Comparative tissue distribution of conformationally restricted radioiodinated vesamicol receptor ligands

• Published in: Nuclear medicine and biology 1995-05, Vol.22 (4), p.437-444
• Main Authors: Efange, S.M.N., Khare, A.B., Langason, R.B.
• Format: Article
• Language: English
• Subjects: Analysis of Variance; Animals; Biological and medical sciences; Contrast media. Radiopharmaceuticals; Iodine Radioisotopes - pharmacokinetics; Male; Medical sciences; Molecular Conformation; Neuromuscular Depolarizing Agents - pharmacokinetics; Pharmacology. Drug treatments; Piperidines - metabolism; Piperidines - pharmacokinetics; Radioligand Assay; Rats; Rats, Wistar; Receptors, Cholinergic - analysis; Receptors, Cholinergic - metabolism; Spiro Compounds - pharmacokinetics; Structure-Activity Relationship; Tissue Distribution
• ISSN: 0969-8051; 1872-9614
• DOI: 10.1016/0969-8051(94)00135-7

Three conformationally restricted analogs of vesamicol, 1′-[1-(3-iodobenzyl)-4-hydroxypiperidin-3-yl]spirol[1H-indene-1,4′-piperidine] ( 5), 1′-[1-(3-iodobenzyl)-4-hydroxypiperidin-3-yl]-3,4-dihydrospiro[indene-1,4′-piperidine] ( 6) and 1′-[1-(3-iodobenzyl)-4-hydroxypiperidin-3-yl)-3,4-dihydrospiro[naphthalene-1(2H),4′-piperidine] ( 7), were labelled with iodine-125 and evaluated as potential radioligands for mapping vesamicol receptor (VR) density and cholinergic function in vivo. All compounds showed similar kinetics in most tissues. However, differences were observed in the brain. Although comparable levels of each corresponding enantiomeric pair were obtained initially in the brain, the levels of the dextrorotatory enantiomers (+)- 5, (+)- 6 and (+)- 7 were found to decrease by 72–82% over a period of 3 h. In contrast, the brain levels of the corresponding levorotatory isomers were maintained throughout the duration of the experiment. Among the dextrorotatory isomers, (+)- 6 showed the highest brain extraction, while (+)- 7 showed the lowest. In tissue dissection experiments, the levels of (+)- 5, (+)- 6 and (+)- 7 were highest in the striatum and moderate to low in the cortex and cerebellum. Co-administration of haloperidol with (+)- 6 decreased the levels of the latter in the striatum by 27%, while the levels in the cortex and cerebellum were each reduced by 60%. In addition, haloperidol failed to affect the regional distribution of (+)- 7 in the brain. However, both haloperidol and spiperone increased the striatal levels of (+)- 5 by 67 and 76%, respectively, suggesting that the binding of this radioligand is related to cholinergic function. Furthermore, haloperidol reduced the concentration of (+)- 5 in the cortex and cerebellum by 25 and 33%, respectively, thereby implicating the sigma site as a secondary target for this ligand in the cortex.