Absorption Fate And Excretion Of Muscarinic Antagonists

belladonna alkaloids and the tertiary synthetic and semisynthetic derivatives are absorbed rapidly from the GI tract and mucosal surfaces; absorption from intact skin is limited, although efficient absorption does occur in the postauricular region for some agents, allowing delivery via transdermal patch. Systemic absorption of inhaled or orally ingested quaternary muscarinic antagonists is minimal, even from the conjunctiva of the eye; the quaternary agents do not cross the blood-brain barrier.

Atropine has a tJ/2 of -4 hours; hepatic metabolism accounts for the elimination of about half of a dose; the remainder is excreted unchanged in the urine. Ipratropium is administered as an aerosol or solution for inhalation; tiotropium is administered as a dry powder. As with most drugs administered by inhalation, -90% of the dose is swallowed and appears in the feces. After inhalation, maximal responses usually develop over 30-90 minutes, with tiotropium having the slower onset. The effects of ipratropium last for 4-6 hours; tiotropium's effects persist for 24 hours, and the drug is amenable to once-daily dosing.

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