Cholinergic Antagonists: Pharmacokinetics

Cholinergic antagonists are administered through multiple routes including oral formulations, topical eye drops, transdermal patches, inhalational aerosols or powders, and injections. Most antimuscarinics are available as oral formulations, while scopolamine is delivered as a topical patch. Ipratropium and tiotropium are available as inhalation aerosols or powders, and atropine, tropicamide, and cyclopentolate are topically instilled in the eye.

Quaternary antimuscarinics are ionic with low lipid solubility, resulting in poor absorption from the gut, skin, or eye and restricted penetration to the blood-brain barrier. Tertiary derivatives have higher lipid solubility and are well-absorbed both centrally and peripherally. When inhaled, quaternary antimuscarinics have limited systemic absorption due to their reduced lipid solubility.

Antimuscarinics undergo hepatic metabolism through hydrolysis or conjugation and are primarily excreted via urine. Atropine rapidly metabolizes in the liver and is excreted unchanged in the urine. Most swallowed ipratropium and tiotropium are excreted through feces, while elimination of quaternary compounds is generally slower than tertiary agents.

Antimuscarinics are classified based on their duration of action as short-acting or long-acting agents. Short-acting antimuscarinics include ipratropium, while long-acting agents include glycopyrrolate, tiotropium, and aclidinium. This classification helps determine dosing frequency and clinical application for different therapeutic conditions and patient needs.

Antimuscarinics interact with tricyclic antidepressants, antihistamines, antianxiety agents, and antipsychotics, exacerbating antimuscarinic side effects due to additive anticholinergic properties. Additionally, antacids can increase stomach pH and form complexes with antimuscarinics, reducing their absorption. These interactions require careful monitoring during co-administration.

Antimuscarinics partially suppress peristaltic movement of the stomach, delaying gastric emptying and slowing the absorption of co-administered drugs. This effect can significantly impact the bioavailability and onset of action of other medications taken concurrently with antimuscarinics, requiring dose timing adjustments.

Quaternary antimuscarinics are ionic compounds with low lipid solubility, which restricts their ability to cross biological membranes. When inhaled, this reduced lipid solubility results in limited systemic absorption, allowing these agents to act primarily at local respiratory sites with minimal systemic effects and reduced side effects.