Trastuzumab and Exemestane in Treating Postmenopausal Women With Metastatic or Locally Advanced Breast Cancer

RATIONALE: The monoclonal antibody trastuzumab can locate breast cancer cells that have HER2 on their surface and either kill them or deliver tumor-killing substances to them without harming normal cells. Estrogen can stimulate the growth of breast cancer cells. Hormone therapy using exemestane may fight breast cancer by reducing the production of estrogen. Combining trastuzumab with exemestane may be an effective treatment for breast cancer with high amounts of HER2. PURPOSE: Phase II trial to study the effectiveness of combining trastuzumab with exemestane in treating postmenopausal women who have metastatic or locally advanced HER2-positive breast cancer.