Inhaled anticholinergics are a class of medications that reduce bronchial smooth muscle tone. In clinical practice, they are generally considered adjunctive therapy, as their bronchodilator potency is objectively inferior to $\beta_2$-adrenergic agonists.
Site of ActionEffectively relieves spasms in large bronchi, but has a weak effect on small airways
Side EffectMakes mucus viscous due to the suppression of bronchial gland secretion
SafetyDoes not cause systemic atropine-like effects due to low absorption
DurationVaries from 5–6 hours (ipratropium) to 12 hours (tiotropium)
Why Are Anticholinergics Weaker Than $\beta_2$-Agonists?
The relatively moderate efficacy of this drug class is due to three physiological factors:
Anatomical factor (receptor topography). The density of muscarinic receptors in the respiratory tract is distributed unevenly. They are abundant in large bronchi and sparse in distal airways. Consequently, these drugs effectively eliminate large airway spasms but have little effect on small bronchi, where obstruction most frequently localizes.
Synaptic feedback mechanism. The primary therapeutic goal is to block postsynaptic M3 muscarinic receptors on smooth muscle to decrease tone. However, non-selective agents also block presynaptic M2 muscarinic receptors (autoreceptors). Normally, these receptors act as a brake, limiting neurotransmitter release. Blockade of M2 receptors leads to an increased release of acetylcholine into the synaptic cleft. This creates a paradox: a high concentration of endogenous acetylcholine begins to competitively displace the drug from its bond with target M3 receptors, diminishing the bronchodilator effect.
Effect on secretion. Anticholinergics suppress bronchial gland activity. Reducing the volume of secretions makes mucus thick, viscous, and difficult to clear. This side effect is particularly undesirable in bronchial asthma.
Pharmacology of Ipratropium Bromide (Atrovent)
This drug is a classic representative of inhaled anticholinergics.
Physicochemical properties: The molecule contains a quaternary nitrogen atom, conferring low lipophilicity. The drug crosses biological membranes extremely poorly.
Pharmacokinetics: When administered via inhalation, it acts exclusively locally and is practically not absorbed into the systemic circulation.
Time parameters: The bronchodilator effect begins 30 minutes after inhalation, peaks in 1.5–2 hours, and lasts approximately 5–6 hours.
Safety profile: The only potential local side effect is dry mouth. Systemic side effects typical of atropine are completely absent due to low absorption.
Pharmacology of Tiotropium Bromide (Spiriva)
Tiotropium bromide is a more modern agent that differs from ipratropium bromide by an improved receptor-blocking profile.
Selectivity mechanism: The drug blocks postsynaptic M3 muscarinic receptors to a significantly greater extent than presynaptic M2 muscarinic receptors. This avoids competitive displacement of the drug by acetylcholine and ensures a more effective reduction in bronchial tone.
Pharmacodynamics: Onset of action develops faster than that of ipratropium bromide, although the maximum effect is also achieved in 1.5–2 hours.
Dosing regimen: The drug has a long duration of action (about 12 hours), allowing for once-daily inhalation administration.
General Evaluation of Atropine-Like Agents
Virtually all drugs in the atropine-like class exhibit a bronchodilator effect. However, the use of classic systemic agents as bronchodilators is considered inappropriate in modern medicine. The main clinical limitation is the excessively broad spectrum of action and a consistently high incidence of systemic side effects.
Mnemonic
The “M3-M2” rule: M3 relaxes bronchi (our goal), M2 inhibits acetylcholine (do not touch, otherwise the neurotransmitter will displace the drug). Tiotropium “knows” this and targets M3, whereas ipratropium blocks both.
Frequently asked questions
What systemic side effects are characteristic of classic atropine-like drugs?
Classic atropine-like agents are characterized by a wide spectrum and high incidence of systemic side effects. Using such drugs as bronchodilators is inappropriate due to pronounced generalized actions on the body, unlike modern inhaled formulations with low systemic absorption.
What other drugs belong to the group of inhaled anticholinergics besides ipratropium and tiotropium?
In addition to ipratropium and tiotropium, the inhaled anticholinergic group includes aclidinium bromide and glycopyrronium bromide.
In which respiratory diseases is the use of inhaled anticholinergics indicated?
Inhaled anticholinergics are indicated for several major obstructive respiratory diseases. These drugs are effective in treating chronic obstructive pulmonary disease (COPD), and are also used in bronchial asthma and during exacerbations of chronic bronchitis.
Which drug classes are combined with anticholinergics in inhalers to enhance the bronchodilator effect?
To mutually enhance bronchodilation, anticholinergics are combined with other pharmacological classes in inhalers. Clinical practice utilizes fixed-dose combinations with long-acting $\beta_2$-agonists (LABAs), as well as triple therapy additionally including inhaled corticosteroids (ICS).
Why doesn't ipratropium bromide cause systemic effects typical of atropine?
Its structure contains a quaternary nitrogen atom, which ensures low lipophilicity. Upon inhalation, the drug acts locally and is barely absorbed into the systemic circulation.
How do anticholinergics affect the consistency of mucus?
They suppress bronchial gland secretion. As a result, the volume of secretion decreases, and mucus becomes viscous and difficult to expectorate.
What is the main pharmacodynamic advantage of tiotropium bromide?
It exhibits selectivity: it more potently blocks target postsynaptic M3 receptors while sparing presynaptic M2 receptors less, and it has a duration of action up to 12 hours.
Go deeper
Competitive antagonism of acetylcholine and anticholinergics
The role of presynaptic M2 autoreceptors in synaptic transmission
Effect of the quaternary nitrogen atom on cell membrane permeability
Systemic side effects of classic atropine-like drugs