Kinetic analysis of interactions between alkylene-linked bis-pyridiniumaldoximes and human acetylcholinesterases inhibited by various organophosphorus compounds

The therapeutic approach of organophosphorus compound (OP) intoxications is to reactivate the inhibited enzyme acetylcholinesterase (AChE). Numerous studies demonstrated a limited efficacy of standard oxime-based reactivators against different nerve agents such as tabun and cyclosarin. This emphasizes research for more effective oximes. In the present study, reactivation kinetics of tabun-, sarin-, cyclosarin-, VX- or paraoxon-ethyl-inhibited human AChE (hAChE) with a homologous series of bis-ortho-pyridiniumaldoximes, Ortho-4 - Ortho-9, was investigated with a robot-assisted setting, allowing determination of second-order reactivation rate constants as well as model calculations. The reactivation constants of Ortho-4 - Ortho-9 resulted in marked differences of affinity and reactivity depending on the OP structure and the linker length of the oximes. In general, the KD values decreased with increasing linker length. Reactivity increased from Ortho-4 to Ortho-6 for PXE- and VX-inhibited hAChE and from Ortho-4 to Ortho-7 for GA-inhibited hAChE and decreased again with Ortho-8 and Ortho-9. In contrast, kr decreased with increasing linker length for sarin- and cyclosarin-inhibited hAChE. In view of the pronounced decrease of KD from Ortho-4 to Ortho-9, the kr2 values increased with all tested OP. Hence, the ratios of KI/KD and of KI/kr2 showed that in almost all cases the affinity of Ortho-N to the native hAChE was higher than to OP-inhibited enzyme. Model calculations indicated that Ortho-6 - Ortho-9 could be superior to obidoxime in reactivating tabun-inhibited hAChE. Finally, these data emphasize the need to develop oximes with a higher selective affinity towards OP-inhibited hAChE in order to minimize possible side effects.

Basic structure and structure–activity-relationship of the second-order rate constant kr2 for the reactivation of OP-inhibited AChE and Ortho oxime linker length.Figure optionsDownload as PowerPoint slide