Izumi Uno

Syntheses and in vitro evaluation of decalinvesamicol analogues as potential imaging probes for vesicular acetylcholine transporter (VAChT)

A series of vesamicol analogues, o-iodo-trans-decalinvesamicol (OIDV) or o-bromo-trans-decalinvesamicol (OBDV), were synthesized and their affinities to vesicular acetylcholine transporter (VAChT) and σ receptors (σ-1, σ-2) were evaluated by in vitro binding assays using rat cerebral or liver membranes. OIDV and OBDV showed greater binding affinity to VAChT (K(i) = 20.5 ± 5.6 and 13.8 ± 1.2 nM, respectively) than did vesamicol (K(i) = 33.9 ± 18.1 nM) with low affinity to σ receptors. A saturation binding assay in rat cerebral membranes revealed that [(125)I]OIDV had a single high affinity binding site with a K(d) value of 1.73 nM and a B(max) value of 164.4 fmol/mg protein. [(125)I]OIDV revealed little competition with inhibitors, which possessed specific affinity to each σ (σ-1 and σ-2), serotonin (5-HT(1A) and 5-HT(2A)), noradrenaline, and muscarinic acetylcholine receptors. In addition, BBB penetration of [(125)I]OIDV was verified in in vivo. The results of the binding studies indicated that OIDV and OBDV had great potential to be VAChT imaging probes with high affinity and selectivity.

Regional brain imaging of vesicular acetylcholine transporter using o-[125I]iodo-trans-decalinvesamicol as a new potential imaging probe

In this study, the regional rat brain distribution of radioiodinated o‐iodo‐trans‐decalinvesamicol ([125I]OIDV) was determined in vivo to evaluate its potential as a single‐photon emission computed tomography (SPECT) imaging probe for vesicular acetylcholine transporter (VAChT). Following intravenous injection, [125I]OIDV passed freely across the blood–brain barrier and accumulated in rat brain. The accumulation of [125I]OIDV in rat brain was significantly reduced by coadministration of (+/−)‐vesamicol (0.125 µmol). In contrast, the coadministration of σ‐receptor ligands, such as (+)‐pentazocine (0.125 µmol) as a σ‐1 receptor ligand and (+)−3‐(3‐hydroxyphenyl)‐N‐propylpiperidine (0.125 µmol) as a σ‐1 and σ‐2 receptor ligands, barely affected the accumulation of [125I]OIDV in rat brain. These findings in vivo were corroborated by autoradiographic analysis ex vivo. The authors found that the tracer binds with pharmacological selectivity to VAChT in rat brain and predicted that it may likewise serve in translational SPECT imaging studies of this marker in the integrity of cholinergic innervations. Synapse 68:107–113, 2014.

The potential of o-bromo-trans-decalinvesamicol as a new PET ligand for vesicular acetylcholine transporter imaging.

We investigated the characteristics of the regional rat brain distribution of radio‐brominated o‐bromo‐decalinvesamicol (OBDV) in vivo to evaluate its potential as a PET ligand for vesicular acetylcholine transporter (VAChT). In in vivo biodistribution study, the specific brain regional accumulation of [77Br]OBDV was revealed 30 min after intravenous injection. The specific brain regional accumulation of [77Br]OBDV was significantly inhibited by co‐injection of (+/−)‐vesamicol. In contrast, no significant inhibition of the uptake of [77Br]OBDV in all brain regions was observed with co‐injection of (+)‐pentazocine (selective σ‐1 receptor agonist) and (+)−3‐(3‐hydroxyphenyl)‐N‐propylpiperidine, [(+)−3‐PPP] (σ‐1 and σ‐2 receptor agonist) with [77Br]OBDV. [77Br]OBDV accumulation in VAChT‐rich brain regions was observed in ex vivo autoradiography. These results showed that [77Br]OBDV selectively bound to VAChT with high affinity in rat brain in vivo. Hence, OVBDV radiolabelled with more suitable 76Br was suggested to be a potent VAChT ligand for PET. Synapse 68:445–453, 2014.

In Vivo Differences between Two Optical Isomers of Radioiodinated o-iodo-trans-decalinvesamicol for Use as a Radioligand for the Vesicular Acetylcholine Transporter

Purpose To develop a superior VAChT imaging probe for SPECT, radiolabeled (-)-OIDV and (+)-OIDV were isolated and investigated for differences in their binding affinity and selectivity to VAChT, as well as their in vivo activities. Procedures Radioiodinated o-iodo-trans-decalinvesamicol ([125I]OIDV) has a high binding affinity for vesicular acetylcholine transporter (VAChT) both in vitro and in vivo. Racemic [125I]OIDV was separated into its two optical isomers (-)-[125I]OIDV and (+)-[125I]OIDV by HPLC. To investigate VAChT binding affinity (Ki) of two OIDV isomers, in vitro binding assays were performed. In vivo biodistribution study of each [125I]OIDV isomer in blood, brain regions and major organs of rats was performed at 2,30 and 60 min post-injection. In vivo blocking study were performed to reveal the binding selectivity of two [125I]OIDV isomers to VAChT in vivo. Ex vivo autoradiography were performed to reveal the regional brain distribution of two [125I]OIDV isomers and (-)-[123I]OIDV for SPECT at 60 min postinjection. Results VAChT binding affinity (Ki) of (-)-[125I]OIDV and (+)-[125I]OIDV was 22.1 nM and 79.0 nM, respectively. At 2 min post-injection, accumulation of (-)-[125I]OIDV was the same as that of (+)-[125I]OIDV. However, (+)-[125I]OIDV clearance from the brain was faster than (-)-[125I]OIDV. At 30 min post-injection, accumulation of (-)-[125I]OIDV (0.62 ± 0.10%ID/g) was higher than (+)-[125I]OIDV (0.46 ± 0.07%ID/g) in the cortex. Inhibition of OIDV binding showed that (-)-[125I]OIDV was selectively accumulated in regions known to express VAChT in the rat brain, and ex vivo autoradiography further confirmed these results showing similar accumulation of (-)-[125I]OIDV in these regions. Furthermore, (-)-[123I]OIDV for SPECT showed the same regional brain distribution as (-)-[125I]OIDV. Conclusion These results suggest that radioiodinated (-)-OIDV may be a potentially useful tool for studying presynaptic cholinergic neurons in the brain.