The purpose of this study is to evaluate the efficacy and safety of tezacitabine when given alone or in combination with 5-fluorouracil (5-FU) to subjects who have advanced esophageal or gastric adenocarcinoma.

Inclusion Criteria:

• Males and females greater than or equal to 18 years of age.
• Histologically confirmed recurrent (inoperable) or metastatic adenocarcinoma of the esophagus, gastroesophageal junction, or stomach.
• At least one bidimensionally measurable lesion, not previously irradiated, with a diameter that meets RECIST criteria, that can be serially measured. Intraluminal tumors, evaluable only by endoscopy, are not acceptable as target lesions.
• Karnofsky Performance Score greater than or equal to 70%.
• Subjects must be in the second line therapy setting, thus they must have experienced progression following treatment with one (and only one) prior chemotherapy regimen used for the treatment of unresectable locally advanced, recurrent, or metastatic disease.
• Recovery from any serious (grade 3 or higher) toxic effects of prior radiation therapy.
• Adequate hematologic profile: absolute neutrophil count greater than or equal to 1,500/mm3; hemoglobin greater than or equal to 9 g/dL; hematocrit greater than or equal to 30% (transfusion allowed); and platelet count greater than or equal to 100,000/mm3.
• Adequate hepatic function: bilirubin less than or equal to 1.5 mg/dL; AST and ALT less that 2.5 X ULN (5 X ULN if liver involved with tumor); alkaline phosphatase less than 5 X ULN
• Adequate renal function; serum creatinine less than or equal to 1.5 mg/dL or calculated creatinine clearance greater than 50 mL/min.
• Both male and female subjects of childbearing potential must be using a contraceptive method that is medically acceptable to the investigative center.
• Disease-free from a prior malignancy, other than non-melanoma skin cancer or carcinoma in-situ of the cervix, for greater than 5 years.

Exclusion Criteria:

• Unstable angina or class III or IV New York Heart Association heart disease.
• CNS metastases.
• Pregnant or breast-feeding.
• Uncontrolled seizure disorder.
• Ongoing (grade 3 or higher) stomatitis, esophagopharyngitis, or uncontrolled diarrhea.
• Impaired nutritional status as evidenced by a serum albumin of 2.7 mg/dL or less.
• Major surgery, chemotherapy, immunotherapy, or radiotherapy during the 28 days preceding the first study treatment. Concomitant anticancer therapy (chemotherapy, immunotherapy, or radiation) or other investigational agents during study participation or 28 days prior to study participation.