Antimuscarinic Activity of Oxybutynin in the Human Plasma Quantitated by a Radioreceptor Assay

: The antimuscarinic activity of oxybutynin was measured as oxybutynin equivalents by a radioreceptor assay (RRA). The activity was studied in plasma samples of five volunteers after a single oral dose (10 mg) or after a single intravenous dose (28 μg/kg) of oxybutynin hydrochloride. The results were compared to the concentrations of the drug measured by gas liquid chromatography (GLC). Following oral administration, the maximum concentration measured by RRA was significantly higher (706 nmol/1) than that by GLC (38 nmol/1). In contrast, equal concentrations were measured after intravenous administration by both methods. Metabolites with antimuscarinic activity are possibly formed through first‐pass metabolism after orally administered oxybutynin. The total antimuscarinic activity of oxybutynin and its metabolites are measured by RRA, but only the parent drug by GLC.