In vitro evaluation of neutral oximes as reactivators of parathion-inhibited electric eel acetylcholinesterase

Abstract

Organophosphorus (OP) compounds are irreversible inhibitors of acetylcholinesterase (AChE) commonly used as pesticides and, unfortunately, as nerve agents in terrorist attacks. These compounds are highly soluble easily crossing the blood-brain barrier (BBB). Clinically, oximes such as pralidoxime and obidoxime are used for the reactivation of AChE. These oximes are not effective to reactivate AChE inhibited by different OPs besides the fact that they are permanently charged and do not readily cross the BBB. This work evaluated the ability of ten neutral oximes to reactivate parathion-inhibited eel AChE. Because oximes can bind to AChE as reversible inhibitors, this property was also evaluated, with pralidoxime (2-PAM) used as a reference compound. Unlike 2-PAM, which inhibited AChE in a concentration-dependent way, neutral oximes did not follow the linear order of AChE inhibition. Neutral ligands can present affinity for the periferic anionic site (PAS) site. Neutral oximes 1 and 2 (200 µM) reactivated parathion-inhibited eel AChE by 9 per cent and 11 per cent, respectively; but neither of them surpassed the reactivation efficacy of 2-PAM (25 per cent). Neutral oximes 1 and 2 reactivated AChE at a safe concentration for humans. Both neutral oximes 1 and 2 are good non-quaternary moieties for the synthesis of conjugates with enhanced reactivation potency and BBB penetration.Keywords: Acetylcholinesterase; Eel AChE; Reactivator; Oxime; Pralidoxime; Parathion RESEARCH PAPERDefence Life Science Journal, Vol. 2, No. 3, July 2017, pp. 363-369, DOI : 10.14429 LIMA, et al.: DEf. LIfE SCI. J., VOL. 2, NO. 3, JULy 2017, DOI : 10.14429/dlsj.2.10730 or allosteric) 14 . Although approved as antidotes, these oximes are not sufficiently effective to reactivate AChE inhibited by the different OPs 15,16 . The mono-quaternary oxime 2-PAM is very efficient in reactivating AChE inhibited with sarin or VX 17 but is not effective against tabun or soman 18 . Obidoxime is the most potent and most efficacious oxime in reactivating AChE inhibited by various classes of OP insecticides and tabun, but was inferior to oxime HI-6 against soman, sarin, cyclosarin and VX 19. A significant drawback to these oximes is they are permanently charged and do not readily cross the blood brain barrier (BBB) 20 . As a result, they show only limited activity in the CNS, which is a major target of OPs. Thus, effective reactivators as antidotes are increasingly needed against a broader spectrum of nerve agents 21 . Introducing non-quaternary organic compounds has been a novel method of efficiently penetrate the BBB for reactivation of brain AChE 22. Non-charged oximes have previously been developed with improved BBB penetration [23][24][25][26][27][28][29] and sufficient reactivation of AChE inhibited by nerve agents and insecticides 24,30 . Even with improved BBB penetration and reactivation of AChE, the specific oxime structures with superior reactivating potency to those in use remain unknown.The work proposed here evaluated the ability of...

Recent developments on oximes to improve the blood brain barrier penetration for the treatment of organophosphorus poisoning: a review

“…49,50 2-PAM and obidoxime were also reported to be less effective against soman, cyclosarin, and Russian VX. 51,52 Trimedoxime has been shown to have better efficacy in treating poisoning with tabun. 53 HI-6 has since been considered as one of the better quaternary oximes for OP nerve agent poisoning so far 54 as it was the rst quaternary oxime that demonstrated reactivation efficiency for soman-inhibited AChE.…”

“…However, conjugates containing isonicotinamide have been reported to give unsatisfactory results against sarin-and tabun-inhibited AChE. Lima et al (2017) too developed several uncharged oximes as AChE reactivators of parathion-inhibited AChE in eels. 51 The presence of neutral ligands as PSL is expected to improve the affinity towards the inhibited AChE.…”

“…Lima et al (2017) too developed several uncharged oximes as AChE reactivators of parathion-inhibited AChE in eels. 51 The presence of neutral ligands as PSL is expected to improve the affinity towards the inhibited AChE. It was reported that these developed uncharged oximes showed a lower affinity for AChE compared to 2-PAM.…”

Straightforward, Economical Procedures for Microscale Ellman’s Test for Cholinesterase Inhibition and Reactivation

“…This is a limitation of Ellman's test, depending on the type of compounds to be screened. It is also known that oximes may act as weak cholinesterase inhibitors, [24][25][26] which may explain the reduced reactivation potential for pralidoxime and obidoxime observed at 1000 µmol L -1 in comparison to other assayed concentrations. This outcome depends on different oxime structural features, such as number of pyridinium rings, linker length, etc.…”

“…Nevertheless, assays at higher concentrations are valuable for further development of cholinesterase inhibitors for neurodegenerative diseases. [25][26][27][28][29] With our procedure, spontaneous reactivation and enzymatic aging may also be accessed. For instance, Soman (O-pinacolyl methylphosphonofluoridate) results in a phosphonylated enzyme, which spontaneously disproportionate to a stable, ionized phosphonate derivative and, thus, is aged.…”