Synthesis of 3-[(1-[11C]methyl-2(S)-pyrrolidinyl) methoxy]pyridine and 3-[(1-[11C]methyl-2(R)-pyrrolidinyl) methoxy]pyridine: Radioligands for in vivo studies of neuronal nicotinic acetylcholine receptors

Synthesis and In-Vivo Evaluation of [11C]p-PVP-MEMA as a PET Radioligand for Imaging Nicotinic Receptors

Within the class of (4-pyridinyl)vinylpyridines developed by Abbott laboratories as potent neuronal nicotinic acetylcholine receptor ligands, p-PVP-MEMA ({(R)-2-[6-chloro-5-((E)-2-pyridin-4-ylvinyl)pyridin-3-yloxy]-1-methylethyl}methylamine) is the lead compound of a novel series that do not display the traditional nicotinic-like pyrrole-ring but still possessing high subnanomolar affinity (Ki 0.077 nm—displacement of [3H](–)cytisine from whole rat brain synaptic membranes). In the present study, p-PVP-MEMA and its nor-derivative ({(R)-2-[6-chloro-5-((E)-2-pyridin-4-ylvinyl)pyridin-3-yloxy]-1-methylethyl}methylamine) as precursor for labelling with the short-lived positron-emitter carbon-11 (T1/2 20.4 min) were synthesized in 10 chemical steps from 2-hydroxy-5-nitropyridine and Boc-d-alanine. N-Alkylation of nor-p-PVP-MEMA with [11C]methyl iodide afforded [11C]p-PVP-MEMA (>98% radiochemically pure, specific activity of 86.4 GBq μmol–1) in 2% (non-decay corrected and non-optimized) radiochemical yield, in 34 min (including HPLC purification and formulation). Preliminary positron emission tomography (PET) results obtained in a Papio hamadryas baboon showed that [11C]p-PVP-MEMA is not a suitable PET-radioligand.

Whole-body radiation dosimetry of 2-[18F]Fluoro-A-85380 in human PET imaging studies

2-[18F]Fluoro-A-85380 (2-[18F]fluoro-3-(2(S)-azetidinylmethoxy)pyridine, 2-[18F]FA) is a recently developed PET radioligand for noninvasive imaging of nicotinic acetylcholine receptors. Previous radiation absorbed dose estimates for 2-[18F]FA were limited to evaluation of activity in only several critical organs. Here, we performed 2-[18F]FA radiation dosimetry studies on two healthy human volunteers to obtain data for all important body organs. Intravenous injection of 2.9 MBq/kg of 2-[18F]FA was followed by dynamic PET imaging. Regions of interest were placed over images of each organ to generate time-activity curves, from which we computed residence times. Radiation absorbed doses were calculated from the residence times using the MIRDOSE 3.0 program (version 3.0, ORISE, Oak Ridge, TN). The urinary bladder wall receives the highest radiation absorbed dose (0.153 mGy/MBq, 0.566 rad/mCi, for a 2.4-h voiding interval), followed by the liver (0.0496 mGy/MBq, 0.184 rad/mCi) and the kidneys (0.0470 mGy/MBq, 0.174 rad/mCi). The mean effective dose equivalent is estimated to be 0.0278 mSv/MBq (0.103 rem/mCi), indicating that radiation dosimetry associated with 2-[18F]FA is within acceptable limits.

Synthesis and evaluation of a novel series of 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine analogues as potential positron emission tomography imaging agents for nicotinic acetylcholine receptors

Reportedly, 2-[(18)F]fluoro-A-85380, 1, a promising radiotracer for imaging the nicotinic acetylcholine receptor (nAChR) by positron emission tomography (PET) in humans, exhibits slow penetration through the blood-brain barrier (BBB) due to its low lipophilicity. A ligand for nAChRs with greater lipophilicity than that of 1 would be potentially more favorable for PET imaging of nAChR due to its faster penetration through the BBB. Herein, a novel series of compounds has been developed based on the high affinity ligand for nAChRs, 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine, 3b. The in vitro binding affinities for the new series were found to be in the range of K(i) = 9-331 pM. A molecular modeling study showed differences in the comformational profiles and the electronic properties of these compounds, which provides further insight into the structure-activity relationships at nAChR. Lipophilicities of the compounds 3b-6b have been found to be substantially higher than that of 1. As a result, compounds 3b-6b might exhibit a faster penetration through the BBB than the less lipophilic 1. The N-methyl derivatives 3b and 6b demonstrated very high affinities at nAChRs (K(i) = 28 and 23 pM, respectively) and will be targets for development of (11)CH(3)-labeled derivatives as radiotracers for PET imaging of nAChRs.

Synthesis of a fluorine-18 labeled derivative of epibatidine for in vivo nicotinic acetylcholine receptor PET imaging

Epibatidine (exo-2-(2'-chloro-5'-pyridyl)-7-azabicyclo[2.2.1]heptane), a natural compound isolated from the skin of the Ecuadorian poison frog Epipedobates tricolor, is the most potent nicotinic acetylcholine receptor (nAChR) agonist reported to date. In order to visualize and quantify in vivo these receptors in human brain using Positron Emission Tomography (PET), [18F]norchlorofluoroepibatidine (exo-2-(2'-[18F]fluoro-5'-pyridyl)-7-azabicyclo[2.2.1]heptane), a fluorine-18 (t(1/2): 110 min) radiolabeled derivative of epibatidine has been designed. The corresponding 2'-bromo-, 2'-iodo- and 2'-nitro exo-2-(5'-pyridyl)-7-azabicyclo[2.2.1]heptane analogues as labeling precursors, as well as norchlorofluoroepibatidine as a reference compound have been synthesized by reductive, stereoselective, palladium-catalyzed Heck-type coupling between an N-Boc protected azanorbornene and the corresponding halopyridine. [18F]Norchlorofluoroepibatidine has been radiolabeled with fluorine-18 by nucleophilic aromatic substitution from the corresponding Boc-protected halo- and nitro precursors using [18F]FK-K222 complex in DMSO by conventional heating (at 150-180 degrees C for 10 min) or microwave activations (at 100 Watt, for 1 to 2.5 min), followed by TFA-removal of the protective group. Typically, using the microwave activation procedure, 60-80 mCi (2.22-2.96 GBq) of pure [18F]norchlorofluoroepibatidine could be obtained in less than 2 h (110-115 min) from the bromo labeling precursor, with specific radioactivities of 1.5-2.5 Ci/micromol (55.5-92.5 GBq/micromol) calculated for End of Bombardment. The preliminary PET experiments in baboon (Papio papio) with [18F]norchlorofluoroepibatidine show a high uptake and a rapid accumulation of the radiotracer into the brain within 30 min. In the thalamus, a nAChR rich area, uptake of radioactivity reached a maximum at 40 min (10% I.D./100 mL tissue). The ratio of radioactivity thalamus/cerebellum (the latter being a nAChR poor area) was 2 at 40 min and increased with time, up to 4.3 at 160 min. Its specific regiodistribution and its high ratio of specific-to-nonspecific binding confirm the ideal profile of [18F]norchlorofluoroepibatidine as a suitable radioligand for PET imaging of nAChRs in the brain.

2-, 5-, and 6-Halo-3-(2(S)-azetidinylmethoxy)pyridines: synthesis, affinity for nicotinic acetylcholine receptors, and molecular modeling

3-(2(S)-Azetidinylmethoxy)pyridine (A-85380) has been identified recently as a ligand with high affinity for nicotinic acetylcholine receptors (nAChRs). Here we report the synthesis and in vitro nAChR binding of a series of 10 pyridine-modified analogues of A-85380. The novel compounds feature a halogen substituent at position 2, 5, or 6 of the 3-pyridyl fragment. Those with the substituents at position 5 or 6, as well as the 2-fluoro analogue, possess subnanomolar affinity for nAChRs in membranes from rat brain. For these ligands, Ki values range from 11 to 210 pM, as measured by competition with (+/-)-[3H]epibatidine. In contrast, 2-chloro, 2-bromo, and 2-iodo analogues exhibit substantially lower affinity. AM1 quantum chemical calculations demonstrate that the bulky substituents at position 2 cause notable changes in the molecular geometry. The high-affinity members of the series and (+)-epibatidine display a tight fit superposition of low-energy stable conformers. The new ligands with high affinity for nAChRs may be of interest as pharmacological probes, potential medications, and candidates for developing radiohalogenated tracers to study nAChRs.

Pharmacological evaluation of [11C]A-84543: an enantioselective ligand for in vivo studies of neuronal nicotinic acetylcholine receptors

[11C]A-84543, 3-[(1-[11C]methyl-2(S)-pyrrolidinyl)methoxy]pyridine, is a specific and enantioselective neuronal nicotinic acetylcholine receptor (nAChR) radiotracer. The in vivo biodistribution of this radiotracer in mice showed high brain uptake and a distribution consistent with the density of nAChRs. Highest uptake was observed in the thalamus (9.6 %ID/g), cortex (9.9 %ID/g), superior colliculus (7.6 %ID/g) and hippocampus (7.6 %ID/g) at 5 min followed by clearance. As a measure of specificity, the thalamus/cerebellar ratio reached a maximum of 2.3 at 30 min post-injection. Radioactivity in the thalamus and superior colliculus was reduced by 33% by pre-administration of unlabeled A-84543. The nAChR agonists (-)nicotine, cytisine, and (+) epibatidine reduced the radioactivity due to [11C]A-84543 in the superior colliculus by 41%, 38%, and 27%, respectively, while lobeline, which also interacts with central nAChRs, produced a 24% inhibition. The noncompetitive nAChR ligand, mecamylamine displayed no inhibitory effect on [11C]A-84543 accumulation in any brain region. Ketanserin (5-HT2/5-HT2C), scopolamine (mAChR antagonist), (+)butaclamol (DA receptor antagonist), and haloperidol (D2/sigma) also displayed no inhibitory effect in any brain region studied. With the pharmacologically less active enantiomer, 3-[(1-[11C]methyl-2(R)-pyrrolidinyl)methoxy] pyridine, high brain uptake was also observed, but with a low thalamus/cerebellar ratio of 1.4 at 30 min post-injection. [11C]A-84543 displays enantioselectivity for nAChRs and may deserve further investigation as a possible PET radiotracer.