• Safety and efficacy of letrozole 2.5 mg/day monotherapy as second-line endocrine therapy in postmenopausal patients with advanced breast cancer who received previous anti-estrogen treatment
• To investigate changes in blood drug concentrations and blood hormone kinetics.
• To investigate gene polymorphisms of CYP2A6, an enzyme involved in the metabolism of letrozole

Inclusion Criteria:

• Patients with histologically or cytologically documented breast cancer
• Patients with progressive breast cancer (advanced breast cancer, locoregional recurrence not operative, or metastatic breast cancer)
• Patients with hormone receptor (ER and/or PgR) positive or both unknown.
• Postmenopausal patients between ages 20 and 79 years, inclusive
• Patients with a history of adjuvant therapy or advanced breast cancer treated with anti-estrogens
• Patients with documented measurable or evaluable lesions.
• Patients with sufficient organ function to evaluate the safety
• Patients whose performance status (PS) is 0～2

Exclusion Criteria:

• Patients with diffuse lymphangitis carcinomatosa of the lung or CNS involvement, liver metastasis occupying more than one third of the liver, or inflammatory breast cancer

• Patients with other concurrent or previous malignant disease (excluding contralateral breast cancer, in situ carcinoma of cervix uteri, and adequately treated basal or squamous cell carcinoma of the skin)

• Patients in whom one of the following is the sole manifestation of disease: hilar enlargement, pleural effusion and ascites

• Patients with only blastic bone metastases or a mixed blastic and lytic bone metastases at the same site and no other measurable or evaluable lesions

• Patients with serious current disease such as uncontrolled cardiac diseases and/or uncontrolled diabetes mellitus by any medications (including a historical serious cardiac disease)

• Patients with adrenal insufficiency (treated or untreated) or Cushing's syndrome

• Patients with any of the following previous treatments

  1. Chemotherapy for metastatic and/or locoregional recurrent disease
  2. Previous adjuvant endocrine therapy other than ovariectomy, anti-estrogen treatment, LH-RH analogues or radiation castration
  3. Previous first-line endocrine therapy (e.g., aromatase inhibitors and gastagens) for the treatment of metastatic and/or locoregional recurrent breast cancer other than anti-estrogen or LH-RH analogues treatment
  4. Patients who have not recovered from toxicity caused by previous therapy
  5. For patients on investigational drugs, adequate wash-out periods of at least 7 days in the case of topical investigational drugs and at least 30 days in the case of systemic
  6. Previous bisphosphonate therapy started within 6 months without any other measurable or evaluable lesions
  7. Patients who have not stopped treatment with other anti-estrogen or anti-cancer drugs (other than bisphosphonates) before starting the trial medication

Other protocol-defined inclusion/exclusion criteria may apply.