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The metabolism of Gefitinib is via the cytochrome P450 isoenzyme CYP3A4 (predominantly) and via CYP2D6.
Active substances that may increase Gefitinib plasma concentrations: Substances that inhibit CYP3A4 may decrease the clearance of Gefitinib. Concomitant administration with potent inhibitors of CYP3A4 activity (e.g. ketoconazole, posaconazole, voriconazole, protease inhibitors, clarithromycin, telithromycin) may increase Gefitinib plasma concentrations. The increase may be clinically relevant since adverse reactions are related to dose and exposure. The increase might be higher in individual patients with CYP2D6 poor metaboliser genotype. In situations of concomitant treatment with potent inhibitors of CYP3A4 the patient should be closely monitored for Gefitinib adverse reactions. Concomitant treatment with an inhibitor of CYP2D6 might cause increased plasma concentrations of Gefitinib in CYP2D6 extensive metabolisers. If concomitant treatment with a potent CYP2D6 inhibitor is initiated, the patient should be closely monitored for adverse reactions.
Active substances that may reduce Gefitinib plasma concentrations: Substances that are inducers of CYP3A4 activity may increase metabolism and decrease Gefitinib plasma concentrations and thereby reduce the efficacy of Gefitinib. Concomitant medicinal products that induce CYP3A4 (e.g. phenytoin, carbamazepine, rifampicin, barbiturates or St John's wort/Hypericum perforatum), should be avoided. Substances that cause significant sustained elevation in gastric pH may reduce Gefitinib plasma concentrations and thereby reduce the efficacy of Gefitinib. High doses of short-acting antacids may have a similar effect if taken regularly close in time to administration of Gefitinib
Active substances that may have their plasma concentrations altered by Gefitinib: When the use of CYP2D6 substrates are considered in combination with Gefitinib, a dose modification of the CYP2D6 substrate should be considered especially for products with a narrow therapeutic window.
