Study of Eniluracil + 5-Fluorouracil (5-FU) + Leucovorin Versus Capecitabine in Metastatic Breast Cancer

Inclusion Criteria: * Histologically or cytologically confirmed metastatic (Stage IV) adenocarcinoma of the breast * Prior exposure to anthracyclines either in the neoadjuvant/adjuvant setting, or as treatment for metastatic disease * Either evidence of a recurrence or development of metastatic disease at least 12 months after the last dose of a taxane as neoadjuvant/adjuvant therapy, or evidence of disease progression while receiving a taxane for metastatic disease * ECOG Performance Status of 0 or 1 * Measurable disease according to RECIST 1.1 Criteria * Adequate renal, hematologic, and hepatic function * Negative pregnancy test and willing to use effective contraception * Willing to avoid any other dose or form (iv, oral, or topical) of 5 FU or related derivatives for 8 weeks following the last dose of eniluracil * Willing to be closely monitored for changes in coagulation parameters (prothrombin time and/or international normalized ratio \[INR\] values) if receiving concomitant warfarin Exclusion Criteria: * Pregnant or lactating females * Prior treatment with capecitabine * More than one prior chemotherapy regimen for metastatic disease * Prior radiation must not have included ≥ 30% of major bone marrow-containing areas (pelvis, lumbar spine). If prior radiation was \< 30%, then a minimum interval of 6 weeks must be allowed between the last radiation treatment and administration of either study arm. * Currently receiving anti-cancer therapy * Residual ≥ Grade 2 clinically significant side effects (excluding alopecia) associated with prior radiotherapy, chemotherapy, and investigational treatments * Unstable CNS metastases. However, subjects that are asymptomatic and off systemic steroids and anticonvulsants for at least 3 months are not excluded. * Malabsorption syndrome, disease significantly affecting gastrointestinal function, resection of the stomach or small bowel, ulcerative colitis, recent history of GI bleeding or perforation * History of other malignancy, except subjects who have been disease-free for 5 years or subjects with a history of completely resected non-melanoma skin cancer or successfully treated in situ carcinoma * Concurrent disease or condition that would make the subject inappropriate for study participation, or any serious medical disorder that would interfere with the subject's safety * Known history or clinical evidence of leptomeningeal carcinomatosis * Active or uncontrolled infection * Dementia, altered mental status, or any psychiatric condition that would prohibit the understanding or rendering of informed consent * Known history of uncontrolled or symptomatic angina, arrhythmia or congestive heart failure * Concurrent treatment with an investigational agent * Use of an investigational drug within 30 days or 5 half-lives, whichever is longer, preceding the first dose of study medication * Taking phenytoin * Known immediate or delayed hypersensitivity reaction or idiosyncrasy to drugs chemically related to capecitabine, fluorouracil, leucovorin, or any excipients * Known dihydropyrimidine dehydrogenase (DPD) deficiency