Potent M3 muscarinic receptor antagonist that displays ~ 120-fold selectivity over M2 receptors (Ki values are 4.2, 19 and 490 nM for human M3, M1 and M2 receptors respectively). Exhibits > 250-fold bronchial selectivity; inhibits ACh-induced bronchoconstriction but not ACh-induced bradycardia (KB values are 3.3 and 170 nM for rat trachea M3 and rat right atria M2 receptors respectively). Purity 99%. MW (observed): 500.63