The purpose of this study is to examine the efficacy and safety as well as the characteristics of the female hormone and study medications after administration in pre-menopausal women with estrogen receptor positive advanced breast cancer who were randomised in a 1:1 ratio to either of the two treatment groups; the ZD9393 3.6 mg depot group or ZD9393 10.8 mg depot group, both given in combination with tamoxifen tablets.

Inclusion Criteria:

• Female ≥20 years and pre-menopausal.Pre-menopausal defined as 1) last menses within 1 year of randomisation, and 2) E2 ≥10 pg/mL and FSH ≤ 30 mIU/mL within 4 weeks of randomisation.
• Hormone sensitivity (ER positive) of primary or secondary tumour tissue.
• Histological/cytological confirmation of breast cancer and are candidates to receive hormonal therapy as therapy for advanced breast cancer.

Exclusion Criteria:

• Patients who have received tamoxifen or other hormonal therapies as adjuvant therapy for breast cancer within 24 weeks before randomisation and/or who have received prior treatment with hormonal therapies for advanced breast cancer
• Patients who have received LHRHa as adjuvant therapy for breast cancer within 48 weeks before randomisation
• Patients who have relapsed during adjuvant hormonal therapy or within 48 weeks after completion of adjuvant hormonal therapy and/or