Anticholinergics mechanism of action in bronchodilation

Some additional information from Goodman & Gilman’s

Anticholinergic Agents

With the advent of inhaled beta adrenergic agonists, use of anticholinergic agents declined. Renewed interest in anticholinergic agents paralleled the realization that parasympathetic pathways are important in bronchospasm in some asthmatics and the availability of ipratropium bromide (ATROVENT), a quaternary muscarinic receptor antagonist that has better pharmacological properties than prior drugs. A particularly good response to ipratropium may be seen in the subgroup of asthmatic patients who experience psychogenic exacerbations (Neild and Cameron, 1985).

The cholinergic receptor subtype responsible for bronchial smooth muscle contraction is the muscarinic receptor 3 (M3). Although iprotropium and related compounds block all five muscarinic receptor subtypes with similar affinity, it is likely that M3-receptor antagonism alone accounts for the bronchodilating effect. The bronchodilation produced by ipratropium in asthmatic subjects develops more slowly and usually is less intense than that produced by adrenergic agonists. Some asthmatic patients may experience a useful response lasting up to 6 hours. The variability in the response of asthmatic subjects to ipratropium presumably reflects differences in the strength of parasympathetic tone and in the degree to which reflex activation of cholinergic pathways participates in generating symptoms in individual patients. Hence the utility of ipratropium must be assessed on an individual basis by a therapeutic trial. The pharmacological properties and therapeutic uses of ipratropium have been reviewed (Gross, 1988; see also Symposium, 1986b).

Combined treatment with ipratropium and b2 adrenergic agonists results in slightly greater and more prolonged bronchodilation than with either agent alone in baseline asthma (Bryant and Rogers, 1992). In acute bronchoconstriction, the combination of a b2 adrenergic agonist and ipratropium is more effective than either agent alone and more effective than simply giving more b2 adrenergic agonist (Bryant, 1985; Bryant and Rogers, 1992). A large multicenter study showed that the asthmatic subjects with the worst initial lung function benefited most from combination therapy (Rebuck et al., 1987). Thus the combination of a selective b2 adrenergic agonist and ipratropium should be considered in acute treatment of severe asthma exacerbations. Ipratropium is available in metered-dose inhalers and as a nebulizer solution. A metered-dose inhaler containing a mixture of ipratropium and albuterol (COMBIVENT) also is available in the United States. In Europe, metered-dose inhalers containing a mixture of ipratropium and fenoterol are available (DUOVENT, BERODUAL).

Recently, tiotropium (SPIRIVA), a structural analogue of ipratropium, has been approved for the treatment of COPD and emphysema. Like ipratropium, tiotropium has high affinity for all muscarinic receptor subtypes, but it dissociates from the receptors much more slowly that ipratropium (Barnes, 2000). In particular, binding and functional studies indicate that tiotropium dissociates from muscarinic M3 receptors more slowly than from muscarinic M2 receptors. The high affinity of tiotropium for muscarinic receptors, combined with its very slow dissociation rate, permits once-daily dosing. The slow dissociation rate also provides a theoretical advantage in that it limits the capacity of large concentrations of the endogenous agonist acetylcholine to surmount the receptor blockade. Tiotropium is provided as a capsule containing a dry-powder formulation that is intended only for oral inhalation using the HandiHaler inhalation device.

Related keywords: Tiotropium bromide (spiriva), ipratropium (Combivent and Duoneb), MOA, mechanism of action, bronchodilation, bronchodilator, pharmacology, asthma, COPD.

Source:Goodman And Gilman’s The Pharmacological Basis of Therapeutics

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