Tiotropium provides once-daily bronchodilation in COPD despite being an antagonist at muscarinic receptor subtypes that mediate physiological cholinergic tone. The property responsible for its long duration of action is:

• A Irreversible covalent binding to the M3 receptor
• B Kinetic selectivity due to very slow dissociation from the M3 receptor ✓
• C Active metabolites that accumulate over several days
• D Complete resistance to acetylcholine because it occupies the allosteric site

Explanation

Tiotropium binds reversibly but dissociates extremely slowly from M3 receptors, giving functional 24-hour blockade from a single inhalation. This is termed kinetic selectivity. Its dissociation from M2 receptors is comparatively faster, sparing presynaptic M2 autoreceptors that would otherwise enhance acetylcholine release. It is not irreversible, unlike older organophosphate-type anticholinergics, and has no active metabolites driving its duration.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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