This study is being carried out to see if treatment with ZD1839 (Gefitinib) combined with Arimidex (Anastrozole) has improved efficacy over Arimidex alone in preventing progression of metastatic breast cancer.

This is a phase II, multicenter, double-blind, randomized, placebo-controlled trial. Patients will be randomized in a double-blind manner in a 1:1 ratio to receive either 250 mg/day ZD1839 or matched placebo in combination with 1 mg/day anastrozole. The dose of ZD1839 has been selected based on clinical studies in other tumor types suggesting identical efficacy together with a better toxicity profile in patients treated at 250 mg compared to 500 mg/day. The 1 mg dose of anastrozole is the standard approved dose.

Inclusion Criteria:

• Female postmenopausal patients aged 18 years or older with newly diagnosed metastatic breast cancer.
• Patients with recurrent disease during or after adjuvant tamoxifen or patients who are hormone therapy naïve are eligible for this trial.
• A paraffin embedded tumor tissue block or slides from either the metastatic or primary tumor site is required.

Exclusion Criteria:

• Patients cannot be on hormone replacement therapy while on study.
• Prior chemotherapy received for metastatic disease is not allowed.
• Previous treatment with tyrosine kinase inhibitors or aromatase inhibitors is not allowed.
• Patients who have evidence of an active interstitial lung disease are not eligible.