Stereoselective Synthesis and Biodistribution of Potent [<sup>11</sup>C]-Labeled Antagonists for Positron Emission Tomography Imaging of Muscarinic Receptors in the Airways
作者:Ton J. Visser、Aren van Waarde、Twan J. H. Jansen、Gerben M. Visser、Thom W. van der Mark、Jan Kraan、Kees Ensing、Willem Vaalburg
DOI:10.1021/jm960374w
日期:1997.1.1
Quantitation of muscarinic receptors in the lungs in vivo with positron emission tomography (PET) is of clinical interest. For that purpose we decided to develop [C-11]-labeled ligands with a high affinity (K-D < 0.1 nM). Three quaternary muscarinic antagonists, racemic N-methylpiperidin-4-yl 2-cyclohexy-2-2-hydroxy-2-phenylacetate methiodide 1a (pK(B) = 10.39), its (R)-isomer 1b (pK(B) = 11.08), and (R,R)-quinuclidin-3-yl 2-cyclohexyl-2-hydroxy-2-phenylacetate methiodide 2 (pK(B) = 11.28), were labeled by reacting [C-11]CH3I With their tertiary amine precursors. The enantiomerically pure tertiary amine precursors were prepared by stereoselective synthesis starting from (R)-(-)-mandelic acid. In vitro binding assay of 1b and 2 demonstrated that both ligands bind with very high affinity to the muscarinic receptor subtypes M(1), M(2), and M(3). They are more potent than the muscarinic antagonist (R)-N-methylquinuclidinyl benzilate ((R)-MQNB). Distribution studies with la, 1b, and 2 in control and atropine-treated male Wistar rats demonstrated significant specific binding (90-99% of total issue uptake) in tissues containing cholinoceptors (heart, intestine, lung, pancreas, spleen, stomach, submandibular gland). Because the tissue/plasma concentration ratios of 1b are most favorable, this ligand was used for further evaluation. Analysis of plasma samples showed a very rapid clearance (t(1/2) = 0.3 min) of the radioligand 1b and a relatively slow appearance of a hydrophilic metabolite. At 15 min postinjection of 1b, analysis of heart, lungs, and liver showed that respectively 99%, 88%, and 8% of the tissue radioactivity corresponded with the parent compound. Ligand 1b appears to be an excellent candidate for PET studies of mAChR receptors in heart and lungs.