Trastuzumab Emtansine in Treating Patients With HER2-Positive Metastatic or Locally Advanced Breast Cancer That Cannot Be Removed by Surgery

PRIMARY OBJECTIVES: I. To assess change in thrombokinetics (platelet circulation life span). SECONDARY OBJECTIVES: I. Benefit rate (as defined by stable disease, partial response, or complete response by Response Evaluation Criteria in Solid Tumors \[RECIST\] v 1.1) at the end of study activities. II. To evaluate the safety of trastuzumab emtansine (ado-trastuzumab emtansine) (non-platelet toxicity). III. To evaluate the pharmacokinetics of ado-trastuzumab emtansine. OUTLINE: Patients receive trastuzumab emtansine intravenously (IV) over 30-90 minutes on day 1. Treatment repeats every 21 days for 3 courses in the absence of disease progression or unacceptable toxicity. Patients achieving response may continue treatment. After completion of study treatment, patients are followed up periodically.