Selective muscarinic receptor antagonists for airway diseases.

Airway tone and airway hyperreactivity are mediated by the parasympathetic nerves that release acetylcholine onto muscarinic receptors (M1-M5). Stimulation of M1 and M3 muscarinic receptors causes bronchoconstriction. The M1 muscarinic receptor is excitatory, and facilitates neuronal transmission at the parasympathetic ganglion. The M2 receptor is an inhibitory prejunctional autoreceptor. The discovery of discrete muscarinic receptor subtypes prompted development of selective muscarinic receptor antagonists. Selective M3 receptor antagonists and antagonists selective for M1 and M3 receptors have recently entered clinical trials and offer much promise for the treatment of airways diseases.