The extreme toxicity of VX is partly due to the fact that the inhibitor was designed to be an excellent structural mimic for the transition state of the natural substrate (acetylcholine) of acetylcholinesterase. VX has a very high "on-rate" to react with the target enzyme and form a stable P-O-C bond (phosphorylation). However, compared with other highly toxic nerve agents like soman or sarin, VX undergoes relatively slow "aging". Aging is a time-dependent side reaction (loss of an alkoxyl group) that occurs on nerve agents after phosphonylation and renders the nerve agent-acetylcholinesterase complex highly resistant to regeneration by any known antidote. Slower aging by VX suggests it should be possible to develop more effective antidotes and treatments.