 Certain cancers require the amino acid arginine. Arginine deiminase ADI is an enzyme from microbes that degrade arginine. ADI has been formulated with polyethylene glycol, and has been used to treat patients that have cancers that require arginine. In this study, ADI will be combined with the well known chemotherapy Doxorubicin and the safety and potential efficacy of this combination will be explored in patients with HER2 Negative Metastatic Breast Cancer. 