Drug Interaction Report

ADJUST DOSE: Coadministration with potent inhibitors of CYP450 3A4 may significantly increase the plasma concentrations of darifenacin, which is partially metabolized by the isoenzyme. Darifenacin is also metabolized by CYP450 2D6, but 3A4 is the major pathway of metabolism in so-called poor metabolizers of 2D6 (approximately 7% of Caucasians and 2% of Asians and those of African descent). In a drug interaction study involving 10 extensive metabolizers and 1 poor metabolizer of 2D6, coadministration with the potent 3A4 inhibitor ketoconazole (400 mg) increased the mean steady-state peak plasma concentration (Cmax) and area under the concentration-time curve (AUC) of darifenacin (extended-release 7.5 mg once a day) approximately 5-fold each compared to values previously reported for extensive metabolizers of 2D6. In the poor metabolizer, Cmax and AUC of darifenacin increased approximately 13-fold each compared to values previously reported for poor metabolizers. When a 15 mg daily dose of extended-release darifenacin was given with ketoconazole, mean darifenacin Cmax and AUC in 3 extensive metabolizers increased approximately 12-fold compared to historical values, while Cmax and AUC increased approximately 6-fold in the poor metabolizer compared to historical values.

MANAGEMENT: The dosage of darifenacin should not exceed 7.5 mg/day when used with potent CYP450 3A4 inhibitors. Some authorities recommend avoiding concomitant use of darifenacin during and for 2 weeks after treatment with itraconazole.

The consumption of grapefruit juice may be associated with increased plasma concentrations of darifenacin. The mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. The clinical significance is unknown.