Abstract
The β&sub2;-adrenergic receptor (β&sub2;-AR) agonist [(3)H]-(R,R')-methoxyfenoterol was employed as the marker ligand in displacement studies measuring the binding affinities (Ki values) of the stereoisomers of a series of 4'-methoxyfenoterol analogs in which the length of the alkyl substituent at α' position was varied from 0 to 3 carbon atoms. The binding affinities of the compounds were additionally determined using the inverse agonist [(3)H]-CGP-12177 as the marker ligand and the ability of the compounds to stimulate cAMP accumulation, measured as EC₅&sub0; values, were determined in HEK293 cells expressing the β&sub2;-AR. The data indicate that the highest binding affinities and functional activities were produced by methyl and ethyl substituents at the α' position. The results also indicate that the Ki values obtained using [(3)H]-(R,R')-methoxyfenoterol as the marker ligand modeled the EC₅&sub0; values obtained from cAMP stimulation better than the data obtained using [(3)H]-CGP-12177 as the marker ligand. The data from this study was combined with data from previous studies and processed using the Comparative Molecular Field Analysis approach to produce a CoMFA model reflecting the binding to the β&sub2;-AR conformation probed by [(3)H]-(R,R')-4'-methoxyfenoterol. The CoMFA model of the agonist-stabilized β&sub2;-AR suggests that the binding of the fenoterol analogs to an agonist-stabilized conformation of the β&sub2;-AR is governed to a greater extend by steric effects than binding to the [(3)H]-CGP-12177-stabilized conformation(s) in which electrostatic interactions play a more predominate role.