5-Fluorouracil Combined With CoFactor (5-10 Methylenetetrahydrofolate) in Treating Advanced Breast Cancer Patients

Inclusion Criteria: * Signed informed consent. * Women with a histologically/cytologically proven diagnosis of advanced adenocarcinoma of the breast. * Measurable disease. At least one unidimensionally measurable non-bony lesion with a diameter \>=10 mm using spiral CT scans (use of spiral CT must be documented in medical records and used consistently throughout the study) or \>= 20 mm using conventional CT or MRI scans outside the irradiated area according to RECIST criteria. * Patients having failed both prior anthracycline and taxane derivative chemotherapy regimens. Anthrocycline and Taxane failure definitions as defined in the protocol. * No more than two prior chemotherapy regimens for advanced disease. * Performance status (ECOG) \<= 2 or Karnofsky \>= 70 * Age \>= 18 years. * Life expectancy \>= 12 weeks. * Adequate organ function as shown by the following: 1. WBCs \>= 3.0 x 109/L, absolute neutrophil count (ANC)\>= 1.5 x 109/L, platelets \>= 100 x 109/L, hemoglobin \>= 9 g/dL 2. Bilirubin \<= 1.25 x the upper limit of normal (ULN),aspartate aminotransferase(AST or alanine aminotransferase(ALT),= 3 x ULN (or \<= 5 x ULN in case of liver metastases) 3. Serum calcium within normal limits 4. Serum albumin within the normal range for the study site 5. Creatinine clearance \>= 60 mL/min (Cockroft and Gault) 6. Left ventricular ejection fraction (LVEF) within normal limits as shown by echocardiography or scintigraphy (multiple-gated acquisition scan). * Patients of childbearing potential must be using medically acceptable contraception for 4 weeks before start of study treatment and 4 months after study treatment completion. Patients of childbearing potential must have a negative pregnancy test result within 7 days prior to study treatment initiation. * Patients with parenchymal brain metastases must be either adequately controlled following resection or completed appropriate radiation therapy and be receiving a stable corticosteroid dose prior to study entry. Patients with leptomeningeal involvement should not be included in the study. Exclusion Criteria: * Her2/neu positive tumor (2+ or 3+). * Pregnancy or lactation * Systemic cytotoxic anti-cancer therapy within \<=4 weeks of study entry, or 6 weeks if the systemic therapy contains a nitrosourea or mitomycin C. Hormonal anti-cancer treatment must be ongoing, or must have been discontinued \>3 months before study entry. * Prior 5-FU- and/or capecitabine-based palliative chemotherapy. * Extensive prior radiotherapy affecting more than 30% of bone marrow reserves, or bone marrow / stem cell transplantation. * Participation in clinical studies of non-approved experimental agents or procedures within \<=4 weeks of study entry. * History of other malignancy, unless cured and the patient has been disease-free for \>=2 years. Patients with a history of cervical carcinoma in situ or cured stage I cervical cancer, or of epidermal or basal cell skin cancer, may be eligible for enrollment after discussion with the sponsor. * Previous unexpected reaction to fluoropyrimidines, with or without documented deficiency of dihydropyrimidine dehydrogenase, or known hypersensitivity to 5-FU. * Psychologic, familial, sociologic or geographic conditions which do not permit compliance with the study protocol and/or study. * Significant cardiac disease, including symptomatic ventricular arrhythmia, congestive heart failure, myocardial infarction within 12 months before study entry. * Concomitant treatment with any experimental drug or anti-cancer drug, except hormone therapy or bisphosphonates.