Resurrection and Reactivation of Acetylcholinesterase and Butyrylcholinesterase

Abstract

Organophosphorus (OP) nerve agents and pesticides present significant threats to civilian and military populations. OP compounds include the nefarious G and V chemical nerve agents, but more commonly, civilians are exposed to less toxic OP pesticides, resulting in the same negative toxicological effects and thousands of deaths on an annual basis. After decades of research, no new therapeutics have been realized since the mid-1900s. Upon phosphylation of the catalytic serine residue, a process known as inhibition, there is an accumulation of acetylcholine (ACh) in the brain synapses and neuromuscular junctions, leading to a cholinergic crisis and eventually death. Oxime nucleophiles can reactivate select OP-inhibited acetylcholinesterase (AChE). Yet, the fields of reactivation of AChE and butyrylcholinesterase encounter additional challenges as broad-spectrum reactivation of either enzyme is difficult. Additional problems include the ability to cross the blood brain barrier (BBB) and to provide therapy in the central nervous system. Yet another complication arises in a competitive reaction, known as aging, whereby OP-inhibited AChE is converted to an inactive form, which until very recently, had been impossible to reverse to an active, functional form. Evaluations of uncharged oximes and other neutral nucleophiles have been made. Non-oxime reactivators, such as aromatic general bases and Mannich bases, have been developed. The issue of aging, which generates an anionic phosphylated serine residue, has been historically recalcitrant to recovery by any therapeutic approach-that is, until earlier this year. Mannich bases not only serve as reactivators of OP-inhibited AChE, but this class of compounds can also recover activity from the aged form of AChE, a process referred to as resurrection. This review covers the modern efforts to address all of these issues and notes the complexities of therapeutic development along these different lines of research.

Keywords: acetylcholinesterase; butyrylcholinesterase; organophosphorus; reactivation; resurrection.

Figure 1

Select examples of G-series organophosphorus nerve agents.

Figure 2

Select examples of V-series organophosphorus nerve agents.

Figure 3

Select examples of organophosphorus pesticides in their oxon form.

Figure 4

Catalytic active site of AChE (S203, H447, E334), the oxyanion hole (G121, G122, A204), and cation-binding pocket (W86, E202) without ACh being bound (left) and with ACh (right).

Figure 5

Hydrolysis of acetylcholine by native AChE.

Figure 6

Aromatic residues (left) composing the gorge bottleneck and a space filling model (right) showing the width of the gorge.

Figure 7

Inhibition and aging of AChE with a model phosphonate nerve agent.

Figure 8

Examples of oximes administered for treatment of organophosphorus poisoning.

Figure 9

Visual comparison of the different widths and volumes of the (left) AChE and (right) BChE gorges.

Figure 10

Novel lipophilic pyridinium oximes.^[95]

Figure 11

Novel oxime structures synthesized using the Ugi multicomponent reaction.^[96]

Figure 12

Tetroxime structure as studied by Musilek and co-workers.^[97]

Figure 13

Bispyridinium compounds capable of reactivating AChE and BChE with (ethyl) paraoxon.^[98]

Figure 14

(E)-2-Butene-linked bispyridinium oximes for the reactivation of tabun-inhibited AChE.^[99]

Figure 15

Two novel K-oximes showing some efficiency for the reactivation of ethyl paraoxon-inhibited hAChE. ^[100]

Figure 16

ZINC database oximes determined from virtual screening which are capable of reactivating multiple OP pesticides.^[101]

Figure 17

Hydroxyiminoacetamide nucleophiles using structural components of HI-6 for reactivation of sarinand VX-inhibited AChE.^[102]

Figure 18

Imidazolium oxime for the reactivation of sarinand VX-inhibited AChE.^[103]

Figure 19

Hydroxyiminoacetamide nucleophiles using structural components of HI-6 for reactivation of sarinand VX-inhibited AChE.

Figure 20

Novel oxime structures based on salicylaldoximes.^[106]

Figure 21

Novel oxime structure that shows significant tabun reactivation.^[107]

Figure 22

Novel oxime structures that show some soman reactivation.^[108]

Figure 23

Novel oxime structures based on the Alzheimer’s disease drug donepezil by Renard and co-workers.^[109]

Figure 24

Uncharged reactivators of cholinesterases with the potential to cross the blood–brain barrier.^[110]

Figure 25

Tacrine-linked reactivators designed on the basis of X-ray crystallographic data.^[111]

Figure 26

Vitamin-B6-based oximes for the reactivation of hAChE and hBChE.^[112]

Figure 27

Non-oxime reactivators.^{[113, 114]}

Figure 28

Additional reactivator classes.^[91]

Figure 29

More reactive Mannich base for the reactivation of AChE.^[116]

Figure 30

Imidazolium-based oximes for the reactivation of OP-inhibited BChE.^[139]

Figure 31

Cinchona-based oxime showing broad scope reactivation potential. ^[140]

Figure 32

Mannich base for the selective reactivation of paraoxon-inhibited BChE.^[91]

Figure 33

Edrophonium-derived derivative 53 for the reactivation of hBChE.^[141]

Figure 34

Imidazole-based oximes found by conducting structural modifications from the originally tested imidazole aldoximes by Radic and co-workers.^[142]

Figure 35

K Oximes capable of reactivating tabun-inhibited BChE.^[143]

Figure 36

Sulfonate alkylator in first attempt to alkylate phosphonate anions.^[150]

Figure 37

Strong alkylator groups used unsuccessfully in vitro to re-alkylate soman-aged AChE.^[149]

Figure 38

N-Methyl-2-methoxypyridnium structure and model phosphonate anion used to study methyl transfer by Quinn and co-workers.^[147]

Figure 39

Various core frameworks and peripheral site ligands employed for hAChE inhibition by Quinn and co-workers.^[148]

Figure 40

Computationally investigated sulfonium re-alkylators of aged AChE.^[146]

Figure 41

Computationally investigated aminoalcohol re-alkylators of aged AChE.^[145]

Figure 42

Initial reaction showing the potential of QM alkylation of phosphodiesters.^[154]

Figure 43

Subsequent study showing the potential of QM alkylation of phosphodiesters.^[155]

Figure 44

QMP structures used to study the aspects of QM reaction by Rokita and co-workers.^[156]

Figure 45

QMP structures used in reaction with nucleophiles for proof of principle alkylation.^[159]

Figure 46

Nerve agent analogues used in biological assays.^[160]

Figure 47

Lead QMP structure for resurrection of aged AChe.^[160]