Since the osmotic gradient remains constant, drug delivery remains essentially constant. The biologically inert components of the tablet remain intact during gastrointestinal transit and are eliminated in the feces as an insoluble shell. Oxybutynin chloride exhibits only one-fifth of the anticholinergic activity of atropine on the rabbit detrusor muscle, but four to ten times the antispasmodic activity. Oxybutynin chloride relaxes bladder smooth muscle. Oxybutynin thus decreases urgency and the frequency of both incontinent episodes and voluntary urination.
Antimuscarinic activity resides predominantly in the R-isomer.
A metabolite, desethyloxybutynin, has pharmacological activity similar to that of oxybutynin in in vitro studies. The rate and extent of absorption and metabolism of oxybutynin are similar under fed and fasted conditions. Distribution Plasma concentrations of oxybutynin decline biexponentially following intravenous or oral administration. The volume of distribution is 193 L after intravenous administration of 5 mg oxybutynin chloride. Metabolism Oxybutynin is metabolized primarily by the cytochrome P450 enzyme systems, particularly CYP3A4 found mostly in the liver and gut wall.