CP-126998

Acetylcholinesterase (AChE) is an important cholinergic marker for the diagnosis of Alzheimer's disease (AD). A recent study has demonstrated that C-labelled 5,7-dihydro-7-methyl-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol-6-one (CP-126,998) shows promising results. The demethylated form of this ligand (CP-118,954) is a more potent and selective inhibitor than CP-126,998. In this study, therefore, CP-118,954 was labelled with F and evaluated for the in vivo mapping of AChE.

The 4-fluoro (1). and 2-fluoro (2). derivatives of CP-118,954 were synthesized from 4-methyl-3-nitroanisole in 11 steps. Their in vitro binding affinities to AChE were measured using Ellman's method. The preparation of [F]-1 was carried out by reductive alkylation of the piperidine precursor with 4-[F]-fluorobenzaldehyde, followed by high-performance liquid chromatography (HPLC) purification. In vitro autoradiography was performed by incubating rat brain coronal slices with [F]-1. Tissue distribution studies were performed in mouse brain and the data were expressed as the percentage of the injected dose per gram of tissue (%ID x g).

Two fluorine-substituted AChE inhibitors were synthesized and their in vitro binding data showed that the 4-fluoro and 2-fluoro derivatives (1 and 2) had similar or superior binding affinity to that of the unsubstituted ligand, CP-118,954. The F-labelled ligand was synthesized in 20-35% radiochemical yield (EOS) and with high effective specific activity (36-42 GBq x micromol). Autoradiography showed high uptake of [F]-1 in the striatum and this striatal uptake was completely inhibited by the unlabelled ligand 1. Tissue distribution studies demonstrated that high radioactivity was accumulated in the striatum, an AChE-rich region.

This study demonstrates that [F]-1 may hold promise as a radioligand for the in vivo mapping of AChE.

Several cholinesterase (ChE) inhibitors have been labeled with carbon-11 for visualizing binding sites on acetylcholinesterase (AChE) by positron emission tomography (PET). Following intravenous injection of 1,2,3,4-tetrahydro-9-[(11)C]methylaminoacridine or [(11)C]donepezil, however, the radioactivity distribution does not reflect the regional distribution of AChE in the brain of animals, probably because these compounds have high non-specific binding and/or other specific binding sites in vivo in the brain. PET studies with [(11)C]physostigmine and [(11)C]CP-126,998 in the brain of healthy subjects have shown a radioactivity distribution corresponding to the regional distribution of AChE activity measured in postmortem human brains. These radiotracers may be useful for measuring the occupancy of binding sites on AChE by AChE inhibitors, and for investigating the cerebral pharmacokinetics of such therapeutic drugs. An alternative approach to map AChE is the use of acetylcholine analogue substrates. We have developed N-methylpiperidinyl esters labeled with carbon-11 for quantitative measurement of AChE activity. Currently, two N-[(11)C]methylpiperidine esters, N-[(11)C]methylipiperidin-4-ylacetate (MP4A) and N-[(11)C]methylpiperidin-4-yl propionate (MP4P or PMP), have been used for clinical studies of Alzheimer's disease and other neurodegenerative diseases. Both [(11)C]MP4A- and [(11)C]MP4P-PET have demonstrated not only the reduction of AChE activity in the cerebral cortex of patients with Alzheimer's disease (AD) but also the inhibitory effects of donepezil and rivastigmine on AChE activity in the brain of AD patients. AChE imaging should prove useful for therapeutic monitoring of the effects of ChE inhibitors, including determination of the appropriate clinical doses of newly developed compounds, and can thus prompt the development of novel drugs targeting AChE.