Comparison of inhibition kinetics of several organophosphates, including some nerve agent surrogates, using human erythrocyte and rat and mouse brain acetylcholinesterase

- Inhibition kinetic assays were conducted with several highly relevant surrogates for sarin, VX and cyclosarin and insecticidal organophosphates in rat brain, mouse brain and purified human erythrocyte acetylcholinesterase.
- With the exception of chlorpyrifos-oxon, the nerve agent surrogates were more potent inhibitors than the other organophosphates.
- The potencies of the nerve agent surrogates were proportional to the toxicities of the nerve agents, cyclosarin > VX > sarin.

Because testing of nerve agents is limited to only authorized facilities, our laboratory developed several surrogates that resemble nerve agents because they phosphylate the acetylcholinesterase (AChE) with the same moiety as the actual nerve agents. The inhibition kinetic parameters were determined for AChE by surrogates of cyclosarin (NCMP), sarin (NIMP, PIMP and TIMP) and VX (NEMP and TEMP) and other organophosphorus compounds derived from insecticides. All compounds were tested with rat brain and a subset was tested with mouse brain and purified human erythrocyte AChE. Within the compounds tested on all AChE sources, chlorpyrifos-oxon had the highest molecular rate constant followed by NCMP and NEMP. This was followed by NIMP then paraoxon and DFP with rat and mouse brain AChE but DFP was a more potent inhibitor than NIMP and paraoxon with human AChE. With the additional compounds tested only in rat brain, TEMP was slightly less potent than NEMP but more potent than PIMP which was more potent than NIMP. Methyl paraoxon was slightly less potent than paraoxon but more potent than TIMP which was more potent than DFP. Overall, this study validates that the pattern of inhibitory potencies of our surrogates is comparable to the pattern of inhibitory potencies of actual nerve agents (i.e., cyclosarin > VX > sarin), and that these are more potent than insecticidal organophosphates.