Potent M 3 muscarinic receptor antagonist that displays ~ 120-fold selectivity over M 2 receptors (K i values are 4.2, 19 and 490 nM for human M 3 , M 1 and M 2 receptors respectively). Exhibits > 250-fold bronchial selectivity; inhibits ACh-induced bronchoconstriction but not ACh-induced bradycardia (K B values are 3.3 and 170 nM for rat trachea M 3 and rat right atria M 2 receptors respectively).