The purpose of this early Phase II multicenter trial is to determine the objective clinical response to GPX-100, an anthracycline similar to doxorubicin, in up to 40 patients with newly diagnosed metastatic breast cancer. GPX-100 is unique among anthracyclines because it is not converted to doxorubicinol during metabolism in the body. This metabolite has been shown to be a major cause of damage to the heart (cardiotoxicity) in laboratory studies. Eligible patients who are enrolled in this study will receive GPX-100 as a single agent at the beginning of as many as 8 three week long cycles of chemotherapy. Objective measurements of tumor response will be made by computed tomography (CT) scans.