17.01.4.2 Muscarinic Receptor Antagonists

Ipratropium bromide (discussed previously in eChapter 7) and tiotropium are muscarinic receptor antagonists used in the treatment of asthma. The main advantage ipratropium has over other anti-muscarinics (e.g. atropine) is limited absorption and, therefore, limited systemic side effects. The mechanism of action of ipratropium and of tiotropium is as non-selective muscarinic receptor antagonists, which prevents acetylcholine from stimulating muscarinic receptors and causing bronchoconstriction. Effectively, the action observed with ipratropium/tiotropium is relaxation of smooth muscle. Tiotropium has a longer half-life than ipratropium and only requires once a day dosing. Ipratropium and tiotropium are occasionally used as bronchodilators in asthma. They are sometimes used in combination with a β₂-adrenoceptor agonist to give an additive effect. The muscarinic receptor antagonists can also be used with glucocorticoids as alternatives to β₂-adrenoceptor agonists.