Mechanism of Action and Chemical Structure
Fenoterol selectively stimulates \u03b2\u2082-adrenergic receptors and, to a lesser extent, \u03b2\u2081-receptors.
Structurally, it differs from endogenous catecholamines. While natural mediators have hydroxyl groups at positions 3 and 4, fenoterol (like terbutaline) has them at positions 3 and 5. This is a critical clinical distinction: this chemical configuration makes the molecule resistant to degradation by COMT (catechol-O-methyltransferase). Consequently, the drug provides a longer duration of pharmacological action compared to endogenous catecholamines.
Unlike long-acting agents (e.g., salmeterol), the fenoterol molecule lacks long lipophilic side chains, 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 pharmacokinetic profile is as follows:
- Onset of action: Very rapid, occurring within 1–3 minutes.
- Peak effect: Reached in 30–60 minutes.
- Duration: Lasts 3 to 6 hours.
By comparison, long-acting agents work for up to 12 hours but may have a delayed onset (such as salmeterol at 15–30 minutes).
The primary pharmacological effects of fenoterol depend on the target tissue:
- Respiratory tract: Relaxes bronchial smooth muscle (bronchodilation).
- Uterus: Exerts a pronounced tocolytic effect—reducing tone and contractile activity of the myometrium via stimulation of uterine \u03b2\u2082-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 exercise- or allergen-induced bronchospasm (administered via inhalation or orally 30–40 minutes before expected exposure to a trigger).
In Obstetrics (as a tocolytic):
- Threatened miscarriage.
- Prevention of preterm labor.
- Uterine dyscoordination.
- Excessive uterine hypertonus.
- Uterine relaxation required during surgical interventions in pregnant patients.
Administration Features and Risks
Combination Therapy for COPD: Rational combination principles apply—pairing \u03b2\u2082-agonists with muscarinic antagonists (anticholinergics). A key representative of this combination is Berodual (containing fenoterol and ipratropium bromide). The goal is synergistic enhancement of bronchodilation, allowing a lower dose of the \u03b2\u2082-agonist and thereby minimizing adverse effects. Kinetics of the combination: onset within 5–10 minutes, duration 3–4 hours.
Obstetric Risks: In obstetrics, drugs are typically administered orally, or intravenously in emergency settings. It is important to note that fenoterol crosses the placental barrier, carrying a risk of fetal tachycardia and arrhythmia. However, fenoterol (along with hexoprenaline) remains a preferred tocolytic due to a more favorable fetal risk profile compared to salbutamol.
Formulations and Dosages
Fenoterolum is available in several formulations for different routes of administration:
- Tablets: 5 mg (administered orally).
- Ampoules: 0.005% solution — 10 mL (administered via intravenous infusion).
- Metered-dose aerosol (trade name Berotec): 1 actuation delivers 0.2 mg of the drug.
- Solution in vials (for nebulizers): 0.1% — 20 mL.