The treatment options in
soman poisoning are very limited due to rapid aging of the inhibited
acetylcholinesterase, which makes the
enzyme essentially intractable. Hence,
oxime treatment probably comes too late in realistic scenarios. As an alternative, protecting part of the
enzyme by reversible inhibition prior to
soman exposure has been proposed. This strategy was successfully tested in animal experiments, but its efficacy still awaits complete understanding. In particular, it is unclear whether survival is improved by a higher residual activity of
acetylcholinesterase during the acute phase, when the reversible and irreversible inhibitors are present together. In previous experiments with
carbamate pre-treatment and
paraoxon challenge we noticed an increased residual activity of erythrocyte
acetylcholinesterase compared to non-pre-treatment. This result was encouraging to also test for comparable effects when using
soman. Immobilized human erythrocytes were continuously perfused for real-time measurement of
acetylcholinesterase activity by a modified Ellman method using 0.45mM
acetylthiocholine. After having established the inhibition rate constant of
soman, we tested the prophylactic potential of
physostigmine,
pyridostigmine and
huperzine A. Pre-treatment with the reversible inhibitors inhibited the
enzyme by 20-95%. Additional perfusion with 10nM
soman for 30min resulted in a residual activity of 1-5%, at low and high pre-inhibition, respectively. The residual activity was markedly higher than in the absence of reversibly blocking agents (0.1%). After discontinuation of
soman and the reversible
inhibitors, enzyme activity recovered up to 30% following pre-inhibition by 50%. The experimental data agreed with computer simulations when feeding the kinetic-based model with the established rate constants. The results with
soman essentially agreed with those obtained previously with
paraoxon.