Oxybutynin is an antimuscarinic agent used to relax the bladder and prevent the urge to void. It can be taken either orally or intravenously. The bioavailability and the elimination half-life are both increased in the elderly. When taken orally, oxybutynin enters the intestine and is transported into the intestinal wall by the transporter solute carrier family 15 member 1. It is heavily metabolized by cytochrome P450 3A4 in both the intesine wall and the liver. In the endoplasmic reticulum of the intestine wall oxybutynin is metabolized by cytochrome P450 3A4 into N-desethyloxybutynin, an active metabolite. Oxybutynin is also metabolized into 2-cyclohexyl-2-phenylglycolic acid or phenylcyclohexylglycolic acid also by cytochrome P450 3A4. Both metabolites and the remaining original oxybutynin are transported into the blood by organic cation transporters. N-Desethyloxybutynin is an active metabolite that circulates the blood along with Oxybutynin. N-Desethyloxybutynin is active at muscarinic receptors in the bladder and salavary glands as well. Oxybutynin travels to the liver where it is transported in via organic cation transporters and metabolized again in the endoplasmic reticulum in the same way it was in the intestine wall. Oxybutynin is heavily cleared by the liver. The majority is metabolized into the inactive metabolite phenylcyclohexylgycolic acid. The metabolites are transported out of the liver and back into the blood where they are eliminated renally. Less than 0.1% is excreted as the active metabolite N-desethyloxybutynin, and less than 0.1% of the dose is excreted as the original oxybutynin. The majority is excreted as the inactive metabolite 2-cyclohexyl-2-phenylglycolic acid.

Oxybutynin Metabolism References