Bioorganic & Medicinal Chemistry Letters 23 (2013) 5786–5789

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Bioorganic & Medicinal Chemistry Letters

journal homepage: www.elsevier .com/ locate/bmcl

Reversible inhibition of human acetylcholinesterase bymethoxypyridinium species

0960-894X/$ - see front matter � 2013 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.bmcl.2013.09.008

⇑ Corresponding author.E-mail address: daniel-quinn@uiowa.edu (D.M. Quinn).

Joseph J. Topczewski, Alexander M. Lodge, Sumana N. Yasapala, Maurice K. Payne, Pedrom M. Keshavarzi,Daniel M. Quinn ⇑Department of Chemistry, The University of Iowa, Iowa City, IA 52242, United States

a r t i c l e i n f o

Article history:Received 12 July 2013Revised 30 August 2013Accepted 3 September 2013Available online 8 September 2013

Keywords:AcetylcholinesteraseReversible inhibition2-Methoxypyridinium compounds

a b s t r a c t

The irreversible inhibition of acetylcholinesterase (AChE) by organophosphorous chemical warfare agentsnecessitates that antidotes be administered for effective treatment. Currently no antidote is known thatresurrects the phosphyl–AChE complex once aging has occurred. This report characterizes the affinities ofover 30 new AChE inhibitors which could act as resurrecting agents for the aged AChE-OP adduct.

� 2013 Published by Elsevier Ltd.

Acetylcholinesterase (AChE) is a crucial enzyme, whose physio-logical role is to terminate the nerve impulse at neuromuscularjunctions and interneuronal synapses.1 The hydrolysis of acetyl-choline by AChE allows the membrane voltage to reestablish aftersynaptic transmission. Thus, the inhibition or modulation of AChEactivity can have a significant impact on physiology. Partial inhibi-tion of AChE, which can restore cognition, is a leading treatment forAlzheimer’s disease.2 The total inhibition of AChE results in a vari-ety of conditions, most of which lead to death. Organophosphorous(OP) compounds are the most infamous irreversible AChE inhibi-tors and they have been used both as pesticides and chemical war-fare agents (CWAs).3–6

The threat of stockpiled OP-CWAs (e.g., Sarin) has been an issueof ongoing controversy.4 Due to the simplicity and availability ofOP-CWAs as well as their demonstrated potential for evil, theyare considered a threat either in the hands of terrorists or hostilegovernments.5 This has spawned efforts to identify and optimizeantidotes to OP-CWAs. Since OP-CWAs inhibit AChE by covalentlyphosphylating the active site serine residue to produce an AChE-OPadduct, the reactivation of AChE requires that an antidote bind tothe AChE-OP adduct and liberate the active site serine. Pyridiniumoximes have long been recognized as AChE-OP reactivators;7,8

however, no universally efficacious oxime has been identified.5

Additionally, oximes are incapable of reactivation once aging hasoccurred by solvolytic dealkylation of the AChE-OP adduct.5,9 Aging

is a key problem for soman, as the soman AChE-OP adduct ageswith a half-life of less than 2 min in vivo.10

To address the aged adduct, we conceived of using 2-meth-oxypyridinium species as methyl transfer agents which could re-verse aging in a model system.11 We termed this concept‘resurrection’ of the aged adduct to differentiate it from oximereactivation. It is required that any resurrecting agent does not in-hibit free AChE covalently, through methyl transfer, or reversiblywith potent kinetics or else it would be a self-defeating antidote.Before these compounds could be evaluated as resurrecting agentsof aged AChE, we sought to establish that they were reversibleinhibitors of AChE with moderate IC50 values. Described here isthe synthesis of several families of AChE inhibitors and theirin vitro potency against human AChE.

Several families of inhibitors were prepared for evaluation. Thesimplest compounds were prepared by methylation of commer-cially available pyridines by exposure to trimethoxonium tetra-fluoroborate11 or methyl triflate (see Supplementary data fordescription of synthetic procedures).

The second inhibitor family was prepared by deprotonation ofmethylpyridine 1 with strong base (Scheme 1). The subsequent an-ion was quenched by addition of various dibromoalkanes, whichresulted in bromoalkylpyridines 2–5 and the dimerized byproducts7–10. Notably, side product 6 could be isolated in varying yieldfrom these alkylation reactions, and is believed to be the productof a transmetallation reaction between the incipient lithium met-hide of methylpyridine 1 and the 1,m-dibromo reagent. This wouldgenerate a bromomethylpyridine which would then couple withthe lithium methide of methylpyridine 1 to form compound 6.

N OCH3Br

nN OCH3H3CBuLi, 0 oC

35% - 53%2-5, n = 4-7

Br Br

m

N No

+

16 - 38%6-10, o = 2, 5 - 8

H3CO OCH3

MeOTf

50 - 81%11-15, o = 2, 5 - 8

N No

H3CO OCH3

CH3 CH3

N

H3CO

H3CO

NH2

H3CO

H3COCl

N OCH3n47 - 71%18-21, n = 4 - 7

K2CO3

N

H3CO

H3CO N OCH3nCH3

CH379 - 92%22-25, n = 4 - 7

MeOTf

OTfOTf

OTfOTf

OH3CO

H3CO

NOCH3

n

OH3CO

H3CO

NOCH3

nCH3

OTf

29-31, n = 5-7, 64-95%26-28, n = 5-7, 3-20%

MeOTf

OH3CO

H3CONaHMDS,NaI, DMF

1

1716

m = 3-6

Scheme 1. Synthesis of C-linked inhibitors.

J. J. Topczewski et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5786–5789 5787

Methylation of the dimers with methyl triflate afforded inhibitors11–15. Alternatively, the bromoalkylpyridines could be furtherreacted with cores 16 or 17. After methylation of intermediates18–21 or 26–28 with MeOTf, inhibitors 22–25 and 29–31 wereisolated.

The N-linked inhibitor families were prepared by use of alkyltriflates (Scheme 2). Numerous attempts to use the correspondinghalides were unproductive. In most cases, the use of alkyl halidesresulted in auto-catalytic pyridone formation. This could be ratio-nalized if the primary halide is less electrophilic than the meth-oxypyridinium. After trace product was formed, methyl transferfrom the product would generate the thermodynamically favoredN-methylpyridone (33). Formation of the bis-alkyl triflates wasaccomplished by exposing the diols to Tf2O followed by slow addi-tion of 2,6-lutidine and methoxypyridine 32. From this reaction, di-mers and mono substituted alcohols could be isolated afterpreparative reversed-phase chromatography.

The inhibition of AChE with coumarins has been observed12,13

and several coumarin based inhibitors were prepared (Scheme 2).Exposure of compound 37 to t-BuOK resulted in a yellow anionwhich could be quenched by exposure to bromoalkyl alcohols(38–40). For the n = 3 case, the TBS protected alcohol was used.The TBS group was removed by TsOH catalyzed hydrolysis. The tri-flates were prepared in situ from the alcohols 41–43 and displacedby the methoxypyridine. The products 44–49 could be isolated inreasonable yield after reversed-phase chromatography. These syn-theses provided a total of 32 inhibitors for biological evaluation.

OO OH

CH3

t-BuOKOO O

CH3

OH

n

Br ORn

38, n = 3, R = TBS39, 40 n = 4 or 5, R = H 22% - 68%

41-43, n = 3 - 5

N Tf2O, 2,6-lutid

HO OHn

NCH3

O

F3C

Br Brn

VariousConditions

33

OCH3

F3C 32

37

Scheme 2. Synthesis of

The inhibitors prepared in this study were evaluated againsthuman AChE using the colorimetric assay developed by Ellman.14

Inhibitors were evaluated at ten concentrations and IC50 valueswere determined by non-linear fit of data to the equation Vi =V0[I]/([I] + IC50), where Vi and V0 are initial rates in the presenceand absence of inhibitor. Because these inhibitors are electrophilic,they were evaluated at an initial time point and again after 2 hincubation with AChE to observe if either inhibitor hydrolysis orirreversible inhibition occurred. Table 1 summarizes these results.

The compounds presented here represent several different clas-ses of inhibitors. The simplest are analogous to the AChE-OP oximereactivator 2-PAM.7 The IC50 values for the simple pyridiniumspecies (50–59)11 ranged over an order of magnitude dependingon substituents from about 7 lM to 70 lM, which is slightly morepotent on average as reversible inhibitors than 2-PAM.

A number of dimeric oximes have been reported to be betterreactivators of AChE-OP adducts than 2-PAM due to increasedAChE binding.5,6 Inhibitors 11–15, 35, 36, and 60 were developedwith this precedent in mind. The C2-linked dimers 11–15 showedincreased AChE binding relative to the simple compounds and allhad IC50 values below 1 lM. The most potent inhibitor of this classwas 13 with a 70 nM IC50. The N-linked dimers 35, 36, and 60(IC50 = 11, 22, and 80 lM respectively) were assayed and are muchless potent then the C-2 linked compounds as was the hydroxypy-ridinium 34 (IC50 = 98 lM). Although it is unclear at this time if thedecreased potency is due to the varied substitutents or to the link-ing pattern, it is clear that these N-linked compounds offer little

1) Tf2O, 2,6-lutidine2) Methoxy Pyridine

N

OTf

On

O

CH3

OOCH3

R

6% - 57%

ine N OHn

N Nn

+

OTfOTf

OTfOCH3

F3C F3C

OCH3

OCH3

CF3

34, n = 535, n = 336, n = 5

4-OCH3, R = 3,5-diF47, n = 3,48, n = 4,49, n = 5,

2-OCH3, R = 5-CF344, n = 3,45, n = 4,46, n = 5,

N-linked inhibitors.

Table 1IC50 values for AChE inhibitors

N

CH3

OCH3

RBF4

I

N

CH3

OCH3

OTf

II

Rn

N

OTf

Rn

OH3CO

H3COIII OO

CH3

O

N

H3CO

H3CO

CH3

OTf

V

VI

OCH3

F

F

NOTf

Rn IV

H3CO

CF3

VII

Entry Inhibitor Core R = n = IC50 at t = 0 h (lM) IC50 at t = 2 h (lM)

1 11 II II 2 0.81 ± 0.07 1.1 ± 0.12 12 II II 5 0.10 ± 0.02 0.11 ± 0.023 13 II II 6 0.07 ± 0.01 0.07 ± 0.014 14 II II 7 0.12 ± 0.02 0.12 ± 0.065 15 II II 8 0.18 ± 0.05 0.19 ± 0.056 22 II VI 4 5.2 ± 0.8 5.5 ± 0.87 23 II VI 5 0.50 ± 0.07 0.46 ± 0.058 24 II VI 6 0.15 ± 0.02 0.19 ± 0.039 25 II VI 7 0.049 ± 0.006 0.11 ± 0.02

10 29 II V 5 0.12 ± 0.01 0.13 ± 0.0411 30 II V 6 0.07 ± 0.01 0.10 ± 0.0212 31 II V 7 0.19 ± 0.02 0.27 ± 0.0313 34 IV OH 5 98 ± 4 131 ± 814 35 IV IV 3 10.5 ± 0.5 3.6 ± 0.115 36 IV IV 5 22 ± 1 22 ± 116 44 IV VII 3 3.6 ± 0.3 3.6 ± 0.217 45 IV VII 4 0.76 ± 0.08 1.11 ± 0.0818 46 IV VII 5 0.72 ± 0.05 1.03 ± 0.0619 47 III VII 3 1.7 ± 0.1 1.9 ± 0.220 48 III VII 4 0.69 ± 0.07 0.53 ± 0.0321 49 III VII 5 0.48 ± 0.02 0.47 ± 0.0222 50 I H 37 ± 4 46 ± 323 51 I 3-F 70 ± 10 110 ± 2024 52 I 5-F 16 ± 2 17 ± 325 53 I 5-CF3 23 ± 1 31 ± 126 54 I 6-CF3 36 ± 2 46 ± 327 55 I 5-NO2 7.5 ± 0.5 35 ± 628 56 I 4-CN 51 ± 8 61 ± 929 57 I 5-CN 43 ± 5 64 ± 730 58 I 6-CN 14 ± 2 18 ± 231 59 III H 1 54 ± 4 98 ± 532 60 III III 3 21 ± 1 28 ± 2

5788 J. J. Topczewski et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5786–5789

advantage over the simple N-methyl compounds in terms of AChEbinding affinity.

Inhibitors that can bind to the AChE peripheral site, span theactive site gorge, and bind to the active site were designed. Sev-eral reports have demonstrated that potent inhibitors can resultfrom this ’gorge spanning‘ strategy.15,16 The increased specificityfor AChE is expected to be necessary for successful resurrectiongiven the potential liability of presenting methylating agents toother biological targets and the slow rate of methyl transferwhich can be expected. The dimethoxyindanone motif is a knownperipheral site binder present in Donepezil, the Alzheimer’sdrug.2,17 Inhibitors 29–31, which contain the indanone motif,were effective AChE inhibitors with IC50 = 120, 74, and 190 nM,respectively. Tetrahydroisoquinolines have been used as periphe-ral site AChE binding motifs.15,16 Inhibitors 22–25 contain thiscore and show a 100 fold increase in potency from 5 lM to50 nM as the linker length increases. Inhibitor 25 is the most po-tent assayed in this study.

Finally, the use of a coumarin peripheral site binding motifproved to be more synthetically tractable then either the indanoneor tetrahydroisoquinolines in terms of providing more substitutedpyridiniums, which will likely be necessary for successful resurrec-tion of the aged AChE-OP adduct. These inhibitors 44–49(IC50 = 0.5–1.7 lM) proved to be more potent than the simple

pyridiniums but less potent than the other established peripheralsite binding motifs.

As Table 1 demonstrates, none of the inhibitors inspectedshowed marked time dependence, with the possible exception of55, which hydrolyzes in the reaction buffer to the correspondingpyridone.11 This indicates that the majority of the inhibitors arestable to the buffer system over the time course of the experimentand that they are reversible inhibitors of AChE. Both of these crite-ria are needed for resurrecting agents.

The compounds described here demonstrated varying potentialfor the inhibition of human AChE. A range of 2000 fold in IC50 val-ues was observed within the compounds presented. For the pur-pose of resurrecting the aged AChE-OP adduct, it will likely benecessary to have moderate to strong and reversible AChE binding.Many of the compounds presented here fit this requirement, how-ever none has shown the ability to resurrect the aged adduct in apreliminary screen. Our continued efforts in this regard will be re-ported in due course.

Acknowledgements

This work was supported by the CounterACT Program, NationalInstitutes of Health Office of the Director, and the National Insti-tute of Neurological Disorders and Stroke, Grant 5 R21

J. J. Topczewski et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5786–5789 5789

NS076430. The content is solely the responsibility of the authorsand does not necessarily represent the official views of the Na-tional Institutes of Health.

Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.bmcl.2013.09.008.

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