlying increased UV-degraded NQ toxicity in fish. Subchronic 7-d toxicity test using P. promelas was conducted in accordance with the USEPA (2002). Larval P. promelas (2-d old within 24-h in age) from a single batch of offspring were acclimated to experimental conditions for 24 h. Each exposure concentration consisted of 5 replicates (250-mL glass beakers containing 10 fish and 200 mL test solution). The target exposure concentrations for the parent MeNQ and the UV-treated MeNQ exposures were: 0 (control), 7, 22, 67, 200, and 600 mg/L. The UV-treated MeNQ was exposed for 4 hours in a a fan cooled Rayonet Reactor RPR-1000 (Southern New England Ultraviolet Company) equipped with a carousel and sixteen 14 W UV lamps (SNE Ultraviolet Company k = 300 ± 50 nm) to create a photo-intensity of 0.05 mW cm-2 nm-1 was used for all UV-treatment experiments. This duration of UV exposure correlated to the UV-content of 96 hours of summertime sunlight in Vicksburg, Mississippi, USA. Experiments with the parent and UV-treated MeNQ was initiated within 3 hours of completion of the UV-treatment, where standard exposure methods were conducted absent of UV-light.