Salmeterol provides bronchodilation for about 12 hours despite having a short duration of action at the beta-2 receptor itself. The molecular explanation for its long duration is:

• A Irreversible covalent bonding to the receptor active site
• B Binding to an exosite adjacent to the active site, causing slow dissociation ✓
• C Inhibition of its own metabolism by CYP enzymes in the airway
• D Formation of a depot in pulmonary surfactant with zero-order release

Explanation

Salmeterol is highly lipophilic and carries a long lipophilic tail that anchors it to an exosite beside the beta-2 receptor active site. The head oscillates between the active site, giving repeated receptor activation, while dissociation from the anchoring exosite is extremely slow, producing a functional half-life around 12 hours. It is not covalently bound and its onset is correspondingly slow, which is why it must never be used as rescue monotherapy. Formoterol binds conventionally and acts faster.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.