Stereoselective synthesis and biodistribution of potent [11C]-labeled antagonists for positron emission tomography imaging of muscarinic receptors in the airways

Article abstract of DOI:10.1021/jm960374w

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 [¹¹C]-labeled ligands with a high affinity (K(D) < 0.1 nM). Three quaternary muscarinic antagonists, racemic N-methylpiperidin-4-yl 2-cyclohexyl-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 [¹¹C]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 M₁, M₂, and M₃. They are more potent than the muscarinic antagonist (R)-N-methylquinuclidinyl benzilate ((R)-MQNB). Distribution studies with 1a, 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 lb 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.