10.1021/jm9904001.s001 John W. Daly John W. Daly Tara H. Gupta Tara H. Gupta William L. Padgett William L. Padgett Xue-Feng Pei Xue-Feng Pei 6β-Acyloxy(nor)tropanes:  Affinities for Antagonist/Agonist Binding Sites on Transfected and Native Muscarinic Receptors American Chemical Society 2000 rat heart membranes Native Muscarinic Receptors affinity transfected cell membranes muscarinic agonists quinuclidinyl benzilate binding rat brain membranes transfected m 1 m 4 K i values agonist activity muscarinic receptors M 2 M 1 transfected m 4 transfected m 3 transfected m 2 2000-05-18 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/6_-Acyloxy_nor_tropanes_Affinities_for_Antagonist_Agonist_Binding_Sites_on_Transfected_and_Native_Muscarinic_Receptors/3684435 A series of esters of 6β-hydroxynortropane and the <i>N</i>-methyl analogue 6β-tropanol were synthesized and screened versus binding of an antagonist (quinuclidinyl benzilate) and an agonist (oxotremorine-M) at sites on human m<sub>1</sub>-, m<sub>2</sub>-, m<sub>3</sub>-, and m<sub>4</sub>-muscarinic receptors in transfected cell membranes and on native M<sub>1</sub>-muscarinic receptors in rat brain membranes and native M<sub>2</sub>-muscarinic receptors in rat heart membranes. Most 6β-acyloxy(nor)tropanes had higher affinity versus oxotremorine-M binding compared to quinuclidinyl benzilate binding at transfected m<sub>1</sub>- and native M<sub>1</sub>-receptors, indicative of agonist activity. 6β-Acetoxynortropane had <i>K</i><sub>i</sub> values versus oxotremorine-M binding at m<sub>1</sub>-, m<sub>2</sub>-, and m<sub>4</sub>-receptors ranging from 4 to 7 nM. <i>N</i>-Methylation reduced affinity greatly as did increasing the size of the acyl moiety. The affinity of 6β-benzoyloxynortropane and other analogues with larger acyl moieties was little affected by <i>N</i>-methylation or in some cases was increased. 6β-Acyloxy(nor)tropanes and classical muscarinic agonists, such as muscarine and oxotremorine, had higher affinity versus oxotremorine-M binding compared to quinuclidinyl benzilate binding at native M<sub>2</sub>-muscarinic receptors of heart, but not at transfected m<sub>2</sub>-muscarinic receptors. Antagonist/agonist binding ratios were not obtained for transfected m<sub>3</sub>-receptors, since significant oxotremorine-M binding could not be detected. 6β-Acyloxy(nor)tropane, two other (nor)tropanes, and the classical muscarinic agonists had higher affinity versus agonist binding compared to antagonist binding for transfected m<sub>4</sub>-receptors. The antagonist/agonist binding ratio method is clearly not always reliable for predicting agonist activity at muscarinic receptors.