iressa and Cerebyx Interactions

Interactions between iressa(gefitinib) and Cerebyx (fosphenytoin)

fosphenytoin and gefitinib (Moderate Drug-Drug)

ADJUST DOSE: Coadministration with inducers of CYP450 3A4 may decrease the plasma concentrations of gefitinib, which is primarily metabolized by the isoenzyme. According to the manufacturer, rifampin reduced the mean area under the plasma concentration-time curve of gefitinib by 85% in healthy male volunteers.

MANAGEMENT: Pharmacologic response to gefitinib should be monitored more closely whenever a CYP450 3A4 inducer is added to or withdrawn from therapy. In patients receiving potent inducers such as rifampin or phenytoin, gefitinib product labeling states that a dosage increase to 500 mg daily should be considered in the absence of severe adverse reactions, and additional dosage adjustments be made based on clinical response. Gefitinib dosage should be decreased accordingly if these agents are discontinued.