Trastuzumab and Letrozole in Treating Postmenopausal Women With Progressive Advanced Breast Cancer

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed breast cancer * Advanced disease * Measurable disease, defined as ≥ 1 unidimensionally measurable lesion outside previously irradiated areas that is ≥ 20 mm OR ≥ 10 mm if the slice thickness of the CT scan or MRI is ≤ 5 mm * No nonmeasurable lesions as the only site of measurable disease, including any of the following: * Osteoblastic bone metastases * Ascites * Pleural or pericardial effusions * Carcinomatous lymphangitis of the lung * Progressive disease after prior treatment with a nonsteroidal aromatase inhibitor (e.g., letrozole or anastrozole) in an adjuvant or advanced disease setting * HER-2 amplification ≥ 2 by fluorescence in situ hybridization * No clinical symptoms or history of CNS or leptomeningeal metastases (no imaging is required) * No visceral involvement with risk for organ dysfunction * Hormone receptor status: * Estrogen receptor- and/or progesterone receptor-positive tumor PATIENT CHARACTERISTICS: Age * 18 and over Sex * Female Menopausal status * Postmenopausal, defined by 1 of the following: * At least 55 years of age * Less than 55 years of age with spontaneous cessation of menses for ≥ 1 year * Less than 55 years of age with spontaneous cessation of menses within the past year, but amenorrheic with biochemical evidence of postmenopausal status * Underwent prior bilateral oophorectomy * Radiation or chemically induced menopause (treatment with luteinizing hormone-releasing hormone antagonists must continue during study treatment) Performance status * WHO 0-1 Life expectancy * Not specified Hematopoietic * WBC ≥ 3,000/mm\^3 * Platelet count ≥ 100,000/mm\^3 Hepatic * AST and ALT ≤ 2 times upper limit of normal Renal * Creatinine clearance \> 30 mL/min Cardiovascular * No uncontrolled cardiac disease, including any of the following: * Unstable angina * Arrhythmia * Hypertension * No history of congestive heart failure * No myocardial infarction within the past 6 months * LVEF \> 50% by echocardiogram Pulmonary * No severe dyspnea at rest Other * No other malignancy within the past 5 years except adequately treated carcinoma in situ of the cervix or localized nonmelanoma skin cancer * No psychiatric disability that would preclude study participation or giving informed consent * No active autoimmune disease * No uncontrolled diabetes * No other serious underlying medical condition that would preclude study participation PRIOR CONCURRENT THERAPY: Biologic therapy * No prior trastuzumab (Herceptin®) Chemotherapy * Prior neoadjuvant or adjuvant chemotherapy allowed * No prior palliative chemotherapy Endocrine therapy * See Disease Characteristics Radiotherapy * See Disease Characteristics Surgery * Not specified Other * More than 1 month since prior experimental drugs on another clinical trial * No concurrent drugs that contraindicate study treatment * No other concurrent anticancer drugs * No other concurrent investigational drugs