CHARACTERIZATION OF [H-3] BA-679-BR, A SLOWLY DISSOCIATING MUSCARINICANTAGONIST, IN HUMAN LUNG - RADIOLIGAND BINDING AND AUTORADIOGRAPHIC MAPPING

Ba 679 BR [7(S)-(1 alpha,2 beta,4 beta,5 alpha,7 beta)-7-[(hydroxydi(2 -thienyl) imethyl-3-oxa-9-azoniatricyclo[3.3.1.0(2,4)]nonane bromide] is a new long-acting muscarinic antagonist developed as a bronchodilat or drug. In this study, we have evaluated its affinity, its selectivit y, and the distribution of its binding sites in human lung. [H-3]Ba 67 9 BR binds to a homogeneous population of muscarinic receptors in huma n lung membranes, with affinities in the subnanomolar concentration ra nge. Like ipratropium bromide, Ba 679 BR showed no selectivity in its interactions with rat cerebrocortical M(1) receptors (labeled with [H- 3]telenzepine) or heart M(2) and salivary gland M(3) receptors [labele d with [N-methyl-H-3]scopolamine ([H-3]NMS)]. Ba 679 BR displayed 620- fold higher affinity, compared with ipratropium bromide. We also studi ed the rate of Ba 679 BR and ipratropium bromide dissociation from hum an lung muscarinic receptors, by monitoring [H-3]NMS association. Unli ke ipratropium bromide (100 nM), which dissociated so quickly that the re was little difference in the [H-3]NMS association, compared with ve hicle-treated membranes, Ba 679 BR (1 nM) had a strong protective effe ct against [H-3]NMS binding (>70%) that lasted for 90 min. Kinetic exp eriments conducted with [H-3]Ba 679 BR confirmed the slow dissociation profile of this compound. The dissociation rate constant (k(-1)) for [H-3]Ba 679 BR was 3.29 +/- 0.18 x 10(-3) min(-1), correspending to a half-life of the complex of 212 +/- 11 min. Autoradiographic studies r evealed that [3H]Ba 679 BR binding sites were densely distributed in a lveolar walls and submucosal glands. These results suggest that the sl ow dissociation profile of Ba 679 BR from human lung muscarinic recept ors might be the underlying mechanism by which this drug achieves its long duration of action observed in functional tests.