β-Aminoalcohols as Potential Reactivators of Aged Sarin-/Soman-Inhibited Acetylcholinesterase

Abstract

Organophosphate nerve agents inhibit the enzyme acetylcholinesterase (AChE), which is involved in nerve signal transduction, by forming covalent adducts with its catalytic serine residue. AChE adducts with soman and sarin nerve agents undergo dealkylation, a process known as aging, within a few minutes and a few hours, respectively. This transformation is detrimental because it precludes reactivation of AChE with known oxime-based antidotes. Here, we designed a b-aminoalcohol molecule for aged AChE reactivation, using a multitiered computational approach. This approach includes highquality quantum mechanical/molecular mechanical calculations, providing reliable reactivation steps and energetics. The calculations suggest that the designed b-aminoalcohol can selectively reactivate aged sarin-/soman-inhibited AChE. Furthermore, unlike existing antidotes, the designed b-aminoalcohol lacks a permanent charge, making it potentially active in the central nervous system. The mechanistic insights of this study can help guide the development of new AChE reactivators with improved access to the central nervous system.

Resurrection and Reactivation of Acetylcholinesterase and Butyrylcholinesterase

“…Despite many attempts anda cross decades of effort, aging was considered irreversible, and until 2018, there were no in vitro or in vivo reports of any recovery of the activity of aged AChE.T he enzyme was hypothesized to adopt as tabilized aged conformation,p erhaps by forming as trong salt bridge between thep hosphylated oxyanion at the serine residue and the histidinium ion of the catalytic triad, and perhaps also due to charge-charge repulsion between the phosphylated oxyanion and typical reactivating nucleophiles, such as pyridinium oximates or their neutral forms.W hether the nucleophile possessesafull negative chargeo rn ot, the increase in electron density leading to its nucleophilicity generatesarepulsive interaction. [145,146] Multiple attempts were made to convert the phosphylated oxyanion aged form to ar e-alkylated (i.e.,i nhibited) form of AChE using av arietyo fa lkylating reagents,i ncluding sulfonates, haloketones, methoxypyridniums,a nd sulfoniums. [88,[145][146][147][148][149][150][151] However,w hile some of these alkylating strategies were effective in model systems, thesea ttempts proved to be unsuccessfulw hen they were attempted in vitro with aged AChE.…”

“…[145,146] Multiple attempts were made to convert the phosphylated oxyanion aged form to ar e-alkylated (i.e.,i nhibited) form of AChE using av arietyo fa lkylating reagents,i ncluding sulfonates, haloketones, methoxypyridniums,a nd sulfoniums. [88,[145][146][147][148][149][150][151] However,w hile some of these alkylating strategies were effective in model systems, thesea ttempts proved to be unsuccessfulw hen they were attempted in vitro with aged AChE.…”

“…Wallqvist and co-workers used computational tools to develop novel reactivators of methylphosphonate-aged AChEi n which the reactivation was direct and did not proceed through ar e-alkylated intermediate. [145] Drawing inspiration from their research on CapD,w hich functions similar to serine proteases but having as ingle threoniner esiduer ather than ac atalytic triad, they noted that b-aminoalcohols may activate the catalytic threonine residue. Since b-aminoalcohols are neutral and become chargedl ate into the nucleophilic attack, it was hypothesized that this functionality would serve as as ufficient nucleophile for aged AChE reactivation due to its proton-transfer ability to quencht he negative charge of the phosphylated Ser,w hile being neutrals hould allow for BBB permeability.T o achieve selectivity for the AChEa ctive site, two molecules were then designed, 77 placed the b-aminoalcohol group onto ap yridiniumr ing, drawing inspiration from the oxime 2-PAM, and 78 was ac onstitutional isomer removing the nitrogen from the aromatic ring and placing it on the periphery (Figure 41).…”

“…The enzyme was hypothesized to adopt a stabilized aged conformation, perhaps by forming a strong salt bridge between the phosphylated oxyanion at the serine residue and the histidinium ion of the catalytic triad, and perhaps also due to charge–charge repulsion between the phosphylated oxyanion and typical reactivating nucleophiles, such as pyridinium oximates or their neutral forms. Whether the nucleophile possesses a full negative charge or not, the increase in electron density leading to its nucleophilicity generates a repulsive interaction . Multiple attempts were made to convert the phosphylated oxyanion aged form to a re‐alkylated (i.e., inhibited) form of AChE using a variety of alkylating reagents, including sulfonates, haloketones, methoxypyridniums, and sulfoniums .…”

“…[145,164] Of the computationally proposed molecules, Mannich bases are simplya romatic b-aminoalcohols which were reported to disrupt this H-bonding network. [145] The very specific orientation of H-bond donors and acceptors may then explain the high affinity for the active site of AChE and the rapid rate of reactivation in addition to the ability to resurrectthe anionic aged Ser residue.…”