Drug Interaction Report

ADJUST DOSE: Coadministration with drugs that are potent inhibitors of CYP450 3A4 may significantly increase the plasma concentrations of tolterodine, which is partially metabolized by the isoenzyme. Tolterodine is primarily metabolized by CYP450 2D6 in most patients (referred to as "extensive metabolizers" or "EMs") to an equipotent, active metabolite, 5-hydroxymethyl tolterodine (5-HMT). However, in patients who are CYP450 2D6-deficient, or so-called "poor metabolizers" or "PMs" of CYP450 2D6 (approximately 7% of Caucasians and less than 2% of Asians and individuals of African descent), tolterodine is primarily metabolized by CYP450 3A4 to N-dealkylated tolterodine. Coadministration of tolterodine with ketoconazole 200 mg daily, a potent CYP450 3A4 inhibitor, increased the tolterodine mean peak plasma concentration (Cmax) by 2-fold and the mean systemic concentrations (AUC) by 2.5-fold in PMs. Data are not available for coadministration of tolterodine with CYP450 3A4 inhibitors in EMs or less potent CYP450 3A4 inhibitors.

MANAGEMENT: The manufacturer recommends a maximum tolterodine dosage of 2 mg once daily (extended-release capsule) and 1 mg twice daily (immediate-release and extended-release tablet) in patients receiving potent CYP450 3A4 inhibitors. Some authorities recommend avoiding concomitant use of tolterodine and potent CYP450 3A4 inhibitors. Clinical and laboratory monitoring, including QTc interval and serum electrolytes, is advised. Patients should have regular ECGs and be monitored for arrhythmias when the QTc interval is prolonged. If the QTc interval becomes markedly prolonged or symptoms of arrhythmia occur, drug discontinuation should be considered. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope. Patients should be advised to notify their physician if they experience new or worsening side effects of tolterodine including severe blurry vision, difficulty urinating, dry mouth, headache, drowsiness, dizziness, or GI upset.

MONITOR: Coadministration with drugs that are inhibitors of CYP450 3A4 may increase the plasma concentrations of tolterodine, which is partially metabolized by the isoenzyme. Tolterodine is primarily metabolized by CYP450 2D6 in most patients (referred to as "extensive metabolizers" or "EMs") to an equipotent, active metabolite, 5-hydroxymethyl tolterodine (5-HMT). However, in patients who are CYP450 2D6-deficient, or so-called "poor metabolizers" or "PMs" of CYP450 2D6 (approximately 7% of Caucasians and less than 2% of Asians and individuals of African descent), tolterodine is primarily metabolized by CYP450 3A4 to N-dealkylated tolterodine. Coadministration of tolterodine with ketoconazole 200 mg daily, a potent CYP450 3A4 inhibitor, increased the tolterodine mean peak plasma concentration (Cmax) by 2-fold and the mean systemic concentrations (AUC) by 2.5-fold in PMs. Data are not available for coadministration of tolterodine with CYP450 3A4 inhibitors in EMs or less potent CYP450 3A4 inhibitors. As tolterodine causes concentration-dependent QT interval prolongation, an increase in its AUC could increase the possibility of experiencing this adverse effect. Likewise, this risk may be further increased if the CYP450 3A4 inhibitor being used also carries a risk of QT prolongation (e.g., asciminib, bepridil, ciprofloxacin, clofazimine, crizotinib, erythromycin, fluconazole, lapatinib, pazopanib, rucaparib).

MANAGEMENT: Caution is advised when tolterodine is used with CYP450 3A4 inhibitors. Clinical and laboratory monitoring, including QTc interval and serum electrolytes, is advised. Patients should have regular ECGs and be monitored for arrhythmias when the QTc interval is prolonged. If the QTc interval becomes markedly prolonged or symptoms of arrhythmia occur, drug discontinuation should be considered. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope. Patients should be advised to notify their physician if they experience new or worsening side effects of tolterodine including severe blurry vision, difficulty urinating, dry mouth, headache, drowsiness, dizziness, or GI upset.

ADJUST DOSING INTERVAL: Food enhances the absorption and oral bioavailability of darunavir administered in combination with low-dose ritonavir. The mechanism is unknown. When administered with food, the peak plasma concentration (Cmax) and area under the plasma concentration-time curve (AUC) of darunavir were approximately 30% higher than when administered in the fasting state. Darunavir exposure was similar for the range of meals studied. The total caloric content of the various meals evaluated ranged from 240 Kcal (12 grams fat) to 928 Kcal (56 grams fat).

MANAGEMENT: To ensure maximal oral absorption, darunavir coadministered with ritonavir should be taken with food. The type of food is not important.