Inclusion Criteria:

• Postmenopausal females >18 years old
• Has histologically or cytologically confirmed diagnosis of carcinoma of the breast.
• Has locally advanced or locally recurrent inoperable or metastatic breast carcinoma with documented disease progression. Note: Patients with a history of adequately treated central nervous system (CNS) metastasis in addition to other measurable disease are allowed to participate in the study.
• Has laboratory documentation of positive ER and/or PgR status, and has responded to the standard first or second line hormonal anti-tumor therapy given (at least stable on hormone after 6 months qualifies as response to prior hormonal therapy).
• Has received and whose disease progressed after one or two prior systemic hormonal anti-tumor therapies, excluding prior hormonal or combined chemo/hormonal therapy given in adjuvant or neo-adjuvant therapy; one regimen of prior single-agent or combination chemotherapy for advanced metastatic disease is allowed if this was given before the last hormonal therapy.
• Has performance status of greater than 2 on the Zubrod scale.
• Has a predicted life expectancy of greater than 12 weeks.
• Must give written informed consent as per institutional and federal regulatory requirements.
• Has measurable disease according to the RECIST criteria.
• Has an absolute granulocyte count of >1,500/microL, a platelet count >75,000/mL, and a hemoglobin of >10.0 g/dL.
• Must have adequate liver and renal function defined by a bilirubin of <1.5 times the upper limit of normal (ULN) and a creatinine of <1.5 times the ULN, respectively. Transaminases must be <2.5 times the ULN except for patients with liver metastasis who may have transaminases <5 times the ULN.
• Must have prothrombin time (PT) of <1.5 times the ULN.
• According to the judgment of the investigator, the patient has recovered from all previous anti-cancer treatment related toxicities to at least Grade 1 (except alopecia, pigmentation changes, and nail changes).
• All previous investigational drugs, regardless of indication, must be stopped at least four weeks before commencement of treatment with TAS-108.

Exclusion Criteria:

• Has untreated or symptomatic brain metastasis, extensive liver involvement, or lymphangitic lung metastasis.
• Has inflammatory breast cancer.
• Supplemental estrogen or progesterone within three weeks of start of the study.
• If the patient has had more than two prior hormonal anti-tumor therapies for metastatic disease or more than one prior chemotherapy for metastatic disease, the patient would not be eligible for this study (adjuvant or neoadjuvant chemotherapy does not count as prior chemotherapy for purposes of this study).
• If the patient's last (most recent) therapy received was chemotherapy, the patient would not be eligible for this study

• Has any other serious illness or medical condition(s) including, but not limited to, the following:

  • congestive heart failure or angina pectoris even if medically controlled; previous history of myocardial infarction within one year from study entry, uncontrolled hypertension or arrhythmias
  • active concurrent malignancy other than breast cancer. Patients with prior history of cancer will be allowed only if biopsy-proven breast cancer with ER/PgR positive at time of enrollment.
  • active infection
  • history of disseminated intravascular coagulation (DIC) or coagulation factor deficiency syndrome
  • unstable diabetes mellitus
  • interstitial pneumonitis
  • psychiatric disorder that may interfere with consent and/or protocol compliance
• Known hypersensitivity to any of the constituents of the study drug.