Drug Interaction Report

MONITOR: Coadministration with inducers of CYP450 3A4 may decrease the plasma concentrations of gefitinib, which is primarily metabolized by the isoenzyme. When a single 500 mg dose of gefitinib was administered following pretreatment with the potent CYP450 3A4 inducer rifampin (600 mg once daily for 16 days) in healthy male volunteers, mean gefitinib systemic exposure (AUC) decreased by 83% compared to administration of gefitinib alone. The interaction has not been studied with other, less potent inducers.

MANAGEMENT: The potential for diminished pharmacologic effects of gefitinib should be considered during coadministration with CYP450 3A4 inducers. Pharmacologic response to gefitinib should be monitored more closely whenever a CYP450 3A4 inducer is added to or withdrawn from therapy, and the gefitinib dosage adjusted as necessary.