Toxicokinetics of (±)-VX in hairless guinea pigs and marmosets - in vitro metabolism of (±)-VX

Although the toxicokinetics of several nerve gases, such as sarin and soman have been thoroughly investigated, the toxicokinetics of (±)-VX are still poorly understood. However, it is important to study the toxicokinetics of (±)-VX because there are several reasons to assume that the behavior of (±)-VX will differ from that of the phosphofluoridates. Levels of (±)-VX higher than 30 pg (±)-VX/ml blood were quantified by means of gas chromatography, whereas the ratio of the enantiomers of (±)-VX in blood could be determined at levels higher than 1 ng/ml blood using normal phase chiral HPLC. Toxicokinetic studies in hairless guinea pigs after intravenous administration of (±)-VX at a dose corresponding with 2 LD50 (56 μg/kg) showed levels in blood at 120 min after administration that are one order of magnitude higher than those of C(±)P(-)-soman at the same time point after administration of an equivalent dose of C(±)P(±)-soman, indicating the in vivo persistence of (±)-VX. In contrast with soman, hardly any stereospecificity was observed in the sequestration of the two enantiomers of (±)-VX up to 64 min after administration. At 6 h after intoxication (±)-VX levels in blood of marmosets were found to be threefold higher than those found in hairless guinea pigs challenged with the same intravenous dose (i.e. 28 μg/kg).
Toxicokinetics of (±)-VX after percutaneous application (dose 125 μg/kg) on hairless guinea pigs were also studied. Intact (±)-VX in blood was detected after a lag time of one hour. The levels increased gradually and reached a maximum of approx. 150 pg/ml blood at 6 h after exposure. A correlation between (±)-VX concentration in blood and AChE inhibition could be established. O-ethyl methylphosphonic acid (EMPA) was found as a metabolite of (±)-VX and could be determined in almost all blood samples taken during the toxicokinetic experiments. The concentration of EMPA in a blood sample, taken 6 hours after administration was found to be at least one order of magnitude higher than the concentration of intact (±)-VX in that particular blood sample.
For further metabolic studies, 35S-(±)-VX and [14CH3-P]-(±)-VX have been synthesized and were used for in vitro experiments. Labeled (±)-VX was incubated in liver homogenate and plasma from humans, marmosets and hairless guinea pigs. The toxic O-hydrogen S-(2- diisopropylaminoethyl) methylphosphonothioate was found in all plasma samples but not in liver homogenate. The leaving group containing sulfur transformed into a variety of metabolites and was found to bind to proteins.
This work was supported by the U.S. Army Medical Research and Materiel Command under Cooperative Agreement DAMD17-97-2-7001