IN-VITRO EVALUATION OF C-11 LABELED (S)-NICOTINE, (S)-3-METHYL-5-(1-METHYL-2-PYRROLIDINYL)ISOXAZOLE, AND (R,S)-1-METHYL-2-(3-PYRIDYL)AZETIDINE AS NICOTINIC RECEPTOR LIGANDS FOR POSITRON-EMISSION-TOMOGRAPHY STUDIES

The binding characteristics of the novel C-11-labeled nicotinic ligand s (R, S)-1-methyl-2-(3-pyridyl) azetidine (MPA) and (S)-3-methyl-5-(1- methyl-2-pyrrolidinyl) isoxazole (ABT-418) were investigated in compar ison with those of (S)-[C-11]nicotine in vitro in the rat brain to be able to predict the binding properties of the new ligands for positron emission tomography studies in vivo. The data from time-resolved expe riments for all ligands indicated fast binding kinetics, with the exce ption of a slower dissociation of [C-11]MPA in comparison with (S)-[C- 11]nicotine and [C-11]ABT-418. Saturation experiments revealed for all ligands two nicotinic receptor binding sites with affinity constants (K-D values) of 2.4 and 560 nM and binding site densities (B-max value s) of 65.5 and 223 fmol/mg of protein for (S)-[C-11]nicotine, K-D valu es of 0.011 and 2.2 nM and B-max values of 4.4 and 70.7 fmol/mg of pro tein for [C-11] MPA, and K-D values of 1.3 and 33.4 nM and B-max value s of 8.8 and 69.2 fmol/mg of protein for [C-11]ABT-418, In competing w ith the C-11-ligands, epibatidine was most potent, followed by cytisin e. A different rank order of potencies was found for (-)-nicotine, (+) -nicotine, MPA, and ABT-418 displacing each of the C-11-ligands. Autor adiograms displayed a similar pattern of receptor binding for all liga nds, whereby [C-11]MPA showed the most distinct binding pattern and th e lowest nonspecific binding. We conclude that the three C-11-labeled nicotinic ligands were suitable for characterizing nicotinic receptors in vitro. The very high affinity of [C-11]MPA to nicotinic acetylchol ine receptors, its low nonspecific binding, and especially the slower dissociation kinetics of the [C-11] MPA from the putative high-affinit y nicotinic acetylcholine receptor binding site compared with (S)-[C-1 1]nicotine and [C-11]ABT-418 raise the level of interest in [C-11] MPA for application in positron emission tomography.