Medical Aspects of Chemical Warfare (2008) - The Black Vault

Medical Diagnosticshalf-life rates (16.5 min for guinea pigs; 9 min formarmosets). 8 Inhalation experiments using nose-onlyexposure of guinea pigs to 0.8 × LCt 50(the vapor oraerosol exposure that is lethal to 50% of the exposedpopulation) agent demonstrate terminal half-lives ofapproximately 36 and 9 minutes for sarin and soman,respectively. 9 In contrast, similar studies with VX inhairless guinea pigs and marmosets indicate VX ismore persistent than the G agents. 10 These studiesshow that VX can be found at acutely toxic levels for10 to 20 hours following intravenous administrationat a dose one or two times the LD 50, with terminalelimination rates of 98 minutes (1 times the LD 50inhairless guinea pigs), 165 minutes (2 times the LD 50),and 111 minutes in marmosets (at a dose equivalentto 1 LD 50in hairless guinea pigs). Percutaneous administrationof the LD 50of VX to hairless guinea pigsdemonstrated relatively low blood levels (140 pg/mL), which reached a maximum after approximately6 hours. 10 Because the route of human exposure toVX would most likely occur percutaneously, the timeframe of 6 hours may be the more relevant assessmentof its persistence in blood. This allows very limitedtime for sample collection and analysis. Others havedemonstrated that VX can be assayed from spikedrat plasma. 11 These authors noted that 53% of the VXwas lost in spiked plasma specimens after 2 hours.The disappearance was attributed to the enzymeaction of the OP hydrolase splitting or to cleavageof the sulfur-phosphorus bond to form diisopropylaminoethanethiol (DAET) and ethyl methylphosphonicacid (EMPA). 11Assay of Hydrolysis CompoundsAnalytical MethodsAn alternative approach to direct assay of parentnerve agents is to measure metabolic or hydrolysisproducts in specimens. These compounds are producedin vivo as a result of hydrolysis or detachmentfollowing spontaneous regeneration of the AChEenzyme. Studies of parent nerve agents with radioisotopicallylabeled phosphorus ( 32 P) or hydrogen ( 3 H) inanimals suggest that agents are rapidly metabolizedand hydrolyzed in the blood and appear in the urineas their respective alkyl MPAs. 12–15 This observationled to the development of assays for alkyl MPAs inbiological samples, 16 the applicability of which wassubsequently demonstrated in animals exposed tonerve agents. 17 The common products found areisopropyl methylphosphonic acid (IMPA), pinacolylmethylphosphonic acid, cyclohexyl methylphosphonicacid, and EMPA derived from sarin, soman, cyclosarin,and VX, respectively (Figure 22-2). Additionally forVX, hydrolysis of the sulfur-phosphorus bond occurs,yielding DAET and EMPA. The formation and assayof DAET has been reported in rat plasma spiked withVX. 11 Furthermore, the presence of diisopropyl aminoethylmethyl sulfide, presumably resulting from thein-vivo methylation of DAET, has been reported in humanexposures. 18 To date, numerous variations of thealkyl MPA assay for biological fluids, such as plasmaand urine, have been developed. These include GCseparations with MS, 18–20 tandem MS (MS-MS), 18,19,21,22and flame photometric detection. 23,24 Other methodsinvolving LC with MS-MS 25 and indirect photometricdetection 26 have also been reported (Table 22-1).Application to Human ExposuresThe utility of some methodologies has been demonstratedin actual human exposure incidents. Mostinvolve assays of urine and plasma or serum. Tsuchihashiet al 18 demonstrated the presence of EMPA inthe serum of an individual assassinated with VX inOsaka, Japan, in 1994. As mentioned earlier, theseauthors also reported the presence of diisopropylaminoethyl methyl sulfide, which resulted from thein-vivo methylation of DAET subsequent to cleavage ofthe sulfur-phosphorus bond. Reported concentrationsin serum collected 1 hour after exposure were 143 ng/mL diisopropyl aminoethyl methyl sulfide and 1.25μg/mL for EMPA.The Aum Shinrikyo cult attacked citizens twice inJapan using sarin. The first was in an apartment complexin Matsumoto City, where approximately 12 litersof sarin were released using a heater and fan. Accordingto police reports, 600 inhabitants in the surroundingarea were harmed, including 7 who were killed.In the second attack, sarin was released into the Tokyosubway, resulting in more than 5,000 casualties and 10deaths. 27 Assay of hydrolysis products as a definitivemarker were used to verify that sarin was the agent employedin these events. Minami et al 23 and Nakajima etal 24 demonstrated the presence of IMPA or MPA in victims’urine following sarin exposure in the Tokyo andMatsumoto attacks, respectively. These methods usedGC separations of the prepared urine matrix coupledwith flame photometric detection. In the Matsumotoincident, urinary concentrations of IMPA and MPA,as well as the total dose of the sarin exposure, werereported. 24 For one victim, MPA concentrations were0.14 and 0.02 ug/mL on the first and third days afterexposure, and 0.76, 0.08, and 0.01 ug/mL for IMPA,respectively, on the first, third, and seventh days afterexposure. 24 In this case, the individual was estimatedto have been exposed to 2.79 mg of sarin.695