Up-and-down procedure (UDP) determinations of acute oral toxicity of nitroso degradation products of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

Abstract

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a widely used military explosive and soil and ground water contaminant of munitions manufacturing and artillery training sites, undergoes microbial nitroreductase metabolism to hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX). Human occupational and accidental exposures to RDX, as well as acute oral exposures in rats, result in seizures, but little is known about the toxicity of the RDX degradation products. The main objective of the present study was to determine the oral LD50 of the most potent RDX N-nitroso product in female Sprague-Dawley rats using the recently validated up-and-down procedure (UDP). With only 26 rats, MNX was identified as the most potent metabolite and a maximum likelihood estimate of 187 mg kg(-1) (95% confidence interval 118-491 mg kg(-1)) for its LD50 was established and found equivalent to that of RDX determined with the same protocol. CNS toxicity, manifested as forelimb clonic seizures progressing to generalized clonic-tonic seizures, was the critical adverse effect. Further, confirmation of the UDP LD50 for MNX with a fixed-dose design enabled identification of 94 mg kg(-1) as the highest nonlethal dose. An ED50 of 57 mg kg(-1) was determined for neurotoxicity, while splenic hemosiderosis and decreased blood hematocrit and hemoglobin concentration occurred with a threshold at 94 mg kg(-1) in 14-day survivors. These studies, while providing new toxicity data necessary for the management of RDX-contaminated sites, illustrate the efficiency of the UDP for comparative acute toxicity determinations and its value in guiding further characterization of dose dependency of identified adverse effects.

Electron shuttle-stimulated RDX mineralization and biological production of 4-nitro-2,4-diazabutanal (NDAB) in RDX-contaminated aquifer material

“…1b-d). The nitroso metabolites are toxic and accumulation is a concern (Meyer et al 2005;Pitot and Dragan 1996). Several reports indicate that the nitroso metabolites are unstable and will spontaneously degrade (Gregory et al 2004;Oh et al 2001;Sheremata et al 2001).…”

Investigations of transcript expression in fathead minnow (Pimephales promelas) brain tissue reveal toxicological impacts of RDX exposure

“…Seizures have been observed in rats (Meyer et al, 2005), miniature swine (Schneider et al, 1977), mice (Dilley et al, 1978), Northern bobwhite quail (Johnson et al, 2007) and humans (Stone et al, 1969;Woody et al, 1986) within hours after exposure to RDX. Gust et al (2009) hypothesized that the mode of action underlying RDX-induced seizures in Northern bobwhite is the inhibition of neuronal membrane depolarization back to the baseline voltage after an action potential thereby resulting in hyper-excitation.…”

“…Observed effects of RDX exposure in the ecotoxicological model species fathead minnow include lethality, impaired growth and reduced reproduction (Burton et al, 1994;Talmage et al, 1999). Evidence that the central nervous system is the primary toxicological target for RDX include observation of RDX-induced convulsions in rats (Meyer et al, 2005), miniature swine (Schneider et al, 1977), mice (Dilley et al, 1978), Northern bobwhite (Johnson et al, 2007;Quinn et al, 2009;Gust et al, 2009) and humans (Stone et al, 1969;Woody et al, 1986). Neurotoxicogenomic investigations in rat and Northern bobwhite indicate that RDX-induced seizures are likely driven by various impacts on genes and pathways involved in the neurophysiology of brain tissue (Bannon et al, 2009;Gust et al, 2009 Given that RDX may accumulate in surface waters, the potential for impacts on CNS in fish is of concern.…”

Nitramine anion fragmentation: A mass spectrometric and Ab initio study

“…The toxicity of MNX to SpragueDawley rats was recently shown to be similar in magnitude and adverse effects to that of RDX [16]. Studies with deer mice suggest potential reproductive effects arising from TNX exposure through drinking water [17].…”