TY - JOUR T1 - A Novel Multivalent Ligand That Bridges the Allosteric and Orthosteric Binding Sites of the M<sub>2</sub> Muscarinic Receptor JF - Molecular Pharmacology JO - Mol Pharmacol SP - 291 LP - 302 DO - 10.1124/mol.106.033746 VL - 72 IS - 2 AU - Tod Steinfeld AU - Mathai Mammen AU - Jacqueline A. M. Smith AU - Richard D. Wilson AU - Jeffrey R. Jasper Y1 - 2007/08/01 UR - http://molpharm.aspetjournals.org/content/72/2/291.abstract N2 - THRX-160209 is a potent antagonist at the M2 muscarinic acetylcholine (ACh) receptor subtype that was designed using a multivalent strategy, simultaneously targeting the orthosteric site and a nearby site known to bind allosteric ligands. In this report, we describe three characteristics of THRX-160209 binding that are consistent with a multivalent interaction: 1) an apparent affinity of the multivalent ligand for the M2 receptor subtype (apparent pKI = 9.51 ± 0.22) that was several orders of magnitude greater than its two monovalent components (apparent pKI values &lt; 6.0), 2) specificity of THRX-160209 for the M2 receptor subtype compared with the closely related M4 (apparent pKI = 8.78 ± 0.24) and M1,M3, and M5 receptors (apparent pKI values ≤ 8.0), and 3) acceleration (&gt;10-fold) of the dissociation rate of tritium-labeled THRX-160209 from M2 receptors by competing monovalent ligands that are known to interact with either the orthosteric site (e.g., atropine) or a well characterized allosteric site (e.g., obidoxime) on the receptor. In complementary kinetic studies assessing allosteric modulation of the receptor, unlabeled THRX-160209 retarded dissociation of [3H]N-methyl scopolamine (NMS). The effects of THRX-160209 on retardation of [3H]NMS dissociation were competitively inhibited by obidoxime, suggesting that obidoxime and THRX-160209 bind to an overlapping region coincident with other typical muscarinic allosteric agents, such as 3-methyl-5-[7-[4-[(4S)-4-methyl-1,3-oxazolidin-2-yl]phenoxy]heptyl]-1,2-oxazole (W84) and gallamine. Taken together, these data are consistent with the hypothesis that THRX-160209 binds in a multivalent manner to the M2 receptor, simultaneously occupying the orthosteric site and a spatially distinct allosteric site. ER -