Fenoterol

Mechanism of action and chemical structure

Fenoterol selectively stimulates β₂-adrenergic receptors and, to a lesser extent, β₁-receptors.

By chemical structure, it differs from the endogenous catecholamines. While the natural mediators have hydroxyl groups at positions 3 and 4, fenoterol (like terbutaline) has them at positions 3 and 5. This is a critically important clinical difference: this structure makes the molecule resistant to the degrading action of the enzyme COMT (catechol-O-methyltransferase). As a result, the drug provides a longer-lasting pharmacological effect compared to the endogenous catecholamines.

Unlike long-acting agents (e.g., salmeterol), the fenoterol molecule lacks long lipophilic "tails," which determines its rapid yet relatively short duration of action.

Pharmacokinetics and effects

The drug is a typical short-acting agent. When administered via inhalation, its pharmacokinetics are as follows:

• Onset of action: very rapid, with the effect developing within 1–3 minutes.
• Peak effect: reached in 30–60 minutes.
• Duration: lasts from 3 to 6 hours.

For comparison, long-acting drugs work for up to 12 hours, but may have a delayed onset (such as salmeterol — 15–30 minutes).

The main pharmacological effects of fenoterol depend on its site of action:

• In the airways: causes relaxation of bronchial smooth muscle (bronchodilation).
• In the uterus: exerts a pronounced tocolytic effect — decreasing myometrial tone and contractile activity by stimulating uterine β₂-receptors.

Indications for use

In clinical practice, fenoterol is used in two completely different fields: pulmonology and obstetrics.

In pulmonology (as a "rescue" medication):

• Relief of acute bronchospasm attacks.
• Prevention of bronchospasm (administered by inhalation or orally 30–40 minutes before expected contact with a triggering factor).

In obstetrics (as a tocolytic):

• Threatened miscarriage.
• Prevention of preterm labor.
• Dysfunctional labor.
• Excessive uterine tension.
• Need for uterine relaxation during surgical interventions in pregnant women.

Administration features and risks

Combination therapy for COPD: The principle of rational combination is applied — combining β₂-adrenergic agonists with anticholinergics (muscarinic antagonists). The key drug of such a combination is Berodual (containing fenoterol and ipratropium bromide). The goal of this combination is the mutual enhancement of the bronchodilating effect (synergism) and the possibility of reducing the dose of the β₂-adrenergic agonist, leading to fewer side effects. Kinetics of the combination: onset in 5–10 minutes, duration 3–4 hours.

Obstetric risks: In obstetrics, drugs are usually prescribed in tablets, and in emergency situations — intravenously. It is important to remember that fenoterol crosses the placental barrier. There is a risk of fetal tachycardia and arrhythmia. However, fenoterol (along with hexoprenaline) is the drug of choice, as it carries a lower risk for the fetus compared to salbutamol.

Dosage forms and dosages

According to the prescription reference book, Fenoterolum is available in several forms for different routes of administration:

• Tablets: 0.005 g each (prescribed orally, 0.005 g).
• Ampoules: 0.005% solution — 10 ml (administered intravenously as a drip at a dose of 0.0005 g).
• Aerosol (trade name Berotec): 1 actuation contains 0.0002 g of the drug (administered by inhalation, 0.0002 g).
• Solution in vials (for inhalers/nebulizers): 0.1% — 20 ml (administered by inhalation, 0.5 ml of the 0.1% solution).

Class and exam questions

What else to study on this topic

• Comparative pharmacokinetics of β₂-adrenergic agonists: salbutamol, fenoterol, salmeterol, and formoterol.
• Mechanism of action of anticholinergics (using ipratropium bromide as an example).
• The role of the enzyme COMT in the metabolism of adrenergic agonists and catecholamines.
• Tocolytics: comparing the safety profile of hexoprenaline, fenoterol, and salbutamol.
• Principles of maintenance therapy and acute attack management in COPD and bronchial asthma.

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