Ira Hill

Clinical aspects of percutaneous poisoning by the chemical warfare agent VX: effects of application site and decontamination.

O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothioate (VX) is an extremely toxic organophosphate nerve agent that has been weaponized and stockpiled in a number of different countries, and it has been used in recent terrorist events. It differs from other well-known organophosphate nerve agents in that its primary use is as a contact poison rather than as an inhalation hazard. For this reason, we examined the effects of application site and skin decontamination on VX toxicity in anesthetized domestic swine after topical application. VX applied to the surface of the ear rapidly resulted in signs of toxicity consistent with the development of cholinergic crisis, including apnea and death. VX on the epigastrium resulted in a marked delayed development of toxic signs, reduced toxicity, and reduction in the rate of cholinesterase depression compared with animals exposed on the ear. Skin decontamination (15 minutes post-VX on the ear) arrested the development of clinical signs and prevented further cholinesterase inhibition and death. These results confirm earlier work that demonstrates the importance of exposure site on the resultant toxicity of this agent and they also show that decontamination postexposure has the potential to be an integral and extremely important component of medical countermeasures against this agent.

Clinical manifestations of VX poisoning following percutaneous exposure in the domestic white pig

Nerve agents are a class of organophosphorus chemicals that inhibit certain cholinesterase enzymes (ChE). If untreated, percutaneous exposure to nerve agents, such as VX (O-ethyl-S-[2(diisopropylamino)ethyl] methylphosphonothioate) can cause paralysis, apnoea and death. Much of the information concerning the percutaneous absorption and subsequent toxicity of nerve agents has been obtained using various rodent models. However, the most relevant ‘skin model’ is arguably the pig. Therefore, the purpose of this study was to examine the clinical manifestations of VX intoxication in the domestic white pig following a 2 LD50 (120 mg/kg) percutaneous challenge. There was a consistent onset of signs (where present) in each animal: mastication was followed by miosis, salivation, fasciculations and apnoea. Whilst ChE activity did not correlate with the onset of signs, there was a qualitative relationship in that mastication preceded substantial ChE inhibition, miosis lagged behind the linear decrease in acetylcholinesterase (AChE) activity and fasciculations and apnoea occurred after maximum ChE inhibition had been attained (5 / 10% of normal). These observations may be of use for the triage of patients exposed to VX. In comparison with similar studies with GD, VX did not affect glucose utilization. However, VX was similar to GD in that it caused a mild hyperkalaemia and hyperphosphataemia, although the significance of this observation was not clear. There was substantial lateral diffusion of the initial droplet of VX over the application site, indicating that, when decontaminating exposed skin, attention should also be directed to areas peripheral to the original site of exposure.

Chromatographic resolution, characterisation and quantification of VX enantiomers in hemolysed swine blood samples

The present study was initiated to develop a sensitive and highly selective method for the analysis of the enantiomers of the nerve agent VX (O-ethyl S-[2(diisopropylamino)ethyl] methylphosphonothioate) in blood samples for toxicokinetic and therapeutic research. To achieve this goal, analytical and semi-preparative enantioseparation of VX were carried out with gas and liquid chromatography. The GC chiral stationary phase was HYDRODEX-beta-TBDAc (beta cyclodextrin), on which VX was baseline-resolved. On the chiral HPLC phase CHIRALCEL OD-H the enantiomers of VX were isolated with enantiomeric excess >99.99%. They were characterised by specific optical rotation (+/-25.8 deg ml dm(-1)g(-1) at 20 degrees C and 589 nm) and by determination of cholinesterase inhibition rate constants. For the quantitative chiral detection of VX the enantioresolution was realized on the HPLC chiral phase CHIRAL AGP. A specific procedure was developed to isolate VX from swine blood samples thereby stabilising its enantiomers. The limit of detection was 200 fg per enantiomer on column. The absolute recovery of the overall sample preparation procedure was 75%. After an intravenous and percutaneous administration of a supralethal dose of VX in anesthetised swine (+)-VX and (-)-VX could be quantified up to 720 min.

In vivo skin absorption and distribution of the nerve agent VX (O-ethyl-S-[2(diisopropylamino)ethyl] methylphosphonothioate) in the domestic white pig

The purpose of this study was to characterize the skin absorption and distribution of VX (Oethyl–S–[2(diisopropylamino)ethyl] methylphosphonothioate) in the domestic pig in order to evaluate the animal as a potential model for assessing pretreatments against toxic anti–cholinesterase compounds. A liquid droplet (equivalent to a 2 LD50 dose) of radiolabelled VX was applied to the inner ear–skin of each anaesthetized animal. Blood and tissue samples (liver, lung, kidney, heart and skin exposure sites) were obtained post–mortem. The amount of radioactivity in each sample was measured by liquid scintillation counting, from which the skin absorption rate and dose distribution of VX were calculated. A substantial proportion (229±3%) of the applied dose remained within the skin at the site of application. It is conceivable that strategies to minimize or remove this reservoir may be of benefit in the early treatment of VX–exposed casualties. Image analysis of autoradiographs of exposed skin sites indicated that each milligram of radioactive VX covered an area of 1.29±0.5 cm2. The average skin absorption rate of 14C-VX was 6619±126 mg/cm2 per hour. Comparison of these data with previous studies suggests that human skin is less permeable to VX than pig skin, but VX spreads over a greater surface area when applied to human skin. Thus, paradoxically, while pig-ear skin is more permeable than human skin, the difference in skin surface spreading may lead to the absorption of an equivalent systemic dose.

Simultaneous quantification of VX and its toxic metabolite in blood and plasma samples and its application for in vivo and in vitro toxicological studies.

The present study was initiated to develop a sensitive and highly selective method for the simultaneous quantification of the nerve agent VX (O-ethyl S-[2(diisopropylamino)ethyl] methylphosphonothioate) and its toxic metabolite (EA-2192) in blood and plasma samples in vivo and in vitro. For the quantitative detection of VX and EA-2192 the resolution was realized on a HYPERCARB HPLC phase. A specific procedure was developed to isolate both toxic analytes from blood and plasma samples. The limit of detection was 0.1 pg/ml and the absolute recovery of the overall sample preparation procedure was 74% for VX and 69% for EA-2192. After intravenous and percutaneous administration of a supralethal doses of VX in anaesthetised swine both VX and EA-2192 could be quantified over 540 min following exposure. This study is the first to verify the in vivo formation of the toxic metabolite EA-2192 after poisoning with the nerve agent VX. Further toxicokinetic and therapeutic studies are required in order to determine the impact of EA-2192 on the treatment of acute VX poisoning.

Toxicokinetics of tabun enantiomers in anaesthetized swine after intravenous tabun administration.

In the present study, we report the first in vivo toxicokinetic study of tabun (O-ethyl-N,N-dimethylphosphoramidocyanidate). The toxicokinetics of the enantiomers of tabun were investigated in anesthetized swine after intravenous administration of 3xLD(50) (161.4mug/kg) tabun. Blood samples were taken for gas chromatographic-mass spectrometric determination of the tabun enantiomers and for measurement of the activity of red blood cell acetylcholinesterase (AChE) and plasma butyrylcholinesterase (BChE). The tabun enantiomers could be quantified in swine blood to a minimum concentration of 3.0pg/ml (18.5pM) and could be detected to a minimum concentration of 1.0pg/ml (6.2pM). The concentration-time profiles of both tabun enantiomers were best described by a bi-exponential equation. The elimination of (+)-tabun and (-)-tabun were comparable in the initial phase. In the terminal phase a remarkable difference was found, with terminal half lives of 11.5min for (+)-tabun and 23.1min for (-)-tabun. (+)-Tabun showed a markedly longer persistence in vivo than (+)-enantiomers of other G-type nerve agents and could be detected in all swine at least up to 30min post-injection, (-)-tabun at least up to 90min post-injection. These results demonstrate a rather rapid elimination of tabun enantiomers in vivo and may provide a toxicokinetic basis for the further development and optimization of medical countermeasures against this nerve agent.