Glycopyrrolate Structure — FRCA Primary MCQ

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ModeratePharmacology - AutonomicGlycopyrrolate StructureFRCA Primary

Which property of glycopyrrolate (glycopyrronium bromide) most directly accounts for its negligible central nervous system effects at usual anaesthetic doses, compared with atropine?

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Correct answer: A — A quaternary ammonium compound with poor lipid-membrane penetration

Explanation lettering: E = shown as A · A = shown as B · B = shown as E

Glycopyrrolate is a quaternary ammonium antimuscarinic and is therefore highly polar and poorly lipid soluble. It penetrates lipid membranes, including the blood–brain barrier, poorly, so central antimuscarinic effects are negligible at usual doses. In contrast, atropine is a tertiary amine that crosses the blood–brain barrier. Glycopyrrolate is not an M2-selective antagonist; its antimuscarinic actions are peripheral and non-selective. It also causes less initial tachycardia than atropine when paired with neostigmine and has longer vagal-blocking and antisialagogue effects, making options B and D incorrect.

Reference: Martindale Pharmaceuticals Ltd. Glycopyrronium Bromide 200 micrograms/ml Solution for Injection, Summary of Product Characteristics, sections 4.4 and 5.1; eMC, updated 3 January 2025. https://www.medicines.org.uk/emc/product/3389/smpc