In vitro and in vivo toxicological studies of V nerve agents: molecular and stereoselective aspects

In vitro inhibition data of cholinesterases (ChEs) and reactivation with HI 6 are presented for separated VX and VR enantiomers with high purity (enantiomer excess >99.999%). Inhibition rate constants for (-)-VR were fourfold higher than for (-)-VX. Marked higher stereoselectivity of ChEs inhibition was observed for VR compared with VX enantiomers. Low/no reactivation was determined for respective (+)-enantiomers. Results were related to orientation of (-)- and (+)-enantiomers in ChEs active sites. In vivo in swine, absorption rate constants were practically identical for VX and VR enantiomers after percutaneous application of 3xLD₅₀ underlining relevance of amine group and postulated equilibria shifts between charged, uncharged, open and cyclic form (skin depot). In vivo toxicokinetics of VX and VR enantiomers differed markedly after 4h. Elimination of VX was much slower compared with VR. Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibition in vivo differed for VX and VR. In vivo spontaneous reactivation was not observed for VX-inhibited AChE while VR-inhibited AChE was much faster spontaneously reactivated than expected and AChE inhibition by VR was slower than expected. Progredient BChE inhibition was detected after VX application while VR inhibited BChE weakly. Possible explanation may be impact of the agents on hemodynamics and different metabolisms. Thus, due to increase of the V agents' blood concentration after atropine administration (depot release) the present standard therapy should be thoroughly reconsidered.