Drug Interactions between lapatinib and tolterodine

ADJUST DOSE: Coadministration with drugs that are inhibitors of CYP450 3A4 may increase the plasma concentrations of tolterodine, which is partially metabolized by the isoenzyme. The possibility of prolonged and/or increased pharmacologic effects of tolterodine should be considered. Although tolterodine is primarily metabolized by CYP450 2D6, there is some evidence that CYP450 3A4 may play a minor role, thus any alteration in its activity levels could conceivably affect the metabolism of tolterodine. The clinical significance of this interaction is yet unknown but 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).

MANAGEMENT: The manufacturer recommends a maximum tolterodine dosage of 1 mg twice daily in patients receiving concomitant CYP450 3A4 inhibitors. Close clinical and laboratory monitoring is advised whenever a CYP450 3A4 inhibitor is added to or withdrawn from therapy. Patients should be advised to notify their physician if they experience an irregular heartbeat, severe blurry vision, difficulty urinating, dry mouth, headache, drowsiness, dizziness, or GI upset.