Iressa Drug Interactions

In vitro studies have shown that the metabolism of gefitinib is predominantly via CYP3A4.
Co-administration with rifampicin (a known potent CYP3A4 inducer) in healthy volunteers reduced mean gefitinib AUC by 83% of that without rifampicin (see Precautions).
Co-administration with itraconazole (a CYP3A4 inhibitor) resulted in an 80% increase in the mean AUC of gefitinib in healthy volunteers. This increase may be clinically relevant since adverse experiences are related to dose and exposure. Although interaction studies with other CYP3A4 inhibitors have not been performed it is expected that drugs such as ketoconazole, clotrimazole, ritonovir would also inhibit gefitinib metabolism.
Co-administration of ranitidine at a dose that caused sustained elevations in gastric pH (≥ 5), resulted in a reduced mean gefitinib AUC by 47% in healthy volunteers (see Precautions and Pharmacology: Pharmacokinetics under Actions).
INR elevations and/or bleeding events have been reported in some patients taking warfarin (see Precautions).