A drug used to treat non-small cell lung cancer that has spread to other parts of the body and has not already been treated with other anticancer therapy. It is used in patients whose cancer has certain mutations (changes) in a gene called epidermal growth factor receptor (EGFR). It is also being studied in the treatment of other types of cancer. Iressa blocks certain proteins made by the mutated EGFR gene, which may help keep cancer cells from growing. It may also prevent the growth of new blood vessels that tumors need to grow. Iressa is a type of tyrosine kinase inhibitor and a type of antiangiogenesis agent. Also called gefitinib and ZD1839.