Drug Interaction Report

MONITOR: Coadministration with potent inducers of CYP450 3A4 may decrease the plasma concentrations of fesoterodine's active metabolite, 5-hydroxymethyl tolterodine, which is partially metabolized by the isoenzyme. Following induction of CYP450 3A4 with rifampin 600 mg once a day, 5-hydroxymethyl tolterodine peak plasma concentration (Cmax) and systemic exposure (AUC) decreased by approximately 70% and 75%, respectively, after oral administration of fesoterodine 8 mg. The effects of less potent CYP450 3A4 inducers were not examined. In addition, when two or more medications with similar adverse effect profiles are given concurrently, the likelihood of experiencing these adverse reactions may be increased. For example, coadministration with other agents that can prolong the QT interval (e.g., apalutamide, encorafenib, enzalutamide) may result in additive effects and an increased risk of ventricular arrhythmias like torsade de pointes.

MANAGEMENT: No dosing adjustments are recommended when fesoterodine is used with CYP450 3A4 inducers. However, the possibility of diminished therapeutic effects should be considered. If the CYP450 3A4 inducer also carries a risk of prolonging the QT interval, then obtaining more frequent electrocardiograms (ECGs) to monitor the QT interval may be advisable. Patients should be counseled to seek immediate medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, syncope, palpitations, irregular heartbeat, and/or shortness of breath. The prescribing information for the concomitant potent CYP450 3A4 inducers should be consulted for specific recommendations.

MONITOR: Coadministration with moderate inhibitors of CYP450 3A4 such as grapefruit juice may increase the plasma concentrations of fesoterodine's active metabolite, 5-hydroxymethyl tolterodine, which is partially metabolized by the isoenzyme. The possibility of prolonged and/or increased pharmacologic effects of fesoterodine should be considered. Because 5-hydroxymethyl tolterodine is also metabolized by CYP450 2D6, the clinical significance of the interaction may be greater in patients who are CYP450 2D6-deficient, or so-called poor metabolizers of CYP450 2D6 (approximately 7% of Caucasians and less than 2% of Asians and individuals of African descent) who may rely more on the 3A4 metabolic pathway for clearance of the drug.

MANAGEMENT: Caution is advised if fesoterodine is administered with grapefruit or grapefruit juice. Patients should be advised to notify their physician if they experience potential adverse effects of fesoterodine such as irregular heartbeat, blurry vision, difficulty urinating, dry mouth, headache, drowsiness, dizziness, gastrointestinal upset, or constipation.