The purpose of the study is to determine if eniluracil/5-FU/leucovorin in metastatic breast cancer (MBC) may have efficacy and tolerability advantages over capecitabine monotherapy.

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