Background

[PubMed]

Acetylcholinerase (AChE) is an enzyme involved in terminating nerve impulses by hydrolyzing the neurotransmitter acetylcholine (ACh). It plays an important role in heart rate and cardiac contractions, for example, by decreasing the ACh levels associated with cardiac parasympathetic responses (1).

AChE, and more exactly the inhibition of its enzymatic function, also seems to play an important role in poisoning from organophosphorus (OP) agents (2). Current treatments consisting of a reversible covalent AChE inhibitor (pyridostigmine (3)), a receptor antagonist atropine (ATR), and a AChE reactivator (pralidoxime chloride) have shown good protection against OP intoxication in animals. Research efforts are being made to find drugs with multiple protective functions. For example, Leader et al. (4) synthesized and evaluated a group of pyridophen analogues, binary pyridogstigmine-aprophen prodrugs with differential inhibition of AChE, butyrylcholinesterase (BChE), and muscarinic receptors (5).

Most of the AChE heart tracers developed for cardiac neurotransmission using positron emission tomography (PET) have shown low selectivity of AChE over BChE, and non-specific binding in AChE enzyme overexpressed regions (6). [¹¹C]Nedrophonium and [¹¹C]Neostigmine, originally developed as potential PET heart imaging agents, have shown either high nonspecific binding or poor myocardial uptake and were therefore unsuitable for PET imaging of the heart vagal system.

[¹¹C]pyridostigmine and its para- and ortho-analogs, as well as N-[¹¹C]methyl-3-[[(dimethylamino)carbonyl]oxy]-2-(2',2'-diphenylpropionoxymethyl)pyridinium ([¹¹C]MDDP), were synthesized by Wang et al. (7) as potential PET agents for imaging heart AChE in vivo. Leader et al. (4) showed that MDDP iodide selectively inhibited AChE more than BChE. However, the first rodent studies performed by Wang et al. (7) using PET- [¹¹C]MDDP did show the presence of nonspecific binding (see the Rodents section for details).

Synthesis

[PubMed]

A multi-step synthesis procedure for [¹¹C]MDDP was reported by Wang et al. (7) in 2005. The authors used a modified version of a method by Leader et al. (4) to obtain the precursor and reference standard. Briefly, the preparation of [¹¹C]MDDP involved an acylation of the key intermediate 2-hydroxymethyl-3-dimethylaminocarbonyl-oxypyridine with 2,2-diphenylpropionyl chloride to produce the carbamyl-ester precursor 3-[[(dimethylamino)carbonyl]oxy]-2-(2',2'-diphenylpropionoxymethyl)pyridine in 51% yield.

The key intermediate used in this synthetic method was obtained in 31% yield from carbamylation of 3-hydroxy-2-hydroxymethylpyridine HCl with dimethylcarbamyl chloride; the acylation agent was produced in 96% yield from 2,2-diphenylpropionic acid with thionyl chloride in 96% yield.

The tertiary pyridine precursor was labeled with [¹¹C]methyl triflate to provide the quaternary pyridinium tracer [¹¹C]MDDP in 40–65% radiochemical yield (decay corrected at end of bombardment). The final product [¹¹C]MDDP was eluted with an aqueous solution of 2% acetic acid (containing up to 8% ethanol to enhance recovery of some [¹¹C-methyl]quaternary cations. The total synthesis time reported by Wang et al. (7) was 10–15 min. The chemical and radiochemical purities (determined by analytical HPLC) were >95% and >99% respectively. The specific radioactivity of the tracer [¹¹C]MDDP was ~0.05 MBq/mol (1.0–1.5 Ci/μmol) at end of the synthesis.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In vitro studies performed by Leader et al. (4) showed that MDDP iodide preferentially inhibited AChE over BChE, and that the inhibition of both enzymes was at least as strong as that obtained with pyridogstigmine (8). The mean values of the affinity constant for the initial reaction obtained for AChE and BChE were 5.62 x 10^-6 M and 3.14 x 10^-6 M respectively. In comparison, for pyridostigmine, those values were 1.97 x 10^-5 M (AChE) and 1.14 x 10^-4 M (BChE).

Animal Studies

Human Studies

[PubMed]

No publication is currently available

References

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Gomez JL , Moral-Naranjo MT , Campoy FJ , Vidal CJ . Characterization of acetylcholinesterase and butyrylcholinesterase forms in normal and dystrophic Lama2dy mouse heart. J Neurosci Res. 1999;56(3):295–306. [PubMed: 10336259]

2.

Duysen EG , Li B , Xie W , Schopfer LM , Anderson RS , Broomfield CA , Lockridge O . Evidence for nonacetylcholinesterase targets of organophosphorus nerve agent: supersensitivity of acetylcholinesterase knockout mouse to VX lethality. J Pharmacol Exp Ther. 2001;299(2):528–535. [PubMed: 11602663]

3.

Dirnhuber P , French MC , Green DM , Leadbeater L , Stratton JA . The protection of primates against soman poisoning by pretreatment with pyridostigmine. J Pharm Pharmacol. 1979;31(5):295–299. [PubMed: 37298]

4.

Leader H , Wolfe AD , Chiang PK , Gordon RK . Pyridophens: binary pyridostigmine-aprophen prodrugs with differential inhibition of acetylcholinesterase, butyrylcholinesterase, and muscarinic receptors. J Med Chem. 2002;45(4):902–910. [PubMed: 11831902]

5.

Leader H , Smejkal RM , Payne CS , Padilla FN , Doctor BP , Gordon RK , Chiang PK . Binary antidotes for organophosphate poisoning: aprophen analogues that are both antimuscarinics and carbamates. J Med Chem. 1989;32(7):1522–1528. [PubMed: 2738887]

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Ryu EK , Choe YS , Park EY , Paik JY , Kim YR , Lee KH , Choi Y , Kim SE , Kim BT . Synthesis and evaluation of 2-[18F]fluoro-CP-118,954 for the in vivo mapping of acetylcholinesterase. Nucl Med Biol. 2005;32(2):185–191. [PubMed: 15721764]

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Wang JQ , Miller MA , Mock BH , Lopshire JC , Groh WJ , Zipes DP , Hutchins GD , Zheng QH . Facile synthesis and PET imaging of a novel potential heart acetylcholinesterase tracer N-[11C]methyl-3-[[(dimethylamino)carbonyl]oxy]-2-(2',2'-diphenylpropionoxy methyl)pyridinium. Bioorg Med Chem Lett. 2005;15(20):4510–4514. [PubMed: 16112863]

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Main AR , Hastings FL . Carbamylation and binding constants for the inhibition of acetylcholinesterase by physostigmine (eserine). Science. 1966;154(747):400–402. [PubMed: 5917090]