Ipratropium bromide relieves bronchospasm in COPD without producing the central effects seen with atropine. What property of ipratropium accounts for this safety advantage?

• A It is metabolised entirely within the lung to inactive products
• B Its quaternary ammonium structure prevents significant absorption across membranes, including the blood-brain barrier ✓
• C It is a partial agonist at muscarinic receptors with very low intrinsic activity systemically
• D It undergoes rapid first-pass metabolism by plasma cholinesterase after any absorption occurs

Explanation

Ipratropium carries a permanent positive charge because it is a quaternary ammonium derivative of atropine. Charged molecules cross lipid membranes poorly, so inhaled ipratropium stays largely in the airway, producing local bronchodilation with minimal systemic or central anticholinergic effects. It is not extensively metabolised in the lung, it is a full antagonist rather than a partial agonist, and plasma cholinesterase does not degrade it, ruling out the other options.

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

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