Which of the following beta-2 agonists has the longest duration of action and is classified as a long-acting beta-2 agonist (LABA)?

Explanation

## Long-Acting Beta-2 Agonists (LABAs) **Key Point:** LABAs are designed for maintenance therapy in chronic asthma and COPD, with a duration of action of 12–24 hours, compared to short-acting beta-2 agonists (SABAs) which last 4–6 hours. ### Classification of Beta-2 Agonists | Agent | Type | Duration | Clinical Use | |-------|------|----------|---------------| | Salbutamol (Albuterol) | SABA | 4–6 hours | Acute relief, rescue therapy | | Terbutaline | SABA | 4–6 hours | Acute relief, subcutaneous use | | Isoproterenol | Non-selective β-agonist | 1–2 hours | Rarely used; non-selective | | Salmeterol | LABA | 12 hours | Maintenance therapy | | Formoterol | LABA | 12 hours | Maintenance therapy | | Vilanterol | LABA | 24 hours | Once-daily maintenance | **High-Yield:** Salmeterol achieves its long duration through lipophilicity, allowing depot formation in airway tissue and gradual release. It has a slow onset (10–20 minutes) and should NEVER be used as a rescue agent. **Clinical Pearl:** LABAs are always prescribed with an inhaled corticosteroid (ICS) in asthma to reduce mortality risk. Monotherapy with LABA increases asthma-related deaths. **Mnemonic:** **SABA vs LABA** — "**S**hort **A**ction, **L**ong **A**ction" — SABAs (Salbutamol, Terbutaline) for quick relief; LABAs (Salmeterol, Formoterol) for steady control. ### Why Salmeterol? Salmeterol's lipophilic side chain allows it to bind to airway tissue depots, creating a reservoir that sustains bronchodilation over 12 hours. This makes it ideal for twice-daily maintenance therapy in chronic obstructive airway disease.