In vitro activity: Oxybutynin N-deethylation in human liver microsomes in vitro is potently inhibited by ketoconazole (IC50 4.5 mM), less and variably by itraconazole and not by quinidine or several other reference inhibitors, suggesting that CYP3A enzymes are predominant catalysts of the reaction. Oxybutynin inhibits CYP3A4- and CYP2D6- associated activities (testosterone 6 beta-hydroxylase and dextromethorphan O- demethylase, respectively) in human liver microsomes. Oxybutynin is predominantly metabolized by CYP3A4 and CYP3A5 but not by CYP2D6. Oxybutynin (30, 100 nM) competitively antagonizes acetylcholine-induced contractions but does not alter those induced by histamine. Oxybutynin (up to 10 mM) induces a non-competitive depression of responses to both agonists and causes a parallel shift to the right of the Ca2+-induced contractions in taenia caeci strips bathed in a Ca2+-free, high-K+ medium. Oxybutynin (1-10 mM) impairs rhythmic muscular contractions in normal medium and after CaCl2 addition in Ca2+-free medium. Oxybutynin increases the perfusion pressure starting at 100 mM in perfused rat liver. Oxybutynin also increases the perfusion pressure in the hepatic artery.

Pharmacol Toxicol. 1998 Apr;82(4):161-6; Life Sci. 2010 Jul 31;87(5-6):175-80.