Synthesis and in Vitro and in Vivo Evaluation of ¹⁸F-Labeled Positron Emission Tomography (PET) Ligands for Imaging the Vesicular Acetylcholine Transporter

A new class of vesicular acetylcholine transporter inhibitor that incorporates a carbonyl group into the benzovesamicol structure was synthesized, and analogues were evaluated in vitro. (±)-trans-2-Hydroxy-3-(4-(4-[18F]fluorobenzoyl)piperidino)tetralin (9e) has Ki values of 2.70 nM for VAChT, 191 nM for σ1, and 251 nM for σ2. The racemic precursor (9d) was resolved via chiral HPLC, and (±)-[18F]9e, (−)-[18F]9e, and (+)-[18F]9e were respectively radiolabeled via microwave irradiation of the appropriate precursors with [18F]/F− and Kryptofix/K2CO3 in DMSO with radiochemical yields of ∼50−60% and specific activities of >2000 mCi/μmol. (−)-[18F]9e uptake in rat brain was consistent with in vivo selectivity for the VAChT with an initial uptake of 0.911 %ID/g in rat striatum and a striatum/cerebellum ratio of 1.88 at 30 min postinjection (p.i.). MicroPET imaging of macaques demonstrated a 2.1 ratio of (−)-[18F]9e in putamen versus cerebellum at 2 h p.i. (−)-[18F]9e has potential to be a PET tracer for clinical imaging of the VAChT

Synthesis and <i>in Vitro</i> Biological Evaluation of Carbonyl Group-Containing Inhibitors of Vesicular Acetylcholine Transporter

To identify selective high-affinity inhibitors of the vesicular acetylcholine transporter (VAChT), we have interposed a carbonyl group between the phenyl and piperidyl groups of the prototypical VAChT ligand vesamicol and its more potent analogues benzovesamicol and 5-aminobenzovesamicol. Of 33 compounds synthesized and tested, 6 display very high affinity for VAChT (Ki, 0.25−0.66 nM) and greater than 500-fold selectivity for VAChT over σ1 and σ2 receptors. Twelve compounds have high affinity (Ki, 1.0−10 nM) and good selectivity for VAChT. Furthermore, 3 halogenated compounds, namely, trans-3-[4-(4-fluorobenzoyl)piperidinyl]-2-hydroxy-1,2,3,4-tetrahydronaphthalene (28b) (Ki = 2.7 nM, VAChT/sigma selectivity index = 70), trans-3-[4-(5-iodothienylcarbonyl)piperidinyl]-2-hydroxy-1,2,3,4-tetrahydronaphthalene (28h) (Ki = 0.66 nM, VAChT/sigma selectivity index = 294), and 5-amino-3-[4-(p-fluorobenzoyl)piperidinyl]-2-hydroxy-1,2,3,4,-tetrahydronaphthalene (30b) (Ki = 2.40 nM, VAChT/sigma selectivity index = 410) display moderate to high selectivity for VAChT. These three compounds can be synthesized with the corresponding radioisotopes so as to serve as PET/SPECT probes for imaging the VAChT in vivo

2-(2-Piperidyl)- and 2-(2-Pyrrolidyl)chromans as Nicotine Agonists: Synthesis and Preliminary Pharmacological Characterization

As part of an effort to develop a new class of subtype selective nicotine agonists, we have synthesized and tested a group of 12 hydroxylated 2-(2-piperidyl)- and 2-(2-pyrrolidyl)chromans. In rat brain membranes, all 12 compounds displayed poor affinity for [125I]-α-bunagarotoxin binding sites. In contrast, three compounds, 17c, 24, and 26, displayed moderate to high affinity for [3H]cytisine binding sites, while three (17b, 18b,c) and six (17a,d,e and 18a,d,e) compounds showed weak and poor affinity, respectively, for these same sites. In subsequent studies, compounds 17a and 17c were found to stimulate the efflux of 86Rb+ from rat cortical synaptosomes, an indication of agonist activity. Further, both 17c and 26 displayed high intrinsic activity in stimulating the release of [3H]dopamine from striatal synaptosomes; however, only 17c was effective at stimulating the release of [3H]acetylcholine from cortical synaptosomes, suggesting differential selectivity. In cloned human nicotinic acetylcholine receptors (nAChR) expressed in Xenopus oocytes, both 17c and 26 activated α7 and α3β2 receptor subtypes in a dose-dependent manner, but 26 was clearly the more potent agonist. Last, neither compound displayed dose-dependent activation of α4β2 nAChRs. We conclude that 2-(2-azacyclic)chromans appear to be a promising new class of nicotine agonists

Summary of selected promising potent compounds derived from African medicinal plants and currently included in AfroDb.

<p>Summary of selected promising potent compounds derived from African medicinal plants and currently included in AfroDb.</p

Comparison of property distribution for the two datasets by percentage distributions.

<p>(A) MW, (B) log <i>P</i>, (C) HBA and (D) HBD. DNP in red and AfroDb in blue. For subfigure B, the <i>x</i>-axis label is the lower limit of binned data, e.g. −2 is equivalent to −2 to −1.</p

Distribution curves for #stars within the AfroDb library, along with the standard “drug-like”, “lead-like” and “fragment-like” subsets.

<p>Blue = AfroDb library, red = “drug-like” subset, green = “lead-like” subset and violet = “fragment-like” subset.</p

Bar chart showing the distribution of the compounds within AfroDb by region of collection.

<p>Bar chart showing the distribution of the compounds within AfroDb by region of collection.</p

A simple descriptor-based comparison of the AfroDb database and the ChemBridge Diversity database.

<p>Comparison of typical physico-chemical property distributions (MW, HBA, HBD, NCC, NO, NRB, log <i>P</i>, NR and TPSA) in the AfroDb (green) and ChemBridge Diverset (red) database. All histograms and scatterplots were generated with the R software <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078085#pone.0078085-R1" target="_blank">[85]</a>.</p

2D structures of the three compounds with log <i>P</i> values >14, included in AfroDb.

<p>2D structures of the three compounds with log <i>P</i> values >14, included in AfroDb.</p

MCSS panel in AfroDb, featuring the most common cyclic structures included in the database.

<p>MCSS panel in AfroDb, featuring the most common cyclic structures included in the database.</p